Review—Recent Advances and Challenges in Indium Gallium Nitride (InxGa1-xN) Materials for Solid State Lighting

Ravinder Kour; Sandeep Arya; Sonali Verma; Anoop Singh; Prerna Mahajan; Ajit Khosla

doi:10.1149/2.0292001JSS

Wide bandgap group III nitrides and their alloys are the most promising class of semiconductor materials for Solid State Lighting purposes and are studied extensively for their use in short wavelength optoelectronic devices for the visible spectrum. The binary semiconductor materials from group III-V like Aluminium Nitride (AlN), Gallium Nitride (GaN), Indium Nitride (InN), Gallium Arsenide (GaAs), ternary like Indium Gallium Nitride (In_xGa_1-xN), Aluminium Gallium Nitride (AlGaN), Indium Aluminium Nitride (InAlN) and quaternary like In_xAl_yGa_1-x-yN have been investigated for their potential for Solid State Lighting purposes. The ternary Indium Gallium Nitride (In_xGa_1-xN) is a group III-V semiconductor material composed of a mixture of x parts of Indium Nitride (InN) and (1-x) part of Gallium Nitride (GaN). It can have Wurtzite or Zinc blende structure. Wurtzite structure of Gallium Nitride (GaN) is thermodynamically more stable. It has a hexagonal close packing lattice with AB atomic repeating pattern. Also, since nitrogen is more electronegative than other group-V elements, therefore nitride semiconductors have a high degree of ionicity. The crystal structures with high degree of ionicity has tendency to go for wurtzite structure.¹ It can also occur in zinc blende structure in case of certain deposition conditions.² Zinc blende has a face centred structure with ABC atomic repeating pattern. The most significant feature of ternary wurtzite Indium Gallium Nitride (In_xGa_1-xN) alloy is the presence of direct bandgap that is one of the peculiar properties required for the efficient light absorption. Also, their energy bandgap can be engineered by varying the content of Indium in the alloy to cover the whole electromagnetic spectrum from visible region to the ultraviolet region.

Enabling Attributes of In_xGa_1-xN

Ternary Wurtzite Indium Gallium Nitride (In_xGa_1-xN) alloy is a group III-V semiconductor material that do possess a direct bandgap due to which direct interband transitions can occur in it without the help of phonons which are required for the conservation of linear momentum.² The films of In_xGa_1-xN grown by plasma enhanced evaporation in larger grain or nanocolumn sizes under specific deposition conditions have exhibited direct bandgap with very large absorption coefficients.³ It is the presence of direct bandgap that makes them suitable for use in various optoelectronic devices viz. Light emitting diodes, Laser diodes and for optical communication.
For Ternary In_xGa_1-xN alloy, the lattice constants follow the Vegard's law.
i.e. a(In_xGa_1-xN) = x a(InN) + (1-x) a(GaN).
In an ideal semiconductor, the absorption coefficient α(E), as a function of energy can be expressed as
where E_g(x) is bandgap of In_xGa_1-xN, E is energy of photons and α₀ for GaN is approx. 2 × 10⁵cm⁻¹. The In_xGa_1-xN alloys have large value of absorption coefficients³ and wide range of tunable bandgap. The α₀ for In_xGa_1-xN is assumed to be same as that of GaN.⁴
The bandgap of In_xGa_1-xN alloy is in the range of 0.7eV–3.4eV. This range covers 90% of the available photon energy of the visible spectrum. Therefore, In_xGa_1-xN alloy films have the ability to absorb 90% of the photons lying above bandgap within the first 500 nm thickness of the film.⁵
In_xGa_1-xN alloy has low effective mass of charge carriers due to which they do possess high electron mobility.⁶ They do possess high heat capacity, high saturation velocity,⁷ high thermal conductivity and large breakdown field. They do have high drift velocity, temperature and radiation resistance.^8–10
In_xGa_1-xN alloy is a hard semiconductor material that has got the ability to withstand higher doses of radiation while retaining its optoelectronic properties than other photovoltaic materials.⁸
The multijunction photovoltaic devices of In_xGa_1-xN alloy have enormous potential for research and development due to their large absorption coefficients, wide range of tunable bandgap, high saturation velocity and thermal conductivity, high temperature and radiation resistance. These attributes make them potential for use in high speed and high power devices. Due to high saturation velocity, they find application in lasers.
They have large spontaneous and piezoelectric polarization resulting in strong internal fields in the active region, that's why in order to decrease recombination, quantum wells are used. The variation of spontaneous polarization with temperature i.e. pyroelectric coefficients of nitrides is very low that makes these materials useful for high power and high temperature application.¹¹
In_xGa_1-xN alloys are inconsiderate to the presence of large dislocation densities as the strong internal fields due to the presence of spontaneous and piezoelectric may counter the effect of dislocations.^12,13
As a consequence of high value of absorption coefficients, even the thin light harvesting layer of In_xGa_1-xN alloy in photovoltaic devices is capable to harness the photons of solar radiation, thereby, reducing the cost of fabrication of the device and its size.

Growth Techniques

The semiconductor planar and nanostructures of In_xGa_1-xN based devices are fabricated by employing the various basic growth techniques which can synthesize the very sharp interfaces of one or few monolayers. The various epitaxial techniques are reported for the growth of these alloys. The High quality Indium rich Wurtzite In_xGa_1-xN films are grown on sapphire substrate by sputtering techniques,^14,15 molecular beam epitaxy (MBE),¹⁶ metal organic vapor phase epitaxy (MOVPE),^17–20 metal organic chemical vapor deposition (MOVCD),^21,22 hydride vapor phase epitaxy (HVPE),^23–26 high pressure chemical vapor deposition (CVD)²⁷ and aerosol assisted chemical vapor deposition (AACVD).²⁸ However, MOVPE and MOCVD are different nomenclature for the same growth technique. Many of these deposition techniques are difficult to achieve over a large area and are expensive as the sapphire substrate is used. Also, the substrates are to be chosen carefully as there can be a large mismatch in the lattice constants and thermal expansion coefficients for many common substrates.²⁹ Due to difference in lattice spacing between InN and GaN, it is difficult to deposit homogenously In_xGa_1-xN films on substrate.^30–32 The difference in lattice spacing results in phase segregation that deteriorates the crystalline and optical quality of films.^33–35 The films of Indium rich In_xGa_1-xN alloy are difficult to synthesize as there exists a solid phase miscibility gap between InN and GaN phases.³⁶ The various factors and conditions are taken into account while growing these materials. The techniques that are most commonly used to synthesize In_xGa_1-xN based planar and nanostructures based LEDs have been outlined.

Epitaxy

The conductivity in a crystalline material is controlled by the process of epitaxy. During epitaxial growth process, the layers of crystalline material are grown on a substrate. This technique can be used to grow the complicated heterostructures for In_xGa_1-xN based LEDs.

Molecular beam epitaxy (MBE)

It is a high vacuum technique in which a beam of atoms or molecules are incident on a substrate which is heated to grow the crystalline layer. The atoms react on the surface of the substrate at a growth pressure of around 10⁻¹⁰ torr; the material grows layer by layer in two dimensions. The atomic source of Nitrogen is provided by the plasma. Due to the presence of Nitrogen source, the nitride material can be grown at a very low temperature which further decreases the thermal stress. The growth temperature in MBE are much lower than that required for the MOCVD growth as InN is grown at a temperature of approx. 550°C³⁷ whereas GaN is grown at a temperature below approx. 800°C.³⁸ Also, there is a possibility that the level of growth to a single atomic layer is possible due to the maintenance of high vacuum conditions. However, it is difficult to control the substrate temperature during high growth temperature conditions where high thermal radiation losses can occur. Also, it is difficult to get uniform temperature across the substrate wafer due to the high radiation losses at its edges and due to the refractory material coated on the back of the substrate. This technique is employed to improve the interfaces in the Multiple Quantum Wells (MQWs).

However, due to low dissociation temperature and high vapor pressure of Nitrogen over InN, the Indium rich In_xGa_1-xN is difficult to grow under thermodynamic equilibrium. The Plasma Assisted MBE which involves the use of Nitrogen plasma source to dissociate the Nitrogen molecule is employed for the production of In_xGa_1-xN alloys with high Indium content.³⁷ In this technique, the growth rates are very low in comparison to the MOCVD growth process but the ultra high vacuum conditions facilitates the growth of semiconductor epitaxial layers with a high purity. As a consequence of ultrahigh vacuum environment for its growth process, various monitoring techniques such as reflection high energy electron diffraction (RHEED) and gas analysis process can be employed into the growth chamber. These monitoring growth techniques help in the characterization and optimization of the parameters of growth.

Sputtering

In this technique, the Nitrogen or Nitrogen-Argon mixtures are bombarded on the Ga target, moving both Ga and N to the substrate to cause GaN layer. It is a low cost, versatile method that can be employed for the scalable production.

Hydride vapor phase epitaxy (HVPE)

In this technique, hydrochloride in vapor form is passed over the molten Gallium leading to the formation of GaCl which further reacts with NH₃ at the substrate temperature leading to the formation of GaN, HCl and H₂ gas.

Metal organic chemical vapor deposition (MOCVD)

It is the most suitable technique for the growth of In_xGa_1-xN alloys as it ensures the high uniformity of grown layer, high growth rates, high flexibility of the growing material, the capability for large scale production, selective and in-situ growth. During MOCVD growth process, the metal alkyl and ammonia precursors are transported in vapor form to a heated substrate. The growth process is conducted at a high temperature because NH₃ is decomposed at its best above 1000°C.³⁷ The TMGa (CH₃)₃Ga, TEGa (C₂H₅)₃Ga, TMIn (CH₃)₃In and TMAl (CH₃)₃Al are the common precursors (molecules) used to introduce the metal into the MOCVD reactor. A carrier gas like H₂ or N₂ is used to transfer the precursors into the reactor as they have high vapor pressure at the room temperature. The NH₃ is employed as the Nitrogen source. Bicyclopentadienyl Magnesium Cp₂Mg is used to carry Mg which is a p-type dopant while Silane is used to carry Si which is a n-type dopant.

The growth for the In_xGa_1-xN LEDs is conducted in an Emcore MOCVD D-125 rotating disk reactor with a short jar configuration. The c-plane sapphire has been used as a substrate for the growth of GaN templates. Firstly, the Sapphire substrate is heated to 1100°C for 3–4 hours in Hydrogen rich atmosphere to enhance the surface quality. Then, NH₃ is flown into the reactor to cause nitridation. This is followed by a low temperature buffer GaN layer of thickness 20–40 nm grown at 550°C. The temperature is then again increased to 1030°C in NH₃ rich atmosphere to reduce the defects that can arise due to the lattice mismatch. The GaN layer is grown at 1050°C offering a high quality surface with very less number of dislocations per unit volume. The increase in the Indium concentration during growth of In_xGa_1-xN results in phase segregation, defect density and difficulty in achieving p-type doping. The formation of ohmic contacts has to be taken into account during the growth of the In_xGa_1-xN based LEDs. Almost whole of the color spectrum can be covered by using MOCVD technique by combining green and blue nitride LEDs with phosphor LEDs. The TEGa flow rate has a marked effect on the growth rate of In_xGa_1-xN epitaxy and composition of In_xGa_1-xN alloy. At high TEGa flow rates, the Indium composition decreases and a secondary phase separated domain is produced. The strain in the material changes to such an extent that it gives rise to the secondary In_xGa_1-xN phase. At a temperature of 720°C, TEGa flow rate of 30 sccm, highly crystalline In_xGa_1-xN layers with Indium composition of 7% are produced.

The growth of Indium rich In_xGa_1-xN requires low temperature due to the low dissociation temperature of InN which is approx. 550°C but at this low temperature, the rate of decomposition of NH₃ becomes very low and limits the MOCVD growth.³⁸ Also, the drawback of MOCVD growth technique is that it involves the use of hazardous materials such as NH₃, H₂ and Silane.

The polycrystalline thin films of In_xGa_1-xN can also be fabricated on a large scale by employing the aerosol assisted chemical vapor deposition (AACVD) technique in which aerosol droplets are used to transfer the precursors to the heated surface. It is an effective technique as there is no need for the precursor volatility and thermal stability.^39–41 This is the most simple, low cost technique as it permits control over the morphology of thin films grown and particle size. In this method, the solvent, the carrier gas flow rate, precursor concentration and aerosol droplet size can be varied very easily.^42–44

Issues related with growth of In_xGa_1-xN alloys

As a consequence of lattice mismatch between InN and GaN, phase separation and stress relaxation occurs during the growth of In_xGa_1-xN layers.⁴⁵ The phase separation leads to the origin of a high density of defects which are responsible for the non-radiative recombination in the material.⁴⁶ Also, the dependence of bandgap on the alloy composition is strongly influenced by the presence of stress.⁴⁷ There is a strong influence of strain on the band structure of In_xGa_1-xN based materials. The strained In_xGa_1-xN has lower Indium incorporation efficiency than the stress-free In_xGa_1-xN.⁴⁸ With an increase in Indium concentration; the quality of the crystalline In_xGa_1-xN material deteriorates. The InN decomposes at a low growth temperature of approx. 400–500°C than the GaN which strongly effects the In_xGa_1-xN alloy growth as the bond energy of In-N is smaller than the In-In and N-N bonds by 6 and 24 times respectively.⁴⁵ As a consequence of it, there is a formation of Indium droplets on the growth surface and Indium nanoparticles within the InN semiconductor. That's why there is a need to grow In_xGa_1-xN at a very low temperature but the low temperature causes insufficient ammonia cracking and subsequent degradation of the crystal quality.⁴⁹

Bandgap Engineering

Earlier it was established that the Wurtzite Indium Nitride (InN) has a bandgap of 1.9 eV.^15,50 Later, it was reported that InN has a bandgap of approx. 0.9eV in year 2002.^51–54 The study of bandgap of Wurtzite InN in the range x = 0.36 to x = 1 have proved the existence of a bandgap of 0.9 eV in these materials.¹ The recent scientific research has proved that InN exhibit a direct bandgap of 0.77 eV at room temperature^1,5,55–57 whereas GaN posses a bandgap of 3.4 eV at room temperature.^58–60 Therefore, the bandgap of ternary Wurtzite Indium Gallium Nitride (In_xGa_1-xN) alloys can be tuned from 0.77 eV to 3.4 eV by adjusting the composition of In and Ga within the alloy.^1,61,62 Due to this inherent property, these alloys have potential for use in photovoltaic devices.^63,64

The bandgap of In_xGa_1-xN can be calculated on the basis of Density Functional Theory (DFT) by applying first principle calculations.⁶⁵ The studies regarding optical properties of In_xGa_1-xN have demonstrated a strong dependence of the fundamental bandgap on the alloy composition. If x is varied between 0 and 0.63, a direct bandgap from 0.7eV to 1.9 eV is produced which matches to the part of solar spectrum used by the solar light harvesters such as Ge (0.66eV), GaAs (1.43 eV), InP (1.27eV) and GaInP (1.9eV) that depicts their potential as the light harvesters. The bandgap of In_xGa_1-xN decreases significantly from the GaN value of 3.4 eV to approx. 2.3 eV for In_0.4Ga_0.6N.^22,66–68

In many semiconductor alloys, a linear function using the lattice parameter of the semiconductor alloy at the end points may be used to determine the bandgap provided it is a function of the composition which is called as Vegard's law. However, the bandgap of most of the semiconductors does not vary linearly with composition and a deviation is observed which is known as bowing. To account for this deviation, a correction term known as bowing parameter b is added. Thus bowing parameter b is a significant factor for determining the bandgap of semiconductor alloys.^69,70 The composition dependence of bandgap of In_xGa_1-xN at room temperature can be determined by using the standard formula,⁷¹

where E_g is bandgap of InGaN, 3.42 is bandgap of GaN, 0.77 is bandgap of InN, 1.43 is the constant bowing parameter b, x is the concentration of Ga and 1-x is the concentration of In.

The bandgap as a function of composition plot is a well fit with a bowing parameter approx. 1.4 eV. The value of bowing parameter was large for the case in which a bandgap of approx. 1.9 eV was considered for InN, the lower energy end point. In this case, a bowing parameter of 2.63 eV is required to accommodate the composition dependence on the Ga rich side.^72,73

However, the quality of the optical material, its physical condition and composition greatly impacts the value of the bowing parameter. There exists a variation in the reported values of the bowing parameter which is attributed to the presence of strain in the layers of the optical material. The biaxial strain has a significant effect on the bowing parameter of In_xGa_1-xN alloy and its optical properties. A bowing parameter of 3.2 eV is reported for strained case of In_xGa_1-xN, alloy, when the epitaxial layers of In_xGa_1-xN alloy are pseudomorphically strained to the underlying GaN layers.

A large bowing parameter indicates the strong influence of alloy composition on the energy gaps in addition to the changes in lattice constants. As the concentration of InN in In_xGa_1-xN alloy increases, there exists a stokes shift between the energy positions for the samples as determined by PT and PL spectra. The recent studies have reported a strong bandgap bowing in the range of 2–5eV.^67,74–76

The bowing parameter of In_xGa_1-xN alloy is a function of composition of the alloy. If content of Indium in the alloy increases, it will decrease.⁷⁷ It ranges from approx. 1.5 eV for large Indium composition to over 5.0eV for small (x<0.1) compositions. For low Indium composition, a large bowing parameter 3.8–4.4eV is reported.^17,75

The bandgap bowing is dominated by the structural defect.⁷² The methods that are being employed for the preparation of the material and for measuring the bandgap energy have a significant effect on the value of the bowing parameter. The bandgap energy determined through Cathode luminescence (CL) OR Photoluminescence (PL) undergoes stokes shifting in contrast to the techniques like optical absorption, reflection or transmission. As a result, due to this shifting, the bandgap energy of a particular sample is overestimated.^{16,21,74,78,79}

The composition dependence of the indium rich In_xGa_1-xN bandgap is studied through electron probe micro analysis and Cathode luminescence (CL) spectroscopy. Also, the composition of In_xGa_1-xN alloy is studied by using X-Ray diffraction characterization technique. However, XRD can produce overestimated values and therefore, Rutherford backscattering is employed to specify the concentration of Indium in the sample.

The variation of bandgap of In_xGa_1-xN alloys as a function of Indium for the wurtzite and Zinc blende structures is shown in the Fig. 1.⁷³

**Figure 1.** Variation of bandgap of In_xGa_1-xN as a function of Indium for the wurtzite and Zinc blende structures.⁷³ (Copyright (2001) American Physical Society).

The studies have revealed that the peak energy of the emission band shows a difference for the samples grown by MBE and MOVPE when composition is varied in similar amount from 0 up to 0.4.⁸⁰ The increase in indium content in In_xGa_1-xN polycrystalline composite thin films have exhibited a shift in absorption onset to longer wavelengths. The bandgap values can be correlated to with the Indium content in the In_xGa_1-xN to get the calibrated values which can further help in getting a desired optical absorption edge in the resulting thin film after deposition.²⁸

The bandgap of In_xGa_1-xN alloy varies indirectly with Indium composition. The films have exhibited imperfect crystallinity and defect complexes with strong phonon coupling that has a significant effect on the emission process.⁸¹ However, the bandgap of In_xGa_1-xN polycrystalline composite thin films does not depend on its thickness.²⁸

The bandgap of In_xGa_1-xN alloy varies with variation in temperature. The luminescence peak undergoes redshift and blueshift alternatively to follow a phenomenon known as S-Shaped dependence, thus violating the Varshni's equation of semiconductors. The S-Shaped temperature dependence of luminescence peaks is attributed to the redistribution of carriers due to their transfer between different localised states and the escape of carriers from the higher energy states.⁸²

The effect of pressure variation on the emission and absorption spectra of In_xGa_1-xN alloys has revealed that when the concentration of Indium increases, the emission energy gets decreased and the pressure coefficient becomes very small becoming zero at approx. 2 eV.⁷³ The variation of the deformation potential and pressure coefficient of InxGa1_-xN as a function of Indium composition for the wurtzite and Zinc blende structures is shown in Fig. 2.⁷³

**Figure 2.** Variation of deformation potential and pressure coefficient of In_xGa_1-xN as a function of Indium for the wurtzite and Zinc blende structures. Reprinted with permission from Ref. 73 (Copyright (2001) American Physical Society).

Challenges

Although a lot of research has been carried out in development of In_xGa_1-xN bulk and nano devices and a vast number of optoelectronic devices based on these materials are devised with significant properties, but still there are some challenges that have to be overcome for efficient use of these materials for solar energy conversion and solid state lighting.

Phase segregation

The films of In_xGa_1-xN exhibits segregation of indium from them at the surface in small clusters within the material.⁸³ This happens due to (i) the presence of a solid phase miscibility gap³⁶ in the films which arises due to a lattice mismatch between GaN and InN⁸⁴ (ii) the difference in formation enthalpy between GaN and InN.⁸⁴ The phenomenon of phase segregation deteriorates the quality and compositional uniformity of the crystal.⁸⁵ The optical properties are influenced by the tendency of alloy for phase segregation and ordering. In the solid miscibility gap of In_xGa_1-xN, multiple phases are observed in photoluminescence (PL) spectra⁸⁵ and X-Ray diffraction material characterization.³⁶ It is quite challenging to grow Indium rich In_xGa_1-xN films due to (i) the high vapor pressure of InN in comparison to GaN leading to higher rate of evaporation of InN at the surface of growing film (ii) low thermodynamic stability of InN at higher temperatures. The only way to enhance the content of Indium in In_xGa_1-xN films is to maintain low temperature and pressure during the growth process. Higher growth rates also increase the concentration of Indium in the In_xGa_1-xN films. However, lower temperature and higher growth rates deteriorate the quality and compositional uniformity of the optical material. The Indium clusters formed due to phase segregation can act as recombination centres for excitons. Due to approx. 10% lattice mismatch between InN and GaN, the growth of Indium rich In_xGa_1-xN films results in relaxed layers with high structural defects which decrease the efficiency of the fabricated device by reducing the lifetime of the minority carriers. In triple junction solar cells, even a very low lattice mismatch of 0.01% can have a significant effect on the output efficiency.⁸⁶ The impacts of lattice mismatch can be reduced by using a thin GaN buffer layer between the substrate and the film which reduces the strain during growth and enhances the optical properties of In_xGa_1-xN films.^87–89 The AlN buffer layer has also been used to grow high quality n-type In_0.47Ga_0.53N/GaN films on Si (111) substrate by using radio frequency nitrogen plasma- Assisted Molecular Beam Epitaxy.⁸⁴ The synthesis of InGaN quantum wells by plasma-assisted MBE has exhibited the phenomenon of phase segregation of Indium during growth that have resulted into quantum wells with smooth interfaces but large width of bands, thereby, causing blue-shifted transition energies and poor quantum efficiency. The segregation of Indium can be alleviated by N-stable conditions but it results into rough interfaces.⁹⁰ However, phase segregation at nanometre scale can account for extraordinary light emission properties in In_xGa_1-xN alloys despite the high dislocation densities in the device's active layers.

Substrate

One of the major challenges which are hindering the progress of In_xGa_1-xN LEDs is the unavailability of the suitable substrate. The substrates that are being employed have large mismatches in lattice constant and thermal expansion coefficients. Lattice mismatch accounts for stacking faults, inversion domain boundaries and dislocations whereas the thermal expansion coefficients mismatch leads to cracks in epitaxial layer during cooling. The presence of these defects gives rise to non radiative recombination centres which lower the quantum efficiency of the device. The high density of defects decreases the mobility life time and the rate of diffusion of the charge carriers which lowers the quantum efficiency of the device. The presence of very high dislocation densities in the range 10⁷–10¹⁰cm⁻² effects the device applications and is a big challenge for the advancement of the technology in this field.^29,91–93 For the growth of a good quality optical material, the mismatch in lattice constant and thermal expansion coefficient between the substrate and the epilayers of the In_xGa_1-xN should be very small. Thus 6H-SiC is more suitable for the growth of In_xGa_1-xN films than the most commonly used substrate sapphire (Al₂O₃). However, 6H-SiC is more costly than Sapphire which is most favoured substrate for the growth of In_xGa_1-xN films as it possesses (i) hexagonal symmetry and exhibits stability at higher temperature upto1000°C (ii) a wide bandgap of about 9.9 eV that permits easily the photons of light from ultraviolet to near infrared spectrum allowing excellent optical transmission with a very scanty optical absorption⁹⁴ (iii) excellent resistance to abrasion (iv) excellent thermal shock properties (v) Mohs scale hardness of 9 in comparison to 10 for diamond⁹⁴ (vi) low cost (vii) easy availability for use. Wurtzite nitrides have been grown successfully on a, c, r, m-planes of Sapphire substrate,^95–99 MgO,^100,101 ZnO,⁹⁶ Si¹⁰² and SiC.⁹⁶ The Silicon is a potential substrate for growing In_xGa_1-xN films due to its availability in large sizes at a very low cost.¹⁰² The Si cut perpendicular to (111) direction is feasible for growing Wurtzite nitrides.^103,104 The Crystallographic data of various materials that can act as a substrate is shown in Table I.

Table I. Crystallographic data of various materials that can act as a substrate for In_xGa_1-xN films.

Material	Lattice Parameter a Å	Lattice Parameter c Å	c/a	Thermal Expansion Coefficient along a (10⁻⁶⁰ c⁻¹)	Thermal Expansion Coefficient along c (10⁻⁶⁰ c⁻¹)
GaN	3.189	5.185	1.626	5.6	3.2
InN	3.545	5.703	1.609	5.7	3.7
Sapphire	4.758	12.991	2.730	7.5	8.5
6H-SiC	3.081	15.092	4.898	4.2	4.7
Si(Cubic)	5.431			3.59
ZnO(Wurtzite)	3.25	5.206	1.601	4.75	2.9
MgO	4.216			10.5
GaAs(Cubic)	5.653			6

Doping

Intrinsic GaN is an n-type semiconductor due to the presence of nitrogen vacancies and impurities like Si, O which may be present during the epitaxial growth process that result in its n-type behavior.^105,106 Also, O along with N and Si along with Ga have low value of formation energies than the nitrogen vacancies which form a thin donor layer in GaN.¹⁰⁷ The nitrogen vacancies and the interstitial gallium are the potential candidates for the n-type behavior of bulk GaN samples because of their low formation energy. Nitrogen vacancy is more significantly present than the interstitial Gallium.¹⁰⁸ Due to highest electron affinity, InN has a tendency for n-type semiconductor.⁵⁰ Since the properties of binary GaN and InN effects the characteristics of In_xGa_1-xN, it also has n-type behavior. Thus p-type doping of In_xGa_1-xN has to be done to remove the n-type charge. Achieving high p-type behavior is very significant in order to form p-n junctions for use in solar cells with maximum open circuit voltage V_OC and low series resistance.¹⁰⁹ The p-type doping of In_xGa_1-xN is very difficult to achieve due to the presence of extremely high density of defects. The first successful evidence of p-type doping of InN is done by irradiating its films by 2 MeV He²⁺ ions.¹¹⁰ In_xGa_1-xN alloy has background n-type doping of 10¹⁶ cm⁻³ caused by the high density of defects in the material, therefore there is a scope to dope it further up to 10¹⁷ to 10²⁰ cm⁻³.¹⁰⁹ The samples of In_xGa_1-xN doped with Mg have exhibited p-type behavior upon annealing at high temperature up to Indium content above 50%.¹¹¹ The surface of In_xGa_1-xN films are resistant to the p-type doping to the extent that even if these films have reported bulk p-type conductivity up to x = 0.3 but still they display n-type Hall measurements due to high surface charge accumulation.¹⁰⁹ Mg is reported to be used as a successful dopant for p-type doping of GaN but hydrogen and nitrogen vacancies play a significant role in p-type doping of GaN. The H-atoms passivates the Mg atoms by forming neutral complexes with it that renders Mg atoms electrically inactive. Thus there is a need to break Mg-H bonds by annealing at high temperature in an inert gas atmosphere.¹¹² P-type doping is also required due to the phenomenon of Yellow Luminescence (YL). It arises from the defects inside grains and at low angle grain boundaries, impurities, and the presence of deep acceptor level caused by Ga vacancy.¹⁰⁷ YL is a defect induced transition in GaN centred around 2.2–2.3 eV that has low formation energy under n-type conditions, mainly reported in films or crystals grown by MBE, MOCVD, and HVPE.^113,114 YL deteriorates the efficiency of the optoelectronic devices. It can be suppressed by adding acceptors like Mg in GaN to make it p-type^113,114 or by reducing Ga vacancies by forming complexes with donor impurities.¹⁰⁷ As a consequence of large n-type behavior of In_xGa_1-xN, it is difficult to synthesize good quality contacts to p-type material. The poor concentration of acceptors in it leads to a very high contact resistance. To search for a particular material/metal that can make ohmic contact to p-type In_xGa_1-xN with high work function is very challenging. There has been a wide research to provide the low resistance ohmic contacts on the metals such as Ni,¹¹⁵ Au,¹¹⁵ Pt,¹¹⁶ Ni/Au,¹¹⁷ Pt/Au,^116,118 Cr/Au,¹¹⁷ Pd/Au,¹¹⁷ Pd/Pt/Au,¹¹⁶ Ni/Cr/Au,¹¹⁹ Ni/Pt/Au¹¹⁹ and Pt/Ni/Au.¹²⁰ Ni/ITO and ITO has also been investigated for p-type contacts.⁸³ The reported experiments provide low resistance ohmic contacts to p-type GaN by the application of surface treatment.¹²¹

Polarization

It has been theoretically calculated and experimentally proved that In_xGa_1-xN is highly polar due to non-centric symmetry of charges in wurtzite structure and large ionicity of covalent bonds. The polarization induced internal fields, potential barriers and band banding can significantly affect the efficiency of optoelectronic devices.¹²² The presence of polarization in the active light harvesting layer can decrease the charge carrier collection significantly.¹²³ The polarization is composed of spontaneous polarization and piezo-electric polarization and can be calculated experimentally without any information about the structure of the alloy.¹²⁴ The polarization of In_xGa_1-xN alloy depends non-linearly on the composition of alloy. The spontaneous polarization P_S of In_xGa_1-xN is related to its binary constituents InN and GaN by the relation

where b_InGa is the bowing parameter, and b_InGa= 2P(InN) + 2P(GaN) − 4P(In_0.5Ga_0.5N), when x = 0.5 for ternary alloys. If bowing parameter is known, the spontaneous polarization can be calculated at any composition of In_xGa_1-xN alloy.¹²⁵ The lattice mismatch and thermal expansion coefficients mismatch of substrate and the epitaxial layers of In_xGa_1-xN alloy also attribute to the presence of piezoelectric polarization.¹²⁶ The wurtzite nitrides are grown pseudomorphically and are under biaxial strain that causes a change in polarization leading to piezo-electric polarization. The piezo-electric polarization of In_xGa_1-xN alloy depends on the composition of the alloy. The piezo-electric polarization P_p of In_xGa_1-xN is related to its binary constituents InN and GaN by the relation

In the first approximation, the piezo-electric polarization P_p of In_xGa_1-xN is related to its binary constituents InN and GaN for symmetry conserving in plane and axial strains by the relation

The piezo-electric polarization P_p of In_xGa_1-xN is not a function of microscopic structure or structural parameters and depends on non-linear strain only.¹²⁵ The polarization can cause polar discontinuities at nitride hetero-interfaces where fixed charges occur leading to free charge accumulation.^127,128 The optical shifts in quantum wells have supported these findings experimentally.¹²² The designing of low dimensional nanostructures is significantly affected by the macroscopic polarization of wurtzite In_xGa_1-xN alloys.^128,129 The use of strained n-GaN window layer in can enhance the tunnelling of holes for p-type InGaN junction due to the presence of piezo-electrically induced sheet charge and strong band bending at the heterointerface. Thus the phenomenon of piezoelectricity, if taken into account while designing, can have a positive marked effect on the collection efficiency of p-i-n InGaN solar cells.¹³⁰

In_xGa_1-xN Based Solid State Devices

A lot of research has been carried out worldwide on the use of In_xGa_1-xN alloys in optoelectronic devices. The p-n junctions based on In_xGa_1-xN alloys are extensively investigated and reported.^{7,16,131–135} The In_xGa_1-xN Schottky devices^136,137 and p-i-n and p-n In_xGa_1-xN Solar devices,^{7,132,138–143} p-n junction In_xGa_1-xN^109,144 materials are reported. The blue, green and violet LEDs of In_xGa_1-xN are reported for solid state lighting industry.^{138,139,145–147} The In_xGa_1-xN alloys are used in Laser diodes.^148–151 The internal and external quantum efficiencies of greater than 70% and 60% of these devices are reported.¹⁰⁹

Issues with In_xGa_1-xN based LEDs

The In_xGa_1-xN alloys have exhibited efficient LED performance for the blue and violet LED emission. However, an efficient In_xGa_1-xN LED emitting green light has not been reported so far. This issue refers to Green Cap.^152,153 The green emission from In_xGa_1-xN requires high concentration of Indium in the alloy which reduces the crystalline quality of the In_xGa_1-xN crystals. Also, there is an enhancement in Auger recombination's leading to the issue of green cap at higher Indium concentrations.¹⁵³

Another issue with In_xGa_1-xN LEDs is the efficiency droop i.e. at higher current densities, the efficiency of the LEDs decreases. The high defect density, Auger recombination and heating effect at higher current densities are responsible for the droop in the efficiency of In_xGa_1-xN LEDs.^154–156

The clustering of Indium leads to the formation of localization centers in which the charge carriers get trapped due to their low bandgap which causes bright luminescence due to the localization of excitons for a time greater than the radiative lifetime.^157,158 However, the localization of excitons takes place away from the location of high density dislocations.¹⁵⁹ This localization due to the Indium clustering can enhance the Auger recombination which can further reduce the efficiency of LEDs.¹⁶⁰ Thus the presence of defects and carrier delocalization, polarization fields, Auger recombination, carrier leakage¹⁶¹ and poor hole transport^162,163 have an influence over the efficiency of the In_xGa_1-xN based LEDs.

In_xGa_1-xN Based Nanostructures for Solid State Lighting

The In_xGa_1-xN based nanostructures like p-n, p-i-n nanowires/nanorods/nanotubes,^7,164–169 quantum wells,^170–172 quantum dot¹⁷³ can play a pivotal role in comparison to the bulk semiconductors due to their extraordinary optical properties and electrical structures which can find immense applications in the development of optoelectronic devices at the nanolevel. The nanostructures of In_xGa_1-xN have displayed more light output efficiency than the bulk materials.^174,175 Nitride nanowires and nanorods are investigated due to their significant attributes as possibility of growth over two dissimilar substrates, almost zero dislocation density, strain alleviation through radial relaxation, enhanced optical absorption.^176,177 The nitride nanowires have surmounted the various challenges in Solid State Lighting (SSL) such as the green cap, efficiency droop and the quark-confined Stark effect induced degradation in the device.^173–180

The nano LEDs have exhibited excellent optical properties within the spectral range due to relaxation in strain,¹⁸¹ the easy growth of sample with higher Indium content and enhanced light harvesting capabilities.^182,183 The In_xGa_1-xN LEDs can produce a linearly polarized light by incorporating a dielectric layer with low refractive index between the multilayer nanogratings and a fluorescent ceramic.¹⁸⁴ The nanowires of In_xGa_1-xN have exhibited exceptional optical properties by overcoming the drop in efficiency when the concentration of indium in the sample is increased.^51,185

The nanowires do have higher aspect ratios and the free sidewalls of the nanowires allows the elastic relaxation of strained axial insertions in contrast to planar structures where strain has to be relieved plastically above a critical composition thickness.^186–188 The elastic strain release causes reduced piezoelectric fields, thereby, causing more electron hole overlapping which leads to more internal quantum efficiency.¹⁸⁹

There is a significant effect of doping in the nanowires of In_xGa_1-xN. The presence of donors has significant effect on the electronic properties of axial In_xGa_1-xN nanowire heterostructures.¹⁹⁰ The presence of localised trap states in nanowires of In_xGa_1-xN lead to a slower electron release rate that leads to a slower growth of SE signal. The numerical simulations have revealed that the doping of Si in the nanowires of In_xGa_1-xN by 4D S-UEM results in a faster decay of SE signal due to the scattering of secondary electron beam with an increased number of charge carriers.¹⁹¹ The near band edge shift occurs in Yb doped In_xGa_1-xN nanorods which depends upon the variation in content of Indium, strain relaxation and quantum confinement.¹⁹²

Growth Techniques for In_xGa_1-xN Based Nanostructures

The nanostructures of In_xGa_1-xN for use in LEDs can be synthesized by using Vapour-Solid-Liquid (VLS), Vapour-Solid-Solid (VSS) and selective Area Growth (SAG) processes.

VLS and VSS mechanisms

This method has been employed for the growth of nanowires. The VLS growth involves the use of a catalyst such as Au, Ni or Fe.¹⁹³ The metal alloys are deposited on the surface of substrate which is followed by the formation of liquid droplets on the surface of substrate. The crystal nucleation takes place in the liquid droplets which lead to the axial growth of nanowires. The length of nanowires can be determined by the rate of growth and time taken whereas the diameter depends upon the position and size of the metal particles, temperature and pressure.¹⁹⁴ The Silicon, GaAs and GaP can be used as a substrate for the growth of nanowires.¹⁹⁵ The Quartz tube furnance is employed for the VLS mechanism of growth of the nanowire structures¹⁹⁶ and in low pressure MOVPE technique.¹⁹⁵ However, for metal catalyst assisted nanowires, there is a chance that the metal catalyst may diffuse into the nanowires and can cause impurities. Generally, the metal catalyst remains on the top of nanowires after growth from where it can be easily removed.^197,198 The VLS growth mechanism involves the setting of temperature above the eutectic point of the catalysts. The metal droplets may get solidified if the growth temperature is less than the eutectic point of the catalysts and the said growth mechanism is known as VSS. The nitride nanowires synthesized by using VSS growth mechanism are reported.^199,200

SAG epitaxy

The nanowires can also be grown by using SAG epitaxy, in which the growth of nanowire takes place through masking of templates by the dielectric material. The SiO₂, Au, Ti and Al templates are employed for the SAG mechanism.²⁰¹ The growth of nanowires can be controlled by controlling the substrate temperature. The nanowires with desired structures can be grown by using the templates of different sizes and patterns. The high quality nanowires are grown by using the MOCVD technique with SAG mechanism. The a-axis or m-axis oriented GaN nanowires grown by using metal catalysts by employing VLS approach are reported.^202–204 The c-axis oriented nanowires can be grown catalyst free via selective area growth^205–207 or they can be grown by etching from the standard c-plane GaN epilayers by using top-down approaches.^208–210 This makes it more feasible for control over arrangement, yield, diameter and alignment. The hexagonal c-axis oriented nanowires have exhibited a symmetric, uniform geometry in comparison to the a-axis or m-axis oriented nanowires.²¹¹ These distinct sidewall facets have different polarization and structural properties and different In_xGa_1-xN growth rates which obstructs in the development of devices from triangular a-axis or m-axis oriented GaN nanowires.²¹¹ It is difficult to synthesize In_xGa_1-xN/GaN heterostructures with higher Indium content due to the tendency for the phase segregation and lattice mismatch between InN and GaN which decreases the growth of In_xGa_1-xN layers on GaN with a high Indium content. That's why GaN nanowires with axial In_xGa_1-xN insertions are used for RGB LEDs. The GaN nanowires with In_xGa_1-xN insertions are synthesized by employing MBE and MOCVD techniques.^212–219 The GaN nanowires can be grown on any substrate without any degradation in their crystal quality,^220,221 substrate bending or dislocations.²²²

The nanowire In_xGa_1-xN LEDs can also be fabricated by making use of top-down and bottom-up fabrication methods. In the top-down approach, etching is used to fabricate the nanowire structures. The etching of as-grown planar heterostructures by employing focused ion-beam, wet, chemical, induced couple plasma or reactive ion beam etching has been investigated and reported.^223–225 The etch defects are mitigated by using ex-situ annealing process.²²⁶ By employing dry and wet etching processes, In_xGa_1-xN nanoLEDs are reported with enhanced PL intensity in comparison to the planar LED heterostructures.²²⁷

The top-down fabrication technique is used to fabricate the nanowire LEDs with a high output power approx. 6.8 W/cm² at an injection current of 32A/cm² and External Quantum Efficiency of approx. 16.6%.²²⁸ However, the performance of the device synthesized by this method is limited due to the presence of dislocations and etches induced defects on the lateral surfaces of the nanowires. The bottom up approach method involves the techniques such as the direct reaction of Ga or In metals with NH₃,²²⁹ CVD,²³⁰ MBE²³¹ and HVPE.⁶³ The bottom up nanowire LEDs include core-shell nanowire,²³² dot-in-a-wire heterostructures.²³³

The single crystalline nanowires of In_xGa_1-xN (0≤x≤1) have also been synthesized by employing low temperature halide chemical vapor deposition.⁶² An extensive research on the suitable substrate and contact for InGaN/GaN nanowires has been in progress. In order to reduce substrate absorption and heat dissipation, the InGaN/GaN nanowire LEDs can be first grown on Si (100) and, later can be transferred to the copper substrate.²³⁴ The optical transparency of rough surface of sapphire can be enhanced by coating it by a glass layer through vitrification process resulting in a high transparency maintained even after annealing up to 1000°C at UV-Visible-IR regions.²³⁵ It is reported that ITO is a better contact for InGaN nanowires than Ni or Au leading to better electrical properties and external quantum efficiency.²³⁶ The PL emission and specially resolved CL measurement on InGaN nanostructures grown by plasma-assisted-MBE shows that the percentage of Indium incorporation depends upon the crystal plane considered.²³⁷ The SEM pictures of InGaN nanocrystals are shown in Fig. 3.²³⁷

**Figure 3.** SEM images of InGaN nanocrystals in top, crosssection and emission color. Reprinted with permission from Ref. 237 (Copyright (2013) Applied Physics Letters).

The nanowire LEDs make feasible for more extraction of light due to their light guiding features.²³⁸ The MBE technique is used for the growth of catalyst free GaN nanowires²³⁹ followed by the addition of InGaN/GaN heterostructures promising better optical properties for the InGaN based LEDs.^215,240 The high quality homogenous c-axis GaN wires are obtained by using the MOVPE technique.²⁴¹ The piezoelectric fields can be reduced in c-axis longitudinal super lattices due to the relaxation in strain.²⁴² This causes reduction in QCSE in piezoelectric polarization along the polar c-axis growth direction.

The c-axis oriented hexagonal core-shell nanowires of GaN with InGaN MQWs are investigated for use in LEDs.^243–252 The InGaN/GaN QWs grown on non polar m-plane sidewalls of the c-axis oriented hexagonal nanowires eliminate the quantum confined stark effect resulting in higher quantum efficiency and thermal stability²⁵³ which in turn gives rise to enhanced overlap of electron and hole wave function leading to a larger momentum matrix element for the non-polar QWs.²⁵⁴ The synthesis of c-longitudinal InGaN MQWs in nanowires is reported by using MOHVPE without the use of catalyst to obtain nanowire based MQWLEDs.¹⁷⁵

The InGaN/GaN MQW structures are grown by using epitaxial technique MOCVD. The TMGa, TMIn, NH₃ and SiH₄ (Silane) are employed as a source of Ga, In, N and Si respectively. The growth process involves the heating of substrate under NH₃ followed by the growth of Si-doped n-GaN layer at 1100°C. The temperature is further reduced to 697–777°C to grow InGaN/GaN MQWs in the active layer. The InGaN QWs as well as GaN cap layer were grown at the same temperature of 697°C.²⁵⁵ The MOCVD,^256,257 MBE²⁵⁸ growth techniques are employed for the fabrication of InGaN MQW heterostructures.

The InGaN Quantum Dots (QDs) with Indium content more than 0.05 are fabricated by employing deposition on GaN buffer layer by Stranski-Krastanov growth method. A homogenous wetting layer is formed with a thickness of 1–2 nm which is followed by the formation of InGaN clusters localized on the surface dislocation.^259–261 The content of Indium within QDs can be varied by controlling the amount of raw material in the reactor and the temperature. At low temperature, the content of Indium within the alloy saturates whereas at temperature above 700°C, the content of Indium is not incorporated fully within the crystalline structure.^261,262

Recent Advances in In_xGa_1-xN Based Nanostructures for Solid State Lighting

In_xGa_1-xN nanocolumn LEDs fabricated by MBE technique emitting light in blue, green and yellow spectral range is reported.²⁶³ A highly directional yellow (572nm) beam with a radiation angle of ±20⁰has been produced by them.²⁶⁴ The structurally graded In_xGa_1-xN nanocolumnar photonic crystals have exhibited a broad multi wavelength lasing spectrum with more than 10 peaks with a full width at half maximum of 27 nm at 505 wavelength as well as lowering of polarization angle.²⁶⁵ There is an effect of substrate temperature on In_xGa_1-xN nanocolumnar crystal structures deposited by using plasma-enhanced evaporation on SiO₂. The more crystalline, thinner and closely packed nanoclumnar structures are possible at low Indium composition and at low substrate temperature.²⁶⁶ The white light is produced by In_xGa_1-xN based high density nanocrystal array with umbrella shaped crystals of diameter approx. 200–700 nm.²⁶⁷ The selective area growth of GaN nanocolumn with InGaN insertions results in decrease in high quality stacking faults and a sharp luminescent emission at 3.473 pointing to a high quality strain free material.²⁶⁸ The SEM pictures of InGaN nanostructures and RT-CL spectra of a single nanostructure and of an ensemble of around 2000 InGaN nanostructures is shown in the Fig. 4.²⁶⁸

**Figure 4.** SEM images of InGaN nanostructures in (a) top view (b) cross-sectional view (c) InGaN- related emissions (d) InGaN- related emissions (e) RT-CL spectra of a single nanostructure and of an ensemble of around 2000 InGaN nanostructures. Reprinted with permission from Ref. 268 (Copyright (2013) Applied Physics Letters).

The transition metal dichalcogenides (TMDCs) are investigated for the device design and applications in nanowire LEDs of In_xGa_1-xN. The first high power drop free nanowire LEDs on large layered transition metal dichalcogenides (TMDCs)/metal substrates are reported with a light output power upto1.5mW, emitting light beyond the green cap without efficiency droop.²⁶⁹

The geometry of nanowire in InGaN LEDs has an effect on the charge carrier transport. The studies have revealed that the hole transport is a function of nanowire geometry.²⁷⁰ The light harvesting capability of the nanowire can be enhanced by the proper tuning of its diameter in comparison to planar LEDs.²⁷¹ In In_xGa_1-xN/GaN nanowires, a significant reduction in built in electrostatic potential is reported for sufficiently thin nanowires in comparison to planar hetero structures of the same composition of the Indium, thickness and orientation.²⁷² In order to study the size at the nanometric scale by electron tomography, it is possible to separate a single nanowire from the Silicon substrate displaying the same thickness in all directions normal to the tilt axis, allowing a sample tilt of ± 90⁰.²⁷³ The growth of nanorods with Silane flux has reported improvement in their properties by increasing edge emission and suppressing yellow luminescence.²⁷⁴

The 3D strain has a significant effect on the emission properties of LEDs based on the N-polar (In, Ga)N/GaN nanowires. The electroluminescence spectra exhibit a double-peak structure due to the presence of a stronger quantum-confined Stark effect occurring in the first and last Quantum Well of the N-polar heterostructures. The relative intensity of two peaks with injected current arises due to the presence of 3D strain variation that results from the elastic relaxation at the free sidewalls of the nanowires.²⁷⁵

The non-radiative surface recombination in the optoelectronic devices lead to droop in the efficiency. The number of pathways for non-radiative recombination has been reduced in InGaN/GaN nanorod LEDs with suspended Graphene transparent electrodes after alumina passivation.²⁷⁶ The non-radiative surface recombination can be removed during the growth of AlGaN barrier in InGaN/ AlGaN dot-in-a-wire core shell nanowire arrays due to the formation of a large bandgap shell on the side of the nanowire. The nano LEDs based on mesoscopic In_0.9Ga_0.1N structure in a vertical device layout has exhibited high frequency operation within the telecommunication wavelength range without undergoing any degradation for long term use.²⁷⁷

The effect of various types of strain and concentration of Indium in the optoelectronic properties of GaN nanowires with embedded In_xGa_1-xN nanodisks is investigated and reported.²⁷⁸ The Q disks within the nanowires are the regions with excellent tenability due to the absence of defects and high potential to tolerate the strain.²⁷⁹ The Q disks within In_xGa_1-xN nanowires have displayed 62% internal quantum efficiencies.²⁸⁰ The carrier and trapping processes are studied extensively in In_xGa_1-xN quantum disks in order to enhance their efficiencies.²⁸¹ The stress has an influence on the optical transitions in GaN nanorods containing a single InGaN quantum disk. The optical transitions depend upon the contribution of deformation potential and the quantum-confined Stark effect to the transition energy when the pseudomorphic strain is varied radially.²⁸²

The InGaN/AlGaN dot-in-a-wire heterostructure LEDs can produce color rendering complex values of 94–98 in warm and cool white regions.²⁸³ There is an increase in the strong emission in InGaN/GaN dot-in-a-wire heterostructures on exposure to flowing out glow of a nitrogen MW plasma because of the passivation of grown-in defects and annealing due to the presence of plasma-generated N atoms.²⁸⁴ The p-type doping in InGaN dot-in-a-wire LEDs can enhance their Internal Quantum Efficiency to approx. 56.8% as a consequence of 3D carrier confinement provided by the electronically coupled dot-in-a-wire heterostructures.²⁸⁵ The improvement in carrier injection efficiency in p-type doped nanowires are reported as a consequence of surface band bending and in InGaN dot-in-a-wire Core-Shell nanoscale heterostructure attributed to the superior carrier confinement as a result of large bandgap AlGaN shell.²³³

A lot of research is being carried out in InGaN Quantum Wells which consist of a well within two barrier layers and the charge carriers are confined to move within the well. A variety of defects lead to poor crystalline quality in InGaN Quantum Wells (QWs). Due to recombination of localized states, the highly effective light emission can happen in InGaN QWs.^286,287 The defects, dislocations, strain and other effects present in the material gives rise to non-radiative recombination centres.^288–291 The polarization fields in QWs grown on c-plane substrates separates the wave functions of electron and holes. As a result, the rate of radiative recombination gets slowed down leading to efficiency droop.^189,292,293 Since in the nanostructures, the polarization field is significantly relaxed, the nanostructured quantum wells are studied to enhance the performance of LEDs.^294–296 The non-polar quantum wells have reported lower transparency carrier density than the c-plane QWs²⁹⁷ due to which the optical gain for the non polar InGaN QWs is larger than for polar c-plane QW at a given charge carrier density.²⁹⁷

The strain can have a major effect on the optical properties in InGaN QWs. The formation of a nanorod structure is introduced to reduce the strain due to free surface presented by the sidewalls enabling the lattice to relax. The efficiency drop caused by density activated defect recombination can be suppressed in nanorod structures of a-plane InGaN QWs by an effect wherein the lateral carrier confinement overcomes the increased surface trapping that comes into existence during fabrication of the material.^180,298

The effects of variation in the composition in InGaN QWs on their spontaneous emission properties lead to the variation in optical matrix elements and the emission energy.¹⁷⁰ The behavior of ground state transition momentum matrix elements with Indium content is found to correlate well with experimentally extracted spontaneous recombination parameters in InGaN QWs. The composition fluctuations cause changes in the strength of optical transitions.¹⁷⁰

The multiple quantum well LEDs (MQWLEDs) consist of p-i-n structures in which a quantum well layer is inserted between the two barrier layers, therefore, the whole intrinsic layer comprises of alternate quantum well and barrier layers. The charge carriers are confined to move in a well in MQWLEDs. The width and the difference of bandgap of quantum well and barrier determines the number of charge carriers confined within the well. The confinement of charge carriers within the well is subject to the obeyance of de-Broglie equation i.e. the width of the quantum well should be comparable to the wavelength of the confined charge carriers. The MQWs are employed to enhance the open circuit voltage V_oc, Short circuit current density J_sc and photovoltaic efficiency of the device. If the number of InGaN MQWs is increased by two times, the peak external quantum efficiency gets doubled for Indium concentration at 0.10 and 0.19 in InGaN MQWs grown on sapphire.¹⁷²

Recently, it is investigated to grow semiconductor materials on the biaxially textured substrates having low cost. The biaxial GaN films on biaxial Mo and W buffer layers are used to grow InGaN MQWs. The nanostructured metal buffer layer can act as a metal contact and also as a biaxial seed layer.²⁹⁹ The substrate can be coated with high performance broadband optical coatings in InGaN MQWs in order to improve the light extraction capability.¹⁶⁴ The increase in thickness of GaN cap layer has an influence on the performance of the InGaN MQWs. The InGaN MQWs are reported which is grown by a two step GaN barrier growth methodology by growing a lower temperature GaN cap layer on the top of the quantum well followed by higher temperature barrier layers of GaN. The increase in the thickness of low temperature GaN cap layer enhanced the performance of the solar cell as a result of improvement in uniformity and larger Indium content of the quantum well.³⁰⁰ The cross sectional architecture of the epitaxial structure of 10X InGaN MQWs and its PL spectra as a function of GaN Cap layer thickness is shown in the Fig. 5.³⁰⁰

**Figure 5.** (a) cross sectional architecture of the epitaxial structure of 10X InGaN MQWs (b) PL spectra as a function of GaN Cap layer thickness. Reprinted with permission from Ref. 300 (Copyright (2012) Applied Physics letters).

A GaN based structure consisting of 5-period InGaN MQWs active layers for light emission in UV region is reported which can be used in medical field applications. The M-plane core-shell InGaN MQWs on GaN wires are reported for the electroluminescent devices.³⁰¹

The non porous InGaN MQWs are reported with high thermal activation energy than that of as grown sample and less non-radiative recombination caused by defects and dislocations.³⁰² In InGaN QWs, the photo excited electrons and holes are strongly bounded by the excitons as a consequence of their large exciton binding energy whereas free electron-hole recombination is dominant in nanorods due to its efficient exciton dissociation. The sidewalls of the nanorods provide a pathway for exciton dissociation, thereby, enhancing the optical absorption in InGaN MQWs.²⁴⁴ The non-polar InGaN Core shell nanowires have exhibited Auger constants two orders of magnitude less than for the planar c-plane MQWs³⁰³ which helps to reduce the efficiency droop in the LEDs. The doping of layers of InGaN/GaN MQWs can influence its luminescence properties. There is an influence of Si doping in modifying the tilt of the band structure. The n-type doping of the buffer layer increases the light emission properties of InGaN/GaN MQWs.³⁰⁴

The optical sensor systems employing InGaN/GaN nanowire heterostructures are investigated that emit green light.³⁰⁵ The use of In_xGa_1-xN alloys for Semiconductor single nanowire lasers is significant as they are compact in size, low power requirement, low threshold potential which makes them suitable for large scale optical integrated circuits, detectors, high speed communications and optical probes.^305–308 The p-i-n junction nanowire LED's with axial InGaN QWs have exhibited the phenomenon of spontaneous emission.^{175,215,240,286,309} Recently, Non-polar p-i-n InGaN/GaN MQW core-shell single-nanowire lasers have exhibited the lasing action through the optical pumping at room temperature.³¹⁰ The p-i-n InGaN/GaN MQW core-shell-nanowire lasers at room temperature are investigated and reported that work by using optical pumping.³¹¹

Conclusions

Indium Gallium Nitride (In_xGa_1-xN) alloy is a group III-V semiconductor material that do possess a direct bandgap with very large absorption coefficients, wide range of bandgap from 0.7–3.4 eV, high mobility of charge carriers, high saturation velocity, high thermal conductivity and temperature and radiation resistance that make them suitable candidate for use in PV devices for energy conversion. The very low variation of spontaneous polarization with temperature makes them potential for use in ultrafast devices. The bandgap of In_xGa_1-xN alloys can be engineered from 0.77 eV to 3.4 eV by adjusting the composition of In and Ga within the alloy to meet the specific requirement of the device. The bowing parameter b is significant factor for determining the bandgap of In_xGa_1-xN alloys and is a function of optical quality of the material, its physical condition and composition. It has not been possible to obtain good optical quality films at large Indium in In_xGa_1-xN alloys due to the presence of high structural defects. The temperature and pressure has a significant effect on the width of bandgap in In_xGa_1-xN alloys. The High quality Indium rich Wurtzite In_xGa_1-xN films are grown on sapphire substrate by sputtering techniques, MBE, MOVPE, HVPE, CVD and AACVD. The lattice mismatch and the difference in formation enthalpy between GaN and InN gives rise to phenomenon of phase segregation that deteriorates its quality and compositional uniformity. The use of a thin buffer layer of AlN between the substrate and film reduces the strain and enhances the optical properties of In_xGa_1-xN films. So far, Sapphire is used as a substrate for In_xGa_1-xN alloys due to its excellent physical and optical properties but 6H-SiC is a better substrate than it. The presence of nitrogen vacancies and interstitial gallium in In_xGa_1-xN alloys accounts for its n-type behavior due to their low formation energy. The n-type behavior can be changed into p-type by doping of Mg in them. It is difficult to get good quality contacts to p-type material in In_xGa_1-xN alloys. The impacts of polarization can lead to free charge accumulation on the surface of In_xGa_1-xN alloys and can reduce their efficiencies. The In_xGa_1-xN alloys are successfully employed in nanostructures. In nanostructures, the polarization field is significantly relaxed, so they are used to enhance the properties of LEDs. The In_xGa_1-xN nanocolumnar LEDs fabricated by MBE emit blue, green and yellow light. The more crystalline, thinner and closely packed nanoclumnar structures are possible at low Indium composition and at low substrate temperature. The substrate transfer is employed to enhance the efficiency of the In_xGa_1-xN alloys by reducing losses. The number of pathways for the non-radiative recombination can be reduced in InGaN nanorod LEDs with suspended Graphene transparent electrodes after alumina passivation.

The p-i-n junction nanowire LED's with axial InGaN quantum wells have exhibited the phenomenon of spontaneous emission. Recently, Non-polar p-i-n InGaN/GaN multi-quantum well core-shell single-nanowire lasers have exhibited the lasing action through the optical pumping at room temperature. The optical sensor systems employing InGaN/GaN nanowire heterostructures are investigated that emit green light. The use of InxGa1_-xN alloys for Semiconductor single nanowire lasers is significant as they are compact in size, low power requirement, low threshold potential which makes them suitable for large scale optical integrated circuits, detectors, high speed communications and optical probes.

Future Scope

Although a lot of scientific research is carried out on the study of wide bandgap group-III In_xGa_1-xN alloys with regard to their inherent properties that has led to the development of vast number of devices for solid state lighting purposes, but still there is a scope for future research work in this exciting class of materials. The future scientific research for In_xGa_1-xN alloys has to be focused on

To achieve excellent solid state behavior for these materials at higher Indium.
To curtail the dependence of properties of In_xGa_1-xN alloys due to the presence of stress, strain, defects or dislocations within the optical material.
To obtain excellent p-type conductivity at higher Indium content in the alloy.
To look for more efficient dopants for p-type doping and develop effective means of doping in nanostructures of these materials.
To develop better ohmic contacts for p-type In_xGa_1-xN alloys.
To develop new techniques of fabrication of In_xGa_1-xN based LEDs so as to reduce the presence of defects, dislocations or other crystal inhomogenities.
To look for a better substrate that can overcome the challenges due to lattice mismatch and thermal conductivity mismatch with excellent optical absorption.
To introduce new ways so that the polarization can have a positive effect on the charge collection efficiency in In_xGa_1-xN based LEDs.
To look for means so as to reduce efficiency droop, Auger recombination, on-radiative surface recombination, thereby, enhancing the efficiency of In_xGa_1-xN based LEDs.
To optimize new designs and features in MQWs by varying the number of QWs, Indium composition and the thickness of well and barrier to achieve more efficiency in In_xGa_1-xN based nanoLEDs.

ORCID

Sandeep Arya 0000-0001-5059-0609

Ajit Khosla 0000-0002-2803-8532

Review—Recent Advances and Challenges in Indium Gallium Nitride (In_xGa_1-xN) Materials for Solid State Lighting

Article metrics

Author affiliations

Author notes

ORCID iDs

Dates

Abstract

Enabling Attributes of In_xGa_1-xN