Improved InGaN/GaN light-emitting diodes with a p-GaN/n-GaN/p-GaN/n-GaN/p-GaN current-spreading layer

: This work reports both experimental and theoretical studies on the InGaN/GaN light-emitting diodes (LEDs) with optical output power and external quantum efficiency (EQE) levels substantially enhanced by incorporating p-GaN/n-GaN/p-GaN/n-GaN/p-GaN (PNPNP-GaN) current spreading layers in p-GaN. Each thin n-GaN layer sandwiched in the PNPNP-GaN structure is completely depleted due to the built-in electric field in the PNPNP-GaN junctions, and the ionized donors in these n-GaN layers serve as the hole spreaders. As a result, the electrical performance of the proposed device is improved and the optical output power and EQE are enhanced.

Meanwhile, besides the technical issues in charge separation, carrier injection/confinement efficiency and the crystal quality of the QWs, the current crowding at the p-contact is also regarded as one of the main obstacles [17][18][19], especially for conventional devices grown on insulating substrates (e.g., sapphire) [20].The driven current tends to flow through the path with smaller resistivity, resulting in current crowding at the pcontact of these InGaN/GaN LEDs.The current crowding effect generates highly localized carrier density and heat underneath the p-electrode and thus leads to a non-uniform light emission of the InGaN/GaN LEDs [21].In order to improve the current spreading, a resistive layer is generally added into the LED architecture [22].For example, Tsai et.al. has proposed a current blocking layer formed by silicon dioxide nanoparticles [23] and the patterned Al 2 O 3 on p-GaN for a better current spreading [24], while n-type InGaN was also proposed and inserted underneath the InGaN/GaN active region to improve the current spreading [25].On the other hand, tunneling junction has also been proposed as a current spreading layer [26].Most recently, more efforts have been invested to the single-crystal ZnO as the current spreading layer [27].In addition, the use of vertical LEDs has also been investigated for suppressing the current crowding in nitride-based LEDs [24,28,29].In this work, we propose and demonstrate a lattice-matched p-GaN/n-GaN/p-GaN/n-GaN/p-GaN (PNPNP-GaN) epitaxial current spreading layer in the p-type cap region of InGaN/GaN LEDs.In these devices, with this PNPNP-GaN epi-structure, the electrical performance is also improved.Further enhancement of the optical output power and external quantum efficiency (EQE) has also been achieved, which is attributed to the improved current spreading effect in the proposed epi-structure.

Experiments
In our study, two sets of InGaN/GaN LED wafers (i.e., Reference LED and PNPNP-GaN LED sketched in Fig. 1) were grown by an AIXTRON close-coupled showerhead metalorganic chemical vapour deposition (MOCVD) reactor on c-plane patterned-sapphire substrates [30].In our experiment, TMGa/TEGa/TMIn and NH 3 were used as the group-III sources and the group-V sources, respectively.Diluted Cp 2 Mg and SiH 4 were used as the dopant precursors for p-type and n-type GaN, respectively.The growth was initiated from a 30 nm low-temperature GaN buffer layer, then followed by a 2 µm u-GaN layer and a 4 µm n-GaN layer (N d = 5 × 10 18 cm −3 ), which were both grown at 1050 °C.In the grown LED episamples, the active layer consists of five-period In 0.15 Ga 0.75 N/GaN multiple quantum wells (MQWs), with well and barrier thickness of 3 nm and 12 nm, respectively.The wells and barriers were grown at the same temperature of 730 °C.The Reference LED has a 0.2 µm thick p-GaN layer with a hole concentration of 3 × 10 17 cm −3 .By inserting two 20 nm n-GaN (N d = 2 × 10 17 cm −3 ) layers into p-GaN layer evenly, we obtained the PNPNP-GaN structure (0.067 µm /20 nm /0.067 µm /20 nm /0.067 µm) with the same thickness of the p-GaN layer in the reference device.The PNPNP-GaN layers were all grown at 1020 °C, which was lower than the temperature (1050 °C) used for the growth of u-GaN and n-GaN layers to suppress the out-diffusion of the InN from the InGaN quantum wells.Specifically, the growth pressure for the n-GaN and p-GaN in the PNPNP-GaN structure was set to 150 mbar.The V/III ratio (NH 3 /TMGa) during PNPNP-GaN growth was kept to be 4100.After the epi-wafers were grown, we performed the thermal annealing to activate the Mg dopants for p-GaN layers.The annealing was conducted for 10 min in the ambient of N 2 at a temperature of 720 °C for both the Reference LED and PNPNP-GaN LED wafers.After the LED wafers were grown, four sets of LED chips were fabricated by following the standard fabrication process.The epi-wafers were patterned into mesas of size 350 µm × 350 µm through reactive ion etch (RIE).Ni/Au film (10 nm/150 nm) was first deposited as the p-electrode, and then the p-contact was annealed in the mixture of O 2 and N 2 at 515 °C for 5 min under the atmosphere pressure.Finally, Ti/Au (30 nm/150 nm) was deposited as the nelectrode.We have prepared two sets of devices without indium tin oxide (ITO) coating (i.e., the Reference LED without ITO and PNPNP-GaN LED without ITO).The other two sets were prepared with ITO coating as the transparent current spreading layer (i.e., Reference LED with ITO and PNPNP-GaN LED with ITO, which are shown in the supplementary material).The transparent current spreading layer (i.e., ITO) was sputtered and annealed in the ambient of N 2 at 500 °C for 120 s before depositing electrodes.Also, we performed numerical simulations for the four sets of devices in APSYS [31], which solves the Poisson equation, continuity equation and Schrödinger equation with proper boundary conditions.The Auger recombination coefficient used in these simulations was 1 × 10 −30 cm 6 s −1 [32], and the energy band offset between the conduction band and the valence band was 70/30 [33].Considering the crystal relaxation by generating dislocations, 40% of the theoretical polarization charge was assumed [34].Other parameters can be found elsewhere [35].

Results and discussion
As indicated in Fig. 1, the PNPNP-GaN LEDs (i.e., PNPNP-GaN LED without ITO and PNPNP-GaN LED with ITO) own two layers of thin n-GaN (20 nm), sandwiched between 67 nm thick p-GaN layers.Each thin n-GaN forms two junctions at the interfaces (i.e., F 1 and F 2 ).When the InGaN/GaN LED is forward biased, F 1 is also forward biased, whereas F 2 is reversely biased.As the applied bias increases, the depletion region of F 1 shrinks, while F 2 extends its depletion region across the n-GaN.Thereby, the depletion region in the entire n-GaN layer can be retrieved, thus with the ionized Si donors here acting as the hole spreaders.With the doping concentration of n-GaN (2 × 10 17 cm −3 ) and p-GaN (3 × 10 17 cm −3 ), the built- , e is the elementary electronic charge, and n i = 1.9 × 10 −10 cm −3 for GaN).The total depletion region thickness is , ε r = 8.9 for GaN, and ε o is the absolute dielectric constant) [36], which consists of the depletion region in n-GaN and p-GaN of 97.70 and 65.13 nm, respectively, provided that n-GaN and p-GaN have infinite lengths.Therefore, the n-GaN layer, which has a thickness of only 20 nm, is fully depleted in PNPNP-GaN LEDs.Meanwhile, according to the principle of charge neutrality in the depletion region of a homojunction, the actual depletion width in p-GaN of the PNP-GaN junction is only about 13.33 nm.On the other hand, the diffusion length (L D ) of holes in the n-GaN can be obtained by using / Here, the minority carrier (hole) lifetime in n-GaN with a doping concentration of 2 × 10 17 cm −3 is assumed to be 0.8 ns, while the hole mobility is set to be 26 cm 2 /Vs [37].Thus, the diffusion length for holes is calculated to be ~231.9nm, which is much larger than the thickness of n-GaN (20 nm) in each PNP-GaN junction of the PNPNP-GaN LED.As a result, there will be no hole loss across the n-GaN region.Furthermore, there exists a reach-through breakdown voltage for the reversely biased junction (i.e., F 2 ), and the reach-through breakdown voltage is 0.08V in this case ( , and W N is the width of n-GaN) [38].It is worth mentioning that this BV RT is smaller than the built-in potential within the p-GaN/n-GaN junctions, and this also manifests itself in the full depletion of n-GaN by leaving behind ionized Si dopants as the hole spreaders.Therefore, after the depletion region extends through the whole n-GaN region, any increase in the applied bias will promote the injection of minority carriers (i.e., holes in our case) from the forward biased junction (i.e., F 1 ) to produce a high-current flow.For InGaN/GaN LEDs grown on insulating substrates with lateral current-injection scheme, the current travels both vertically and laterally as indicated in Fig. 2(a).As the sheet resistance of n-GaN is much smaller than the sheet resistance of p-GaN, the current tends to flow through the low-resistivity n-GaN layer (Fig. 2(a)), giving a non-uniform current distribution in p-GaN (i.e., I 1 > I 2 > I 3 > I 4 > ..... > I n ), well known as the current crowding effect [22].This current crowding effect can be suppressed if PNPNP-GaN is employed in the LED architecture.A simplified equivalent circuit of InGaN/GaN LEDs with the embedded PNPNP-GaN homojunctions is depicted in Fig. 2(b), in which the total current is divided into vertical current (J 1 ) and horizontal current (J 2 ).The total voltage drop consists of those in the p-contact, p-GaN, MQW region, n-GaN as well as n-contact.Based on the equivalent circuit in Fig. 2(b), Eq. ( 1) (for current path 1) and Eq. ( 2) (for current path 2) are obtained.
( ) ( ) where l represents the length of the lateral current path, l 0 is the distance from the mesa edge to the center of the n-contact, and w is the length of the stripped p-contact.The thickness of p-GaN and n-GaN is t p and t n , respectively; ρ p-GaN and ρ n-GaN is the resistivity for p-GaN and n-GaN, respectively; V pn denotes the junction voltage drop of multiple quantum wells in InGaN/GaN LED; and V p-contact and V n-contact are the voltage drops across the p-contact and ncontact, respectively.ρ PNP is the specific interfacial resistivity induced by the barrier height in each PNP-GaN junction.N is the total number of PNP-GaN junction, and in our device, there are two PNP-GaN junctions (i.e., PNPNP-GaN), and thus N is 2 (i.e., the total interfacial specific resistivity is 2 × ρ PNP ).By equating Eq. ( 1) and Eq. ( 2), Eq. ( 3) is derived.However, l is in the order of device mesa size, which is 350 µm × 350 µm, while t p is the scale of p-GaN thickness, which is ~200 nm.Since t p << l, then Eq. ( 3) can be simplified into Eq.( 4).  has to be reduced.It is also feasible to increase the lateral current flow by increasing the p-GaN layer thickness (t p ).Furthermore, the PNPNP-GaN will not have very abrupt interfaces because of the dopant diffusion, especially the Mg diffusion [39].However, one still can maintain the PNPNP-GaN junctions by properly increasing the Si doping concentration and/or the n-GaN thickness.Through this, the junction barrier (i.e., ρ PNP ) in each PNP-GaN can be formed, and Eq.(3), Eq. ( 4), Eq. ( 8) and Eq. ( 9) are still valid to explain the current spreading.Meanwhile, the n-GaN doping and n-GaN thickness have to follow the design guidelines, which were addressed previously, such that n-GaN has to be completely depleted and the reversed junction F 2 will not block any current.
In order to probe the embedded PNPNP-GaN in terms of spreading current, we present the current as a function of voltage, as shown in Figs.3(a) and 3(b).In the low forward voltage regime (0 ~3V in Figs.3(a) and 3(b)), the PNPNP-GaN LED without ITO coating exhibits lower leakage current with respect to the Reference LED without ITO film in Fig. 3(a), while the same conclusion can be made for PNPNP-GaN LED with ITO coating with respect to its Reference LED with ITO according to Fig. 3(b).The Reference LEDs (i.e., Reference LEDs without and with ITO) and PNPNP-GaN LEDs (i.e., PNPNP-GaN LEDs without and with ITO) have the similar crystal quality, as their full-width at half-maximum (FWHM) of ( 102) and (002) X-ray diffraction spectra are both about 213.5 and 216.0 arcsec, respectively.Thus the suppressed leakage current in PNPNP-GaN LEDs comes from the increased junction barrier height.As the PNPNP-GaN junctions are embedded in such InGaN/GaN LEDs with multiple heterojunctions, it is therefore difficult to extract the barrier height for PNPNP-GaN.Here, we calculate the effective overall barrier height by the following, where Ø B is the overall barrier height within the LED device, and n is the ideality factor for the diodes [40,41].Ø B is calculated to be 1.10V for Reference LED without ITO and 1.31V for PNPNP-GaN LED without ITO, respectively.It is clearly revealed that a higher overall barrier height is obtained when PNPNP-GaN feature is integrated in the p-type layer.The ideality factor is 5.32 and 4.19 for Reference LED without ITO and PNPNP-GaN LED without ITO, respectively.An improved current spreading in PNPNP-GaN LED is responsible for the reduced ideality factor [42].Furthermore, because of the improved current spreading effect in PNPNP-GaN LED without ITO, the electrical performance is improved compared to Reference LED without ITO when the applied bias is higher than 3V.Similarly, Ø B is determined to be 1.33V for Reference LED with ITO and 1.44V for PNPNP-GaN LED with ITO, respectively.Moreover, the ideality factor is 6.52 and 4.51 for Reference LED with ITO and PNPNP-GaN LED with ITO, respectively.We also observed the increased overall energy barrier height in PNPNP-GaN LED with ITO, which in the meanwhile features the reduced ideality factor and the improved electrical properties compared to those in Reference LED with ITO.This is well attributed to the improved current spreading effect by the incorporation of PNPNP-GaN homojunctions.show a red shift as the injection current level increases, which is due to a gradually increasing junction temperature during testing [43].However, a less pronounced red shift observed in the EL spectra of Reference LED with ITO and PNPNP-LED with ITO in Figs.4(c) and 4(d) is attributed to the significantly improved current spreading after ITO incorporation, which suppresses the high local heat caused by current crowding [21].We can also see the EL intensity of PNPNP-GaN LEDs without and with ITO is enhanced compared to that of Reference LEDs without and with ITO, respectively.We also measured the integrated optical output power and EQE (Fig. 5(a)), which is compared with the simulated results for the four sets of studied devices (Fig. 5(b)).We can see an improved optical output power and EQE for PNPNP-GaN LEDs without and with ITO in both experiments and simulations.Compared to Reference LED without ITO in Fig. 5(a), PNPNP-GaN LED without ITO shows a power enhancement of 10.19% and 12.16% at 20 mA and 100 mA, respectively, while PNPNP-GaN LED with ITO enhances the output power by 16.98% and 14.37% at 20 mA and 100 mA, respectively compared to Reference LED with ITO.The improved device performance in PNPNP-GaN LEDs without and with ITO is attributed to the reduced current crowding effect.This in turn suppresses the high local carrier concentration, resulting in the reduced non-radiative Auger recombination in the multiple quantum wells [44].
The energy band diagrams of Reference LEDs (i.e., Reference LED without and with ITO) and PNPNP-GaN LEDs (i.e., PNPNP-GaN LED without and with ITO) are shown in Figs.6(a) and 6(b), respectively.The holes in the Reference LEDs experience no barriers when transporting through p-GaN according to Fig. 6(a).In contrast, for the PNPNP-GaN LEDs, there are two hole energy barriers, which are due to the ionized Si donors in the n-GaN layers.With the aid of the hole barriers, hole spreading is enhanced, and this alleviates the hole crowding effect in InGaN/GaN LEDs and leads to an improved lateral hole distribution.

Conclusions
In conclusion, a promising epitaxial current spreading technology based on a lattice-matched PNPNP-GaN current spreading layer was proposed and investigated both theoretically and experimentally for InGaN/GaN LEDs.The proposed PNPNP-GaN current spreading layer can be directly achieved in a MOCVD chamber during the LED growth, avoiding the need for additional post-growth treatments, unlike those typically necessary for special current spreading layers (e.g., silicon dioxide nanoparticles used as the current spreading layer).Besides, n-GaN is lattice matched with p-GaN, which reduces the growth difficulty during MOCVD epitaxial process, as opposed to the growth of InGaN layers when used as a current spreading layer.In the proposed epitaxial current-spreading layer, the thin n-GaN layers between p-GaN layers are completely depleted, leaving behind positively ionized Si donors, which spread the injection current.This mitigates the current crowding effect and resulting in 12.16% improvement in the optical output power and 10.95% enhancement in EQE for the InGaN/GaN LEDs without using ITO as the transparent current spreading layer.Besides, the reduced current crowding effect was also observed for PNPNP-GaN LEDs even when using ITO coating compared with Reference LEDs with ITO films.In this case, the enhancement factors of 14.37% in the optical output power and 13.54% in EQE have been obtained.Also, a theoretical model was proposed to illustrate and explain the mechanism of the enhanced lateral current through incorporating PNPNP-GaN into the p-type layer.The simulation results were in good agreement with the experimental measurements.The enhanced current spreading effect was found to improve the electrical property even if the overall barrier height is increased due to the PNPNP-GaN junctions.As a result, the luminous efficacy (in lm/W) can also be increased.These findings indicate that the proposed PNPNP-GaN current spreading layer improves the InGaN/GaN LED performance both electrically and optically in any case, when using or not using an ITO coating as the transparent current spreading layer.We believe that PNPNP-GaN homojunction current-spreading holds great promise for energy-saving LEDs.

Supplementary material
We have also fabricated the InGaN/GaN LEDs coated with ITO films as the external transparent current spreading layers (i.e., Reference LEDs with ITO coatings and PNPNP-GaN LED with ITO coatings), which are shown in Figs.7(a) and 7(b).A 200 nm thick ITO film was sputtered on each device and then annealed in the ambient environment of N 2 at 500 °C for 120 s.In these devices, the deposited Ti/Au (30 nm/150 nm) contacts were used as the p-electrode and n-electrode, respectively.
Figures 7(a) and 7(b) depict the simplified equivalent circuit of InGaN/GaN LEDs.In the case of PNPNP-GaN incorporated as the current spreading layer, we divide the total current into the vertical portion (J 1 ) and the horizontal portion (J 2 ).Similar to the devices without transparent current spreading layer, the total voltage drop consists of those between ITO and Ti/Au in the p-contact, p-GaN, MQW region, n-GaN as well as the n-contact.Based on the equivalent circuit in Fig. 7(b), Eq. ( 6) (for current path 1) and Eq. ( 7) (for current path 2) are obtained.( ) where l represents the length of the lateral current path, l 0 is the distance from the mesa edge to the center of the n-contact, and w is the width of the device mesa.t ITO and w ITO is the thickness and width of the ITO film (in our case, w = 350 µm and w ITO = 330 µm), respectively, and ρ ITO is the ITO resistivity.The thickness of p-GaN and n-GaN is t p and t n , respectively; ρ p-GaN and ρ n-GaN is the resistivity for p-GaN and n-GaN, respectively; V pn denotes the junction voltage drop of multiple quantum wells in InGaN/GaN LED; and V p- contact and V n-contact are the voltage drops across the p-contact (Ti/Au on ITO) and n-contact, respectively.ρ PNP is the specific interfacial resistivity induced by the barrier height in each PNP-GaN junction.N is the total number of PNP-GaN junction, and in our device, we have two PNP-GaN junctions (i.e., PNPNP-GaN), and thus N is 2 (i.e., the total interfacial specific resistivity is 2 × ρ PNP ).
By equating Eq. ( 6) and Eq. ( 7), Eq. ( 8) is derived.However, l, w and w ITO are in the order of the device mesa size, which is 350 µm × 350 µm, while t ITO is 200 nm (t ITO << l), then Eq. ( 8) can be simplified into Eq.( 9). .Meanwhile, the current spreading effect will also be improved by properly increasing the p-GaN thickness (t p ).

Fig. 1 .
Fig. 1.Schematic diagrams of the studied devices (Reference LED without ITO coating and PNPNP-GaN LED without ITO coating), shown along with the band diagram of one PNP-GaN junction in the PNPNP-GaN LED.
p is the diffusion constant of holes, correlated with µ p by Einstein relationship, and k is the Boltzman constant).

Fig. 2 .
Fig.2.(a) Equivalent circuit of an InGaN/GaN LED grown on an insulating substrate (e.g., sapphire) with lateral current-injection scheme (I 1 > I 2 > I 3 > I 4 > ..... > I n ), and (b) a simplified equivalent circuit of this InGaN/GaN LED with possible current paths (J 1 and J 2 ) when a PNPNP-GaN current spreading layer is embedded.ITO is not used in these two devices.

Fig. 3 .
Fig.3.Liner-plot of experimentally measured current as a function of the applied voltage for (a) Reference LED without ITO coating and PNPNP-GaN LED without ITO coating (along with a semi-log plot inserted in the inset) and (b) Reference LED with ITO coating and PNPNP-GaN LED with ITO coating (again with a semi-log plot given in the inset).The electroluminescence (EL) spectra (Figs.4(a), 4(b), 4(c) and 4(d)) were collected under 10, 20, 30, 40 and 50 mA of the injection current for both Reference LEDs without and with ITO and PNPNP-GaN LEDs without and with ITO.Both the EL spectra of Reference LED without ITO and PNPNP-LED without ITO in Figs.4(a) and 4(b) show a red shift as the injection current level increases, which is due to a gradually increasing junction temperature during testing[43].However, a less pronounced red shift observed in the EL spectra of Reference LED with ITO and PNPNP-LED with ITO in Figs.4(c) and 4(d) is attributed to the significantly improved current spreading after ITO incorporation, which suppresses the high local heat caused by current crowding[21].We can also see the EL intensity of PNPNP-GaN LEDs without and with ITO is enhanced compared to that of Reference LEDs without and with ITO, respectively.We also measured the integrated optical output power and EQE (Fig.5(a)), which is compared with the simulated results for the four sets of studied devices (Fig.5(b)).We can see an improved optical output power and EQE for PNPNP-GaN LEDs without and with ITO in both experiments and simulations.Compared to Reference LED without ITO in Fig.5(a), PNPNP-GaN LED without ITO shows a power enhancement of 10.19% and 12.16% at 20 mA and 100 mA, respectively, while PNPNP-GaN LED with ITO enhances the output power by 16.98% and 14.37% at 20 mA and 100 mA, respectively compared to Reference LED with ITO.The improved device performance in PNPNP-GaN LEDs without and with ITO is attributed to the reduced current crowding effect.This in turn suppresses the high local carrier concentration, resulting in the reduced non-radiative Auger recombination in the multiple quantum wells[44].The energy band diagrams of Reference LEDs (i.e., Reference LED without and with ITO) and PNPNP-GaN LEDs (i.e., PNPNP-GaN LED without and with ITO) are shown in Figs.6(a) and 6(b), respectively.The holes in the Reference LEDs experience no barriers when transporting through p-GaN according to Fig.6(a).In contrast, for the PNPNP-GaN LEDs, there are two hole energy barriers, which are due to the ionized Si donors in the n-GaN layers.With the aid of the hole barriers, hole spreading is enhanced, and this alleviates the hole crowding effect in InGaN/GaN LEDs and leads to an improved lateral hole distribution.

Fig. 5 .Fig. 6 .
Fig. 5. (a) Experimentally measured optical output power and EQE as a function of the current injection, and (b) numerically simulated optical output power and EQE as a function of the current for Reference LEDs without and with ITO coatings and PNPNP-GaN LEDs without and with ITO coatings.

Fig. 7 .
Fig. 7. (a) Equivalent circuit of an InGaN/GaN LED grown on an insulating substrate (e.g., sapphire) using ITO top coating with lateral current-injection scheme (I 1 > I 2 > I 3 > I 4 > ..... > I n ), and (b) a simplified equivalent circuit of this InGaN/GaN LED with possible current paths (J 1 and J 2 ) when a PNPNP-GaN current spreading layer is embedded, along with additional ITO film used as the transparent current spreading layer on the top.