Enhanced electro-optic performance of surface-treated nanowires : origin and mechanism of nanoscale current injection for reliable ultraviolet light-emitting diodes

Self-assembled nanowires are posed to be viable alternatives to conventional planar structures, including the nitride epitaxy for optoelectronic, electronic and nano-energy applications. In many cases, current injection and extraction at the nanoscopic scale are essential for marked improvement at the macroscopic scale. In this investigation, we study the mechanism of nanoscale current injection and the origin of improvement of the flow of charged carriers at the group-III nitride semiconductor surface and metal-semiconductor interfaces. Conductive atomic force microscopy (c-AFM) and Kelvin probe force microscopy (KPFM) enable a rapid analysis of the electrical and morphological properties of single and ensemble nanostructures. The surface potential and current injection of AlGaN nanowirebased LEDs are spatially mapped before and after surface treatment with KOH solution. Treated-nanowires showed an improved current spreading and increased current injection by nearly 10×, reduced sub-turn-on voltage (as low as 5 V), and smaller series resistance. The reduced contact potential confirms the lower semiconductor/metal barrier, thus enabling larger carriers flow, and correlates with the 15% increase in injection efficiency in macroscopic LEDs. The improvement leads to the normalization of nanoscale electrical conducting properties of UV AlGaN-based nanowire-LEDs and lays the foundation for the realization of practical nanowire-based device applications. © 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement


Introduction
AlGaN-based UV nanowires light emitters have recently attracted attention due to their unique properties [1][2][3] that partly overcome the drawbacks of the more mature planar technology, such as spontaneous and piezoelectric polarization fields [4], high dislocation density [5], low p-doping efficiency [6], and light extraction efficiency [7].Moreover, group-III nitride nanowires can be grown on CMOS-compatible and cost-effective substrate such as Si [8,9] and highly conductive metal substrates [10][11][12][13][14].This allows better electrical and heat dissipation compared to the commercial AlN and sapphire substrates, and improves the overall device performance [15,16].Therefore, they offer a promising future for applications in air purification, water disinfection, curing, phototherapy and the next generation of high resolution displays [17,18].However, limitations in eventual electro-optical conversion devices remain elusive to the nanostructure community, in particular when practical electrical devices are developed.For instance, nanostructures suffer from the large density of surface states, caused by the high surface-to-volume ratio.The large depletion layer resulting from the Fermi level pinning decreases the density of free electrons toward the surface, increasing the non-radiative recombination and affecting the quantum efficiency of the devices [19].Moreover, the small contact area and the self-assembled growth of nanowires, lead to nonuniformity and low carrier injection which subsequently lowers the injection efficiency.
Various approaches have been employed to overcome these issues, such as sulfur-based compounds [20][21][22] and large band-gap encapsulation layers [23].Recently, the microscopic enhancement of luminescence of AlGaN-based nanowires LEDs by using KOH solution has been reported [24].However, the nanoscale origin and the mechanism of such enhancement are yet to be clarified.In fact, the results do not elucidate how the uniform injection can be obtained in such inhomogeneous ensemble of nanowires.Without this information, the utilization of nanowires for electrical devices is therefore hampered.
Conductive atomic force microscopy (c-AFM) has been previously utilized to study the nanoscale behavior of single nanowires and to visualize the electrical inhomogeneity [25][26][27][28][29].This powerful technique allows a systematic investigation of rapid electrical properties without directly fabricating the device, hence enabling an immediate feedback on the grown sample quality.Moreover, it is applicable to a large variety of nanostructures and nanomaterials with high aspect ratio for studying the optical and electrical interaction with the surface.Various single nanowire electrical measurements have been performed to analyze material properties such as specific contact resistivity [30], doping concentration [31] [2], and current-voltage characteristics [32].However, most of these efforts are based on single nanowire which requires complicated and time-consuming fabrication processes, e.g.e-beam lithography, preventing an overall device evaluation in a relatively large scale.Thus, c-AFM highlights the advantage of electrical measurements not only in single nanowire but also a cluster of nanowires with overall picture of their electrical performance.Despite the insightful work using this technique to measure the single nanowire and to understand the failure mechanisms, a thorough study of the electrical performance relative to the poor injection current caused by the nanowires surface in large scale is still lacking, and requires significant investigation to advance this field and bridge the gap that currently exists with the thin film technology.
In this work we address the nanoscale current injection limitation of UV AlGaN nanowires by analyzing, for the first time, the origin that prevents the efficient carrier flow and distribution uniformity.c-AFM and KPFM techniques were used to measure the single nanowires, and to map the current and the contact potential difference distributions of ensembles of nanowires.KOH-treatment was utilized to remove oxides and flatten the contact surface hence enabling a larger number of nanowires to be electrically injected and contribute to the overall device characteristics enhancement.Higher injection current, lower sub-turn-on voltage and smaller series resistance were achieved for the KOH-treated samples, as well as more than 2 × lower contact potential, confirming the lower surface band-bending.Full device structures exhibited current density as high as 240 A/cm 2 compared to an average of 140 A/cm 2 for the as-grown samples.Moreover, 15% improvement in injection efficiency was achieved for the treated device samples.This study aims to give a deep understanding of the nanometer scale current injection mechanism among the self-assembled AlGaN-based nanowires, and provides insights towards enhanced current spreading, injection efficiency and overall UV device performance demonstration.

Experimental section
The AlGaN-based nanowires were grown on a Ti-TaN metal bi-layer [33] deposited on a phosphorus-doped <100> silicon substrate (10 −1 Ω•cm) under nitrogen-rich condition using a Veeco GEN 930 plasma-assisted molecular beam epitaxy (PA-MBE) system.The samples were thermally degassed in the MBE load-lock at 200 °C, in the buffer chamber at 600 °C and subsequently in the growth chamber at 900 °C.The growth was initiated with n-GaN seeds at a temperature of 485 °C for 10 min at a beam equivalent pressure (BEP) of 6.2 × 10 −8 Torr and silicon effusion cell at 1165 °C.The plasma RF power and N 2 flow were kept at 350 W and 1.0 sccm, respectively for the whole growth process.To improve the crystal quality, a higher temperature (585 °C) n-GaN layer was grown, therefore achieving a total thickness of ~80 nm.An approximately 50 nm n-AlGaN cladding layer was grown at 640 °C and it was followed by 15 stacks of alternating Al y Ga (1-y) N quantum barrier (Al and Ga BEP of 1.5 × 10 −8 and 3 × 10 −8 Torr) and Al x Ga (1-x) N quantum disks (Al and Ga BEP of 0.75 × 10 −8 and 4.5 × 10 −8 Torr) (where x < y) grown at 640 °C and with thicknesses of 4 nm and 3 nm respectively.The p-AlGaN layer was then grown at 620 °C (Al and Ga BEP of 1.5 × 10 −8 and 2 × 10 −8 Torr), Mg temperature at 440 °C with a final thickness of ~50 nm.Finally, a p + -AlGaN contact layer was grown with a thickness of ~20 nm (growth temperature of 580 °C and Mg temperature of 450 °C).After growth, two samples were treated in 10% KOH solution for 10 s and 40 s and a third sample was left untreated as a reference.
The surface morphology is measured using AFM (Agilent 5500 SPM with a scan range of 90 μm × 90 μm × 8 μm and a vertical noise of 0.016 nm) and c-AFM equipped with a conductive Pt/Ir-coated tip (BRUKER CONTV-PT, spring constant = 0.2 N/m).All tests are performed at room temperature.The KPFM measurements were takes using the same equipment.The contact potential difference (CPD) between the conductive tip and the AlGaN-based nanowires surface is measured by nullifying the tip-sample CPD, hence allowing almost zero surface band-bending caused by the tip.Both c-AFM and KPFM scans were taken at a speed of 0.8 inch/s and 512 points/line.
The UV AlGaN-based nanowires-LEDs were fabricated with the lithography technique.The KOH-treated sample was immersed in 10% KOH solution prior to deposit a 5 nm/5 nm Ni/Au ohmic contact.The evaporation is performed using the angle-deposition (45°) technique in an electron-beam evaporation chamber, therefore guaranteeing the metal to be deposited only on the top p-AlGaN contact.The samples are then annealed at 550 °C for 1 min (2 cycles) in ambient environment.The top Ni/Au (10 nm/200 nm) metal contact is then evaporated.The sample is subsequently loaded in the inductively coupled plasma reactive-ion etching (ICP-RIE) chamber to remove the back silicon dioxide and immediately loaded in the sputtering chamber for the deposition of Ti/Au (10 nm/150 nm) n-contact.The powercurrent -voltage (L-I-V) and the electroluminescence (EL) measurements are performed in a setup similar to the PL with a current source meter (Keithley 2116B) and a silicon photodetector (Newport 818-UV) connected to a Newport 1936-C power meter.Figure 1(a) depicts the AlGaN nanowire structure grown on a metal bi-layer designed to improve the electrical injection and the heat dissipation.In this regard, we obtained lower junction temperature of samples grown on a metal-bilayer compared to silicon and AlGaN planar LEDs grown on sapphire [15].The p-AlGaN/p + -AlGaN layers were grown instead of the commonly used p-GaN, to reduce light absorption, therefore increasing light extraction.However, this constitutes a trade-off as the doping ionization efficiency reduces with increasing Al concentration, resulting in less free holes.In addition, we were able to achieve a hole concentration of 1.3 × 10 19 cm −3 with large injection current [34].

Results and discussion
The normalized PL and EL spectra of the as-grown AlGaN-based nanowires and fabricated device are shown in Fig. 1(b).The emission wavelengths match very well with the peak position centered at 325 nm.The full-width at half maximum (FWHM) is also measured to be 23 nm and 16 nm respectively.The slight greater FWHM in the PL measurement might be due to the high number of states that are excited by the high energy laser source as well as the large number of quantum disks that leads to inhomogeneous broadening of the spectrum and deviates from the theoretical ∆λ = 1.8 kTλ 2 /hc value, where ∆λ, k, h and c are the FWHM, Boltzmann constant, Planck constant and speed of light.However, these values align well with previously reported AlGaN-based nanowires-LEDs [16] [35].Furthermore, we recently reported PL and EL emission enhancement due to the increased radiative recombination as a consequence of the surface states passivation [24].It is noted that the KOH-treated samples showed a longer carrier lifetime (slower non-radiative recombination) compared to as-grown samples at room temperature.This indicates a better radiative recombination (reduction of surface states).The small hump in the PL spectrum at ~290 nm is attributed to the higher Al composition barrier.Such peak, however, does not appear in the EL spectrum, confirming the high quantum confinement in the active region.We then measured the I-V curves of the three samples by sweeping the voltage from −6 to 10 V as shown in Fig. 3(a).Single nanowires were identified and tested, and the measurement was performed on different areas of the sample to have a better understanding of the inhomogeneity throughout the sample.One can notice that the as-grown sample shows the lowest current at the same voltage, while the values increase with KOH treatment time.A maximum current of ~1.4 nA was obtained for the 40 s KOH sample that corresponds to more than 1 order of magnitude higher than the as-grown sample.The 10 s KOH sample characteristics fall between the other two samples, meaning that the treatment time is not enough to obtain the highest device performance.All the three samples show a relatively large operation range, signifying that the non-uniformity among single nanowire plays an important role in the I-V curves [27].Despite this, clear improvement was achieved by treating the nanowire samples with KOH.This can be explained by both surface oxide etching and flat surface (Fig. 1(b)) that allow better metal-semiconductor contact and current injection, as well as the removal of surface states at the nanowires sidewalls.It is worth mentioning that some electrical shorts exist in the sample, hence carrying most of the current injected [28].However, the I-V characteristics of such nanowires have been removed for a more representative comparison.
The sub-turn-on voltages (non-abrupt V th ) are extracted by linearly fitting the high current (between 7.5 V and 10 V) segment of the curve (see blue dotted line in Fig. 3(a)).On average, lower V th (~5 V) were obtained for the 40 s KOH samples, while similar values are shown for the other two samples.The V th is slightly larger than the theoretical value of V th ≈V D ≈E g /e, (where V D , E g and e are the diffusion voltage, energy bandgap and elementary charge, respectively) that is due to the common III-V semiconductor disadvantages such as bandgap discontinuities and contact resistance that cause additional voltage drop.Our results align well with previous report that showed an average turn-on voltage of 4.6 V for single AlGaNbased nanowire adopting a tunnel junction structure [27].Therefore, we believe that by combining the tunnel junction and the KOH treatment, we would be able to reduce the operation voltage even further.Despite the deviation from the ideal I-V characteristics, lower series resistance was achieved for the 40 s KOH sample meaning that the contact resistance is reduced.The parallel resistance also plays a crucial role in nanowires due to the large surfaceto-volume ratio.By decreasing the surface states with KOH, such resistances are reduced and improved I-V curves can be measured.The figure of merits extracted from the I-V curves are shown as histograms in Fig. 3(b).To study the structural and electrical properties of the AlGaN-based nanowires surface and interface, we utilized KPFM technique.KPFM allows the measurement of the surface potentials and offer indication on nanoscopic structural and chemical variations as well as on surface band bending, being the latter closely related to the potential profile when an electric field is applied.The contact potential difference measured by KPFM corresponds to the work function difference between the conductive tip and the sample.As reported by J. Bardeen, in a metal-semiconductor junction, such difference in work functions gives rise to a space charge region that in turn gives an electrostatic potential energy rise at the surface, i.e. the band bending [37].Figure 4 shows the topography images and the corresponding CPD measurement on a 2 × 2 μm 2 area of the three samples.The red circles show the same spot for the specific sample.According to the scale bar on the right, a higher CPD is observed for the as-grown sample compared to the 10 s and 40 s KOH treated ones, which show lower CPD.This confirms the reduced potential barrier at the nanowires surface for the KOH-treated samples, compared to the as-grown sample where the presence of surface oxides clearly hinders the current flow (Fig. 3(a)).A line scan comparison over a 2 μm length is depicted in Fig. 4(g).It is noted that the spikes observed for the as-grown sample are attributed to the inhomogeneity within the as-grown single nanowires CPD.An overlap of the CPD mapping and the line scan is presented in Fig. 4(d) for the as-grown sample, showing the spikes caused by the inhomogeneous nanowires.However, these are not observed in the treated sample, meaning that a more uniform potential is obtained as a consequence of the KOH treatment that flatten th the three sam values, which thickness of different, whi noise level of which are es oxide/p-AlGa measurements the c-AFM m significantly compared to t   [24].Moreover, the turn-on voltage is lower than other AlGaN studies, where values ranging from 6 V to 8 V have been reported [40] [9].These results are even more remarkable considering that the common p-GaN top contact has been replaced by the more resistive p-AlGaN.
The relative EQE of an average sample was calculated by measuring the optical power and dividing it by the current injection, as depicted in Fig. 5(c).Higher values were obtained for the KOH-treated sample, confirming the better performances of the full device.The EQE increases with the current until 100 A/cm 2 after which it reaches a plateau.As mentioned above, the poor p-doping efficiency and the contact-barrier heights in GaN-based lightemitters highly hampers the carrier injection, reducing the overall EQE [41][42][43].Therefore, to have a deeper understanding on the better current injection of the KOH-treated samples and to shed light on the electrical device performance, we studied the injection efficiency by employing the ABC model as follow [44].We selected two devices (40 s KOH and as-grown) that showed an average current injection and EQE.
The injection efficiency can be calculated as a function of the EQE: .
where η int , η le and η inj are the internal quantum efficiency, light extraction efficiency and injection efficiency.
The η int can be expressed as: where A, B, C and n are the rate coefficients for the Shockley-Read-Hall, radiative and Auger recombination, and carrier concentration respectively.In steady state condition, the recombination rate con be considered equivalent to the carrier injection rate: where the term on the left is the recombination rate at steady state and the term on the right is the carrier injection rate, q is the elementary charge and V QDs is the volume of the nanowires quantum disks; we can then find a relation between the current density (J) and η inj .
( ) . inj QDs J An Bn Cn ql where l QDs is the quantum disks thickness.By inserting Eq. (2) into Eq.( 1) and by considering Eq. ( 4), we can extract the carrier concentration as: .
We can then obtain a cubic polynomial relation between J and EQE J ⋅ by inserting Eq. ( 5) into Eq.( 4): Considering the J vs EQE curve in Fig. 5(c) we can then use the polynomial function to fit the J vs EQE J ⋅ curve in Fig. 5(d), and extract the coefficients.It has been recently reported that the η le of AlGaN-based nanowires can be enhanced for both TE and TM polarization by varying the tapering angle [3].A maximum η le for TE polarization was achieved with a tapering angle of 5.25 ° that perfectly represents our nanowires due to the same MBE growth conditions used.The η inj • η le can then be determined from the coefficient of the quadratic term.We considered a total η le of 41.9% for our calculation and we obtained a 15% increment in η le for the 40 s KOH sample compared to the as-grown.It is noted that other approaches have been adopted to calculate the η inj and are based on EL, PL and IQE, and they all agreed that the low-current η inj is equal to 1.This was confirmed by both simulation and experimental results.In fact, at low injection current the electron quasi Fermi level is low and the heating effects are not pronounced, therefore resulting in negligible carrier overflow.Despite the η inj of the AlGaN-based nanowires remains low even after the KOH treatment (~1.3%), the improvement is evident, and it sets the foundation for improving the carrier injection characteristics of AlGaN-based nanowires devices.

Summary
Understanding the origin and mechanism of nanoscale current injection in nanostructures constitutes a critical effort towards the exploitation of a vast range of devices that rely on surface dynamics.We studied the AlGaN nanowires-based LEDs surface treatment by treating the samples with KOH solution to remove the surface oxide and reduce the semiconductor/metal potential barrier.By using c-AFM and KPFM, we provided a rapid statistical feedback on the nanoscale electrical and structural properties without the need of full devices fabrication.The single nanowire electrical characteristics clearly demonstrated the effectiveness and applicability of the facile and adaptable process, which can be applicable to a multitude of nanostructures.More specifically, KOH revealed a pristine and smooth semiconductor contact-layer with the end effect of lowering the contact potential barrier for the overall improvement in uniform metal-semiconductor interface and current injection.We achieved ~10 times higher current injection, reduced sub-turn-on voltage of 5 V and a lower average series resistance of 13 nΩ, compared to those of the un-treated as-grown samples (6.7 V and 170 nΩ).Finally, the characterization of fabricated LEDs correlates well with the observations, and an improvement of ~15% on the injection efficiency was measured.These results and techniques shed light on the central role of the nanoscale electrical measurement towards the realization of high-efficiency electro-optical nano-and macro-devices.

Fig. 1 .
Fig. 1.(a) AlGaN multi quantum-disk (MQdisk) schematic nanowire device structure used in the experiment.(b) PL and EL spectrum.Top-view SEM images of: (c) as-grown AlGaNbased nanowires sample characterized by rough surface, and (d) AlGaN-based nanowires sample after 40 s KOH treatment showing smooth surface.
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Fig. 3 .
Fig. 3. (a) I-V curves of the three samples under test.(b) Histograms of the extracted figures of merit (maximum current, turn-on resistance, sub-turn-on voltage) of the three samples and the respective average values.
Fig. 4 KOH the CP μm, a variou The full d performance device with th of the two sa KOH AlGaN current compl it can be see current is com nanowires dev lower metal/s area of 0.5 × measurement.that is half o previously Al reduced by 1 for the two sa nanowires com overall inhom some nanowi voltage.Desp

Fig. 5 .
Fig. 5. (a) I-V curves of AlGaN-based nanowires-ensemble devices treated with 40 s KOH and that of the as-grown.(b) Comparative statistical analysis of the turn-on resistance and turn-on voltage extracted from the I-V curves.(c) Relative EQE comparison.(d) ' J EQE ⋅ vs J plot and fitting lines showing an injection efficiency enhancement of 15% for the 40 s KOH-treated AlGaN-based nanowire-ensemble device.