GaN Regions Research Articles

High performance electron blocking layer-free InGaN/GaN nanowire white-light-emitting diodes.

We investigated the effect of coupled quantum wells to reduce electron overflow in InGaN/GaN dot-in-a-wire phosphor-free white color light-emitting diodes (white LEDs) and to improve the device performance. The light output power and external quantum efficiency (EQE) of the white LEDs with coupled quantum wells were increased and indicated that the efficiency droop was reduced. The improved output power and EQE of LEDs with the coupled quantum wells were attributed to the significant reduction of electron overflow primarily responsible for efficiency degradation through the near-surface GaN region. Compared to the commonly used AlGaN electron blocking layer between the device active region and p-GaN, the incorporation of a suitable InGaN quantum well between the n-GaN and the active region does not adversely affect the hole injection process. Moreover, the electron transport to the device active region can be further controlled by optimizing the thickness and bandgap energy of this InGaN quantum well. In addition, a blue-emitting InGaN quantum well is incorporated between the quantum dot active region and the p-GaN, wherein electrons escaping from the device active region can recombine with holes and contribute to white-light emission. The resulting device exhibits high internal quantum efficiency of 58.5% with highly stable emission characteristics and virtually no efficiency droop.

Red‐Emitting InGaN‐Based Nanocolumn Light‐Emitting Diodes with Highly Directional Beam Profiles

Various InGaN/GaN pn‐junction nanocolumn arrays arranged in a triangle lattice are grown on the same substrate while changing the nanocolumn diameters (Dn‐GaN) of the underlying n‐side GaN region under the lattice constant (L) of 340 nm. The nanocolumn diameter increases during the growth of the active InGaN and p‐side GaN regions. The periodic arrangement of nanocolumns leads to a photonic crystal effect. Redshift of the band edge wavelength, from 573 to 629 nm, is observed as Dn‐GaN increases from 159 to 282 nm. This phenomenon can be explained by the fact that the larger filling factor increases the effective refractive index of the nanocolumn system, resulting in a redshift of the band edge. The light diffraction at the photonic band edge induces the directional radiation beam from the surface of the nanocolumn system. Using the nanocolumn array with Dn‐GaN = 260 nm, the red‐emitting (λ = 637 nm) nanocolumn light‐emitting diodes with the radiation angle of ±30° are demonstrated.

High ${I}_{\text{ON}}$ and ${I}_{\text{ON}}$ /${I}_{\text{OFF}}$ Ratio Enhancement-Mode Buried ${p}$ -Channel GaN MOSFETs on ${p}$ -GaN Gate Power HEMT Platform

Enhancement-mode (E-mode) buried ${p}$ -channel GaN metal-oxide-semiconductor field-effect-transistors ( ${p}$ -GaN-MOSFET’s) with threshold voltage ( ${V}_{\text {TH}}$ ) of −1.7 V, maximum ON-state current ( ${I}_{\text {ON}}$ ) of 6.1 mA/mm and ${I}_{\text {ON}}/{I}_{\text {OFF}}$ ratio of 107 are demonstrated on a standard ${p}$ -GaN/AlGaN/GaN-on-Si power HEMT substrate. An oxygen plasma treatment (OPT) was deployed to the gated ${p}$ -GaN region where a relatively thick (i.e. 31 nm) GaN is retained without aggressive gate recess. The OPT converts the top portion of the GaN layer to be free of holes so that only the bottom portion remains ${p}$ -type while being spatially separated from the etched GaN surface and gate-oxide/GaN interface. As a result, E-mode operation is enabled while a high-quality ${p}$ -channel is retained. Multi-energy fluorine ion implantation was implemented for planar isolation of GaN ${p}$ -channel FETs with mesa edges and sidewalls eliminated. Consequently, high ${I}_{\text {ON}}/{I}_{\text {OFF}}$ ratio is obtained.

On the determination of alloy composition using optical spectroscopy in MOVPE grown InGaN layers on Si(111)

Abstract Alloy composition of InxGa1-xN layer is investigated using photoluminescence (PL), and photoluminescence excitation (PLE) techniques. InGaN layers were grown on Si(111) substrates using metal organic vapour phase epitaxy (MOVPE) technique under variable flow of either ammonia (NH3) or trimethyl gallium (TMGa) with corresponding V/III ratios in the range of 3.6 × 103 to 1.2 × 104. We observed that though room temperature PL measurements give an indication about the growth of InGaN, it doesn't help in the estimation of band gap or alloy composition of layer. It is restricted mainly by the presence of multiple peaks in the PL spectra which is rather severe at low temperature. A comparison of PL and reflectivity spectra confirmed that such peaks are attributed to interference oscillations and are not related with multiple compositions of InGaN. Further, in such a scenario PLE is found to be quite effective where the measured PLE spectrum is consisted of a clear onset related to InGaN layer in the sub band gap region of GaN. It is quite helpful in a precise determination of band gap and hence the alloy composition of InGaN layer. The indium composition is estimated to be in the range of 3.5% ≤ x ≤ 20.5%, however its trends with respect to V/III ratio as a function of NH3 or TMGa flow are found to be rather opposite.

Deep traps localization in AlGaN/GaN MIS-HEMTs by a comparative study using capacitance and current deep level transient spectroscopies

In this study, deep traps in multiple-finger normally-off AlGaN/GaN metal-insulator-semiconductor high-electron mobility transistors (MIS-HEMTs) were identified. The localization of these traps has been established by a comparative study using capacitance and current deep level transient spectroscopies (DLTS). The Cgd-DLTS measurements cover the GaN buffer region between the gate and drain contacts. On the other hand, the IDS-DLTS measurements cover the channel region in GaN including the zone under the gate. Two electron traps, E2 (0.31eV) and E4 (0.5eV) have been detected. They are respectively attributed to reactive ion etching (RIE) induced surface damage. These two traps are more likely located in the GaN channel close to the gate. Two other deep electron traps E5 (0.64eV) and E6 (0.79eV) have also been detected and are localized in the GaN buffer layer.

Comparison of defect structure in Si and Ge ion implanted GaN epilayers by RBS/channeling

In this work, we compared ion implantation n-type doping processes in GaN using Si+ or Ge+ ions, by conducting structural and electrical characterization. In the case of Ge+ implantation processes, we compared both the standard single implantation and the sequential doping technique. Analysis of RBS/c spectra and AFM images did not show surface degradation after each technological step (RMS < 1 nm). RBS/c studies showed that the sequential Ge+ implantation/annealing process leads to lowering of the defect concentration as compared to the single implantation process. All those doping processes resulted in low specific ohmic contact resistance RC < 1 Ω·mm. The obtained sheet resistivity of Si-implanted GaN region was RSH = 120 Ω/□, as compared to RSH of 500–530 Ω/□ for Ge-implanted GaN regions.

Atomic-scale identification of crystalline GaON nanophase for enhanced GaN MIS-FET channel

In this work, we demonstrate an easy channel-engineering method using oxygen-plasma treatment followed by in-situ annealing before gate dielectric deposition on GaN. A crystalline GaON nanophase was identified to serve as an optimized channel layer in the gate region of E-mode GaN metal-insulator-semiconductor field-effect transistors (MIS-FETs). The atomic-scale element distribution and crystalline structure of the GaON nanophase were revealed by aberration-corrected scanning transmission electron microscopy. First-principles calculations further correlate the enhanced thermal stability with the atomic observation of the refined structure in the GaON nanophase. Owing to the atomically sharp gate-dielectric/channel interface and low interface trap density (Dit), the boosted performance of the E-mode MIS-FET was achieved with the GaON channel. This study not only validates an approach of local channel modification for high-performance normally off GaN MIS-FETs but also opens possibilities of utilizing this crystalline GaON nanophase as a promising channel material in various GaN devices.

Analysis of defect structure in GaN epilayers doped with implanted Si+ by RBS/c method

In this work, the Si+ doping of undoped GaN (n ∼1 × 1016 cm−3) on sapphire by a single step implantation or sequential implantation/recrystallization processes, to a total fluence of 1 × 1016 at/cm2 and subsequent activation annealing at 1100 °C for 2 min, were demonstrated and analysed. We proved that the multicycle implantation/recrystallization process is better in terms of structural and electrical parameters of implanted region than conventional single step implantation. Rutherford backscattering spectrometry in channeling geometry (RBS/c), McChasy simulations and atomic force microscopy (AFM) studies showed, that the proposed process does not deteriorate surface morphoplogy and leads to a significantly lower defect concentrations. Low resistivity ohmic contacts, with RC < 0.1 Ω mm, to low sheet resistivity Si-implanted GaN region (RSH = (36.9 ± 0.7) Ω/sq.), were obtained with the use of sequential implantation with 30 s recrystallization annealing.

Insertion of NiO electron blocking layer in fabrication of GaN–organic heterostructures

We report the fabrication of a NiO thin film on top of an n-type GaN epitaxial layer. The electron-blocking capability of NiO in a hybrid organic/inorganic heterostructure consisting of n-GaN/NiO/poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) is discussed. Surface morphology, crystallography orientation, bandgap, and fermi level information of NiO films were investigated in detail. A rectifying property consistent with the proposed band diagram was observed in the current–voltage measurement. Theoretical analysis also demonstrated the effective electron blocking due to band alignment and a more balanced carrier distribution inside the GaN region with NiO inserted into the n-GaN/PEDOT:PSS heterostructure. This work provides a promising approach to the fabrication of high-efficiency hybrid optoelectronic devices.

Direct evidence of recombination between electrons in InGaN quantum discs and holes in p-type GaN.

Intense emission from an InGaN quantum disc (QDisc) embedded in a GaN nanowire p-n junction is directly resolved by performing cathodoluminescence spectroscopy. The luminescence observed from the p-type GaN region is exclusively dominated by the emission at 380 nm, which has been usually reported as the emission from Mg induced impurity bands. Here, we confirm that the robust emission from 380 nm is actually not due to the Mg induced impurity bands, but rather due to being the recombination between electrons in the QDisc and holes in the p-type GaN. This identification helps to get a better understanding of the confused luminescence from nanowires with thin QDiscs embedded for fabricating electrically driven single photon emitters.

Defect quasi Fermi level control-based CN reduction in GaN: Evidence for the role of minority carriers

Compensating point defect reduction in wide bandgap semiconductors is possible by above bandgap illumination based defect quasi Fermi level (dQFL) control. The point defect control technique employs excess minority carriers that influence the dQFL of the compensator, increase the corresponding defect formation energy, and consequently are responsible for point defect reduction. Previous studies on various defects in GaN and AlGaN have shown good agreement with the theoretical model, but no direct evidence for the role of minority carriers was provided. In this work, we provide direct evidence for the role of minority carriers in reducing point defects by studying the predicted increase in work done against defect (CN−1) formation with the decrease in the Fermi level (free carrier concentration) in Si doped GaN at a constant illumination intensity. Comparative defect photoluminescence measurements on illuminated and dark regions of GaN show an excellent quantitative agreement with the theory by exhibiting a greater reduction in yellow luminescence attributed to CN−1 at lower doping, thereby providing conclusive evidence for the role of the minority carriers in Fermi level control-based point defect reduction.

The structure of InAlGaN layers grown by metal organic vapour phase epitaxy: effects of threading dislocations and inversion domains from the GaN template.

Defects in quaternary InAlGaN barriers and their effects on crystalline quality and surface morphology have been studied. In addition to growth conditions, the quality of the GaN template may play an important role in the formation of defects in the barrier. Therefore, this work is focused on effects caused by threading dislocations (TDs) and inversion domains (IDs) originating from the underlying GaN. The effects are observed on the crystalline quality of the barrier and characteristic surface morphologies. Each type of TDs is shown to affect the surface morphology in a different way. Depending on the size of the corresponding hillock for a given pinhole, it was possible to determine the dislocation type. It is pointed out that the smallest pinholes are not connected to TDs whereas the large ones terminate either mixed type or edge type TDs. At sufficiently large layer thickness, the IDs originating from the GaN template lead to the formation of concentric trenches at the layer surface, and this is related to the change in growth kinetics on top and at the immediate surroundings of the ID.

Hole-Trapping Process at Al2O3/GaN Interface Formed by Atomic Layer Deposition

The hysteresis of the capacitance–voltage (C–V) characteristics of an Al2O3/n-GaN metal–insulator–semiconductor structure was evaluated under light (white LED)-irradiation and dark conditions. The hysteresis was not observed under the dark condition but was observed under the light-irradiation condition. The GaN surface was completely depleted in negative bias under the dark condition. The C–V characteristics indicated that the hysteresis is caused by hole trapping under the LED irradiation condition and that the holes are generated in the n-GaN surface by LED irradiation and subsequently injected into the Al2O3 films. When the holes are generated in the depletion region of GaN for any reason, such as a short generation lifetime, they can be trapped in the Al2O3 films.

Relaxation rate and polarization charge density model for AlN/Al$_{x}$Ga$_{1 - x}$N/AlN heterostructures

This work describes the strain-relaxation--dependent carrier concentration ($n_{s})$ profile model using spontaneous and piezoelectric polarization for AlN/Al$_{x}$Ga$_{1 - x}$N/AlN HEMTs in all mole fraction ($x)$ interpolations. As $x$ varies, the Aluminum Gallium Nitride (AlGaN) channel shows strain relaxation with the Aluminum Nitride (AlN) barrier. The degree of relaxation is modeled from AlN to GaN regions in the channel. It shows that the AlN barrier and buffer relaxation and strain recovery occurs due to the gradual crystal quality degradation from barrier/buffer to the channel interface. These combination devices show less drain current degradation with temperature variation from 300 K to 573 K. This model shows a good agreement with experimental data with a carrier density of $n_{s}$ = 2.8 $\times $ 10$^{13}$/cm$^{2}$.

On the Importance of the Polarity for GaN/InGaN Last Quantum Barriers in III-Nitride-Based Light-Emitting Diodes

In this paper, we investigate the electron injection efficiency in terms of different polarities and different polarization charge densities at the ${\rm{GaN/ InGaN}}$ interface. We find that the growth orientation for the ${\rm{GaN/ InGaN}}$ -type last quantum barrier is essentially vital, i.e., the ${\rm{GaN/ InGaN}}$ -type last quantum barrier is not able to effectively reduce the electron leakage and will degrade the light-emitting diode (LED) performance when the ${\rm{GaN/ InGaN}}$ interface is [000–1] polarized. However, a suppressed electron leakage and enhanced optical power can be obtained for III-nitride LEDs grown along the [0001] orientation when the ${\rm{GaN/ InGaN}}$ interface possesses polarization-induced negative charges. We conclude that the polarization-induced negative charges at the [0001] oriented ${\rm{GaN/ InGaN}}$ interface facilitate the surface depletion in the GaN region, i.e., the conduction band of the GaN region is bent in the way of favoring electron depletion and contributes to an enhanced conduction band barrier height for electrons.

Semipolar (202̅1) III-Nitride P-Down LEDs with in situ anneal to reduce the Mg memory effect

P-down LEDs (PDLEDs) have the potential to open up new design schemes for III-nitride LEDs compared to conventional n-down LEDs (NDLEDs). For light emitters operating above 480nm, the PDLED design enables the epitaxial advantages of semipolar (202̅1) and gains the polarization benefits of semipolar (202̅1̅). Here, we investigated semipolar (202̅1) InGaN-based PDLEDs in terms of their photoluminescence (PL) spectra and compositional profile. Despite concerns of the Mg memory effect degrading PDLED performance due to Mg-related non-radiative recombination centers, the PL intensities were nearly identical between the NDLED and PDLEDs, which emitted at wavelengths centered near 500nm. Secondary ion mass spectrometry revealed that the Mg doping levels in the multiple quantum well (MQW) active region were comparable for each structure, with average values of 2.9×1018cm−3 for the NDLED and 1.8×1018cm−3 for the PDLED. Prior to growing the active region MQW, a 700°C in situ anneal was carried out to reduce the average Mg concentration in the PDLED MQW to 3.7×1017cm−3. Its hydrogen concentration remained at 5×1019cm−3 in the p-type GaN region, which suggests that hydrogen passivation occurs during the growth of subsequent epitaxial layers in ammonia.

Three-dimensional current collapse imaging of AlGaN/GaN high electron mobility transistors by electric field-induced optical second-harmonic generation

Three-dimensional (3D) current collapse imaging of Schottky gate AlGaN/GaN high electron mobility transistor devices was achieved by a combination of two-dimensional (2D) and depth directional electric field-induced optical second-harmonic generation (EFISHG) measurements. EFISHG can detect the electric field produced by trapped carriers, which causes the current collapse. In the 2D measurement, the strong second-harmonic (SH) signals appeared within 1 μm from the gate edge on the drain side at 0.8 μs after the transition from the off- to no bias- state in both unpassivated and passivated samples. In the depth measurement, the SH signals were generated mainly from the AlGaN surface region of the unpassivated sample due to the presence of high-density trap sites in the AlGaN layer, and SH signals from bulk GaN region were also detected at 50 μs after the transition from the off- to no bias- state in the passivated sample. The origin of the traps is presumably the nitrogen vacancies in the GaN buffer layer.

GaN nanowire arrays by a patterned metal-assisted chemical etching

We developed an one-step and two-step metal-assisted chemical etching method to produce self-organized GaN nanowire arrays. In one-step approach, GaN nanowire arrays are synthesized uniformly on GaN thin film surface. However, in a two-step etching processes, GaN nanowires are formed only in metal uncovered regions, and GaN regions with metal-covering show nano-porous sidewalls. We propose that nanowires and porous nanostructures are tuned by sufficient and limited etch rate, respectively. PL spectra shows a red-shift of band edge emission in GaN nanostructures. The formation mechanism of nanowires was illustrated by two separated electrochemical reactions occur simultaneously. The function of metals and UV light was illustrated by the scheme of potential relationship between energy bands in Si, GaN and standard hydrogen electrode potential of solution and metals.

Improved Vertical GaN Schottky Diodes with Ion Implanted Junction Termination Extension

The realization of selectively implanted p-type regions in GaN as well as an understanding of processing effects that cause carrier type conversion are key enabling steps for vertical GaN devices. Here, GaN Schottky barrier diodes (SBDs) with edge termination formed by either a field plate or junction termination extension (JTE) achieved by ion implanted and symmetric multicycle rapid thermal annealing (SMRTA) are presented. The devices with JTE exhibited substantially reduced leakage currents and improved turn-on characteristics. This is attributed to the elimination of the plasma process steps associated with the deposition and patterning of the field oxide layer required in a field plate process. The breakdown characteristics were studied by electroluminescence imaging, and is indicative of avalanche behavior. The realization of vertical GaN devices with low reverse leakage and ion implanted termination regions represents a key step for future power electronic devices.

Dislocation confinement in the growth of Na flux GaN on metalorganic chemical vapor deposition-GaN

We have demonstrated a GaN growth technique in the Na flux method to confine c-, (a+c)-, and a-type dislocations around the interface between a Na flux GaN crystal and a GaN layer grown by metalorganic chemical vapor deposition (MOCVD) on a (0001) sapphire substrate. Transmission electron microscopy (TEM) clearly revealed detailed interface structures and dislocation behaviors that reduced the density of vertically aligned dislocations threading to the Na flux GaN surface. Submicron-scale voids were formed at the interface above the dislocations with a c component in MOCVD-GaN, while no such voids were formed above the a-type dislocations. The penetration of the dislocations with a c component into Na flux GaN was, in most cases, effectively blocked by the presence of the voids. Although some dislocations with a c component in the MOCVD-GaN penetrated into the Na flux GaN, their propagation direction changed laterally through the voids. On the other hand, the a-type dislocations propagated laterally and collectively near the interface, when these dislocations in the MOCVD-GaN penetrated into the Na flux GaN. These results indicated that the dislocation propagation behavior was highly sensitive to the type of dislocation, but all types of dislocations were confined to within several micrometers region of the Na flux GaN from the interface. The cause of void formation, the role of voids in controlling the dislocation behavior, and the mechanism of lateral and collective dislocation propagation are discussed on the basis of TEM results.