GaN Decomposition Research Articles

Role of Capping Material and GaN Polarity on Mg Ion Implantation Activation

Ion implantation of magnesium for p‐type GaN presents many opportunities; however, activation has proven difficult due to the decomposition of GaN at relevant annealing temperatures. Herein, testing the efficacy of multiple in situ and ex situ caps based on aluminum nitride and silicon nitride for GaN protection during annealing is presented. Photoluminescence shows better activation for in situ metal organic chemical vapor deposition (MOCVD)‐grown aluminum nitride caps compared with ex situ sputtered aluminum nitride and the best performance by ex situ plasma‐enhanced chemical vapor deposition (PECVD) silicon nitride. Furthermore, only samples annealed at the highest temperatures tested show preferential growth of UV luminescence to yellow‐green luminescence reinforcing the need for better capping solutions and higher temperature annealing.

FeGa2O4 nanowires preparation after milling and annealing of Fe doped GaN samples

In this work, FeGa2O4 nanowires were prepared using high-energy ball milling and subsequent annealing from GaN samples doped with Fe. The structural and chemical characterization were studied by X-ray diffraction (XRD), scanning electron microscopy (SEM), differential scanning calorimetry (DSC-TGA), and transmission electron microscopy (TEM). The XRD patterns showed that GaN samples doped with Fe reach nanocrystalline sizes after the process of high-energy ball milling. The DSC-TGA studies confirmed the GaN thermal decomposition and weight loss of the milled samples. XRD patterns reveal the formation of the FeGa2O4 compound with an fcc crystal lattice in GaN: Fe milled and subsequently annealed at 1100 °C. The TEM analysis of annealed powders showed the formation of FeGa2O4 nanowires with diameters of around 50 to 60 nm and lengths up to 2 μm. High-resolution transmission electron microscopy (HRTEM) images provide additional evidence of the fcc lattice structure of the nanowires. The decomposition of GaN produced FeGa2O4 nanowires since molten Ga at high temperature is combined with the doping element of Fe and then with the O2 in the air.

Role of RF power on physical properties of RF magnetron sputtered GaN/p-Si(1 0 0) thin film

Abstract GaN thin films were deposited on p-Si(1 0 0) substrates using RF magnetron sputtering at various RF powers. Influence of RF power on morphological, optical and structural properties of GaN thin films were investigated and presented in detail. XRD results proved that the films were polycrystalline in structure with (1 0 0) and (1 1 0) planes of hexagonal GaN. It was found that increasing RF power led to deterioration of crystal structure of the films due to increased decomposition of GaN. Stress in GaN thin films was calculated from XRD measurements and the reasons for this stress were discussed. Furthermore, it was analyzed and interpreted whether the experimental measurement results support each other. E2 (high) optical phonon mode of hexagonal GaN was obtained from the analysis of Raman results. UV-Vis spectroscopy results showed that optical band gap of the films varied by changing RF power. The reasons of this variation were discussed. AFM study of the surfaces of the GaN thin films showed that some of them were grown in Stranski-Krastanov mode and others were grown in Frank-Van der Merwe mode. AFM measurements revealed almost homogeneous, nanostructured, low-roughness surface of the GaN thin films. SEM analysis evidenced agglomerations in some regions of surface of the films and their possible causes have been discussed. It has been inferred that morphological, optical, structural properties of GaN thin film can be changed by controlling RF power, making them a potential candidate for LED, solar cell, diode applications.

Stimulated emission of AlGaN layers grown on sapphire substrates using ammonia molecular beam epitaxy

By optimising the growth temperature of the AlGaN layers and using high-temperature AlN buffer layers, high-quality AlxGa1 – xN layers (x = 0.15, 0.21, 0.26, and 0.3) were obtained, in which stimulated emission in the UV spectral range 330 – 297 nm was implemented with the threshold intensity of excitation Ith ≈ 0.7 – 1.4 MW cm−2, respectively. It is found that the threshold value of the stimulated emission of AlGaN layers grown by molecular beam epitaxy is largely determined by the intensity of the process of thermal decomposition of GaN, which affects the surface morphology and, consequently, the amount of optical scattering loss. It is shown that no pronounced localisation of nonequilibrium charge carriers occurs in the AlGaN layers, which is manifested in the absence of a large Stokes shift and in the realisation of optical amplification at transitions in an electron-hole plasma, and also indicates a relatively homogeneous material composition.

Effect of Thermal Cleaning Prior to p-GaN Gate Regrowth for Normally Off High-Electron-Mobility Transistors

This work studied the effect of thermal cleaning in metal-organic chemical vapor deposition (MOCVD) prior to p-GaN gate regrowth for normally off high-electron-mobility transistors. X-ray photoelectron spectroscopy, capacitance-voltage measurement, and atomic force microscopy were employed to identify the effects of thermal cleaning before p-GaN regrowth. It was found that the residual damage was hardly repaired at a relatively low thermal cleaning temperature, while GaN decomposition would occur at an excessively high temperature. Thermal cleaning at 850 °C for 2 min in MOCVD can effectively remove the surface contamination and alleviate the etch damage without causing any significant deterioration of the AlGaN barrier. In addition, the density of interface states ( Dit) in the p-GaN gate was reduced from 1012-1013 to 1011-1012 eV-1·cm-2, resulting in a low gate reverse leakage of 0.1 nA/mm @ VDS-OFF = 180 V, a high Ion/ Ioff ratio of 4 × 1010, and a relatively high threshold voltage of +1.7 V @ ID = 10 μA/mm.

Implementation of an artificial neural network to predict properties of MOVPE-grown AlGaN layers

A feed-forward artificial neural network was employed in order to predict the growth rate and composition of AlxGa1-xN metalorganic vapor deposition-grown layers depending on the flow of a precursor gas (trimethylaluminum, trimethylgallium, and ammonia). The results showed a good agreement with experimental data. The obtained dependencies seem to be reasonable in the context of well-known effects: the presence of parasitic reactions between trimethylaluminum and ammonia and GaN decomposition at low ammonia (high hydrogen) flow.

Energy band structure and electrical properties of Ga-oxide/GaN interface formed by remote oxygen plasma

The energy band structure of a Ga-oxide/GaN structure formed by remote oxygen plasma exposure and the electrical interface properties of the GaN metal–oxide–semiconductor (MOS) capacitors with the SiO2/Ga-oxide/GaN structures with postdeposition annealing (PDA) at various temperatures have been investigated. Reflection high-energy electron diffraction and X-ray photoelectron spectroscopy clarified that the formed Ga-oxide layer is neither a single nor polycrystalline phase with high crystallinity. We found that the energy band offsets at the conduction band minimum and at the valence band maximum between the Ga-oxide layer and the GaN surface were 0.4 and 1.2 ± 0.2 eV, respectively. Furthermore, capacitance–voltage (C–V) characteristics revealed that the interface trap density (Dit) is lower than the evaluation limit of Terman method without depending on the PDA temperatures, and that the SiO2/Ga-oxide stack can work as a protection layer to maintain the low Dit, avoiding the significant decomposition of GaN at the high PDA temperature of 800 °C.

GaN decomposition after high-energy ball milling and its transformation to BN hollow spheres in annealing samples doped with B

In this work, GaN powders doped with B and Si were treated by high-energy ball milling, and subsequently, annealing at different temperatures. Transmission electron microscopy (TEM) observations showed the in-situ GaN decomposition in the undoped GaN specimen milled for 5 h. TEM also revealed that the chemical decomposition of GaN was more affected in samples doped with B. Thermo-gravimetric analysis indicates that an undoped GaN specimen decomposes at 1120 °C, Si-doped GaN samples disintegrate at 1150 °C, and finally, B-doped GaN samples decompose at the lower temperature of 940 °C. TEM and SEM results show that the annealing treatment of the as-milled B-doped GaN samples at 1150 °C for 15 min leads to the GaN decomposition and BN hollow nanostructures formation under an N2 atmosphere. The synthesized product consists of mesoporous powders with variable particle sizes from 500 nm to 1 μm having variable cavities in the range from 60 nm to 800 nm.

Micro-structure changes induced by femtosecond laser on the surface of GaN multilayer film grown on Si substrate

The light-emitting-diode (LED) surface of GaN epilayers grown on Si substrate was irradiated by a femtosecond laser (λ = 800 nm). The morphology of LED surface after laser irradiation was measured by optical microscope and scanning electron microscopy. It was found that many radial cracks appeared in the film on account of thermal expansion and then became more numerous as the number of laser pulses increased. In addition, the changes of the micro structure of GaN and Si substrate were studied by means of Micro Raman Spectroscopy. The results showed the presence of polycrystalline silicon and the decomposition of GaN had taken place in the laser spot or the region with the large laser fluence. Comparatively, in the areas with the low laser fluence or the edge of laser spot, the characteristic peaks of GaN films and the AO phonon peak of Si had an obvious blueshift, which most likely was the result of compressive stress between GaN layers and Si substrate. The maximum values of the compressive stress were 0.74 GPa and 1.4 GPa, respectively. Furthermore, with the increase of the laser fluence, the original Si crystalline structure recovered gradually. The above study showed that the stress, the transformation and the deformation play a predominant role in the femtosecond laser etching process of LED.

Observation of the early stages of GaN thermal decomposition at 1200 °C under N2

We investigated the partial decomposition of GaN layers grown in an atmospheric pressure metal organic vapor phase epitaxy (AP-MOVPE) vertical reactor at 1200 °C under N2 ambient. In these conditions, the early stages of GaN thermal decomposition were studied. The GaN decomposition was monitored by in-situ laser reflectometry (LR). The properties of the as grown and decomposed GaN samples were analyzed by scanning electron microscope (SEM). We show that GaN decomposition starts by the formation of GaN nano-grains at the early stages. Then, due to the anisotropy of the decomposition, the GaN nano-grains are more easily etched than the GaN (00.2) surface. The lateral etching may results in local smooth GaN surface formation. After that the depth etching starts again on the etched pits. Room temperature cathodoluminescence (CL) study revealed better optical properties of the GaN grains when compared to the whole GaN surface.

Effects of hydrogen etching on stress control in AlN interlayer inserted GaN MOVPE on Si

Clarification and control of the hydrogen etching of GaN is essential to achieving high-quality GaN-on-Si virtual substrates and devices based on it produced by metalorganic vapor phase epitaxy. This phenomenon and its effects on GaN-on-Si stress control were studied in this work. Without deliberate protection, voids with lateral sizes on the micrometre level underneath AlN interlayers emerged in GaN. Such voids made stress balancing in GaN-on-Si systems by inserting AlN interlayers less efficient. By flowing a protective large flow rate of ammonia, voids were eliminated while GaN decomposition still happened, which led to AlGaN alloy in the interlayers. Interfaces of Ga incorporated AlN interlayers grown under large-scale varied conditions were characterised by scanning transmission electron microscopy and energy dispersive spectrometry. Higher growth temperatures caused more Ga in the interlayers and weakened their capacity to induce compressive stress in the overlying GaN.

Nanoscale morphology and optical property evolution of Pt nanostructures on GaN (0 0 0 1) by the systematic control of annealing temperature and duration with various Pt thickness

By the controlled fabrication of Pt nanostructures, various surface morphology dependent electronic, catalytic and optical properties can be exploited for a wide range of applications. In this paper, the evolution of Pt nanostructures on GaN (0 0 0 1) by the solid-state dewetting of Pt thin films is investigated. Controlling the annealing temperature, time and film thickness allows us to fabricate distinct size, density and configurations of Pt nanostructures. For 10 nm Pt thickness, tiny voids and Pt hillocks up to 550 °C, extensive void expansion and Pt nanostructure evolution between 600 °C–750 °C and finally Pt nanostructures assisted nanoholes penetration on GaN surface above 800 °C are demonstrated. Furthermore, comparatively elongated Pt nanostructures and NHs are resulted with 20 nm Pt thickness and voids growth and connected Pt nanostructure are formed by annealing duration control. The transformation of Pt films to nanostructures is governed by the surface diffusion, Rayleigh instability, Volmer–Weber growth and energy minimization mechanism whereas NHs penetration is commenced by the decomposition of GaN, Pt–Ga alloying and nitrogen desorption at high temperature. In addition, the optical characteristic of Pt nanostructures on GaN (0 0 0 1) by reflectance, photoluminescence (PL) and Raman spectroscopy demonstrate the surface morphology dependent spectral response.

On the Role of AlN Insertion Layer in Stress Control of GaN on 150-mm Si (111) Substrate

In this study, low-temperature (LT) and high-temperature (HT) AlN insertion layers (ILs) grown at 680 and 970 °C were integrated with 3.7-μm GaN-based heterostructure grown on 150-mm Si (111) substrates by metalorganic chemical vapor deposition. Under a V/III flow ratio of 1960, the GaN epilayer with a continuous interface resulting from the LT AlN IL was subject to a compressive stress of −0.109 GPa. However, the GaN epilayer with discontinuous interfaces resulting from the HT AlN IL growth under the same flow ratio was subject to a tensile stress of 0.174 GPa. To realize continuous interfaces between the GaN epilayer and HT AlN IL, a higher V/III ratio of 5960 was utilized to suppress the decomposition of GaN. It results in changing the stress state of the GaN-based heterostructure from tensile to compressive. This strategic finding indicates that a stress-controllable GaN on Si can be achieved via the incorporation of HT AlN ILs. A minimum curvature at 5 km−1 is demonstrated for the 3.7-μm GaN-based heterostructure on a 150-mm Si (111) substrate, which has high potential for power switching device applications.

The influence of MOVPE growth conditions on the shell of core-shell GaN microrod structures

A core-shell geometry is employed for most next-generation, three-dimensional opto-electric devices based on III–V semiconductors and grown by metal organic vapor phase epitaxy (MOVPE). Controlling the shape of the shell layers is fundamental for device optimization, however no detailed analysis of the influence of growth conditions has been published to date.We study homogeneous arrays of gallium nitride core-shell microrods with height and diameter in the micrometer range and grown in a two-step selective area MOVPE process. Changes in shell shape and homogeneity effected by deliberately altered shell growth conditions were accurately assessed by digital analysis of high-resolution scanning electron microscope images.Most notably, two temperature regimes could be established, which show a significantly different behavior with regard to material distribution. Above 900°C of wafer carrier temperature, the shell thickness along the growth axis of the rods was very homogeneous, however variations between vicinal rods increase. In contrast, below 830°C the shell thickness is higher close to the microrod tip than at the base of the rods, while the lateral homogeneity between neighboring microrods is very uniform. This temperature effect could be either amplified or attenuated by changing the remaining growth parameters such as reactor pressure, structure distance, gallium precursor, carrier gas composition and dopant materials. Possible reasons for these findings are discussed with respect to GaN decomposition as well as the surface and gas phase diffusion of growth species, leading to an improved control of the functional layers in next-generation 3D V–III devices.

Fabrication of InGaN/GaN MQW nano‐LEDs by hydrogen‐environment anisotropic thermal etching

III‐nitride semiconductor nanostructures are attractive materials for the realization of high‐performance and highly functional optoelectronic devices. In this study, we fabricated InGaN/GaN multiple quantum well (MQW) nanostructured light‐emitting diodes (nano‐LEDs) using hydrogen‐environment anisotropic thermal etching (HEATE). HEATE is a low‐damage anisotropic etching technique that is based on the thermal decomposition of (In)GaN under a low‐pressure hydrogen environment. The InGaN/GaN MQW nano‐LEDs showed good rectification characteristics and blue emission without any post treatment to remove all etching damage. The narrowest MQW lateral sizes of successfully fabricated nanowall and nanopillar LEDs were 87 and 70 nm, respectively. These LEDs are among the smallest InGaN/GaN‐based nano‐LEDs fabricated via a top–down process.Electroluminescence images of InGaN/GaN nano‐LEDs operated at 3 V. (a) An a‐axis honeycomb nanowall, (b) an m‐axis nanowall array, (c) an a‐axis nanowall array, and (d) a nanopillar array.

Migration mechanism of a GaN bicrystalline grain boundary as a model system

Using in situ high-resolution transmission electron microscopy, we have explored migration mechanism of a grain boundary in a GaN bicrystal as a model system. During annealing at 500 °C, the grain-boundary region underwent a decrease in thickness, which occurred by decomposition or sublimation of GaN during annealing at 500 °C coupled with electron-beam sputtering. The decrease in thickness corresponds to an increase in the driving force for migration, because the migration of the grain boundary was driven by the surface energy difference. As the driving force increased with annealing time, the grain-boundary morphology turned from atomically smooth to rough, which is characterized by kinetic roughening. The observations indicate that a grain boundary exhibits a nonlinear relationship between driving force for migration and migration velocity, in discord with the general presumption that a grain boundary follows a linear relationship.

Dependence of adhesion strength between GaN LEDs and sapphire substrate on power density of UV laser irradiation

Selective laser lift-off (SLLO) is an innovative technology used to manufacture and repair micro-light-emitting diode (LED) displays. In SLLO, laser is irradiated to selectively separate micro-LED devices from a transparent sapphire substrate. The light source used is an ultraviolet (UV) laser with a wavelength of 266nm, pulse duration of 20ns, and repetition rate of 30kHz. Controlled adhesion between a LED and the substrate is key for a SLLO process with high yield and reliability. This study examined the fundamental relationship between adhesion and laser irradiation. Two competing mechanisms affect adhesion at the irradiated interface between the GaN LED and sapphire substrate: Ga precipitation caused by the thermal decomposition of GaN and roughened interface caused by thermal damage on the sapphire. The competition between these two mechanisms leads to a non-trivial SLLO condition that needs optimization. This study helps understand the SLLO process, and accelerate the development of a process for manufacturing micro-LED displays via SLLO for future applications.

Nanoparticles to Nanoholes: Fabrication of Porous GaN with Precisely Controlled Dimension via the Enhanced GaN Decomposition by Au Nanoparticles

Porous GaN exhibits unique optoelectronic, chemical, and physical properties such as shift of band gap, increased surface area ratio, excellent chemical, mechanical, and thermal stability as well as efficient luminescence as compared to its bulk counterpart. Herein, we demonstrate a precise, efficient, and still cost-effective method of the fabrication of porous GaN through the enhanced GaN decomposition by using Au nanoparticles (NPs) as a catalyst, in which the size, density, and shape of the pores (nanoholes, NHs) can be precisely controlled. By the thermal annealing assisted with the Au NPs, the NHs are successfully fabricated, and the existence of Au NPs significantly accelerate the GaN decomposition at the interface between the NPs and GaN due to the Ga absorption by the Au NPs. We systematically study the formation mechanism of NHs assisted by the Au NPs by means of annealing temperature, duration, and Au deposition amount, and the results are systematically analyzed and discussed.

Effect of High-Temperature Annealing on Yellow and Blue Luminescence of Undoped GaN **Supported by the National Basic Research Program of China under Grant Nos 2011CB301900, 2012CB619200 and 2012CB619304, the High-Technology Research and Development Program of China under Grant Nos 2014AA032605 and 2015AA033305, the National Natural Science Foundation of China under Grant Nos 60990311, 61274003, 61422401, 51461135002, 60936004, 61176063 and 61334009,

The effect of high-temperature annealing on the yellow and blue luminescence of the undoped GaN is investigated by photoluminescence (PL) and x-ray photoelectron spectroscopy (XPS). It is found that the band-edge emission in the GaN apparently increases, and the yellow luminescence (YL) and blue luminescence (BL) bands dramatically decrease after annealing at 700°C. At the annealing temperature higher than 900°C, the YL and BL intensities show an enhancement for the nitrogen annealed GaN. This fact should be attributed to the increment of the Ga and N vacancies in the GaN decomposition. However, the integrated PL intensity of the oxygen annealed GaN decreases at the temperature ranging from 900°C to 1000°C. This results from the capture of many photo-generated holes by high-density surface states. XPS characterization confirms that the high-density surface states mainly originate from the incorporation of oxygen atoms into GaN at the high annealing temperature, and even induces the 0.34eV increment of the upward band bending for the oxygen annealed GaN at 1000°C.

Influence of high-temperature postgrowth annealing under different ambience on GaN quantum dots grown via Ga droplet epitaxy

The influence of high-temperature postgrowth annealing (PGA) under NH3/H2, NH3/N2 ambience on the morphologies and nitridation degree of GaN/AlN QDs grown via droplet epitaxy is investigated. The results show that the size and density of GaN QDs changes with different ambiences and the NH3/N2 ambience is demonstrated as a necessary condition for maintaining optimal QD morphology by suppressing the migration and evaporation of Ga atoms and preventing the GaN decomposition. Moreover, the PGA process can effectively enhance the nitridation and crystallization of GaN QDs and the photoluminescence performance has been effectively improved after annealed under NH3/N2 ambience.