High-temperature annealing of bulk GaN layers

The influence of high-temperature (1010°C) annealing in a flow of gaseous ammonia on the properties of bulk GaN layers, grown by chloride-hydride VPE and then separated from SiO₂ substrates, was studied using atomic force microscopy. The bulk (∼360-μm-thick) epitaxial GaN layers were synthesized in two steps: a first stage of nucleation and growth at a low temperature (530°C) followed by epitaxy at a high temperature (970°C). It was found that the annealing increases the nanorelief height and activates the donor-acceptor recombination on the surface of a GaN layer grown at the lower temperature and decreases the intensity of photoluminescence from the layer grown at the higher temperature.     

Effects of annealing on the structural properties of indium rich InGaN films

Indium rich (In-rich) InGaN films were grown on Ge (111) substrate by plasma assisted molecular beam epitaxy with thin GaN as a buffer layer. The effects of annealing temperature and annealing time on the structural properties of In-rich InGaN films were investigated by X-ray diffraction (XRD). XRD results indicate that the as-grown InGaN films annealed at different temperatures for 1 min and 1 h respectively did not improve the film crystalline quality. But with the annealing at 750 °C and 800 °C for 1 min respectively the metallic indium was desorbed from the InGaN structure. The InGaN films annealed at higher than 660 °C for 1 h also showed the indium desorption. The InGaN film has the best film quality after annealed at 660 °C for 6 h with the full-width at half-maximum of InGaN (002) peak to be 879 arcsec. The InGaN crystalline quality started to degrade after annealed at the temperatures higher than 660 °C for 6 h.     

Improved growth of GaN layers on ultra thin silicon nitride/Si (1 1 1) by RF-MBE

High-quality GaN epilayers were grown on Si (1 1 1) substrates by molecular beam epitaxy using a new growth process sequence which involved a substrate nitridation at low temperatures, annealing at high temperatures, followed by nitridation at high temperatures, deposition of a low-temperature buffer layer, and a high-temperature overgrowth. The material quality of the GaN films was also investigated as a function of nitridation time and temperature. Crystallinity and surface roughness of GaN was found to improve when the Si substrate was treated under the new growth process sequence. Micro-Raman and photoluminescence (PL) measurement results indicate that the GaN film grown by the new process sequence has less tensile stress and optically good. The surface and interface structures of an ultra thin silicon nitride film grown on the Si surface are investigated by core-level photoelectron spectroscopy and it clearly indicates that the quality of silicon nitride notably affects the properties of GaN growth.     

采用阳极氧化铝做掩膜生长氮化镓膜

采用均匀的多孔阳极氧化铝做掩膜在氢化物气相外延设备中生长出高质量的氮化镓膜.采用扫描电镜观察了氮化镓膜的界面性质并用阴极发光谱表征了截面上氮化镓层在不同位置的的发光性质,发现随着厚度的增加,其发光特性得到改善,而且由于掩膜结构的引入,外延膜中的压应力得到一定程度的释放.     

Stress reduction in GaN films on (111) silicon-on-insulator substrate grown by metal-organic chemical vapor deposition

The GaN film was grown on the (111) silicon-on-insulator (SOI) substrate by metal-organic chemical vapor deposition and then annealed in the deposition chamber. A multiple beam optical stress sensor was used for the in-situ stress measurement, and X-ray diffraction (XRD) and Raman spectroscopy were used for the characterization of GaN film. Comparing the characterization results of the GaN films on the bulk silicon and SOI substrates, we can see that the Raman spectra show the 3.0 cm^− 1 frequency shift of E₂(TO), and the full width at half maximum of XRD rocking curves for GaN (0002) decrease from 954 arc sec to 472 arc sec. The results show that the SOI substrates can reduce the tensile stress in the GaN film and improve the crystalline quality. The annealing process is helpful for the stress reduction of the GaN film. The SOI substrate with the thin top silicon film is more effective than the thick top silicon film SOI substrate for the stress reduction.     

Growth of continuous GaN films on ZnO buffer layer by chemical vapor deposition for ultraviolet photodetector

In this work, c-axis-oriented continuous GaN films have been synthesized by the chemical vapor deposition (CVD) method using ZnO material as the intermediate buffer layer. The GaN films with different growth temperatures exhibit high crystal quality and small surface roughness due to the same crystal structure and low lattice mismatches rate between GaN and ZnO materials. Meanwhile, the UV photodetector based on the CVD-grown GaN film exhibits a relatively high responsivity, fast rise and decay time, and good thermal stability. Our work provides a simple and promising CVD method to fabricate continuous GaN film for electronic and optoelectronic devices.

     

GaN epilayer on separately deposited buffer layer by hot wall epitaxy

High quality GaN epilayers were grown on a sapphire substrate using a hot wall epitaxy method. We have investigated the crystal, optical, and electrical properties of GaN epilayers grown as functions of the nitridation condition of the substrate and the growth condition of GaN buffer layer. In order to study an effective method to grow a buffer layer for the growth of high quality GaN epilayer, the buffer layers were formed on the nitridated substrate using two different methods. One is separately deposited buffer layer (SDBL), and the other is co-deposited buffer layer (CDBL). It was observed that the growth condition of the buffer layer had a strong influence on the crystal and optical properties of GaN epilayer. A strong band edge emission peak at 3.474 eV was observed from the photoluminescence spectrum measured at 5 K for GaN epilayer grown at the optimum condition of the buffer layer. The carrier concentration and mobility of undoped GaN epilayer grown with a growth rate of 0.5 μm h⁻¹ were 2 × 10¹⁸ cm⁻³ and >50 cm³ V⁻¹ s⁻¹ at room temperature, respectively.     

多晶硅薄膜的铝诱导晶化法制备及其晶粒的择优取向特性

采用铝诱导非晶硅薄膜晶化技术制备了多晶硅薄膜,并研究了多晶硅的成核和生长特性。非晶硅薄膜采用等离子体增强化学气相沉积法制备,其表面沉积铝薄膜后经不同温度的氮氛围退火处理。结果表明,退火后的硅薄膜层与铝层发生置换,所生长的多晶硅颗粒的平均尺寸约为150nm。X射线衍射分析结果揭示,薄膜的晶向显著依赖于退火温度,较低温度下,铝诱导晶化速率较慢,薄膜的优化晶向与非晶硅薄膜中团簇的初始原子排列趋势紧密相关。而较高温度下,铝诱导晶化促使多晶硅(111)择优成核及随后的固相生长。     

Microstructural and electrical characteristics of rapidly annealed Ni/Mo Schottky rectifiers on cleaned n-type GaN (0001) surface

We have investigated the electrical and microstructural properties of Ni/Mo Schottky rectifiers to n-type GaN by current–voltage (I–V) and transmission electron microscopy (TEM) before and after annealing at 600 °C. The obtained barrier height for as-deposited Ni/Mo contact is 0.66 eV. It is observed that the barrier height increases with annealing temperature up to 500 °C, reaching a maximum value of 0.75 eV at this temperature. However, the Schottky barrier height of the Ni/Mo Schottky contact slightly decreased to 0.67 eV (I–V) when the contact was annealed at 600 °C. According to the HRTEM, STEM and EDX analysis, the formation of Ga-Ni interfacial layer at the interface results in the accumulation of gallium vacancies near the surface of the GaN layer. This could be the reason for increase in barrier heights upon annealing at elevated temperatures. The variation in the measured barrier height after annealing at 600 °C may be due to the formation of native oxide layer at the interface compared to the 500 °C annealed contact.     

Growth and thermoelectric properties of multilayer thin film of bismuth telluride and indium selenide via rf magnetron sputtering

A bismuth telluride (BT)/indium selenide (IS) multilayer film was deposited at room temperature by rf magnetron sputtering on a sapphire substrate in order to investigate how the multilayered structure affects the microstructure and thermoelectric properties. The effect of annealing at different temperatures was also studied. The results were compared with those from a BT film with the same thickness. A TEM study showed that the interface between the BT and IS layers became vague as the annealing temperature increased, and the BT layer crystallized while the IS layer did not. The presence of thin IS layers can help to limit the evaporation of Te from the BT/IS multilayer film, thus increasing the amount of Bi2Te3 phase in the multilayer film as compared with that of the BT film. An abrupt increase in the Seebeck coefficient of the multilayer film was observed when annealed at 300 degrees C, and the resistivity of the annealed multilayer film was high compared to that of the BT film. This result can also be explained by the proposed role of the IS layer, which limits the evaporation of Te at high temperature. The highest power factor of -3.9 x 10(-6) W/K2 cm was obtained at room temperature from the multilayer film annealed at 300 degrees C.     

Microstructure evolution and development of annealed Ni/Au contacts to GaN nanowires

The development of Ni/Au contacts to Mg-doped GaN nanowires (NWs) is examined. Unlike Ni/Au contacts to planar GaN, current-voltage (I-V) measurements of Mg-doped nanowire devices frequently exhibit a strong degradation after annealing in N(2)/O(2). This degradation originates from the poor wetting behavior of Ni and Au on SiO(2) and the excessive void formation that occurs at the metal/NW and metal/oxide interfaces. The void formation can cause cracking and delamination of the metal film as well as reduce the contact area at the metal/NW interface, which increases the resistance. The morphology and composition of the annealed Ni/Au contacts on SiO(2) and the p-GaN films were investigated by scanning electron microscopy (SEM), energy-dispersive x-ray spectroscopy (EDS) and x-ray diffraction (XRD) measurements. Adhesion experiments were performed in order to determine the degree of adhesion of the Ni/Au films to the SiO(2) as well as observe and analyze the morphology of the film's underside by SEM. Device degradation from annealing was prevented through the use of a specific adhesion layer of Ti/Al/Ni deposited prior to the nanowire dispersal and Ni/Au deposition. I-V measurements of NW devices fabricated using this adhesion layer showed a decrease in resistance after annealing, whereas all others showed an increase in resistance. Transmission electron microscopy (TEM) on a cross-section of a NW with Ni/Au contacts and a Ti/Al/Ni adhesion layer showed a lack of void formation at the contact/NW interface. Results of the XRD and TEM analysis of the NW contact structure using a Ti/Al/Ni adhesion layer suggests Al alloying of the Ni/Au contact increases the adhesion and stability of the metal film as well as prevents excessive void formation at the contact/NW interface.     

Study of GaP single crystal layers grown on GaN by MOCVD

The performance of GaN based devices could possibly be improved by utilizing the good p-type properties of GaP layer and it provides the possibility of the integration of InAlGaN and AlGaInP materials to produce new devices, if high quality GaP compounds can be grown on III-nitride compounds. In this paper, the growth of GaP layers on GaN by metalorganic chemical vapor deposition (MOCVD) has been investigated. The results show that the GaP low temperature buffer layer can provide a high density of nucleation sites for high temperature GaP growth. Using a 40 nm thick GaP buffer layer, a single crystal GaP layer, whose full-width at half-maximum of the (1 1 1) plane measured by double crystal X-ray diffraction is 580″, can be grown on GaN. The V/III ratio plays an important role in the GaP layer growth and an appropriate V/III ratio can improve the quality of GaP layer. The GaP:Mg layer with hole carrier concentration of 4.2 × 10¹⁸ cm⁻³ has been obtained.     

Growth process from amorphous GaN to polycrystalline GaN on Si (111) substrates

Amorphous GaN (a-GaN) films on Si (111) substrates have been deposited by RF magnetron sputtering with GaN powder target. The growth process from amorphous GaN to polycrystalline GaN is studied by XRD, SEM, PL and Raman. XRD data mean that annealing under flowing ammonia at 850-950 °C for 10 min converts a-GaN into polycrystalline GaN (p-GaN). The growth mechanism can be mostly reaction process through N³⁻ in amorphous GaN replaced by N³⁻ of NH₃. Annealing at 1000 °C, the appearance of GaN nanowires can be understood based on the vapor-liquid-solid (VLS) mechanism. In addition, XRD, PL and Raman measurement results indicate that the quality of GaN films increases with increasing temperature. The tensile stress in the films obtained at 1000 °C is attributable to the expansion mismatch between GaN and Si, with the gallium in the film playing a negligible role.     

Structural and electrical properties of Au and Ti/Au contacts to n-type GaN

Au and Ti/Au layers were deposited on n-GaN. The samples were annealed at 400, 700 and 900 °C for 10 min in vacuum. The contacts were rectifying up to 700 °C and the highest Schottky barrier height of 1.07 eV was obtained for an Au single layer by current-voltage measurements. A binary phase of Au₂Ga was identified at the interface of the n-GaN/Ti/Au contact after annealing at 900 °C. The formation of Ti₂N and TiN (twin) phases epitaxially grown on GaN was also observed in the same contact as well as some gold diffusion into the topmost region of the GaN epilayer.     

Controlling of the electrical resistivity of GaN layer using AIN nucleation layer

The sheet resistance (Rs) of undoped GaN films on AIN/c-plane sapphire substrate was investigated. The Rs was strongly dependent on the AIN layer thickness and semi-insulating behavior was observed. To clarify the effect of crystalline property on Rs, the crystal structure of the GaN films has been studied using X-ray scattering and transmission electron microscopy. A compressive strain was introduced by the presence of AIN nucleation layer (NL) and was gradually relaxed as increasing AIN NL thickness. This relaxation produced more threading dislocations (TD) of edge-type. Moreover, the surface morphology of the GaN film was changed at thicker AIN layer condition, which was originated by the crossover from planar to island grains of AIN. Thus, rough surface might produce more dislocations. The edge and mixed dislocations propagating from the interface between the GaN film and the AIN buffer layer affected the electric resistance of GaN film.     

Synthesis and characterization of heteroepitaxial GaN films on Si(111)

We report crack-free and single-crystalline wurtzite GaN heteroepitaxy layers have been grown on Si (111) substrate by metal-organic chemical vapor deposition(MOCVD). Synthesized GaN epilayer was characterized by X-ray diffraction(XRD), atomic force microscope (AFM) and Raman spectrum. The test results show that the GaN crystal reveals a wurtzite structure with the <0001> crystal orientation and XRD ω-scans showed a full width at half maximum (FWHM) of around 583 arcsec for GaN grown on Si substrate with an HT-AlN buffer layer. In addition, the Raman peaks of E₂^high and A₁(LO) phonon mode in GaN films have an obvious redshit comparing to bulk GaN eigen-frequency, which most likely due to tensile strain in GaN layers. But the AO phonon mode of Si has a blueshit which shows that the Si substrate suffered a compressive strain. And we report that the AlN buffer layer plays a role for releasing the residual stress in GaN films.     

Structural properties of GaN grown on AlGaN/AlN stress mitigating layers on 100-mm Si (111) by ammonia molecular beam epitaxy

The structural properties of GaN grown on AlGaN/AlN stress mitigating layers on 100-mm diameter Si (111) substrate by ammonia molecular beam epitaxy have been reported. High resolution X-ray diffraction, micro-Raman spectroscopy, transmission electron microscopy and secondary ion mass spectroscopy have been used to study the influence of AlN thickness and AlGaN growth temperature on the quality of GaN. GaN grown on thicker AlN showed reduced dislocation density and lesser tensile strain. Three-dimensional growth regime was observed for GaN grown at lower AlGaN growth temperature while higher AlGaN growth temperature resulted in two-dimensional growth mode. The dislocation bending and looping at the AlGaN/AlN interface was found to have significant influence on the dislocation density and strain in the GaN layer. The evolution and interaction of threading dislocations play a major role in determining the quality and the strain states of GaN.     

Effect of high temperature AlGaN buffer thickness on GaN Epilayer grown on Si(111) substrates

Crack-free GaN epitaxial layer was obtained through inserting 80 nm graded AlGaN buffer layer between GaN epilayer and high temperature AlN buffer on 2-in Si(111) substrates by metal organic chemical vapor deposition. This paper investigated the influence of AlGaN buffer thickness on the structural properties of the GaN epilayer. It was confirmed from the optical microscopy and scanning electronic microscopy that the graded AlGaN buffer with optimized thickness had a remarkable effect on introducing relative compressive strain to the top GaN layer and preventing the formation of cracks. X-ray diffraction and atomic force microscopy analysis showed that AlGaN buffer with proper thickness could improve the crystal quality and surface morphology of the GaN film. Transmission electron microscopy analysis revealed that a significant reduction in threading dislocations was achieved in GaN epilayer by the insertion of graded AlGaN buffer.     

Employing Al buffer layer with Al droplets-distributed surface to obtain high-quality and stress-free GaN epitaxial films on Si substrates

Al buffer layers with Al droplets-distributed surface have been employed to grow high-quality and stress-free GaN epitaxial films on Si substrates. The Al droplets are proved to efficiently improve the quality of as-grown GaN. On the one hand, they can act as nucleation seeds to facilitate the epitaxial growth, improving the crystalline quality and surface morphology of as-grown GaN epitaxial films. On the other hand, they also can compensate the huge compressive stress produced by Al buffer layer during the cooling process, achieving stress-free film. The density and volume of Al droplets greatly impact the properties of as-grown GaN epitaxial films. The GaN epitaxial film grown on the Al buffer layer with many small Al droplets uniformly distributed on it shows the best crystalline quality with the full-width at half maximum (FWHM) of GaN(0002) and GaN(10–12) as 0.5° and 0.7°, respectively, and flat surface with the smallest surface root-mean-square roughness of 3.8 nm. In addition, it also exhibits relatively better photoelectric properties with an FWHM of near band gap emission peak of 18 nm, carrier concentration of 2.0 × 10¹⁷ cm^?3, and mobility of 137.1 cm²/Vs. This work has revealed the advantages of Al buffer layer and the important effects of buffer layer surface on achieving high-quality GaN by PLD, which is of significance for various applications of GaN-based devices.     

Improvement of quality and strain relaxation of GaN epilayer grown on SiC substrate by in situ SiNx interlayer

Improved structural quality and tensile stress releasing were realized in GaN thin films grown on 6H–SiC by metal organic chemical vapor deposition using an in situ porous SiN_x interlayer. The SiN_x was formed in situ in the growth chamber by simultaneous flow of diluted silane and ammonia, leading to the formation of a randomly distributed mask layer and induced lateral overgrowth similar to conventional epitaxial lateral overgrowth of GaN. The full width at half maximum (FWHM) of X-ray diffraction peaks decreases dramatically by the SiN_x interlayer, indicating an improved crystalline quality. Also, it was found that the biaxial tensile stress in the GaN film was significantly reduced by in situ SiN_x interlayer from Raman spectra. Low temperature photoluminescence spectra exhibited a narrower FWHM by the SiN_x interlayer.