Defect structure in GaN pyramids

High-quality GaN/AlN layers grown on (111) Si substrates have been used as the seeding layer for lateral epitactic overgrowth of GaN. The selective overgrowth was controlled by depositing a Si₃N₄ mask on the GaN seed layer. Growth of additional GaN resulted in the formation of GaN pyramids above the apertures in the patterned Si₃N₄ mask. Transmission electron microscopy showed that the GaN pyramids, the GaN seed layer, and the AlN buffer layer in the samples have the following epitactic relationship with respect to the silicon substrate: and . The pyramids were found to consist of a defective core region and a nearly defect-free outer region. In the core of the pyramid (at, or above, the aperture in the mask), numerous dislocations thread through the pyramid perpendicular to the interface plane. Some of these threading dislocations, which originated from the GaN/AlN seed layer, bend abruptly through 90° at the edge of this core region. In the outer part of the GaN pyramid, the density of vertically propagating dislocations was much lower. Most of the dislocations in this region are closely parallel to the original (0001) substrate plane. The top few microns of material are found to be essentially defect-free. The growth mechanism of the GaN pyramids is discussed in light of this defect structure.     

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.     

Deposition of GaN films on (1 1 1) Si substrates by alternate supply of TMG and NH3

GaN films were grown on (1 1 1) Si substrates at 1000 °C by separate admittances of trimethylgallium (TMG) and ammonia (NH₃). To achieve high quality GaN films, the optimization in growth temperature and layer thickness of AlN buffer layer between GaN film and Si substrate is required. Cross-sectional transmission electron microscopic observations of the GaN/(1 1 1)Si samples show a nearly parallel orientation relationship between the (0 0 0 1) planes of GaN film and the (1 1 1) planes of Si substrate. Room temperature photoluminescence spectra of high quality GaN films show a strong near band edge emission and a weak yellow luminescence. The achievement of high quality GaN films on (1 1 1) Si substrates is believed to be attributed to enhancement in surface mobilities of the adsorbed surface species and adequate accommodation of lattice mismatch between high temperature AlN buffer layer and Si substrate.     

Flip-Chip p(GaN)-i(GaN)-n(AlGaN) Narrowband UV-A Photosensors

Flip-chip p(GaN)-i(GaN)-n(AlGaN) photosensors with extremely low dark currents were fabricated and characterized. It was found that the sensor with a 0.5-$muhboxm$-thick Si-doped$hboxn^+hbox-Al_0.15hboxGa_0.85hboxN$layer could only detect optical signals with wavelength in between 325 and 360 nm. With an incident wavelength of 355 nm, the authors achieved a peak responsivity of 0.16 A/W at zero bias, which corresponds to an external quantum efficiency of 56%.     

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.     

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.     

Structure and Lattice Dynamics of R $$_2$$ 2 Sn $$_2$$ 2 O $$_7$$ 7 and R $$_2$$ 2 Zr $$_2$$ 2 O $$_7$$ 7 (R = La–Lu) Crystals: Ab Initio Calculation

Journal of Superconductivity and Novel Magnetism - Crystal structure and phonon spectrum of rare-earth stannates R $$_2$$ Sn $$_2$$ O $$_7$$ and zirconates R $$_2$$ Zr $$_2$$ O $$_7$$ (R =...     

The properties of AlGaN films and AlGaN/GaN heterostructures grown on (11\bar{2}0) sapphire substrates

Al _x Ga_1-x N films and Al _x Ga_1-x N/GaN heterostructures were prepared on the ( ) sapphire substrates at elevated temperatures by alternate supply of trimethylgallium (TMG)/trimethylaluminum (TMA) and ammonia (NH₃) in an inductively heated quartz reactor. X-ray studies reveal the monocrystalline nature of these Al-containing structures. The results of absorption measurements of the Al _x Ga_1-x N films exhibit clear cut-off energies of the films. Based on the investigations of transmission electron microscopy (TEM), Al _x Ga_1-x N films and Al _x Ga_1-x N/GaN structures were found to deposit on the ( ) sapphire substrates with < 0001 > _AlGaN and being parallel to and , respectively.     

Size effects of nano-pattern in Si(1 1 1) substrate on the selective growth behavior of GaN nanowires by MOCVD

We report on the size effects of nano-patterned Si(1 1 1) substrates on the selective growth of GaN nanowires (NWs) using metal organic chemical vapor deposition. The nano-patterns on Si(1 1 1) substrates were fabricated by etching process of Au nano-droplets. The size of nano-patterns fabricated on Si(1 1 1) substrates were corresponding to size of Au nano-droplets, and the diameter of GaN NWs grown on nano-patterns was similar to the size of nano-pattern. Dense and well-oriented GaN NWs were grown on Si(1 1 1) substrates corresponding to the nano-patterns with an average diameter of about 50 nm. However, only a few GaN bulk grains, and mixed phase of a few NWs and bulk crystal of GaN were grown on the nano-patterned Si(1 1 1) having too small and large diameter, respectively, compare to the nano-patterns with diameter of 50 nm. Our results suggested that the selective growth of GaN NWs is strongly affected by the size of nano-patterns and its related mechanism.     

GaN薄膜的制备及其振动光谱的密度泛函理论研究

以氧化镓为镓源, 用溶胶-凝胶和高温氨化二步法, 在Si(111)衬底上制备出GaN薄膜. X射线衍射(XRD)分析表明制备的GaN薄膜是六角纤锌矿结构; 扫描电子显微镜(SEM)图片显示GaN晶粒的尺寸<100nm; 薄膜的红外光谱(FTIR)中有GaN的E1 (TO)声子模式. 用密度泛函理论(DFT)计算了氮化镓小团簇的振动频率. 结果表明: 富镓氮化镓团簇的振动频率在六方晶系纤锌矿结构GaN的光学声子峰值附近; 富氮氮化镓团簇中的N--N键的振动频率为2200cm^-1. 用氮化镓团簇的频谱对所制薄膜的红外光谱作了进一步分析.     

Study of using aqueous NH3 to synthesize GaN nanowires on Si(1 1 1) by thermal chemical vapor deposition

High-quality GaN nanowires (NWs) and zigzag-shaped NWs were grown on catalyst-free Si(1 1 1) substrate by thermal chemical vapor deposition (TCVD). Gallium (Ga) metal and aqueous NH₃ solution are used as a source of materials. Ga vapor was directly reacts with gaseous NH₃ under controlled nitrogen flow at 1050 °C. Scanning electron microscopy (SEM) images showed that the morphology of GaN displayed various densities of NWs and zigzag NWs depending on the gas flow rate, and increased nitrogen flow rate caused density reduction. The GaN NWs exhibited clear X-ray diffraction analysis (XRD) peaks that corresponded to GaN with hexagonal wurtzite structures. The photoluminescence spectra showed that the ultraviolet band emission of GaN NWs had a strong near band-edge emission (NBE) at 361–367 nm. Yellow band emissions were observed at low and high flow rates due to nitrogen and Ga vacancies, respectively. Moderate N₂ flow resulted in a strong NBE emission and a high optical quality of the NWs. This study shows the possibility of low-cost synthesis of GaN nanostructures on Si wafers using aqueous NH₃ solution.     

Structural Properties of Doped GaN on Si(111) Studied by X-Ray Diffraction Techniques

X-ray diffraction (XRD) is a non-destructive technique which is widely used in material characterization, particularly to determine structure, crystalline quality and orientation of samples. Two representative samples are used in this work, these samples are sample I (Si-doped GaN/AlN/Si) nominally consisted of 284 nm AlN followed by 152 nm of Si-doped GaN, and sample II (Mg-doped GaN/AlN/Si), grown with 194 nm AlN followed by 136 nm of Mg-doped GaN. Both doped GaN films were investigated by high resolution X-ray diffraction (HRXRD) with rocking curve (RC) measurement around the symmetrical (0002) and asymmetrical (10 $\overline{1}$ 2) diffraction peaks. The phase analysis result revealed that monocrystalline GaN was obtained. The XRD pattern show sharp and well separated (000l) reflections of doped GaN and AlN indicating complete texture with GaN[0?0?0?2] ∥ AlN[0?0?0?2] ∥ Si[1?1?1]. From the HRXRD RC ω/2θ scans of (10 $\overline{1}$ 2) and (0002) plane, we determined both a and c lattice parameters of the doped GaN. The symmetrical and asymmetrical RC full width at half maximum (FWHM) of doped GaN were obtained.     

Synthesis of vertically oriented GaN nanowires on a LiAlO2 substrate via chemical vapor deposition

Vertically oriented nanowires (NWs) of single-crystalline wurtzite GaN have been fabricated on γ-LiAlO₂ (100) substrate coated with a Au layer, via a chemical vapor deposition process at 1000 °C using gallium and ammonia as source materials. The GaN NWs grow along the nonpolar [100] direction with steeply tapering tips, and have triangular cross-sections with widths of 50–100 nm and lengths of up to several microns. The GaN NWs are formed by a vapor-liquid-solid growth mechanism and the tapering tips are attributed to the temperature decrease in the final stage of the synthesis process. The aligned GaN NWs show blue-yellow emission originating from defect levels, residual impurities or surface states of the GaN NWs, and have potential applications in nanotechnology.     

GaN材料的横向外延过生长(LEO)及其特性研究

对用LEO技术生长GaN材料的选择生长和横向生长速率进行了实验研究。结果表明，作为LEO生长基板的GaN层的表面质量是实现选择生长的关键，而图形方向对横向生长速率与纵向生长速率之比（L／V）也有重要的影响。通过选择合适的工艺条件，实现了GaN材料的LEO外延生长，所得样品的X射线衍射峰宽比用常规MOCVD法生长的样品减小了1／3。     

Optical properties of GaN epilayers grown on Si (111) and Si (001) substrates

We report the observation of bright photoluminescence (PL) emission from two types of GaN epilayers grown by molecular beam epitaxy (MBE). Wurtzite phase GaN/Si (111) epilayers are grown by gas source MBE process, whereas cubic phase GaN epilayers are grown on (001) Si covered by thin SiC film in the process of Si annealing in propane prior to the GaN growth. PL emissions are identified based on the results of detailed PL and time-resolved PL investigations. For the wurtzite phase GaN we observe an efficient up in the energy transfer from bound to free excitons. This process is explained by a large difference in the PL decay times for two types (free and bound (donor, acceptor)) of excitonic PL emissions. For cubic phase GaN we confirm recent suggestion that acceptors have smaller thermal ionization energies than those in the wurtzite phase GaN.     

用MOCVD法在LiGaO2(001)上生长GaN的研究

LiGaO₂单晶是目前所知的GaN最为理想的衬底材料,本研究用金属有机物气相沉积法(MOCVD)在LiGaO₂(001)衬底上进行了外延生长GaN膜的试验,生长出了表面较为平整的GaN外延膜.应用原子力显微镜(AFM)、X射线粉末衍射(XRD)和高分辨X射线双晶衍射分别对衬底对外延膜和衬底材料进行了分析测试.结果表明,用MOCVD法可以在LiGaO₂(001)衬底上生长出较高质量的无掺杂GaN(0001)外延膜.但由于MOCVD法是在高温还原气氛中生长GaN外延膜的,LiGaO₂在这种气氛中不够稳定,实验发现衬底材料在生长过程中部分样品发生开裂,但没有发生相变.     

GaN epitaxial layers prepared on nano-patterned Si(001) substrate

We report the growth of GaN epitaxial layer on Si(001) substrate with nano-patterns prepared by dry etching facility used in integrated circuit (IC) industry. It was found that the GaN epitaxial layer prepared on nano-patterned Si(001) substrate exhibits both cubic and hexagonal phases. It was also found that threading dislocation observed from GaN prepared on nano-patterned Si(001) substrate was significantly smaller than that prepared on conventional unpatterned Si(111) substrate. Furthermore, it was found that we can reduce the tensile stress in GaN epitaxial layer by about 78% using the nano-patterned Si(001) substrate.     

Pd/GaN(0001) interface properties

This report concerns the properties of an interface formed between Pd films deposited onto the surface of (0001)-oriented n-type GaN at room temperature (RT) under ultrahigh vacuum. The surface of clean substrate and the stages of Pd-film growth were characterized in situ by X-ray photoelectron spectroscopy (XPS), scanning tunneling microscopy (STM), ultraviolet photoelectron spectroscopy (UPS), and low energy electron diffraction (LEED). As-deposited Pd films are grainy, cover the substrate surface uniformly and reproduce its topography. Electron affinity of the clean n-GaN surface amounts to 3.1 eV. The work function of the Pd-film is equal to 5.3 eV. No chemical interaction has been found at the Pd/GaN interface formed at RT. The Schottky barrier height of the Pd/GaN contact is equal to 1.60 eV.     

Conversion between hexagonal GaN and beta-Ga(2)O(3) nanowires and their electrical transport properties

We have observed that the hexagonal GaN nanowires grown from a simple chemical vapor deposition method using gallium metal and ammonia gas are usually gallium-doped. By annealing in air, the gallium-doped hexagonal GaN nanowires could be completely converted to beta-Ga(2)O(3) nanowires. Annealing the beta-Ga(2)O(3) nanowires in ammonia could convert them back to undoped hexagonal GaN nanowires. Field effect transistors based on these three kinds of nanowires were fabricated, and their performances were studied. Because of gallium doping, the as-grown GaN nanowires show a weak gating effect. Through the conversion process of GaN nanowires (gallium-doped) --> Ga(2)O(3) nanowires --> GaN nanowires (undoped) via annealing, the final undoped GaN nanowires display different electrical properties than the initial gallium-doped GaN nanowires, show a pronounced n-type gating effect, and can be completely turned off.     

Microstructure of a-pl ane ($$2\b ar{1}\b ar{1}0$$) G aN ELOG stripe p atterns with different in-pl ane orient ation

Nonpolar epitaxial lateral overgrown (ELOG) GaN ridges with different in-plane orientations were studied using cathodoluminescence (CL) and micro-Raman spectroscopy. The high density of threading dislocations (TD) and basal plane stacking faults (BSF) in a-plane GaN on r-plane sapphire requires a defect reduction, e.g. via ELOG, in order to realize efficient emission from light emitting diodes (LEDs) or lasers. The distribution of TDs and BSFs is derived from plan view and cross sectional spectrally resolved CL micrographs. For [0001] stripe orientations BSFs and TDs spread into the laterally overgrown material (wing). In [\(0\bar{1}11\)] oriented stripes TDs are mostly restricted to the region on top of the mask opening (window), whereas BSFs extend into the wing region. A reduction of BSF and TDs in the wing region was observed for [\(01\bar{1}0\)] oriented stripes. Cross sectional Raman mapping of an [\(01\bar{1}0\)] oriented GaN ridge shows reduced strain and improved crystalline quality in the wing regions.