Si(111)外延GaN的表面形貌及形成机理研究

用高温AlN作缓冲层在Si(111)上外延生长出GaN薄膜.通过对薄膜表面扫描电子显微镜(SEM)和高分辨率双晶X射线衍射(DCXRD)的分析,确定缓冲层对外延层形貌的影响,分析解释了表面形貌中凹坑的形成及缓冲层生长温度对凹坑的影响.结果表明:温度的高低通过影响缓冲层初始成核密度和成核尺寸来影响外延层表面形貌.     

Characterization of silicon layers epitaxially grown on metallurgical-grade polycrystalline substrates

The structural and electrical properties of silicon layers epitaxially grown on metallurgical-grade polycrystalline silicon substrates are examined to clarify the effect of grain boundaries, crystal defects and impurities in the substrates. Chemical etching of the epitaxial layer reveals that all the grain boundaries continue from the substrate into the epitaxial layer, whereas lines of high density etch pits do not always continue. The polycrystalline thin film solar cells are fabricated on the metallurgical-grade silicon substrates by successive deposition of p and n⁺ layers. These cells show short circuit current densities around 70% of that of the conventional single crystal cell. This reduction of the short circuit current is caused mainly by the short minority carrier diffusion length in the grains probably due to impurities involved in the epitaxial layers. The origins of such impurities are discussed by considering autodoping and solid-state diffusion from the substrate during growth of epitaxial layers.     

Defects in epitaxial layers of silicon-germanium grown on silicon substrates

Crystal defects of various kinds found in epitaxially grown Si/Ge alloy layers on Si substrate, may be either inherent to the material and originating from atomic radii misfit, or can be traced to the growth process and controlled or eliminated by varying its parameters. A network of slip lines, becoming more pronounced with increased Ge content, indicates plastic deformation resulting from partial relief of stresses during the high temperature growth process. Electron microprobe and X-ray diffraction analysis indicate some Ge segregation in the fault vicinity, and a slight anisotropy in the lattice constant expansion due to the Ge.     

Fourier analysis of scratches generated on m‐GaN substrates during polishing

Generation of scratches on surface of m‐plane GaN substrates due to polishing was studied by atomic force microscopy (AFM). For epi‐ready substrates AFM images confirm a flat surface with the atomic step roughness while a lot of scratches are visible in AFM images for partially polished GaN substrates. The Fourier analysis of AFM images show that scratches propagate easier along {c‐plane} and {a‐plane} directions on m‐plane GaN surface. This observation is an evidence of anisotropy of mechanical properties of GaN crystals in the micro‐scale. This anisotropy is directly correlated with the symmetry and atomic arrangement of m‐plane GaN.     

Structural properties of bulk GaN substrates: Impact of structural anisotropy on non‐polar and semi‐polar crystals

In this paper we show the structural parameters and structural anisotropy of the bulk GaN substrates of various crystallographic orientations (00.1), (10.0), (11.0) and (20.1) obtained by ammonothermal method. The structural parameters were investigated using high resolution X‐Ray Diffraction. Perfect crystalline structure manifests in very narrow peaks in X‐ray rocking curves. The full width at half maximum (FWHM) values of 16 and 18 arcsec for the symmetrical and asymmetrical peaks, respectively, have been observed. In addition, we observed structural anisotropy in the non‐polar and semi‐polar crystals, depending on the orientation of the sample relative to the X‐ray beam. It is conducted that this anisotropy is a intrinsic property of non‐polar and semi‐polar GaN substrates.     

FTIR,powder XRD,TEM and dielectric studies of pure and zinc doped nano‐hydroxyapatite

Hydroxyapatite, Ca₁₀(PO₄)₆(OH)₂ or HAP, is an important bio‐material, which is having application in bone implants and dentistry. In the present study, zinc doped nano‐hydroxyapatite (Zn‐HAP) was synthesized via chemical precipitation route using surfactant mediated approach. The doping of zinc was confirmed by EDAX. The powder X‐ray diffraction (XRD) pattern revealed the typical hydroxyapatite pattern with broadening and extra peaks were observed for higher concentration. The average crystallite size was calculated by applying the Scherrer's formula to powder XRD pattern and was found in the range of 16 to 33 nm. The morphology of synthesized nano‐particles was also confirmed using TEM. FTIR spectroscopy was used to confirm the presence of various bonds. The dielectric study was carried out at room temperature within the frequency range from 10² Hz to 10⁷ Hz and the variations of dielectric constant with frequency of applied field as well as with the concentration of zinc were studied. It was found that as the concentration of zinc increased the dielectric constant increased. The variations of dielectric loss and a.c. conductivity with frequency of applied field were studied. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Transmission electron microscopy study of hexagonal GaN film grown on GaAs (0 0 1) substrate by using AlAs nucleation layer

Hexagonal gallium nitride (h-GaN) films have been grown on AlAs nucleation layer by using radio frequency (RF) plasma source-assisted molecular beam epitaxy on GaAs (0 0 1) substrate. Transmission electron microscopy (TEM) techniques are used to characterize such h-GaN epilayers. TEM results show that (0 0 0 1) atom planes of h-GaN are parallel to (0 0 1) atom planes of the GaAs substrate. Defects, such as stacking faults and dislocations, have also been observed.     

InGaN/GaN微米阵列结构的生长及发光性能研究

采用金属有机化学气相外延技术,在图形化蓝宝石衬底上通过选区外延可控生长了三种不同形貌的InGaN/GaN微米阵列结构,阵列尺寸均为6μm.其中,六方片状阵列结构以(0001)c面为主,高度为0.6μm;六棱台状阵列结构包括一个(0001)c面和六个等效的(10-11)半极性面,高度为1.2μm;六棱锥状阵列结构以(10...     

Crystallography and cathodoluminescence of ultra‐long GaN nanowires

Ultra‐long GaN nanowires have been synthesized via a simple thermal evaporation process by heating mixed GaN and Ga₂O₃ powders in a conventional resistance furnace under ammonia gas at 1150 °C. The average length of GaN nanowires is estimated to be more than 100 μm after 30‐min growth, corresponding to a fast growth rate of more than 200 μm/h. Scanning electron microscope (SEM) observation indicated that the diameter of GaN nanowires was rather uniform along the growth direction and in the range of 100–200 nm. X‐ray diffraction (XRD) and transmission electron microscope (TEM) measurements confirmed that the GaN nanowires are crystalline wurtzite‐type hexagonal structure. Room‐temperature cathodoluminescence (CL) measurement indicated that an obvious red‐shift of the near band‐edge emission peak centered at 414 nm of the ultra‐long GaN nanowires and a wide shoulder in the range of 600–700 nm were observed. Possible reasons responsible for the red‐shift of the near band‐edge emission of the ultra‐long GaN nanowires was discussed. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Well parameters of two‐dimensional electron gas in Al0.88In0.12N/AlN/GaN/AlN heterostructures grown by MOCVD

Resistivity and Hall effect measurements were carried out as a function of magnetic field (0‐1.5 T) and temperature (30‐300 K) for Al_0.88In_0.12N/AlN/GaN/AlN heterostructures grown by Metal Organic Chemical Vapor Deposition (MOCVD). Magnetic field dependent Hall data were analyzed by using the quantitative mobility spectrum analysis (QMSA). A two‐dimensional electron gas (2DEG) channel located at the Al_0.88In_0.12N/GaN interface with an AlN interlayer and a two‐dimensional hole gas (2DHG) channel located at the GaN/AlN interface were determined for Al_0.88In_0.12N/AlN/GaN/AlN heterostructures. The interface parameters, such as quantum well width, the deformation potential constant and correlation length as well as the dominant scattering mechanisms for the Al_0.88In_0.12N/GaN interface with an AlN interlayer were determined from scattering analyses based on the exact 2DEG carrier density and mobility obtained with QMSA. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Homo-epitaxial GaN growth on exact and misoriented single crystals: suppression of hillock formation

GaN single crystals were used as substrates for MOCVD growth. The ( ) plane of the substrate crystals was polished to obtain off-angle orientations of 0, 2, and 4° towards the [ ] direction. The highest misorientation resulted in a reduction of the hexagonal hillock density by nearly two orders of magnitude as compared with homo-epitaxial films grown on the exact ( ) surface. The features that are still found on the 4° off-angle sample after growth can be explained by a model involving the interaction of steps, introduced by the misorientation, and the hexagonal hillocks during the growth process. Following from this explanation it could be concluded that surface diffusion is found to be not important during growth on the N-side. The material quality of the N-side was examined by photoluminescence (PL) measurements. The PL spectrum measured at 5 K shows dominant donor bound excitons with a FWHM of 1.4 meV as well as free excitonic transitions.     

低温AlN插入层降低硅基GaN膜微裂

为了降低MOCVD外延硅基GaN膜层中的应力、减少硅基厚GaN层的微裂;在高温GaN层中插入低温AlN.低温AlN插入层可平衡HT-GaN生长和降温过程引起的张应力,降低厚膜外延层的微裂,已研制出厚度超过1.8微米无微裂GaN外延层.本文重点研究了低温AlN生长温度对HT-GaN材料的影响,给出了较佳的LT-AlN生长温度.采用扫描电子显微镜(SEM),原子力显微镜(AFM)和高分辨率双晶X射线衍射(DCXRD),对样品进行了测试分析.试验和测试结果表明低温AlN的生长温度至关重要,生长温度过低影响GaN晶体质量,甚至不能形成晶体;生长温度过高同样会影响GaN结晶质量,同时降低插入层的应力平衡作用;实验结果表明最佳的LT-AlN插入层的生长温度为680℃左右.     

Microstructure and cathodoluminescence study of GaN nanowires without/with P‐doping

In this work, P‐doped GaN nanowires were synthesized in a co‐deposition CVD process and the effects of P‐doping on the microstructure and cathodoluminescence (CL) of GaN nanowires were studied in details. SEM observation and CL measurments demonstrated that P‐doping has led to a rough morphology evolution and a depression of the band‐gap emission of GaN nanowires, whereas the visible emission of GaN nanowires was obviously enhanced. Finally, the corresponding morphology transition and optical properties of GaN nanowires with P‐doping were discussed. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

过渡层结构和生长工艺条件对Si基GaN的影响

为了提高MOCVD外延硅基GaN材料的质量，在硅（111）衬底上以HT-AlN为缓冲层，在缓冲层上再生长变组份过渡层后外延生长GaN。过渡层为多层复合结构，分为高温变组分AlGaN、GaN、低温AlN、高温变组分AlGaN。在高温生长AlGaN和GaN层中插入一层低温生长AlN以缓解降温过程中应力对厚GaN层的影响，为了缓慢释放热应力、采用合适的慢降温工艺。当外延层的厚度小于1．7微米时GaN外延层无龟裂，而厚度不断增加时，GaN外延层产生龟裂。本文研究了AlN缓冲层生长温度、高温变组分AlGaN生长过程中生长时间的变化对所生长GaN材料的影响。采用三维视频显微镜、高分辨率双晶X射线衍射（DCXRD）、扫描电子显微镜（SEM）、原子力显微镜（AFM）和室温光致荧光光谱（RT-PL）对样品进行了测试分析。测试结果表明所研制的硅基GaN表面光亮、平整，过渡层的引入有利于降低外延层中应力，提高GaN结晶质量。     

Characterization of polycrystalline GaN grown on silica glass substrates

The structural, optical, and electrical properties of GaN films grown on silica glass substrate by metalorganic chemical vapor deposition were studied. X-ray diffraction showed that the films were grown in hexagonal structure with a predominant (0 0 0 2) peak. A broad and strong band-edge emission and very weak yellow luminescence in photoluminescence (PL) spectra were observed. And the temperature dependence of the PL spectra was extensively studied. The thermal quenching activation energy was found to be very close to the donor activation energy determined from the temperature dependence of the carrier concentration. Longitudinal optical phonons were found to be responsible for the PL broadening above 100 K.     

Lateral confined epitaxy of GaN layers on Si substrates

We developed a novel, simple procedure for achieving lateral confined epitaxy (LCE). This procedure enables the growth of uncracked GaN layers on a Si substrate, using a single, continuous metalorganic chemical vapor deposition (MOCVD) run. The epitaxial growth of GaN is confined to mesas, defined by etching into the Si substrate prior to the growth. The LCE-GaN layers exhibit improved morphological and optical properties compared to the plain GaN-on-Si layers grown in the same MOCVD system. By performing a set of LCE growth runs on mesas of varying lateral dimensions, we specified the crack-free range of GaN on Si as 14.0±0.3 μm.     

缓冲层厚度对MOCVD法生长GaN外延薄膜性能的影响

本文研究了低温GaN(LT-GaN)缓冲层表面形貌,其随厚度的变化规律及对随后生长GaN外延膜各项性能的影响.用场发射扫描电镜(SEM)和原子力显微镜(AFM)研究LT-GaN缓冲层表面形貌,发现随着厚度的增加,其表面由疏松、粗糙变得致密、平整,六角GaN小晶粒的数量减少,且取向较为一致.用X光双晶衍射(XRD)、AFM和Hall测量研究1μm厚本征GaN外延薄膜的结晶质量、表面粗糙度、背底载流子浓度和迁移率等性能,发现随着LT-GaN缓冲层厚度的增加:XRD的半高宽FWHMs增大,表面粗糙度先减小后又略有增大,背底载流子浓度则随之减少,而迁移率的变化则不明显.通过分析进一步确认LT-GaN缓冲层的最优生长时间.     

Growth behaviour of bulk GaN single crystals grown with various flux ratios using solvent‐thermal method

Bulk GaN single crystals were grown using a solvent‐thermal method. They were grown for 200 h at 600 °C and 800 °C using 8 MPa of N₂ gas and 1–3 mm sized pyramid GaN single crystals. Pure Na, NaN₃ and Ca were used as the flux. The mole fraction of the [flux]/([flux] + [Ga]) was 0.30–0.67. The growth behavior differed according to the flux ratio. The quality of the bulk GaN single crystals was improved by increasing the flux ratio. The bulk GaN single crystals formed by spontaneous nucleation were deposited on the BN crucible wall and bottom during the first step of synthesis. The wurtzite structure of the GaN grown single crystal was confirmed by x‐ray diffration. The chemical composition was analyzed by electron probe microanalysis. The quality and optical properties of the GaN single crystal were examined by Raman spectroscopy and photoluminesence analysis. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

低温GaN形核层的形核速率对GaN外延薄膜晶体质量的影响

采用金属有机化学气相沉积方法系统研究了在蓝宝石衬底上低温GaN形核层的形核速率对GaN外延薄膜晶体质量的影响机理.利用高分辨X射线衍射仪、原子力显微镜、光致发光光谱和Hall测试仪表征材料的位错密度、表面形貌以及光、电学性能.研究结果表明随着形核速率的增加,GaN形核层更倾向于三维生长模式;当形核速率达到1.92(A)/s时退火后生成尺寸为100 nm宽、32 nm高的均匀形核岛,随后生长的未掺杂GaN外延薄膜层的螺型和刃型位错密度以及黄带峰强度达到最小值,并且其具有最高的载流子迁移率和最低的载流子浓度.