4H-SiC高速同质外延研究

研究了生长温度为1400℃时4H-SiC同质外延膜的生长速率、表面形貌及缺陷. 拉曼表征并结合KOH腐蚀表明外延膜中未出现3C-SiC多晶, 为单一的4H-SiC晶型. 通过KOH腐蚀发现, 低生长速率和高C/Si比有利于衬底表面的基平面位错(BPDs)转变成露头刃位错(TEDs). 在高生长速率下, 外延膜的表面三角形缺陷和位错密度会显著增加. 通过引入界面层, 可以实现生长初期的平滑过渡, 极大地降低高生长速率下外延膜的缺陷密度.     

分子束外延PbTe/Cd 0.98 Zn 0.02 Te异系材料的微结构特性研究

采用分子束外延方法在Ⅱ-Ⅵ族Cd0.98Zn0.02Te(111)衬底上实现了异系Ⅳ-Ⅵ族半导体(PbTe)的外延生长.原子力显微镜(AFM)的表面形貌表征表明,PbTe表面形貌主要由三角形台阶线和螺旋形台阶面构成;理论计算表明,螺旋形台阶面的分布受到滑移位错弹性应变能的影响.通过高分辨透射电镜(HRTEM)观察,发现在PbTe和Cd0.98Zn0.02Te界面处存在Frank位错.分析表明,这些Frank位错在运动过程中会形成不同的位错组态,位错组态的相互作用是表面上形成三角形台阶线和螺旋形台阶面的主要原因.     

低位错密度8英寸导电型碳化硅单晶衬底制备

碳化硅具有优异的物理化学性能,在电动汽车、轨道交通、高压输变电、光伏、5G通信等领域具有广泛应用前景。8英寸(1英寸=2.54 cm)SiC衬底在降低器件单位成本、增加产能供应方面具有巨大的潜力,成为行业重要的技术发展方向。近期山东大学与广州南砂晶圆半导体技术有限公司在8英寸SiC衬底位错缺陷控制方面取得了重大突破,使用物理气相传输法(Physical vapor transport,PVT)制备了低位错密度8英寸导电型4H-SiC单晶衬底,其中螺位错(Threading screw dislocation,TSD)密度为0.55 cm^-2,基平面位错(Basal plane dislocation,BPD)密度为202 cm^-2。     

GaAS/Si(100)异质外延4°偏角衬底对外延层结晶质量的提高(英文)

用低压金属有机物化学汽相沉积法(MOCVD)在Si(100)无偏角和Si(100)4°偏角衬底上外延生长GaAs层。异质外延采用两步生长法,并分别优化了两种衬底上的非晶低温缓冲层的生长条件。用X射线双晶衍射(XRD)和透射电子显微镜(TEM)对两种衬底上的GaAs外延层进行了结构表征,其中Si(100)4°偏角衬底上1.8μm厚GaAs的(004)面XRD衍射半高全宽338 arcsec,同比在无偏角衬底上的半高全宽为494arcsec,TEM图片显示4°偏角衬底上外延层中的位错密度大大降低。     

蓝宝石衬底的斜切角对GaN薄膜特性的影响

利用金属有机物化学气相沉积技术在具有斜切角度的蓝宝石衬底(0～0.3°)上生长了非故意掺杂GaN薄膜,并采用显微镜、X射线双晶衍射、光荧光及霍尔技术对外延薄膜的表面形貌、晶体质量、光学及电学特性进行了分析.结果表明,采用具有斜切角度的衬底,可以有效改善GaN外延薄膜的表面形貌、降低位错密度、提高GaN的晶体质量及其光电特性,并且存在一个衬底最优斜切角度0.2°,此时外延生长出的GaN薄膜的表面形貌和晶体质量最好.     

不同厚度对分子束外延生长GaSb薄膜的影响

采用分子束外延(MBE)在GaAs衬底上生长GaSb薄膜,为了减小因晶格失配度较大所引起的位错密度,采用低温GaSb作为缓冲层.通过X射线双晶衍射仪和原子力显微镜分析得出,当低温GaSb缓冲层的厚度为20nm时,GaSb外延层中的位错密度最小,晶体质量最好.此外,缓冲层和外延层的厚度共同对GaSb薄膜晶体质量和表面形貌产生影响.     

GaN基LED图形衬底的性能研究

蓝宝石图形衬底可以降低外延位错密度并增强背散射光, 已经成为制备高亮LED有效技术手段。本研究运用时域有限差分(FDTD)法模拟和比较了GaN基微纳米图形衬底LED几种衬底图形结构对光的提取效率的影响。模拟结果显示纳米图形衬底(NPSS)对光效的提高明显优于微米图形衬底(MPSS)。在对圆柱、圆孔、圆台、圆锥和曲面锥等纳米结构的研究中, 圆台柱结构的纳米图形衬底对光提取效果最好。通过进一步模拟优化, 得到圆台结构的最佳参数, 此时相对于普通衬底LED光的提取效率提高了96.6%。试验中, 采用软模压印技术在蓝宝石基片上大面积制备出纳米圆台图形衬底, 并测得外延生长GaN层后的外延片的PL强度增加了8倍, 可见纳米图形衬底对提高LED的出光效率有显著效果。     

衬底温度对Si(111)衬底上MBE异质外延3C-SiC薄膜的影响

利用固源分子束外延(SSMBE)技术, 在Si(111)衬底上异质外延生长3C-SiC单晶薄膜, 通过RHEED、XRD、AFM、XPS等实验方法研究了衬底温度对薄膜结构、形貌和化学组分的影响. 研究结果表明, 1000℃生长的样品具有好的结晶质量和单晶性. 在更高的衬底温度下生长, 会导致大的孔洞形成, 衬底和薄膜间大的热失配使降温过程中薄膜内形成更多位错, 从而使晶体质量变差. 在低衬底温度下生长, 由于偏离理想的化学配比也会导致薄膜的晶体质量降低.     

MBE方法在GaAs(001)衬底上外延生长GaSb膜

利用分子束外延方法(MBE)在GaAs(001)衬底上外延生长了GaSb薄膜,利用高分辨透射电子显微镜(HRTEM)、原子力显微镜(AFM)、Hall效应(HallEffect)和低温光荧光谱(LTPL)等手段对薄膜的晶体质量、电学性能和光学质量进行了研究。发现直接生长的GaSb膜表面平整,空穴迁移率较高。引入GaSb/AlSb超晶格可有效阻断进入GaSb外延层的穿通位错,对应的PL谱强度增强,材料的光学质量变好。     

用VTE方法在蓝宝石衬底上生长γ-LiAlO2薄膜

利用气相传输平衡技术(VTE)和后退火处理工艺在(0001)蓝宝石衬底上获得了高度[100]取向的γ-LiAlO2薄膜．X射线衍射表明是由单相的γ-LiAlO2所组成,此薄膜经850～900℃／120h空气中退火处理后显示出高度的[100]取向,这一实验结果意味着有望通过VTE方法制备用于GaN基器件外延生长的γ-LiAlO2(100)-Al2O3(0001)复合衬底.     

Effect of crystallographic dislocations on the reverse performance of 4H-SiC p-n diodes

A quantitative study was performed to investigate the impact of crystallographic dislocation defects, including screw dislocation, basal plane dislocation, and threading edge dislocation, and their locations in the active and JTE region, on the reverse performance of 4H-SiC p-n diodes. It was found that higher leakage current in diodes is associated with basal plane dislocations, while lower breakdown voltage is attributed to screw dislocations. The above influence increases in severity when the dislocation is in the active region than in the JTE region. Furthermore, due to the closed-core nature, the impact of threading edge dislocation on the reverse performance of the p-n diodes is less severe than that of other dislocations although its density is much higher.     

Liquid-phase epitaxy of 2H-SiC film on a (0001) 4H-SiC substrate in Li-Si melt

We first applied a liquid-phase epitaxy (LPE) method to the growth of 2H-SiC to obtain an oriented 2H-SiC layer. The LPE growth was performed on a (0001) 4H-SiC substrate in a Li-Si-C melt at 850 °C. A LPE layer grew on the carbon face of the (0001) 4H-SiC substrate with the area of about 1 × 1 mm². The thickness and morphology of the LPE layer were confirmed by scanning electron microscope (SEM), and the polytype of the LPE layer was determined using a high-resolution transmission electron microscope (HR-TEM) and a Raman spectroscopy. SEM measurements revealed that the thickness of the LPE layer was 50 μm and that this layer developed with a hexagonal shape. HR-TEM observations and Raman spectral analysis showed that 2H-SiC layer grows on 4H-SiC substrate with the relationship of (0001) 2H-SiC (0001) 4H-SiC. In this study, we first succeeded to grow <0001>-axis-oriented 2H-SiC films and concluded that the LPE technique is applicable to the growth of oriented 2H-SiC films in the Li-Si melt.     

Oxidation stacking faults in epitaxial silicon crystals

Oxidation stacking faults (SFs) in epitaxial silicon crystals have been studied by means of preferential etch and X-ray topography. SFs lie on {111} planes and are surrounded by a partial Frank dislocation. Nucleation takes place at crystallographic defects located near the surface of epitaxial layers. These defects appear as etch hillocks after preferential etch. SF length as a function of temperature and time for different thermal treatments is reported. The SF length is controlled by an activation energy of 2.6 eV. This energy, which is nearly half the self diffusion activation energy of silicon, supports a SF growth mechanism controlled by vacancy emission.     

4H-SiC晶体中的层错研究

采用物理气相传输法(PVT法)在4英寸(1英寸=25.4 mm)偏<11¯20>方向4°的4H-SiC籽晶的C面生长4H-SiC晶体。用熔融氢氧化钾腐蚀4H-SiC晶体, 并利用光学显微镜研究了晶体中的堆垛层错缺陷的形貌特征和生长过程中氮掺杂对4H-SiC晶体中堆垛层错缺陷的影响。结果显示, 4H-SiC晶片表面的基平面位错缺陷的连线对应于晶体中的堆垛层错, 并且该连线的方向平行于<1¯100>方向。相对于非故意氮掺杂生长的4H-SiC晶体, 氮掺杂生长的4H-SiC晶体中堆垛层错显著偏多。然而, 在氮掺杂生长的4H-SiC晶体的小面区域, 虽然氮浓度高于其他非小面区域, 但是该小面区域并没有堆垛层错缺陷存在, 推测这主要是由于4H-SiC晶体小面区域特有的晶体生长习性导致的。     

A Study on Crystal Defects in Epitaxial GaN Film by High-Order Weak-Beam Electron Microscopy

Various crystal defects in epitaxial GaN film were studied by high-order bright-field and dark-field electron microscopy. The results revealed that the film is composed of small grains in nano-meter scale. Distinct shapes of the grain boundaries and a great number of threading dislocations were shown in the experimental g/3g weak-beam images. Diffraction contrast analysis verified that the majority of threading dislocations is 1/3<11 0> edge dislocation. A unique (11 0) planar defect was observed in the specimen. The defect was identified to have a similar structure to the incipient 1/3[11 0] edge dislocation.     

Thermal stability of LiF thin films on 6H-SiC(0001) surface

The morphology and the temperature induced changes of LiF thin layers deposited on three different kinds of 6H-SiC(0001) surfaces have been investigated by the atomic force microscopy technique. As the substrates used were: a single crystal of Si-terminated on-axis oriented additionally hydrogen-etched, off oriented 3.5° from basal plane and with an epitaxial adlayer grown on the Si- or on the C-face surface. For all the systems investigated, independently of the substrate, the LiF grows uniformly at room temperature, and even for rather thick films (estimated to be about 5 nm), terraces are still visible. However, in each case the LiF layers are not temperature stable, and when the samples are heated (starting from ∼600 K), regular grains with radius up to 40 nm and with height up to 9 nm are formed (for initial layers thickness 2.5 nm). Subsequent sample heating procedures cause LiF desorption and consequently, a decrease of the dimensions of the islands.     

Investigation of Ni/4H-SiC diodes as radiation detectors with low doped n-type 4H-SiC epilayers

The development of SiC minimum ionising particle (MIP) detectors imposes severe constrains in the electronic quality and the thickness of the material due to the relatively high value of the energy required to produce an electron–hole pair in this material by MIP against the value for Si. In this work, particle detectors were made using semiconductor epitaxial undoped n-type 4H-SiC as the detection medium. The thickness of the epilayer is on the order of 40 μm and the detectors are realised by the formation of a nickel silicide on the silicon surface of the epitaxial layer (Schottky contact) and of the ohmic contact on the back side of 4H-SiC substrate. The low doping concentration (6×10¹³ cm⁻³) of the epilayer allows the detector to be totally depleted at relatively low reverse voltages (100 V). We present experimental data on the charge collection properties by using 5.486 MeV -particles impinging on the Schottky contact. A 100% charge collection efficiency (CCE) is demonstrated for reverse voltages higher than the one needed to have a depletion region equal to the -particle extrapolated range in SiC. The diffusion contribution of the minority change carriers to CCE is pointed out. By comparing measured CCE values to the outcomes of drift–diffusion simulation, values are inferred for the hole lifetime within the neutral region of the charge carrier generation layer.     

Material quality improvements for high voltage 4H-SiC diodes

The influence of thin 4H-SiC buffer layers grown by liquid phase epitaxy (LPE) on structural quality of 4H-SiC low-doped epitaxial layers, grown by chemical vapor deposition (CVD) was investigated in detail. A dramatic defect density reduction in CVD epitaxial layers grown on commercial wafers with buffer LPE layer was detected. P⁺n junctions were formed on these CVD layers by high dose Al ion implantation followed by rapid thermal anneal. It was shown that both the increase of diffusion lengths of minority carriers (Lp) in CVD layers and the forming of p⁺-layers after Al ion implantation and high temperature anneal lead to superior device characteristics.     

Plasma-assisted molecular beam epitaxy of ZnO on in-situ grown GaN/4H-SiC buffer layers

Plasma-assisted molecular beam epitaxy (MBE) was used to grow ZnO(0001) layers on GaN(0001)/4H-SiC buffer layers deposited in the same growth chamber equipped with both N- and O-plasma sources. The GaN buffer layers were grown immediately before initiating the growth of ZnO. Using a substrate temperature of 440°C–445°C and an O₂ flow rate of 2.0–2.5 sccm, we obtained ZnO layers with smooth surfaces having a root-mean-square roughness of 0.3 nm and a peak-to-valley distance of 3 nm shown by AFM. The FWHM for X-ray rocking curves recorded across the ZnO(0002) and ZnO(101ˉ5) reflections were 200 and 950 arcsec, respectively. These values showed that the mosaicity (tilt and twist) of the ZnO film was comparable to corresponding values of the underlying GaN buffer. It was found that a substrate temperature >450°C and a high Zn-flux always resulted in a rough ZnO surface morphology. Reciprocal space maps showed that the in-plane relaxation of the GaN and ZnO layers was 82.3% and 73.0%, respectively and the relaxation occurred abruptly during the growth. Room-temperature Hall-effect measurements showed that the layers were intrinsically n-type with an electron concentration of 10¹⁹ cm^–3 and a Hall mobility of 50 cm²·V^–1·s^–1.