LEC法生长富磷掺铁InP单晶晶格应变与残留应力研究

运用原位磷注入合成法在高压单晶炉内合成富磷的InP熔体,并利用液封直拉法(LEC)生长出了3英寸富磷掺Fe的InP单晶.运用高分辨率X射线衍射技术、偏振差分透射谱测试技术、光致荧光谱技术对富磷掺Fe的InP晶片进行了结构、应力及发光特性测试.结果表明,晶格的应变导致了PL发光峰峰位的变化,晶格应变与残留应力测试结果相一致,说明材料生长过程中的热应力是导致样品晶格常数分布不均匀的主要因素.     

水热法制备Ti1-xFexO2稀磁半导体

本论文采用水热法制备Fe掺TiO2稀磁半导体Ti1-xFexO2(x=0-0.04),并对其形貌、结构及磁性能进行表征.X射线衍射分析结果表明,掺杂Fe离子后产物为TiO2锐钛矿结构,其晶体晶格常数随掺人铁离子含量的增加而略有增大.透射电镜和x射线能谱测试结果表明,样品形貌规整,平均粒径约为20 nm.利用振动样品磁强计对其磁性能进行测量,结果表明样品在室温条件下存在铁磁性.     

单晶Si_3N_4纳米带可控Al掺杂研究

采用有机前驱体热解法制备了Al掺杂的单晶Si3N4纳米带,并实现了单晶纳米带Al掺杂浓度的调控。采用SEM、XRD、TEM和HRTEM等对所合成的Al掺杂单晶Si3N4纳米带进行了系统分析和表征。纳米带平均厚度约为几十纳米,平均宽度为几百纳米,具有完整的晶体结构,生长方向为[011],固-液-气-固(SLGS)生长机理。对Al掺杂的纳米带的光学性能进行了初步检测。结果表明:Al掺杂对单晶Si3N4纳米带的光学性能具有显著的影响,通过调节Al掺杂浓度,可以成功实现对纳米带光学性能的调控。     

微下拉法生长炉研制及其在BGSO混晶生长上的应用

基于垂直凝固原理设计并研制了适合中低温晶体生长的微下拉法生长炉.采用该生长设备成功生长出不同掺杂浓度的锗硅酸铋纤维晶体,讨论了工艺参数以及掺杂浓度对晶体生长的影响,所得纤维晶体的最大长度达到195 mm.测试了晶体XRD和发射光谱.结果表明,随着SiO2含量增加,Bi4(Ge1-xSix)3O12单晶的晶胞参数随着Si含量增加而减小,而Si掺杂使晶体主发射峰轻微地向短波方向移动.     

c向蓝宝石单晶纳米压痕尺度效应研究

本文以c向蓝宝石单晶为对象采用纳米压痕方法测试了不同载荷下蓝宝石单晶的硬度与杨氏模量.分析表明,当载荷P=25±2 mN时出现了第一次位移突变现象,且位移突变量与最大压入载荷无关;试样的硬度与杨氏模量值随着压入深度的增大而减小,呈现出明显的压痕尺度效应;利用Nix-Gao尺度效应模型对测试数据进行了拟合,最终得出试样的硬度值与杨氏模量为30.956 GPa与382.316 GPa,与前人分析结果符合程度较高,证明Nix-Gao模型适用于蓝宝石单晶纳米压痕测试尺度效应的模拟与分析.     

Cr∶LiNbO_3及Cr,Fe∶LiNbO_3晶体热释电系数提高的研究

生长了掺杂量分别为0.2mol%、0.5mol%的掺铬同成分配比铌酸锂晶体和掺铬0.2mol%、铁0.04mol%的双掺同成分配比铌酸锂晶体。利用动态电流法测试了它们的热释电系数。在同样的测试条件下,与未掺杂的同成分铌酸锂晶体相比较,掺铬铌酸锂(Cr∶LiNbO3)晶体和铬、铁双掺铌酸锂(Cr,Fe∶LiNbO3)晶体的热释电系数明显提高。同时对掺杂提高晶体热释电系数的机理进行了探讨。     

β-Ga2O3体单晶X射线光电子能谱分析

通过对导模法制备的非故意掺杂、Si掺杂β-Ga2 O3晶体和Si掺杂后退火处理的β-Ga2 O3晶体进行了X射线光电子能谱分析(XPS),对比分析不同样品的Ga3d、Ga2p、O1s特征峰位和峰强度变化,并结合文献报道中β-Ga2O3薄膜及单晶材料的报道结果,进一步确认β-Ga2 O3体单晶特征峰峰值.同时,通过对各峰强度的变化进行对比分析,对次峰产生的原因进行推测,获得Si掺杂及退火对晶体表面及晶体内部个特征峰的变化规律.     

γ-LiAlO2晶体的生长及掺镓研究

由于与GaN晶格失配小(约1.4;),γ-LiAlO2单晶有望成为GaN外延衬底材料.本文首先使用提拉法生长出了尺寸达φ45×50mm3的γ-LiAlO2单晶,然后采用Ga2O3作为掺杂剂,仍用提拉法生长出了三种不同掺镓浓度的LiAl1-xGaxO2(x=0.1,0.2,0.3)晶体,并用X射线粉末衍射(XRPD)分别对晶体及坩锅中剩余的熔体的成份进行了表征.结果表明LiAl-xGaxO2(x=0,0.1,0.2,0.3)晶体归属于γ-LiAlO2结晶结构,Ga3+离子部分地取代Al3+离子,发生分凝且分凝系数小于1.     

GGA+U方法研究C与Ti掺杂GaN的电子结构和光学性质

作为一种优良的半导体材料,GaN所具有的宽禁带导致其只能吸收可见光中的紫光,因此如何增加GaN材料对可见光的利用率是一个值得研究的问题,掺杂是解决这个问题常用的手段。本文利用第一性原理的方法对本征GaN,C单掺、Ti单掺、C-Ti共掺GaN四种体系的电子结构和光学性质做了计算和分析,结果表明:掺杂后的体系都具有良好的稳定性;掺杂后各体系的体积均增大,说明杂质的引入使体系晶格发生畸变,对光生空穴-电子对的分离有促进作用,进而提高材料的光催化性能;杂质元素的引入使体系能级发生劈裂,电子跃迁更加容易;掺杂后各体系的介电函数虚部主峰均向低能区移动,吸收谱均红移至可见光区域,其中共掺体系在蓝绿光区域的吸收系数最大,由此可以推测C-Ti共掺有助于提高GaN的光催化性能。     

Mg掺杂浓度对GaN电子结构和光学性质的影响

基于密度泛函理论的第一性原理,分析了Mg掺杂浓度对GaN晶格参数、能带结构、电子态密度和光学性质的影响.研究表明:Mg掺杂GaN体系,晶格常数增大,禁带宽度增加,而且禁带宽度随着Mg含量的增加而增加,同时N2p和Mg2p态电子轨道的相互杂化,从而在费米能级附近引入受主能级,随着Mg含量的增加,费米能级进入价带的位置加深,同时Mg掺杂浓度越高,价带和导带带宽越窄.掺Mg后在介电函数和光学吸收谱的低能区和高能区均出现了新的介电峰,这些峰的出现和禁带中的杂质能级到导带底的跃迁有关,由于带隙的增加使介电峰向高能量方向发生偏移.     

Evidence of free carrier concentration gradient along the c-axis for undoped GaN single crystals

Results of measurements of infrared reflectivity and micro-Raman scattering on the undoped GaN high pressure grown single crystals are reported. These crystals have usually a high electron concentration due to unintentional doping by oxygen. We show, by the shift of the plasma edge (infrared reflectivity measurements), that the free electron concentration is always higher on the (0 0 0  )N face of the GaN single crystal than on the (0 0 0 1)Ga face. In order to determine the profile of the free carrier concentration, we performed transverse micro-Raman scattering measurements along the (0 0 0 1) c-axis of the crystal with spatial resolution of 1 μm. Micro-Raman experiments give a quantitative information on the free carrier concentration via the longitudinal optical phonon–plasmon (LPP) coupling modes. Thus, by studying the behavior of the LPP mode along the c-axis, we found the presence of a gradient of free electrons. We suppose that this gradient of electrons is due to the gradient of the main electron donor, in undoped GaN single crystals, i.e. oxygen impurity. We propose a growth model which explains qualitatively the incorporation of oxygen during the growth of GaN crystal under high pressure of nitrogen.     

Growth and characterization of pure and lithium doped L‐alanine single crystals for NLO devices

Single crystals of pure and lithium substituted L‐alanine are successfully grown by slow evaporation method at constant temperature of 32°C. The effect of lithium dopant on crystal properties has been studied. Powder and single crystal XRD analysis confirms the structure and change in lattice parameter values for the doped crystals. The crystals were characterized by solubility studies, density, melting point measurements, FTIR and UV‐Vis‐NIR techniques. Thermal and mechanical stability of crystals were tested by TGA/DTA and micro hardness analysis. NLO activity of the crystals is found to be increased in the presence of lithium ions. The dielectric constant and dielectric loss of the crystals were studied as a function of frequency. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Fe掺杂CoSb3化合物的热电性能及电子结构研究

采用机械合金化结合冷等静压的方法制备了Co4-xFexSb12化合物,探讨了Fe掺杂对化合物热电性能的影响;利用基于密度泛函理论赝势平面波的方法对Fe掺杂前后的CoSb3的电子结构进行了计算.结果表明:在Co4-xFexSb12化合物中Fe的固溶极限x在0.3～0.5之间;CoSb3的费米而位于导带和价带之间,其电阻率随温度的升高而降低,为非简并半导体;Fe掺杂后费米面进入价带,使其成为P型简并半导体,电阻率较掺杂前大大降低并随温度的升高而增加;本实验条件下,Co37Fe0.3Sb12化合物的功率因子在600 K时出现1406.31μW/ m·K2的最高值,是未掺杂试样的功率因子最高值的7.4倍.     

Comparative analysis of characteristics of Si, Mg, and undoped GaN

We have grown undoped, Si- and Mg-doped GaN epilayers using metalorganic chemical vapor deposition. The grown samples have electron Hall mobilities (carrier concentrations) of 798 cm²/V s (7×10¹⁶ cm⁻³) for undoped GaN and 287 cm²/V s (2.2×10¹⁸ cm⁻³) for Si-doped GaN. Mg-doped GaN shows a high hole concentration of 8×10¹⁷ cm⁻³ and a low resistivity of 0.8 Ω cm. When compared with undoped GaN, Si and Mg dopings increase the threading dislocation density in GaN films by one order and two orders, respectively. Besides, it was observed that the Mg doping causes an additional biaxial compressive stress of 0.095 GPa compared with both undoped and Si-doped GaN layers, which is due to the incorporation of large amount of Mg atoms (4–5×10¹⁹ cm⁻³).     

4 inch低位错密度InP单晶的VGF生长及性质研究

采用高压垂直温度梯度凝固法(VGF)生长了非掺、掺硫和掺铁的4 inch直径(100)InP单晶,获得的单晶的平均位错密度均小于5000 cm-2.对4 inch InP晶片上进行多点X-射线双晶衍射测试, 其(004)X-射线双晶衍射峰的半峰宽约为30弧秒且分布均匀.与液封直拉法(LEC)相比, VGF-InP单晶生长过程的温度梯度很低,导致其孪晶出现的几率显著增加.然而大量晶体生长结果表明VGF-InP晶锭上出现孪晶后,通常晶体的生长方向仍为(100)方向,这确保从生长的4 inchVGF-InP(100)晶锭上仍能获得相当数量的2~4 inch(100)晶片.由于铁在InP中的分凝系数很小,掺Fe-InP单晶VGF生长过程中容易出现组份过冷,导致多晶生长.通过控制生长温度梯度及掺铁量,可获得较高的掺铁InP单晶成晶率.对VGF-InP单晶的电学性质、位错密度及位错的分布特点、晶体完整性等进行了研究.     

Influence of Fe-doping on GaN grown on sapphire substrates by MOCVD

The influence of Fe-doping on GaN grown on sapphire substrates by MOCVD was investigated using microscopy, in situ optical monitoring, double-crystal X-ray, Hall and photoluminescence. The growth from 3-D mode to 2-D mode for undoped GaN, and the growth from 2-D mode to 3-D mode for Fe-doped GaN was observed, respectively. The 2-D mode during the initial stage of the Fe-doped GaN buffer growth suggests that Fe plays a role of surfactant. A slight Fe-doping did not significantly degrade the crystalline quality of GaN buffer, confirmed by the FWHM of X-ray rocking curves. More than 4 orders of magnitude increase in the resistivity of Fe-doped GaN was achieved as compared to the undoped GaN. As a deep energy level acceptor, the compensation of Fe atom at the vacancy of Ga atom can be explained as the results of increased resistivity and suppressed yellow luminescence.     

Structural and electrical properties of Si-doped a-plane GaN grown on r-plane sapphire by MOVPE

Approximately 2-μm-thick Si-doped a-plane GaN films with different doping concentrations were grown on approximately 8-μm-thick undoped a-plane GaN/r-sapphire by metal organic vapor phase epitaxy (MOVPE). The structural and electrical properties of the Si-doped a-plane GaN films were investigated by high-resolution X-ray diffraction (HRXRD), atomic force microscopy (AFM) and temperature-dependent Hall measurement. The results showed that a small amount of Si doping can improve the surface morphology and decrease the density of pits. Upon increasing the CH₃SiH₃ flow rate, the crystalline quality of the (0 0 0 2) plane was slightly improved. The highest room-temperature mobility of 83.4 cm²/Vs was obtained at a carrier density of 6.2×10¹⁸ with a CH₃SiH₃ flow rate of 10 sccm.     

Influence of semi-insulating InP substrates on InAlAs epilayers grown by molecular beam epitaxy

Tensile-strained InAlAs layers have been grown by solid-source molecular beam epitaxy on as-grown Fe-doped semi-insulating (SI) InP substrates and undoped SI InP substrates obtained by annealing undoped conductive InP wafers (wafer-annealed InP). The effect of the two substrates on InAlAs epilayers and InAlAs/InP type II heterostructures has been studied by using a variety of characterization techniques. Our calculation data proved that the out-diffusion of Fe atoms in InP substrate may not take place due to their low diffusion coefficient. Double-crystal X-ray diffraction measurements show that the lattice mismatch between the InAlAs layers and the two substrates is different, which is originated from their different Fe concentrations. Furthermore, photoluminescence results indicate that the type II heterostructure grown on the wafer-annealed InP substrate exhibits better optical and interface properties than that grown on the as-grown Fe-doped substrate. We have also given a physically coherent explanation on the basis of these investigations.     

Effect of ammonium malate on growth rate,crystalline perfection,structural, optical,thermal, mechanical,dielectric and NLO behaviour of ammonium dihydrogen phosphate crystals

The effect of ammonium malate on the growth rate, structural, optical, thermal, mechanical, dielectric properties, crystalline perfection and second harmonic generation (SHG) efficiency of ammonium dihydrogen phosphate single crystals grown by the slow cooling method has been investigated. The lattice parameters of the grown crystals were obtained by the single-crystal X-ray diffraction analysis. Fourier transform infrared studies confirm the functional groups of the crystals. UV–vis study shows that the transparency is increased much by the dopant. Thermal analysis was performed to study the thermal stability of the grown crystals. Vickers hardness measurements reveal the higher hardness of the doped crystals. Low dielectric losses were observed from the dielectric measurements for the doped ADP crystals. The high-resolution X-ray diffraction studies show that the crystalline perfection of the crystals is good. The relative SHG efficiency measurements revealed that the dopant has enhanced the efficiency.     

Dielectric and microhardness studies on L‐citrulline and L‐ascorbic acid admixture TGS crystals

Single crystals of amino acid doped (L–citrulline, L‐ascorbic acid) triglycine sulphate were grown by slow evaporation technique. The lattice parameters and crystalline quality were confirmed by powder X‐ray diffraction studies. The presence of functional groups in the grown crystals was confirmed by Fourier transform infrared spectrum analysis. The dielectric studies were carried out to identify the phase transition temperature and the dielectric constant was found to be less for both the doped crystals than pure triglycine sulphate crystal. Micro hardness studies were carried out using Vickers pyramidal indentation technique at room temperature showed a decrease in hardness due to doping. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)