MOCVD生长AlN/GaN化学反应路径的量子化学研究

应用量子化学的密度泛函理论,对MOCVD生长GaN/AlN薄膜的反应路径进行理论计算和分析,特别是针对Ⅲ族TMX(X=Ga,Al)与V族NH3的反应路径与温度的关系进行研究.计算结果表明:当温度T≤473.15 K时,反应自由能△G＜0,TMX与NH3自发生成配位加合物TMX∶ NH3;当T≥573.15 K时,△G＞0,TMX∶ NH3将重新分解为TMX和NH3.在473.15 K≤T≤573.15 K区间,将存在△G=0,即加合反应达到平衡,反应为双向可逆.随着温度的升高,从加合物变为氨基物DMX∶ NH2的反应概率加大.TMX和MMX的直接热解反应均需要高温激活,而DMX变为MMX则较容易发生.当T＞873.15 K时,DMGa变为MMGa的热解反应将自发进行;当T＞1273.15 K时,DMAl变为MMAl的热解反应将自发进行.在自由基CH3参与下,TMX→DMX(X=Ga、Al)的能垒仅为TMX直接热解能垒的一半,约为30 ～ 40 kcal/mol;在自由基H参与下,TMGa和TMAl的热解反应能垒更低,约为16～ 20kcal/mol.因此,自由基H的产生将大大促进TMX的热解.     

加合反应对MOCVD生长GaN化学反应路径的影响

针对分隔进口垂直高速转盘式(RDR)MOCVD反应器生长GaN的气相化学反应路径进行数值模拟研究.分别考虑TMG与一个NH3的加合反应(模型1)和TMG: NH3与第二个NH3的加合反应(模型2)两种情况,通过对比两种情况下衬底表面附近主要反应前体的浓度大小,判断GaN生长的主导反应路径.通过分析模拟结果发现:只考虑TMG与一个NH3的加合反应时,GaN生长主要遵循TMG热解路径;在考虑TMG: NH3与第二个NH3的加合反应时,GaN生长主要遵循加合路径.由此可见,TMG: NH3与第二个NH3的反应对于MOCVD生长GaN的化学反应路径的选择具有很大的影响.     

MOVPE生长AlN的气相反应机理的密度泛函理论研究

利用量子化学的密度泛函理论,对MOVPE生长AlN的气相反应路径进行理论计算和分析,特别针对氨基物DMAlNH2形成后的多聚反应、多聚物消去甲烷反应、以及温度的影响关系进行研究.通过对不同反应路径的吉布斯自由能和反应能垒的计算,分别从热力学和动力学上确定最可能的末端气相反应前体.研究发现,氨基物通过与NH3的双分子碰撞,很容易越过较低的能垒,形成稳定的Al(NH2)3.在385 K＜T＜616 K,三聚物(DMAlNH2)3消去CH4变成(MMAlNH)3的反应容易发生.在641 K＜T＜1111 K,二聚物(DMAlNH2)2消去CH4变成(MMAlNH)2的反应容易发生.而(MMAlNH)2、(MMAlNH)3继续消去CH4生成(AlN)2、(AlN)3的反应,由于吉布斯自由能差都大于零,而且能垒也很大,故很难发生.因此,在AlN的MOVPE过程中,Al(NH2)3、(MMAlNH)2和(MMAlNH)3是最可能的三种末端气相反应前体,它们将决定AlN的表面反应生长.     

MOVPE生长GaN薄膜的表面吸附和扩散研究

利用量子化学计算方法,对MOVPE生长GaN薄膜的表面反应进行研究.特别针对反应前体GaCH3(简称MMG)在理想、H覆盖和NH2覆盖GaN(0001)面的吸附和扩散进行计算分析.通过建立3×3 超晶胞模型,优化计算了MMG在三种不同覆盖表面的稳定吸附位、吸附能和电子布居,搜寻了MMG在稳定吸附位之间的扩散能垒.计算结果表明:对于三种表面,MMG的稳定吸附位均为T4位和H3位,H3位比T4位略微稳定.MMG在NH2覆盖表面吸附能最大,在H覆盖表面吸附能最小,在理想表面吸附能居中.MMG中的Ga与不同的表面原子形成的化学键的键强的大小顺序为:Ga-N>Ga-Ga>Ga-H.相比于理想表面和H覆盖表面,MMG在NH2覆盖表面的扩散能垒最大,因此表面过量的NH2会抑制MMG的扩散.     

喷淋式MOCVD反应器中AlN的生长速率和化学反应路径的数值模拟研究

利用数值模拟方法,结合反应动力学和气体输运过程,研究喷淋式MOCVD反应器中AlN的生长速率和气相反应路径与反应前体流量(NH3和H2)、进口温度、压强、腔室高度等参数的关系.研究发现:薄膜生长前体和纳米粒子前体的浓度决定了不同的生长速率和气相反应路径.在低Ⅴ/Ⅲ比(2000)、高H2流量(12 L/min)、高进口温...     

自由基对气相化学反应生长GaN的影响研究

结合化学反应动力学模型,对MOCVD反应器中自由基对GaN生长的化学反应路径的影响进行数值模拟研究。通过对比加入自由基前后RDR反应器中Ga浓度变化,来分析自由基对化学反应热解路径的影响。同时改变压强,分析操作参数的变化对自由基活性的影响。研究发现:在不考虑自由基的反应路径,薄膜生长的主要前体为DMG;而考虑自由基的反应路径,主要的生长前体为MMG。自由基的存在加速了DMG向MMG的热解,使得DMG分解为MMG速度远大于TMG分解为DMG的速度,导致衬底上方的MMG浓度高于DMG。而操作压强的变化仅仅对流动边界层产生了影响,对热解路径影响不大。     

放射状三水碳酸镁晶体合成及生长机理研究

以NH4HCO3与MgSO4·7H2O为反应物料,采用低温液相法合成了放射状三水碳酸镁晶体,采用XRD,SEM、FHR对合成的试样进行了测试表征,重点讨论了NH4HCO3反应初始浓度及不同反应时间对晶体结晶形貌的影响.研究结果表明:当NH4HCO3的反应初始浓度为0.2 ～ 0.5 mol/L,反应初始浓度越小,越有利于放射状晶体的结晶生长.反应体系中Mg(NH3)y2+络合物的形成成为放射状晶体形成的主要原因.在此基础上,探讨了放射状晶体的结晶生长机理.     

Cu3V2O7(OH)2·2H2O纳米线的制备及光吸收性能

以CuSO4·5H2O和 NH4VO3为原料,采用水热法制备了Cu3V2O7(OH)2·2H2O纳米线.采用X射线衍射(XRD)和场发射扫描电子显微镜(FE-SEM)对样品的组成和表面形貌进行了表征,结果显示:Cu3V2O7(OH)2·2H2O纳米线直径约80 nm,长度达到几个微米.对纳米线形成机理研究表明:该纳米线的形成主要取决于反应温度和反应体系pH值等因素.紫外-可见光吸收测试显示Cu3V2O7(OH)2·2H2O纳米线具有较宽的紫外-可见光吸收范围,计算其带隙宽度为1.94 eV.     

SiH4在Si(001)-(2×1)表面吸附的第一性原理研究

用第一性原理密度泛函理论研究SiH4在Si(001)-(2×1)表面的分裂吸附及吸附后的结构、能量和成键特性,计算了反应物、过渡态及生成物三个状态的能量,吸附能和反应能.计算结果表明:SiH4在Si(001)-(2×1)重构表面吸附的可能反应路径是由于SiH4中Si-H键的拉长和二聚体键的断裂导致的,形成产物Si(001)-(2×1)(SiH3:H);由SiH4分裂的能量势垒0.78 eV说明所推测的反应路径是合理的.     

改进的碳铵沉淀法中氧化钇粉体尺寸及形貌的调控研究

以商业氧化钇(Y2 O3)为原材料,采用改进的碳酸盐沉淀工艺合成钇碳酸盐前驱体,经随后的焙烧工艺获得Y2O3产品.X衍射技术(XRD)和扫描电镜(SEM)表征表明:合成的碳酸盐前驱体Y2(CO3)3·2H2O具有类球形形貌、粒度分布均匀,经950℃焙烧2 h,前驱体受热分解为Y2 O3粉体,粉体完整保留了前驱体的形貌特征.同时,基于对NH4 HCO3/NH4+不同摩尔比条件下NH4 HCO3/NH4 NO3混合溶液中pH值变化的理论计算,在保持其他条件不变化条件下,通过改变NH4 HCO3/NH4 NO3混合溶液的加入量以获得不同的稀土碳酸盐沉淀终点pH值,当碳酸盐沉淀过程中pH值从6.0降低至4.7时,可实现超细至大颗粒不同尺寸氧化钇粉体的制备.同时,公斤级的扩大试验证实该工艺具有良好的可推广性.进一步提出NH4+在碳酸氢铵沉淀制备氧化钇工艺中的作用机理:(1)改变反应体系中阳离子和阴离子的组成;(2)显著影响前驱体的晶型;(3)显著影响产品颗粒尺寸大小.     

High quality GaN grown by MOVPE

Several nitrogen precursors have been used for the growth of GaN in MOVPE, but so far the best results were obtained using NH₃, even though NH₃ does not produce a significant amount of active species at the growing interface. To produce active species from N₂ or NH₃, a remote plasma-enhanced chemical vapour deposition (RPECVD) process has been implemented. In addition, nitrogen metalorganic precursors, triethylamine and t-butylamine, were also used. To accurately control the critical parameters of the MOVPE of GaN, we have implemented a laser reflectometry equipment, which allows a real-time in situ monitoring of the different steps of the growth, i.e. nitridation of the substrate, nucleation, heat treatment, and deposition. Using an appropriate buffer layer, GaN grown on sapphire using NH₃ as nitrogen precursor, shows sharp low temperature photoluminescence lines (4 meV at 9 K), whereas other nitrogen precursors did not lead to comparable electronic quality.     

Effect of heat treatment on phase transformation of aluminum nitride ultrafine powder prepared by chemical vapor deposition

Ultrafine aluminum nitride (AlN) powders were obtained by chemical vapor deposition via AlCl₃–NH₃–N₂ system operated at various temperatures and at a same 200 cm³/min flow rate of NH₃ and N₂, respectively. It has been shown that when the reaction temperature of AlCl₃ and NH₃ was above 600°C, then crystalline AlN powder can be formed; whereas, amorphous AlN was obtained with NH₄Cl if reacted in a lower-temperature zone of the reaction chamber. The amorphous AlN powder was heat treated at 1400°C under NH₃ and N₂ atmosphere for 2 h, then the crystalline phases of the obtained powder belong to a single phase of AlN; a mixture of AlN and Al₂O₃ and only AlON, respectively. On the other hand, if crystalline AlN powder is heat treated at 1400°C under N₂ atmosphere for 2 h, the crystalline phases were composed of the major phase of AlN and a minor phase of Al₂O₃. The morphology, particle size and agglomerate size of the AlN powder were strongly dependent on the heat-treatment temperature. The particle size of AlN powder increases from 28.1 to 45.0 nm, as the heat treatment temperature increases from 800 to 1400°C.     

Enhanced two-dimensional growth of MOVPE InN films on sapphire (0 0 0 1) substrates

MOVPE growth of InN on sapphire substrates is compared using two different designs of horizontal reactor. The major difference between the two designs is a variation in the reactant-gas flow-spacing between the substrate and the ceiling of the quartz chamber: 33 mm for the Type A reactor and 14 mm for Type B. Compared with the Type A reactor, the Type B reactor brings about InN films with a larger grain size. This is especially true when InN is grown at 600°C using the Type B reactor, in which case the two-dimensional (2D) growth of InN is found to be extremely enhanced. An investigation of the NH₃/TMIn molar ratio dependence of the surface morphology of grown InN films using the two reactors suggests that the enhanced 2D growth is attributed to the decrease in the effective NH₃/TMIn ratio in the growth atmosphere. Even using the Type A reactor, a film with enhanced 2D growth can be obtained when the NH₃/TMIn ratio is considerably low (1.8×10⁴). The enhanced 2D growth results in a smaller XRC-FWHM (full-width at half maximum of the X-ray rocking curve) (1500 arcsec), than that for a 3D-grown film (5000 arcsec).     

Effect of NH3 on the growth characterization of TiN films at low temperature

Titanium nitride (TiN) films were obtained by the atmospheric pressure chemical vapor deposition method of the TiCl₄–N₂–H₂ system with various flow rates of NH₃ at 600°C. The growth characteristics, morphology and microstructure of the TiN films deposited were analyzed by X-ray diffraction, scanning electron microscopy and transmission electron microscopy. Without NH₃ addition, no TiN was deposited at 600°C as shown in the X-ray diffraction curve. However, by adding NH₃ into the TiCl₄–N₂–H₂ system, the crystalline TiN was obtained. The growth rate of TiN films increased with the increase of the NH₃ flow rate. The lattice constant of TiN films decreased with the increase of the NH₃ flow rate. At a low NH₃ flow rate, the TiN (2 2 0) with the highest texture coefficient was found. At a high NH₃ flow rate, the texture coefficient of TiN (2 0 0) increased with the increase of the NH₃ flow rate. In morphology observation, thicker plate-like TiN was obtained when the NH₃ flow rate was increased. When the flow rate of NH₃ was 15 sccm, Moiré fringes were observed in the TiN film as determined by TEM analysis. The intrinsic strain was found in the TiN film as deposited with 60 sccm NH₃.     

双甜菜碱为客体包合物的三维氢键网格的构筑

利用硫脲和双甜菜碱制备了一种新型的包合物[-OOCCH2N+(CH3)2]2(CH2)3·4(NH2)2CS,用X射线单晶衍射方法测定其晶体结构.结果表明,晶体属三斜晶系,P 1空间群,其中a=0.8845(2)nm,b=0.9367(2)nm,c=0.9464(3)nm,α=91.591(2)°,β=91.591(2...     

MOMBE growth characteristics of antimonide compounds

This paper describes the MOMBE (metalorganic molecular beam epitaxy) growth characteristics of antimonide compounds using TMIn (trimethylindium), TEGa (triethylgallium) and TIBAl (triisobutylaluminium) as group III sources, and As₄, Sb₄, TEAs (triethylarsine) and TESb (triethylstibine) as group V sources. Large differences in the growth characteristics of GaAs and GaSb MOMBE are observed. These are explained, using a theoretical consideration of the growth mechanism, by the difference in the effective surface coverage of excess As and Sb atoms during the growth. The use of TEAs and TESb instead of As₄ and Sb₄ alters the growth rate variation of both GaAs and GaSb with substrate temperature (T_sub), which results from the interaction of alkyl Ga species with the alkyl radicals coming from the thermally cracked TEAs and TESb. The alkyl exchange reaction process is observed in the growth of AlGaSb using TIBAl and TEGa, where the incorporation rate of Al is suppressed by the coexistence of TEGa on the growth surface, in the low T_sub region. This is caused by the formation of an ethyl-Al bond which is stronger than the isobutyl-Al bond. The composition and the growth rate variations of InGaSb with T_sub are similar to those of InGaAs, which are closely related to the MOMBE growth process and are quite different from those of MBE (molecular beam epitaxy) and MOVPE (metalorganic vapor phase epitaxy) growth. In the MOMBE growth of InAsSb and GaAsSb using TEAs and TESb, the composition variation with T_sub is weaker than that of MBE. This is a superior point of MOMBE growth for the composition control. The electrical and optical properties of MOMBE grown films as well as the quantum well structures are also described.     

化学水浴沉积法制备硫化镉薄膜的微结构和性能

采用化学水浴沉积法在不同氨水用量下制备了Cu(In,Ga)Se2太阳能电池的缓冲层CdS薄膜,根据化学平衡动力学计算出混合溶液中反应粒子的初始浓度、pH值和离子积,利用台阶仪、扫描电子显微镜(SEM)、X射线衍射仪(XRD)、量子效率测试仪(EQE)和IV测试仪对制备样品的薄膜厚度、表面形貌、晶体结构、量子效率和光电转...     

MOVPE growth of GaInP/GaAs hetero-bipolar-transistors using CBr4 as carbon dopant source

Carbon doping of GaAs with carbon tetrabromide (CBr₄) in low pressure MOVPE has been investigated and applied to the fabrication of GaInP/GaAs HBTs. Especially the hydrogen incorporation and the associated acceptor passivation has been studied. The hydrogen found in single GaAs:C layers is predominantly incorporated during cooling the sample under AsH₃ after growth, n-Type capping layers can block this H indiffusion and GaAs:C base layers in HBTs show much lower H concentrations than GaAs:C single layers without a cap. A further reduction of acceptor passivation is possible by optimization of the growth procedure. First HBTs processed from layers with a base that was doped using CBr₄ show promising DC and HF performance (β = 45, for 2 × 20 μm² devices).     

Incorporation of phosphorus during gas phase epitaxy

The PH₃ desorption rate can be reduced and the decomposition rate increased, thereby increasing the P incorporation efficiency by replacing TMIn with InCl created by the pyrolysis of DEIn. InCl generated by cracking DEInCl and uncracked PH₃ could be used for CBE growth of InP provided that H on the PH₃ can be used to remove Cl from the InCl. Evidence is presented that this is possible. Evidence is also provided that P is more readily incorporated during OMVPE growth using TBP than PH₃ because PH₂ is a primary pyrolysis product, and it is less likely to desorb and more likely to decompose than PH₃.