Effects of piezoelectricity and spontaneous polarization on electronic and optical properties of wurtzite III-V nitride quantum wells

A theoretical model accounting for the macropolarization effects in wurtzite III-V nitrides quantum wells (QWs) is presented. Energy dispersions and exciton binding energies are calculated within the framework of effective-mass theory and variational approach, respectively. Exciton-associated transitions (EATs) are studied in detail. An energy redshift as high as 450 meV is obtained in Al0.25GaN0.75/GaN QWs. Also, the abrupt reduction of optical momentum matrix elements is derived as a consequence of quantum-confined Stark effects. EAT energies are compared with recent photoluminescence (PL) experiments and numerical coherence is achieved. We propose that it is the EAT energy, instead of the conduction-valence-interband transition energy that is comparable with the PL energy. To restore the reduced transition rate, we apply an external electric field. Theoretical calculations show that with the presence of the external electric field the optical matrix elements for EAT increase 20 times. (C) 2001 American Institute of Physics.

Point defects in III-V compound semiconductors

Point defects in III-V compound semiconductors were analyzed systematically in this paper. The effects of substitutes, antisites, interstitials, and vacancies on lattice parameters in III-V compound semiconductors were calculated with a simple model. The formation energies of vacancies in compound semiconductors can be obtained by this calculation. A practical technique established on this model has been utilized for measuring the stoichiometry in GaAs. The relationship between stoichiometry and deep level centers in GaAs was also investigated.

Studies of high DC current induced degradation in III-V nitride based heterojunctions

We report experiments on high de current stressing in commercial III-V nitride based heterojunction light-emitting diodes. Stressing currents ranging from 100 mA to 200 mA were used. Degradations in the device properties were investigated through detailed studies of the current-voltage (I-V) characteristics, electroluminescence, deep-level transient Fourier spectroscopy and flicker noise. Our experimental data demonstrated significant distortions in the I-V characteristics subsequent to electrical stressing. The room temperature electro-luminescence of the devices exhibited a 25% decrement in the peak emission intensity. Concentration of the deep-levels was examined by deep-level transient Fourier spectroscopy, which indicated an increase in the density of deep-traps from 2.7 x 10(13) cm(-3) to 4.2 x 10(13) cm(-3) at E-1 = E-C - 1.1 eV. The result is consistent with our study of 1/f noise, which exhibited up to three orders of magnitude increase in the voltage noise power spectra. These traps are typically located at energy levels beyond the range that can be characterized by conventional techniques including DLTS. The two experiments, therefore, provide a more complete picture of trap generation due to high dc current stressing.

Kinetic study of MOCVD III-V quaternary antimonides

The kinetics of MOCVD GaInAsSb and AlGaAsSb was studied by the growth rate as a function of growth temperature and partial pressure of III and V MO species. The diffusion theory was used to explain the mass transport processes in MOCVD III-V quaternary antimonides. On the basis of the discussion about their growth kinetics and epilayer properties, the good quality multi-epilayers of these two quaternary antimonides and their photodetectors and arrays with wavelength of 1.8 similar to 2.3 mu m and detectivities of D* > 10(9) cm Hz(1/2) W-1 were obtained.

Raman scattering studies of III-V semiconductors grown on [n11]-oriented substrates

The room temperature Raman spectra of the Ga(0.5)Al(0.5)AS and the In0.52Al0.48As epilayer grown on [n11]-oriented substrates were measured in various back scatterng geometries, The relative intensity of TO modes and LO modes in those samples shows a regular Variation with differently oriented substrates in the experiments. By comparing experimental data with Raman scattering selection rules for the zincblende structure epilayer grown on [n11]-oriented substrates, it was found that the present calculations are in good agreement with the experimental results.

Characterization of hot-electron effects on flicker noise in III-V nitride based heterojunctions

We report experiments on hot-electron stressing in commercial III-V nitride based heterojunction fight-emitting diodes. Stressing currents ranging from 100 mA to 200 mA were used. Degradations in the device properties were investigated through detailed studies of the I-V characteristics, electroluminescence, Deep-Level Transient Fourier Spectroscopy and flicker noise. Our experimental data demonstrated significant distortions in the I-V characteristics. The room temperature electroluminescence of the devices exhibited 25% decrement in the peak emission intensity. Concentration of the deep-levels was examined by measuring the Deep-Level Transient Fourier Spectroscopy, which indicated an increase in the density of deep-traps from 2.7 x 10(13) cm(-3) to 4.21 x 10(13) cm(-3) at E-1 = E-C - 1.1eV. The result is consistent with our study of 1/f noise, which exhibited up to three orders of magnitude increase in the voltage noise power spectra. Our experiments show large increase in both the interface traps and deep-levels resulted from hot-carrier stressing.

Characterization of GaSb substrate wafers for MOCVD III-V antimonides

The qualities of GaSb substrates commonly used for the preparation of III-V antimonide epilayers were studied before and after growing GaInAsSb multi-layers by MOCVD using PL, FTIR and DCXD together with the electrical properties and EPD value. The correlation between the substrate qualities and epilayer properties was briefly discussed. The good property epilayers of GaInAsSb and, then, the high preformance of 2.3 um photodetectors were achieved only using the good quality GaSb wafers as the substrates.

III-V族化合物半导体超晶格的喇曼散射研究

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Controlled growth of III-V compound semiconductor nano-structures and their application in quantum-devices

Various techniques on the growth of self-assembled compound semiconductor nano-structures (quantum dots, QDs) have been tried to enhance the controlling on size, density, emitting wavelength, uniformity in size and ordering in location of the QDs. Optimized growth conditions have been used in the application of the QD materials in opto-electronic devices. High-power long-lifetime quantum-dot laser-diodes (QD-LDs) emitting near 1 mu m, QD-LDs emitting in red-light range, 1.3 mu m QD-LDs on GaAs substrate and quantum-dot super-luminescent diodes (QD-SLDs) have successfully been achieved.

离子液体在铈（IV）钍及稀土（III）萃取分离中的应用研究

本文对1-辛基-3-甲基咪唑六氟磷酸盐离子液体（[C8mim]PF6）在包头稀土矿分离钍以及攀西矿分离铈工艺中的应用作了初步探索。论文首先研究了伯胺N1923溶于离子液体对硫酸体系钍的萃取分离行为，考察了酸度、盐析剂浓度、萃取剂浓度等对N1923/IL萃取钍的影响，并与庚烷体系的萃取规律作了对比。在此基础上对萃取机理进行了探讨，提出了不同于分子溶剂体系的萃取机理，认为N1923/IL在水相有硫酸钠作为盐析剂时对钍的萃取为“胶束增溶”萃取机理。此外，钍对稀土的分离考察结果认为，虽然N1923/IL对钍的萃取效率降低，但在一定条件下钍对稀土仍能保持较高的分离系数。由此本文对N1923/IL应用于钍的分离实践作了初步评估。 论文的另外一部分工作研究了纯[C8mim]PF6及其与2-乙基己基膦酸二（2-乙基己基）酯（DEHEHP）的混合体系对硝酸铈（IV）的萃取性能。研究发现纯[C8mim]PF6本身对硝酸铈（IV）有良好的萃取选择性，机理考察结果认为铈（IV）的萃取是由于Ce4+离子在富硝酸根溶液中形成络阴离子Ce(NO3)62-并与离子液体的阴离子PF6—发生交换的结果。一种新型含铈离子液体[C8mim]2Ce(NO3)6的合成进一步证实了这一机理。反萃考察发现被萃入离子液相的铈（IV）用水可完全反萃，且萃取和反萃过程中因离子交换机理导致的离子液体流失可以通过添加适量离子液体组分的方法予以抑制。此外，本文考察了DEHEHP溶于[C8mim]PF6对于含氟硝酸体系铈（IV）的萃取行为，发现在一定条件下DEHEHP对铈（IV）的萃取能获得与分子溶剂中等同的萃取机理。在此基础上，采用DEHEHP/IL从氟碳铈矿分解液中进行分离铈和氟的研究，获得纯度为99.9%以上的纳米CeF3产品。由此我们提出一个将离子液体应用于攀西氟碳铈矿铈钍稀土分离流程的模型，为离子液体在稀土清洁分离流程中实现溶剂绿色化的目标提供了一个范例。

膜基萃取稀土研究－中空纤维膜器中RE(III)、Th(IV)的萃取传质动力学及分离

首次使用两种新的聚偏氟乙烯中空纤维膜萃取法研究了HEH／EHP体系中钕、钐及铈及Cyanex272体系中镱、铒及钍的膜基萃取及分离。研究了膜材料性能。提出了膜萃取反应与液液萃取相同，且均为一级反应；高料液pH值条件下的H~+优先于NH_4~+的传质规律；氨化萃取剂可降低传质阻力；膜萃取机理为伴有界面反应的扩散控制机理及膜孔径对萃取机理的影响。利用钍的传质速率远大于镱的传质速率的特点，实现动力学竞争分离钍和镱。通过界面反应动力学研究，得到了各组分的反应动力学方程、反应速率常数K及相关组分的分离系数。将氨离子选择电极与流动注射分析相结合，实现了氨离子的自动在线分析。为膜萃取技术的工业化提供有价值的基本参数。

Cr(III)盐在Pt电极和PbO_2电极上阳极氧化为络酸盐的机理的研究

关于Cr(III)阳极氧化为Cr(VI)的过程，在工业应用方面已有许多工作，但对机理尚不清楚。本文对Pt电极和PbO_2电极上的这一过程进行了动力学的研究，首次得到了Cr(VI)在这两种电极上阳极形成的真实动力学数据，并提出了与实验基本相符的反应机理。用“分解极化曲线法”和稳态极化曲线的测量，得到了[H_2SO_4]和[SO_4~=]恒定的不同浓度Cr_2(SO_4)_3溶液中Cr(VI)在光滑Pt电极上阳极形成的真实动力学数据，可用如下方程式来表达：在较低电位下φ = a + 0.25 log i_2 - 0.23 log [Cr(III)]在较高电位下φ = a' + 0.48 log i_2 - 0.44 log [Cr(III)]同时得到了氧的阳极发生的动力学数据，Tafel线性区的斜率接近于2.303 RT/ΔF (Δ ≈ 0.5)。动力学方程式的推导与实验的比较表明，在光滑Pt电极上Cr(VI)是由Cr(II)通过“活性氧”的氧化形成的，提出了如下反应机理：H_2O → (OH)_(ad) + H~+ + e~- 2(OH)_(ad) → (O)_(ad) + H_2O 2(O)_(ad) →O_2 (OH)_(ad) + [Cr(H_2O)_6]~(3+) → (CrO_2)_(ad) + 3H~+ + 5H_2O (CrO_2)_(ad) + H_2O → (CrO_3~-)_(ad) + 2H~+ + e~- (CrO_3~-)_(ad) + H_2O → HCrO_4~- + H~+ + e~- 2HCrO_4~- <-> Cr_2O_7~(2-) + H_2O 在[H_2SO_4]和[SO_4~(2-)]恒定的不同浓度Cr_2(SO_4)_3溶液中，测得了Cr(VI)在Δ-PbO_2电极上阳极形成的动力学数据：在较低电位下φ = a + 0.28 log i_2 - 0.30 log [Cr(III)]在较高电位下φ = a' + 0.55 log i_2 - 0.51 log [Cr(III)]氧的极化曲线的Tafel线性区斜率也为2.303RT/ΔF (Δ approx= 0.5)。PbO_2电极和Pt电极上分解极化曲线的比较表明，Cr(VI)在前一电极上阳极形成的过电位远低于在后一电极上，这可能是两电极上电流效率显著差别的原因。测得了PbO_2电极上不同过电位下电极反应的有效活化能，其数值均在10 Kcal mol~(-1)以上，且随着极化的增大而减小，据此在动力学处理中可以忽略扩散的作用，交流阻抗的研究进一步证实了这一点。溶液pH的增大或[H_2SO_4]的减小会降低Cr(VI)阳极形成的过电位，使反应加速。在[H_2SO_4]和[SO_4~(2-)]恒定的不同浓度Cr_2(SO_4)_3溶液中，测得了在不同电位极化下PbO_2电极的阻抗频谱。在较高电位下阻抗谱呈现明显的两个半圆，表明电极过程包括了中间吸附物的形成。求得的吸附电容Cad比双层电容Cd大1-2个数量级；Cd比通常光滑电极表面的Cd大得多，这可能与SO_4~(2-)，HCrO_4~-, Cr_2O_7~-以及[_((SO_4))~((H_2O)_2) Cr_((SO_4))~((OH)_2) Cr_((SO_4))~((OH_2)_2)]~(2-)等阳离子的特性吸附有关。从动力学的推导与实验的比较得出，在PbO_2电极上Cr(VI)也按“活性氧”机构形成，可能的机理如下：H_2O → (OH)_(ad) + H~+ + e~- (OH)_(ad) + H_2O → (O)_(ad) + H_3O~+ + e~- 2(O)_(ad) → O_2 [Cr(H_2O)_6]~(3+) + (O)_(ad) → (CrO_3~-)_(ad) + 4H~+ + 4H_2O (CrO_3~-)_(ad) + H_2O → HCrO_4~- + H~+ +e~- 2HCrO_4~- <-> Cr_2O_7~(2-) + H_2O.