Electron paramagnetic resonance and optically-detected magnetic resonance of donors in AlxGa1-xAs

The Electron Paramagnetic Resonance (EPR) and Optically-Detected Magnetic Resonance (ODMR) work on Si-donors in Al _{x Ga1- x} As is reviewed in the context of the shallow-deep bistability (DX) problem. Three donor states are important. Little work has been published on donors tied to theT-minimum. However, there are many results forX-donors. In AlAs/GaAs heterostructures, well-resolved spectra reveal a donor state comprised of independentX _x andX _y valleys with theXz valley unpopulated due to the hetero-epitaxial strain. As Al mole fraction decreases, intervalley coupling is evident from the line positions and linewidths. The published attempts to observe and identify the deep (relaxed) state are inconclusive. Some suggestions for future work are presented.     

Confirmation of large lattice relaxation of theDX center by extended photo-ionization cross-section measurements

Although the large lattice relaxation model (LLR) for electron capture at the donor relatedDX center in Al_xGa_1-xAs has seen wide acceptance over the last 12 years, there have been some recent proposals which have attempted to explain the experimental data with models that only require small lattice relaxation (SLR). One key piece of evidence supporting LLR is the large observed difference (in the case of Si-doped Al_xGa_1-xAs) between the optical (∼1.4 eV) and thermal (∼0.2 eV) ionization energies. The SLR model proposed that the lowest energy optical ionization was a very weak process, and that the optical transition which had been observed previously is a transition to a higher band. These arguments were supported by photoconductivity data showing a finite photo-ionization rate at energies as low as 200 meV. To resolve this question we have measured the photo-ionization cross section over 7 to 8 orders of magnitude using a tunable infrared laser as a source. A consistent optical ionization energy of about 1.4 eV was observed for 4 samples of differing alloy compositions and doping levels.In no case was there any detectable photo- ionization below 0.8 eV. A detailed discussion of these experiments examines the difficulty in obtaining such a large dynamic range optical spectrum. Of particular relevance are the issues of ionization detection, and the brightness of purity of the optical source. A thorough review of these issues and their impact on previous studies of theDX center is presented.     

DX centers in AlxGa1-xAs bulk alloy,AlAs/GaAs ordered,and disordered superlattices

Properties of theDX centers in Al_0.5Ga_0.5As bulk alloy (b-AL), (AlAs)₂ (GaSa)₂ ordered superlattice (o-SL) and (AlAs) _m (GaAs) _n disordered superlattice (d-SL) (m = 1, 2, 3,n = 1, 2, 3) with the same macroscopic composition were measured and compared. By deconvolution of deep level transient spectroscopy (DLTS) spectrum due to theDX center, we have found a decrease in the number of separate peaks in DLTS spectrum in an intentionally atomic ordered arrangement. Visiting Scholar of the Japan Society for the Promotion of Science. On leave from Department of Electrical Engineering, San Jose State University, San Jose, California 95192-0084, USA.     

Analysis of the vacancy-interstitial model ofDX centers

I describe the vacancy-interstitial model ofDX centers, in which the substitutional donor atoms relax toward interstitial sites when they capture electrons. This appears to be the only known model ofDX centers which can account for the structure observed by Mooneyet al. in DLTS spectra of dilute Al _x Ga_1-x As alloys. By comparing its predictions with the DLTS and Hall data from Al _x Ga_1-x As alloys of low Al content one can establish values for many of the relevant parameters, such as the enthalpies and entropies (degeneracies) associated with sites surrounded by differing numbers of Al atoms. The data also show that the distribution of donor configurations among the four available neighboring interstitial sites does not attain thermal equilibrium among states of differing energies before being released, although transitions (through tunneling) among states of equal energy may be possible. The model is seen to be consistent with those treated theoretically by Chadi and Chang forDX and by Dabrowski and Scheffler for EL2 centers.     

Effect of local alloy disorder on the emission kinetics of deep donors (DX centers) in AlxGa1-xAs

In this paper we summarize our recent studies of the effects of local alloy disorder on the properties ofDX levels. A single emission rate is observed in GaAs where all Si-donors have identical local environments. In contrast, three discrete emission rates are observed in dilute AlGaAs alloys, suggesting that the group IV donor moves towards the interstitial site, thereby “selecting” three of the twelve surrounding group III atoms. We present evidence for an ordering of theDX levels consistent with Morgan’s model of a deepening potential well for theDX level as Al atoms are subsequently substituted for Ga atoms near the relaxed donor. These conclusions are consistent with earlier calculations of Chadi and Chang.     

Effect of surface encapsulation and As4 overpressure on Si diffusion and impurity-induced layer disordering in GaAs,AlxGa1-xAs,and AlxGa1-xAs-GaAs quantum well heterostructures

Data are presented demonstrating that the surface encapsulant and the As₄ overpressure strongly affect Si diffusion in GaAs and Al_xGa_1-xAs, and thus are important parameters in impurity-induced layer disordering. Increasing As₄ overpressure results in anincrease in diffusion depth in the case of GaAs, and adecrease in diffusion depth for Al_xGa_1-xAs. In addition, the band-edge exciton is observed in absorption on an Al_xGa_1-xAs-GaAs superlattice that is diffused with Si and is converted to bulk crystal Al_yGa_1-yAs via impurity-induced layer disordering. In contrast, the exciton is not observed in absorption on GaAs diffused with Si in spite of the high degree of compensation. These data indicate that the Si diffusion process, and the properties of the diffused material, are different for GaAs and for Al_xGa_1-xAs-GaAs superlattices converted into uniform Al_yGa_1-yAs (0 ≤y ≤x ≤ 1) via impurity-induced layer disordering with the amphoteric dopant Si.     

Two electron transitions of the exciton bound at the Si Donor confined in GaAs/AlxGa1-xAs quantum wells

The exciton bound to the shallow Si-donor confined in a 100A wide GaAs quantum well has been studied in selective photoluminescence (SPL) and photoluminescence excitation (PLE) spectroscopy. The transition from the ground state, ls(Γ₆), to the first excited state, 2s(Γ₆), of the confined Si donor has been observed via two-electron transitions (TETs) of the donor bound exciton observed in SPL for the first time to the best of our knowledge. The interpretation of the TET peaks is confirmed by PLE measurements. Further, from Zeeman measurements, the magnetic field dependence of the donor ls(Γ₆)-2s(Γ₆) transition energy has been determined.     

Hall analysis of the electrical properties of n-type AlxGa1-xAs grown by MOVPE

Variable temperature Hall measurements were used to study the electrical properties of undoped and Se-doped Al_xGa_1-xAs (0 <x < 0.4) layers grown by metalorganic vapour phase epitaxy (MOVPE). It is shown that the deep donor activation energy measured in undoped AlGaAs exhibits a similar dependency upon composition as that reported for Si-doped AlGaAs grown by MBE. For Al_xGa_1-xAs, doping with selenium is found to reduce the activation energy from 66 meV (forn = 4.1 x 10¹⁶/cm³), to 9 meV (forn = 1.6 × 10¹⁸/cm³).     

Selective etching characteristics of HF for AlxGa1-xAs/GaAs

The first detailed report of the selective etching characteristics of an AlxGa1-xAs/GaAs DH wafer (x?0.4) in an HF system is given. Selective etching at a cleaved facet and a calculation of the energy of activation are included.     

Influence of V/III variation on AlxGa1-xAs quantum well lasers grown by metalorganic chemical vapor deposition

The role of the V-III ratio during growth on the optical and electrical properties of Al_xGa_1-x As laser material grown by metalorganic chemical vapor deposition has been investigated. Controlled studies involving more than twenty growth runs show that this parameter has a profound influence on both device performance and reliability.     

AlxGa1-xAs/GaAs分布布拉格反射镜的湿法氧化

对垂直腔面发射激光器(VCSEL)的AlxGa1-xAs/GaAs分布布拉格反射镜(DBR)进行了高温湿法氧化.由于AlxGa1-xAs层的氧化产生了应力而导致Al2O3/GaAs界面处出现了孔洞.这些孔洞反过来又缓解了应力而使氧化层的厚度只收缩了8%而不是理论上的20%.并且,随着氧化时间的延长,湿法氧化反应中的反应物和产物沿着多孔界面在氧化物中的传输越充分,从而使AlGaAs层的氧化进行的越完全,氧化质量就越好.     

Wet Oxidation of AlxGa1－xAs/GaAs Distributed Bragg Reflectors

The wet oxidation of AlGaAs with high Al content in a distributed Bragg reflectors (DBR) is studied by scanning electron microscopy (SEM) and transmission electron microscopy (TEM).Some voids distribute along the oxide/GaAs interfaces due to the stress induced by the wet oxidation of the AlGaAs layers.These voids decrease the shrinkage of the Al2O3 layers to 8% instead of the theoretical 20% when compared to the unoxidized AlGaAs layers.With the extension of oxidation time,the reactants are more completely transported to the front interface and the products are more completely transported out along the porous interfaces.As a result,the oxide quality is better.     

Atomic structure ofDX centers: Theory

A structural model forDX centers in Al _x Ga_1-x As alloys which explains their unusual properties in terms of two distinct bonding configurations for donor impurities is proposed. The results of ourab initio self-consistent pseudopotential calculations showDX to be a “negative-U” defect center. It results from a large lattice relaxation which is different for a Group IV than a Group VI donor. The proposed structural model provides a satisfactory explanation of the properties ofDX centers.     

Al_xGa_(1-x)As/GaAs调制掺杂结构的MBE优化工艺生长

通过对MBE工艺中影响GaAs和AlGaAs材料质量的生长关键工艺实验研究,优化了MBE生长AlxGa1-xAs/GaAs调制掺杂结构工艺。用GEN-ⅡMBE设备生长AlxGa1-xAs/GaAs调制掺杂结构材料,得到了高质量的AlxGa1-xAs/GaAs调制掺杂结构材料。用范德堡法研究材料特性,得到材料参数的典型值:二维电子气浓度在室温时为5.6×1011cm-2,电子迁移率为6000cm2/V·s;在77K低温时浓度达3.5×1011cm-2,电子迁移率为1.43×105cm2/V·s。用C-V法测量其浓度分布表明,分布曲线较陡。典型的器件应用结果为:单管室温直流跨导达280mS/mm,在12GHz时均有8dB以上的增益。     

Investigation on resonant tunneling in GaAs/AlxGa1-xAs DBD using tunneling spectroscopy

 Experimental investigation on resonant tunneling in various GaAs/Al_xGa_(1-x)As double barrier single well structures has been performed by using tunneling spectroscopy at different temperatures.The results show that in addition to resonant tunneling via GaAs well state confined by Al_xGa_(1-x)As Γ-point barrier there exists resonant tunneling via GaAs well state confined by Al_xGa_(1-x)As X-point barrier for both indirect(x>0.4)and direct(x<0.4)cases.     

Process parameter dependence of impurity-free interdiffusion in GaAs/AlxGa1−xAs and InxGa1−yAs/GaAs multiple quantum wells

The dependence of the impurity-free interdiffusion process on the properties of the dielectric cap layer has been studied, for both unstrained GaAs/Al_xGa_1−xAs and pseudomorphic In_y Ga_1−yAs/GaAs MQW structures grown by molecular beam epitaxy. The influence of the cap layer thickness, composition, and deposition technique on the degree of interdiffusion were all systematically investigated. Electron-beam evaporated SiO₂ films of varying thickness, chemical-vapor-deposited SiO_xN_y films of varying composition, and spin-on SiO₂ films were used as cap layers during rapid thermal annealing (850-950°C). Photoluminescence at 10K has been employed to determine the interdiffusioninduced bandgap shifts and to calculate the corresponding Al-Ga and In-Ga interdiffusion coefficients. The latter were found to increase with the cap layer thickness (e-beam SiO₂) up to a limit determined by saturation of the outdiffused Ga concentration in the SiO₂ caps. A maximum concentration of [Ga] = 4–7 ×10¹⁹ cm⁻³ in the SiO₂ caps was determined using secondary ion mass spectroscopy profiling. Larger band-edge shifts are also obtained when the oxygen content of SiO_xN_y cap layers is increased, although the differences are not sufficiently large for a laterally selective interdiffusion process based on variations in cap layer composition alone. Much larger differences are obtained by using different deposition techniques for the cap layers, indicating that the porosity of the cap layer is a much more important parameter than the film composition for the realization of a laterally selective interdiffusion process. For the calculated In_0.2Ga_0.8As/GaAs interdiffusion coefficients, activation energies E_A and prefactors D_o were estimated to ranging from 3.04 to 4.74 eV and 5 × Kh⁻³ to 2 × 10⁵ cm²/s, respectively, dependent on the cap layer deposition technique and the depth of the MQW from the sample surface.     

磁场对GaAs/AlxGa1-xAs异质结系统中束缚极化子的影响

对GaAs/AlxGa1-xAs单异质结系统引入三角势近似异质结势,同时考虑体纵光学(LO)声子和有效近似下两支界面光学(IO)声子的影响,采用变分法讨论了外界恒定磁场对束缚于近界面杂质的光学极化子结合能的影响.利用改进的Lee-Low-Pines(LLP)中间耦合方法处理电子-声子和杂质-声子的相互作用,计算了杂质态结合能随杂质位置、磁场强度、电子面密度的变化关系.结果表明,极化子结合能随磁场呈现增加的趋势,其中LO声子对结合能的负贡献受磁场影响显著,而IO声子的负贡献受磁场的影响并不明显,但当杂质靠近界面时,杂质-IO声子相互作用对磁场的影响很敏感.结果还表明,导带弯曲作用不容忽略;电子像势对结合能的影响很小,可以忽略.     

磁场对GaAs/Al_xGa_(1-x)As异质结系统中束缚极化子的影响

对GaAs/AlxGa1-xAs单异质结系统引入三角势近似异质结势,同时考虑体纵光学(LO)声子和有效近似下两支界面光学(IO)声子的影响,采用变分法讨论了外界恒定磁场对束缚于近界面杂质的光学极化子结合能的影响.利用改进的Lee-Low-Pines(LLP)中间耦合方法处理电子-声子和杂质-声子的相互作用,计算了杂质态结合能随杂质位置、磁场强度、电子面密度的变化关系.结果表明,极化子结合能随磁场呈现增加的趋势,其中LO声子对结合能的负贡献受磁场影响显著,而IO声子的负贡献受磁场的影响并不明显,但当杂质靠近界面时,杂质-IO声子相互作用对磁场的影响很敏感.结果还表明,     

GaAs MISFET制备中柠檬酸/双氧水溶液对GaAs/AlxGa1-xAs结构的选择湿法腐蚀

用α台阶仪和原子力显微镜(AFM)研究了不同体积比的柠檬酸(50%)/双氧水溶液对GaAs/AlxGa1-xAs系统的选择湿法腐蚀特性,对AlxGa1-xAs停止层的组分和腐蚀液体积比进行了优化.当腐蚀液体积比在1.5∶1到2∶1范围时获得了最好的选择腐蚀效果.在25℃.温度条件下,当腐蚀液体积比为1.5∶1时,对Al摩尔分数x为0.2、0.3和1的GaAs/AlxGa1-xAs系统腐蚀比分别为45、74和大于200,表面腐蚀形貌均匀平整.将这种选择腐蚀技术用于GaAsMISFET的栅槽工艺,获得了良好的阈值电压均匀性.