Compositional inhomogeneity at the epitaxial layer and substrate interface of AlGaAs/GaAs heterostructures

Carrier concentration spikes at the epilayer/substrate interface were observed in some two-dimensional electron gas AIGaAs/GaAs structures grown by low pressure organometallic vapor phase epitaxy. Using secondary ion mass spectroscopy, the carrier spikes were correlated with indium. Under certain growth conditions an anomalous interfacial layer, which is compositionally inhomogeneous, is formed producing an enhanced carrier density. Procedures are described which reduce the presence of indium at the epilayer/substrate interface and eliminate the carrier spike.     

Effects of GaAs buffer layer and lattice-matching on deep levels in Zn(S)Se/GaAs heterostructures

The effects of GaAs buffer layer and lattice-matching on the nature of deep levels involved in Zn(S)Se/GaAs heterostructures are investigated by means of deeplevel transient spectroscopy (DLTS). The heterojunction diodes (HDs) where nZn(S)Se is grown on p⁺-GaAs by metalorganic vapor phase epitaxy are used as a test structure. The DLTS measurement reveals that when ZnSe is directly grown on a GaAs substrate, there exist five electron traps A-E at activation energies of 0.20, 0.23, 0.25, 0.37, and 0.53 eV, respectively. Either GaAs buffer layer and lattice-matching may reduce the incorporation of traps C, D, and E, implying that these traps are ascribed to surface treatment of GaAs substrate and to lattice relaxation. Concentration of trap B, which is the most dominant level, is proportional to the donor concentration. However, in the ZnSSe/GaAs sub. HD, another trap level, instead of trap B, locates at the almost same position as that of trap B, and it shows anomalous behavior that the DLTS peak amplitude changes drastically as changing the rate windows. This is explained by the defect generation through the interaction between sulfide and a GaAs substrate surface. For the trap A, the concentration is a function of donor concentration and lattice mismatch, and the origin is attributed to a complex of donor induced defects and dislocations.     

Effect of GaAs surface pretreatment on electrical properties of MBE-ZnSe/GaAs substrate interfaces

Interface properties of MBE-grown ZnSe/GaAs substrate systems formed on variously pretreated GaAs surfaces, which include standard chemically etched (5H₂SO₄:1H₂O₂: 1H₂O), (NH₄)₂S_x-, NH₄I-, and HF-pretreated surfaces, are investigated by capacitance-voltage (C-V) and deep level transient spectroscopy (DLTS) measurements. A HF-pretreated and annealed ZnSe/p-GaAs sample showed marked reduction of interface state density, N_ss, with N_ss,_min below 4 x 10¹¹cm^-2 eV^-1 near E_c- E_FS= 1.0 eV. The value is about one order of magnitude smaller than that of the standard chemically etched interface, and comparable to (NH₄)₂S_x- pretreated interface. Nevertheless, C-V characteristics of ZnSe/nGaAs samples, which were measured for the first time, indicate that interface Fermi level, E_FS, is not completely unpinned due to the interface states located above the midgap. A consistent result was obtained by DLTS method in determining E_FS position. The influence of N_ss distribution on vertical current conduction is also analyzed. It is found that U-shaped interface states with N_ss(E) > 1 x 10¹³ cm^-2 eV^-1 above the midgap may cause an excess voltage drop larger than a few volts at the interface.     

AlGaAs/GaAs 三角形量子势阱与方形量子阱的界面粗糙散射的比较

We have theoretically studied the mobility limited by interface roughness scattering on two-dimensional electrons gas（2DEG） at a single heterointerface（triangle-shaped quantum well）.Our results indicate that,like the interface roughness scattering in a square quantum well,the roughness scattering at the Al_xGa_1-xAs/GaAs heterointerface can be characterized by parameters of roughness height A and lateral A,and in addition by electric field F.A comparison of two mobilities limited by the interface roughness scattering between the present result and a square well in the same condition is given.     

Digital deep level transient spectroscopy considered for discrimination of traps closely spaced in emission coefficients in semiconductors

A method is presented for analysis of digitally recorded capacitance transients to give activation energies and capture cross-sections of two deep levels which would yield overlapping peaks in conventional DLTS spectra. It is shown that baseline errors can be overcome by proper analysis of the data. The accuracy of the method is examined by simulation with parameters representative of a typical DLTS system. The addition of various noise levels is also simulated and the effects of averaging of transients on the ability to discriminate closely spaced traps is examined quantitatively.     

Deep level electronic structure of ZnSe/GaAs heterostructures

We performed 1—2 keVcathodoluminescence measurements and He-Ne and HeCd excited photoluminescence studies of ZnSe/GaAs( 100) heterostructures grown by molecular beam epitaxy. Our goal was to investigate the deep level electronic structure and its connection with the heterojunction band offsets. We observed novel deep level emission features at 0.8, 0.98, 1.14, and 1.3 eV which are characteristic of the ZnSe overlayer and independent in energy of overlayer thickness. The corresponding deep levels lie far below those of the near-bandedge features commonly used to characterize the ZnSe crystal quality. The relative intensity and spatial distribution of the deep level emission was found to be strongly affected by the Zn/Se atomic flux ratio employed during ZnSe growth. The same flux ratio has been shown to influence both the quality of the ZnSe overlayer and the band offset in ZnSe/GaAs heterojunctions. In heterostructures fabricated in Se-rich growth conditions, that minimize the valence band offset and the concentration of Se vacancies, the dominant deep level emission is at 1.3 eV. For heterostructures fabricated in Zn-rich growth conditions, emission by multiple levels at 0.88,0.98, and 1.14 eV dominates. The spectral energies and intensities of deep level transitions reported here provide a characteristic indicator of ZnSe epilayer stoichiometry and near-interface defect densities.     

Cathodoluminescence spectroscopy of deep defect levels at the ZnSe/GaAs interface with a composition-control interface layer

In this work we investigate ZnSe/GaAs heterostructures with an additional 2 nm controlled interfacial layer (CIL) of Se- or Zn-rich composition to modify the band offset. The samples are analyzed as a function of annealing temperature by cathodoluminescence spectroscopy. The as-prepared samples show defect luminescence at ∼ 0.9 eV. With staged annealing at increasing temperatures, both the Zn-rich as well as the Se-rich interfacial layer exhibits luminescence at ∼ 1.9 eV, indicative of defect formation with an onset temperature of ∼400°C. Excitation-dependent spectroscopy provides evidence for defect formation near the interface, which extends into the ZnSe epilayer at higher temperatures. Compared to earlier work, where the threshold temperature for defect formation in bulk samples fabricated under Se-rich growth conditions occurs at temperatures as low as 325°C, the resistance to defect formation has now been improved to that of stoichiometric ZnSe. These results demonstrate that epitaxially grown CILs provide a means to alter ZnSe/GaAs band offsets without degrading the heterojunction’s resistance to defect formation at elevated temperatures.     

GaAs/Si/AlAs异质结的带阶和GaAs生长温度的影响

用分子束外延 (MBE)设备制备了 Ga As/ Al As和 Ga As/ Si/ Al As异质结 ,通过 XPS分别研究了异质结界面处 Si层厚度为 0 .5 ML 和 1ML 对异质结带阶的调节 ,得到最大调节量为 0 .2 e V;通过 C- V法研究了异质结的Ga As层在不同温度下生长对 0 .5 ML Si夹层的影响 ,得到 Si夹层的空间分布随 Ga As层生长温度的升高而扩散增强的温度效应 ,通过深能级瞬态谱 (DL TS)研究了在上述不同温度下生长的 Ga As层的晶体质量 .     

Radiative recombination in modulation-doped GaAs/AlGaAs heterostructures in the presence of an electric field

The radiative recombination processes involving two dimensional (2D) carriers from the notch potential formed at the interface of modulation doped GaAs/AlGaAs heterostructures have been studied by means of photoluminescence (PL) and photoluminescence excitation spectroscopy in the presence of an external electric field applied perpendicular to the layers via a gate electrode. Two PL bands related to the 2D electron gas are interpreted as the radiative recombination between 2D electrons and holes from the valence band (HB1) and from residual acceptors (HB2), respectively. The band bending in the active layer, which determines the energy positions of these H-bands, can be controlled by applying an external electric field. However, also the separation between the Fermi edge, E_F, and the second 2D electron subband is deliberately varied by applying an electric field. At a sufficiently small separation, an efficient scattering path near k=0 is available for electrons at the Fermi energy. This can be observed in the PL spectra as a striking enhancement of the many-body excitonic transition, usually referred to as the Fermi edge singularity (FES). The enhancement of the FES is usually explained in terms of an efficient scattering for electrons at the Fermi edge via the nearly resonant adjacent subband. The efficiency of this process is dependent on the separation between the Fermi edge, E_F, and the next subband, which can be controlled via the applied field in our experiments.     

Localized deposition of GaAs/GaInP heterostructures using LP-MOVPE

GaInP layers were grown selectively by low pressure MOVPE in patterned SiO₂ masks on GaAs (100) substrates. The variation of the composition and spontaneous ordering phenomena were analysed by Raman spectroscopy and photoluminescence. In contrast to GaInAs, the composition of GalnP shows only a very weak dependence on the size of the structures. On the other hand, there is a shift of the bandgap energy up to 40 meV with decreasing size of the stripes caused by ordering of the Ga and In atoms. Based on these findings lattice matched GaAs/GalnP multilayers were grown to delineate the growth history of the structures. It was demonstrated that the growth habit of deposition in narrow slits (>1μm) can be used to produce mesa-like stripes with dimensions below 100 nm on top of the mesa. Results of GaAs/GaInP quantum wells selectively grown on top of a mesa are presented.     

The role of the V/III ratio in the growth and structural properties of metalorganic vapor phase epitaxy GaAs/Ge heterostructures

GaAs epitaxial layers have been grown on (001) 6† off-oriented toward (110) Ge substrates by metalorganic vapor phase epitaxy. In order to study the influence of V/III ratio on the growth mechanisms and the structural properties of the layers, the input flow of arsine was changed over a wide range of values, while keeping constant all other experimental settings. Optical microscopy in the Nomarski contrast mode, x-ray topography and high resolution diffractometry, transmission electron microscopy and Rutherford backscattering have been used to investigate the epilayers. It has been found that the growth rate increases and the surface morphology worsens with increasing V/III ratio. The abruptness of the layer-substrate interface has also been found to strongly depend on the V/III ratio, the best results being obtained under Ga-rich conditions. The main structural defects within the layers are stacking faults and misfit dislocations. Layers grown under As-rich conditions only contain stacking faults, probably originated by a growth island coalescence mechanism, whereas layers grown under Ga-rich conditions contain both misfit dislocations and stacking faults generated by dissociation of threading segments of interfacial dislocations. In spite of the different defects, the strain relaxation has been found to follow the same trend irrespective of the V/III ratio. Finally, the relaxation has been found to start at a thickness exceeding the theoretical critical value.     

Identification of the acceptor level V3+/2+ in GaAs and a new experimental observation of V2 in “low spin” ground state in GaAs

The results of optical absorption and deel level transient spectroscopy on various V-doped GaAs materials (n type Bridgman, n and p type liquid encapsulated Czochralski,n andp type liquid phase epitaxy) are reported. It is definitively shown that the single acceptor state of isolated vanadium is located atE _c -0.14 eV and that no mid gap-level related to isolated vanadium which could explain same reported semi-insulating properties of V-doped GaAs has been detected. From the analysis of the absolute photoionization cross section gs/sk _n ^o of the above level it is shown that V²⁺ is in a low spin state in accordance with recent theoretical predictions.     

关于Ge/GaAs(110)界面的费米能级钉扎

采用饱和的平板模型及半经验的紧束缚方法计算了吸附Ge原子的GaAs(110)表面在不同覆盖度情况下的电子态密度.结合以前对Ge/GaAs(110)界面系统电子态的理论与实验研究工作,提出Ge/GaAs(110)界面的费米能级钉扎位置主要决定于GaAs表面层与Ge吸附层之间的电子电荷转移.     

Role of interface chemistry and growing surface stoichiometry on the generation of stacking faults in ZnSe/GaAs

The existence of Zn-As and vacancy-contained Ga-Se interfacial layers are suggested by transmission electron microscopy of Zn-and Se-exposed (or - reacted) ZnSe/GaAs interfaces, respectively. A very low density of faulted defects in the range of ∼10⁴cm² was obtained in samples with Zn passivation on an Asstabilized GaAs-(2 × 4). However, the density of As precipitates increases as the surface coverage of c(4 × 4) reconstruction increased on the Zn-exposed Asstabilized GaAs-(2 × 4) surface and this is associated with an increase of the density of extrinsic-type stacking faults bound by partial edge dislocations with a core structure terminated on additional cations. On the other hand, densities of extrinsic Shockley-and intrinsic Frank-type stacking faults are of ∼5 × 10⁷/cm² in samples grown on Se-exposed Ga-rich GaAs-(4 × 6) surfaces. Annealing on this Se-exposed Ga-rich GaAs-(4 × 6) generated a high density of vacancy loops (1 × 10⁹/cm²) and an increase of the densities of both Shockley-and Frank-type stacking faults (>5 × 10⁸/cm²) after the growth of the films. Furthermore, we have studied the dependence of the generation and structure of Shockley-type stacking faults on the beam flux ratios in samples grown on Zn-exposed As-stabilized GaAs-(2 × 4) surfaces. Cation-and anion-terminated extrinsic-type partial edge dislocations were generated in samples grown under Zn-and Se-rich conditions, respectively. However, an asymmetric distribution on defect density under varied beam flux ratios (0.3 ≤ P_Se/P_Zn ≤ 10) is obtained.     

Influence of deep states on CdTe and GaAs metal interface formation

Luminescence and soft x-ray photoemission measurements of clean metal interfaces with (110) surfaces of CdTe from several sources as well as (100) MBE vs (110) LEC GaAs reveal that the quality of the semiconductor can have a major effect on the interface Fermi level stabilization. Crystals with low optical emission from bulk deep levels show reduced state formation at their metal interfaces and improved adherence to the work function model of barrier height. Furthermore, atomic-scale interface chemistry can inhibit the formation of deep traps due to interdiffusion, thereby improving control over barrier formation.     

Investigation of DX centers in modulation-doped field-effect transistor-type Al0.3Ga0.7As/GaAs heterostructures using a fourier-transform deep level transient spectroscopy system

Al_0.3Ga_0.7As:Si/GaAs modulation-doped field-effect transistor-type heterostructures were grown using two different growth temperatures (500 and 620°C) and three doping modes (δ-doping, pulse-doping, and uniform-doping). Deep level transient spectroscopy (DLTS) measurements were performed on these structures using a new Fourier-analysis method. Up to four DLTS peaks, related to the different possible configurations of the nearest Al and Ga neighbors around each DX site, were observed. Both the growth temperature and the doping-mode are found to affect the DLTS spectra, in particular the number of observed peaks and their width. These results are interpreted in terms of the different mobilities of the Si doping atoms on the surface during growth.     

The impact of deep levels on the photocurrent transients in semi-insulating GaAs

The photocurrent transients, I_PC(t), were studied in semi-insulating (SI) GaAs during a low-temperature (low-T) illumination. Unusual transients were explained by the model, relating I_PC(t) to the deep levels/traps and their occurpancy. Such traps were actually detected and characterized by the independent measurements of the thermally stimulated currents (TSCs). The processes of the generation, recombination, and capture were described by a set of coupled differential equations and solved numerically. The I_PC(t), calculated without any free parameter, well reproduced (through eight orders of magnitude) the experimental transients over a wide range of the photon energies and intensities. The best-fit parameters agreed well with those determined from the TSC measurements.     

Microstructural analysis of NiInGe ohmic contacts for n-type GaAs

NiInGe ohmic contact materials, which are attractive to use in future GaAs devices, were previously developed in our laboratories. Although the NiInGe contacts provided low contact resistances of about 0.3 Ω-mm and excellent thermal stability, further reduction of the contact resistance (R_C) of the NiInGe contacts was mandatory to use these contacts in submicron devices. In this paper, the microstructural parameters, which influence the R_C values, were investigated by correlating the R_C values with the microstructure at the interface between the contact materials and the GaAs substrate. The R_C values of the NiInGe contacts were found to depend strongly on the volume fraction and the In concentration (x) of the In_xGa_1−xAs compound semiconductor layers, which were formed at the metal/GaAs interface. Both the volume fraction and the In concentration of the In_xGa_1−xAs layers were found to depend on the thickness of the In layer used in the NiInGe contact and the annealing temperature to form the ohmic contact. A R_C value of 0.18 Ω-mm was obtained for the Ni (18 nm)/In (13 nm)/Ge (30 nm) contact (where a slash “/” indicates the deposition sequence) after annealing at temperature of 650°C for 5 sec.     

On the carrier profiling of GaAsSb/GaAs heterostructures

Carrier profiles of MBE grown Ga(As,Sb)/GaAs heterostructures were studied. In low Sb content samples of Ga(As,Sb)/GaAs grown by MBE the experimentally measured carrier profiles exhibit double dips in concentration, whereas a single large dip is ob-tained for higher Sb content with larger lattice-mismatched samples. The capacitance voltage(C-V) carrier profile of a Schottky barriern- N heterojunction of Au/n-GaAsSb/N-GaAs has been modeled and double dips occur when nonuniform doping is present which may explain the experimentally observed double dips in a GaAs_0.95Sb_0.05/GaAs specimen. For large lattice mismatch and therefore large heterointerface charge density in the model, the accumulation peak due to ΔE_c is shown to be overwhelmed by a pro-nounced single dip as observed in higher Sb content samples such as GaAs_0.91Sb_0.09/ GaAs and GaAs/GaAs_0.9Sb_0.1.     

GaAs/InP的键合界面热应力分析

采用双层条状金属热应力模型,用MATLAB方法对退火过程中GaAs/InP晶片间的应力和双轴弹性形变能进行模拟和分析。结果表明:将InP剪薄至200μm,GaAs剪薄至175μm,剪应力取得了一个相对的小值,而剥离应力更是被完全消除,这时正应力也相对较小。而按照一定的比例适度剪薄两侧晶片的厚度,可以使得两侧的双轴形变能减小到原来的一半以下。通过减薄键合晶片的厚度可以得到较好的键合质量。另外,不管那一种应力都随退火温度的升高而快速增加,所以实验中一定保持低的退火温度,通常小于300°C为宜。