Nominal Substrates Research Articles

Photoluminescence under pressure of ultrathin AlAs layers grown on GaAs vicinal surfaces: A search for lateral confinement effects.

In ${\mathrm{Al}}_{0.3}$${\mathrm{Ga}}_{0.7}$As/GaAs quantum wells bordered by ultrathin (one or two monolayers) AlAs layers, the AlAs layer has been shown to provide a well for electrons at high pressure [Phys. Rev. B 45, 11 846 (1992)]. In this work, the same type of structures, but grown on (001) substrates misoriented towards (111)Ga, are studied by photoluminescence as a function of pressure, and a comparison is made with those grown on nominal substrates in order to investigate the influence of surface steps on electronic levels. Particular emphasis is given to indirect type-II transitions. An important blueshift of indirect transitions is observed in the case of two AlAs monolayers grown on a vicinal substrate. The size of this shift may be explained considering diffusion of carriers by the interface steps, or also lateral confinement. Inversely, a redshift is observed in the one AlAs monolayer sample. We suggest that this last point may be due to terrace length fluctuations.

Photoluminescence Fatigue in Laterally-Ordered (GaP)2/(InP)2 Short-Period-Superlattices (SPS) Grown by Molecular Beam Epitaxy

AbstractSpontaneously laterally ordered (GaP)2/(InP)2 short period superlattices (SPS) grown by Molecular Beam Epitaxy (MBE) on nominal (100) GaAs substrates have been studied by photoluminescence (PL) spectroscopy. The samples studied included SPS comprising 110 pairs of (GaP)2/(InP)2 (total thickness σ90 nm) and multiquantum well structures in which quantum wells comprising 12 pairs of (GaP)2/(InP)2 SPS layers (thickness σ10 nm) are alternated with lattice-matched GaInP random alloy barrier layers. The 5K PL spectra include a σ1760 meV nearband edge band, and a much broader, lower energy (σ1670 meV) luminescence band that exhibits an unusual fatiguing behavior; its intensity diminishes monotonically during continuous illumination by the exciting light. This fatigued PL state is metastable at low temperatures. In the quantum well structure, although the relative intensity of the lower energy band is significantly weaker in comparison to the higher one, the fatiguing behavior still exists. However the fatiguing rate is slower in quantum well structures than that observed in the thick SPS film.

Magneto-optics of narrow GaAs/AlxGa1-xAs quantum wells grown on vicinal substrates.

Magnetoluminescence measurements performed on GaAs/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As quantum wells grown on a vicinal substrate show a blueshift of the excitonic transitions compared with those obtained on a reference sample grown on nominal substrates. This blueshift is interpreted in terms of the lateral modulation induced by the steps array resulting from the use of vicinal surfaces.

Formation of quantum well wire-like structures by MBE growth of AlGaAs/GaAs superlattices on GaAs (110) surfaces

We report on molecular beam epitaxial (MBE) growth of AlGaAs/GaAs superlattices on several GaAs (110) substrates, i.e. nominal (110) substrates and vicinal (110) substrates misoriented toward (111)A and (111)B. MBE growth of the superlattices on vicinal (110) substrates misoriented toward (111)B has been found to produce quantum well wire-like structures being coherently aligned toward the 〈110〉 direction with almost equal spacing. We discuss the formation mechanism of the quantum well wire-like structures which is closely related with the MBE growth mechanism on (110) surfaces.

Atomic ordering in GaAsP

CuPt type ordering, which consists of a monolayer compositional modulation along one of the 4 〈111〉 directions in the lattice, was studied using transmission electron microscopy for GaAs1−xPx with values of x extending from 0.25 to 0.85. The samples were grown by organometallic vapor phase epitaxy on nominal (001) GaAs substrates that were misoriented by varying amounts in three directions. No CuPt type ordering was observed for GaAs1−xPx with x ≤0.35, while ordering was found to occur for 0.4≤x≤0.85. The direction of substrate misorientation has a major effect on the determination of which of the four possible CuPt variants are formed for 0.4≤x≤0.85. Two variants, with ordering on the (1̄11) and (11̄1) planes, appear for epilayers grown on substrates oriented exactly on the (001) plane and for substrates misoriented by 6° towards the [110] direction. Only one variant, with ordering on the (1̄11) plane, appears for epilayers grown on substrates misoriented by 6° towards [1̄10]. These ordering-induced spots observed in transmission electron diffraction (TED) patterns for GaAsP occur only for the [110] cross section. From TED studies of GaInP grown on similar substrates, we conclude that the CuPt variants in GaAsP are exactly the same as for GaInP. Further evidence supporting this conclusion was obtained by growing first a layer of GaInP followed by a layer of GaAsP. High-resolution dark field electron micrographs show domains of the same variants in both layers. A mechanism describing the formation of the specific ordered variant for both GaAsP and GaInP is proposed. From studies of ordering in a strain-layer superlattice, the strain due to lattice mismatch was found to play no significant role in the propagation of ordered domains. Microtwins, also generated due to lattice mismatch, can act as domain boundaries and prevent the propagation of the ordered domains.

Substrate Misorientation Effect on Be Transport during MBE Growth of GaAs

The Be transport during MBE growth of GaAs has been studied. The Be surface segregation was clearly separated from the Be diffusion by using misoriented as well as nominal (100) substrates. The Be profiles measured by SIMS showed that the surface segregation was suppressed by the substrate misorientation even when the anomalous diffusion took place. The CAT observation revealed that the Be-induced disordering of superlattices occurred for 6×1019 cm-3 doping. The width of the disordered region was found to depend on the substrate misorientation, which was due to the misorientation dependence of the surface segregation. The step exchange model has been successfully employed to account for these results.

Deep‐Level Dominated Electrical Characteristics of Au Contacts on β ‐ SiC

Current‐voltage characteristics of Au contacts formed on films grown heteroepitaxially on both nominally (100) oriented and off‐axis (100) silicon substrates have been investigated. These contact diodes are rectifying, and very low reverse leakage currents are observed, particularly in off‐axis silicon substrates. The diode ideality factor is between 1.3 and 2.0 in all cases except in nominal (100) silicon substrates where it is greater than two. Logarithmic plots of the I‐V characteristics in the forward direction indicate space charge limited current conduction through the active volume of the diodes. The films grown on nominally (100) oriented substrates show the presence of two deep levels located approximately between 0.26 and 0.38 eV below the conduction bandedge. In some films on nominal (100) substrates, the I‐Vcharacteristics are also influenced by additional traps which are exponentially distributed in energy with a maximum occurring at the conduction bandedge. In contrast, the films deposited on off‐axis substrates have only one deep level located at approximately 0.49 eV for the 2° off (100) substrates and 0.57 eV for the 4° off (100) substrates. Previous microstructural analysis revealed that the nature and density of defects in the heteroepitaxial films on both nominal and off‐axis (100) silicon are similar except that the films on nominal (100) substrates have a high density of antiphase domain boundaries. Therefore, the presence of the shallower deep‐level states observed in the films grown on nominal (100) substrates is speculated to be due to the electrical activity of antiphase domain boundaries.

Anisotropy in the doping characteristics of dimethylcadmium in GaAs grown by MOVPE on (100) GaAs

GaAs layers have been grown by MOVPE on nominal and vicinal (100) GaAs substrates. The effects of doping using dimethylcadmium have been studied as a function of misorientation of the substrate. The hole concentration N A is found to increase exponentially with the DMCd flow rate for the samples grown at 720°C on (100)±0.5°, (100) 2° off towards (111)B and (110) planes, but it increases linearly for the samples grown at 720°C on (100) 2° off towards the (111)A plane. N A for the layers grown at 680°C is, however, proportional to the DMCd flow rate for all the substrates. These results are explained in terms of step pinning due to kink poisoning by the impurity atoms, and the resultant increase in the supersaturation for two-dimensional growth and impurity-induced nucleation, taking account of the anisotropy in the steps on (100) GaAs.

Deep-Level Dominated Electrical Characteristics of Au Contacts on β-SiC

ABSTRACTCurrent-voltage characteristics of Au contacts formed on β-SiC films grown heteroepitaxially on both nominally (100) oriented and off-axis (100) silicon substrates have been investigated. Logarithmic plots of the I-V characteristics in the forward direction indicate space charge limited current conduction through the active volume of the diodes. The β-SiC films grown on nominally (100) oriented substrates show the presence of two deep levels located approximately between 0.26 eV and 0.38 eV below the conduction band edge. In some films on nominal (100) substrates, the I-V characteristics are also influenced by additional traps which are exponentially distributed in energy with a maximum occurring at the conduction band edge. In contrast, the films deposited on off-axis substrates have only one deep level located at approximately 0.49 eV for the 2° off (100) substrates and 0.57 eV for the 4° off (100) substrates. Previous microstructural analysis revealed that the nature and density of defects in the β-SiC heteroepitaxial films on both nominal and off-axis (100) silicon are similar except that the films on nominal (100) substrates have a high density of antiphase domain boundaries. Therefore, the presence of the shallower deep-level states observed in the β-SiC films grown on nominal (100) substrates is speculated to be due to the electrical activity of antiphase domain boundaries.

Wave morphologies on the surfaces of GaAs and Ga 0.65Al 0.35As grown LPE

It is important to understand and hence have the means to control the intrinsic morphologies of materials grown by the LPE process. This is particularly so if the method is utilized to make devices, which, for efficient operation rely upon planar interfaces between the layers of a multilayer structure. The double heterostructure laser is such a device and the materials with which this paper is concerned are commonly used in the fabrication of this device. The particular morphology with which this paper is concerned is that of waves on the surfaces of grown layers. A brief discussion will be presented of the distinction between waves and terraces, or microfacets. An LPE system will be described in which the growth rate is essentially time independent. In this apparatus layers of both GaAs and Ga 0.65Al 0.35As have been grown both on nominal (100) substrates and on substrates 1° off that orientation at a range of growth rates. The wavelength of the surface waves has been found to be proportional to the reciprocal of the square root of the growth rate. The observed wavelengths vary in the range between 40 μm at a growth rate of 0.7x10 -7 cm s -1 and 6 μm at 1.8x10 -6 cm s -1. On (100) substrates it is observed that terraces and waves may coexist. The terrace edges are lines of intersection of the mean plane of the surface with the singular surface. The waves are found to be parallel to the terraces. This may be either due to the terraces presenting a regular series of initial perturbations to the equilibrium form of the surface or due to the strong anisotropy in the surface tension near a singularity. First order morphological stability theory has been applied to the situation. This confirms the observed dependence of wavelength on growth rate and gives a reasonable agreement with the constant of proportionality which has been observed.