Vicinal GaAs Substrates Research Articles

Direct evidence of lateral bandgap patterning on stepped structures grown on non-planar, vicinal GaAs surfaces by cathodoluminescence investigations

A novel technique for lateral bandgap patterning on non-planar GaAs substrates is introduced. When a grating is lithographically etched on a vicinal (100) GaAs substrate prior to organometallic chemical vapor deposition (OMCVD), growth results in a regular saw-tooth-type surface morphology consisting of macroscopic steps. These steps expose two distinct crystallographic orientations to the gas phase, i.e. (100) and (111)A. In this paper, we demonstrate that it is possible to take advantage of the selective deposition of reactants from the gas phase onto the two crystallographic facets to pattern the thickness and the corresponding effective bandgap of a quantum well. Spatially and spectrally resolved cathodoluminescence was employed to image directly the bandgap patterning, which was found to be in good agreement with transmission electron microscopy data on the structures. The ability to control the bandgap patterning with growth conditions is discussed. Finally, this study, which was first performed with 3.5 μm period gratings, was extended to submicron gratings where the step height becomes small enough to be in the range where confinement effects in two dimensions can be anticipated. This demonstrates that non-planar vicinal substrates can be used to grow lateral arrays of quantum wires by OMCVD.

Optical Anisotropy in a GaAs/AlAs Quantum Wire Array Grown on Vicinal Substrate

ABSTRACTThe optical properties of a quantum wire array which was successfully realized in a 4-nm-thick (GaAs)1/2 (AlAs)1/2 fractional-layer superlattice (FLS) are described. The FLS sample was grown by metalorganic chemical vapor deposition on a (001) vicinal GaAs substrate tilted 2 ° towards [110]. The nominal cross sectional size of the GaAs wires is 4×4 nm. Optical anisotropy in absorption, refractive index and photoluminescence was clearly observed by polarization spectroscopy at room temperature as well as lower temperatures.

Application of organometallic chemical vapor deposition mechanisms to lateral band-gap patterning on stepped surfaces

We report the study of organometallic chemical vapor deposition (OMCVD) on the stepped surfaces obtained by crystal growth on nonplanar vicinal (100) GaAs substrates. Scanning and transmission electron microscopy investigations were combined to identify two distinct diffusion mechanisms in OMCVD: one mechanism involves the gas phase and allows us to selectively distribute the incoming flux of volatile reactants between two adjacent facets; the other one involves surface diffusion of nonvolatile species over distances in the hundred nm range. Spectrally and spatially resolved cathodoluminescence imaging of the light emissions from a single quantum well structure gives direct evidence that the quantum well thickness variations, which can be controlled over a wide range (up to 1:5) by growth parameters, produce effective band-gap variations.

Step-flow growth on strained surfaces: (Al,Ga)Sb tilted superlattices

We have demonstrated the molecular beam epitaxial growth of (Al,Ga)Sb tilted superlattices (TSLs) on 2° vicinal (100) GaSb and GaAs substrates. The TSLs grown on GaSb substrates exhibit good AlSb/GaSb separation and a uniform short-range superlattice period. The TSLs grown on GaAs substrates are similar, except for the presence of threading dislocations and a decreased uniformity. The existence of TSLs proves that step-flow growth can occur in this material system, and in the presence of strain. Lateral fluctuations in the tilt angle of the superlattice are observed and are found to be caused by a nonuniform adatom distribution which is correlated with the surface step density.

Photoluminescence line shape due to arrayed steps at the interfaces of GaAs/AlGaAs single quantum wells grown on vicinal surfaces by molecular beam epitaxy

Systematic variations in excitonic photoluminescence spectral features were observed for growth-interrupted GaAs/AlGaAs SQW's on GaAs(100) vicinal substrates for a variation in the misorientation angle (toward the (111)A) from 0.5° to 4.0°. These were explained by a structural model of hetero-interfaces composed of staircases which have monolayer straight steps and terraces with a fluctuating width characteristic of the angles. An analysis, which includes the effect of the fluctuation in terrace widths relative to the exciton size is given to explain the observed narrowing of the luminescence linewidth for the highest misorientation angle examined.

Sublattice ordering in GaInP and AlGaInP: Effects of substrate orientations

GsAs substrate surface orientation effects on the CuPt type 〈 1 21 1 21 1 2 〉 superlattice (SL) on the group III sublattice in GaInP and AlGaInP grown by metalorganic vapor phase epitaxy were described in detail. Among the four equivalent direction SL variants, whose directions are [ 1 11]: v 1, [1 11 ]: v 2, [11 1 ]: v 3 and [ 111 ]: v 4, a TED intensity relation, I(v 3) = I(v 4) = 0, was consistently observed for epilayers grown on exact (001) and vicinal (001) GaAs substrates, where I(v i ) denotes the intensity of v i direction SL variant. As for I(v 1) and I(v 2), symmetric ( I(v 1) ≅ I(v 2)) and asymmetric ( I(v 1) ≠ I(v 2)) intensity relations were observed for epilayers grown on GaAs substrates with symmetric and asymmetric surface orientation relations with v 1 and v 2 directions, respectively. A slight misorientation towards the v 1 direction enhanced the v 1 direction SL formation. Ga 0.5In 0.5P epilayers grown on ( 111 ) and (110) GaAs substrates did not show any 〈 1 21 1 21 1 2 〉 type SLs. These experimental results are discussed.

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.

Spontaneous growth of coherent tilted superlattice on vicinal (100) GaAs substrates

Periodic Al composition modulations have been observed to occur spontaneously during molecular beam epitaxy of AlGaAs on vicinal (100) substrates. The formation of the spontaneous Al modulation requires (a) the migration-enhanced epitaxy deposition and (b) one monolayer deposition of Al and Ga atoms per cycle, hence, the denomination of coherent tilted superlattice (C-TSL). Cross-sectional transmission electron microscopy clearly shows that the C-TSL has the periodicity of the surface steps. We also show that the Al-rich regions of the C-TSL form at the bottom of steps before the As flux is established. These results indicate that growth kinetics dominates the C-TSL formation.

Structure of AlAs-GaAs interfaces grown on (100) vicinal surfaces by molecular beam epitaxy

Hyperfine control of the interface structure and composition between GaAs and AlAs films grown by molecular beam epitaxy has been achieved by deposition on vicinal (100) GaAs substrates. This control is demonstrated by producing (GaAs)m-(AlAs)n submonolayer (m and/or n<1) superlattices over a wide temperature range. Analysis of these submonolayer superlattices by transmission electron microscopy shows that the layer growth regime is dominant and that layer nucleation is initiated preferentially at the step edges on the (100) vicinal surface. Potential applications of submonolayer superlattices including the growth of superlattice layers perpendicular to the substrate surface are described.