K. Mahalingam

Substrate temperature dependence of arsenic precipitate formation in AlGaAs and GaAs

GaAs epilayers which are grown by molecular‐beam epitaxy under ‘‘normal’’ group III–V fluxes but at very low substrate temperatures contain as much as 1% excess arsenic. Upon annealing these epilayers at a temperature of 600 °C, the excess arsenic forms precipitates. We have undertaken a systematic study of the substrate growth temperature dependence of this incorporation of excess arsenic in both GaAs and Al0.3Ga0.7As epilayers. The substrate growth temperature was varied in increments of 25 °C from 225 to 375 °C after every 0.25 μm of film growth for a GaAs and an Al0.3Ga0.7As epilayer. Both epilayers were grown using a dimer arsenic source and a group V to total group III beam equivalent pressure ∼20. After growth the films were annealed for 1 h in the As2 flux at a temperature of 600 °C. Cross‐sectional samples were than prepared by the ion thinning technique and examined by transmission electron microscopy (TEM). Both epilayers contained arsenic precipitates; this is the first observation of arsenic ...

Formation of two‐dimensional arsenic‐precipitate arrays in GaAs

GaAs epilayers were grown by molecular beam epitaxy under normal conditions, except a substrate temperature of 250 °C was used instead of the normal 600 °C. This results in an excess of arsenic of about 1.5% in the epilayer. The epilayers also contained regions that were delta doped with silicon, beryllium, and indium. Samples were annealed for 30 s at 600, 700, and 800 °C to investigate the effects of the Si, Be, and In impurities on the precipitation of the excess As. It was found that the As precipitates form preferentially on planes of Si while forming preferentially between planes of Be. The isoelectronic impurity In appeared to have no effect on the precipitation process.

GaAs buffer layers grown at low substrate temperatures using As2 and the formation of arsenic precipitates

Abstract We have grown GaAs layers by molecular beam epitaxy at low substrate temperatures (250°C) using the dimer arsenic source As 2 . Following a one hour anneal at 600°C, the GaAs layers were examined with transmission electron microscopy. The GaAs layers contained arsenic precipitates of average diameter 100 A and density of 10 17 cm −3 .

Arsenic cluster dynamics in doped GaAs

We have studied the formation of As precipitates in doped GaAs structures that were grown by molecular beam epitaxy at low substrate temperatures and subsequently annealed. We find that the As precipitates form preferentially on the n side of such fabricated GaAs pn junctions. As the coarsening process proceeds, there is a gradual increase in the amount of As in precipitates in the n‐GaAs region and a decrease in the p‐GaAs region; the depletion region between the pn junction becomes free of As precipitates. These observations can be understood qualitatively based on the charge states of the As interstitial and using thermodynamic arguments in which the crystal attempts to minimize the chemical potential during the anneal. The presence of the excess As results in a stable Be profile even to anneals of 950 °C. Finally, a temperature cycling technique to grow arbitrarily thick GaAs epilayers containing As precipitates was demonstrated.

Arsenic precipitates in Al0.3Ga0.7As/GaAs multiple superlattice and quantum well structures

Arsenic precipitates in Al0.3Ga0.7As/GaAs multiple superlattices and quantum well structures which were grown at low substrate temperatures by molecular beam epitaxy were studied by transmission electron microscopy. Novel precipitate microstructures were observed in annealed samples, including confinement of precipitates in GaAs wells and nearly complete depletion of precipitates in a short period superlattice. It is shown that these observed microstructures can be explained as a result of the difference of precipitate/matrix interfacial energies in GaAs and Al0.3Ga0.7As.

Arsenic precipitate accumulation and depletion zones at AlGaAs/GaAs heterojunctions grown at low substrate temperature by molecular beam epitaxy

AlxGa1−xAs/GaAs heterojunctions were grown by molecular beam epitaxy under normal growth conditions except that the substrate temperature was 250 °C. After a 1 h anneal at 600 °C, a narrow precipitate depletion (PD) zone was observed on the AlxGa1−xAs sides of the heterojunctions, while a high density precipitate accumulation (PA) zone was observed on the GaAs sides. The formation of these PD and PA zones is explained as a result of diffusion of excess As atoms across the interface from the AlxGa1−xAs layer to the GaAs layer. No significant difference in PD and PA zones was found at interfaces between AlxGa1−xAs grown on GaAs and GaAs grown on AlxGa1−xAs, indicating a negligible effect due to the growth sequence. Widths of PDs were about 250 A, exhibiting a weak dependence on the Al concentration of the AlxGa1−xAs layers.

GaAs, AlGaAs, and InGaAs epilayers containing As clusters: Semimetal/semiconductor composites

Abstract A new kind of semiconductor composite material is demonstrated with a dispersion of semimetallic particles with properties common to high-quality single crystals. These materials are arsenides such as GaAs, AlGaAs, and InGaAs containing arsenic clusters. These composites are formed by incorporating excess As in the semiconductor, which precipitate with anneal. The incorporation of the excess As in accomplished by molecular beam epitaxy at low substrate temperatures. We demonstrate that the cluster densities can be controlled with the coarsening anneal. Furthermore, we demonstrate that heterojunctions and doping can be used to control the positioning of the As clusters.

TEM study of the effect of growth interruption in MBE of InGaP/GaAs superlattices

The influence of growth interruption during the MBE growth of (100) In0.5Ga0.5P/GaAs superlattices is investigated by cross-sectional TEM. A roughening of the growth front is observed during an interruption after the exchange of the group-V molecular beams. The roughening of growth front occurs due to a spontaneous change in the growth orientation of the superlattice from [100] to 〈311〉 directions. This change in growth orientation is characterized by an initial formation of V-shaped grooves with {311} facets on the GaAs growth front which eventually lead to the formation of regions of {311} superlattice structures. The direction of V-shaped grooves is along the [011] axis, which is parallel to the surface dangling bonds of the group V atoms in the unreconstructed (100) plane. The most critical stage for the spontaneous change of the growth orientation is the interruption after the growth of a GaAs layer with the P2 flux.

Growth and characterization of digital alloy quantum wells of CdSe/ZnSe

We report a study of digital alloy quantum wells of CdSe/ZnSe grown by migration enhanced epitaxy. The quantum well regions consist of various numbers of periods of one monolayer of CdSe and three monolayers of ZnSe, and the barriers are ZnSe. It will be shown that the optical properties of such quantum wells are greatly affected by the structural quality of the digital alloy. Both structural and optical properties will be discussed. Such digital alloy quantum wells are shown to have excellent room temperature optical characteristics.

Two-dimensional arsenic-precipitate structures in GaAs

Abstract A technique is demonstrated to control the incorporation of excess arsenic and subsequent positioning of As clusters with coarsening anneals in AlGaAs/GaAs heterostructures. By growing at low substrate temperatures using molecular beam epitaxy, excess As can be incorporated in GaAs and AlGaAs epilayers. By switching the growth mode at these low substrate temperatures to migration enhanced epitaxy, close to stoichiometric epilayers can be obtained when the As flux is As4. Upon anneal, the As precipitates form preferentially in the GaAs regions, even if the as-grown GaAs regions were highly-stoichiometric and the as-grown AlGaAs regions contained an excess of As. However, the excess As can be contained in the AlGaAs regions, where it will form clusters with anneal, if thin AlAs As-diffusion barriers clad the AlGaAs regions.