C.M. Reaves

Direct formation of quantum‐sized dots from uniform coherent islands of InGaAs on GaAs surfaces

The 2D–3D growth mode transition during the initial stages of growth of highly strained InGaAs on GaAs is used to obtain quantum‐sized dot structures. Transmission electron micrographs reveal that when the growth of In0.5Ga0.5As is interrupted exactly at the onset of this 2D–3D transition, dislocation‐free islands (dots) of the InGaAs result. Size distributions indicate that these dots are ∼300 A in diameter and remarkably uniform to within 10% of this average size. The areal dot densities can be varied between 109 and 1011 cm−2. The uniformity of the dot sizes is explained by a mechanism based on reduction in adatom attachment probabilities due to strain. We unambiguously demonstrate photoluminescence at ∼1.2 eV from these islands by comparing samples with and without dots. The luminescent intensities of the dots are greater than or equal to those of the underlying reference quantum wells.

Formation of coherently strained self-assembled InP quantum islands on InGaP/GaAs(001)

Abstract We have investigated non-hydride metalorganic chemical vapor deposition (MOCVD) for the growth of self-assembled InP islands on InGaP surfaces lattice-matched to 0° and 2° misoriented GaAs(001) substrates. Atomic force microscopy (AFM) and transmission electron microscopy (TEM) have been used to investigate the self-assembling nature of quantum-size InP islands on InGaP/GaAs(001) surfaces. From the structural analysis of the surface by AFM, we have observed three types of InP island evolve during the growth process. At low coverages of InP deposited at 650°C on a (001) surface, coherently strained islands with irregular dimensions in the plane of the growth surface but of uniform height (2.5 nm), are observed. With increasing InP deposition of uniform coherently strained islands are observed with a typical diameter of 120 nm with a dramatic increase in height (23.5 nm).

Conduction band offsets in ordered‐GaInP/GaAs heterostructures studied by ballistic‐electron‐emission microscopy

Ordered‐GaInP/GaAs heterostructures have been studied using ballistic‐electron‐emission microscopy (BEEM). The GaInP/GaAs conduction band offset was found to decrease with increasing order. Samples were grown simultaneously on different misoriented substrates to vary the degree of order in the GaInP. Concurrent scanning tunneling microscopy and BEEM images show ridge structures in the topography and contrast in the BEEM current that may correspond to ordered domains in the GaInP. Room temperature conduction band offsets of 137 and 86 meV were measured using BEEM spectroscopy for GaInP with 2 K band gaps of 1.97 and 1.89 eV, respectively.

Characterization of MOCVD-grown InP on InGaPGaAs(001)

Abstract Atomic force microscopy and transmission electron microscopy have been used to investigate the metalorganic chemical vapor deposition of InP on nominally flat InGaP GaAs (001) substrates. From a detailed structural analysis of the deposited InP, we observe coherent Stranski-Krastanov growth, i.e., after the initial formation of a two-dimensional layer, two types of coherently strained islands appear above a defined thickness. At higher coverages a third type of larger, incoherent and defected island is observed. The two types of coherently strained islands can be characterized by their distinct sizes. For low coverages the coherently strained islands possess irregular dimensions in the plane of the substrate and have a maximum height of 6 nm. At higher coverages the second type of coherently strained islands appears. These are cap-shaped and have a rather narrow distribution of base diameter (120 ± 10 nm) and height (22.5 ± 2.5 nm). An appreciable density of incoherent and defected relaxed islands also becomes evident at increased coverages. These have a hexagonal shape exposing clear facets and are elongated along the [110] crystallographic direction of the substrate.

Formation of self‐assembled InP islands on a GaInP/GaAs(311)A surface

The coherent Stranski–Krastanov growth mode is used to create selfassembled InP islands on GaInP/GaAs(311)A surfaces. The resulting islands on (311)A surfaces have a base width distribution peaked in the range of 600–800 A in contrast to a distribution peaked at 1200 A for islands on (100) surfaces. In addition on the (311)A surfaces, there is a bimodal island height distribution peaked at 15 and 50 A. For the (311)A surfaces, the islands are significantly smaller and more dense (∼1010 islands/cm2) than the islands formed on (100) surfaces (∼109 islands/cm2). Despite these differences in the islands formed on the two surfaces, the growth occurs similarly for the two surfaces, with the formation of three different types of islands distinguished primarily by height.

InP islands on InGaP/GaAs(001): island separation distributions

Abstract Heteroepitaxy of InP on InGaP/GaAs(001) has been studied using atomic force microscopy (AFM) as a function of substrate temperature and misorientation. Stranski-Krastanov growth of InP on GaAs results in three types and sizes of islands. The smallest and medium-sized islands are coherently strained to the substrate, whereas dislocations are seen in the largest islands. Growth, morphology and island separation distributions have been studied for the small and medium islands. Scaling in the radial distribution function indicates a uniformly random growth of medium-sized InP dots on all the substrates. However, on substrates grown at 650°C and misoriented by 2°, clustering of islands is also observed. We suggest that is due to the preferential growth of mediumsized islands at the step edges of the substrate. Separation distributions along and perpendicular to the step edges of this surface reveal that the step edges are decorated by clusters of 6–8 islands having an average period of 2 μm between them. These clusters exhibit long-range ordering and have a potential for device applications. Radial separation distribution studies indicate that the substrate morphology and topography are crucial for achieving a desirable distribution of islands. Moreover, the small InP islands and the dislocated islands are uniformly distributed on all the substrates irrespective of the growth temperature or misorientation, suggesting a different nucleation mechanism than for the medium-sized InP islands.

Surface scienceCharacterization of MOCVD-grown InP on InGaPGaAs(001)

Abstract Atomic force microscopy and transmission electron microscopy have been used to investigate the metalorganic chemical vapor deposition of InP on nominally flat InGaP GaAs (001) substrates. From a detailed structural analysis of the deposited InP, we observe coherent Stranski-Krastanov growth, i.e., after the initial formation of a two-dimensional layer, two types of coherently strained islands appear above a defined thickness. At higher coverages a third type of larger, incoherent and defected island is observed. The two types of coherently strained islands can be characterized by their distinct sizes. For low coverages the coherently strained islands possess irregular dimensions in the plane of the substrate and have a maximum height of 6 nm. At higher coverages the second type of coherently strained islands appears. These are cap-shaped and have a rather narrow distribution of base diameter (120 ± 10 nm) and height (22.5 ± 2.5 nm). An appreciable density of incoherent and defected relaxed islands also becomes evident at increased coverages. These have a hexagonal shape exposing clear facets and are elongated along the [110] crystallographic direction of the substrate.

Control of III–V epitaxy in a metalorganic chemical vapor deposition process: impact of source flow control on composition and thickness

Abstract A procedure for developing a real-time automated control system for improving epitaxial growth of semiconductors is presented and applied to GaInAs growth by metalorganic chemical vapor deposition. Vapor concentration variations of the gallium source are identified as a primary disturbance. An analog control system that regulates the supply of gallium is designed and implemented. Key issues in the controller synthesis are outlined. The controller performance is investigated by growing GaInAs InP superlattices. Results of growths performed under normal operating conditions and also under large perturbations are presented. These results include X-ray diffraction from the samples as well as real-time data from the concentration monitor and the flow measurement. High quality superlattices that display up to eight orders of satellite peaks are obtained under closed loop control, demonstrating improved layer-to-layer reproducibility of thickness and composition.

Effects of deposition rate on the size of self-assembled InP islands formed on GaInP/GaAs(100) surfaces

Utilizing the Stranski-Krastanov growth mode, three-dimensional InP islands are formed on a GalnP/GaAs surface using metalorganic chemical vapor deposition. The islands have been investigated with atomic force microscopy, and the effect of the deposition rate on the shape of the islands has been quantified. The height of the islands varies with deposition rate, whereas the basediameters are nearly constant around 1260 ± 35Å. The island height is 290 ± 12 Å at a high (2.6 ML/s) deposition rate and decreases to approximately 250 ± 16 Å for low (0.1 ML/ s) and moderate (0.8 ML/s) deposition rates.

Characterization of InP islands on InGaPGaAs(001): effect of deposition temperature

Abstract Atomic force microscopy has been used to investigate the metalorganic chemical vapor deposition of InP on nominally flat InGaP GaAs (001) substrates as a function of deposition temperature. In particular, the deposition time required for island formation as a function of deposition temperature is determined at a constant reactant flux, and the effect of InP deposition temperature on the island shape and size distributions is explored. The qualitative behavior of the growth changes very little with temperature, whereas the size and spatial distributions are significantly affected. In addition, the effect of InGaP surface morphology on the size and density of InP islands is compared at constant deposition times. From a detailed analysis of the islands on the various surfaces, the island density and base area are observed to be strong functions of surface morphology, while the height of the islands remains unchanged. These observations indicate that the size, uniformity, and density of the coherent islands are not dominated by strain energy, and, thus, they can be manipulated by changes in substrate morphology and growth conditions.