A. Salokatve

All-solid-source molecular beam epitaxy for growth of III–V compound semiconductors

Abstract All-solid-source molecular beam epitaxy (SS-MBE) is a novel variant of MBE. It allows for toxic-gas-free growth of III–V compound semiconductors, including phosphides. We have examined the quality of SS-MBE-grown phosphorus containing heterostructures and laser diodes. The results discussed in this article show that state-of-the-art quantum well (QW) materials and lasers, covering a wide range of bandgaps from red to infrared, can be prepared by SS-MBE.

New approach to growth of high‐quality GaAs layers on Si substrates

GaAs films were grown by molecular beam epitaxy (MBE) on Si (100) substrates using a two‐step growth process of a 300 °C GaAs buffer layer followed by a 600 °C device layer. The films were examined by Rutherford backscattering and x‐ray diffraction methods. A significant reduction in the defect density near the GaAs/Si interface and in the bulk of these films was observed when the buffer layer was deposited by alternately supplying Ga atoms and As4 molecules to the substrate, rather than applying conventional MBE. Possible reasons for this reduction of crystal defects are discussed.

Molecular beam epitaxy growth of MgZnSSe/ZnSSe Bragg mirrors controlled by in situ optical reflectometry

In situ optical reflectometry at the wavelength of 488 nm was employed to control the growth of MgZnSSe/ZnSSe Bragg mirror stacks for the blue‐green spectral region. 10‐ and 20‐period layer structures of MgZnSSe/ZnSSe were grown on GaAs (100) epilayers by molecular beam epitaxy. A room‐temperature peak reflectance of 86% was obtained for the 20‐period structure at the central wavelength of 474 nm. The results show that, in general, in situ optical monitoring of growth is a viable and simple method for real‐time layer thickness control of MgZnSSe/ZnSSe quarter‐wave stacks.

Reduction of surface defects in GaAs grown by molecular beam epitaxy

A method to reduce the density of oval defects originating from pregrowth surface particulates and other contaminants for GaAs layers grown by molecular beam epitaxy (MBE) is presented. It appears that if a thin GaAs buffer layer is deposited by alternately supplying Ga atoms and As4 molecules to a GaAs substrate, prior to further growth by MBE, the density of the oval defects in the final layer is reduced reproducibly by a factor of 7, from about 490 to 70 cm−2, when compared with that obtained using MBE alone under closely similar conditions. The improved surface morphology produced by the pulsed beam method is thought to be related to initial film growth which proceeds likely in a two‐dimensional layer‐by‐layer fashion.

All solid source molecular beam epitaxy growth of strained‐layer InGaAs/GaInAsP/GaInP quantum well lasers (λ=980 nm)

We report on the growth of 980‐nm strained‐layer InGaAs/GaInAsP/GaInP separated confinement quantum well lasers using all solid source molecular beam epitaxy. Valved cracker cells were employed for both phosphorus and arsenic. Fabricated lasers exhibited excellent performance that is comparable to similar lasers grown by gas source molecular beam epitaxy in our laboratory. A maximum output power of 450 mW and over 250 mW in single mode operation was achieved for ridge waveguide lasers with AR/HR coated facets.

High performance laser diode bars with aluminum-free active regions.

We present operating and lifetest data on 795 and 808 nm bars with aluminum-free active regions. Conductively cooled bars operate reliably at CW power outputs of 40 W, and have high efficiency, low beam divergence, and narrow spectra. Record CW powers of 115 W CW are demonstrated at 795 nm for 30% fill-factor bars mounted on microchannel coolers. We also review QCW performance and lifetime for higher fill-factor bars processed on identical epitaxial material.

Epitaxial lift-off of ZnSe based II-VI structures

The epitaxial lift‐off technique is applied to II–VI based structures. Epilayers of 255 nm thickness containing quantum wells are lifted off their substrates and redeposited onto polyimide coated GaAs. The technique has also been applied to II–VI samples onto which dielectric films had been deposited. Photoluminescence measurements show that the material quality has not been degraded during the processing. The success of this technique with II–VI’s opens up many possibilities for the integration of these materials with metals and dielectrics in vertical structure devices.

Growth mode and electronic structure of copper films on aluminium substrates

The growth mode, geometric and electronic structure of thin copper films deposited onto aluminium single crystal substrates have been studied by LEED, AES, angle-resolved UPS and work function changes. Growth occurs in a quasi-layer-by-layer fashion on Al(111). Temperature dependent interdiffusion leads to formation of a mixed Cu/Al interface upon which ordered epitaxial films eventually grow. The ordered structure is rotationally commensurate with the Al(111) substrate with an interatomic distance typical of bulk Cu. Electronic structure of films grown at 300 K converges to that of a bulk Cu single crystal, convergence being essentially complete at a coverage of 10 monolayers. In contrast, growth occurs in a long-range disordered fashion up to relatively high coverages on Al(100) probably in the form of densely packed Cu clusters above a mixed Cu/Al interfacial region. For submonolayer coverages the work function remains close to the clean Al values, increasing to 4.7 and 4.5 eV for (111) and (100) faces, respectively, in the range from 1 to 4 monolayers. Additional studies carried out at elevated (375 K) and low temperature (120 K) allow postulation of a fully coherent model of the complex growth and electronic properties of this particular metal-on-metal system.

Growth and characterization of an epitaxially grown ZnSSe/MnZnSSe distributed Bragg reflector

A Bragg reflector consisting of a 25‐period MnZnSSe/ZnSSE Bragg stack is reported. The II–VI semiconductor structure was grown by molecular beam epitaxy on a GaAs (100) epilayer. Structural characterization of the Bragg reflector was performed with double crystal x‐ray diffraction and transmission electron microscopy. These studies indicated that the epitaxial II–VI structure, whose total thickness is about 2150 nm, remains pseudomorphic with the GaAs substrate. The Bragg stack has a maximum reflectance of 81% at 468 nm. This result shows that fabrication of high reflectance mirrors from epitaxial ZnSe‐based II–VI compounds is possible in spite of relatively small refractive index differences between constituent II–VI layers.

Properties of strained In0.2Ga0.8As/GaAs superlattices with various barrier thicknesses

In0.2Ga0.8As/GaAs strained layer superlattices were grown by molecular‐beam epitaxy with various GaAs barrier thicknesses to study how this affects the properties of nominally 90‐A‐thick InGaAs wells. Double‐crystal x‐ray diffraction, photoconductivity, and photoluminescence gave similar results regarding superlattice degradation with decreasing barrier thickness. The optical measurements showed that at 58‐A‐GaAs thickness, the strained layer superlattice was relaxed with concomitant deterioration of its optical properties.