Spontaneous Lateral Composition Modulation in III-V Semiconductor Alloys
- PDF / 4,383,393 Bytes
- 7 Pages / 576 x 777.6 pts Page_size
- 54 Downloads / 209 Views
of phase separation. The term refers to the spontaneous formation of a phaseseparated, self-organized periodic structure, as appears in Figure 1. Composition modulation in epitaxial growth has been observed to occur either parallel to (vertical modulation) or perpendicular to
(lateral modulation) the growth direction.1 Vertical modulation has been observed in III-V materials such as InAsSb2 and in II-VI materials such as ZnSeTe.3 Lateral modulation has been observed for a wide range of compound semiconductors during homogeneous growth,1 when all components of the alloy are deposited simultaneously. This phenomenon has also been observed as a result of short-period superlattice growth,4"8 when alternating thin layers of each binary component of the alloy are deposited by molecular-beam epitaxy. The situation resulting from short-period superlattice growth is a particularly interesting case to study since the modulation is very strong and regular. This article will describe the microstructure and optical properties of lateral composition modulation in short-period superlattices, and will present a number of models proposing mechanisms for why it occurs in these films. Microstructure Lateral composition modulation in short-period superlattices shows similar microstructure over a range of material systems, including InPm/GaPn on GaAs,4'7 InAs m /GaAs n , w and AlAsra/InAs,,8 on InP (where m and n are the number of monolayers of each binary compound deposited). Figure 2a shows a crosssectional dark-field transmissionelectron-microscopy (TEM) micrograph with g = 002 of a compositionally modulated short-period superlattice of AlAsi 44/
Short-Period Superlattice
Lateral Composition Modulation
•L.
Figure 1. Schematic diagrams of the deposited and resulting structures. The upper figure (a) shows the as-deposited short-period superlattice with alternating layers along the growth direction z. The lower figure (b) shows the resulting compositionally modulated film. The average composition across such a modulated film varies along some crystallographic direction, in this case along x, with some characteristic modulation wavelength A and amplitude A.
Downloaded from https:/www.cambridge.org/core. Cornell University Library, on 24 May 2017 at 13:04:22, subject to the Cambridge Core terms of use, available at BULLETIN/JULY 1997 38 MRS https:/www.cambridge.org/core/terms. https://doi.org/10.1557/S088376940003339X
Spontaneous Lateral Composition Modulation in Ill-V Semiconductor Alloys
InAsi56 in the [110] projection. The very strong and regular contrast variation across the film corresponds to modulation in composition. Figure 3a shows a corresponding x-ray reciprocal space map around the (224) reflection_with the sample aligned such that the [110] direction is perpendicular to the diffraction plane, as recorded using Cu-Ka x-rays and a position-sensitive detector.9 Distinct lateral satellites arising from composition modulation are visible on either side of the Bragg reflection of the shortperiod superlattice, with a wavel
