Surface Roughening and Composition Modulation of ZnSe-related II-VI epitaxial films
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Abstract We have investigated ZnSSe and ZnMgSSe epitaxial layers lattice-matched to GaAs (001) substrates grown by molecular beam epitaxy using atomic force microscopy and transmission electron microscopy. Under 11-rich conditions with c(2x2) surface reconstruction, surface morphology exhibited corrugation aligned in the [1I0] direction and composition modulation was observed in the same [1T0] direction. Under VI-rich condition with (2xl) surface reconstruction, the surface morphology becomes rounded grain-like and composition modulation was not observed. The formation of composition modulation is associated with the surface corrugated structures.
1. Introduction Ternary ZnSSe and quaternary ZnMgSSe alloy materials lattice matched to GaAs substrates have been playing significant role as guiding and cladding layers in ZnSe based blue/green II-VI laser diodes [1]. Composition modulation has been observed in many III-V alloy epilayers such as InGaAsP. However, to date, there have been only a few reports on TEM investigations of composition modulation of quaternary ZnMgSSe alloys [2,3]. In this paper, we report for the first time the relationship between the surface morphology and composition modulation. We will show that the surface roughening is responsible for the composition modulated structures.
2. Experimental The samples investigated in this study were ZnSe, ternary ZnSSe alloy and quaternary ZnMgSSe alloy epitaxial layers and laser structures containing these alloy layers grown on semi-insulating GaAs (001) substrates using MBE using elemental Zn, Se, Mg and compound ZnS as source materials. The growth temperatures ranged from 250 0C to 350 0C. Growth was performed under group II-rich conditions with (2xl) surface reconstruction and under group VI-rich conditions with c(2x2) surface reconstruction. The detailed II-VI laser structures have been reported elsewhere [1]. Atomic Force Microscope (AFM) measurements of the surface morphology were performed with a Digital Instrument Nanoscope II, in contact mode. AFM images were obtained for a 5 gm2 area in the middle of each sample. The average compositions of the epilayers was determined using the double crystal X-ray rocking curve and electron-probe microanalysis (EPMA). Transmission Electron Microscope (TEM) experiments were performed using 159
Mat. Res. Soc. Symp. Proc. Vol. 448 ©1997 Materials Research Society
Hitachi H-700H, HF-2000 and JEOL 4000FXS electron microscopes operating at 200 kV and 400kV. Nano-probe X-ray energy dispersive spectrometer (EDS) analysis was also carried out with 1 nm probe size in the HF-2000. Crosssectional and plan-view TEM samples were prepared by mechanical polishing followed by ion-milling and chemical etching. 3. Results and Discussion Figure l(a) and (b) shows AFM images of ZnSe and ZnSo.07Seo.93 alloy layers grown on GaAs (001) at 280 °C under II-rich conditions. A c(2x2) surface reconstruction was observed in the reflection high-energy electron diffraction during growth. The image shows the formation of elongated corrugati
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