Strain Relaxation and Surface Roughness as a Function of Growth Temperature in Linearly Graded In x Al 1-x as (x=0.05 to
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INTRODUCTION Optoelectronic devices operating at -1.3 gsm have several important applications. These applications include devices using the Nd:YAG laser line at 1.32 gtm and devices taking advantage of the dispersion minimum in silica fiber-optics near 1.3 gtm. InxGal-xAs alloys with x-0.4 have the desired bandgap for these applications, but growth of these materials is difficult because of the large lattice mismatch between In0.4Ga0. 6 As and the commonly available substrates GaAs and InP. The general growth method for highly lattice-mismatched materials is to interpose a buffer layer between the substrate and active layers of the device. The buffer layer is grown thick enough so that it becomes favorable for the film to decrease its strain energy by the introduction of misfit dislocations which allow the buffer to relax toward its free standing lattice parameter. 1 An ideal buffer layer relaxes completely and does not propagate threading dislocations formed during relaxation into the active region of the device. To achieve high-quality relaxed buffers, many different growth schemes have been tried. Studies suggest that devices grown on linearly-graded buffers result in materials with superior optical and electrical properties compared to the same devices grown on step-graded buffers. 2 ,3 In addition, linearly-graded buffers have reduced threading dislocation densities. 4 -6 Modulators operating at 1.3 itm in the In(GaAI)As material system have been successfully fabricated using linearly-graded buffers. 2 ,7 Low temperature growths of the buffer layers are preferred to minimize 7 the nucleation of dislocations and problems with the surface segregation of indium. ,8 This paper examines the influence of growth temperature on relaxation and surface morphology of InxAll-xAs linearly-graded buffer layers intended for use in the fabrication of 1.3 jIm modulators. Double-crystal x-ray diffraction with a position sensitive detector (PSD) was used to measure the relaxation and composition of the materials. The surface morphology was examined using AFM. EXPERIMENTAL TECHNIQUE The epitaxial multilayers used in this study were prepared in a computer-controlled RIBER 32P 395 Mat. Res. Soc. Symp. Proc. Vol. 326. ©1994 Materials Research Society
molecular-beam epitaxy system which has an idling base pressure of -3x10-10 torr. Growth rates were calibrated using reflection high-energy electron diffraction, and the substrate temperatures were monitored using an Ircon infrared pyrometer calibrated to the silicon-aluminum eutectic. The substrates were 2-inch diameter, epi-ready, (100), n+ GaAs, mounted in indium-free holders, rotating at 20 RPM. Undoped structures were grown using As 2 (from a valved cracker) with a V/III beam equivalent pressure ratio of approximately 11. The epitaxial growth consisted of a .5 gm GaAs layer, followed by a linearly-graded buffer graded from In.05 A1. 95 As to In.25 AI. 75 As of over 2.5 jtm, and finally, a .3 jim cap layer whose composition matched the final composition the graded buffer. Samples
