Comparison Between Homo-And Hetero-Epitaxial Layers by Photoreflectance Spectroscopy
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COMPARISON BETWEEN HOMO- AND HETERO-EPITAXIAL LAYERS BY PHOTOREFLECTANCE SPECTROSCOPY
K.L. JIAO, Z.Q. SHI, AND WA. ANDERSON Center for Electronic & Electro-optic Materials, Department of Electrical & Computer Engineering, State University of New York at Buffalo, Amherst, NY 14260
ABSTRACT MOCVD grown GaAs and InP epitaxial layers have been studied using photoreflectance spectroscopy. Homogeneous and heterogeneous structures were employed to investigate the influence of the mismatch induced strain and dislocations. All the tested spectra contained a sharp peak related to the fundamental absorption edge and a pronounced Franz-Keldysh oscillation. The data analysis revealed a consistent difference in bandgap, temperature coefficients of the bandgap, and surface electric field, in the order of the degree of mismatch. For GaAs/GaAs and GaAs/Si samples, the bandgaps derived from the three point method were 1.436 and 1.324eV, respectively. Values of 1.334, 1.325, and 1.294 eV for the bandgap were found for InP/InP, InP/GaAs, and InP/GaAs/Si, respectively. For GaAs epitaxial layers, the intensity of the surface field bore a ratio of 1.18:1 between GaAs and Si substrates. For InP epitaxial layers, the ratio was 1.23:1.12:1 in the sequence of InP, GaAs, and GaAs/Si substrates. Such a measure must be related to the mismatch in the heteroepitaxy structures. A shoulderlike peak, 18-23 meV below the gap-energy peak (Eo), was found for GaAs samples, which could be impurity related. A broad shoulder-like peak 30 meV below the fundamental absorption edge was also observed only for InP/GaAs/Si and hence attributed to some shallow defect levels induced by antiphase disorder. INTRODUCTION The growth of III-V compound semiconductors on Si is gaining interest because it can combine the mature Si microelectronic technology with the unique electrical and optical properties of III-V compounds so that optoelectronic integrated circuits (OEICs) will be feasible. In addition, using Si as substrates has the advantages of low cost, large wafer size, high mechanical strength, and good thermal conductivity. Most of the work on the heteroepitaxy of II-V compounds has been focused on GaAs on Si [1,2], although the growth of InP on Si has attracted more attention in the past four years [3-6]. Up to now, the crystal quality of the heteroepitaxy films is still inferior to that obtained from bulk or homogeneous-epitaxy due to some inevitable incompatibilities such as lattice constant mismatch and difference in the thermal expansion coefficients. The lattice mismatch, 4.1% between GaAs and Si, or 8.06% between InP and Si, will cause thread dislocations which often propagate to the active layer. The different thermal expansion coefficients will result in residual stress which can lead to wafer bowing and film cracking if too thick. The stress may also cause changes in bandgap. Moreover, placing a polar compound semiconductor like GaAs or InP onto a non-polar material such as Si will introduce the problem of antiphase domains and antiphase boundaries.
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