Investigation of electric fields, interface charges, and conduction band offsets at ZnSe/GaAs heterojunctions with a nov
- PDF / 290,308 Bytes
- 6 Pages / 414.72 x 648 pts Page_size
- 47 Downloads / 185 Views
D. J. Dougherty, S.B.Fleischer, E. L. Warlick, J. L. House, G. S. Petrich, E. Ho, L. A. Kolodziejski, and E. P. Ippen Research Laboratory of Electronics, MIT 36-325, 50 Vassar St., Cambridge, MA 02139
ABSTRACT
ZnSe/GaAs heterojunctions were investigated by contactless electroreflectance and photoreflectance techniques. Negative surface charge densities on the order of 1012 cm-2 were observed for films grown on n-type GaAs indicating a large contribution to the conduction band barrier between the materials due to band bending. The conduction band offset was also measured using a new photoreflectance technique involving a tunable pump laser. INTRODUCTION Because its lattice constant is well matched to ZnSe and it can be produced in high purity wafers, GaAs is a natural substrate material for ZnSe based opto-electronic devices. Parameters such as interface charges, electric fields, and even the conduction band offsets which determine the effective barrier height between the materials, and therefore transport properties, have either not been characterized or appear to depend on the growth conditions. Surface second harmonic studies' for ZnSe grown on semi-insulating GaAs have shown a positive interface charge which causes band bending on the GaAs side of the junction leading to interface quantum well states and the absence of any effective conduction band barrier. X-ray photoemission experiments indicate that the valence band offset can vary from 1.2 eV to 0.6 eV with growth conditions 2. Device designers have invoked up to 0.6 eV conduction band offsets to explain their measured I-V curves3.3 These results suggest that it would be desirable to have a technique for measuring effective barrier heights due to both the band offsets as well as band bending for understanding the effects of growth conditions. We have used contactless electroreflectance (CER) and photoreflectance (PR), which are powerful methods for measuring electric fields4 , to determine the band bending at the heterojunction. In addition, we have measured the conduction band offsets for several ZnSe films grown on n-type GaAs using a tunable pump PR techniq1ue which is similar to internal photemission (IPE) photoconductive methods. ELECTROREFLECTANCE RESULTS The ZnSe films used in this study were grown by MBE on semi-insulating and n-
type GaAs buffer layers. The buffer layers were typically 2-3 gm thick and were grown
475 Mat. Res. Soc. Symp. Proc. Vol. 448 01997 Materials Research Society
CER
.•
1.3
1.4
1.5
1.6
1.7
1.8
probe energy ( eV)
Figure I PR and CER data at the GaAs bandedge for a 0.12 gtm ZnSe film grown on n-type GaAs on either n-type or semi-insulating substrates. The nucleation of the ZnSe was done on GaAs c(4x4) reconstructed surfaces. Figure 1 shows conventional CER and PR spectra of the GaAs bandedge for a 0.12 gtm ZnSe film on a 7 x 1017 cm-3 n-type GaAs buffer layer. The long period FranzKeldysh oscillations in the reflectivity spectrum indicate a electric field on the GaAs side of the junction. The shorter period signal
Data Loading...
