Photoreflectance and X-Ray Photoelectron Spectroscopy in Lt MBE GaAs
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PHOTOREFLECTANCE AND X-RAY PHOTOELECTRON SPECTROSCOPY IN LT MBE GaAs D.C. LOOK*, J.E. HOELSCHER*, J.T. GRANT**, C.E. STUTZ***, K.R. EVANS*** AND M. NUMAN**** *University Research Center, Wright State University, Dayton, OH 45435 "**Research Institute, University of Dayton, Dayton, OH 45469 ***Wright Laboratory (WL/ELRA), Wright-Patterson AFB, OH 45433 ****Indiana University of Pennsylvania, Dept. of Physics, Indiana, PA 15705 ABSTRACT It has recently been shown that a 1000A cap layer of molecular beam epitaxial MBE) GaAs grown at 2000C passivates the surface of a GaAs active layer (n_2x10I7cm-3) in the sense of reducing the free-carrier depletion which arises from surface acceptor states. The same phenomenon holds for active-layer concentrations up to 7x1018cm- 3, for caps as thin as 14A, and for either As2 or As 4 anion species. In an attempt to understand these effects, we have applied photoreflectance (PR) and x-ray photoelectron spectroscopy (XPS). In general, the PR shows contributions from the surface, cap/active-layer interface, and active-layer/buffer-layer interface, because each of these regions can have a different electric field. In fact the various field strengths can be determined from Franz-Keldysh oscillations (FKO), and good agreement with Hall-effect measurements is usually found. However, for 2000C material, no PR is seen, suggesting that there is no surface charge (no surface acceptor states below the Fermi level) or at least no surface-charge modulation by the light. The XPS data, which arise only from the near-surface (-30A) region, show that the binding energies in the capped samples are increased (i.e., surface Fermi pinning energy decreased) by 0.2 eV with respect to those in the uncapped samples. These data are discussed in relation to a passivation model. INTRODUCTION Although the first applications of low-temperature molecular-beam epitaxial (LTMBE) GaAs used the material as a buffer layer underneath an active layer, more recent applications, such as the metal-insulator-semiconductor field-effect transistor (MISFET and the photoconductive (PC) switch, have used it as a cap layer, on top of the active layer or the substrate [1,2]. Furthermore, the material may have a role simply as a passivation layer, since it is known to reduce free-carrier depletion in the active layer and to enhance the surface breakdown voltage. Thus, it is important to know how the bulk states, surface states, and interface states in the cap interact with the donor states in the active layer. We have earlier used the Hall effect [3] to investigate this problem, and in this work we report, for the first time, the application of x-ray photoelectron spectroscopy (XPS) and photoreflectance (PR) spectroscopy. We also solve the Poisson equation or various possible models to see which ones are consistent with the experimental data. SAMPLES In this study, we will primarily deal with five, representative samples, grown sequentially in a Varian Gen II apparatus over the course of a single day, and ea h having the following ba
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