151 COMPREHENSIVE INVESTIGATION OF TRAPS IN GaAs/A1GaAs HETEROSTRUCTURES AND SUPERLATTICES BY DLTS G. PAPAIOANNOU*, G. KIRIAKIDIS**, A. GEORGAKILAS** and A. CHRISTOU*** * University of Athens, Physics Department, Athens, Greece ** Foundation for Research & Technology-Hellas, P.O. Box Heraklion, Crete, Greece *** Naval Research Laboratory, Code 6830, Washington, DC 20375, U.S.A. ABSTRACT The electron traps of GaAs/AlGaAs heterostructures or superlattices (S.L.) have been investigated in a series of device structures which ranged from simple Schottky diodes to high electron mobility transistors. For the S.L. structure traps investigated, emission into the El miniband as well as A1GaAs trapping was considered. In simple heterostructures such as HEMTs, DLTS investigation of electron trapping from the 2DEG channel revealed a number of new traps, which were previously undetected with CTS (Current Transient Spectroscopy) investigations of the 2DEG. Interface traps in GaAs/AIGaAs heterojunctions were also investigated and distinguished from previously reported traps. INTRODUCTION Selectively doped AIGaAs/GaAs heterostructures receive considerable attention because of their applications to high electron mobility transistors (HEMTs) [1] and optoelectronic devices [2,3]. However, traps can limit the performance of both minority and majority carrier devices (with or without superlattice structures). Thus it is important to detect and control the presence of electrically active defects. Studies of defects in superlattices have shown that both the width of each layer and the number of periods involved may affect the concentration of deep levels and their location within the band gap [41. The study of trap levels in HEMTs has also revealed [5] a number of process induced traps as well as the presence of DX centers [6]. In this paper we have investigated the electron traps in heterojunctions and superlattices. The study consisted of a series of device structures which ranged from simple Schottky diode test structures to high electron mobility transistors. In the case of S.L. structures, the deep traps activation energy has been interpreted by the method of conduction in order to identify properly each contribution in the DLTS spectra. In the case of simple heterostructures such as HEMTs both gate capacitance DLTS and current transient spectroscopy (CTS) of the 2DEG have been used. The different number of traps obtained with these methods is discussed by taking into account the peculiarities of each one. Finally interface-like traps have been investigated and distinguished from the bulk traps by using previously published methods [7]. EXPERIMENTAL RESULTS I. Superlattice Structure In order to investigate inherent traps in the MBE material used in the present investigation, undoped superlattice layers were assessed by DLTS. The samples were grown by molecular beam epitaxy (MBE). The layers consisted of a GaAs buffer (n*) layer grown on top of a conducting GaAs substrate, followed by a S.L Mat. Res. Soc. Symp. Proc. Vol. 144. '1989 Materials Research Society