881 Cathodoluminescence Spectroscopy of Deep Defect Levels at the ZnSe/GaAs Interface with a Composition-Control Interface Layer 881 Regular Issue Paper Journal of Electronic Materials, Vol. 28, No. 7, 1999 (Received August 13, 1998; accepted February 12, 1999) Cathodoluminescence Spectroscopy of Deep Defect Levels at the ZnSe/GaAs Interface with a Composition-Control Interface Layer J. SCHÄFER, 1,2 A.P. YOUNG, 3 T.M. LEVIN, 3 L.J. BRILLSON, 1,3,4 J.J. PAGGEL, 5,6 L. VANZETTI, 5 and A. FRANCIOSI 5,7 1.—Center for Materials Research. 2.—Present address: Lawrence Berkeley Lab, Advanced Light Source, Mailstop 7-222, 1 Cyclotron Road, Berkeley, CA 94720. 3.—Department of Electrical Engineering, The Ohio State University, 2015 Neil Avenue, Columbus, OH 43210. 4.—Department of Physics, The Ohio State University, 2015 Neil Avenue, Columbus, OH 43210. 5.—Laborotorio Nazionale TASC-INFM, Area di Ricerca, Padriciano 99, Trieste, I-34012, Italy. 6.—Present address: Department of Physics, University of Illinois, at Urbana-Champaign, Urbana, IL 61801. 7.—Dipartimento di Fisica, Universitá di Trieste, I-34127 Trieste, Italy and Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, MN 55455 In this work we investigate ZnSe/GaAs heterostructures with an additional 2 nm controlled interfacial layer (CIL) of Se- or Zn-rich composition to modify the band offset. The samples are analyzed as a function of annealing temperature by cathodoluminescence spectroscopy. The as-prepared samples show defect lumi- nescence at ~ 0.9 eV. With staged annealing at increasing temperatures, both the Zn-rich as well as the Se-rich interfacial layer exhibits luminescence at ~1.9 eV, indicative of defect formation with an onset temperature of ~400ºC. Excitation- dependent spectroscopy provides evidence for defect formation near the inter- face, which extends into the ZnSe epilayer at higher temperatures. Compared to earlier work, where the threshold temperature for defect formation in bulk samples fabricated under Se-rich growth conditions occurs at temperatures as low as 325°C, the resistance to defect formation has now been improved to that of stoichiometric ZnSe. These results demonstrate that epitaxially grown CILs provide a means to alter ZnSe/GaAs band offsets without degrading the heterojunction’s resistance to defect formation at elevated temperatures. Key words: ZnSe/GaAs, interface stoichiometry, heterovalent interfaces, deep level spectroscopy INTRODUCTION The near lattice-matched (0.27%) ZnSe/GaAs(001) heterojunction represents a model system for the study of band offsets and their modification with near-interface atomic bonding. Such band offsets are critical to the efficiency of charge transport across junctions used in optoelectronic applications such as blue light emitters. In such devices, hole injection from the III-V substrate into the ZnSe layer with its wide band gap is hindered by the large (up to 1.0 eV) valence band discontinuity. However, for heterovalent semiconductor junctions with polar orientation, a wide range of band offsets can be derived from neutral interfaces with different atomic reconstructions. 1,2 Nicolini and coworkers 3 demonstrated that the va- lence band offset can be increased or lowered by varying the chemical composition of the ZnSe overlayer. They reported valence band discontinuities as low as 0.58 eV and as high as 1.20 eV by varying the ZnSe beam pressure ratio (BPR) during molecular beam epitaxy (MBE) growth from Se-rich to Zn-rich, respectively. From this work it thus appears that layers obtained under Se-rich growth conditions are desirable for improved hole injection. Although hole injection can be optimized by tuning the band alignment with growth conditions, variation of the BPR also affects the electronic and optical properties of the ZnSe layer as a whole. 4,5 It is in near- stoichiometric samples that point defects and other extended defects are at a minimum concentration. Fabricating Se-rich interfaces with desirable low va-