PHYSICAL REVIEW B VOLUME 38, NUMBER 12 15 OCTOBER 1988-II Photolusniaescence study of confined donors in GaAs/Al Gal As quantum wells X. Liu, A. Petrou, and B. D. McCombe State University of ¹wYork at Buffalo, Buffalo, 1Vew York 14260 J. Ralston and G. Wicks Cornell University, Ithaca, 1Vew York 14853 (Received 2 May 1988) The photolumiuescence from doped GaAs quantum wells in GaAs/Al, Gal- As multiple- quantum-well structures has been studied at very low laser excitation intensity. Under these con- ditions, new impurity-associated features appear at energies both below and above that of the impurity-related transition reported in earlier work. The new feature at lower energy is attributed to transitions between electrons on neutral Si donors at the centers of the wells and confined heavy holes, while the feature reported in earlier work is attributed to confined ionized donor bound excitons and the new high-energy feature to confined neutral donor bound excitons. With this assignment, binding energies for Si donors at the centers of the wells agree with values de- duced from far-infrared experiments. Confined impurity states in GaAs/Al, Gat —, As quan- tum wells have attracted considerable recent attention. The binding energies of donors' and acceptors have been calculated as functions of well width and impurity position. Photoluminescence s Raman scattering ' and far-infrared (FIR) magnetoabsorption"' spectro- scopies have been used for the study of donors and accep- tors in these structures. Photoluminescence associated with Si donors in tt-type GaAs/AI, Ga|-, As quantum wells was first reported by Shanabrook and Comas. ' In this work, a donor-impurity-associated feature below the ground-state heavy-hole exciton was observed and attri- buted to transitions between electrons on Si donors at the centers of the wells and heavy holes in the topmost valence-band confinement state [Si(c) ~ VB]. This inter- pretation yielded donor binding energies significantly lower than those inferred from FIR measurements on samples with similar characteristics, as well as with calcu- lated values that are in good agreement with the FIR data. In this paper we present a systematic photolumines- cence study of high-quality, Si-doped multiple-quantum- well samples, carried out under experimental conditions that reveal two new features at energies slightly below and slightly above the line previously reported. The lowest- energy feature is ascribed to the Si(c)~ VB transition; the previously reported photoluminescence feature is identified as the confined ionized donor bound exciton, and the new high-energy feature as due to the confined neutral donor bound exciton. Comparison of the spectra from quantum wells doped at the center with those from edge-doped structures having the same dimensions sup- ports this interpretation. The experimental values for the binding energies of donors at the center of the wells de- duced from our photoluminescence data are in good agreement with those determined from FIR magnetospec- troscopy on the same samples, and the theory of Refs. 1 and 2. ' In addition, the binding energies of excitons on donor impurities are in good agreement with recent calcu- TABLE I. Sample characteristics. Sample Well width (A) 375 210 210 210 210 140 80 Doping Bottom Center Center Bottom —, ' Top Center Center Eso (meV) 10. 3 10. 3 5. 6 12.1 13. 0 Ex-si (meV) 1.1 1. 3 1. 3 1. 5 1. 9 lations. ' Several molecular-beam-epitaxy-grown GaAs/Al„- Gat „As multiple-quantum-well structures were used in this study; well widths varied from 375 to 80 A. The sam- ples were doped with Si donors (1 x10' cm ') either in the central one-third of the GaAs layers, or in the bottom or top one-third of the wells. "Bottom" and "top" are defined as regions of the GaAs wells grown immediately after the AI„Gat —, As barrier and immediately before the AI, Gat-, As barrier, respectively. All samples had bar- rier widths of 125 A. Relevant sample characteristics are shown in Table I. The samples were situated on a copper mounting block in a variable temperature optical cryostat, and the photoluminescence spectra were excited with the 6328-A line of a helium-neon laser, the 4880-A line of an argon-ion laser, and the 3250-A. line of a helium-cadmium laser. The emitted light was analyzed by a double mono- chromator equipped with a cooled photomultiplier tube and standard photon-counting electronics. Laser powers smaller than 0. 5 mW were used in these experiments, and the exciting laser beam was focused to a spot of diameter =400 pm. Under these conditions new impurity-associated features appeared in the photoluminescence spectra. The various features were better resolved at shorter laser 8522 1988 The American Physical Society