Superlattices and Microstructures, Vol. IO, No. 2, 7991 zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIH PHOTOCURRENT AND PHOTOREFLECTANCE SPECTRA OF GaAs SAWTOOTH DOPING SUPERLATTICES 131 V.L.Alperovich, A.S.Jaroshevich, D.I.Lubyshev, V.P.Migal. V.V.Preobrazhenskii, B.R.Semjagin Institute of Semiconductor Physics Academv of Sciences of the USSR, Siberian Branch 430090 Novosibirsk 90, USSR (Received 12 August 1990) Photocurrent and photoreflectance spectra of GaAs Cj-doped sawtooth superlattices of various periods are studied experimentally. Both the monotonic exponential part of the photocurrent tail below the bandgap and oscillations of the photoreflectance above the bandgap stem from the Franz-Keldysh effect in the SL built-in electric field. Regular changes of the tail as well as of the above-bandgap oscillations with decreasing SL period are explained by the strengthening of the built-in field. Impurity-related peaks are observed in the photocurrent spectra along with weaker shoulders; the latter may be tentatively attributed to optical transitions between quantum levels in the SL potential. Possible reasons for the blurring of quantum-confined optical transitions and for the impurity-related peaks are discussed. 1 .Introduction Semiconductor doping n-1-p-i superlattices (SL) are promising for creating electrooptical switches. tunable light emitting diodes, and other devices [l-5]. Investigations of optical properties of sawtooth doping superlattices, where doping impurities are placed -in the alternating n- and p-type h-doped layers separated by undoped regions, were started in [3-61. Due to the minimal influence of the impure ty potential fluctuations on the electronic spectrum of &-doped SLs, it 1s possible to observe spectral features associated with quantum-confined optical transitions (4-61. In this paper we present experimentally measured photocurrent (PC) and photoreflectance [71 (PR) spectra of 6 -doped n-i-p-i SLs with various periods. Both the monotonic part of the low-energy tail of the PC and oscillations of the PR above the bandgap are explained by Franz-Keldysh effect in the built-in electric field. Z.Experimental The structures have been grtwn by molecular-beam epitaxy at T<530 C on semiinsulating (100) GaAs substrates and consist of 0.5 pm thick undoped GaAs buffer layer, alternating n(Si) and p(Be)-type 8-layers, separated by undoped GaAs regions, and 0.08 pm cap layer. The period zp and number of periods k in each SL are listed in the Table. The designed concentrations of Si and Be in 6-layers are approximately equal : Nifsjl.3XlO 13 -2 N;+SWO 13 -2 cm , cm . A computer-controlled dual-beam experimental set-up with grating monochromator is used for measuring the PC and PR spectra. The photocurrent Ipo(0.l) is proportional t:o the absorption coefficient a(u), provided an -1 overall thickness of the SL kzp<a . 3.Results and Discussion Depending on the period zp, the SLs are “semimetallic” (with free electrons and holes) or “dielectric” (containing only compensated charged donor and acceptor impuritv ions in respective g-layers). For increasing zp the electric field in a dielectric g-doped SL remains constant and homogeneous (E=eN2D/2x), while the amplitude of the modulation potential zyxwvutsrqpon 0749-6036/91/060131+04$02.00/0 0 1991 Academic Press Limited