Pergamon zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA Solid&w Ekc~ronics Vol. 40. NOS l-8, pp. 683-686. 1996 Copyright C I996 Elsevier Science Ltd 0038-1101(9spo386-x Printed in Great Britain. All rights rescrwd 0038-I 101/96 515.00 + 0.00 MANY BODY EFFECTS AND CHARGE CARRIER KINETICS STUDIED BY ELECTRO-OPTICAL EXPERIMENTS IN TYPE-I HETERO n-i-p-i STRUCTURES WITH SELECTIVE CONTACTS J. SCHULTZ’, S. MALZER’, M. KNEISSL’, U. PFEIFFER’, P. KIESEL’, J. S. SMITH2 and G. H. DGHLER’ ‘Institut fiir Technische Physik, UniversitHt Erlangen-Niimberg, Erlangen, Germany *Electronics Research Laboratory, University of California, Berkeley, CA, U.S.A. zyxwvutsrqponmlkjihgfedcb Abstract-We report on a comparative study of the electro-optical properties of a tunable two-dimensional electron plasma by transmission, photocurrent (PC), and photoluminescence (PL) measurements. The sheet electron density nt2’in the pseudomorphic strained InGaAs quantum wells of the investigated type-1 hetero zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA n- i- p - i structure can be tuned between zero and more than 5.10” cm-* by applying a voltage U,, between the selective and ohmii n- and pcontacts. A direct relation between n(*’and zyxwvutsrqponmlkjihgfedcbaZYXWVUTSR Urn can be determined by capacitance measurements. Many particle effects (bleaching of the exciton, shift of the absorption edge due to bandfilling, renormalization of the bandgap) have been studied as a function of the sheet electron density n(*’ in the quantum wells. The carrier dynamics in the type-1 hetero zyxwvutsrqponmlkjihgfedcbaZYXWV n-i-p-i structure was investigated by a detailed comparison of absorption, PL and PC measurements as a function of the applied voltage Up. A photogenerated hole in the quantum well can either recombine in the quantum well or escape into the p-layers by tunnelling or thermally assisted hopping. The first process is monitored by PL, while the escape process, which varies strongly with the UP bias dependent potential barrier height and width, contributes to the PC. Both recombination paths are compared with the e-h generation rate determined by the absorption coefficient, which is extracted from transmission experiments. INTRODUCHON Many body effects in (GaAs/AlGaAs, InGaAs/ (Al)GaAs) quantum well (QW) structures have been the object of a number of fundamental investigations [1,2]. Their understanding is also of great import- ance for practical use in semiconductor devices, for example in (strained) hetero QW structure lasers[3], modulators based on state filling in blockaded reservoir and quantum well electron transfer structures (BRAQWETS)[4], or in hetero n- i- p - i bandfilling modulator@]. The properties of a electron (hole) plasma were investigated to study many particle effects like bleaching of the excitonic absorption in n-type[6] and p-type modulation doped structures[q, in un- doped QWs[8] as well as in doped QWs[9], and band- gap renormalization in undoped QWs[lO] and in modulation doped QWs(1 11. However, as the carrier density in these experiments is tuned by optical excitation, a quantitative comparison with theory is rather difficult[l2]. A direct control of the carrier density could be achieved by using electrical contacts. Andrews et 0/.[13] have been able to tune the carrier density in single QWs up to 2.10” cmT2 by means of a Schottky gate. Recently, we have shown that the electron density in a type-I hetero n- i- p - i structure can be tuned from zero to more than 5. lOi2cme2[14] by applying a voltage to the grown-in selective con- tacts[lS]. The electron density in this system is well related to the applied voltage and many body effects can be studied within the same sample over a wide electron density range. Moreover, the dynamics of the carriers can be investigated by a detailed com- parison of transmission, PC and PL measurements. EXPERIMENTAL The investigated sample was grown with the epi- taxial shadow mask technique providing excellent selective and ohmic contacts to the n- and p-layers [15] of the GaAs type-1 hetero n- i- p - i structure. The 8 nm wide wells are separated from the 6-n doped layers by 10 nm wide spacer layers. A 70 nm thick intrinsic GaAs layer has been grown between the 6-n doped (3.10’2cm-2) and the 6-p doped (1.2.10” cm-*) layers. A scheme of the bandstructure in real space is shown in Fig. l(a) for a reverse bias of C$,,, = - 3 V with depleted QWs and at zero bias [Fig. I(b)], where nc2’ z 4.3. 1Oi2 cm-* in the QWs. For the electrical and electro-optical measurements at 2 K a pumped helium bath cryostat was used. The probe light from a tungsten lamp was focused on 75 pm x 210 pm large devices and detected by a CCD camera after dispersal in a 0.64m mono- chromator. A tunable Ti:Sapphire laser was ustd for optical excitation. The excitation density of 1.5 W cme2 was constant throughout the PL and PC exper- iments for all applied voltages Upn . It should be noted 683