Solid State Communications, Vo1.59,No.8, Pp.53?-540, 1986. Printed in Great Britain. 0038-1098/86 $3.00 + .oo Pergamon Journals Ltd. ELECXXNCRAHM~ ERa4tIARBcNAcm HuLTIPu3(Y$mmJHNELLs IN UMDOPHD GaAs-All_xCaxAs K.T. Tsen Department of Physics, Arizona State University, Tempe, Arizona 85287 J. Klem and H. Morkop Coordinated Science Laboratory, University of Illinois at Urbana-Champaign Urbana, Illinois 61801 (Received 10 February, 1986 by J. Taut) Electronic Raman scatterinq related to carbon acceptor quantum well structures. The experimentallf&Z sFZ%tZ~ states was observed in unhoped GaAs-Al the largest contributions involve transitions from P (heavy hole) ground state to P state at the center of the we1 P (heavy hole) first excite 8 , and are in good agreement with recent theoretical calculations. Recently, the properties of shallow implrities in GaAs-All_x Ga As x multiple quantum wells (MQW) have attracted tremendous attention. ‘-lo The effect of two dimensional confinement on impurities manifests itself in two ways: (1) the binding energy depends on the position of the impurity in the well, (2) the binding energy increases appreciably, in particular, when the thickness of the well becomes comparable to or smaller than the Hohr radius of the impurity. Carbon impurities are believed to play an important role in the transport properties of GaAs and fll_xGaxAs* Mqw gr- by MHE; for instance, recent studies have suggested that the carbon impurities in unintentionally doped GaAs grown by MBE, might be responsible for limiting the low temperature electron mobility. 11 An extrinsic carbon layer at All_xGaxAsW interface has recently been observed and has helped to explain the mystery of the difficulty in obtaining the mobility enhancement in modulation doped structures grown by MRE when the GaAs layer is on the top. l2 Electronic Raman Scattering (RRS) has &en successfully employed to measure the transitions involving Si donor states lo and He acceptor states 13 in intentionally doped --All_xGaxAs MQW. In this cormaunication, we would like to report the observation of ERS from carbon acceptors in undoped GaAs-All_xGaxAs W. The samples studied in this work were grown by MHE on (001) oriented undoped GaAs substrates. They consist of -30 periods of lo&-thick Al l_xGaxAs (x-0.7) and GaAs layers of various thickness (L) as indicated in Table I. The carbon acceptors (which will be identified below from our experimental results) in these undoped MQW presumably come from the unavoidable carbon background in MHE-grown GaAs. 6,11,14,15 The samples were either mounted on the cold finger of a liquid nitrogen dewar or innnersed in liquid He. In the former configuration, the effective temperature at the laser irradiated area was estimated to be -lOOoK from the ratio of Stokes to anti-Stokes signals of first order Raman lines. Light scattering experiments were carried out in backscattering geometry with laser excitation lines close to the Eo+ A0 energy gap of GaAs. The average power denisty was -lOaU/~m~. Raman spectra were taken in the 2(x,x)8 and Z(X,Y)Z configurations, where 2 is normal to the layers and x,y are along the orthogonal [lo01 537