Solid State Communications, Vol. 64, No. 3, pp. 379-382, 1987. Prir[ted in Great Britain. 0038-1098/87 $3.00 + .00 © 1987 Pergamon Journals Ltd. BAND EDGE OFFSETS IN STRAINED (InGa)As--(A1Ga)As HETEROSTRUCTURES T.G. Andersson, Z.G. Chen, V.D. Kulakovskii*, A. Uddint and J.T. Vallin Department of Physics, Chalmers University of Technology, S-412 96 G6teborg, Sweden (Received 30 March 1987 by L. Hedin) The excitonic transitions between the ground electron and hole quantum well sublevels in strained InxGa~_xAs-AlyGal_yAs multiple quantum well structures (x = 0.12-0.35 and y = 0.2-0.35) have been investigated by means of photoluminescence and photoconductivity measurements. The molecular beam epitaxy grown structures contained an AlyGal_yAs matrix with one unstrained GaAs and three strained InxGal_xAs quantum wells one of which was in the GaAs cladding layers. The ratio of the conduction band edge line up to the band gap offset for the strained In~Ga~_xAs-unstrained AlyGal_yAs interface has been found to be 0.67 _+ 0.08 for the studied regions of x and y. UNDERSTANDING THE PHYSICS of strained quantum wells (QW) and superlattices will largely extend the number of useful heterostructures from the common (AIGa)As-GaAs system. The InxGa~_xAs- GaAs and InxGat_xAs-AlyGal_yAs system are important heterostructures for high speed and opto- electronic device applications and for investigating the physics of strained QW structures and superlattices. Despite the lattice mismatch (~ 7%) between GaAs (AlAs) and InAs, high quality strained single (SQW) and multiple (MQW) quantum well structures as well as superlattices can be grown provided the thickness of the strained layers is kept small enough to avoid a misfit dislocation generation [1-5]. In such layers the lattice mismatch is entirely taken up by elastic strain. The critical thickness, Lc, for dislocation generation has been found [4, 5] to be approximately inversely proportional to the In content and being about 200 A for an In0 ~sGa0ssAs QW in GaAs. The substitution of Ga in GaAs by Al should not influence critical thick- ness as the AlAs and GaAs lattice mismatch is only 0.1%. One of the basic parameters characterizing the properties of heterostructures is the conduction (valence) band offset, AEc(AEv), across the interface. It has been found [6-8] that at the GaAs-AlxGa~ _xAs interface the ratio AE~/AEg ~ 0.64 (AEg = AEc + AE, is the band gap discontinuity). The repor- * On leave from the Solid State Physics Institute of Academy of Sciences of the USSR, Chernogolovka, USSR. t On leave from the Atomic Energy Centre, Dhaka, Bangladesh. ted values of AE,/AEg for the interface between strained InxGa~_xAs and unstrained GaAs are 0.7 [9] and 0.8 [10]. In the present Communication the band edge offsets have been investigated for the first time in the strained InxGa~ xAs-unstrained AlyGal_yAs sys- tem by means of photoluminescence (PL) and photo- conductivity (PC) measurements. The use of optical methods for the band edge offset determination is based on comparing measured excitonic transition energies in a QW with those cal- culated from a particle-in-box model with a single adjustable parameter AE<IAE~. Unfortunately, in general the necessary accuracy in the QW thickness and the (InGa)As and (AlGa)As compositions cannot be obtained from X-ray measurements. To determine all the characteristics of the heterostructure by means of precise optical methods we have studied specially designed MQW structures shown in Fig. 1. They con- tain an AlyGa~_yAS matrix with one GaAs and two InxAs QW's. In addition there is one InxGa~ _xAs (just below the critical thickness) QW separated from the AlyGa~_yAs matrix by 150 A GaAs layers. The GaAs GaAs GaAs (InGa)As cladding (AlGa)As QW QW°s layers matrix GaAs (buffer) ! Fig. 1. The energy structure of the investigated MQW samples and the observed QW emission transitions. 379