Electron and hole microwave cyclotron resonance in photoexcited undoped GaAs/Al0.3Ga0.7As multiple quantum wells

We studied the microwave cyclotron resonance (CR) of photoexcited free and weakly localized electrons and holes in undoped GaAs/Al0.3Ga0.7As multiple quantum wells (MQW's) of various well widths. The photoinduced microwave ab sorption was measured at a frequency of omega(mw) = 35.6 GHz and at various lattice temperatures in the range of T-L = 4.2-300 K. The interband excita tion intensity was very low, so that the density of photogenerated electron s and holes was of the order of n less than or equal to 10(8) cm(-2). In al l the studied QW's, an electron CR was observed, while a heavy hole CR was measured only in narrow QW's. By model fitting the CR line shape, the elect ron and hole cyclotron masses and the electron scattering rate dependence o n TL and on the microwave power were obtained. Assuming that the electron i n-plane mobility at omega(mw) is proportional to the inverse scattering rat e, we find that it varies in the range of (0.8-8) x 10(5) cm(2)V(-1) sec(-1 ) for 100 Angstrom and 200 Angstrom MQW's. This is less than the mobility m easured in modulation doped QW's of similar widths. We present a detailed a nalysis of the temperature dependence of the electron scattering rate by co mbining the electron-phonon, electron-impurity, and electron-interface roug hness scattering rates. The latter is found to be an important scattering m echanism in undoped MQW's at low temperatures. The CR analysis also shows t hat the electron cyclotron mass varies (in the range of 0.055-0.070m(0)) wi th increasing either T-L or the microwave power. These variations are inter preted in terms of weak electron localization in large area, in-plane poten tial fluctuations arising from interface roughness. [S0163-1829(99)06547-9] .