Electron and hole microwave cyclotron resonance in photoexcited undoped ${\mathrm{G}\mathrm{a}\mathrm{A}\mathrm{s}/\mathrm{A}\mathrm{l}}_{0.3}{\mathrm{Ga}}_{0.7}\mathrm{As}$ multiple quantum wells

We studied the microwave cyclotron resonance (CR) of photoexcited free and weakly localized electrons and holes in undoped ${\mathrm{G}\mathrm{a}\mathrm{A}\mathrm{s}/\mathrm{A}\mathrm{l}}_{0.3}{\mathrm{Ga}}_{0.7}\mathrm{As}$ multiple quantum wells (MQW’s) of various well widths. The photoinduced microwave absorption was measured at a frequency of ${\omega }_{\mathrm{mw}}=35.6\mathrm{}\mathrm{}\mathrm{GHz}$ and at various lattice temperatures in the range of $T_{\mathrm{L}}=4.2\mathrm{}–300\mathrm{}\mathrm{K}.$ The interband excitation intensity was very low, so that the density of photogenerated electrons and holes was of the order of $n<~{10}^8{\mathrm{cm}}^{\mathrm{-}2}.$ In all 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 electron and hole cyclotron masses and the electron scattering rate dependence on $T_{\mathrm{L}}$ and on the microwave power were obtained. Assuming that the electron in-plane mobility at ${\omega }_{\mathrm{mw}}$ is proportional to the inverse scattering rate, we find that it varies in the range of $(0.8\mathrm{}–8)\times {10}^5{\mathrm{cm}}^2{\mathrm{V}}^{-1}{\sec }^{-1}$ for 100 Å and 200 Å MQW’s. This is less than the mobility measured in modulation doped QW’s of similar widths. We present a detailed analysis of the temperature dependence of the electron scattering rate by combining the electron-phonon, electron-impurity, and electron-interface roughness scattering rates. The latter is found to be an important scattering mechanism in undoped MQW’s at low temperatures. The CR analysis also shows that the electron cyclotron mass varies (in the range of $0.055–{0.070m}_0)$ with increasing either $T_{\mathrm{L}}$ or the microwave power. These variations are interpreted in terms of weak electron localization in large area, in-plane potential fluctuations arising from interface roughness.