Photoconductivity effects in extremely high mobility modulation-doped (Al,Ga)As/GaAs heterostructures

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Abstract
Single period modulation doped Al0.35Ga0.65 As/GaAs heterostructures were grown by molecular beam epitaxy. The mobilities and sheet carrier concentrations were measured as a function of lattice temperature in the dark and in room light. Mobilities as high as 8490, 105 000, and 221 000 cm2/Vs at 300, 78, and 10 K, respectively, were obtained for samples measured in the dark. When measured in light, these mobilities increased to 9090, 136 000, and 286 000 cm2/Vs at the same respective temperatures. In all cases the sheet carrier concentrations were between 4.5×1011 and 9.5×1011 cm−2. These values represent one of the best dark values reported to date and are significant with respect to field effect transistor applications. The 300 K mobility of 9090 is equivalent to the best mobilities obtained in ultrapure GaAs (n⩽1013 cm−3). The change in the mobility and sheet carrier concentration is the result of a persistent photoconductivity effect which is attributed to the ionization of electrons from traps in the (Al,Ga)As layer. Electron hole pair generation, though smaller, also affects the electron mobility because of increased screening.