High mobilities in AlxGa1−xAs-GaAs heterojuntions

Abstract

Electron mobilities as high as 6485 cm²/V sec at 300 and 57 260 cm²/V sec at 78 K were obtained in Al_0.33Ga_0.67As‐GaAs modulation‐doped single‐period heterojunctions with an average electron concentration of ∼10¹⁷ cm⁻³. The achievement of such high mobility is the result of leaving part of the AlGaAs near the interface undoped. By varying the thickness of this undoped region, electron mobilities much greater than for uniformly doped AlGaAs in modulation‐doped structures have been achieved. The electron mobility enhancement has been observed to increase with the increase of the undoped region, reach a maximum, and then decrease. Mobility enhancement was still observed up to the point of leaving a 200‐Å undoped region in the AlGaAs. The peak mobilities for an AlAs mole fraction of 33% has been found to occur for an undoped region of 50 Å while for 25% the mobility peaks for 100 Å. The Al_0.25Ga_0.75As‐GaAs modulation‐doped structure had a slightly lower peak mobility, namely 50 234 cm²/V sec at 78 K. These results indicate a mobility improvement of about a factor of 2 over previously reported modulation‐doped AlGaAs‐GaAs structures at 78 K.