Very low density two-dimensional hole gas in an inverted GaAs/AlAs interface

Two-dimensional electron or hole gas ~2DEG and 2DHG, respectively! structures of AlGaAs/GaAs are widely used for the study of physics of low dimensional electronic systems and quantum transport. A particularly versatile realization of 2DEGs is the inverted-semiconductor-insulatorsemiconductor ~ISIS! structure, where the carriers are accumulated in an undoped GaAs layer on top of an undoped AlGaAs barrier ~thus ‘‘inverted’’!, grown over an n conducting layer. In ISIS devices, the sheet carrier concentration can easily be modulated by the underlying conductor layer and by surface Schottky gates, thereby increasing the range of possible measurements and allowing patterned gate structures on the surface while having additional and separate control of n by means of the back gate. In this work, we study ISIS structures grown on ~311!A oriented GaAs substrates where a 2DHG is formed in an analogous fashion. This p-ISIS structure allows us to vary the sheet hole density p over a wide range, and in particular to achieve and to measure extremely low densities. In conventional modulation doped 2DEGs ~2DHGs!, placing the donors ~acceptors! far from the channel generally leads to improved mobility, particularly at low densities. Indeed, very low density 2DEGs with high mobility have been realized using spacers of order 300 nm. In an ISIS structure, since the carriers are generated by field effect rather than by modulation doping, the spacing between the channel and any intentional doping can be increased at will. Furthermore, increasing the depth of the channel below the surface does not lead to major difficulty in the formation of Ohmic contacts, due to the absence of an AlGaAs barrier between the surface and the channel. For this reason, the ISIS is an attractive device for the realization of high quality, low density 2DHG systems. Given that the carriers are generated by a field effect, one might ask why ~311!A substrates are at all necessary for accumulating a 2DHG in a p-ISIS. The answer lies in the pinning of the surface potential and the resulting need for a p-type cap layer which, although depleted by the surface states, has a crucial role in bringing the valence band close to the Fermi level. This point will be elaborated below. Work on ~100! n-type ISIS structures failed to match the high mobility of conventional heterostructure 2DEGs at the low densities. The inferior mobility was attributed to the relatively poor quality of the interface due to the incorporation of background impurities which tend to ride towards the surface and accumulate at the inverted interface, as well as