Spin-orbit coupling correlates orbital degrees of freedom of a charge carrier with its spin. Emerging helicity of electronic eigenstates makes spin-orbit materials attractive for both applied and fundamental research. In conventional gaped 2D electronic systems, interplay of spin-orbit coupling, Coulomb interactions and disorder opens up an intriguing possibility of experimental access to novel aspects of single-particle and many-body localization-delocalization transition. The realization of such systems, however, depends on the availability of high-quality spin-orbit coupled materials with low carrier density n (a few 1010 cm-2). 2D electron gases (2DEG) in In0.75Ga0.25As/In0.75Al0.25As quantum wells (QWs) are ideal candidates, as they exhibit large g factor and Rashba coupling, along with carrier mean-free paths larger than a few µm. However, background doping in the InAlAs barriers populates the InGaAs 2DEG with n around 3X1011 cm-2, partly dependent on the growth conditions, which makes difficult to achieve diluted 2DEGs below ~1X1011 cm-2 by top gating.
In this work, we have grown In0.75Ga0.25As/In0.75Al0.25As 2DEGs where C doping was introduced in the InAlAs barriers. Since C acts as an acceptor, such modulation doping partly compensates the intrinsic n doping of the barriers and helps reducing 2DEG electron population. We grew 20-nm thick In0.75Ga0.25As/In0.75Al0.25As metamorphic QWs, at a 200nm depth from the surface. The C-doped layer was placed 40nm above the QW. Similarly to Hirmer, APL2011, we have verified that C acts as a donor in InxAl1-xAs with x=0.75, thus we reduced the composition of the doped layer to x=0.4 by growing a 4.5nm-thich (AlAs/In0.75Ga0.25As) digital C-doped alloy. We have measured the charge density in the 2DEG in the dark at 300K and 4.2K as a function of C doping. At 300K, we were able to reduce n from 3.64X1011cm-2 in an undoped 2DEG to 8.7X1010cm-2 with 1.8X1013cm-2 C atom density. The corresponding electron mobility dropped from about 1.1X104cm2/Vs to 1.5X103cm2/Vs. Higher C dopings yielded unmeasurable samples. At 4.2K, a 2DEG could be measured only up to about 6X1012cm-2 C doping, reducing the 2DEG population from 3.3 to 1.1X1011cm-2, with corresponding mobilities dropping from about 2X105cm2/Vs to 6X104cm2/Vs. The 3-fold charge reduction induced by C doping could favor the achievement of 2DEGs in the low-1010cm-2 density range by appropriate gating.