Ultrahigh doping of GaAs by carbon during metalorganic molecular beam epitaxy

Abstract

Recent advances in heterostructure bipolar transistor technology have created a need for p-type doping at levels ≥1020 cm−3. Furthermore, such levels may eliminate the need for alloying during ohmic contact formation. We have achieved p-type doping levels as high as 5×1020 cm−3 using an unconventional dopant, C, derived from the gaseous source chemical, trimethylgallium (TMG), during metalorganic molecular beam epitaxial (MOMBE) growth of GaAs. We have controllably achieved doping levels between 1019 and 5×1020 cm−3 by diluting the TMG flow with another metalorganic, triethylgallium (TEG). By utilizing the so-called δ-doping or atomic planar doping method we have also been able to grow C-doped spikes with hole concentrations as high as 7×1019 cm−3, with a full width at half maximum of ∼50 Å at 300 K. This doping level is the highest yet reported for planar doping, and the narrow width indicates that the C atoms are restricted to one or two atomic planes. By switching out the TMG, and switching in the TEG to continue the growth of C-free GaAs we have grown sandwich-type structures with C levels of 1020 cm−3, which fall off within 210 Å to C levels of <1017 cm−3. High-temperature annealing of such structures reveals a C diffusion coefficient of ≤10−16 cm2 s−1 at 950 °C, in agreement with other reports. This is at least three orders of magnitude less than for the other conventional p-type dopants, Be and Zn. Finally, we report the presence of strain in the highly C-doped layers, detected by x-ray diffraction. The lattice constant obtained corresponds roughly to that calculated by assuming a Vegard’s law mixture of GaAs and 0.7% GaC. This distortion of the GaAs lattice has not been previously measured.