Control of doping in semiconductor nanowires is a fundamental issue in view of their integration in optoelectronic devices. n-type doping of GaAs nanowires (NWs) has demonstrated to be to be impossible using Si, the typical n-type dopant in III-As molecular beam epitaxy (MBE), due to its amphoteric character that is enhanced by the peculiar growth conditions used for the nanowires.Tellurium is a possible alternative n-type dopant material, and his efficiency in standard epitaxy has been demonstrated for arsenides and phosphides.
We demonstrate the efficiency of our doping method with the fabrication of single-wire FET devices and by measuring the transport characteristics and evaluating carrier concentration and mobility. As transport measurements are limited to high doping levels and are affected by nanocontact issues, we have used complementary contact-less techniques, namely Raman scattering and photoluminescence, to make a systematic study of Te doping. The results obtained from optical methods were then compared with the transport measurements.
We demonstrate that we control the Te doping from very low concentrations up to carrier densities well inside the 1019 cm-3 range.We also demonstrate that Raman spectroscopy, requiring no sample processing, can be used as a fast and easy method to get quantitative information about nanowire doping as far as the carrier density is sufficiently high to give rise to a depletion length shorter than the penetration length of the exciting laser. A comparison between PL and Raman measurements also show that the semi-quantitative information about the doping obtained by PL is in very good agreement with the Raman result. This feature allows us to use PL as a contactless method to study Te doping for carrier densities below the threshold value useful for Raman analysis and well below the densities needed for device fabrication.
Our data show that the doping attained in the nanowire is very similar to that measured in reference two-dimensional epitaxial layers, demonstrating that our doping strategy is very effective. The devices fabricated with the most doped nanowires, transport measurements show carrier concentrations higher than those obtained by Raman. The difference is attributed to the uncertainty on the mobility obtained in the transport measurements.
As for the NW growth, we found that Te affects Ga mobility during the growth,as shown by the decrease of the size of the Ga nanoparticle at the NW tip and of the NW diameter, with the Te flux. Moreover the incorporation of Te atoms increases the probability to find wurtzite sections in the nanowires, a peculiar feature of III(As,P) NWs, not observed in bulk material.
This work has received funding from the European FP7 for research, technological development and demonstration, under grant agreement no. 316751 (NanoEmbrace).