Effect of Arsenic Depletion on the Silicon Doping of Vapor–Liquid–Solid GaAs Nanowires

Abstract

It is well known that chemical potential driving the vapor-liquid-solid growth of nanowires oscillates in synchronization with the monolayer growth. In III-V nanowires, this occurs due to depletion of group V atoms in a catalyst droplet. The amphoteric behavior of silicon doping, which often changes from n-type in planar GaAs layers to p-type in nanowires, is attributed to low arsenic concentrations. Herein, we present an analytical model which quantifies the doping oscillations over the monolayer formation cycle, and its impact on the electron-tohole ratio for the silicon doping of GaAs nanowires. It is shown that arsenic depletion can easily double the amphoteric effect and strongly favor the tendency for p-type doping. On a more general ground, the nanowire doping process appears highly sensitive to the chemical potential oscillations related to a restricted amount of material in a nanoscale catalyst.

Oscillations of As Concentration and Electron-to-Hole Ratio in Si-Doped GaAs Nanowires

“…Clearly, is the coverage equivalent to refill, which is proportional to the growth time , and is the ratio of the island growth time over the deposition time . As discussed above, the previous results of Reference [ 29 ] correspond to the limiting case of instantaneous growth at . Equation (2) for shows that the fractional ML progresses at the rate in this normalization when , and it stops growing when .…”

“…As discussed in detail in Reference [ 29 ], the main contribution to is logarithmic [ 3 , 32 , 33 ], and it can be put as . Here, we present the As concentration in a catalyst droplet in terms of the effective coverage equivalent to the arsenic content in liquid, denoted , by normalizing the total number of As atoms in liquid to the number of As atoms (or GaAs pairs) in the full ML of a GaAs NW.…”

“…Periodic oscillations of the As concentration over the ML growth cycle further decrease the average electron-to-hole ratio achievable with Si doping of VLS GaAs NWs and therefore increase the tendency for p-type doping. This effect has recently been studied in Reference [ 29 ] under the assumption of instantaneous ML growth until reaching the “stopping” size. Instantaneous development of the ML in the initial stage requires that the characteristic time of island growth is much shorter than the equivalent deposition time of 1 ML , which is inversely proportional to the As deposition rate.…”

Thermodynamics of the Vapor–Liquid–Solid Growth of Ternary III–V Nanowires in the Presence of Silicon

“…In addition, a small fraction of foreign species, such as Si atoms, can also be added to the droplet. This can be achieved intentionally when doping of NWs is required [ 19 , 20 , 21 , 22 , 23 ], or unintentionally when a catalyst droplet absorbs foreign atoms from the underlying substrate. The latter process often occurs in the initial nucleation step of III–V NWs grown on (oxidized) silicon substrates, and is enhanced at higher temperatures [ 24 , 25 , 26 ].…”

Calculation of Hole Concentrations in Zn Doped GaAs Nanowires

“…In addition, the chemical potential of As is known to oscillate during growth due to depletion effects correlated with monolayer nucleation and growth. This As depletion has been found to have a strong effect on the Si doping of GaAs nanowires grown by the VLS mechanism [ 13 ]. Moreover, Dubrovskii et al [ 14 ] have developed a model based on growth thermodynamics where the doping level of VLS grown nanowires is related to the doping level of a thin film grown from the vapor phase under corresponding conditions.…”