Encoding Abrupt and Uniform Dopant Profiles in Vapor–Liquid–Solid Nanowires by Suppressing the Reservoir Effect of the Liquid Catalyst

Abstract

Semiconductor nanowires (NWs) are often synthesized by the vapor-liquid-solid (VLS) mechanism, a process in which a liquid droplet-supplied with precursors in the vapor phase-catalyzes the growth of a solid, crystalline NW. By changing the supply of precursors, the NW composition can be altered as it grows to create axial heterostructures, which are applicable to a range of technologies. The abruptness of the heterojunction is mediated by the liquid catalyst, which can act as a reservoir of material and impose a lower limit on the junction width. Here, we demonstrate that this "reservoir effect" is not a fundamental limitation and can be suppressed by selection of specific VLS reaction conditions. For Au-catalyzed Si NWs doped with P, we evaluate dopant profiles under a variety of synthetic conditions using a combination of elemental imaging with energy-dispersive X-ray spectroscopy and dopant-dependent wet-chemical etching. We observe a diameter-dependent reservoir effect under most conditions. However, at sufficiently slow NW growth rates (≤250 nm/min) and low reactor pressures (≤40 Torr), the dopant profiles are diameter independent and radially uniform with abrupt, sub-10 nm axial transitions. A kinetic model of NW doping, including the microscopic processes of (1) P incorporation into the liquid catalyst, (2) P evaporation from the catalyst, and (3) P crystallization in the Si NW, quantitatively explains the results and shows that suppression of the reservoir effect can be achieved when P evaporation is much faster than P crystallization. We expect similar reaction conditions can be developed for other NW systems and will facilitate the development of NW-based technologies that require uniform and abrupt heterostructures.

Impact of Dopant Compensation on Graded p–n Junctions in Si Nanowires

“…The p-type and intrinsic segments were grown at 460 °C and the n-type segment were at 420 °C. 7,27 The temperature started to decrease with the initiation of phosphine flow, and it decreased and stabilized at 420 °C within ~3 minutes. The pressure was kept at 40 Torr for the entire growth.…”

“…Recently, groups have made improvements in junction abruptness through adjustments in growth conditions by reducing the solubility of dopant species in the catalyst 8,22 or even slowing down the growth rate to allow dopant evaporation from the catalyst during growth. 7 However, it is in general difficult to achieve growth conditions that are optimal for both dopant species in a homojunction. Furthermore, surface doping from the background gases in the reactor is a common outcome, leading to the formation of a surface region that is enriched in dopant.…”

Progress in doping semiconductor nanowires during growth

“…Switching dopant gases can result in dopant modulation 158,159 or p-n junction formation in NWs. 13,97,160,161,162 We briefly overview here two techniques for profiling NW junctions, the scanning capacitance microscopy (SCM) and off-axis electron holography despite the availability of other compelling techniques such as electron-beam induced current (EBIC).…”

Contactless Electrical and Structural Characterization of Semiconductor Nanowires with Axially Modulated Doping Profiles

“…Central to the promise of the VLS mechanism is the ability to change gas‐phase composition during growth, thus allowing segments with distinct structures or compositions to be axially (or radially) encoded. Direct control of crystal structure, dopant profile, alloy composition, and morphology along the nanowire length have all been reported.…”