A low-energy metal-ion source for primary ion deposition and accelerated ion doping during molecular-beam epitaxy

Abstract

The design, operation, and use of a single-grid, electron impact, ultrahigh vacuum (UHV) compatible, low-energy ion gun capable of operating with a low vapor pressure solid source material such as In are described. The gun consists of a single chamber which combines the functions of an effusion cell, a vapor transport tube, and a glow discharge ionizer. It has been operated over a wide range of In pressures from 10−5 to 10−3 Torr (10−3 to 10−1 Pa) and provides total current densities of up to 300 μA cm−2, without magnetic discharge confinement, at ion energies Ei from 50 to 400 V. Accelerated In+ (Ei=100 eV) doping during Si(100) crystal growth by molecular-beam epitaxy (MBE) resulted in increases in the In incorporation probability ranging from approximately one to four orders of magnitude above values obtained for thermal In doping at film growth temperatures between 570 and 800 °C. Accelerated beam doping profiles were also much more abrupt than thermal beam profiles. Initial results of experiments designed to investigate the role of ion/surface interactions during nucleation and the early stages of crystal growth in UHV showed that for deposition on amorphous substrates, the use of a partially ionized In+ beam (Ei=150–300 eV) resulted in a progressive shift towards larger average island sizes, a decreased rate of secondary nucleation, and a more uniform island size distribution.