Theoretical and experimental studies of the broadening of dilute delta‐doped Si spikes in GaAs during SIMS depth profiling

Abstract

AbstractDelta‐doped structures represent a powerful class of test structures to investigate the experimental and fundamental factors limiting the depth resolution obtainable in SIMS sputter depth profiling.In this work, theoretical studies of the effects on the broadening of an Si delta spike in GaAs as a function of the energy (1.4–4.4 keV) and angle of incidence (2°, 45° and 60° off‐normal) of the O2 sputter probe beam have been compared with recent experimental data. The theoretical calculations were carried out using the newly developed IMPETUS computer code, which simulates the depth profiling process by taking into account the combined effects of ballistic mixing (treating collisional mixing as a diffusion process), projectile incorporation into the matrix and sputtering. All of these are processes that always occur in any practical sputter depth profiling situation.The IMPETUS model can reproduce low‐energy Si depth profiles with great accuracy by using the well‐established TRIM calculated range, energy deposition and sputtering data and by making reasonable assumptions for the threshold energy for diffusion in addition to assuming a beam‐ and sputter statistics‐induced surface microtopography, which is described by a Gaussian area versus height distribution having a standard deviation σ = 0.8 nm. Significantly, it is shown that the effects of these parameters on the shape of the sputter profile are largely independent, with σ (accounting for microroughness) mainly affecting the leading edge and the threshold energy (determining mixing processes) the trailing edge of the sputter profile. Good agreement on the energy dependence of the broadening is also obtained. The expected improvement in depth resolution with increasing off‐normal bombardment angle is confirmed and can be quantified. The error in the experimental depth scale calibration based on a constant sputter rate, ignoring transient sputtering, is evaluated. Finally, the sputter depth profile observed for an Si delta spike in GaAs subjected to thermal annealing during growth by molecular beam epitaxy (MBE) can be reproduced accurately by considering diffusion broadening of the initial spike followed by a sputter profiling simulation.