Characterization of subsurface hydrogen in diamond films by high-resolution elastic recoil detection analysis

Abstract

High-resolution elastic recoil detection analysis (HERDA) was carried out to characterize the subsurface structure in highly oriented diamond films that had been treated by hydrogen/deuterium plasma before and after a conventional wet oxidation. The results were compared with those obtained by high-resolution Rutherford backscattering spectrometry (HRBS) and secondary ion mass spectroscopy (SIMS). The results of HERDA revealed that residual deuterium of 3.8×10¹⁴ cm⁻² was present after the wet oxidation, while the film surface had been covered with a monolayer deuterium of 1.5×10¹⁵ cm⁻² before the treatment. On the other hand, deuterium was not observed in the film annealed at 1000 °C before the oxidation. In accordance with these results, the deuterium content in the subsurface region of the film annealed at 1000 °C before the oxidation (down to 50 nm from the surface) decreased by two orders of magnitude compared to that for the films without annealing, as revealed by SIMS. It is thus concluded that the subsurface hydrogen cannot be removed by the wet oxidation, which could cause the surface Fermi level pinning as commonly observed in oxidized diamond films.

Introduction

The understanding of subsurface structure in thin films is a challenging task and particularly important to realize practical electronic devices because electronic properties of semiconductors are strongly influenced by impurities incorporated in the subsurface as well as on the surface. The techniques such as elastic recoil detection analysis (ERDA) and Rutherford backscattering spectrometry (RBS) are widely used for depth profiling measurements [1]. They have non-destructive, rapid, and quantitative features. However, the depth resolution of conventional ERDA and RBS is typically 10 nm, which is too large to characterize the subsurface regions. Secondary ion mass spectroscopy (SIMS) is known to have better depth resolution, but the surface transient and the matrix effects, observed in the SIMS analysis, often cause the uncertainty in the very shallow region [2], [3]. The techniques using medium energy ions, namely, high-resolution ERDA (HERDA) and high-resolution RBS (HRBS) are suitable for investigating the elements on and near the surface region [4]. These techniques have a much better resolution of 0.2 nm than conventional techniques, and undertake simultaneous observations of both the surface and the subsurface regions.

It has been reported that the hydrogen-rich environment for diamond growth by chemical vapor deposition (CVD) induces a p-type highly conducting layer at the diamond film surface, which can be removed by surface oxidation using acid solutions or annealing in oxygen ambient [5]. The resulting surfaces after oxidation are terminated by oxygen atoms, and the surface Fermi level pinning is commonly observed in oxidized diamond films. Recently, Chen et al. [6] observed using Schottky junctions that the pinning of the surface Fermi level for oxidized diamond can be partially released after high-temperature annealing above 1000 °C. They also pointed out that a high-density space charge in the subsurface region was not observed after the high-temperature annealing [6], [7]. However, the correlation between the electric properties and the subsurface structure, including incorporated impurities, has not been clarified yet. In the present study, we carried out HERDA to characterize the subsurface structure in highly oriented diamond (HOD) films that had been treated by hydrogen/deuterium plasma before and after a conventional wet oxidation. The results were compared with those obtained by HRBS and SIMS. Current–voltage (I–V) measurements were also done to examine the relationship of the subsurface hydrogen with the highly conducting layer existing in the near-surface of hydrogenated diamond films.