Improved depth resolution of secondary ion mass spectrometry profiles in diamond: A quantitative analysis of the delta-doping
Introduction
The development of diamond growth technology has largely improved the fabrication of homo and heterostructures with abrupt interfaces such as superlattices and quantum wells [1]. Consequently, the request for a very accurate characterization has become more demanding even though the analysis of such structures is difficult and sometimes a challenge of its own (nanometer scale, low concentration of light atoms, hard material, and so on). Secondary ion mass spectrometry (SIMS) is commonly used to obtain depth profiles of dopants over many orders of magnitude in concentration. However, below 100 nm in thickness, SIMS induced ion mixing is no longer negligible; it affects strongly the depth profile measurements by broadening and distortion, so that the raw SIMS profile differs from the dopant profile, up to the point where thickness values and atom peak concentrations in multilayer stacks become erroneous. Other alternative and promising techniques like atom probe tomography [2] are not yet so commonly available, and in fact not yet demonstrated on the diamond material.
This work is dedicated to the potentiality of SIMS applied to the characterization of nanoscale diamond embedded heterogeneous structures. Diamond has several excellent properties, in most cases superior to those of other semiconductors, e.g., Si and SiC. Actually, two types of application require the availability of very thin layers (boron or nitrogen-doped) in the range of nanometer thickness, the so-called “delta structures” [3], [4], [5], as well as the possibility to characterize such ultrathin epilayers. These applications are related to high breakdown voltage/high temperature electronic devices [6] aimed at the development of next-generation high power devices, but also to colour centers, e.g., NV centers in diamond [3], [7], a very active research field of photonics and spintronics, more in line with the optical properties of diamond.
Technically, during a SIMS analysis, the experimental depth profile is the convolution of the dopant depth profile and of the depth resolution function (DRF) [8]. Evaluation of this DRF (which depends on the probed atom) is a key issue in nm-range secondary ion mass spectrometry. Deconvolution analysis using such a DRF provides accurate measurements on abrupt dopant depth profiles over many orders of magnitude in concentration. The best tool to estimate quantitatively the influence of ion mixing during the SIMS analysis is the local isotopic substitution (or “isotopically pure growth”). This has already been demonstrated with silicon superlattices (28Si/30Si) [9]. The atomic substitution by an isotope is the best approach to extract the experimental response, i.e. the DRF, because it introduces only a negligible difference in mass (same recoiling effect) and ionization threshold as well as no additional crystalline strain (same lattice parameter). Once the DRF expression is known for carbon in diamond, we can apply this function to determine a genuine dopant depth profile for nitrogen, or boron, or phosphorus.
However, the requirements to record an accurate DRF are stringent. The embedded layer has to be in the same thickness range as the lattice parameter. The fabrication of such structure requires strict conditions such as flat interface, no chemical diffusion in the matter, and a single crystalline substrate [10]. Diamond epitaxial multilayer stacks fulfill these requirements.
Section snippets
Diamond sample growth
Two diamond single crystalline samples were grown in this study. A first sample, composed of a synchronized boron- and carbon-13-doped layer, was used to extract the DRF from the 13C signal intensity. Furthermore, the fitting process was applied on the boron profile, in order to qualify the possibility to deconvolve the boron concentration and the layer thickness. The second sample was constituted of a double boron-doped delta layers in order to analyse the growth uniformity and the interface
SIMS profile fitting
Several authors have reported that a SIMS profile can be modelled by convolving the genuine atom profile with the SIMS depth resolution function, a response which depends on instrumental and fundamental aspects as well (convolution model). In the 90s, Dowsett et al. [10] have demonstrated that for delta-doped layers characterized by few atomic layers and hence below the SIMS resolution, an excellent approximation of depth resolution function (DRF) can be obtained by convolving a double
Discussion
The deconvolved boron profile of the first sample was found to be 5 to 7 times sharper: the initially measured 1.5 nm/decade rising edge became 0.3 nm/decade (see Fig. 6). The best fit seemed to justify the presence of a finite thickness for the interface located between boron-doped and intrinsic diamond layer. Nevertheless, such interface thickness was composed of two points only. The sampling (Δz = 0.9 nm) was not rich enough to really conclude about an exponential or linear dependence on the
Conclusion
By a local isotope enrichment of diamond, we were able to extract the instrument response of the SIMS and to characterize the incorporation of both carbon and boron atoms. These treatments allowed to increase the SIMS resolution, in order to subtract the broadening and the distortion induced by ion-mixing and to reach the nanometer-range. This procedure yielded a more reliable characterization by SIMS of nanometer thin diamond embedded layers containing specific impurities over a wide range of
Acknowledgments
The financial support of Agence Nationale de la Recherche under contract ANR08-BLAN-0195 and la Région Rhône-Alpes for the bourse de mobilité Explora'Doc is gratefully acknowledged. This work was also supported by the Strategic International Collaborative Research Project from the Japan Science and Technology Agency, Japan and Grant-in-Aid for Scientific Research from the Japan Society for the Promotion of Science, Japan (No. 23360143).
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