Spectroscopic ellipsometry: A sensitive tool to monitor domains formation during the bias enhanced nucleation of heteroepitaxial diamond

https://doi.org/10.1016/j.diamond.2021.108246Get rights and content

Highlights

  • The analysis by ellipsometry proved its sensitivity to the formation of domains.

  • Sequential analysis by FE-SEM, AFM and XPS allows to build the most relevant ellipsometric model keeping a physical meaning.

  • This study opens the way to in situ monitoring of early stages of heteroepitaxial diamond growth during BEN.

Abstract

Surface morphology and surface chemistry of epitaxial Ir/SrTiO3/Si(001) pseudo-substrates were systematically characterized by Field Emission Scanning Electron Microscopy (FE-SEM), Atomic Force Microscopy (AFM) and X-ray Photoelectron Spectroscopy (XPS) before and after the Bias Enhanced Nucleation (BEN) of diamond. Based on these characterizations, an ellipsometric model was built taking into account both the roughening of the iridium surface and the formation of domains occurring during the BEN process. Sequential spectroscopic ellipsometry (SE) characterizations allowed estimating the roughening of the iridium surface, the thickness of the amorphous carbon layer and the coverage ratio of domains. By annealing the sample in air after BEN, domains were successfully removed without changing the iridium surface morphology. From these analyses, we demonstrate that SE is enough sensitive and discriminative to monitor both the formation of domains and the roughening of iridium surface, distinctly. SE can then be viewed as a powerful in situ tool of critical importance to study, monitor and finally enhance the diamond nucleation and growth on iridium for various advanced applications.

Introduction

Single crystal diamond presents unique properties, such as a wide bandgap, a high breakdown voltage or a high mobility for both charge carriers. With the possibility to accurately control the doping, this material is a promising candidate for power electronics or radiation detection [1]. However, the size of single crystal diamond synthesized by High Pressure High Temperature (HPHT) or by homoepitaxial growth by Chemical Vapor Deposition (CVD) available for such applications is still limited (<10 × 10 mm2). Today, diamond heteroepitaxy on iridium appears as a credible alternative to overcome this issue while maintaining a high crystalline quality [[2], [3], [4]].

The few groups involved in diamond heteroepitaxy use iridium surfaces which allows the highest epitaxial rate (>90%). The iridium epilayer is deposited onto scalable substrates [5,6]. Due to the specific reactivity of iridium under ion bombardment, the nucleation mechanism of diamond differs from the one occurring on silicon and silicon carbide. Indeed, tridimensional isolated crystals are observed on silicon and silicon carbide [7,8] contrary to bidimensional structures obtained on iridium, known as domains [9,10]. A model including involved mechanisms and kinetics of propagation of domains has been proposed [11]. Sequential approach combining a MPCVD reactor connected to a UHV set-up allowing surface analysis was previously reported to investigate surface modifications induced by Bias Enhanced Nucleation (BEN) on different heterosubstrates [12,13]. However, in situ characterizations could be much more efficient for having a deeper comprehension of phenomena implied in nucleation. Additionally, it may help the tuning of BEN parameters to improve the quality of diamond/iridium interface. Laser reflectometry has already been investigated to monitor in real time diamond nucleation on silicon. The light scattering induced by the formation of diamond nanocrystals is used to estimate their size and their density (number per cm2) [14]. This method is not usable for diamond nucleation on iridium because of the bidimensional structure of domains and of surface modifications induced by the BEN process via a surface roughening (iridium balls, furrows) [15,16]. Spectroscopic ellipsometry (SE) could overcome this issue because of its sensitivity to the optical index differences at interfaces. It was already used to deduce in situ the growth rate and the boron concentration in monocrystalline diamond [17] but has never been investigated for diamond nucleation. However, it is necessary to ensure the sensitivity of SE to domains before making in situ characterizations.

In this paper, we report on ex situ characterizations by SE before and after BEN by building an ellipsometric model based on sequential characterizations by X-ray Photoelectron Spectroscopy (XPS), Field Emission Scanning Electron Microscopy (FE-SEM) and Atomic Force Microscopy (AFM). By performing annealing in air after BEN to remove domains without affecting the iridium surface morphology, we demonstrate the potential of SE as a sensitive in situ tool to monitor both the iridium surface roughening and the domain formation.

Section snippets

SrTiO3 pseudo-substrate on silicon (001)

The deposition of a thin film of epitaxial SrTiO3 layer on a 500 μm thick silicon oriented (001) was achieved by Molecular Beam Epitaxy (MBE) using Knudsen effusion cells. Experimental details can be found elsewhere [6,[18], [19], [20], [21]]. A thickness of around 40 nm has been grown to make further iridium depositions. XRD analysis showed a mosaicity of 0.3° for the SrTiO3/Si(001) heterosubstrates used in this work.

Iridium epitaxy on SrTiO3/Si(001)

Epitaxial iridium layers were deposited by e-beam evaporation on two SrTiO3

Results

In order to make the most complete, simplest and most relevant ellipsometric model, it is required to first characterize the surface morphology and the surface chemistry of samples. For that purpose, FE-SEM, AFM and XPS analyses were carried out before and after the BEN step. First, the evolution of the surface morphology and the surface chemistry of the pseudo-substrate Ir/SrTiO3/Si(001) is presented, leading to the creation of an ellipsometric model (3.1 Evolution of the surface morphology,

Sequential analysis and conception of an ellipsometric model

The two first proposed models present a low value of χ2 (<1). Nevertheless, the visible difference between the experimental data and simulations for Is and Ic shows the necessity to take into account both phenomena, i.e. roughening and domain formation to build a more robust model conserving a physical meaning. In this third model, diamond nuclei included in domains were not considered. Indeed, N. Vaissiere et al. demonstrated with combined characterizations by FE-SEM, XPS and Nano-Auger

Conclusion

The complex evolution of the iridium surface occurring during the BEN with mechanisms competing with heteroepitaxial diamond nucleation explains the high interest for in situ monitoring. This approach could lead to an accurate optimization of the BEN parameters for diamond heteroepitaxy on large Ir/SrTiO3/Si(001) wafers (2 in. or more). Such experimental approach will also deepen the knowledge in the formation of domains. Considering its sensitivity to the optical index differences at

CRediT authorship contribution statement

Julien Delchevalrie: Conceptualization, Methodology, Investigation, Writing - Original Draft.

Romain Bachelet: Methodology, Review & Editing.

Guillaume Saint-Girons: Methodology, Review & Editing.

Samuel Saada: Supervision, Conceptualization, Methodology, Visualization, Investigation, Methodology, Review & Editing.

Jean-Charles Arnault: Supervision, Project administration, Funding acquisition, Resources, Conceptualization, Methodology, Visualization, Review & Editing.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

Authors would like to thank the French National Agency (ANR) and DGA for funding this work under the project DIAMWAFEL (Large and conducting DIAMondWaFers for industrial applications in power ELectronics), Grant No. ANR-15-CE08-0034. Authors would like to acknowledge Nicolas Tranchant for useful discussions.

References (25)

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