Elsevier

Applied Surface Science

Volume 541, 1 March 2021, 148548
Applied Surface Science

Full Length Article
Peculiarities of the AlN crystalline phase formation in a result of the electron-stimulated reconstruction transition (√31×√31)R ± 9° − (1 × 1)

https://doi.org/10.1016/j.apsusc.2020.148548Get rights and content

Highlights

  • Reconstructed (√31×√31) R ± 9° sapphire surface under an ammonia flux is not nitridated.

  • In the presence of an electron beam a reconstruction transition (√31×√31)R ± 9°→(1 × 1) occurs.

  • Aluminum deposition on the sapphire surface do not lead to the (√31×√31)R ± 9° superstructure formation.

  • Electron beam destroys the (√31×√31)R ± 9° superstructure by the electron-stimulated oxygen desorption mechanism.

Abstract

In the present work, it was found for the first time that an electron beam used in reflection high-energy electron diffraction technique stimulates a reconstruction transition from a sapphire (√31×√31)R ± 9° superstructure which is chemically inert under an ammonia flux to an unreconstructed (1 × 1) structure with subsequent surface nitridation. The electron beam initiates electron-stimulated oxygen desorption from the sapphire surface followed by formation of oxygen vacancies, which are potential energetically accessible centers for the primary nuclei formation of the AlN crystalline phase.

Introduction

After Isamu Akasaki [1], Hiroshi Amano [2], and Shuji Nakamura [3] success in obtaining on a sapphire substrate a first bright blue LED based on III-nitride semiconductor compounds, these semiconductors are considered as a very promising materials for opto- and electronic applications. Despite the fact that, today native AlN and GaN substrates are gradually beginning to be used for growing III-nitride structures, foreign substrates are still widely applied in industrial technology. One of the most common substrates is a sapphire (Al2O3) substrate. Sapphire is transparent in the visible and UV wavelength ranges, has a rather high thermal conductivity (40 W/m∙K at 300 K), is thermally stable, has high crystalline perfection and much cheaper than SiC or native nitride substrates.

On the Al2O3 surface, after being placed in the loading chamber, adsorbed carbon and hydroxyl groups are retained [4], [5], [6], and the surface may consist of mixed cationic/anionic areas [7]. Since the Al2O3 substrate for epitaxial growth is initially “epi-ready” prepared, it is preliminary annealed before loading to the growth chamber without additional chemical treatment. Annealing leads to the surface cleaning from residual impurity gases and improving the structural properties of the substrate surface, increasing the terraces size between steps [8], [9], [10], and decreasing the roughness [9]. When the samples are heated to high temperatures (over 1150 °C), a reconstruction transition (1 × 1) → (√31×√31)R ± 9° on the sapphire surface occurs [11], [12]. The sapphire (√31×√31)R ± 9° surface reconstruction was investigated in the classical work of French and Somorjai (1970) by the low energy electron diffraction technique (LEED) [11]. The reconstruction model is shown in Fig. 1a. It was found that the (√31×√31)R ± 9° superstructure is formed upon high-temperature annealing, and the reverse reconstruction transition (√31×√31)R ± 9° → (1 × 1) is possible when the sapphire surface is exposed to oxygen (at high temperatures about of 1000–1200 °C). It was also indicated that the (√31×√31)R ± 9° superstructure can be obtained by direct Al deposition on a clean unreconstructed sapphire surface and further annealing of the surface at a temperatures of about 800 °C. The surface reconstructed (√31×√31)R ± 9° layer has been described as an oxygen-depleted layer or, in other words, as partially reduced aluminum oxide (i.e., containing suboxides AlO, Al2O). A model of a surface reconstructed monolayer (ML) as an suboxide with a rectangular lattice 4.40 × 4.57 Å2 (Fig. 1c) rotated by 9° relative to the sapphire lattice (Fig. 1b) was proposed. Later, the (√31×√31)R ± 9° reconstruction was investigated using X-ray diffraction in [12], [13], [14], [15]. Based on the obtained diffraction data, the authors of these works rejected the Somorjai model, proposing a model in the form of a hexagonal structure consisting of two metallic Al crystal planes (1 1 1) with certain structural distortions and partial disordering in comparison with the ideal hexagonal structure of the bulk aluminum crystal (1 1 1) planes (Fig. 1d).

It is expected that such an aluminum-enriched surface is more suitable for nitridation - an integral part of the III-nitrides growth technology on sapphire to form a thin AlN crystalline layer. The AlN layer is formed as a result of chemical transformation of the Al2O3 surface during the heated substrate exposure to the active nitrogen flux. The attractiveness of the (√31×√31)R ± 9° reconstructed surface for nitridation is due to a number of reasons. First, the (√31×√31)R ± 9° reconstruction should facilitate the nitridation process due to the high chemical activity of metallic aluminum according to the Renaud model [13]. Secondly, the effective mismatch between the lateral parameters of the AlN crystal unit cell and the characteristic periodic structure manifested in the reconstruction (√31×√31)R ± 9° is small compared with the effective mismatch with the lateral lattice parameter of the unreconstructed (1 × 1) sapphire. It was shown in [16] that AlN crystalline nuclei have a lateral lattice constant of 3.01 Å, which then increases to 3.08 Å. At the same time, the value of 3.01 Å was also found for the reconstructed (√31×√31)R ± 9° (0001) Al2O3 surface in [17]. Meanwhile, on the unreconstructed sapphire surface, the crystalline AlN unit cell formed upon nitridation turns by 30° relative to the Al2O3 unit cell, which reduces the mismatch of the lateral parameters of Al2O3 (aAl2O3 = 4.76 Å) and AlN (bulk value aAlN = 3.11 Å) from (aAl2O3 − aAlN)/aAlN = 53.1% [18], [19] down to only ~ 9%. Third, it is believed that the presence of a high-temperature reconstruction indicates a clean surface and a higher atoms ordering that improves the adhesion of growth components [13].

It was demonstrated in [4] that the presence of a superstructure (√31×√31)R ± 9° on the sapphire surface is the preferred condition for the subsequent successful formation of the AlN layer in the ammonia flux. From the relative changes in the intensities between the XRD peaks O1s and Al2p on the reconstructed and unreconstructed surfaces, the authors concluded that the reconstructed (√31×√31)R ± 9° surface is characterized by an oxygen deficiency, enriching the surface with aluminum ions with a higher reactivity. However, despite the fact that chemical analysis of the surface confirmed the presence of Al – N bonds after nitridation of the reconstructed (√31×√31)R ± 9° sapphire surface, crystalline AlN was not detected by the LEED technique.

In this work, the crystalline AlN formation during sapphire nitridation with a reconstructed (√31×√31)R ± 9° surface was investigated. Despite expectations, it was found that crystalline AlN does not form on the (√31×√31)R ± 9° superstructure. Its formation occurs only upon (√31×√31)R ± 9° superstructure destruction by a high-energy electron beam, which, apparently, is accompanied by the oxygen vacancies generation, which are potential nucleation centers for the AlN crystalline phase formation.

Section snippets

Experimental equipment and samples

The experiments were carried out on a Riber CBE-32(P) molecular beam epitaxy machine, adapted for ammonia MBE. The residual pressure in the growth chamber was 1.0 × 10-9 Torr. As a source of active nitrogen, high-purity ammonia of 99.999% in combination with additional Entegris purification filters with an ammonia purification degree higher than 99.999999% was used. In the experiments, epi-ready (0001) sapphire substrates with a 2 in. diameter were used. Before the experiment, sapphire

Experimental results and discussion

The aim of the experiment was to compare the diffraction patterns (DP) from the reconstructed (√31×√31)R ± 9° and unreconstructed sapphire surfaces before and after exposure to ammonia flux in order to detect the newly formed AlN crystalline phase. In Fig. 2a the sapphire (0001) hexagonal crystal lattice with the unit cell parameter aAl2O3 = 4.76 Å and the AlN hexagonal crystal lattice (0001) located on it with the unit cell parameter aAlN = 3.11 Å are schematically shown. Solid purple circles

Conclusion

It was experimentally established that the sapphire reconstructed (√31×√31) R ± 9° surface under an ammonia flux is not nitridated at all, or nitridated at a low rate, in contrast to the unreconstructed (1 × 1) sapphire surface. In the presence of a high-energy electron beam a reconstruction transition (√31×√31) R ± 9°→(1 × 1) occurs, followed by nitridation of the sapphire surface (1 × 1), as a result the registration of the AlN crystalline phase becomes feasible.

This work presents 3

CRediT authorship contribution statement

D.S. Milakhin: Methodology, Formal analysis, Investigation, Writing - original draft, Visualization. T.V. Malin: Investigation, Resources. V.G. Mansurov: Methodology, Formal analysis, Writing - review & editing. G. Galitsyn Yu: Methodology, Formal analysis, Writing - review & editing. A.S. Kozhukhov: Investigation, Visualization. D.E. Utkin: Investigation, Visualization. K.S. Zhuravlev: Conceptualization, Writing - review & editing, Supervision.

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

This work was financially supported by the Russian Foundation for Basic Research (Grants No. 21-52-46001 and No. 21-52-15009). The SEM and AFM diagnostics were performed on the equipment of Core Facilities Centre «Technologies for nanostructuring of semiconductor, metal, carbon, bioorganic materials and analytical methods for their study at the nanoscale» (CKP «Nanostructure»).

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