硅表面的化学处理对碳化硅外延薄膜质量和结构的影响

Abstract—The fundamentals of a new technique for the cleaning and passivation of (111), (110), and (100) silicon wafer surfaces by hydride groups, which ensure a high surface purity and smoothness at the nanoscale upon long-term storage of the wafers at room temperature in air, are discussed. A new composition of the passivation solution for the long-term antioxidation protection of silicon surfaces is developed. The proposed solution is suitable for the long-term storage and repeated passivation of silicon wafers. The composition of the passivation solution and the conditions of passivation of the silicon wafers in it are described. Silicon wafers treated using the proposed technique can be used for growing epitaxial semiconductor films and different nanostructures. It is shown that only silicon surfaces prepared in this way allow SiC epitaxial films on silicon to be grown by atom substitution. The experimental dependences of the SiC and GaN film structures grown on silicon on the silicon-surface etching conditions are presented. The developed technique for silicon cleaning and passivation can both be used under laboratory conditions and easily adapted for the industrial production of silicon wafers with an oxidation-resistant surface coating.

As we showed in [2–4], reaction (1) occurs in two stages. At the first stage, an intermediate compound forms, which represents an activated complex Si vacancy–C atom–Si matrix. At this stage, C atoms are located in the silicon matrix interstitials. At the second stage of the reaction, the activated complex transforms to SiC and the released vacancies merge in pores forming under the SiC layer. The technique is intended for synthesizing nanoscale SiC epitaxial films on substrates with different crystallographic orientations and conductivity types. The crystal structure of the SiC films grown on Si by atom substitution was studied by different methods, including photoluminescence, Raman spectroscopy, ellipsometry, high-resolution microscopy, and transmission electron microscopy. The results of these investigations were generalized in reviews [4, 5]. It was found that at the first stage of reaction (1), during the formation of an intermediate compound Si vacancy–C atom–Si matrix, the future crystal structure of a SiC film is grounded. The Si vacancy and C atom in the Si interstitial interact by means of the elastic mechanical energy [2–4]. They attract along some crystallographic directions and repulse along others. The Si vacancy and C atom are attracted to one another in the Si [111] direction with the formation of one object consisting of a pair of point defects. Analogously to the electric dipole, we called this object a dilatation dipole [2–4]. It appeared that the formation of an ensemble of dilatation dipoles leads to the formation of a single-crystal SiC film on Si. If such an interaction were absent, the films would be polycrystalline.

The formation of etching pits at the places of dislocation output should be avoided. Etching pits can start intensively developing and intergrowing through the SiC film with the formation of through channels from Si to the SiC surface. The intergrowth and merging of dislocations with the formation of nanotubes destroys the SiC surface layer. These nanotubes transmit the gaseous SiO reaction product with the formation of structures on the SiC film surface, which resemble volcanos at Earth’s surface [4, 5]. It was found [8, 9] that even if the nanotubes do not directly terminate at the SiC film surface, but lie near it, the occurring elastic energy significantly affects the initial stages of the nucleation of aluminum nitride (AlN) and gallium nitride (GaN) films. The dislocation nanotubes lead to the formation of so-called V defects on the AlN and GaN film surface.

An important condition for Si-surface preparation before SiC growth is the absence of any impurity particles, including metallic ones, on the Si surface. This especially concerns organic contaminants since the latter can be involved in spurious reactions during SiC synthesis.

Thus, the aim of this study is to develop a universal technique for chemical pretreatment of a wafer surface that would be suitable for both synthesizing SiC on Si and growing SiC films by the standard chemical vapor deposition (CVD) method, as well as AlN, GaN, and ZnO semiconductor films, specifically, growing nanostructures and whisker nanocrystals on Si. As we show below, the technique described in this work can be used in the production of various semiconductor devices on Si substrates.

Based on the data from [10, 18, 20], we developed a solution passivating Si by hydride groups [21], which has a composition different from that of analogous solutions proposed in [10, 18, 20]. In addition, we determined the optimal temperature and time of passivation for the Si(100), (110), and (111) surfaces. Initially, before passivation, we removed hydrated Si oxide with a thickness from 0.6 to 1.6 nm formed at the first etching and cleaning stages. For this purpose, we used, as in [10, 18, 20], a HF solution with a HF volume fraction of 5%. The etching time was 1–2 min. Etching was performed at room temperature. After this procedure, we passivated the Si surface using buffer solutions of two compositions, BR-1 and BR-2, with strictly specified pH values. These solutions were prepared in the following way.

After exposure of the Si wafers to BR-2 or BR-1, they are washed twice with deionized water stepwise, for 15–20 s at each step, and dried with a paper filter under standard conditions. The Si wafer surface becomes hydrophobic. After that, the wafers can be used and stored in open air at a temperature of 25°C for 7 days without noticeable degradation of their properties. We note that we investigated the effect of this cleaning and passivation method not only on the Si(111) and (100) wafers, but also on the Si(110) wafers, vicinal (111) surfaces deviated by 2°, 4°, 6°, and 8° from the (111) basal plane and vicinal (100) planes deviated by 2°, 4°, and 7° from the basal (100) plane to epitaxial film growth. All these surfaces yielded an unambiguously good hydrophobic coating with hydride groups; later, single-crystal SiC, AlN, and GaN films were grown on their surface.