2.1 Preparation of the YSZ Substrates

The single YSZ crystals, consisting of ZrO₂ stabilized with 10, 15 and 21 mole per cent of Y₂O₃, were grown by the skull melting technique Reference Aleksandrov, Osiko, Prokhorov and Tatarintsev[5] at 3000°C in air (RF power – 160 kW, frequency – 1.67 MHz). The YSZ crystal block dimensions were large (40-80 mm in cross section and 80-100 mm in length). As-grown YSZ crystals can have both block crystalline and single crystalline structure. We used only single crystalline YSZ with the best structural perfection for wafer preparation. YSZ (100) and (111) oriented wafers were prepared by a multistage technique Reference Shabunov, Shengurov and Buzynin[6] using ultra dispersed diamond at the mechanical treatment stage.

The chemical composition of the YSZ crystals has been confirmed by x-ray wavelength dispersion analysis using the “Camebax 50SX” SEM. The analysis of impurity compositions of both bulk YSZ crystals and the surface layer of wafers has been performed by x-ray fluorescence spectral analysis using the “VRA-3” instrument. The results of this investigation (Table 2) showed that the mechanical treatment of crystals resulted in the increase of some impurity concentration (Al, Ca, Na, K, Si). The wafers were subjected to additional annealing in order to reduce the concentration of these impurities and to optimize the physical-chemical state of wafer surface before the epitaxy. Recrystallization annealing of YSZ wafers resulted in both removal of impurities and improvement of crystal perfection [7].

Table II Impurity concentration in YSZ single crystals, YSZ-wafers and GaAs/YSZ films

2.2 Capillary epitaxy of GaAs on YSZ

The capillary epitaxy technique Reference Busynin, Buzynin and Osiko[4] is based on covering the YSZ wafer with the thin film of A component, and subsequently saturating the film with the corresponding B component until the formation of III-V continuous layer. After that, III-V films were grown to the needed thickness. This technique was studied for the example of GaAs heteroepitaxial growth on YSZ substrates. It was found that the GaAs layers could be grown only within a narrow temperature range of about 550-600°C. When the buffer Ga layer is absent the films have polycrystalline structure and a rough surface. The minimum thickness of these films was about 1,5-2μ. In contrast, thin (50 nm) GaAs films can be obtained after Ga layer formation using the “capillary epitaxy” technique Reference Busynin, Buzynin and Osiko[4].

Figure 1 shows SEM micrographs of GaAs films prepared on YSZ by both conventional MOCVD (Figure 1a) and the capillary epitaxy technique (Figure 1b) at the initial stage of growth, and NH₄J graphoepitaxy on an amorphous Al substrate (Figure 1c,1d). An analogy between the helpful action of capillary forces for the new capillary epitaxy technique and for graphoepitaxy was suggested earlier Reference Sheftal and Buzynin[8]. The formation of layers in both capillary epitaxy and the graphoepitaxy technique take place from creation and coalescence of three-dimensional nuclei. As in the case of graphoepitaxy using surface active substances Reference Sheftal and Buzynin[8], the use of capillary forces at the first heteroepitaxial stage of the GaAs/YSZ film formation resulted in the following consequences:

1. 1. epitaxial nuclei density increased;

2. 2. the nucleus dimension decreased;

3. 3. the nucleus habit became more flat;

4. 4. the epitaxial orientation of the nuclei improved.

Figure 1 SEM micrographs of GaAs films prepared using both conventional MOCVD (a) and capillary epitaxy MOCVD(b) at the initial stage of epitaxy on YSZ substrates (60 and 600s after procedure beginning, 20000x) and optical micrographs of NH₄J flat crystals prepared by the graphoepitaxy technique on amorphous Al substrates without (c) and with(d) a surface active substance (100x).

As a result, the minimum GaAs/YSZ film thickness was decreased to 50 nm, and the structural perfection and surface morphology of the films were improved. Single crystalline 50-500 nm thick GaAs(100) films on YSZ(100) were prepared. The submicron GaAs films grown by the capillary epitaxy technique had high resistance. For n-type GaAs films, SiH₄ was used as dopant source. Electron concentration in the films was in the range of 5·10¹⁶ to 5·10¹⁷cm^-3. SIMS data showed that background impurity concentration was similar to that in GaAs/GaAs films. The layer -by-layer SIMS analysis indicated a uniform distribution of impurities in the GaAs films and a sharp distribution of Zr, O and Y across the film-substrate interface.

According to figure 1, better wetting of substrate led to better film structure both for graphoepitaxy of NH₄ on amorphous Al and capillary epitaxy of GaAs on YSZ.

2.3 GaN Films on YSZ

GaN films on YSZ substrates were grown both by conventional MOVPE Reference Ramer and Malloy[9] and by the “capillary epitaxy” Reference Aleksandrov, Osiko, Prokhorov and Tatarintsev[5] technique similarly to the GaAs films. The thin (∼50nm) Ga layer on the YSZ substrate was pre-deposited by the MOD technique before the GaN MOVPE growth in TMG-NH₃-H₂ system at 1025°C in the case of the “capillary epitaxy” technique. The GaN films had a mosaic structure. The film-substrate orientation relationships are GaN(0001)/YSZ(100) and GaN(0001)/YSZ(111). Typical examples of PL spectra of undoped n-GaN films are shown in Figure 2.

Figure 2. Luminescence spectra of undoped GaN films on a YSZ substrate.

The work on improvement of the structural perfection of GaN films on YSZ substrates is in progress.