Hydrothermally grown ZnO crystals and structural quality

The size and the structural quality of the obtained ZnO crystals depend on the growth environment that defines the growth rate and growth morphology, and on the growth time. High growth rates lead to poor structural quality of the crystals; therefore, low growth rates, typically below 1 mm/h, are used. To obtain ZnO crystals of sufficient thickness (around or above 1 cm), growth time must last for a number of days, usually between 15 and 30 days. The crystal diameter further depends on the seed size and the volume of the autoclave. A successful development of the hydrothermal method enabled to obtain 2- and 3-inch diameter and about 1 cm thick ZnO crystals, as shown in Fig. 3.39.

The structural quality of the obtained ZnO crystals by the hydrothermal method is relatively high. This can be concluded from the FWHM of the rocking curve and from the etch pit density (EPD) revealed by etching or by dislocation density visualized by X-ray topography. The FWHM of the rocking curve of the (0002) reflection of polished (0001) wafers was measured as low as 8-45 arcsec [71,

209, 210, 215, 219, 226]. Etch pits can be revealed by etching, e.g., in HC1, H3PO4, HN0₃, CH3COOH aqueous solution, or NaOH. The (0001) face is etched much faster as compared with the (0001) face and may become matt while the latter smooth and flat. As low values of the dislocation density as 10²-10³ cm'² for as-grown (0001) faces were reported by Dem'yanets et al. [216] and EPD of 300 cm"² was reported by Maeda et al. [219], which decreased to the value of 80 cm"² after annealing. The lowest dislocation density of 1-5 cm"² was reported by Dem'yanets and Lyutin [214] by using "turning seeds." The etch pits on the (0001) surface (Fig. 3.40) have a form of negative hexagonal prisms that keep crystallographic orientation of the crystal.

Figure 3.39 ZnO crystals obtained by the hydrothermal method: (a) 2-inch- diameter and (b) 3-inch-diameter crystal and wafer. Figure (a) reprinted from Ref. [214], Copyright 2008, and (b) reprinted from Ref. [71], Copyright 2006, both with permission from Elsevier.

Figure 3.40 Etch pits on the (0001) surface of hydrothermally grown ZnO crystal. Reprinted from Ref. [214], Copyright 2008, with permission from Elsevier.

Due to the anisotropy of impurity incorporation, different sectors of the crystals reveal not only different coloration, but also different structural quality. The higher Li⁺ incorporation at the (0001) face leads to a higher dislocation density as compared to the (0001) face [216]. Due to the lowest impurity incorporation, m-planes are regarded as planes of high structural quality as a result of the lowest density of defects.

Effect of annealing on the surface quality

The (OOOl)-oriented wafers can be polished (usually by chemical- mechanical polishing, CMP) to a high flatness to values of about 0.155 nm of the root mean square (RMS) roughness [71]. Mechanical polishing only produced worst surface quality, as the FWHM of the rocking curve was about twice larger as compared to CMP [208]. In addition to polishing and etching, the wafer surface quality can be further modified by annealing at elevated or high temperatures in the presence of oxygen. After CMP, the (0001) and (0001) surfaces do not show any step or terrace structure, but high flatness, with RMS roughness < 1 nm [219]. The terraces are formed by annealing in the presence of oxygen at 1050-1100°C for 2-4 h [219].

The step height depends on annealing temperature and a face orientation. After annealing at 1050°C and 1100°C for 2 h, the steps are monoatomic (about 0.5 nm) and diatomic (about 1 nm) on the c-planes, respectively, and multi-atomic (50 nm) on the m-plane, which requires lower thermal activation [71]. Annealing may also cause the formation of voids of 1-10 pm on the polished surface (0001).

Ко et al. [228] showed that annealing of the (0001) surface in oxygen at 1000-1100°C for 1 h decreased both the FWHM of the rocking curve by a factor of almost 3 and the RMS roughness by factor of about 6 (Fig. 3.41) [228].

Figure 3.41 FWHM of the rocking curve and roughness of (0001) surface as a function of annealing temperature in pure oxygen for 1 h. Reconstructed basing on data from Ref. [228], with some modifications and different visualization.

Borseth et al. [229] stated that the annealing of (OOOl)-oriented wafers in oxygen at 1 bar and 680-960°C improved the FWHM a bit but increased the roughness of both faces. On the other hand, annealing at a lower pressure, 0.5 bar, and 500-1050°C led to an increase in both FWHM and roughness. Annealing in air at the ambient pressure did not change the FWHM but improved the roughness. Further, annealing in ZnO powder substantially did not change the FWHM, but made the +c-plane flat with terraces. The surface improvement or degradation upon annealing depends on the equilibrium between Zn and О partial pressures in the crystals and partial pressures of Zn and О in the annealing environment.