Sandwich growth method

In this Datareview, bulk growth of GaN and AIN by a sublimation method and of GaN by a sublimation sandwich method is described. The source powder was analysed. The bulk GaN obtained was characterised by XRD (X-ray diffraction), TEM (transmission electron microscopy), and so on. 

The sublimation sandwich method (SSM) is similar to the sublimation method, except for the small distance between source powder and substrate, 2-5 mm [10-12], Ga or GaN was used as a source and the substrate was sapphire or 6H-SiC. The optimal growth temperature was around 1200°C as temperatures lower than 1150°C caused the formation of structural defects such as voids and micropipes. Polycrystalline GaN was grown under 1050°C. At higher temperatures, GaN was thermally decomposed. The growth rate is much higher, up to 300 pm/hr [10] or 0.2 - 1.1 mm/hr [12], and crystalline GaN with the maximum thickness of 500 pm could be obtained. 

Bulk growth of GaN and AIN has been achieved by a sublimation method and a sublimation sandwich method. Bulk GaN and AIN bulk crystals were proved to have high crystallinity. It will improve the quality of nitride-based optoelectronic devices, if these bulk crystals are used as substrates for homoepitaxial growth. The size of the bulk GaN, however, is not large enough at this moment, and enlargement of bulk GaN may be necessary. 

Epitaxial growth was carried out by modifying the sublimation method, the so-called sandwich method [8,56-60]. In this method, source and substrate are separated by a small gap of about 1 mm. The growth is carried out close to equilibrium in this arrangement. In this method, the substrate is thermally etched before sublimation begins. This makes it possible to grow, at low rates, thin homo-epitaxial layers on 6H and 4H a-SiC with residual impurity concentration. This method is used for device fabrication. 

A new approach to the directed crystallisation of silicon carbide has been proposed by Vodakov and Mokhov [6]. Their idea was to exclude the conditions which could permit any uncontrolled nucleation. They employ a nearly flat source positioned close to the substrate and perform the growth under near-equilibrium conditions. This has ensured a high quality of the grown material. The method was named the sublimation sandwich method and it appeared to be very effective. 

The studies of growth by the sandwich method have provided a better understanding of the sublimation growth peculiarities and they have formed the basis of the new approach to the bulk crystal growth of silicon carbide. The first successful results in this direction were reported by Tairov and Tsvetkov [7,8]. Currently, similar studies are being performed by a number of research groups and rather impressive progress has been achieved thus far see Datareview 8.1. 

Fig. 4.33. Experimentally measured rate of growth of the delamination along the TiN-Si02 interface in 45% relative humidity laboratory air environment as a function of the energy release rate Q for the symmetric multilayer structure shown in Figure 4.32 with the thickness of the Al—Gu layer of 0.65 um. The thin film multilayer structure, sandwiched between two identical Si layers, was tested for interface delamination growth using a symmetric four-point flexure test method, similar to that shown in Figure 4.31. Adapted from Dauskardt et al. (1998).

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