Sapphire dislocation density

One of the major hurdles in the epitaxial growth of high quality GaN thin films is the unavailability of suitable substrates. The lack of suitable substrates leads to poor quality epitaxial films with dislocation densities in the range of 1010-1012 cm-2 for GaN grown on sapphire [1,2] and SiC [3]. Such high defect densities are detrimental to the device performance conventional III-V (like GaAs) devices have defect densities about four orders of magnitude lower than presently achievable in GaN. 

Heteroepitaxial growth of GaN is usually performed on sapphire or SiC (13 % and 3.4% lattice mismatch, respectively, with GaN). Such a lattice mismatch between GaN and these substrates results in a high dislocation density in the epitaxial films. A variety of techniques have been employed in the past to reduce this high dislocation density and one of the common methods has been to engineer the substrate surface to control, and thus inhibit, the formation of threading dislocations. 

S. Sakai, T. Wang, Y. Morishima and Y. Naoi, A new method of reducing dislocation density in GaN layer grown on sapphire substrate by MOVPE , J. Cryst. Growth, 221, 334-337 (2000). 

Figure 4.51 indicates that the yielding in sapphire undergoing basal slip is a consequence of dislocation multiplication and is not due to the unpinning of a Cottrell-type atmosphere, where dislocation pinning results from impurities. The study of two types of sapphires, with different initial dislocation densities, was meant to point out the difference in their surface dislocation densities and the consequent differences in their yield phenomena. 

In this section, as examples of nonpolar GaN on lattice-mismatched substrates, the surface morphology and microstructure of a-plane GaN on an r-plane sapphire substrate and m-plane GaN on m-plane 4H-SiC are presented. Next, the SELO method of reducing threading-dislocation and stacking-fault densities is described in detail. This is followed by a description of the properties of the conductivity control of n-type andp-type nonpolar GaN, and the growth of the heterostructure/quantum well structure. Finally, the performances of the violet and green LEDs on nonpolar GaN are discussed with respect to the threading-dislocation density dependence of the output power. 

Table 5.1 Summary of surface roughness, threading-dislocation density (TDD) and stacking-fault density (SFD) of template and SELO-grown nonpolar GaN on r-plane sapphire and m-plane 4H-SiC.

To reduce the strains and dislocation density in epitaxial ZnO and related films, closely lattice-matched substrates are favored for growth. Sapphire substrates are commonly used for ZnO heteroepitaxial growth, primarily on the (0001) orientation (basal or c-plane), and also on the (11 20) o-plane. In addition, ZnO and related oxides have been grown on Si [20], SiC [39], GaAs [21, 22], Cap2 [19], and ScAlMg04 [23]. Lattice parameters of several substrate materials frequently used for ZnO growth and their mismatch to ZnO are listed in Table 2.3. 

The dislocation density within the first 0.5 xm of the GaN film on the vicinal 6H-SiC(0001)si substrate was approximately 1x10 cm", as determined from initial plan view TEM analysis by counting the number of dislocations per unit area. This value is approximately an order of magnitude lower than that reported (43) for thicker GaN films deposited on sapphire(OOOl) substrates using low-temperature buffer layers. The dislocation density of the GaN film deposited on the vicinal 6H-SiC(0001)si substrate decreased rapidly as a function of thickness. In contrast, the on-axis wafers had less step and terrace features thus, the HT-AIN buffer layers on these substrates were of higher microstructural quality with smoother surfaces and fewer inversion domain boundaries. Consequently, the microstructural quality of the GaN films were better for on-axis growth as shown by the DCXRC data noted below. 

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