SEMIPOLAR INTERFACES AND EPILAYER PROPERTIES
4.3 Structural properties of PAMBE-grown semipolar InN epilayers
4.3.2 Structural characterization of semipolar s-plane InN on r-plane sapphire
Figure 4.22 illustrates AFM, and HRXRD results for sample G1508. HRXRD showed the presence of semipolar (1011)as well as sphalerite cubic phases. The AFM observations showed a double domain morphology.
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Figure 4.22: (a) 5×5 µm2 AFM image of sample G1508 showing 3.34 nm rms roughness. (b) HRXRD (ω-2θ) scan showing (002) and (1011) phases (courtesy of Prof. A. Georgakilas).38
Figure 4.23(a) shows an overall image of the polycrystalline epilayer structure.
The semipolar orientation was found to exist in two variants due to the symmetry of the sapphire nucleating plane. One variant is projected along [1120]InN and the other along [2113]InN. The two variants give distinct contrast since, along the [1120]InNprojection, the c-axis is edge-on and the region is manifested by multiple SFs. In the second variant, ascending TDs are discernible. Regions without defect contrast are attributed to the cu- bic phase. It is seen that both s-plane and cubic crystallites originate from the interface with sapphire.
The direct growth of s-plane InN from r-plane sapphire is illustrated in the HRTEM image of Figure 4.23(b). This image is taken along [2021]Al O2 3z.a. and with the InN viewed along the [1120]InN. The angle that the c-axis forms with the interface con- firms the (1101)orientation of the epilayer (62o). However, the epilayer is too heavily faulted and the FFT gives a very streaky pattern (inset). The spacing between the streaks in the FFT gives the d-spacing of (1100)planes which is equal to
2
3 2 2 d= a
+ Λ , where α is the lattice constant of InN. We obtained d(1100)= 0.301 nm, thus the InN lat- tice constant was measured at α = 0.348 nm. The SFs introduce local transformations from the …ABAB… stacking towards …ABC… stacking, i.e. the epilayer can be considered as intermediate between wurtzite hexagonal and sphalerite cubic. Also big cubic pock- ets are identified inside both semipolar variants.
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Figure 4.23: (a) BF CTEM image showing the overall structure of the s-plane film. S-plane vari- ants are indicated by arrows. (b) HRTEM image along [1210] z.a. showing the interfacial region between s-plane InN and sapphire. The inset is the corresponding FFT of the s-plane material.38
Figure 4.24(a) is a HRTEM image of the InN/sapphire interface. The InN is hex- agonal viewed along [1120] z.a. Figure 4.24(b) is the corresponding lattice strain map obtained by geometrical phase analysis using the g 1014Al O2 3/ 0002InNspatial periodici- ties. From this the strain was measured to be 12.6% ± 1.5%. From the lattice strain, the c lattice parameter of InN was calculated at 0.574 nm ± 0.008 nm.
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Figure 4.24: (a) HRTEM image along [2021]Al O2 3 z.a. from the InN/sapphire interface. (b) Cor- responding (1014)Al O2 3/ (0002)InNlattice strain map obtain by GPA using a g/2 mask size.
The OR relationship between sapphire and s-plane InN, as well as the lattice con- stants of InN, have been studied in detail by electron diffraction in plan-view and in XTEM geometry.43 The OR is described as (1101)InN||(1 102)Al O2 3, [1120]InN||[2021]Al O2 3, [1102]InN~||[0221]Al O2 3, and is illustrated schematically in Figure 4.25. The misfit along [1120]InN||[2021]Al O2 3is 0.7% with respect to sapphire, taking a 2:1 coincidence be- tween lattice vectors 1/3[1120]InNand 1/3[2021]Al O2 3. That corresponds to a 1:1 bond- ing between Al atoms from sapphire, and nitrogen atoms from InN.
Figure 4.25: Schematic illustration of the OR between s-plane InN and r-plane sapphire in plan- view. The InN and sapphire unit cells are drawn side-by-side instead of being superimposed.
The matching lattice vectors at the heteroepitaxial interface are illustrated. The two orientation variants of InN unit cells that are interrelated by the (1120)glide mirror of sapphire are also indicated.44
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There is a 4.3o misorientation between [1102]InNand [0221]Al O2 3. In addition, matching of lattice vectors requires a 1:2 coincidence between [1102]InNand 1/3 [0221]Al O2 3 and the misfit is -6.1%. This misfit is relaxed by inclined single (0002)InN ex- tra half planes that lead to BSFs since the hexagonal …ABAB… stacking is disrupted, as has been described in detail previously for the case of nonpolar growth.36,45 The geo- metrically necessary BSF density is 1.9×106 cm-1, which exceeds the corresponding one of a-plane GaN by one order of magnitude. Further BSFs are introduced in order to ac- commodate the 4.3o twist angle as detailed elsewhere.43 This very high BSF density was verified by the TEM observations as can be seen in Figures 4.23 and 4.24. As shown in Figure 4.25, the two orientation variants are interrelated by the (1120)Al O2 3glide mirror.
The HRTEM image of Figure 4.26 shows the coexistence of the two variants and the continuity of lattice planes is indicated. Interestingly the relative rotation between the two variants is described by the 90o [1210] rotation which leads to a high order of shared symmetry as described elsewhere in this thesis. As described already, the ener- gies of prominent GBs have been calculated for InN using a Tersoff empirical interatom- ic potential and have been found to be lower than those of GBs in GaN or AlN.46 The lowest energy was calculated for the (0002)|| (0110)GB. This GB orientation is seen to comprise part of the interface between the two variants depicted in the HRTEM image of Figure 4.26.
Figure 4.27(a) is a HRTEM image along the [0 221]Al O2 3 z.a. If this area was hex- agonal, then the SFs would lie vertical to the c-axis, as in Fig 4.23(b). But in this image we have SFs along two orientations at ~70.5o angle. This could be possible if the SFs were on two crystallographically equivalent {111}planes of cubic InN viewed along [110]InN. This is further confirmed from angle measurement on the FFT [Figure 4.27(c)], where the 3-indices correspond to cubic InN and 4-indices denote sapphire. Also in Fig- ure 4.27(a), sapphire protrusions are denoted near the InN-sapphire interface. In Figure 4.27(b), the image of Figure 4.27(a) is filtered, so only the sapphire periodicities are present, to highlight the sapphire islands inside InN. In Figure 4.28, a large nanocrystal is delineated by a dotted line. In its base a sapphire island is also indicated. Inside the crystal and near the sapphire island, the InN is a distorted mixture of hexagonal struc- ture viewed along [1120] and cubic viewed along [110] .
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Figure 4.26: HRTEM image showing the coexistence of the two s-plane variants and parallel lattice planes.44
Figure 4.27: (a) HRTEM image of a cubic region of the InN epilayer. The {111} planes are de- noted. Sapphire protrusions are also denoted. In (b) the InN reflections have been filtered, so only the sapphire periodicities are clear. The sapphire islands now can be distinguished from the InN epilayer. (c) is the FFT diffractogram of (a). Three-indices denote cubic InN and the 4- index system is used for the sapphire.
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Figure 4.28: HRTEM image of an InN nanocrystal containing both hexagonal and cubic InN. A sapphire protrusion is also denoted at the base of the nanocrystal.