For systems with such a large lattice mismatch, the well established lattice matching epitaxy (LME), where films grow by one-to-one matching of lattice constants or pseudomorphically across the film-substrate interface, is not the
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favorable mechanism. Instead, domain matching epitaxy (DME) [7], whereintegral multiples of lattice planes containing densely packed rows are matched across the interface, provides a nice description of the interfacial structure of these systems.
Fig. 6-3 (a) Schematic of atomic arrangement of O sub-lattice in Y2O3 (111) planes, where the filled circles are O atoms and the open circles denote O vacnacies.
The dashed arrows are (111) projection of the basis vectors of Y2O3 cubic lattice. (b) Illustration of the lattice alignment of ZnO basal plane (small hexagon) with O sub-lattice in Y2O3 (large hexagon).
The planar spacing ratio of (1120)ZnO to parallel (440)Y2O3, which coincides with the (1120) planes of O sub-lattice in Y2O3, 1.6292/1.875 falls between 6/7 and 7/8;
this implies a matching of 7(8) planes of ZnO with 6(7) planes of Y2O3 across the
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interface along this direction. The large lattice mismatch is thus accommodated by the misfit dislocations localized at the interface with a periodicity of 6(7) times of
3
) 2
0 4 4
( YO inter-planar spacing, leading to a significant reduction of residual strain down to ~1%. To verify this interfacial structure, we performed cross-sectional TEM measurements. Figure 6-4(a) is the TEM micrograph along [112]Si projection which shows atomically sharp ZnO/Y2O3 and Y2O3/Si interfaces; no intermediate reaction layer is observed in both interfaces. The periodic contrast variation along the ZnO/Y2O3 interface with an average spacing of ~1.2 nm found in the high resolution TEM images, shown in Fig. 6-4(b), was attributed to the misfit dislocations induced strain field. The nearly periodically arranged extra (1120)ZnO half planes with a spacing of 6 or 7 (440)Y2O3 planes are clearly seen in the Fourier filtered image shown in Fig. 6-4(c); this confirms the DME of ZnO on Y2O3 (111).
To further characterize the crystalline quality of the grown film, θ-rocking curves and radial scans of ZnO normal reflections (000n) with n = 2 and 4 and in-plane reflections (n n0 0) where n = 1, 2 and 3 were measured. Plotting the rocking curve width Δθ versus the diffraction order n, an analog to the Williamson and Hall plot, we obtained the tilt and twist angle of the film to be 0.27° and 0.52°, respectively. The line widths of the radial scans across ZnO surface reflections yield its lateral domain size of ~110 nm. It is interesting that the twist angle and domain size of the ZnO
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layer are significantly better than that of the Y2O3 buffer layer, 0.67° and 20.5 nm, even though we do see the positive correlation between the structure perfection of buffer layer and that of the ZnO layer. Be aware that both the crystalline quality and optical properties of ZnO epi-layers are known to be significantly improved with increasing film thickness [8]. For a film as thin as 0.21 μm, the obtained tilt angle approaches that of ZnO epi-films of similar thickness grown on c-plane sapphire (0.05°) and its twist angle is even smaller than the later films (0.58°) [9], demonstrating the high crystalline quality of the ZnO films grown on Y2O3.
Fig. 6-4 (a) Cross-sectional TEM micrograph recorded along [112]Si projection.
The high resolution image of the ZnO/Y2O3 interface is shown in (b). The Fourier filtered image of the area enclosed by the dashed rectangle in (b) is displayed in (c), on which the number of(440)Y O2 3 planes between adjacent extra (1120)ZnO half planes are marked below.
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6.4 Photoluminescence
We performed PL measurements at both 300 K (RT) and 13 K (LT) to examine the optical performance of the 0.21 μm thick ZnO films. The RT-PL spectrum, shown in Fig. 6-5(a), exhibits a very weak deep-level emission (DLE) near 2.2 eV and a narrow near-band edge (NBE) emission at 3.296 eV, that is dominated by the free exciton emission. Both low DLE signals as well as the narrow and intense NBE emission are signatures of good optical performance. In comparison with the ZnO films grown on Si using other buffer layers such as LT-ZnO (NBE FWHM/film thickness: 130 meV/0.7 μm) [10], ZnS (>100 meV/0.35 μm) [11], and Mg/MgO (95
meV/1 μm) [12], our ZnO film exhibits equally good or even better optical quality (100 meV/0.21 μm). Its performance is also comparable to ZnO films grown on c-plane sapphire (117 meV/0.4μm) [13] and CaF2 (~98 meV/1.3μm) [14].
Fig. 6-5 PL spectra of the ZnO film on Y2O3/Si (111) measured at 300 K (a) and 13 K (b). The inset is the extended spectrum of NBE emission in (b).
Figure 6-5(b) illustrates the LT-PL spectrum and the enlarged spectrum of the NBE region together with the peak assignment is shown in the inset. The peak for NBE emission shifts to the low energy side from LT to RT, shown in Fig. 6-5, is due to the decrease of the band gap caused by the change of lattice constants or interaction with phonon [15, 16]. The dominant LT luminescence line at 3.358 eV with a FWHM of ~9.1 meV and the lines around 3.368 eV were ascribed to the recombination of A-exciton bound to the neutral donors, Do2XA and Do1XA, respectively. By fitting the temperature-dependent intensity variation of the free A-exciton (FXA) line at 3.371 eV using the Arrhenius expression [17], we obtained
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the binding energy of A-exciton 56.57 ± 6.53 meV, in good agreement with the 60 meV for bulk ZnO crystal. The other strong line at 3.329 eV (Do2XA)2e originates from the transitions involving radiative recombination of an exciton bound to a neutral donor (Do2XA) and leaving the donor in the excited state, also known as two-electron satellite (TES). We made such assignment based on the ratio of donor binding energy to exciton binding energy ~ 0.34 as reported by Teke et al [18]. The FXA, Do2XA and TES emission accompanied with single phonon (FXA-1LO, Do2XA-1LO and TES-1LO) replica were observed at 3.301, 3.288 and 3.26 eV, respectively, and the peak at ~3.22 eV is the donor-acceptor pair (DAP) transition.
These results demonstrated the superior optical properties of the ZnO thin film on Si using a Y2O3 buffer layer.
6.5 Summary
In conclusion, high quality c-plane ZnO epitaxial films have been successfully grown by pulsed-laser deposition on Si (111) substrates with a thin Y2O3 buffer layer.
Two (111) oriented domains with 180° in-plane rotation exist in the Y2O3 buffer layer and the B-type orientation domain prevails over the A-type one. The in-plane epitaxial relationship between the wurtzite ZnO and cubic Y2O3 follows
2110 ZnO|| 101 Y O2 3
< > < > . The growth of ZnO on Y2O3 can be well described by
124
125
domain matching epitaxy. The photoluminescence spectra of ZnO epi-films exhibit superior optical properties at room temperature even for films of thickness as thin as 0.21 μm. Our results demonstrate that the Y2O3 layer well serves as a template for integrating ZnO based optoelectronic device with Si substrate.
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