Chapter 7 Conclusions and Prospects
7.1 Conclusions
In this dissertation, high-quality c-oriented ZnO film has been epiaxially grown by utilizing PLD on the sapphire (0001), and Si (111) substrates with a nano-thick γ-Al2O3 or Y2O3 buffer layer, respectively. XRD results show a 30° offset between the {2020} reflections of ZnO and sapphire verifies the in-plane epitaxial relationship of [1010] sapphire || [1120] ZnO and [1120]sapphire || [0110]ZnO; the great disparity of X-ray diffraction line widths between the normal and in-plane reflections reveals the specific threading dislocation (TD) geometry of ZnO. The calculated TDs densities from XRD and TEM indicate most TDs are pure edge dislocations. From a combination of scattering and microscopic results, it is found that the TDs are not uniformly distributed in the ZnO films, but the ZnO films consist of columnar epitaxial cores surrounded by annular regions of edge threading dislocations at a large density. The shift of flatband voltage and the raise of potential barrier at the aggregation of TDs observed by scanning capacitance microscope and conduction atomic force microscope were attributed to the interface trap densities caused by the existence of high-density edge threading dislocations. On the other hand, because the distribution of the screw TDs is much less than that of the edge TDs, we cannot identify the location of the screw TDs and their electrical properties.
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The structural analysis of c-oriented ZnO epitaxial films on Si(111) substrates with a thin γ-Al2O3 buffer layer reveals that epitaxial γ-Al2O3 buffer layer consists of two (111) oriented domains rotated 60o from each other against the surface normal and the in-plane epitaxial relationship among ZnO layer, γ-Al2O3 buffer and Si buffer follows
2 3 2 3
(1 010 )Z nO || {22 4}γ−A l O or {4 2 2}γ−A l O || {2 2 4}S i . Studies on the crystalline quality and optical properties of ZnO epi-layers by XRD and PL measurements clearly indicate the intensity ratio of deep-level emission (DLE) to near-band edge emission (NBE) of ZnO films correlates with the width of φ−scan across off-normal reflection and the NBE linewidth is strongly dependent on the width of ZnO (0002) rocking curve. These observations manifest that the (IDLE/INBE) ratio is dominantly affected by edge TDs and the line width of NBE emission is mainly related to screw TDs.
Both high-quality structural and optical properties of ZnO epi-film on Si (111) substrates using a nano-thick high-k oxide Y2O3 buffer layer was verified by XRD, TEM, and PL measurements. The nano-thick Y2O3 epi-layer serves not only as a buffer layer to ensure the growth of ZnO epi-film of high structural perfection but also as an insulator layer between ZnO and Si. Determined by XRD and TEM the
epitaxial relationship between ZnO and Y2O3 follows
3
oxygen (O) sub-lattice in Y2O3 and the interfacial structure can be well described by domain matching epitaxy with 7 or 8 ZnO {1120} planes matching 6 or 7 {440} planes of Y2O3; the large lattice mismatch is thus accommodated by the misfit dislocations (MDs) 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.
Superior photoluminescence were obtained even for ZnO-films as thin as 0.21μm.
Our results demonstrate that the Y2O3 layer well serves as a template for integrating ZnO based optoelectronic devices with Si substrate.
In all three studied systems, the lattice of ZnO is always aligned with the hexagonal O sub-lattice in the oxide layer underneath. The lattice constant ao of 2D hexagonal oxygen sub-lattice are 2.75, 2.80, 3.75 Å for sapphire, γ-Al2O3 and Y2O3, respectively. As compared with the lattice constant a of ZnO (3.249 Å), compressive strain along in-plane direction is expected for ZnO epi-film grown on sapphire (0001) and γ-Al2O3 (111). In contrast, the expected lateral strain is tensive for ZnO epi-film on Y2O3 (111). However, compressive lateral strain is only observed for ZnO epi-layers grown on sapphire. On both γ-Al2O3 and Y2O3 buffer layers, ZnO epi-films bear tensile strain. In fact, for ZnO epi-film grown on Si(111) using other oxide buffer layers, including Gd2O3, and Y2O3 doped HfO2, all ZnO epi-film suffers tensile strain along in-plane direction. Moreover, high density of MDs at
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ZnO/oxide-buffer interface should accommodate most of the strain caused by lattice mismatch. It is noted that the thermal expansion coefficient of ZnO (α ~ 4-6.5 × 10-6 K-1) is less than that of sapphire (8 × 10-6 K-1) but larger than that of Si (2.6-3.6 × 10-6 K-1). The trend agrees with the observed strain state of ZnO layer grown on sapphire and Si. This observation strongly suggests that the strain of the ZnO-epi layers is dictated by the thermal stress built up during the post-growth cooling. Because of the nano-thickness of the employed buffer layers, the influence coming from the buffer is negligible in these cases.
From these studies, we concluded that the major structural defect for ZnO epi-films on sapphire(0001), γ-Al2O3/Si(111), or Y2O3 /Si(111) substrates is TDs.
Both XRD and TEM results indicate most TDs belong to pure edge dislocations.
Table 7-1 summarizes the influence of the two types of TDs on the electrical and optical properties of ZnO epitaxial films. As for the pratical applications of ZnO-based photoelectronic devices, the ZnO/sapphire(0001) system is still the better choice for LED because of low TDs density and high otpical performance.
However, considering the cost and the potential of integrating with well-established Si electronics for active and inactive photoelectronic devices on one chip, the depositon of ZnO epi-fim on Si has its unique merit. The ZnO grwon on Si(111) with a Y2O3
buffer layer is the more favroable choice than using a γ-Al2O3 buffer layer because the
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better crystalline quality and otpical performance.
Recently, the selective growth methods, such as epitaxial lateral overgrowth [1]
and Pendeo epitaxy [2], where epitaxial layer is deposited on the pattern substrate, have attracted much attention becasue of the effective reduction of TDs. Therefore, desirability of deposited ZnO on patterned substrates will be an important approach to further eliminate TDs for the future applications of ZnO thin films.
Table 7-1. The influence of TDs on electrical and optical properties of ZnO epitaxial film in this studies
TDs type Electrical property Optical property Screw TDs undetermined Degradation of PL width
at NBE
Edge TDs
Extra negative charge due to Dit, nonconductive, lower carrier concentration
Enhancement of PL intensity at DLE
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