The roles of buffer layer thickness on the properties of the ZnO epitaxial films
Introduction
The development of technologies to improve the material quality is always a hot topic in the research of a specific functional material. The optimization is unlimited since defects and dislocations always exist. For the ZnO material studied in this article, the material quality is far from the device-ready level [1]. The control of defects and dislocations is always problematic. Thanks to the similarity between GaN and ZnO in terms of structure and properties, the study on ZnO always uses the successful experience of GaN for reference. For instance, the epitaxial growth of GaN on the lattice-mismatched sapphire substrates has achieved a tremendous success, which have led to the commercialization of GaN-based blue light-emitting diodes (LEDs) and laser diodes (LDs). Therefore, sapphire has been an economic and most popularly used substrate for the hetero-epitaxial growth of ZnO [2], [3].
Due to the large lattice mismatch and distinctive thermal expansion coefficient between sapphire and GaN [4], a low-temperature buffer layer is indispensably employed for the epitaxy of GaN on sapphire in order to buffer the strain and dislocations induced by the lattice mismatch [5], [6]. For the epitaxy of ZnO on sapphire, various buffer layers, like GaN, MgO, ZnS, and ScAlMgO4, have been utilized [4], [7], [8], [9]. However, the ZnO homogeneous buffer is always preferred since the epitaxial growth on an optimized ZnO homo-buffer would be quite similar as a homo-epitaxy [10].
The lattice mismatch could be released by strains and forming dislocations. It is easy to understand that, when the mismatch between buffer and substrate is sufficiently small, the buffer that can fully release the mismatch by strains should be relatively thick. On the contrary, when the mismatch is large, the strain energy would quickly reach a critical value wherein dislocations are energetically favorable to be formed. In this case, the buffer with no dislocations should be relatively thin. Therefore, a critical thickness has been defined as the thickness at which the first dislocation is formed. The existence of this critical thickness was first detailed by Van der Merwe [11], and modulated by Matthews [12], People and Bean [13]. For ZnO buffer deposited on c-plane sapphire, there is a significant lattice mismatch (18.3%) between them with a 30° domain orientation (ZnO [10]//Al2O3 [11], [12], [13], [14], [15], [16], [17], [18], [19], [20]) [14]. Utilizing the calculation method reported by Fischer et al. [15], [16], [17], the critical thickness for ZnO is estimated to be only 0.63 nm, corresponding to 1–2 layers of atoms. As a result, the formation of dislocations in the buffer is unavoidable.
It is obvious that the properties of the buffer layer will influence those of the epi-layer. Especially for the thickness, if the buffer layer is too thin, high density of dislocations and rough surface will be formed which is not ideal for the following epitaxy. While if the buffer layer is too thick, the surface is also rough and the dislocations will grow larger due to the over-matured grains and suppressed horizontal growth. Therefore, an appropriate buffer layer thickness is of great importance for the quality of the epi-layer. In fact, several research groups have done works on the optimization of the buffer thickness for further ZnO epitaxy [10], [18], [19], [20], [21], [22], [23], [24], [25]. Table 1 lists their brief results. It can be noticed that the optimized values for the thickness vary among different reports, which is possibly due to distinctive growth method and conditions. Therefore, the roles of buffer thickness on the properties of the ZnO epi-layers are still unclear and require further investigation.
In this article, the morphological, optical, structural, and electrical properties of the buffers with three different thickness and the epi-layers grown on the corresponding buffers have been extensively investigated, respectively. The surface smoothness, rather than the other properties, has been proposed as the dominate factor for the optimized growth for the ZnO epi-layer. Besides, the correlation between the formation of the morphological pits (V-defects) observed on the epi-layers and the existence of threading dislocations in the buffers has been established. The formation and properties of the V-defects have also been proposed.
Section snippets
Sample preparation
Samples discussed in this article were grown on c-plane sapphire substrate by a home-built metal-organic chemical vapor deposition (MOCVD) system with close coupled-showerhead configuration. The details of the MOCVD system have been reported and can be referenced elsewhere [26]. A four-step process for the high-temperature epitaxial growth of ZnO has been established as follows. (1) The as-received sapphire substrates were pretreated for 5 min at 1100 °C in nitrogen and hydrogen environment in
The properties of the ZnO buffers with different thickness
The thickness data for samples A1–C1 are 78, 133, and 295 nm, respectively, and the corresponding growth rates are calculated as 13, 7.5, and 9.8 nm per min. The non-constant growth rate indicates that the growth mode is not unique during the 30 min time. In order to reveal the reason for the different growth rate, the surface morphology for the buffer samples are thus measured and shown in Fig. 1(a)–(c). Fig. 1(a), representing the 5-min growth, shows the morphology just after initial nucleation
Summary
The morphological, optical, structural, and electrical properties have been shown and discussed for the ZnO buffers with three different thickness and the corresponding epi-layers grown on the buffers. The growth process of the buffers including the initial nucleation, lateral coalescence of grains, and vertical growth along grains, has been clearly explained with the help of the morphological analysis. Via the comprehensive evaluation of the properties, the epi-sample grown on the buffer with
Acknowledgments
This research was supported by the State Key Program for Basic Research of China under Grant No. 2011CB302003, National Natural Science Foundation of China (Nos. 61025020, 61274058, 61322403, 61274058, 61504057, and 61574075), the Natural Science Foundation of Jiangsu Province (Nos. BK2011437, BK20130013, and BK20150585), the Six Talent Peaks Project in Jiangsu Province (2014XXRJ001), and the Priority Academic Program Development of Jiangsu Higher Education Institutions.
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