Molecular beam epitaxy (MBE) was adopted to grow zinc oxide (ZnO) film on sapphire substrate and improve the quality of ZnO thin film epitaxy using a magnesium oxide (MgO) buffer layer and a two-segment temperature scheme for ZnO thin film growth. The influence of thermal annealing of different layers on the optical and crystalline features, stress expression, as well as surface morphology of ZnO thin film was examined. SEM images showed smooth surfaces were formed, and these surfaces allowed the low-temperature ZnO buffer layer to have better epitaxial environment at the very beginning. X-ray diffraction (XRD) analysis revealed that a lower thermal annealing temperature more effectively released the stress of materials. The thermally annealed MgO buffer layer had 26% less strain than the purely thermally annealed, high-temperature-grown ZnO (HT-ZnO), and 33% less strain than the unannealed samples. Atomic force microscopy results showed that the root-mean-square of surface roughness of thermally annealed MgO was 0.255 nm, which was 80% lower than that of thermally annealed HT-ZnO (1.241 nm). Photoluminescence measurement showed that the thermally annealed MgO buffer layer had the highest strength for near-band-edge emission because of improved crystalline quality. HRTEM results showed that the stress caused by the mismatch between the sapphire lattice was effectively released because the MgO buffer layer was annealed at a high temperature. The surface of the MgO buffer layer became smooth and the stress mismatching with the ZnO lattice did not obviously extend upwards. When MBE was used to grow ZnO thin film, a lower thermal annealing temperature for the MgO buffer layer more effectively controlled stress accumulation and produced high-quality ZnO thin film.
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The extraordinary properties of zinc oxide (ZnO) in the ultraviolet region of the electromagnetic spectrum have made this compound a research hotspot in recent years. For example, Zhu et al.  found the optimum conditions for MOCVD to grow ZnO on sapphire by controlling the concentration of hydrogen and growth temperature. They discovered that low-concentration hydrogen and high reaction pressure can produce ZnO with smoother surface, bigger crystalline grains, and near-band-edge emission. Samanta et al.  applied oxygen partial pressure and annealing to improve p-type Sb-doped ZnO thin film, achieve high conductivity, and reduce defects in thin film. Oyola et al.  adopted the reactive evaporation method to produce large ZnO transparent conducting oxide film for solar cells. Medina-Montes  also used ZnO thin film as the channel layer on thin film transistors to improve its efficiency. The use of ZnO/ZnMgO quantum wells with graded barriers  had also been reported for achieving improved electron-hole wave function overlap for suppressing the charge separation in polar quantum wells, which are similar to the concept widely studied and demonstrated in the InGaN QWs with large overlap designs [6–8]. Indeed, the application of ZnO for material growth and assembly application is important and widely used. In addition to the ZnO-based approach, significant advances have been reported for addressing high performance mid–deep UV sources by using AlGaN-based [9–12] and AlInN-based [13,14] material systems.
Sapphire substrate has the advantages of low cost, large wafer size, and high quality, and is thus the best substrate for ZnO thin-film growth . However, ZnO growth on sapphire often has two problems. First, the lattice constant of ZnO, which has a hexagonal wurtzite structure, is a = 3.249 Å, and the lattice constant of sapphire is a = 4.76 Å. This 18% mismatch between the two materials results in a high-intensity dislocations during ZnO thin-film growth. Second, ZnO and sapphire substrate largely differ in their thermal expansion coefficients (αZnO = 6.51 × 10−6 K−1 vs. αsapphire = 8 × 10−6 K−1). Strain results from high-temperature ZnO epitaxy. Two methods of solving these problems to obtain high-quality ZnO thin film have been reported. First, Chen et al.  grew ZnO thin film using a two-segment temperature scheme. In the first segment, when the epitaxy turns into a nucleation layer, a low temperature is adopted to grow a ZnO buffer layer (LT-ZnO) and reduce the axial growth rate. Consequently, a neat arrangement of crystalline grain is produced in the early growth period of ZnO thin film. The second segment adopts a high temperature to grow ZnO layer (HT-ZnO) in which ZnO grows with a neat arrangement along the nucleation layer and the axial growth rate increases, thereby enlarging the crystalline grain of ZnO thin film, making the crystallinity direction more consistent, and improving further the quality of ZnO thin film. A recent works by the authors related to the growths of ZnO by MBE with two-step temperature variation growth of ZnO buffer layers was recently published . The second reported method improves the lattice-constant mismatch between ZnO and substrate through a buffer layer. Gallium nitride (GaN)  and magnesium oxide (MgO)  are often used as buffer layer materials for the epitaxy of ZnO thin layer on sapphire substrate. GaN and ZnO both have wurtzite structures. Their lattice constants have only 1.8% mismatch (aGaN = 3.189 Å), and their thermal expansion coefficients are also close (αGaN = 5.59 × 10−6 K−1) . Hence, they are suitable buffer layer materials for ZnO. When ZnO grows on sapphire substrate, the lattice-constant mismatch causes high-intensity dislocations. The lattice constant of ZnO is smaller than that of sapphire substrate. Consequently, tensile stress is produced during epitaxy and this stress produces cracks on the ZnO thin-film surface. The tensile stress transforms into compressive stress by adding a GaN buffer layer whose crystalline constant is slightly smaller than that of ZnO, which solves the problem of cracks on the thin-film surface and reduces the intensity of dislocations . Ting et al.  pointed out that an MgO buffer layer can more effectively reduce the intensity of dislocations than a GaN buffer layer. When the MgO buffer layer is used, tensile stress transforms into extremely small compressive stress, which reduces the roughness of ZnO thin film by 58%. Chen et al. [15,23,24] adopted a thin MgO buffer layer to improve the poor quality of ZnO thin film resulting from dislocations. When MgO film is extremely thin, the upper-layer ZnO is subjected to layer-by-layer two-dimensional growth such that dislocations do not extend along the C-axis. Experiments have revealed that ZnO dislocations are only concentrated within 20 nm of the ZnO/MgO junction, which improves the dislocation intensity of the epitaxy layer. The MgO buffer layer mainly has two mechanisms. First, when MgO grows at high temperatures (750–850 °C), MgO interacts with the sapphire substrate, Mg2+ and Al3- diffuse to form a spinel structure, and the mismatch among MgAl2O4, MgAl2O4, and sapphire substrate is 2.6%. The mismatch between MgAl2O4 and MgO is 5.6%, dispersing an 18% mismatch between the original ZnO and sapphire substrate, and the remaining 9.2% mismatch occurs at the ZnO/MgO junction. Experiments have shown that the dislocation intensity can be reduced five times after adding an MgO buffer layer . Second, when the growth temperature is low (~600 °C), the MgO lattice has a stereo structure, with the mismatch between MgO and sapphire substrate being 9.1% and that between ZnO and MgO being 8.4%. Hence, the lattice mismatch influence is reduced and the crystallinity quality of ZnO is improved. Moreover, the polarity of the ZnO epitaxy layer can be controlled through the depth of the MgO buffer layer. When the MgO layer is thin, ZnO has O polarity. With increased MgO depth, ZnO changes to Zn polarity and the growth rate increases twofold . The optimal MgO buffer layer is about 8 nm thick . If two MgO/LT-ZnO layers are added, the dislocation intensity is further reduced by 30% .
Annealing is a material-processing technology for metal smelting that has extensive applications. The underlying principle is the removal by heat energy of some defects that cause internal stress inside objects. In the semiconductor industry, mixing foreign substances such as boron, phosphor, or arsenic into semiconductors results in a chaotic arrangement of atoms and an abrupt change in semiconductor materials. Hence, semiconductors require annealing to restore the crystalline structure and remove defects, which also helps move foreign atoms from an interstitial position to a displacement position. The applied energy increases the vibration and diffusion of lattice atoms and defects in objects, and then resort the arrangement and combination of atoms. Consequently, objects can be recrystallized or even become monocrystals after the defects disappear. Fang et al. [27,28] pointed out that the ZnO thin film helps reduce defects in ZnO thin film because the energy provided by a high annealing temperature increases the migration rate of atoms and further reduces the occurrence of defects. Atoms have sufficient energy to move to the correct lattice position, and large crystalline grains are produced because of the low surface energy. Given that the MgO buffer layer and sapphire have a 9.1% lattice mismatch, an MgO layer full of dislocations and rough MgO surface is produced, thereby affecting the growth of MgO thin film. Setiawan et al.  pointed out that high-temperature annealing of an MgO buffer layer can effectively release the strain of the MgO buffer layer to achieve low surface energy, strengthen the migration of surface-adsorbed atom by annealing the MgO buffer layer, and make the surface smoother. Consequently, the dislocation intensity of the ZnO layer is reduced by up to 65% and the growth of high-quality ZnO thin film is benefited.
The growth of ZnO thin film has been extensively studied in recent years. Annealing the thin film or buffer layer can release stress and obtain high-quality thin film. However, only a few studies have compared the difference between the releasing stress of annealing the thin film and buffer layer. In this paper, molecular beam epitaxy (MBE) was adopted to grow ZnO thin film on sapphire substrate and anneal different buffer layers. Then, the optical features of ZnO were observed through heterothermal photoluminescence (PL), X-ray diffraction (XRD), and rocking curve on HT-ZnO, LT-ZnO, and MgO layers to calculate the crystalline quality and stress change of ZnO. The influence of stress on the thin-film surface was also examined by scanning electron microscopy (SEM) and atomic force microscopy (AFM) to understand the influence of layered annealing on ZnO epitaxy quality by comparing the annealing of different buffer layers.
2. Growth conditions and structures of samples
A control sample, sample A, was prepared without any thermal treatment. First, an 8 nm MgO buffer layer was grown on c-plane sapphire substrate at 550 °C with an Mg target temperature of 380 °C and O2 flow rate of 3 sccm. Then, MBE was used to grow an LT-ZnO layer at a temperature of 300 °C, a Zn target temperature of 300 °C, an O2 flow rate of 3 sccm, and a thickness of 6 nm. For the HT-ZnO layer, the growth temperature was 600 °C, the Zn target temperature was 320 °C, the O2 flow rate was 1.9 sccm, and the thickness was 150 nm. Samples B, C, and D were prepared similarly as A under the following conditions. Sample B only had one layer annealed, and HT-nZnO was annealed for 20 min at 950 °C. Sample C had two layers annealed; LT-ZnO was annealed for 10 min at 780 °C and HT-nZnO was annealed for 20 min at 950 °C. Sample D had three layers annealed, and MgO was annealed for 10 min at 950 °C. LT-ZnO was annealed for 10 min at 780 °C and HT-nZnO was annealed for 20 min at 950 °C. Figure 1 summarizes the layer structures of the samples. Table 1 lists the details of the growth of the four samples and the layer-by-layer annealing parameters.
3. SEM and AFM images
Figures 2(a) and 2(b) are the front and side SEM views of sample A (reference sample), respectively. The surface of sample A had many holes because ZnO thin film was not annealed and the thin film was not recrystallized to improve the epitaxy quality, and also because of stress at the bottom of the MgO buffer layer. Figures 2(c) and 2(d) are the front and side SEM views of sample B, respectively. The sample surface became smooth because the HT-ZnO thin film was annealed at a high temperature. Sample B had an apparent hexagonal facet. This observation can be due to the low-temperature ZnO buffer layer being too thin, which resulted in the density of the LT-ZnO seed layer being too low and the grains incompletely joined with adjacent grains to form a hexagonal shape. This finding was similar to that using GaN [30,31]. Figures 2(e) and 2(f) are the front and side SEM views of sample C, respectively. Some parts of the surface became rough possibly because the low-temperature ZnO buffer layer improved the size of the crystalline grains after annealing and the HT-ZnO surface became rough. Figures 2(g) and 2(h) are the front and side SEM views of sample D, respectively. A mirror-like surface possibly caused by the stress release of the MgO layer after annealing was observed. Hence, a smooth surface was formed, and this surface allowed the low-temperature ZnO buffer layer to have better epitaxial environment at the very beginning. Consequently, the MgO stress was released after high-temperature annealing to improve the epitaxy quality of HT-ZnO. Figures 3(a) –3(d) show the measurement results of the AFM of samples A to D. The root-mean-square (RMS) values of surface roughness were found to be 1.330, 1.241, 2.550, and 0.255 nm, respectively. As shown in the AFM images in Fig. 3(d), sample D had a terrace-like surface similar to a previous result . A comparison of the AFM images revealed that the surface evenness of sample D was 80% lower than that of sample A. Thus, annealing of the MgO buffer layer was crucial to the growth of high-quality ZnO thin film.
4. XRD and XRC measurements
Figure 4(a) shows the XRD measurement results of samples A to D. The peak at about 34.5° was the diffraction peak of ZnO (002). The annealing of different layers induced changes in the crystalline lattice constant of HT-ZnO, which changed the compressive stress or tensile stress in ZnO thin film. The ZnO (002) diffraction peak of samples A-D had a slight shift. When the distance d between lattice planes narrowed, the XRD peak shifted toward a high angle. When the distance d between the lattice planes widened, the XRD peak migrated toward a low angle. Figure 4(b) shows the measurement result of the ZnO (0002) peak rocking curve, and the full-width at half-maximum (FWHM) at the arrow was quite narrow. This finding indicated that the samples had a certain crystalline quality. To examine further the stress and strain endured by the materials as well as the possible grain size and lattice constant, calculations using the Sherrer equation (Eq. (1)) were made [33–36]:
Here D, λ, β, and θ are the grain size, beam wavelength (0.15405 nm), FWHM, and diffraction angle of the XRD measurement, respectively.
Here dhkl is the interplanar distance and h, k, l are the Miller Indices.
Table 2 shows the material parameters of ZnO thin-film samples obtained from XRD measurement. For sample A, the lattice constant of the MgO buffer layer was greater than that of ZnO (aZnO = 3.249 Å vs. aMgO = 4.13 Å); thus, ZnO was subjected to a tensile stress of 2.527 Gpa and strain of 42.278 microstrain when no thermal treatment was performed. Sample B was the HT-ZnO layer annealed for 20 min at 950 °C. It showed a stress drop from 2.527 Gpa to 1.424 Gpa caused by the high-temperature annealing, a released tensile stress of 44%, and a strain drop to 38.046 microstrain. Sample C comprised a low-temperature ZnO buffer layer annealed at 780 °C and an HT-ZnO annealed at 950 °C. The tensile stress of MgO was released because the low-temperature ZnO buffer layer was annealed. The low-temperature ZnO epitaxy layer did not effectively serve as the buffer layer because the seed layer grains did not completely join with adjacent grains given its 6 nm thickness . However, high-temperature annealing still effectively releases the stress of MgO, and the tensile stress of sample C was found to change into a compressive stress of −2.875 Gpa with a strain drop of 35.869 microstrain compared with sample B. Sample D comprised an MgO buffer layer annealed at 950 °C, a low-temperature ZnO buffer layer annealed at 780 °C, and a thin film annealed at 950 °C. Calculations showed that MgO stress was released because the MgO buffer layer was annealed at high temperature. Consequently, the tensile stress of LT-ZnO and HT-ZnO dropped and the stress of sample D to decrease to −1.307 Gpa, which was 55% lower than that of sample C, approaching stress-free ZnO. This sample showed a rather complete release of stress because two buffer layers and thin film had been annealed at a high temperature. Moreover, the rocking curve FWHM was 71.65 arcsec, which was 34% lower than sample A. These data showed that the annealing of the MgO buffer layer was crucial to the release stress of ZnO thin film.
5. Temperature-dependent PL measurements
Figure 5 shows the heterothermal PL measurement results of samples A to D. As shown in the top left graph, the samples in this experiment had no apparent illumination from defects, indicating that all samples reached a certain epitaxy quality. The heterothermal PL measurement showed that the bandgap energy decreased with increased temperature, causing a common band-gap shrinkage. Figure 6 shows the integrated PL intensity ratio and PL peak positions of the samples as functions of temperature. The solid line is normalized integrated intensity. The normalized ratio of the integrated intensity of the near-band-edge emission (NBE; 2.91–3.76 eV) and that at 10 K were used as the basis for evaluation. NBE is the emission of various exciton recombinations in ZnO, such as donor's bound exciton, receptor's bound exciton, free exciton, etc. A strong NBE intensity indicated that the emission of various excitons was safer. A higher ratio resulted in better quality of ZnO. In general, the optical quality of sample D was higher than that of the other samples. The dotted line shows the relationship between the temperature and bandgap energy. The graph shows that the bandgaps of samples B to D had red shifts compared with sample A. When studying the growth of ZnO thin film on glass substrate, R. Hong  mentioned that when the compressive stress changes to tensile stress, a blue shift results because of the accumulation of tensile stress. In this study, the NBE at room temperature shifted from 3.27 eV to 3.25 eV, which also proved that it resulted from the release of tensile stress and the transformation into compressive stress, consistent with the aforementioned XRD results. Figure 7 shows the relationship between the grain size and FWHM of NBE. The PL FWHM decreased with increased grain size of samples A to D because sufficient energy was provided after each layer was annealed, inducing the atoms to move to stable regions and foreign substances to move to the grain boundary. Consequently, the defect density decreased . In addition, the epitaxy layer had fewer defects caused by stress when the stress was released because of the high-temperature annealing. Hence, the grain size of the sample increased and the optical quality improved because of the fewer defects.
6. Interface characteristics by high-resolution transmission electron microscopy
Figure 8 shows the transmission electron microscopy (TEM) images of samples A to D. Taking sample A as an example, apparent dislocations were observed to extend from LT-ZnO because no thermal treatment was performed and the surface was rough, consistent with the aforementioned AFM measurement results. Sample B showed lower intensity of dislocations because of certain stress release after HT-ZnO was annealed. Samples C and D showed no apparent dislocations at HT-ZnO. Figure 9 shows the HRTEM images of joints formed by the sapphire substrate with the MgO buffer layer and LT-ZnO buffer layer in samples A to D. The interface between the MgO buffer layer and LT-ZnO buffer layer in samples A to C had great stress and strain, which affected the growth of the top-layer LT-ZnO buffer layer. In particular, Fig. 9(c) clearly shows that the rough surface of MgO affected the growth of the top-layer LT-ZnO buffer layer, and LT-ZnO surface layer was not affected by stress with annealed at a high temperature. Figure 9(d) clearly shows that the stress caused by the mismatch between the sapphire lattice was effectively released because the MgO buffer layer was annealed at a high temperature. The surface of the MgO buffer layer became smooth and the stress mismatching with the ZnO lattice did not obviously extend upwards. In addition, the low-temperature buffer layer was annealed at a high temperature, and sample D achieved better epitaxy quality and smooth surface.
MBE was adopted to grow ZnO thin film on sapphire substrate and reduce dislocations caused by lattice mismatch using an MgO buffer layer and a two-segment temperature scheme for ZnO thin film growth. The stress between layers was released through layer-by-layer annealing to improve the quality of ZnO thin film. XRD analysis revealed that the crystalline quality of samples A to D decreased as D > C > B > A. After annealing of the MgO buffer layer, the XRC FWHM of sample D became 34% less than that of sample A that was not annealed. This finding was due to the fact that LT-ZnO and HT-ZnO had good epitaxy environments, and the MgO buffer layer had a smooth surface layer and lower stress after annealing. PL measurement results were consistent with the XRD results, and sample D had the optimum optical properties. SEM and AFM indicated that HT-ZnO thin film had a smooth surface after annealing; stress was released and transformed into compressive stress, and HT-ZnO became rougher after LT-ZnO was annealed. When the MgO buffer layer was also annealed, the tensile stress of MgO was significantly reduced. Consequently, HT-ZnO obtained an even surface. Compared with sample A, AFM RMS was 80% less than that of sample A (without annealing). Therefore, annealing of the MgO buffer layer was crucial to controlling stress accumulation and preparing high-quality ZnO with a flat surface.
This research was supported by the National Science Council, The Republic of China, under grants NSC 99-2221-E-194-020, 100-2221-E-194-043, and 101-2221-E-194-049, and the U.S. Air Force Office of Scientific Research, under contract AOARD-10-4049.
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