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Abstract
The optical properties and film quality for a series of high-In composition InGaN films grown on ZnO substrate by metal-organic chemical vapor deposition (MOCVD) are characterized by using high resolution X-ray diffraction (HRXRD), Rutherford backscattering (RBS), X-ray photoelectron spectroscopy (XPS), photoluminescence (PL), and Raman scattering spectroscopy (RSS). The In composition is evaluated by analyzing the RBS and PL emission spectra. The XPS measurements revealed the diffusion of Zn atoms from the substrate into InGaN films. All the analyses of experimental measurements have shown that the growth temperature played an important role in indium composition as well as of film quality. An optimum growth temperature is a necessary condition for obtaining high-quality films.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
The binary group III-Nitrides (GaN, InN, AlN) and their alloys (InGaN, AlGaN and AlInGaN) are semiconductors with spectral ranges varying from deep ultraviolet (DUV) to ultraviolet (UV) to visible and to infrared regions. These materials are the basis for many and emerging modern electronic and optoelectronic devices. For example, the 2014 Nobel Prize in Physics was awarded for the invention of “efficient blue light-emitting diodes to enable white light sources to be bright and energy-saving”. The discovery of light emitting diodes (LEDs) has been revolutionary in enabling new light sources and the optoelectronic industry based on III-Ns is rapidly developing [1–3].
The impact of GaN-based materials is not limited to optical emitters and is currently making contributions in the next generation of solar cells [4]. It is necessary to find alternative materials for photovoltaic applications due to a low efficiency of the silicon-based solar cells [5–8]. InGaN alloys are one of the promising materials for designing high efficiency solar cells therefore their material properties need to be better understood [9–15]. The primary advantage of using InGaN alloy is that its band gap can be tuned between 0.65eV to 3.42eV by changing Indium (In) composition and this covers almost the entire solar spectrum. The InGaN materials are beneficial because they also have a high absorption coefficient, high radiation resistance and the possibility of making 50% efficiency of solar cells by using GaN/InGaN heterojunctions [16]. Obviously, the interfaces of InGaN/GaN based heterostructures play an important role for realizing high quality electronic and optoelectronic devices [17]. The biggest challenge of achieving high efficiency solar cells is to grow, homogeneous, defect free InGaN layers with a high In content. Traditionally GaN based epitaxial layers are generally grown on sapphire, silicon and/or SiC substrates. However, the considerable lattice mismatch between III-N films and sapphire substrate leads to high density of crystalline defects [18–21]. ZnO has attracted attention due to its potentially superior performance and lower cost than other substrate technologies [22].
The number of publications on the growth of nano-structured materials for device applications using ZnO as a substrate has recently shown increased interest. ZnO is a wide band gap semiconductor which complements the growth of III-Ns and SiC [23–26]. ZnO material exhibits wurtzite (wz) structure whose a-axis lattice mismatch with wz GaN is 1.8%. Using Vegard’s law, the InxGa1-xN alloy for x = 0.18 has a perfect lattice-match with ZnO in the a-axis resulting in low or little possibility of mismatch related dislocations in InGaN grown films [27–30]. In addition, the thermal expansion coefficient of ZnO is also very similar to GaN so there should be no or very little thermal strain between ZnO and GaN [31]. Moreover, the optical and physical properties of ZnO such as high refractive index, high thermal and electrical conductivity, are beneficial for improving the performance of LEDs. It should be noted that the growth of InGaN on ZnO has led to the development of higher In incorporation (>30%) without phase separation for the production of green (555 nm) LEDs [32].
In this paper, several InGaN/GaN device structure samples having high-In composition of InGaN layers, on ZnO substrates (cf. Sec. 2) by using (cf. Sec. 2.1) a metallic organic chemical vapor deposition (MOCVD) technique, under different growth conditions have been prepared. Comprehensive characterization results are reported (cf. Sec. 2.2) by employing different methods (viz., high resolution X-ray diffraction (HRXRD), Rutherford backscattering (RBS), X-ray photoelectron spectroscopy (XPS), Photoluminescence (PL), and Raman scattering spectroscopy (RSS)). In InxGa1-xN, the In composition x is evaluated by analyzing the HRXRD, RBS and PL emission spectra. The XPS measurements have confirmed the presence of a small amount of Zn at the surfaces of all samples, probably due to the diffusion of Zn atoms from ZnO substrate to InGaN films. All analyses of experimental measurements (cf. Sec. 3) have shown that the growth temperature played an important role for the incorporation of indium composition in InGaN alloys as well as achieving film quality. An optimum growth temperature is a necessary condition for obtaining the high-quality InGaN films on ZnO substrate.
2. Experimental
2.1 Sample preparation
The InGaN/GaN device structure material samples were grown on ZnO by MOCVD method in an improved double injection block rotating disc reactor. Trimethylgallium (TMGa) and ammonia (NH3) were used as the gallium and nitrogen sources, the GaN buffer layer was prepared at 530°C with a thickness of ~30-60nm. The InGaN layer were grown by introducing trimethylindium (TMIn) and triethylgallium (TEGa) over a temperature range of 656°C to 736°C. Dimethyl-hydrazine (DMHy) was used in the recrystallization process to prevent GaN decomposition and in order to avoid etching of the ZnO surface. N2 carrier gas rather than H2 was also used throughout the growth process for a similar reason. The structures of five InGaN samples (S1-S5) prepared at different temperatures (cf. Table 1) of various In compositions were studied.
Table 1. RBS fitting sample’s information
2.2 Characterization techniques
The InGaN/GaN samples were carefully characterized by using several methods including HRXRD, RBS, PL, RSS and XPS. The HRXRD measurements were performed by a Broker D8 XRD system, for wide and fine scans. The RBS experiments were carried out on two accelerator systems. For samples S1, S3 and S5 a 2 × 1.7 MV tandem accelerator, with a collimated 2.023 MeV He+ was used and for the other two samples S2, and S5 a second accelerator using a 1.57 MeV He+ beam was employed. The sample were installed in the vacuum chamber, and the backscattered particles accepted by an Au-Si barrier detector. The detection angle was 165° (for three samples S1, S3 and S4) and 168.2° (for two samples S2 and S5) and the energy resolution of the detector was set to ~15 keV.
The 325nm He-Cd lasers with 30 mW output power was used to excite the samples. The PL spectra were detected by a micro region Raman/PL spectrometer (Finder one, Zolix, China) with an ANDOR Newton CCD with 0.09 nm resolution rate. In this study, we recorded Raman-scattering spectra at room temperature (RT) by using a Renishaw micro-Raman spectrometer equipped with a laser of 633 nm and a charge-coupled device (CCD) for detection. Here, we reported the surface properties of InxGa1-xN samples after analyzing the XPS measurements carried out in vacuum by using an ESCALAB 250XI + (Thermo Fisher Scientific Company, United States of America) system at vacuum degree < 5 × 10−10 Mbar. The Al Kα (1500eV) radiation was used as an excitation source.
3. Results and discussion
3.1 High-resolution X-ray diffraction (HRXRD)
Figure 1(A) shows the HRXRD 2θ – ω wide scans recorded on InGaN/GaN five samples prepared on ZnO substrate (S1, S2, S3, S4 and S5) with growth temperatures maintained at 656°C, 680°C, 700°C, 720°C and 720°C, respectively. The strongest peaks are from ZnO substrate with the 1st order (0006) 2θ at near 34.5°, the 2nd (00012) at ~72.5° and the 3rd (00018) at ~125°. It is seen that the samples S2, S3 and S4 exhibited the 1st, 2nd and 3rd order diffraction peaks, while samples S1 and S5 showed only 1st order peaks. The weaker bands in the right side of the strong 1st and 2nd order ZnO peaks are the diffraction peaks from InGaN layers. Figure 1(B) shows the HRXRD 2θ – ω fine scans recorded on InGaN/GaN/ZnO five samples (S1, S2, S3, S4 and S5). The InGaN peak shifted from ~34° (2θ) to ~32.3° with the increase of x(In) from S5 up to S1, i.e., with an increased aviation from the ZnO band at ~34.5° (2θ). This band indeed included a contribution, appeared as a weak shoulder, from the GaN thin (about 30-50 nm) buffer layer between the InGaN top layer and ZnO substrate. If a much thicker (0.2-0.5μm) GaN layer was grown on ZnO, with no InGaN on top, see Fig. 1(C), two distinguished bands from the GaN film and ZnO substrate were observed, with the GaN and ZnO (0006) peaks at 34.61° and 34.49°, respectively. Some weaker peaks located near 31.5° indicated some InN which is not usual in these types of samples and will discussed further in the XPS analysis.
Fig. 1 (A) Experimental HRXRD 2-theta wide scans for the InGaN/GaN/ZnO samples (S1, S2, S3, S4 and S5) grown at 656°C, 680°C, 700°C, 720°C and 720°C, respectively. (B) Experimental HRXRD 2-theta fine scans for the InGaN/GaN/ZnO samples (S1, S2, S3, S4 and S5). (C) 2θ – ω HR-XRD scanning curve of GaN films grown on ZnO substrate.
3.2 Rutherford backscattering (RBS)
RBS was applied to determine the layer thickness and composition as well as element inter-diffusion for our samples. Figure 2(A) shows the results of random RBS spectra recorded on three InGaN/GaN samples prepared on ZnO substrate (S1, S3 and S4) with growth temperatures maintained at 656°C, 700°C and 720°C, respectively. Figure 2(B) shows the random RBS spectra for the other two samples (S2 and S5) of InGaN with growth temperatures kept at 680°C and 736°C, respectively.
Fig. 2 Random RBS spectra of three and two InGaN/GaN/ZnO samples with InGaN layer were grown at A (656, 700 and 720°C) and B (680 and 736°C), respectively.
The simulated RBS spectrum was obtained using SIMNRA software program (Fig. 3(A)) for sample S1 (growth temperature 656°C). Figure 3(B) shows both random and simulated spectra for the InGaN/GaN/ZnO sample S2 where the growth temperature of InGaN layer was kept at 680°C. It was noticed from the RBS simulation that the In composition in InGaN layer remained uneven with layer contents decreasing while the contributions of Zn increasing. Figures. 3A and 3B clearly revealed that at these growth temperatures, the decomposition of ZnO substrate led to the increase of Zn and O diffusion – resulting in poor epitaxial growth with degradation of the film quality. However, it was not clear yet if the source of the Zn and O was only from diffusion through the InGaN/GaN as it could also be from the direct evaporation of the ZnO substrate onto the sample surface.
Fig. 3 Random and simulated RBS spectra of InxGa1-xN/GaN/ZnO samples with InGaN grown at (A) (656°C, x = 0.65) and (B) (680°C, x = 0.65).
The thickness and composition of the five samples grown from 656°C to 736°C were acquired from the simulation on the RBS experimental spectra with the results listed in Table 1. The thickness of the GaN buffer to be around 28-57 nm, while the thicknesses of InGaN layers fall between 52nm to 91nm. The contents of In at InGaN layers are 0.65, 0.52, 0.49, 0.37, and 0.21, respectively, and their growth temperatures were 656°C, 680°C, 700°C, 720°C, and 736°C, respectively.
The RBS simulation results show that the growth temperature has a significant impact on the incorporation of indium. The relationship between growth temperature and In content is displayed in Fig. 4. This clearly shows that the In content is higher when the growth temperature is lower. This correlation, as expected, can be associated with the higher incorporation of indium with the decrease of temperature, i.e. more indium desorbs at higher temperatures.
3.3 Photoluminescence (PL)
The room-temperature photoluminescence (RT-PL) spectra of the three InGaN samples with indium composition of 49%, 37% and 52% grown at 700°C, 720°C and 680°C, respectively, are shown in Fig. 5(A). Their PL emission peaks are observed at 2.12, 2.23 and 2.36 eV, respectively.
Fig. 5 (A) The PL spectra of three different Indium fractions at room temperature, and (B) Raman spectrum of InGaN/GaN/ZnO structure with Indium fraction of 0.52 (680°C).
Assuming that the InxGa1-xN layer is strained the composition dependent bandgap can be found by using the following equation [33]:
where x is the indium composition. The values of x(In) obtained by RBS simulation are 49%, 37% and 52% respectively, and the corresponding calculated PL peak energies estimated are at 2.25, 2.38 and 2.23 eV, respectively. Obviously, the calculated results are found different from the experimental PL data of 2.12, 2.23, and 2.36 eV, respectively (see: Fig. 5(A)). The calculated PL shifts assessed from the peaks of the experimental results fall between 0.11 and 0.15eV for each sample. By comparison the simulated and the experimental results of Eg (x) for InxGa1-xN showed positions of the PL peaks in the presence of Zn atoms cause energy gap to become smaller [9,34]. It is likely that the lower PL energy signals are a result of Zn and O diffusion into InGaN layers – resulting in the formation of impurity levels within the energy gap. The diffusion of Zn and O atoms in InGaN are also contributing factors to the broad full width at half maximum observed in the PL measurements.3.4 Raman scattering (RS)
The micro-Raman imaging is a contrast of the intensity caused by the intrinsic vibrational spectral characteristics of the materials. It is highly sensitive to the composition, structure and the local chemical environment of the materials. This characterization technique has been widely used in material research owing to its non-destructive and non-contact approach. Here, the quality of InGaN on ZnO substrate by using the Raman scattering spectroscopy is analyzed.
In Fig. 5(B), the vibrational spectrum of InGaN/GaN/ZnO with indium composition of 0.52 is displayed using RSS. In the back scattering Z(X, X)Z geometry, the room temperature Raman measurement was recorded with a laser source of λexc = 633nm where the Z-direction is along the c-axis. The prominent phonon characteristic of the bare ZnO substrate observed at 437 cm−1 corresponds to the E2 (high) frequency mode while the peak at 332 cm−1 is related to the two-phonon line. Moreover, A1(LO) modes of GaN phonon peaks are also clearly identified in the spectrum [35].
Figure 6(A) shows the room temperature Raman spectra with A1 (LO) mode of InxGa1-xN samples having In compositions ranging from 0.21 to 0.52. The A1 (LO) phonon mode can be used to indirectly evaluate the strain in the layers. Figure 6(A) also reveals that the In content increases with the decrease of growth temperature, and the low frequency shift of A1 (LO) shows a reduction of the compressive strain [36].
Fig. 6 (A) Raman spectrum of A1(LO) mode for Four different In fractions in InGaN epilayers. (B) Observed frequency for A1(LO) mode of InGaN alloy versus In fraction.
Figure 6(B) exhibits frequency shift of the observed A1 (LO) mode for InxGa1-xN alloys as a function of In composition. The mode frequency of A1 (LO) increases with the decrease of concentration. The best fit results from the experimental data using a second order polynomial leads to the relationship expressed by the following equation:
where Y is the frequency of the A1 (LO) mode in the InxGa1-xN alloys.3.5 XPS survey, fine scans and analyses
Wide scan XPS measurements have also been completed on these InxGa1-xN/GaN/ZnO samples. The typical results for the sample S5 with x(In) ~0.21 are displayed in Fig. 7, showing the relevant peak assignments and positions. The XPS fine scans are presented in Figs. 8 and 9 for Ga 3d, In 3d5/2, N 1s and Zn 2P3/2, respectively.
The software package XPSPEAK 4.1 was used to fit the XPS data. Each XPS scan is matched with a Gaussian-Lorentzian (20%-80%) function and the details of the fitted results are reported in Table 2. For five InGaN/GaN/ZnO samples, the In compositions at the surface region are obtained from XPS data analyses. While, the results from XPS studies are, in general, found consistent with the x(In) values assessed by RBS, however, it was noticed that for sample S4 the x(In = 0.42) the value in the film is higher from the surface x(In = 0.37) than the measurement using RBS. For the other two films S2 and S5, the surface x(In) values are slightly lower than those of the corresponding values obtained in the film. On the other hand, the surface x(In) values for S1 and S3 are almost the same with those obtained in the film. Moreover, from the XPS simulations, the influence of growth temperature on the incorporation of In is still apparent.
Table 2. The fitting details of XPS fine scans data simulated by Lorentzian-Gaussian fitting method about the peaks Ga 3d, In 3d5/2 and N 1s for five InGaN films.
Figure 8(A) shows that the peak of Ga 3d is mixed with In 4d and N 2s. Only the peak in the binding energy 20.0 eV belongs to Ga 3d. Figure 8(B) is the appropriate XPS fine scan results of In 3d5/2 fitted with two components, InN and In2O3. In Fig. 9(A), the peak of N 1s is fitted with five components, Ga Auger, GaN, InN, N-H and N-O. In samples S3 and S4, no N-O components are observed. Figure 9(B) shows the fitting results of Zn 2P3/2, and in all samples a certain amount of Zn detected. This indicates that Zn from the ZnO substrate was diffused into the InGaN film.
Figure 8(B) showed In 3d5/2 has different degrees of oxidation on the surface of each samples, but for the S5 grown at the highest temperature, oxidation is particularly apparent. This is also observed in Fig. 9(B) by the Zn 2P3/2 emission. The issue of serious surface oxidation should be considered when growing InGaN at higher temperatures such as 736 °C. According to the calculation results in Table 3, the amount of Zn increases with the increase of growth temperature (except S2), and the Zn content of S4 is slightly less than S3, because the GaN layer of S4 is greatly thicker than S3. Therefore, at higher growth temperatures, higher diffusion of Zn and O into InGaN would be expected and this could cause a deterioration of the quality of the films.
Table 3. The fitting details of XPS fine scans data simulated by Lorentzian-Gaussian fitting method about the peaks Zn 2P3/2 for five InGaN films.
4. Summary
The results of comprehensive analyses on a series of high-In composition InGaN thin films grown on ZnO substrate by MOCVD under different growth conditions by using HRXRD, RBS, XPS, PL and RSS are reported. The quality of films and their optical properties are accurately determined for assessing important material characteristics.
HRXRD confirmed the single crystalline lattice structure for the nano-meter scale InGaN thin films, with high In-compositions, on single crystalline ZnO substrate with the nanoscale GaN buffer, grown from MOCVD. By examining RBS data, we found an increase of Zn/O atoms decomposing from the substrate with the increase of growth temperature. The increase of Zn/O contribution in the InGaN layer is accompanied by the decrease of the In component, which leads to the formation of impurity level causing poor quality epitaxial growth of layers. The In composition is evaluated by analyzing RBS and PL emission spectra. The XPS measurements revealed a small amount of Zn in all samples and established diffusion of Zn atoms from the substrate to InGaN films as the most likely mechanism. All the analyses of experimental measurements have shown that the growth temperature plays an important role on the incorporation of indium composition in InGaN alloys as well as of achieving the film quality. Suitable growth temperature is a necessary condition for obtaining the high-quality films grown on ZnO substrate. The present study has not only deepened our understanding of the InGaN thin films materials grown by MOCVD through the comprehensive experimental data analysis, but also it paves the way for the preparation of high quality InGaN for solar cell applications.
Funding
National Natural Science Foundation of China (51868002, 61367004); Guangxi Key Laboratory through the Relativistic Astrophysics-Guangxi Natural Science Creative Team funding (2017JJA170740y).
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