High quality of GaN film growth on atomically stepped lithium niobate (LiNbO3) substrates using molecular beam epitaxy (MBE)

Abstract. High temperature thermal treatment has been used to obtain atomically flat surfaces and to remove surface damage caused by mechanical polishing of as-received lithium niobate (LiNbO3) substrates. Annealing at 1000 ◦C for 2 hours produces optimal surface smoothness. The micro steps are nearly parallel and periodic almost all over the sample. The step height on z-cut substrates was 0.212 nm, which was well in accordance with the distance between oxygen layers along the c-axis of the hexagonal unit cell of LiNbO3 crystals. The step terrace width is about 217 nm, and the surface roughness is 0.111 nm, for a 5 µm × 5 µm area unit. We grew high quality GaN films on these atomically-flat LiNbO3 substrates with AlN buffer layers using molecular beam epitaxy (MBE), and then investigated their structural properties. The full-width-at-half maximum (FWHM) value of the XRD (0002) GaN rocking curve was 122.14 arcsec for GaN film grown on the positive side of LiNbO3 substrates (+z-LiNbO3. The morphology surface of GaN films was investigated using atomic force microscopy (AFM). The typical PL spectrum of GaN film grown on the both side of LiNbO3 substrate reveals a strong band-edge emission peak at 360 nm (3.45 eV).

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JOURNAL OF SCIENCE OF HNUE Mathematical and Physical Sci., 2014, Vol. 59, No. 7, pp. 126-134 This paper is available online at HIGH QUALITY OF GaN FILM GROWTH ON ATOMICALLY STEPPED LITHIUM NIOBATE (LiNbO3) SUBSTRATES USING MOLECULAR BEAM EPITAXY (MBE) Man Hoai Nam1, Nguyen Vu1 and Woochul Yang2 1Institute of Materials Science, Vietnam Academy of Science and Technology 2Department of Physics, Dongguk University, Seoul 100-715, KOREA Abstract. High temperature thermal treatment has been used to obtain atomically flat surfaces and to remove surface damage caused by mechanical polishing of as-received lithium niobate (LiNbO3) substrates. Annealing at 1000 ◦C for 2 hours produces optimal surface smoothness. The micro steps are nearly parallel and periodic almost all over the sample. The step height on z-cut substrates was 0.212 nm, which was well in accordance with the distance between oxygen layers along the c-axis of the hexagonal unit cell of LiNbO3 crystals. The step terrace width is about 217 nm, and the surface roughness is 0.111 nm, for a 5 m× 5 m area unit. We grew high quality GaN films on these atomically-flat LiNbO3 substrates with AlN buffer layers using molecular beam epitaxy (MBE), and then investigated their structural properties. The full-width-at-half maximum (FWHM) value of the XRD (0002) GaN rocking curve was 122.14 arcsec for GaN film grown on the positive side of LiNbO3 substrates (+z-LiNbO3. The morphology surface of GaN films was investigated using atomic force microscopy (AFM). The typical PL spectrum of GaN film grown on the both side of LiNbO3 substrate reveals a strong band-edge emission peak at 360 nm (3.45 eV). Keywords: LiNbO3, GaN film, molecular beam epitaxy, atomic force microscopy, root mean square. 1. Introduction Gallium nitride (GaN) and its related compounds have attracted much attention because of their excellent optical and electrical properties for applications such as light emitting diodes, laser diodes, blue, violet and ultraviolet light emitting devices [1-3, 7]. For the fabrication of these optical devices, high quality GaN thin film is required [4]. However, a number of drawbacks including a lack of a lattice-matched substrate and high Received October 16, 2014. Accepted October 27, 2014. Contact Man Hoai Nam, e-mail address: nammh@ims.vast.ac.vn 126 High quality of GaN film growth on atomically stepped lithium niobate (LiNbO3) substrates... growth temperature are present. Currently, no commercially available native substrate exists for GaN. Therefore sapphire and silicon have become the dominate substrate for GaN thin film [5, 11, 14]. Each of these substrates has advantages for common devices developed over the last decade. But, it is difficult to grow high quality GaN thin film with flat surfaces because of the large lattice mismatch and large difference in thermal expansion between GaN and these substrates [1, 12, 13]. Therefore, it is necessary to find a suitable substrate to replace sapphire and silicon substrates. In this article, we report on the use of a new substrate for GaN epitaxy. Employment of z- cut LiNbO3 substrates for the growth of GaN film provide a number of advantages such as smaller lattice mismatch (∼ 6.8%), and little difference of thermal expansion compared with that of sapphire (16%) to GaN, and over again it can be used to control the polarity of GaN films grown [8-10]. In addition, the use of LiNbO3 substrates is attractive, because large-diameter wafers (> 4 inch) are commercially available at reasonable cost, as well as its potential for optoelectronic applications such as optical switching devices and second harmonic generators due to its large nonlinear-optical coefficients [11, 13]. Hence, the successful epitaxial growth of GaN on LiNbO3 could lead to the fabrication of optoelectronic integrated circuits that utilize both GaN lasers and LiNbO3 optical switches [9, 12]. In the latter, we report that high temperature thermal treatment, which has been used to obtain an atomically flat surface and to remove surface damage caused by mechanical polishing of as-received lithium niobate (LiNbO3) wafer [7, 11]. We also report on the growth of high quality GaN thin films on LiNbO3 substrates with the AlN buffer layer using a molecular beam epitaxy (MBE) system, and we investigate their structural properties. 2. Content 2.1. Experiments The LiNbO3 substrates were degreased sequentially in acetone for 10 minutes and methanol for 10 minutes in an ultrasonic bath. Next, the substrate was dipped in deionized water (D.I water) for 10 minutes, and then blown dry with nitrogen gas. After that the LiNbO3 substrates were put on a quartz board and loaded onto a quartz tube of furnace system for annealing in the air at 1000 ◦C for several hours. The transparent nature of LiNbO3 requires a back side metal coating prior to MBE growth to absorb radiation from the substrate heater. The anomalously high thermal expansion coefficient of LiNbO3 requires a special mounting method to prevent substrate cracking. On the backside of cleaned substrates a triple metal coating of titanium, aluminum, and titanium (Ti: 30 nm, Al: 40 nm, Ti: 200 nm) was deposited by sputtering to enhance radiation absorption from the heater. In this structure, the first layer of titanium and the layer of aluminum layer act as the thermal buffer layer. They become an alloy at high growth temperatures, increasing the melting point. This method allows for higher 127 Man Hoai Nam, Nguyen Vu and Woochul Yang temperature growth conditions. We have success with this method and have not had a problem of samples braking when the temperature is increased to reach the temperature needed. This is one of the notable points of this paper. The high quality GaN films described in this paper were grown by MBE. The flat of z-cut LiNbO3 substrates which have been prepared above will be cleaned prior to entering the MBE chamber. The substrates were cleaned with organic solvent EKC 830 (Posistrip positive photoresist remover filtered to 10 microns) at 60 ◦C for 25 minutes. As the substrates are removed from the EKC 830 organic solvent, these substrates are embedded and rinsed with de-ionized D.I water for 15 minutes, and then blown dry with nitrogen gas. After that the substrates were loaded in a preparation chamber for outgassed in vacuum at 200 ◦C for 1 hour before being loaded into the growth chamber of MBE. High quality GaN thin films were grown on both side of LiNbO3 substrate with an AlN buffer layer using a Veeco Applied EPI nitrogen plasma source. The AlN buffer layer was grown at 600 ◦C to a 50 nm thickness with 0.8 SCCM (standard cubic centimeters per minute) nitrogen at 250 W and provides for predominantly Ga-polar GaN films. The GaN film was grown with two different temperature conditions, 630 ◦C for 30 nm thickness and 690 ◦C for the remaining 500 nm thickness, both with 4.5 SCCM nitrogen at 300W. During the growth, reflected high energy electron diffraction (RHEED) was monitored. The morphology surface of GaN films was investigated using atomic force microscopy (AFM). The crystal structural properties of the films were characterized by X-ray diffraction (XRD). The optical properties of these samples were characterized by photoluminescent (PL) measurement using a He-Cd laser (325 nm) as the excitation source at room temperature. 2.2. Results and discussions Figure 1(a) shows the AFM image and the cross sectional profile of the surface morphology of as-received z- cut LiNbO3 substrate. The surface of as-received LiNbO3 substrate has a lot of surface damage such as scratches and corrugations on the atomic scale on the topmost surface of the substrate due to mechanical polishing. As shown in Figure 1(a) the surface of the as-received substrate is rugged, and many irregular small corrugations are observed. The existence of such small corrugations implies that many atomic planes which are different from the original plane exist on the surface. Accordingly it is very difficult to analyze the atomic species and their alignments on the surface of the substrate [9]. This damage also decreases the adhesion of GaN thin films to the substrate. The surface valley roughness on the surface has values of 0.47 nm in root mean square (rms), for a 5 µm × 5 µm area. Figure 1(b) represents the surface image and cross sectional profile on z-cut LiNbO3 substrate annealed at 1000 ◦C for 2 hours in air. It showed that, all of the surface damages were removed. The roughness of the surface drastically improved to become an atomically flat type. Irregular small corrugations disappeared, while atomic 128 High quality of GaN film growth on atomically stepped lithium niobate (LiNbO3) substrates... steps and atomically flat terraces appeared. The annealed substrates were as ultra smooth as the atomic steps so that could be observed. These micro steps are nearly parallel and periodic on the surface. The step height on z-cut substrates was 0.212 nm, which was well in accordance with the distance between oxygen layers along the c-axis of the hexagonal unit cell of LiNbO3 crystals [7]. The step terrace width is about 217 nm, and the surface roughness is 0.111 nm, for a 5 µm × 5 µm area unit. This mean that the surface roughness on the annealed substrate was improved over four times compared with that of the as-received substrates. The improvement of LiNbO3 surface quality has been attributed to diffusion and re-growth processes. When the LiNbO3 substrates were annealed at high temperature, the topmost surface is thermodynamically unstable and is transformed to the equilibrium crystal surface by the rearrangement of the surface atoms [14], leading to atomic flatness and the removal of scratches from the surface. The step heights it could be supposed the atoms constitute the topmost layer on the surfaces. Using substrates with this surface, it is expected that there could be a development of high quality optoelectronic devices based on LiNbO3 substrates. Figure 1. AFM images and cross sectional profile of the surface on: (a) as-received z-cut LiNbO3 substrate, (b) z-cut LiNbO3 substrate annealed at 1000 ◦C for 2 hours in air Figure 2 shows RHEED patterns of the surface of a positive side of z-LiNbO3 substrate, an AlN buffer layer grown at 600 ◦C, and a GaN film grown on the AlN buffer layer at 690 ◦C, respectively. The sharp, streaky pattern that was obtained for the LiNbO3 substrate in Figure 2(a) is quite consistent with the atomically-flat morphology shown in Figure 1(b). The RHEED image of the AlN buffer layer consists of streaks with spots, as shown in Figure 2(b) which indicates that single-crystal AlN grows epitaxially on LiNbO3 and the surface morphology is slightly roughened, probably due to stress in the film. The broadness of the diffraction lines suggests that the AlN buffer layer is very thin and highly strained. It is also very likely that this film is not continuous. It is consists of isolated AlN 129 Man Hoai Nam, Nguyen Vu and Woochul Yang islands. We believe that this AlN layer will better for the growth of GaN film because of the smaller lattice mismatch between AlN and GaN. The RHEED patterns for GaN grown on this layer are shown in Figure 2(c). The streakiness and sharpness of the diffraction lines suggest that the GaN film has atomically smooth surface morphology with excellent crystalline quality. It also indicates that high quality GaN (0001) can be grown using this technique and the surface flatness is restored, probably due to stress reduction. Figure 2. RHEED patterns for (a) +z-LiNbO3 substrate, (b) 50 nm thick AlN buffer layer grown at 600 ◦C, and(c) 500 nm thick GaN film grown at 690 ◦C on the AlN buffer layer Figure 3. AFM image of GaN thin film grown at 690 ◦C on the positive side (+z-LiNbO3) Figure 3 shows the surface morphologies of GaN films grown on positive side (+z-LiNbO3) substrate. The atomic steps are visible and the surface is rather flat with a rms roughness of 1.067 nm, over a 4 µm2 area. In this image we can see the spiral hillocks around threading dislocations with a screw component. The films with morphologies similar to that seen in Figure 3 must be grown under Ga-stable conditions near the 130 High quality of GaN film growth on atomically stepped lithium niobate (LiNbO3) substrates... transition to Ga droplet formation. The films grown under slightly less Ga rich conditions, while still quite flat, It is not present visible atomic steps. Our results indicate that GaN films grown on LiNbO3 substrates via the MBE system exhibit surface morphologies comparable to those GaN film achieved by MBE growth on other substrates. Figure 4. Bright field cross sectional TEM micrograph of 600 mm thick GaN film grown on -z-LiNbO3 substrate recorded in the (100) zone axis To further understand the interfacial structure and threading dislocations in the epitaxial layers, we have carried out quantitative TEM studies. Figure 4 shows the cross sectional TEM images of GaN film grown on -z-LiNbO3 substrate with an AlN buffer layer at 690 ◦C. The TEM images clearly show that an approximately 600 nm highly uniform GaN film was formed on the 40 nm thick AlN buffer layer. The high resolution TEM of the interface also shows the atomic scale roughness at the AlN buffer layer/substrate and the AlN/GaN layer interface. The amplitude of the roughness is typically a few monolayers. The interfaces appear to be fully crystalline (no amorphous interface region) We also performed XRD measurements to investigate the structural properties of GaN films. Figure 5 shows the x-ray diffraction profile and its rocking curve from the GaN film with a thickness of 500 nm. The (0001) plane of GaN is parallel to the (0001) plane of the LiNbO3 substrate. The peak located at 39.012◦ came from (0006) reflections of the z-LiNbO3 substrate. The peak located at 34.462◦ originated from the (0002) reflection of the wurtzite GaN. We obtain a c-axis lattice constant of 5.208 A˚which is almost the same with a single crystal of hexagonal GaN [1, 8]. The full width at half-maximum (FWHM) obtained for the (0002) diffraction from the 500 nm thick GaN sample is 122.14 arcsec (see the inset of Figure 5). This value is better than the values reported by K. K. Lee et al. about FWHM of GaN peaks on the LiNbO3 and LiTaO3 which were 1500 arcsec and 1601 arcsec, repectively, for (0002) GaN diffraction grown by MBE method 131 Man Hoai Nam, Nguyen Vu and Woochul Yang on Refs.[8, 10]. The GaN films grown on sapphire substrate by conventional HVPE or H-MOVPE technique typically show FWHM values that are higher than that obtained in this study, even though a MOCVD-GaN template was incorporated in those cases. The experimental results for above sample demonstrated that by using the MBE system we obtained high quality GaN on both side of z-LiNbO3 substrates. Our sample is of very high quality compared with other groups as reported in journals. We found the optimal conditions for processing LiNbO3 substrates before growing GaN films, as well as optimal conditions for growing GaN films by MBE system. Figure 5. X-ray diffraction profile and its rocking curve of the (0002) reflection for the 500 mm thick GaN film grown on +z-LiNbO3 substrates (in the inset of figure) Figure 6. Photoluminescence spectrum of GaN film grown on +z-LiNbO3 substrates at 690 ◦C with the same AlN buffer layer of 50 nm grown at 600 ◦C Optical properties of GaN thin films on both sides of z-LiNbO3 substrates were characterized by photoluminescent (PL) measurement using an He-Cd laser (20 mW, 325 nm) as the excitation source at room temperature. Figure 6 shows the room 132 High quality of GaN film growth on atomically stepped lithium niobate (LiNbO3) substrates... temperature PL spectrum of high quality GaN film grown on both sides of z-LiNbO3 substrate with the AlN buffer layer 50 nm thick. As seen from Figure 6, the PL spectrum shows that a strong band-edge emission peak is situated at 360 nm (3.45 eV). On the other hand, the PL spectrums are not showing any peaks related to the yellow band at around 2.25 eV. This mean that the GaN films obtained in this study are of very high quality. 3. Conclusion Atomically smooth surfaces with atomic step structure were obtained on LiNbO3 substrates by high temperature annealing at 1000 ◦C for 2 hours in air. The step height on z-cut substrates was 0.212 nm, which was well in accordance with the distance between oxygen layers along the c-axis of the hexagonal unit cell of LiNbO3 crystals. The step terrace width is about 217 nm, and the surface roughness is 0.111 nm, for a 5 µm × 5 µm area unit. The surface roughness on the annealed substrate was improved over four times compared with that on the as-received substrates. We have succeeded with a method of using a triple metal coating of titanium, aluminum, and titanium (Ti: 30 nm, Al: 40 nm, Ti: 200 nm) on the backside (-z-surface) to absorb radiation from the substrate. High quality of GaN films were grown on atomically-flat LiNbO3 substrates with AlN buffer layers using molecular beam epitaxy (MBE). The full-width-at-half maximum (FWHM) values of the XRD (0002) GaN rocking curves were 122.14 arcsec. The morphology surface of GaN films was investigated by atomic force microscopy (AFM). The RMS of GaN film was about 1.067 nm. The HTEM images clearly showed that an approximately 600 nm thick high uniform GaN film was formed on a 40 nm thick AlN buffer layer. The high resolution TEM of the interface also shows the atomic scale roughness at the AlN buffer layer/substrate and the AlN/GaN layer interface. The amplitude of the roughness is typically a few monolayers. The optical properties of these samples were characterized by photoluminescent (PL) measurement using a He-Cd laser (325 nm) as the excitation source at room temperature. The typical PL spectrum of GaN film grown on both sides of LiNbO3 substrate reveals a strong band-edge emission peak at 360 nm (3.45 eV). Acknowledgements. 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