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We introduced the use of solution-processed few-layer hexagonal boron nitride (h-BN) stripe patterns embedded in the lateral epitaxial overgrowth (LEO) of AlN grown on sapphire substrates using high temperature metal organic chemical vapor deposition (MOCVD). This straightforward usage of h-BN film contributes to reducing the lattice mismatch and almost entirely terminates the threading dislocations originating from the AlN/sapphire interface, which results in a low pit density and the absence of air-voids in the AlN template. Compared with AlN templates grown on conventional sapphire substrates, the full width at half maximum of the AlN template grown on the h-BN pattern in the (0002) and (10-12) planes decreased from 376 arcsec to 227 arcsec and from 495 arcsec to 398 arcsec, respectively. For device applications, AlGaN-based visible-blind UV photodetectors fabricated using the as-obtained high quality AlN templates show one order of magnitude reduction of the dark leakage current and 50% increase in the responsivity. Our results suggest that the h-BN pattern plays a promising role in the growth of a high quality AlN template, leading to the improvement of performance of AlGaN-based optoelectronic devices.
© 2017 Optical Society of America
1. Introduction
Single crystal AlN is promising as an ideal substrate for AlGaN-based deep ultraviolet optoelectronic device applications because of its high thermal conductivity, wide and direct band gap, and compatibility with other III-V nitride materials [1,2]. However, AlN substrates are often grown by solution and sublimation-recondensation methods which produce small sized substrates at a high cost. In order to overcome the disadvantages of bulk AlN substrates, heteroepitaxial growth techniques such as hydride vapor phase epitaxy, metal organic chemical vapor deposition (MOCVD), and molecular beam epitaxy have been developed to prepare AlN templates on sapphire substrates as an alternative. However, the large mismatches of lattice constants and thermal conductivities, as well as poor Al adatom migration on sapphire substrates, lead to low quality AlN templates with extremely high threading dislocation densities (TDD) [3].
One important application of high quality AlN templates is for high Al-composition AlxGa1-xN (x≥0.4) visible-blind photodetectors. It has been shown that high dislocation density in the epilayers is the primary reason for the deteriorating leakage current in AlGaN-based photodiodes. Therefore, several methods have been used to improve the AlN quality by enhancing migration during deposition and reducing the gas phase reactions between trimethylaluminum (TMA) and ammonia (NH3) [3,4], such as changing growth temperature, using epitaxial lateral overgrowth (ELO) on patterned sapphire [5], and adopting a low-pressure flow-modulated MOCVD method [6]. Among them, ELO of AlN on patterned sapphire substrates in combination with the recently emergent technology of growing thick AlN layers (≥1μm) is an effective approach for the reduction of the TDD down to the low 108 cm−2 range [7–10], and provide a preferred solution for obtaining high quality AlGaN epilayers and high performance visible-blind UV photodetectors [11].
Some groups have reported on the use of hexagonal boron nitride (h-BN) in fabricating GaN- and AlGaN-based optoelectronics devices [12–14]. Single-layer h-BN, an analog of graphene, has a hexagonal structure with lattice constant a = 2.50 Å, compatible to that of AlN (20% lattice mismatch), and a high thermal conductivity (~600 Wm−1K−1) also close to that of AlN (~285 Wm−1K−1) [3,15]. Motivated by these considerations, in this letter we report on the first use of a few-layer h-BN stripe pattern in improving the crystalline quality of an AlN template grown using high temperature MOCVD. Herein, we covered a 2 inch sapphire wafer with a stripe-patterned film of an h-BN pristine flake solution. The h-BN stripe-patterned film helped to reduce the lattice constant mismatch between AlN and sapphire, and enabled the growth of an epitaxial AlN layer with high crystalline quality. The AlGaN-based visible-blind UV photodetectors subsequently fabricated using the as-obtained high quality AlN template shows improved performance.
2. Experimental details
First, a stripe-patterned photoresist with a width of 2 µm and a period of 6 µm was fabricated on a 2-inch sapphire substrate using standard photolithography. Then, 5 mL of commercial h-BN pristine flakes (Graphene Supermarket) was spray-coated on top of the substrate using an airbrush system with nitrogen as the carrier gas. The flakes have lateral size of 50-200nm and thickness of 1-5 monolayers, and are dispersed in ethanol/DI water with a concentration of 5.4 mg/L. The airbrush inlet pressure of nitrogen, substrate temperature, and spraying rate were 2 bar, 130°C, and 3 mL min−1, respectively. Then, the photoresist was removed with acetone to form the stripe-patterned h-BN film. The resulting sapphire substrate covered with h-BN was cleaned with methanol and deionized water. We also prepared a conventional sapphire substrate for comparison. Both substrates were set into a vertical showerhead MOCVD (Phaethon 100 series: 6 pocket × 2 inch MOCVD, from Topengnet Co., Ltd, Korea).
Figure 1(a) shows the steps to grow the AlN template on the sapphire substrate in the MOCVD reactor. First, we performed a thermal cleaning of the sapphire substrate at 1350°C for 30 s with hydrogen carrier gas. Second, we performed a trimethlyaluminum (TMA) pre-treatment with a flow rate of 13 µmol min−1 at 1350°C and under a pressure of 30 Torr for 30 s. It has been found that introducing only TMA precursor into the reactor at the beginning is necessary for high quality AlN since it can promote two-dimensional growth [16]. Next, we deposited the first AlN nucleation layer with a V/III mass flow ratio of 1736 at 1350°C for 150 s, and the second AlN nucleation layer with a V/III ratio of 7639 at 1050°C for 300 s.
Fig. 1 (a) Key steps in growing the AlN template using MOCVD, (b) schematic structure of the AlGaN Schottky photodiodes grown on the AlN template, (c) top view image of the as-fabricated AlGaN photodiodes.
During the last step, to get high lateral growth and planarize the epilayer, we changed the V/III ratio to 243 and increased the temperature to 1350°C for 6000 s. We estimated a lateral overgrowth rate to a vertical growth rate ratio of 0.57, and a fully coalesced AlN epilayer after 1 hour have been achieved. Our process is a typical epitaxial lateral overgrowth process for growing AlN using MOCVD. The AlN templates grown on the stripe-patterned h-BN film and the conventional sapphire substrate are denoted as BN-AlN and C-AlN, respectively. After the MOCVD growth, both the BN-AlN and the C-AlN samples were characterized using atomic force microscopy (AFM, Digital Instruments; Nanoscope IV A) in tapping mode. X-ray diffraction (XRD, X’PERT-MRD) and scanning electron microscopy (SEM, HITACHI-S-4300SE) analyses were also performed. Transmission electron microscopy (TEM, FEI, Techani G2 300 KeV) was used to verify the existence of h-BN at the sapphire/AlN interface.
High-quality AlN templates grown on the sapphire substrates can be used to fabricate many optoelectronic devices, such as light-emitting diodes, laser diodes, and photodetectors (PDs). As an example of a device application, we fabricated AlGaN-based Schottky photodiodes on top of the BN-AlN and C-AlN templates, shown in Fig. 1(b) and 1(c) [7,17]. Forty periods of AlN/Al0.4Ga0.6N superlattices (0.7 µm thick) were grown on top of the 5 µm-thick AlN template, followed by a 1 µm-thick n-doped Al0.4Ga0.6N layer and a 0.5 µm-thick unintentionally doped Al0.4Ga0.6N layer. The n-type gas precursor for n-doped Al0.4Ga0.6N is SiH4 (200 ppm) diluted in H2. For the metal contacts, we used Ni/Au for the anode and Cr/Au for the cathode. The chip size is 0.4 mm × 0.4 mm, and the active area is 0.076 mm2. The photodiodes fabricated on the BN-AlN and C-AlN templates are denoted BN-PD and C-PD, respectively. The I-V characteristics were measured using a Keithley 4200 semiconductor characterization system, with a Xenon lamp (450 W) and Oriel Cornerstone 130 1/8 m monochromator as the light source.
3. Results and discussion
Figure 2(a) shows the AFM image of the surface of as-prepared h-BN stripe-patterned film on the sapphire substrate. The graph in Fig. 2(a) shows that the average thickness of the h-BN film is approximately 1.5 nm, which corresponds to 2-4 layers of h-BN [18]. The uniform height profile implied that h-BN sheets randomly overlapped each other to form a continuous film after the spray-coating process. Figure 2(b) shows the first-order Raman spectrum of the h-BN film, and we obtained a Raman peak at 1370 cm−1. This peak corresponds to the E2g symmetry and is consistent with previous reports on the Raman signature of h-BN [19–22].
Fig. 2 (a) AFM image of the surface of the h-BN stripe-patterned film and line scan indicating thickness, (b) Raman spectrum of the as-prepared h-BN film.
Figure 3 shows the cross-sectional TEM image and material analysis at the sapphire/h-BN/AlN interface after MOCVD growth. We observe the existence of boron atoms at the interface between AlN and sapphire. The figure reveals that some boron atoms diffused into the sapphire and AlN layers after the high temperature MOCVD growth.
We performed AFM imaging of the C-AlN and BN-AlN surfaces, as shown in Fig. 4(a) and 4b, respectively. The surface of the C-AlN sample is characterized by a larger density of surface defects and via holes. In comparison, the surface of the BN-AlN sample is smooth with clear atomic steps, has mostly very small pits (defects) and no via holes. The atomic step on the surface of the BN-AlN sample has an average height of 0.425 nm. This height is slightly lower than the c lattice constant of bulk AlN (0.498 nm) possibly because of surface tension. We also measured the root mean square (RMS) roughness on an area of 4 × 4 µm2, and obtained RMS values of 0.702 nm and 0.134 nm for the C-AlN and BN-AlN surfaces, respectively.
Fig. 4 AFM images of (a) C-AlN and (b) BN-AlN surfaces. SEM images of the etched surfaces of (c) C-AlN and (d) BN-AlN were used to evaluate the etch pit density.
In order to assess the dislocation density, we perform an etch pit density (EPD) study on the C-AlN and BN-AlN samples. The samples were etched for 13 min in a H3PO4 solution heated to 120°C. The etched surfaces of the C-AlN and BN-AlN samples were measured by SEM, and the results are shown in Fig. 4(c) and 4(d), respectively. EPD values of 2 × 109 cm−2 and 1.8 × 108 cm−2 were obtained from the SEM images of the etched surfaces of C-AlN and BN-AlN, respectively. The pits formed in the C-AlN epilayer are known to be mainly due to the propagation of threading dislocations. The lower EPD value of BN-AlN is attributed to the fact that dislocations in the substrate beneath the h-BN nanosheets are blocked and cannot propagate into the lateral epitaxial layer. In other words, the threading dislocations are terminated when they encounter the h-BN pattern, resulting in a decrease in the threading dislocation density of the lateral overgrowth AlN on patterned h-BN.
Dislocation reduction of AlN by the embedded h-BN film was further studied by cross-sectional TEM and SEM imaging. Figure 5(a) and 5(b) give the cross-sectional TEM images at the sapphire/AlN interface of the C-AlN and BN-AlN samples, respectively, with the diffraction vector g = (0002). It is clear that for the C-AlN sample, many dislocations and defects originate from the interface and spread towards the center of the AlN epilayer. On the other hand, for the BN-AlN sample, defects were significantly reduced and limited to the region near the interface. Figures 5(c) and 5(d) give the cross-sectional TEM images at the AlN surface of the C-AlN and BN-AlN samples, respectively, with the diffraction vector g = (0002). There are very few dislocation lines that reached the surface of the AlN epilayer of the BN-AlN sample, compared to the C-AlN sample.
Fig. 5 (a, b) Cross-sectional TEM images at the sapphire/AlN interface of C-AlN and BN-AlN, respectively. (c, d) Cross-sectional TEM images at the AlN surface of the C-AlN and BN-AlN, respectively. (e, f) Cross-sectional SEM images of the entire AlN epilayer of C-AlN and BN-AlN, respectively.
Figures 5(e) and 5(f) show the cross sectional SEM images of the entire AlN epilayer of the C-AlN and BN-AlN samples, respectively. We obtained the same thickness of 5 µm for both samples, which indicates that the stripe-patterned h-BN film did not affect the thickness of the AlN epilayer. An unintentional air-void is clearly seen in the C-AlN sample, which is usually formed when using the LEO technique for AlN epilayers thicker than 2 μm because of the relaxation of strain. However, we do not observe any void formations in the BN-AlN sample under identical growth conditions, as shown in Fig. 5(f). A possible explanation for the absence of air voids in the BN-AlN sample is the lateral epitaxial growth mechanism which only occurs at the sapphire windows and extends over the h-BN coated areas until coalescence and smoothing take place [12]. Low EPD and absence of air-voids indicate that the threading dislocations (TDs) are significantly reduced in the AlN epilayer on the patterned h-BN, which also agrees with the TEM and XRD results discussed below.
The XRD rocking curves of the C-AlN and BN-AlN samples were measured, and are shown in Fig. 6. The XRD full width at half maximum values (FWHMs) of the (0002) reflection of the C-AlN and BN-AlN samples are 376 arcsec and 227 arcsec, respectively. The XRD FWHMs of the (10-12) reflection of the C-AlN and BN-AlN samples are 495 arcsec and 398 arcsec, respectively.
The densities of screw TDs and edge TDs are estimated based on the following equations [23]:
where and are the Burgers vector sizes of the screw TDs ( = 0.4982 nm) and edge TDs ( = 0.3112 nm), and are the XRD FWHM values of the (0002) and (10-12) reflections, respectively. The calculated dislocation densities are 3.1 × 108 cm−2 (screw) and 1.4 × 109 cm−2 (edge) for the C-AlN sample, and 1.1 × 108 cm−2 (screw) and 8.8 × 108 cm−2 (edge) for the BN-AlN sample.The influence of the high quality AlN template on the performance of the subsequently grown AlGaN-based UV photodetector is demonstrated as follows. Figure 7(a) shows the responsivity as a function of wavelength (at zero bias) of the C-PD photodiodes (grown on the C-AlN template) compared to the BN-PD photodiodes (grown on the BN-AlN template). At the peak responsivity at a wavelength λ = 270 nm, C-PD and BN-PD have responsivities of 0.04 AW−1 and 0.06 AW−1, which correspond to quantum efficiencies of 18% and 28%, respectively. Figure 7(b) shows the current-voltage (I-V) curves in the dark and under illumination (UVC lamp, 1 mWcm−2) of the C-PD and BN-PD photodiodes, respectively, and it shows good rectifying behavior with a turn-on voltage of 0.3 V. The BN-PD photodiode exhibits reverse dark currents around one order of magnitude lower than the C-PD photodiode in the entire measured voltage range of 0 to −5 V. Specifically, at a reverse bias of −5V, the dark current density was 1.01 × 10−5 Acm−2 for the C-PD photodiode and 9.14 × 10−7 Acm−2 for the BN-PD photodiode.
Fig. 7 (a) Responsivity of C-PD and BN-PD photodetectors as a function of wavelength, (b) I-V curves in the dark and under illumination (UVC lamp, 1 mWcm−2) of C-PD and BN-PD photodetectors.
The dynamic resistances at zero bias, , are deduced from by exponential fitting to both the forward and reverse I-V curves [24]. The resulting values of the C-PD and BN-PD photodiodes are 1.45 × 1011 Ω and 1.75 × 1013 Ω, respectively. The specific detectivities at λ = 270 nm are estimated based on
where is the responsivity at λ = 270 nm, k is the Boltzmann’s constant, T = 298.15 K is the operating temperature, and A = 7.6 × 10−4 cm2 is the detecting area. The resulting values of the C-PD and BN-PD photodiodes are 3.27 × 1012 cm Hz1/2 W−1 and 5.39 × 1013 cm Hz1/2 W−1, respectively.The above data indicate that the h-BN stripe-patterned film reduced the dark leakage current and improved the electrical properties of the photodiode. The increase in the performance is attributed to the improvement of the crystalline quality of the AlGaN epilayers by the high quality BN-AlN template [24,25]. Our solution-based method of embedding h-BN stripe-patterned films in an AlN template provides an effective, inexpensive, and simple method to improve the crystalline quality and performance of AlGaN-based UV photodetectors as well as other nitride optoelectronic devices.
4. Conclusions
In summary, the high temperature MOCVD growth of AlN templates was conducted on sapphire substrates covered with h-BN stripe-patterned films. The crystalline quality of the BN-AlN template was investigated, and compared to the AlN template grown on a conventional sapphire substrate. The AlN epilayer in the BN-AlN sample exhibited lower FWHMs of the XRD rocking curves: 227 arcsec (0002 reflection) and 398 arcsec (10-12 reflection) compared to 376 arcsec (0002 reflection) and 495 arcsec (10-12 reflection) of the C-AlN sample. The RMS roughness of the BN-AlN sample is as low as 0.134 nm, compared to 0.702 nm of the C-AlN sample. We also fabricated and compared the performance of AlGaN-based UV photo diodes grown on top of C-AlN and BN-AlN templates. The BN-PD exhibits 50% higher responsivity, at 0.06 AW−1, and a lower dark leakage current than the C-PD. These results indicated that the inclusion of an h-BN stripe-patterned film at the sapphire/AlN interface reduced the dislocation density and improved the crystalline quality of the AlN epilayer. Moreover, this improvement in the crystalline quality of the AlN template translates to a considerable enhancement in performance of the AlGaN-based photodiodes grown on top of the AlN template.
Funding
This work was supported by Basic Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (NRF-2014R1A1A2008104) and the Materials and Components program of MOTIE/KEIT [K10067283, AlGaN-based APDs (avalanche photodiodes) for flame detector].
References and links
1. H. Hirayama, S. Fujikawa, J. Norimatsu, T. Takano, K. Tsubaki, and N. Kamata, “Fabrication of a low threading dislocation density ELO-AlN template for application to deep-UV LEDs,” Phys. Status Solidi Curr. Top. Solid State Phys. 6(SUPPL. 2), 2–5 (2009).
2. X. Hu, J. Deng, N. Pala, R. Gaska, M. S. Shur, C. Q. Chen, J. Yang, G. Simin, M. A. Khan, J. C. Rojo, and L. J. Schowalter, “AlGaN/GaN heterostructure field-effect transistors on single-crystal bulk AlN,” Appl. Phys. Lett. 82(8), 1299–1301 (2003). [CrossRef]
3. H. Wang, H. Xiong, Z.-H. Wu, C.-H. Yu, Y. Tian, J.-N. Dai, Y.-Y. Fang, J.-B. Zhang, and C.-Q. Chen, “Occurrence and elimination of in-plane misoriented crystals in AlN epilayers on sapphire via pre-treatment control,” Chin. Phys. B 23(2), 028101 (2014). [CrossRef]
4. O. Reentilä, F. Brunner, A. Knauer, A. Mogilatenko, W. Neumann, H. Protzmann, M. Heuken, M. Kneissl, M. Weyers, and G. Tränkle, “Effect of the AIN nucleation layer growth on AlN material quality,” J. Cryst. Growth 310(23), 4932–4934 (2008). [CrossRef]
5. S. Newman, D. S. Kamber, T. J. Baker, Y. Wu, F. Wu, Z. Chen, S. Namakura, J. S. Speck, and S. P. Denbaars, “Lateral epitaxial overgrowth of (0001) AlN on patterned sapphire using hydride vapor phase epitaxy,” Appl. Phys. Lett. 94(12), 1–4 (2009). [CrossRef]
6. D. Huantao, H. Yue, and Z. Jincheng, “Effect of a high temperature AlN buffer layer grown by initially alternating supply of ammonia on AlGaN/GaN heterostuctures,” J. Semicond. 30(9), 093001 (2009). [CrossRef]
7. H. Jiang, T. Egawa, and H. Ishikawa, “AlGaN solar-blind Schottky photodiodes fabricated on 4H-SiC,” IEEE Photonics Technol. Lett. 18(12), 1353–1355 (2006). [CrossRef]
8. T. Shibata, K. Asai, S. Sumiya, M. Mouri, M. Tanaka, O. Oda, H. Katsukawa, H. Miyake, and K. Hiramatsu, “High-quality AlN epitaxial films on (0001)-faced sapphire and 6H-SiC substrate,” Phys. Status Solidi C Conf. 2026(7), 2023–2026 (2003). [CrossRef]
9. N. Kuwano, T. Tsuruda, Y. Kida, H. Miyake, K. Hiramatsu, and T. Shibata, “TEM analysis of threading dislocations in crack-free AlxGa 1-xN grown on an AlN(0001) template,” Phys. Status Solidi C Conf. 2447(7), 2444–2447 (2003).
10. B. T. Tran, H. Hirayama, N. Maeda, M. Jo, S. Toyoda, and N. Kamata, “Direct Growth and Controlled Coalescence of Thick AlN Template on Micro-circle Patterned Si Substrate,” Sci. Rep. 5(October), 14734 (2015). [CrossRef] [PubMed]
11. R. Jain, W. Sun, J. Yang, M. Shatalov, X. Hu, A. Sattu, A. Lunev, J. Deng, I. Shturm, Y. Bilenko, R. Gaska, and M. S. Shur, “Migration enhanced lateral epitaxial overgrowth of AlN and AlGaN for high reliability deep ultraviolet light emitting diodes,” Appl. Phys. Lett. 93(5), 1–3 (2008). [CrossRef]
12. L. Zhang, X. Li, Y. Shao, J. Yu, Y. Wu, X. Hao, Z. Yin, Y. Dai, Y. Tian, Q. Huo, Y. Shen, Z. Hua, and B. Zhang, “Improving the quality of GaN crystals by using graphene or hexagonal boron nitride nanosheets substrate,” ACS Appl. Mater. Interfaces 7(8), 4504–4510 (2015). [CrossRef] [PubMed]
13. Y. Kobayashi, K. Kumakura, T. Akasaka, T. Makimoto, and T. Makimoto, “Layered boron nitride as a release layer for mechanical transfer of GaN-based devices,” Nature 484(7393), 223–227 (2012). [CrossRef] [PubMed]
14. H. X. Jiang and J. Y. Lin, “Hexagonal boron nitride for deep ultraviolet photonic devices,” Semicond. Sci. Technol. 29(8), 084003 (2014). [CrossRef]
15. N. Coudurier, R. Boichot, F. Mercier, R. Reboud, S. Lay, E. Blanquet, and M. Pons, “Growth of boron nitride on (0001) AlN templates by High Temperature- Hydride Vapor Phase Epitaxy (HT-HVPE),” Phys. Procedia 46, 102–106 (2013). [CrossRef]
16. X. Sun, D. Li, Y. Chen, H. Song, H. Jiang, Z. Li, G. Miao, and Z. Zhang, “In situ observation of two-step growth of AlN on sapphire using high-temperature metal–organic chemical vapour deposition,” CrystEngComm 15(30), 6066 (2013). [CrossRef]
17. G. Hellings, J. John, A. Lorenz, P. Malinowski, and R. Mertens, “AlGaN schottky diodes for detector applications in the UV wavelength range,” IEEE Trans. Electron Dev. 56(11), 2833–2839 (2009). [CrossRef]
18. K. K. Kim, A. Hsu, X. Jia, S. M. Kim, Y. Shi, M. Dresselhaus, T. Palacios, and J. Kong, “Synthesis and characterization of hexagonal boron nitride film as a dielectric layer for graphene devices,” ACS Nano 6(10), 8583–8590 (2012). [CrossRef] [PubMed]
19. S. Reich, A. C. Ferrari, R. Arenal, A. Loiseau, I. Bello, and J. Robertson, “Resonant Raman scattering in cubic and hexagonal boron nitride,” Phys. Rev. B 71(20), 205201 (2005). [CrossRef]
20. G. Lu, T. Wu, Q. Yuan, H. Wang, H. Wang, F. Ding, X. Xie, and M. Jiang, “Synthesis of large single-crystal hexagonal boron nitride grains on Cu-Ni alloy,” Nat. Commun. 6, 6160 (2015). [CrossRef] [PubMed]
21. Y. Stehle, H. M. Meyer III, R. R. Unocic, M. Kidder, G. Polizos, P. G. Datskos, R. Jackson, S. N. Smirnov, and I. V. Vlassiouk, “Synthesis of Hexagonal Boron Nitride Monolayer: Control of Nucleation and Crystal Morphology,” Chem. Mater. 27(23), 8041–8047 (2015). [CrossRef]
22. A. Eckmann, J. Park, H. Yang, D. Elias, A. S. Mayorov, G. Yu, R. Jalil, K. S. Novoselov, R. V. Gorbachev, M. Lazzeri, A. K. Geim, and C. Casiraghi, “Raman fingerprint of aligned graphene/h-BN superlattices,” Nano Lett. 13(11), 5242–5246 (2013). [CrossRef] [PubMed]
23. W. Zhang, A. Y. Nikiforov, C. Thomidis, J. Woodward, H. Sun, C.-K. Kao, D. Bhattarai, A. Moldawer, L. Zhou, D. J. Smith, and T. D. Moustakas, “Molecular beam epitaxy growth of AlGaN quantum wells on 6H-SiC substrates with high internal quantum efficiency,” J. Vac. Sci. Technol. B Microelectron. Nanom. Struct. 30(2), 2119 (2012).
24. H. Jiang and T. Egawa, “High quality AlGaN solar-blind Schottky photodiodes fabricated on AIN/sapphire template,” Appl. Phys. Lett. 90(12), 121121 (2007). [CrossRef]
25. B. Potì, M. A. Tagliente, and A. Passaseo, “High quality MOCVD GaN film grown on sapphire substrates using HT-AlN buffer layer,” J. Non-Cryst. Solids 352(23–25), 2332–2334 (2006). [CrossRef]
