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Abstract
The hexagonal boron nitride (hBN) and BAlN films were prepared by RF-sputtering, which were used as the low and high refractive index layers. A series of hBN/BAlN distributed Bragg reflectors (DBRs) were prepared on sapphire substrate. The reflectivity of 9-pair hBN/BAlN (39 nm/33 nm) DBR reached 90% at 300 nm with a bandwidth of 45 nm, and which of 6-pair hBN/BAlN (35 nm/29 nm) reached 52% at 280 nm. The hBN/BAlN DBRs can be used to achieve higher reflectivity in shorter UV bands with the improvement of BAlN material quality through the growth condition optimization.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Highly reflective distributed Bragg reflectors (DBRs) are essential components for the development of deep-UV photonics devices such as vertical-cavity surface-emitting lasers (VCSELs) [1] and resonant-cavity light-emitting diodes (RCLEDs) [2]. Fabricating AlGaN with high Al content is the frequent method to obtain deep-UV DBRs [3]. Due to insufficient reflectivity and a limited crystal quality of III-nitride semiconductor based DBRs, which are often hindered by large lattice mismatch, thermal expansion mismatch, composition variations, and low refractive-index contrast [4]. Up to now, there are few reports on the demonstration of III-nitride deep-UV DBRs.
At first, based on AlGaN materials, Mitrofanov et al. prepared 25-pair DBRs at the central wavelength of 350 nm. The reflectivity reached to 99% with the bandwidth of 26 nm [5]. Later, Feltin et al. fabricated 35-pair DBRs with a reflectivity of 99% at 347 nm and a bandwidth of 20 nm [6]. In recent years, the DBRs in solar blind bands were prepared by combining AlN with AlGaN. Franke et al. have grown 25.5-pair AlN/AlGaN DBRs with a central wavelength of 273 nm by metal organic chemical vapor deposition (MOCVD), obtaining a reflectivity of 97.7% [7]. T. Detchprohm et al. used epitaxial 30.5-pair AlxGa1-xN/AlN super-lattice structures grown on AlN/sapphire templates via MOCVD for peak reflectivity between 220 nm and 250 nm, and the reflectivity reached as high as 96%–97% [8]. Nowadays, Abid et al. doped B elements into AlN and prepared 24-pair BAlN/AlN DBRs with a central wavelength of 286 nm and a reflectivity of 60% [9]. This initiated the application of boron nitride (BN) film in DBRs.
It has been the goal of researchers to achieve high reflectivity in deep-UV bands with less pair numbers. To avoid the difficulty of AlGaN growth with high Al content and the stress problem caused by lattice mismatch and thermal mismatch, new materials and methods are expected. With the development of ultra wide band gap semiconductor materials, the deep-UV DBRs have a new material system for the fabrication. Hexagonal boron nitride (hBN), which exhibits a similar structure to graphene, has gained significant amount of attention due to its unique properties such as electrical insulator with a wide band gap of ∼5.9 eV, high thermal conductivity, inherently flexibility, and high temperature stability [10 –12]. The DBRs prepared by hBN have three advantages: (i) it can be combined with III-nitride devices by growing AlN as a buffer layer; (ii) hBN has excellent thermal performance and the application of ultraviolet DBR based on hBN material to optoelectronic devices will improve the thermal performance and improve the heat dissipation performance; and (iii) hBN has graphene like structure. The DBR prepared by hBN has great application in stripping. It can be used in flexible devices and high power devices with higher optical efficiency. So, hBN is highly expected to be applied to deep-UV photoelectric devices.
In this paper, hBN and BAlN films were prepared by RF-sputtering. Firstly, hBN thin films have been prepared at high temperature (600°C) and high vacuum (2.4×10−4 Pa), and the properties of the films have been characterized. Then, BAlN film via adding the Al element into hBN by dual-power magnetron sputtering was proposed, which were deposited by co-sputtering of high-purity hBN target and Al target. The refractive index and band gap of the films with different Al components were analyzed. Lastly, hBN film and BAlN film were used as the low refractive index layer and the high refractive index layer, respectively. A series of hBN/BAlN DBRs were prepared on sapphire. The reflectivity of 9-pair hBN/BAlN DBR reached 90% at the central wavelength of 300 nm with a bandwidth of 45 nm, and the reflectivity of 6-pair hBN/BAlN DBR reached 52% at the central wavelength of 280 nm. We believe that the hBN/BAlN DBRs can be used to achieve higher reflectivity in shorter UV bands with the improvement of BAlN material quality through the growth condition optimization.
2. Fabrication procedures and characterization methods
hBN films are prepared by a magnetron sputtering technology, where a radio-frequency (RF) sputtering power of 150 W and a gas flow ratio of nitrogen (N2) to argon (Ar) of 10/40 sccm are used. During the sputtering deposition processes, a background pressure is typically 2.4×10−4 Pa and a working pressure is 0.6 Pa. The sputtering target is 4” hBN with a purity of 99.99%. Firstly, the growth temperature reaches 600°C, followed by 60 minutes stabilization which ensures a uniform temperature across the substrate. Subsequently, the growth is conducted.
As we know, the DBR structure consists of a high refractive index film and a low refractive index film. The fabricated hBN film was used as a low refractive index layer with the refractive index of 1.69, and Al element was added into BN film to form a high refractive index layer. The BAlN films were prepared via double-power magnetron sputtering, using an hBN target and Al target to sputter together. The chamber temperature and background pressure were consistent with that of sputtered hBN film. Here, keeping the sputtering power of hBN unchanged, only the power of Al-target was adjusted. The RF-power of the hBN and Al targets was set to be 150 W and 50/75/100/125 W, respectively. After 60 min deposition, the target power and the center heater were turned off. The sample was taken out when the chamber came back to room temperature naturally.
The quality of the films has been characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), atomic force microscopy (AFM), Raman spectra and X-ray photoelectron spectroscopy (XPS). The refractive index and thickness of all the samples were characterized by ellipsometry. The reflectivity of all the fabricated DBR structures was measured by an ultraviolet (UV) spectrophotometer.
3. Results and discussion
Figure 1(a) shows a typical SEM image of the hBN film on a sapphire substrate prepared by the magnetron sputtering technology. With 60 minutes deposition, an hBN film with a thickness of ∼65 nm has been obtained as shown in the insert. TEM was utilized to characterize the structure of the hBN film. Figure 1(b) displays a typical top-view high-resolution TEM (HRTEM) image from the hBN transferred onto Cu grids. The image clearly reveals the characteristic hBN lattice constant of 0.25 nm, indicating that the hBN film is well crystallized [13 –15]. The insert in Fig. 1(b) shows the typical AFM image obtained by scanning an area of 4 μm×4 μm, indicating a root-mean-square (RMS) surface roughness of ∼2.9 nm. This illustrated that the prepared hBN films can achieve good smoothness in a large area.
Fig. 1. (a) The top view SEM and the cross-sectional SEM images of hBN film. (b) The HRTEM and AFM images (4 μm×4μm) of the hBN film.
In order to further verify the properties of hBN films, Raman spectroscopy and XPS analysis were used. The Raman spectrum exhibits a peak at 1378 cm-1 (see Supplement 1, Fig. S1) with the full width at half maximum (FWHM) value of 46 cm-1. XPS core level spectra can provide valuable information on the bonding mechanism of boron in hBN film. The B 1s and N 1s peaks from the XPS survey were deconvoluted (see Supplement 1, Fig. S2). From the XPS spectra, the main binding energies of the B 1s and N 1s orbital are 190.78 eV and 398.30 eV, respectively, which confirm the formation of hBN [16].
To characterize the optical properties of the hBN film, a 2 inch wafer-scale sample with the thickness of 65 nm was prepared on sapphire, as shown in the insert-I of Fig. 2. The UV-Vis absorption spectrum of the hBN film was shown in Fig. 2. A strong peak is observed at 202 nm in the absorption spectrum and the corresponding wavelength of absorption edge is 214 nm. By using the formula for a direct band semiconductor [17], the optical band gap is determined to be 5.86 eV as shown in the inset-II of Fig. 2, which is very close to theoretical calculations and the previous experimental data [18–19]. The measured band gap also suggests the as-grown hBN film to be highly transparent and electrically insulating [20].
Fig. 2. (a) The UV-Vis absorption spectrum of hBN film. The insert-I is the picture of hBN film fabricated on a 2 inch substrate of sapphire. The insert-II shows optical band gap (5.86 eV) analysis of hBN from the absorbance spectrum.
The prepared BAlN films were sputtered with dual power sources, and an Al target sputtering was added to the process for preparing hBN. The refractive index and thickness of all the BAlN samples were characterized by ellipsometry with a helium-neon laser (the wavelength of 633 nm). The detailed results are shown in Table 1, which shows the thickness, RMS, reflective index, and optical band-gap of BAlN films. The Al content was obtained by XPS analysis. With the increase of Al target power, the content of the Al component in the corresponding BAlN films increased significantly. This is due to the high power of Al target, that is, a higher negative bias pressure was added to Al target, which increased the ionization rate of the gas around the target and generated more Ar ions to bombard the target surface to sputter more Al, thereby increasing the probability of Al binding to N and the content of the Al component in BAlN film. The thickness and refractive index were also increased with the increase of Al target power. The difference of refractive index reached to 0.34, which made it possible to prepare DBR. The absorption spectra for a series of BAlN films were measured and the band gaps were calculated. The absorption spectra and band gap curves were presented in Supplement 1. The absorption peaks of the four BAlN samples were all in the ultraviolet region (Fig. S3), with peaks at 196 nm, 195 nm, 193 nm and 192 nm, respectively. Moreover, with the increase of the Al component, the absorption peak of BAlN has a blue shift. After calculating the band gap of the samples, according to their absorption spectra, we found the band gap of BAlN narrowed with the increase of the Al-component content (Fig. S4). The first principle was used to calculate the band gap of the BAlN film with Al doped into BN and maintain the graphene-like structure (Fig. S5, Supplement 1) via establishing a model in Materials Studio. With the increase of Al content (Fig. S6), the band gap of BAlN film decreased (Fig. S7), which was consistent with the experimental results (Table 1). However, changing the Al content can only adjust the BAlN band gap in a small range. The BAlN films are also suitable for the preparation of deep-UV DBRs.
Table 1. The characteristic parameters of BAlN films under different Al-powers.
As shown in Fig. 3, the refractive indexes of four samples in the band of 250 nm to 1000 nm were tested to explore the influence of Al content on the refractive index of BAlN films. Quite evidently, the higher Al content corresponds to a higher refractive index and the refractive index is between hBN (n=1.69) and AlN (n=2.2). Thus, introducing Al can significantly change the optical properties of hBN and establish a certain foundation for its optical application as a functional film. In deep ultraviolet bands (<270 nm), the absorption of materials is unusual, the refractive index may appear abnormal, just as curve B.
As a result of the high refractive index contrast (exceeding 0.375 at 300 nm) and the band gap of BAlN (Table 1), improved DBRs based on hBN/BAlN structures with high reflectivity can be theoretically achieved in the deep UV region. Based on hBN (sample-A in Table 1) and BAlN (sample-D in Table 1) as the low/high refractive index layer and the film thicknesses were designed to be quarter-wave layers, the hBN/BAlN DBRs structures were prepared with a target center wavelength located around 287 nm. The reflectivity of DBR was calculated based on R=1- 4(n L/n H)N. (where N is the number of periods composing the DBR. The n L and n H represent low refractive index and high refractive index, respectively.) [21] Next, a series of hBN/BAlN DBRs were grown on sapphire substrate directly.
Figure 4(a) shows the SEM image of a 9-pair DBR structure. The bottom is the sapphire substrate and the upper is the hBN/BAlN DBR structures. A pair was made up of a dark layer (hBN) and a bright layer (BAlN). The thickness of hBN layer is 39 nm and that of BAlN layer is 33 nm. The interface is not very clear in a pair, but the boundary between pair and pair is very clear. When BAlN film is deposited on hBN thin film, the RF-power and chamber conditions of the hBN target are unchanged, and then the sputtering of Al-target is added. With Al doping, the process is connected smoothly and naturally. Therefore, the interface is not obvious. When a pair is completed, the power of the Al-target is turned off and all baffles including the hBN-target are closed. The hBN-target keeps sputtering for 5 min to remove the surface fouling layer, and then opens the baffle of the hBN. In this way, the boundary is clearer. Generally speaking, the thickness of each period is uniform and the interface is clear.
Fig. 4. (a) Cross section of a 9-pair hBN/BAlN stack grown on sapphire substrate. (b) Experimental reflectivity spectrum of 3/6/9-pair hBN/BAlN DBR. The spectra are centered around at 300 nm.
Figure 4(b) shows the experimental reflectivity of the fabricated 3/6/9-pair DBR structures. The reflectivity increases gradually with the increase of pair number. A reflectivity of 90% and a bandwidth of 45 nm at the central wavelength of 300 nm were obtained for only 9 pairs. At the same time, the central wavelength has a red shift with the increase of pair number, which is from 289 nm of 3 pairs to 294 nm of 6 pairs, and finally to 300 nm of 9 pairs. The main reason is that the several nm of surface roughness of each layer is accumulated, the more the number of DBR pairs, the larger the deflection of whole DBR thickness, resulting in the shift of central wavelength.
From the above results, the hBN/BAlN DBRs could achieve a high reflectivity of 90% in a wide bandwidth of deep-UV region with a minimum number of pairs. In order to further realize the high reflectivity in the solar blind band, 6-pair hBN/BAlN DBRs were prepared by optimizing the film thickness. The thickness of the hBN layer and BAlN layer is 35 nm and 29 nm, respectively. Figure 5 shows the experimental and calculated reflectivity spectra of 6-and 16-pair hBN/BAlN DBR structures. At 280 nm, a reflectivity of 52% was obtained under 6 pairs. Under the conditions of this hBN/BAlN film, the reflectivity was simulated by using the finite difference time domain (FDTD) solution. The 16-pair DBRs exhibit a peak reflectivity of 84.3% and a bandwidth of 30 nm.
Fig. 5. Experimental and simulated reflectivity spectrum of 6/16-pair hBN/BAlN DBRs. The spectra are all centered at 280 nm.
4. Conclusion
In summary, we have reported on the growth and characterization of hBN/BAlN DBRs for peak reflectivity at 280 nm and 300 nm. Using the method of RF-sputtering, high-quality hBN films were prepared and the refractive index of BAlN films was adjusted. The reflectivity at 280 nm was reached 52% by only using 6-pair hBN/BAlN (35 nm/29 nm) DBRs, and the reflectivity at 300 nm was reached 90% with 9-pair hBN/BAlN (39 nm/33 nm) DBRs. Therefore, we believe that the hBN/BAlN DBRs can be used to achieve higher reflectivity in shorter UV bands with the improvement of BAlN material quality through the growth condition optimization. This DBR prepared via RF-sputtering technology will greatly promote the development of deep-UV optoelectronic devices.
Funding
Fundamental Research Funds for the Central Universities (XJJ2017011); National Key Research and Development Program of China (2016YFB0400801).
Acknowledgments
The SEM work was done at International Center for Dielectric Research (ICDR), Xi’an Jiaotong University.
Disclosures
All the authors declare no competing interests.
Data Availability
The data that support the findings of this study are available from the corresponding author upon reasonable request.
See Supplement 1 for supporting content.
References
1. S. Kako, T. Someya, and Y. Arakawa, “Observation of enhanced spontaneous emission coupling factor in nitride-based vertical-cavity surface-emitting laser,” Appl. Phys. Lett. 80(5), 722–724 (2002). [CrossRef]
2. M. Diagne, Y. He, H. Zhou, E. Makarona, and A. V. Nurmikko, “Vertical cavity violet light emitting diode incorporating an aluminum gallium nitride distributed Bragg mirror and a tunnel junction,” Appl. Phys. Lett. 79(22), 3720–3722 (2001). [CrossRef]
3. H. Klausing, F. Fedler, J. Dänhardt, R. Jaurich, A. Kariazine, S. Günster, D. Mistele, and J. Graul, “Electron Beam Pumped Nitride Vertical Cavity Surface Emitting Structures with AlGaN/AlN DBR Mirrors,” Phys. Status Solidi C 194(2), 428–432 (2002). [CrossRef]
4. X. L. Ji, R. L. Jiang, B. Liu, Z. L. Xie, J. J. Zhou, L. Li, P. Han, R. Zhang, Y. D. Zheng, and J. G. Zheng, “Structural characterization of AlGaN/AlN Bragg reflector grown by metalorganic chemical vapor deposition,” Phys. Status Solidi A 205(7), 1572–1574 (2008). [CrossRef]
5. O. Mitrofanov, S. Schmult, M. J. Manfra, T. Siegrist, N. G. Weimann, A. M. Sergent, and R. J. Molnar, “High-reflectivity ultraviolet AlGaN∕AlGaN distributed Bragg reflectors,” Appl. Phys. Lett. 88(17), 171101 (2006). [CrossRef]
6. E. Feltin, J. F. Carlin, J. Dorsaz, G. Christmann, R. Butté, M. Laügt, M. Ilegems, and N. Grandjesn, “Crack-free highly reflective AlInN/AlGaNbragg mirrors for UV applications,” Appl. Phys. Lett. 88(5), 051108 (2006). [CrossRef]
7. A. Franke, M. P. Hoffmann, L. Hernandez-Balderrama, F. Kaess, I. Bryan, S. Washiyama, M. Bobea, J. Tweedie, R. Kirste, M. Gerhold, and R. Collazo, and, Z. Sitar, “Strain engineered high reflectivity DBRs in the deep UV,” Proc. SPIE 9748, 97481G (2016). [CrossRef]
8. Y.-S. Liu, Z. Lochner, T.-T. Kao, M. Satter, L. Mahbub, R. Xiao-Hang, S. Jae-Hyun, Y. Shyh-Chiang, P. Douglas, T. Detchprohm, R. D. Dupuis, Y. Wei, H. Xie, A. Fischer, and F. Ponce, “Optically pumped AlGaN quantum-well lasers at sub-250 nm grown by MOCVD on AlN substrates,” Phys. Status Solidi C 11(2), 258–260 (2014). [CrossRef]
9. M. Abid, T. Moudakir, G. Orsal, S. Gautier, A. EnNaciri, Z. Djebbour, J. H. Ryou, G. Patriarche, L. Largeau, H. J. Kim, Z. Lochner, K. Pantzas, D. Alamarguy, F. Jomard, R. D. Dupuis, J. P. Salvestrini, P. L. Voss, and A. Ougazzaden, “Distributed Bragg reflectors based on diluted boron-based BAlN alloys for deep ultraviolet optoelectronic applications,” Appl. Phys. Lett. 100(1), 011902 (2012). [CrossRef]
10. J. Li, Z. Y. Fan, R. Dahal, M. L. Nakarmi, J. Y. Lin, and H. X. Jiang, “200 nm deep ultraviolet photodetectors based on AlN,” Appl. Phys. Lett. 89(21), 213510 (2006). [CrossRef]
11. Y. Kobayashi, T. Akasaka, T. Makimoto, Y. Kobayashi, T. Akasaka, and T. Makimoto, “Hexagonal boron nitride grown by MOVPE,” J. Cryst. Growth 310(23), 5048–5052 (2008). [CrossRef]
12. Y. Kubota, K. Watanabe, O. Tsuda, and T. Taniguchi, “Deep ultraviolet light-emitting hexagonal boron nitride synthesized at atmospheric pressure,” Science 317(5840), 932–934 (2007). [CrossRef]
13. J. H. Meng, X. W. Zhang, H. Liu, Z. G. Yin, D. G. Wang, Y. Wang, J. B. You, and J. L. Wu, “Synthesis of atomic layers of hybridized h-BNC by depositing h-BN on graphene via ion beam sputtering,” Appl. Phys. Lett. 109(17), 173106 (2016). [CrossRef]
14. L. F. Wang, B. Wu, J. S. Chen, H. T. Liu, P. A. Hu, and Y. Q. Liu, “Monolayer Hexagonal Boron Nitride Films with Large Domain Size and Clean Interface for Enhancing the Mobility of Graphene-Based Field-Effect Transistors,” Adv. Mater. 26(10), 1559–1564 (2014). [CrossRef]
15. Xixi Tao, Lei Zhang, Xiaohong Zheng, Hua Hao, Xianlong Wang, Lingling Song, Zhi Zeng, and H Guo, “h-BN/graphene van der Waals vertical heterostructure: a fully spin-polarized photocurrent generator,” Nanoscale 10(1), 174–183 (2018). [CrossRef]
16. Tse-An Chen, Chih-Piao Chuu, Chien-Chih Tseng, Chao-Kai Wen, and Lain-Jong Li, “Wafer-scale single-crystal hexagonal boron nitride monolayers on Cu(111),” Nature 579(7798), 219–223 (2020). [CrossRef]
17. T. H. Yuzuriha and D. W. Hess, “Structural and optical properties of plasma-deposited boron nitride films,” Thin Solid Films 140(2), 199–207 (1986). [CrossRef]
18. A. Ismach, H. Chou, D. A. Ferrer, Y. Wu, S. McDonnell, H. C. Floresca, A. Covacevich, C. Pope, R. Piner, M. J. Kim, R. M. Wallace, L. Colombo, and R. S. Ruoff, “Toward the Controlled Synthesis of Hexagonal Boron Nitride Films,” ACS Nano 6(7), 6378–6385 (2012). [CrossRef]
19. Chaohua Zhang, Lei Fu, Shuli Zhao, Yu Zhou, Hailin Peng, and Zhongfan Liu, “Controllable Co-segregation Synthesis of Wafer-Scale Hexagonal Boron Nitride Thin Films,” Adv. Mater. 26(11), 1776–1781 (2014). [CrossRef]
20. H. Wang, X. Zhang, H. Liu, Z. Yin, J. Meng, J. Xia, X. Meng, J. Wu, and J. You, “Synthesis of Large-Sized Single-Crystal Hexagonal Boron Nitride Domains on Nickel Foils by Ion Beam Sputtering Deposition,” Adv. Mater. 27(48), 8109–8115 (2015). [CrossRef]
21. P. Lova, G. Manfredi, and D. Comoretto, “Advances in Functional Solution Processed Planar 1D Photonic Crystals,” Adv. Opt. Mater. 6(24), 1800730 (2018). [CrossRef]
