Abstract
1. Introduction
AlN is of great potential in the implantation of deep ultraviolet (DUV) emitters because of its wide bandgap (~6.2 eV), excellent UV transparency and high thermal conductivity[
2. Experiment
In this experiment, AlN layers with a target thickness of 200 nm were deposited on 2-inch c-plane sapphire by radio frequency (RF) reactive magnetron sputtering. High-purity Al (> 99.9999 at%) was used as the target. The RF power was set to 2000 and 3000 W. N2 was used as the sputtering gas in the range of 100–180 sccm, and the substrate temperature was changed from 550 to 700 °C. The chamber background pressure was lower than 3.2 × 10–5 Pa, and the process pressure was varied from 0.31 to 0.55 Pa as the increase of N2 flux. The deposition rate of the sputtered particles was in the range of 0.20–0.35 nm/s, which is mainly determined by RF power[
3. Results and discussion
3.1. The effect of substrate temperature
Fig. 1 shows the (0002)- and (10
Figure 1.(Color online) The (a) (0002)- and (b) (10
To better comprehend the impact of substrate temperature on the crystallinity of annealed AlN films, AlN films with various substrate temperatures are applied. Fig. 2 shows the effect of substrate temperature on the FWHM values of (10
Figure 2.(Color online) The FWHM values of (10
The relationship between the FWHM values of (10
Figure 3.(Color online) The (0002)-plane XRCs of the AlN films sputtered at (a) 2000 and (b) 3000 W with various substrate temperatures before HT annealing.
Figs. 4(a) and 4(b) show the Raman spectrum of AlN films sputtered at various temperatures before annealing. The E2(high) peak frequency is presented in Fig. 4(c). The peak intensity of strain-free AlN films was reported at the phonon frequency of 657.4 cm–1[
Figure 4.(Color online) Raman spectrum of the AlN films sputtered at (a) 2000 and (b) 3000 W, a N2 flux of 100 sccm, and various substrate temperatures before annealing. (c) The
Fig. 5 shows AFM images (5 × 5 μm2) of the AlN films sputtered at 2000 W and a N2 flux of 100 sccm with different substrate temperatures before and after HT annealing. The surface morphologies of the sputtered AlN films show high-density and uniform column structures in Fig. 5(a)[
Figure 5.(Color online) 5 × 5
Figure 6.The effect of N2 flux on the FWHM values of (10
3.2. The effect of N2 flux
N2 has two functions during the sputtering process, involving that (1) N2 is ionized as N source to bombard the Al target and form AlN and (2) N2 acts as environment gas and collides with sputtered particles. To reveal the effect of N2 flux during the sputtering process on the crystallinity of annealed AlN films, three different N2 fluxes of 100, 150, 180 sccm are used to deposit AlN at 600 °C. Fig. 6 shows the FWHM values of (10
To further analyze the crystallinity evolution of annealed AlN films with the increase of N2 flux during sputtering, the tilt component of sputtered AlN films is investigated. Fig. 7 shows the (0002)-plane XRCs of the AlN films sputtered with various N2 fluxes at 600 °C, 2000 W and 3000 W before annealing. For the AlN films sputtered at 2000 W, all samples have side peaks. Furthermore, the normalized intensity of side peaks decrease with decreasing N2 flux, which means the less tilt component. The 3000 W case has a similar tendency. During sputtering, the process pressure is decreased as the decrease of N2 flux. This will result in a longer mean free path of particles, which means that sputtered Al particles from Al target have less possibility to collide with N2 molecules from target to substrate. Thus, less energy of the sputtered Al will be lost in this process and AlN will deposit on the surface with higher energy, which will lead to less tilt component. This lower tilt component will possibly improve the vacancy diffusion. Hence, the FWHM values of (10
Figure 7.(Color online) The (0002)-plane XRCs of the AlN films sputtered at (a) 2000 and (b) 3000 W before annealing with various N2 fluxes.
Fig. 8 presents AFM images (5 × 5 μm2) of the sputtered AlN films sputtered at 2000 W, 600 °C with various N2 fluxes before and after HT annealing. Before annealing, the size and altitude of column structures are increased as the increase of N2 flux and the RMS values are increased from 1.51 to 1.93 nm. After HT annealing, step-bunching structures are formed due to the coalescence of column structures. Moreover, high-altitude columns tend to disappear with the increase of N2 flux. However, lots of voids are observed for the annealed AlN films with a N2 flux of 180 sccm, which are ascribed to the remained residual spaces after the coalescence of domains[
Figure 8.(Color online) AFM images of the AlN films sputtered at 2000 W and 600 °C with N2 fluxes of (a) 100, (b) 150 and (c) 180 sccm before annealing and (d) 100, (e) 150 and (f) 180 sccm after annealing.
3.3. The effect of RF power
The effect of RF power can be analyzed by comparing the 2000 and 3000 W cases in the previous parts. Fig. 3 and Fig. 7 show that the 3000 W case has narrower (0002)-plane XRC of sputtered AlN films than 2000 W case with the same other sputtering conditions. It is because the higher RF power provides high kinetic energy for AlN during sputtering which lead to less tilt component. However, the FWHM values of (10
By further reducing the RF power to 1000 W, the color of sputtered AlN films will change from transparent to light yellow due to the lower deposition energy (not shown here). The FWHM values of (10
4. Conclusion
The effect of sputtering parameters on the annealed AlN films is comprehensively studied. The high-quality annealed AlN films are obtained at lower substrate temperature, lower N2 flux, and lower RF power. The lower substrate temperature and lower RF power provide less energy for AlN deposition during sputtering, which leads to the increased tilt component and high vacancy concentration. A high vacancy will increase dislocation annihilation by improving dislocation climb during HT annealing. However, a vacancy concentration that is too high or too much of a tilt component will prevent the vacancy from diffusion or stop the dislocation climb. The transformation from compressive stress to tensile stress of sputtered AlN films may also be useful for AlN columns to twist. Lower N2 flux reduces the energy loss by decreasing the collision possibility of sputtered Al particles and N2 molecules, and thus improves the tilt component, which will improve vacancy diffusion. Finally, the FWHM of the (0002) and (10
Acknowledgements
This work was supported by the National Key R&D Program of China (Nos. 2016YFB0400800 and 2017YFB0404202), the National Natural Sciences Foundation of China (Grant Nos. 61527814, 61674147, 61904176, U1505253), Beijing Nova Program Z181100006218007 and Youth Innovation Promotion Association CAS 2017157.
References
[1] J M Li, Z Liu, Z Q Liu et al. Advances and prospects in nitrides based light-emitting-diodes. J Semicond, 37, 061001(2016).
[2] Z H Wu, J C Yan, Y N Guo et al. Study of the morphology evolution of AlN grown on nano-patterned sapphire substrate. J Semicond, 40, 122803(2019).
[3] P Dong, J C Yan, Y Zhang et al. AlGaN-based deep ultraviolet light-emitting diodes grown on nano-patterned sapphire substrates with significant improvement in internal quantum efficiency. J Cryst Growth, 395, 9(2014).
[4] D Ehrentraut, Z Sitar. Advances in bulk crystal growth of AlN and GaN. MRS Bull, 34, 259(2009).
[5] R Dalmau, B Moody, R Schlesser et al. Growth and characterization of AlN and AlGaN epitaxial films on AlN single crystal substrates. J Electrochem Soc, 158, H530(2011).
[6] F J Xu, L S Zhang, N Xie et al. Realization of low dislocation density AlN on a small-coalescence-area nano-patterned sapphire substrate. CrystEngComm, 21, 2490(2019).
[7] J C Yan, J X Wang, N X Liu et al. High quality AlGaN grown on a high temperature AIN template by MOCVD. J Semicond, 30, 103001(2009).
[8] J C Yan, J X Wang, Y Zhang et al. AlGaN-based deep-ultraviolet light-emitting diodes grown on High-quality AlN template using MOVPE. J Cryst Growth, 414, 254(2015).
[9] X Chen, Y Zhang, J C Yan et al. Deep-ultraviolet stimulated emission from AlGaN/AlN multiple-quantum-wells on nano-patterned AlN/sapphire templates with reduced threshold power density. J Alloy Compd, 723, 192(2017).
[10] P Dong, J C Yan, J X Wang et al. 282-nm AlGaN-based deep ultraviolet light-emitting diodes with improved performance on nano-patterned sapphire substrates. Appl Phys Lett, 102, 241113(2013).
[11] X Chen, J C Yan, Y Zhang et al. Improved crystalline quality of AlN by epitaxial lateral overgrowth using two-phase growth method for deep-ultraviolet stimulated emission. IEEE Photonics J, 8, 1(2016).
[12] Z J Du, R F Duan, T B Wei et al. Producing deep UV-LEDs in high-yield MOVPE by improving AlN crystal quality with sputtered AlN nucleation layer. J Semicond, 38, 113003(2017).
[13] C Huang, P Wu, K Chang et al. High-quality and highly-transparent AlN template on annealed sputter-deposited AlN buffer layer for deep ultra-violet light-emitting diodes. AIP Adv, 7, 055110(2017).
[14] N Susilo, S Hagedorn, D Jaeger et al. AlGaN-based deep UV LEDs grown on sputtered and high temperature annealed AlN/sapphire. Appl Phys Lett, 112, 041110(2018).
[15] M X Wang, F J Xu, N Xie et al. Crystal quality evolution of AlN films via high-temperature annealing under ambient N2 conditions. CrystEngComm, 20, 6613(2018).
[16] S Tanaka, K Shojiki, K Uesugi et al. Quantitative evaluation of strain relaxation in annealed sputter-deposited AlN film. J Cryst Growth, 512, 16(2019).
[17] S Y Xiao, R Suzuki, H Miyake et al. Improvement mechanism of sputtered AlN films by high-temperature annealing. J Cryst Growth, 502, 41(2018).
[18] T Kumada, M Ohtsuka, K Takada et al. Influence of sputter power and N2 gas flow ratio on crystalline quality of AlN layers deposited at 823 K by RF reactive sputtering. Phys Status Solidi C, 9, 515(2012).
[19] H Miyake, C H Lin, K Tokoro et al. Preparation of high-quality AlN on sapphire by high-temperature face-to-face annealing. J Cryst Growth, 456, 155(2016).
[20] H Fukuyama, H Miyake, G Nishio et al. Impact of high-temperature annealing of AlN layer on sapphire and its thermodynamic principle. Jpn J Appl Phys, 55, 05FL02(2016).
[21] S Washiyama, Y Guan, S Mita et al. Recovery kinetics in high temperature annealed AlN heteroepitaxial films. J Appl Phys, 127, 115301(2020).
[22] T Kumada, M Ohtsuka, H Fukuyama. Influence of substrate temperature on the crystalline quality of AlN layers deposited by RF reactive magnetron sputtering. AIP Adv, 5, 017136(2015).
[23] F Medjani, R Sanjinés, G Allidi et al. Effect of substrate temperature and bias voltage on the crystallite orientation in RF magnetron sputtered AlN thin films. Thin Solid Films, 515, 260(2006).
[24] S B Yang, R Miyagawa, H Miyake et al. Raman scattering spectroscopy of residual stresses in epitaxial AlN films. Appl Phys Express, 4, 031001(2011).
[25] Q X Guo, K Yahata, T Tanaka et al. Low-temperature growth of aluminum nitride on sapphire substrates. J Cryst Growth, 257, 123(2003).
[26] L Zhao, K Yang, Y J Ai et al. Crystal quality improvement of sputtered AlN film on sapphire substrate by high-temperature annealing. J Mater Sci: Mater Electron, 29, 13766(2018).
Set citation alerts for the article
Please enter your email address