Low temperature growth of (AlGa)₂O₃ films by oxygen radical assisted pulsed laser deposition

Abstract

Low temperature growth of β-(AlGa)₂O₃ films has been realized by oxygen radical assisted pulsed laser deposition. The prepared films show a good (−201) orientation perpendicular to (0001) sapphire substrates even at a deposition temperature as low as 200 °C. The influences of the substrate temperature on the structural and optical properties of the films have been systematically investigated. All the films grown at substrate temperatures from 100 to 500 °C exhibit a high transmittance of over 90% in the ultraviolet and visible range. Abrupt bandgap value variation has been observed for films deposited at substrate temperatures higher than 100 °C, which agrees with the amorphous to crystalline transition temperature evidenced by X-ray diffraction. The speed of the film thickness decrease with substrate temperature is much slower for (AlGa)₂O₃ films grown with oxygen radical assistance, indicating the suppression of the evaporation of volatile species with the help of oxygen radical species. The low temperature growth of β-(AlGa)₂O₃ films could be compatible with the established lithography of semiconductor microfabrication processes.

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1. Introduction

(AlGa)₂O₃, which has a tunable bandgap between 4.9 (Ga₂O₃) and 8.7 eV (α-Al₂O₃) and high stability in air at elevated temperatures with respect to other nitride and oxide systems, has recently emerged as a promising material for optoelectronic applications.¹ Okumura et al. have demonstrated the assembly of lateral field-effect transistors using Sn-doped β-(AlGa)₂O₃.² Feng et al. have found that (AlGa)₂O₃ can be a good candidate for solar-blind photodetectors.³ A modulation-doped two dimensional electron gas at a β-(Al_0.2Ga_0.8)₂O₃/Ga₂O₃ heterojunction has been realized by Krishnamoorthy et al.⁴

The above mentioned (AlGa)₂O₃-based solid-state devices are almost based on material structures created by thin film deposition. Thus, the deposition technology can be regarded as one of the keys to the fabrication of (AlGa)₂O₃-based devices. Various growth techniques have been explored to deposit (AlGa)₂O₃ films, including sputtering,⁵ molecular beam epitaxy,^6,7 mist chemical vapor deposition,⁸ metal–organic chemical vapor deposition⁹ and pulsed laser deposition (PLD).^10–12 The reported growth temperature for the crystalline (AlGa)₂O₃ film was varied from 350 to 800 °C, depending on the deposition technology mentioned above. However, in order to be compatible with the established lithography of semiconductor microfabrication processes, a lower growth temperature is preferred.^13,14

Among various deposition methods, PLD is a good candidate technology for low temperature growth of (AlGa)₂O₃ films because the ablated species generated in the PLD process have relatively high kinetic energies. In another aspect, the oxygen radical has been found to be an effective assistant species for decreasing the growth temperature of thin films. Thus, the combination of PLD and oxygen radical assistance should be a very effective method for low temperature film growth. Shao et al.¹⁵ found that oxygen radicals are effective in realizing low temperature growth of nanocrystalline ZnO thin films by PLD. Room-temperature deposition of Al-doped ZnO films by oxygen radical-assisted PLD has been reported by Matsubara et al.¹⁶ We have found that β-Ga₂O₃ thin films with a (−201) orientation can be fabricated at a substrate temperature as low as 200 °C by using plasma-assisted PLD. However, low temperature growth of (AlGa)₂O₃ films has not been reported till now, to the best of our knowledge.

In this work, we report the growth of (AlGa)₂O₃ using oxygen radical-assisted PLD. The substrate temperature influence on the structural and optical properties of (AlGa)₂O₃ has been discussed in detail.

2. Experimental

(AlGa)₂O₃ films were deposited by the PLD method on (0001) sapphire substrates. Prior to deposition, the substrates were cleaned using an (a) ultrasonic methanol bath, followed by an ultrasonic acetone bath, (b) with a mixture of phosphoric acid and sulfuric acid, and finally, with (c) deionized water. A 99.99% pure (AlGa)₂O₃ ceramic with an Al₂O₃ weight content of 20 wt% was set as a target. The (AlGa)₂O₃ films were deposited using a laser pulse energy of 225 mJ, a repetition rate of 2 Hz, and an oxygen pressure of 0.1 Pa, and the distance between the substrate and the target is 5 cm. Oxygen radicals were generated by passing high purity oxygen gas (99.999%) through an oxygen plasma cell with an RF power set at 300 W. Five samples of the (AlGa)₂O₃ films were fabricated at substrate temperatures of 100, 200, 300, 400, and 500 °C. The deposition time was 2 hours for all the samples.

The thickness of the films was determined using a step profile analyzer. X-ray diffraction (XRD) was carried out on a PANalytical MRD Pro system in order to investigate the crystal structure of the samples. The optical transmission spectra were recorded using a UV-visible spectrometer. X-ray photoelectron spectroscopy (XPS) measurements were performed using an Al Kα X-ray source. The position of the C 1s peak was used as a standard reference. The surface morphologies were studied by atomic force microscopy (AFM) under ambient conditions.

3. Results and discussion

Fig. 1 shows the AFM images of the (AlGa)₂O₃ films grown at substrate temperatures from 100 to 500 °C. The surface morphologies of the films change with the increase of substrate temperature. Smooth surfaces are obtained for the films deposited at substrate temperatures lower than 300 °C while the surfaces of the films deposited at substrate temperatures of 400 and 500 °C are composed of elongated islands.

Fig. 1 AFM images of (AlGa)₂O₃ films grown at different substrate temperatures of (a) 100, (b) 200, (c) 300, (d) 400 and (e) 500 °C, respectively.

Fig. 2 shows the thickness of the (AlGa)₂O₃ films grown at different substrate temperatures. The film thickness shows a decreasing tendency with the increase of substrate temperature. The decrease of film thickness with increasing substrate temperature is often related to the morphologies of the films. The morphology of the film changes with the homologous temperature T_s/T_m (T_s: substrate temperature, T_m: melting temperature of the film material), according to the empirical structure zone model.¹⁷ Based on this model, the film becomes dense which results in the decrease of the film thickness with the increase of the substrate temperature.^18–21 However, this model seems to be not the main factor because the films deposited at low temperatures show smooth surfaces in our case. The decrease of film thickness with increasing substrate temperature in this work is attributed to the re-evaporation of the adsorbed species on the surface of the substrate. The decrease of film thickness with increasing substrate temperature has also been observed on Ga₂O₃ films as well as on (AlGa)₂O₃ films grown by PLD without oxygen radical assistance in our previous work.^22,23 However, the speed of film thickness variation ((T₃₀₀–T₅₀₀)/T₃₀₀, where T₃₀₀ and T₅₀₀ are the thickness values of films deposited at 300 and 500 °C, respectively) in this work (5.3%) is much slower than that of (AlGa)₂O₃ films grown without oxygen radical assistance (25%).²³ The relatively smaller variation of the growth rate with substrate temperature in this work indicates the suppression of the evaporation of volatile Ga₂O species with the help of oxygen radical species.

Fig. 2 Temperature dependence of film thickness of (AlGa)₂O₃ films deposited by oxygen radical-assisted PLD.

In order to investigate the stoichiometry of the (AlGa)₂O₃ films, XPS measurements were carried out. Fig. 3(a) shows the stacked XPS survey scans of the (AlGa)₂O₃ films deposited at different substrate temperatures. From the spectra, elements Ga, O, C and Al have been observed. No other elements can be found from the survey scans, indicating that high purity (AlGa)₂O₃ films are obtained. The Ga 2p core level spectra of the (AlGa)₂O₃ films deposited at different substrate temperatures are shown in Fig. 3(b). In all cases, the binding energy of Ga 2p remains at 1117.7 eV and has been attributed to Ga–O bonding.²⁴ The Al 2p core level spectra of the (AlGa)₂O₃ films deposited at different substrate temperatures are shown in Fig. 3(c). The Al 2p peaks for the films deposited from 200 to 500 °C are all located at 74.1 eV. The Al 2p peak for the film deposited at 100 °C is located at about 73.9 eV. These values are smaller than that of the pure Al₂O₃ film (74.5 eV),²⁵ indicating the formation of Al–O–Ga bonds. Hu et al.²⁶ have found that the Al 2p peak shifts to a higher binding energy with increasing Al composition. We have investigated the XPS spectra of the (AlGa)₂O₃ films with different Al contents and also found that the Al 2p peak shifts towards a higher binding energy with the increase of Al content.²⁵ The Al content (Al/(Ga + Al)) and the oxygen content were estimated from the XPS core level peak area after applying an atomic sensitivity factor, as shown in Fig. 3(d). The Al contents in the films deposited at substrate temperatures from 200 to 500 °C are all about 0.126 while the Al content in the film deposited at 100 °C is 0.14, which agrees well with the peak shift of Al 2p observed in Fig. 3(c). It is noticeable that the oxygen contents in the films deposited at substrate temperatures of 100, 200, 300, 400 and 500 °C are about 0.60, 0.58, 0.58, 0.55 and 0.56, respectively, as shown in Fig. 3(d), verifying the formation of the (AlGa)₂O₃ films.

Fig. 3 (a) XPS survey scans, (b) Ga 2p core level, (c) Al 2p core level and (d) Al and oxygen contents of (AlGa)₂O₃ films grown at different substrate temperatures by oxygen radical-assisted PLD.

In order to investigate the crystal structure of the grown (AlGa)₂O₃ films, XRD patterns were recorded in the 2θ range of 10 to 70° as shown in Fig. 4. For the film grown at 100 °C, no obvious (AlGa)₂O₃ peak can be observed, indicating the amorphous nature of the film. For the film deposited at a substrate temperature of 200 °C, two obvious peaks located at 2θ values of about 38° and 59° have been observed, which has been ascribed to the (−402) and (−603) faces of monoclinic structured β-(AlGa)₂O₃. For the film deposited at a substrate temperature of 300 °C, one more peak located at 19° appears, which has been ascribed to the (−201) face of β-(AlGa)₂O₃. The existence of the sloping background that increases towards the small angles in the XRD patterns suggests that the films have amorphous components.²⁷ Further increasing the substrate temperature to 400 and 500 °C caused the appearance of additional XRD peaks located at 31° and 45°, which can be ascribed to the (−401) and (−601) faces of the β-(AlGa)₂O₃ film. From the above results, it is obvious that (−201) oriented β-(AlGa)₂O₃ films have been obtained at substrate temperatures from 200 to 300 °C. The degradation of orientation for films grown at higher temperatures has been observed by several groups.^28,29 Jones et al.²⁸ reported the degradation of film orientation with substrate temperature. Increasing the substrate temperature will increase the surface mobility of the deposited atoms and tend to favor the development of an equilibrium structure.^21,28,30 It is noticeable that the substrate temperature variation has caused simultaneous microstructure evolution and preferred orientation change as observed in Fig. 1 and 4. These variations are similar to radio frequency magnetron sputtered ZnO thin films reported by Lee et al.²⁹ Films deposited at lower temperatures have a smooth structure with a high orientation, which is generally thought to be developed because of the low adatom mobility and a shadowing effect.^31–33 It is clear that the adatoms already have suitable kinetic energy and mobility for growing highly oriented films at low substrate temperatures. Thus, the adatoms at high substrate temperatures may have sufficient adatom mobility to overcome the self-shadowing effect, resulting in the growth of large elongated grains with degraded orientation.^29,34

Fig. 4 XRD patterns of (AlGa)₂O₃ films deposited at different substrate temperatures by oxygen radical-assisted PLD.

The transmission spectra of the (AlGa)₂O₃ films deposited at different substrate temperatures are shown in Fig. 5(a). The presence of interference fringes indicates good surface homogeneity. All the films exhibit a high transmittance of over 90% in the ultraviolet and visible region. No multiple absorption edges, a typical characteristic of phase separation, can be observed for all the films. A sharp absorption edge near 230 nm, which is caused by the fundamental absorption of light for (AlGa)₂O₃ films, has been observed for all the films. The absorption edges of the (AlGa)₂O₃ films grown at temperatures from 200 °C to 500 °C are similar to each other while that of the film deposited at 100 °C has shifted toward a longer wavelength region. In order to determine the bandgap values of the (AlGa)₂O₃ films, the Tauc equation was applied: (αhν) = A(αhν − E_g)^1/2, where E_g represents the bandgap energy, hν is the photon energy and α is the absorption coefficient. A linear character of (αhν) vs. hν has been observed in Fig. 5(b), confirming the direct transition type of the obtained (AlGa)₂O₃ films.²⁵ By entrapping the linear part of the Tauc curves intercepting the energy axis (at αhν = 0), the bandgap values have been obtained. The obtained bandgap values of the (AlGa)₂O₃ films deposited at substrate temperatures from 200 to 500 °C are 5.34 eV while the bandgap value of the (AlGa)₂O₃ film deposited at 100 °C is 5.09 eV with uncertainties of 0.01 eV. We have investigated the evolution of the optical properties and band structures from amorphous to crystalline Ga₂O₃ films. We found an abrupt bandgap value variation when amorphous to crystalline transition took place.³⁵ The bandgap variation of (AlGa)₂O₃ films in this work is similar to that of our previously reported Ga₂O₃ films; the bandgap value of the (AlGa)₂O₃ films changed abruptly when the substrate temperature was above 100 °C, at which amorphous to crystalline transition took place.

Fig. 5 (a) Transmission spectra and (b) (αhν)²versus hν plots of (AlGa)₂O₃ films deposited at different substrate temperatures by oxygen radical-assisted PLD.

From the above results, it is evidenced that the crystalline temperature for β-(AlGa)₂O₃ films grown by oxygen radical-assisted PLD can be as low as 200 °C. We have also deposited (AlGa)₂O₃ films by PLD without radical assistance at various substrate temperatures and found that the crystalline substrate temperature for the (AlGa)₂O₃ film is about 350 °C.²³ Shao et al.¹⁵ found that the growth temperature of c-axis oriented ZnO films can be lowered to 100 °C using plasma-assisted pulsed laser deposition. Lim et al.³⁶ found that the SiO₂ films can be obtained at temperatures below 250 °C by plasma enhanced atomic layer deposition. The oxygen radical should benefit the low temperature growth of β-(AlGa)₂O₃ films by two aspects in this work: (1) providing reactive oxygen species for the formation of (AlGa)₂O₃ precursors through the reaction with the ablation species and (2) enhancing the mobility of the adatoms on the substrate surface, which could obtain enough energy to occupy energy-favored locations in the lattice for oriented crystallographic growth.¹⁵

Conclusions

(AlGa)₂O₃ films were grown on (0001) sapphire substrates by oxygen radical-assisted PLD. The prepared films show a good (−201) orientation perpendicular to the substrates even at a deposition temperature as low as 200 °C. All the films deposited from 100 to 500 °C show a high transmittance of over 90% in the UV and visible range. Abrupt bandgap value variation has been observed for films deposited at substrate temperatures higher than 100 °C, which agrees with the amorphous to crystalline transition temperature evidenced by XRD. The speed of the film thickness decrease with substrate temperature is much slower for (AlGa)₂O₃ films grown with oxygen radical assistance, indicating the suppression of the evaporation of volatile species with the help of oxygen radical species.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was partially supported by the Japan Society for Promotion of Science (JSPS) for providing grants (KAKENHI Grant No. 19K04492 and. 18K04681) and the Partnership Project for Fundamental Technology Researches of the Ministry of Education, Culture, Sports, Science and Technology, Japan. One of the authors (Fabi Zhang) was sponsored by the Guangxi Scholarship Fund of Guangxi Education Department and the Guangxi Science and Technology Base and Talent Special Project (AD18281084).

References

1. B. W. Krueger, C. S. Dandeneau, E. M. Nelson, S. T. Dunham, F. S. Ohuchi and M. A. Olmstead, J. Am. Ceram. Soc., 2016, 99, 2467–2473.
2. H. Okumura, Y. Kato, T. Oshima and T. Palacios, Jpn. J. Appl. Phys., 2019, 58, SBBD12.
3. Q. Feng, X. Li, G. Han, L. Huang, F. Li, W. Tang, J. Zhang and Y. Hao, Opt. Mater. Express, 2017, 7, 1240–1248.
4. S. Krishnamoorthy, Z. Xia, C. Joishi, Y. Zhang, J. McGlone, J. Johnson, M. Brenner, A. R. Arehart, J. Hwang, S. Lodha and S. Rajan, Appl. Phys. Lett., 2017, 111, 023502.
5. C.-C. Wang, S.-H. Yuan, S.-L. Ou, S.-Y. Huang, K.-Y. Lin, Y.-A. Chen, P.-W. Hsiao and D.-S. Wuu, J. Alloys Compd., 2018, 765, 894–900.
6. T. Oshima, T. Okuno, N. Arai, Y. Kobayashi and S. Fujita, Jpn. J. Appl. Phys., 2009, 48, 070202.
7. E. Ahmadi, Y. Oshima, F. Wu and J. S. Speck, Semicond. Sci. Technol., 2017, 32, 035004.
8. K. Kaneko, K. Suzuki, Y. Ito and S. Fujita, J. Cryst. Growth, 2016, 436, 150–154.
9. R. Miller, F. Alema and A. Osinsky, IEEE Trans. Semicond. Manuf., 2018, 31, 467–474.
10. X. Wang, Z. Chen, F. Zhang, K. Saito, T. Tanaka, M. Nishio and Q. Guo, AIP Adv., 2016, 6, 015111.
11. R. Wakabayashi, T. Oshima, M. Hattori, K. Sasaki, T. Masui, A. Kuramata, S. Yamakoshi, K. Yoshimatsu and A. Ohtomo, J. Cryst. Growth, 2015, 424, 77–79.
12. Q. Feng, Z. Feng, Z. Hu, X. Xing, G. Yan, J. Zhang, Y. Xu, X. Lian and Y. Hao, Appl. Phys. Lett., 2018, 112, 072103.
13. D. Huang, F. Liao, S. Molesa, D. Redinger and V. Subramanian, J. Electrochem. Soc., 2003, 150, G412–G417.
14. V. Jousseaume, J. Cuzzocrea, N. Bernier and V. T. Renard, Appl. Phys. Lett., 2011, 98, 123103.
15. J. Shao, Y. Q. Shen, J. Sun, N. Xu, D. Yu, Y. F. Lu and J. D. Wu, J. Vac. Sci. Technol., B, 2008, 26, 214–218.
16. K. Matsubara, P. Fons, K. Iwata, A. Yamada and S. Niki, Thin Solid Films, 2002, 422, 176–179.
17. J. A. Thornton, J. Vac. Sci. Technol., A, 1975, 12, 830–835.
18. V. Torrisi and F. Ruffino, Surf. Coat. Technol., 2017, 315, 123–129.
19. S. Mukherjee and D. Gall, Thin Solid Films, 2013, 527, 158–163.
20. Y. Y. Choi and D. J. Choi, CrystEngComm, 2013, 15, 6963–6970.
21. L. Yu, L. Zhang, H. Song, X. Jiang and Y. Lv, CrystEngComm, 2014, 16, 3331–3340.
22. F. B. Zhang, K. Saito, T. Tanaka, M. Nishio and Q. X. Guo, J. Cryst. Growth, 2014, 387, 96–100.
23. X. Wang, Z. Chen, F. Zhang, K. Saito, T. Tanaka, M. Nishio and Q. Guo, Ceram. Int., 2016, 42, 12783–12788.
24. T. Mathew, Y. Yamada, A. Ueda, H. Shioyama and T. Kobayashi, Appl. Catal., A, 2005, 286, 11–22.
25. F. Zhang, K. Saito, T. Tanaka, M. Nishio, M. Arita and Q. Guo, Appl. Phys. Lett., 2014, 105, 162107.
26. Z. Hu, Q. Feng, J. Zhang, F. Li, X. Li, Z. Feng, C. Zhang and Y. Hao, Superlattices Microstruct., 2018, 114, 82–88.
27. T. D. Shen, C. C. Koch, T. L. McCormick, R. J. Nemanich, J. Y. Huang and J. G. Huang, J. Mater. Res., 1995, 10, 139–148.
28. M. I. Jones, I. R. McColl and D. M. Grant, Surf. Coat. Technol., 2000, 132, 143–151.
29. Y. E. Lee, J. B. Lee, Y. J. Kim, H. K. Yang, J. C. Park and H. J. Kim, J. Vac. Sci. Technol., A, 1996, 14, 1943–1948.
30. S. Maiti, S. Pal and K. K. Chattopadhyay, CrystEngComm, 2015, 17, 9264–9295.
31. F. Ruffino, I. Crupi, F. Simone and M. G. Grimaldi, Appl. Phys. Lett., 2011, 98, 023101.
32. J. A. Venables, G. D. T. Spiller and M. Hanbucken, Rep. Prog. Phys., 1984, 47, 399–459.
33. J. Carrey and J. L. Maurice, Phys. Rev. B: Condens. Matter Mater. Phys., 2001, 63, 245408.
34. P. Sundara Venkatesh and K. Jeganathan, CrystEngComm, 2014, 16, 7426–7433.
35. F. Zhang, H. Li, Y.-T. Cui, G.-L. Li and Q. Guo, AIP Adv., 2018, 8, 045112.
36. J. W. Lim, S. J. Yun and J. H. Lee, ETRI J., 2005, 27, 118–121.

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