High transmittance β-Ga2O3 thin films deposited by magnetron sputtering and post-annealing for solar-blind ultraviolet photodetector
Introduction
Solar-blind photodetectors, which merely sense deep ultraviolet (DUV) light having a cut-off wavelength of 280 nm, are usually very sensitive to detect weak signals, even under sun or artificial illumination due to their extremely low natural background [1,2]. Owing to this unique advantage, solar-blind photodetectors have a wide range of military and civil applications, such as missile tracking, secure communications, ozone hole detection, UV astronomy, chemical/biological analysis, fire detection, and corona detection, and so on [[3], [4], [5]]. Current researches in solar-blind photodetectors focus mostly on wide bandgap semiconductors (Eg > 4.4 eV) such as AlxGa1-xN, MgxZn1-xO, diamond, and gallium oxide (Ga2O3), etc. [[6], [7], [8], [9]]. However, the quality deterioration and phase segregation phenomena are often present AlxGa1-xN and MgxZn1-xO with high Mg and Al contents, respectively [10]. And for diamond, it is costly and its detection range is constrained to shorter than 225 nm due to a fixed bandgap of 5.5 eV [11]. Therefore, challenges still exist for these materials to be applicable for solar-blind photodetectors.
Ga2O3 is intrinsically suitable for solar-blind photodetection and avoids the complex and unmanageable alloying process due to the appropriate bandgap (4.5–5.0 eV), critical breakdown field (∼8 MV/cm), good transparency, high thermal and chemical stability [[12], [13], [14]]. Among the five phases known to Ga2O3 (α, β, γ, δ and ε), the stable monoclinic Ga2O3 (β-Ga2O3) belong to C2/m space group with a bandgap of 4.9 eV, and the lattice parameters of a = 12.23 Å, b = 3.04 Å, c = 5.80 Å and β = 103.7°, which has been extremely considered to fabricate DUV solar-blind photodetectors [15,16]. β-Ga2O3 materials including bulk crystals, thin films and various nanostructures have been exploited as the active layer of solar-blind photodetectors [6,[17], [18], [19], [20]]. A thin-film-type photodetector is more concerned due to its good repeatability, easy growth, more convenient for practical application, and so on. Therefore, numerous reported works have been focused on the β-Ga2O3 thin-film-based photodetectors [5,21,22]. At present, there are many methods to deposit β-Ga2O3 thin films, including physical vapor deposition (MBE [23], PLD [24], sputtering [25], etc.), chemical vapor deposition (MOCVD [26], ALD [27], HVPE [28], LPCVD [29], etc.) and also sol-gel coating [5]. Among these technologies, sputtering method is a low-cost, easy control, harmless and scalable to deposit Ga2O3 films with a relatively fast growth rate. Arora et al. [21] demonstrated an ultrahigh-performance and self-powered β-Ga2O3 thin film solar-blind photodetector fabricated on a cost-effective Si substrate using radio frequency magnetron sputtering. Qian et al. [30] reported on the p-type high insulating thin films were obtained by doping Mg into β-Ga2O3 thin film by radio frequency magnetron sputtering, and the β-Ga2O3:Mg film ultraviolet solar-blind photodetector showed good performance. Because the Ga2O3 thin films prepared directly by magnetron sputtering are amorphous, most other reports focused on the amorphous Ga2O3 films solar blind photodetectors [31,32]. Evidently, sputtering is one of the most popular deposition methods to grow Ga2O3 thin films for DUV solar blind photodetectors.
In this work, the amorphous Ga2O3 thin films with various sputtering powers were deposited on (0001) sapphire substrates by radio frequency magnetron sputtering. β-Ga2O3 thin films were obtained by post-annealing treatment. The structural and optical properties of β-Ga2O3 films deposited at various sputtering powers were investigated by various testing techniques. A metal-semiconductor-metal (MSM) structured solar-blind photodetector based β-Ga2O3 film with optimal structural and optical properties was fabricated, and solar-blind ultraviolet photodetector characteristics were discussed in details.
Section snippets
Experiment procedure
Gallium Oxide (Ga2O3) thin films were prepared on (0001) sapphire substrates by radio frequency (RF) magnetron sputtering at room temperature. A high-purity ceramic Ga2O3 (99.99% purity) disk (100 mm in diameter) was employed as the sputtering target. The sputtering chamber was first evacuated to a base pressure below 5.0 × 10−4 Pa with a turbo molecular pump, and then filled with Ar (99.999% purity) up to 2.0 Pa. The sputtering time is 1.5 h, and the sputtering powers are 60 W, 100 W, and 150 W
Results and discussion
Fig. 1(a)-(c) shows the top-view (left) and cross-sectional (right) FE-SEM images of the annealed Ga2O3 thin films deposited at various sputtering powers. It can be seen from the surface morphology that, when the sputtering power is low (60 and 100 W), the surface of the films is rough, and when the sputtering power is high (150 W), the surface of the film is relatively flat and the roughness is remarkably lowered. As we known, the sputtering power determines the energy of the ion bombardment
Conclusion
In summary, β-Ga2O3 thin films were successfully prepared by RF magnetron sputtering and post-annealing method. The structure and optical properties of the β-Ga2O3 thin films were investigated. The SEM images, XRD patterns and Raman spectra showed that all the films are pure β-Ga2O3 with single (01) preferred orientation, β-Ga2O3 film deposited at a high sputtering power had a relatively high crystal quality and flat surface structure. The UV–visible transmittance spectra demonstrated
Acknowledgments
The authors gratefully acknowledge support from the National Natural Science Foundation of China (Grant No. 51472038), the Natural Science Foundation of Chongqing (Grant Nos. cstc2016jcyjA0390), the Science and Technology Research Project of Chongqing Education Committee (Grant No. KJ1500319), the Foundation for the Creative Research Groups of Higher Education of Chongqing (No. CXTDX201601016).
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