Prerpints.org logo
Preprint
Article

Impact of Amorphous-to-Crystalline Transition on the Upconversion Luminescence in Er3+-Doped Ga2O3 Thin Films

Altmetrics

Downloads

112

Views

59

Comments

0

A peer-reviewed article of this preprint also exists.

Submitted:

05 February 2024

Posted:

05 February 2024

You are already at the latest version

Alerts
Abstract
Gallium oxide (Ga2O3) is an emerging wide bandgap semiconductor promising a wide range of important applications. However, mass production of high-quality crystalline Ga2O3 still suffers from limitations associated with poor reproducibility and low efficiency. Low-temperature grown amorphous Ga2O3 demonstrates comparable performance with its crystalline counterparts. Lanthanide Er3+-doped Ga2O3 (Ga2O3: Er) possesses great potential for developing light-emitting devices, photodetectors and optical waveguides. The host circumstance can exert a crystal field around the lanthanide dopants and strongly influence their photoluminescence properties. Here we present a systematical study of the impact of amorphous-to-crystalline transition on the upconversion photoluminescence in Ga2O3: Er thin films. Through controlling the growth temperature of Ga2O3: Er films, the upconversion luminescence of as-grown thin films are strongly enhanced over 100 times. Moreover, the variation of photoluminescence reflects the amorphous-to-crystalline transformation of the Ga2O3: Er thin films. These results will aid further design of favorable optoelectronic devices integrated with lanthanide-doped Ga2O3 thin films.
Keywords: 
Subject: Chemistry and Materials Science  -   Electronic, Optical and Magnetic Materials

1. Introduction

Gallium oxide (Ga2O3) as an emerging wide bandgap semiconductor has sparked intense interest during the past years for its superior chemical and thermal stability, radiation hardness, and notably high breakdown electric field (~ 8 MV/cm) [1]. Owing to the unique electric and optical characteristics, Ga2O3 has been explored as the workhorse for applications such as solar-blind ultraviolet photodetectors, power electronic devices, etc [2–4]. Given the ultrawide bandgap, Ga2O3 is regarded as an ideal host material for incorporating lanthanide element [5]. Lanthanide ions, referred to as magic dopants, are highly effective in promoting the electrical properties of the host materials [6–8], moreover, lanthanide dopants can introduce an additional luminescent functionality to the semiconductor [9]. The most stable oxidation state of lanthanide elements is the trivalent state (Ln3+), which exhibits efficient upconversion and downconversion luminescence. They demonstrate unique luminescent characteristics, including abundant energy levels, sharp emission bandwidths, large Stokes shift, and high photoresistance, making them an important class of phosphor materials [10–12].
Among lanthanide elements, Er3+-doped phosphors exhibit strong visible and near-infrared emissions from its intra-4f shell transition of Er3+ ions, enabling diverse applications including full color displays, biological imaging, optical storage, optical communications [13–16]. So far, Er3+-doped Ga2O3 has been developed for electroluminescence (EL) devices, light-emitting devices (LEDs), and photodetectors [17–20]. In terms of fundamental research and real applications, luminescent thin films are of great importance from both scientific and technological aspects [21]. To date, Er3+-doped Ga2O3 thin films have been fabricated using pulsed laser deposition (PLD) and radio frequency magnetron sputtering methods on various substrates. Ga2O3 has five different polymorphs, including corundum (α), monoclinic (β), defective spinel (γ), cubic (δ), orthorhombic (ε) structures. Among them, β-Ga2O3 is the most stable form under ambient condition, and has been extensively investigated for various applications [22,23]. However, fabrication of single-crystalline β-Ga2O3 with a perfect stoichiometry remains a daunting challenge. Recently, low-temperature deposited amorphous Ga2O3 (a-Ga2O3) has demonstrated comparable optoelectronic properties with its crystalline counterparts [24]. Moreover, low-temperature fabricated a-Ga2O3 is compatible with mature CMOS technologies and favorable for flexible devices. It has been proven that a-Ga2O3 is qualified for high-performance X-ray and solar-blind photon detectors [25,26]. To date, there is no investigation on the luminescent properties of lanthanide-doped a-Ga2O3. The host circumstance can exert a crystal field around the lanthanide dopants and strongly influence their photoluminescence properties. According to the Judd-Ofelt theory, local symmetry around the doped lanthanide ions can render their radiative transition probabilities [27]. Previous studies have shown that growth temperature has a huge impact on the crystallinity of Ga2O3 [28]. Herein, we present an experimental investigation on the crystallinity of Ga2O3 host impact on the upconversion Photoluminescence (PL) from Ga2O3: Er thin films. The variation of photoluminescence clearly reflects the amorphous-to-crystalline transformation of the Ga2O3: Er thin films. We demonstrate a strong enhancement of the upconversion emissions associated with the improved crystallinity of Ga2O3 by increasing the growth temperature. These results identify the interplay between the crystallinity and the dopant lanthanide luminescence, which paves the way for developing lanthanide-doped Ga2O3 based optoelectronic devices.

2. Experiment

Target Synthesis: Er3+-doped Ga2O3 targets were synthesized via high temperature solid-state reaction method using reagent grade Ga2O3 and Er2O3 powders as raw materials. Er3+-doped Ga2O3 powders with different doping concentrations of 0.5 at%, 0.75 at%, 1 at%, 1.25 at% and 1.5 at% were prepared. The weighted Ga2O3 and Er2O3 powders were milled using alcohol for 3 h. And then, the obtained slurry was dried at 80 ºC. After that, the dried compounds were granulated with 10 wt% polyvinyl alcohol binder, pressed into disk-shaped pellets and sintered at 1450 ºC in air for 4 h.
Thin films Fabrication: Ga2O3: Er thin films were deposited on (0001) sapphire substrates by PLD using a 248-nm KrF excimer laser (Coherent COMPex205). Before deposition, sapphire substrates were ultrasonically cleaned in organic solvents, and rinsed in deionized water and blown dried with nitrogen gas. Then, the substrates were inserted into the growth chamber. A KrF excimer laser was used with a fluence of 1.5 J cm-2 and a repetition frequency of 5 Hz. The distance between target and substrate was set at 50 mm, the basic vacuum was below 1×10-4 Pa controlled by a turbo molecular pump. During the growth, the oxygen pressure was controlled to be 1 Pa. These films were achieved by setting different substrate temperatures including room temperature (RT), 300 ºC, 400 ºC, and 500 ºC. After the deposition of an hour, except for RT grown sample, other samples were slowly cooled down to room temperature.
Characterization and Luminescence Measurement: The X-ray diffraction (XRD) measurement was carried out with the Bruker D8 Advanced X-ray diffractometer under CuKα1 (λ = 1.5406 Å) radiation. The Scanning Electron Microscopy (SEM) was measured using a field emission scanning electron microscope (Zeiss Sigma 500). The UV/Vis absorption spectra were recorded with the Hitachi U-3900 UV/Vis spectro-photometer. The Photoluminescence spectrum was measured by an Acton SpectraPro 300i spectrophotometer. All upconversion spectra were excited by a 980 nm diode laser. The spectrum ranges from 480nm to 700nm. Transient decay curves were measured with a Tektronix DPO3034 oscilloscope combined with photomultiplier tube (H11902-04, Hamamatsu). All measurements were carried out at room temperature.

3. Results and Discussion

The primary advantage of the PLD technology is the stoichiometric transfer of the target material to the grown film on the substrate. In order to filter out the most efficient luminescent composition, we prepared Er3+-doped Ga2O3 powders with different doping concentrations from 0.5% to 1.5%. Figure 1a shows the XRD θ-2θ patterns of Er3+-doped Ga2O3 powders with different doping concentrations. The characteristic diffraction peaks correspond to the monoclinic (β) phase of Ga2O3. The results imply that Er3+ ions are efficiently doped into Ga2O3 host. Besides the diffraction peaks of Ga2O3, small amount of secondary phase Er3Ga5O12 (ErGG) appears along with the Ga2O3 phase. Note that the (420) peak at 32.6° of the secondary ErGG phase increases along with the concentration of Er3+ dopant. Compared with pure Ga2O3 shown in Figure 1a, there are minor left shifts of the diffraction peak (111) for the Er3+-doped Ga2O3 samples as seen in the inset of Figure 1a. Considering that the ionic radius (0.881 Å) of the Er3+ ion is larger than that of Ga3+ ion (0.62 Å), it means that the lattice constant of Er3+-doped Ga2O3 expands, contributing to the diffraction peaks shift to lower angles. Figure 1b shows the upconversion PL spectra of Ga2O3 powders doped with different Er3+ ion concentrations. The typical upconversion spectra of Er3+ consist of strong green and red emission bands. Green emissions located at 524 nm and 552 nm corresponding to 2H11/2/4S3/2→4I15/2 transitions, respectively. Red emission band between 645 nm to 678 nm ascribed to 4F9/2→4I15/2 transition of the Er3+ ion. It can be found that the emission intensity increases monotonically from 0.5% to 1.25% Er3+ doping concentration, reaching its maximum value at 1.25%. And then the PL intensity decreases when the doping concentration arrives at 1.5%. Such concentration quenching is associated with the competition between the radiative and non-radiative transitions of the luminescent centers.
Taking both structural and luminescent factors into account, Ga2O3:1%Er target was chosen for growing Ga2O3: Er thin films as well as for investigating the effect of amorphous-to-crystalline transition on the luminescent properties. It is well known that the growth temperature has a great influence on the crystallinity of Ga2O3 during the PLD growth. Figure 2a shows the XRD patterns of the PLD-grown Ga2O3: Er thin films on (0001) sapphire substrate at RT, 300 ºC, 400 ºC, and 500 ºC. The diffraction peaks at 20.4°, 41.7°, and 64.5° correspond to the (0003), (0006), and (0009) planes of sapphire, respectively. Besides the diffraction peaks of the sapphire substrate, no diffraction peaks corresponding to β-Ga2O3 can be found in the XRD patterns of Ga2O3: Er thin films grown at substrate temperatures of RT, 300 ºC, and 400 ºC. Thus, the low temperature grown Ga2O3: Er films tend to be an amorphous phase. As the temperature increases, three diffraction peaks located at 30.1°, 38.2° and 58.9° corresponding to β-Ga2O3 (400), ( 4 ¯ 02 ) and ( 6 ¯ 03 ) were observed at a substrate temperature of 500 ºC, indicating the formation of a crystalline β-Ga2O3 phase. Figure 2b illustrates the cross-sectional SEM image of the Ga2O3: Er thin film grown at 500 ºC. The Ga2O3: Er thin film grown at high temperature exhibits a smooth and flat surface, with a uniform thickness of 480 nm. Figure 2c,d show the surface morphology images of the Ga2O3: Er thin films grown at RT and 500 ºC. The RT-grown Ga2O3: Er thin film exhibits amorphous or poorly crystallized. And the Ga2O3: Er thin films grown at 500 ºC turns into crystalline phase.
Figure 3a shows the normalized ultraviolet-visible (UV-Vis) transmittance spectra of Ga2O3: Er thin films. All Ga2O3: Er thin films exhibit a sharp intrinsic transmittance edge at the wavelength of around 280 nm. The broad absorption band before 300 nm corresponds to the valence-to-conduction band transition of Ga2O3. There also exist some absorption bands centered at about 460 nm, 532 nm, and 660 nm derived from Figure 3a, which can be ascribed to the 4f-4f transition of Er3+ ions as well as the defects absorption. The bandgap Eg can be calculated by extrapolating the linear region of the plot (αhv)2 versus hv, as shown in Figure 3b, where h is Planck’s constant, α is the linear absorption coefficient and ν is the transition frequency of incident photon. It can be found that Eg increases from 4.10 eV to 4.22 eV for Ga2O3: Er thin films grown from RT to 500 ºC, respectively. As the growth temperature increases, the Eg gradually increases, though it remains smaller than that of pristine Ga2O3. One possible reason to this reduction of Eg due to the formation of oxygen vacancies under low-temperature growth. Moreover, Er3+ doping may have contributed to the reduction of the Ga2O3 Eg.
Figure 4a shows the upconversion PL spectra from Ga2O3: Er thin films grown under different temperatures (RT, 300 ºC, 400 ºC, and 500 ºC). The PL spectra of the Ga2O3: Er films were investigated under 980 nm laser diode excitation. It was found that the RT-grown Ga2O3: Er thin film showed almost no PL emission, suggesting that amorphous environment hinders dopant Er3+ ions from emitting light within an amorphous host. In other upconversion spectra, similar to the spectra of Ga2O3: Er targets, green emissions located at 525 nm and 549 nm arise from 2H11/2/4S3/2→4I15/2 transitions of Er3+ ions. While, the red emission around 659 nm corresponds to 4F9/2→4I15/2 transition of Er3+ ions. As seen from Figure 4a, the emission intensities increase sharply with growing temperature. Notably, the emission intensity of the Ga2O3: Er film grown at 500 ºC was significantly enhanced compared to that of the Ga2O3: Er film grown at 300 ºC, at a ratio of 127 times. This means that growth at higher temperature improves the crystallinity which in turn enhances the upconversion emission. Since the 4f-4f transitions of free lanthanide ions are Laporte forbidden, the incorporation of uneven components of the crystal field when doping into a crystalline lattice allows for intra-configurational transitions to occur [29]. Thus, compared with amorphous Ga2O3 host, the high-temperature grown crystalline Ga2O3: Er film exerts larger crystal field around the doped Er3+ ions, contributing to the observed enhanced upconversion emissions [30].
For further exploring the upconversion processes in Ga2O3: Er thin films, the pump power dependence of the upconversion emissions were investigated, as shown in Figure 4b. The number of photons can be calculated from the relationship between upconversion (UC) emission intensity (I) and incident pump power (P). Using the formula of I ∝ P n , where n is the number of near-infrared photons. We give the double logarithmic plot of emission intensity variation for three peaks (525 nm, 549 nm, and 659 nm) with excitation power from 0.4 to 1.8 W, which are recorded based Ga2O3: Er film grown at 500 ºC. Within the power range, the slope values of green and red bands can be obtained as 2.39 (525 nm), 2.31 (549 nm), 2.32 (659 nm) by a linear fit, respectively (Figure 4c). This indicates that all these upconversion emissions belong to three-photon processes in Er3+ ions under 980 nm excitation. Figure 4d illustrates the possible up-conversion processes through the energy level diagram of Er3+ ions. Upon 980 nm excitation, Er3+ ion can populate into 4F7/2 energy level via the excited state absorption or energy transfer processes. Subsequently, the Er3+ ions relax nonradiatively to the 2H11/2 and 4S3/2 energy levels by the multiphoton relaxation, contributing to the green emissions at 525 nm, 549 nm, respectively. Some electrons of excited Er3+ ions can further relax to the 4F9/2 level, which gives rise to the red emissions at 659 nm.
In the end, we investigated the fluorescence lifetime of Ga2O3: Er thin film grown at 500 ºC. Figure 5a shows that the lifetime at 549 nm of Ga2O3: Er target is about 173 μs, which is in the same order of reported Er3+-doped oxide bulks. While, as shown in Figure 5b the lifetime of Ga2O3: Er thin film grown at 500 ºC has greatly reduced to ~4.3 μs. The strongly reduced fluorescence lifetime observed in Ga2O3: Er thin film may arise from the appearance of abundant defects in the as-grown thin film. These defects introduce nonradiative relax channels, resulting in the reduced lifetime of the Ga2O3: Er thin film.

4. Conclusions

In summary, we have investigated the amorphous-to-crystalline transition impact on the upconversion properties of lanthanide Er3+-doped Ga2O3 thin films. Ga2O3: Er thin films were grown on (0001) sapphire substrate at different temperatures by PLD methods. With increasing growth temperature, Ga2O3: Er thin films undergo the amorphous-to-crystalline transformation, and β-Ga2O3 phase gradually dominates in the Ga2O3: Er films. There is almost no upconversion emission observed in amorphous Ga2O3: Er thin film. The PL intensity of the β-Ga2O3 phase at 500 ºC is strongly enhanced. The observed remarkable PL enhancement with increasing growth temperature is associated with the improved crystallinity of the as-grown Ga2O3 thin films. The appearance of abundant defects give rise to the strongly reduced lifetime in the Ga2O3: Er thin film. The interplay between the upconversion emission and the crystallization condition provides a feasible method to explore the intrinsic crystallinity state of Ga2O3. Our finding offers more insight in developing lanthanide-doped wide bandgap semiconductor Ga2O3 materials, which holds promise for multi-functional optoelectronic applications.

Author Contributions

Conceptualization, Y.Z., and Z.W.; methodology, H.C., J.G., X.D., T.B. and F.Z.; formal analysis, Y.L., Y.Z., D.D., J.G. and T.B.; investigation, Y.L., Y.Z., J.G., X.D. and F.Z.; data curation, Y.L. and D.D.; writing—original draft preparation, Y.L.; writing—review and editing, Y.Z. and Z.W.; supervision, Y.Z. and Z.W.; funding acquisition, Y.Z. and Z.W. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the National Natural Science Foundation of China (No. 12274243, 52233014, 51172208 and 12074044), the Fund of State Key Laboratory of Information Photonics and Optical Communications (IPOC2021ZT05), and the Open Fund of IPOC (BUPT, IPOC2022A02).

Data Availability Statement

The data presented in this study are available on request from the corresponding author.

Conflicts of Interest

The authors declare no conflicts of interest.

References

  1. Pearton, S.J.; Yang, J.; Cary, P.H.; Ren, F.; Kim, J.; Tadjer, M.J.; Mastro, M.A. A review of Ga2O3 materials, processing, and devices. Appl. Phys. Rev. 2018, 5, 011301. [Google Scholar] [CrossRef]
  2. Zhang, Q.; Li, N.; Zhang, T.; Dong, D.; Yang, Y.; Wang, Y.; Dong, Z.; Shen, J.; Zhou, T.; Liang, Y.; Tang, W.; Wu, Z.; Zhang, Y.; Hao, J. Enhanced gain and detectivity of unipolar barrier solar blind avalanche photodetector via lattice and band engineering. Nat. Commun. 2023, 14, 418. [Google Scholar] [CrossRef]
  3. Hu, Z.; Cheng, Q.; Zhang, T.; Zhang, Y.; Tian, X.; Zhang, Y.; Feng, Q.; Xing, W.; Ning, J.; Zhang, C.; Zhang, J.; Hao, Y. Solar-blind photodetectors fabricated on β-Ga2O3 films deposited on 6° off-angled sapphire substrates. J. Lumin. 2023, 255, 119596. [Google Scholar] [CrossRef]
  4. Zhang, J.; Dong, P.; Dang, K.; Zhang, Y.; Yan, Q.; Xiang, H.; Su, J.; Liu, Z.; Si, M.; Gao, J.; Kong, M.; Zhou, H.; Hao, Y. Ultra-wide bandgap semiconductor Ga2O3 power diodes. Nat. Commun. 2022, 13, 3900. [Google Scholar] [CrossRef]
  5. Chen, Z.; Saito, K.; Tanaka, T.; Guo, Q. Efficient pure green emission from Er-doped Ga2O3 films. CrystEngComm 2017, 19, 4448–4458. [Google Scholar] [CrossRef]
  6. Luo, W.; Liu, Y.; Chen, X. Lanthanide-doped semiconductor nanocrystals: electronic structures and optical properties. Sci. China Mater. 2015, 58, 819–850. [Google Scholar] [CrossRef]
  7. Perumal, R.N.; Athikesavan, V. Structural and electrical properties of lanthanide-doped Bi0.5(Na0.80K0.20)0.5TiO3–SrZrO3 piezoelectric ceramics for energy-storage applications. J. Mater. Sci. Mater. Electron. 2020, 31, 4092–4105. [Google Scholar] [CrossRef]
  8. Bian, T.; Zhou, T.; Zhang, Y. Preparation and Applications of Rare-Earth-Doped Ferroelectric Oxides. Energies 2022, 15, 8442. [Google Scholar] [CrossRef]
  9. Marin, R.; Jaque, D. Doping Lanthanide Ions in Colloidal Semiconductor Nanocrystals for Brighter Photoluminescence. Chem. Rev. 2021, 121, 1425–1462. [Google Scholar] [CrossRef] [PubMed]
  10. Xie, Y.; Song, Y.; Sun, G.; Hu, P.; Bednarkiewicz, A.; Sun, L. Lanthanide-doped heterostructured nanocomposites toward advanced optical anti-counterfeiting and information storage. Light Sci. Appl. 2022, 11, 150. [Google Scholar] [CrossRef] [PubMed]
  11. Dong, F.; Chen, H.; Dong, Z.; Du, X.; Chen, W.; Qi, M.; Shen, J.; Yang, Y.; Zhou, T.; Wu, Z.; Zhang, Y. Mechanically induced enhancement and modulation of upconversion photoluminescence by bending lanthanide-doped perovskite oxides. Opt. Lett. 2022, 47, 706–709. [Google Scholar] [CrossRef]
  12. Zhao, Y.; Yuan, Y.; Chen, H.; Chen, W.; Du, X.; Gong, C.; Lin, L.; Luo, W.; Liu, W.; Zhang, Y. In Situ Probing of Surface Acoustic Waves by Interfacing with Lanthanide Emitters. Adv. Opt. Mater. 2021, 9, 2001760. [Google Scholar] [CrossRef]
  13. Zhang, P.; Ke, J.; Tu, D.; Li, J.; Pei, Y.; Wang, L.; Shang, X.; Guan, T.; Lu, S.; Chen, Z.; Chen, X. Enhancing Dye-Triplet-Sensitized Upconversion Emission Through the Heavy-Atom Effect in CsLu2F7: Yb/Er Nanoprobes. Angew. Chem. Int. Ed. 2022, 61, e202112125. [Google Scholar] [CrossRef] [PubMed]
  14. Chen, H.; Dong, Z.; Chen, W.; Sun, L.; Du, X.; Zhao, Y.; Chen, P.; Wu, Z.; Liu, W.; Zhang, Y. Flexible and Rewritable Non-Volatile Photomemory Based on Inorganic Lanthanide-Doped Photochromic Thin Films. Adv. Opt. Mater. 2020, 8, 1902125. [Google Scholar] [CrossRef]
  15. Chen, H.; Dong, Z.; Zhao, Y.; Li, S.; Du, X.; Wu, Z.; Liu, W.; Zhang, Y. In-situ tailoring upconversion processes from lanthanide ions doped ferroelectric films through piezoelectric strain. J. Lumin. 2020, 219, 116914. [Google Scholar] [CrossRef]
  16. Zhang, G.; Dang, P.; Lian, H.; Xiao, H.; Cheng, Z.; Lin, J. Er3+/Yb3+-Based Halide Double Perovskites with Highly Efficient and Wide Ranging Antithermal Quenching Photoluminescence Behavior for Light-Emitting Diode Applications. Laser & Photonics Rev. 2022, 16, 2200078. [Google Scholar]
  17. Nogales, E.; García, J.A.; Méndez, B.; Piqueras, J.; Lorenz, K.; Alves, E. Visible and infrared luminescence study of Er doped β-Ga2O3 and Er3Ga5O12. J. Phys. D 2008, 41, 065406. [Google Scholar] [CrossRef]
  18. Khartsev, S.; Hammar, M.; Nordell, N.; Zolotarjovs, A.; Purans, J.; Hallén, A. Reverse-Bias Electroluminescence in Er-Doped β-Ga2O3 Schottky Barrier Diodes Manufactured by Pulsed Laser Deposition. Phys. Status Solidi (a) 2022, 219, 2100610. [Google Scholar] [CrossRef]
  19. Wu, Z.; Bai, G.; Qu, Y.; Guo, D.; Li, L.; Li, P.; Hao, J.; Tang, W. Deep ultraviolet photoconductive and near-infrared luminescence properties of Er3+-doped β-Ga2O3 thin films. Appl. Phys. Lett. 2016, 108, 211903. [Google Scholar] [CrossRef]
  20. Deng, G.; Huang, Y.; Chen, Z.; Pan, C.; Saito, K.; Tanaka, T.; Guo, Q. Yellow emission from vertically integrated Ga2O3 doped with Er and Eu electroluminescent film. J. Lumin. 2021, 235, 118051. [Google Scholar] [CrossRef]
  21. Zhang, Y.; Kämpfe, T.; Bai, G.; Mietschke, M.; Yuan, F.; Zopf, M.; Abel, S.; Eng, L.M.; Hühne, R.; Fompeyrine, J.; Ding, F.; Schmidt, O.G. Upconversion photoluminescence of epitaxial Yb3+/Er3+ codoped ferroelectric Pb(Zr,Ti)O3 films on silicon substrates. Thin Solid Films 2016, 607, 32–35. [Google Scholar] [CrossRef]
  22. Liu, Z.; Zhi, Y.; Zhang, S.; Li, S.; Yan, Z.; Gao, A.; Zhang, S.; Guo, D.; Wang, J.; Wu, Z.; Li, P.; Tang, W. Ultrahigh-performance planar β-Ga2O3 solar-blind Schottky photodiode detectors. Sci. China Technol. Sci. 2021, 64, 59–64. [Google Scholar] [CrossRef]
  23. Jamwal, N.S.; Kiani, A. Gallium Oxide Nanostructures: A Review of Synthesis, Properties and Applications. Nanomaterials 2022, 12, 2061. [Google Scholar] [CrossRef] [PubMed]
  24. Kim, J.; Sekiya, T.; Miyokawa, N.; Watanabe, N.; Kimoto, K.; Ide, K.; Toda, Y.; Ueda, S.; Ohashi, N.; Hiramatsu, H.; Hosono, H.; Kamiya, T. Conversion of an ultra-wide bandgap amorphous oxide insulator to a semiconductor. NPG Asia Mater. 2017, 9, e359–e359. [Google Scholar] [CrossRef]
  25. Qin, Y.; Li, L.-H.; Yu, Z.; Wu, F.; Dong, D.; Guo, W.; Zhang, Z.; Yuan, J.-H.; Xue, K.-H.; Miao, X.; Long, S. Ultra-High Performance Amorphous Ga2O3 Photodetector Arrays for Solar-Blind Imaging. Adv. Sci. 2021, 8, 2101106. [Google Scholar] [CrossRef]
  26. Liang, H.; Cui, S.; Su, R.; Guan, P.; He, Y.; Yang, L.; Chen, L.; Zhang, Y.; Mei, Z.; Du, X. ; Flexible X-ray Detectors Based on Amorphous Ga2O3 Thin Films. ACS Photonics 2019, 6, 351–359. [Google Scholar] [CrossRef]
  27. Hao, J.; Zhang, Y.; Wei, X. Electric-Induced Enhancement and Modulation of Upconversion Photoluminescence in Epitaxial BaTiO3: Yb/Er Thin Films. Angew. Chem. Int. Ed. 2011, 50, 6876–6880. [Google Scholar] [CrossRef]
  28. Wang, Y.; Li, H.; Cao, J.; Shen, J.; Zhang, Q.; Yang, Y.; Dong, Z.; Zhou, T.; Zhang, Y.; Tang, W.; Wu, Z. Ultrahigh Gain Solar Blind Avalanche Photodetector Using an Amorphous Ga2O3-Based Heterojunction. ACS Nano 2021, 15, 16654–16663. [Google Scholar] [CrossRef] [PubMed]
  29. Fu, H.; Peng, P.; Li, R.; Liu, C.; Liu, Y.; Jiang, F.; Hong, M.; Chen, X. A general strategy for tailoring upconversion luminescence in lanthanide-doped inorganic nanocrystals through local structure engineering. Nanoscale 2018, 10, 9353–9359. [Google Scholar] [CrossRef]
  30. Chen, B.; Zhang, X.; Wang, F. Expanding the Toolbox of Inorganic Mechanoluminescence Materials. Acc. Mater. Res. 2021, 2, 364–373. [Google Scholar] [CrossRef]
Preprints 98196 g001
Figure 1. (a) XRD θ-2θ scan of Er3+-doped Ga2O3 targets with different concentrations. Dashed lines represent the positions of the characteristic bulk Ga2O3: Er (111) reflections. (b) PL spectra of the Ga2O3: Er targets with different Er doping concentrations at room temperature.
Figure 1. (a) XRD θ-2θ scan of Er3+-doped Ga2O3 targets with different concentrations. Dashed lines represent the positions of the characteristic bulk Ga2O3: Er (111) reflections. (b) PL spectra of the Ga2O3: Er targets with different Er doping concentrations at room temperature.
Preprints 98196 g001
Preprints 98196 g002
Figure 2. (a) XRD θ-2θ scan of Ga2O3: Er thin films grown at different temperatures. (b) The cross-sectional SEM image of the Ga2O3: Er thin film grown at 500 ºC. (c) The surface SEM image of the Ga2O3: Er thin film grown at RT. (d) The surface SEM image of the Ga2O3: Er thin film grown at 500 ºC.
Figure 2. (a) XRD θ-2θ scan of Ga2O3: Er thin films grown at different temperatures. (b) The cross-sectional SEM image of the Ga2O3: Er thin film grown at 500 ºC. (c) The surface SEM image of the Ga2O3: Er thin film grown at RT. (d) The surface SEM image of the Ga2O3: Er thin film grown at 500 ºC.
Preprints 98196 g002
Preprints 98196 g003
Figure 3. (a) Transmittance spectra of Ga2O3: Er thin films grown at different substrate temperatures. (b) The square of optical coefficient versus photon energy of Ga2O3: Er thin films, revealing the bandgap of different samples.
Figure 3. (a) Transmittance spectra of Ga2O3: Er thin films grown at different substrate temperatures. (b) The square of optical coefficient versus photon energy of Ga2O3: Er thin films, revealing the bandgap of different samples.
Preprints 98196 g003
Preprints 98196 g004
Figure 4. (a) The upconversion PL spectra of Ga2O3: Er thin films at different grown temperatures. (b) The upconversion emission spectra of the Ga2O3: Er thin film grown at 500 ºC under 980 nm laser excitation with different power densities. (c) Double logarithmic plot of upconversion emission intensity at 525 nm, 549 nm and 659 nm, respectively. (d) The Energy scheme of Er3+ ions and the mechanism to produce the upconversion spectra.
Figure 4. (a) The upconversion PL spectra of Ga2O3: Er thin films at different grown temperatures. (b) The upconversion emission spectra of the Ga2O3: Er thin film grown at 500 ºC under 980 nm laser excitation with different power densities. (c) Double logarithmic plot of upconversion emission intensity at 525 nm, 549 nm and 659 nm, respectively. (d) The Energy scheme of Er3+ ions and the mechanism to produce the upconversion spectra.
Preprints 98196 g004
Preprints 98196 g005
Figure 5. (a) PL decay curve of the Ga2O3: Er target with 1% concentration. (b) PL decay curve of the Ga2O3: Er thin film grown on sapphire substrate at 500 ºC.
Figure 5. (a) PL decay curve of the Ga2O3: Er target with 1% concentration. (b) PL decay curve of the Ga2O3: Er thin film grown on sapphire substrate at 500 ºC.
Preprints 98196 g005
Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.
Copyright: This open access article is published under a Creative Commons CC BY 4.0 license, which permit the free download, distribution, and reuse, provided that the author and preprint are cited in any reuse.
Prerpints.org logo

Preprints.org is a free preprint server supported by MDPI in Basel, Switzerland.

facebook logotwitter logolinkedin logo
MDPI Initiatives
Important Links
Subscribe

Disclaimer

Terms of Use

Privacy Policy

© 2024 MDPI (Basel, Switzerland) unless otherwise stated