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

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 ²H_11/2/⁴S_3/2→⁴I_15/2 transitions, respectively. Red emission band between 645 nm to 678 nm ascribed to ⁴F_9/2→⁴I_15/2 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), ($\overline{4}02$) and ($\overline{6}03$) 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 ²H_11/2/⁴S_3/2→⁴I_15/2 transitions of Er³⁺ ions. While, the red emission around 659 nm corresponds to ⁴F_9/2→⁴I_15/2 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 $I\propto P^n$, 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 ⁴F_7/2 energy level via the excited state absorption or energy transfer processes. Subsequently, the Er³⁺ ions relax nonradiatively to the ²H_11/2 and ⁴S_3/2 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 ⁴F_9/2 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.