Crucial Role of Oxygen Vacancies in Scintillation and Optical Properties of Undoped and Al-Doped β-Ga₂O₃ Single Crystals

Abstract

In this paper, the effects of oxygen vacancy and gallium vacancy on the optical and scintillation properties of undoped β-Ga₂O₃ crystal and 2.5 mol % Al doped gallium oxide were investigated. For the undoped β-Ga₂O₃, the transmittance is improved after annealing in oxygen or nitrogen atmosphere. After the introduction of Al element, the absorption cutoff appears slightly blue shift, and the band gap increases. For the undoped as-grown β-Ga₂O₃ single crystals, the decay time consists of a fast component (τ₁) of the order of nanoseconds, and two slow components (τ₂, τ₃) of tens to hundreds of nanoseconds. The contribution of the fast decay time component in the decay times is 2.78%. While for Al-doped β-Ga₂O₃, the faster (τ₁) time is 2.33 ns for the as-grown one, and the contribution is 68.02%. However, the pulse height spectrum shows that the introduction of 2.5 mol % Al will reduce the light yield of the β-Ga₂O₃ crystal.

1. Introduction

The scintillation detection system has the advantages of high detection efficiency and fast time response, and has important applications in high-energy physics experiment, nuclear physics experiment, nuclear medicine imaging and other fields [1]. The scintillation materials play an important role in scintillation detection systems. They can absorb the energy of high-energy particles or rays, convert them into visible light or near-ultraviolet light, and then detect visible or near-ultraviolet light through photomultiplier tubes or CCD and other photodetectors. Therefore, scintillation materials can be used to detect high-energy particles or X-rays and gamma rays. The performance of scintillation materials is closely related to the performance of scintillation detection systems. Excellent scintillation materials should have the characteristics of high light yield, fast scintillation decay time, excellent radiation resistance, matching emission spectrum with the response band of photodetector, and stable physical and chemical properties [2]. Scintillation materials can be divided into organic scintillation materials and inorganic scintillation materials. Inorganic scintillation materials usually have higher effective atomic number, larger densities, and have a strong ability to cut off high-energy particles or high-energy rays. Single crystal inorganic scintillation materials also tend to have higher luminous efficiency. Compared with dielectric scintillators, among single crystal inorganic scintillators, wide band gap semiconductor scintillators have the advantages of ultrafast scintillation decay time and better energy resolution. However, the self-absorption effect of semiconductor scintillation materials limits their wide application in the field of scintillation. For example, the Stokes shift of CuI at room temperature is small, and there is a serious self-absorption effect, which has an adverse effect on the light yield [3].

The β-Ga₂O₃ single crystal is a new type of ultra-wide bandgap(~4.9 eV) semiconductor material, which has important application value in high-power electronic devices and solar-blind ultraviolet detection [4]. In addition, the scintillation properties of β-Ga₂O₃ have attracted extensive research in recent years and other fields, and has attracted extensive research in recent years. The fast scintillation properties of β-Ga₂O₃ single crystals were first reported by Yanagida et al. in 2016, the X-ray excited emission band was around 380 nm with two exponential decay components 8 ns and 977 ns, while the ¹³⁷Cs 662 keV γ-ray induced light yield was 15,000 ± 1500 ph/MeV [5]. In 2017, Usui et al. reported scintillation features of Ce-doped β-Ga₂O₃, the emission peak was around 420 nm due to the 5d–4f transition of Ce³⁺, other than 380 nm in undoped β-Ga₂O₃, meanwhile, the decay time became slower [6]. In 2019, Mykhaylyk et al. calculated the theoretical light yield of gallium oxide single crystal, which can reach 40,800 ph/MeV [7]. In 2020, Galazka et al. studied undoped, singly (Ce, Si, Al), doubly (Ce + Si, Ce + Al), and triply (Ce + Si + Al) doped β-Ga₂O₃ crystals, the highest light yield was 7040 ph/MeV for low carrier concentration samples, while the energy resolution was around 10% [8]. All these results show that β-Ga₂O₃ is a promising ultrafast scintillator which needs further research.

For the luminous mechanism, Binet et al. reported [9] that β-Ga₂O₃ has ultraviolet (UV) and blue-band (BB) emissions ascribed to the recombination of self-trapped excitons and donor-acceptor pairs, respectively. UV emission is considered an important factor because of its fast decay time constant is several nanoseconds [10], indicating that it can be used as an ultrafast scintillator in the nuclear radiation detection. Moreover, we cannot deny the main role of intrinsic defects in changing their optical and scintillation properties, especially the oxygen and gallium vacancy defects. Therefore, this work focuses on how to manage the energy band gap to give greater flexibility to design and improve the performance of these devices. In this article, the effects of annealing in different atmospheres (oxygen, nitrogen) and the introduction of Al elements on the scintillation and optical properties of β-Ga₂O₃ single crystals were investigated.

2. Experimental Procedure

Undoped and 2.5 mole% Al-doped β-Ga₂O₃ crystals were prepared using the floating zone method by Quantum Design IRF01-001-00 infrared image furnace, as described in the previous report [11]. The samples used in this work have two types, one is as-grown crystals, and the second is as-annealed in the oxygen and nitrogen at 1400 °C for 20 h, respectively. The optical properties were performed by a PerkinElmer Lambda 750 UV/VIS/NIR Spectrometer (Massachusetts, USA). X-ray excited luminescence (XEL) spectra, photoluminescence (PL) spectrum and decay time profiles were carried by a fluorescence spectrometer (Edinburgh Instrument FLS1000, Edinburgh, UK). The X-ray source with Ag target operating at 50 kV and 15 μA was used as an excitation source. Pulse height spectra was implemented using a ¹³⁷Cs γ ray source, an ORTEC digiBASE 1024 channel analyzer and a photomultiplier tube (PMT; Hamamatsu R878 with the shaping time of 0.75 μs, Hamamatsu, Japan). All measurements were performed at room temperature.

3. Experimental Results and Discussion

Transmission spectra as a function of incident wavelength (200–1300 nm) for undoped and 2.5 mole% Al-doped β-Ga₂O₃ single crystals as-grown and as-annealed in oxygen and nitrogen are shown in Figure 1. The transmittance of undoped β-Ga₂O₃ samples after annealing in oxygen or nitrogen atmosphere is higher than that of Al-doped β-Ga₂O₃ treated with the same annealing conditions. Under the same annealing conditions (oxygen, nitrogen), the transmittance of undoped β-Ga₂O₃ samples is higher than that of Al-doped β-Ga₂O₃ samples, and the transmittance of undoped gallium oxide samples annealed by oxygen is the highest among all samples. The absorption edge of β-Ga₂O₃ has a slight blue shift with 2.5 mole% Al-doped. Moreover, all transmittances were characterized by downtrends in the IR region [12], except the O₂-annealed of Al-doped β-Ga₂O₃ crystal. The inset of Figure 1 reveals the optical band gap estimated by a Tauc plot [13], and tabulated in

Table 1. XEL (X-ray excited luminescence), PL (photoluminescence), energy gap and PL decay time constants of undoped and 2.5 mole% Al-doped β-Ga₂O₃ (Ex. 270 nm, Em. 380 nm).

Sample x	β-Ga2O3 as-Grown	β-Ga2O3 as-O2 Annealed	β-Ga2O3 as-N2 Annealed	Al:β-Ga2O3 as-Grown	Al:β-Ga2O3 as-O2 Annealed	Al:β-Ga2O3 as-N2 Annealed
X-ray excited luminescence
XEL-UV(eV), Contr.%	3.62, 31.51	3.61, 36.59	3.64, 28.52	3.69, 33.3	3.678, 19.19	3.68, 27.90
XEL-UV′(eV), Contr.%	3.25, 12.60	3.26, 21.73	3.27, 16.5	3.26, 34	3.27, 29.16	3.30, 28.83
XEL-Blue(eV), Contr.%	3.16, 55.87	3.06, 41.66	3.15, 54.92	2.87, 32.25	2.93, 51.63	3.009, 43.25
Photoluminscence
PL-UV(eV), Contr.%	3.51, 12.05	3.48, 17.65	3.37, 18.95
PL-UV′(eV), Contr.%	3.20, 25.82	3.10, 45.35	2.93, 31.50	3.01 44.54	3.02 43.98	3.07 49.24
PL-Blue(eV), Contr.%	2.52, 62.12	2.57, 36.99	2.50, 49.53	2.60 55.46	2.47 56.02	2.72 50.76
Optical transmission
Eg(eV)	4.69	4.71	4.71	4.76	4.76	4.76
PL decay time
τ1 (ns), Contri. %	5.22 2.78	6.85 0.98	7.72 2.54	2.33 68.02	3.34 62.45	4.26 9.31
τ2 (ns), Contri. %	54.25, 17.61	65.70 11.04	55.17 27.79	41.58 6.35	38.14 6.87	41.64 20.48
τ3 (ns), Contri. %	206.5 79.61	271.69 87.98	153.7 69.66	276.77 25.63	220.19 30.69	170.81 70.22

.

By comparing the transmittance spectra of undoped β-Ga₂O₃ and Al-doped β-Ga₂O₃ samples, it can be found that the doping of Al element can increase the band gap of β-Ga₂O₃. The band gap increased from 4.7 eV to 4.76 eV with 2.5 mole% Al doping, which is consistent with references [14,15]. These can be interpreted as follows:

Defective chemistry reactions of crystals can be illustrated using the Kröger–Vink notation:

$$\frac{1}{2}{\mathrm{O}}_2\to {\mathrm{V}}_{\mathrm{Ga}}^{″}+{\mathrm{O}}_{\mathrm{O}}^{\mathrm{x}}+2\dot{\mathrm{h}}\mathrm{Excess}\mathrm{Oxygen}$$

(1)

$$2\mathrm{GaO}+\frac{1}{2}{\mathrm{O}}_2\to {\mathrm{Ga}}_2{\mathrm{O}}_3\mathrm{Oxidation}\mathrm{of}{\mathrm{Ga}}^{2+}\mathrm{ions}$$

(2)

$${\mathrm{O}}_{\mathrm{O}}\to \frac{1}{2}{\mathrm{O}}_2+\ddot{{\mathrm{V}}_{\mathrm{O}}}+2{\mathrm{e}}^{\prime }\mathrm{Oxygen}\mathrm{deficiency}$$

(3)

$${\mathrm{Al}}_2{\mathrm{O}}_3\stackrel{{\mathrm{Ga}}_2{\mathrm{O}}_3}{\to }2\ddot{{\mathrm{Al}}_{\mathrm{Ga}}}+{\mathrm{V}}_{\mathrm{Ga}}^{″}+3{\mathrm{O}}_{\mathrm{O}}\mathrm{Charge}\mathrm{equilibrium}\mathrm{for}\mathrm{Al–doped}\mathsf{\beta }-{\mathrm{Ga}}_2{\mathrm{O}}_3$$

(4)

$${({\mathrm{V}}_{\mathrm{O}}{\mathrm{V}}_{\mathrm{Ga}})}^{\mathrm{X}}+{\mathrm{V}}_{\mathrm{o}}^{\mathrm{X}}\to ({\mathrm{V}}_{\mathrm{O}},{\mathrm{V}}_{\mathrm{G}}{)}^{\prime }+{\mathrm{V}}_{\mathrm{o}}^{\prime }+\mathrm{hv}\hspace{1em}\mathrm{Simple}\mathrm{blue}\mathrm{emission}\mathrm{mechanism}$$

(5)

In these equations only oxygen and gallium vacancies, as well as a pair of charge (V_O, V_Ga) vacancies will be taken into consideration.

Equation (1) elucidates that the O₂-annealing process increases the oxidation of Ga²⁺ ions (GaO_x) to form Ga³⁺ ions (Ga₂O₃, Equation (2)) [16] and Ga vacancies V_Ga [17], as well as filling the oxygen vacancies Vo with oxygen and forming O²⁻ [18]. V_Ga serves as deep acceptors [19], thereby compensating the unintentionally doped shallow donors. The increase in V_Ga leads to a reduction in the free electron concentrations [17] that cause an increase in the band gap. The transmittance of the crystals after oxygen annealing in the near-infrared region also increases due to the increase in the concentration of V_Ga.

Regarding the increase in the energy gap due to Al doping, 27Al MAS NMR spectra [20] revealed that the percentage of Al was greater in the octahedral than in the tetrahedral sites due to the lower formation energy. Since the ion radius of Al³⁺ (0.0675 nm) at the octahedral site is smaller than the radius of Ga³⁺ (0.076 nm) at the site, the lattice size will shrink after doping with Al element. Hence, the inter-atomic distance of the ions will decrease and increase the binding force will increase resulting in enhancement of the bandgap.

Besides, as seen in Equation (4), an increase in the concentration of V_Ga will lead to an increase in the band gap and a blue shift of the absorption cutoff edge. In the O₂-annealing process, as described above, the V_Ga concentration of Al-doped β-Ga₂O₃ increases more than that of undoped β-Ga₂O₃. This agrees with Ma et al., report [21], where they have revealed that the formation energy of O vacancy in the Al-doped β-Ga₂O₃ is larger than which in the undoped β-Ga₂O₃. On the other hand, in the N₂-annealing process, the increment rate of V_O is greater than V_Ga, which causes a slight enhancing of the band gap.

Figure 2 shows the X-ray excited luminescence (XEL) spectra of undoped β-Ga₂O₃ and Al-doped β-Ga₂O₃ single crystals as-grown and as-annealed in oxygen and nitrogen at room temperature. The XEL spectra presented mainly consist of two ultraviolet (UV~3.6 eV, UV′~3.25 eV) and blue-band (BB~3.06 eV) emissions after well-fitted by Gaussian analysis. The luminous intensities of O₂ or N₂ annealed β-Ga₂O₃ crystals are apparently higher than the unannealed one, while it is contrary in Al doped crystals, which is caused by the combined action of self-absorption and luminous efficiency. In undoped β-Ga₂O₃ crystals, the transmission of as-grown and annealed samples is almost the same, while in Al-doped β-Ga₂O₃ crystals, the transmission in the range of luminous wavelength of the as-grown, O₂ annealed and N₂ annealed sample is about 72%, 55%, and 69%, respectively. For undoped β-Ga₂O₃ crystals, the O₂ annealed sample possesses the highest XEL intensity. However, for Al doped crystals, the XEL peak intensity at 345 nm of the as-grown, O₂ annealed and N₂ annealed sample is about 12.2 × 10⁴, 8.5 × 10⁴, and 7.5 × 10⁴, respectively. Thus, we can conclude that O₂ annealing can increase the luminous intensity for undoped β-Ga₂O₃. Besides, by comparing Figure 2a,b, we can see that the as-grown Al-doped β-Ga₂O₃ has a higher XEL spectral intensity than the as-grown undoped β-Ga₂O₃ sample. The introduction of Al element is beneficial to improve the XEL spectrum intensity of the as-grown β-Ga₂O₃.

As can be seen from Table 1, for Al-doped β-Ga₂O₃, XEL spectra also consist of two ultraviolet (UV~3.69 eV, UV′~3.3 eV) and blue-band (BB~2.87 eV) emissions after well-fitted by Gaussian analysis. The percentage of (UV and UV′) contribution to total luminescence decrease while BB increases under both annealing processes, besides UV′ and BB peak positions have a slight shift to higher energy after annealing. Since UV emission is an intrinsic property of crystals, it generally does not depend on specific impurities and appears due to recombination of free electrons and self-trapped hole (STHs) that are confined to O_(I) and O_(II) sites [22]. On the other hand, BB (BB~3.06 eV) is an extrinsic property that strongly depends on the impurities and heat treatment of crystals and is attributed to the donor-acceptor recombination assign to (V_Ga + V_O) defects (with a slight contribution of V_Ga) [9].

It can be seen from the above that the luminescence peak positions and intensity of the UV band and the BB bands of the β-Ga₂O₃ single crystal can be controlled by means of Al element doping and oxygen or nitrogen annealing. Therefore, it can be interpreted that as follows:

Intrinsic UV emission increases further with the O₂-annealing process since more carriers in impurity levels can be liberated to conduction and valence bands that are captured again via self-trapped excitons accompanied by a UV photon emission [23,24]. Al doping effect decreases the carrier concentrations that make the electrons in bands are more easily excited to form an exciton resulting in a higher exciton density. Thereby, reducing the excitation energy under the O₂-annealing process leads to a red shift, while in the N₂-annealing process the excitation energy increase leads to a blue shift. According to the Equation (4), extrinsic BB emissions occur by transferring an electron trapped on a donor site (V_O) to a hole created on an acceptor (V_Ga or V_O, V_Ga) after the acceptor’s excitation [9]. For the Al-doped β-Ga₂O₃, the decrease in the luminescence intensity of the blue band after annealing in oxygen atmosphere is caused by the increase in the concentration of V_Ga and the decrease in the concentration of V_O [25]. In addition, the increased probability of non-radiative transition recombination of excited carriers is also the reason for the decrease in the luminescence intensity of the blue band.

Figure 3 shows PL spectra of undoped and Al-doped β-Ga₂O₃ single crystals as-grown and annealed in oxygen and nitrogen under an excitation wavelength of 270 nm. After Gaussian fitting, two ultraviolet (UV~3.5 eV, UV′~3.2 eV) and blue-band (BB~2.5 eV) emissions were observed for undoped one, while for Al-doped β-Ga₂O₃ is observed ultraviolet (UV′~3.0 eV) and blue-band (BB~2.5 eV) emissions. This is due to the increase of band gap by Al doping. The excitation energy is 4.59 eV (270 nm), which is lower than the band gap of the undoped β-Ga₂O₃ (4.69 eV~4.71 eV) and the Al-doped β-Ga₂O₃ (4.76 eV). Thus, the intrinsic UV emission disappear in Al doped crystals, and decrease in undoped one.

PL spectra have the same trend as in the XEL spectra. Moreover, the difference between these spectra is that the PL can easily detect defect points more than XEL, besides X-ray generates more free charges that contribute to generating more self-trapped excitons causing the increased appearance of UV emission.

PL decay curve profiles are represented in Figure 4. Decay curves were well fitted with 3-exponential functions for undoped and Al doped β-Ga₂O₃ using the most accurate time-resolved analysis as follows [26];

$$\mathrm{I}\left(\mathrm{t}\right)={\mathrm{a}}_0+{\displaystyle \sum }_{\mathrm{i}=1}^3{\mathrm{a}}_{\mathrm{i}}\exp (\frac{-\mathrm{t}}{{\mathsf{\tau }}_{\mathrm{i}}})$$

(6)

where a₀ is background signal, a_i and τ_i are intensity and decay time. Besides, the contribution of each decay component can be determined by

$${\mathrm{f}}_{\mathrm{i}}=\frac{{\mathrm{a}}_{\mathrm{i}}{\mathsf{\tau }}_{\mathrm{i}}}{{\displaystyle \sum }{\mathrm{a}}_{\mathrm{i}}{\mathsf{\tau }}_{\mathrm{i}}}$$

(7)

The decay time constants of undoped have two components faster (τ₁ and τ₂) and slower (τ₃) with approximate order nanoseconds, and for Al-doped have two faster components (τ₁ and τ₂) and a slower component(τ₃) with approximate values of order nanoseconds and several hundred nanoseconds, respectively. For undoped as-grown β-Ga₂O₃ single crystal, the decay time constants are about 5.22, 54.25, and 206.55 ns, and, also, have similar values reported in the previous work [5]. For the undoped β-Ga₂O₃, the O₂/N₂-annealing will increase the fast component (τ₁), and, also, decrease the proportion of the fast component in the decay time. The increase in the decay time of the undoped β-Ga₂O₃ crystals after annealing is affected by many factors, and the increase in the concentration of V_Ga will also lead to an increase in the decay time. For the as-grown Al-doped β-Ga₂O₃, the fast component (τ₁) time is even faster than the undoped β-Ga₂O₃ and the contribution is as large as 68%. In turn, the O₂/N₂-annealing process slightly decreases the fast component.

Pulse height spectra profiles are represented in Figure 5. It can be seen from Figure 5a that the light yield of the undoped β-Ga₂O₃ single crystal after annealing is increased compared with that of the as-grown sample. However, in Figure 5b, the light yield of the annealed Al-doped sample decreased compared with the as-grown sample. This is basically consistent with the trend of XEL and PL spectra. Besides, the undoped β-Ga₂O₃ annealed in the same atmosphere have higher light yields than Al-doped β-Ga₂O₃, which is consistent with the trend reported by Galazka et al. [8] that the introduction of Al element may reduce the light yields of β-Ga₂O₃. The mechanism behind this result needs further study.

Summary, based on the experimental data of PL, XEL and pulse height spectra techniques, the UV-blue paradigm of undoped β-Ga₂O₃ under the O₂-annealing process is constructed and presented in Figure 6. The O₂-annealing process affects the following:

• Increases Ga³⁺ ions and V_Ga acceptors, as well as reduces V_O donors by filling the oxygen vacancies V_O with oxygen and forming O₂⁻ (process num. 1).
• Increases release of more charge carriers (electron, holes) that are in impurity levels (donors, acceptors) to conduction and valence bands, as they are captured again via self-trapped excitons accompanied by a UV photon emission (process num. 2).
• UV′ and blue emissions occur by transferring an electron trapped on a donor site (V_O) to a hole created on an acceptor (V_Ga or V_O, V_Ga) after the acceptor’s excitation (process num. 3)
• Decrease in the blue luminescence intensity has two reasons, the first is due to an increase in V_Ga acceptors and a decrease in V_O donors at the expense of (V_O, V_Ga) pairs, and the second is due to the excited carriers in levels have a high probability to non-radiative recombination transfer (process num 4).

4. Conclusions

This paper has explained in more details the role of both oxygen and gallium vacancies in X-ray excited luminescence (XEL) and photoluminescence properties during the oxygen and nitrogen annealing process of the undoped β-Ga₂O₃ and the Al-doped β-Ga₂O₃ crystals, which were grown by floating zone method. According to the transmittance spectrum, an enhancement in the band gap of the β-Ga₂O₃ crystal through the O₂-annealing process or by doping Al element, and in the shape of the shoulder that appears near the absorption edge owing to its anisotropic electronic structure. Luminescence spectra revealed two emission bands in the UV and blue regions are ascribed to the recombination of self-trapped excitons and donor-acceptor pairs, respectively. The total luminescence can be managed by UV and BB emission under Al-doping or annealing process. The fast decay time constants (τ₁) of undoped β-Ga₂O₃ are about 5 ns, and the contribution of the fast decay time is 2.78%. Moreover, for Al-doped β-Ga₂O₃, the fast (τ₁) time is only 2.33 ns, even faster than the undoped one, the contribution of the fast component is higher than 68%, which indicates that Al-doped β-Ga₂O₃ has a great potential as fast crystal scintillator. The pulse height spectrum shows that oxygen annealing is more beneficial to improve the light yield of undoped β-Ga₂O₃, and Al element doping has an adverse effect on the light yield of β-Ga₂O₃.