A high-quality bulk single crystal of β-Ga2O3 has been grown by the Czochralski method and its basic scintillation characteristics (light yield, energy resolution, proportionality, and scintillation decay times) have been investigated. All the samples cut from the crystal show promising scintillation yields between 8400 and 8920 ph/MeV, which is a noticeable step forward compared to previous studies. The remaining parameters, i.e. the energy resolution slightly above 10% (at 662 keV) and the scintillation mean decay time just under 1 μs, are at the same level as we have formerly recognized for β-Ga2O3. The proportionality of yield seems not to deviate from standards determined by other commercial scintillators

Drozdowski, Winicjusz

Galazka, Zbigniew

Irmscher, Klaus

Makowski, Michał

Schewski, Robert

Witkowski, Marcin E.

Wojtowicz, Andrzej J.

English

Repositorium für Naturwissenschaften und Technik  (TIB Hannover)

Research Article Vol. 11, No. 8 / 1 August 2021 / Optical Materials Express 2488Recent progress in the development of β-Ga2O3scintillator crystals grown by the CzochralskimethodWINICJUSZ DROZDOWSKI,1,* MICHAŁ MAKOWSKI,1 MARCIN E.WITKOWSKI,1 ANDRZEJ J. WOJTOWICZ,1 KLAUS IRMSCHER,2ROBERT SCHEWSKI,2 AND ZBIGNIEW GALAZKA21Institute of Physics, Faculty of Physics, Astronomy and Informatics, Nicolaus Copernicus University inToruń, ul. Grudziądzka 5, 87-100 Toruń, Poland2Leibniz-Institut für Kristallzüchtung, Max-Born-Str. 2, 12489 Berlin, Germany*wind@fizyka.umk.plAbstract: A high-quality bulk single crystal of β-Ga2O3 has been grown by the Czochralskimethod and its basic scintillation characteristics (light yield, energy resolution, proportionality,and scintillation decay times) have been investigated. All the samples cut from the crystal showpromising scintillation yields between 8400 and 8920 ph/MeV, which is a noticeable step forwardcompared to previous studies. The remaining parameters, i.e. the energy resolution slightlyabove 10% (at 662 keV) and the scintillation mean decay time just under 1 µs, are at the samelevel as we have formerly recognized for β-Ga2O3. The proportionality of yield seems not todeviate from standards determined by other commercial scintillators.© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement1. Introductionβ-Ga2O3, one of the most perspective representatives of transparent semiconducting oxides, isconsidered as a promising candidate for manifold applications, most of them related to novel(opto)electronic devices [1]. Since it has been observed that β-Ga2O3 produces scintillation lightwhen excited with ionizing radiation [2–5], it has also attracted attention of the scintillator marketand scientists carrying out their research on this class of materials. Besides pure β-Ga2O3 crystals,doped ones have also been investigated [6–8], aiming at maximizing the scintillation yield andshortening the scintillation decay times. Nevertheless, based on the most recent publications[8–10], none of the dopants considered so far has led to an increase of the scintillation yield,which has been indicated to be strongly related to the free electron concentration in the material.For Czochralski-grown crystals the highest yield of 7040 ph/MeV has been reported up till now[9].Our previous works have been aimed at identifying factors that affect the scintillation yield ofβ-Ga2O3. For that purpose, we have grown and studied a wide spectrum of crystals using differentgrowth conditions and intentional doping. From that picture we have concluded that the highestvalues of the scintillation yield are confined to a relatively narrow range of the free electronconcentration (at an order of 1016 cm−3), which is the focus of the present manuscript. Thereinwe present the results of pulse height and scintillation time profile measurements performedon a series of samples prepared from a new bulk, Czochralski-grown, β-Ga2O3 crystal, whichturns out to have much better performance as compared to any previous ones with respect to thescintillation yield: all the samples display clearly higher yields (by ∼25%) than the above-cited7040 ph/MeV. In addition to the standard 662 keV excited pulse height spectra we show for thefirst time the proportionality of the scintillation yield, examined between 59.6 and 1275 keV.Next, for the scintillation time profiles, which look very similar to those reported before [8,9],#431340 https://doi.org/10.1364/OME.431340Journal © 2021 Received 12 May 2021; revised 24 Jun 2021; accepted 29 Jun 2021; published 13 Jul 2021Research Article Vol. 11, No. 8 / 1 August 2021 / Optical Materials Express 2489we propose a modified method of analysis, which sheds a new light on the question of decaycomponents (their number and decay times).2. Materials and experimentDetails of an impact of the growth conditions on the electrical properties are already recognizedand described by Galazka et al. [10,11], which enabled to grow, by the Czochralski method,a high-quality bulk β-Ga2O3 single crystal with a low level of free electron concentration thatwould generate the highest scintillation yield, as concluded from our previous studies [7–10].In particular, knowing the origin of the electrical conductivity in β-Ga2O3 (mainly siliconand hydrogen), as well as main compensation defects (gallium vacancies), we could balanceshallow donor and acceptors by adjusting the oxygen concentration in the growth atmosphere(note that the concentration of Ga vacancies increases with O2 concentration) without anyintentional doping. By using a given Ga2O3 powder of 5N purity (containing traces of Si < 2wt. ppm) and a gas atmosphere (Ar+O2, 5N purity) we found out for given growth conditionsan oxygen concentration in the growth atmosphere of 1.9% that resulted in a low free electronconcentration, which indeed led to the highest scintillation yield as compared with our previousworks [8–10]. To prepare plate samples necessary for pulse height spectra and scintillation timeprofile measurements, first (010)-oriented 5 mm thick slabs were cut from the bulk crystal, fromwhich bars with 5 × 5 mm2 cross-sections parallel to the (100) plane (which is an easy cleavageplane) were prepared. Then, about 0.5-0.6 mm thick (100)-oriented samples were cleaved fromthe bars. The advantage of freshly cleaved surfaces was the avoidance of any contamination andsurface damage that could arise from polishing. The samples had a free electron concentrationand an electron mobility of about 2.5·1016 cm−3 and 130 cm2 V−1 s−1, respectively, according toHall effect measurements.Room temperature pulse height spectra were measured under 137Cs (662 keV, 210 kBq)γ-excitation. The output signal from a Hamamatsu R878 photomultiplier (PMT) biased with avoltage of 1250 V was processed by a Canberra 2005 integrating preamplifier, a Canberra 2022spectroscopy amplifier (working with a 2 µs shaping time), and a TUKAN-8K-USB multichannelanalyzer. To improve the light collection efficiency, a thin layer of Viscasil grease was injectedbetween the sample and the PMT window, and the sample was covered with several layers ofTeflon tape. To provide the most accurate values of the photoelectron yield, pulse height spectraof each sample were recorded twice (i.e. sticking both sides of the plate to the PMT window; fromthe two values, quite close to each other, the higher one was credited as the photoelectron yield ofthe sample), as well as single photoelectron spectra were taken prior to and after examination ofeach sample. The photoelectron yields (expressed in phe/MeV) were afterward converted into thescintillation yields (in ph/MeV), considering the spectral matching of the β-Ga2O3 luminescenceto the PMT characteristics.The delayed coincidence single photon counting method proposed by Bollinger and Thomas[12] was chosen for scintillation time profile measurements. The setup was constructed from twoHamamatsu PMTs (R1104 and R928 for “starts” and “stops”, respectively), a Canberra 2145time-to-amplitude converter, and a TUKAN-8K-USB multichannel analyzer. The same 137Cssource as specified above was used for excitation.3. Results and discussion3.1. Scintillation yield and energy resolution137Cs pulse height spectra of four of the investigated samples (U2a,b,d,e) are presented in Fig. 1(the spectrum of the remaining U2c sample appears in Fig. 2). All of them display a clearlyresolved full energy peak, which is essential for determination of the basic scintillation parameters:scintillation yield and energy resolution. Their values are summarized in Table 1 and alsoResearch Article Vol. 11, No. 8 / 1 August 2021 / Optical Materials Express 2490included in Fig. 1. It turns out that this series consists of the brightest β-Ga2O3 samples evercharacterized at our laboratory. The yields from 8400 to 8920 ph/MeV indicate a significant stepforward compared to our previous reports [8,9], in which the highest yield was equal to 7040ph/MeV. A small spread of the yield values reflects the differences in heights [7] and possibly insurface qualities. The resolution of the new samples is maintained at the same level as of the bestsamples from the preceding series [8,9].Fig. 1. 137Cs pulse height spectra of β-Ga2O3 (samples: U2a,b,d,e).Table 1. Summary of properties of the studied β-Ga2O3 samples (Y - scintillation yield, R - energyresolution at 662 keV, τi - scintillation decay time constants with their contributions in brackets,τmean - scintillation mean decay time)aID size (mm3) Y (ph/MeV) R (%) τi (ns) τmean (ns)U2a 5 × 5 × 0.62 8400 11.1 20.0 (4.8%), 121 (15.4%), 1215 (79.8%) 989U2b 5 × 5 × 0.58 8480 10.3 17.7 (2.6%), 113 (14.6%), 1187 (82.8%) 1000U2c 5 × 5 × 0.49 8770 10.5 16.7 (3.5%), 110 (15.2%), 1169 (81.3%) 968U2d 5 × 5 × 0.52 8810 10.9 16.2 (3.1%), 104 (14.5%), 1065 (82.4%) 894U2e 5 × 5 × 0.56 8920 10.7 16.9 (3.1%), 107 (14.6%), 1182 (82.3%) 989aThe uncertainties of Y and R are below 5%, whereas those of τi and τmean are specified in Fig. 3.Research Article Vol. 11, No. 8 / 1 August 2021 / Optical Materials Express 2491Fig. 2. 241Am, 22Na, and 137Cs pulse height spectra of β-Ga2O3 (sample: U2c), and thescintillation yield proportionality plot.In order to assess the proportionality of scintillation yield of β-Ga2O3, besides the standard137Cs pulse height spectra we have performed measurements with two other radioactive sources:22Na (511 and 1275 keV) and 241Am (59.6 keV). In Fig. 2 and Table 2 we show the results forone selected sample (U2c). The corresponding proportionality plot (Fig. 2) reveals a rathercommon tendency, i.e. the yield increases with excitation energy, likewise for e.g. LaBr3:Ce [13],Lu2SiO5:Ce [14] or Lu3Al5O12:Pr [15]. We note that almost identical plots have been obtainedfor the remaining β-Ga2O3 samples, hence for conciseness we limit the presentation of the yieldproportionality to a single sample.Table 2. Scintillation yield proportionality of β-Ga2O3 based on the U2c sample (E - excitationenergy, Y - scintillation yield, Yrel - scintillation yield normalized to 1.000 for 1275 keV excitation, R -energy resolution at particular excitation energy)aID source E (keV) Y (ph/MeV) Yrel R (%)U2c 241Am 59.6 6710 0.761 40.122Na 511 8640 0.980 22.6137Cs 662 8770 0.994 10.522Na 1275 8820 1.000 8.95aThe uncertainties of Y and R determination are below 5%.Research Article Vol. 11, No. 8 / 1 August 2021 / Optical Materials Express 24923.2. Scintillation time profilesScintillation time profiles of all the studied β-Ga2O3 samples are shown in Fig. 3. Additionally, thelowest right window illustrates our new approach to the data analysis. It is based on an assumptionthat an observed profile is in fact a convolution of a genuine profile with an instrumental response(which is a profile recorded with the same apparatus settings, but without a sample between thePMTs). For each sample such a deconvolved genuine profile has been fitted with a 3-exponentialdecay function (included in Fig. 3), providing a reasonably good matching with one componentless than when fitting an observed profile, like it was done in previous works [8,9]. We note thatit is the fastest component (a few ns) that does not appear in genuine profiles, hence it must beassociated with some instrumental response.The decay times of the three other components, as well as the mean decay times, calculated as:τmean =∑︁3i=1 Aiτ2i∑︁3i=1 Aiτi(Ai and τi - amplitudes and decay time constants of each decay component, respectively),specified in Fig. 3 and Table 1, roughly agree with the previously obtained values for undopedsemiconducting β-Ga2O3 crystals with lower free electron concentrations [8,9]. Of course, dueto the absence of the fastest component, the contributions are now accordingly higher: ∼5% forthe prompt component (16.2-20.0 ns), ∼15% for the middle one (104-121 ns), and ∼80% for theslow one (∼ 1 µs). Although the samples have very similar free electron concentrations, there aresome differences amongst their time profiles and related decay times. To explain this observationwe point out that the samples come from different parts of the same crystal. They can thus differby some local trace impurities (electrically neutral) that could possibly introduce some additionaltrapping or scattering, affecting the scintillation kinetics (possibly also the scintillation yields,but in a lesser degree). Most of such trace impurities have a segregation coefficient different thanunity, hence their concentration is not the same in various parts of the crystal [11]. This results insomewhat different scintillation time profiles of particular samples.Research Article Vol. 11, No. 8 / 1 August 2021 / Optical Materials Express 2493Fig. 3. Scintillation time profiles of β-Ga2O3 (samples: U2a,b,c,d,e; in addition, for thecase of the U2e sample three curves have been shown to explain the way we analyze thescintillation time profiles).Research Article Vol. 11, No. 8 / 1 August 2021 / Optical Materials Express 24944. ConclusionsThe present study confirms our previous findings that the scintillation yield of semiconductingβ-Ga2O3 crystals is mainly driven by the free electron concentration. Adjusting the free electronconcentration in a crystal grown by the Czochralski method without any intentional doping, wehave obtained high values of yield close to 9000 ph/MeV, accompanied by energy resolutions of10-11% (at 662 keV), and mean decay times below 1 µs. The scintillation yield proportionalityhas revealed a similar tendency to that of electrically insulating oxide scintillators. Furtheradjustments of the electrical properties of semiconducting β-Ga2O3 single crystals combined withyet higher chemical purity may bring a subsequent enhancement of their scintillation properties,the more that we are still far below a theoretical upper limit for the scintillation yield of β-Ga2O3,estimated by Mykhaylyk et al. [16] at about 40000 ph/MeV.Funding. Narodowe Centrum Nauki (2016/23/G/ST5/04048); Deutsche Forschungsgemeinschaft (GA 2057/2-1).Acknowledgments. We would like to thank Mike Pietsch (IKZ) for measurements of electrical properties and Dr.Tobias Schulz (IKZ) for critical reading of the present manuscript.Disclosures. The authors declare that there are no conflicts of interest related to this article.Data availability. Data underlying the results presented in this paper are not publicly available at this time but maybe obtained from the authors upon reasonable request.References1. Z. Galazka, “β-Ga2O3 for wide-bandgap electronics and optoelectronics,” Semicond. Sci. Technol. 33(11), 113001(2018).2. T. Yanagida, G. Okada, T. Kato, D. Nakauchi, and S. Yanagida, “Fast and high light yield scintillation in the Ga2O3semiconductor material,” Appl. Phys. Express 9(4), 042601 (2016).3. D. Szalkai, Z. Galazka, K. Irmscher, P. Tutto, A. Klix, and D. Gehre, “β-Ga2O3 solid state devices for fast neutrondetection,” IEEE Trans. Nucl. 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Recent progress in the development of β-Ga2O3 scintillator crystals grown by the Czochralski method

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Recent progress in the development of β-Ga2O3 scintillator crystals grown by the Czochralski method

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