The luminescence spectra in -Ga2O3 (4N) and -Ga2O3:Si single crystals excited with the band-edge energy consist of UV/blue and blue/green broad bands. The luminescence with decay times (>1 s) is called persistent phosphorescence (PP). The PP spectra observed in both -Ga2O3 single crystals at 15 K are in agreement with the blue/green band. The PP intensities decrease gradually by increasing the temperature from 15 K and disappear above ~150 K. The decay curves of the luminescence in wide time range between 4 and 100 s and between 1 and 103 s at 15 K fit a power function of t-n with n~1.1 and 0.9, respectively. The decay occurs through the same mechanism as the recombination of donors and acceptors in semiconductors. Thermal excitation of shallowly trapped electrons in -Ga2O3 into the conduction band leads to the decrease of the PP intensity and the increase of the electrical conductivity and the photocurrent.
© 2016 Optical Society of America
-Ga2O3 crystals belong to the group of transparent conductive oxides (TCO), for example, indium tin oxide (ITO), with a wide band gap and high electrical conductivity. ITO has been extensively investigated due to important applications such as in flat panel displays, thermal windows, and solar cells . As -Ga2O3 crystals exhibit a large band gap of Eg = 4.8 eV, the transparency extends from the visible into UV regions . The electrical conductivity exhibits a semiconductor behavior even at low temperatures . The band-to-band excitation (E>4.8 eV) produces UV/blue and blue/green luminescence [2,4]. The decay curves of the blue luminescence in -Ga2O3 single crystals were observed in time ranges from 0.5 s to 10 ms . The fastest decay component gives a lifetime of 1.5 s, which is assigned to self-trapped excitons composed of self-trapped holes and electrons . The other decay components in the range from 10 s to 10 ms may be due to distant electron-hole pairs, which are created by the band-to-band excitation. Recombination of distant electron-hole pairs with lifetimes longer than 1 s, called persistent phosphorescence, was observed at low temperatures below 150 K .
The basic mechanism responsible for the persistent phosphorescence is very similar to the donor-acceptor recombination process in semiconductors . If donors and acceptors, being uniformly distributed in semiconductors, recombine radiatively through tunnelling, the decay curve of the recombination luminescence can be described by a t-1 power function [6, 7]. The decay times for the donor-acceptor recombination and the persistent phosphorescence have distributions in the ranges of 10−4 to 1 s and 1 to 104 s, respectively. The difference in time scales is mainly due to separation distance and energy depth of the trapped charges .
In this paper, we discuss the relationship between the persistent phosphorescence and the electrical conductivity observed in -Ga2O3 single crystals, that is, how shallowly trapped electrons contribute to both the persistent phosphorescence as insulators and the electrical conductivity as semiconductors.
2. Experimental procedure
-Ga2O3 has a monoclinic structure with the space group C2/m . The lattice parameters at room temperature are a = 1.223 nm, b = 0.304 nm, and c = 0.5807 nm and the unique axis = 103.7° . Ga ions are surrounded by O ions in either a tetrahedral or an octahedral coordination. A separated chain structure of the two Ga sites is formed along the b-axis .
Single crystals of -Ga2O3 were grown from starting powders of 4N and 6N Ga2O3 by the floating zone technique. -Ga2O3 single crystals doped with Si were also grown. Si doping concentrations were ~1019 cm−3 and carrier densities as donors was estimated to be (1-2) 1018 cm−3 from the Hall measurement with the Van der Pauw configuration [9–11]. The details of the growth procedure were described in previous papers [2, 3, 9–11]. Samples were prepared by cleaving along the (100) and (001) planes, with approximate dimensions of 10100.5 mm3 for optical and electrical measurement, and 20.40.2 mm3 for electron spin resonance (ESR) measurement.
Optical absorption, luminescence and excitation spectra were measured in the temperature range between 10 and 300 K using the tunable source (100-700 nm) in the UVSOR facility at Institute for Molecular Science at Okazaki. Optical absorption and luminescence spectra were also measured using a Perkin Elmer Lambda950 spectrophotometer and an Ocean Optics HR4000 spectrophotometer, respectively. Decay curves of luminescence intensities were measured using a Hamamatsu Photonics Quantaurus-Tau C11367 spectrometer.
Persistent phosphorescence was measured as follows. A sample was excited for five minutes with 266-nm laser light with a width of ~10 ns from a Spectra-Physics GCR100 pulsed Nd:YAG laser. Measurement of whole phosphorescence intensities started at 1 s after removal of the excitation light. Optical signals were detected using a photomultiplier and a Keithley 428 current amplifier. Signals were sampled, integrated in a period of 1 s, changed to digital signals, and stored in a personal computer. Sample temperatures between 15 and 300 K were achieved using an Iwatani CA201 cryo-refrigerator.
ESR measurement was carried out using a Bruker EMX10/12 X-band spectrometer with ~9.45 GHz and 100 kHz field modulation in the temperature range of 110-500 K.
Photocurrent measurement was carried out using 266-nm pulsed laser light at 300 K. Photocurrent was amplified using a Keithley 428 current amplifier with a filter rise time of 10 s and a filter frequency response of 35 kHz.
3. Experimental results
3.1 Optical spectra
Figure 1 shows the optical absorption spectra at 300 K in the wavelength range between 200 and 2500 nm for the -Ga2O3 (4N), (6N) and -Ga2O3:Si single crystals. The band edges are observed approximately at 275 nm for these samples . The absorption coefficients for the 4N pure and Si-doped samples gradually increase in the infrared (IR) region from 1000 to 2500 nm. The IR absorption is associated with the plasma oscillation, which is determined by free carrier densities . Concerning the Si-doped sample, the carrier density of ~1018 (cm−3) estimated from the Hall measurement gives rise to a wavelength (33 µm) corresponding to the plasma frequency [9,12]. On the other hand, the optical absorption spectrum for the 6N pure sample is flat and negligibly small in the range between 300 and 2500 nm. The lack of the plasma absorption/reflection in this range is due to a further shift of the plasma frequency to the far-infrared, which is linked to the decrease in the carrier densities.
Figure 2(a) shows the luminescence spectra in -Ga2O3:Si excited in the range from 210 to 270 nm at 20 K. Excitation below 260 nm produces UV/blue broad bands with double peaks at 350 and 390 nm, whereas the UV/blue bands excited above 260 nm are weakened and a blue/green band appears around 520 nm [2,12]. The blue/green band, denoted by a dot-dashed line in Fig. 2(a), is decomposed through subtracting the UV/blue bands with the 210 nm excitation from the luminescence spectrum with 270 nm excitation. The luminescence spectra observed at 300 K in Fig. 2(b) are independent of the excitation wavelengths and coincident with those observed at 20 K with excitation wavelengths of below 260 nm in Fig. 2(a). Such observation is related to red-shift of the absorption edge due to the lattice expansion .
Figures 3(a) and (b) show temperature dependence of the luminescence spectra excited at 210 and 270 nm for -Ga2O3:Si, respectively. Although the luminescence intensity excited at 210 nm decreases drastically above 160 K, the line shape does not change as shown in Fig. 3(a). The 20 K luminescence spectrum excited at 270 nm is decomposed into the weak UV/ blue and relatively strong blue/green bands, denoted by two dot-dashed lines Fig. 3(b). In increasing temperatures, the blue/green band gradually decreases, and disappears above 220 K. On the other hand, the intensities of the UV/blue bands are nearly constant below 180 K, and increase above 180 K. The intensity at 300 K is three times larger than that at 20 K. These results suggest that the emission centers associated with the blue/green band are unstable above 180 K and energy transfer occurs from the blue/green-band to the UV/blue-bands centers.
Figure 4 shows the persistent phosphorescence (PP) spectra for -Ga2O3 (4N) and -Ga2O3:Si excited at 266 nm at 15 K when the spectra were observed at 2 s after removal of 266 nm UV light. The UV/blue bands for both samples as shown in Fig. 2(a) completely disappear. The spectrum for -Ga2O3:Si is slightly red-shifted. The intensity for -Ga2O3:Si is one order of magnitude weaker than that for -Ga2O3 (4N). The reason will be discussed later. In addition, the PP intensities for -Ga2O3 (4N) decrease gradually in increasing the temperature above 15 K and completely disappear above 150 K .
Figure 5(a) shows the decay curves of the UV/blue band fixed at 390 nm in the -Ga2O3:Si observed in the time range between 0 and 400 s. The decay curves cannot fit a single exponential function, but are composed of, at least, two exponential components. The fastest decay component with a decay time of 1.5 s, denoted by a dotted line in Fig. 5(a), is assigned to a self-trapped exciton composed of electron and self-trapped hole . The other decay time is ten times longer than the fastest one and may be due to excitons bound to Si impurities. The decay curves are independent of temperature from 20 to 300 K. Figure 5(b) shows the decay curves of the blue/green band fixed at 520 nm when measured at 50 and 300 K. The non-exponential curves indicate a distribution of the decay times in the range from ~10 to 400 s. The fastest component below 5 s is associated with the tail of the UV/blue bands extended to longer wavelengths.
In order to confirm the distribution model of the decay time, the decay curves -Ga2O3:Si are redrawn in log-log scales as shown in Fig. 6(a). The decay curves at 50 and 300 K fit a function of t-n with n = 1.14 and 1.32, respectively, except for the fastest decay components below 5 s. When temperature increases, the enhancement of the slope of the decay curve at 300 K indicates that a nonradiative decay process is added in the deexcitation process. In comparison, the decay curves of the whole persistent phosphorescence intensities in -Ga2O3 (4N) in the time range between 1 and 103 s are shown in Fig. 6(b). The decay curves in the temperature range from 15 to 110 K fit a function of t-n (n~0.8-0.9). The initial intensities at t = 1 s drastically decrease in increasing temperature. Above 150 K, the intensities disappear.
3.2 ESR spectra
The ESR measurement gives information on carriers of electrical conductivity. The anisotropic g values of less than 2 estimated from the sharp ESR line in -Ga2O3 (4N) deduce that electrons move in the chain structure along the b-axis direction . On the other hand, the ESR signals for -Ga2O3 (6N) are two orders of magnitude weaker than those for -Ga2O3 (4N). The decrease of the ESR signals corresponds to the decrease of free electron densities, that is, the disappearance of the absorption/reflection in the near-IR region as shown in Fig. 1.
The ESR spectra for -Ga2O3:Si are coincident with those for -Ga2O3 (4N) except the intensity [2,3]. Inset in Fig. 7 shows the typical ESR line due to conduction electrons. The line widths in Fig. 7 drastically increase above 200 K. Such line-broadening can be explained by the spin-lattice relaxation process . There are three processes; (1) direct process, (2) Raman process, and (3) Orbach process . The Orbach process is dominant at high temperatures. The line width is proportional to the inverse of the longitudinal spin-lattice relaxation time (T1). The width (), defined as the peak-to-peak width, is given in the form of Fig. 7 calculated using Eq. (1) and parameters of a = 0.053, b = 0.5 and = 1430 K( = 120 meV) fits the experimental data points very well. The activation energy of = 120 meV is associated with thermal excitation of shallowly trapped electrons into the bottom of the conduction band. When temperatures are below or above ~150 K, the localized state or thermally delocalized state of the shallowly trapped electrons gives rise to the persistent phosphorescence or the electrical conductivity, respectively.
Electrical conductivity of -Ga2O3 single crystals is in the range from 0.02 to 50 (−1cm−1), which is determined by concentrations of oxygen vacancies as donors [2, 3, 9, 10]. On the other hand, the conductivity of -Ga2O3:Si can be intentionally controlled over three orders of magnitude by Si doping [9, 10]. The related free-carrier concentrations, varying between 1016 and 1018 cm−3, correspond to a 25%–50% effective Si donors and the remains are trapped electrons.
Figure 8 shows photocurrent responses produced in -Ga2O3 (4N) by 266 nm pulsed laser light with ~10 ns. The pulse width was determined by a current amplifier with a filter frequency response (<35kHz) of a low pass filter. The photocurrent is independent of applied voltage, and proportional to light power. The photocurrent for -Ga2O3 (6N) could not be observed even at high power of 8 mJ because lifetimes of electron and hole, close to the decay time (1.5 s) of the self-trapped excitons, are too short to be detected as photocurrent. In consequence, the observed photocurrent is strongly associated with distant electron and hole pairs.
4. Mechanism of persistent phosphorescence
The intrinsic luminescence spectrum for -Ga2O3 consists of the UV/blue and blue/green bands. The broadband feature is ascribed to strong electron-phonon interaction, that is, localized states of electron and hole. The UV/blue bands are due to self-trapped excitons which are composed of self-trapped hole bound to electron [4, 15]. The decay curves of the bands at 390 nm with the 210 nm excitation do not change in the temperature range between 50 and 300 K as shown in Fig. 5(a). This result suggests the self-trapped excitons are stable up to 300 K. The blue/green band excited around the band tail in the range from 260 to 290 nm is responsible for the persistent phosphorescence as shown in Fig. 4. In a previous paper , the ESR results for persistent phosphor Ca2SiAl2O7 doped with Ce3+ were reported that electron and hole created by the UV excitation are trapped at oxygen vacancies and Al lattice sites in tetrahedra, respectively. Taking account of these results, holes in -Ga2O3 are expected to be self-trapped at Ga sites in tetrahedra and electrons are trapped at oxygen vacancies of sites sharing two octahedra and one tetrahedron . In consequence, the persistent phosphorescence occurs through tunneling recombination of self-trapped holes and nearby trapped electrons. With an assumption that trapped electrons and self-trapped holes are distributed uniformly in the crystal, the decay curve is approximately represented by the t−1 power function , and is consistent with the decay curves of the persistent phosphorescence for -Ga2O3 (4N) and -Ga2O3:Si with different time scales of 1-103 s and 1-102 s in Fig. 6 (b) and (a), respectively.
Next, we consider the difference in the decay times of the persistent phosphorescence in -Ga2O3 (4N) and -Ga2O3:Si. The theoretical calculation of the decay curve using the recombination model of a nearest neighbor pair consisting of self-trapped electron and self-trapped hole in PbBr2 crystals, proposed by Iwanaga et al. , and given in the form of,Fig. 6(a) fits also a thick solid curve calculated using Eq. (2) with parameters of 2 = 2s, and 1>>400 s in the observation time from 3 to 400s. The value of 2 is comparable to the lifetime of self-trapped exciton. In the same way, the decay curve of -Ga2O3 (4N) at 15 K in Fig. 6(b) fits to a curve calculated using Eq. (2) with parameters of 2<1 s, and 1>103 s in the observation time from 1 to 103 s, being approximated to be t−1. However, the persistent phosphorescence intensity for -Ga2O3 (4N) is one order of magnitude larger than that for -Ga2O3:Si as shown in Fig. 4.
The difference in the persistent phosphorescence intensities in -Ga2O3 (4N) and -Ga2O3:Si is explained by a distribution model of electrons trapped at oxygen ion vacancies, or Si-impurities . The concentration of trapped electrons for -Ga2O3:Si can be controlled by Si-doping level and is larger than that for -Ga2O3 (4N) . In increasing the concentration of trapped electrons, the average distance between self-trapped hole and trapped electron decreases. In consequence, the recombination time of self-trapped hole and trapped electron is shortened, that is, the value of 2 is decreased and approaches the lifetime of self-trapped excitons . The concentration is low enough to increase the recombination times in the range from s to ms or s. This expectation is in agreement with the fact that recombination lifetimes of donors and acceptors in semiconductors, for example, GaP:S:C, are distributed in the range between 1 s and 1 ms .
The observation of persistent phosphorescence spectra below 150 K for -Ga2O3 (4N) and -Ga2O3:Si indicates that there are stably trapped electrons and self-trapped holes at low temperatures. The concentration of trapped electrons in the -Ga2O3 determines the recombination times of electron and hole from sub-millisecond to second. The persistent phosphorescence observed in the time range between 1 and 103 s is due to electron-hole pairs with the long average distances.
The temperature dependence of the ESR line widths of the conduction electrons, the electrical resistivity, and the persistent phosphorescence intensities is observed in the -Ga2O3 (4N). These results suggest the existence of electrons trapped at ~120 meV below the bottom of the conduction band. In increasing temperatures, electrons trapped at low temperatures are excited thermally into the conduction band, and move in the crystal. The electron migration reduces the persistent phosphorescence intensity and enhances the electric conductivity and the photocurrent. In consequence, the transparent conductive oxide-Ga2O3 single crystal behaves not only like a wide-gap semiconductor, but also like an insulator with persistent phosphorescence properties.
The present results have shown the important role of the intrinsic deficits associated with oxygen vacancies and Si-doping in the host crystal. The oxygen vacancies are formed through evaporation in GaO and O2 from the molten zone. It is very difficult to control the concentration of the oxygen vacancies through the growth conditions. On the other hand, the electrical conductivity can be intentionally controlled through the Si-doping level. The shallowly trapped electrons in -Ga2O3 contribute to both the persistent phosphorescence below 150 K and the high conductivity above 200 K. In order to observe the persistent phosphorescence at room temperature, deep electron-trapped centers are required. However, they do not contribute to the conductivity below room temperature.
At last, a good understanding of the formation of these defects centers leads to high electrical conductivity and/or intense persistent phosphorescence in wide-gap semiconductors for the desired optical and electrical response to special applications.
One of the authors (M. Yamaga) is indebted to the Koshiyama Foundation for promotion of Science and Technology for a research grant.
References and links
1. B. G. Lewis and D. C. Paine, “Application and processing of transparent conducting oxides,” MRS Bull. 25(08), 22–27 (2000). [CrossRef]
2. E. G. Villora, M. Yamaga, T. Inoue, S. Yabasi, Y. Masui, T. Sugawara, and T. Fukuda, “Optical spectroscopy study on, ” Jpn. J. Appl. Phys. 41(2), L622–L625 (2002). [CrossRef]
3. M. Yamaga, E. G. Villora, K. Shimamura, N. Ichinose, and M. Honda, “Donor structure and electric transport mechanism in, ” Phys. Rev. B 68(15), 155207 (2003). [CrossRef]
4. L. Binet and D. Gourier, “Origin of the blue luminescence of,” J. Phys. Chem. Solids 59(8), 1241–1249 (1998). [CrossRef]
5. M. Yamaga, Y. Kitoh, Y. Ohsumi, E. G. Villora, and K. Shimamura, “Long-lasting phosphorescence in -Ga2O3 transparent conductive oxide,” J. Ceram. Process. Res. 12(S1), s26–s29 (2011).
6. P. Y. Yu and M. Cardona, Fundamentals of Semiconductors (Springer-Verlag, 1999), Chap. 7.
7. S. W. S. Mckeever, Thermoluminescence of Solids (Cambridge University Press, 1985), pp143–148.
8. M. Orita, H. Hiramatsu, H. Ohta, M. Hirano, and H. Hosono, “Preparation of highly conductive, deep ultraviolet transparent, ” Thin Solid Films 411(1), 134–139 (2002). [CrossRef]
9. E. G. Víllora, K. Shimamura, Y. Yoshikawa, T. Ujiie, and K. Aoki, “Electrical conductivity and carrier concentration control in, ” Appl. Phys. Lett. 92(20), 202210 (2008). [CrossRef]
10. E. G. Víllora, K. Shimamura, T. Ujiie, and K. Aoki, “Electrical conductivity and lattice expansion of, ” Appl. Phys. Lett. 92(20), 202118 (2008). [CrossRef]
11. K. Shimamura, E. G. Víllora, T. Ujiie, and K. Aoki, “Excitation and photoluminescence of pure and Si-doped, ” Appl. Phys. Lett. 92(20), 201914 (2008). [CrossRef]
12. C. Kittel, Introduction to Solid State Physics (John Wiley & Sons, 1996) p. 274.
13. M. Yamaga, T. Ishikawa, M. Yoshida, T. Hasegawa, E. G. Villora, and K. Shimamura, “Polarization of optical spectra in transparent conductive oxide, ” Phys. Status Solidi 8(9), 2621–2624 (2011). [CrossRef]
14. A. Abragam and B. Bleaney, Electron Paramagnetic Resonance of Transition Ions (Clarendon Press, 1970) pp 60–74.
15. K. S. Song and R. T. Williams, Self-Trapped Excitons (Springer-Verlag, 1993), Chaps. 2 and 7.
16. M. Yamaga, Y. Tanii, N. Kodama, T. Takahashi, and M. Honda, “Mechanism of long-lasting phosphorescence process of Ce3+-doped Ca2Al2SiO7 melilite crystals,” Phys. Rev. B 65(23), 235108 (2002). [CrossRef]
17. M. Iwanaga, M. Watanabe, and T. Hayashi, “Charge separation of excitons and the radiative recombination process in PbBr2 crystals,” Phys. Rev. B 62(15), 10766–10773 (2000). [CrossRef]