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Carrier transport and photoluminescence properties of Ga-doped ZnO films grown by ion-plating and by atmospheric-pressure CVD
Thin Solid Films 549 (2013) 12–17
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Carrier transport and photoluminescence properties of Ga-doped ZnO films grown by ion-plating and by atmospheric-pressure CVD T. Terasako a,⁎, Y. Ogura a, S. Fujimoto a, H. Song b, H. Makino b, M. Yagi c, S. Shirakata a, T. Yamamoto b a b c
Graduate School of Science & Engineering, Ehime University, 3 Bunkyo-cho, Matsuyama-shi, Ehime 790-8577, Japan Materials Design Center, Research Institute, Kochi University of Technology, 185 Miyanokuchi, Tosayamada-cho, Kami-shi, Kochi 782-8502, Japan Kagawa National College of Technology, 551 Koda, Takuma-cho, Mitoyo-shi, Kagawa 769-1192, Japan
a r t i c l e
i n f o
Available online 26 June 2013 Keywords: Ga-doped ZnO Ion plating with dc arc discharge Chemical vapor deposition Carrier transport Photoluminescence Photoluminescence excitation spectra
a b s t r a c t Ga-doped ZnO (GZO) films with carrier concentrations ranging from 5.2 × 1017 to 2.9 × 1020 cm−3 were grown on r-plane sapphire substrates by atmospheric-pressure CVD (AP-CVD). The gradients of Hall mobility (μ) — temperature (T) curves (denoted by Δμ/ΔT) for the GZO films grown by AP-CVD (CVD-GZO films) plotted as a function of carrier concentration n obey those for the GZO films deposited by ion plating with dc arc discharge (IP-GZO films). This suggests that the dominant carrier scattering mechanism limiting the carrier transport is common between the CVD-GZO and IP-GZO films at any given n. The CVD-GZO films with low n exhibited the Ga-related neutral donor bound exciton and two-electron satellite (TES) lines at low temperature. With increasing n, the above two characteristic lines shifted higher energies accompanied by broadening. The IP-GZO films exhibited two emission lines with opposite n dependences; with increasing n, one line shifted towards higher energies, whereas another line shifted towards lower energies. With an increase in n caused by the donor doping, the relaxation of the momentum-conservation law brought the remarkable changes to the PL and PLE spectra of both the IP-GZO and CVD-GZO films with n of more than 4.0 × 1019 cm−3 that is close to the Mott density. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Necessity of an alternative material for tin (Sn) doped indium (In) oxide (ITO), widely used as transparent conducting oxide (TCO) films in electrodes of flat panel displays and in window layers of thin-film solar cells, is growing because of limited availability of In species. Gallium (Ga)-doped zinc oxide (GZO) films with low electrical resistivity (ρ) and high optical transmittance in visible to near-infrared regions is one of the most conceivable candidates for the alternative TCO films. In addition, the GZO films have advantages over the other candidates in their low deposition temperatures (typically b200 °C) and low material costs. A variety of deposition methods, such as sputtering [1,2], ion plating with dc arc discharge (IP) [3,4], molecular beam epitaxy [5,6], chemical vapor deposition (CVD) [7,8], sol-gel [9], chemical spray pyrolysis [10,11], and pulsed laser deposition [12,13], have been reported for the preparation of GZO thin films so far. Among these methods, the IP method with the high density plasma beam generated by dc arc discharge has attracted much attention because of its higher growth rate, typically 170 nm/min, than that of the DC-magnetron sputtering [14]. Previously, the successful fabrication of 3-inch-thin-film-transistor liquid crystal display panels (TFT LCDPs) and 20-inch TFT LCD TV utilizing GZO based highly transparent common electrodes on the RGB-color-filter ⁎ Corresponding author. Tel.: +81 89 927 9789; fax: +81 89 927 9790. E-mail address: [email protected] (T. Terasako). 0040-6090/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.tsf.2013.06.039
substrates by commercial compatible mass production line was reported using the DC-magnetron sputtering [15]. Note that the IP method can be adapted easily to the production of large area films in very short time, leading to the reduction of the process cost demanded for the display manufacturing. In addition, the relative change in electrical resistivity before and after reliability test for 500 h under damp heat conditions (60 °C and a relative humidity of 95%) was found to be less than 10% [16]. In our previous paper, for GZO films with carrier concentration n from 3 × 1018 to 1 × 1021 cm−3 deposited by the IP, we reported that the grain boundary scattering mechanism plays a minor role in carrier transport. Temperature (T) dependent Hall mobility (μ) measurements of the GZO films showed a continuous transition in dominant scattering in intra-grain from ionized impurity scattering mechanism [from nondegenerate (3 × 1018 b n b 4 × 1019 cm−3) to degenerate (4 × 1019 b n b 3 × 1020 cm−3)] to thermal lattice vibration scattering mechanism (n N 3 × 1020 cm−3) with increasing n [17]. Shirakata et al. have reported that the photoluminescence (PL) spectra of the GZO films (Ga2O3 contents of 2–4 wt.% in the target resource materials) exhibited a dominant and very broad near-bandedge (NBE) emission [18]. Moreover, we found that the PL intensity of the NBE emission depends not only on the Ga2O3 contents, but also on the flow rates of oxygen gas (O2) introduced into the growth chamber during the film-growth process [19]. Beside the IP-GZO films, this paper deals with the GZO films grown by atmospheric-pressure CVD (AP-CVD). The AP-CVD is effective in
T. Terasako et al. / Thin Solid Films 549 (2013) 12–17
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obtaining high quality films at low process cost. Recently, we have reported the successful growth of epitaxial undoped ZnO films on r-plane sapphire substrates and the controllability of PL spectra strongly related to native defects introduced by the deviation from the stoichiometric composition and the residual impurities [20]. In this study, to address routes toward better understanding of factors limiting carrier transport in GZO films, a comparison of effects of Ga-doping on electrical and optical properties between IP-GZO films and GZO-films by AP-CVD (CVD-GZO films) has been made. 2. Experimental 2.1. Sample preparation Polycrystalline GZO films with thicknesses of approximately 200 nm were deposited on alkali-free glass substrates (100 × 100 mm2) at a substrate temperature (TS) of 200 °C by IP. Details of the deposition system can be seen in ref. [21]. Sintered ZnO tablets with different Ga2O3 contents ranging from 0.003 to 4 wt.% were used as resources. During the growth process, O2 was introduced into the deposition chamber to compensate oxygen deficiencies. The O2 gas flow rates were varied in the range from 0 to 30 SCCM as a deposition parameter. For all the samples, the wurtzite GZO(002) peak was observed to be much higher than all the other peaks. The GZO films with thicknesses of 0.36–5 μm were grown on r-plane sapphire substrates by AP-CVD using Zn, H2O and GaCl3 as source materials. TS was kept at 700 °C. The source temperatures of Zn, H2O and GaCl3 (denoted hereafter by “TZn”, “TH2O” and “TGaCl3”, respectively) were changed in the ranges of 650–700 °C, 54–65 °C and 50–80 °C, respectively. The carrier gas flow rates for Zn (and GaCl3) and H2O were maintained at 120 and 320 SCCM, respectively. It was observed that X-ray diffraction patterns of the GZO films grown by AP-CVD were dominated by a (110) peak. This indicated preferential growth in a-axis direction.
Fig. 1. (a) Resistivity ρ, (b) carrier concentration n, and (c) Hall mobility μ as a function of temperature T for the CVD-GZO films grown at TH2O = 54 and 65 °C (TS = 700 °C, TZn = 675 °C and TGaCl3 = 60 °C).
2.2. Characterizations Hall effect measurements were carried out using van der Pauw method at 83–343 K under the magnetic field of 0.47 T. The samples were cut into squared shape with the dimension of 5 × 5 mm2. The indium or aluminum dot electrodes were formed at four corners on the surface of each sample. For optical transmittance measurements, a double-beam spectrometer (Hitachi, U-4000) was used. The optical gap energy Eopt of films was determined using the Tauc's model for the direct band gap semiconductors [22]. Room temperature (RT) photoluminescence (PL) and photoluminescence excitation (PLE) spectra were taken by spectrofluorometer with a 450 W Xe lamp and two double-grating monochromators (HORIBA, Fluorolog-3 Model FL-3-22). Low temperature PL spectra were measured for GZO films attached to a coldfinger of a cryogenic refrigerator (Daikin, Cryo Kelvin UV202CL or V20C6L) at 10–300 K. The excitation light source for low temperature PL measurements was the 325 nm line from a He-Cd laser (Melles Griot, 3056-NIKN-A01, Output: 5 mW or Kinmon IK3452RF Output: 45 mW). PL from the sample was dispersed by a monochromator (Ritsu Oyo Kogaku, MC-100 N or SPEX, model 1608B) and detected by a photomultiplier (Hamamatsu, H8259-02 or R955). 3. Results and discussion 3.1. Temperature dependences of ρ, n and μ of CVD-GZO films The variations of ρ, n and μ as a function of temperature (T) for the CVD-GZO films grown at TH2O = 54 and 65 °C are shown in Fig. 1. ρ of the CVD-GZO film grown at TH2O = 54 °C decreases slightly with
increasing T, while that of the CVD-GZO film grown at TH2O = 65 °C is independent of T. The carrier concentrations for both the CVD-GZO films grown at TH2O = 54 and 65 °C remain almost constant at 3.0 × 1019 and 9.4 × 1019 cm−3, respectively, indicating that both the films are n-type degenerate semiconductors. This result suggests that the higher TH2Os conditions, i.e. oxygen-excess conditions, are favorable for effective doping of Ga species, Zn-substitution dopant. μ for the CVD-GZO film grown at TH2O = 54 °C slightly increases with T, whereas μ for the CVD-GZO film grown at TH2O = 65 °C is independent of T: The values of the gradients of the μ-T curves (defined as “Δμ/ΔT” [17]) for the CVD-GZO films grown at TH2O = 54 and 65 °C are 0.005 and 0, respectively. This finding shows that the dominant scattering mechanism in the CVD-GZO film grown at TH2O = 54 °C differs from that in the CVD-GZO film grown at TH2O = 65 °C. Recently, it has been reported that GZO films with different carrier concentrations grown by plasma enhanced MBE showed the different temperature dependences, i.e. different Δμ/ΔT values [23]. In our previous paper, we found that for the IP-GZO films the variation of Δμ/ΔT as a function of n exhibited a characteristic behavior of dominant scattering mechanism, reflecting the continuous transformation; (1) ionized impurity scattering mechanism in the non-degenerate system (Region I), (2) ionized impurity scattering mechanism in the degenerate system (Region II), and (3) thermal lattice vibration scattering mechanism (Region III) [17]. Fig. 2 shows T-μ and T-Δμ/ΔT plots for GZO films deposited by various growth methods by adding those results of the CVD-GZO films obtained in the present study and those of the MBE-GZO films after ref. [23] to the data of the IP-GZO films (Fig. 4 in ref. [17]). It can be clearly seen that the Δμ/ΔT values for the CVD-GZO and MBE-GZO films follow the trend of those for the IP-GZO films.
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Fig. 3. Photoluminescence (PL) and photoluminescence excitation (PLE) spectra of the IP-GZO films deposited with Ga2O3 contents of 0.003–2 wt.% at an O2 flow rate of 10 SCCM. The arrows indicate the knees (see in the body) of the PLE spectra. The vertical broken line on each spectrum indicates the optical gap (Eopt) determined using Tauc's model from the corresponding transmittance spectrum.
Fig. 2. (a) Carrier concentration n vs. Hall mobility μ and (b) carrier concentration n vs. gradient of μ — T curve (Δμ/ΔT) for GZO films. This figure is a revised version of Fig. 4 in ref. [17] by adding experimental results of the CVD-GZO films obtained in the present study and the reported values of the GZO films grown by MBE [23].
This proves the invariance of the n-Δμ/ΔT plot regardless of the growth method
sharply, which differ clearly from the behavior observed for the L-MBE-GZO films, as shown in Fig. 4. Note that the n of 4 × 1019 cm−3 corresponds to the critical point between Regions I and II. The remarkable change in the curve slope of the n vs. FWHM plot in the vicinity of n = 4 × 1019 cm−3 can be due to the transformation from the nondegenerate system to the degenerate system. Note that there is a possibility that the NBE emissions of the IP-GZO films are composed of at least two emissions with different peak energies. As described in Section 3.5, the NBE emissions of the IP-GZO films can be resolved into two components at low temperature of 10 K. To obtain information on deeper impurity or defect levels, PLE spectra were taken by monitoring at 390 or 395 nm, longer wavelengths than the peak positions of the NBE emissions. The vertical
3.2. Photoluminescence and photoluminescence excitation spectra of IP-GZO films Fig. 3 shows PL and PLE spectra of the IP-GZO films deposited with Ga2O3 contents ranging from 0.003 to 2 wt.% at an O2 flow rate of 10 SCCM. At a Ga2O3 content of 0.003 wt.%, the NBE emission has a peak at 380 nm with full-width at half-maximum (FWHM) value of 124 meV. With increasing Ga2O3 contents, the NBE emission shifts towards shorter wavelengths accompanied by widening of the FWHM value. At the highest Ga2O3 content of 2 wt.%, the NBE emission has an asymmetric spectral shape with a long tail extending to lower energies. The variation of the FWHM values of the NBE emissions observed for the IP-GZO films at RT as a function of n is shown in Fig. 4, together with the reported results for the GZO films grown by laser molecular beam epitaxy (denoted hereafter by “L-MBE-GZO films”) [24]. Asymmetric and broad PL spectra of the L-MBE-GZO films were fitted to a model that takes into account LO phonon replicas, in which the FWHM values of each individual emissions have been determined mainly from the concentration fluctuation of Ga dopants [24,25]. Up to n ~ 4 × 1019 cm−3, we found the same tendency of the FWHM values between the IP-GZO films and L-MBE-GZO films; they increase with increasing n. The finding indicates that the broadening mechanism and its contribution to the FWHM values are common between the GZO films deposited by the two different kinds of growth methods. With further increasing n, the FWHM values for the IP-GZO films increase
Fig. 4. Full-width at half-maximum (FWHM) values of the NBE emissions plotted as a function of carrier concentration n. Closed and open circles indicate the FWHM values for the IP-GZO films and those for the L-MBE-GZO films [24], respectively.
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broken lines indicate the optical gap energies (Eopt) determined from the transmittance spectra data using Tauc's model [22] in Fig. 3. No peaks can be observed on the PLE spectra of the IP-GZO films. Each PLE spectrum shows a tail extending to longer wavelengths than the corresponding Eopt. The intersection of two asymptotes on each PLE spectrum is defined as “PLE knee” and indicated by an upward arrow in Fig. 3. With increasing Ga2O3 content, the PLE knee shifts towards shorter wavelengths and the tail becomes more gentler.
3.3. Photoluminescence and photoluminescence excitation spectra of CVD-GZO films Fig. 5 shows PL and PLE spectra of the CVD-GZO films with different carrier concentrations. As with the case of the IP-GZO films, PLE spectra were taken by monitoring at 390 or 395 nm. The CVD-GZO film with lowest n of 5.2 × 1017 cm−3 shows an emission at 383 nm. An increase in n from 5.2 × 1017 to 5.6 × 1019 cm−3 splits the emission into two emissions with peaks at 376 and 387 nm. The former and latter emissions are denoted hereafter by “UH” and “UL”, respectively. The intensity ratio of the UH to UL emissions becomes larger with increasing n. At n = 9.5 × 1019 cm−3, the peak wavelengths of the UH and UL emissions are 373 and 385 nm, respectively. The CVD-GZO film with the highest n of 3.0 × 1020 cm−3 exhibits a very broad emission with a peak at 380 nm. The band gap of undoped ZnO at RT (3.37 eV [26]) is indicated by the broken line in Fig. 5. For the CVD-GZO film with n = 5.2 × 1017 cm−3, the knee of the PLE spectrum appears at 375 nm, which is ~65 meV lower than the band gap energy. A remarkable change in spectral shape can be observed between the CVD-GZO films with n = 5.2 × 1017 and 5.6 × 1019 cm−3. The PLE spectrum of the GZO film with n = 7.2 × 1019 cm−3 exhibits a peak at 376 nm and a hump at around 355 nm. This indicates that two different excitation processes contribute to the PL process. We find that the PLE spectra of the CVD-GZO films with carrier concentrations ranging from 5.6 × 1019 to 9.5 × 1019 cm−3 are completely different from those of the IP-GZO films. At the highest n of 2.9 × 1020 cm−3, the PLE spectrum has a long tail extending to lower energies and its knee is located at 346 nm, which is 320 meV higher than the PL peak.
Fig. 5. Photoluminescence (PL) and photoluminescence excitation (PLE) spectra of the CVD-GZO films with different carrier concentrations. The vertical broken line indicates the reported band gap energy of undoped ZnO [26].
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3.4. Comparison of PL and PLE properties of IP-GZO films with those of CVD-GZO films Fig. 6 shows the optical gap energy, PL peaks and PLE knees for the IP-GZO films, and the PL and PLE peaks for the CVD-GZO films as a function of n. The solid line indicates that the calculated band gap energy (denoted by “Eg curve”) taking into account both band gap widening caused by filling of the lowest states of the conduction band, the so-called Burstein–Moss shift [27–29], and band gap narrowing caused by the electron-electron and electron-impurity interactions [30]. For the IP-GZO films, the PLE knee energy at the lowest n is in accordance with the Eg curve, whereas both the optical gap and PL peak energies are ~80 meV lower than the Eg curve. With increasing n, the optical gap energy approaches the Eg curve, but the value of the PLE knee leaves from the Eg curve. In the range from 3.2 × 1018 to 3 × 1020 cm−3, however, the differences in photon energy between the PL peaks and the Eg curve for the IP-GZO films remain almost constant. Note that in the range from 3 × 1019 to 8 × 1019 cm−3, especially at the critical point between Regions I and II, the PLE knee values of the IP-GZO films exhibit relatively large separations from the Eg curve upward. Fig. 6 shows that the PL and PLE peak energies of the CVD-GZO films are quite different from the PL peak and PLE knee energies of the IP-GZO films.
3.5. Low temperature PL spectra of IP-GZO and of CVD-GZO films Fig. 7a shows low temperature PL spectra for the IP-GZO films deposited with Ga2O3 contents of 0.003, 0.03 and 0.3 wt.% at O2 flow rates ranging from 0 to 30 SCCM. All the IP-GZO films exhibit the NBE emission at around 3.35 eV. Analysis of the data shows that the NBE emissions from the IP-GZO films deposited with Ga2O3 contents of 0.003 and 0.03 wt.% are composed of two emissions with different peak energies: One emission with higher photon energy is denoted by “UIP-H” and another is denoted by “UIP-L”. An increase in the O2 flow rate leads to the reduction in the energy separation between the UIP-H and UIP-L emissions. On the other hand, it can be seen that for the IP-GZO films deposited with a higher Ga2O3 content of 0.3 wt.% we find an asymmetric broad emission with a long tail. Further analysis of
Fig. 6. Optical gap, PL peak and PLE knee energies for the IP-GZO films, and PL peak and PLE peak energies for the CVD-GZO films plotted as a function of carrier concentration n. The solid line indicates the theoretical band gap energy.
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Fig. 7. (a) Low temperature (10 K) photoluminescence (PL) spectra of the IP-GZO films deposited with Ga2O3 contents of 0.003, 0.03 and 0.3 wt.% at O2 flow rates ranging from 0 to 30 SCCM (UIP-H emission: △, UIP-L emission: ▽). (b) Low temperature (10–13 K) photoluminescence (PL) spectra of the CVD-GZO films with different carrier concentrations (UH emission: ▲, UL emission: ▼).
the results shows that the NBE emissions of the IP-GZO films deposited with higher Ga2O3 content are close to the UIP-H emissions rather than the UIP-L emissions of the IP-GZO films deposited with lower Ga2O3 contents above, and can be assigned to the UIP-H emission. Fig. 7b shows PL spectra, taken at 10–13 K, of the CVD-GZO films with various carrier concentrations. It shows that the PL spectra of the CVD-GZO films with n of lower than 7.2 × 1019 cm−3 are found to be composed of two peaks. Temperature dependences of the PL spectra of the CVD-GZO films revealed that these two peaks change continuously to the UH and UL emissions at RT. The two peaks at 3.363 and 3.322 eV observed on the PL spectrum of the CVD-GZO film with the lowest n of 5.2 × 1017 cm−3 are denoted by “UH” and “UL”, respectively. The photon energy of the former is very close to that of the Ga-related bound exciton line (3.362 eV) of the GZO film grown by MBE [5]. With increasing n up to 7.2 × 1019 cm−3, both the UH and UL peaks shift towards higher energies and the intensity ratio of the UH to UL peaks becomes larger. Fig. 7b clearly shows that with further increasing n, at n = 9.5 × 1019 cm−3, the UL emission is found to be hidden by the tail of the UH emission. Fig. 8 shows n dependences of low temperature PL peak energies of the UH and UL emissions for the CVD-GZO films and those of the UIP-H and UIP-L emissions of the IP-GZO films. As n increases up to 3 × 1019 cm−3, the photon energies of the UH and UL emissions remain almost constant, followed by both of the emission energies shift drastically towards higher energies. The energy separations between the two emissions remain approximately constant over the n range from 5.2 × 1017 to 2.9 × 1020 cm−3. In contrast to the above common n dependence behavior of both the UH and UL emissions for the CVDGZO films, the variation of the UIP-H emission differs from that of the UIP-L emission. With increasing n, the former shifts dramatically to higher energies, whereas the latter shifts slightly to lower energies. Above n ~ 4 × 1019 cm−3, the recombination processes for both UH and UIP-H emissions are not capable of being distinguished. We find that the same applies to both the UL and UIP-L emissions. Taking into account the fact that the UH emission at the lowest n is assigned to the Ga-related bound exciton line for the CVD-GZO film, the small change in photon energy up to n = 4 × 1019 cm−3, as shown in Fig. 8, allows us to regard the UH emission as the excitonic emission. Always the UL emission appears together with the UH emission. In addition, the n dependence of the peak energy of the UH emission is similar to that of the UL emission. These experimental results strongly implies that both the UH and UL emissions are related to the same recombination centers or recombination mechanism. The
UL emission appears at the region in which the two-electron satellite (TES) recombination lines of the neutral donor bound exciton lines can be seen [31–33]. The energy difference between the UH and UL emissions of ~ 41 meV corresponds to the difference between the donor energies in the 1s and 2s/2p states, which is 3/4 of the donor binding energy (ED). We, thus, obtain the ED value of 54 meV, which agrees well with the reported ED value of Ga related donor level [32]. The extremely large change of the peak energy in Region II, however, forces us to cancel the excitonic picture for the UH emission. We note that the most conceivable reason for the remarkable change is the decrease of the exciton binding energy due to the screening of the Coulomb interaction occurs in the heavily doped donor-doped semiconductors with a large number of carrier electrons [34]. The n dependences of the emission peak energies which similar to those of the UIP-H and UIP-L emissions were observed for the donor-tovalence-band and the conduction-band-to-acceptor transitions from the heavily doped GaAs crystals [35,36]. Assuming that the UIP-H emission (the UIP-L emission) is due to the donor-to-valence-band transition (the
Fig. 8. Photoluminescence peak energies of the UH, UL, UIP-H and UIP-L emissions as a function carrier concentration n.
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conduction-band-to-acceptor transition), the upward shift of the UIP-H emission (the downward shift of the UIP-L emission) can be explained by the broadening of the donor levels (the acceptor levels) followed by tailing of the states [37]. We confirm the formation of the tail states on the PLE spectra of the IP-GZO films in Fig. 3. One of the most conceivable candidates for the acceptor related to the conduction-band-to-acceptor transition is a p-type defect associated with Zn vacancy. Alves et al. reported that the recombination at 3.335 eV, which is close to the UIP-L emissions, is due to the localized recombination at extended defects such as dislocation loops [33]. Since the IP-GZO films exhibiting the UIP-L emissions have polycrystalline structure, there is a possibility that extended defects with high concentrations are formed. Therefore, this is an important and undeniable point. As mentioned above, the drastic changes in PL and PLE spectra of the IP-GZO and CVD-GZO films occur at the critical point of n = 4 × 1019 cm−3 between Regions I and II. It is very interesting to find that the n value of the critical point is very close to the Mott density of 3.7 × 1019 cm−3 [38]. The PL spectral shape similar to the NBE emissions of the GZO films with very high n have been observed for CdS:Cl [39] and CdS:I [40] crystals with doping concentrations of more than Mott density. The spectral shape could be explained by the convolution of the density of occupied states in the conduction band and the density of the empty states in the valence band as a result of the relaxation of the momentum conservation law caused by the electron-electron or electron-impurity scatterings. This is the so-called “non-k-conserving band-to-band emission”. We, thus, believe that the drastic changes in PL and PLE spectra at n = 4 × 1019 cm−3 are due to the change into the non-k-conserving band-to-band emission. 4. Conclusion Electrical and photoluminescence (PL) properties of GZO films grown by atmospheric-pressure CVD (AP-CVD) and ion plating with dc arc discharge (IP) were studied. Main conclusions are as follows: (1) the gradients of Hall mobility (μ) — temperature (T) curves (denoted by Δμ/ΔT) for the GZO films grown by AP-CVD (CVD-GZO) plotted as a function of carrier concentration n obey those for the GZO films deposited by IP (IP-GZO films), (2) low temperature PL spectra of the CVD-GZO films with low carrier concentrations showed the Ga-related neutral donor bound exciton and two-electron satellite (TES) lines, (3) the IPGZO films exhibited two characteristic emissions, which move away from each other with increasing n, and (4) remarkable changes were observed on the PL and PLE spectra for both the IP-GZO and CVD-GZO films with n of ~4 × 1019 cm−3, which is close to the Mott density, are probably due to the relaxation of the momentum conservation law caused by the highly introduced donors and free electrons. Acknowledgments T.T. and S.S. would like to thank Mr. A. Miyata, Mr. T. Shimada and the alumni of Semiconductor Laboratory of Ehime University who contributed to this study for their technical assistance. This work is partly supported by grants for the development of indium substitute materials for a transparent conducting electrode in Rare Metal
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Thin Solid Films journal homepage: www.elsevier.com/locate/tsf
Carrier transport and photoluminescence properties of Ga-doped ZnO films grown by ion-plating and by atmospheric-pressure CVD T. Terasako a,⁎, Y. Ogura a, S. Fujimoto a, H. Song b, H. Makino b, M. Yagi c, S. Shirakata a, T. Yamamoto b a b c
Graduate School of Science & Engineering, Ehime University, 3 Bunkyo-cho, Matsuyama-shi, Ehime 790-8577, Japan Materials Design Center, Research Institute, Kochi University of Technology, 185 Miyanokuchi, Tosayamada-cho, Kami-shi, Kochi 782-8502, Japan Kagawa National College of Technology, 551 Koda, Takuma-cho, Mitoyo-shi, Kagawa 769-1192, Japan
a r t i c l e
i n f o
Available online 26 June 2013 Keywords: Ga-doped ZnO Ion plating with dc arc discharge Chemical vapor deposition Carrier transport Photoluminescence Photoluminescence excitation spectra
a b s t r a c t Ga-doped ZnO (GZO) films with carrier concentrations ranging from 5.2 × 1017 to 2.9 × 1020 cm−3 were grown on r-plane sapphire substrates by atmospheric-pressure CVD (AP-CVD). The gradients of Hall mobility (μ) — temperature (T) curves (denoted by Δμ/ΔT) for the GZO films grown by AP-CVD (CVD-GZO films) plotted as a function of carrier concentration n obey those for the GZO films deposited by ion plating with dc arc discharge (IP-GZO films). This suggests that the dominant carrier scattering mechanism limiting the carrier transport is common between the CVD-GZO and IP-GZO films at any given n. The CVD-GZO films with low n exhibited the Ga-related neutral donor bound exciton and two-electron satellite (TES) lines at low temperature. With increasing n, the above two characteristic lines shifted higher energies accompanied by broadening. The IP-GZO films exhibited two emission lines with opposite n dependences; with increasing n, one line shifted towards higher energies, whereas another line shifted towards lower energies. With an increase in n caused by the donor doping, the relaxation of the momentum-conservation law brought the remarkable changes to the PL and PLE spectra of both the IP-GZO and CVD-GZO films with n of more than 4.0 × 1019 cm−3 that is close to the Mott density. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Necessity of an alternative material for tin (Sn) doped indium (In) oxide (ITO), widely used as transparent conducting oxide (TCO) films in electrodes of flat panel displays and in window layers of thin-film solar cells, is growing because of limited availability of In species. Gallium (Ga)-doped zinc oxide (GZO) films with low electrical resistivity (ρ) and high optical transmittance in visible to near-infrared regions is one of the most conceivable candidates for the alternative TCO films. In addition, the GZO films have advantages over the other candidates in their low deposition temperatures (typically b200 °C) and low material costs. A variety of deposition methods, such as sputtering [1,2], ion plating with dc arc discharge (IP) [3,4], molecular beam epitaxy [5,6], chemical vapor deposition (CVD) [7,8], sol-gel [9], chemical spray pyrolysis [10,11], and pulsed laser deposition [12,13], have been reported for the preparation of GZO thin films so far. Among these methods, the IP method with the high density plasma beam generated by dc arc discharge has attracted much attention because of its higher growth rate, typically 170 nm/min, than that of the DC-magnetron sputtering [14]. Previously, the successful fabrication of 3-inch-thin-film-transistor liquid crystal display panels (TFT LCDPs) and 20-inch TFT LCD TV utilizing GZO based highly transparent common electrodes on the RGB-color-filter ⁎ Corresponding author. Tel.: +81 89 927 9789; fax: +81 89 927 9790. E-mail address: [email protected] (T. Terasako). 0040-6090/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.tsf.2013.06.039
substrates by commercial compatible mass production line was reported using the DC-magnetron sputtering [15]. Note that the IP method can be adapted easily to the production of large area films in very short time, leading to the reduction of the process cost demanded for the display manufacturing. In addition, the relative change in electrical resistivity before and after reliability test for 500 h under damp heat conditions (60 °C and a relative humidity of 95%) was found to be less than 10% [16]. In our previous paper, for GZO films with carrier concentration n from 3 × 1018 to 1 × 1021 cm−3 deposited by the IP, we reported that the grain boundary scattering mechanism plays a minor role in carrier transport. Temperature (T) dependent Hall mobility (μ) measurements of the GZO films showed a continuous transition in dominant scattering in intra-grain from ionized impurity scattering mechanism [from nondegenerate (3 × 1018 b n b 4 × 1019 cm−3) to degenerate (4 × 1019 b n b 3 × 1020 cm−3)] to thermal lattice vibration scattering mechanism (n N 3 × 1020 cm−3) with increasing n [17]. Shirakata et al. have reported that the photoluminescence (PL) spectra of the GZO films (Ga2O3 contents of 2–4 wt.% in the target resource materials) exhibited a dominant and very broad near-bandedge (NBE) emission [18]. Moreover, we found that the PL intensity of the NBE emission depends not only on the Ga2O3 contents, but also on the flow rates of oxygen gas (O2) introduced into the growth chamber during the film-growth process [19]. Beside the IP-GZO films, this paper deals with the GZO films grown by atmospheric-pressure CVD (AP-CVD). The AP-CVD is effective in
T. Terasako et al. / Thin Solid Films 549 (2013) 12–17
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obtaining high quality films at low process cost. Recently, we have reported the successful growth of epitaxial undoped ZnO films on r-plane sapphire substrates and the controllability of PL spectra strongly related to native defects introduced by the deviation from the stoichiometric composition and the residual impurities [20]. In this study, to address routes toward better understanding of factors limiting carrier transport in GZO films, a comparison of effects of Ga-doping on electrical and optical properties between IP-GZO films and GZO-films by AP-CVD (CVD-GZO films) has been made. 2. Experimental 2.1. Sample preparation Polycrystalline GZO films with thicknesses of approximately 200 nm were deposited on alkali-free glass substrates (100 × 100 mm2) at a substrate temperature (TS) of 200 °C by IP. Details of the deposition system can be seen in ref. [21]. Sintered ZnO tablets with different Ga2O3 contents ranging from 0.003 to 4 wt.% were used as resources. During the growth process, O2 was introduced into the deposition chamber to compensate oxygen deficiencies. The O2 gas flow rates were varied in the range from 0 to 30 SCCM as a deposition parameter. For all the samples, the wurtzite GZO(002) peak was observed to be much higher than all the other peaks. The GZO films with thicknesses of 0.36–5 μm were grown on r-plane sapphire substrates by AP-CVD using Zn, H2O and GaCl3 as source materials. TS was kept at 700 °C. The source temperatures of Zn, H2O and GaCl3 (denoted hereafter by “TZn”, “TH2O” and “TGaCl3”, respectively) were changed in the ranges of 650–700 °C, 54–65 °C and 50–80 °C, respectively. The carrier gas flow rates for Zn (and GaCl3) and H2O were maintained at 120 and 320 SCCM, respectively. It was observed that X-ray diffraction patterns of the GZO films grown by AP-CVD were dominated by a (110) peak. This indicated preferential growth in a-axis direction.
Fig. 1. (a) Resistivity ρ, (b) carrier concentration n, and (c) Hall mobility μ as a function of temperature T for the CVD-GZO films grown at TH2O = 54 and 65 °C (TS = 700 °C, TZn = 675 °C and TGaCl3 = 60 °C).
2.2. Characterizations Hall effect measurements were carried out using van der Pauw method at 83–343 K under the magnetic field of 0.47 T. The samples were cut into squared shape with the dimension of 5 × 5 mm2. The indium or aluminum dot electrodes were formed at four corners on the surface of each sample. For optical transmittance measurements, a double-beam spectrometer (Hitachi, U-4000) was used. The optical gap energy Eopt of films was determined using the Tauc's model for the direct band gap semiconductors [22]. Room temperature (RT) photoluminescence (PL) and photoluminescence excitation (PLE) spectra were taken by spectrofluorometer with a 450 W Xe lamp and two double-grating monochromators (HORIBA, Fluorolog-3 Model FL-3-22). Low temperature PL spectra were measured for GZO films attached to a coldfinger of a cryogenic refrigerator (Daikin, Cryo Kelvin UV202CL or V20C6L) at 10–300 K. The excitation light source for low temperature PL measurements was the 325 nm line from a He-Cd laser (Melles Griot, 3056-NIKN-A01, Output: 5 mW or Kinmon IK3452RF Output: 45 mW). PL from the sample was dispersed by a monochromator (Ritsu Oyo Kogaku, MC-100 N or SPEX, model 1608B) and detected by a photomultiplier (Hamamatsu, H8259-02 or R955). 3. Results and discussion 3.1. Temperature dependences of ρ, n and μ of CVD-GZO films The variations of ρ, n and μ as a function of temperature (T) for the CVD-GZO films grown at TH2O = 54 and 65 °C are shown in Fig. 1. ρ of the CVD-GZO film grown at TH2O = 54 °C decreases slightly with
increasing T, while that of the CVD-GZO film grown at TH2O = 65 °C is independent of T. The carrier concentrations for both the CVD-GZO films grown at TH2O = 54 and 65 °C remain almost constant at 3.0 × 1019 and 9.4 × 1019 cm−3, respectively, indicating that both the films are n-type degenerate semiconductors. This result suggests that the higher TH2Os conditions, i.e. oxygen-excess conditions, are favorable for effective doping of Ga species, Zn-substitution dopant. μ for the CVD-GZO film grown at TH2O = 54 °C slightly increases with T, whereas μ for the CVD-GZO film grown at TH2O = 65 °C is independent of T: The values of the gradients of the μ-T curves (defined as “Δμ/ΔT” [17]) for the CVD-GZO films grown at TH2O = 54 and 65 °C are 0.005 and 0, respectively. This finding shows that the dominant scattering mechanism in the CVD-GZO film grown at TH2O = 54 °C differs from that in the CVD-GZO film grown at TH2O = 65 °C. Recently, it has been reported that GZO films with different carrier concentrations grown by plasma enhanced MBE showed the different temperature dependences, i.e. different Δμ/ΔT values [23]. In our previous paper, we found that for the IP-GZO films the variation of Δμ/ΔT as a function of n exhibited a characteristic behavior of dominant scattering mechanism, reflecting the continuous transformation; (1) ionized impurity scattering mechanism in the non-degenerate system (Region I), (2) ionized impurity scattering mechanism in the degenerate system (Region II), and (3) thermal lattice vibration scattering mechanism (Region III) [17]. Fig. 2 shows T-μ and T-Δμ/ΔT plots for GZO films deposited by various growth methods by adding those results of the CVD-GZO films obtained in the present study and those of the MBE-GZO films after ref. [23] to the data of the IP-GZO films (Fig. 4 in ref. [17]). It can be clearly seen that the Δμ/ΔT values for the CVD-GZO and MBE-GZO films follow the trend of those for the IP-GZO films.
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Fig. 3. Photoluminescence (PL) and photoluminescence excitation (PLE) spectra of the IP-GZO films deposited with Ga2O3 contents of 0.003–2 wt.% at an O2 flow rate of 10 SCCM. The arrows indicate the knees (see in the body) of the PLE spectra. The vertical broken line on each spectrum indicates the optical gap (Eopt) determined using Tauc's model from the corresponding transmittance spectrum.
Fig. 2. (a) Carrier concentration n vs. Hall mobility μ and (b) carrier concentration n vs. gradient of μ — T curve (Δμ/ΔT) for GZO films. This figure is a revised version of Fig. 4 in ref. [17] by adding experimental results of the CVD-GZO films obtained in the present study and the reported values of the GZO films grown by MBE [23].
This proves the invariance of the n-Δμ/ΔT plot regardless of the growth method
sharply, which differ clearly from the behavior observed for the L-MBE-GZO films, as shown in Fig. 4. Note that the n of 4 × 1019 cm−3 corresponds to the critical point between Regions I and II. The remarkable change in the curve slope of the n vs. FWHM plot in the vicinity of n = 4 × 1019 cm−3 can be due to the transformation from the nondegenerate system to the degenerate system. Note that there is a possibility that the NBE emissions of the IP-GZO films are composed of at least two emissions with different peak energies. As described in Section 3.5, the NBE emissions of the IP-GZO films can be resolved into two components at low temperature of 10 K. To obtain information on deeper impurity or defect levels, PLE spectra were taken by monitoring at 390 or 395 nm, longer wavelengths than the peak positions of the NBE emissions. The vertical
3.2. Photoluminescence and photoluminescence excitation spectra of IP-GZO films Fig. 3 shows PL and PLE spectra of the IP-GZO films deposited with Ga2O3 contents ranging from 0.003 to 2 wt.% at an O2 flow rate of 10 SCCM. At a Ga2O3 content of 0.003 wt.%, the NBE emission has a peak at 380 nm with full-width at half-maximum (FWHM) value of 124 meV. With increasing Ga2O3 contents, the NBE emission shifts towards shorter wavelengths accompanied by widening of the FWHM value. At the highest Ga2O3 content of 2 wt.%, the NBE emission has an asymmetric spectral shape with a long tail extending to lower energies. The variation of the FWHM values of the NBE emissions observed for the IP-GZO films at RT as a function of n is shown in Fig. 4, together with the reported results for the GZO films grown by laser molecular beam epitaxy (denoted hereafter by “L-MBE-GZO films”) [24]. Asymmetric and broad PL spectra of the L-MBE-GZO films were fitted to a model that takes into account LO phonon replicas, in which the FWHM values of each individual emissions have been determined mainly from the concentration fluctuation of Ga dopants [24,25]. Up to n ~ 4 × 1019 cm−3, we found the same tendency of the FWHM values between the IP-GZO films and L-MBE-GZO films; they increase with increasing n. The finding indicates that the broadening mechanism and its contribution to the FWHM values are common between the GZO films deposited by the two different kinds of growth methods. With further increasing n, the FWHM values for the IP-GZO films increase
Fig. 4. Full-width at half-maximum (FWHM) values of the NBE emissions plotted as a function of carrier concentration n. Closed and open circles indicate the FWHM values for the IP-GZO films and those for the L-MBE-GZO films [24], respectively.
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broken lines indicate the optical gap energies (Eopt) determined from the transmittance spectra data using Tauc's model [22] in Fig. 3. No peaks can be observed on the PLE spectra of the IP-GZO films. Each PLE spectrum shows a tail extending to longer wavelengths than the corresponding Eopt. The intersection of two asymptotes on each PLE spectrum is defined as “PLE knee” and indicated by an upward arrow in Fig. 3. With increasing Ga2O3 content, the PLE knee shifts towards shorter wavelengths and the tail becomes more gentler.
3.3. Photoluminescence and photoluminescence excitation spectra of CVD-GZO films Fig. 5 shows PL and PLE spectra of the CVD-GZO films with different carrier concentrations. As with the case of the IP-GZO films, PLE spectra were taken by monitoring at 390 or 395 nm. The CVD-GZO film with lowest n of 5.2 × 1017 cm−3 shows an emission at 383 nm. An increase in n from 5.2 × 1017 to 5.6 × 1019 cm−3 splits the emission into two emissions with peaks at 376 and 387 nm. The former and latter emissions are denoted hereafter by “UH” and “UL”, respectively. The intensity ratio of the UH to UL emissions becomes larger with increasing n. At n = 9.5 × 1019 cm−3, the peak wavelengths of the UH and UL emissions are 373 and 385 nm, respectively. The CVD-GZO film with the highest n of 3.0 × 1020 cm−3 exhibits a very broad emission with a peak at 380 nm. The band gap of undoped ZnO at RT (3.37 eV [26]) is indicated by the broken line in Fig. 5. For the CVD-GZO film with n = 5.2 × 1017 cm−3, the knee of the PLE spectrum appears at 375 nm, which is ~65 meV lower than the band gap energy. A remarkable change in spectral shape can be observed between the CVD-GZO films with n = 5.2 × 1017 and 5.6 × 1019 cm−3. The PLE spectrum of the GZO film with n = 7.2 × 1019 cm−3 exhibits a peak at 376 nm and a hump at around 355 nm. This indicates that two different excitation processes contribute to the PL process. We find that the PLE spectra of the CVD-GZO films with carrier concentrations ranging from 5.6 × 1019 to 9.5 × 1019 cm−3 are completely different from those of the IP-GZO films. At the highest n of 2.9 × 1020 cm−3, the PLE spectrum has a long tail extending to lower energies and its knee is located at 346 nm, which is 320 meV higher than the PL peak.
Fig. 5. Photoluminescence (PL) and photoluminescence excitation (PLE) spectra of the CVD-GZO films with different carrier concentrations. The vertical broken line indicates the reported band gap energy of undoped ZnO [26].
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3.4. Comparison of PL and PLE properties of IP-GZO films with those of CVD-GZO films Fig. 6 shows the optical gap energy, PL peaks and PLE knees for the IP-GZO films, and the PL and PLE peaks for the CVD-GZO films as a function of n. The solid line indicates that the calculated band gap energy (denoted by “Eg curve”) taking into account both band gap widening caused by filling of the lowest states of the conduction band, the so-called Burstein–Moss shift [27–29], and band gap narrowing caused by the electron-electron and electron-impurity interactions [30]. For the IP-GZO films, the PLE knee energy at the lowest n is in accordance with the Eg curve, whereas both the optical gap and PL peak energies are ~80 meV lower than the Eg curve. With increasing n, the optical gap energy approaches the Eg curve, but the value of the PLE knee leaves from the Eg curve. In the range from 3.2 × 1018 to 3 × 1020 cm−3, however, the differences in photon energy between the PL peaks and the Eg curve for the IP-GZO films remain almost constant. Note that in the range from 3 × 1019 to 8 × 1019 cm−3, especially at the critical point between Regions I and II, the PLE knee values of the IP-GZO films exhibit relatively large separations from the Eg curve upward. Fig. 6 shows that the PL and PLE peak energies of the CVD-GZO films are quite different from the PL peak and PLE knee energies of the IP-GZO films.
3.5. Low temperature PL spectra of IP-GZO and of CVD-GZO films Fig. 7a shows low temperature PL spectra for the IP-GZO films deposited with Ga2O3 contents of 0.003, 0.03 and 0.3 wt.% at O2 flow rates ranging from 0 to 30 SCCM. All the IP-GZO films exhibit the NBE emission at around 3.35 eV. Analysis of the data shows that the NBE emissions from the IP-GZO films deposited with Ga2O3 contents of 0.003 and 0.03 wt.% are composed of two emissions with different peak energies: One emission with higher photon energy is denoted by “UIP-H” and another is denoted by “UIP-L”. An increase in the O2 flow rate leads to the reduction in the energy separation between the UIP-H and UIP-L emissions. On the other hand, it can be seen that for the IP-GZO films deposited with a higher Ga2O3 content of 0.3 wt.% we find an asymmetric broad emission with a long tail. Further analysis of
Fig. 6. Optical gap, PL peak and PLE knee energies for the IP-GZO films, and PL peak and PLE peak energies for the CVD-GZO films plotted as a function of carrier concentration n. The solid line indicates the theoretical band gap energy.
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Fig. 7. (a) Low temperature (10 K) photoluminescence (PL) spectra of the IP-GZO films deposited with Ga2O3 contents of 0.003, 0.03 and 0.3 wt.% at O2 flow rates ranging from 0 to 30 SCCM (UIP-H emission: △, UIP-L emission: ▽). (b) Low temperature (10–13 K) photoluminescence (PL) spectra of the CVD-GZO films with different carrier concentrations (UH emission: ▲, UL emission: ▼).
the results shows that the NBE emissions of the IP-GZO films deposited with higher Ga2O3 content are close to the UIP-H emissions rather than the UIP-L emissions of the IP-GZO films deposited with lower Ga2O3 contents above, and can be assigned to the UIP-H emission. Fig. 7b shows PL spectra, taken at 10–13 K, of the CVD-GZO films with various carrier concentrations. It shows that the PL spectra of the CVD-GZO films with n of lower than 7.2 × 1019 cm−3 are found to be composed of two peaks. Temperature dependences of the PL spectra of the CVD-GZO films revealed that these two peaks change continuously to the UH and UL emissions at RT. The two peaks at 3.363 and 3.322 eV observed on the PL spectrum of the CVD-GZO film with the lowest n of 5.2 × 1017 cm−3 are denoted by “UH” and “UL”, respectively. The photon energy of the former is very close to that of the Ga-related bound exciton line (3.362 eV) of the GZO film grown by MBE [5]. With increasing n up to 7.2 × 1019 cm−3, both the UH and UL peaks shift towards higher energies and the intensity ratio of the UH to UL peaks becomes larger. Fig. 7b clearly shows that with further increasing n, at n = 9.5 × 1019 cm−3, the UL emission is found to be hidden by the tail of the UH emission. Fig. 8 shows n dependences of low temperature PL peak energies of the UH and UL emissions for the CVD-GZO films and those of the UIP-H and UIP-L emissions of the IP-GZO films. As n increases up to 3 × 1019 cm−3, the photon energies of the UH and UL emissions remain almost constant, followed by both of the emission energies shift drastically towards higher energies. The energy separations between the two emissions remain approximately constant over the n range from 5.2 × 1017 to 2.9 × 1020 cm−3. In contrast to the above common n dependence behavior of both the UH and UL emissions for the CVDGZO films, the variation of the UIP-H emission differs from that of the UIP-L emission. With increasing n, the former shifts dramatically to higher energies, whereas the latter shifts slightly to lower energies. Above n ~ 4 × 1019 cm−3, the recombination processes for both UH and UIP-H emissions are not capable of being distinguished. We find that the same applies to both the UL and UIP-L emissions. Taking into account the fact that the UH emission at the lowest n is assigned to the Ga-related bound exciton line for the CVD-GZO film, the small change in photon energy up to n = 4 × 1019 cm−3, as shown in Fig. 8, allows us to regard the UH emission as the excitonic emission. Always the UL emission appears together with the UH emission. In addition, the n dependence of the peak energy of the UH emission is similar to that of the UL emission. These experimental results strongly implies that both the UH and UL emissions are related to the same recombination centers or recombination mechanism. The
UL emission appears at the region in which the two-electron satellite (TES) recombination lines of the neutral donor bound exciton lines can be seen [31–33]. The energy difference between the UH and UL emissions of ~ 41 meV corresponds to the difference between the donor energies in the 1s and 2s/2p states, which is 3/4 of the donor binding energy (ED). We, thus, obtain the ED value of 54 meV, which agrees well with the reported ED value of Ga related donor level [32]. The extremely large change of the peak energy in Region II, however, forces us to cancel the excitonic picture for the UH emission. We note that the most conceivable reason for the remarkable change is the decrease of the exciton binding energy due to the screening of the Coulomb interaction occurs in the heavily doped donor-doped semiconductors with a large number of carrier electrons [34]. The n dependences of the emission peak energies which similar to those of the UIP-H and UIP-L emissions were observed for the donor-tovalence-band and the conduction-band-to-acceptor transitions from the heavily doped GaAs crystals [35,36]. Assuming that the UIP-H emission (the UIP-L emission) is due to the donor-to-valence-band transition (the
Fig. 8. Photoluminescence peak energies of the UH, UL, UIP-H and UIP-L emissions as a function carrier concentration n.
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conduction-band-to-acceptor transition), the upward shift of the UIP-H emission (the downward shift of the UIP-L emission) can be explained by the broadening of the donor levels (the acceptor levels) followed by tailing of the states [37]. We confirm the formation of the tail states on the PLE spectra of the IP-GZO films in Fig. 3. One of the most conceivable candidates for the acceptor related to the conduction-band-to-acceptor transition is a p-type defect associated with Zn vacancy. Alves et al. reported that the recombination at 3.335 eV, which is close to the UIP-L emissions, is due to the localized recombination at extended defects such as dislocation loops [33]. Since the IP-GZO films exhibiting the UIP-L emissions have polycrystalline structure, there is a possibility that extended defects with high concentrations are formed. Therefore, this is an important and undeniable point. As mentioned above, the drastic changes in PL and PLE spectra of the IP-GZO and CVD-GZO films occur at the critical point of n = 4 × 1019 cm−3 between Regions I and II. It is very interesting to find that the n value of the critical point is very close to the Mott density of 3.7 × 1019 cm−3 [38]. The PL spectral shape similar to the NBE emissions of the GZO films with very high n have been observed for CdS:Cl [39] and CdS:I [40] crystals with doping concentrations of more than Mott density. The spectral shape could be explained by the convolution of the density of occupied states in the conduction band and the density of the empty states in the valence band as a result of the relaxation of the momentum conservation law caused by the electron-electron or electron-impurity scatterings. This is the so-called “non-k-conserving band-to-band emission”. We, thus, believe that the drastic changes in PL and PLE spectra at n = 4 × 1019 cm−3 are due to the change into the non-k-conserving band-to-band emission. 4. Conclusion Electrical and photoluminescence (PL) properties of GZO films grown by atmospheric-pressure CVD (AP-CVD) and ion plating with dc arc discharge (IP) were studied. Main conclusions are as follows: (1) the gradients of Hall mobility (μ) — temperature (T) curves (denoted by Δμ/ΔT) for the GZO films grown by AP-CVD (CVD-GZO) plotted as a function of carrier concentration n obey those for the GZO films deposited by IP (IP-GZO films), (2) low temperature PL spectra of the CVD-GZO films with low carrier concentrations showed the Ga-related neutral donor bound exciton and two-electron satellite (TES) lines, (3) the IPGZO films exhibited two characteristic emissions, which move away from each other with increasing n, and (4) remarkable changes were observed on the PL and PLE spectra for both the IP-GZO and CVD-GZO films with n of ~4 × 1019 cm−3, which is close to the Mott density, are probably due to the relaxation of the momentum conservation law caused by the highly introduced donors and free electrons. Acknowledgments T.T. and S.S. would like to thank Mr. A. Miyata, Mr. T. Shimada and the alumni of Semiconductor Laboratory of Ehime University who contributed to this study for their technical assistance. This work is partly supported by grants for the development of indium substitute materials for a transparent conducting electrode in Rare Metal
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