- Email: [email protected]
Electronic properties of amorphous zinc tin oxide films by junction capacitance methods
Available online at www.sciencedirect.com
Journal of Non-Crystalline Solids 354 (2008) 2801–2804 www.elsevier.com/locate/jnoncrysol
Electronic properties of amorphous zinc tin oxide films by junction capacitance methods Peter T. Erslev a,*, Hai Q. Chiang b, David Hong b, John F. Wager b, J. David Cohen a b
a University of Oregon, Department of Physics, Eugene, OR 97403, USA Oregon State University, Department of Electrical Engineering and Computer Science, Corvallis, OR 97331-5501, USA
Available online 12 February 2008
Abstract Amorphous zinc tin oxides have recently shown sufficiently high mobilities that fully transparent electronic devices with these materials are now considered promising. We report the first detailed characterization of the electronic properties of these materials using the methods of admittance spectroscopy, drive-level capacitance profiling (DLCP), and transient photocapacitance (TPC) spectroscopy. We have examined a series of 1–3 lm thick zinc tin oxide films (at a composition of 1:1 ZnO:SnO2) incorporated into metal–insulator–semiconductor (MIS) devices. These were annealed at 400, 500, and 600 °C. The DLCP measurement is largely insensitive to interface defects and thus provides a good measurement of the free carrier and defect densities within the bulk region of these films. For the best samples (annealed at 600 °C), our results indicate a free carrier density of 5 1014 cm3 plus a deep defect density 1.5 1015 cm3. The TPC spectra disclose optical transitions between defect levels and the conduction and/or valence bands. They reveal two primary features in these materials: A dominant gaussian shaped deep defect band with an optical threshold near 1.6 eV relative to the conduction band, and an exponential band-tail with a characteristic (Urbach) energy near 120 meV. The Urbach energy indicates the degree of disorder in semiconducting materials, and is directly correlated to the anneal temperature for this series of samples. Published by Elsevier B.V. PACS: 73.20.r; 73.61.Le; 71.23.k; 71.55 Keywords: Thin-film transistors; Heterojunctions; Defects; Absorption; Indium tin oxide and other transparent conductors
1. Introduction Recent progress in the development of transparent thinfilm transistors (TTFTs) has brought the idea of fully transparent electronics much closer to reality. Amorphous oxides with heavy-metal cations are interesting because they possess higher electron mobilities than might be expected from their amorphous character. Such high mobilities are thought to be the result of a conduction band derived from spherically symmetric s-orbitals with large radii [1]. Mobilities as high as 50 cm2 V1 s1 have been reported for zinc tin oxide (ZTO), and the fabrication of
*
Corresponding author. E-mail address: [email protected] (P.T. Erslev).
0022-3093/$ - see front matter Published by Elsevier B.V. doi:10.1016/j.jnoncrysol.2007.09.062
TTFTs from this material has already been achieved [2]. Because of the good mobilities and also a relatively low processing temperature, ZTO is currently considered a strong candidate material for the development of flexible transparent electronics [3]. We have carried out a detailed examination of the electronic properties of amorphous ZTO with a 1:1 ZnO:SnO2 composition for films incorporated into metal–insulator– semiconductor (MIS) devices. We employed junction capacitance methods including admittance spectroscopy, drive-level capacitance profiling (DLCP), and transient photocapacitance spectroscopy (TPC). These methods have previously been successfully applied to photovoltaic devices incorporating disordered semiconducting materials such as a-Si:H and CuInSe2 and they are generally applicable to the study of most semiconductors provided one can
2802
P.T. Erslev et al. / Journal of Non-Crystalline Solids 354 (2008) 2801–2804
observe a junction capacitance that responds to an applied bias. The application of such methods to ZTO has already proven to be an extremely informative. For example, using admittance spectroscopy, we are able to directly measure the oxide/ZTO interface potential and observe how it varies with the applied gate voltage across the device. Our DLCP measurements indicate a carrier density near 5 1014 cm3 for the material with the highest mobility and this appears to increase with decreasing anneal temperature. Somewhat surprisingly, however, the TPC spectra indicate that the structural disorder of these samples, as assessed by the Urbach energy, increases with increasing anneal temperature. 2. Samples The MIS device structures were fabricated by depositing ZTO onto a highly doped Si substrate capped with 100 nm SiO2. A semi-transparent gold contact was then evaporated onto the ZTO. The 1–3 lm thick ZTO was deposited via rf magnetron sputtering in an Ar/O2 environment. The devices were subsequently annealed for one hour in air at temperatures of 400, 500, and 600 °C. X-ray diffraction measurements of ZTO thin films prepared in this manner reveal the films to be amorphous (or nanocrystalline) with a maximum crystal size not exceeding 5 nm as estimated, using the Scherrer equation, from the width of two very broad XRD peaks [2]. In contrast, ZTO films annealed above 650 °C display a multiplicity of sharp XRD peaks, indicating a dramatic increase in the crystalline nature of such films [2]. ZTO thin-film transistors processed in an identical manner as to this experiment are found to possess uniform properties across the substrate. Thus, the films being examined in this study are believed to be quite homogeneous. 3. Experimental procedures Our junction capacitance characterization methods included admittance spectroscopy, drive-level capacitance profiling, and transient photocapacitance spectroscopy. Admittance spectroscopy examines the complex electrical response of a sample to an AC perturbing signal over a range of frequencies and temperatures. The frequency and temperature of the measurement determine an emission energy Ee = kBT ln(m/2pf) where T is the temperature of the measurement, m is the thermal emission prefactor, and f is the measurement frequency. As the emission energy encounters the energetic level of a defect as the sample temperature is increased, a capacitive step is observed. When one examines these capacitive steps over a range of frequencies, the activation energy of the defect can be extracted. Drive-level capacitance profiling (DLCP) determines the density of the responding charge by examining the dependence of the capacitance to the amplitude, dV, of the applied oscillating voltage: C = C0 + C1(dV) + C2(dV)2 + The
drive-level density, NDL, is obtained from the coefficients C0 and C1 and it is equal to the free carrier density plus an integral over the density of deep defect states to an energy Ee below EC, and near the spatial position hxi = eA/C0 from the barrier junction. Spatial profiles are then obtained by varying the dc reverse bias. DLCP is superior to conventional capacitance vs. voltage (C–V) profiling because one can tune to a particular thermal emission energy by controlling the measurement frequency and temperature. In addition, this method is primarily sensitive to bulk defects, discriminating against the interfacial defect states since they tend to respond too slowly to follow the higher frequency voltage oscillation utilized by the DLCP method [4]. The transient photocapacitance (TPC) method is used to obtain sub-band-gap optical absorption-like spectra. A sample is held at reverse bias and is periodically pulsed to zero or slightly forward bias to ‘reset’ the occupation of defects within the sample; i.e., to let majority carriers occupy the previously empty traps in the depletion region. After the pulse is removed, a capacitance transient is observed as the carriers are thermally emitted from the occupied traps. The TPC signal is obtained by comparing the transient while the sample is illuminated with subband-gap monochromatic light to the transient while the sample is in the dark. By normalizing the difference to the photon flux and varying the wavelength of the monochromatic light we obtain a TPC spectrum. Such spectra are closely related to an integral over the density of states in the band-gap, as has been described in detail elsewhere [5]. 4. Results Admittance spectra were acquired for samples subjected to all three anneal temperatures (400, 500, and 600 °C). In each sample the admittance spectra showed an activated step with a characteristic energy between 500 and 700 meV. These activation energies increase with increasing anneal temperature. However, the activation energies of the dominant features were also found to be very dependent on the applied DC voltage bias. This indicates that these admittance steps are predominantly not due to the response of a bulk defect level but instead arise in large part from defects at the semiconductor–oxide interface. In that case the activation energy simply reveals the potential at that interface. Other features in the capacitance response may reflect the presence of bulk defects; however, here we are focusing on the most obvious step. Fig. 1 shows the 0 V bias admittance spectra for the 500 °C annealed sample, and the inset shows how dramatically the activation energy of the step changes with applied bias. Thus, we may directly determine that, for the range applied biases from 2 V to +4 V, that the semiconductor–oxide interface potential changes by roughly 600 mV. The sample annealed at 400 °C also exhibited an additional very small step with an activation energy of 20 meV. This step was not present in the admittance
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Fig. 1. Admittance Spectra of 500 °C anneal ZTO under 0 V bias. The activation energy of the step is directly dependent on applied bias, as shown in the inset. This suggests the step is not measuring a bulk defect, more likely measuring the interface potential.
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spectra of samples annealed at higher temperatures. We note that Ko et al. [6] have observed an activation in the Hall mobility of 22–25 meV in ZTO samples and attributed it to grain boundary potential barriers in crystallized ZTO films. While it is suggestive that our activation energies are similar to that found for the mobility in this study, we do not believe this interpretation applies to our films since they are amorphous. Drive-level capacitance and capacitance–voltage (C–V) profiles could only be obtained for the higher temperature anneals of 500 and 600 °C and these are compared in Fig. 2. Samples annealed at lower temperatures showed evidence of a very large defect density which would not allow us to profile into the bulk of the films. The profiles indicate free carrier densities of 5 1014 and 1 1015 cm3 for the 600 and 500 °C annealed samples, respec-
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tively. The profiles also show deep defect densities of 1.5 1015 and 5 1015 cm3 for the respective samples. Thus higher temperature annealing makes the ZTO more intrinsic and lowers the deep defect density. The higher temperature profiles are most likely influenced by the response from the interface charges which we observe in admittance spectroscopy, and so can not provide a good estimate of the bulk film free carrier density or deep defect density at that temperature. However, we note that the maximum defect density in the high temperature DLC profile closely matches the low temperature C–V density. This agrees with our basic interpretation of the differences between drive-level and C–V profiling, and so we are confident in the information contained in the low temperature profiles. Transient photocapacitance spectra were obtained for ZTO samples annealed at 400, 500, and 600 °C. One important feature in TPC spectra such as those shown in Fig. 3 is the exponential distribution of band-tail states near the band edge. Note that the spectra have been offset vertically for clarity. The characteristic energy for these band-tail states is the Urbach energy and typically indicates the degree of structural disorder in the sample [7]. The Urbach energies of these samples of 110, 120 and 140 meV for the 400, 500, and 600 °C anneals, respectively, are very high compared to previous lower band-gap disordered materials that we have investigated with TPC. For example, the Urbach energy of high-quality amorphous silicon lies in the range of 40–50 meV and the Urbach energy of highperformance CIGS solar cells usually lies below 20 meV. However, these Urbach energies are lower than those measured for the amorphous In2O3–SnO2 system, a more comparable material, which were in the range of 160–210 meV [8]. In addition to the exponential band-tail, we observe the shoulder-like features in Fig. 3 for optical energies below about 2.5 eV. These features indicate a band of optically
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P.T. Erslev et al. / Journal of Non-Crystalline Solids 354 (2008) 2801–2804
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Fig. 2. DLC and C–V profiles for 500 and 600 °C ZTO samples. The lower temperature DLC and C–V profiles show the free carrier and deep defect densities, respectively. The higher temperature DLC profiles are likely influenced by the response of the interface states.
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structural disorder, and would likely only influence the minority carrier (hole) mobility.
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Fig. 3. Comparison of TPC spectra for ZTO samples annealed at different temperatures. The spectra have been offset vertically for clarity. Interestingly, the sample with the highest mobility (600 °C) has the broadest bandtail.
excited transitions from filled deep defects to the conduction band. The defect is centered at 1.6, 1.9, and 2.05 eV for the 600, 500, and 400 °C anneals, respectively; however, there was not a strong enough defect feature for the 400 °C anneal to be entirely confident in the fit. One somewhat surprising aspect of the TPC spectra for these ZTO samples is that the Urbach energy increases with increasing anneal temperature, as shown in Fig. 3, while the electron mobility of ZTO thin-film transistors also increases with increasing annealing temperature. The increasing mobility trend could be a result of the decreasing deep defect density as revealed by DLCP. The effect of increasing structural disorder with increasing annealing temperature as reflected in the Urbach energy has also been quite well established for the case of amorphous silicon [9]. The same trend was also observed by Anwar et al. [8] in the In2O3–SnO2 system. The situation with regard to electron transport is less clear. The mobility of TCO’s with heavymetal cations is thought to not be strongly affected by structural disorder due to the spherical symmetry of the s-orbitals with large radii which define the conduction band. Since the Urbach energy measured by sub-bandgap optical measurements only reveals the band-tail with the broadest slope, we believe that the band-tail distributions revealed in these spectra are associated with the valence band rather than the conduction band. In these materials, the valence band is believed to be comprised of O 2p atomic orbitals, which are more susceptible to
Our preliminary investigation into the electronic properties of ZTO with a 1:1 ZnO:SnO2 ratio using junction capacitance methods has revealed some very important properties of this material. Drive-level capacitance profiling measurements reveal that increasing the ZTO annealing temperature from 400 to 600 °C leads to a reduction in the free carrier density and the deep defect density. This reduction in the deep defect density is most likely responsible for a corresponding increase in the mobility observed in ZTO thin-film transistors with increasing annealing temperature. We have also found that the bias-dependent oxide/ZTO interface potential can be directly measured by admittance spectroscopy, and if we estimate the change in interface potential with applied bias obtained from the DLC profiles, we observe consistent results. Transient photocapacitance spectroscopy has revealed the presence of a very deep defect band which moves further from the conduction band with decreasing annealing temperature over the range of 400–600 °C. Also, TPC measurements show an increase in the disorder, as estimated by the Urbach energy, of the ZTO film with increasing anneal temperature. The observed increase in Urbach energy of the ZTO film with increasing anneal temperature is a very interesting result, and requires further investigation. Acknowledgements This work was supported in part by the Hewlett–Packard Company and by Defense Advanced Research Projects Agency (MEMS/NEMS: Science and Technology Fundamentals). P.E. and D.H. were supported as IGERT fellows by the N.S.F. under Grant No. 0549503. H.Q.C. was funded as an Intel PhD Fellow by the Intel Foundation PhD Fellowship Program. References [1] H. Hosono, M. Yasukawa, H. Kawazoe, J. Non-Cryst. Solids 203 (1996) 334. [2] H.Q. Chiang, J.F. Wager, R.L. Hoffman, J. Jeong, D.A. Keszler, Appl. Phys. Lett. 86 (2005) 013503. [3] W.B. Jackson, G.S. Herman, R.L. Hoffman, C. Taussig, S. Braymen, F. Jeffery, J. Hauschildt, J. Non-Cryst. Solids 352 (2006) 1753. [4] J.T. Heath, J.D. Cohen, W.N. Shafarman, J. Appl. Phys. 95 (2004) 1000. [5] J.D. Cohen, J.T. Heath, W.N. Shafarman, in: U. Rau, S. Siebentritt (Eds.), Wide Gap Chalcopyrites, Springer, Berlin, 2006, p. 69. [6] J.H. Ko, I.H. Kim, D. Kim, K.S. Lee, T.S. Lee, J.-H. Jeong, B. Cheong, Y.J. Baik, W.M. Kim, Thin Solid Films 494 (2006) 42. [7] S.M. Wasim, C. Rincon, G. Marin, P. Bocaranda, E. Hernandez, I. Bonalde, E. Medina, Phys. Rev. B 64 (2001) 195101. [8] M. Anwar, I.M. Ghauri, S.A. Siddiqi, Czech J. Phys. 55 (2005) 1013. [9] G.D. Cody, T. Tiedje, B. Abeles, B. Brooks, Y. Goldstein, Phys. Rev. Lett. 47 (1981) 1480.
Journal of Non-Crystalline Solids 354 (2008) 2801–2804 www.elsevier.com/locate/jnoncrysol
Electronic properties of amorphous zinc tin oxide films by junction capacitance methods Peter T. Erslev a,*, Hai Q. Chiang b, David Hong b, John F. Wager b, J. David Cohen a b
a University of Oregon, Department of Physics, Eugene, OR 97403, USA Oregon State University, Department of Electrical Engineering and Computer Science, Corvallis, OR 97331-5501, USA
Available online 12 February 2008
Abstract Amorphous zinc tin oxides have recently shown sufficiently high mobilities that fully transparent electronic devices with these materials are now considered promising. We report the first detailed characterization of the electronic properties of these materials using the methods of admittance spectroscopy, drive-level capacitance profiling (DLCP), and transient photocapacitance (TPC) spectroscopy. We have examined a series of 1–3 lm thick zinc tin oxide films (at a composition of 1:1 ZnO:SnO2) incorporated into metal–insulator–semiconductor (MIS) devices. These were annealed at 400, 500, and 600 °C. The DLCP measurement is largely insensitive to interface defects and thus provides a good measurement of the free carrier and defect densities within the bulk region of these films. For the best samples (annealed at 600 °C), our results indicate a free carrier density of 5 1014 cm3 plus a deep defect density 1.5 1015 cm3. The TPC spectra disclose optical transitions between defect levels and the conduction and/or valence bands. They reveal two primary features in these materials: A dominant gaussian shaped deep defect band with an optical threshold near 1.6 eV relative to the conduction band, and an exponential band-tail with a characteristic (Urbach) energy near 120 meV. The Urbach energy indicates the degree of disorder in semiconducting materials, and is directly correlated to the anneal temperature for this series of samples. Published by Elsevier B.V. PACS: 73.20.r; 73.61.Le; 71.23.k; 71.55 Keywords: Thin-film transistors; Heterojunctions; Defects; Absorption; Indium tin oxide and other transparent conductors
1. Introduction Recent progress in the development of transparent thinfilm transistors (TTFTs) has brought the idea of fully transparent electronics much closer to reality. Amorphous oxides with heavy-metal cations are interesting because they possess higher electron mobilities than might be expected from their amorphous character. Such high mobilities are thought to be the result of a conduction band derived from spherically symmetric s-orbitals with large radii [1]. Mobilities as high as 50 cm2 V1 s1 have been reported for zinc tin oxide (ZTO), and the fabrication of
*
Corresponding author. E-mail address: [email protected] (P.T. Erslev).
0022-3093/$ - see front matter Published by Elsevier B.V. doi:10.1016/j.jnoncrysol.2007.09.062
TTFTs from this material has already been achieved [2]. Because of the good mobilities and also a relatively low processing temperature, ZTO is currently considered a strong candidate material for the development of flexible transparent electronics [3]. We have carried out a detailed examination of the electronic properties of amorphous ZTO with a 1:1 ZnO:SnO2 composition for films incorporated into metal–insulator– semiconductor (MIS) devices. We employed junction capacitance methods including admittance spectroscopy, drive-level capacitance profiling (DLCP), and transient photocapacitance spectroscopy (TPC). These methods have previously been successfully applied to photovoltaic devices incorporating disordered semiconducting materials such as a-Si:H and CuInSe2 and they are generally applicable to the study of most semiconductors provided one can
2802
P.T. Erslev et al. / Journal of Non-Crystalline Solids 354 (2008) 2801–2804
observe a junction capacitance that responds to an applied bias. The application of such methods to ZTO has already proven to be an extremely informative. For example, using admittance spectroscopy, we are able to directly measure the oxide/ZTO interface potential and observe how it varies with the applied gate voltage across the device. Our DLCP measurements indicate a carrier density near 5 1014 cm3 for the material with the highest mobility and this appears to increase with decreasing anneal temperature. Somewhat surprisingly, however, the TPC spectra indicate that the structural disorder of these samples, as assessed by the Urbach energy, increases with increasing anneal temperature. 2. Samples The MIS device structures were fabricated by depositing ZTO onto a highly doped Si substrate capped with 100 nm SiO2. A semi-transparent gold contact was then evaporated onto the ZTO. The 1–3 lm thick ZTO was deposited via rf magnetron sputtering in an Ar/O2 environment. The devices were subsequently annealed for one hour in air at temperatures of 400, 500, and 600 °C. X-ray diffraction measurements of ZTO thin films prepared in this manner reveal the films to be amorphous (or nanocrystalline) with a maximum crystal size not exceeding 5 nm as estimated, using the Scherrer equation, from the width of two very broad XRD peaks [2]. In contrast, ZTO films annealed above 650 °C display a multiplicity of sharp XRD peaks, indicating a dramatic increase in the crystalline nature of such films [2]. ZTO thin-film transistors processed in an identical manner as to this experiment are found to possess uniform properties across the substrate. Thus, the films being examined in this study are believed to be quite homogeneous. 3. Experimental procedures Our junction capacitance characterization methods included admittance spectroscopy, drive-level capacitance profiling, and transient photocapacitance spectroscopy. Admittance spectroscopy examines the complex electrical response of a sample to an AC perturbing signal over a range of frequencies and temperatures. The frequency and temperature of the measurement determine an emission energy Ee = kBT ln(m/2pf) where T is the temperature of the measurement, m is the thermal emission prefactor, and f is the measurement frequency. As the emission energy encounters the energetic level of a defect as the sample temperature is increased, a capacitive step is observed. When one examines these capacitive steps over a range of frequencies, the activation energy of the defect can be extracted. Drive-level capacitance profiling (DLCP) determines the density of the responding charge by examining the dependence of the capacitance to the amplitude, dV, of the applied oscillating voltage: C = C0 + C1(dV) + C2(dV)2 + The
drive-level density, NDL, is obtained from the coefficients C0 and C1 and it is equal to the free carrier density plus an integral over the density of deep defect states to an energy Ee below EC, and near the spatial position hxi = eA/C0 from the barrier junction. Spatial profiles are then obtained by varying the dc reverse bias. DLCP is superior to conventional capacitance vs. voltage (C–V) profiling because one can tune to a particular thermal emission energy by controlling the measurement frequency and temperature. In addition, this method is primarily sensitive to bulk defects, discriminating against the interfacial defect states since they tend to respond too slowly to follow the higher frequency voltage oscillation utilized by the DLCP method [4]. The transient photocapacitance (TPC) method is used to obtain sub-band-gap optical absorption-like spectra. A sample is held at reverse bias and is periodically pulsed to zero or slightly forward bias to ‘reset’ the occupation of defects within the sample; i.e., to let majority carriers occupy the previously empty traps in the depletion region. After the pulse is removed, a capacitance transient is observed as the carriers are thermally emitted from the occupied traps. The TPC signal is obtained by comparing the transient while the sample is illuminated with subband-gap monochromatic light to the transient while the sample is in the dark. By normalizing the difference to the photon flux and varying the wavelength of the monochromatic light we obtain a TPC spectrum. Such spectra are closely related to an integral over the density of states in the band-gap, as has been described in detail elsewhere [5]. 4. Results Admittance spectra were acquired for samples subjected to all three anneal temperatures (400, 500, and 600 °C). In each sample the admittance spectra showed an activated step with a characteristic energy between 500 and 700 meV. These activation energies increase with increasing anneal temperature. However, the activation energies of the dominant features were also found to be very dependent on the applied DC voltage bias. This indicates that these admittance steps are predominantly not due to the response of a bulk defect level but instead arise in large part from defects at the semiconductor–oxide interface. In that case the activation energy simply reveals the potential at that interface. Other features in the capacitance response may reflect the presence of bulk defects; however, here we are focusing on the most obvious step. Fig. 1 shows the 0 V bias admittance spectra for the 500 °C annealed sample, and the inset shows how dramatically the activation energy of the step changes with applied bias. Thus, we may directly determine that, for the range applied biases from 2 V to +4 V, that the semiconductor–oxide interface potential changes by roughly 600 mV. The sample annealed at 400 °C also exhibited an additional very small step with an activation energy of 20 meV. This step was not present in the admittance
o
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Fig. 1. Admittance Spectra of 500 °C anneal ZTO under 0 V bias. The activation energy of the step is directly dependent on applied bias, as shown in the inset. This suggests the step is not measuring a bulk defect, more likely measuring the interface potential.
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Drive Level Defect Density (cm )
spectra of samples annealed at higher temperatures. We note that Ko et al. [6] have observed an activation in the Hall mobility of 22–25 meV in ZTO samples and attributed it to grain boundary potential barriers in crystallized ZTO films. While it is suggestive that our activation energies are similar to that found for the mobility in this study, we do not believe this interpretation applies to our films since they are amorphous. Drive-level capacitance and capacitance–voltage (C–V) profiles could only be obtained for the higher temperature anneals of 500 and 600 °C and these are compared in Fig. 2. Samples annealed at lower temperatures showed evidence of a very large defect density which would not allow us to profile into the bulk of the films. The profiles indicate free carrier densities of 5 1014 and 1 1015 cm3 for the 600 and 500 °C annealed samples, respec-
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tively. The profiles also show deep defect densities of 1.5 1015 and 5 1015 cm3 for the respective samples. Thus higher temperature annealing makes the ZTO more intrinsic and lowers the deep defect density. The higher temperature profiles are most likely influenced by the response from the interface charges which we observe in admittance spectroscopy, and so can not provide a good estimate of the bulk film free carrier density or deep defect density at that temperature. However, we note that the maximum defect density in the high temperature DLC profile closely matches the low temperature C–V density. This agrees with our basic interpretation of the differences between drive-level and C–V profiling, and so we are confident in the information contained in the low temperature profiles. Transient photocapacitance spectra were obtained for ZTO samples annealed at 400, 500, and 600 °C. One important feature in TPC spectra such as those shown in Fig. 3 is the exponential distribution of band-tail states near the band edge. Note that the spectra have been offset vertically for clarity. The characteristic energy for these band-tail states is the Urbach energy and typically indicates the degree of structural disorder in the sample [7]. The Urbach energies of these samples of 110, 120 and 140 meV for the 400, 500, and 600 °C anneals, respectively, are very high compared to previous lower band-gap disordered materials that we have investigated with TPC. For example, the Urbach energy of high-quality amorphous silicon lies in the range of 40–50 meV and the Urbach energy of highperformance CIGS solar cells usually lies below 20 meV. However, these Urbach energies are lower than those measured for the amorphous In2O3–SnO2 system, a more comparable material, which were in the range of 160–210 meV [8]. In addition to the exponential band-tail, we observe the shoulder-like features in Fig. 3 for optical energies below about 2.5 eV. These features indicate a band of optically
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P.T. Erslev et al. / Journal of Non-Crystalline Solids 354 (2008) 2801–2804
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Fig. 2. DLC and C–V profiles for 500 and 600 °C ZTO samples. The lower temperature DLC and C–V profiles show the free carrier and deep defect densities, respectively. The higher temperature DLC profiles are likely influenced by the response of the interface states.
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structural disorder, and would likely only influence the minority carrier (hole) mobility.
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Fig. 3. Comparison of TPC spectra for ZTO samples annealed at different temperatures. The spectra have been offset vertically for clarity. Interestingly, the sample with the highest mobility (600 °C) has the broadest bandtail.
excited transitions from filled deep defects to the conduction band. The defect is centered at 1.6, 1.9, and 2.05 eV for the 600, 500, and 400 °C anneals, respectively; however, there was not a strong enough defect feature for the 400 °C anneal to be entirely confident in the fit. One somewhat surprising aspect of the TPC spectra for these ZTO samples is that the Urbach energy increases with increasing anneal temperature, as shown in Fig. 3, while the electron mobility of ZTO thin-film transistors also increases with increasing annealing temperature. The increasing mobility trend could be a result of the decreasing deep defect density as revealed by DLCP. The effect of increasing structural disorder with increasing annealing temperature as reflected in the Urbach energy has also been quite well established for the case of amorphous silicon [9]. The same trend was also observed by Anwar et al. [8] in the In2O3–SnO2 system. The situation with regard to electron transport is less clear. The mobility of TCO’s with heavymetal cations is thought to not be strongly affected by structural disorder due to the spherical symmetry of the s-orbitals with large radii which define the conduction band. Since the Urbach energy measured by sub-bandgap optical measurements only reveals the band-tail with the broadest slope, we believe that the band-tail distributions revealed in these spectra are associated with the valence band rather than the conduction band. In these materials, the valence band is believed to be comprised of O 2p atomic orbitals, which are more susceptible to
Our preliminary investigation into the electronic properties of ZTO with a 1:1 ZnO:SnO2 ratio using junction capacitance methods has revealed some very important properties of this material. Drive-level capacitance profiling measurements reveal that increasing the ZTO annealing temperature from 400 to 600 °C leads to a reduction in the free carrier density and the deep defect density. This reduction in the deep defect density is most likely responsible for a corresponding increase in the mobility observed in ZTO thin-film transistors with increasing annealing temperature. We have also found that the bias-dependent oxide/ZTO interface potential can be directly measured by admittance spectroscopy, and if we estimate the change in interface potential with applied bias obtained from the DLC profiles, we observe consistent results. Transient photocapacitance spectroscopy has revealed the presence of a very deep defect band which moves further from the conduction band with decreasing annealing temperature over the range of 400–600 °C. Also, TPC measurements show an increase in the disorder, as estimated by the Urbach energy, of the ZTO film with increasing anneal temperature. The observed increase in Urbach energy of the ZTO film with increasing anneal temperature is a very interesting result, and requires further investigation. Acknowledgements This work was supported in part by the Hewlett–Packard Company and by Defense Advanced Research Projects Agency (MEMS/NEMS: Science and Technology Fundamentals). P.E. and D.H. were supported as IGERT fellows by the N.S.F. under Grant No. 0549503. H.Q.C. was funded as an Intel PhD Fellow by the Intel Foundation PhD Fellowship Program. References [1] H. Hosono, M. Yasukawa, H. Kawazoe, J. Non-Cryst. Solids 203 (1996) 334. [2] H.Q. Chiang, J.F. Wager, R.L. Hoffman, J. Jeong, D.A. Keszler, Appl. Phys. Lett. 86 (2005) 013503. [3] W.B. Jackson, G.S. Herman, R.L. Hoffman, C. Taussig, S. Braymen, F. Jeffery, J. Hauschildt, J. Non-Cryst. Solids 352 (2006) 1753. [4] J.T. Heath, J.D. Cohen, W.N. Shafarman, J. Appl. Phys. 95 (2004) 1000. [5] J.D. Cohen, J.T. Heath, W.N. Shafarman, in: U. Rau, S. Siebentritt (Eds.), Wide Gap Chalcopyrites, Springer, Berlin, 2006, p. 69. [6] J.H. Ko, I.H. Kim, D. Kim, K.S. Lee, T.S. Lee, J.-H. Jeong, B. Cheong, Y.J. Baik, W.M. Kim, Thin Solid Films 494 (2006) 42. [7] S.M. Wasim, C. Rincon, G. Marin, P. Bocaranda, E. Hernandez, I. Bonalde, E. Medina, Phys. Rev. B 64 (2001) 195101. [8] M. Anwar, I.M. Ghauri, S.A. Siddiqi, Czech J. Phys. 55 (2005) 1013. [9] G.D. Cody, T. Tiedje, B. Abeles, B. Brooks, Y. Goldstein, Phys. Rev. Lett. 47 (1981) 1480.