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Effect of UV and visible light radiation on the electrical performances of transparent TFTs based on amorphous indium zinc oxide
Journal of Non-Crystalline Solids 352 (2006) 1756–1760 www.elsevier.com/locate/jnoncrysol
Effect of UV and visible light radiation on the electrical performances of transparent TFTs based on amorphous indium zinc oxide P. Barquinha *, A. Pimentel, A. Marques, L. Pereira, R. Martins, E. Fortunato Department of Materials Science/CENIMAT, Faculty of Sciences and Technology, New University of Lisbon and CEMOP-UNINOVA, Campus da Caparica, 2829-516 Caparica, Portugal Available online 18 April 2006
Abstract Insensitivity to light irradiation is desirable for conventional applications of thin-film transistors, i.e., the active matrices of displays. However, if one produces a device presenting controlled sensitivity to light, many other applications can benefit or can even be created. In this work it is shown the influence of the photon energy on the optoelectronic properties presented by n-type bottom-gate thin-film transistors based on indium zinc oxide. In the dark, the devices present very good electrical properties, working in the enhancement mode, exhibiting on–off ratios higher than 107 and channel mobility above 30 cm2/V s. Remarkable results were achieved when the devices were exposed to light radiation, the most striking one is the possibility to switch between enhancement (in the dark) and depletion (illuminated) modes, with different threshold voltages and on/off ratios, function of the light power density and wavelength used. This type of behavior permits to design circuits where one can have the same transistor working either in the enhancement or depletion modes, function of the light beam and intensity impinging on it, highly important for short wavelength detector applications. 2006 Elsevier B.V. All rights reserved. PACS: 71.23.Cq; 73.61.Jc Keywords: Amorphous semiconductors; Thin film transistors; Sputtering; Optical properties; Photoinduced effects
1. Introduction Multicomponent amorphous oxides mainly based on combinations with oxygen of metallic elements like indium, zinc, gallium and/or tin are starting to become a very promising new class of materials for application in transparent electronics [1–4]. Despite their amorphous structure, these materials present high mobilities, which could be explained by a different carrier transport mechanism than the one found in amorphous covalent semiconductors: in these multicomponent amorphous oxides, the electrical behavior is a consequence of a conduction band primarily derived from spherically symmetric heavy-metal cation ns
*
Corresponding author. E-mail address: [email protected] (P. Barquinha).
0022-3093/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2006.01.068
orbitals with (n 1)d10ns0 (n P 4) electronic configuration. These ns orbitals have large radii and so there is a large overlap between the adjacent orbitals, forming a well defined path for carriers, almost insensitive to the degree of disorder of the film [4]. Besides that, amorphous semiconductors are preferred over polycrystalline ones, since the processing temperature is lower and the uniformity of film’s characteristics is better [1]. Indium zinc oxide (IZO) is one of these new materials, and it can be used not only as a substitute of conventional transparent conductive oxides (TCOs) but also as a transparent semiconductor, since it is possible to change the resistivity more than four orders of magnitude, only by modifying the deposition conditions [4]. This permits the application of IZO simultaneously as the active layer (more resistive) and as the electrodes (less resistive) of transparent TFTs (TTFTs). Besides IZO, other multicomponent amorphous oxides have already been used to produce TTFTs
P. Barquinha et al. / Journal of Non-Crystalline Solids 352 (2006) 1756–1760
The TFTs were produced using 2.5 · 2.5 cm glass substrates coated with indium tin oxide (ITO, 200 nm) and aluminum titanium oxide (ATO, 220 nm), supplied by Planar Systems. ITO was used as the gate electrode and presents an average transmittance in the visible of 85%, a resistivity of 2.3 · 10 4 X cm, carrier concentration of 7.7 · 1020 and hall mobility of 36 cm2/V s. Concerning ATO, it was used as the gate insulator of the transistors, presenting an average capacitance of 60 nF/cm2 and a dielectric constant of 16. radio-frequency (rf, 13.56 MHz) magnetron sputtering was the deposition technique used to produce the active layer (80 nm) and the source/drain electrodes (100 nm) at room temperature, both based on IZO. Regarding the deposition conditions, it was used as the starting material an IZO ceramic target from Super Conductor Materials, Inc, power densities of 5 and 9 W/ cm2, O2/Ar flow ratios of 0.15 and 0.02 (for the active layer and the source/drain electrodes, respectively) and a deposition pressure of 0.15 Pa. The patterning of the channel and the source/drain electrodes was performed by lift-off, and the used width-to-length ratio (W/L) was 12, with L being 125 lm. The films thicknesses were measured with a surface profilometer Sloan Tech Dektak 3. The electrical characterization was performed using an Alessi microprobe station and a semiconductor parameter analyzer Agilent 4155C, controlled by the software Metrics ICS. The effect of light irradiation on the TFTs was analyzed by the transfer characteristics (i.e., drain current versus gate voltage, with a constant drain voltage) in the saturation regime, always making double sweep measurements in order to investigate the hysteresis of the devices. Light illumination was performed with a xenon lamp with stabilized source energy and a Kratos Analytical GMA 301 monochromator in the range of 380–650 nm, being the light directed to the TFTs by an optical fiber. The optical power densities, Pd, were measured by a detector connected to an International Light IL 1400 radiometer. Two different experimental procedures were employed: first, the wavelength, k, was changed between 380 and 650 nm, always maintaining a Pd of 1 mW/cm2; second, k was maintained at 380 nm and Pd was changed between 1 and 0.17 mW/cm2.
3.1. Influence of wavelength Fig. 1(a) shows the transfer characteristics of the TTFTs measured in the dark. Even with room temperature processing, the devices show very interesting electrical properties in the dark state, largely better than a-Si:H TFTs, like an on/off ratio higher than 108, saturation mobility (lsat, 1=2 calculated by the slope of the I D vs: V G plot [11]) of 45.0 cm2/V s, turn-on voltage, Von, of 6.50 V (Von is included in the analysis for being a less ambiguous parameter than VT [12]), threshold voltage in the saturation regime, VT, around 9 V (that can be made smaller increasing the active layer’s thickness) and a gate voltage swing, S, lower than 0.40 V/dec (see Table 1). When the transfer characteristics are measured with incident monochromatic light and as k decreases, several changes in the electrical properties of the TTFTs are verified, as can be seen in Fig. 1(b) (and also Fig. 3(b), measured in UV) and Table 1. The red light (k = 650 nm) does not affect noticeably the electrical parameters, since the energy of the photons (less than 2 eV) is too low to be absorbed by the IZO film, that has an optical bandgap around 3.7 eV [4]. The general
10-2 10-3
Drain current, ID (A)
2. Experimental details
3. Results and discussion
10-4 dark Pd=1.00 mW/cm2
10-5 10-6 10-7 10-8 10-9 10-10
VD=20 V
10-11 10-12 -20
-10
(a)
0 10 Gate voltage, VG (V)
20
10-2 10-3
Drain current, ID (A)
[1–3], and the obtained electrical properties seem to be even more promising than the first reported TTFTs, that were mainly based on ZnO [5–8]. In addition to the more immediate applications of TTFTs, like transparent active matrices for displays and general transparent electronic circuits, other possibilities exist for these devices, because their electrical properties can be significantly changed when exposed to light with different wavelengths and/or optical power densities [9]. Furthermore, this change in the electrical properties is controlled, reproducible and recoverable, as opposed to the well know effects of permanent light degradation of amorphous and polycrystalline silicon TFTs [10].
1757
10-4
green (λ=550 nm) Pd =1.00 mW/cm2
10-5 10-6 10-7 10-8 10-9
VD=20 V
10-10 10-11
(b)
-20
-10
0 10 Gate voltage, VG (V)
20
Fig. 1. Transfer characteristics for a IZO TTFT measured in double sweep: (a) in the dark and (b) irradiated with a monochromatic light of 550 nm (green) with optical power density of 1.00 mW/cm2.
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Table 1 Influence of the radiation’s wavelength on the electrical parameters of the TTFTs, with constant optical power density of 1.00 mW/cm2 Wavelength (nm) Dark
Red (650)
Green (550)
1.15 · 108 45.0 6.50 9.24 0.37
1.24 · 108 38.2 6.50 8.74 0.38
8.44 · 107 24.2 2.00 3.84 0.42
The values refer to the transfer characteristic measured in
Blue (450)
UV (380)
1.46 · 107 15.6 13.0 11.4 0.42
5.45 · 106 20.5 18.0 16.9 0.38
VG to +VG direction.
trend as k decreases is a decrease in the on/off ratio, despite the natural increase on the on-current as k decreases due to the photo induced carriers [9] present in the IZO film (oncurrent goes from about 1.5 mA in the dark to 2.5 mA with UV). However, the increment on the off-current is much higher (from 10 11 to 5 · 10 10 A) because in the on state the channel already has a very high density of electrons even without considering the photo induced ones; in contrast, when the transistor is in the off state, the channel is depleted of electrons and so the photogain effect is more pronounced [9]. Von (and consequently VT) also decreases for lower k, because the higher energy photo induced carriers can occupy more defects and surface states that are necessary to fill before an appreciable flow of electrons can occur in the IZO/ATO interface, thus facilitating the formation of the conductive channel. The most remarkable factor that arises from here is that the same transistor can work both in enhancement (VT > 0) depletion (VT < 0) modes, function of the incident light’s wavelength. Concerning lsat, it also decreases for lower k, since the ionized atoms and Coulomb scattering tend to be predominant with very high carrier concentrations [13]: in spite of IZO films seem to have some defects acting as acceptor-like
centers in a n-type semiconductor and so the mobility tends to increase as the carrier concentration increases (to compensate the scattering induced by these defects) [4], in the conduction channel the carrier concentration is much higher than in the remaining semiconductor and increases even more when k is lower (by photo induced carriers), so it is probable that the ionic and Coulomb scattering are the predominant scattering mechanisms (and naturally, the scattering due to the IZO/ATO interface traps) in the TFT channel, leading to a decrease in lsat. The obtained results are in agreement with the evolution of the absorption coefficient of the IZO film, as presented in Fig. 2, where is visible that the absorption coefficient starts to 10-2 10-3
Drain current, ID (A)
On/off ratio lsat (cm2/V s) Von (V) VT (V) S (V/dec)
10-4 10-5 10-6 10-7
UV (λ=380 nm) Pd =0.17 mW/cm2
10-8 10-9
VD =20 V
10-10
Photon wavelength, λ (nm) 750
500
10-11
250
-20
2.5
0 10 Gate voltage, VG (V)
10-3
1.5 1.0 0.5 0.0
10-4 10-5 UV (λ=380 nm) Pd =1.00 mW/cm2
10-6 10-7 10-8
VD=20 V
10-9
1.5
2.0
2.5
20
10-2
IZO film thickness ~120 nm
2.0
1.0
-10
(a)
3.0
Drain current, ID (A)
Absorption coefficient, α (105 cm-1)
1000
3.0
3.5
4.0
4.5
5.0
5.5
10-10
Photon energy, E (eV)
-20
(b) Fig. 2. Evolution of the absorption coefficient with the incident photon energy and wavelength. The deposition parameters used for the analyzed IZO film are the same of the ones used for the TFT’s active layer (see description in Section 2), with a thickness of 120 nm.
-10
0
10
20
Gate voltage, VG (V)
Fig. 3. Transfer characteristics for a IZO TTFT measured in double sweep, irradiated with a monochromatic light of 380 nm (UV), with optical power density of 0.17 mW/cm2 (a) and 1.00 mW/cm2 (b).
P. Barquinha et al. / Journal of Non-Crystalline Solids 352 (2006) 1756–1760
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Table 2 Influence of the optical power density on the electrical parameters of the TTFTs, with constant wavelength of 380 nm Optical power density (mW/cm2) 0.17 On/off ratio lsat (cm2/V s) Von (V) VT (V) S (V/dec)
2.59 · 107 19.9 10.5 8.87 0.36
0.25
0.50
2.14 · 107 20.1 12.0 10.3 0.39
The values refer to the transfer characteristic measured in
1.10 · 107 19.7 15.5 14.0 0.40
0.75 7.54 · 106 21.5 17.0 15.5 0.37
1.00 5.45 · 106 20.5 18.0 16.9 0.38
VG to +VG direction.
increase around 450 nm (blue), which corresponds to a significant variation of the electrical properties, mainly on/off ratio and Von. 3.2. Influence of power density with UV radiation Maintaining the incident light with k = 380 nm, Pd was changed between 0.17 and 1 mW/cm2 to analyze the effect on the transfer characteristics and electrical properties of the TTFTs. Fig. 3 shows the effect of the two Pd limits (Pd = 0.17 and Pd = 1.00 in Fig. 3(a) and (b), respectively). The obtained results (Fig. 3 and Table 2) show that there is a tendency for the transistors to return to their off-state characteristics as Pd is decreased, as expected. The diminishing number of photo induced carriers as Pd becomes lower is the most plausible explanation for this trend. At this stage, it is important to explain that the effect of varying Pd is quite different of the effect of varying k: while the variations of k used here are responsible to ‘find’ different energy levels within the semiconductor optical bandgap, characteristic of different defects such as deep or shallow states (in the limit, when the incident radiation has an energy higher than the bandgap, the photons are readily absorbed by the semiconductor), a variation in Pd is fixed always in the same energy level, changing only the number of photo induced charges that can be created within that energy level [13]. Thus, in this particular case, it seems that even with a low Pd, if k is kept near 380 nm (3.30 eV), the illumination is capable of exciting electrons to energy states below the conduction band that can then pass to the conduction band by field excitation, i.e., when the gate field increases [14]. Increasing Pd will allow for more energy states to be filled and so to increase the conductivity of the IZO film (thus, to increase the off-current and decrease the on/off ratio) and facilitate the formation of the conduction channel, which is formed for lower Von (or VT) as Pd increases. Note that, for the Pd range used herein, the TFTs always work in depletion mode, but they return to enhancement mode as soon as the light source is turned off. 3.3. Evolution of hysteresis with k and Pd To end the analysis, a final paragraph about something that was not discussed above, the variation of the hysteresis in Figs. 1 and 3. The magnitude of hysteresis, taken as the difference between Von when the transfer characteristics are
measured from VG to +VG (1) and from +VG to VG (2), increases with decreasing k and increasing Pd, and the variation is even higher in TFTs with thicker IZO films (not shown), as expected by classical semiconductor theory. However, when (2) is measured, its shape and characteristics are only strongly affected when k = 380 nm and Pd P 0.75 mW/cm2, not like the characteristics measured in the (1) direction, whose evolution is much more gradual, as seen in Tables 1 and 2. With the limit conditions (k = 380 nm and Pd = 1.00 mW/cm2), in the (2) direction, one obtains an abrupt raise of S (goes up to 3.26 V/dec) and off-current (8 · 10 7 A). Although none concrete physical explanation is know at the moment for this phenomenon, it surely is related with charging and discharging effects of existing traps. A detailed analysis of these results will be published in the near future and will certainly help to understand the peculiarities of IZO material, in general, and IZO TTFTs, particularly. 4. Conclusions In this paper were presented the first results regarding the influence of k and Pd in fully transparent TFTs. In spite of the transistors characteristics were significantly changed when exposed to low k and/or high Pd, they keep working perfectly as field-effect devices. Moreover, the obtained results showed that it is possible to have the same TFT working as an enhancement or depletion mode device, only by controlling the k of the incident light, being the process reversible, which is quite promising for future optoelectronic applications like optical keys or optical memories. Acknowledgements The authors thank Planar Systems, Inc., Espoo, Finland, for supplying the ITO/ATO glass substrates. The authors would also like to thank Portuguese Science Foundation (FCT-MCTES) for the fellowships SFRH/BD/ 17970/2004 and SFRH/BD/6215/2001 given to two of the authors (Pedro Barquinha and Luı´s Pereira) and by the projects POCTI/CTM/38924 and POCI/CTM/55942. References [1] K. Nomura, H. Ohta, A. Takagi, T. Kamiya, M. Hirano, H. Hosono, Nature 432 (2004) 488.
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[2] H.Q. Chiang, J.F. Wager, R.L. Hoffman, J. Jeong, D.A. Keszler, Appl. Phys Lett. 86 (2005) 1. [3] N.L. Dehuff, E.S. Kettenring, D. Hong, H.Q. Chiang, J.F. Wager, R.L. Hoffman, C.-H. Park, D.A. Keszler, J. Appl. Phys. 97 (2005) 1. [4] R. Martins, P. Barquinha, A. Pimentel, L. Pereira, E. Fortunato, Phys. Stat. Sol. (a) 202 (9) (2005) R95. [5] E. Fortunato, P. Barquinha, A. Pimentel, A. Gonc¸alves, A. Marques, L. Pereira, R. Martins, Adv. Mat. 17 (2005) 590. [6] R.L. Hoffman, B.J. Norris, J.F. Wager, Appl. Phys. Lett. 82 (2003) 733. [7] P.F. Carcia, R.S. McLean, M.H. Reilly, G. Nunes, Appl. Phys. Lett. 82 (2003) 1117.
[8] B.J. Norris, J. Anderson, J.F. Wager, D.A. Keszler, J. Phys. D: Appl. Phys. 36 (2003) L105. [9] H.S. Bae, M.H. Yoon, J.H. Kim, S. Im, Appl. Phys. Lett. 83 (2003) 5313. [10] S. Masuda, K. Kitamura, Y. Okumura, S. Miyatake, H. Tabata, T. Kawai, J. Appl. Phys. 93 (2003) 1624. [11] E. Fortunato, P. Barquinha, A. Pimentel, A. Gonc¸alves, A. Marques, R. Martins, L. Pereira, Appl. Phys. Lett. 85 (2004) 2541. [12] R.L. Hoffman, J. Appl. Phys. 95 (2004) 5813. [13] E.S. Yang, Microelectronic Devices, McGraw-Hill, Singapore, 1988. [14] A.C. Tickle, Thin-Film Transistors – A New Approach to Microelectronics, Wiley, USA, 1969.
Effect of UV and visible light radiation on the electrical performances of transparent TFTs based on amorphous indium zinc oxide P. Barquinha *, A. Pimentel, A. Marques, L. Pereira, R. Martins, E. Fortunato Department of Materials Science/CENIMAT, Faculty of Sciences and Technology, New University of Lisbon and CEMOP-UNINOVA, Campus da Caparica, 2829-516 Caparica, Portugal Available online 18 April 2006
Abstract Insensitivity to light irradiation is desirable for conventional applications of thin-film transistors, i.e., the active matrices of displays. However, if one produces a device presenting controlled sensitivity to light, many other applications can benefit or can even be created. In this work it is shown the influence of the photon energy on the optoelectronic properties presented by n-type bottom-gate thin-film transistors based on indium zinc oxide. In the dark, the devices present very good electrical properties, working in the enhancement mode, exhibiting on–off ratios higher than 107 and channel mobility above 30 cm2/V s. Remarkable results were achieved when the devices were exposed to light radiation, the most striking one is the possibility to switch between enhancement (in the dark) and depletion (illuminated) modes, with different threshold voltages and on/off ratios, function of the light power density and wavelength used. This type of behavior permits to design circuits where one can have the same transistor working either in the enhancement or depletion modes, function of the light beam and intensity impinging on it, highly important for short wavelength detector applications. 2006 Elsevier B.V. All rights reserved. PACS: 71.23.Cq; 73.61.Jc Keywords: Amorphous semiconductors; Thin film transistors; Sputtering; Optical properties; Photoinduced effects
1. Introduction Multicomponent amorphous oxides mainly based on combinations with oxygen of metallic elements like indium, zinc, gallium and/or tin are starting to become a very promising new class of materials for application in transparent electronics [1–4]. Despite their amorphous structure, these materials present high mobilities, which could be explained by a different carrier transport mechanism than the one found in amorphous covalent semiconductors: in these multicomponent amorphous oxides, the electrical behavior is a consequence of a conduction band primarily derived from spherically symmetric heavy-metal cation ns
*
Corresponding author. E-mail address: [email protected] (P. Barquinha).
0022-3093/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2006.01.068
orbitals with (n 1)d10ns0 (n P 4) electronic configuration. These ns orbitals have large radii and so there is a large overlap between the adjacent orbitals, forming a well defined path for carriers, almost insensitive to the degree of disorder of the film [4]. Besides that, amorphous semiconductors are preferred over polycrystalline ones, since the processing temperature is lower and the uniformity of film’s characteristics is better [1]. Indium zinc oxide (IZO) is one of these new materials, and it can be used not only as a substitute of conventional transparent conductive oxides (TCOs) but also as a transparent semiconductor, since it is possible to change the resistivity more than four orders of magnitude, only by modifying the deposition conditions [4]. This permits the application of IZO simultaneously as the active layer (more resistive) and as the electrodes (less resistive) of transparent TFTs (TTFTs). Besides IZO, other multicomponent amorphous oxides have already been used to produce TTFTs
P. Barquinha et al. / Journal of Non-Crystalline Solids 352 (2006) 1756–1760
The TFTs were produced using 2.5 · 2.5 cm glass substrates coated with indium tin oxide (ITO, 200 nm) and aluminum titanium oxide (ATO, 220 nm), supplied by Planar Systems. ITO was used as the gate electrode and presents an average transmittance in the visible of 85%, a resistivity of 2.3 · 10 4 X cm, carrier concentration of 7.7 · 1020 and hall mobility of 36 cm2/V s. Concerning ATO, it was used as the gate insulator of the transistors, presenting an average capacitance of 60 nF/cm2 and a dielectric constant of 16. radio-frequency (rf, 13.56 MHz) magnetron sputtering was the deposition technique used to produce the active layer (80 nm) and the source/drain electrodes (100 nm) at room temperature, both based on IZO. Regarding the deposition conditions, it was used as the starting material an IZO ceramic target from Super Conductor Materials, Inc, power densities of 5 and 9 W/ cm2, O2/Ar flow ratios of 0.15 and 0.02 (for the active layer and the source/drain electrodes, respectively) and a deposition pressure of 0.15 Pa. The patterning of the channel and the source/drain electrodes was performed by lift-off, and the used width-to-length ratio (W/L) was 12, with L being 125 lm. The films thicknesses were measured with a surface profilometer Sloan Tech Dektak 3. The electrical characterization was performed using an Alessi microprobe station and a semiconductor parameter analyzer Agilent 4155C, controlled by the software Metrics ICS. The effect of light irradiation on the TFTs was analyzed by the transfer characteristics (i.e., drain current versus gate voltage, with a constant drain voltage) in the saturation regime, always making double sweep measurements in order to investigate the hysteresis of the devices. Light illumination was performed with a xenon lamp with stabilized source energy and a Kratos Analytical GMA 301 monochromator in the range of 380–650 nm, being the light directed to the TFTs by an optical fiber. The optical power densities, Pd, were measured by a detector connected to an International Light IL 1400 radiometer. Two different experimental procedures were employed: first, the wavelength, k, was changed between 380 and 650 nm, always maintaining a Pd of 1 mW/cm2; second, k was maintained at 380 nm and Pd was changed between 1 and 0.17 mW/cm2.
3.1. Influence of wavelength Fig. 1(a) shows the transfer characteristics of the TTFTs measured in the dark. Even with room temperature processing, the devices show very interesting electrical properties in the dark state, largely better than a-Si:H TFTs, like an on/off ratio higher than 108, saturation mobility (lsat, 1=2 calculated by the slope of the I D vs: V G plot [11]) of 45.0 cm2/V s, turn-on voltage, Von, of 6.50 V (Von is included in the analysis for being a less ambiguous parameter than VT [12]), threshold voltage in the saturation regime, VT, around 9 V (that can be made smaller increasing the active layer’s thickness) and a gate voltage swing, S, lower than 0.40 V/dec (see Table 1). When the transfer characteristics are measured with incident monochromatic light and as k decreases, several changes in the electrical properties of the TTFTs are verified, as can be seen in Fig. 1(b) (and also Fig. 3(b), measured in UV) and Table 1. The red light (k = 650 nm) does not affect noticeably the electrical parameters, since the energy of the photons (less than 2 eV) is too low to be absorbed by the IZO film, that has an optical bandgap around 3.7 eV [4]. The general
10-2 10-3
Drain current, ID (A)
2. Experimental details
3. Results and discussion
10-4 dark Pd=1.00 mW/cm2
10-5 10-6 10-7 10-8 10-9 10-10
VD=20 V
10-11 10-12 -20
-10
(a)
0 10 Gate voltage, VG (V)
20
10-2 10-3
Drain current, ID (A)
[1–3], and the obtained electrical properties seem to be even more promising than the first reported TTFTs, that were mainly based on ZnO [5–8]. In addition to the more immediate applications of TTFTs, like transparent active matrices for displays and general transparent electronic circuits, other possibilities exist for these devices, because their electrical properties can be significantly changed when exposed to light with different wavelengths and/or optical power densities [9]. Furthermore, this change in the electrical properties is controlled, reproducible and recoverable, as opposed to the well know effects of permanent light degradation of amorphous and polycrystalline silicon TFTs [10].
1757
10-4
green (λ=550 nm) Pd =1.00 mW/cm2
10-5 10-6 10-7 10-8 10-9
VD=20 V
10-10 10-11
(b)
-20
-10
0 10 Gate voltage, VG (V)
20
Fig. 1. Transfer characteristics for a IZO TTFT measured in double sweep: (a) in the dark and (b) irradiated with a monochromatic light of 550 nm (green) with optical power density of 1.00 mW/cm2.
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Table 1 Influence of the radiation’s wavelength on the electrical parameters of the TTFTs, with constant optical power density of 1.00 mW/cm2 Wavelength (nm) Dark
Red (650)
Green (550)
1.15 · 108 45.0 6.50 9.24 0.37
1.24 · 108 38.2 6.50 8.74 0.38
8.44 · 107 24.2 2.00 3.84 0.42
The values refer to the transfer characteristic measured in
Blue (450)
UV (380)
1.46 · 107 15.6 13.0 11.4 0.42
5.45 · 106 20.5 18.0 16.9 0.38
VG to +VG direction.
trend as k decreases is a decrease in the on/off ratio, despite the natural increase on the on-current as k decreases due to the photo induced carriers [9] present in the IZO film (oncurrent goes from about 1.5 mA in the dark to 2.5 mA with UV). However, the increment on the off-current is much higher (from 10 11 to 5 · 10 10 A) because in the on state the channel already has a very high density of electrons even without considering the photo induced ones; in contrast, when the transistor is in the off state, the channel is depleted of electrons and so the photogain effect is more pronounced [9]. Von (and consequently VT) also decreases for lower k, because the higher energy photo induced carriers can occupy more defects and surface states that are necessary to fill before an appreciable flow of electrons can occur in the IZO/ATO interface, thus facilitating the formation of the conductive channel. The most remarkable factor that arises from here is that the same transistor can work both in enhancement (VT > 0) depletion (VT < 0) modes, function of the incident light’s wavelength. Concerning lsat, it also decreases for lower k, since the ionized atoms and Coulomb scattering tend to be predominant with very high carrier concentrations [13]: in spite of IZO films seem to have some defects acting as acceptor-like
centers in a n-type semiconductor and so the mobility tends to increase as the carrier concentration increases (to compensate the scattering induced by these defects) [4], in the conduction channel the carrier concentration is much higher than in the remaining semiconductor and increases even more when k is lower (by photo induced carriers), so it is probable that the ionic and Coulomb scattering are the predominant scattering mechanisms (and naturally, the scattering due to the IZO/ATO interface traps) in the TFT channel, leading to a decrease in lsat. The obtained results are in agreement with the evolution of the absorption coefficient of the IZO film, as presented in Fig. 2, where is visible that the absorption coefficient starts to 10-2 10-3
Drain current, ID (A)
On/off ratio lsat (cm2/V s) Von (V) VT (V) S (V/dec)
10-4 10-5 10-6 10-7
UV (λ=380 nm) Pd =0.17 mW/cm2
10-8 10-9
VD =20 V
10-10
Photon wavelength, λ (nm) 750
500
10-11
250
-20
2.5
0 10 Gate voltage, VG (V)
10-3
1.5 1.0 0.5 0.0
10-4 10-5 UV (λ=380 nm) Pd =1.00 mW/cm2
10-6 10-7 10-8
VD=20 V
10-9
1.5
2.0
2.5
20
10-2
IZO film thickness ~120 nm
2.0
1.0
-10
(a)
3.0
Drain current, ID (A)
Absorption coefficient, α (105 cm-1)
1000
3.0
3.5
4.0
4.5
5.0
5.5
10-10
Photon energy, E (eV)
-20
(b) Fig. 2. Evolution of the absorption coefficient with the incident photon energy and wavelength. The deposition parameters used for the analyzed IZO film are the same of the ones used for the TFT’s active layer (see description in Section 2), with a thickness of 120 nm.
-10
0
10
20
Gate voltage, VG (V)
Fig. 3. Transfer characteristics for a IZO TTFT measured in double sweep, irradiated with a monochromatic light of 380 nm (UV), with optical power density of 0.17 mW/cm2 (a) and 1.00 mW/cm2 (b).
P. Barquinha et al. / Journal of Non-Crystalline Solids 352 (2006) 1756–1760
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Table 2 Influence of the optical power density on the electrical parameters of the TTFTs, with constant wavelength of 380 nm Optical power density (mW/cm2) 0.17 On/off ratio lsat (cm2/V s) Von (V) VT (V) S (V/dec)
2.59 · 107 19.9 10.5 8.87 0.36
0.25
0.50
2.14 · 107 20.1 12.0 10.3 0.39
The values refer to the transfer characteristic measured in
1.10 · 107 19.7 15.5 14.0 0.40
0.75 7.54 · 106 21.5 17.0 15.5 0.37
1.00 5.45 · 106 20.5 18.0 16.9 0.38
VG to +VG direction.
increase around 450 nm (blue), which corresponds to a significant variation of the electrical properties, mainly on/off ratio and Von. 3.2. Influence of power density with UV radiation Maintaining the incident light with k = 380 nm, Pd was changed between 0.17 and 1 mW/cm2 to analyze the effect on the transfer characteristics and electrical properties of the TTFTs. Fig. 3 shows the effect of the two Pd limits (Pd = 0.17 and Pd = 1.00 in Fig. 3(a) and (b), respectively). The obtained results (Fig. 3 and Table 2) show that there is a tendency for the transistors to return to their off-state characteristics as Pd is decreased, as expected. The diminishing number of photo induced carriers as Pd becomes lower is the most plausible explanation for this trend. At this stage, it is important to explain that the effect of varying Pd is quite different of the effect of varying k: while the variations of k used here are responsible to ‘find’ different energy levels within the semiconductor optical bandgap, characteristic of different defects such as deep or shallow states (in the limit, when the incident radiation has an energy higher than the bandgap, the photons are readily absorbed by the semiconductor), a variation in Pd is fixed always in the same energy level, changing only the number of photo induced charges that can be created within that energy level [13]. Thus, in this particular case, it seems that even with a low Pd, if k is kept near 380 nm (3.30 eV), the illumination is capable of exciting electrons to energy states below the conduction band that can then pass to the conduction band by field excitation, i.e., when the gate field increases [14]. Increasing Pd will allow for more energy states to be filled and so to increase the conductivity of the IZO film (thus, to increase the off-current and decrease the on/off ratio) and facilitate the formation of the conduction channel, which is formed for lower Von (or VT) as Pd increases. Note that, for the Pd range used herein, the TFTs always work in depletion mode, but they return to enhancement mode as soon as the light source is turned off. 3.3. Evolution of hysteresis with k and Pd To end the analysis, a final paragraph about something that was not discussed above, the variation of the hysteresis in Figs. 1 and 3. The magnitude of hysteresis, taken as the difference between Von when the transfer characteristics are
measured from VG to +VG (1) and from +VG to VG (2), increases with decreasing k and increasing Pd, and the variation is even higher in TFTs with thicker IZO films (not shown), as expected by classical semiconductor theory. However, when (2) is measured, its shape and characteristics are only strongly affected when k = 380 nm and Pd P 0.75 mW/cm2, not like the characteristics measured in the (1) direction, whose evolution is much more gradual, as seen in Tables 1 and 2. With the limit conditions (k = 380 nm and Pd = 1.00 mW/cm2), in the (2) direction, one obtains an abrupt raise of S (goes up to 3.26 V/dec) and off-current (8 · 10 7 A). Although none concrete physical explanation is know at the moment for this phenomenon, it surely is related with charging and discharging effects of existing traps. A detailed analysis of these results will be published in the near future and will certainly help to understand the peculiarities of IZO material, in general, and IZO TTFTs, particularly. 4. Conclusions In this paper were presented the first results regarding the influence of k and Pd in fully transparent TFTs. In spite of the transistors characteristics were significantly changed when exposed to low k and/or high Pd, they keep working perfectly as field-effect devices. Moreover, the obtained results showed that it is possible to have the same TFT working as an enhancement or depletion mode device, only by controlling the k of the incident light, being the process reversible, which is quite promising for future optoelectronic applications like optical keys or optical memories. Acknowledgements The authors thank Planar Systems, Inc., Espoo, Finland, for supplying the ITO/ATO glass substrates. The authors would also like to thank Portuguese Science Foundation (FCT-MCTES) for the fellowships SFRH/BD/ 17970/2004 and SFRH/BD/6215/2001 given to two of the authors (Pedro Barquinha and Luı´s Pereira) and by the projects POCTI/CTM/38924 and POCI/CTM/55942. References [1] K. Nomura, H. Ohta, A. Takagi, T. Kamiya, M. Hirano, H. Hosono, Nature 432 (2004) 488.
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