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Origin of High Photoconductive Gain in Fully Transparent Heterojunction Nanocrystalline Oxide Image Sensors and Interconnects Sanghun Jeon,* Ihun Song, Sungsik Lee, Byungki Ryu, Seung-Eon Ahn, Eunha Lee, Young Kim, Arokia Nathan,* John Robertson, and U-In Chung
Ever-evolving advances in oxide semiconductor materials and devices[1–5] continue to drive leading-edge developments in transparent electronics,[6–9] thanks to new integration processes, enabling large-area processing on rigid and flexible substrates. In transparent electronics, the key materials are wide bandgap semiconductors, such as oxide semiconductors.[8] This family of semiconductors offer a host of advantages such as low cost and high scalability, in addition to seamless heterogeneous integration with many other inorganic and organic materials in view of their low thermal budget in processing which provides integration flexibility. This has spawned a wealth of applications ranging from high frame-rate interactive displays with embedded imaging to flexible electronics,[10–15] where speed and transparency are essential requirements.[3,16] Interest in oxide semiconductors stem from a number of attributes primarily their ease of processing, high field-effect mobility, and wide bandgap.[1–3] In particular, transparency is a desirable attribute that enables the seamless embedding of electronics for smart, immersive ambients.[17] Indeed, oxide semiconductors are less disordered due to their ionic bonding, giving rise to high field-effect mobility even in the amorphous phase.[1–3] Therefore, transparent electronic systems, which have been once viewed as science fiction, can now become a reality. Prof. S. Jeon Department of Display and Semiconductor Physics, Korea University, 2511 Sejongro, Sejong 339–700, Korea E-mail: [email protected] Prof. S. Jeon Department of Applied Physics, Korea University, 2511 Sejongro, Sejong 339–700, Korea Dr. I. Song, Dr. S.-E. Ahn, Dr. E. Lee, Y. Kim, Dr. U-I. Chung Samsung Advanced Institute of Technology Samsung Electronics Corporations Yongin-Si, Gyeonggi-Do 446–712, Korea Dr. S. Lee, Prof. A. Nathan, Prof. J. Robertson Centre for Advanced Photonics and Electronics Department of Engineering Cambridge University Cambridge CB3 0FA, UK E-mail: [email protected] Dr. B. Ryu Korea Electrotechnology Research Institute 12, Bulmosan-ro 10 beon-gil Seongsan-gu, Changwon-si, Gyeongsangnam-do 642–120, Korea
DOI: 10.1002/adma.201401955
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However, the oxide semiconductor has one weakness, a light-induced threshold voltage instability.[18,19] Despite that, the light-induced instability can be put to good use as the basis of a high-gain photo-image sensor, with higher sensitivity than the amorphous silicon equivalent. We analyze the photo conduction mechanism in oxide semiconductor and use this to maximize the performance of image sensors to provide ultra-high photoconductive gain. Previously, we reported a phototransistor embedded in a display pixel,[20] in which gate operation is used to accelerate recovery from photocurrent level to the dark state. In this work, we describe the origin of ultra-high quantum efficiencies in an all invisible imaging array with photosensors based on nanocrystalline oxide heterojunction thin-film transistor (TFT) where we use well-known oxide materials such as InZnO[21–29] and HfInZnO.[30] While the InZnO is known to have optical transparency it has a significant number of subgap states associated with oxygen vacancies leading to persistent photoconductivity.[21–24] In order to understand the origin of the high photocurrent of a device, we evaluated the influence of a light spot from source to drain side on the photoconductive gain of photosensor array. A three-dimensional device simulation tool was employed. Also, a set of pulse measurement experiments and first-principles calculations based on hybrid density functional theory were performed to study the effect of applying gate bias on the recovery mechanism of photocurrent. Three primary factors are believed to be responsible for the high quantum efficiency. Introducing a buried layer with a higher density of oxygen vacancies (Vo), as an integral part of the TFT channel, leads to significant visible-light absorption. This coupled with a favorable band offset along with gate-modulated band bending leads to transfer of photogenerated electrons to the photo-TFT channel where the Vo concentration is small. Because of hole localization, recombination is retarded, giving rise to an extended electron lifetime, and hence, high quantum efficiency. In addition, scaling down the channel length of the TFT reduces the carrier transit-time from source to drain, yielding a higher efficiency. This work presented here demonstrates the first invisible, high sensitivity image sensor along with quantitative analysis of the quantum efficiency in the heterojunction TFT taking into account the optical absorption, electron lifetime, and transit time. In order for the photosensor element to meet the requirements of both light sensitivity and manageable dark state device characteristics, a bilayer HfInZnO (HIZO)/ [(In2O3)-(ZnO)] (IZO) active semiconductor was used, as shown
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COMMUNICATION Figure 1. Material and photoresponsive characteristics of nanocrystalline oxide semiconductor. a) cross-sectional high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) image. b) Energy-dispersive spectroscopy (EDS) data for bilayer active region used in the photosensor TFTs. c) Transfer characteristics of HfInZnO (HIZO) single layer TFT in the dark and under illumination with light wavelengths, where the HIZO TFT shows negligible light sensitivity. The inset shows a cross-sectional view of HIZO TFT. d) Transfer characteristics of HIZO–IZO bilayer photo-TFT in the dark and under illumination with light-wavelengths, where the IZO shows significant light absorption. The inset shows a crosssectional view of HIZO–IZO TFT.
in Figure 1a and 1b. In order to verify the microstructure of semiconductor, we performed nanobeam diffraction analysis throughout the red straight line along SiO2 gate insulator and oxide semiconductor in cross-sectional high angle annular dark field scanning transmission electron microscopy (HAADFSTEM) image (Figure 1a). In comparison to well-known amorphous SiO2 gate insulator, the oxide semiconductor presents a characteristic line and tiny bright dots (see the inset), indicating that this material consists of a nanocrystalline phase in an amorphous matrix. The back channel IZO region in the thickness direction exhibits further improvement in the crystallinity. As seen in Figure 1b, energy dispersive spectroscopy data clearly verifies bi-layer active region such as HIZO–IZO layer used in the photosensor. In active semiconductor region, the front HIZO layer plays the role as a threshold voltage (VT) adjusting layer, increasing the VT of the device toward positive gate bias direction as presented in Section S1 and Figure S1 in the Supporting Information. On the other hand, the IZO layer is primarily responsible for optical absorption. As seen
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in Section S2–S3 in the Supporting Information, the density of sub-gap states in the IZO layer is much higher than that of the HIZO layer, leading to significant sub-gap absorption in the IZO layer under light illumination. In order to examine device characteristics in the dark and under illumination, we performed transfer IDS–VGS measurements of the HIZO and HIZO–IZO bi-layer transistors in the dark and under illumination with various light wavelengths (400–600nm) as seen in Figure 1c and 1d. The tested device structure is an inverted staggered architecture with bottom gate and top source/drain (S/D) Mo electrodes. An etch stopper (E/S) was used to protect any possible damage of back channel during S/D etch process. In the dark state, as compared with HIZO TFT (the µsat of 3.6 cm2 eV−1 s−1), HIZO–IZO bilayer transistor presents a high mobility (the µsat of 106 cm2 eV−1 s−1) mainly due to the buried IZO channel with high crystallinity. Also, the HIZO–IZO bilayer transistor exhibits much higher photocurrent rather than the HIZO single-layer device. Considering the fact that the optical
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Figure 2. Various photosensor performances and three dimensional band diagram of transparent IZO and opaque Mo electrode integrated devices under illumination. a) IDS and photoinduced barrier lowering (Δφb) of transparent In-rich IZO and Mo electrode integrated devices as a function of light-wavelength. b) External quantum efficiency (EQE) and corrected carrier density (ncoll) extracted and compared with the transistor with metal electrodes. c) Three-dimensional band diagram of the active channel for transparent In-rich IZO electrode integrated phototransistor under illumination. d) Three-dimensional band diagram of the active channel for opaque Mo electrode integrated phototransistor under illumination.
bandgaps of HIZO and IZO are 3.2 and 2.9 eV, respectively, the drastic increase in photocurrent of the HIZO–IZO bilayer transistor under 400–550nm (2.2–3.1 eV) light can be ascribed to carrier generation due to high optical absorption in the IZO layer. In particular, at photon energies smaller than the bandgap energy (Eg) of IZO, the absorption is mainly due to ionized Vo rather than band-to-band excitation and takes place in the IZO layer even under visible illumination.[20] In order to make an entirely transparent photosensor TFT array, transparent conducting oxide should be used for bottom gate and top S/D electrodes. In addition, the responsivity of the photosensor TFT with transparent S/D electrode needs to be probed. To this end, we used In-rich IZO [In/(In+Zn):Zn/ (In+Zn) = 10:1] as a transparent electrode and compared with conventional Mo metal electrode. The photocurrent levels for both devices were measured at various wavelengths and VGS of −2 V. The IDS for a TFT with transparent conducting electrode shows a factor of improvement compared to a metal electrode transistor, as can be seen in Figure 2a. Electrons are injected from source to the channel across a potential barrier whose height is modulated by the light. For the tested devices, S/D electrodes are formed on top of the etch stopper. The channel regions under opaque metal S/D electrodes
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are not directly exposed to the incident light beam, while transparent electrodes allow light to reach the whole active region including both source and drain sides. In particular, the light exposure at the source side crucially influences the carrier transport and thus the improvement in photocurrent of a transparent electrode transistor, as can be seen in Figure 2c. This is mainly due to the source-to-channel barrier lowering (SBL), i.e., Δφb, at the source side, and can be explained by the following relation:[31,32] ⎛ qΔφb ⎞ I DS = I DS0 exp ⎜ α ⎝ kT ⎟⎠
(1)
where IDS0 is a reference value without barrier lowering, kT is the thermal energy, and α is a constant. Note that the SBL is mainly affected by the increased electrostatic image force at metal/semiconductor junction.[33] This type of force is proportional to the carrier concentration in semiconductor.[34] In the presented work, it can be argued that the SBL is mainly due to the increased electron density which corresponds to the number of the ionized oxygen vacancies. Based on the values of IDS in Figure 2a, the external quantum efficiency (EQE) can be retrieved using the following equation:[35]
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I DS /q Pinc /hv
(2)
Here, Pinc is power density of the incident illumination (ca. 100 µW/cm2) and hv the photon energy. Also, the corrected carrier density (ncoll) can be extracted as follows:[30] ncoll =
I DS qμFE (W /L ) tSVDS
(3)
where µFE is the field-effect mobility and tS channel layer thickness. Both EQE and ncoll are proportional to IDS, as can be seen in Equation 2 and 3. Based on Equation 2 and 3, the EQE and ncoll are extracted and compared with the transistor with metal electrodes, as can be seen in Figure 2b. It is found that both EQE and ncoll for a transparent conducting electrode transistor are about 10-times higher compared to the metal electrode transistor, and stems from the increase of photocurrent IDS shown in Figure 2a. This can be attributed to sourceto-channel barrier lowering (Δφb)[36,37] at the back channel of the transistor with transparent electrodes, as seen in Equation 1, which is consistent with the results of the 3D device simulation seen in Figure 2c and 2d (details regarding device simulation are seen in Section S4 in the Supporting Information). As shown in Figure 2c, the source side barrier in the transistor with transparent electrode is reduced whereas that of the transistor with metal electrode remains almost the same (see Figure 2d). This suggests that light incident on the source side gives rise to the ionized Vo. The ionized Vo (Vo++) is positive, and its accumulation on the source side reduces barrier height.[36,37] To further verify the effect of beam position on photocurrent (IDS) and barrier lowering (Δφb), incident light with the same optical power is locally spotted (see Figure 3a), and the
photocurrent and 3dimensional device simulation results for each case are measured as shown in Figure 3b and Figure 3c, respectively (see the method in Section S4 in the Supporting Information). When beam is positioned on source side, IDS is at least a few orders larger than other cases where incident light is absorbed at other locations, e.g., at drain or mid-channel. This implies that barrier lowering at source side is more effective to increase photocurrent. Indeed, the simulated conduction band profiles show significant barrier lowering at the source (A), middle (B & C) and drain side (D), as shown in Figure 3c. This suggests a higher injection of electrons (ninj) from source, resulting in higher photocurrent.[38] From Equation 1 and 3, we can deduce: ⎛ qΔφb ⎞ n inj = n0 exp ⎜ α ⎝ kT ⎟⎠
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EQE =
(4)
where n0 is a reference carrier density. In order to fabricate photosensor array and satisfy the requirements of different light sensitivity for a switch and a sensor, we employed a unique fabrication process using separation photomask pattern and wet processes presented in Figure 4a and the details are found in Section S5 in the Supporting Information. The device structure of both elements is TFT based inverted staggered architecture. In order to ensure a fully transparent device array with low ρ electrode, multilayer transparent conducting materials[38] such as [10(In2O3)-(ZnO)] (In-rich IZO)/Au/In-rich IZO materials were used for gate, source and drain (S/D) electrodes. In our investigation, we used a 14 nm-thick Au layer inserted between the IZO layers. The sheet resistance of the In-rich IZO electrode measured by a four point probe was 105 䊐 Ω/square䊐 . At Au thickness of less than 8 nm, the In-rich IZO/Au/In-rich IZO still showed a fairly high sheet resistance (85 䊐 Ω/square䊐 ) probably due to the form of randomly disconnected islands at nucleation.
Figure 3. Photon-exposure location dependent photo current and three dimensional band diagram of channel. a) Schematics of the optical scanning microscope measurement of photosensor device with illumination location. b) IDS with respect to beam position. c) Three-dimensional band diagram of the channel under source-side illumination, under middle-side illumination, and under drain-side illumination. Here, (A) and (D) represent the source-side beam exposure and drain side beam exposures, respectively; (B) and (C) are the cases in between.
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Figure 4. Process integration procedure, photo-view and materials data of photosensor pixel. a) The fabrication procedure of photosensor pixel comprising of sensor and switch elements where a single HIZO layer is used for a switch and HIZO–IZO bilayer is used for a sensor. b) The photo-image of fully fabricated entirely see-through photosensor pixel array. The photo image of cactus and related texts[39] are clearly seen behind photosensor array due to entire transparency of the device. The bottom inset shows the transparency of In-rich IZO–Au–In-rich IZO and Mo electrode integrated photosensor array. c) Cross-sectional HAADF-STEM images of a single layer and bi-layer active semiconductor. d) Cross-sectional HAADF-STEM image of In-rich IZO–Au–In-rich IZO and nano beam diffraction pattern analysis on In-rich IZO–Au–In-rich IZO. e) Cross-sectional HAADF-STEM images of In-rich IZO–Au–In-rich IZO and Mo electrode integrated photosensor array.
However, the insertion of Au layer with a thickness of 10 nm led to a significant reduction in the sheet resistance (Au 14 nm: 䊐 4.2 Ω/square䊐, Au 20 nm: 3.6 䊐 Ω/square䊐, Au 24 nm: 䊐 3.2 Ω/square䊐). Considering the sheet resistance and transmittance, the chosen thickness of Au inserted between In-rich IZO was 14 nm. The active semiconductor for the sensor is bi-layer HIZO–IZO and that for the switch is a single layer HIZO. As a final outcome, we fabricated the photosensor arrays comprising of hybrid active channel for the switch and sensor as seen in Figure 4b. The photo image of a fully fabricated device clearly shows the entirely see-through photosensor pixel array. Here, the photo image of cactus and some texts[39] are clearly seen behind photosensor array due to entire transparency of the device. It should be noted that the transparency of In-rich IZO/Au/In-rich IZO integrated photosensor array is pronounced in comparison to Mo based photosensor array as seen in the bottom inset. Figure 4c shows cross-sectional HAADF-STEM image of active semiconductor for a sensor and a switch, respectively, depicting that the active layer of photo sensor is the HIZO–IZO double layer, and that of switch is the HIZO single layer. Cross-sectional HAADFSTEM image and energy dispersion spectrum in Figure 4d verifies the composition of multilayer transparent electrodes comprising of IZO and Au. Figure 4e shows cross-sectional HAADF STEM images of photosensor with different electrodes. The details regarding structural and materials analyses
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on sensor array are presented in Section S6–S10 in the Supporting Information. It is known that Zn-series oxide semiconductors have one weakness, namely persistent photoconductivity (PPC), which leads to a certain amount of current flow even after the illumination has stopped. The PPC mechanism for ZnO-series semiconductors is relevant to the ionization of oxygen vacancy in ZnO from the ionized states.[39,40] The gradual increase in photocurrent under 425–550 nm (2.8–2.2 eV) light is attributed to carrier generation as a result of gate field screening by the ionized oxygen vacancies in the channel, as previously demonstrated in Figures 2 and 3. As expected, high energy photon exposure leads to a high degree of ionization from Vo to Vo++ and leaves high density of Vo++ in oxide channel. Due to resulting change in cell structure, neighboring Zn atoms are displaced in the outward direction,[41,42] which lowers the recovery time to Vo which is stable in the dark. The PPC issue should be solved in order to improve the switching speed and the frame rate in interactive display. To this end, a series of positive biases were tested to clarify the optimum reset bias condition as seen in Figure 5a. The standby and read states were set to VGS of −5 V and VDS of 5 V. A sufficiently high gate bias as for reset operation is needed to remove the PPC state. Possible explanation is given by Figure 5b. When applying negative bias on the gate in the dark state, the channel is fully depleted. In this circumstance, when the device is exposed to the light
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COMMUNICATION Figure 5. Fermi-level control of oxide-semiconductor to eliminate persistent photoconductivity phenomena. a) Drain-source current as a function time when TFTs are subjected to light pulses. Drain–source current as a function time when TFTs are subjected to light and/or the magnitude of positive bias pulses. Sufficiently positive bias needs to be applied to remove persistent photoconductivity issue, revealing the positive-bias-assisted PPC recovery mechanism. b) The schematics of the TFT when TFTs are subjected to light and/or the magnitude of positive bias pulses. c) Band energies of a-IZO. The defect level of Vo is denoted by ED, EC(k) and EC+1(k) represent the calculated first and second conduction band energies at k = (¼ ¼ ¼). Closed and open circles represent the occupied and unoccupied states, respectively. CBM and VBM are indicated by horizontal solid lines. d) Contour plot of charge densities. The left figure presents the atomic structure of a-IZO which is comprised of occupied defect state of Voo while the right figure shows that of a-IZO which is comprised of the ionized defect state, Vo++ and electrons at the conduction band.
illumination, the ionization of Vo occurs in the channel. Since the gate bias for read operation is negative, the positively charged Vo++ is located in the front channel while the generating free electrons are at the back channel. Additionally, after turning the light off, the rate of recombination between Vo++ in the front channel and free electrons at the back channel is lowered by a negative gate bias, which does not solve the PPC problem. Thus band banding in the opposite direction achieved by a positive gate pulse will force recombination of electrons with ionized oxygen vacancies. In order to facilitate recombination more effectively, a sufficiently high gate bias is required. Then, when we apply negative bias on the gate for read operation again, the channel is totally depleted, providing sufficiently low dark current. To examine the effect of applying a positive gate bias on the recovery of positively ionized Vo++ defect to the neutral Voo defect, we performed hybrid density functional calculations of Vo defects in amorphous In-Zn-O (a-IZO).[43,44] First, we prepared the 132-atom supercell of a-IZO, which is generated by
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melt-and-quench first principle molecular dynamics simulations. The initial structure of Vo in a-IZO is constructed by removing one O atom from the bulk a-IZO. The negative bias and light illumination (NBI) and the positive bias (PB) conditions are modeled by adjusting the electron chemical potential. The band energies of the conduction bands and the Vo defect level are traced and plotted versus the atomic structure simulations under various light and bias conditions as seen in Figure 5c. The defect state of neutral Voo is localized and located within the band gap. The atomic structure of Voo in a-IZO is presented in Figure 5d (left). However, under NBI condition, the neutral Voo defects lose the electrons and the defect states become ionized and resonant with the conduction bands. The atomic structure of this state is presented in Figure 5d (right), similar to the Vo++ defects in a-IGZO.[45,46] When the illumination is stopped and the gate bias is switched from negative to positive, depending on the condition of materials processed, the Vo defect becomes either shallow Vo++, deep Voo, or transferable state between Vo++ and
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Voo as proposed as follows. For shallow Vo++, some ionized Vo++ defects do not return to the neutral states, always presenting the conducting behavior. In another case, a certain ionized Vo++ can easily capture the conduction electrons, becoming Voo without any activation energy and there is no PPC phenomenon. In the other case, i.e. transferable state, other Vo++ defects can return to the neutral state only under positive bias condition where these Vo++ defects combine with the conduction electrons, forming the deep sub gap state in a-IZO. Consistent with experimental data, our calculated data present that Voo and Vo++ defects in a-IZO are transferable. Therefore, deep Voo defects are easily ionized under negative bias illumination condition and recovered from Vo++ to Voo by providing activation energy through gate bias, without thermal annealing. In this work, we explored the origin of high photoconductive gain in phototransistors based on transparent heterojunction nanocrystalline oxide semiconductor thin-film transistor integrated with transparent interconnection technologies. The photosensor array with a HIZO–IZO heterostructure with transparent conducting electrodes yields high quantumefficiency and high transmittance in the visible range. Based on three-dimensional simulations along with light position dependent photocurrent measurements, the transparent electrodes facilitates relatively high injection of photogenerated carriers due to significant lowering of the Schottky barrier at the source side in comparison to metal electrodes. Also, a set of pulse measurement experiments and first-principles calculations based on hybrid density functional theory provide in-depth understanding on the effect of gate pulse bias on recovery of the persistent photocurrent. The results presented here expand the scope of applications of oxide semiconductors. Indeed transparent electronic systems, once viewed as science fiction, can now become a reality.
Experimental Section In order to fabricate photosensor pixel using hybrid active channel, we used single HIZO layer for a switch element and bilayer HIZO–IZO for a photosensor. All these oxide semiconductor films were deposited by reactive radio-frequency magnetron sputtering under controlled Ar and O2 mixed plasma. For both switch and sensor elements, we used the inverted and staggered thin-film transistor (TFT) structure having In-rich IZO–Au–In-rich IZO gate, source, and drain electrodes where In-rich IZO film was prepared by sputtering method and Au film was prepared by e-beam evaporation. For comparison, Mo gate, source, and drain electrode was deposited by DC sputtering in an Ar rich atmosphere and patterned. In order to warrant high capacitance coupling with the channel while attaining comparatively low leakage current density through gate insulator, bilayer, SiN-SiO2 was used. During source/drain etching process, etch stopper layer was used to prevent any possible damage of back channel. First-principles calculations based on hybrid density functional theory (DFT) were performed to investigate the effect of applying positive gate bias on the recovery of an ionized oxygen vacancy (Vo++) defect to a neutral one (Voo) in amorphous oxide semiconductors. The projectoraugmented wave (PAW) pseudopotentials, the generalized gradient approximation (GGA), and the screened hybrid functional of Heyd, Scuseria, and Ernzerhof (HSE) were used, as implemented in VASP. The hybrid functional contains 25% of Hartree–Fock exchange energy and 75% of PBE exchange energy and the screening parameter of 0.2 Å−1 was used. To correct the deep nature of metal d bands in oxides, the on-site
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Coulomb energies (U) were included. The U parameters of 3.9 and 4.5 eV for In d and Zn d orbitals well reproduce the position of metal d bands. To reduce the computational cost, we used the energy cutoff of 300 eV with the special k-point of (¼, ¼, ¼). The 132-atom cubic supercell was used to model the defect properties of Vo in amorphous In-Zn-O (a-IZO). The a-IZO was generated through melt-and-quench ab initio molecular dynamics (MD) simulations. The O-vacancy structure was modeled by removing one O atom from bulk a-IZO and we considered five Vo defect configurations in a-IZO
Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements This work was supported by the National Research Foundation of Korea(NRF) grant funded by the Korea government(MEST) (No. 2014R1A2A2A01006541). S.J. designed this work. S.J. and A. Nathan prepared the manuscript. The experiment and electrical analysis were carried out by S.J., S.-E.A., and I.S. E.L. performed TEM analysis work. S.L. developed the device model for simulation and analysis of the two and three-dimensional energy band diagrams, and worked for extracting photosensor parameters. B.R. performed ab-initial calculation and interpreted the modeling data. A.N. and J.R. did the quantitative analysis on photoresponse and quantum efficiency. All the authors discussed the results and implications and commented on the manuscript at all stages. Note: Figure 2 was revised after initial publication online. Received: April 30, 2014 Revised: August 5, 2014 Published online: September 15, 2014
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[31] B. G. Streetman, S. Banerjee, Solid State Electronic Devices, 5th ed., Prentice Hall, Upper Saddle River, NJ, USA, 1981. [32] S. M. Sze, Physics of Semiconductor Devices, 2nd ed., Wiley, Hoboken, NJ, USA, 1981. [33] E. H. Rhoderick, Metal–Semiconductor Contacts, Clarendon, Oxford, UK, 1978, pp. 35–38. [34] G. P. Lousberg, H. Y. Yu, B. Froment, E. Augendre, A. De Keersgieter, A. Lauwers, M.-F. Li, P. Absil, M. Jurczak, IEEE Electron. Dev. Lett. 2007, 28, 123. [35] S.-E. Ahn, I. Song, S. Jeon, Y. W. Jeon, Y. Kim, C. Kim, B. Ryu, J.-H. Lee, A. Nathan, S. Lee, G. T. Kim, U.-I. Chung, Adv. Mater. 2012, 24, 2631. [36] T.-C. Chen, T.-C. Chang, T.-Y. Hsieh, C.-T. Tsai, S.-C. Chen, C.-S. Lin, M.-C. Hung, C.-H. Tu, J.-J. Chang, P.-L. Chen, Appl. Phys. Lett. 2010, 97, 192103. [37] Y. Kamada, S. Fujita, T. Hiramatsu, T. Matsuda, M. Furuta, T. Hirao, Solid-State Electron. 2010, 54, 1392. [38] K. H. Choi, J. Y. Kim, Y. S. Lee, H. J. Kim, Thin Solid Films 1999, 341, 152. [39] A. Janotti, Van de Walle, Phys. Rev. B. 2007, 76, 162502. [40] T. Kamiya, K. Nomura, M. Hirano, H. Hosono. Phys. Status Solidi C 2008, 5, 3098. [41] K. Nomura, T. Kamiya, H. Yanagi, E. Ikenaga, K. Yang, K. Kobayashi, M. Hirano, H. Hosono, Appl. Phys. Lett. 2008, 92, 202117. [42] A. Janotti, Van de Walle. Appl. Phys. Lett. 2005, 87, 122102. [43] J. Heyd, G. E. Scuseria, M. Ernzerhof, J. Chem. Phys. 2003, 118, 8207. [44] G. Kresse, J. Furthmüller, Phys. Rev. B 1996, 54, 11169. [45] B. Ryu, H.-K. Noh, E.-A. Choi, K. J. Chang, Appl. Phys. Lett. 2010, 97, 022108. [46] H.-K. Noh, K. J. Chang, B. Ryu, W. J. Lee, Phys. Rev. B 2011, 84, 115205.
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[15] S.-C. Wu, H.-T. Feng, M.-J. Yu, I-T. Wang, T.-H. Hou, IEEE Electron Device Lett. 2013, 34, 1265. [16] J. F. Wager, D. A. Keszler, R. E. Presley, Transparent Electronics, Springer, New York, 2008. [17] T. Kamiya, H. Hosono, NPG Asia Mater. 2010, 2, 15. [18] J. Robertson, Phys. Status Solidi B 2008, 245, 1026. [19] P. Görrn, M. Lehhardt, T. Riedl, W. Kowalsky, Appl. Phys. Lett. 2007, 91, 193504, [20] S. Jeon, S.-E. Ahn, I. Song, C. J. Kim, U.-I. Chung, E. Lee, I. Yoo, A. Nathan, S. Lee, J. Robertson, K. Kim, Nat. Mater. 2012, 11, 301. [21] P. Barquinha, G. Gonçalves, L. Pereira, R. Martins, E. Fortunato, Thin Solid Films 2007, 515, 8450. [22] D. C. Paine, B. Yaglioglu, Z. Beiley, S. Lee, Thin Solid Films 2008, 516, 5894. [23] R. Martins, P. Almeida, P. Barquinha, L. Pereira, A. Pimentel, I. Ferreira, E. Fortunato, J. Non-crystalline Solids 2006, 352, 1471. [24] H. Choi, S. Jeon, Appl. Phys. Lett. 2014, 104, 023505. [25] H. Choi, S. Jeon, Appl. Phys. Lett. 2014, 104, 133507. [26] S.-E. Ahn, S. Jeon, Y. W. Jeon, C. Kim, M.-J. Lee, C.-W. Lee, J. Park, I. Song, A. Nathan, S. Lee, U-I. Chung, Adv. Mater. 2013, 25, 5549. [27] S. Lee, S.-E. Ahn, Y. Jeon, J.-H. Ahn, I. Song, S. Jeon, D.-J. Yun, J. Kim, H. Choi, U-i. Chung, J. Park, Appl. Phys. Lett. 2013, 103, 251111. [28] S. Park,S. Park, S.-E. Ahn, I. Song, W. Chae, M. Han, J. Lee, S. Jeon, J. Vac. Sci. Technol. 2013, 31, 050605. [29] H. Choi, S. Jeon, H. Kim, J. Shin, C. Kim, U. Chung, Appl. Phys. Lett. 2012, 100, 173501. [30] H. Choi, S. Jeon, H. Kim, J. Shin, C. Kim, U. Chung, Appl. Phys. Lett. 2011, 99, 183502.
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Origin of High Photoconductive Gain in Fully Transparent Heterojunction Nanocrystalline Oxide Image Sensors and Interconnects Sanghun Jeon,* Ihun Song, Sungsik Lee, Byungki Ryu, Seung-Eon Ahn, Eunha Lee, Young Kim, Arokia Nathan,* John Robertson, and U-In Chung
Ever-evolving advances in oxide semiconductor materials and devices[1–5] continue to drive leading-edge developments in transparent electronics,[6–9] thanks to new integration processes, enabling large-area processing on rigid and flexible substrates. In transparent electronics, the key materials are wide bandgap semiconductors, such as oxide semiconductors.[8] This family of semiconductors offer a host of advantages such as low cost and high scalability, in addition to seamless heterogeneous integration with many other inorganic and organic materials in view of their low thermal budget in processing which provides integration flexibility. This has spawned a wealth of applications ranging from high frame-rate interactive displays with embedded imaging to flexible electronics,[10–15] where speed and transparency are essential requirements.[3,16] Interest in oxide semiconductors stem from a number of attributes primarily their ease of processing, high field-effect mobility, and wide bandgap.[1–3] In particular, transparency is a desirable attribute that enables the seamless embedding of electronics for smart, immersive ambients.[17] Indeed, oxide semiconductors are less disordered due to their ionic bonding, giving rise to high field-effect mobility even in the amorphous phase.[1–3] Therefore, transparent electronic systems, which have been once viewed as science fiction, can now become a reality. Prof. S. Jeon Department of Display and Semiconductor Physics, Korea University, 2511 Sejongro, Sejong 339–700, Korea E-mail: [email protected] Prof. S. Jeon Department of Applied Physics, Korea University, 2511 Sejongro, Sejong 339–700, Korea Dr. I. Song, Dr. S.-E. Ahn, Dr. E. Lee, Y. Kim, Dr. U-I. Chung Samsung Advanced Institute of Technology Samsung Electronics Corporations Yongin-Si, Gyeonggi-Do 446–712, Korea Dr. S. Lee, Prof. A. Nathan, Prof. J. Robertson Centre for Advanced Photonics and Electronics Department of Engineering Cambridge University Cambridge CB3 0FA, UK E-mail: [email protected] Dr. B. Ryu Korea Electrotechnology Research Institute 12, Bulmosan-ro 10 beon-gil Seongsan-gu, Changwon-si, Gyeongsangnam-do 642–120, Korea
DOI: 10.1002/adma.201401955
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However, the oxide semiconductor has one weakness, a light-induced threshold voltage instability.[18,19] Despite that, the light-induced instability can be put to good use as the basis of a high-gain photo-image sensor, with higher sensitivity than the amorphous silicon equivalent. We analyze the photo conduction mechanism in oxide semiconductor and use this to maximize the performance of image sensors to provide ultra-high photoconductive gain. Previously, we reported a phototransistor embedded in a display pixel,[20] in which gate operation is used to accelerate recovery from photocurrent level to the dark state. In this work, we describe the origin of ultra-high quantum efficiencies in an all invisible imaging array with photosensors based on nanocrystalline oxide heterojunction thin-film transistor (TFT) where we use well-known oxide materials such as InZnO[21–29] and HfInZnO.[30] While the InZnO is known to have optical transparency it has a significant number of subgap states associated with oxygen vacancies leading to persistent photoconductivity.[21–24] In order to understand the origin of the high photocurrent of a device, we evaluated the influence of a light spot from source to drain side on the photoconductive gain of photosensor array. A three-dimensional device simulation tool was employed. Also, a set of pulse measurement experiments and first-principles calculations based on hybrid density functional theory were performed to study the effect of applying gate bias on the recovery mechanism of photocurrent. Three primary factors are believed to be responsible for the high quantum efficiency. Introducing a buried layer with a higher density of oxygen vacancies (Vo), as an integral part of the TFT channel, leads to significant visible-light absorption. This coupled with a favorable band offset along with gate-modulated band bending leads to transfer of photogenerated electrons to the photo-TFT channel where the Vo concentration is small. Because of hole localization, recombination is retarded, giving rise to an extended electron lifetime, and hence, high quantum efficiency. In addition, scaling down the channel length of the TFT reduces the carrier transit-time from source to drain, yielding a higher efficiency. This work presented here demonstrates the first invisible, high sensitivity image sensor along with quantitative analysis of the quantum efficiency in the heterojunction TFT taking into account the optical absorption, electron lifetime, and transit time. In order for the photosensor element to meet the requirements of both light sensitivity and manageable dark state device characteristics, a bilayer HfInZnO (HIZO)/ [(In2O3)-(ZnO)] (IZO) active semiconductor was used, as shown
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COMMUNICATION Figure 1. Material and photoresponsive characteristics of nanocrystalline oxide semiconductor. a) cross-sectional high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) image. b) Energy-dispersive spectroscopy (EDS) data for bilayer active region used in the photosensor TFTs. c) Transfer characteristics of HfInZnO (HIZO) single layer TFT in the dark and under illumination with light wavelengths, where the HIZO TFT shows negligible light sensitivity. The inset shows a cross-sectional view of HIZO TFT. d) Transfer characteristics of HIZO–IZO bilayer photo-TFT in the dark and under illumination with light-wavelengths, where the IZO shows significant light absorption. The inset shows a crosssectional view of HIZO–IZO TFT.
in Figure 1a and 1b. In order to verify the microstructure of semiconductor, we performed nanobeam diffraction analysis throughout the red straight line along SiO2 gate insulator and oxide semiconductor in cross-sectional high angle annular dark field scanning transmission electron microscopy (HAADFSTEM) image (Figure 1a). In comparison to well-known amorphous SiO2 gate insulator, the oxide semiconductor presents a characteristic line and tiny bright dots (see the inset), indicating that this material consists of a nanocrystalline phase in an amorphous matrix. The back channel IZO region in the thickness direction exhibits further improvement in the crystallinity. As seen in Figure 1b, energy dispersive spectroscopy data clearly verifies bi-layer active region such as HIZO–IZO layer used in the photosensor. In active semiconductor region, the front HIZO layer plays the role as a threshold voltage (VT) adjusting layer, increasing the VT of the device toward positive gate bias direction as presented in Section S1 and Figure S1 in the Supporting Information. On the other hand, the IZO layer is primarily responsible for optical absorption. As seen
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in Section S2–S3 in the Supporting Information, the density of sub-gap states in the IZO layer is much higher than that of the HIZO layer, leading to significant sub-gap absorption in the IZO layer under light illumination. In order to examine device characteristics in the dark and under illumination, we performed transfer IDS–VGS measurements of the HIZO and HIZO–IZO bi-layer transistors in the dark and under illumination with various light wavelengths (400–600nm) as seen in Figure 1c and 1d. The tested device structure is an inverted staggered architecture with bottom gate and top source/drain (S/D) Mo electrodes. An etch stopper (E/S) was used to protect any possible damage of back channel during S/D etch process. In the dark state, as compared with HIZO TFT (the µsat of 3.6 cm2 eV−1 s−1), HIZO–IZO bilayer transistor presents a high mobility (the µsat of 106 cm2 eV−1 s−1) mainly due to the buried IZO channel with high crystallinity. Also, the HIZO–IZO bilayer transistor exhibits much higher photocurrent rather than the HIZO single-layer device. Considering the fact that the optical
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Figure 2. Various photosensor performances and three dimensional band diagram of transparent IZO and opaque Mo electrode integrated devices under illumination. a) IDS and photoinduced barrier lowering (Δφb) of transparent In-rich IZO and Mo electrode integrated devices as a function of light-wavelength. b) External quantum efficiency (EQE) and corrected carrier density (ncoll) extracted and compared with the transistor with metal electrodes. c) Three-dimensional band diagram of the active channel for transparent In-rich IZO electrode integrated phototransistor under illumination. d) Three-dimensional band diagram of the active channel for opaque Mo electrode integrated phototransistor under illumination.
bandgaps of HIZO and IZO are 3.2 and 2.9 eV, respectively, the drastic increase in photocurrent of the HIZO–IZO bilayer transistor under 400–550nm (2.2–3.1 eV) light can be ascribed to carrier generation due to high optical absorption in the IZO layer. In particular, at photon energies smaller than the bandgap energy (Eg) of IZO, the absorption is mainly due to ionized Vo rather than band-to-band excitation and takes place in the IZO layer even under visible illumination.[20] In order to make an entirely transparent photosensor TFT array, transparent conducting oxide should be used for bottom gate and top S/D electrodes. In addition, the responsivity of the photosensor TFT with transparent S/D electrode needs to be probed. To this end, we used In-rich IZO [In/(In+Zn):Zn/ (In+Zn) = 10:1] as a transparent electrode and compared with conventional Mo metal electrode. The photocurrent levels for both devices were measured at various wavelengths and VGS of −2 V. The IDS for a TFT with transparent conducting electrode shows a factor of improvement compared to a metal electrode transistor, as can be seen in Figure 2a. Electrons are injected from source to the channel across a potential barrier whose height is modulated by the light. For the tested devices, S/D electrodes are formed on top of the etch stopper. The channel regions under opaque metal S/D electrodes
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are not directly exposed to the incident light beam, while transparent electrodes allow light to reach the whole active region including both source and drain sides. In particular, the light exposure at the source side crucially influences the carrier transport and thus the improvement in photocurrent of a transparent electrode transistor, as can be seen in Figure 2c. This is mainly due to the source-to-channel barrier lowering (SBL), i.e., Δφb, at the source side, and can be explained by the following relation:[31,32] ⎛ qΔφb ⎞ I DS = I DS0 exp ⎜ α ⎝ kT ⎟⎠
(1)
where IDS0 is a reference value without barrier lowering, kT is the thermal energy, and α is a constant. Note that the SBL is mainly affected by the increased electrostatic image force at metal/semiconductor junction.[33] This type of force is proportional to the carrier concentration in semiconductor.[34] In the presented work, it can be argued that the SBL is mainly due to the increased electron density which corresponds to the number of the ionized oxygen vacancies. Based on the values of IDS in Figure 2a, the external quantum efficiency (EQE) can be retrieved using the following equation:[35]
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I DS /q Pinc /hv
(2)
Here, Pinc is power density of the incident illumination (ca. 100 µW/cm2) and hv the photon energy. Also, the corrected carrier density (ncoll) can be extracted as follows:[30] ncoll =
I DS qμFE (W /L ) tSVDS
(3)
where µFE is the field-effect mobility and tS channel layer thickness. Both EQE and ncoll are proportional to IDS, as can be seen in Equation 2 and 3. Based on Equation 2 and 3, the EQE and ncoll are extracted and compared with the transistor with metal electrodes, as can be seen in Figure 2b. It is found that both EQE and ncoll for a transparent conducting electrode transistor are about 10-times higher compared to the metal electrode transistor, and stems from the increase of photocurrent IDS shown in Figure 2a. This can be attributed to sourceto-channel barrier lowering (Δφb)[36,37] at the back channel of the transistor with transparent electrodes, as seen in Equation 1, which is consistent with the results of the 3D device simulation seen in Figure 2c and 2d (details regarding device simulation are seen in Section S4 in the Supporting Information). As shown in Figure 2c, the source side barrier in the transistor with transparent electrode is reduced whereas that of the transistor with metal electrode remains almost the same (see Figure 2d). This suggests that light incident on the source side gives rise to the ionized Vo. The ionized Vo (Vo++) is positive, and its accumulation on the source side reduces barrier height.[36,37] To further verify the effect of beam position on photocurrent (IDS) and barrier lowering (Δφb), incident light with the same optical power is locally spotted (see Figure 3a), and the
photocurrent and 3dimensional device simulation results for each case are measured as shown in Figure 3b and Figure 3c, respectively (see the method in Section S4 in the Supporting Information). When beam is positioned on source side, IDS is at least a few orders larger than other cases where incident light is absorbed at other locations, e.g., at drain or mid-channel. This implies that barrier lowering at source side is more effective to increase photocurrent. Indeed, the simulated conduction band profiles show significant barrier lowering at the source (A), middle (B & C) and drain side (D), as shown in Figure 3c. This suggests a higher injection of electrons (ninj) from source, resulting in higher photocurrent.[38] From Equation 1 and 3, we can deduce: ⎛ qΔφb ⎞ n inj = n0 exp ⎜ α ⎝ kT ⎟⎠
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EQE =
(4)
where n0 is a reference carrier density. In order to fabricate photosensor array and satisfy the requirements of different light sensitivity for a switch and a sensor, we employed a unique fabrication process using separation photomask pattern and wet processes presented in Figure 4a and the details are found in Section S5 in the Supporting Information. The device structure of both elements is TFT based inverted staggered architecture. In order to ensure a fully transparent device array with low ρ electrode, multilayer transparent conducting materials[38] such as [10(In2O3)-(ZnO)] (In-rich IZO)/Au/In-rich IZO materials were used for gate, source and drain (S/D) electrodes. In our investigation, we used a 14 nm-thick Au layer inserted between the IZO layers. The sheet resistance of the In-rich IZO electrode measured by a four point probe was 105 䊐 Ω/square䊐 . At Au thickness of less than 8 nm, the In-rich IZO/Au/In-rich IZO still showed a fairly high sheet resistance (85 䊐 Ω/square䊐 ) probably due to the form of randomly disconnected islands at nucleation.
Figure 3. Photon-exposure location dependent photo current and three dimensional band diagram of channel. a) Schematics of the optical scanning microscope measurement of photosensor device with illumination location. b) IDS with respect to beam position. c) Three-dimensional band diagram of the channel under source-side illumination, under middle-side illumination, and under drain-side illumination. Here, (A) and (D) represent the source-side beam exposure and drain side beam exposures, respectively; (B) and (C) are the cases in between.
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Figure 4. Process integration procedure, photo-view and materials data of photosensor pixel. a) The fabrication procedure of photosensor pixel comprising of sensor and switch elements where a single HIZO layer is used for a switch and HIZO–IZO bilayer is used for a sensor. b) The photo-image of fully fabricated entirely see-through photosensor pixel array. The photo image of cactus and related texts[39] are clearly seen behind photosensor array due to entire transparency of the device. The bottom inset shows the transparency of In-rich IZO–Au–In-rich IZO and Mo electrode integrated photosensor array. c) Cross-sectional HAADF-STEM images of a single layer and bi-layer active semiconductor. d) Cross-sectional HAADF-STEM image of In-rich IZO–Au–In-rich IZO and nano beam diffraction pattern analysis on In-rich IZO–Au–In-rich IZO. e) Cross-sectional HAADF-STEM images of In-rich IZO–Au–In-rich IZO and Mo electrode integrated photosensor array.
However, the insertion of Au layer with a thickness of 10 nm led to a significant reduction in the sheet resistance (Au 14 nm: 䊐 4.2 Ω/square䊐, Au 20 nm: 3.6 䊐 Ω/square䊐, Au 24 nm: 䊐 3.2 Ω/square䊐). Considering the sheet resistance and transmittance, the chosen thickness of Au inserted between In-rich IZO was 14 nm. The active semiconductor for the sensor is bi-layer HIZO–IZO and that for the switch is a single layer HIZO. As a final outcome, we fabricated the photosensor arrays comprising of hybrid active channel for the switch and sensor as seen in Figure 4b. The photo image of a fully fabricated device clearly shows the entirely see-through photosensor pixel array. Here, the photo image of cactus and some texts[39] are clearly seen behind photosensor array due to entire transparency of the device. It should be noted that the transparency of In-rich IZO/Au/In-rich IZO integrated photosensor array is pronounced in comparison to Mo based photosensor array as seen in the bottom inset. Figure 4c shows cross-sectional HAADF-STEM image of active semiconductor for a sensor and a switch, respectively, depicting that the active layer of photo sensor is the HIZO–IZO double layer, and that of switch is the HIZO single layer. Cross-sectional HAADFSTEM image and energy dispersion spectrum in Figure 4d verifies the composition of multilayer transparent electrodes comprising of IZO and Au. Figure 4e shows cross-sectional HAADF STEM images of photosensor with different electrodes. The details regarding structural and materials analyses
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on sensor array are presented in Section S6–S10 in the Supporting Information. It is known that Zn-series oxide semiconductors have one weakness, namely persistent photoconductivity (PPC), which leads to a certain amount of current flow even after the illumination has stopped. The PPC mechanism for ZnO-series semiconductors is relevant to the ionization of oxygen vacancy in ZnO from the ionized states.[39,40] The gradual increase in photocurrent under 425–550 nm (2.8–2.2 eV) light is attributed to carrier generation as a result of gate field screening by the ionized oxygen vacancies in the channel, as previously demonstrated in Figures 2 and 3. As expected, high energy photon exposure leads to a high degree of ionization from Vo to Vo++ and leaves high density of Vo++ in oxide channel. Due to resulting change in cell structure, neighboring Zn atoms are displaced in the outward direction,[41,42] which lowers the recovery time to Vo which is stable in the dark. The PPC issue should be solved in order to improve the switching speed and the frame rate in interactive display. To this end, a series of positive biases were tested to clarify the optimum reset bias condition as seen in Figure 5a. The standby and read states were set to VGS of −5 V and VDS of 5 V. A sufficiently high gate bias as for reset operation is needed to remove the PPC state. Possible explanation is given by Figure 5b. When applying negative bias on the gate in the dark state, the channel is fully depleted. In this circumstance, when the device is exposed to the light
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COMMUNICATION Figure 5. Fermi-level control of oxide-semiconductor to eliminate persistent photoconductivity phenomena. a) Drain-source current as a function time when TFTs are subjected to light pulses. Drain–source current as a function time when TFTs are subjected to light and/or the magnitude of positive bias pulses. Sufficiently positive bias needs to be applied to remove persistent photoconductivity issue, revealing the positive-bias-assisted PPC recovery mechanism. b) The schematics of the TFT when TFTs are subjected to light and/or the magnitude of positive bias pulses. c) Band energies of a-IZO. The defect level of Vo is denoted by ED, EC(k) and EC+1(k) represent the calculated first and second conduction band energies at k = (¼ ¼ ¼). Closed and open circles represent the occupied and unoccupied states, respectively. CBM and VBM are indicated by horizontal solid lines. d) Contour plot of charge densities. The left figure presents the atomic structure of a-IZO which is comprised of occupied defect state of Voo while the right figure shows that of a-IZO which is comprised of the ionized defect state, Vo++ and electrons at the conduction band.
illumination, the ionization of Vo occurs in the channel. Since the gate bias for read operation is negative, the positively charged Vo++ is located in the front channel while the generating free electrons are at the back channel. Additionally, after turning the light off, the rate of recombination between Vo++ in the front channel and free electrons at the back channel is lowered by a negative gate bias, which does not solve the PPC problem. Thus band banding in the opposite direction achieved by a positive gate pulse will force recombination of electrons with ionized oxygen vacancies. In order to facilitate recombination more effectively, a sufficiently high gate bias is required. Then, when we apply negative bias on the gate for read operation again, the channel is totally depleted, providing sufficiently low dark current. To examine the effect of applying a positive gate bias on the recovery of positively ionized Vo++ defect to the neutral Voo defect, we performed hybrid density functional calculations of Vo defects in amorphous In-Zn-O (a-IZO).[43,44] First, we prepared the 132-atom supercell of a-IZO, which is generated by
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melt-and-quench first principle molecular dynamics simulations. The initial structure of Vo in a-IZO is constructed by removing one O atom from the bulk a-IZO. The negative bias and light illumination (NBI) and the positive bias (PB) conditions are modeled by adjusting the electron chemical potential. The band energies of the conduction bands and the Vo defect level are traced and plotted versus the atomic structure simulations under various light and bias conditions as seen in Figure 5c. The defect state of neutral Voo is localized and located within the band gap. The atomic structure of Voo in a-IZO is presented in Figure 5d (left). However, under NBI condition, the neutral Voo defects lose the electrons and the defect states become ionized and resonant with the conduction bands. The atomic structure of this state is presented in Figure 5d (right), similar to the Vo++ defects in a-IGZO.[45,46] When the illumination is stopped and the gate bias is switched from negative to positive, depending on the condition of materials processed, the Vo defect becomes either shallow Vo++, deep Voo, or transferable state between Vo++ and
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Voo as proposed as follows. For shallow Vo++, some ionized Vo++ defects do not return to the neutral states, always presenting the conducting behavior. In another case, a certain ionized Vo++ can easily capture the conduction electrons, becoming Voo without any activation energy and there is no PPC phenomenon. In the other case, i.e. transferable state, other Vo++ defects can return to the neutral state only under positive bias condition where these Vo++ defects combine with the conduction electrons, forming the deep sub gap state in a-IZO. Consistent with experimental data, our calculated data present that Voo and Vo++ defects in a-IZO are transferable. Therefore, deep Voo defects are easily ionized under negative bias illumination condition and recovered from Vo++ to Voo by providing activation energy through gate bias, without thermal annealing. In this work, we explored the origin of high photoconductive gain in phototransistors based on transparent heterojunction nanocrystalline oxide semiconductor thin-film transistor integrated with transparent interconnection technologies. The photosensor array with a HIZO–IZO heterostructure with transparent conducting electrodes yields high quantumefficiency and high transmittance in the visible range. Based on three-dimensional simulations along with light position dependent photocurrent measurements, the transparent electrodes facilitates relatively high injection of photogenerated carriers due to significant lowering of the Schottky barrier at the source side in comparison to metal electrodes. Also, a set of pulse measurement experiments and first-principles calculations based on hybrid density functional theory provide in-depth understanding on the effect of gate pulse bias on recovery of the persistent photocurrent. The results presented here expand the scope of applications of oxide semiconductors. Indeed transparent electronic systems, once viewed as science fiction, can now become a reality.
Experimental Section In order to fabricate photosensor pixel using hybrid active channel, we used single HIZO layer for a switch element and bilayer HIZO–IZO for a photosensor. All these oxide semiconductor films were deposited by reactive radio-frequency magnetron sputtering under controlled Ar and O2 mixed plasma. For both switch and sensor elements, we used the inverted and staggered thin-film transistor (TFT) structure having In-rich IZO–Au–In-rich IZO gate, source, and drain electrodes where In-rich IZO film was prepared by sputtering method and Au film was prepared by e-beam evaporation. For comparison, Mo gate, source, and drain electrode was deposited by DC sputtering in an Ar rich atmosphere and patterned. In order to warrant high capacitance coupling with the channel while attaining comparatively low leakage current density through gate insulator, bilayer, SiN-SiO2 was used. During source/drain etching process, etch stopper layer was used to prevent any possible damage of back channel. First-principles calculations based on hybrid density functional theory (DFT) were performed to investigate the effect of applying positive gate bias on the recovery of an ionized oxygen vacancy (Vo++) defect to a neutral one (Voo) in amorphous oxide semiconductors. The projectoraugmented wave (PAW) pseudopotentials, the generalized gradient approximation (GGA), and the screened hybrid functional of Heyd, Scuseria, and Ernzerhof (HSE) were used, as implemented in VASP. The hybrid functional contains 25% of Hartree–Fock exchange energy and 75% of PBE exchange energy and the screening parameter of 0.2 Å−1 was used. To correct the deep nature of metal d bands in oxides, the on-site
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Coulomb energies (U) were included. The U parameters of 3.9 and 4.5 eV for In d and Zn d orbitals well reproduce the position of metal d bands. To reduce the computational cost, we used the energy cutoff of 300 eV with the special k-point of (¼, ¼, ¼). The 132-atom cubic supercell was used to model the defect properties of Vo in amorphous In-Zn-O (a-IZO). The a-IZO was generated through melt-and-quench ab initio molecular dynamics (MD) simulations. The O-vacancy structure was modeled by removing one O atom from bulk a-IZO and we considered five Vo defect configurations in a-IZO
Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements This work was supported by the National Research Foundation of Korea(NRF) grant funded by the Korea government(MEST) (No. 2014R1A2A2A01006541). S.J. designed this work. S.J. and A. Nathan prepared the manuscript. The experiment and electrical analysis were carried out by S.J., S.-E.A., and I.S. E.L. performed TEM analysis work. S.L. developed the device model for simulation and analysis of the two and three-dimensional energy band diagrams, and worked for extracting photosensor parameters. B.R. performed ab-initial calculation and interpreted the modeling data. A.N. and J.R. did the quantitative analysis on photoresponse and quantum efficiency. All the authors discussed the results and implications and commented on the manuscript at all stages. Note: Figure 2 was revised after initial publication online. Received: April 30, 2014 Revised: August 5, 2014 Published online: September 15, 2014
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