Origin of High Photoconductive Gain in Fully Transparent Heterojunction Nanocrystalline Oxide Image...

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ON 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

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

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 conduc-tion mechanism in oxide semiconductor and use this to maxi-mize 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 effi -ciencies in an all invisible imaging array with photosensors based on nanocrystalline oxide heterojunction thin-fi lm tran-sistor (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 signifi cant number of sub-gap states associated with oxygen vacancies leading to persis-tent photoconductivity. [ 21–24 ] In order to understand the origin of the high photocurrent of a device, we evaluated the infl uence of a light spot from source to drain side on the photoconduc-tive gain of photosensor array. A three-dimensional device sim-ulation tool was employed. Also, a set of pulse measurement experiments and fi rst-principles calculations based on hybrid density functional theory were performed to study the effect of applying gate bias on the recovery mechanism of photocur-rent. Three primary factors are believed to be responsible for the high quantum effi ciency. Introducing a buried layer with a higher density of oxygen vacancies (V o ), as an integral part of the TFT channel, leads to signifi cant visible-light absorption. This coupled with a favorable band offset along with gate-mod-ulated band bending leads to transfer of photogenerated elec-trons to the photo-TFT channel where the V o concentration is small. Because of hole localization, recombination is retarded, giving rise to an extended electron lifetime, and hence, high quantum effi ciency. In addition, scaling down the channel length of the TFT reduces the carrier transit-time from source to drain, yielding a higher effi ciency. This work presented here demonstrates the fi rst invisible, high sensitivity image sensor along with quantitative analysis of the quantum effi ciency in the heterojunction TFT taking into account the optical absorp-tion, 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)/[(In 2 O 3 )-(ZnO)] (IZO) active semiconductor was used, as shown

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 pro-cesses, enabling large-area processing on rigid and fl exible sub-strates. 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 pro-vides integration fl exibility. This has spawned a wealth of appli-cations ranging from high frame-rate interactive displays with embedded imaging to fl exible electronics, [ 10–15 ] where speed and transparency are essential requirements. [ 3,16 ] Interest in oxide semiconductors stem from a number of attributes pri-marily their ease of processing, high fi eld-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 semiconduc-tors are less disordered due to their ionic bonding, giving rise to high fi eld-effect mobility even in the amorphous phase. [ 1–3 ] Therefore, transparent electronic systems, which have been once viewed as science fi ction, can now become a reality.

Adv. Mater. 2014, 26, 7102–7109

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in Figure 1 a and 1 b. 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 fi eld scanning transmission electron microscopy (HAADF-STEM) image (Figure 1 a). In comparison to well-known amor-phous SiO 2 gate insulator, the oxide semiconductor presents a characteristic line and tiny bright dots (see the inset), indi-cating 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 crys-tallinity. As seen in Figure 1 b, energy dispersive spectroscopy data clearly verifi es 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 ( V T ) adjusting layer, increasing the V T 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

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 signifi cant 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 I DS – V GS measurements of the HIZO and HIZO–IZO bi-layer transistors in the dark and under illumi-nation with various light wavelengths (400–600nm) as seen in Figure 1 c and 1 d. 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 cm 2 eV −1 s −1 ), HIZO–IZO bilayer transistor presents a high mobility (the µsat of 106 cm 2 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 1. Material and photoresponsive characteristics of nanocrystalline oxide semiconductor. a) cross-sectional high-angle annular dark-fi eld scan-ning 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 signifi cant light absorption. The inset shows a cross-sectional view of HIZO–IZO TFT.

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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 ( E g ) of IZO, the absorption is mainly due to ionized V o 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 V GS of −2 V. The I DS for a TFT with transparent conducting elec-trode shows a factor of improvement compared to a metal electrode transistor, as can be seen in Figure 2 a. Electrons are injected from source to the channel across a potential bar-rier 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

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 infl uences the car-rier transport and thus the improvement in photocurrent of a transparent electrode transistor, as can be seen in Figure 2 c. This is mainly due to the source-to-channel barrier lowering (SBL), i.e., Δ φ b , at the source side, and can be explained by the following relation: [ 31,32 ]

I I

q

kTexpDS DS0

bαφ

=Δ⎛

⎝⎜⎞⎠⎟

(1)

where I DS0 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 propor-tional 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 I DS in Figure 2 a, the external quantum effi ciency (EQE) can be retrieved using the following equation: [ 35 ]

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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) I DS and photoinduced barrier lowering (Δ φ b ) of transparent In-rich IZO and Mo electrode integrated devices as a function of light-wavelength. b) External quantum effi ciency (EQE) and corrected carrier density ( n coll ) extracted and compared with the transistor with metal elec-trodes. 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.


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EQE

I q

P hv

/

/DS

inc

=

(2)

Here, P inc is power density of the incident illumination (ca. 100 µW/cm 2 ) and hv the photon energy. Also, the corrected car-rier density ( n coll ) can be extracted as follows: [ 30 ]

n

I

q W L t V/coll

DS

FE S DSμ ( )=

(3)

where µ FE is the fi eld-effect mobility and t S channel layer thickness. Both EQE and n coll are proportional to I DS , as can be seen in Equation 2 and 3 . Based on Equation 2 and 3 , the EQE and n coll are extracted and compared with the transistor with metal electrodes, as can be seen in Figure 2 b. It is found that both EQE and n coll for a transparent conducting electrode transistor are about 10-times higher compared to the metal electrode transistor, and stems from the increase of photocur-rent I DS shown in Figure 2 a. This can be attributed to source-to-channel barrier lowering (Δ φ b ) [ 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 2 c and 2 d (details regarding device simulation are seen in Section S4 in the Supporting Information). As shown in Figure 2 c, 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 2 d). This suggests that light incident on the source side gives rise to the ionized V o . The ionized V o (V o ++ ) is positive, and its accumulation on the source side reduces barrier height. [ 36,37 ]

To further verify the effect of beam position on photocur-rent ( I DS ) and barrier lowering (Δ φ b ), incident light with the same optical power is locally spotted (see Figure 3 a), and the

photocurrent and 3dimensional device simulation results for each case are measured as shown in Figure 3 b and Figure 3 c, respectively (see the method in Section S4 in the Supporting Information). When beam is positioned on source side, I DS 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 effec-tive to increase photocurrent. Indeed, the simulated conduction band profi les show signifi cant barrier lowering at the source (A), middle (B & C) and drain side (D), as shown in Figure 3 c. This suggests a higher injection of electrons ( n inj ) from source, resulting in higher photocurrent. [ 38 ] From Equation 1 and 3 , we can deduce:

n n

q

kTexpinj 0

bαφ

=Δ⎛

⎝⎜⎞⎠⎟

(4)

where n 0 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 sep-aration photomask pattern and wet processes presented in Figure 4 a and the details are found in Section S5 in the Sup-porting 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(In 2 O 3 )-(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 � Ω/square� . At Au thickness of less than 8 nm, the In-rich IZO/Au/In-rich IZO still showed a fairly high sheet resistance (85 � Ω/square� ) probably due to the form of randomly disconnected islands at nucleation.

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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) I DS 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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However, the insertion of Au layer with a thickness of 10 nm led to a signifi cant reduction in the sheet resistance (Au 14 nm: � 4.2 Ω/square�, Au 20 nm: 3.6 � Ω/square�, Au 24 nm: � 3.2 Ω/square�). Considering the sheet resist-ance 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 fi nal outcome, we fabricated the photosensor arrays comprising of hybrid active channel for the switch and sensor as seen in Figure 4 b. 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 4 c shows cross-sectional HAADF-STEM image of active semicon-ductor 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 HAADF-STEM image and energy dispersion spectrum in Figure 4 d verifi es the composition of multilayer transparent electrodes comprising of IZO and Au. Figure 4 e shows cross-sectional HAADF STEM images of photosensor with different elec-trodes. The details regarding structural and materials analyses

on sensor array are presented in Section S6–S10 in the Sup-porting Information.

It is known that Zn-series oxide semiconductors have one weakness, namely persistent photoconductivity (PPC), which leads to a certain amount of current fl ow even after the illu-mination 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 attrib-uted to carrier generation as a result of gate fi eld 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 V o to V o ++ and leaves high density of V o ++ 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 V o 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 pos-itive biases were tested to clarify the optimum reset bias con-dition as seen in Figure 5 a. The standby and read states were set to V GS of −5 V and V DS of 5 V. A suffi ciently high gate bias as for reset operation is needed to remove the PPC state. Pos-sible explanation is given by Figure 5 b. 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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Figure 4. Process integration procedure, photo-view and materials data of photosensor pixel. a) The fabrication procedure of photosensor pixel com-prising 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.


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illumination, the ionization of V o occurs in the channel. Since the gate bias for read operation is negative, the positively charged V o ++ is located in the front channel while the gener-ating free electrons are at the back channel. Additionally, after turning the light off, the rate of recombination between V o ++ 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 recombina-tion more effectively, a suffi ciently high gate bias is required. Then, when we apply negative bias on the gate for read opera-tion again, the channel is totally depleted, providing suffi ciently low dark current.

To examine the effect of applying a positive gate bias on the recovery of positively ionized V o ++ defect to the neutral V o o defect, we performed hybrid density functional calculations of V o defects in amorphous In-Zn-O ( a -IZO). [ 43,44 ] First, we pre-pared the 132-atom supercell of a -IZO, which is generated by

melt-and-quench fi rst principle molecular dynamics simu-lations. The initial structure of V o 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) condi-tions are modeled by adjusting the electron chemical poten-tial. The band energies of the conduction bands and the V o defect level are traced and plotted versus the atomic structure simulations under various light and bias conditions as seen in Figure 5 c. The defect state of neutral V o o is localized and located within the band gap. The atomic structure of V o o in a -IZO is presented in Figure 5 d (left).

However, under NBI condition, the neutral V o o 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 5 d (right), similar to the V o ++ 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 V o defect becomes either shallow V o ++ , deep V o o , or transferable state between V o ++ and

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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. Suffi ciently 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 V o is denoted by E D , E C ( k ) and E C+1 ( k ) represent the calculated fi rst 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 fi gure presents the atomic structure of a -IZO which is comprised of occupied defect state of V o o while the right fi gure shows that of a -IZO which is comprised of the ionized defect state, V o ++ and electrons at the conduction band.


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V o o as proposed as follows. For shallow V o ++ , some ionized V o ++ defects do not return to the neutral states, always presenting the conducting behavior. In another case, a certain ionized V o ++ can easily capture the conduction electrons, becoming V o o without any activation energy and there is no PPC phe-nomenon. In the other case, i.e. transferable state, other V o ++ defects can return to the neutral state only under positive bias condition where these V o ++ defects combine with the conduc-tion electrons, forming the deep sub gap state in a -IZO. Con-sistent with experimental data, our calculated data present that V o o and V o ++ defects in a -IZO are transferable. Therefore, deep V o o defects are easily ionized under negative bias illumination condition and recovered from V o ++ to V o o by providing activa-tion energy through gate bias, without thermal annealing.

In this work, we explored the origin of high photoconduc-tive gain in phototransistors based on transparent heterojunc-tion nanocrystalline oxide semiconductor thin-fi lm transistor integrated with transparent interconnection technologies. The photo sensor array with a HIZO–IZO heterostructure with transparent conducting electrodes yields high quantum-effi ciency and high transmittance in the visible range. Based on three-dimensional simulations along with light position dependent photocurrent measurements, the transparent elec-trodes facilitates relatively high injection of photogenerated carriers due to signifi cant lowering of the Schottky barrier at the source side in comparison to metal electrodes. Also, a set of pulse measurement experiments and fi rst-principles cal-culations 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 fi ction, 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 fi lms were deposited by reactive radio-frequency magnetron sputtering under controlled Ar and O 2 mixed plasma. For both switch and sensor elements, we used the inverted and staggered thin-fi lm transistor (TFT) structure having In-rich IZO–Au–In-rich IZO gate, source, and drain electrodes where In-rich IZO fi lm was prepared by sputtering method and Au fi lm 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-SiO 2 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 (V o ++ ) defect to a neutral one (V o o ) in amorphous oxide semiconductors. The projector-augmented 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

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 V o 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 fi ve V o defect confi gurations 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 effi ciency. 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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