1040 IEEE ELECTRON DEVICE LETTERS, VOL. 35, NO. 10, OCTOBER 2014
Integration of a-IGZO Thin-Film Transistor and
Crystalline-Si Interdigitated Back Contact
Photovoltaic Cell With 3D Stacking
Structure as Self-Powered Solar Switch
Yen-Ming Juan, Han-Ting Hsueh, Shoou-Jinn Chang, T. H. Chang, K. C. Lai,
T. C. Cheng, Y. D. Lin, C. J. Chiu, and Wen-Yin Weng
Abstract— In this letter, a Ta2O5/a-IGZO thin film
transis-tor (TFT) was directly stacked on a crystalline-Si interdigitated back contact (IBC) photovoltaic (PV) cell to create a self-powered solar switch. The a-IGZO TFT and IBC PV cell were integrated into a single chip without an external circuit. This device exhibits switching property induced by illumination. The results show that it can be switched even under a low solar illumination of 300 W/m2due to the low threshold voltage of the a-IGZO TFT (0.25 V). The ON/OFF current contrast ratio was measured to be ∼20 under 1-sun illumination. The fabrication process and characteristics of this device make it suitable and practicable for use as a self-powered solar switch.
Index Terms— Interdigitated back contact (IBC), a-IGZO,
self-powered device.
I. INTRODUCTION
O
XIDE semiconductor-based thin-film transistors (TFTs) have made impressive progress for application in trans-parent electronics and displays. Among them, amorphous indium gallium zinc oxide (a-IGZO) is considered one of the most promising materials as the channel layer for TFTs due to its good carrier mobility, which is considerably higher than that of a-Si:H TFTs [1], [2].Recently, self-powered devices which could function well without the external power unit have attracted a lot of atten-tion, especially for the issue of energy shortage. Yang et al. fabricated a self-powered device by connecting a single fiber nanowire hybrid-structured microbial fuel cell (MFC) with a single CdS nanowire photosensor in series [3]. However, their MFC was separated from the photosensor by an external circuit, which was not easily to integrate. Shen et al. fabri-cated CIGS PV cells and microcrystalline-Si TFTs as self-powered electronics on the same plane [4]. Their group also demonstrated a hybrid Si-based thin film PV cell/transistor Manuscript received May 26, 2014; revised August 5, 2014; accepted August 7, 2014. Date of publication September 4, 2014; date of current version September 23, 2014. The review of this letter was arranged by Editor A. Ortiz-Conde.
Y.-M. Juan, S.-J. Chang, T. H. Chang, K. C. Lai, C. J. Chiu, and W.-Y. Weng are with the Department of Electrical Engineering, Institute of Microelectronics, National Cheng Kung University, Tainan 701, Taiwan.
H.-T. Hsueh and Y. D. Lin are with the National Nano Device Laboratories, Tainan 741, Taiwan (e-mail: [email protected]).
T. C. Cheng is with the Department of Mechanical Engineering, National Kaohsiung University of Applied Science, Kaohsiung 807, Taiwan.
Color versions of one or more of the figures in this letter are available online at http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/LED.2014.2347039
device on a flexible substrate [5]. However, the PV cell and the transistor have separate areas on the same plane, leading to large device dimension. The devices reported above are discrete or have a large area, and thus they are not easy to integrate. To solve this problem, three-dimensional (3D) integration has been a subject of intense research, as it allows miniaturization, high performance, low power dissipation, high density and heterogeneous integration of devices.
In this study, a self-powered solar switch was fabricated by integrating a crystalline-Si interdigitated back contact (IBC) PV cell and a-IGZO TFTs with a direct 3D stacking structure into a single chip. Such a stacking structure efficiently reduces the device dimensions and allows integration without an exter-nal circuit. This is the first study to use direct 3D stacking of TFTs on a PV cell, to our knowledge. Different from other PV cells, crystalline-Si IBC PV cell have all their electrodes on the rear side, which makes the front surface optimal for optical performance, avoiding shadowing problems. The rear electrodes of the IBC PV cell also benefit the process of vertical 3D stacking, which makes the device easy to integrate and small.
II. EXPERIMENTS
For the fabrication of crystalline-Si IBC PV cells, n-type Czochralski wafer was used. After diffusion process of p+and n+area, the sample surface was textured by the mixed solution of KOH and IPA. The sheet resistance of the following n+ front surface field was around 140/. A 80-nm-thick SiNx was then deposited on the surface as the anti-reflection layer by PECVD. To prevent the recombination of the carriers, the ratio of metal contact opening was around 5.2%, and then, 2-μm-thick Al was deposited to form interdigitated electrodes by e-beam evaporation.
To achieve vertical stacking of a-IGZO TFT on the IBC PV cell, a 2-μm-thick SiO2 layer was firstly deposited on the rear Al electrodes of the IBC PV cell by PECVD to serve as the isolating layer. This SiO2 layer was pat-terned to open the contact holes for the underlayered IBC PV electrodes (500μm × 500μm). The Al electrode was then deposited in the middle of SiO2 surface as the bottom gate by sputtering. A 100-nm-thick Ta2O5 gate dielectric was subsequently deposited on the bottom gate by e-beam evaporation. A 50-nm-thick a-IGZO (target In:Ga:Zn=1:1:1 in atomic ratio) active layer was then deposited via RF sputtering. Finally, in order to form ohmic contact [6], an Al layer was deposited onto the a-IGZO film to serve as the source and 0741-3106 © 2014 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission.
1042 IEEE ELECTRON DEVICE LETTERS, VOL. 35, NO. 10, OCTOBER 2014
Fig. 4. I-V characteristics of a-IGZO TFT measured under various illumi-nation intensities by B1500 analyzer. The inset shows transfer characteristics.
Fig. 5. (a) I-V output characteristics of self-powered solar switch driven by IBC PV cell. (b) Transient response of self-powered switch under 1000 W/m2 was switched on and off.
to 9.3 × 10−11 A when the TFT was illuminated under 1000 W/m2. Chiu et al. found that for a Ta2O5/a-IGZO UV sensor, the off-current increased with decreasing wavelength of illumination [8]. Their results showed that the increase of off-current was significant when the illumination wavelength was shorter than 280 nm. It is known that the bandgap of Ta2O5 and a-IGZO are around 4.5 and 3.0 eV, respectively. When the photon energy is higher than 3.0 eV, electron-hole pairs are generated from band excitation transitions and defect states in a-IGZO film. This energy is too low to be absorbed by Ta2O5. Thus, the measured off-current at zero gate bias increased slightly when the device was illuminated at wavelengths of 280nm<λ<410nm. Furthermore, when the photon energy is higher than 4.5 eV, the light is absorbed by Ta2O5 film. Electron-hole pairs are produced by Ta2O5, and injected into the channel. Thus, the off-current can be greatly increased. In our experiment, a xenon lamp AM 1.5G simulator was used to illuminate the device, which almost has no spectra when the wavelength is shorter than 300 nm. Thus, the results in Fig.4 show that only 5× 10−11 A off-current was increased.
To realize the self-powered device with 3D stacked inte-gration, the conductive paths were connected by depositing Al from the IBC PV cell’s electrodes to the gate and source electrodes of the a-IGZO TFT. Two probes of B1500 analyzer were used to measure the drain and source electrodes with solar illumination upon the IBC PV cell’s surface. Fig. 5(a) shows the a-IGZO TFT output characteristics of the self-powered device under various illumination levels. It can be clearly observed that the current was very low in the dark environment. With solar illumination, the saturation current increased to around 9× 10−8 A. The plots also show that the saturation currents slightly increased when the solar intensity was changed from 300 to 1000 W/m2. A comparison with
Fig. 4 indicates that this is due to the increasing Voc of the PV cell by raising the solar illumination. These performances proved that the a-IGZO TFT which was driven by IBC PV cell with directly stacking was practicable.
Fig. 5(b) shows the measured transient response of the fab-ricated self-powered a-IGZO TFT, obtained with the solar illu-mination switched on and off with the intervals of 20 seconds. The probes of B1500 with a constant Vd= 1V were applied to the source and drain electrodes. The current response was fast for both turning on and off the solar illumination. The stability of stress time of a-IGZO TFT had been discussed in previous reports. Hoshino et al. reported that the decrease of Id was only ∼10% of the original value while the constant voltage bias (Vd = Vg = 30V) was applied for a period of 105s on their SiO2/IGZO TFT [9]. Such report also suggests that the device could be used under long time operation. Under 1000 W/m2 illumination, the dynamic response of the device was stable and reproducible with an on/off current contrast ratio of around 20. The contrast ratio can be further increased by connecting multiple PV cells in series to increase Voc, as it could provide more gate bias to drive the a-IGZO TFT with higher Id.
IV. CONCLUSION
This study reported the integration of an IBC PV cell and a Ta2O5/a-IGZO TFT into a single chip, as a self-powered solar switch via a direct 3D stacking method. For the IBC PV cell, the Vocvariations with different illumination is relatively small and thus the variation of the TFT output characteristics with different solar illumination is negligible. The PV cell and the TFT were vertically connected without an external circuit. The integrated device shows TFT characteristics under solar illumination. This self-powered switch with a single PV cell under 1-sun illumination has an on/off current contrast ratio of 20 and a stable response, indicating that the proposed self-powered solar switch with a 3D stacking structure is potentially useful.
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