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Design of a Multifunctional Double-Active-Layer Thin-Film Transistor for Photosensing Applications Sang Youn Han, Kyung Sook Jeon, Seung Mi Seo, Mi Seon Seo, and Suk-Won Jung
Abstract—Photocurrent generation from infrared and thinfilm transistor (TFT) driving properties were simultaneously achieved with a hydrogenated amorphous silicon/a-SiGe:H double-active-layer structure. The proposed double layer showed electrical performance degradation due to the large series resistance from the thick active structure. By reducing the thickness of a-SiGe:H only in the driving TFT, a compatible field-effect mobility of 0.29 cm2 /V · s was achieved, which can fully drive the TFT. In the photosensor TFT, a higher photosensitivity was obtained with a thick a-SiGe:H layer. This implies that the multifunctional double active layer and new process effectively realized the two different properties at the same time. Index Terms—Double active layer, photosensor, phototransistor, thin-film transistor (TFT).
I. I NTRODUCTION
H
YDROGENATED amorphous silicon (a-Si:H) has been widely used as an active layer of the active-matrix liquid crystal display due to its simple process and sufficient fieldeffect mobility for pixel thin-film transistor (TFT) driving. Furthermore, there is much interest in the generation of a photocurrent under illumination in the application of photovoltaic and photosensor devices [1]. For infrared (IR) photosensing, however, a-SiGe:H draws much attention due to its higher light absorption in the near-IR region due to the reduced optical energy gap [2]. In particular, the IR photosensing circuit embedded in a display panel generally consists of a driving TFT and a photosensor TFT. Therefore, a-SiGe:H must be used as a photosensor TFT when high mobility is not required in the normal operation but superior photogeneration is crucial. On the contrary, for the driving TFT, sufficient mobility is important, so it should be fabricated with a a-Si:H layer. Thus, to fabricate the IR photosensing circuit in the display panel, two different active materials of a-Si:H and a-SiGe:H should be deposited and patterned separately. These increase the process steps and the number of masks, which increase the cost and reduce the yield. One solution to such a problem is stacking a-Si:H and a-SiGe:H consecutively in one active layer. By first depositing a-Si:H to a channel layer and then a-SiGe:H above a-Si:H, the photon can easily be absorbed from the outside. This structure can efficiently reduce the process steps. No study has been conducted on such structure yet, though. Manuscript received September 18, 2012; accepted October 6, 2012. Date of publication November 26, 2012; date of current version December 19, 2012. The review of this letter was arranged by Editor O. Manasreh. The authors are with the LCD R&D Center, Samsung Display Company, Ltd., Yongin 446-711, Korea (e-mail:
[email protected]). 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.2012.2223811
Fig. 1. I–V characteristics of a a-Si:H/a-SiGe:H double layer with and without PT and a a-Si:H single-layer TFT structure.
In this letter, a double active layer [3], [4] made of a-Si:H and a-SiGe:H is reported. Through current–voltage (I–V ) curve analysis, the relevant parameters of the double-layer TFT were extracted. Moreover, a new method of decreasing the series resistance in TFT was adopted to realize higher field-effect mobility. From this, the basis of a multifunctional double-layer structure in TFT is proposed. II. E XPERIMENTAL The device structure used in this study was a conventional bottom-gate TFT, which is shown in the inset of Fig. 1. The detailed fabrication process is described elsewhere [2], [5]. The double active layer of a-Si:H/a-SiGe:H was deposited via plasma-enhanced chemical vapor deposition at 320 ◦ C. From the depth profile monitoring in the X-ray photoelectron spectroscopy, the Ge content of the a-SiGe:H film was found to be about 20%. The optical energy bandgap, which was measured with a UV/Vis spectrometer, was determined by extrapolating the linear region of the curves, following the Tauc model [6], and was evaluated as 1.78 and 1.64 eV for a-Si:H and a-SiGe:H, respectively. For the IR photocurrent measurement, the incident photon was provided by an IR LED with a wavelength of 850 nm, and its optical radiation power was 13.75 mW/cm2 . III. R ESULTS AND D ISCUSSION Fig. 1 shows the transfer I–V curves of various active-layer structures. Their resulting electrical parameters are summarized in Table I. When designing a double active layer, the
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HAN et al.: DESIGN OF A MULTIFUNCTIONAL DOUBLE-ACTIVE-LAYER TFT
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TABLE I S UMMARY OF THE E LECTRICAL PARAMETERS E XTRACTED F ROM THE T RANSFER I–V C URVES OF VARIOUS ACTIVE -L AYER S TRUCTURES
required thickness of the a-SiGe:H layer for the generation of a photocurrent should be higher to maximize the number of the absorbed photons. From the previous studies [2], to obtain a photosensitivity (defined as the photocurrent divided by the dark-state current) of around 600, the a-SiGe:H layer should be deposited at over 4000 Å. Moreover, an at least 1000-Å-thick a-Si:H layer was needed to form the channel region. The initial −8 ON -current of the a-Si:H/a-SiGe:H TFT (Ids = 9.2 × 10 A at Vgs = 20 V) was much lower than that of the a-Si:H TFT (Ids = 6.7 × 10−6 A at Vgs = 20 V), which means that the double active layer suffers from a large series resistance [7]. The field-effect mobility that was obtained from the a-Si:H/ a-SiGe:H double layer (0.01 cm2 /V · s) was much lower than that from the a-Si:H-only layer (0.41 cm2 /V · s). The a-Si:H/ a-SiGe:H double layer showed a higher subthreshold swing and OFF-state current than the a-Si:H TFT. These properties can be attributed to the poor film quality of a-SiGe:H among the double layers. The field-effect mobility obtained from a-SiGe:H (0.005 cm2 /V · s) was much lower than that from a-Si:H, which can be attributed to the number of defects contained in the a-SiGe:H active layer. These were confirmed by the higher subthreshold swing of 2.2 V/dec in a-SiGe:H TFT than ∼0.9 V/dec in a-Si:H TFT. Moreover, the defect densities (Dit ) between gate insulator and active layer were estimated [8], and the Dit ’s of a-SiGe:H and a-Si:H were 3.78 × 1012 and 1.49 × 1012 cm−2 , respectively. In addition, it has been reported that a-SiGe:H has a high ratio of defects in the film, which reduces the ON-state current and the field-effect mobility [9]. These confirmed that the low mobility of a-SiGe:H TFT was originated from the defect states at the interface as well as the bulk area. This infers that there is a tradeoff between the electrical (driving) and optoelectric (photogeneration) properties in the double-layer structure. The photoresponses of a-SiGe:H TFT for different IR wavelengths of 890 and 940 nm were also measured. The optical power of each wavelength was calibrated for comparison. When the photoresponse at the wavelength of 850 nm was set as reference, the photoresponses were 50%–65% at 890 nm and 21%–35% at 940 nm. This is deeply related to the optical bandgap of a-SiGe:H where the electron–hole pairs were generated by absorbed photons. Additionally, the visible light interference is critical in the IR photosensor due to the high response of a-SiGe:H to the visible light. Thus, the thin-film optical filter layer, which could cut off the incidence of the ambient visible light into the sensor materials, was introduced. The detailed characteristics were described elsewhere [10]. On the other hand, the hydrogen plasma treatment (PT) before the deposition of the a-SiGe:H layer was con-
Fig. 2. Schematic cross-sectional diagrams of the process flow for the fabrication of (a) the switching TFT and (b) the photosensor TFT.
ducted to reduce the interface resistance between the a-Si:H and a-SiGe:H layers. As shown in Fig. 1, the ON-state current and the field-effect mobility were improved, which implies that the H2 PT effectively reduced the resistance at the interface of the a-Si:H and a-SiGe:H layers. The mobility of the a-Si:H/ a-SiGe:H double-layer structure was insufficient to drive the TFT, however, so a new fabrication process was proposed to increase the mobility in the driving TFT. Fig. 2 shows the proposed fabrication processes for the driving and photosensor TFTs. For the active-layer patterning in the driving TFT, the slit mask was used to decrease the thickness of the patterned photoresist (PR), but in the case of the photosensor TFT, a regular PR thickness was formed with a normal mask. Then, dry etching was performed to confine the TFT active layer, after which PR ashing was carried out using O2 plasma to eliminate the residual PR on the surface of the active layer in the driving TFT. Thus, the a-SiGe:H layer in the driving TFT was exposed. Then, the a-SiGe:H of the driving TFT was partially etched to reduce the thickness of the a-SiGe:H layer. For the photosensor TFT, however, no etching of the active layer was done due to the remaining PR on the surface of the a-SiGe:H layer. Before the deposition of n+ a-Si, the native oxide was removed with the buffered oxide etchant. Then, n+ a-Si and the source/drain electrode were formed to complete the TFT. With these processes, the thickness of the a-SiGe:H layer in the driving TFT can be effectively reduced without increasing the number of masks. The device simulation from Silvaco TCAD was performed to predict the driving TFT performance with respect to the etching thickness in a-SiGe:H. The Poisson equation and the basic mobility/drift model were applied, and the density of states of each a-Si:H and a-SiGe:H was used. Fig. 3(a) shows the simulated linear I–V curves, depending on the residual thickness of the a-SiGe:H layer in the driving TFT. From the simulation analysis, the field-effect mobility was expected to decrease as the remaining thickness of a-SiGe:H increased. To attain a field-effect mobility of over 0.3 cm2 /V · s, it was estimated that 2500-Å dry etching was needed in the a-SiGe:H layer of the driving TFT. The driving and photosensor TFTs were fabricated simultaneously, following the procedures shown in Fig. 2, and the
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successfully obtained, even though the electrical properties are insufficient. IV. C ONCLUSION We have demonstrated the TFT driving and IR photogeneration properties simultaneously using the a-Si:H/a-SiGe:H double-active-layer structure. The large series resistance from the thick active structure was resolved by reducing the thickness of a-SiGe:H in the driving TFT. In the photosensor TFT, higher photosensitivity was attained in this structure. This multifunctional double active layer and new process effectively developed the two different properties at the same time. R EFERENCES
Fig. 3. (a) Simulated I–V characteristics of the switching TFT by varying the partial etching thickness. Their field-effect mobilities are also shown in the inset. The measured transfer I–V characteristics of (b) the driving TFT and (c) the photosensor TFT, which were fabricated using the process shown in Fig. 2.
measured transfer I–V curves are shown in Fig. 3(b) and (c). From the simulation results, the partial etching of the a-SiGe:H layer was determined to have been 2500–2800 Å. From the curves in Fig. 3(b), the field-effect mobility was evaluated to be ∼ 0.29 cm2 /V · s, and the subthreshold swing was evaluated to be 1.2 V/dec. These results agree with the simulated ones. Fifteen different TFTs were measured, and their performance levels were uniform over the entire glass. In the case of the photosensor TFT under illumination at the wavelength of 850 nm, an increase in the drain current at the OFF state was observed, which shows a higher photosensitivity of around 800 at Vgs = −13 V. It must be noted that, in the a-Si:H/a-SiGe:H doubleactive-layer structure, a higher photosensing signal can be
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