Highly Repeatable and Recoverable Phototransistors Based on

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Highly repeatable and recoverable phototransistors based on multi-functional channels of photoactive CdS, fast charge transporting ZnO, and chemically durable AlO layers 2

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Cheol Hyoun Ahn, Won Jun Kang, Ye Kyun Kim, Myeong Gu Yun, and Hyung Koun Cho ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b04482 • Publication Date (Web): 03 Jun 2016 Downloaded from http://pubs.acs.org on June 7, 2016

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ACS Applied Materials & Interfaces

Highly repeatable and recoverable phototransistors based on multi-functional channels of photoactive CdS, fast charge transporting ZnO, and chemically durable Al2O3 layers Cheol Hyoun Ahn, Won Jun Kang, Ye Kyun Kim, Myeong Gu Yun, and Hyung Koun Cho* School of Advanced Materials Science and Engineering, Sungkyunkwan University, 2066 Seobu-ro, Jangan-gu, Suwon, Gyeonggi-do, 16419, Republic of Korea e-mail: [email protected] Keywords: Phototransistor, Cadmium-sulphide, Oxide semiconductor, Charge transport layer, multi-functional channels, visible-light

Abstract Highly repeatable and recoverable phototransistors were explored using a “multi-functional channels” structure with multi-stacked chalcogenide and oxide semiconductors. These devices were made of i) photoactive CdS (with a visible band gap), ii) fast charge transporting ZnO (with a high field-effect mobility), and iii) a protection layer of Al2O3 (with high chemical durability). The CdS TFT without the Al2O3 protection layer did not show a transfer curve due to the chemical damage that occurred on the ZnO layer during the chemical bath deposition (CBD) process used for CdS deposition. Alternatively, compared to CdS phototransistors with long recovery time and high hysteresis (∆ = 19.5 ), our “multi-functional channels” phototransistors showed an extremely low hysteresis loop (∆ = 0.5 ) and superior photosensitivity with repeatable high photo-responsivity (52.9 A/W @ 400 nm). These improvements are likely caused by the physical isolation of the sensing region and charge transport region by the insertion of the ultra-thin Al2O3 layer. This approach successfully addresses some of the existing problems in CdS phototransistors, such as the high gate-interface trap site density and high absorption of molecular oxygen, which originate from the polycrystalline CdS. 1 ACS Paragon Plus Environment


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1. Introduction Visible-to-ultraviolet

photodetectors

are

widely

used

in

imaging,

optical

communication/interconnects/switches, security, environmental monitoring, and consumer optoelectronics.1,2 Recently, metal chalcogenide compound semiconductors have received much attention for visible light detection due to their direct band gap, excellent photoconductivity, processability, low cost, and good chemical/thermal stability.3,4 Among various chalcogenide materials, cadmium sulphide (CdS) compound semiconductors have a direct band gap of 2.42 eV at room temperature, which makes them suitable for visible light absorption. Resistive-type photodetectors using CdS thin films have generally been investigated with two-terminal structures for visible-light detection.5,6 In attempts to improve photosensitivity in the visible region, several research groups have reported that resistive-type photodetectors using nanostructure-based CdS showed higher sensitivity than CdS thin film photodetectors. 7,8

However, the nanostructure arrangement (with large-area uniformity) is a critical issue,

and the sensitivity for visible-light detection is still quite low. Moreover, for resistive-type photodetectors using only CdS films, there is a trade-off between the fundamental sensing parameters, such as responsivity and selectivity. In contrast to conventional resistive-type photodetectors, thin-film transistor-based photodetectors (phototransistors) enable easier control of the responsivity and selectivity, which would allow increased amplification of lower detection signals. Much research has been carried out to fabricate CdS-based thin-film transistor (TFT) devices as an alternative to amorphous silicon (a-Si); this area of work was first reported by Weimer at the RCA Laboratories in 1962,9 but these devices still exhibit insufficient electrical properties and poor device reliability, despite being studied for such a long period of time. However, the versatile fundamental properties of CdS, such as its direct visible band gap, high refraction index,

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relatively low work function, good chemical thermal stability, and piezoelectricity, continue to make it appealing for electronic and optoelectronic applications.10 However, the innovative improvement of CdS-based phototransistors, due to their potential for highly repeatable and recoverable photo-sensing, has rarely been reported (despite their obvious significance). This lack of research is related with unavoidable problems that originate from polycrystalline CdS films, which have a large number of grain boundaries that provide excess trap sites in the channel of the transistor. Alternatively, ZnO-based oxide semiconductors are considered to be the most feasible semiconducting materials for transparent/flexible optoelectronic devices; this appeal is due to their unique high mobility, transparency in the visible region, and low-temperature processing capabilities.11,12 It was recently reported that oxide semiconductors are more suitable for the sensing materials of photo-TFTs than conventional a-Si. The photo-response of oxide-based photo-TFTs is largely attributed to the photo-excitation of subgap states, such as ionized oxygen vacancies, and the subsequent liberation of electrons.13-15 However, the photo-excitation of ionized oxygen vacancies in oxide-based photo-TFTs is accompanied by lattice relaxation, which subsequently results in a persistent photocurrent, even after the light has been turned off. Unfortunately, this usually leads to long recovery times for oxide-based photo-TFTs, as compared to those of typical II-VI semiconductors.16,17 In this work, the channel layers of chalcogenide CdS phototransistors were composed of multi-stacked constituent layers with significantly different physical properties. These were designed to produce high photo-sensing performance by combining the various characteristics of individual layers. Our proposed channel structure is based on multi-layers consisting of Al2O3, CdS, and ZnO. Here, the CdS and ZnO in the channel were functionally designed as a photoactive sensing layer to absorb visible light and a fast transport layer of photo-induced charges, respectively. The Al2O3 layer was included for chemical protection of



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the ZnO surface, to ensure that the charge transport efficiency during CdS deposition is not degraded. Consequently, each layer has a particular function in the channel structure. We refer to this channel as a “multi-functional channel”; our “multi-functional channel” structure exhibits greatly enhanced sensing performance in CdS-based phototransistors

2. Experimental details Growth of photoactive channel layers. Photoactive chalcogenide CdS thin films were simply synthesized by a conventional chemical bath deposition (CBD) process. Here, an aqueous solution (pH: 9 ~ 10) containing 2 mM cadmium salt (CdSO4), 10 mM thiourea ((NH2)2CS, and 1M ammonia (NH4OH) was used at 60 oC for 15 min for the photoactive CdS coating. CdS thin films were grown on heavily-doped p-type Si substrates with a thermally-grown SiO2 layer (200 nm) and Corning glass substrates to make the phototransistors and the thin films for characterization, respectively. The bath was rigorously stirred during the synthesis to ensure chemically-homogeneous precursor distribution in the aqueous solution. In order to improve the sensing performance of the phototransistors, the Al2O3 (3.6 nm)/ZnO (5 nm)/Al2O3 (3.6 nm) layers, which act as charge transport layers, were continuously deposited on the Si/SiO2 substrate at 150 oC by atomic layer deposition (ALD). The high TFT performance of this structure was reported in a previous study. Diethylzinc (DEZn), trimethylaluminum (TMA), and high-purity H2O were used as the precursors of zinc, aluminum, and oxygen, respectively. The containers of all precursors were held at 10 oC, and N2 gas (100 SCCM) was continuously used to deliver the precursors into the reaction chamber. The exposure and purge times for all precursors were set to be 0.1 s and 10 s, respectively. In this study, the best phototransistor, which exhibited periodic repeatability and fast recovery, had a multi-stacked channel structure of Al2O3/CdS/Al2O3/ZnO/Al2O3.


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Fabrication of phototransistors. For fabrication of the CdS-based phototransistors, the CdS and multi-stacked channels on the Si/SiO2 substrate was patterned by conventional lithography and wet-etching using an HCl solution. Then, the Mo metal was deposited as the source and drain electrodes by dc magnetron sputtering at room-temperature. Electrodes were patterned via a lift-off process using an AZ5214 photoresist. Here, the width and length of the channel were 500 µm and 100 µm, respectively, and the TFT devices were finally annealed at 150 oC for 2 h to reduce the contact resistance. Analysis of characterizatio on photoactive layer and phototransistors. The structural and optical properties were investigated by scanning electron microscopy (SEM, JEOL JSM6700F) and UV/visible/near-IR spectroscopy (Cary5000). The electrical and photosensing performances of all the phototransistor devices were measured using an HP4145B semiconductor parameter analyzer. For monochromatic light, a 150 W Xe arc lamp and monochromator were used. The optical power of the light was calibrated using a UVenhanced Si detector and was controlled to be 0.1 mW/cm2.

3. Results and Discussion CBD techniques that can be employed for large-area batch processing or continuous deposition are typically used to synthesize binary chalcogenide CdS in simple solution containers due to their low-cost/low-temperature process. These methods form stable, adherent, and hard films on various substrates. In CBD processes, CdS thin films can be uniformly produced by the decomposition reaction of (NH2)2CS in an alkaline solution of CdSO4 according to the following reaction:18 Cd( ) + ( ) + 2 → + + 4 + 2 O

As is well-known, CdS synthesis using the CBD process occurs either by ion-by-ion condensations of Cd and S ions or by the adsorption of colloidal particles of CdS. First, to



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illustrate the fundamental properties of CdS layers deposited by the CBD process, their microstructural and optical properties were investigated. As shown in Fig. 1(a), the CdS film on the SiO2/Si substrate showed granular morphology consisting of nanocrystals with grain sizes typically between 50 and 100 nm. In addition, the optical band edge of the CdS film on a glass substrate was observed to be 2.26 eV [Fig. 1(b)], which is estimated from the relationship of ( !")# versus energy in the transmittance curves. This indicates that the photo-response for the CBD-processed CdS films will intensively occur from light ≳ 2.26 eV (green light, ≲ 550 nm). To confirm the photosensitivity of the active CdS film for visible light, we fabricated a topcontact, bottom-gated thin-film transistor with a CdS channel. The electrical performance of the CdS phototransistors was characterized with/without visible light, as shown in Fig. 2. The typical field-effect mobility was evaluated from a linear curve using the following equation: *+ 1 102 -) -) &'( = ) ,+ . ∙ 0 132 Here, *+ , ,+ , and . are the channel length, channel width, and capacitance per unit area of the gate dielectric, respectively. The subthreshold swing (SS) was extracted from the equation SS=(3 /567102 ). The threshold voltage (Vth) was determined as the gate voltage showing a drain current of *+ /,+ × 10 nA . The threshold voltage, field-effect mobility, and subthreshold swing for the CdS transistor were 0.19 V, 0.02 cm2/Vs, and 0.33 V/dec, respectively. It can be clearly seen that the observed low &'( is related to the high density of grain boundaries in the granular-shaped CdS channels due to the existence of charge trapping and scattering sites. Also, the CdS TFT shows a significantly larger clockwise hysteresis loop (∆; = 19.5 ), as shown in Fig. 2(a). In general, it has been suggested that a positive Vth shift in polycrystalline TFTs is due to frequent electron charge trapping in the pre-existing trap sites located at the channel/gate dielectric interface and from grain boundaries or natural defects in the channel materials. Here, negative charge carriers were generally trapped at the 6 ACS Paragon Plus Environment

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channel/gate dielectric interface by an induced gate-bias field under the forward sweep mode, and the polycrystalline channels with many grain boundaries provide an excess amount of trap sites in the gate interface and within the channel bulk. Furthermore, the solution-based CBD process can supply unwanted process impurities, such as OH- species and S-H bonding, which act as trap sites in the CdS films due to insufficient reaction in aqueous solutions.19 The hydroxyl groups between the dielectric and channel layers may also be responsible for the large hysteresis. From the hysteresis voltage, we can roughly estimate the charge trap density at the channel/gate dielectric interface, which shows a large value (2.1×1012/cm2). After the forward sweep, the gate electrode is switched “off”, as a ground state, in order to observe the recovery behavior. With increasing recovery time, the Vth slowly returns to the original position [Fig. 2(b)]; this behavior is associated with the tardy de-trapping of previously trapped negative charges. Although the Vth was nearly recovered to its initial state after about 3600 s, the complete recovery time in CdS transistors is too long; this is due to the high defect density. Figure 2(c) shows the photo-induced variations in the transfer curves of CdS phototransistors as a function of the light wavelength. The CdS phototransistor did not show a distinct response above 550 nm, as expected. In contrast, a large negative Vth shift in the CdS phototransistors was confirmed under lower wavelengths of light (≤ 550 nm). The ∆ of phototransistors under visible light was 22.9 V for green light (550 nm) and 48.1 V for blue light (450 nm), and the off-current of CdS phototransistors gradually increased by more than two orders of magnitude. This increase is attributed to the presence of trapped charges in the polycrystalline CdS layer, as well as to photo-generated free carriers, which can be vigorously activated by light exposure. Although CdS phototransistors exhibit high photosensitivity, the photo-induced negative Vth of phototransistors was very slowly recovered after the light was turned off, as shown in Fig. 2(d). The slow recovery of CdS



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phototransistors is caused by the high charge trap density that is induced by the nanocrystalline CdS layers. In addition, it is also suggested that the absorption of molecular oxygen in the channel surface may also be an origin of the Vth shift.20 Moreover, it is wellknown that the absorption of molecular oxygen can increase the recovery time in visible light detection. To improve the electrical performance of phototransistors for visible-light detection, phototransistors with a functional channel structure are designed with multi-stacked channel layers, as shown in Fig. 3(a). Here, the ZnO, Al2O3, and CdS layers are functionally used as carrier transport, buffer/passivation, and photo-absorber layers, respectively. In order to enhance the electrical conductivity of the charge transport layer, the Al2O3/ZnO/Al2O3 multilayer structure was deposited by a thermal ALD process. In a previous study, we illustrated that the Al2O3 buffer layer promoted layer-by-layer growth of the ZnO layers, resulting in improved crystallinity.21,22 Subsequently, in situ deposition of an ultra-thin Al2O3 passivation layer on the ZnO helps to reduce the potential barrier height/width between nanocrystalline ZnO grains by suppressing the diffusion of oxygen molecules into the crystal surface.23 As a result, the high-quality Al2O3/ZnO/Al2O3 multi-layer deposited on the gate dielectric layer in CdS-based phototransistors is as an effective charge transport layer of the carriers that are photogenerated in CdS (as opposed to being transported via the CdS). Consequently, by separating the photoactive and charge transport layers, we can expect both fast photoresponsivity and considerably reduced recovery time. However, a significant and unavoidable processing problem exists in the formation of oxide semiconductor/CdS multilayers. Typically, ZnO-based materials are amphoteric in nature. These can undergo chemical reactions in both acidic and basic solutions, even at low temperatures, and are readily dissolved by the following example reactions:24 ?@(ABC.D) + 2 = ?@ + (C.EF.D) G@ HℎJ KLG M65NHG6@ 8 ACS Paragon Plus Environment

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?@(ABC.D) + (C.EF.D) + 2 = ?@() G@ HℎJ OKMJ M65NHG6@ Since conventional deposition of CdS is generally carried out with strong base solutions (pH 9~13), direct CdS coating on the charge transport layer (ZnO) should be avoided in the CBD process. Alternatively, our strategy uses an Al2O3 protection film, with a very thin thickness, which does not degrade the charge transport efficiency, as shown in Fig. 3(a). The Al2O3 film shows high chemical durability, requiring a thickness of only a few nm, in solution 3 < pHs < 11.25 Figures 3(b) and (c) show the typical electrical performance of phototransistors with/without an Al2O3 protection layer (3.6 nm). The TFT devices without the Al2O3 protection layer did not show any meaningful transfer curves due to the chemical damage that occurred on the ZnO layer during the CBD process at pH 9 ~ 10 used for CdS deposition. Interestingly, the preferential use of a 3.6-nm-thick Al2O3 protection layer, before CdS coating, results in significantly improved TFT performance with a high &'( (8.6 cm2/Vs) and a good SS value (0.25 V/dec). In particular, the shape of this transfer curve, which strongly depends on the mobility, SS, and on/off ratio, shows similar characteristics with that of ZnObased oxide TFTs (but not the CdS TFT). This signifies that the TFT performance of this device is dominantly influenced by the conduction characteristics of the oxide semiconductors. It also implies that the ultra-thin Al2O3 film protects the ZnO surface from being chemically damaged by the strong base solution during the CBD process. Remarkably, in contrast to the results of the CdS transistor (∆
= QR. S

Highly Repeatable and Recoverable Phototransistors Based on

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