Simple Hydrogen Plasma Doping Process of Amorphous Indium

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Simple Hydrogen Plasma Doping Process of Amorphous Indium Gallium Zinc Oxide-based Phototransistors for Visible Light Detection Byung Ha Kang, Won-Gi Kim, Jusung Chung, Jin Hyeok Lee, and Hyun Jae Kim ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b17897 • Publication Date (Web): 06 Feb 2018 Downloaded from http://pubs.acs.org on February 6, 2018

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ACS Applied Materials & Interfaces

Simple Hydrogen Plasma Doping Process of Amorphous Indium Gallium Zinc Oxide-based Phototransistors for Visible Light Detection Byung Ha Kang, Won-Gi Kim, Jusung Chung, Jin Hyeok Lee, and Hyun Jae Kim* School of Electrical and Electronic Engineering, Yonsei University, 50 Yonsei-ro, Seodaemungu, Seoul 03722, Republic of Korea

KEYWORDS a-IGZO, phototransistor, visible light detection, hydrogen plasma doping, sub-gap state

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ABSTRACT

A homojunction-structured amorphous indium gallium zinc oxide (a-IGZO) phototransistor that can detect visible light is reported. The key element of this technology is an absorption layer composed of hydrogen-doped a-IGZO. This absorption layer is fabricated by simple hydrogen plasma doping (HPD), and sub-gap states are induced by increasing the amount of hydrogen impurities. Those sub-gap states, which lead to a higher number of photo-excited carriers and aggravate the instability under negative bias illumination stress, enabled the detection of a wide range of visible light (400–700 nm). The optimal condition of hydrogendoped absorption layer (HAL) is fabricated at hydrogen partial pressure ratio of 2%. As a result, the optimized a-IGZO phototransistor with HAL exhibits high photoresponsivity of 1,932.6 A/W, photosensitivity of 3.85 × 106, and detectivity of 6.93 × 1011 Jones under 635 nm light illumination.


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1. Introduction As the era of Internet of Things (IoT) emerges, various types of sensors are being developed and embedded in electronic devices. Among these, the photosensor is considered as one of the most significant devices, because it enables the detection of objects and can monitor the interaction between objects and users via optical signal. According to indispensable roles of photosensors, they have been widely used and will be utilized in many applications, such as automotive, robot, mobile device, interactive display, smart home, and healthcare.1-4 For the realization of commercially viable applications, the following conditions are highly required for photosensors: high performance, low power consumption, flexibility, transparency, largearea applicability, and low fabrication cost. Therefore, extensive research has been conducted to develop appropriate materials and structures for photosensors that satisfy those prerequisites. Recently, amorphous oxide semiconductor (AOS)-based thin film transistors (TFTs) are promising candidates for photosensor applications because of their outstanding properties such as high field effect mobility, low leakage current, high transparency, and good uniformity over large areas.5-8 Among AOS materials, especially amorphous indium gallium zinc oxide (aIGZO)-based TFTs have been reported to exhibit remarkable performance and stability. Consequently, they have been applied in many different types of devices, including active matrix flat panel displays, logic circuits, integrated sensor arrays, and phototransistors.9-11 However, there is a critical limitation on detecting visible light, a wavelength ranging from 400 nm (~3.1 eV) to 700 nm (~1.7 eV), by an a-IGZO phototransistor because of its wide band gap of more than 3 eV. For this reason, a-IGZO-based phototransistors are generally used to detect ultraviolet (UV) or blue visible light.12-15 Additional light absorbing material is currently essential for a-IGZO-based phototransistors to perceive longer wavelengths of visible light. Therefore, most a-IGZO-based phototransistors have adopted a heterojunction structure consisting of a-IGZO for the active layer and other materials for the absorption layer. Various 3 ACS Paragon Plus Environment


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materials for the absorption layer of heterojunction structure have been evaluated, such as organic polymers, nanostructures, carbon composites, two-dimensional layered transition metal dichalcogenides, and quantum dots.16-25 Although those materials absorb visible light successfully, there remain many issues concerning device stability and fabrication processes, i.e., poor uniformity, low film quality at the interface, difficulty in material synthesis, and process complexity. Unlike previous researches, we suggest a facile fabrication method for a homojunctionstructured a-IGZO-based visible light phototransistor which has high performance and stability. This method is for the purpose of increasing sub-gap states, and we apply the theory that a higher density of sub-gap states lead to improved optical absorption in oxide semiconductors.2629

The fabrication process has considerable advantages from the perspective of mass production

and device performance. Furthermore, the current study discusses the unsolved issue concerning the effect of hydrogen impurities in a-IGZO.

2. Experimental Section 2.1 Fabrication of a-IGZO phototransistors Active layer deposition was conducted on heavily boron-doped silicon wafers with 120 nm thick thermally grown silicon dioxide (SiO 2 ) layer. The heavily doped silicon was used as the gate electrode, and the SiO 2 layer was used as the gate insulator. A 40 nm thick a-IGZO active layer was deposited by radiofrequency (RF) magnetron sputtering at room temperature. The working pressure and oxygen partial pressure ratio were 5 mTorr of Ar plasma and 0%, respectively. The IGZO target was composed of In 2 O 3 :Ga 2 O 3 :ZnO at 1:1:1 ratio. After active layer deposition, it was annealed at 300°C in air for 1 h. A 20 nm thick hydrogen-doped absorption layer (HAL) was deposited on the active layer to provide a visible light absorbing layer, using RF magnetron sputtering with a mixed Ar and H 2 plasma. We named this process 4 ACS Paragon Plus Environment

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as hydrogen plasma doping (HPD); this process is explained in detail in Section 2.2. After HPD, post-annealing was conducted at 200°C in air for 20 min. Finally, 200 nm thick Al electrodes were deposited by thermal evaporation via shadow masks. The length and width of the channel was defined as 150 μm and 1,000 μm, respectively. Figure 1 schematically shows the phototransistor construction.

Figure 1. Device schematics of a-IGZO phototransistors (a) without HAL and (b) with HAL.

2.2 Method of hydrogen plasma doping HALs are composed of hydrogen-doped a-IGZO (a-IGZO:H) and their film characteristics are varied in accordance with the degree of HPD. We used an Ar/H 2 gas mixture to generate the hydrogen plasma during sputter deposition. To control the degree of HPD, the hydrogen partial pressure ratio (H 2 /[Ar + H 2 ]) was adjusted from 0 to 1, 2, 3, and 4% HPD by adjusting the gas infolw amount. 2.3 Measurement and analysis The electrical characteristics of phototransistors were measured with semiconductor parameter analyzer (model HP 4156C; Agilent Technologies) in the dark under ambient 5 ACS Paragon Plus Environment


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conditions. Various diode laser sources of different wavelengths (405, 532, and 635 nm), at power intensities of 1, 3, 5, and 10 mW/mm2, were used to provide illumination condition during the electrical measurements. The time-dependent photoresponse of a phototransistor was observed by applying 0.1 Hz red laser illumination (635 nm, 10 mW/mm2). The distribution of hydrogen ions in a-IGZO phototransistors was analyzed by time-of-flight secondary ion mass spectroscopy (TOF-SIMS) (model TOF SIMS 5; ION TOF). Spectroscopic visible photocurrent-voltage measurement was carried out using a xenon lamp for optical illumination and a stepping motor controlled monochromator to change the wavelength from 500 to 800 nm. The electronic band structures and the sub-gap states enrichment of the films were determined and compared using ultraviolet photoelectron spectroscopy (UPS) (model Axis Supra; Kratos) and ultraviolet-visible spectroscopy (UV-Vis spectroscopy) (model V-650; JASCO).

3. Results and Discussion An a-IGZO phototransistor lacking an absorption layer hardly detects visible light. By this reason, HAL was devised for detecting visible light in this research. We applied HPD during sputter deposition, which can induce the formation of sub-gap states that facilitated visible light absorption. In order to optimize the absorption layer properties, we controlled the HPD concentration under various conditions. Then, we compared the performance of phototransistors made with different conditions of HALs under white light emitting diode (LED) illumination by noting threshold voltage (V th ) shift and difference between drain current under illumination (I illuminated ) and in the dark (I dark ) at a gate voltage (V G ) of –5 V. As shown in Figure 2, the better light detectability was realized with increasing HPD concentration. However, the phototransistors fabricated at 4% HPD displayed the off current (I off ) that was unacceptably high for a switching device. Therefore, we defined 2% HPD as the optimal HAL 6 ACS Paragon Plus Environment

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condition. HALs of a-IGZO phototransistors in subsequent experiments were made with this optimal condition.

Figure 2. Transfer characteristics of a-IGZO phototransistors with various conditions of HAL by (a) 0% HPD, (b) 2% HPD, and (c) 4% HPD under white LED illumination.

Figure S1 plots the output characteristics of the a-IGZO phototransistor with HAL in the dark. The phototransistor exhibited n-type transistor characteristics with current saturation. We defined a constant drain voltage (V D ) of 10.1 V where the device operates in saturation mode, and then measured all transfer characteristics in this study at that fixed V D . Figure 3 (a), (c), and (e) show the transfer characteristics of a-IGZO phototransistors without HAL under red (635 nm), green (532 nm), and blue (405 nm) light illumination at V D = 10.1 V, respectively. Those a-IGZO phototransistors absorbed blue light (near-UV wavelength) quite well, and there was almost no change in the transfer characteristics under red illumination, and little change under green illumination, as reported previously.21,30 In the case of a-IGZO:H (same as the composition of HAL) phototransistors and a-IGZO phototransistors with HAL, the transfer characteristics varied considerably under the same illumination conditions, such as more negative shift of V th and large difference between I illuminated and I dark (see Figure S2 and Figure 7 ACS Paragon Plus Environment


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3 (b), (d), and (f)). From these results, we confirmed that HAL improved the visible light detectability. Furthermore, the a-IGZO phototransistor with HAL showed more improved optoelectronic characteristics than the a-IGZO:H phototransistor, due to more considerable difference of I D between in the dark state and under illumination state.


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Figure 3. Transfer characteristics of a-IGZO phototransistors without HAL under (a) red, (c) green, and (e) blue light illumination at different intensities. Transfer characteristics of a-IGZO phototransistors with HAL under (b) red, (d) green, and (f) blue light illumination at different intensities.

Three figures of merit are widely used to evaluate the performance of phototransistors: photoresponsivity (R), photosensitivity (PS), and detectivity (D*). Note that, we assumed the shot noise from the dark current is the major contribution, in calculating D*. The figures of merit are defined as follows: R = J photo /P

(1)

PS = I photo /I dark D* = R/(2qJ dark )1/2

(2) (3)

where J photo is the photocurrent density, P is the incident laser power density, I photo = I illuminated – I dark , q is the absolute value of the electron charge (1.6 × 10–19 C), and J dark is the dark current density.23,31-34 The calculated figures of merit were used to compare the optoelectronic characteristics of the fabricated phototransistors in detail. In the dark condition, the electrical characteristics of the a-IGZO phototransistor with HAL were superior to those of a-IGZO phototransistor without HAL, because the hydrogen doping increased the number of carriers.35,36 As the effect of stacking the HAL, R, PS, and D* were improved by 0.12 to 1,008.8 A/W, 4.35 × 102 to 2.32 × 106, and 4.30 × 106 to 2.41 × 1011 Jones, respectively, under red light illumination at V D = 10.1 V. Similarly, they were improved by 242.07 to 1,932.6 A/W, 1.35 × 105 to 3.85 × 106, and 5.50 × 109 to 6.93 × 1011 Jones, respectively, under green light illumination at V D = 10.1 V. The parameters are summarized in Table 1; the data listed are the maximum values of R, PS, and D*. Figure S3 and Figure S4 show further R, PS, and D* data 9 ACS Paragon Plus Environment


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of the phototransistors with HAL measured at various light intensities and as a function of V G at a V D = 10.1 V. The optoelectronic characteristics of a-IGZO phototransistors with HAL showed high performance among the other a-IGZO-based photodetectors, even though its absorption layer is also composed of an oxide semiconductor. (see Supporting Information Table S1). Also, the device in our study showed wider detection range in visible light region.

Table 1. Optoelectronic characteristics of a-IGZO based phototransistors. In dark Sample

Under red light (635 nm)

Under green light (532 nm)

μ FE (cm2/V·s)

V th (V)

Max. R (A/W)

Max. PS

Max. D* (Jones)

Max. R (A/W)

Max. PS

Max. D* (Jones)

a-IGZO

7.73

3.20

0.12

4.35 × 102

4.30 × 106

242.07

1.35 × 105

5.50 × 109

a-IGZO:H (HAL)

12.64

0.21

105.0

15.2

1.41 × 108

505.55

4.82 × 102

1.36 × 109

a-IGZO w/ HAL

16.15

2.29

1,008.8

2.32 × 106

2.41 × 1011

1,932.6

3.85 × 106

6.93 × 1011

However, there may be issues that the enhanced light sensibility was not due to the effect of hydrogen doping, but rather to increased thickness or the multilayer structure. To address this concern, we fabricated an a-IGZO phototransistor with a non-HPD treated layer which has the same thickness as the HAL. Structure and transfer characteristics of the device under same illumination conditions are shown in Figure S3. They showed almost identical visible light detectability compared to a-IGZO phototransistors without absorption layer. This indicated that the main cause of enhanced visible light detection was the hydrogen doping effect. TOF-SIMS analysis was conducted to examine the uniformity of the hydrogen ion distribution in the HAL. The hydrogen ion distribution was monitored as a function of depth (see Figure 4). An a-IGZO phototransistor with a non-HPD treated layer which has the same thickness as the HAL contained quite a small amount of hydrogen ions throughout the device. These were 10 ACS Paragon Plus Environment

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inevitably acquired from the ambient atmosphere during the fabrication process. On the contrary, in a-IGZO phototransistor with HPD treated layer, much higher content of hydrogen ions existed in the HAL region than other layers. Only few hydrogen ions in the HAL diffused into other layers or out of the device, therefore, unintentional effects or severe degradation of stability were not generated. This result is due to the tendency that hydrogens in the sputtered a-IGZO layer mostly form –OH bonds and hardly shift.37 Furthermore, hydrogen doping by plasma treatment at room temperature results in the formation of –OH bonds, which exist as sub-gap states acting as electron donors.38,39 Thus, we could infer that sub-gap states were generated in the HAL, according to distribution of hydrogen ions.

Figure 4. SIMS depth profiles for a-IGZO phototransistors (a) with non-HPD treated layer and (b) with HPD treated layer.

Spectroscopic visible photocurrent-voltage measurement was carried out to verify that hydrogen ions in the HAL practically led to the formation of sub-gap states. Interpreting this qualitative analysis was based on the principle that sub-gap states are excited and generate photocurrent, when light illumination is applied.40,41 We applied 1 V as the gate bias and 11 ACS Paragon Plus Environment


collected I D from V D of 27–30 V for wavelengths of 500–800 nm with 10 nm steps. In order to normalize the current values, I D at dark was subtracted from I D at the different wavelengths and divided by the power of the light source at each wavelength. Figure 5 shows that the phototransistor without HAL generated extremely low photocurrent under sub-bandgap light illumination, indicating few sub-gap states. In contrast, the phototransistor with HAL showed considerably high photocurrent due to the increased number of photo-excited sub-gap states. As a result, photo-excitation was clearly observed, confirming that the number of sub-gap states had increased via the incorporation of hydrogen ions. Furthermore, this result supported our devised phototransistor could operate at consecutive and wide-ranging visible light wavelengths, not only at the specific wavelengths of 405, 532, and 635 nm.

Figure 5. Normalized current with respect to consecutive visible light wavelengths of a-IGZO phototransistors without HAL and with HAL.

In the preceding results, we found that hydrogen incorporation increased the number of carriers and sub-gap states. However, in general, the role of hydrogen in oxide semiconductor is not clearly revealed due to its various chemical states.42 So far, the most widely known role of hydrogen impurities (interstitial hydrogen and substitutional hydrogen) in n-type oxide semiconductor is that they work as shallow donors by generating electrons.39,43 Additionally, 12 ACS Paragon Plus Environment

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from the perspective of stability issue, most previous studies reported that sub-gap states induced by hydrogen impurities deteriorate stability of oxide semiconductor TFTs, such as positive bias thermal stress (PBTS) and negative bias illumination stress (NBIS) stabilities.44 It was this stability related controversy that drove us to determine stacking HAL on the active layer instead of directly doping the active layer. Our sub-gap state engineering was intended to enhance the responsivity of visible light by increasing both the number of carriers and NBIS instability. The influence of hydrogen impurities in this study was almost consistent with those two widely known hypotheses. So as to confirm that how hydrogen impurities degraded the PBTS stability of the phototransistor, we observed the V th shift under specific PBTS condition. In Figure S6, there was somewhat degradation of PBTS stability when the HAL was stacked on the a-IGZO phototransistor. On the other hand, there was almost no difference of V th shift under negative bias stress condition (V G = –20 V, 3,600 s) and the values were negligible (∆V th = –0.04 V). These results imply that PBTS instability was not due to the movement of hydrogen ions, but mainly due to a number of trapped electrons at the interface between active layer and HAL. The degradation was not that critical, so there was no problem using it as a device.45,46 The time-dependent photoresponse characteristics of the a-IGZO phototransistor with the HAL were evaluated by switching the illumination of the red laser (635 nm) at a light intensity of 10 mW/mm2 (see Figure 6 (a)). The red laser was periodically switched on and off at 0.1 Hz, and I D was measured at V G = –1 V and V D = 10.1 V. Since red light is more difficult to detect than green light by a-IGZO phototransistors, the measurement results under red light illumination have more significance than other conditions. However, there are somewhat increase in both I dark and I illuminated as the bias time flows, due to persistent photoconductance (PPC) originating from the slow recombination in trap sites, as observed in oxide semiconductors like IGZO.9,11 The PPC occurred in this device mainly derives from the defective HAL and at the interface between active layer and HAL. There would be a number 13 ACS Paragon Plus Environment


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of trapped electrons which were photo-excited from HAL and confined at the interface when they shifted toward active layer. Additionally, these trapping behavior at the interface leads to relatively slow response time and recovery time. Although, this PPC issue was resolved by applying a short positive gate pulse (+30 V for 5 ms) on the device which induces recombination and detrapping of the photo-excited charges and trapped electrons at the interface between HAL and active layer (see Figure 6 (b)).27 In Figure 6 (c), to demonstrate that the aforementioned stability issue with hydrogen incorporation is not a problem for practical phototransistor, we conducted this measurement at V G = –0.1 V and V D = 10.1 V for longer times (switching the same red laser illumination condition at 0.02 Hz over 3,000 s) than previous studies (mostly below 1,000 s). As a result, the device exhibited highly durable and reversible characteristics between the on and off states.


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Figure 6. (a) Time-dependent photoresponse characteristics of the a-IGZO phototransistor with HAL under periodic dark and red light (635 nm) illumination conditions. (b) Comparison of one cycle photoresponse characteristics depending on gate pulse bias. (c) Endurance test with red light (635 nm) illumination at 0.02 Hz over 3,000 s.

In order to comprehend overall operation mechanism of the a-IGZO phototransistor with HAL under visible light, we analyzed band alignment of the active layer and HAL. In Figure S7, bandgap (E g = E C – E V ) was calculated from Tauc plots based on UV-Vis spectroscopy data. The E g of the active layer and HAL were 3.67 and 3.65 eV, respectively. The work function (∅) and band offset between Fermi energy level (E F ) and valence band maximum (VBM) energy level (E FV = E F – E V ) were calculated from the UPS spectra shown in Figure 7. Moreover, the band offset between conduction band minimum (CBM) energy level and E F (E CF = E C – E F ) could be inferred by subtracting E FV from E g . As shown in the inset image of the Figure 7, the intensity of the sub-gap states located near the valence band edge in the HAL was higher than in the active layer. Additionally, E FV of the active layer and HAL was measured to be 3.19 and 3.54 eV, E CF of the active layer and HAL was calculated to be 0.48 and 0.11 eV, respectively, indicating that the E F of the HAL was closer to the conduction band. The aforementioned data, plotted in Figure 8 reveals the band alignment of the films, which explains the visible light absorption mechanism. The HPD treatment hardly changed E g , but induced the formation of sub-gap states near-VBM and CBM.47 There are several factors that increase the number of near-VBM states: oxygen vacancies with voids, undercoordinated oxygens, hydrogen anions, and –OH bonds.38,39,48-50 As HPD was conducted on the film, –OH bonds were generated, which affected the increased number and the expanded distribution of near-VBM states. Near-CBM shallow states were also increased due to hydrogen doping, as 15 ACS Paragon Plus Environment


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mentioned above. When visible light shines, there are difficulties in photo-excitation with visible light because of wide band gap and fewer states exist in the a-IGZO layer. On the contrary, a number of near-VBM states located at higher energy levels, and a narrowed energy level difference between E F and CBM, provide various photo-excitation routes for sub-gap states under the light having low photon energy.26 These donor-like sub-gap states are excited above the CBM energy level and generate more carriers; lots of photo-excited electrons accumulate at the interface between HAL and active layer, which leads to decreased effective potential barrier height, then they could transport to the active layer and amplify the optoelectronic characteristics.20,51 By adjusting the HPD concentration, formation of sub-gap states was controlled, and enabled the oxide semiconductor to detect a wide range of visible light wavelengths without adopting opaque light absorbing materials. Thus, our devised process can be developed into manufacture of fully transparent a-IGZO visible light phototransistors (see Supporting Information Figure S8).

Figure 7. UPS spectra of the active layer and HAL.


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Figure 8. Schematic diagram of the energy level alignment of (a) a-IGZO for the active layer and (b) a-IGZO:H for the absorption layer.

4. Conclusion In conclusion, we proposed an a-IGZO-based phototransistor that could detect a wide spectrum of visible light region by stacking a-IGZO:H layer, named HAL. We applied the property of HAL which has high density of sub-gap states and imperfect metal–oxygen bonds for absorbing visible light. Unlike previously reported phototransistors, our phototransistor has a homojunction structure composed of a-IGZO-based layers. As a consequence, our device has numerous advantages, such as high transparency, easy fabrication, and excellent compatibility with current sputter-based large-area processes. This research demonstrated that a-IGZO-based optoelectronic devices without using heterojunction materials can be widely applied in various fields. Furthermore, this study confirmed that hydrogen doping in a-IGZO increase the number 17 ACS Paragon Plus Environment


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of near-CBM and -VBM states, which generate more carriers and aggravate NBIS instability, respectively. In future work, we will investigate totally transparent and flexible phototransistors fabricated by novel technology combined with the method introduced in this paper.


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ASSOCIATED CONTENT Supporting Information The following files are available free of charge. An output curve of a-IGZO phototransistor with HAL; transfer characteristics of aIGZO:H phototransistor under light illumination conditions; optoelectronic characteristics of a-IGZO phototransistor with HAL as a function of gate voltage; comparison of optoelectronic characteristics with previously reported a-IGZO based phototransistors; transfer characteristics of a-IGZO phototransistor with non-HPD treated layer under light illumination conditions; PBTS results of a-IGZO based phototransistors; Tauc plots of active layer and HAL; photograph and schematic of transparent phototransistor entirely composed of oxide materials

AUTHOR INFORMATION Corresponding Author *E-mail: (H.J.K.) [email protected]

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by the National Research Foundation of Korea (NRF) Grant funded by the Korea government (MSIT) (No. 2017R1A2B3008719) 19 ACS Paragon Plus Environment


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Table of contents


Simple Hydrogen Plasma Doping Process of Amorphous Indium

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