Study on Optoelectronic Characteristics of ZnGa2O4 Thin-Film

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Study on optoelectronic characteristic of ZnGa2O4 thin-film phototransistors Yuan-Chu Shen, Chun-Yi Tung, Chiung-Yi Huang, Yu-Chang Lin, Yan-Gu Lin, and Ray Hua Horng ACS Appl. Electron. Mater., Just Accepted Manuscript • DOI: 10.1021/acsaelm.9b00128 • Publication Date (Web): 03 May 2019 Downloaded from http://pubs.acs.org on May 9, 2019

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ACS Applied Electronic Materials

Study on optoelectronic characteristic of ZnGa2O4 thin-film phototransistors Yuan-Chu Shen,1 Chun-Yi Tung,1 Chiung-Yi Huang,1 Yu-Chang Lin,2 Yan-Gu Lin2 and Ray-Hua Horng1,* 1

Institute of Electronics, National Chiao Tung Univeristy, Hsinchu 300, Taiwan, ROC

2

National Synchrotron Radiation Research Center, Hsinchu 300, Taiwan, ROC

*

[email protected]

ABSTRACT Deep ultra-violet (DUV) phototransistors with high photoresponsivity are fabricated on ZnGa2O4 grown by metalorganic chemical-vapor deposition. Owing to transistor actions, the photodetector meets to a large photocurrent and optical response. When illuminated with photon wavelength within the range of 200 to 250 nm, the ZnGa2O4-based phototransistor presented a large responsivity, especially 1.51 × 106 A/W as the incident light at 210 nm with 1.73 μW/cm2. It was also observed that the photocurrent/dark current ratio and rising time can be improved by gate control, which is related to threshold voltage shifting when under illumination. These results demonstrate ZnGa2O4-based phototransistor is a very promising candidate for DUV optoelectronic devices applications.

KEYWORDS : ZnGa2O4, DUV phototransistors, photodetectors, phototransistors, metalorganic chemical-vapor deposition

1. Introduction In recent years, internet of things (IoT) is the major development in new technology. IoT has been used to analysis and utilize the big data immediately from sensors. Therefore, the sensing ability, immediacy and reproducibility are the matters of concern. In the different sensing materials and structures of devices, photodetectors (PDs) can be used to detect various wavelengths of light. ACS Paragon Plus Environment

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Especially, deep ultraviolet (DUV) PDs have been widely applied in biochemical sensing, ozone-hole monitoring,1

flame

detection,2,3

environmental

monitoring,

astronomy,4

advanced

optical

communications5-7 …etc. Detectors only sensing radiations with wavelength shorter than 280 nm can be regarded as solar-blind DUV photodetectors.8-10 It possess inherently high UV-to-visible rejection ratio, ultralow dark current, and high-temperature operation. In order to obtain the high UV-to-visible rejection ratio, a new generation of wide bandgap semiconductor-based UV PDs has now emerged, which involve metal oxide semiconductors such as βGa2O3, ZnMgO and ZnGa2O4.9-13 Among them, ZnGa2O4 has a potential to provide the superior performance as a wide-bandgap semiconductor material (at room temperature) due to its ~5 eV bandgap, which is provided with outstanding optical properties.14 Moreover, the ZnGa2O4 epilayers have been successfully grown by metal-organic chemical vapor deposition (MOCVD), and the metalsemiconductor-metal (MSM) PD and metal-oxide-semiconductor field effect transistors (MOSFET) made of ZnGa2O4 epilayers have been reported in our previous study.15-16 Owing to the limited improvement of responsivity, response time and power consumption issues in MSM PDs, several researches of phototransistors have been studied, including amorphous indium gallium zinc oxide (α-IGZO), amorphous hafnium indium zinc oxide (α-HIZO) and indium gallium oxide (α-IGO).17-20 However, the cutoff wavelength of these phototransistors is always above 280 nm and the performance of these phototransistors is not good enough due to the oxide materials with amorphous structure. Furthermore, phototransistors possess higher sensitivity and lower noise than general photodiodes do. Therefore, the development of phototransistors is necessary for the future prospects. In this paper, the phototransistor was successfully fabricated on ZnGa2O4 epilayer grown on the sapphire substrate. The ZnGa2O4 epilayers were as a channel material and light absorption layer. The electrical properties of phototransistors will be studied, and the effects of illumination and the optoelectronic properties of the MOSFET will be reported in this work.

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2. Experimental section (1) Phototransistor Fabrication A ZnGa2O4 thin film was grown on a c-plane (0006) sapphire substrate at 650°C through MOCVD. Diethylzinc (DEZn) and triethylgallium (TEGa) were used as Zn and Ga precursors, respectively. The ZnGa2O4 epilayer was an n-type epilayer. Our previous study reported the growth parameters and material quality of the thin film.21 After the epilayer was grown, the isolation process was started with an inductively-coupled plasma reactive ion etching system by using Cl2/Ar. Multilayer metals with Ti/Al/Ni (50/75/25 nm) were patterned on the isolated ZnGa2O4 film area as source and drain electrodes by using an e-beam gun evaporator. Then, a 20-nm-thick Al2O3 dielectric layer was deposited using an atomic layer deposition system at 250°C. Trimethylaluminum (TMA) and H2O vapor were used as an Al precursor and oxidant, respectively, for the growth of an Al2O3 dielectric layer. Finally, a 150-nm-thick nickel layer was patterned on the dielectric layer as a gate electrode by using an e-beam gun evaporator. Nickel was used as the gate electrode. Figure 1 (a) displays the threedimensional structure of the phototransistor used in this study. The channel width (W) and length (L) were 250 and 18 μm, respectively. The channel length was divided into LG, LGS, and LGD, which were 3, 5, and 10 μm, respectively. An asymmetric architecture was designed in LGS and LGD so that the phototransistor could endure a high breakdown voltage.22 Figure 1 (b) displays the image observed with the optical microscope of the phototransistor.

(2) Characterization The current–voltage (I–V) characteristics were measured using an Agilent-4155B semiconductor parameter analyzer in a black box at room temperature. The light source was a 30-W deuterium lamp and irradiated on the top of the devices through optic fibers. The wavelength was modulated using an Action Series SP2150i monochromator. The energy levels of the valence-band maximum (EVBM) and work function (Φ) for ZnGa2O4 were recorded with ultraviolet photoelectron spectra (UPS) in an ACS Paragon Plus Environment


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ultrahigh vacuum system (1×10−9 Torr) at beamline 24 A of Taiwan Light Source at National Synchrotron Radiation Research Center (NSRRC). In terms of UPS, the excited photon energy was 40 eV, calibrated with the Fermi edge of clean standard gold. To avoid the low-energy secondary cut-off signals of samples, bias voltage −5.0 V was applied to the samples. (b)

(a)

Figure 1 (a) Three-dimensional structure and (b) Image observed with the optical microscope of the ZnGa2O4 phototransistor.

3. Results and Discussion (1) Transistor characteristics without illumination The phototransistor device exhibited typical n-type behavior [Figure 2 (a)]. The field-effect mobility and threshold voltage (VT) were obtained from equation 1, which represents linear regime transfer characteristics.

I DS =

1 W µ n ⋅ C ox ⋅ 2(V GS − V DS )V DS − V DS 2  2 L

(1)

where μn and COX are the field-effect mobility (cm2/V · s) and dielectric oxide capacitance (F), respectively; W and L are the channel width and length of the device, respectively; VGS is an applied gate-to-source bias; and VDS is an applied drain-to-source bias. As displayed in Figure 2 (a), IDS (in dark) was as a function of VGS when VDS was 5 V. Figure 2 (b) displays the IDS–VDS curve at various gate voltages with a step of 1 V. The VT (at 1 nA) was


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approximately −3 V. A depletion-mode (D-mode) and normally ON ZnGa2O4 transistor was used as a phototransistor. Moreover, the phototransistor was pinched off at VDS = VGS - VT. Through analysis, the transconductance (gm), field-effect mobility (μn), and subthreshold swing (SS) were determined to be 1.65 × 10-2 mS/mm, 5.97 cm2/V·s, and 1.51 × 10-1 V/dec, respectively. The transistor exhibited a high ON/OFF ratio of approximately 107. Nevertheless, it was found that the output curves do not saturate at the high VDS region. It could be result from some oxygen vacancies existing in the ZnGa2O4 epilayer. These defects can provide extra path (Vo+ or Vo2+) for free carriers in the channel, thus they extend the depletion region of channel. Therefore, the IDS will not saturate at high VDS. (b)

(a)

Figure 2 (a) Transfer characteristic behavior and (b) Output characteristic behavior of the phototransistor in the dark environment.

(2) Photosensitive properties The photosensitive properties of the phototransistor must be investigated through irradiation with various wavelengths. Figure 3 displays the transfer characteristics of the phototransistor measured at a VDS of 5 V in the dark and under irradiation with various wavelengths. Because the incident wavelengths were longer than 300 nm, IDS marginally increased in the OFF state (red rectangular region). By contrast, the saturated IDS increased from 9.53 × 10-6 to 3.54 × 10-5A due to the transistor being operated in an ON state with a VGS of 10 V (blue rectangular region). Note that the incident wavelengths

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were shorter than 250 nm, the phototransistor was normally ON at a VGS of less than −20 V and a VDS of 5 V. Figure 4 displays the transfer characteristics of the ZnGa2O4 transistor measured in the dark and under 230-nm illumination at various VDS values (1–5 V). In the dark, VT changed from −3 to −4 V when VDS was biased from 1–5 V. This change was observed because an increased number of electrons were induced by VDS. An increased gate voltage depleted these electrons when the transistor was turned off. The transistor was turned on when VGS was between −10 and 10 V and VDS was biased at 1–5 V under 230-nm illumination. VT had a lower value when the phototransistor was illuminated as compared with that when the phototransistor was in the dark environment. The VT values exhibited a negative shift when the phototransistor was illuminated by a short wavelength light.

Figure 3 Transfer characteristics in the dark and under various levels of illumination.

Figure 4 Transfer characteristics in the dark under 230-nm illumination at various drain biases.


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In our previous study, the bandgap of the ZnGa2 O4 phototransistor is approximately 5.2 eV.21 To investigate the possible positions of the band level with respect to the vacuum level, the energy levels of ZnGa2O4 were determined using an inelastic secondary electron cutoff of UPS energy distribution curve. Figure 5(a) presented the typical UPS data, in which the valence band maximum energy (EVBM) and work functions can be estimated. The Fermi level (EFermi) with respect to EVBM is about 3.97 eV. The work function of ZGO measured from UPS cutoff spectra is about 3.54 eV as shown in Figure 5(b). As consider the band gap of ZnGa2O4 is 5.2 eV, the energy between the conduction band minimum (ECBM) and EFermi is about 1.23 eV. Thus the energy diagram of ZnGa2O4 is schematically proposed in Figure 5(c). From the band diagram, obviously, no response should be observed for light with a wavelength longer than 240 nm. In previous studies,23 defects such as oxygen vacancies [(VO”) and (VO, VGa)’] and anion interstitial defects (ZnGa)’ have been observed in epilayers. The transfer characteristics of the phototransistor indicated the presence of some defect levels in the ZnGa2O4 epilayer. Under illumination, electron–hole pairs can be generated. Because a limited number of electrons can be obtained from electron–hole pairs (for wavelength > 300 nm), VT marginally shifts toward a negative direction. A marginal increase in IDS was observed when the transistor was operated in the OFF state because electrons were easily depleted in this situation [Figure 6 (a)]. When the transistor was operated in the ON state, the generated electrons contributed to the current [Figure 6 (b)]. When the incident wavelengths were shorter than 250 nm, most electrons were contributed from the band-to-band transition, which transformed the channel into a conductor under a VGS of −20 V. An increased negative voltage results in the breakdown of the gate oxide. Thus, phototransistors can be operated at a VGS between approximately −10 and −20 V for DUV light sensing. The aforementioned operating characteristic enables a PD to have sensitive detection. The degree of increment in the saturation drain current gradually decreased in a strong inversion region with low incident wavelengths



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because under low incident wavelengths, the gate bias was in a relatively strong inversion region, which enabled the appropriate inversion of free carriers and reduced the effect of illumination.

Figure 5(a) Valence band spectra of ZnGa2O4 in the region of the valence band maximum. The binding energy scale is with respect to the Fermi level (EFermi). The valence band maximum occurs at the intersection of a line fit to the linear portion of the leading edge and the extended background line between the valence band maximum and the Fermi level. (b) The work function of ZnGa2O4 measured from UPS cutoff spectra. (c) Schematic illustration of the band structure of the ZnGa2O4.

Figure 6 Band diagram of the device in the (a) OFF state and (b) ON state under illumination.

Figure 7 displays the variation in the VT and SS under illumination with different wavelengths. The VT abruptly decreased with a decrease in the wavelength. Thus, the incident energy was high. This phenomenon can generate photoelectron–hole pairs because of illumination. Because photoelectron– hole pair generation effectively improves the free-carrier density in the channel, the gate bias required to operate the device, which is an n-channel D-mode phototransistor, in an inversion regime is reduced. 8 ACS Paragon Plus Environment

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Under distinct illumination, particularly in the range of 200–280 nm, photoelectrons were generated such that additional negative gate bias was required to deplete photoelectrons for forming a channel. This mechanism shifted the VT further in a negative direction. The SS, which can be considered as the gate control ability over the channel, did not show obviously change (only small change from 0.12 V/dec to 0.18 V/dec) at wavelengths higher than 300 nm. The SS increased rapidly from 0.12 V/dec to 0.22 V/dec at wavelengths less than 280 nm. Thus, gate as well as illumination behavior was applied to the channel, which caused an abrupt change in the free-carrier density. This effect is more severe when the incident energy progressively exceeds the bandgap of the ZnGa2O4 film and the gate loses the channel control. This phenomenon is described in the section on response time.

Figure 7 Variation in VT and the SS under different incident wavelengths.

Figure 8 illustrates the responsivity of the ZnGa2 O4 phototransistor under VGS = −9 V and VDS = 5 V with varying light power density (~1.73, 12.8 and 30.1 μW/cm2) at 230 nm. It was found that a high responsivity was observed within the range of 200–250 nm. The lowest light power density resulted in the highest responsivity. The highest responsivity of 1.51 × 106 A/W can be obtained as the incident light at 210 nm with 1.73 μW/cm2. Because the transistor was at OFF state under VGS = −9 V and VDS = 5 V in the dark box (showing in Fig. 4). It suggested that the channel was totally depleted by the bias. As the 1.73 μW/cm2 light power density with 230 nm irradiated to the phototransistor, almost all carriers 9 ACS Paragon Plus Environment


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were created by light due to the epilayer being too thin. Further increasing the incident power density can not excite more free carriers. It resulted in the responsivity decreasing as increasing the light power density. Moreover, the small peak in the visible region from 375 to 425 nm can be attributed to defects such as oxygen vacancies and sublevels in the bandgap, which cause visible light to excite photoelectrons and produce photocurrent. It is worthy to mention that the energy (@ 350 nm) is not enough to pump electron from valence band (EVBM) to conduction band (ECBM). But it has the ability to pump electrons from the valence band to defect levels. Thus, (ZnGa’) and (Vo”) levels would accumulate electrons. After, the responsivity would increase as the optical energy is enough to pump electrons from (ZnGa) and (Vo”) levels to conduction band. Thus, there exists small peak responsivity in the visible region from 375 to 425 nm. Table 1 shows the responsivity of various metal-oxide semiconductors phototransistors fabricated by different methods. Among these materials, ZnGa2O4 shows the highest responsivity, indicating that the ZnGa2O4 epilayer has high potential in DUV optoelectronic applications.

Figure 8 Responsivity of the ZnGa2O4 phototransistor under VGS = −9 V and VDS = 5 V with varying light power density.

Figure 9 displays the response time under 230-nm illumination at VGS values of −10, −3, and 0 V, which represent the OFF state, subthreshold region, and strong inversion region, respectively. In the OFF state, the gate bias was set at −10 V to avoid a defect-induced barrier lowering effect (“D”IBL).16 When the DUV light was ON, the rising time was 0.83 and 0.72 s in the OFF state and subthreshold 10 ACS Paragon Plus Environment

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circumstances, respectively. The ON/OFF ratio was approximately 108 in the OFF state. The inset in Figure 9 indicates curve floating at a different gate biases when the DUV light was OFF. This observation indicates that the rising time is almost unaffected by the distinct operating regime, whereas the falling time is dependent on the gate bias. Moreover, the SS increases considerably when the incident energy is higher the bandgap of ZnGa2O4 (230 nm). Therefore, the gate control considerably deteriorates. A suitable gate bias was used to obtain the comparative responsivity between visible light and DUV light, which is the basic requirement of a regular photodetector. The selection of the gate bias considerably influences the response time (falling time). Because the MOSFET is n-channel in this study and the more negative gate voltage will replete the electrons (excited by DUV light) more quick which results in shorter falling time.

Figure 9 Response time under 230-nm illumination at various gate biases.

4. Conclusion The electrical performance was studied, and the phototransistor was fabricated on ZnGa2O4 grown by MOCVD. Under illumination with various wavelengths, the VT shifted in a negative direction. Moreover, the phototransistor exhibited a high and selective responsivity under VGS = −9 V. The 11 ACS Paragon Plus Environment


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ON/OFF ratio ranged from 107 to 108, and the rising time was approximately 0.83 s after 230-nm illumination. The highest ON/OFF ratio and fastest rising time for the MSM photodetector were 107 and 1.8 s, respectively. These values indicate that the performance of the phototransistor improved when the gate control was used. Although some issues must be overcome, the ZnGa2O4 thin-film phototransistor is highly favorable in DUV optoelectronic applications.

Table 1. Characteristics comparison of various metal oxide-based ultraviolet phototransistors.

Material

Vth (V)

SS (V/dec)

Mobility 2 (cm /V-s)

On/off ratio

Rejection ratio

α-IGZO

0.5

0.49

3

1.1×104

α-IGO

0.5

0.23

7.6

-

-

-4.7

0.71

ZnO NRs/CVD Cr MgInO

Responsivity (A/W) Response time (sec)

Ref.

104

3.2(@250 nm)

-

[18]

105

4×104

0.18(@280 nm)

-

[20]

-

-

-

2.5×106(@365 nm)

-

[24]

0.96

109

-

104(@300 nm)

-

[25]

6

6

Ta2O5/αIGZO

1.25

0.49

48.5

~10

10

6.93(@250 nm)

-

[26]

MgZnO

3.1

0.8

5.65

4.4×105

6.55×105

3.12(@290 nm)

-

[27]

ZnGa2O4

-3

0.15

5.97

~107

2.74×107

1.51×106(@210 nm)

Trise: 0.83

This work

Acknowledgement This study was supported by the Ministry of Science and Technology (MOST), Taiwan, R.O.C., under the grants MOST 105-2221-E-009-183-MY3, 107-2221-E-009-117-MY3, 107-2218-E-009-056 and 107-3017-F009-00 as well as the Ministry of Education, Taiwan (SPROUT Project-Center for Emergent Functional Matter Science of National Chiao Tung University). We thank the National Nano Device Laboratory for allowing us to use their facilities. We thank the numerous people that improved this study by offering their expertise and skills.


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Author contributions statement S. Y. C and T. C. Y designed the experiments and wrote the paper. H. C. Y contributed to the growth of the ZnGa2O4 films through MOCVD. L. Y. C. and L. Y. G. contributed to measure and analyze the ultraviolet photoelectron spectra. H. R. H. designed the experiments, analyzed, and verified the data, and wrote the paper. All the authors read and approved the final version of the manuscript to be submitted. Author information Corresponding Author: Ray-Hua Horng *Tel: +886-35722121 ext 54138, E-mail:[email protected]. ORCID: Ray-Hua Horng : 0000-0002-1160-6775 Notes The authors declare no competing financial and non-financial interests.

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Study on Optoelectronic Characteristics of ZnGa2O4 Thin-Film

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