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Article

Improved-Performance Amorphous Ga2O3 Photodetectors Fabricated by Capacitive Coupled Plasma-Assistant Magnetron Sputtering

by
Yiming Liu
,
Chong Peng
,
Chang Liu
,
Cong Yu
,
Jiarui Guo
,
Yiyang Chang
and
Yi Zhao
*
State Key Laboratory of Integrated Optoelectronics, College of Electronic Science and Engineering, Jilin University, Changchun 130012, China
*
Author to whom correspondence should be addressed.
Coatings 2024, 14(9), 1204; https://doi.org/10.3390/coatings14091204
Submission received: 9 August 2024 / Revised: 14 September 2024 / Accepted: 17 September 2024 / Published: 19 September 2024
(This article belongs to the Collection Feature Paper Collection in Thin Films)
Browse Figures
Figure 1
<p>(<b>a</b>) Schematic of Ga<sub>2</sub>O<sub>3</sub> MSM photodetector. (<b>b</b>) The microscopy image of the Ga<sub>2</sub>O<sub>3</sub> photodetectors (<b>c</b>) Schematic of CCP-assistant magnetron sputtering system.</p> "> Figure 2
<p>(<b>a</b>) XRD patterns of S0–S60 as well as c-sapphire substrate. (<b>b</b>) AFM topographies of a-Ga<sub>2</sub>O<sub>3</sub> under different RF-bias powers.</p> "> Figure 3
<p>(<b>a</b>) Transmission spectra, (<b>b</b>) plots of (αhν)<sup>2</sup> versus hν, (<b>c</b>) E<sub>g</sub> and E<sub>u</sub> of Ga<sub>2</sub>O<sub>3</sub> films deposited on c-plane sapphire substrate under different RF-bias powers.</p> "> Figure 4
<p>I-V characteristics in log coordinates of S0–S60 (<b>a</b>) in the dark and (<b>b</b>) illuminated with 254 nm UV light. (<b>c</b>) I<sub>photo</sub>/I<sub>dark</sub>, responsivity, and detectivity plotted as functions of RF-bias power. (<b>d</b>) Normalized spectral response of S0–S60 under 20 V bias.</p> "> Figure 5
<p>(<b>a</b>) O 1 s and (<b>b</b>) Ga 2p<sub>3/2</sub> core-level spectra of amorphous Ga<sub>2</sub>O<sub>3</sub> films deposited under different RF-bias powers.</p> "> Figure 6
<p>(<b>a</b>) Normalized transient response of S0–S60 photodetectors in one single cycle. (<b>b</b>) Transient response of S0–S60 photodetectors with repetition test.</p> ">
Versions Notes

Abstract

:
Ga2O3 has received increasing interest for its potential in various applications relating to solar-blind photodetectors. However, attaining a balanced performance with Ga2O3-based photodetectors presents a challenge due to the intrinsic conductive mechanism of Ga2O3 films. In this work, we fabricated amorphous Ga2O3 (a-Ga2O3) metal–semiconductor–metal photodetectors through capacitive coupled plasma assisted magnetron sputtering at room temperature. Substantial enhancement in the responsivity is attained by regulating the capacitance-coupled plasma power during the deposition of a-Ga2O3. The proposed plasma energy generated by capacitive coupled plasma (CCP) effectively improved the disorder of amorphous Ga2O3 films. The results of X-ray photoelectron spectroscopy (XPS) and current-voltage tests demonstrate that the additional plasma introduced during the sputtering effectively adjust the concentration of oxygen vacancy effectively, exhibiting a trade-off effect on the performance of a-Ga2O3 photodetectors. The best overall performance of a-Ga2O3 photodetectors exhibits a high responsivity of 30.59 A/W, a low dark current of 4.18 × 10−11, and a decay time of 0.12 s. Our results demonstrate that the introduction of capacitive coupled plasma during deposition could be a potential approach for modifying the performance of photodetectors.

1. Introduction

Solar-blind photodetectors (SBPDs) based on wide-bandgap (WBG) semiconductors have received extensive attention owing to their compact size, being entirely solid-state, intrinsic solar-blindness, high radiation hardness, and other notable advantages [1,2]. Among many WBG semiconductors, Ga2O3 possesses suitable bandgaps of 4.5–5.3 eV without any complex bandgap engineering, making it an ideal material for the preparation of solar-blind photodetectors [3,4,5]. However, defects such as oxygen vacancies and structural dislocations, present both within the material and on its surface, invariably impair the performance of Ga2O3 SBPDs [6]. To achieve detection performance enhancement, many effective methods boosting the performance of a-Ga2O3 PDs have been reported, including but not limited to increasing oxygen concentration during the deposition of Ga2O3 [7], annealing under oxygen conditions [8], plasma treatment [9], using two-dimensional (2D) materials [10], and dielectric passivation [11]. Plasma treatment is widely recognized as an effective method for tailoring the surface properties of various semiconductor materials, such as ZnO, GaN [12,13], and Ga2O3 [14,15]. However, plasma treatment can only affect the surface of materials; there is a limited effect on the inside of the material [16]. Hence, introducing plasma into the deposition process of Ga2O3 holds the potential to efficiently alter the oxygen vacancy profile across the entire thin film. Capacitance coupled plasma (CCP) technology has effectively improved the quality of thin films in chemical vapor deposition (CVD) and pulsed laser deposition (PLD). Until now, capacitive coupled plasma-assisted magnetron sputtering has not been used in the preparation of high-performance Ga2O3-based PDs.
In this paper, we introduce additional plasma in the sputtering process of Ga2O3 thin films through the radio frequency bias (RF-bias) power supply connected with a matching network and the corresponding metal–semiconductor–metal (MSM) solar-blind UV photodetectors prepared. The influences of RF-bias power on the morphology and optical response of the amorphous Ga2O3 films have been systematically investigated. A series of different plasma powers effectively regulate the concentration of oxygen vacancy in amorphous Ga2O3, which exhibits a trade-off effect on the performance of Ga2O3 PDs. This work provides a promising way to fabricate high-performance Ga2O3 solar-blind ultraviolet detectors.

2. Materials and Methods

Ga2O3 films were deposited on a c-plane sapphire substrate at room temperature utilizing the customized CCP-assistant magnetron sputtering system (PVD Products Co. Ltd., Wilmington, MA, USA), which is schematically illustrated in Figure 1b. A 4 in. high-purity (99.99%) Ga2O3 ceramic target was used. Before deposition, the vacuum system achieved a base pressure of 1 × 10−7 torr. Subsequently, 20 sccm high-purity argon (Ar) sputtering gas was introduced into the chamber and maintained at a pressure of 5 mtorr. The thickness of the Ga2O3 film deposited on the sapphire substrate was controlled to about 300 nm, employing a sputtering power of 200 W. During the deposition of the Ga2O3 films, the CCP RF-bias power was fixed at 0 W, 20 W, 40 W, and 60 W. The corresponding samples were labeled S0, S20, S40, and S60, respectively. Finally, 100 nm thick interdigital Au electrodes were deposited on the Ga2O3 films via vacuum evaporation to form a Ga2O3 MSM photodetector, where the length of the interfinger electrode was 900 μm, the electrode width was 80 μm, and the spacing of the electrode fingers was 100 μm. A microscopy image of the Ga2O3 photodetectors is shown in Figure 1a. The photoelectric performances of Ga2O3 photodetectors were measured using Keithley 2636B. A 150 W UV-enhanced Xe lamp (GLORIA-X150A, Zolix, Beijing, China), in conjunction with a monochromator (Omni-λ3047i, Zolix), served as the light source. The transmittance spectrum was measured using a Shimadzu UV-1700 spectrophotometer (Shimadzu, Tokyo, Japan). To investigate the material morphology, X-ray diffraction (XRD) measurements (TTRIII, Rigaku, Tokyo, Japan) were conducted with a scanning range of 10–80°. The chemical status of the thin films was analyzed via X-ray photoelectrons (XPS, Nexsa, Thermo Scientific, Waltham, MA, USA), with all the XPS data calibrated based on the C1s binding energy at 284.8 eV.

3. Results and Discussion

The XRD patterns of S0–S60 deposited on sapphire substrates at different RF-bias power were measured and are shown in Figure 2a. The diffraction peaks at 2θ = 21.0° and 41.7° correspond to the (0003) and (0006) plants of c-plant sapphire. The peak at 2θ = 37.5° (marked with an asterisk) represents the Kβ line associated with the 00.6 Bragg reflection [17]. Beyond that, no other observable diffraction peaks can be found in Figure 2a, indicating the amorphous feature of these four samples. Atomic force microscopy (AFM) topographies of the a-Ga2O3 film are shown in Figure 2b. The images indicate that the a-Ga2O3 film without RF-bias exhibits numerous protrusions. As the RF-bias power increases, both the number and size of these protrusions decrease. The roughness of the a-Ga2O3 film without RF-bias shows a higher roughness value of Rrms ~0.98 nm. As RF-bias power increases from 0 W to 60 W, the Rrms significantly decreases from 0.98 nm to 0.13 nm. While the alternative electric field is applied on the substrate through the RF-power supply and match network, a DC self-bias potential difference builds up at the surface of the substrate. This DC potential difference accelerates positively charged ions, which then collide with the atoms sputtered from the targets, transferring essential energy for the structural modification of the sputtered particles. Consequently, the a-Ga2O3 films deposited with RF-bias power exhibit smooth and featureless surfaces.
Figure 3a presents the optical transmittance spectra of Ga2O3 samples deposited under various substrate RF-bias powers ranging from 0 W to 60 W. A distinct absorption edge at approximately 280 nm demonstrates the high visible light transmittance and selective absorption against solar-blind light of Ga2O3. The Tauc relation was used to determine the Eg of Ga2O3 films by the linear extrapolation of the absorption edge, which is show in Figure 3b. The values of Eg were extracted using Equation (1) [18]:
α h ν = A ( h ν − E g ) 1 / 2
where α is the absorption coefficient, hν is the photon energy, and A is a constant. In Figure 3b, the plot illustrates the variation in Eg as a function of the RF-bias powers, revealing values close to 5.05 eV with a minor variation within 0.1 eV. In addition, the Urbach tail (EU) serves as a crucial indicator for analyzing the structures of amorphous semiconductors, and it is usually associated with the structural disorder of amorphous semiconductors. Greater disorder in the structure corresponds to a broader local electron band tail within the pseudogap, leading to an increase in the magnitude of EU [19], which is described by
ln ⁡ α = h ν / E U + ln ⁡ A
where A is a constant. Accordingly, EU can be extracted from the inverse of the slope based on a linear fit for a plot of lnα versus hν. As shown in Figure 3c, the EU of Ga2O3 films without RF-bias is 299 meV. When the RF-bias power adds to 20 W, EU is slightly reduced to 290 meV, and then increased with the RF-bias power, reaching 447 meV at 60 W. A larger EU indicates a higher density of structural disorder and defects [20]. Thus, the application of 20 W plasma power slightly reduces the EU, suggesting that an appropriate plasma power can improve the structural disturbances. During the plasma-assisted deposition of Ga2O3, the high-energy plasma could provide a specific energy for the sputtered particles, and it would be helpful for the atoms move to a stable position, thereby promoting a more orderly structure of the films. However, the elevated ion’s bombardment energy can disrupt the chemical bonds within the film, resulting in the formation of additional defects and consequently inducing increased disorder in the Ga2O3 film.
The photoelectric performance was also measured both in the dark and under 254 nm DUV radiation, as shown in Figure 4a,b, respectively. It is obvious that the photoelectric characteristics of the a-Ga2O3 PDs are critically sensitive to the RF-bias power. The a-Ga2O3 PDs without RF-bias exhibited the lowest photocurrent (Iphoto), which was only 1.12 × 10−8 A at a bias voltage of 20 V. Then, the Iphoto continues to rise with the augmentation of RF-bias power, reaching values from 1.20 × 10−6 A to 8.64 × 10−6 A. The variation trend in the dark current (Idark) is similar to that of Iphoto, with Idark experiencing a significant boost following the application of substrate RF-bias, and the current continues to escalate with the increasing RF-bias power [21]. The responsivity (R) of the device was calculated using Equation (3) [22] and is shown in Figure 4c:
R = ( I p h o t o − I d a r k ) / P λ S
where Pλ is the irradiation power density, and S is the effective illuminated area. Furthermore, EQE is defined as [21]
E Q E = R h c / q λ
where h is Planck’s constant, c is the velocity of light, q is the elementary charge, and λ is the wavelength of the incident light. In addition, detectivity (D*) is another key figure of merit, which was employed to evaluate the ability of the photodetector to sense light signals from noise. It is usually calculated using the following formula [23]:
D * = R S / 2 q I dark a r k
The highest D* appears at an RF-bias power of 20 W, which is attributed to high responsivity, and at a Iphoto/Idark of S20. The performances of all a-Ga2O3 photodetectors with and without substrate RF-bias during sputtering are summarized in Table 1. Although the S40 and S60 have a larger Iphoto compared with S20, the similarly larger Idark implies that because of the larger background noise in these two PDs, the D* of the PDs deteriorates after the RF-bias power exceeds 20 W. The response properties of S0–S60 are shown in Figure 4d. The peak response of S0–S60 slightly red-shift from 248 nm (S0) to 254 nm (S20) and 260 nm (S40 and S60), all in the solar-blind region, indicating the good wavelength selectivity of the Ga2O3 PDs.
In general, the concentration of oxygen vacancies in Ga2O3 films is a key factor affecting the performance of Ga2O3-based photodetectors [8,16]; therefore, the X-ray photoelectron spectroscopy (XPS) measurement was performed on the samples of S0–S60. As shown in Figure 5a, the O1s core-level spectrums were deconvoluted into three components based on Gaussian fitting analysis: (OI) 530.6 eV, which corresponds to the Ga-O bond in Ga2O3 film; (OII) 531.2 eV, attributed to the O2− ions in an oxygen-deficient region, which is generally considered to be the oxygen vacancies in Ga2O3; and (OIII) 532.2 eV, usually related to chemisorbed species on the surface of the film [15,24]. Generally, the intensity ratios of OII/(OI + OII) are used to reflect the density of oxygen vacancies in the film, and the oxygen vacancies of S0–S60 were determined to be 33.35%, 34.47%, 40.18%, and 43.44%, respectively. The spectral analysis of Ga2p3/2 reveals a similar trend. It can be seen from Figure 5b that the intensity area ratio of Ga3+/(Ga+ + Ga3+) of S0–S60 exhibits a continuous decrease from 63.52% to 49.21%, indicating a significant increase in the concentration of Vo. Clearly, the introduction of substrate plasma and the application of a higher RF-bias power result in a substantial increase in the concentration of Vo within the Ga2O3 film. According to a previous report [25], the deep-level neutral oxygen vacancies (Vo) can be excited by illumination, thus generating ionized oxygen vacancies (Vo2+) and the photogenerated electron that excites the conduction band. Due to the existence of energy barriers, Vo2+ does not immediately recombine with photogenerated electrons, resulting in a long lifetime of Vo2+ and photogenerated electrons. The longer the lifetime of free photogenerated electrons, the larger the gain and responsivity, as well as the longer photocurrent rise and decay times. Consequently, the introduction of plasma could enhance the device performance by effectively modifying the concentration of oxygen vacancies in Ga2O3. During the sputtering process, ion bombardment can peel off the molecules with weak bonding, resulting in the destruction of some Ga-O bonds and the increase in Vo within the film.
Response time is another important parameter for photodetectors, so a time-dependent response test of a-Ga2O3 photodetectors was performed at 20 V bias to investigate the PDs’ response speed and repeatability. The normalized transient responses of S0–S60 are shown in Figure 6a, for which 254 nm UV light was periodically on/off during the test. The rise time (τr) is defined as the time required for the current to transform from 10% of the peak value to 90%, and the decay time (τd) is defined as the time required for the current to transform from 90% of the peak value to 10% [15]. The τr/τd of these four samples were 1.49 s/0.24 s, 1.15 s/0.12 s, 3.13 s/6.05 s, and 4.99 s/>10 s, respectively. Notably, an appropriate RF-bias power contributes to the improvement in response speed, potentially linked to the earlier-discussed enhancement in the Ga2O3 structure. However, the τr/τd experiences a significant increase with the further elevation of RF-bias power. In addition, it can be seen that S0 has no significant persistent photoconductivity (PPC) effect after introducing the substrate RF-bias in the magnetron sputtering process; only S20 still shows the sharp falling edge, and S40 and S60 both exhibit severe PPC phenomenon. Overall, in comparison with other reported Ga2O3 PDs, as listed in Table 2, both PDs in this work exhibited faster response and recovery. According to the XPS results, there are higher concentrations of oxygen vacancy in S40 and S60. Generally, the PPC phenomenon in oxide semiconductors is attributed to the increase in oxygen vacancy [26]. Upon turning off the light, conduction band electrons and valence band holes undergo rapid recombination [27,28]. However, photogenerated carriers captured by deep-level Vo will gradually release, causing a gradual decline in the device current, resulting in the phenomena of PPC [26]. A repetition test of the four samples was conducted, and the results are shown in Figure 6b. All Ga2O3 photodetectors exhibited good cyclicity during the measurement of their time-dependent response, demonstrating excellent reliability.

4. Conclusions

In summary, amorphous Ga2O3 solar-blind photodetectors were fabricated with CCP-assistant magnetron sputtering at room temperature. The effects of the RF-bias power on the morphology of Ga2O3 films and the performances of photodetectors were investigated. Through increasing the RF power from 0 W to 60 W in the sputtering process, the responsivity exhibited a robust improvement, while the response speed continued to slow down. Appropriate RF power contributes to the overall enhancement of photodetector performance. According to the XPS results, the increase in oxygen vacancy concentration caused by plasma is the reason for the change in device performance. The best overall performance of a-Ga2O3 photodetectors fabricated at 20 W exhibits a high responsivity of 30.59 A/W, a low dark current of 4.18 × 10−11 A, and a decay time of 0.12 s. This research prepared a high-performance a-Ga2O3 based photodetector at room temperature and provided a promising way to balance the device performance.

Author Contributions

Conceptualization, Y.L.; methodology, Y.L.; software, Y.L. and C.P.; validation, C.P. and Y.L.; formal analysis, Y.L., C.Y. and C.L.; investigation, Y.L. and Y.C.; resources, Y.Z.; data curation, J.G.; writing—original draft preparation, Y.L.; writing—review and editing, Y.L. and Y.Z; supervision, Y.Z. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the National Natural Science Foundation of China (No. 61675089).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Data are contained within the article.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. (a) Schematic of Ga2O3 MSM photodetector. (b) The microscopy image of the Ga2O3 photodetectors (c) Schematic of CCP-assistant magnetron sputtering system.
Figure 1. (a) Schematic of Ga2O3 MSM photodetector. (b) The microscopy image of the Ga2O3 photodetectors (c) Schematic of CCP-assistant magnetron sputtering system.
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Figure 2. (a) XRD patterns of S0–S60 as well as c-sapphire substrate. (b) AFM topographies of a-Ga2O3 under different RF-bias powers.
Figure 2. (a) XRD patterns of S0–S60 as well as c-sapphire substrate. (b) AFM topographies of a-Ga2O3 under different RF-bias powers.
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Figure 3. (a) Transmission spectra, (b) plots of (αhν)2 versus hν, (c) Eg and Eu of Ga2O3 films deposited on c-plane sapphire substrate under different RF-bias powers.
Figure 3. (a) Transmission spectra, (b) plots of (αhν)2 versus hν, (c) Eg and Eu of Ga2O3 films deposited on c-plane sapphire substrate under different RF-bias powers.
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Figure 4. I-V characteristics in log coordinates of S0–S60 (a) in the dark and (b) illuminated with 254 nm UV light. (c) Iphoto/Idark, responsivity, and detectivity plotted as functions of RF-bias power. (d) Normalized spectral response of S0–S60 under 20 V bias.
Figure 4. I-V characteristics in log coordinates of S0–S60 (a) in the dark and (b) illuminated with 254 nm UV light. (c) Iphoto/Idark, responsivity, and detectivity plotted as functions of RF-bias power. (d) Normalized spectral response of S0–S60 under 20 V bias.
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Figure 5. (a) O 1 s and (b) Ga 2p3/2 core-level spectra of amorphous Ga2O3 films deposited under different RF-bias powers.
Figure 5. (a) O 1 s and (b) Ga 2p3/2 core-level spectra of amorphous Ga2O3 films deposited under different RF-bias powers.
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Figure 6. (a) Normalized transient response of S0–S60 photodetectors in one single cycle. (b) Transient response of S0–S60 photodetectors with repetition test.
Figure 6. (a) Normalized transient response of S0–S60 photodetectors in one single cycle. (b) Transient response of S0–S60 photodetectors with repetition test.
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Table 1. The performance parameters of amorphous Ga2O3 photodetectors fabricated under different RF-bias powers a.
Table 1. The performance parameters of amorphous Ga2O3 photodetectors fabricated under different RF-bias powers a.
DevicePowerRD*EQEτrτd
(W)(A/W)(Jones)(%)(s)(s)
S000.348.31 × 10131.66 × 1021.490.24
S202030.596.20 × 10141.49 × 1041.150.12
S4040120.049.32 × 10135.87 × 1043.136.05
S6060160.066.11 × 10137.83 × 1044.99>10
a The bias voltage was 20 V.
Table 2. Performance comparison of the Ga2O3-based solar-blind UV photodetectors prepared using different deposition techniques to date.
Table 2. Performance comparison of the Ga2O3-based solar-blind UV photodetectors prepared using different deposition techniques to date.
Method of DepositionBias (V)R (A/W)D* (Jones)τd (s)Ref.
sputtering2030.596.20 × 10140.12This work
MOCVD1038.829.0 × 10150.50[16]
exfoliation301.683.73 × 10100.24[29]
PA-MBE20170.21.3 × 10142.1[30]
Sol–gel150.0285.41 × 10110.04[31]
PECVD02.6 × 10−46.67 × 10100.013[14]
MOCVD57.9 × 10−2-0.80[32]
sputtering80.786.22 × 10100.154[33]
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Liu, Y.; Peng, C.; Liu, C.; Yu, C.; Guo, J.; Chang, Y.; Zhao, Y. Improved-Performance Amorphous Ga2O3 Photodetectors Fabricated by Capacitive Coupled Plasma-Assistant Magnetron Sputtering. Coatings 2024, 14, 1204. https://doi.org/10.3390/coatings14091204

AMA Style

Liu Y, Peng C, Liu C, Yu C, Guo J, Chang Y, Zhao Y. Improved-Performance Amorphous Ga2O3 Photodetectors Fabricated by Capacitive Coupled Plasma-Assistant Magnetron Sputtering. Coatings. 2024; 14(9):1204. https://doi.org/10.3390/coatings14091204

Chicago/Turabian Style

Liu, Yiming, Chong Peng, Chang Liu, Cong Yu, Jiarui Guo, Yiyang Chang, and Yi Zhao. 2024. "Improved-Performance Amorphous Ga2O3 Photodetectors Fabricated by Capacitive Coupled Plasma-Assistant Magnetron Sputtering" Coatings 14, no. 9: 1204. https://doi.org/10.3390/coatings14091204

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