Article pubs.acs.org/journal/apchd5
Ultrahigh-Responsivity, Rapid-Recovery, Solar-Blind Photodetector Based on Highly Nonstoichiometric Amorphous Gallium Oxide Ling-Xuan Qian,*,†,‡ Ze-Han Wu,†,‡ Yi-Yu Zhang,†,‡ P. T. Lai,§ Xing-Zhao Liu,*,†,‡ and Yan-Rong Li†,‡ †
State Key Laboratory of Electronic Thin Films and Integrated Devices, University of Electronic Science and Technology of China, Chengdu 610054, China ‡ School of Microelectronics and Solid-State Electronics, University of Electronic Science and Technology of China, Chengdu 610054, China § Department of Electrical and Electronic Engineering, The University of Hong Kong, Pokfulam Road, Hong Kong, Hong Kong ABSTRACT: Recently, Ga2O3-based, solar-blind photodetectors (PDs) have been extensively studied for various commercial and military applications. However, to date, studies have focused only on the crystalline phases, especially β-Ga2O3, and the crystalline quality must be carefully controlled because of its strong impact on device characteristics. Based on previous reports, amorphous-semiconductorbased PDs can also be expected to exhibit excellent photodetection characteristics. In this work, amorphous gallium oxide thin films were deposited by radio frequency (RF) magnetron sputtering, and the metal−semiconductor− metal (MSM) PD was fabricated and compared with a βGa2O3 film prepared side-by-side as the control sample. The as-sputtered film possessed a high density of defects, including structural disorders, oxygen vacancies, and likely, dangling bonds, resulting in record-high responsivity (70.26 A/W) for a thin-film-type gallium oxide PD due to a high internal gain and the contribution of extrinsic transitions despite a relatively large dark current. The high sensitivity was further confirmed by a high 250 nm/350 nm rejection ratio exceeding 105, the specific detectivity as large as 1.26 × 1014 Jones, and a cutoff wavelength of 265.5 nm. A rapid recovery (0.10 s) rather than a strong, persistent photoconductivity was observed and attributed to effective surface recombination. Our findings contribute to a more comprehensive understanding of highly nonstoichiometric amorphous gallium oxide thin films and reveal additional pathways for the development of high-performance, solar-blind PDs that are inexpensive, large-area, and suitable for mass production. KEYWORDS: solar-blind photodetector, gallium oxide, responsivity, transient response, sputtering, surface recombination
S
ductivity. Accordingly, gallium oxide-based PDs have been intensively explored in the past few years, with particular focus on the monoclinic phase (β-Ga2O3), the most stable of this material’s five different crystalline structures. In practical applications, thin films are the most popular forms because they can be easily adapted to device fabrication processes. Several techniques have been employed for the growth of βGa2O3 thin films, including molecular beam epitaxy (MBE),7 metal−organic chemical vapor deposition (MOCVD),8 pulsed laser deposition (PLD),9 and the furnace oxidization of GaN.10 However, the solar-blind photodetection characteristics strongly depend on the crystalline quality of the β-Ga2O3 film, which is readily influenced by various factors, including thermal-expansion/lattice match with the substrate, base pressure, growth temperature, deposition rate, and annealing conditions.11 As a result, the fabrication of β-Ga2O3 PDs usually exhibits poor reproducibility, high cost, and low efficiency, and
olar-blind PDs, which sense only deep ultraviolet (DUV) light with wavelengths shorter than 280 nm, are usually very sensitive, even under sun or room illumination due to their extremely low natural background. This feature is an advantage for a wide range of applications, such as flame detection, missile early warning systems, and secure optical communication.1,2 Currently, Si-based photodiodes are the most commonly used type in the commercial market because of their high compatibility with the highly mature silicon processes.3 However, expensive and cumbersome Wood’s optical filters are required because Si is sensitive to infrared, visible, and nearUV lights due to its small bandgap (1.1−1.3 eV).1,2 Therefore, PDs based on wide-bandgap (Eg) semiconductors are regarded as more promising alternatives. Among these materials, gallium oxide, which has an Eg of 4.9 eV,4 is intrinsically suitable for solar-blind photodetection and avoids the complex and unmanageable alloying process required for other widebandgap semiconductors, such as AlGaN5 and ZnMgO.6 In addition, gallium oxide has good chemical and thermal stability, high visible-light transparency, and excellent thermal con© 2017 American Chemical Society
Received: April 5, 2017 Published: August 9, 2017 2203
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thus, is far from satisfying the requirements of mass production. In addition, the performance of β-Ga2O3 PDs, for example, responsivity (R), remains lower than expected, hindering their replacement of currently available commercial products, especially for the detection of extremely weak signals. Very recently, amorphous gallium oxide has also attracted considerable attention because of its exciting potential for applications in various electron and photonic devices, and thus, its material properties have been investigated in depth. For instance, Kumar et al. comprehensively studied the structure, morphology, and optical properties of amorphous and nanocrystalline Ga2O3 thin films.12 Heinemann et al. revealed that amorphous gallium oxide thin films deposited at low temperatures and low oxygen partial pressures by PLD exhibited a high oxygen deficiency; as a result, a defect band was generated below the conductionband minimum (CBM), and the optical Eg was affected.13 Moreover, amorphous Ga2O3 thin films have been successfully employed as electron transport layers in Cu(In,Ga)Se214 and Cu2O15 solar cells. Martin et al. first reported a chemically driven insulator−metal transition16 and the memristive behavior induced by bulk mixed ion electron conduction17 in nonstoichiometric amorphous gallium oxide (a-GaOx). However, despite this progress, no reports have focused on amorphous gallium oxide-based PDs to date, and therefore, their solar-blind photodetection properties remain less understood than those of the well-explored β-Ga2O3 PDs. In fact, amorphous semiconductors have great potential for highperformance photodetection. For example, amorphous Se MSM blue-light PDs have been observed to exhibit high responsivity (0.45 A/W) and high response speed up to 2 kHz.18 Similarly, amorphous Si MSM green-light PDs can achieve a high responsivity of 0.280 A/W while maintaining a low dark-current density of 15 pA/mm.19 In this work, MSM PDs based on amorphous gallium oxide were fabricated and comprehensively investigated. RF magnetron sputtering was used to deposit the film because it has many advantages, such as high degree of freedom on substrate selection, high deposition rate, good film adhesion, and excellent process reproducibility, as compared with other techniques. Additionally, for clear comparison, another MSM PD based on the β-Ga2O3 thin film prepared by plasma-assisted MBE was also fabricated as a control sample. Both devices were subjected to the same fabrication processes and measurement conditions, side-by-side, except for the deposition of gallium oxide. The a-GaOx PD possessed ultrahigh solar-blind photodetection characteristics, including a record-high responsivity of 70.26 A/W for a thin-film-type gallium oxide PD, a large DUVto-UV rejection ratio exceeding 105, a high specific detectivity of 1.26 × 1014 Jones, and a surprisingly rapid recovery within 0.10 s, despite a relatively large dark current (338.6 pA at 10 V). The inner mechanisms were discussed in depth based on the analysis of the material properties. Together with the simple fabrication process, these results suggest that a-GaOx MSM PD is a promising candidate for highly sensitive, solar-blind photodetection.
Figure 1. (a) Schematic diagram and (b) photograph of a typical gallium-oxide MSM PD fabricated on a c-plane sapphire substrate.
Figure 2. (a) XRD patterns and (b) HR-TEM bright-field images of the gallium oxide films deposited by RF magnetron sputtering and plasma-assisted MBE, respectively.
film, three clear peaks were located at 18.88°, 38.26°, and 58.96°, but not from the sapphire substrate. All of these peaks were attributed to β-Ga2O3 (2̅01) and the higher-order diffractions (JCPDS CARD No. 43−1012), which revealed a single-phase (20̅ 1)-oriented crystalline formation resulting from the similar oxygen atom arrangements of the sapphire c-plane and the β-Ga2O3 (2̅01) plane. In contrast, no peak appeared for the sputtering-deposited gallium oxide, except for the sapphire substrate. This finding confirmed the presence of the amorphous phase resulting from the lower substrate-heating temperature, higher deposition rate (200 nm per hour), and relatively poor vacuum environment of sputtering as compared with MBE. The HR-TEM bright-field images of gallium oxide films deposited by the two techniques are displayed in Figure 2b. The sputtering-prepared gallium oxide film did not exhibit any crystalline patterns. In contrast, the crystalline lattice of the MBE-grown film was highly aligned in the growth direction, and the d-spacing was 4.64 Å, corresponding to the β-Ga2O3 (2̅01) plane. This result confirms the findings of the XRD analysis. Figure 3a presents the AFM 3D surface images of the gallium oxide films deposited by the two techniques. A more obvious nanoparticulate morphology was observed on the surface of the MBE-deposited film, where the conical-shaped grains were uniformly distributed. In contrast, no crystalline grains emerged on the surface of the sputtering-deposited film. This result is in good agreement with the XRD data, further confirming that the as-deposited gallium oxide film obtained by sputtering was amorphous. Moreover, a larger root-mean-
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RESULTS AND DISCUSSION Figure 1a presents a schematic diagram of the gallium-oxide MSM PD fabricated on a c-plane sapphire substrate and a photograph of a typical PD, and the sample fabrication and measurement will be introduced in details later. Figure 2a exhibits the out-of-plane XRD patterns of the sputtering- and MBE-deposited gallium oxide films. For the MBE-deposited 2204
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and carbonate species chemisorbed on the film surface.20 The peak area is proportional to the intensity of the corresponding component. Accordingly, the intensity ratio of OII/(OI + OII) reflects the density of oxygen vacancies and was determined to be 67.4% and 31.6% for the prepared amorphous and crystalline gallium oxide films, respectively. Clearly, the sputtering-deposited film contained more than twice the oxygen vacancies than the MBE-grown one. Moreover, these oxygen-deficient defects were located in more than half of the regions containing sputtering-deposited a-GaOx film. The O 1s peak position of the a-GaOx film (located at 531.3 eV) shifted more than 0.6 eV to a higher binding energy relative to that of the β-Ga2O3 film (located at 530.7 eV), which also suggested a much higher oxygen deficiency. The XPS Ga 2p3/2 core-level spectra were collected, as shown in Figure 4b, to further confirm the oxygen deficiency. Similarly, the Ga 2p3/2 peak of each sample could be fit using two components centered at 1117.7 and 1118.3 eV, which were associated with the two gallium oxidation states of Ga1+ (Ga2O) and Ga3+ (Ga2O3), respectively.21,22 As a result, the percentages of Ga2O species in the sputtering- and MBE-deposited gallium oxide films were 69.7% and 33.2%, respectively. A clear shift of more than 0.4 eV to a lower binding energy was also observed for the Ga 2p3/2 peak values. These results were consistent with the analysis of the O 1s spectra, further demonstrating the poorer stoichiometry of the sputtering-deposited a-Ga2O3 film. According to the O 1s and Ga 2p3/2 peak intensities, the O/ Ga ratio of the sputtering-deposited a-GaOx film was normalized to 0.83, which was far from the stoichiometric value of 1.5. Figure 5a shows the optical transmission spectra of the prepared a-GaOx and β-Ga2O3 films. Compared to the β-Ga2O3 film, the a-GaOx film exhibited lower transmittance in the measured wavelength range and a flatter absorption edge, which was attributed to its amorphous structure as revealed by XRD analysis and the presence of low valence species (i.e., Ga2O) as supported by XPS data.9,23 However, the a-GaOx film still exhibited good transparency in the medium-wave UV (280− 320 nm), long-wave UV (320−400 nm), and visible (400−760
Figure 3. (a) AFM three-dimensional (3D) surface images and (b) top-view FE-SEM images of the gallium oxide films deposited by RF magnetron sputtering and plasma-assisted MBE, respectively.
square (RMS) surface roughness value (2.102 nm) was achieved for the sputtering-deposited film compared to that of the MBE-deposited film (0.622 nm), indicating that more surface defect states might be generated during the sputtering process. Figure 3b depicts the top-view FE-SEM images of the prepared gallium oxide films, in which the sputtering-prepared film had larger surface particles with a more inhomogeneous distribution than the MBE-grown film, further confirming the poor surface morphology of the former. Figure 4a displays the high-resolution XPS spectra of the O 1s core level of our prepared gallium oxide films. All the binding energies were corrected by referencing the C 1s line at 284.6 eV and considering the sample-charging effect. For each sample, the O 1s core-level spectrum could be resolved into three separate components based on Gaussian fitting analysis; these components were centered at (1) 530.6 eV (OI), related to the O2− ions in the regions without oxygen vacancy (Vo); (2) 531.2 eV (OII), characterizing the O2− ions in the oxygen-deficient regions; and (3) 532.1 eV (OIII), corresponding to the hydroxyl
Figure 4. (a) XPS O 1s and (b) Ga 2p3/2 core-level spectra of the gallium oxide films deposited by RF magnetron sputtering (red) and plasmaassisted MBE (blue), respectively. 2205
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Figure 5. (a) Transmission spectra and (b) logarithm dependence of the absorption coefficient on the photon energy for the a-GaOx (red) and βGa2O3 (blue) thin films.
Figure 6. (a) I−V characteristics in the dark and under 254 nm DUV illumination on a semilogarithmic scale, (b) Idark−V characteristics on a linear scale, (c) spectral response on a semilogarithmic scale, and (d) normalized spectral response of the MSM PDs based on a-GaOx (red) and β-Ga2O3 (blue) thin films. The inset of (d) shows the cutoff wavelengths of the two PDs.
the crystalline one.25 These structural disorders might generate localized tail states above the VBM, thereby extending the valence band into the forbidden gap. Moreover, it can lead to an exponentially decaying tail of the absorption coefficient toward lower photon energies, as shown in the insets of Figure 5a. It should be noted that the CBM is related to the metal s orbitals, which are isotropic, and, thus, is almost insensitive to structural disorder.25 The width of the exponentially decaying tail in the absorption spectrum is often called the Urbach energy (EU), which is described by
nm) spectral regions with transmittance values between 70% and 80%, indicating the potential of this material for achieving solar-blind photodetection. As shown in the insets, the bandgaps of the samples were determined by the Tauc method, that is, extrapolating the linear region of the incident photon energy (hν) versus (αhν)2 to the horizontal axis for the crystalline material and plotting hν versus (αhν)1/2 for the amorphous film.24 The absorption coefficient α was evaluated using the following equation: α = 1/t ln(1/T )
(1)
ln(α) = hν /E U + ln(A)
where t is the film thickness and T is the film transmittance. Consequently, the bandgaps of the a-GaOx and β-Ga2O3 films were determined to be 4.83 and 4.92 eV, respectively, in good agreement with previously reported values.9 The bandgap of the a-GaOx film was slightly smaller than that of the β-Ga2O3 film. In general, the valence-band maximum (VBM) of n-type metal-oxide semiconductors consists of antibonding oxygen p orbitals, which possess random orientations and, accordingly, more bonding angle tilts for the amorphous phase relative to
(2)
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 ln(α) versus hν.26,27 As shown in Figure 5b, the EU values of a-GaOx and β-Ga2O3 films were determined to be 335.91 and 98.27 meV, respectively. a-GaOx had a longer valence band tail and, accordingly, more localized tail states above the VBM than βGa2O3. These results verified our proposed model, confirming 2206
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Figure 7. Schematic energy band diagrams of the MSM PDs based on a-GaOx and β-Ga2O3 thin films: (a) in dark and (b) under DUV illumination. Note that the exact CBM/VBM location as well as the band bending for each device have not been identified in this work.
that the relatively small bandgap of the a-GaOx film is attributed to the presence of high-density structural disorder. Figure 6a presents the semilogarithmic I−V plots of a-GaOx and β-Ga2O3 PDs characterized in the dark and under UV illumination. The dark current (Idark) of the a-GaOx PD was much larger than that of the β-Ga2O3 PD, with differences of 1−2 orders of magnitude under the same bias voltage. For example, the Idark values of both PDs under the bias voltage of 10 V were 338.6 and 9.7 pA, respectively. As shown in Figure 6b, the Idark−V characteristics were also analyzed in a linear coordinate system, and symmetric nonlinear exponential and strict linear relations were observed for the a-GaOx and βGa2O3 PDs, respectively. Thus, homogeneous back-to-back ohmic (i.e., corresponding to photoconductive operation) contacts were formed at the metal/a-GaOx interface rather than Schottky (i.e., corresponding to photovoltaic operation) barriers between the metal and β-Ga2O3. To explore the origin of the ohmic contacts and the large dark current, the schematic energy band diagram of the a-GaOx MSM PD is illustrated in Figure 7a. Since oxygen vacancies act as donor states in gallium oxide, an a-GaOx thin film with a high density of oxygen vacancies has a high intrinsic carrier concentration, which can lower the effective barrier height at the metal/a-GaOx interface relative to that of the metal/β-Ga2O3 interface and facilitate the direct tunneling of electrons at the reverse-biased Schottky barrier (process 1). In contrast, the sputtering-deposited aGaOx film exhibited low crystallinity, rough surface, and poor stoichiometry, which could generate a large number of defects, including structural disorders, dangling bonds, and oxygen vacancies. These defects, especially the deep-level ones, might act as efficient trapping states and promote trap-assisted tunneling.13,28 Indeed, the presence of traps allows electrons to tunnel through the Schottky barrier to unoccupied states (process 2), probably with the assistance of thermal excitation, and then through the remaining barrier (process 3). Note that both the interface states and bulk states within tunneling distance from the interface can contribute to these processes. As a result, the current transport in an a-GaOx PD is dominated by field emission and thermionic field emission rather than thermionic emission (process 4), resulting in the formation of an ohmic contact and a large dark current.29,30 Additionally, the high electrical conductivity of the a-GaOx thin film, which results from the high intrinsic carrier concentration, also causes a relatively large dark current and the formation of ohmic contact.30
After exposure to 254 nm and 70.5-μW/cm2 illumination, both PDs exhibited considerable photoresponses with significant current increases, as shown in Figure 6a. Under the same bias voltage, a larger photocurrent (Iphoto) was observed for the a-GaOx PD. Figure 6c shows the spectral responses of the aGaOx and β-Ga2O3 PDs under a 10-V bias plotted on a semilogarithmic scale, whereas Figure 6d presents the normalized spectra on a linear scale. The peak responses of the two PDs both occur at 250 nm with responsivities of 70.26 and 4.21 A/W. This high responsivity of the a-GaOx PD can be ascribed to the internal gain (G) mechanism and the trapping of minority carriers, which often exist in MSM PDs.28 G can be estimated: G = R ℏc /ηqλ
(3)
where ℏ is the Planck’s constant, c is the velocity of light, η is the quantum efficiency, and λ is the wavelength of the incident light.31 It was assumed that the incident photons were completely absorbed; that is, η = 100%. The corresponding G values were 343.66 and 20.59 for the a-GaOx and β-Ga2O3 PDs, respectively. Taking the n-type semiconductor as an example, the trapping of photogenerated holes induces the sweep-out and reinjection of electrons to maintain charge neutrality, leading to the generation of multiple electrons per collected photon and the subsequent introduction of internal gain.28 In our case, electron−hole pairs were generated by illumination excitation with photon energies above the bandgap of the a-GaOx thin film, that is, the intrinsic transition (band-toband, process 5) and separated by the electric field in the depletion region, as shown in Figure 7b. When these photogenerated holes drifted toward the metal/a-GaOx interface, many were trapped (process 7) because of the high density of defects in the a-GaOx thin film, leading to a large internal gain. These defects were most likely structural disorders and gallium−oxygen vacancy pairs, which can act as acceptor states in gallium oxide.13,32 In addition, the contribution of the extrinsic transitions from the valence-band tail states and/or other defect levels to the conduction band (process 6 in Figure 7b) cannot be ignored.33,34 The rejection ratio (R250/R350), which is defined as the ratio of the responsivity values at 250 and 350 nm, of the a-GaOx PD was 5 orders of magnitude greater than those of our samples and many other previously reported β-Ga2O3 PDs. This excellent wavelength selectivity helps guarantee true, highly sensitive solar-blind photodetection. As shown in Figure 6d, the -3-dB cutoff wavelength of the a-GaOx PD (265.5 nm) was 2207
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Table 1. Performance Comparison among Solar-Blind PDs Based on Gallium-Oxide Thin Films with Various Crystalline Statesa
a
material
growth
Idark (pA)
R (A/W)
R250/R350
τr1/τr2 (s)
τd1/τd2 (s)
reference
a-GaOx β-Ga2O3 β-Ga2O3 β-Ga2O3 β-Ga2O3 β-Ga2O3 β-Ga2O3 β-Ga2O3 β-Ga2O3 Si-doped β-Ga2O3 α-Ga2O3
sputtering MBE PLD MBE MBE MBE MBE MOCVD MOCVD MOCVD MBE
338.6 9.7 12@5 V 1.4 × 103 1.28 × 106 300@1 V
70.26 4.21 0.903@5 V 0.037
1.15 × 105 1.61 × 104 7.87 × 103
0.41/2.04 0.41/1.98
