ZnGa2O4 Films Research Articles

Deep-ultraviolet n-ZnGa2O4/p-GaN heterojunction photodetector fabricated by pulsed laser deposition.

Zinc gallium oxide (ZnGa2O4) has attracted considerable interest in deep-ultraviolet photodetectors, due to the ultrawide bandgap, high transmittance in the ultraviolet (UV) region, and excellent environmental stability. In this study, ZnGa2O4 thin films were deposited on p-GaN epi-layers using pulsed laser deposition, resulting in improved crystalline quality. The ZnGa2O4 film exhibited a bandgap of 4.93 eV, calculated through absorption spectra. A heterojunction photodetector (PD) was constructed, demonstrating a rectification effect, an on/off ratio of 12,697 at -5.87 V, a peak responsivity of 14.5 mA/W, and a peak detectivity of 1.14 × 1012 Jones (262 nm, -6 V). The PD exhibited a fast response time (39 ms) and recovery time (30 ms) under 262 nm illumination. The band diagram based on the Anderson model elucidates the photoresponse and carrier transport mechanism. This work paves the way for advancing next-generation optoelectronics.

Preparation of ZnGa2O4-based deep ultraviolet photodetector with high photodetectivity by magnetron sputtering

Ultra-wide bandgap semiconductors are frequently utilized materials in the fabrication of deep ultraviolet photodetectors (DUVPDs). However, it is imperative to enhance both the photoresponsivity and response speed of these detectors. Herein, we deposited high-quality (111)-oriented ZnGa2O4 films with a bandgap of approximately 4.75 eV onto c-plane sapphire (0001) substrates using magnetron sputtering. Our findings illuminate the pivotal role of pressure in shaping their structural properties, chemical compositions, and photoelectric characteristics. Importantly, DUVPDs based on ZnGa2O4 films exhibited a photo/dark current ratio of 1.46 × 104. Furthermore, we observed the highest responsivity to be 72.2 A/W, complemented by a photodetectivity of 4.21 × 1014 Jones, an external quantum efficiency of 4.04 × 104 %, and rise and decay times of 2.32 s and 0.055 s, respectively. This study underscores the commendable photoresponsivity and rapid response time of the ZnGa2O4 photodetector, positioning it as a compelling candidate for the advancement of deep ultraviolet devices.

Growth optimization, optical, and dielectric properties of heteroepitaxially grown ultrawide-bandgap ZnGa2O4 (111) thin film

Ultrawide bandgap ZnGa2O4 (ZGO) thin films were grown on sapphire (0001) substrates at various growth temperatures with a perspective to investigate the electrical and optical characteristics required for high-power electronic applications. Due to the variation in the vapor pressure of Zn and Ga, severe loss of Zn was observed during pulsed laser deposition, which was solved by using a zinc-rich Zn0.98Ga0.02O target. A pure phase single-crystalline ZGO thin film was obtained at a deposition temperature of 750 °C and an oxygen pressure of 1 × 10−2 Torr. The out-of-plane epitaxial relationship between the sapphire and ZGO thin film was obtained from φ-scan. The x-ray rocking curve of the ZGO thin film grown at 750 °C exhibits a full width at half maximum of ∼0.098°, which indicates a good crystalline phase and quality of the thin film. Core-level x-ray photoelectron spectroscopy of ZGO grown at 750 °C indicated that Zn and Ga were in the 2+ and 3+ oxidation states, respectively, and the atomic ratio of Zn/Ga was estimated to be ∼0.48 from the fitted values of Zn-2p3/2 and Ga-2p3/2. The high-resolution transmission electron microscopy images revealed a sharp interface with the thickness of the ZGO film of ∼265 nm, and the signature of minor secondary phases was observed. The bandgap of the ZGO film at different growth temperatures was calculated from the ultraviolet-diffuse reflectance spectroscopy spectra, and its value was obtained to be ∼5.08 eV for the 750 °C grown sample. The refractive index (n) and the extinction coefficient (k) were determined to be ∼1.94 and 0.023 from the ellipsometric data, respectively, and the real dielectric function (ɛr) was estimated to be ∼6.8 at energy 5 eV. The ultrawide bandgap and dielectric function of ZGO recommend its possible potential applications in deep-ultraviolet optoelectronic devices and high-power electronics.

Comprehensive enhancement of ZnGa2O4-based solar blind photodetector performance by suppressing defects in oxygen-rich atmosphere

ZnGa2O4 is a potential candidate material for next-generation solar-blind UV photodetectors (PDs) owing to its intrinsically wide bandgap and various favorable physical properties. The photoelectric properties of metal oxides are susceptible to oxygen. This study investigated the effect of oxygen flow on the properties of ZnGa2O4 thin films. Furthermore, photovoltaic devices with Au as Schottky contacts were fabricated to compare the performance. The ZnGa2O4 films grown in an oxygen-rich atmosphere exhibit better crystallinity and fewer oxygen vacancies than those grown in an oxygen-poor atmosphere, and the corresponding device demonstrates excellent performance parameter at a bias voltage of 10 V: a responsivity (R), external quantum efficiency (EQE) and detectivity (D*) up to 15.62 A/W, 7626%, ∼1015 Jones, respectively. Additionally, the suppression of oxygen vacancies prevents persistent photoconductivity under low light irradiation. The device possesses satisfactory rise/decay times of 0.125/0.025 s and maintains excellent stability and repeatability. Our study confirms that sputtering in an oxygen-rich atmosphere is an effective strategy for producing ZnGa2O4 thin films that can be applied for fabricating ZnGa2O4 solar-blind PDs with excellent comprehensive performance.

Surface Morphology of Thin ZnGa2O4 Films Obtained by Different Methods

Surface Morphology of Thin ZnGa2O4 Films Obtained by Different Methods

Growth and Characterization of Sputtered ZnO:ZnGa2O4 Dual-Phase Films on Sapphire Substrates for NO Gas-Sensing Applications

Nitric oxide (NO) released from combustion facilities and automobiles adversely affects the respiratory systems and environment. Hence, the development of a NO gas sensor with high sensing response to measure NO gas levels is significantly commendable. Herein, we report the growth of ZnO:ZnGa2O4 dual-phase films on the c-plane sapphire substrates using radio-frequency magnetron sputtering for NO gas sensors. The microstructures and morphology of ZnO:ZnGa2O4 films were systematically investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM), and atomic force microscopy. The XRD patterns revealed the polycrystalline nature of the ZnO:ZnGa2O4 film with (222) preferred orientation. The SEM micrographs showed the presence of grain boundaries in the nanostructures over the surface of the film due to the recrystallization of grains upon annealing the film at 700 °C. This ZnO:ZnGa2O4 film was employed to fabricate the NO gas sensor, which demonstrates significantly high sensor response of 28.6 at 400 °C operating temperature with the quick response time of 8 s and recovery time of 140 s upon exposure to 100 ppm NO gas concentration. The ZnO:ZnGa2O4-based gas sensor is found to be highly selective for NO gas among NO, CO2, and SO2 gases. The gas-sensing mechanism behind the high response of ZnGa2O4-based NO gas sensors is described. The results achieved in this study are particularly important in terms of the development of technologies for the early detection of poisonous and hazardous gases from polluting industries and metropolises, which is crucial for environmental security.

ZnGa2O4 and ZnGa2O4:N thin films applied as sensors for detection of acetaldehyde in ethanol

Acetaldehyde, a byproduct of fermentation, is a highly toxic contaminant. Therefore, it is important to develop cost-effective, fast-response and portable acetaldehyde sensors to monitor the formation of this contaminant in the process. Herein, we have evaluated the potential use of ZnGa2O4 and ZnGa2O4:N (referred as ZnGaON) films as vapor sensors of ethanol containing acetaldehyde. Sensors based on ZnGa2O4 films were successfully prepared by PLD on Al2O3 substrates containing interdigitated Pt electrodes. The nitridation was carried out under NH3 gas flow at 700 °C, to obtain ZnGaON films. The prepared sensors presented high sensitivity to acetaldehyde, allowing identification of acetaldehyde in ethanol (300 ppm). This acetaldehyde concentration corresponds to the limit established by Brazialian legislation to sugarcane alcoholic beverages (cachaça). The ZnGaON sensor should be operated at moderate temperature (450 °C), and response time of 43 s and recovery time of 26 s are quite fast. The results strongly suggest that ZnGaON may be considered a potential candidate for the development of an efficient screening test for a highly sensitive and selective identification of acetaldehyde in beverages, foods, or fuel.

Effect of thermal annealing on the structural and optical properties of ZnGa2O4 films deposited on sapphire by magnetron sputtering

ZnGa2O4 films were prepared on α-Al2O3 (0006) wafer at room temperature using magnetron sputtering. The impact of thermal annealing on the crystallization quality, epitaxial relationship and optical property of the films were studied in detail. XRD results showed that the crystalline quality of film was significantly improved by annealing, and the 500 °C-annealed sample presented a monocrystalline film with the best crystallization quality. The results of XRD and HRTEM revealed an epitaxial relationship of ZnGa2O4 (111)//α-Al2O3 (0006) with ZnGa2O4 [011̅]//α-Al2O3 <1̅100 > . The average transmittance of the ZnGa2O4 film in the visible region surpassed 83.1 %. The optical bandgaps of the ZnGa2O4 films annealed at 400, 500 and 600 °C were calculated to be 5.13 eV, 5.12 eV and 5.15 eV, respectively, which were much higher than the value of the as-deposited one (4.65 eV).

Influence of Al doping on crystal structure, optical, and photoluminescence characteristics of ZnGa2O4 films

Al-doped ZnGa2O4 (ZAGO) thin film phosphors were grown on the c-plane sapphire substrates by co-sputtering of ceramic ZnGa2O4 (ZGO) and metallic Al targets with the Al concentration in the range of 0–5.35%. A systematic investigation on the effects of Al dopant contents on the crystal structure and optical characteristics of ZAGO films has been conducted. First principle calculations were performed to investigate the preferable site of Al atoms in ZAGO supercell in detail. The high-crystalline nature of 1.65% ZAGO film was evidenced by X-ray diffraction patterns and transmission electron microscopy. Al doping revealed the enhanced bandgap of 5.32 eV for ZAGO films. The X-ray photoelectron spectroscopy was utilized in order to determine the chemical states as well as the stoichiometric Zn/Ga ratio in the ZAGO films. The crystallinity, optical, and photoluminescence (PL) characteristics of the ZAGO films are found strongly influenced by Al doping, O/(Zn + Ga + Al), and Zn/Ga ratio. The PL spectra exhibited the broad bands expanding from 380 to 600 nm with the peak located at 525 nm. The improved PL characteristics of ZAGO phosphors with trace Al (1.65%) demonstrate the promise of a relatively new semiconductor material system ZAGO for high-crystalline thin film phosphors.

Pulsed laser deposition grown non-stoichiometry transferred ZnGa2O4 films for deep-ultraviolet applications

It is generally known that Zn volatilization would result in a serious degradation in film quality and device performance during a non-stoichiometric ZnGa2O4 target ablation. This work highlights the effect of various substrate temperatures and partial oxygen pressures on the microstructural and optoelectronic properties of stoichiometric ZnGa2O4 films. First-principle studies have been performed to calculate the formation energy per atom of pure Ga2O3, ZnGa2O4, and ZnO to understand the mechanism of preparing ZnGa2O4 by pulsed laser deposition, revealing the stability of Ga2O3 > ZnGa2O4 > ZnO under various growth parameters. Sufficient oxygen partial pressure should be introduced to deposit a large amount of ZnO, and then the Ga:Zn ratio can be adjusted to the most suitable 2:1 by controlling the substrate temperature and post-annealing heat treatment. The X-ray diffraction analysis revealed the high crystalline nature of stoichiometric ZnGa2O4 film. The average transmittance of above 80% and wide energy gap of 5.18 eV revealed the potential of stoichiometric ZnGa2O4 film in deep-ultraviolet photodetectors. This stoichiometric ZnGa2O4 film exhibited the photo/dark current ratio of 3.63 × 104 and high responsivity of 1.97 A/W at the bias of 5 V under the illumination wavelength of 220 nm for deep-ultraviolet applications.

Structural and photodetector characteristics of Zn and Al incorporated ZnGa2O4 films via co-sputtering

Zn and Al-incorporated ZnGa2O4 films (ZGO:Zn and ZGO:Al) were deposited on sapphire substrates using radio-frequency magnetron sputtering in this work. The effects of Zn and Al incorporation on the microstructural and optoelectronic properties of ZGO films were systematically investigated and compared with each other. This investigation revealed that incorporation of Al enhanced the characteristics of the ZGO film, whereas incorporation of Zn affected the characteristics of the ZGO film. The annealed ZGO:Al film revealed high crystallinity, average transmittance of >82%, and wide-bandgap of 5.18 eV. First principles calculations were performed to investigate the defects associated with the ZGO:Al structure. The Al incorporated ZGO PD exhibited an optimum metal–semiconductor–metal photodetector achievement with a photo/dark current ratio of ∼104 order and responsivity of 3.01 A/W (at 3 V and 220 nm), demonstrating the responsivity of ZGO PD (0.25 A/W) can be increased up to 12 times by incorporating Al in ZGO films. This enhancement in the characteristics of the ZGO:Al film can be ascribed to the high crystalline nature and the formation of significant amount of oxygen vacancies in this film. These results demonstrate the potential of Al incorporated ZGO films in wide-bandgap devices and deep-ultraviolet applications.

The Effect of Annealing Ambience on the Material and Photodetector Characteristics of Sputtered ZnGa2O4 Films.

Spinel ZnGa2O4 films were grown on c-plane sapphire substrates at the substrate temperature of 400 °C by radio-frequency magnetron sputtering. Post thermal annealing was employed at the annealing temperature of 700 °C in order to enhance their crystal quality. The effect of thermal annealing on the microstructural and optoelectronic properties of ZnGa2O4 films was systematically investigated in various ambiences, such as air, nitrogen, and oxygen. The X-ray diffraction patterns of annealed ZnGa2O4 films showed the crystalline structure to have (111) crystallographic planes. Transmission electron micrographs verified that ZnGa2O4 film annealed under air ambience possesses a quasi-single-crystalline structure. This ZnGa2O4 film annealed under air ambience exhibited a smooth surface, an excellent average transmittance above 82% in the visible region, and a wide bandgap of 5.05 eV. The oxygen vacancies under different annealing ambiences were revealed a substantial impact on the material and photodetector characteristics by X-ray photoelectron spectrum investigations. ZnGa2O4 film exhibits optimal performance as a metal-semiconductor-metal photodetector when annealed under air ambience. Under these conditions, ZnGa2O4 film exhibits a higher photo/dark current ratio of ~104 order, as well as a high responsivity of 2.53 A/W at the bias of 5 V under an incident optical light of 240 nm. These results demonstrate that quasi-single-crystalline ZnGa2O4 films have significant potential in deep-ultraviolet applications.

Application of ZnGa₂O₄:Mn Down-Conversion Layer to Increase the Energy-Conversion Efficiency of Perovskite Solar Cells.

The perovskite solar cell is capable of energy conversion in a wide range of wavelengths, from 300 nm to 800 nm, which includes the entire visible region and portions of the ultraviolet and infrared regions. To increase light transmittance of perovskite solar cells and reduce manufacturing cost of perovskite solar cells, soda-lime glass and transparent conducting oxides, such as indium tin oxide and fluorine-doped tin oxide are mainly used as substrates and light-transmitting electrodes, respectively. However, it is evident from the transmittance of soda-lime glass and transparent conductive oxides measured via UV-Vis spectrometry that they absorb all light near and below 310 nm. In this study, a transparent Mn-doped ZnGa₂O₄ film was fabricated on the incident surface of perovskite solar cells to obtain additional light energy by down-converting 300 nm UV light to 510 nm visible light. We confirmed the improvement of power efficiency by applying a ZnGa₂O₄:Mn down-conversion layer to perovskite solar cells.

A study for the influences of temperatures on ZnGa2O4 films and solar-blind sensing performances

In this work, ZnGa2O4 metal–semiconductor–metal solar-blind ultraviolet (UV) photodetectors were fabricated and discussed by using the radio-frequency (r.f.) magnetron sputtering method with considerations of the influences of growing and measuring temperatures on photoelectric performances in detail. With the increase of growing temperature, the grain size and optical bandgap of the films increased. Ohmic contacts were formed between the films and Ti/Au electrodes for all the samples, and the photoconductive photodetectors shown decent solar-blind photoelectric performances. At 10 V biasing voltage, the devices had fast response speed and large photo-to-dark current ratio of ∼107 at 650 °C. Moreover, we measured the dark current (I dark) and photocurrent (I photo) of these devices in the temperature range from 298 K to 598 K. With the increase of temperature, the I dark increased monotonously, while the I photo first decreased and then increased. Furthermore, this study about the effects of temperatures on the ZnGa2O4 films and photodetectors may well provide a meaningful guide for its relevant developments in the future.

Growth and characterization of co-sputtered Al-doped ZnGa2O4 films for enhancing deep-ultraviolet photoresponse

Al-doped ZnGa2O4 (ZGO) films were deposited on c-plane sapphire substrates by co-sputtering of Al and ZGO targets at a substrate temperature of 400 ℃ and thermally annealed at 800 ℃ to enhance their crystalline quality. In this investigation, the DC power for metallic Al target was varied from 0 to 50 W and its effect on structural, optical, and optoelectronic properties of these films were investigated. After annealing, all films exhibited the crystalline nature with the ZGO phase. Further, this investigation revealed that the 800 ℃ annealed Al-doped ZGO film deposited with the 20 W DC power possessed the highest crystalline quality among other films along with the wide-bandgap of 5.18 eV. The X-ray photoemission spectrum measurements revealed from the Ga 2p3/2 and Al 2p core-level spectra that the Al atoms occupy the octahedral sites, which makes Al─O─Ga bonding in the ZGO network. The metal–semiconductor-metal photodetector with this annealed 20 W Al-doped ZGO film exhibited the photocurrent to the dark current ratio of 3.35 × 104 and high responsivity of 3.01 A/W (at 3 V and 220 nm), which shows the enhancement in device performance about 12 times when compared with that of annealed 0 W Al-doped ZGO photodetector.

Epitaxial Growth and Stoichiometry Control of Ultrawide Bandgap ZnGa2O4 Films by Pulsed Laser Deposition

ZnGa2O4 is a promising semiconductor for developing high-performance deep-ultraviolet photodetectors owing to a number of advantageous fundamental characteristics. However, Zn volatilization during the ZnGa2O4 growth is a widely recognized problem that seriously degrades the film quality and the device performance. In this study, we report the synthesis of epitaxial ZnGa2O4 thin films by pulsed laser deposition using a non-stoichiometric Zn1+xGa2O4 target. It is found that supplementing excessive Zn concentration from the target is highly effective to stabilize stochiometric ZnGa2O4 thin films during the PLD growth. The influence of various growth parameters on the phase formation, crystallinity and surface morphology is systematically investigated. The film growth behavior further impacts the resulting optical absorption and thermal conductivity. The optimized epitaxial ZnGa2O4 film exhibits a full width at half maximum value of 0.6 degree for a 120 nm thickness, a surface roughness of 0.223 nm, a band gap of 4.79 eV and a room-temperature thermal conductivity of 40.137 W/(m⋅K). This study provides insights into synthesizing epitaxial ZnGa2O4 films for high performance optoelectronic devices.

Performance enhancement of a self-powered solar-blind UV photodetector based on ZnGa2O4/Si heterojunction via interface pyroelectric effect

The photodetectors based on the wide bandgap semiconductor (WBS)/Si heterojunction have attracted more and more attention in recent years due to their excellent photoelectric characteristics and easy integration capabilities. In this work, we have demonstrated a self-powered solar-blind ultraviolet (UV) photodetector based on the ZnGa2O4/Si heterojunction. A typical rectification characteristic with a rectification ratio exceeding 103 within ±5 V can be obtained. At 0 V bias, the −3 dB cutoff wavelength of ∼255 nm and the UV-visible rejection ratio of ∼3 × 102 show that the device has excellent self-powered solar-blind UV detection performance. In addition, the responsivity and the response speed of ZnGa2O4/Si heterojunction can be efficiently enhanced by a transient spike current at 0 V bias when turning on and off the 254 nm UV light. The interface pyroelectric effect of the ZnGa2O4 film should be responsible for this transient spike photocurrent phenomenon. Our findings in this work pave a feasible way to realize high-performance WBS/Si heterojunction self-powered solar-blind photodetectors.

Wide bandgap tunability of N-alloyed ZnGa2O4 thin films

ZnGa2O4 films with different nitrogen impurity concentrations were deposited using radio frequency magnetron sputtering in an ammonia (NH3)/Ar gas mixture and were post-annealed at different temperatures, ranging from 600 °C to 900 °C, in NH3 atmosphere. The influence of ammonia partial pressure ratio and nitridation temperature on the microstructure, surface morphology, nitrogen doping profile, and optical properties has been investigated. The optical band gap of ZnGa2O4, under in-situ nitrogen-doping, decreases from 4.6 to 4.1 eV but the primitive ZnGa2O4 crystalline structure does not change. On nitridation, the optical band gap gradually decreases from 4.6 to 2.1 eV and the ZnGa2O4 changes into a zinc gallium oxynitride quaternary alloy structure. Secondary ion mass spectrometry analysis revealed that nitrogen atoms were uniformly distributed in the film. The narrowing, by more than 40%, of the optical band gap is attributed to the hybridization of Zn3d and N2p orbits promote p-d repulsion in the top of the valence band, and the formation of hexagonal wurtzite phase.

ZnGa2O4 deep-ultraviolet photodetector based on Si substrate

Deep-ultraviolet photodetectors with fast response speed are urgently required in flame detection and atmospheric monitoring. A method for reducing oxygen vacancies in ZnGa2O4 films by high-temperature and oxygen thermodynamic conditions is proposed, to increase the response speed of ZnGa2O4/Si heterojunction deep-ultraviolet photodetectors. Here, a ZnGa2O4/Si heterojunction deep-ultraviolet photodetector with excellent performance is fabricated, exhibiting a decay time of ~67 ms, a dark current of 0.027 nA and a photo-to-dark current ratio of ~490. These results indicate that the high-temperature and oxygen-rich annealing method proposed here can provide a reference for the fabrication of ZnGa2O4 deep-ultraviolet photodetectors with excellent performance.

Self-powered solar-blind ZnGa2O4 UV photodetector with ultra-fast response speed

A novel self-powered solar-blind ZnGa2O4 ultraviolet (UV) photodetector has been constructed by fabricating asymmetric pairs of Au Schottky electrodes on the ZnGa2O4 film. The current-voltage curve of the device exhibits an obvious rectifying characteristic with a rectification ratio of 11.3 at ±8 V. At 0 V bias, the peak responsivity of our device at 246 nm is about 22.2 mA/W with an UV–vis rejection ratio of 1.3×104, indicating the excellent self-powered solar-blind UV photodetector performance. More interestingly, the rise time and decay time of the device are only 10 ns and 30 ns, respectively, which are much faster than that of any other previously reported self-powered solar-blind UV photodetectors to the best of our knowledge. The ultra-fast response speed of the device should be attributed to the asymmetric Schottky junctions and the high crystalline quality of our ZnGa2O4 film with low trap density. Our work provides a new method to constructing high-performance self-powered solar-blind UV photodetectors.