ZnGa2O4 Thin Films Research Articles

High-performance and low-power consumption deep UV photodetectors based on MOCVD-grown ZnGa2O4epilayers with high temperature functionality

This work reports on the fabrication of excellent quality deep ultraviolet photodetectors (DUV PDs) based on metalorganic chemical vapor deposition (MOCVD) grown spinel ZnGa2O4 thin films on c-plane sapphire. Herein, The DUV PDs by exhibiting an ultra-low dark current of 6.69 fA and a high photo-to-dark-current ratio (PDCR) of >108 at a low-power operation of 0.2 V. While PDCR reached more than 109 on applied bias of 10 V having extremely low dark current of 37 fA. The detector revealed an ultra-high responsivity of 185 and 3.53 A/W under the exposure of 107 μW/cm2 of 245 nm wavelength at room temperature (RT) and applied bias of 10 and 0.2 V, respectively, and exhibited the record-high detectivity of the order of 1017 Jones. The temperature-dependent operation remained stable across a wide temperature range, from 27 °C to 125 °C, maintaining ultra-low dark current. The nearly constant growth and decay time of DUV PDs throughout the temperature range emphasizes the robustness and reliability of PD. The ideality factor in both DUV and dark conditions nearing unity proves the dominance of the thermionic emission mechanism. Also shows the stable and slight improved performance after a year. Therefore, these findings suggest that ZnGa2O4 is a strong contender for deep UV detection that shows robustness even at high temperatures.

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.

Substrate temperature dependent crystal structure and deep-ultraviolet photodetection of ZnGa2O4 thin films * *Project supported by This work was supported by the National Natural Science Foundation of China (No. 12074044), the Fund of State Key Laboratory of Information Photonics and Optical Communications (IPOC2021ZT05), and the Fundamental Research Funds for the Central Universities (BUPT, 2023ZCJH1).

Zinc gallate (ZnGa2O4) is a promising material for deep-ultraviolet (DUV) photodetectors, owing to its wide bandgap and high transparency. However, the effect of substrate temperature on the structural and optical properties of ZnGa2O4 thin films prepared by magnetron sputtering is not well understood. Here, we report a systematic study of the influence of substrate temperature on the crystal quality, stoichiometry, bandgap, and photodetection performance of ZnGa2O4 thin films deposited on sapphire substrates. We find that the films undergo a phase transition from amorphous to polycrystalline at 300 °C, and then to single crystalline at 500 °C, accompanied by an increase in the bandgap from 4.6 to 4.9 eV. We also fabricate metal-semiconductor–metal photodetectors based on the ZnGa2O4 thin films with Ti/Au electrodes, which exhibit excellent Ohmic contact and high light-to-dark current ratio. The photodetectors show remarkable and stable DUV response, with the highest performance achieved at a substrate temperature of 650 °C. Our results demonstrate the crucial role of substrate temperature in tailoring the crystal structure and DUV photodetection of ZnGa2O4 thin films, and provide a facile route for optimizing their performance.

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.

Formation of quaternary [formula omitted] epilayers driven by thermally induced interdiffusion between spinel [formula omitted] epilayer and [formula omitted] substrate

Zinc aluminogallate, Zn(AlxGa1−x)2O4 (ZAGO), a single-phase spinel structure, offers considerable potential for high-performance electronic devices due to its expansive compositional miscibility range between aluminum (Al) and gallium (Ga). Direct growth of high-quality ZAGO epilayers however remains problematic due to the high volatility of zinc (Zn). This work highlights a novel synthesis process for high-quality epitaxial quaternary ZAGO thin films on sapphire substrates, achieved through thermal annealing of a ZnGa2O4 (ZGO) epilayer on sapphire in an ambient air setting. In-situ annealing x-ray diffraction measurements show that the incorporation of Al in the ZGO epilayer commenced at 850 °C. The Al content (x) in ZAGO epilayer gradually increased up to around 0.45 as the annealing temperature was raised to 1100 °C, which was confirmed by transmission electron microscopy (TEM) and energy dispersive x-ray spectroscopy. X-ray rocking curve measurement revealed a small full width at half maximum value of 0.72 °, indicating the crystal quality preservation of the ZAGO epilayer with a high Al content. However, an epitaxial intermediate β–(AlxGa1−x)2O3 layer (β - AGO) was formed between the ZAGO and sapphire substrate. This is believed to be a consequence of the interdiffusion of Al and Ga between the ZGO thin film and sapphire substrate. Using density functional theory, the substitution cost of Ga in sapphire was determined to be about 0.5 eV lower than substitution cost of Al in ZGO. Motivated by this energetically favorable substitution, a formation mechanism of the ZAGO and AGO layers was proposed. Spectroscopic ellipsometry studies revealed an increase in total thickness of the film from 105.07 nm (ZGO) to 147.97 nm (ZAGO/AGO) after annealing to 1100 °C, which were corroborated using TEM. Furthermore, an observed increase in the direct (indirect) optical bandgap from 5.06 eV (4.7 eV) to 5.72 eV (5.45 eV) with an increasing Al content in the ZAGO layer further underpins the formation of a quaternary ZAGO alloy with a tunable composition.

Enhancing the performance of ZnGa2O4 metal-semiconductor-metal ultraviolet solar-blind photodetectors by surface fluorine plasma sensitization

ZnGa2O4 has a wide direct bandgap and excellent optical properties and is an emerging material for use in solar-blind ultraviolet (UV) photodetectors (PDs). Recently, metal-semiconductor-metal (MSM) based devices have gained popularity because of their low production cost, minimal design, flexible integration, and tunable performance. However, such devices suffer from low sensitivity. Herein, we propose a sensitization engineering strategy that involves surface fluorine plasma (SFP) treatment to enhance the performance of a ZnGa2O4 MSM solar-blind UV PD. Through this strategy oxygen vacancy defects are simultaneously passivated and the ratio of adsorbed oxygen species on the surface of the ZnGa2O4 thin film is changed. These modifications optimize the current under both dark and light conditions. In particular, the dark current has a trade-off relationship between the reduction in the oxygen vacancy concentration and changes in the crystal grain boundaries. As a result, the photocurrent is enhanced because of the increased amount of adsorbed oxygen. The PD prepared using a ZnGa2O4 thin film treated by SFP for 15 min exhibited excellent performance, specifically, it showed a large responsivity (42.22 A/W) and good specific detectivity (5.72 × 1014 Jones). Notably, these values are superior to those of most reported MSM solar-blind PDs. Moreover, the excellent rejection ratio and durability during repeated use demonstrate the benefits of SFP treatment for enhancing operating stability. Our study provides an effective avenue for optimizing the optoelectronic performance of oxide semiconductors.

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.

Assessment of large critical electric field in ultra-wide bandgap p-type spinel ZnGa2O4

The spinel zinc gallate ZnGa2O4 stands out among the emerging ultra-wide bandgap (∼5 eV) semiconductors as the ternary complex oxide with the widest gap where bipolar conductivity has been demonstrated. For power and energy electronics, a fundamental property of the material is its critical electric field () although, for ZnGa2O4, is yet unknown. In this work, highly resistive p-type ZnGa2O4 thin films on sapphire and Si substrates were grown by metal organic chemical vapor deposition to determine both, the remote acceptor concentration and vertical breakdown voltage. Hall Effect measurements confirmed a low carrier concentration at room temperature of ∼1011 cm−3. From vertical metal-semiconductor-metal structures the average E CR has been estimated to be of at least 5.3 MV cm−1, which already is significantly larger than the one of SiC and GaN.

Pulsed laser deposition of ZnGa2O4 thin films on Al2O3 and Si substrates for deep optoelectronic devices applications

Due to the ultra-wideband gap, high transmittance in the ultraviolet (UV) region, and excellent environmental stability, zinc gallium oxide (ZnGa2O4) has attracted considerable interest in deep-ultraviolet photodetectors. Here, ZnGa2O4 thin film was fabricated on different substrates by pulsed laser deposition with a post-annealing process under an oxygen atmosphere. It is found that the substrates have a great impact on the morphology, structure, and crystal quality of thin film. After annealing, the thin film quality has been improved. The metal–semiconductor–metal photodetector shows excellent reproducible characteristics and fast response performance, which demonstrates great potential in next-generation optoelectronic devices.

Preparation and photocatalytic activity of ZnGa2O4-β-Ga2O3 thin films

Photocatalytic zinc gallate thin films were fabricated by aerosol-assisted chemical vapor deposition (AACVD). The photocatalytic enhancement of ZnGa2O4-β-Ga2O3 compared with ZnGa2O4 thin films results from heterojunction interface facilitating charge separation.

Fabrication and performance characterization of deep UV photodetectors based on magnetron sputtered ZnGa2O4 thin films

Based on the broad prospects and application needs of deep-ultraviolet in industrial industry, biological evolution and environmental monitoring, research on the development of high-performance deep-ultraviolet photodetector is urgently desired. ZnGa2O4 is considered as an effective material for deep-ultraviolet detection applications due to its impressive advantages in this field. Herein, the ZnGa2O4 thin films were successfully grown on c-plane sapphire substrates by radio frequency magnetron sputtering technology. The ZnGa2O4 photodetectors based on a metal-semiconductor-metal structure exhibited fast overall response time. At the substrate temperature of 700 °C, the device exhibited excellent performance characteristics with a considerable high photo/dark-current ratio (>105 orders, at 5 V bias), an outstanding responsivity (R = 6.89 A/W, under 255 nm with 5 V bias) and rapid response time (τr = 90 ms, τd = 30 ms). Moreover, the unencapsulated device demonstrated high selectivity for UVC band and continued to work stably without degradation after 3 months of storage. These results indicated that the photoelectric performance of the ZnGa2O4 PDs is sensitive to the substrate temperature and the proper deposition conditions are conducive to the electrical properties of the ZnGa2O4 thin films.

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.

Chromium doped ZnGa2O4 thin films: An X-ray absorption near edge structure (XANES) and X-ray excited optical luminesce (XEOL) study

We report the thermally controlled synthesis of Cr doped ZnGa2O4 thin films and the evolution of their morphology, crystallinity, and optical luminescence tracked with X-ray Absorption Near Edge Structure (XANES) and X-ray Excited Optical Luminescence (XEOL). It is found that the as-prepared ZnGa2O4 thin films doped with/without Cr are amorphous and exhibits no luminescence while annealing induces crystallization which markedly improves crystallinity at higher temperatures. SEM, XRD and O K-edge, Zn and Ga L-edge, and Cr K-edge XANES show disorder to order phase transition upon annealing which is accompanied by the appearance of the N2 line (696 nm) characteristic of the luminescence from Cr-doped ZnGa2O4 where Cr3+ occupies the octahedral Ga3+ site. The N2 line correlates with the crystallinity of the sample in that the higher the annealing temperature, the better the crystallinity and the brighter the N2 line. It is also interesting to note that the optical luminescence from the host ZnGa2O4, which appears in the blue (420 nm), is completely quenched and the energy is transferred to the N2 luminescence in the red (696 nm). These results and their implications are discussed.

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.

Influences of Work Function Changes in NO2 and H2S Adsorption on Pd-Doped ZnGa2O4(111) Thin Films: First-Principles Studies

The work function variations of NO2 and H2S molecules on Pd-adsorbed ZnGa2O4(111) were calculated using first-principle calculations. For the bonding of a nitrogen atom from a single NO2 molecule to a Pd atom, the maximum work function change was +1.37 eV, and for the bonding of two NO2 molecules to a Pd atom, the maximum work function change was +2.37 eV. For H2S adsorption, the maximum work function change was reduced from −0.90 eV to −1.82 eV for bonding sulfur atoms from a single and two H2S molecules to a Pd atom, respectively. Thus, for both NO2 and H2S, the work function change increased with an increase in gas concentration, showing that Pd-decorated ZnGa2O4(111) is a suitable material in NO2/H2S gas detectors.

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.

Reliability study on deep-ultraviolet photodetectors based on ZnGa2O4 epilayers grown by MOCVD

Deep ultra-violate (DUV) photodetectors (PDs) fabricated by ZnGa2O4 thin film with back-to-back Schottky type metal–semiconductor-metal (MSM) structure have been studied and presented a superior performance. To further discuss the reliability of DUV device should be the pivot for the DUV application. We were using an accelerating life test (ALT) for UVC PDs by applied in monitoring sterilization light sources during this research. After ALT for one week, the device showed low reliability with a varying current level, and the change of response time and rejection ratio are enormous. Following the XRD and XPS results, we find out that the reliability issue is caused by the surface water adsorption and ozone compensation. Furthermore, the 45-nm-thick Al2O3 passivation layer was deposited by atomic layer deposition (ALD) for isolating the effect of the atmosphere. It resulted in increasing the stability of the current level and reducing the response time for the DUV PDs.

Heteroepitaxial (111) ZnGa2O4 Thin Films Grown on (00.1) Sapphire by Pulsed Laser Deposition

Heteroepitaxial ZnGa2O4 thin films grown on sapphire substrates would be preferable for fundamental studies on properties of this material for device applications. To achieve near‐stoichiometric ZnGa2O4 epitaxial thin films by pulsed laser deposition (PLD), the severe loss of Zn must be overcome. Herein, the fabrication and characterization of epitaxial (111) ZnGa2O4 thin films grown on (00.1) sapphire substrates by PLD using a Zn0.97Ga0.03O target are reported. A deposition temperature of 750 °C and a laser fluence of 3.5 J cm−2 are suitable for growing near‐stoichiometric ZnGa2O4 film. The in‐plane orientation relationship is identified to be ZnGa2O4//[11.0] Al2O3. A 14.2 nm thick (111) ZnGa2O4 epitaxial thin film with a Zn/Ga atomic ratio of about 0.47 shows a narrow full width at half maximum value for the rocking curve of 0.1° and a direct optical bandgap of 4.9 eV.

Influence of Annealing Temperature on the Properties of ZnGa2O4 Thin Films by Magnetron Sputtering

Zinc gallate (ZnGa2O4) thin films were grown on sapphire (0001) substrate using radio frequency (RF) magnetron sputtering. After the thin film deposition process, the grown ZnGa2O4 was annealed at a temperature ranging from 500 to 900 °C at atmospheric conditions. The average crystallite size of the grown ZnGa2O4 thin films increased from 11.94 to 27.05 nm as the annealing temperature rose from 500 to 900 °C. Excess Ga released from ZnGa2O4 during thermal annealing treatment resulted in the appearance of a Ga2O3 phase. High-resolution transmission electron microscope image analysis revealed that the preferential crystallographic orientation of the well-arranged, quasi-single-crystalline ZnGa2O4 (111) plane lattice fringes were formed after the thermal annealing process. The effect of crystallite sizes and lattice strain on the width of the X-ray diffraction peak of the annealed ZnGa2O4 thin films were investigated using Williamson-Hall analysis. The results indicate that the crystalline quality of the deposited ZnGa2O4 thin film improved at higher annealing temperatures.

Deep-ultraviolet Schottky photodetectors with high deep-ultraviolet/visible rejection based on a ZnGa2O4 thin film

A single-crystalline ZnGa2O4 epilayer was successfully grown on a c-plane (0001) sapphire substrate through metalorganic chemical vapor deposition. A metal-semiconductor-metal Schottky deep-ultraviolet (DUV) photodetector based on a ZnGa2O4 thin film was fabricated through a simple process of E-gun evaporation and thermal annealing. At a bias of 10 V, the ZnGa2O4 photodetectors exhibited excellent performance characteristics such as an extremely low dark current (0.86 pA), a responsivity of 0.46 A/W under 230-nm DUV, a high photo/dark current ratio (up to 4.68 × 104), a sharp cutoff wavelength of approximately 270 nm, and short rise and fall times of 0.96 and 0.34 s. The photogenerated holes trapped in the Schottky barrier and the shrinking of the depletion region under DUV illumination enabled high DUV/visible rejection ratio (3–4 orders with a 20-V bias). Therefore, the Fowler–Nordheim field tunneling emission functioned as the main electron transport mechanism under DUV illumination and improved the photoelectric characteristics of the epilayer.