Deep Ultraviolet Photodetectors Research Articles

In situ synthesis of monoclinic β-Ga2O3 nanowires on flexible substrate and solar-blind photodetector

Flexible solar-blind deep ultraviolet (UV) photodetectors based on β-Ga2O3 have important applications in wearable portable UV light monitoring and smart skins devices. However, flexible devices limit the maximum working temperature, and amorphous nature of film grown under low temperature displays instability. In this work, monoclinic β-Ga2O3 nanowires were synthesized in situ on a flexible glass fiber fabric by chemical vapor deposition with a diameter of 40–280 nm and a length of one to tens of microns under a high temperature. The fabricated photodetector based on β-Ga2O3 nanowires network exhibits a good solar-blind UV photoelectric performance such as an Ilight/Idark ratio of 260, a photoresponsivity of 0.71 A/W and a decay time of 0.19 s under 254 nm UV light illumination. Meanwhile, the performance of the device is not affected by bending conditions. Simple fabrication method and excellent photoelectric performance reveal that this device will be a promising candidate for future flexible solar-blind photodetectors with a high working temperature and high stability.

Ultrafast Photovoltaic-Type Deep Ultraviolet Photodetectors Using Hybrid Zero-/Two-Dimensional Heterojunctions.

Deep ultraviolet (DUV) photodetectors have wide-range applications in satellite communications, air purification, and missile-plume detection. However, the critical barriers for the currently available wide-band gap semiconductor film-based DUV photodetectors are their low efficiency, complicated processes, and lattice mismatch with the substrate. Quantum dot (QD) devices prepared using solution-based methods can solve these problems. However, so far, there are no reports on photovoltaic-type DUV photodetectors using QDs. In this study, we propose a novel methodology to construct a hybrid zero-/two-dimensional DUV photodetector (p-type graphene/ZnS QDs/4H-SiC) with photovoltaic characteristics. The device exhibits excellent selectivity for the DUV light and has an ultrafast response speed (rise time: 28 μs and decay time: 0.75 ms), which are much better than those reported for conventional photoconductive photodetectors.

Recent Progress in Solar‐Blind Deep‐Ultraviolet Photodetectors Based on Inorganic Ultrawide Bandgap Semiconductors

AbstractDue to its significant applications in many relevant fields, light detection in the solar‐blind deep‐ultraviolet (DUV) wavelength region is a subject of great interest for both scientific and industrial communities. The rapid advances in preparing high‐quality ultrawide‐bandgap (UWBG) semiconductors have enabled the realization of various high‐performance DUV photodetectors (DUVPDs) with different geometries, which provide an avenue for circumventing numerous disadvantages in traditional DUV detectors. This article presents a comprehensive review of the applications of inorganic UWBG semiconductors for solar‐blind DUV light detection in the past several decades. Different kinds of DUVPDs, which are based on varied UWBG semiconductors including Ga2O3, MgxZn1−xO, III‐nitride compounds (AlxGa1−xN/AlN and BN), diamond, etc., and operate on different working principles, are introduced and discussed systematically. Some emerging techniques to optimize device performance are addressed as well. Finally, the existing techniques are summarized and future challenges are proposed in order to shed light on development in this critical research field.

Synthesis of ternary oxide Zn2GeO4 nanowire networks and their deep ultraviolet detection properties.

Ternary oxide Zn2GeO4 with a wide bandgap of 4.84 eV, as a candidate for fourth generation semiconductors, has attracted a great deal of attention for deep ultraviolet (DUV) photodetector applications, because it is expected to be blind to the UV-A/B band (290–400 nm) and only responsive to the UV-C band (200–290 nm). Here, we report on the synthesis of Zn2GeO4 nanowire (NW) networks by lower pressure chemical vapor deposition and investigate their corresponding DUV detection properties. We find that pure Zn2GeO4 NWs could be obtained at a growth pressure of 1 kPa. The DUV detection tests reveal that growth pressure exerts a significant effect on DUV detection performance. The Zn2GeO4 NW networks produced under 1 kPa show an excellent solar-blind photoresponsivity with fast rise and decay times (trise ≈ 0.17 s and tdecay ≈ 0.14 s).

Catalyst-free growth of two-dimensional hexagonal boron nitride few-layers on sapphire for deep ultraviolet photodetectors

Catalyst-free growth of wafer-scale h-BN few-layers is realized on sapphire substrates by the combination of surface nitridation and N+ sputtering.

Highly preferred orientation of Ga2O3 films sputtered on SiC substrates for deep UV photodetector application

As a wide-bandgap semiconductor oxide material, Ga2O3 has great application prospects in various optoelectronic fields. At present, much attention has been paid to Ga2O3 deep ultraviolet (DUV) detectors, in which Ga2O3 films are often poor-quality poly-crystalline or expensive mono-crystalline. In this paper, Ga2O3 films with highly preferred orientation were deposited on hexagonal SiC (6H-SiC) substrate by magnetron sputtering, and the as-prepared Ga2O3 films were then annealed. The influences of sputtering and annealing parameters on Ga2O3 films were analyzed. Finally, DUV photo-detectors based on the as-obtained Ga2O3 films were fabricated. The experimental results show, when choosing optimal sputtering parameters (RF power: 120 W, gas pressure: 1.5 Pa, film thickness: 400 nm), the sputtered Ga2O3 films with (2¯01) highly preferred orientation after annealing at 800 °C, have good crystalline quality (FWHM = 0.108°) between single-crystal and polycrystal, showing good compromise between optoelectronic characteristics and preparation cost. The fabricated metal-semiconductor-metal (MSM) DUV photodetector operated at a bias voltage of 10 V can achieve excellent comprehensive photo-detection properties for 254 nm ultraviolet light: the responsivity, detectivity and quantum efficiency are high as 2.6 A/W, 1.6 × 1012 Jones and 1265%, respectively, along with short response time of 0.26 s.

Graphene Interdigital Electrodes for Improving Sensitivity in a Ga2O3:Zn Deep-Ultraviolet Photoconductive Detector

Graphene (Gr) has been widely used as a transparent electrode material for photodetectors because of its high conductivity and high transmittance in recent years. However, the current low-efficiency manipulation of Gr has hindered the arraying and practical use of such detectors. We invented a multistep method of accurately tailoring graphene into interdigital electrodes for fabricating a sensitive, stable deep-ultraviolet photodetector based on Zn-doped Ga2O3 films. The fabricated photodetector exhibits a series of excellent performance, including extremely low dark current (∼10-11 A), an ultrahigh photo-to-dark ratio (>105), satisfactory responsivity (1.05 A/W), and excellent selectivity for the deep-ultraviolet band, compared to those with ordinary metal electrodes. The raise of photocurrent and responsivity is attributed to the increase of incident photons through Gr and separated carriers caused by the built-in electric field formed at the interface of Gr and Ga2O3:Zn films. The proposed ideas and methods of tailoring Gr can not only improve the performance of devices but more importantly contribute to the practical development of graphene.

Catalyst-free growth of a Zn2GeO4 nanowire network for high-performance transfer-free solar-blind deep UV detection

Solar-blind deep-ultraviolet (DUV) photodetectors have attracted wide attention because of their extensive military and civil applications. The ternary oxide Zn2GeO4 is an ideal material with a wide bandgap (Eg = 4.69 eV). In this work, DUV photodetectors based on a ternary Zn2GeO4 nanowire (NW) network were fabricated on SiO2/Si substrates. Reactive ion etching of the SiO2/Si wafer was used to grow the NW network to avoid contamination of the Au catalyst during synthesis of the Zn2GeO4 NW network via high-temperature chemical vapor deposition. Photodetectors based the Zn2GeO4 NW revealed fast response and recovery times, which is attributed to the unique cross-junction barrier-dominated conductance of the NW network. Results showed that the ternary oxide-based NW network is an ideal building block for nanoscale solar-blind photodetectors with superior performance.

Deep-Ultraviolet Photodetectors Based on Epitaxial ZnGa2O4 Thin Films

A single-crystalline ZnGa2O4 epilayer was successfully grown on c-plane (0001) sapphire substrate by metal-organic chemical vapor deposition. This epilayer was used as a ternary oxide semiconductor for application in high-performance metal–semiconductor–metal photoconductive deep-ultraviolet (DUV) photodetectors (PDs). At a bias of 5 V, the annealed ZnGa2O4 PDs showed better performance with a considerably low dark current of 1 pA, a responsivity of 86.3 A/W, cut-off wavelength of 280 nm, and a high DUV-to-visible discrimination ratio of approximately 107 upon exposure to 230 nm DUV illumination than that of as-grown ZnGa2O4 PDs. The as-grown PDs presented a dark current of 0.5 mA, a responsivity of 2782 A/W at 230 nm, and a photo-to-dark current contrast ratio of approximately one order. The rise time of annealed PDs was 0.5 s, and the relatively quick decay time was 0.7 s. The present results demonstrate that annealing process can reduce the oxygen vacancy defects and be potentially applied in ZnGa2O4 film-based DUV PD devices, which have been rarely reported in previous studies.

In-situ fabrication of PtSe2/GaN heterojunction for self-powered deep ultraviolet photodetector with ultrahigh current on/off ratio and detectivity

The research of ultraviolet photodetectors (UV PDs) have been attracting extensive attention, due to their important applications in many areas. In this study, PtSe2/GaN heterojunction is in-situ fabricated by synthesis of large-area vertically standing two-dimensional (2D) PtSe2 film on n-GaN substrate. The PtSe2/GaN heterojunction device demonstrates excellent photoresponse properties under illumination by deep UV light of 265 nm at zero bias voltage. Further analysis reveals that a high responsivity of 193 mA·W–1, an ultrahigh specific detectivity of 3.8 × 1014 Jones, linear dynamic range of 155 dB and current on/off ratio of ~ 108, as well as fast response speeds of 45/102 μs were obtained at zero bias voltage. Moreover, this device response quickly to the pulse laser of 266 nm with a rise time of 172 ns. Such high-performance PtSe2/GaN heterojunction UV PD demonstrated in this work is far superior to previously reported results, suggesting that it has great potential for deep UV detection.

Optimizing the performance of a β-Ga2O3 solar-blind UV photodetector by compromising between photoabsorption and electric field distribution

Deep ultraviolet solar blind photodetectors based on an ultra-wide band gap semiconductor of β-Ga2O3 have attracted much attention for their potential applications, e.g., missile tracking, space communication, and ozone hole monitoring. As the active layer of photodetectors, the thickness of β-Ga2O3 films plays an important role in the photoelectric performance of the photodetector because it affects the ultraviolet light photoabsorption and the electric field distribution. Highly oriented (2¯01) direction β-Ga2O3 thin films with different thickness (90 nm-540 nm) were grown on (0001) sapphire substrates using radio frequency magnetron sputtering with a substrate temperature of 750 °C. Based on the different thicknesses of β-Ga2O3 thin films, the MSM structure photodetectors were fabricated and the photoelectric performances were investigated. A photodetector with 303 nm thick thin films has the highest light-to-dark current ratio (Ilight/Idark≈16250) and exhibits the best solar-blind photoelectric performance.

A flexible solar-blind 2D boron nitride nanopaper-based photodetector with high thermal resistance

Flexible electronics are expected to play a key role in connecting human lives with versatile smart electronic devices due to their adaptability to different shapes, surfaces, and even the human body. However, heat management issues found in most flexible devices due to the low thermal conductivity of conventional plastic or paper substrates become significant for large-scale integration or high-temperature applications. In this study, we employed high thermal conductivity nanopaper composed of two-dimensional (2D) hexagonal boron nitride nanosheets and one-dimensional nanofibrillated cellulose to form a flexible deep-ultraviolet photodetector demonstrating superior photodetectivity of up to 8.05 × 1010 cm Hz1/2/W, a short response time of 0.267 s, and excellent flexible durability featuring repeatable ON/OFF photoswitching over 200 bending cycles. Because the boron nitride paper has a high thermal conductivity of 146 W/mK, which is three orders of magnitude larger than plastic or paper substrates, the photodetectors can work at high temperatures of up to 200 °C. The boron nitride paper-based strategy described herein suggests a path for improving heat dissipation in flexible electronics and achieving high-performance deep-ultraviolet photodetectors, which can be applied in wearable applications.

Demonstration of zero bias responsivity in MBE grown β-Ga2O3 lateral deep-UV photodetector

We demonstrate zero-bias spectral responsivity in MBE-grown β-Ga2O3 planar UV-C detector with good linearity up to optical power density of 4.6 mW cm−2. Devices with asymmetrical metal contacts were realized on 150 nm thick β-Ga2O3 films on sapphire. The device exhibited a spectral responsivity of 1.4 mA W−1 at 255 nm under zero-bias condition, dark current <10 nA at 15 V and UV-to-visible rejection ratio ∼105 at 5 V. The demonstrated UV-C detector exhibited an estimated high detectivity of 2.0 × 1012 Jones at 1 V and were found to be very stable and repeatable, suggesting its potential use for focal plane arrays.

Band offset and electronic properties at semipolar plane AlN()/diamond heterointerface**Project supported by the Scholarship Council of China (Grant No. 201508340047), the Postdoctoral Science Foundation of China (Grant No. 2016M601993), the Postdoctoral Science Foundation of Anhui Province, China (Grant No. 2017B215), and the Anhui Province University Outstanding Talent Cultivation Program, China (Grant No. gxfxZD2016077).

Tailoring the electronic states of the AlN/diamond interface is critical to the development of the next-generation semiconductor devices such as the deep-ultraviolet light-emitting diode, photodetector, and high-power high-frequency field-effect transistor. In this work, we investigate the electronic properties of the semipolar plane AlN()/diamond heterointerfaces by using the first-principles method with regard to different terminated planes of AlN and surface structures of diamond (100) plane. A large number of gap states exist at semi-polar plane AlN()/diamond heterointerface, which results from the N 2p and C 2s2p orbital states. Besides, the charge transfer at the interface strongly depends on the surface termination of diamond, on which hydrogen suppresses the charge exchange at the interface. The band alignments of semi-polar plane AlN()/diamond show a typical electronic character of the type-II staggered band configuration. The hydrogen-termination of diamond markedly increases the band offset with a maximum valence band offset of 2.0 eV and a conduction band offset of 1.3 eV for the semi-polar plane N–AlN()/hydrogenated diamond surface. The unique band alignment of this Type-II staggered system with the higher CBO and VBO of the semi-polar AlN/HC(100) heterostructure provides an avenue to the development of robust high-power high-frequency power devices.

Fabrication of high responsivity deep UV photo-detector based on Na doped ZnO nanocolumns

We report a variety of the hydrothermally synthesized ZnO nanostructures with a significant suppression in defect-related emission and huge enhancement in the photo-current to the dark current ratio (approximately six orders of magnitude) upon UV light illumination. Interestingly, the photo-detector shows lower dark current of 1.6 nA with high responsivity of 507 A W−1 at 254 nm. Here, a systematic analysis of the growth process as well as the physical, chemical and electrical properties of as-grown ZnO nanostructures has been performed. We have utilized the duo effect of both the inorganic (KMnO4) and organic (Na3C6H5O7) additives, which has facilitated the precise tuning of the morphology and intrinsic defects in nanostructures that have made an impact on the photo-responsivity, photoluminescence (PL) and adhesivity of the film on to the underlying substrate. PL analysis of as-grown ZnO nanostructures has suggested 11 times improvement in the near band emission (NBE) to defect level emission (DLE) ratio. Interestingly, thermal annealing of the samples has shown a dramatic change in the morphology with significant improvement in the crystallinity. Notably, the band gap was observed to be modulated from 3.3 eV to 3.1 eV after annealing. In addition to UV photo-detector based applications, the work presented here has provided a subtle solution towards the rectification of various problems pertaining to hydrothermal processes like poor adhesivity, feeble UV emission and problem in precise tuning of the morphology along with the bandgap in one go. Therefore, these investigations assume critical significance towards the development of next-generation optoelectronic devices.

High-performance solar-blind flexible deep-UV photodetectors based on quantum dots synthesized by femtosecond-laser ablation

High-performance deep ultraviolet (DUV) photodetectors operating at ambient conditions with < 280 nm detection wavelengths are in high demand because of their potential applications in diverse fields. We demonstrate for the first time, high-performance flexible DUV photodetectors operating at ambient conditions based on quantum dots (QDs) synthesized by femtosecond-laser ablation in liquid (FLAL) technique. Our method is facile without complex chemical procedures, which allows large-scale cost-effective devices. This synthesis method is demonstrated to produce highly stable and reproducible ZnO QDs from zinc nitride target (Zn3N2) without any material degradation due to water and oxygen molecule species, allowing photodetectors operate at ambient conditions. Carbon-doped ZnO QD-based photodetector is capable of detecting efficiently in the DUV spectral region, down to 224 nm, and exhibits high photo-responsivity and stability. As fast-response of DUV photodetector remains significant parameter for high-speed communication; we show fast-response QD-based DUV photodetector. Such surfactant-free synthesis by FLAL can lead to commercially available high-performance low-cost optoelectronic devices based on nanostructures for large scale applications.

Modulated Al2O3-Alloyed Ga2O3Materials and Deep Ultraviolet Photodetectors

To modulate the cutoff wavelength of the metal-semiconductor-metal deep ultraviolet photodetectors (MSM DUV-PDs), the aluminum content of the aluminium oxide-alloyed gallium oxide (Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> :Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> ) absorption layers was deposited using radio frequency (RF) magnetron cosputter system with dual targets, in which the Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> target was sputtered with an RF power of 100 W and the Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> target sputtered with various RF powers. The optical bandgap energy of the Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> :Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> films increased with an increase of the Al content deposited with a larger sputtered RF power of the Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> target. In view of the suppression effect of the oxygen vacancy and the defect by the more Al-O bonds in the Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> :Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> absorption layer, the noise performances and the detectivity of the MSM DUV-PDs were improved. For depositing the Al atomic percentage of 5.9% in the Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> :Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> absorption layer, the resulting MSM DUV-PDs revealed the cutoff wavelength of 230 nm, the noise equivalent power of 2.80 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-12</sup> W, and the detectivity of 1.37 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">11</sup> cmHz0.5W <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> . Furthermore, the dominant low-frequency noise source was the flicker noise.

Influence of post-deposition annealing on structural, optical and transport properties of nanocomposite ZnO-Ag thin films

Thin films of ZnO and ZnO-Ag were grown by rf sputter deposition and post-annealed in air in the 300–600 °C temperature range. Annealing caused variation in strain in ZnO films and significant improvement of ZnO in ZnO-Ag films. The partial oxidation of Ag into AgxO upon annealing of ZnO-Ag films in air resulted in increase of transmittance, absorption due to AgxO phase, weakening of surface plasmon resonance absorption due to Ag nanocrystals and an increase in fluorescence. Surface studies indicate temperature induced particle transport cause agglomeration of nanocrystals in ZnO-Ag films during post-annealing. Annealed ZnO films showed a decrease in dark current and a sixteenfold increase in photocurrent at 3 V applied bias under white light illumination due to the formation of charged acceptors / oxygen vacancy centres, which was comparable to the white light photosensitivity of as-deposited ZnO-Ag film. For low applied bias voltages, Metal-Semiconductor-Metal photoconductive detectors made with annealed ZnO-Ag films showed potential for application as efficient deep ultraviolet (DUV) photodetectors with negligible sensitivity to white light and high sensitivity to DUV light.

Stabilization and enhanced energy gap by Mg doping in ε-phase Ga2O3 thin films

Mg-doped Ga2O3 thin films with different doping concentrations were deposited on sapphire substrates using laser molecular beam epitaxy (L-MBE) technique. X-ray diffraction (XRD), x-ray photoelectron spectroscopy (XPS) and ultraviolet-visible (UV-vis) absorption spectrum were used to characterize the crystal structure and optical properties of the as-grown films. Compared to pure Ga2O3 thin film, the Mg-doped thin films have transformed from the most stable β-phase into ε-phase. The absorption edge shifted to about 205 nm and the optical bandgap increased to ∼ 6 eV. These properties reveal that Mg-doped Ga2O3 films may have potential applications in the field of deep ultraviolet optoelectronic devices, such as deep ultraviolet photodetectors, short wavelength light emitting devices and so on.

Bandgap bowing in Ni1−xMgxO alloy

It is crucial to understand the compositional dependence of the bandgap of the Ni1–xMgxO alloy as it is a promising bandgap-tunable material for ultraviolet (UV) and deep-UV photodetectors. However, the dependence of the bandgap of this material on the Mg content, x, is still a subject of debate, as the experimentally measured optical bandgap of Ni1–xMgxO is 2–3 eV lower than that of pure MgO (7.77 eV), in contrast to that predicted by conventional bandgap bowing theories. Here, we demonstrate that the Ni1−xMgxO alloy has two bandgaps: (i) the Ni-3d bandgap Eg3d, i.e., the bandgap between the valence band (O 2p) and Ni-3d(eg) bands, where the charge-transfer (CT) transition energy weakly depends on the Mg content and (ii) the alloy bandgap EgNi1−xMgxO, i.e., the bandgap between the valence band and the conduction band of the Ni1−xMgxO alloy, which is related to the Ni 4s/Mg 3s states; the alloy bandgap obeys the conventional bandgap bowing model. The Ni1−xMgxO absorption spectra at low Mg contents are difficult to deconvolute because the electronic bands originating from the Ni-3d bands and the alloy conduction band overlap. The band structure described above elucidates the anomalous characteristics of the bandgap of Ni1–xMgxO, i.e., most of the optical bandgap values reported so far are smaller than the expected value corresponding to the transition from the valence band to the alloy conduction band because they were evaluated based on the absorption due to the O 2p-Ni 3d(eg) CT transition.