Masatomo Sumiya

A Comprehensive Review of Semiconductor Ultraviolet Photodetectors: From Thin Film to One-Dimensional Nanostructures

Ultraviolet (UV) photodetectors have drawn extensive attention owing to their applications in industrial, environmental and even biological fields. Compared to UV-enhanced Si photodetectors, a new generation of wide bandgap semiconductors, such as (Al, In) GaN, diamond, and SiC, have the advantages of high responsivity, high thermal stability, robust radiation hardness and high response speed. On the other hand, one-dimensional (1D) nanostructure semiconductors with a wide bandgap, such as β-Ga2O3, GaN, ZnO, or other metal-oxide nanostructures, also show their potential for high-efficiency UV photodetection. In some cases such as flame detection, high-temperature thermally stable detectors with high performance are required. This article provides a comprehensive review on the state-of-the-art research activities in the UV photodetection field, including not only semiconductor thin films, but also 1D nanostructured materials, which are attracting more and more attention in the detection field. A special focus is given on the thermal stability of the developed devices, which is one of the key characteristics for the real applications.

Enhanced performance of InGaN solar cell by using a super-thin AlN interlayer

A super-thin AlN layer is inserted between the intrinsic InGaN and p-InGaN in the InGaN solar cell structure to improve the photovoltaic property. The dark current is markedly decreased by more than two orders of magnitude and the short-circuit current density is increased from 0.77 mA/cm2 to 1.25 mA/cm2, leading to a doubled conversion efficiency compared to the conventional structure. Electrical transport analysis reveals that the forward electrical property is greatly improved in the range of open circuit voltage and the leakage current mechanism changes from defect related Poole-Frenkel emission to interface tunneling emission. The improvement on the electrical and photovoltaic properties is ascribed to insertion of the AlN interlayer, which not only provides a barrier to reduce tunneling for electrons, but also suppresses the nonradiative recombination.

High-performance metal-semiconductor-metal InGaN photodetectors using CaF2 as the insulator

The authors report on the high-performance metal-semiconductor-metal (MSM) photodetectors (PDs) fabricated on high-quality InGaN film by introducing a superwide bandgap calcium fluoride (CaF2) as the insulator. The dark current of the PDs with CaF2 is drastically reduced by six orders of magnitude compared with those without CaF2, resulting in an extremely high discrimination ratio larger than 106 between ultraviolet and visible light. The responsivity at 338 nm is as high as 10.4 A/W biased at 2 V, corresponding to a photocurrent gain around 40. The CaF2 layer behaves as an excellent insulator for the InGaN-based MSM-PDs in dark condition, while it allows the electron injection through the metal/semiconductor interface under ultraviolet illumination, contributing to the photocurrent gain without sacrificing the response time (∼ms).

High-temperature ultraviolet detection based on InGaN Schottky photodiodes

A thermally stable metal-insulator-semiconductor (MIS) Schottky-type photodiode with high performance based on the InGaN film is demonstrated at high temperatures up to 523 K. The reverse leakage current remains at a low level (10−7−10−8 A), while the UV responsivity is as high as 5.6 A/W at −3 V under 523 K, without observing the persistent photoconductivity. The discrimination ratio between ultraviolet (378 nm) and visible light (600 nm) is maintained to be more than 105. The temperature-dependent current-voltage characteristics of the MIS diode were analyzed. The photocurrent gain at reverse biases was interpreted in term of thermionic-field emission (TFE) and field-emission tunneling mechanism from room-temperature to 463 K, while TFE becomes the dominant mechanism at high temperatures.

Temperature-controlled epitaxy of InxGa1-xN alloys and their band gap bowing

InxGa1-xN alloys (0u2009≤u2009xu2009≤u20091) have been grown on GaN/sapphire templates by molecular beam epitaxy. Growth temperature controlled epitaxy was proposed to modulate the In composition so that each InxGa1-xN layer was grown at a temperature as high as possible and thus their crystalline quality was improved. The bandgap energies of the InxGa1-xN alloys have been precisely evaluated by optical transmission spectroscopy, where the effect of residual strain and electron concentration (the Burstein-Moss effect) on the bandgap energy shift has been considered. Finally, a bowing parameter of ∼1.9u2009±u20090.1u2009eV has been obtained by the well fitting In-composition dependent bandgap energy.

Phase Separation Resulting from Mg Doping in p-InGaN Film Grown on GaN/Sapphire Template

The spontaneous formation of a two-layered structure in p-InGaN is observed with increasing Mg doping level. X-ray reciprocal space mappings reveal that the two layers are resulted from phase separation with different In contents and strain states. The bottom p-InGaN layer near the GaN template is totally strained with low-density dislocations, while the top layer is relaxed with high-density dislocations. The average In content in the relaxed layer is much higher than that in the totally strained one. The layered structure caused by phase separation is interpreted in terms of the compressive strain increase induced by Mg doping and the strain relaxation by dislocations.

A Multilevel Intermediate‐Band Solar Cell by InGaN/GaN Quantum Dots with a Strain‐Modulated Structure

Multiple stacked InGaN/GaN quantum dots are embedded into an InGaN p-n junction to develop multilevel intermediateband (MIB) solar cells. An IB transition is evidenced from both experiment and theoretical calculations. The MIB solar cell shows a wide photovoltaic response from the UV to the near-IR region. This work opens up an interesting opportunity for high-efficiency IB solar cells in the photovoltaics field.

Photovoltaic Action in Polyaniline/n-GaN Schottky Diodes

Schottky diodes were fabricated on n-GaN films by coating them with an organic polyaniline layer as transparent conducting electrodes. These diodes have a high Schottky barrier height (1.28 eV) and a low reverse leakage current (2.7×10-9 A/cm2 at an applied bias of -1 V). The photovoltaic action of these diodes (VOC = 0.67 V and external quantum efficiency ~30%) was studied under the illumination of an Air Mass 1.5 solar simulator. The polyaniline/n-GaN Schottky contacts were found to be sensitive to shorter wavelengths, indicating their potential for use as solar cells.

Defects in ZnO transparent conductors studied by capacitance transients at ZnO/Si interface

Deep levels in heavily aluminum-doped zinc oxide (AZO) thin film were studied by transient capacitance (C-t) measurements and photoemission spectroscopy (PES). To study degenerated AZO by C-t measurements, AZO films were deposited on a p-type silicon (p-Si) substrate to form a depletion layer at the AZO/p-Si interface. Analyses of C-t behavior revealed that concentration of a trap level 0.3 eV below the bottom of the conduction was on the order of 1019u2002cm−3, which is 20%–50% of the shallow donor concentration. Such a high concentration of the trap level in AZO was evidenced by subsequent PES measurements.

Hetero-Epitaxial Growth of ZnO Film by Temperature-Modulated Metalorganic Chemical Vapor Deposition

ZnO films were fabricated by metalorganic chemical vapor deposition while a laser substrate heating system was used to rapidly cycle the sample temperature. When a hydrogen gas atmosphere was used together with the temperature modulation, ZnO films deposited from diethyl-zinc and oxygen exhibited oxygen-face polarity and a smooth surface. The quality of ZnO films was good enough to observe phonon replicas corresponding to the transition from donor-bound to free exciton in the temperature dependence of photoluminescence. The growth of ZnO film is discussed in relation to the combined effect of growth temperature modulation and the hydrogen atmosphere.