Industries such as the automotive, aerospace or military, as well as environmental and biological research have promoted the development of ultraviolet (UV) photodetectors capable of operating at high temperatures and in hostile environments. UV-enhanced Si photodiodes are hence giving way to a new generation of UV detectors fabricated from wide-bandgap semiconductors, such as SiC, diamond, III-nitrides, ZnS, ZnO, or ZnSe. This paper provides a general review of latest progresses in wide-bandgap semiconductor photodetectors.

https://iopscience.iop.org/article/10.1088/0268-1242/18/4/201/pdf

Wide-bandgap semiconductor ultraviolet photodetectors

We report the electrical and optical characteristics of avalanche photodiodes fabricated in GaN grown by metalorganic chemical vapor deposition. The current–voltage characteristics indicate a multiplication of >25. Experiment indicates and simulation verifies that the magnitude of the electric field at the onset of avalanche gain is ⩾3 MV/cm. Small-area devices exhibit stable gain with no evidence of microplasmas.

GaN avalanche photodiodes

The wet etching of GaN, AlN, and SiC is reviewed including conventional etching in aqueous solutions, electrochemical etching in electrolytes and defect-selective chemical etching in molten salts. The mechanism of each etching process is discussed. Etching parameters leading to highly anisotropic etching, dopant-type/bandgap selective etching, defect-selective etching, as well as isotropic etching are discussed. The etch pit shapes and their origins are discussed. The applications of wet etching techniques to characterize crystal polarity and defect density/distribution are reviewed. Additional applications of wet etching for device fabrication, such as producing crystallographic etch profiles, are also reviewed.

Wet etching of GaN, AlN, and SiC : a review

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.

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

GaN and related materials (especially AlGaN) have recently attracted a lot of interest for applications in high power electronics capable of operation at elevated temperatures. Although the growth and processing technology for SiC, the other viable wide bandgap semiconductor material, is more mature, the AlGaInN system offers numerous advantages. These include wider bandgaps, good transport properties, the availability of heterostructures (particularly AlGaN/GaN), the experience base gained by the commercialization of GaN-based laser and light-emitting diodes and the existence of a high growth rate epitaxial method (hydride vapor phase epitaxy) for producing very thick layers or even quasi-substrates. These attributes have led to rapid progress in the realization of a broad range of GaN electronic devices, including heterostructure field effect transistors (HFETs), Schottky and p–i–n rectifiers, heterojunction bipolar transistors (HBTs), bipolar junction transistors (BJTs) and metal-oxide semiconductor field effect transistors (MOSFETs). This review focuses on the development of fabrication processes for these devices and the current state-of-the-art in device performance, for all of these structures. We also detail areas where more work is needed, such as reducing defect densities and purity of epitaxial layers, the need for substrates and improved oxides and insulators, improved p-type doping and contacts and an understanding of the basic growth mechanisms.

Fabrication and performance of GaN electronic devices

We report the growth, fabrication, and characterization of AlxGa1−xN (0⩽x⩽0.60) heteroepitaxial back-illuminated solar-blind p-i-n photodiodes on (0001) sapphire substrates. The group III-nitride heteroepitaxial layers are grown by low-pressure metalorganic chemical vapor deposition on double polished sapphire substrates using various growth conditions. The back-illuminated devices exhibit very low dark current densities. Furthermore, they exhibit external quantum efficiencies up to 35% at the peak of the photoresponse (λ∼280 nm). Improvements were made to the growth technique in order to achieve crack-free Al0.4Ga0.6N active regions on a thick Al0.6Ga0.4N window layer and to obtain activated p-type Al0.4Ga0.6N layers.

Back illuminated AlGaN solar-blind photodetectors

Defects were observed in GaN:Mg grown on sapphire substrates using metal–organic chemical-vapor deposition (MOCVD) with Mg-delta doping similar to those previously observed in bulk GaN:Mg grown from Ga solution under high hydrostatic pressure of nitrogen. Pyramidal defects (pinholes) with (1100) hexagonal facets on the (0001) base plane and six {1122} side facets, and defects with a rectangular shape also delineated by planar facets on the basal (0001) planes, were observed for growth with Ga polarity for both of these very different growth methods. The Mg dopant is apparently responsible for their formation since the oxygen concentration in the MOCVD-grown samples was orders of magnitude lower than in the bulk samples. Mg segregation on these planes apparently does not allow uniform continuous growth on these planes leading to these hollow defects. Some defects in the heterolayers also develop into longer nanotubes elongated along the c axis. Change of polarity from Ga to N followed by a change back to...

Mg-doped GaN: Similar defects in bulk crystals and layers grown on Al2O3 by metal–organic chemical-vapor deposition

We have fabricated and investigated high-voltage GaN vertical Schottky-barrier rectifiers grown by metalorganic chemical vapor deposition. A mesageometry Schottky-barrier rectifier having a 5-μm-thick i region, and processed using reactive-ion etching, exhibited a reverse breakdown voltage of −450 V (at 10 mA/cm2) and an on-resistance of 23 mΩ cm2. For comparison, we have also applied wet chemical etching for the fabrication of mesageometry Schottky-barrier rectifiers. The 2-μm-thick i-region GaN mesa-Schottky rectifiers showed a breakdown voltage of −310 and −280 V for wet-etched and dry-etched devices, respectively, and an on-resistance of 8.2 and 6.4 mΩ cm2, respectively. These results indicate that the performance of the wet-etched rectifiers is comparable to or better than that of comparable dry-etched devices.

High-voltage mesa-structure GaN Schottky rectifiers processed by dry and wet etching

We report the growth, fabrication and characterization of Al/sub 0.4/Ga/sub 0.6/N-Al/sub 0.6/Ga/sub 0.4/N back-illuminated, solar-blind p-i-n photodiodes. The peak responsivity of the photodiodes is 27 and 79 mA/W at /spl lambda//spl ap/280 nm for bias voltages of 0 V and -60 V, respectively, with a UV-to-visible rejection ratio of more than three decades (at 400 nm). These devices exhibit very low dark current densities (/spl sim/5 nA/cm/sup 2/ at -10 V). At low frequencies, the noise exhibits a 1/f-type behavior. The noise power density is S/sub 0//spl ap/5/spl times/10/sup -25/ A/sup 2//Hz at -12.7 V and the detectivity (D*) at 0 V is estimated to be in the range of 4/spl times/10/sup 11/-5/spl times/10/sup 13/ cm/spl middot/Hz/sup 1/2//W. Time-domain pulse response measurements in a front-illumination configuration indicate that the devices are RC-time limited and show a strong spatial dependence with respect to the position of the incident excitation, which is mainly due to the high resistivity of the p-type Al/sub 0.4/Ga/sub 0.6/ N layer.

D. J. H. Lambert

Papers

GaN avalanche photodiodes

Back illuminated AlGaN solar-blind photodetectors

Mg-doped GaN: Similar defects in bulk crystals and layers grown on Al2O3 by metal–organic chemical-vapor deposition

High-voltage mesa-structure GaN Schottky rectifiers processed by dry and wet etching

Low-noise back-illuminated Al/sub x/Ga/sub 1-x/N-based p-i-n solar-blind ultraviolet photodetectors