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

Wide-band-gap GaN and Ga-rich InGaN alloys, with energy gaps covering the blue and near-ultraviolet parts of the electromagnetic spectrum, are one group of the dominant materials for solid state lighting and lasing technologies and consequently, have been studied very well. Much less effort has been devoted to InN and In-rich InGaN alloys. A major breakthrough in 2002, stemming from much improved quality of InN films grown using molecular beam epitaxy, resulted in the bandgap of InN being revised from 1.9 eV to a much narrower value of 0.64 eV. This finding triggered a worldwide research thrust into the area of narrow-band-gap group-III nitrides. The low value of the InN bandgap provides a basis for a consistent description of the electronic structure of InGaN and InAlN alloys with all compositions. It extends the fundamental bandgap of the group III-nitride alloy system over a wider spectral region, ranging from the near infrared at ∼1.9 μm (0.64 eV for InN) to the ultraviolet at ∼0.36 μm (3.4 eV for GaN...

When group-III nitrides go infrared: New properties and perspectives

Atomically thin two-dimensional materials have emerged as promising candidates for flexible and transparent electronic applications. Here we show non-volatile memory devices, based on field-effect transistors with large hysteresis, consisting entirely of stacked two-dimensional materials. Graphene and molybdenum disulphide were employed as both channel and charge-trapping layers, whereas hexagonal boron nitride was used as a tunnel barrier. In these ultrathin heterostructured memory devices, the atomically thin molybdenum disulphide or graphene-trapping layer stores charge tunnelled through hexagonal boron nitride, serving as a floating gate to control the charge transport in the graphene or molybdenum disulphide channel. By varying the thicknesses of two-dimensional materials and modifying the stacking order, the hysteresis and conductance polarity of the field-effect transistor can be controlled. These devices show high mobility, high on/off current ratio, large memory window and stable retention, providing a promising route towards flexible and transparent memory devices utilizing atomically thin two-dimensional materials.

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Controlled charge trapping by molybdenum disulphide and graphene in ultrathin heterostructured memory devices

A memory cell array has a first and a second storage area. The first storage area has a memory elements selected by an address signal. The second storage area has a memory elements selected by a control signal. A control circuit has a fuse element. When the fuse element has been blown, the control circuit inhibits at least one of writing and erasing from being done on the second storage area.

Nonvolatile Semiconductor Memory Device

With the increasing requirements for microelectromechanical systems (MEMS) regarding stability, miniaturization and integration, novel materials such as wide band gap semiconductors are attracting more attention. Polycrystalline SiC has first been implemented into Si micromachining techniques, mainly as etch stop and protective layers. However, the outstanding properties of wide band gap semiconductors offer many more possibilities for the implementation of new functionalities. Now, a variety of technologies for SiC and group III nitrides exist to fabricate fully wide band gap semiconductor based MEMS. In this paper we first review the basic technology (deposition and etching) for group III nitrides and SiC with a special focus on the fabrication of three-dimensional microstructures relevant for MEMS. The basic operation principle for MEMS with wide band gap semiconductors is described. Finally, the first applications of SiC based MEMS are demonstrated, and innovative MEMS and NEMS devices are reviewed.

http://iopscience.iop.org/article/10.1088/0022-3727/40/20/S19/pdf

Group III nitride and SiC based MEMS and NEMS: materials properties, technology and applications

GaN metal–semiconductor–metal photoconductive detectors have been fabricated on Si(111) substrates. The GaN epitaxial layers were grown on Si substrates by means of metalorganic chemical-vapor deposition. These detectors exhibited a sharp cutoff at the wavelength of 363 nm and a high responsivity at a wavelength from 360 to 250 nm. A maximum responsivity of 6.9 A/W was achieved at 357 nm with a 5 V bias. The relationship between the responsivity and the bias voltage was measured. The responsivity saturated when the bias voltage reached 5 V. The response time of 4.8 ms was determined by the measurements of photocurrent versus modulation frequency.

Metal–semiconductor–metal GaN ultraviolet photodetectors on Si(111)

Using the Debye model and existing experimental data of the pyroelectric coefficient of AlN, the temperature dependence of the pyroelectric coefficient as well as the spontaneous polarization of AlN is calculated over a wide temperature range from 0to1000K. The pyroelectric coefficient is proportional to T3 at low temperature and increases acutely from 0 to around 400K, and then increases gently from 400to1000K. It makes AlN uniquely suitable for application in high temperature pyroelectric sensors. The spontaneous polarization of AlN changes a little from 0to1000K, which indicates that the features of III-nitrides based devices will hardly be degraded by the change of the spontaneous polarization.

Temperature dependence of the pyroelectric coefficient and the spontaneous polarization of AlN

A GaN-based metal–insulator–semiconductor (MIS) structure has been fabricated by using ferroelectric Pb(Zr053Ti047)O3 instead of conventional oxides as insulator gate Because of the polarization field provided by ferroelectric and the high dielectric constant of ferroelectric insulator, the capacitance–voltage characteristics of GaN-based metal–ferroelectric–semiconductor (MFS) structures are markedly improved compared to those of other previously studied GaN MIS structures The GaN active layer in MFS structures can reach inversion just under the bias of smaller than 5 V, which is the generally applied voltage used in semiconductor-based integrated circuits The surface carrier concentration of the GaN layer in the MFS structure is decreased by one order compared with the background carrier concentration The GaN MFS structures look promising for the practical application of GaN-based field effect transistors

Studies of metal–ferroelectric–GaN structures

We report on the composition pulling effect and strain relief mechanism in AlGaN/AlN distributed Bragg reflectors (DBRs) grown on GaN template/α-Al2O3(0001) by metal organic chemical vapor deposition. The reciprocal space mapping contours reveal that these DBRs are coherently grown. Cross-section transmission electron microscopy image of the AlGaN/AlN DBRs and the energy-dispersive x-ray analysis indicate that an AlGaN layer with gradient Al composition is located between the Al0.4Ga0.6N and AlN layers along the [0001] direction. It is attributed to the fact that Ga atoms in AlGaN are pulled and segregated to the upper layer by the strain. The density of strain energy is estimated to reduce more than one order by forming this quasi-three-sublayer structure comparing to the designed bi-sublayer structure.

Composition pulling effect and strain relief mechanism in AlGaN/AlN distributed Bragg reflectors

at 750 °C and 850 °C. The oxide and interface morphology are characterized by cross-sectional scanning electron microscope images. It is found that the oxidized nanowire following oxidation at 750 °C still keeps its pentagon shape even if it has been oxidized for 19 h. However, the oxidized samples at 850 °C become circular in shape. The oxidation-temperature dependence of the sample shapes is discussed. Our results should be useful in generating silicon nanowires coated with SiO2 in microelectronic technology with careful selection of the SiO2 growth temperatures.

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R. L. Jiang

Papers

Metal–semiconductor–metal GaN ultraviolet photodetectors on Si(111)

Temperature dependence of the pyroelectric coefficient and the spontaneous polarization of AlN

Studies of metal–ferroelectric–GaN structures

Composition pulling effect and strain relief mechanism in AlGaN/AlN distributed Bragg reflectors

Fabrication of silicon nanowires