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Vacuum ultraviolet (VUV) light sensing shows great potential applications in the space science, materials, biophysics, and plasma physics. In this work, an all-optical detection method is proposed for VUV sensing by constructing an optical fiber-end Fabry-Pérot interferometer based on a single aluminum nitride (AlN) microwire. Compared with the traditional electrical devices, this all-optical detection method overcomes the difficulties like the fast response and electromagnetic interference immunity in detecting VUV bands at the present stage, and improves the response speed. The proposed device shows the excellent performance of VUV detection, with the static sensitivity of 1.03 nm/(W·cm−2), response rise time of down to 10 µs, and decay time of 0.64 ms. Beneficial from the excellent radiation resistance of AlN microwires and UV resistance of silica fibers, the proposed device is expected to have the good stability and potential applications in the fields of the solar physics and space exploration. [ABSTRACT FROM AUTHOR]

High-quality aluminum nitride (AlN) crystals are the key material for the development of high-performance solid-state solar-blind vacuum-ultraviolet (VUV) photodetectors. However, the commonly used epitaxial method to grow AlN crystals would limit this development due to the existence of indispensable substrates. This study addresses this issue using free-standing lamellar AlN single crystals that are grown using the physical vapor transport method. The large lateral dimension of the crystal enables the construction of an Au-AlN-graphene van der Waals heterojunction, which can function as a vertical VUV photodetector with the graphene serving as the light window. The asymmetric junctions formed on the two sides of the crystal and the limited penetration of the VUV endow the device with a bias polarity-dependent photoresponse feature arising from different photoelectric processes. Furthermore, the device demonstrates a high responsivity of 5.77 A W−1 and a high specific detectivity of 1.71 × 1013 cm Hz1/2 W−1 under the illumination of a 193 nm laser. The high crystallinity of the AlN guarantees a high spectral selectivity of responsivity with a 193 nm/280 nm rejection ratio of 3 × 102. This work would inspire the development of wide-bandgap-semiconductor-based VUV photodetectors in terms of methodology and mechanism. [ABSTRACT FROM AUTHOR]

Aluminum nitride (AlN) exhibits excellent high-temperature resistance, chemical stability, and a wide bandgap, making it a prime candidate material for deep ultraviolet detectors. In this study, a modified metal–semiconductor–metal (MSM) photodetector using titanium (Ti) electrodes and millimeter-scale AlN bulk polycrystals grown through physical vapor transport is developed, demonstrating photoresponse to light from visible to vacuum ultraviolet. An additional tungsten (W) electrode is designed on the backside of the device, transforming it into a W-MSM device. A proper bias to the W electrode (VW) is found valid to boost the performance of the photodetector. Representatively, with a V of 20 V and VW of −12 V applied, the device achieves improvements in responsivity (R), detectivity (D*), and external quantum efficiency of 112.84%, 30.5%, and 112.84%, respectively, to 532 nm light and 123.18%, 36.84%, and 123.18%, respectively, to 193 nm light. Furthermore, it is found that with the total voltage being instant, optimizing the distribution of voltage between the Ti electrode and the W electrode would induce a better photoresponse than applying voltage solely to the Ti electrode (VW = 0 V). The reason is elaborated through modeling the voltage distribution in the device, revealing the particular role of the bulk semiconductor in this feature. This research provides a facile and innovative approach to developing low-power photodetectors for bulk materials. [ABSTRACT FROM AUTHOR]

Visible-blind ultraviolet-selective photodetection and ultraviolet-visible broad spectral photodetection are two essential functions eagerly pursued in each application area. However, they usually cannot be realized simultaneously in a bare photodetector because their different underlying photoexcitation processes would interfere with each other. In this work, a photodetector integrating the two distinct photodetector characteristics is presented. The device is prepared based on the heterojunction of a large-scale aluminum nitride bulk crystal and monolayer graphene. The visible-blind ultraviolet-selective photodetection and the ultraviolet-visible broad spectral photodetection are separately manifested in the device depending on the bias polarity. Under a negative bias, the device is a visible-blind deep-ultraviolet photodetector, demonstrating a 193/785 nm rejection ratio of over 106 for the photocurrent and a 193/405 nm rejection ratio of over 103 for the signal/noise ratio. Under a positive bias, the device performs as a broad spectral photodetector responding to light from 193 to 785 nm. Systematical characterization reveals that different photodetection manners are the synergistical results of the different photon energies of the incident light, wavelength-dependent penetration depths in AlN, and the different working modes of the device under different bias conditions. This work provides a particular dual-functional photodetector, which is of great significance in terms of both application and device physics. [ABSTRACT FROM AUTHOR]

The below bandgap optical transitions of an aluminum nitride (AlN) crystal grown on a tungsten (W) substrate by physical vapor transport (PVT) are investigated by below-bandgap-excited photoluminescence (PL) spectroscopy and first-principles calculations. Oxygen (O) is the only impurity in the AlN-on-W crystal grown by PVT. By analyzing the excitation-power-, excitation-photon-energy-, and temperature-dependence of the PL spectra, the emission peaks of defect complexes involving aluminum vacancy (VAl) and substitutional oxygen (ON) with different spatial and atomic configurations, i.e., VAl–ON and VAl–2ON with ON featuring axial or basal configurations, are identified. It is revealed that two different charging states coexist in thermal equilibrium for each configuration of VAl–ON complexes. The optical transitions between the conduction band and (VAl–ON)2− and/or (VAl–2ON)1− contribute the UV emissions and those between the valence band and (VAl–ON)1− or (VAl–2ON)0 contribute the red emissions. [ABSTRACT FROM AUTHOR]

A systematic analytical investigation of the charge distribution and gate control of the normally-off GaN double-channel MOS-HEMT (DC-MOS-HEMT) is presented in this paper. Compared to conventional GaN MOS-HEMT, the DC-MOS-HEMT features a thin AlN insertion layer (AlN-ISL) below the original two dimensional electron gas (2DEG) channel, thus forming a second channel at the interface between AlN-ISL and the underlying GaN. This paper reveals the impact of the AlN-ISL on the 2DEG distribution and the gate control of the channels. The sensitivity of ${V}_{\text {th}}$ against the recess depth is also analytically studied and is found to be nearly independent of the recess depth as long as the recess is terminated in the upper channel layer. The analytical results are well supported by numerical device simulations, and the physical mechanisms behind these findings are explained along with the analytical investigations. [ABSTRACT FROM AUTHOR]

A low on-resistance normally-off GaN double-channel metal–oxide–semiconductor high-electron-mobility transistor (DC-MOS-HEMT) is proposed and demonstrated in this letter, which features a 1.5-nm AlN insertion layer (ISL) located 6 nm below the conventional barrier/GaN interface, forming a second channel at the interface between the AlN-ISL and the underlying GaN. With gate recess terminated at the upper channel, normally-off operation was obtained with V\mathrm {th} of +0.5 V at I\mathrm {DS}= 10 ~\mu \text{A} /mm or +1.4 V from the linear extrapolation of the transfer curve. The lower heterojunction channel is separated from the etched surface in the gate region, thereby maintaining its high field-effect mobility with a peak value of 1801 cm2/( \textV\cdot \texts ). The on-resistance is as small as 6.9 \Omega \cdot \text mm for a DC-MOS-HEMT with LG/L\mathrm {GS}/L\mathrm {GD} = 1.5/2/15~\mu \text{m} , and the maximum drain current is 836 mA/mm. A high breakdown voltage (>700 V) and a steep subthreshold swing of 72 mV/decade are also obtained. [ABSTRACT FROM AUTHOR]

AlN/GaN heterostructures with AlN thin film growing by low-thermal-budget (300 °C) plasma-enhanced atomic layer deposition (ALD) were realized on a semi-insulating GaN-on-sapphire template. By applying in situ ALD-grown Al2O3 as the gate dielectric, thin film transistors (TFTs) have been successfully fabricated. The proposed TFTs exhibit effective gate control with a low subthreshold swing of ∼ 85 mV/dec, an ION/ IOFF ratio of 105 and good channel conductive with a low-field mobility of ∼ 27 cm2/(V·s). This mobility is one order of magnitude higher than that achieved in AlN/GaN heterostructures growing by magnetron sputtering. The enhanced device performance is attributed to a high-quality ALD-AlN/GaN interface obtained by in situ remote-plasma pre-deposition cleaning and the single crystal nature of subsequent epitaxial AlN thin film by plasma-enhanced ALD. (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [ABSTRACT FROM AUTHOR]

Seed crystals are the prerequisite for the growth of high quality and large size aluminum nitride (AlN) single crystal boules. The physical vapor transport (PVT) method is adopted to grow AlN seed crystal. However, this method is not available in nature. Herein, the temperature field distribution in the PVT furnace was simulated using the numerical analysis method to obtain free-standing and large-size seeds. The theoretical studies indicate that the temperature distribution in the crucible is related to the crucible height. According to the theory of growth dynamics and growth surface dynamics, the optimal thermal distribution was achieved through the design of a specific crucible structure, which is determined by the ratio of top-heater power to main-heater power. Moreover, in our experiment, a sole AlN single crystal seed with a length of 12 mm was obtained on the tungsten (W) substrate. The low axial temperature gradient between material source and substrate can decrease the nucleation rate and growth rate, and the high radial temperature gradient of the substrate can promote the expansion of crystal size. Additionally, the crystallinity of the crystals grown under different thermal field conditions are analyzed and compared. The Raman results manifest the superiority of the thermal inversion method in the growth of high quality AlN single crystal. [ABSTRACT FROM AUTHOR]

Aluminum nitride (AlN) is a multifunctional semiconductor. AlN doped with rare earth ions has broad application prospects in monochromatic lighting, color display, laser medium, medical treatment, etc. In this paper, an Er3+-doped AlN film has been prepared by radio frequency magnetron sputtering and characterized by surface morphology, chemical composition and photoluminescence (PL), focusing on the emission mechanism of Er3+ in the AlN host. The films show PL in a wide range of the visible (540, 560 and 668 nm) and near-infrared (816, 869, 985 and 1534 nm) ranges. It has been found a certain energy transfer bridge between Er3+ and AlN defect-related optical transitions. At 532 nm excitation, V Al -O N vacancy-oxygen complex in the AlN host efficiently absorbs photons, then, nonradiative energy transfer (NRET) occurs from the defect levels to the levels of Er3+ since they are in resonance. Since there is a resonance of a donor emission level and an acceptor absorption level, Förster resonance energy transfer (FRET) occurs with the transfer efficiency about 51%. This results in a very strong enhancement of excitation of the 4f related PL. On the contrary, optical excitation at other energies results in a low intensity of the Er3+ PL. As a matter of facts, a strong absorption of the AlN host and the FRET-enabling resonance of donor-acceptor levels allow for a great enhancement of the PL efficiency of Er3+ ions. The results show that the impurity defect state of AlN has an important influence on the Er3+ luminescence efficiency. [Display omitted] • AlN:Er thin films grown by magnetron sputtering have visible and infrared luminescence. • Efficient excitation of Er3+ ions occurs via resonant nonradiative energy transfer (NRET) from AlN defects. • Since there is a resonance of AlN defect and Er3+ levels, Förster resonance energy transfer occurs. • A strong absorption of oxygen-defect complex of AlN enhances the luminescence efficiency of Er3+. • NRET leads to a very strong enhancement of excitation of Er3+ ions, with an efficiency about 51%. [ABSTRACT FROM AUTHOR]