Liquid-Metal Synthesized Ultrathin SnS Layers for High-Performance Broadband Photodetectors

Atomically thin materials face an ongoing challenge of scalability, hampering practical deployment despite their fascinating properties. Tin monosulfide (SnS), a low-cost, naturally abundant layered material with a tunable bandgap, displays properties of superior carrier mobility and large absorption coefficient at atomic thicknesses, making it attractive for electronics and optoelectronics. However, the lack of successful synthesis techniques to prepare large-area and stoichiometric atomically thin SnS layers (mainly due to the strong interlayer interactions) has prevented exploration of these properties for versatile applications. Here, SnS layers are printed with thicknesses varying from a single unit cell (0.8 nm) to multiple stacked unit cells (approximate to 1.8 nm) synthesized from metallic liquid tin, with lateral dimensions on the millimeter scale. It is reveal that these large-area SnS layers exhibit a broadband spectral response ranging from deep-ultraviolet (UV) to near-infrared (NIR) wavelengths (i.e., 280-850 nm) with fast photodetection capabilities. For single-unit-cell-thick layered SnS, the photodetectors show upto three orders of magnitude higher responsivity (927 A W-1) than commercial photodetectors at a room-temperature operating wavelength of 660 nm. This study opens a new pathway to synthesize reproduceable nanosheets of large lateral sizes for broadband, high-performance photodetectors. It also provides important technological implications for scalable applications in integrated optoelectronic circuits, sensing, and biomedical imaging

GaN-UV photodetector integrated with asymmetric metal semiconductor metal structure for enhanced responsivity

Fabrication of very thin GaN ultraviolet photodetectors on Si (111) substrate integrated with asymmetric (Pt-Ag, Pt-Cr) metal-semiconductor-metal (MSM) structure have been illustrated. Designed GaN photodetection device displays significant enhancement in responsivity for asymmetric (Pt-Ag) MSM structure (280 mA/W) in comparison to symmetric (Pt-Pt) MSM structure (126 mA/W) at 10 V bias. The fabricated asymmetric and symmetric devices also exhibit fast response time in the range of 30-59 ms. The enhancement in responsivity using asymmetric MSM structure ascribed to large difference in work function which lead to change in Schottky barrier height of the metal semiconductor junction. Additionally, power dependent photoresponse analysis of GaN asymmetric (Pt-Ag) ultraviolet photodetector was showing a responsivity of 116 mA/W at low optical power of 1 mW. Such GaN asymmetric MSM ultraviolet photodetectors having high responsivity can extensively be used for low power, high speed ultraviolet photo detection application

A high-performance hydrogen sensor based on a reverse-biased MoS2/GaN heterojunction

We report a MoS2/GaN heterojunction-based gas sensor by depositing MoS2 over a GaN substrate via a highly controllable and scalable sputtering technique coupled with a post sulfurization process in a sulfur-rich environment. The microscopic and spectroscopic measurements expose the presence of highly crystalline and homogenous few atomic layer MoS2 on top of molecular beam epitaxially grown GaN film. Upon hydrogen exposure, the molecular adsorption tuned the barrier height at the MoS2/GaN interface under the reverse biased condition, thus resulting in high sensitivity. Our results reveal that temperature strongly affects the sensitivity of the device and it increases from 21% to 157% for 1% hydrogen with an increase in temperature (25-150 degrees C). For a deeper understanding of carrier dynamics at the heterointerface, we visualized the band alignment across the MoS2/GaN heterojunction having valence band and conduction band offset values of 1.75 and 0.28 eV. The sensing mechanism was demonstrated based on an energy band diagram at the MoS2/GaN interface in the presence and absence of hydrogen exposure. The proposed methodology can be readily applied to other combinations of heterostructures for sensing different gas analytes

A high-performance hydrogen sensor based on a reverse-biased MoS2/GaN heterojunction

We report a MoS2/GaN heterojunction-based gas sensor by depositing MoS2 over a GaN substrate via a highly controllable and scalable sputtering technique coupled with a post sulfurization process in a sulfur-rich environment. The microscopic and spectroscopic measurements expose the presence of highly crystalline and homogenous few atomic layer MoS2 on top of molecular beam epitaxially grown GaN film. Upon hydrogen exposure, the molecular adsorption tuned the barrier height at the MoS2/GaN interface under the reverse biased condition, thus resulting in high sensitivity. Our results reveal that temperature strongly affects the sensitivity of the device and it increases from 21% to 157% for 1% hydrogen with an increase in temperature (25-150 degrees C). For a deeper understanding of carrier dynamics at the heterointerface, we visualized the band alignment across the MoS2/GaN heterojunction having valence band and conduction band offset values of 1.75 and 0.28 eV. The sensing mechanism was demonstrated based on an energy band diagram at the MoS2/GaN interface in the presence and absence of hydrogen exposure. The proposed methodology can be readily applied to other combinations of heterostructures for sensing different gas analytes

Influence of temperature and A1/N ratio on structural, chemical & electronic properties of epitaxial A1N films grown via PAMBE

The present article investigates structural, chemical and electronic properties of epitaxial AlN films grown via plasma assisted molecular beam epitaxy on atomically clean Si (1 1 1) substrates. An inclusive optimization process of growth parameters by varying the substrate temperature (790-825 degrees C) and Al/N (III/V) ratio is demonstrated. The AlN film grown with optimized parameters yielded an FWHM of 24.6 arcmin, crystallite size of 11.6 nm, screw dislocation density of 4.43 x 10(9)/cm(2) and a surface roughness of 3.11 nm. Besides, the chemical states and electronic structure analysis displayed absence of remnant metallic aluminium and native surface oxide (-2%) with Fermi level (3.0 eV) pinned near its intrinsic value. A growth mechanism has been proposed for the optimized growth of AlN. Further, the high quality AlN film can potentially be used for the fabrication of smart optoelectronics for deep UV application and field emission devices

Impact of thermal oxidation on the electrical transport and chemical & electronic structure of the GaN film grown on Si and sapphire substrates

Gallium oxide (Ga2O3) has emerged as a fourth-generation semiconductor for futuristic device requirements. Integration of Ga2O3 with industry-viable gallium nitride (GaN) can provide a pathway to design efficient device technology. In this paper, we design Ga2O3/GaN heterointerface by using a simple thermal annealing method under an oxygen-rich environment. Thermal annealing at 850 °C for 5 h results in a good yield of Ga2O3 from surficial GaN which was grown on sapphire and Si substrates. Surface morphology revealed a nanorod structure-based Ga2O3 on the GaN surface grown on industrial compatible Si substrate. XPS measurements provide a quantitative understanding of the heterostructure, where 84.62% and 70.92% of surficial GaN is converted into Ga2O3. Besides, the valence band maximum is shifted to a higher energy side in comparison to bare GaN samples. This helps in understanding the device physics of the grown heterostructures. Current-Voltage (I–V) characteristics revealed Schottky behaviour where the Schottky barrier height is increased after thermal annealing of the GaN films. Temperature correlated I–V characteristics suggest that the thermally annealed GaN films are stable up to 300 °C. These material structures can potentially be used in the high-temperature applications for power devices, optoelectronics, and sensing applications

Microstructural evolution of high quality AlN grown by PAMBE under different growth conditions

The morphological evolution of AlN microstructures by varying the growth temperature and Al/N flux ratio on Si (111) substrate via plasma-assisted molecular beam epitaxy has been investigated. The transformations in microstructures of AlN grown along the c-plane were explored as a function of N-2-flow rate, growth temperature and Al-flux. The structural analysis carried out using high resolution X-ray diffraction reveals single crystalline quality with reduced full widths at half maximum value of 15 arcmin corresponding to a screw dislocation density of 8.5 x 10(8) cm(-2). The topographical study of AlN grown by modulating growth conditions revealed an average surface roughness of 6.9 nm. It was exemplified that interplay between higher growth temperature and nitrogen flow rate is desired to prevent condensation of metallic Al on the surface. Also, the AlN pertaining less screw dislocation density leads to lower dark current which can be fruitful for various optoelectronic applications like vacuum-UV photodetectors

Investigating the growth of AlGaN/AlN heterostructure by modulating the substrate temperature of AlN buffer layer

Abstract: We have investigated the impact of AlN buffer layer growth parameters for developing highly single crystalline AlGaN films. The low mobility of Al adatoms and high temperature for compound formation are amongst the major causes that affects the growth quality of AlGaN films. Thus, proper optimization need to be carried out for achieving high quality AlGaN due to an augmented tendency of defect generation compared to GaN films. Thus, growth conditions need to be amended to maximize the incorporation ability of adatoms and minimize defect density. So, this study elaborates the growth optimization of AlGaN/AlN/Si (111) heterostructure with varied AlN buffer growth temperature (760 to 800 °C). It was observed that the remnant Al in low temperature growth of AlN buffer layer resist the growth quality of AlGaN epitaxial films. A highly single crystalline AlGaN film with comparatively lowest rocking curve FWHM value (~ 0.61°) and smooth surface morphology with least surface defect states was witnessed when AlN buffer was grown at 780 °C. From the Vegard’s law, the photoluminescence analysis unveils Aluminium composition of 31.5% with significantly reduced defect band/NBE band ratio to 0.3. The study demonstrates good crystalline quality AlGaN film growth with Aluminium content variation between ~ 30–39% in AlGaN/AlN heterostructure on Si(111) substrate leading to a bandgap range which is suitable for next-generation solar-blind photodetection applications

Liquid-Metal Synthesized Ultrathin SnS Layers for High-Performance Broadband Photodetectors

Atomically thin materials face an ongoing challenge of scalability, hampering practical deployment despite their fascinating properties. Tin monosulfide (SnS), a low-cost, naturally abundant layered material with a tunable bandgap, displays properties of superior carrier mobility and large absorption coefficient at atomic thicknesses, making it attractive for electronics and optoelectronics. However, the lack of successful synthesis techniques to prepare large-area and stoichiometric atomically thin SnS layers (mainly due to the strong interlayer interactions) has prevented exploration of these properties for versatile applications. Here, SnS layers are printed with thicknesses varying from a single unit cell (0.8 nm) to multiple stacked unit cells (≈1.8 nm) synthesized from metallic liquid tin, with lateral dimensions on the millimeter scale. It is reveal that these large-area SnS layers exhibit a broadband spectral response ranging from deep-ultraviolet (UV) to near-infrared (NIR) wavelengths (i.e., 280-850 nm) with fast photodetection capabilities. For single-unit-cell-thick layered SnS, the photodetectors show upto three orders of magnitude higher responsivity (927 A W−1 ) than commercial photodetectors at a room-temperature operating wavelength of 660 nm. This study opens a new pathway to synthesize reproduceable nanosheets of large lateral sizes for broadband, high-performance photodetectors. It also provides important technological implications for scalable applications in integrated optoelectronic circuits, sensing, and biomedical imaging.The authors would like to acknowledge support from the ARC Discovery Project schemes DP180102752 (Y.L., M.J.S.S.), DP180104141 (S.B. and K.C.), DP170102138 (K.K.Z.) and the RMIT Vice Chancellor Fellowships (N.M. and S.W.). K.K.Z. also acknowledges support from the Australian Research Council Centre of Excellence FLEET

Customized Two-Dimensional Nanostructured MoO₃ Inks For Spectrally Selective UV Chromic Patches

Ultraviolet (UV) radiation exposure plays an important role in human health. However, excessive exposure can lead to skin injuries and melanoma. It is crucial to have a technology that can monitor exposure, particularly to UV A, which penetrates the human skin the most and also assists in building healthy sun habits in the general population. Current sensors do not offer the needed combination of narrow optical absorbance, ease of use, and low cost. Here, we report MoO3 photochromic inks that are customized to the skin complexion based on Fitzpatrick’s skin typing system. The ink is subsequently incorporated into bioelastomers to make “stick-on” UV patches that provide a qualitative assessment of UV A exposure to a user on the go. The photochromism of the patch upon UV exposure is correlated to the calculated percentages of the minimal erythema dose (MED) limits. The patch gives a visual indication of the levels of UV A exposure, which will allow the users to make informed choices to avoid overexposure and prevent skin damage. The patch is fabricated using a manufacturing-compatible screen printing process. This technology has the potential to drive behavioral changes in the population and move toward an aware and sun-smart community