Ultraviolet Optoelectronic Devices Research Articles

Deep Ultraviolet Excitation Photoluminescence Characteristics and Correlative Investigation of Al-Rich AlGaN Films on Sapphire.

AlGaN is attractive for fabricating deep ultraviolet (DUV) optoelectronic and electronic devices of light-emitting diodes (LEDs), photodetectors, high-electron-mobility field-effect transistors (HEMTs), etc. We investigated the quality and optical properties of AlxGa1-xN films with high Al fractions (60-87%) grown on sapphire substrates, including AlN nucleation and buffer layers, by metal-organic chemical vapor deposition (MOCVD). They were initially investigated by high-resolution X-ray diffraction (HR-XRD) and Raman scattering (RS). A set of formulas was deduced to precisely determine x(Al) from HR-XRD data. Screw dislocation densities in AlGaN and AlN layers were deduced. DUV (266 nm) excitation RS clearly exhibits AlGaN Raman features far superior to visible RS. The simulation on the AlGaN longitudinal optical (LO) phonon modes determined the carrier concentrations in the AlGaN layers. The spatial correlation model (SCM) analyses on E2(high) modes examined the AlGaN and AlN layer properties. These high-x(Al) AlxGa1-xN films possess large energy gaps Eg in the range of 5.0-5.6 eV and are excited by a DUV 213 nm (5.8 eV) laser for room temperature (RT) photoluminescence (PL) and temperature-dependent photoluminescence (TDPL) studies. The obtained RTPL bands were deconvoluted with two Gaussian bands, indicating cross-bandgap emission, phonon replicas, and variation with x(Al). TDPL spectra at 20-300 K of Al0.87Ga0.13N exhibit the T-dependences of the band-edge luminescence near 5.6 eV and the phonon replicas. According to the Arrhenius fitting diagram of the TDPL spectra, the activation energy (19.6 meV) associated with the luminescence process is acquired. In addition, the combined PL and time-resolved photoluminescence (TRPL) spectroscopic system with DUV 213 nm pulse excitation was applied to measure a typical AlGaN multiple-quantum well (MQW). The RT TRPL decay spectra were obtained at four wavelengths and fitted by two exponentials with fast and slow decay times of ~0.2 ns and 1-2 ns, respectively. Comprehensive studies on these Al-rich AlGaN epi-films and a typical AlGaN MQW are achieved with unique and significant results, which are useful to researchers in the field.

Influence of Growth Temperature on Morphological and Structural Properties of Sputtered- Zinc Oxide Nanorods

Zinc oxide is highly sought after for several applications in biology, optoelectronics, electronics, and sensing. Zinc oxide nanorods exhibit a high exciton binding energy and a wide direct band gap, making them an attractive material for ultraviolet optoelectronic devices in the compound semiconductor field. This investigation focuses mainly on the influence of growing temperature on the crystal structure and surface morphology of ZnO nanorods, specifically for their application as UV photodetectors. Si (100) substrates were utilized for the growth of ZnO nanorods using radio frequency magnetron sputtering. The ZnO nanorods are grown at temperatures of 350 °C, 375 °C, and 400 °C, respectively. The crystal structure and surface morphology were examined using X-ray diffraction and scanning electron microscopy. The samples exhibited a same crystal orientation of the (002) diffraction peak at all growth temperatures. The growth temperatures show a direct influence on the density and length of nanorods, as evidenced by the scanning electron microscopy photographs. The results revealed the significant influence of substrate temperature on the regulation of nanorod length and crystal formation.

Synthesis and characterizations of zinc oxide nanorods by cost effective aqueous hydrothermal technique for solid state lighting applications

Zinc oxide (ZnO) is recognized as the most promising and often utilized semiconducting material due to its exceptional physical, chemical, optical, and relatively low-cost production features, as well as its ecologically beneficial nature. ZnO is a prominent candidate owing to its wide band gap (3.37 eV at room temperature) as well as its high exciton binding energy (60 meV) for the next generation of ultraviolet and blue optoelectronic devices. This would be achieved through the development of hybrid heterostructured LED devices with low manufacturing costs by blending gallium nitride (GaN) with zinc oxide (ZnO), which serves as the most efficient opto-electronic device. Due to its high electron mobility, high transparency in the visible wavelength spectrum, and low work function, ZnO is also a highly investigated transparent conducting oxide thin film for several applications. Besides, their incredible optical capabilities, ZnO-based optical devices, such as light-emitting diodes (LEDs) and laser diodes (LDs), have drawn a great deal of interest. This reported research work demonstrates the growth of high-density, high-aspect-ratio ZnO nanorod (NR) arrays on p-type gallium nitride substrates employing low-cost facial aqueous hydrothermal syntheses with optimised growing conditions. Additionally, a top-down chemical etching technique is adapted for fabricating zinc oxide nanotubes. High-quality ZnO nanorods with high intensity that were a few microns long were found across the GaN substrate with FESEM analysis. The chemical composition of the grown nanorods is confirmed with EDAX analysis. The wurtzite crystalline structure and c-axis orientation of the grown ZnO NR-arrays were identified in X-ray diffraction studies. A Schimadzu UV–Vis–NIR spectrophotometer was used to record the optical curve, and the energy gap was determined to be 3.24 eV by plotting the Tauc curve. The good optical quality of the harvested nanorods is ensured by the narrow near-band-edge (NBE) emission at 384 nm and defect emission peaks detected through photoluminescence analyses. Grown specimens are also subjected to Raman spectrum analyses, and the obtained results are presented.

Enhanced p-type conductivity of hexagonal boron nitride by an efficient two-step doping strategy

The present study proposes a two-step doping strategy for achieving efficient Mg doping of h-BN, involving an additional post-annealing treatment. This approach leads to ∼108-fold enhancement in conductivity of h-BN, compared with the as-doped h-BN grown by low-pressure chemical vapor deposition. The mechanism for large enhancement in h-BN doping efficiency after post-annealing was investigated, providing evidence that this treatment not only facilitates the nanoparticle decomposition and incorporation of Mg atoms into h-BN, but also restores its lattice defects. The efficient two-step doping strategy for p-type h-BN in this study enlightens its promising prospects for ultraviolet optoelectronic devices.

Theoretical insights into the structural and optoelectronic properties of the ternary cesium tetrafluoridobromate semiconductor CsBrF4

This work deals with the first theoretical study of structural, electronic, and optical properties of CsBrF4 ternary compound by using first-principle calculations. The calculated structural parameters and atomic positions are in accordance with the experimental ones declared in the literature. The obtained electronic properties signalize that this compound is a semiconductor material with a direct energy band gap. According to the investigated optical properties, the vast optical absorption range in the ultraviolet region indicates that CsBrF4 semiconductor could be useful for specific applications in ultraviolet optoelectronic devices; consequently, this theoretical study is the potential to support future experimental research.

Insights into the optoelectronic and thermoelectric properties of defect chalcopyrites XAl2Se4 (X = Zn, Cd, and Hg): A density functional theory approach

In the current study, we employed the full-potential linearized augmented plane wave plus local orbitals (FP-LAPW + lo) approach within the density functional theory (DFT) to investigate the physical properties of Defect Chalcopyrites XAl2Se4 (X = Zn, Cd, and Hg). Calculations yield precise structural and electronic parameters in good agreement with experimental data, revealing all three compounds as direct wide-bandgap semiconductors. The modified Becke-Johan potential (TB-mBJ) method accurately estimates energy gaps, which increase with decreasing lattice constants. The calculated values are 3.34 eV, 3.12 eV, and 2.30 eV for ZnAl2Se4, CdAl2Se4, and HgAl2Se4, respectively. The optical properties, including dielectric function and absorption coefficient, indicate potential applications in visible and ultraviolet optoelectronic devices. The high anisotropy of these materials further enhances their suitability for a range of linear and nonlinear optical applications. Moreover, ZnAl2Se4 exhibits the highest Seebeck coefficient at room temperature. The positive Seebeck coefficient confirmed the p-type behavior of the studied compounds. The highest observed power factor demonstrates optimal thermoelectric performance for these materials. High electrical conductivity values suggest their suitability as effective electrical conductors, especially at elevated temperatures. This study encourages further research into defect chalcopyrites for applications in optoelectronics and thermoelectric energy conversion.

Structural and optoelectronic properties of Zn1-x-yBexMgyTe/InP quaternary alloys: A theoretical study

Density functional calculations show that designed zinc blende Zn1-x-yBexMgyTe/InP quaternary alloys are Γ-centered direct band-gap semiconductors and stable within a wide compositional range. Enhancement in the of Be and Mg compositions sum enhances lattice constant, but reduces minimum band-gap of these quaternary alloys. The Te-5p of valence to Zn-5s; Be-2p,3s; Mg-4s, 3p of conduction band HOMO-LUMO electronic excitations collectively control their optical properties. Such transitions show significant optical absorption in the ultra-violet (UV) region of the electromagnetic spectrum. Calculated optical energy-gaps of these alloys reside in the UV region. Optical energy-gap of quaternary alloy is found to be overestimated with respect to the corresponding fundamental band-gap. With enhancement in the Be and Mg compositions sum, both the fundamental band-gap and optical energy-gap exhibit blue-shift. Computed ε1 (0), n (0) and R (0) reduces with growing band-gap and vice versa. These Zn1-x-yBexMgyTe/InP semiconductor quaternary alloys with such optoelectronic features would be compatible for fabricating UV optoelectronic devices.

Insight into Interfacial Heat Transfer of β-Ga2O3/Diamond Heterostructures via the Machine Learning Potential.

β-Ga2O3 is an ultrawide-band gap semiconductor with excellent potential for high-power and ultraviolet optoelectronic device applications. Low thermal conductivity is one of the major obstacles to enable the full performance of β-Ga2O3-based devices. A promising solution for this problem is to integrate β-Ga2O3 with a diamond heat sink. However, the thermal properties of the β-Ga2O3/diamond heterostructures after the interfacial bonding have not been studied extensively, which are influenced by the crystal orientations and interfacial atoms for the β-Ga2O3 and diamond interfaces. In this work, molecular dynamics simulations based on machine learning potential have been adopted to investigate the crystal-orientation-dependent and interfacial-atom-dependent thermal boundary resistance (TBR) of the β-Ga2O3/diamond heterostructure after interfacial bonding. The differences in TBR at different interfaces are explained in detail through the explorations of thermal conductivity value, thermal conductivity spectra, vibration density of states, and interfacial structures. Based on the above explorations, a further understanding of the influence of different crystal orientations and interfacial atoms on the β-Ga2O3/diamond heterostructure was achieved. Finally, insightful optimization strategies have been proposed in the study, which could pave the way for better thermal design and management of β-Ga2O3/diamond heterostructures according to guidance in the selection of the crystal orientations and interfacial atoms of the β-Ga2O3 and diamond interfaces.

Responses Toward Structural, Electrical, Optical, and Mechanical Properties of RbGeF3 Under Pressure: DFT Insights

AbstractDensity functional theory is used in the current study to thoroughly examine the physical properties of perovskites made of halide RbGeF3 under a variety of hydrostatic pressure ranges between 0 and 50 GPa. This research seeks to reduce the electronic bandgap of RbGeF3 under pressure to enhance the optical properties and evaluate the compounds' decency for usage in optoelectronic and electrical uses. The precision of the structural characteristics is relatively high, and they fit well with published earlier research. A higher interaction between atoms is also a result of the large drop in lattice characteristics and link length. Pressurization reveals the ionic and covalent characteristics of the bonds between Rb‐F and Ge‐F, respectively. Both the conductivity and the optical absorbance vary noticeably when hydrostatic pressure is applied. A zero bandgap finally arises via pressure‐induced bandgap shrinkage, improving conductivity and electromagnetic absorption. Based on their optical properties, the materials being studied could be used in a variety of visible and ultraviolet optoelectronic devices. External pressure increases the anisotropy and ductility of the aforementioned perovskites, hence influencing their mechanical behavior. This study describes the changes in physical characteristics brought on by applied stress and offers a thorough examination of those changes.

Ionic liquid gated black phosphorus/SiC-based 2D/3D mixed-dimensional photodetector

The utilization of 2D layered black phosphorus (BP) in ultraviolet (UV) optoelectronic devices has garnered significant attention due to its high hole mobility and strong light–material interaction. However, its practical applications are impeded by its low light absorption efficiency and environmental instability. To address these challenges, constructing an ionic liquid gated BP/SiC mixed-dimensional photodetector by integrating the high ultraviolet light absorption capacity of SiC with the effective electronic property modulation and surface protection of ionic liquid is suggested in this work. Remarkably, the developed device exhibits an improving photoresponsivity of 10.24 A/W at a gate voltage of −2 V and source–drain voltage of 0.5 V, which is ∼73 times larger than that of the BP UV photodetector, as well as an enhanced environmental stability for at least 30 days. This research offers potential strategies for optimizing the performance of advanced functional devices using 2D/3D mixed-dimensional heterojunctions.

Effects of different atomic passivation on conductive and dielectric properties of silicon carbide nanowires

Based on the transport and polarization relaxation theories, the effects of hydrogen, fluorine, and chlorine atom passivation on the conductivity and dielectric properties of silicon carbide nanowires (SiCNWs) were numerically simulated. The results show that passivation can decrease the dark conductivity of SiCNWs and increase its ultraviolet photoconductivity. Among them, the photoconductivity of univalent (H) passivated SiCNWs is better than that of seven-valent (Cl, F) passivated SiCNWs. In terms of dielectric properties, the passivated SiCNWs exhibit a strong dielectric response in both deep ultraviolet and microwave regions. Hydrogen passivation SiCNWs produce the strongest dielectric response in deep ultraviolet, while fluorine passivation SiCNWs produce the strongest dielectric relaxation in the microwave band, which indicates that atomic passivation SiCNWs have a wide range of applications in ultraviolet optoelectronic devices and microwave absorption and shielding.

Probing and controlling oxygen impurity diffusion in h-BN semi-bulk crystals

Combining its unique features of ultrawide bandgap (UWBG) and two-dimensional nature, h-BN has been explored for emerging applications such as deep ultraviolet optoelectronic devices and single photon emitters. One of the unusual applications of h-BN is for solid-state neutron detectors by utilizing the property of high thermal neutron capture cross section of B-10 as well as its UWBG properties. Although a record high detection efficiency of 59% has been attained by h-BN detectors, the understanding/minimization of defects and impurities is still needed to further advance the h-BN material and detector technologies. We report metal organic chemical vapor deposition growth and oxygen (O) impurity diffusion in thick h-BN. The diffusion coefficient (D) of O impurities has been measured via the evolution of an oxygen related emission with the etching depth, providing a value of D of ∼ 2 × 10−13 cm2/s at 1450 °C and supporting the interpretation that oxygen in h-BN is a substitutional donor. A multiple-buffer-layer approach was employed to mitigate to a certain degree the issue of oxygen diffusion from sapphire substrate during growth. It was demonstrated that the performance of h-BN neutron detectors fabricated from the wafer incorporating multiple buffer layers was significantly improved, as manifested by the enhanced thermal neutron detection efficiency. The advancement of the crystal growth technology of h-BN semi-bulk crystals creates applications in optoelectronic and power electronic devices utilizing the UWBG semiconductor properties of h-BN, while high efficiency h-BN neutron detectors have the potential to supplant the traditional He-3 gas detectors in various application areas by offering the obvious advantages of UWBG semiconductor technologies.

A theoretical investigation on optoelectronic and photocatalytic behaviour of Janus X-Ga-Al-Y (X, Y[dbnd]S and Te) monolayers

The structural, electronic, and optical characteristics of hexagonal Janus X-Ga-Al-Y (X, YS and Te) monolayers have been assessed within the theoretical framework by using density functional theory. The negative cohesive energy of all monolayers confirms the energetic stability. The phonon dispersion diagram validates that all Janus X-Ga-Al-Y (X, YS and Te) monolayers are dynamically stable. The obtained indirect type band gap values of monolayers GaS, GaAlS2, GaAlTe2, SGaAlTe, and TeGaAlS are 3.25 eV, 2.87 eV, 2.45 eV, 2.25 eV, and 1.39 eV by utilizing Heyd-Scuseria-Ernzerhof functional respectively which indicates their semiconductor behaviour. All the monolayers possess the utmost absorption in the ultraviolet (UV) portion. Hence, X-Ga-Al-Y (X, YS and Te) monolayers have promising applications as UV absorbers, optoelectronic devices, and UV detectors. The band edge alignments of the X-Ga-Al-Y (X, YS and Te) monolayers also have been analyzed which suggests that X-Ga-Al-Y (X, YS and Te) monolayers are efficient as photocatalysts for hydrogen production.

Influence of oxygen partial pressure on the performance of MoO3-based ultraviolet photodetectors

To save our motherland from the harmful ultraviolet (UV) ray, the essentiality of UV photodetectors is increasing gradually. Herein, the MoO3-x thin films are deposited onto the SiO2/Si substrates through a sputtering technique by varying the oxygen partial pressure (pO2) at a substrate temperature of 200°C to examine the UV photodetector performance. It is observed that the thickness of the thin film is decreased with an increase in the pO2 due to the less ejection of atoms from the oxidized target surface. Structural study confirms the improved crystalline nature of the thin film deposited at 5.0% of pO2 and results in better electrical properties. The photodetector performance under UV illumination is studied as a function of pO2. The linear behavior of the current-voltage (I-V) characteristics attests to the ohmic contact nature between the photoactive layer (MoO3-x) and the metal (Ag) electrode interface. The fabricated photodetector test-device achieves a high photocurrent of 11.22 µA, a fast response time of 100 ms, and a superior responsivity of 1.537 A/W even at a lower optical power density of 0.0292 mW/cm2. Finally, these enhanced figures of merit of the MoO3-based photodetector could show a significant path toward the development of fast UV photodetector and optoelectronic devices.

Ga2O3/Ag/Ga2O3-Laminated Film Fabricated at Room Temperature: Toward Applications in Ultraviolet Transparent Highly Conductive Electrodes

Ga2O3/Ag/Ga2O3-laminated films with high electrical conductivity and ultraviolet (UV) transparency were achieved by radio frequency magnetron sputtering at room temperature (RT) on quartz glass. The influence of annealing temperature and ambient on the structural, electrical and optical properties of Ga2O3/Ag/Ga2O3-laminated films were investigated in detail. As the annealing temperature increases, the optical bandgap of the Ga2O3-laminated films widens. The Ga2O3/Ag/Ga2O3-laminated films exhibited good photoelectric performance with a figure-of-merit (FOM) value of 5.83 × 10−3 Ω−1, a sheet resistance of 12.55 Ω/sq, a transmittance of 95.15% at 325 nm, and an average transmittance of 77.56% (250~300 nm). All these results suggest that RT-fabricated Ga2O3/Ag/Ga2O3-laminated films show great potential in UV transparent conductive electrodes for UV optoelectronic devices and in flexible electronics.

Layer-selective growth of 2D hexagonal boron nitride using two-step chemical vapor deposition

Hexagonal boron nitride (h-BN) has recently become a promising and popular material due to its wide applications, such as deep ultraviolet optoelectronic devices, novel nonvolatile memory technology, room temperature quantum emitter, and dielectric in devices based on two-dimensional (2D) materials. However, realizing uniform h-BN thin films thicker than monolayer remains a major challenge, which is important in both fundamental and practical studies. In this work, we report the selective growth of large-area bilayer h-BN using a two-step chemical vapor deposition (CVD) approach. Through constructing a confined space in the Cu foils/sapphire stack, controlled synthesis of bilayer h-BN is achieved. Systematic characterization verified the formation of bilayer h-BN and the mechanism of CVD growth is also studied. We find that the in situ annealing between the two steps could efficiently facilitate the selective growth, and the local oxidation and reduction process could create preferred nucleation sites for uniform bilayer h-BN. Our results could enrich the understanding of the synthesis mechanism and lay the foundation for controllable synthesis of high-quality h-BN.

Nanoscale phase separation on an AlGaN surface characterized by scanning diffusion microscopy.

AlGaN is an important material for deep ultraviolet optoelectronic devices and electronic devices. The phase separation on the AlGaN surface means small-scale compositional fluctuations of Al, which is prone to degrade the performance of devices. In order to study the mechanism of the surface phase separation, the Al0.3Ga0.7N wafer was investigated by the scanning diffusion microscopy method based on the photo-assisted Kelvin force probe microscope. The response of the surface photovoltage near the bandgap was quite different for the edge and the center of the island on the AlGaN surface. We utilize the theoretical model of scanning diffusion microscopy to fit the local absorption coefficients from the measured surface photovoltage spectrum. During the fitting process, we introduce as and ab parameters (bandgap shift and broadening) to describe the local variation of absorption coefficients α(as, ab, λ). The local bandgap and Al composition can be calculated quantitatively from the absorption coefficients. The results show that there is lower bandgap (about 305 nm) and lower Al composition (about 0.31) at the edge of the island, compared with those at the center of the island (about 300 nm for bandgap and 0.34 for Al composition). Similar to the edge of the island, there is a lower bandgap at the V-pit defect which is about 306 nm corresponding to the Al composition of about 0.30. These results mean Ga enrichment both at the edge of the island and the V-pit defect position. It proves that scanning diffusion microscopy is an effective method to review the micro-mechanism of AlGaN phase separation.

Synthesis and investigation of the physicochemical properties of polymorphic 3C–SiC

Silicon carbide (SiC) nanopowders were synthesized under Ar atmosphere via the carbonization reaction method using silicon powders and expanded graphite as initial materials, and ferric nitrate as catalyst precursor. The effects of heat treatment temperature and the C/Si molar ratio on SiC products were studied. The microstructure, phase composition and chemical state of the products were analyzed by XRD, SEM, XPS, TEM and Raman. The photoluminescence (PL) properties of SiC nanopowders were tested. The results showed that the content of the 3C–SiC product is higher after heat treatment at 1400 °C for 3 h with a molar ratio of C/Si of 1:1. 3C–SiC exists in the form of particles and wires. The formation process of 3C–SiC can be explained by a two-step method, nucleation under catalysis and growth under Vapor-Solid (VS) mechanism. Two ultraviolet light emission peaks are observed at about 318 and 380 nm. The main PL spectra of 3C–SiC nanopowders exhibit larger blueshifts. The blueshift is attributed to the stacking fault, oxygen defects, morphology and quantum confinement effect. It has potential application prospects in ultraviolet detectors, optoelectronic devices and light emitters.

Antireflective GaN Nanoridge Texturing by Metal-Assisted Chemical Etching via a Thermally Dewetted Pt Catalyst Network for Highly Responsive Ultraviolet Photodiodes.

Antireflective (AR) surface texturing is a feasible way to boost the light absorption of photosensitive materials and devices. As a plasma-free etching method, metal-assisted chemical etching (MacEtch) has been employed for fabricating GaN AR surface texturing. However, the poor etching efficiency of typical MacEtch hinders the demonstration of highly responsive photodetectors on an undoped GaN wafer. In addition, GaN MacEtch requires metal mask patterning by lithography, which leads to a huge processing complexity when the dimension of GaN AR nanostructure scales down to the submicron range. In this work, we have developed a facile texturing method of forming a GaN nanoridge surface on an undoped GaN thin film by a lithography-free submicron mask-patterning process via thermal dewetting of platinum. The nanoridge surface texturing effectively reduces the surface reflection in the ultraviolet (UV) regime, which can be translated to a 6-fold enhancement in responsivity (i.e., 115 A/W) of the photodiode at 365 nm. The results demonstrated in this work show that MacEtch can offer a viable route for enhanced UV light-matter interaction and surface engineering in GaN UV optoelectronic devices.

Effect of stress on thermal properties of AlGaN nanofilms

Aluminum gallium nitride (AlGaN) nanofilms have been widely applied as active layers in ultra-violet opto-electronic devices and power electronics. Stress plays essential role in AlGaN based devices, especially in high electron mobility transistor. Therefore, it is necessary to investigate the thermal properties of AlGaN nanofilms with various stresses. In this work, biaxial stressed [0001] oriented AlGaN nanofilms were studied. The phonon dispersion, density of states, velocity and heat capacity were simulated based on the elastic theory. Thermal conductivities of AlGaN nanofilms, which was found 1 ∼ 2 orders of magnitude lower than the bulk materials, were then calculated by Boltzmann transport equation. Due to the modification of phonon dispersion and increasing of group velocity by tensile stress, the thermal conductivities of AlGaN nanofilms increase from compressive stress (−15 GPa) to tensile stress (+15 GPa). Moreover, a phonon energy gap appears in AlN nanofilm of −15 GPa, which disrupts the linear relation between thermal conductivity and stresses. Our work confirmed that the stress could be promising to tune the thermal conductivity of AlGaN nanofilms.