Ultraviolet Optoelectronic Devices Research Articles

Growth temperature impact on film quality of hBN grown on Al2O3 using non-catalyzed borazane CVD

Hexagonal boron nitride (hBN) films were directly grown on c-plane Al2O3 substrates by low pressure thermal chemical vapor deposition (CVD) with single precursor i.e. borazane (NH3-BH3) without employing any extra catalysis. The growth temperature influence on the growth and properties of BN films were investigated. It was found that with a suitable source supply rate, the growth temperature could obviously influence the growth rate, chemical state, crystal structure and optical properties of BN films. High growth temperature depicts low thickness film grown, even there was no BN film deposited when the temperature was higher than or equal to 1400 °C. The B–N bonding type in all the deposited BN films were conformed as sp2 bonding, and films were evidenced to be amorphous BN (aBN) at lower growth temperature (i.e. less than 1350 °C) and hBN in turbostratic phase (tBN) between 1350 and 1400 °C. All the grown BN films on sapphire substrate showed good ultraviolet (UV) absorption edge near 210 nm, according to transmittance spectra. This work presents a feasible route towards combination of large-scale CVD-grown hBN film with traditional III-N compounds for applications in deep UV optoelectronic devices.

Fast-response photoconductive ultraviolet light detectors fabricated using high-quality ZnO films obtained by plasma-assisted molecular beam epitaxy

Abstract ZnO films form the core of numerous ultraviolet (UV) optoelectronic devices. Herein, a ZnO buffer layer produced by thermal plasma oxidation of metallic Zn was used to grow an active ZnO layer, with Zn film and active layer growth achieved by plasma-assisted molecular beam epitaxy. Although low-temperature ZnO buffer layer and Zn film growth was mainly three-dimensional (3D) due to a large lattice mismatch between ZnO/Zn and Si, the use of thermal plasma oxidation changed the growth mode from 3D to 2D, producing a smooth ZnO active layer. The thus fabricated ZnO active layer (sample 2) exhibited a lower tensile stress than that fabricated on a ZnO buffer layer using a conventional method (sample 1). Both samples were used to fabricate UV sensors, with that based on sample 2 exhibiting a faster photoresponse.

Epitaxial growth of α-Ga2O3 thin films on a-, m-, and r-plane sapphire substrates by mist chemical vapor deposition using α-Fe2O3 buffer layers

Epitaxial a-, m-, and r-plane α-Ga2O3 thin films were successfully grown on a-, m-, and r-plane sapphire substrates through the insertion of α-Fe2O3 buffer layers using mist chemical vapor deposition. The α-Fe2O3 buffer layers improved the crystal growth of the α-Ga2O3 thin films. We reveal that the out-of-plane and in-plane orientations of each plane α-Ga2O3 thin film corresponded to that of each plane sapphire substrate. The direct bandgap of the α-Ga2O3 thin films from the optical transmittance and reflectance results of a-, m-, and r-planes was estimated to be 5.15–5.2eV. The epitaxial α-Ga2O3 thin films on the various sapphire substrates are promising materials for power devices and deep ultraviolet region optoelectronic devices.

Reduction of Defects in AlGaN Grown on Nanoscale-Patterned Sapphire Substrates by Hydride Vapor Phase Epitaxy.

In this study, a 3-μm-thick AlGaN film with an Al mole fraction of 10% was grown on a nanoscale-patterned sapphire substrate (NPSS) using hydride vapor phase epitaxy (HVPE). The growth mechanism, crystallization, and surface morphology of the epilayers were examined using X-ray diffraction, transmission electron microscopy (TEM), and scanning electron microscopy at various times in the growth process. The screw threading dislocation (TD) density of AlGaN-on-NPSS can improve to 1–2 × 109 cm−2, which is significantly lower than that of the sample grown on a conventional planar sapphire substrate (7 × 109 cm−2). TEM analysis indicated that these TDs do not subsequently propagate to the surface of the overgrown AlGaN layer, but bend or change directions in the region above the voids within the side faces of the patterned substrates, possibly because of the internal stress-relaxed morphologies of the AlGaN film. Hence, the laterally overgrown AlGaN films were obtained by HVPE, which can serve as a template for the growth of ultraviolet III-nitride optoelectronic devices.

Ultraviolet light-absorbing and emitting diodes consisting of a p-type transparent-semiconducting NiO film deposited on an n-type GaN homoepitaxial layer

Gallium nitride (GaN) and related (Al,Ga,In)N alloys provide practical benefits in the production of light-emitting diodes (LEDs) and laser diodes operating in ultraviolet (UV) to green wavelength regions. However, obtaining low resistivity p-type AlN or AlGaN of large bandgap energies (Eg) is a critical issue in fabricating UV and deep UV-LEDs. NiO is a promising candidate for useful p-type transparent-semiconducting films because its Eg is 4.0 eV and it can be doped into p-type conductivity of sufficiently low resistivity. By using these technologies, heterogeneous junction diodes consisting of a p-type transparent-semiconducting polycrystalline NiO film on an n-type single crystalline GaN epilayer on a low threading-dislocation density, free-standing GaN substrate were fabricated. The NiO film was deposited by using the conventional RF-sputtering method, and the GaN homoepitaxial layer was grown by metalorganic vapor phase epitaxy. They exhibited a significant photovoltaic effect under UV light and also exhibited an electroluminescence peak at 3.26 eV under forward-biased conditions. From the conduction and valence band (EV) discontinuities, the NiO/GaN heterointerface is assigned to form a staggered-type (TYPE-II) band alignment with the EV of NiO higher by 2.0 eV than that of GaN. A rectifying property that is consistent with the proposed band diagram was observed in the current-voltage characteristics. These results indicate that polycrystalline NiO functions as a hole-extracting and injecting layer of UV optoelectronic devices.

Enhanced Emission Efficiency of Deep Ultraviolet Light-Emitting AlGaN Multiple Quantum Wells Grown on an N-AlGaN Underlying Layer

Remarkably enhanced light emission efficiency of AlGaN multiple quantum wells (MQWs) was realized by growing on an n-AlGaN underlying layer (UL). The parasitic peaks emitting from inactive regions can be effectively suppressed, and the nonradiative recombination process in AlGaN MQWs proved to be substantially lessened with the inclusion of the n-AlGaN UL. Numerical simulations showed that the electric field in AlGaN MQWs was reduced by 26%, which efficiently weakened the quantum-confined Stark effect (QCSE). Further analysis attributed this to the reduction of strain-induced piezoelectric electric field in the AlGaN well layer. The enhanced emission efficiency of AlGaN MQWs was therefore due to the greatly suppressed nonradiative recombination and reduced QCSE with the introduction of the n-AlGaN UL. The present research will be helpful to the future promotion of high-performance deep ultraviolet (DUV) optoelectronic devices, as well as to better understanding the recombination mechanism in AlGaN MQWs.

Ultraviolet optoelectronic devices based on AIGaN alloys grown by molecular beam epitaxy

AbstractThis paper reviews progress in ultraviolet (UV) optoelectronic devices based on AIGaN films and their quantum wells (QWs), grown by plasma-assisted molecular beam epitaxy. A growth mode, leading to band-structure potential fluctuations and resulting in AIGaN multiple QWs with internal quantum efficiency as high as 68%, is discussed. Atomic ordering in these alloys, which is different from that observed in traditional III-V alloys, and its effect on device performance is also addressed. Finally, progress in UV-light-emitting diodes, UV lasers, UV detectors, electroabsorption modulators, and distributed Bragg reflectors is presented.

Improvement of p-type conductivity in Al-rich AlGaN substituted by MgGaδ-doping (AlN)m/(GaN)n (m≥n) superlattice

The challenge of p-type doping in Al-rich AlGaN is investigated based on first-principles calculations. We find that the p-type conductivity can be improved by replacing Al-rich AlGaN with MgGaδ-doping (AlN)m/(GaN)n (m ≥ n) superlattice (SL). The formation energy Ef is the lowest and acceptor activation energy EA is the smallest for Mg substituting Ga in the SL. The EA increases if the doping position moves from GaN to AlN layer. Moreover, EA decreases with increasing the number of GaN monolayers. The EA can be reduced from 0.48 eV in AlN to 0.26 eV in (AlN)5/(GaN)1, 0.25 eV in (AlN)1/(GaN)1, 0.24 eV in (AlN)4/(GaN)2 and 0.22 eV in (AlN)3/(GaN)3 SLs. This will lead to a high hole concentration in the order of 1018 cm−3 at room temperature, which is favorable for AlGaN-based deep ultraviolet optoelectronic devices.

Nucleation and growth of (10͞11) semi-polar AlN on (0001) AlN by Hydride Vapor Phase Epitaxy.

Wurtzite AlN is widely used for deep ultraviolet optoelectronic devices (DUV), which are generally grown along the [0001]-direction of the wurtzite structure on currently available substrates. However, huge internal electrostatic fields are presented within the material along [0001] axis induced by piezoelectric and spontaneous polarization, which has limited the internal quantum efficiency of AlN based DUV LEDs dramatically. The internal fields can be strongly reduced by changing the epitaxial growth direction from the conventional polar c-direction into less polar crystal directions. Twinned crystal is a crystal consisting of two or more domains with the same crystal lattice and composition but different crystal orientations. In other words, twins can be induced to change crystal directions. In this work we demonstrated that the epitaxial growth of () semi-polar AlN on (0001) AlN by constructing () and () twin structures. This new method is relative feasible than conventional methods and it has huge prospect to develop high-quality semi-polar AlN.

A class of monolayer metal halogenides MX2: Electronic structures and band alignments

With systematic first principles calculations, a class of monolayer metal halogenides MX2 (M = Mg, Ca, Zn, Cd, Ge, Pb; M = Cl, Br, I) has been proposed. Our study indicates that these monolayer materials are semiconductors with the band gaps ranging from 2.03 eV of ZnI2 to 6.08 eV of MgCl2. Overall, the band gap increases with the increase of the electronegativity of the X atom or the atomic number of the metal M. Meanwhile, the band gaps of monolayer MgX2 (X = Cl, Br) are direct while those of other monolayers are indirect. Based on the band edge curvatures, the derived electron (me) and hole (mh) effective masses of MX2 monolayers are close to their corresponding bulk values except that the me of CdI2 is three times larger and the mh for PbI2 is twice larger. Finally, the band alignments of all the studied MX2 monolayers are provided using the vacuum level as energy reference. These theoretical results may not only introduce the monolayer metal halogenides family MX2 into the emerging two-dimensional materials, but also provide insights into the applications of MX2 in future electronic, visible and ultraviolet optoelectronic devices.

Aluminum incorporation efficiencies in A- and C-plane AlGaN grown by MOVPE**Project supported by the National Natural Science Foundation of China (Grant Nos. 61504128, 61504129, 61274041, and 11275228), the National Basic Research Program of China (Grant No. 2012CB619305), the National High Technology Research and Development Program of China (Grant Nos. 2014AA032603, 2014AA032609, and 2015AA010801), and the Guangdong Provincial Scientific and Technologic

The aluminum incorporation efficiencies in nonpolar A-plane and polar C-plane AlGaN films grown by metalorganic vapour phase epitaxy (MOVPE) are investigated. It is found that the aluminum content in A-plane AlGaN film is obviously higher than that in the C-plane sample when the growth temperature is above 1070 °C. The high aluminum incorporation efficiency is beneficial to fabricating deep ultraviolet optoelectronic devices. Moreover, the influences of the gas inlet ratio, the V/III ratio, and the chamber pressure on the aluminum content are studied. The results are important for growing the AlGaN films, especially nonpolar AlGaN epilayers.

Ultraviolet Photoconductive Detectors Based on A-Plane ZnO Film Grow by Hydrothermal Method

A-plane ZnO film was grown on a-plane GaN/r-sapphire template by using the hydrothermal growth method. The film was characterized for the structural and morphological properties by means of x-ray diffraction, scanning electron microscopy, and atomic force microscopy. The results show the a-plane ZnO film with a very uniform striated morphology was achieved. Meanwhile, an ultraviolet (UV) photoconductive detector based on the as-grown a-plane film was fabricated, and the detector current was increased by more than 17 times under 5 V bias upon UV illumination. Moreover, it also showed good reproducibility and stability, which confirms the film as a good potential material for UV optoelectronic devices.

Growth and properties of electrodeposited transparent Al-doped ZnO nanostructures

Al-doped zinc oxide (AZO) nanostructures were fabricated on fluorine-doped tin-oxide (FTO)- coated glass substrates by using electrodeposition. The effects of the doping concentration of Al on the morphological, microstructural, electrical and optical properties of the nanostructures were investigated. From the field emission scanning electron microscopy (FE-SEM) observation, when the amount of Al was increased in the solution, the grains size was observed to decreases. The observed changes in the morphology indicate that Al acts as nucleation centers in the vacancy sites of ZnO and destroys the crystalline structure at high doping level. Effectively, the X-ray diffraction (XRD) analysis indicated that the undoped and the doped ZnO nanostructures has a polycrystalline nature and a hexagonal wurtzite structure with a (002) preferential orientation. The photoluminescence (PL) room-temperature measurements showed that the incorporation of Al in the Zn lattice can improve the intensity of ultraviolet (UV) emission, thus suggesting its greater prospects for use in UV optoelectronic devices.

Ultraviolet Lasers Realized via Electrostatic Doping Method.

P-type doping of wide-bandgap semiconductors has long been a challenging issue for the relatively large activation energy and strong compensation of acceptor states in these materials, which hinders their applications in ultraviolet (UV) optoelectronic devices drastically. Here we show that by employing electrostatic doping method, hole-dominant region can be formed in wide bandgap semiconductors, and UV lasing has been achieved through the external injection of electrons into the hole-dominant region, confirming the applicability of the p-type wide bandgap semiconductors realized via the electrostatic doping method in optoelectronic devices.

60-fold photoluminescence enhancement in Pt nanoparticle-coated ZnO films: role of surface plasmon coupling and conversion of non-radiative recombination.

Giant 60-fold enhanced ultraviolet (UV) emission is obtained in Pt nanoparticle-assembled ZnO film. Besides surface plasmons coupling, the conversion of non-radiative recombination into UV emission makes great contributions to the enhancement. It paves a new way in designing high-efficiency UV optoelectronic devices without defect-related energy loss.

Deep Ultraviolet Emission from Water-Soluble SnO2 Quantum Dots Grown via a Facile “Top-Down” Strategy

Tin oxide (SnO 2 ) is a promising wide bandgap semiconductor for next generation ultraviolet (UV) non-polar optoelectronic devices applications. The development of SnO 2 -based optoelectronic devices is obsessed by its low exciton emission efficiency. In this study, quantum confined SnO 2 nanocrystals have been fabricated via pulsed laser ablation in water. The SnO 2 quantum dots (QDs) possess high performance exciton emission at 297–300 nm light in water. The exciton emission intensity and wavelength can be slightly tuned by laser pulse energy and irradiation time. Optical gain has been observed in SnO 2 QDs. Therefore, SnO 2 QDs can be a promising luminescence material for the realization of deep UV nano-emitter and lasing devices.

Chemically synthesized nano composite (Zinc/Magnesium) Oxide for tunable band gap devices

Formation of hetro structures in nano structured materials is essential for their potential applications in nano electronics and photonic devices. As a promising candidate for blue and ultraviolet optoelectronic devices, ZnO has attracted much attention due to its wide band gap (3.37eV), large exciton binding energy (60meV), low epitaxial growth temperature and high oxidation resistance. In addition since the ionic radii of Mg+2+(0.57A0) and Zn2+(0.60A0) are quite close, they may alloy by replacing each other in the matrix. The doping of Mg in ZnO is done through a simple and novel technique from metal acetates using ammonium carbonate as precipitant. An organic capping agent (EDTA) is used prevent agglomeration and the addition is done under constant stirring. The carbonate precursor obtained is heated on the basis of TGA to obtain the metal nano composite. The effects of different parameters on particle size and morphology of (Zn-Mg)O nano composite is optimized by "one at a time" method. Under optimum conditions, spongy shaped, uniform and homogeneous structured (Zn-Mg)O nano composite powders with particle size few nano meters are obtained. The optical and structural properties of nano composite prepared by solution techniques are investigated by X-ray diffraction, UV-Visible spectroscopy, and PL, FTIR and electron microscopy techniques. The effect of annealing on the optical properties of this nano composite is also studied

Enhancement of TE polarized light extraction efficiency in nanoscale (AlN)m /(GaN)n (m>n) superlattice substitution for Al-rich AlGaN disorder alloy: ultra-thin GaN layer modulation

The problem of achieving high light extraction efficiency in Al-rich AlxGaN is of paramount importance for the realization of AlGaN-based deep ultraviolet (DUV) optoelectronic devices. To solve this problem, we investigate the microscopic mechanism of valence band inversion and light polarization, a crucial factor for enhancing light extraction efficiency, in Al-rich AlxGaN alloy using the Heyd–Scuseria–Ernzerhof hybrid functional, local-density approximation with 1/2 occupation, and the Perdew–Burke–Ernzerhof functional, in which the spin–orbit coupling effect is included. We find that the microscopic Ga-atom distribution can effectively modulate the valence band structure of Al-rich AlxGaN. Moreover, we prove that the valence band arrangement in the decreasing order of heavy hole, light hole, and crystal-field split-off hole can be realized by using nanoscale (AlN)m/(GaN)n (m>n) superlattice (SL) substituting for Al-rich AlxGaN disorder alloy as the active layer of optoelectronic devices due to the ultra-thin GaN layer modulation. The valence band maximum, i.e., the heavy hole band, has px- and py-like characteristics and is highly localized in the SL structure, which leads to the desired transverse electric (TE) polarized (E⊥c) light emission with improved light extraction efficiency in the DUV spectral region. Some important band-structure parameters and electron/hole effective masses are also given. The physical origin for the valence band inversion and TE polarization in (AlN)m/(GaN)n SL is analyzed in depth.

Reducing Mg acceptor activation-energy in Al(0.83)Ga(0.17)N disorder alloy substituted by nanoscale (AlN)₅/(GaN)₁ superlattice using Mg(Ga) δ-doping: Mg local-structure effect.

Improving p-type doping efficiency in Al-rich AlGaN alloys is a worldwide problem for the realization of AlGaN-based deep ultraviolet optoelectronic devices. In order to solve this problem, we calculate Mg acceptor activation energy and investigate its relationship with Mg local structure in nanoscale (AlN)5/(GaN)1 superlattice (SL), a substitution for Al0.83Ga0.17N disorder alloy, using first-principles calculations. A universal picture to reduce acceptor activation energy in wide-gap semiconductors is given for the first time. By reducing the volume of the acceptor local structure slightly, its activation energy can be decreased remarkably. Our results show that Mg acceptor activation energy can be reduced significantly from 0.44 eV in Al0.83Ga0.17N disorder alloy to 0.26 eV, very close to the Mg acceptor activation energy in GaN, and a high hole concentration in the order of 1019 cm−3 can be obtained in (AlN)5/(GaN)1 SL by MgGa δ-doping owing to GaN-monolayer modulation. We thus open up a new way to reduce Mg acceptor activation energy and increase hole concentration in Al-rich AlGaN.

Ab initio study of electronic structure, quantum transport and optical absorption properties of polyacene quinone radical polymers

Semiempirical quantum mechanical calculations have been implemented to study the molecular structure, electronic structure, optical properties of polyacene quinone radical polymers, and the quantum transport properties are also been calculated through combination of the numerical atomic orbitals basis set method based on density functional theory with the non-equilibrium Green's function formalism. The polyacene quinone radical polymers exhibit similar elastic modulus to the carbon nanotubes. The electronic molecular orbitals and energy bands of polyacene quinone radical polymers show insulator features, with the great conjugated bonds formed in polymer molecules resulting in extreme high polarization and dielectric constants, and the Fermi energy and band-gaps of ultraviolet range are static with the variation of polymerization degree. The aluminum electrodes with crystallographic (001) surfaces have been used in the electronic transport calculations, which indicate the quantum conductance spectrum changes with the varied strain and polymerization degree, however appears no explicit change, representing large open-gap feature of insulators. Ultraviolet-visible absorption spectra show a few characteristic peaks, the positions and number of which varying with polymerization degree, predicting that polyacene quinone radical polymers, as high dielectric functional materials, could be applied to ultraviolet optoelectronic nanoscale quantum devices.