Ultraviolet Optoelectronics Research Articles

Study of ZnO nanostructures grown by a hydrothermal process on GaP/ZnO nanowires

ZnO is a prominent material for creation of nanostructures as a multifunctional material for broad applications in blue and ultraviolet optoelectronics. A lot of efforts have been dedicated to understand the unique properties of nanostructured materials. The main purpose of this work is to verify (1) the technology of thin ZnO films deposition by RF magnetron sputtering to cover round GaP nanowires prepared by metalorganic vapor phase epitaxy, and (2) the hydrothermal growth of core/shell ZnO nanostructures on both GaP/ZnO and GaP/ZnO structures and nanowires deposited by RF magnetron sputtering. SEM, XRD, optical ellipsometry and micro-Raman measurements were employed to investigate the structural and optical properties of GaP/ZnO structures as well as core/shell nanostructures. The thickness, columnar structure and surface layer roughness of the deposited ZnO layers were estimated from cut-edge SEM images and verified by spectral ellipsometry. The measured XRD intensity patterns of (002) ZnO line of hydrothermally grown ZnO nanoclusters on nanowires show a narrow spectral width ∼0.17° which indicates a good crystalline structure in comparison with ZnO films prepared by magnetron sputtering on GaP substrate, even after their annealing. The PL spectra of the hydrothermally grown GaP/ZnO core/shell nanostructure show typical near band emission and very strong yellow broadband emission from Zn(OH)2 or hydroxyl groups for ZnO structures. For the first time we have demonstrated formation of GaP/ZnO core/shell nanostructured arrays on GaP nanowires grown by hydrothermal process.

Structural and optoelectronic properties of the zinc titanate perovskite and spinel by modified Becke–Johnson potential

Structural and electronic properties of the cubic perovskite ZnTiO3 and spinel Zn2TiO4 are theoretically studied by the modified Becke–Johnson (mBJ) potential within the framework of density functional theory (DFT). The calculated lattice constants are found to be consistent with the experimental results. The electronic band structures of both the materials reveal that ZnTiO3 is an indirect band gap while Zn2TiO4 is a direct band gap semiconductor. The calculated fundamental band gaps of these compounds are 2.7eV and 3.18eV, which are consistent with the experimental band gaps of 2.9eV and 3.1eV, respectively. Zn2TiO4 is a wide and direct band gap compound and hence is an attractive material for optoelectronic applications, especially in near ultraviolet (UV) optoelectronics. Keeping in view the importance of Zn2TiO4 in low frequency UV devices its optical properties like dielectric functions, refractive index, reflectivity and energy loss function are also evaluated and discussed in detail.

Carrier dynamics in active regions for ultraviolet optoelectronics grown on thick, relaxed AlGaN on semipolar bulk GaN

AbstractActive regions for mid‐ultraviolet laser diodes grown on bulk AlGaN templates are investigated by time‐resolved photoluminescence. The active regions were grown pseudomorphically on thick, relaxed AlGaN on bulk GaN in the semi‐polar ($20{\bar {2}}1$) orientation where it has been shown that the glide of dislocations create strain relieving defects confined to the AlGaN/GaN interface, away from the active region. The photoluminescence lifetimes were found to have mono‐exponential decays of around 500 ps and calculated radiative and non‐radiative lifetimes are compared to previously reported results for active regions on bulk m‐plane GaN.

Highly Sensitive UV Sensors Based on SMR Oscillators

The interest in ultraviolet (UV) light sensors have been stimulated in the fields of ozone layer and ultraviolet radiation monitoring, satellite communications and UV astronomy. Zinc oxide (ZnO) thin film is an attractive candidate for acoustic wave devices and ultraviolet optoelectronics, due to its high coupling coefficient and the direct energy band gap of approximately 3.2eV at room temperature. In general, thin film bulk acoustic wave (TFBAW) devices, which are solidly mounted resonator (SMR) devices in this study, possess a higher operating frequency and better frequency response than those of surface acoustic wave (SAW) devices. Therefore, ZnO thin films in the SMR devices acting as both the piezoelectric layer and UV sensing layer were adopted to investigate the acoustic and UV properties in this study. The SMR-based oscillator is composed of a high frequency amplifier, the matching networks, and an SMR device. The resonance frequency of the SAW oscillator is 619.31MHz with output power of -18.75 dBm, and the phase noise is -60.63 dBc at 100kHz. The frequency shifts of SMR oscillators show a trend as a function of the illumination intensities of UV light. Finally, the maximum frequency shift of 552kHz could be obtained when the illumination intensity of UV light was 212μW/cm2.

Monolithic Integration of Ultrafast Photodetector and MESFET in the GaN Material System

We have fabricated and characterized ultrafast metal–semiconductor–metal (MSM) photodetectors integrated with metal–semiconductor–field-effect-transistors (MESFETs) integrated in coplanar strip lines in the GaN/AlN/SiC material system. We recorded electrical transients of the single photodetector as short as 0.9 ps wide by optoelectric pump–probe measurements using 360-nm-wavelength and 100-fs-duration laser pulses. Electric photoresponse transients of the photodetector with 6-mV peak amplitude were amplified by the MESFET, resulting in 4-ps-wide and 35-mV peak amplitude signals. This monolithically integrated optoelectronic circuit is presented as a potential candidate for high-speed ultraviolet optoelectronics. <?Pub _bookmark="" Command="[Quick Mark]"?>

Epitaxially grown semiconducting hexagonal boron nitride as a deep ultraviolet photonic material

Hexagonal boron nitride (hBN) has emerged as an important material for various device applications and as a template for graphene electronics. Low-dimensional hBN is expected to possess rich physical properties, similar to graphene. The synthesis of wafer-scale semiconducting hBN epitaxial layers with high crystalline quality and electrical conductivity control has not been achieved but is highly desirable. Large area hBN epitaxial layers (up to 2 in. in diameter) were synthesized by metal organic chemical vapor deposition. P-type conductivity control was attained by in situ Mg doping. Compared to Mg-doped wurtzite AlN, which possesses a comparable energy band gap (∼6 eV), dramatic reductions in Mg acceptor energy level and P-type resistivity (by about six to seven orders of magnitude) have been realized in hBN epilayers. The ability of conductivity control and wafer-scale production of hBN opens up tremendous opportunities for emerging applications, ranging from revolutionizing p-layer approach in III-nitride deep ultraviolet optoelectronics to graphene electronics.

Transparent conductive graphene electrode in GaN-based ultra-violet light emitting diodes

We report a graphene-based transparent conductive electrode for use in ultraviolet (UV) GaN light emitting diodes (LEDs). A few-layer graphene (FLG) layer was mechanically deposited. UV light at a peak wavelength of 368 nm was successfully emitted by the FLG layer as transparent contact to p-GaN. The emission of UV light through the thin graphene layer was brighter than through the thick graphene layer. The thickness of the graphene layer was characterized by micro-Raman spectroscopy. Our results indicate that this novel graphene-based transparent conductive electrode holds great promise for use in UV optoelectronics for which conventional ITO is less transparent than graphene.

GaN Substrates for III-Nitride Devices

Despite the rapid commercialization of III-nitride semiconductor devices for applications in visible and ultraviolet optoelectronics and in high-power and high-frequency electronics, their full potential is limited by two primary obstacles: i) a high defect density and biaxial strain due to the heteroepitaxial growth on foreign substrates, which result in lower performance and shortened device lifetime, and ii) a strong built-in electric field due to spontaneous and piezoelectric polarization in the wurtzite structures along the well-established [0001] growth direction for nitrides. Recent advances in the research, development, and commercial production of native GaN substrates with low defect density and high structural and optical quality have opened opportunities to overcome both of these obstacles and have led to significant progress in the development of several optoelectronic and high-power devices. In this paper, the recent achievements in bulk GaN growth development using different approaches are reviewed; comparison of the bulk materials grown in different directions is made; and the current achievements in device performance utilizing native GaN substrate material are summarized.

Violet to deep-ultraviolet InGaN∕GaN and GaN∕AlGaN quantum structures for UV electroabsorption modulators

In this paper, we present four GaN based polar quantum structures grown on c-plane embedded in p-i-n diode architecture as a part of high-speed electroabsorption modulators for use in optical communication (free-space non-line-of-sight optical links) in the ultraviolet (UV): the first modulator incorporates ∼4–6nm thick GaN∕AlGaN quantum structures for operation in the deep-UV spectral region and the other three incorporate ∼2–3nm thick InGaN∕GaN quantum structures tuned for operation in violet to near-UV spectral region. Here, we report on the design, epitaxial growth, fabrication, and characterization of these quantum electroabsorption modulators. In reverse bias, these devices exhibit a strong electroabsorption (optical absorption coefficient change in the range of 5500–13000cm−1 with electric field swings of 40–75V∕μm) at their specific operating wavelengths. In this work, we show that these quantum electroabsorption structures hold great promise for future applications in ultraviolet optoelectronics technology such as external modulation and data coding in secure non-line-of-sight communication systems.

Heteroepitaxy of AlGaN on bulk AlN substrates for deep ultraviolet light emitting diodes

The authors report the growth of AlGaN epilayers and deep ultraviolet (UV) light emitting diodes (LEDs) on bulk AlN substrates. Heteroepitaxial nucleation and strain relaxation are studied through controlled growth interruptions. Due to a low density of preexisting dislocations in bulk AlN, the compressive strain during AlGaN heteroepitaxy cannot be relieved effectively. The built-up of strain energy eventually induces either an elastic surface roughening or plastic deformation via generation and inclination of dislocations, depending on the stressor interlayers and growth parameters used. AlGaN LEDs on bulk AlN exhibit noticeable improvements in performance over those on sapphire, pointing to a promising substrate platform for III-nitride UV optoelectronics.

Electrical studies on sputtered CuCl thin films

CuCl is a wide-direct band gap semiconductor, lattice matched to Si and it possesses excellent ultra violet (UV) emission properties. It is thus a promising candidate for the next generation Si based UV optoelectronics. CuCl films were deposited using RF magnetron sputtering technique. X-ray diffraction analysis reveals that the grains are strongly oriented. Triangular crystallites of CuCl were observed in the AFM surface topograph. Au–CuCl–Si–Au structures were fabricated and field dependent electrical studies were carried out in the electric field range of 1.25 × 106 to 2.5 × 107 V/m. I–V characteristics show that ohmic conduction prevails in low electric fields up to 2.5 × 106 V/m. In the higher field range, from 2.5 × 106 to 2.5 × 107 V/m, the conduction mechanism was Schottky emission controlled. There was no trap related charge transport observed at higher electric fields. Preliminary electrical studies are reported in this article.

AlGaN deep ultraviolet LEDs on bulk AlN substrates

AbstractWe report the growth of sub‐300 nm ultraviolet light emitting diodes (UV LEDs) on bulk AlN substrates. Heteroepitaxial evolution study through interrupted growth experiments revealed that AlxGa1‐xN (x &gt; 0.5) epilayers can be grown pseudomorphically with well‐defined step‐flow growth mode below a certain critical thickness. The build‐up of compressive strain energy eventually induces a morphological roughening followed by the admission of misfit dislocations. LEDs grown on bulk AlN substrates exhibit noticable improvement over those on sapphire in device impedance, efficiency and thermal characteristics under high‐level injection, pointing to a promising substrate platform for high performance III‐nitride ultraviolet optoelectronics. (© 2007 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Cathodoluminescence of Diffusion Layer Synthesized by Coupling of β-Ga&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;3&lt;/sub&gt; and ZnO

Beta gallium oxide (β-Ga2O3) ceramics was coupled and reacted with zinc oxide (ZnO) ceramics at 1200oC for 24 h. Energy dispersive X-ray spectroscopy (EDS) analysis revealed the existence of diffusion layer near the interface between β-Ga2O3 and ZnO. Furthermore, the layer showed monochromatic and intense cathodoluminescence (CL) at 3.5 eV. On the other hand, very weak emissions were observed from the out of the layer. The compound in the layer is considered to be an attractive material for ultraviolet optoelectronics.

Growth of CuCl thin films by magnetron sputtering for ultraviolet optoelectronic applications

Copper (I) chloride (CuCl) is a potential candidate for ultraviolet (UV) optoelectronics due to its close lattice match with Si (mismatch less than 0.4%) and a high UV excitonic emission at room temperature. CuCl thin films were deposited using radio frequency magnetron sputtering technique. The influence of target to substrate distance (dts) and sputtering pressure on the composition, microstructure, and UV emission properties of the films were analyzed. The films deposited with shorter target to substrate spacing (dts=3cm) were found to be nonstoichiometric, and the film stoichiometry improves when the substrate is moved away from the target (dts=4.5 and 6cm). A further increase in the spacing results in poor crystalline quality. The grain interface area increases when the sputtering pressure is increased from 1.1×10−3to1×10−2mbar at dts=6cm. Room temperature cathodoluminescence spectrum shows an intense and sharp UV exciton (Z3) emission at ∼385nm with a full width at half maximum of 16nm for the films deposited at the optimum dts of 6cm and a pressure of 1.1×10−3mbar. A broad deep level emission in the green region (∼515nm) is also observed. The relative intensity of the UV to green emission peaks decreased when the sputtering pressure was increased, consistent with an increase in grain boundary area. The variation in the stoichiometry and the crystallinity are attributed to the change in the intensity and energy of the flux of materials from the target due to the interaction with the background gas molecules.

Cathodoluminescence of Diffusion Layer Synthesized by Coupling of α-Al&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;3&lt;/sub&gt; and ZnO

Alpha aluminum oxide (α-Al2O3) ceramics was coupled and reacted with zinc oxide (ZnO) ceramics at 1200°C for 24 h. Energy dispersive X-ray spectroscopy (EDS) analysis revealed the existence of step-shaped distribution of Al and Zn near the interface between α-Al2O3 and ZnO. Intense ultraviolet (3.75 eV) emission was clearly observed from the layer. On the other hand, very weak emissions were observed outside the layer near the interface. The compound in the layer is considered to be an attractive material for ultraviolet optoelectronics.

Microstructure of InGaN quantum wells grown on GaN single crystals and sapphire

In the last decade, the III-N compounds have attracted much interest because of their applications in blue, violet and ultraviolet optoelectronics. Most of the devices and research use sapphire as a substrate for epitaxy of nitrides. However, these epi-structures contain a very high dislocation density induced by the 16% lattice mismatch between GaN and sapphire. In our laboratory, we grow single crystals of GaN at a high hydrostatic pressure of 10 kbar of nitrogen. These crystals have an ultra-low dislocation density and are successfully used for construction of violet laser diodes. This work presents experimental data of high resolution x-ray diffractometry and photoluminescence. This work consist of three parts: (i) comparison between GaN substrates and GaN/sapphire templates; (ii) influence of AlGaN layers on bowing of samples; (iii) microstructure of InGaN/GaN multiple quantum wells.

Optical investigations on sputtered CuCl thin films

ABSTRACTCopper (I) chloride (CuCl) is a potential candidate for ultra violet optoelectronics due to the fact that it is closely lattice matched with silicon, which makes it readily integrable with silicon device technology. The structural and optoelectronic properties of CuCl thin films deposited by RF magnetron sputtering are investigated. The crystallinity is studied using X-ray diffraction which confirms the growth of CuCl thin films with cubic zinc blende structure predominantly orientated in &lt;111&gt; direction. Excitonic transitions in the thin films were thoroughly investigated using optical absorbance and luminescence spectroscopies. Room temperature absorption spectroscopic analysis confirms the existence of two exciton peaks namely Z12 and Z3 at 372 and 380 nm respectively. A strong UV emission is observed at room temperature in cathodoluminescence and photoluminescence spectra due to the recombination of Z3 exciton at approximately 384 nm. In the low temperature photoluminescence spectrum, a free exciton (Z3) and a bound exciton (I1) are observed. A variation of 1.3 nm to 10 nm was observed in the Z3 exciton line width from 10 K to 300 K.

Annealing of gallium nitride under high-N 2 pressure

GaN is the key material for blue and ultraviolet optoelectronics. It is a strongly bonded wurztite structure semiconductor with the direct energy gap 3.5 eV. Due to strong bonding, the diffusion processes require high temperatures, above 1300 K. However at this temperature range at ambient pressure, GaN becomes unstable and dissociates into Ga and N 2. Therefore high pressure of N 2 is required to study the diffusion and other annealing related processes. We studied annealing of bulk GaN nitride single crystals grown under high pressure and also annealing of homo- and heteroepitaxial GaN layers grown by MOCVD technique. Annealing at temperatures above 1300 K influences strongly the structural and optical properties of GaN crystals and layers. At this temperature diffusion of the Mg and Zn acceptors have been observed. In spite of very interesting experimental observations the understanding of microscopic mechanisms of these processes is limited.

Monitoring and Controlling of Strain During MOCVD of AlGaN for UV Optoelectronics

The grown-in tensile strain, due to a lattice mismatch between AlGaN and GaN, is responsible for the observed cracking that seriously limits the feasibility of nitride-based ultraviolet (UV) emitters. We report in-situ monitoring of strain/stress during MOCVD of AlGaN based on a wafer-curvature measurement technique. The strain/stress measurement confirms the presence of tensile strain during growth of AlGaN pseudomorphically on a thick GaN layer. Further growth leads to the onset of stress relief through crack generation. We find that the growth of AlGaN directly on low-temperature (LT) GaN or AlN buffer layers results in a reduced and possibly controllable strain.

Monitoring and Controlling of Strain During Mocvd of AlGaN for UV Optoelectronics

AbstractThe grown-in tensile strain, due to a lattice mismatch between AlGaN and GaN, is responsible for the observed cracking that seriously limits the feasibility of nitride-based ultraviolet (UV) emitters. We report in-situ monitoring of strain/stress during MOCVD of AIGaN based on a wafer-curvature measurement technique. The strain/stress measurement confirms the presence of tensile strain during growth of AlGaN pseudomorphically on a thick GaN layer. Further growth leads to the onset of stress relief through crack generation. We find that the growth of AlGaN directly on low-temperature (LT) GaN or AIN buffer layers results in a reduced and possibly controllable strain.