Low-voltage blue light emission from n-ZnO/p-GaN heterojunction formed by RF magnetron sputtering method

High quality n-ZnO/p-GaN heterojunction was fabricated by growing highly crystalline ZnO epitaxial films on commercial p-type GaN substrates via radio frequency (RF) magnetron sputtering. Low-voltage blue light emitting diode with a turn-on voltage of ∼2.5 V from the n-ZnO/p-GaN heterojunction was demonstrated. The diode gives a bright blue light emission located at ∼460 nm and a low threshold voltage of 2.7 V for emission. Based on the results of the photoluminescence (PL) and electroluminescence (EL) spectra, the origins of the EL emissions were studied in the light of energy band diagrams of ZnO–GaN heterojunction, and may attribute to the radiative recombination of the holes in p-GaN and the electrons injected from n-ZnO, which almost happened on the side of p-GaN layer. These results may have important implications for developing short wavelength optoelectronic devices.     

GaN深中心Zn的低温瞬态研究

本文研究了GaN:Zn的低温瞬态过程,同时测量了衰减中的时间分辨光谱,从两者测量的结果得出:2.89eV的光致发光寿命为300ns(2K).实验证实,Zn作为发光中心的同时也引起一些非辐射陷阱.2.89eV的瞬态曲线可分为两部分,在短时范围(1μs)内基本上是指数形式,可归结为导带中光激发的电子和束缚在ZnGa受主上空穴的复合.在长时范围(t>>1μs)内瞬态曲线则偏离了指数规律,相当好地符合Becqureal经验公式.     

The origin of yellow band emission and cathodoluminescence of Au-catalyzed wurtzite GaN nanowires

GaN nanowires with large yield are directly synthesized by simply ammoniating the gallium oxide powders in the presence of ammonia gas at 1000 °C, under the assistance of Au nanocatalysts. The microstructure and crystallinity of as-synthesized GaN nanowires are well studied by using high-resolution transmission electron microscope (HRTEM) and some structural defects such as stacking faults are found in the GaN nano-crystal. Cathodoluminescence measurement shows that a strong near-band-edge (NBE) emission band centered at 384 nm and a broad yellow band in the range of 500–800 nm are observed. Finally, the growth mechanism and possible optical emission process of GaN nanowires are discussed.     

Localised transitions in luminescence of AlN ceramics

The photoluminescence (PL) and thermoluminescence (TL) properties of AlN ceramics revealed under UV irradiation are determined mainly by oxygen-related centres, giving rise to the UV (around 3.18 eV) and the Blue (2.58 eV) bands. It was found that the UV irradiation-generated donor–acceptor pairs (DAPs), responsible for the UV emission band, are randomly distributed with regard to separation distance. Luminescence properties of AlN are interpreted basing on the model of localised recombination involving electron tunnel transitions from the excited state of D to the ground state of A, proposed by Jain et al. (2012). The observed features of PL, afterglow and TL of AlN ceramics are explained by dependence of tunnelling recombination probability on separation distance between D and A implied by the used model.     

Oxygen deficiency effects on recombination lifetime and photoluminescence characteristics of ZnO thin films; correlation with crystal structure

The photoluminescence (PL), recombination lifetime (RL), and X-ray diffraction (XRD) spectra of the samples grown at various O₂ fractions of 0.290 (Zn-rich), 0.585 (moderate), and 0.836 (O-rich) over the total pressures in the growth chamber were investigated. XRD measurements revealed that all the films show highly preferred (0002) orientation. The PL measurements exhibit different dominant emissions in the ultraviolet (UV), violet and blue regions for Zn-rich, moderate and O-rich samples, respectively. Well-known green emission and high intensity of free exciton (FX) transition has been observed in Zn-rich sample after the sample is annealed at vacuum probably due to the oxygen deficiencies. Annealing the moderate sample gives rise to the UV emission at energy of 3.263 eV similar to the observed PL emission spectrum for the Zn-rich thin film. O-rich thin film exhibits a 338 meV acceptor level above the valance band maximum, most probably related to zinc vacancy (V_Zn). Free exciton RL measurements result in 568.23, 397.65, and 797.46 ps for Zn-rich, moderate, and O-rich thin films, respectively. A good correlation was found between crystallite size and the lifetime values.     

Excitation Intensity Dependence of Free-exciton Transitions in GaN Grown by Low-pressure Metalorganic Chemical Vapor Deposition

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分子束外延高Mg掺杂GaN的发光特性

采用分子束外延技术在蓝宝石衬底上制备Ｍｇ掺杂的立方相ｐ－ＧａＮ,并对其不同温度下的光致发光光谱进行了研究．实验观察到高Ｍｇ掺杂ＧａＮ中施主受主对发光的反常温度行为．理论分析表明,高Ｍｇ掺杂ＧａＮ中施主受主对的发光受到陷阱与受主间竞争俘获非平衡空穴过程和空穴隧穿输运过程的影响．     

Time‐resolved free carrier lifetime microscopy in bulk GaN

A novel setup for lifetime microscopy measurements was designed and applied for carrier lifetime mapping in a bulk GaN. Photoexcitation by a picosecond UV pump and detection of time‐resolved free carrier absorption (FCA) images on a CCD camera enabled the mapping of carrier lifetime distribution with a spatial resolution of 5 μm. The spatial variation of lifetime in the bulk HVPE‐grown GaN revealed the presence of different‐size crystalline grains, with lifetime peaking up to 70 ns in the centers of the largest grains (～20 μm in diameter) and dropping to 10 ns in the small ones, while the spatially averaged lifetime was 40 ns. The inhomogeneity was ascribed to the interplay of nonradiative diffusion‐limited recombination at grain boundaries and a bulk lifetime in the crystallite centers. The numerical solution of spatially‐resolved carrier decay rate in the crystallite centers at high injection levels and comparison with experimental data provided a bulk nonradiative recombination time of ～70 ns. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Application of hexagonal boron nitride micropowder for thermoluminescent dosimetry of UV radiation

Thermoluminescence (TL) response of h-BN micropowder, characterized by two peaks near 337 and 584 K, was studied in the 380-nm band after excitation with monochromatic ultraviolet radiation (UVR). On the basis of the obtained results and independent data analysis, assumptions were made on the mechanisms of donor–acceptor recombination with the participation of 1B, O_N, and C_N centers, responsible for the observed thermally activated emission. TL excitation spectra in the 200–380-nm range were analyzed. It was observed that the resolved maxima correspond quite to the characteristic wavelengths of 260 nm (maximum bactericidal efficiency) and 297 nm (maximum skin erythema). It is found that the dose dependence for an integral intensity of high-temperature peak is linear in the suberythemal range of 0.01–0.35 mJ/cm².     

Photoluminescence study of disordering in the cation sublattice of Cu2ZnSnS4

In this study we investigated the influence of the degree of disordering in the cation sublattice on low temperature photoluminescence (PL) properties of Cu₂ZnSnS₄ (CZTS) polycrystals. The degree of disordering was changed by using different cooling rates after post-annealing at elevated temperatures. The results suggest that in the case of higher degree of cation sublattice disorder radiative recombination involving defect clusters dominates at T = 10 K. These defect clusters induce local band gap energy decrease in CZTS. The concentration of defect clusters can be reduced by giving more time for establishing ordering in the crystal lattice. As a result, radiative recombination mechanism changes and band-to-impurity recombination involving deep acceptor defect with ionization energy of about 200 meV starts to dominate in the low temperature PL spectra of CZTS polycrystals.     

Impact of doping on carrier recombination and stimulated emission in highly excited GaN:Mg

Light-induced transient grating and photoluminescence measurements were employed for carrier recombination studies in variously Mg doped GaN layers. Carrier lifetime and ambipolar diffusion coefficient were found to decrease with doping from 210 to 20 ps and from 2.0 to 0.9 cm²/s,respectively, which proved the degradation of electrical quality of the layers. A threshold of stimulated emission was found to depend non-monotonously on doping and had the lowest value of 0.19 mJ/cm² in the most doped layer. This dependence was explained in terms of degeneracy of the hole system.     

Band structure investigations of GaN films using modulation spectroscopy

The paper presents investigation results concerning band structure of gallium nitride and position of intrinsic and associate defect levels. Main optical characteristics (transmission, reflection and luminescence) were measured in both ordinary and λ-modulation mode for epitaxy-grown GaN films, allowing to determine valence band splitting caused by spin-orbital interaction (48 meV) and crystalline field (10 meV). Analysis of photoluminescence spectra made it possible to identify main recombination mechanisms involving donor and acceptor levels formed by intrinsic point defects , and their associates.     

Optical properties of p-type ZnO doped by lithium and nitrogen

A lithium and nitrogen doped p-type ZnO (denoted as ZnO: (Li, N)) film was prepared by RF-magnetron sputtering and post annealing techniques with c-Al₂O₃ as substrate. Its transmittance was measured to be above 95%. Three dominant emission bands were observed at 3.311, 3.219 and 3.346 eV, respectively, in the 80 K photoluminescence (PL) spectrum of the p-type ZnO:(Li, N), and are attributed to radiative electron transition from conduction band to a Li_Zn-N complex acceptor level (eFA), radiative recombination of a donor-acceptor pair and recombination of the Li_Zn-N complex acceptor bound exciton, respectively, based on temperature-dependent and excitation intensity-dependent PL measurement results. The Li_Zn-N complex acceptor level was estimated to be about 126 meV above the valence band by fitting the eFA data obtained in the temperature-dependent PL spectra.     

On the recombination behaviour of iron in moderately boron-doped p-type silicon

The recombination lifetime and diffusion length of intentionally iron-contaminated samples were measured by the Surface Photo Voltage (SPV) and the Elymat technique. The lifetime results from these techniques for intentionally iron-contaminated samples were analysed, in particular for the aspect of the injection-level dependency of recombination lifetime. Based on theoretical considerations, a method for the analysis of deep-level parameters combining constant photon flux SPV and Elymat measurements has been developed. This method is based on a detailed numerical analysis of the Elymat technique, including the Dember electric field, the characteristics of the laser beam, the transport parameters of the semiconductor and multilevel Shockley-Read-Hall (SRH) recombination kinetics. The results of the numerical simulation are applied to the analysis of recombination lifetime measurements on intentionally iron-contaminated samples. We compared numerical simulations and experimental results from SPV and Elymat for p-type samples using the classical acceptor level atE _v +0.1 eV and the donor level of FeB pairs atE _c -0.3 eV as recombination centre. Better consistency in the interpretation of the results has been found in the doping range 10¹⁴–10¹⁶ cm^–3 supposing theE _c -0.3 eV level as predominant recombination centre. An attempt to extract the electron and hole capture cross-sections for this defect is made.     

氮化镓纳米粒子的制备及光致发光研究

采用直接氮化法制备出尺寸不同的GaN纳米粒子,分别利用X射线衍射仪（XRD）、扫描电子显微镜（SEM）和光致发光（PL）谱测试手段对所制样品进行表征和发光特性的研究。结果表明：所制备的两种GaN纳米粒子直径分别为100 nm和300 nm左右。在GaN纳米粒子的PL谱中,中心在357 nm的发射源于本征发光,中心在385 nm的发射带源于浅施主能级到价带的辐射复合,中心在560 nm左右的发射带源于浅施主能级到深受主能级间的施主-受主对辐射发光。     

Time-resolved spectroscopy of freestanding GaN layers grown by halide vapour phase epitaxy

Freestanding GaN layers of various thicknesses grown by HVPE have been studied by time-resolved spectroscopy combined with structural and electrical measurements. We have observed an increase of the PL lifetime with increasing layer thickness; however, a saturation of the recombination times has been detected for the GaN layers thicker than 400 μm. We explain the observed thickness-dependent behavior of the decay times by competition of two nonradiative mechanisms; namely, for layers with thickness less than 400 μm the main nonradiative channel is related to the structural defects, while in thicker layers the recombination decay time is limited by impurities and/or vacancies.     

Photoluminescence dynamics of organic molecule-passivated Si nanoclusters

The size-selected phenyl-passivated Si nanoclusters with the mean diameters of 1.5 and 1.3 nm have been prepared in the solution route. The intense PL observed in Si nanoclusters, in which the band gap energies increase up to approximately 4 eV, originates from electron-hole pair recombination. From time-resolved PL, two kinds of exponential PL decay components with fast and slow lifetime were observed. The identical decays with slow lifetime will be derived from the surface states of Si nanoclusters passivated by phenyl molecules. The size-dependent zero-phonon assisted optical transitions by quantum size effects occur at a high rate of sub-nanosecond timescale.     

GaN薄膜的蓝光和红光发射机理研究

由于生长工艺的不完善，非掺杂GaN薄膜中通常存在未知的杂质和缺陷，产生与这些未知杂质和缺陷能级相关的发光。报道了非掺杂GaN薄膜的692nm红色发光．并研究了非掺杂GaN薄膜的蓝、红色发光的发射机理；利用作者提出的吸收归一化光致发光激发光谱，直接测量出了非掺杂GaN薄膜的蓝、红色发光的初始态能级，确定蓝色发光为施主-价带跃迁复合，而红色发光为施主-受主跃迁复合；给出了黄、蓝、红光的发射模型。所取得的结果对于确定未知杂质和缺陷的种类具有重要的参考价值。     

Radiative life time of an exciton confined in a strained GaN/Ga1-xAlxN cylindrical dot: built-in electric field effects

The binding energy of an exciton in a wurtzite GaN/GaAlN strained cylindrical quantum dot is investigated theoretically.The strong built-in electric field due to the spontaneous and piezoelectric polarizations of a GaN/GaAlN quantum dot is included.Numerical calculations are performed using a variational procedure within the single band effective mass approximation.Valence-band anisotropy is included in our theoretical model by using different hole masses in different spatial directions.The exciton oscillator strength and the exciton lifetime for radiative recombination each as a function of dot radius have been computed.The result elucidates that the strong built-in electric field influences the oscillator strength and the recombination life time of the exciton.It is observed that the ground state exciton binding energy and the interband emission energy increase when the cylindrical quantum dot height or radius is decreased,and that the exciton binding energy,the oscillator strength and the radiative lifetime each as a function of structural parameters (height and radius) sensitively depend on the strong built-in electric field.The obtained results are useful for the design of some opto-photoelectronic devices.     

Radiation-stimulated pulse conductivity of CsBr crystals

The radiation-stimulated pulse conductivity of CsBr crystals is investigated upon picosecond excitation with electron beams (0.2 MeV, 50 ps, 0.1–10 kA/cm²). The time resolution of the measuring technique is ～150 ps. It is shown that the lifetime of conduction band electrons is limited by their bimolecular recombination with autolocalized holes (V _k centers). A delay in the conduction current pulse build-up is revealed. This effect is explained within the proposed model, according to which the Auger recombination of valence band electrons and holes of the upper core band substantially contributes to the generation of conduction band electrons.