A theoretical investigation of the band alignment of type-I direct band gap dilute nitride phosphide alloy of GaN_xAs_yP_(1-x-y)/GaP quantum wells on GaP substrates

The GaP-based dilute nitride direct band gap material Ga（NAsP） is gaining importance due to the monolithic integra- tion of laser diodes on Si microprocessors. The major advantage of this newly proposed laser material system is the small lattice mismatch between GaP and Si. However, the large threshold current density of these promising laser diodes on Si substrates shows that the carrier leakage plays an important role in Ga（NAsP）/GaP QW lasers. Therefore, it is necessary to investigate the band alignment in this laser material system. In this paper, we present a theoretical investigation to optimize the band alignment of type-I direct band gap GaNxAsyP1-x-y/GaP QWs on GaP substrates. We examine the effect of nitrogen （N） concentration on the band offset ratios and band offset energies. We also provide a comparison of the band alignment of type-I direct band gap GaNxAsyP1-x-y/GaP QWs with that of the GaNxAsyP1-x-y/Al2Ga1-2P QWs on GaP substrates. Our theoretical calculations indicate that the incorporations of N into the well and AI into the barrier improve the band alignment compared to that of the GaAsP/GaP QW laser heterostructures.     

Electron-hole liquid in GaP

The luminescence of strongly excited GaP is reported. A new emission band is attributed to the electron-hole liquid. Values of the carrier density and binding energy of the liquid are compared to calculations.     

Electronic structure and optical gain of InAsPN/GaP(N) quantum dots

The electronic structure and optical gain of InAsPN/GaP(N) quantum dots (QDs) are investigated in the framework of the effective-mass envelope function theory. The strain distribution is calculated using the valence force field (VFF) method. With GaP barrier, for smaller InAsPN QDs, the minimum transition energy may occur at a lower phosphorous (P) composition, but for larger QDs, the transition energy increases as P composition increases due to the increased bandgap of alloy QDs. When the nitrogen (N) composition increases, the transition energy decreases due to the stronger repulsion between the conduction band (CB) and the N resonant band, and the transition matrix element (TME) is more affected by the transition energy rather than N–CB mixing. To obtain laser materials with a lattice constant comparable to Si, we incorporated 2% of N into the GaP barrier. With this GaP_0.98N_0.02 barrier, the conduction band offset is reduced, so the quantum confinement is lower, resulting in a smaller transition energy and longer wavelength. At the same time, the TME is reduced and the optical gain is less than those without N in the barrier at a low carrier density, but the peak gain increases faster when the carrier density increases. Finally it can surpass and reach a greater saturation optical gain than those without N in the barrier. This shows that incorporating N into GaP barriers is an effective way to achieve desirable wavelength and optical gain.     

老化产生的深能级对GaP纯绿发光二极管发光效率的影响

测量了GaP纯绿发光二极管老化前后的可见和近红外发光光谱,研究了老化产生的深能级的来源及其对二极管发光效率的影响.在老化后的发光光谱中观测到650nm和1260nm发光带,发现1260nm发光带的发光强度随老化时间的增加而增强.实验结果表明老化产生的与磷相关的深能级严重地影响了GaP纯绿LED的发光效率.     

GaP低阻欧姆接触层的AES和SIMS分析

本文用AES和SIMS分析讨论了p-GaP与三层金属膜Pd/Zn/Pd形成良好欧姆接触层的性质.     

A trend in the energy distributions of Ga dangling-orbital surface states on GaP,GaAs and GaSb (110)

Relaxed (110) surfaces of GaP, GaAs, and GaSb are studied by cluster calculations. Strongly localized surface states near the band gap are presented. It is shown that Ga dangling-orbital surface states can occur within the band gap as a result of a strong decrease in the anti-bonding character of surface Ga orbitals. A trend in the bond length on going from GaSb to GaP explains experimental data. Surface states associated with the group-V atoms are also described.     

Electronic surface properties of uhv-cleaved III–V compounds

Combined CPD and photoemission measurements were performed on uhv-cleaved surfaces of the III–V compounds InAs, GaSb, GaAs and GaP with moderate p-type and n-type dopings. Except for n-type GaP these materials show practically no band bending. N-type GaP exhibits surface Fermi level stabilization at 0.55 eV below the conduction band edge. This is ascribed to an intrinsic empty surface state band in the forbidden zone. On the basis of our experiments together with available data from literature we propose an empirical model for the (110) plane of III–V compounds containing In or Ga as metal and Sb, As or P as non-metal atoms from which the lower edge of the empty surface state band can be predicted. The model indicates that for any of these compounds except for GaP no empty surface state band exist in the band gap on the (110) surface.     

Formation and crystal structure of GaSb/GaP quantum dots

The atomic structure of GaSb/GaP quantum dots grown via molecular beam epitaxy on a (100) GaP surface at epitaxy temperatures of 420–470°C is investigated. It is established that, depending on morphology of the GaP growth surface, the deposition of 1 ML of GaSb leads to the formation of strained Ga(Sb, P)/GaP or fully relaxed GaSb/GaP quantum dots. The obtained heterostructures exhibit high photoluminescence efficiency.     

MOCVD生长的GaN单晶膜的蓝带发光研究

对实验室用ＭＯＣＶＤ方法生长的未掺杂ＧａＮ单晶膜的发光性能进行了研究。结果表明：在室温时未掺杂ＧａＮ单晶出现的能量为２．９ｅＶ左右蓝带发光与被偿度有较强的依赖关系。高补偿ＧａＮ的蓝带发射强,低补偿ＧａＮ的蓝带发射弱。对蓝带发光机理进行了探讨,认为蓝 导带电子过至受主能级的发光（ｅＡ发光）。观察到降低ＧａＮ补偿度能提高ＧａＮ带边发射强度。     

Luminescence properties of BeO optically stimulated luminescence (OSL) detectors

The objective of this work is to investigate basic luminescence properties of BeO optically stimulated luminescence (OSL) detectors, including the OSL emission and stimulation spectrum, the lifetime of the luminescence centers contributing to the OSL signal, and the temperature dependence of the luminescence lifetime and of the luminescence efficiency. The OSL stimulation spectrum shows a continuous increase in OSL intensity with decreasing stimulation wavelength. The emission spectrum indicates two OSL emission bands at ～310 nm and ～370 nm, the latter being the dominant OSL emission band. We also observed that the luminescence centers associated with the OSL signal are strongly quenched above room temperature, resulting in a reduction in luminescence lifetime from ～27 μs at room temperature down to ～800 ns at 140 °C. The activation energy for non-radiative decay of the luminescence center was determined to be E = (0.568 ± 0.023) eV. The ～27 μs luminescence lifetime observed for BeO indicate that POSL technique may be used to improve the signal-to-noise ratio using stimulation pulses of the order of microseconds. The information obtained in this study may help further optimize the BeO dosimetry systems and provide guidance on the timing parameters to be used for POSL measurements of this material.     

基于分子束外延生长的1.05 eV InGaAsP的超快光学特性研究

利用分子束外延方法制备了应用于四结光伏电池的1.05 eV InGaAsP薄膜, 并对其超快光学特性进行了研究. 温度和激发功率有关的发光特性表明: InGaAsP材料以自由激子发光为主. 室温下InGaAsP材料的载流子发光弛豫时间达到10.4 ns, 且随激发功率增大而增大. 发光弛豫时间随温度升高呈现S形变化, 在低于50 K时随温度升高而增大, 在50–150 K之间时减小, 而温度高于150 K时再次增大. 基于载流子弛豫动力学, 分析并解释了温度及非辐射复合中心浓度对样品材料载流子发光弛豫时间S形变化的影响.     

Limits on light emission from silicon Invited Paper

Although silicon is an indirect semiconductor, light emission from silicon is governed by the same gener-alized Planck's radiation law as the emission from direct semiconductors. The emission intensity is given by the absorptance of the volume in which there is a difference of the quasi Fermi energies. A difference of the Fermi energies may rcsult from the absorption of external light (photoluminescence) or from the in-jection of electrons and holes via selective contacts (electroluminescence). The quantum efficiency may be larger than 0.5 for carrier densities below 1015 cm-3. At larger densities, non-radiative recombination, in particular Auger recombination dominates. At all carrier densities, the relation between emission intensity and difference of the quasi Fermi energies is maintained. Since this difference is equal to the voltage of a properly designed solar cell, luminescence is the key indicator of material quality for solar cells.     

生长过程中磷压对MOVPE GaP电致发光谱的影响

对在不同磷压下生长的MOVPE GaP二极管的电致发光(EL)谱进行了测量。参照对相应的肖特基势垒样品进行的结技术测量和对衬底进行的光致发光(PL)测量结果,对观测到的几个强度随生长时所用磷压明显变化的EL带的起因作了讨论。将其中相对强度随磷压增加的两个EL带分别试归因于与反位磷PGa的一次和二次离化态有关的辐甜复合;将相对强度随磷压减小而增大的一个EL带归因于与氧有关。     

Fabrication and characterization of GaP/polymer nanocomposites for advanced light emissive device structures

GaP nanoparticles have been prepared using white P and a mild aqueous synthesis at decreased temperature followed by ultrasonication and stored as the suspension in water–ethanol mixture. They were characterized by standard methods of X-ray diffraction, transmission electron microscopy, Raman light scattering, and photoluminescence. Properties of GaP nanoparticles were compared with industrial and specially grown perfect GaP single crystals. It was shown that the GaP nanoparticles in suspension are the most suitable for high quality GaP/polymers nanocomposites because only they are uniform with dimensions of about 10 nm which is optimal for appearance of the pronounced quantum confinement effect. Polyglycidyl methacrylate (PGMA), polyglycidyl methacrylate-co-polyoligoethyleneglycol methacrylate (PGMA-co-POEGMA), and biphenyl vinyl ether (BPVE) polymers were used to prepare GaP polymer nanocomposites. The thickness of the polymer nanocomposite film was about 250–300 nm defined from AFM scratch experiment. The resulting nanocomposites yielded a bright luminescence at room temperature in a broad band with the maximum ranging from 2.5 to 3.2 eV and showed pronounced quantum confinement effects and other interesting and important for application phenomena leading to dramatic 1 eV expansion of GaP luminescence to the UV spectral region.     

Emission and excitation spectra of ZnO:Ga and ZnO:Ga,N ceramics

The spectral characteristics of ZnO:Ga and ZnO:Ga,N ceramics prepared by uniaxial hot pressing have been investigated. At room temperature, the edge (exciton) band at 3.12 eV dominates in the luminescence spectra of ZnO:Ga, while a wide luminescence band at 2.37 eV, which is likely to be due to zinc vacancies, is observed in the spectra of ZnO:Ga,N. Upon heating, the edge band maximum shifts to lower energies and the bandwidth increases. The extrapolated position of the edge-band maximum at zero temperature, E _m (0) = 3.367 ± 0.005 eV, is in agreement with the data for thin zinc oxide films. The luminescence excitation spectra in the range from 3 to 6.5 eV are reported and the mechanism of energy transfer to excitons and luminescence centers is considered.     

GaP:(Bi,N)晶体中束缚激子的辐射复合及能量转移

本文研究了在4.2K—147K温度范围内GaP:(Bi,N)晶体的光致发光光谱的精细结构,积分强度及其随温度的变化,并将它们与GaP:Bi,GaP:N光谱进行比较。首次发现在GaP:(Bi,N)晶体中激发能量从孤立N中心和NNi对(i≥3)中心到Bi中心的转移,增大了Bi束缚激子态的浓度,使Bi发射带增强。     

An Improved Study of Electronic Band Structure and Optical Parameters of X-Phosphides (X=B,Al, Ga,In) by Modified Becke-Johnson Potential

We report the electronic band structure and optical parameters of X-Phosphides (X=B, Al, Ga, In) by first-principles technique based on a new approximation known as modified Becke-Johnson (mBJ). This potential is considered more accurate in elaborating excited states properties of insulators and semiconductors as compared to LDA and GGA. The present calculated band gaps values of BP, AlP, GaP, and InP are 1.867 eV, 2.268 eV, 2.090 eV, and 1.377 eV respectively, which are in close agreement to the experimental results. The band gap values trend in this study is as: E_g(mBJ-GGA/LDA) > E_g(GGA) > E_g(LDA). Optical parametric quantities (dielectric constant, refractive index, reflectivity and optical conductivity) which based on the band structure are also presented and discussed. BP, AlP, GaP, and InP have strong absorption in between the energy range 4-9 eV, 4-7 eV, 3-7 eV, and 2-7 eV respectively. Static dielectric constant, static refractive index and coefficient of reflectivity at zero frequency, within mBJ-GGA, are also calculated. BP, AlP, GaP, and InP show significant optical conductivity in the range 5.2-10 eV, 4.3-8 eV, 3.5-7.2 eV, and 3.2-8 eV respectively. The present study endorses that the said compounds can be used in opto-electronic applications, for different energy ranges.     

An Improved Study of Electronic Band Structure and Optical Parameters of X-Phosphides (X=B,Al, Ga,In) by Modified Becke-Johnson Potential

We report the electronic band structure and optical parameters of X-Phosphides (X=B, Al,Ga,In) by first-principles technique based on a new approximation known as modified Becke-Johnson (mBJ). This potential is considered more accurate in elaborating excited states properties of insulators and semiconductors as compared to LDA and GGA. The present calculated band gaps values of BP, AlP, GaP, and InP are 1.867 eV, 2.268 eV, 2.090 eV, and 1.377 eV respectively, which are in close agreement to the experimental results. The band gap values trend in this study is as: Eg (mBJ-GGA/LDA)Eg (GGA) Eg (LDA). Optical parametric quantities (dielectric constant, refractive index, reflectivity and optical conductivity) which based on the band structure are also presented and discussed. BP, AlP, GaP, and InP have strong absorption in between the energy range 4-9eV, 4-7eV, 3-7eV, and 2-7eV respectively. Static dielectric constant, static refractive index and coefficient of reflectivity at zero frequency, within mBJ-GGA, are also calculated. BP, AlP, GaP, and InP show significant optical conductivity in the range 5.2-10eV, 4.3-8eV, 3.5-7.2eV, and 3.2-8eV respectively. The present study endorses that the said compounds can be used in opto-electronic applications, for different energy ranges.     

Modelling the thermal quenching mechanism in quartz based on time-resolved optically stimulated luminescence

This paper presents a new numerical model for thermal quenching in quartz, based on the previously suggested Mott-Seitz mechanism. In the model electrons from a dosimetric trap are raised by optical or thermal stimulation into the conduction band, followed by an electronic transition from the conduction band into an excited state of the recombination center. Subsequently electrons in this excited state undergo either a direct radiative transition into a recombination center, or a competing thermally assisted non-radiative process into the ground state of the recombination center. As the temperature of the sample is increased, more electrons are removed from the excited state via the non-radiative pathway. This reduction in the number of available electrons leads to both a decrease of the intensity of the luminescence signal and to a simultaneous decrease of the luminescence lifetime. Several simulations are carried out of time-resolved optically stimulated luminescence (TR-OSL) experiments, in which the temperature dependence of luminescence lifetimes in quartz is studied as a function of the stimulation temperature. Good quantitative agreement is found between the simulation results and new experimental data obtained using a single-aliquot procedure on a sedimentary quartz sample.     

纳米ZnO薄膜的光致发光性质

利用溶胶－凝胶法制备了纳米ZnO薄膜,室温下测量了样品的光致发光谱(PL)、吸收谱(ABS)、X射线衍射谱(XRD)．X射线衍射(XRD)的结果表明：纳米ZnO薄膜呈多晶状态,具有六角纤锌矿晶体结构和良好的C轴取向．观察到二个荧光发射带,中心波长分别位于395 nm的紫带、524 nm的绿带和450 nm附近的蓝带．证实了纳米ZnO薄膜绿光可见发射带来自氧空位(VO)形成的浅施主能级和锌空位(VZn)形成的浅受主能级之间的复合;450 nm附近的蓝带来自电子从VO的浅施主能级到价带顶或锌填隙(Zni) 到价带顶或导带底到VZn的浅受主能级的复合．