High-Efficiency ZnCdSe/ZnSSe/ZnMgSSe Green Light-Emitting Diodes

Green light-emitting diodes (LEDs) were fabricated employing a ZnCdSe/ZnSSe triple quantum-well (TQW) active region surrounded by ZnMgSSe cladding layers grown on an n-type (100) GaAs substrate by molecular beam epitaxy (MBE). A 3.5 mW pure green emission was observed for the surface-emitting LED device at a peak wavelength of 513.3 nm (2.415 eV) with a spectral half-width of 11.7 nm (55 meV) under a 20 mA (4.6 V) direct current at room temperature (25°C). These correspond to an external quantum efficiency of 7.2%, a power conversion efficiency of 3.8%, a luminous current efficiency of 66 lm/A, and a luminous efficiency of 14 lm/W.     

Ohmic Contacts to ZnSe-Based Materials

The formation of ohmic contacts to n- and p-type ZnSe is reviewed. The mechanisms for forming reasonable low-resistance ohmic contacts to n-ZnSe are well understood. This results from the fact that the Fermi energy level of ZnSe is unpinned and metals with sufficiently large work functions can make contact to n-type material. However, the situation is reversed for p-ZnSe where a large band gap and large electron affinity make it impossible to find metals with sufficiently large work functions to create an ohmic contact. Instead, the use of HgSe to form low barrier height Schottky contacts and of ZnSe/ZnTe multiple quantum wells (MQWs) to form ohmic contacts is reviewed. Although the MQWs can be used to form ohmic contacts to p-ZnSe, they degrade at high temperatures and high current densities. This is reviewed and shown to be a serious problem for applications to laser diodes.     

ZnSe相变、电子结构的第一性原理计算

基于第一性原理平面波赝势(PWP)和广义梯度近似(GGA)方法,对闪锌矿结构(ZB)和岩盐结构(RS)的ZnSe在0—20GPa高压下的几何结构、态密度、能带结构进行了计算研究,分析了闪锌矿结构ZnSe和岩盐结构ZnSe的几何结构.在此基础上,研究了ZnSe的结构相变、弹性常数、成键情况以及相变压强下电子结构的变化机理.结果发现:通过焓相等原理得到的ZB相到RS相的相变压强为15.3GPa,而由弹性常数判据得到的相变压强为11.52GPa,但在9.5GPa左右并没有发现简单立方相的出现;在结构相变过程中,sp3轨道杂化现象并未消除,Zn原子的4s电子在RS相ZnSe的导电性中起主要贡献.     

Photoluminescence quantum and surface states of excitons in ZnSe and CdS nanoclusters

The optical spectra of quantum dots (QDs) of CdS and ZnSe grown in borosilicate glass by the sol-gel method are obtained and analyzed. It is found that at concentrations of the two semiconductors x<0.06% the emission spectra are due to annihilation of free (internal) excitons in quantum states. The mean size of the quantum dots for a given concentration of ZnSe and CdS is calculated and found to be in good agreement with the X-ray data, and the exciton binding energy is calculated with allowance for the dielectric mismatch between the semiconductor and matrix. It is proposed that this mismatch may be the cause giving rise to the exciton percolation level that is observed in QD arrays for both systems at x>0.06%. The emission from the surface level of CdS QDs in the region ~2.7 eV, formed by the outer atoms with dangling bonds, is observed for the first time, as is the emission band from surface localized states. The relation between the position of the maximum of this band and the energy of the 1S state of the free exciton is established. It is shown that the properties of surface localized states are largely similar to the analogous properties of localized states of 3D (amorphous semiconductors, substitutional solid solutions of substitution) and 2D (quantum wells and superlattices) structures.     

Optical absorption of zinc selenide doped with cobalt (Zn1−xCoxSe) under hydrostatic pressure

Abstract

The optical absorption of the diluted magnetic semiconductor Zn_1?xCO_xSe (x = 0.02) has been measured at room temperature under hydrostatic pressure up to 14GPa in a membrane diamond-anvil cell. We found two absorption features: (i) an absorption structure in the energy range 1.6?1.8eV, with a negligible pressure shift (i.e., 0.45 ± 0.05 meV/GPa) which we have identified as the Co²⁺(3d⁷) internal transition ⁴A₂(F)→+⁴T₁(P) and (ii) an onset in the energy range 2?2.7eV which redshifts with pressure (?8.1±0.6meV/GPa). We have attributed such absorption edge to charge transfer between the ZnSe valence band and the Co²⁺(3d⁷) levels.     

飞秒激光诱导硒化锌晶体表面自组织生长纳米结构

 以250 kHz高重复频率钛宝石飞秒激光聚焦到硒化锌晶体表面,利用扫描电子显微镜观测飞秒激光辐照后晶体的表面结构。发现线偏振激光辐照的区域形成了自组织周期性纳米结构,其周期为160 nm左右,并且可以通过改变激光的偏振方向调节纳米光栅结构的取向;当晶体相对于激光光束以10 mm/s速度移动,经激光扫描后,在晶体表面形成了长程类布拉格光栅。当飞秒激光光束为圆偏振光时,辐照区域形成均匀的纳米颗粒。     

ZnCdSe/ZnSe非对称双量子阱中的光学特性研究

用LP-MOCVD技术在GaAs衬底上外延生长了ZnCdSe/ZnSe非对称双量子阱(ADQW)结构。通过ps时间分辨光谱、吸收光谱、发射光谱等的研究得到了如下的结果:在弱激发下,观测到ADQW结构中的激子隧穿现象;在强激发下,在ADQW结构中发现了一个内建电场,它将影响激子隧穿;首次观测到由激子隧穿引起的在一定温度范围内宽阱的发光强度随温度上升而增加的现象;首次观测到该ADQW结构中来自宽阱的光泵受激发射。     

ZnCdSe-ZnSe组合超晶格的受激发射

77K下首次观测到了来自ZnCdSe-ZnSe组合超晶格的受激发射过程。在组合超晶格中由于载流子转移过程的存在,受激发射出现在具有宽阱的超晶格中。     

Exciton reflection of ZnSe single crystals with account for the effect of the surface electric field

The mechanism of formation of an exciton reflection band is analyzed using a multilayer model of the near-surface region of ZnSe single crystals. It is shown that the appearance of a fine structure (a spike) in the reflection spectra at large damping parameters of the exciton is due to the surface electric field and the Stark exciton effect. The exciton-resonance energies and the damping parameters of free excitons for temperatures ranging from 12 to 100 K are determined by comparing experimental and predicted contours of the reflection spectrum. The surface concentration of charged centers and the characteristics of the space-charge region are evaluated within the framework of the model employed. Brest Polytechnic Institute, 267, Moskovskaya Str., Brest, 224017, Belarus. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 66, No. 3, pp. 401–407, May–June, 1999.     

ZnSe单晶生长及性能研究

采用物理气相输运法对ZnSe(4N)多晶原料在850℃±10℃进行提纯,再用高压坩埚下降法在1530℃、氩气保护气氛下生长出高质量ZnSe单晶体.研究了提纯过程温度的选择以及氧含量和压力对于晶体生长的影响.对生长出的单晶体进行均匀性测试表明ZnSe单晶完整性和均匀性良好.对ZnSe单晶进行光学性能测试分析表明ZnSe单晶的折射率高,吸收系数低,红外透过率大于70;.