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.     

Photographic diagnosis of crystalline silicon solar cells utilizing electroluminescence

The photographic surveying of electroluminescence (EL) under forward bias was proved to be a powerful diagnostic tool for investigating not only the material properties but also process induced deficiencies visually in silicon (Si) solar cells. Under forward bias condition, solar cells emit infrared light (wavelength around 1000 to 1200 nm) whose intensity reflects the number of minority carriers in base layers. Thus, all the causes that affect the carrier density can be detected, i.e., the minority carrier diffusion length (or in other words, lifetime), recombination velocity at surfaces and interfaces, etc. (intrinsic material properties), and wafer breakage and electrode breakdown, etc. (extrinsic defects). The EL intensity distribution can be captured by Si CCD camera in less than 1 s, and the detection area simply depends upon the optical lens system suitable to the wide range of 1 cm–1.5 m. This fast and precise technique is superior to the conventional scanning method such as the laser beam induced current (LBIC) method. The EL images are displayed as grayscale, which leads to the difficulty of distinguishing the sorts of those deficient areas. Since the intrinsic deficiency is more sensitive to temperature than the extrinsic deficiency, the change in solar cell temperature can offer the difference in EL intensity contrasts. These effects upon the measurement temperature can be applied to categorize the types of deficiency in the crystalline Si solar cell.     

Effect of the postimplantation-annealing temperature on the properties of silicon light-emitting diodes fabricated through boron ion implantation into <Emphasis Type="Italic">n</Emphasis>-Si

The effect of the temperature of postimplantation annealing on the electroluminescence and the electrophysical and structural properties of light-emitting diodes fabricated by the implantation of boron ions into n-Si with a resistivity of 0.5 and 500 Ω cm is studied. All spectra contain strong electroluminescence (EL) peaks associated with band-to-band radiative transitions. An increase in the annealing temperature from 700 to 1100°C is accompanied by a monotonic increase in the quantum efficiency for the dominating EL peak and in the effective minority-carrier lifetime in the base of the light-emitting diodes and by the transformation of extended structural defects. Analysis of the experimental data shows that the extended structural defects formed are most likely to affect the EL properties via the formation or gettering of the radiative or nonradiative recombination centers rather than via preventing the removal of charge carriers to nonradiative recombination centers. The maximum internal quantum efficiency is reached after annealing at 1100°C (where extended structural defects are absent) and is estimated to be 0.4% at 300 K.     

输运层厚度对双层有机器件复合发光的影响

采用ITO/PVK/Alq/Al双层电致发光（EL）结构,制备了三种载流子输运层厚度分别为30、60、120nm,发光层厚度均为300nm的有机薄膜EL器件,测试其EL谱及J-V特性曲线。根据有机EL器件中载流子的产生和输运过程导出了载流子复合几率及电子和空穴密度分布表示式,用以解释其发光强度随输运层厚度的变化关系,用一维无序结构载流子随机跃迁模型讨论输运层厚度对器件电流密度及启动电压的影响,探讨了载流子在薄膜中的输运过程,其理论与实验符合得很好。     

Luminescence of II–VI and III–V nanostructures

Photoluminescence of HgCdTe epitaxial films and nanostructures and electroluminescence of InAs(Sb,P) light-emitting diode (LED) nanoheterostructures were studied. For HgCdTe-based structures, the presence of compositional fluctuations, which localized charge carriers, was established. A model, which described the effect of the fluctuations on the rate of the radiative recombination, the shape of luminescence spectra and the position of their peaks, was shown to describe experimental photoluminescence data quite reasonably. For InAs(Sb,P) LED nanoheterostructures, at low temperatures (4.2–100 K) stimulated emission was observed. This effect disappeared with the temperature increasing due to the resonant ‘switch-on’ of the Auger process involving transition of a hole to the spin-orbit-splitted band. Influence of other Auger processes on the emissive properties of the nanoheterostructures was also observed. Prospects of employing II–VI and III–V nanostructures in light-emitting devices operating in the mid-infrared part of the spectrum are discussed.     

1.54 μm Light emission from Er/O and Er/F doped Si p–i–n diodes grown by molecular beam epitaxy

Various light emitting devices (LED) have been processed using Er/O- and Er/F-doped Si layered structures grown by molecular beam epitaxy (MBE) at low temperature. A comparative study has been carried out in order to provide more understanding of the electroluminescence (EL) excitation and de-excitation mechanisms in particular at a high injection current regime. Comparing the experimental results with model calculations the values of excitation cross section, σ_ex, and effective Auger coefficient, C_A, have been determined for various devices operated at different biases. Time-resolved EL measurements of these Er/O- and Er/F-doped MBE Si structures, using an experimental set-up with a time response of 200 ns, have been performed with different excitation conditions. Besides the spontaneous Er emission (700 μs), some fast EL decay processes associated with the Auger energy transfer via free carriers (4 μs), and the hot carrier effects (200 ns) have been identified.     

Electroluminescence from Si quantum dots/SiO2 multilayers with ultrathin oxide layers due to bipolar injection

Si quantum dots/SiO₂ multilayers with ultrathin oxide layers (2.4 nm) were fabricated on a p-type Si substrate in order to enhance the hole injection. Besides the luminescence band at 900 nm which was also shown in photoluminescence spectra, another strong luminescence band near the infrared region (1200 nm) can be observed in electroluminescence spectra. It can be assigned to the band-edge emission from the quasi 2-dimensional potential well in the Si substrate. Moreover, it is interesting to find the reduction of photoluminescence intensity under biased conditions which can be attributed to the occurrence of non-radiative Auger recombination process in charged Si quantum dots.     

Mechanisms of Auger recombination in semiconducting quantum dots

Microscopic calculation of the probability of Auger recombination of charge carriers localized in a semiconducting quantum dot (QD) is carried out. It is shown that two mechanism of Auger recombination (nonthreshold and quasi-threshold) operate in the QD. The nonthreshold Auger recombination mechanism is associated with scattering of a quasimomentum from a heterobarrier, while the quasi-threshold mechanism is connected with spatial confinement of the wave functions of charge carriers to the QD region; scattering of carriers occurs at the short-range Coulomb potential. Both mechanisms lead to a substantial enhancement of Auger recombination at the QD as compared to a homogeneous semiconductor. A detailed analysis of the dependence of Auger recombination coefficient on the temperature and QD parameters is carried out. It is shown that the nonthreshold Auger recombination process dominates at low temperatures, while the quasi-threshold mechanism prevails at high temperatures. The dependence of the Auger recombination coefficient on the QD radius experiences noticeable changes as compared to quantum wells and quantum filaments.     

硅衬底InGaN/GaN基蓝光发光二极管droop效应的研究

InGaN/GaN基阱垒结构LED当注入的电流密度较大时, LED的量子效率随注入电流密度增大而下降, 即droop效应.本文在Si (111)衬底上生长了 InGaN/GaN 基蓝光多量子阱结构的LED,通过将实验测量的光电性能曲线与利用ABC模型模拟的结果进行对比, 探讨了droop效应的成因.结果显示:温度下降会阻碍电流扩展和降低空穴浓度, 电子在阱中分布会越来越不平衡,阱中局部区域中因填充了势能越来越高的电子而溢出阱外, 从而使droop效应随着温度的降低在更小的电流密度下出现且更为严重, 不同温度下实验值与俄歇复合模型模拟的结果在高注入时趋势相反.这此结果表明,引起 droop效应的主因不是俄歇非辐射复合而是电子溢出,电子溢出的本质原因是载流子在阱中分布不均衡.     

富硅a-SiOxNy：H薄膜的电致发光特性

采用甚高频等离子体增强化学气相沉积方法制备富硅氮氧化硅(a-SiO_0.35N_0.59:H)薄膜,以这层薄膜作为有源层构建发光二极管。实验结果表明器件在室温下可观测到强的电致红光发射,发光峰在715 nm附近,与其光致发光峰位一致。电致发光谱测量还表明器件开启电压为8 V,器件的电致发光强度随注入电流的增大呈线性递增关系。电流-电压特性分析表明器件的载流子输运机制以Pool-Frenkel(P-F)发射模型为主。结合发光有源层的微结构分析,初步认为电致红光发射来自于电子和空穴通过有源层的带尾态的辐射复合。     

PECVD纳米晶粒硅薄膜的可见电致发光

在用等离子体增强化学汽相淀积的嵌有纳米晶粒硅薄膜中观测到电致发光．发光谱处在500—800nm之间，它有两个分别位于630—680nm和730nm附近的峰，两个峰的强度与薄膜的电导率有密切关系．根据这种材料的结构特性对载流子的传导通道进行了讨论，并且对发光机制进行了初步解释 关键词：     

Optical characterizations of GaAs-based spin light emitting diodes using Fe3O4 spin injectors

We fabricated GaAs-based spin-LED (light emitting diode) structures using half-metallic Fe₃O₄ as spin injectors and measured the circular polarization of the electroluminescence (EL). The circular polarization of the EL due to the spin injection was improved by the low temperature growth of the ferromagnetic layer, compared to the room temperature growth. We also studied the excitation wavelength dependence of the photoluminescence (PL) spectra and the variation of the EL spectra with increasing current. The excess carrier dependence of the EL peaks was found to be different from that of the PL peaks, which was explained by the carrier injection into the buffer layer.     

Charge carriers bulk recombination instead of electroplex emission after their tunneling through hole-blocking layer in OLEDs

Charge carriers bulk recombination instead of forming electroplex after their tunneling through a hole-blocking layer, i.e. 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP), in organic electroluminescence (EL) device ITO/poly-(N-vinyl-carbazole)(PVK)/BCP/tris(8-hydroxyquinoline) aluminum (Alq₃)/Al is reported. By changing the thickness of BCP layer, one can find that high electric fields enhance the tunneling process of holes accumulated at the PVK/BCP interface into BCP layer instead of forming “electroplex emission” as reported earlier in literatures. Our experimental data show that charge carriers bulk recombination takes place in both PVK layer and BCP layer, and even in Alq₃ layer when BCP layer is thin enough. Further, it is suggested that PVK is the origin of the emission shoulder at 595 nm in the EL spectra of trilayer device ITO/PVK/BCP/Alq₃/Al.     

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.     

A wide-narrow well design for?understanding the?efficiency droop in?InGaN/GaN light-emitting diodes

The electroluminescence efficiency at room temperature and low temperature (15 K) in a wide-narrow-well InGaN/GaN light-emitting diode with a narrow last well (1.5 nm) and a narrow next-to-last barrier (5 nm) is investigated to study the efficiency droop phenomenon. A reduced droop in the wide wells and a reduced droop at low temperatures reveals that inferior hole transportation ability induced Auger recombination is the root for the droop at high excitation levels.     

Determination of carrier density dependent lifetime and quantum efficiency in semiconductors with a photoluminescence method (application to InGaAsP/InP heterostructures)

The light beam of a laser is focussed near the surface of a semiconductor sample. Thereby the excitation rate can be controlled precisely assuming a Gaussian intensity distribution of the beam. Measuring the recombination light intensity yields the quantum efficiency of the sample. By sinusoidal modulation of the excitation light and measurement of the resulting phase shift of the recombination light, the carrier density dependent lifetime is obtained. By evaluation of measured internal quantum efficiency and phase shift, Auger and radiative recombination coefficients are determined. The analysis takes into account the carrier density dependence of the radiative coefficient and shows that for most experimental conditions carrier diffusion can be neglected. In this case the analysis can be performed without numerical integration. Application of the method to quaternary InGaAsP material yielded values for Auger coefficient and radiative coefficients in accordance with published results.     

Ir(PPY)3掺杂PVK的电致发光机理

近几年来发展起来的电致磷光(electrophosphorescence)是有机发光二极管(OLED)研究的新生长点。对电致磷光发光机理的研究随即得到了人们普遍的关注。比较了不同正向偏压条件下Ir(PPY)₃掺杂聚乙烯基咔唑(PVK)的光致发光(PL)和电致发光(EL)光谱。研究结果显示在电场和注入电流的共同作用下,PL光谱中基质PVK发光的相对强度并没有发生显著的变化。电场或注入载流子不会影响PVK向Ir(PPY)₃的能量传递。磷光掺杂聚合物EL主要是由于载流子在掺杂磷光分子上的直接复合,而不是由基质向磷光掺杂分子的能量传递。     

电致发光成像技术在硅太阳能电池隐性缺陷检测中的应用

论述了一种利用硅太阳能电池在一定偏压下的电致发光(Electroluminescence,EL)成像来检测硅太阳能电池隐性缺陷的方法.硅太阳能电池的EL波长范围为850～1 200 nm.正向偏压下的EL光强反映了少数载流子的浓度及其扩散长度,而反向偏压下的EL区和发光强度对应于电池的缺陷区域和缺陷密度.用硅CCD相机...     

An Electroluminescence Delay Time Model of Bilayer Organic Light-Emitting Diodes

Based on the mechanism of injection, transport and recombination of the charge carriers, we develop a model to calculate the delay time of electroluminescence （EL） from bilayer organic light emitting diodes. The effect of injection, transport and recombination processes on the EL delay time is discussed, and the relationship between the internal interface barrier and the recombination time is revealed. ＂]~he results show that the EL delay time is dominated by the recombination process at lower applied voltage and by the transport process at higher applied voltage. When the internal interface barrier varies from 0.15 eV to 0.3 eV, the recombination delay time increases rapidly~ while the internal interface barrier exceeds about 0.3eV~ the dependence of the recombination delay time on applied voltage is almost undiversified, which may serve as a guideline for designing of a high-speed EL response device.