Enhancement of light-emission efficiency of ultraviolet InGaN/GaN multiple quantum well light emitting diode with InGaN underlying layer

Two ultraviolet InGaN/GaN light emitting diodes (LEDs) with and without InGaN underlying layer beneath the multiple quantum wells (MQWs) were grown by metal-organic vapor phase epitaxy. Based on the photoluminescence excitation measurements, it was found that the Stokes shift of the sample with a 10-nm-thick In_0.1Ga_0.9N underlying layer was about 64 meV, which was smaller than that of the reference sample without InGaN underlying layer, indicating a reduced quantum-confined Stark effect (QCSE) due to the decrease of the piezoelectric polarization field in the MQWs. In addition, by fitting the photon energy dependence of carrier lifetime values, the radiative recombination lifetime of the sample with and without InGaN underlying layer were obtained about 1.22 and 1.58 ns at 10?K, respectively. The shorter carrier lifetime also confirmed that the QCSE in the MQWs was weakened after inserting the InGaN underlying layer. In addition, although the depth of carrier localization in the sample with InGaN underlying layer became smaller, the nonradiative recombination centers (NRCs) inside it decreased, and thus suppressed the nonradiative recombination process significantly according to the electroluminescence measurement results. Compared to the reference sample, the efficiency droop behavior was delayed in the sample with InGaN underlying layer and the droop effect was also effectively alleviated. Therefore, the enhanced light-emission efficiency of ultraviolet InGaN/GaN MQW LEDs could be attributed to the decrease of QCSE and NRCs.     

Carrier recombination under one-photon and two-photon excitation in GaN epilayers

Luminescence properties of 100-mum thick GaN epilayers grown by hydride vapor phase epitaxy (HVPE) over three different substrates: high-pressure grown n-type bulk GaN (HP-n-GaN), high-pressure bulk GaN doped with magnesium (HP-GaN:Mg), and free-standing HVPE lifted-off from sapphire (FS-HVPE-GaN), were compared by means of one-photon and two-photon excitations. The contribution of carrier capture to nonradiative traps was estimated by the analysis of luminescence transients with carrier diffusion taken into account. The estimated values of carrier lifetime of about 3ns and diffusion coefficient of 1cm(2)/s indicate the highest quality of GaN epilayers on FS-HVPE-GaN substrates.     

Gain mechanisms in field‐free InGaN layers grown on sapphire and bulk GaN substrate

The gain mechanisms and recombination dynamics of InGaN layers strongly depend on the structural properties of the substrate material. The 4.5 nm and 9.5 nm thick layers were grown by metal organic chemical vapor deposition on two different substrates (sapphire and GaN) with different dislocation densities. Time‐resolved photoluminescence spectroscopy at high excitation densities identifies the saturation of nonradiative recombination centers through excited carriers as a major gain mechanism. The prime argument is an unusual nonexponential luminescence decay. This was confirmed by a lower threshold of the optical gain for the structures grown on GaN with lower dislocation densities. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

蓝光激光器结构中InGaN/GaN多量子阱界面效应的精细光致发光光谱研究

金属有机化学气相沉积(MOCVD)方法制备InGaN/GaN多量子阱结构时,在GaN势垒层生长的N2载气中引入适量H2,能够有效改善阱/垒界面质量从而提升发光效率.本工作利用光致发光(PL)光谱技术,对蓝光激光器结构中的InGaN/GaN多量子阱的发光性能进行了精细的光谱学测量与表征,研究了通H2生长对量子阱界面的调控...     

双波长自由载流子吸收技术测量半导体载流子体寿命和表面复合速率

提出了采用双波长自由载流子吸收技术同时测量半导体材料载流子体寿命和前表面复合速率的方法.通过数值模拟,定性分析了不同载流子体寿命和前表面复合速率对信号的影响,同时对测量参数的可接受范围和不确定度进行计算并与传统频率扫描自由载流子吸收方法测量结果进行比较.结果表明：提出的双波长自由载流子吸收方法可明显减小载流子体寿命和前表面复合速率的测量不确定度,提高参数测量精度;表面杂质和缺陷越多的样品,其前表面复合速率测量不确定度越小.进一步分析表明,此现象与不同波长激光抽运产生的过剩载流子浓度分布不同有关.     

Thermal activation and deactivation of grown‐in defects limiting the lifetime of float‐zone silicon

By studying the minority carrier lifetime in recently manufactured commercially available n‐ and p‐type float‐zone (FZ) silicon from five leading suppliers, we observe a very large reduction in the bulk lifetime when FZ silicon is heat‐treated in the range 450–700 °C. Photoluminescence imaging of these samples at the wafer scale revealed concentric circular patterns, with higher recombination occurring in the centre, and far less around the periphery. Deep level transient spectroscopy measurements indicate the presence of recombination active defects, including a dominant center with an energy level at ～E_v + 0.5 eV. Upon annealing FZ silicon at temperatures >1000 °C in oxygen, the lifetime is completely recovered, whereby the defects vanish and do not reappear upon subsequent annealing at 500 °C. We conclude that the heat‐treatments at >1000 °C result in total annihilation of the recombination active defects. Without such high temperature treatments, the minority carrier lifetime in FZ silicon is unstable and will affect the development of high efficiency (>24%) solar cells and surface passivation studies.     

Carrier dynamics in GaN at extremely low excited carrier densities

Carrier dynamics in GaN was studied using fluorescence lifetime measurement in the frequency domain technique in the temperature range from 8 to 300 K at very low and very high excitation levels. The study was performed in a high-quality GaN epilayer exhibiting a room-temperature nonequilibrium carrier lifetime of 2 ns, which was determined by a light-induced transient grating (four-wave mixing) technique. The results reveal the roles of donor–acceptor pair recombination and conduction band–acceptor recombination in yellow luminescence band formation.     

调制激光致硅晶圆载流子辐射扫描成像试验研究

建立了调制激光诱发硅晶圆少数载流子密度波一维模型, 仿真分析了少数载流子输运参数对调制激光诱发载流子辐射信号频域响应的影响. 利用调制激光诱发载流子辐射扫描成像系统对含有表面划痕的硅晶圆进行了扫描成像试验研究. 通过少数载流子密度波模型与多参数拟合方法反求得到了扫描区域的输运参数二维分布图. 该方法得到的少数载流子寿命与利用传统光电导方法测量的少数载流子寿命结果相符; 分析了划痕对载流子输运参数造成的影响, 与光电导方法比较, 该方法可以测量不同位置的全部载流子输运参数且分辨率高.     

A combined transient and steady state approach for robust lifetime spectroscopy with micrometer resolution

We present the combination of two complementary micro‐photoluminescence spectroscopic techniques operating in transient and steady state condition, respectively. Introducing the time domain into the well‐established micro‐photoluminescence mapping approach operating under steady state conditions demonstrates a distinct improvement of the robustness and reliability in the determination of charge carrier lifetime measured with micrometer spatial resolution. Lifetimes from 50 ns to above ms are accessible. We elaborate a calibration procedure and apply the combined all‐photoluminescence setup to high‐performance multicrystalline silicon. A lifetime image obtained from the established photoluminescence imaging technique is reconstructed from the microscopic map by considering lateral diffusion and optical blurring, revealing a more detrimental influence of small angle grain boundaries as well as a higher lifetime within grains as may be deduced from the standard imaging technique. (© 2015 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Effect of Nanorod Diameters on Optical Properties of GaN-Based Dual-Color Nanorod Arrays

Dual-color(blue and green) InGaN/GaN nanorod light-emitting diodes(LEDs) with three different nanorod diameters are fabricated. Enhancement of luminescence intensity per area is observed in blue and green wells,to varying degrees. When the diameter is 40 nm, it sharply decreases, which could be explained by the sidewall nonradiative recombination. Time-resolved photoluminescence is conducted to study the carrier lifetime. High recombination rate is observed in nanorod arrays, and is an order of magnitude less than that of the planar LED.When the diameter is 40 nm, the nonradiative lifetime decreases, and this explains the decrease of intensity. The3 D-FDTD simulations show the enhancement of light extraction out of geometry structure by calculating the transmittance of the nanorod arrays.     

Type-II superlattice detectors for free space optics applications and higher operating temperature conditions

The utmost limit performance of interband cascade detectors optimized for the longwave range of infrared radiation is investigated in this work. Currently, materials from the III–V group are characterized by short carrier lifetimes limited by Shockley-Read-Hall generation and recombination processes. The maximum carrier lifetime values reported at 77 K for the type-II superlattices InAs/GaSb and InAs/InAsSb in a longwave range correspond to ～200 and ～400 ns. We estimated theoretical detectivity of interband cascade detectors assuming above carrier lifetimes and a value of ～1–50 μs reported for a well-known HgCdTe material. It has been shown that for room temperature the limit value of detctivity is of ～3–4×10¹⁰ cmHz^1/2/W for the optimized detector operating at the wavelength range ～10 μm could be reached.     

Temporary Surface Passivation for Characterisation of Bulk Defects in Silicon: A Review

Accurate measurements of the bulk minority carrier lifetime in high‐quality silicon materials is challenging due to the influence of surface recombination. Conventional surface passivation processes such as thermal oxidation or dielectric deposition often modify the bulk lifetime significantly before measurement. Temporary surface passivation processes at room or very low temperatures enable a more accurate measurement of the true bulk lifetime, as they limit thermal reconfiguration of bulk defects and minimize bulk hydrogenation. In this article we review the state‐of‐the‐art for temporary passivation schemes, including liquid immersion passivation based upon acids, halogen‐alcohols and benzyl‐alcohols, and thin film passivation usually based on organic substances. We highlight how exceptional surface passivation (surface recombination velocity below 1 cm s^?1) can be achieved by some types of temporary passivation. From an extensive review of available data in the literature, we find p‐type silicon can be best passivated by hydrofluoric acid containing solutions, with superacid‐based thin films showing a slight superiority in the n‐type case. We review the practical considerations associated with temporary passivation, including sample cleaning, passivation activation, and stability. We highlight examples of how temporary passivation can assist in the development of improved silicon materials for photovoltaic applications, and provide an outlook for the future of the field.

     

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.     

Submicron resolution carrier lifetime analysis in silicon with Fano resonances

Defect rich regions in multicrystalline silicon are investigated by Raman spectroscopy at high and low injection levels. By analyzing the Fano type asymmetry and the spectral position of the first order Raman peak crucial properties such as recombination lifetime, doping density and stress can be extracted simultaneously. Due to the small wavelength of the excitation laser the spatial resolution of these measurements is significantly below 1 µm, which gives new insight into the impact of defects on the carrier recombination lifetime. The results are evaluated by comparing them to micro‐photoluminescence and synchrotron X‐ray fluorescence measurements. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

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.     

Recombination dynamics of free and bound excitons in bulk GaN

We report new data on the transient photoluminescence behaviour of free and donor bound excitons in high quality bulk GaN material grown by HVPE. With 266 nm photoexcitation the no-phonon free exciton has a short decay time, about 100 ps at 2 K, assigned to nonradiative surface recombination. The LO replicas of the free exciton have a much longer decay at 2 K, about 1.4 ns, believed to be a lower bound for the bulk radiative lifetimes of the free excitons at 2 K. The donor bound exciton no-phonon lines exhibit a rather short (about 300 ps) nonexponential decay at 2 K, which appears to be dominated by a scattering process. The corresponding LO replicas and the two-electron transitions have a much longer decay. From the latter, the lower bound of the radiative lifetime of the O- and Si-bound excitons are 1800 ps and 1100 ps, respectively.     

Impact of the firing temperature profile on light induced degradation of multicrystalline silicon

Light‐ and elevated temperature‐induced degradation in multicrystalline silicon can reduce the efficiency of solar cells significantly. In this work, the influence of the firing process and its temperature profile on the degradation behaviour of neighbouring mc‐Si wafers is analysed. Five profiles with measured high peak temperatures ≥800 °C and varying heating and cooling ramps are examined. With spatially resolved and lifetime calibrated photoluminescence images, normalized defect concentrations N^*_t are calculated to determine the degradation intensity. Wafers that underwent a fast firing process typical for industrial solar cell production show a significantly stronger degradation than samples that were subjected to the same peak temperature but with slower heating and cooling rates. A spatially resolved analysis of the carrier lifetime in the whole wafer shows that the degradation begins in low lifetime areas around dislocation clusters, spreading into good grains after several hours. By the use of optimized ramp‐up and/or ramp‐down rates during the firing even at very high peak temperatures, light and elevated temperature induced degradation can be suppressed. (© 2016 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

硅基全光宽带太赫兹幅度调制器的研究

提出了一种基于掺金硅的全光学宽带太赫兹波幅度调制器, 研究了金(Au)点阵掺杂后硅(Si)体内的少数载流子寿命及其太赫兹波调制特性. 实验结果表明, 掺杂的Au原子为Si中的光生电子- 空穴对提供了有效复合中心, 使其少数载流子寿命由原来十几微秒降低至110 ns左右. 利用波长915 nm 调制激光作为抽运光源, 在340 GHz载波的动态调制测试中获得4.3 MHz的调制速率和21%的调制深度, 使Si基调制器的调制速率提高了两个数量级. 该全光太赫兹调制器可工作在整个太赫兹频段内, 具有极化不敏感特性, 因而在太赫兹波高速和宽带调控方面具有重要的应用价值, 也是构建光控型Si 基太赫兹功能器件的重要基础.     

Thermal deactivation of lifetime‐ limiting grown‐in point defects in n‐type Czochralski silicon wafers

In this study, we uncover a recombination‐active grown‐in defect reducing the minority carrier lifetime of Czochralski grown n‐type silicon from 5 ms to below 2 ms. We also demonstrate that the defect can be de‐activated by annealing between 300 °C and 360 °C. Our experimental findings suggest that vacancy‐related pairs incorporated during ingot growth may be responsible for the decreased minority carrier lifetime. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Low dose 60Co gamma-irradiation effects on electronic carrier transport and DC characteristics of AlGaN/GaN high-electron-mobility transistors

The impact of internal irradiation with secondary Compton electrons, generated by gamma-photons, on the characteristics of III-N/GaN-based devices was explored. N-channel AlGaN/GaN high-electron-mobility transistors (HEMTs) were exposed to gamma-radiation from a ⁶⁰Co source for doses up to 600?Gy. Temperature-dependent electron beam-induced current (EBIC) was employed to measure minority carrier transport properties. For low doses below ～250?Gy, the minority carrier diffusion length in AlGaN/GaN HEMTs is shown to increase by about 40%. This increase is likely due to longer minority carrier lifetime induced by internal Compton electron irradiation. An associated decrease in activation energy, extracted from temperature-dependent EBIC, was also found. The obtained increase in transconductance and decrease in gate leakage current indicate an improvement in performance of the devices after low doses of irradiation. For high doses of gamma-irradiation, above ～300?Gy, the performance of HEMTs showed a deterioration. The deterioration results from the onset of increased carrier scattering due to additional radiation-induced defects, as is translated in a decrease of minority carrier diffusion length.