Terahertz generation and detection of LT-GaAs thin film photoconductive antennas excited by lasers of different wavelengths

A new method of generating and detecting terahertz waves is proposed. Low-temperature-grown gallium arsenide (LT-GaAs) thin films are prepared by etching a sacrificial layer (AlAs) in a four-layer epitaxial structure constituted with LT-GaAs, AlAs, GaAs, and semi-insulating gallium arsenide (SI-GaAs). The thin films are then transferred to clean silicon for fabricating the LT-GaAs thin film antennas. The quality and transmission characteristics of the films are analyzed by an 800-nm asynchronous ultrafast time domain spectroscopy system, and the degree of bonding between the film and silicon wafer is determined. Two LT-GaAs thin film antennas for generating and detecting the terahertz waves are tested with a 1550-nm femtosecond laser. The terahertz signal is successfully detected, proving the feasibility of this home-made LT-GaAs photoconductive antennas. This work lays a foundation for studying the mechanism of terahertz wave generation in GaAs photoconductive antennas below the semiconductor band gap.     

低温生长砷化镓的超快光抽运-太赫兹探测光谱

本文采用光抽运-太赫兹探测技术系统研究了低温生长砷化镓(LT-GaAs)中光生载流子的超快动力学过程.光激发LT-GaAs薄层电导率峰值随抽运光强增加而增加,最后达到饱和,其饱和功率为54μJ/cm~2.当载流子浓度增大时,电子间的库仑相互作用将部分屏蔽缺陷对电子的俘获概率,从而导致LT-GaAs的快速载流子俘获时间随抽运光强增加而变长.光激发薄层电导率的色散关系可以用Cole-Cole Drude模型很好地拟合,结果表明LT-GaAs内部载流子的散射时间随抽运光强增加和延迟时间(产生光和抽运光)变长而增加,主要来源于电子-电子散射以及电子-杂质缺陷散射共同贡献,其中电子-杂质缺陷散射的强度与光激发薄层载流子浓度密切相关,并可由散射时间分布函数α来描述.通过对光激发载流子动力学、光激发薄层电导率以及迁移率变化的研究,我们提出适当增加缺陷浓度,可以进一步降低载流子迁移率和寿命,为研制和设计优良的THz发射器提供了实验依据.     

太赫兹片上系统中低温砷化镓薄膜光电导天线的研究

太赫兹时域光谱技术是一种在太赫兹频段内,广泛应用的光谱测量技术。这种技术可以用于许多物质的频谱分析,对于研究化学、半导体与生物分子等领域有着无可比拟的作用。然而用该系统进行样品探测时,受回波的影响频谱分辨率较低;受太赫兹波光斑大小以及待测样品与电磁波相互作用距离长短的影响,样品消耗量较多,并且整个系统的占用空间较大,这些局限性都限制了太赫兹时域光谱系统的进一步发展。为了突破太赫兹时域光谱系统的局限性,设计了一种将太赫兹泵浦区、探测区和传输波导集成到一个硅片上的太赫兹片上系统,该系统不仅能够解决上述系统的局限性,还能够省去样品测量前的光路准直环节,使样品的测量过程更加简便,同时集成化的系统也很大程度上提高了太赫兹波传输的稳定性。在太赫兹片上系统中,泵浦区和探测区的光电导天线是由低温砷化镓和金属电极制成,由于受到太赫兹片上系统的高度集成化和低温砷化镓晶体生长条件的限制,如何制备出低温砷化镓半导体薄膜衬底,并将其转移与键合,是太赫兹片上系统研制过程中的关键环节。首先利用分子束外延（MBE）技术制备出由半绝缘砷化镓、砷化镓缓冲层、砷化铝牺牲层和低温砷化镓层构成的外延片,然后利用盐酸溶液与砷化铝和低温砷化镓反应速度差别较大的原理,将200 nm厚的AlAs牺牲层腐蚀掉,从而得到2 μm厚的低温砷化镓薄膜。为了更加高效并且完整地得到低温砷化镓薄膜,研究了盐酸溶液在不同温度和不同浓度下与AlAs牺牲层的选择性腐蚀速率的关系。给出了低温砷化镓薄膜制备过程中盐酸的最佳体积比浓度和最佳温度,即在73 ℃下13.57%的盐酸溶液中进行砷化铝牺牲层的腐蚀。相比于已有工艺,这种腐蚀方法对实验设备的要求较低并且具有较高的安全性。最后,将单层低温砷化镓薄膜转移键合至硅片上,并制成光电导天线的结构。利用飞秒激光脉冲进行激发探测到太赫兹信号。由此说明,低温砷化镓薄膜的获取、转移与键合工艺能够满足芯片级太赫兹系统的制作要求,这为太赫兹片上系统的进一步研制打下了坚实的基础。     

Influence of semiconductor cap layer on the impurity-free vacancy disordering of the In<Subscript>0.2</Subscript>Ga<Subscript>0.8</Subscript>As/GaAs multiquantum-well structure by dielectric capping layers

The feasibility of normal GaAs, low-temperature-grown GaAs (LT-GaAs) and low-temperature-grown InGaAs (LT-InGaAs) as the capping layers for impurity-free vacancy disordering (IFVD) of the In_0.2Ga_0.8As/GaAs multiquantum-well (MQW) structure has been studied. The normal GaAs, LT-GaAs and LT-InGaAs layers were tested as the outermost capping layer and the intermediate cap layer underneath the SiO₂ or Si₃N₄ capping layer. The degree of quantum-well intermixing (QWI) induced by rapid thermal annealing was estimated by the shift of the photoluminescence (PL) peak energy. It was found that the IFVD of the In_0.2Ga_0.8As/GaAs MQW structure using LT-GaAs (LT-InGaAs) as the outermost capping layer was much smaller (larger) than that using a SiO₂ (Si₃N₄) capping layer. It was also observed that the insertion of the normal GaAs, LT-GaAs and LT-InGaAs cap layers below the SiO₂ or Si₃N₄ capping layer reduces the degree of QWI and the PL intensity after the QWI. A plausible explanation for the influence of normal GaAs, LT-GaAs and LT-InGaAs cap layers for the QWI of the InGaAs/GaAs structure is also discussed. PACS 68.55.Ln; 73.20.Dx; 78.55.-m     

Correlation of electrical properties with magnetic properties for ferromagnetic (Ga1−xMnx)As epilayer

The correlation of electrical properties with magnetic properties for the (Ga_1−xMn_x)As epilayer was investigated. For electrical transport measurements, it was clearly observed that the electrical mobility is increased with decreasing temperatures for both the (Ga_0.974Mn_0.026)As/LT-GaAs epilayer and the LT-GaAs:Be epilayer. However, a different behavior was observed at the cryogenic temperature region. The electrical mobility of (Ga_0.974Mn_0.026)As/LT-GaAs epilayer increases with decreasing temperature, while the mobility of LT-GaAs:Be epilayer decreases with decreasing temperature. In Arrhenius plots of carrier mobility for the (Ga_0.974Mn_0.026)As/LT-GaAs epilayer, the critical point is observed at 69 K, and this value is almost the same as the T_C. This result indicates that the carrier transport in ferromagnetic (Ga_1−xMn_x)As epilayers might be related to a spin-ordering effect because the spins will be arranged with the same direction below the T_C, and this will lead to reducing the probability of spin-disorder scattering. Therefore, the observation of a gradual drop below the T_C in the temperature-dependent resistivity curve is expected to be a result of the spin-ordering effect in the ferromagnetic (Ga_0.974Mn_0.026)As/LT-GaAs epilayer.     

光电导开关产生太赫兹电磁波双极特性分析

利用光电导体产生太赫兹电磁波(THz波)，THz远场辐射波形与光电导体材料的载流子寿命、偏置电场以及触发光有直接关系.用不同方法对低温GaAs(LT-GaAs)和半绝缘GaAs(SI-GaAs)光电导开关辐射的THz电磁波所呈现的双极特性进行了模拟计算.结果表明，LT-GaAs光电导开关辐射THz波呈现双极性的主要原因是光生载流子寿命小于一个THz波产生时间；而光生载流子寿命大于100ps的SI-GaAs光电导开关，在不同的实验条件（不同偏置电场、不同光脉冲能量）下，产生的THz波呈现双极特性的主要原因分别是载流子发生了谷间散射和空间电荷电场屏蔽. 关键词： 光电导开关 THz电磁波 载流子寿命 空间电荷屏蔽     

基于LT-GaAs外延片的THz片上系统

太赫兹（THz）波在物质检测方面发挥着巨大的作用,是一种非常有潜力的生化传感工具。但是传统的太赫兹时域光谱系统（TDS）结构复杂,系统的集成度低,占用空间较大。所以,如何对THz波进行有效引导、实现集成化传输并得到高质量光谱就成为太赫兹光谱系统的研究热点。太赫兹片上系统是将THz的产生、传输以及探测都集成到同一芯片上,然后通过相干探测的方法获得THz时域光谱。它可以实现对多种样品的检测,尤其在对难于取样的微量样品探测方面具有广泛的应用价值。它无需光路准直,操作简便,成品率高。两个研究工作都是基于低温砷化镓(LT-GaAs)外延片开展的。首先将一根直径为200 μm的铜线固定在LT-GaAs外延片的上方,通过真空蒸镀的方法制备出天线电极,同时得到天线间隙,研制出基于LT-GaAs外延片的THz天线。利用波长为800 nm的飞秒激光对其进行测试,得到了质量较高的THz信号,验证了天线的实用性。然后在另一外延片上利用光刻微加工工艺制作出传输线和微电极,得到了集成的THz片上系统。使用波长为1 550 nm的飞秒激光分别激发片上系统的太赫兹产生天线和探测天线,天线产生的太赫兹波在传输线上传播,在探测端同样得到了质量较高的THz时域信号,证实了THz片上系统的可行性。该方法省去了腐蚀牺牲层以及LT-GaAs薄膜的转移、键合等步骤,极大地提高了片上系统的成品率,避免了薄膜转移过程中易破碎及腐蚀液存在毒性的问题。最后,研究了外加电压对从片上系统中获得的THz波性能的影响,结果为电压越高,THz波的信号强度越强;另外,通过在传输线上方垂直放置铜箔的方法验证了THz波沿着传输线传播的事实。该研究中采用的基于LT-GaAs外延片的片上系统的制备方法简单,制作周期短,制作过程安全,应用领域广泛,这为将来与微流控芯片相结合实现对液体样品的探测打下了基础。     

大孔径光电导天线产生高功率窄带宽THz辐射特性分析

从大孔径光电导天线产生THz辐射的饱和理论出发,考虑了载流子的瞬变迁移率.分析了脉冲序列激发大孔径光电导天线产生高功率窄带宽THz辐射的特性.对比了单个光脉冲和序列光脉冲激发SI-GaAs和LT-GaAs光电导天线的饱和特性.分析表明,采用序列光脉冲激发载流子寿命小于光脉冲间隔的光电导天线时,可以克服大孔径光电导天线的饱和特性,产生高峰值功率的窄带THz辐射.     

Effect of Gallium Arsenide Charge Carrier Life Time on the Generation and Detection Efficiency of Continuous and Pulsed Terahertz Radiation

Russian Physics Journal - A configuration and test samples of photoconductive dipole antennas based on SI-GaAs:Cr and LT-GaAs for generation and detection of terahertz radiation are developed....     

太赫兹瞬态响应光导开关的实验研究

采用飞秒激光激励光导开关能够产生脉宽皮秒甚至亚皮秒级的太赫兹脉冲,近年来,这项技术成为校准宽带示波器上升时间的有效手段。以低温生长砷化镓（LT-GaAs）为光导开关的基底,在飞秒激光激励下产生太赫兹脉冲,经共面波导传输,通过微波探针耦合为1．85mm同轴输出,然后利用带宽70GHz的取样示波器对其半幅度宽度进行测量。实验获得太赫兹脉冲的半幅度宽度（FWHM）约为7．4ps。     

Microstructure of annealed low-temperature-grown GaAs layers

The crystal structure and orientation of As precipitates in annealed low-temperature GaAs (LT-GaAs) layers have been investigated by transmission electron microscopy. Three types of As precipitates were identified in layers grown by molecular beam epitaxy at substrate temperatures from 180° to 210° C. In the monocrystalline LT-GaAs layers small pseudocubic As precipitates (2–3 nm diameter) coherent with the GaAs lattice were observed. These precipitates lose their coherency when a certain critical size is exceeded. Precipitates of similar sizes are occasionally found for which a TEM lattice image cannot be obtained. These precipitates are believed to be amorphous. Larger As precipitates with a hexagonal structure (>4 nm diameter) were also found in the layers. These hexagonal As precipitates were observed to be largest near structural defects. The effect of these precipitates on the structure and on the electronic properties of the host GaAs is discussed.Dedicated to H.-J. Queisser on the occasion of his 60th birthday     

On-chip pulsed terahertz systems and their applications

The generation and detection of guided wave terahertz (THz) transients in microstrip transmission line systems is demonstrated at both room and cryogenic (∼ 4 K) temperatures using thin film low-temperature-grown GaAs (LT-GaAs) switches, excited by a 100 fs, 80 MHz repetition rate pulsed Ti:Sapphire laser. The characterisation of passive filter elements formed in the microstrip line is reported, together with their response to the application of dielectric loads of varying thickness at room temperature.     

Comprehensive study of structural and optical properties of LT-GaAs epitaxial structures

The results of the comprehensive study of LT-GaAs epitaxial structures on GaAs and Si substrates by high-energy electron diffraction, reflection anisotropy spectroscopy, atomic force microscopy, X-ray diffraction, Raman scattering, and photoluminescence methods are presented. The results obtained indicate the existence of several channels of nonequilibrium carrier recombination, which depend, in particular, on the substrate type and on the complex dependence of the concentration of structural imperfections on the used technology and growth conditions of the structures.     

All-semiconductor laser driven terahertz time-domain spectrometer

We present a terahertz time-domain spectrometer pumped by an all-semiconductor femtosecond laser system which emits 600 fs long optical pulses at 830 nm. Standard LT-GaAs antennas are used to generate and coherently detect THz radiation with a bandwidth of 1.4 THz. The measured time traces are in good agreement with simulations based on the Drude–Lorentz model. Spectroscopic measurements on polymers with different additive contents as well as THz imaging were carried out to confirm the successful operation of the spectrometer. Our approach holds great potential for the development of cost-effective, rugged and portable THz spectrometers for a variety of applications.     

A novel THz source based on a two-color Nd:LSB microchip-laser and a LT-GaAsSb photomixer

A two-color Nd:LSB microchip-laser that simultaneously emits at 1061.3 nm and 1063.9 nm is used for photomixing. Since LT-GaAs which is typically utilized for photomixing cannot be excited efficiently with wavelengths exceeding 1 μm, LT-GaAsSb with a bandgap energy designed to match the laser emission is used here for the first time. With this system THz radiation at 0.7 THz is generated and detected with a bolometer. PACS 84.40Ba; 42.60By; 42.55 Xi; 42.62b     

Q-switched and mode-locked diode-pumped Nd:YAG laser with an LT-GaAs

Simultaneous Q-switching and mode-locking (QML) is accomplished in a diode-pumped Nd:YAG laser using low-temperature GaAs (LT-GaAs) as the saturable absorber, which also acts as an output coupler at the same time. The repetition rate of the Q-switched envelope increased from 25 to 40 kHz as the pump power increased from 2.2 to 6.9 W. The mode-locked pulses inside the Q-switched pulse envelope had a repetition rate of 714 MHz. A maximum average output power of 770 mW was obtained.     

太赫兹波段介质微腔光学特性研究

采用匀胶法制备了厚度在微米量级的 Si/[TiO₂/Al₂O₃]²TiO₂和Si/[TiO₂/MgO]²/TiO₂ 多层介质膜反射镜. 采用太赫兹(THz)时域透射光谱系统获得了多层膜的时域透射谱. 用传输矩阵法模拟了Si/[TiO₂/Al₂O₃]²TiO₂ 和Si/[TiO₂/MgO]²/TiO₂两种分布式布拉格反射镜 (DBR)的反射相移和相位穿透深度等光学特性. 设计了两种结构为 DBR/LT-GaAs/DBR的对称THz光学微腔结构并模拟了腔结构的辐射光谱. 结果表明:通过引入谐振腔, 两种DBR组成的微腔器件在谐振波长处的强度分别提高了19和14倍. 其中Si/[TiO₂/Al₂O₃]²TiO₂/LT-GaAs (12 μm)/ [TiO₂/Al₂O₃]²TiO₂腔的辐射光谱存在两个峰, 分别位于208和248 μm, 并分析了出现两个谐振峰的原因. 探讨了通过引入介质谐振腔实现对THz源的辐射特性进行调控的可行性. 关键词： 分布式布拉格反射镜 光子晶体 穿透深度 太赫兹微腔     

Passively Q-switched and mode-locked diode-pumped Nd:YVO4 laser with LT-GaAs output coupler

We have demonstrated an efficient and compact passively Q-switched and mode-locked (QML) 1064 nm Nd:YVO₄ laser by using a low temperature grown GaAs (LT-GaAs) saturable absorber as well as an output coupler. Stable QML with envelope duration as short as 10 ns and Q-switched repetition rate of 36 kHz was obtained. It is the shortest envelope duration as far as we know, and it is so short that it can be used as Q-switching pulses directly. At 6.9 W of the incident pump power, average output power of 1.24 W was achieved and the corresponding peak power and energy of a single Q-switched pulse were 3.44 kW and 34.4 μJ, respectively. The mode-locked pulses inside the Q-switched pulse envelope had a repetition rate of 780 MHz.     

低温GaAs被动调Q锁模Nd:Gd0.42 Y0.58VO4 混晶激光器特性研究

采用低温生长GaAs晶体作为被动饱和吸收体兼输出镜,实现了Nd:Gd0.42Y0.58VO4混晶激光器的调Q锁模运转.研究了Nd:Gd0.42Y0.588VO4激光器的基频运转特性.在输出镜透射率T=10%、腔长L=40 mm的情况下,当抽运功率为8.6 W时,获得激光输出功率3.78 W,光-光转换效率为43.9%.并测量了Nd:Gd0.42Y0.58VO4混晶被动调Q激光器的输出特性.实验结果表明激光器调Q运转阈值为2 W,当抽运功率为3.7 W时,激光器出现调Q锁模行为;当抽运功率为8.6 W时,激光器调Q锁模深度达70%以上,对应的脉冲包络重复频率为670 kHz,半峰全宽为180 ns,平均输出功率为1.35 W,光-光转换效率为15.7%.     

Low-Temperature Molecular-Beam Epitaxy of GaAs: Effect of Excess Arsenic on the Structure and Properties of the GaAs Layers

The present paper reviews works devoted to control over the properties of epitaxial GaAs by incorporation of excess (non-stoichiometric) arsenic into the GaAs films grown by molecular-beam epitaxy (MBE) at low-temperature (LT). The effect of excess arsenic on the material structure and properties is analyzed for both as-grown and annealed LT-GaAs layers. The effect of doping on the incorporation of excess arsenic is also examined. The data on the effect of excess arsenic on the properties of the Ga_0.47In_0.53As solid solution are presented. The specific features of the mechanism of the excess arsenic incorporation into the solid phase during the low-temperature epitaxial growth are discussed.