Infrared absorption in SiGe/Si multiple quantum wells

Infrared absorption has been used to investigate the subband structures in SiGe/Si quantum wells. The quantum wells are prepared using RRH/VLP-CVD and consist of 20 periods of and 60 periods of . The good periodical and interface sharpness of the SiGe/Si quantum wells have been shown by Auger Electron Spectroscopy (AES). The absorption peaks due to transitions between the hole subbands and the conduction band have been observed in infrared absorption spectra. The transverse photocurrent spectrum parallel to the growth plane have also shown absorption peaks due to transitions between the heavy and light hole band states and the conduction band states in quantum wells.     

Pure spin photocurrents in low-dimensional structures

As is well known, the absorption of circularly polarized light in semiconductors results in optical orientation of electron spins and helicity-dependent electric photocurrent, and the absorption of linearly polarized light is accompanied by optical alignment of electron momenta. Here, we show that the absorption of unpolarized light leads to the generation of a pure spin current, although both the average electron spin and electric current vanish. We demonstrate this for direct interband and intersubband as well as indirect intraband (Drude-like) optical transitions in semiconductor quantum wells.     

Photocurrent response in a double barrier structure with quantum dots–quantum well inserted in central well

We report the photocurrent response in a double barrier structure with quantum dots–quantum well inserted in central well. When this quantum dots–quantum well hybrid heterostructure is biased beyond +1 or −1 V, the photocurrent response manifests itself as a step-like enhancement, increasing linearly with the light intensity. Most probably, at proper bias condition, the pulling down of the X minimum of GaAs at the outgoing interface of the emitter barrier by the photovoltaic effect in GaAs QW will initiate the Γ–X–X tunneling at much lower bias as compared with that in the dark. That gives rise to the observed photocurrent response.     

Tuning photoresponse through size distribution control of silicon quantum dots

We report a detailed experimental and theoretical investigation on the photocurrent characteristics of nanocrystalline Si thin films, with the emphasis on the effect of Si dot size distribution. Broader photocurrent response has been observed in Si quantum dots with larger size dispersion due to the improvement of light harvest. As a result of tunneling loss in the expanded energy distribution, we have demonstrated that there is a tradeoff between the absorption enhancement and reduced transport for the photocurrent intensity. The present work opens new strategy to maximize the photoresponse through size distribution control for quantum dot solar cell application.     

有机量子阱的光学性质

采用多源有机分子气相沉积系统(OMBD)制备了Alq₃，PBD/Alq₃，PBD/Alq₃/PBD单层、双层以及量子阱结构，利用电化学循环伏安法和吸收光谱、荧光光谱研究了量子阱的类型和样品的光致发光特性.电化学循环伏安法和吸收光谱的测量结果表明，PBD/Alq₃有机量子阱为Ⅰ型量子阱结构.荧光光谱的研究结果表明，单层Alq₃的光致发光峰不随Alq₃厚度变化而变化；但是双层PBD/Alq₃结构光致发光峰随Alq₃厚度的减小而发生蓝移；同样对于PBD/Alq₃/PBD量子阱结构光致发光峰随Alq₃厚度的减小而发生蓝移.对引起光谱蓝移的原因进行了讨论. 关键词： 有机量子阱 光谱蓝移     

有机多层量子阱的能量转移

采用多源有机分子气相沉积系统制备了不同类型的有机多量子阱结构，利用电化学循环伏安法和吸收光谱、荧光光谱研究了量子阱的类型、光致发光的特性.电化学循环伏安法和吸收光谱的测量结果表明，PBD/8-羟基喹啉铝(Alq₃)有机量子阱为Ⅰ型量子阱结构，NPB/Alq₃和BCP/Alq₃有机量子阱为Ⅱ型量子阱结构.荧光光谱的研究结果表明，PBD/Alq₃和BCP/Alq₃量子阱结构可以实现PBD，BCP向Alq₃能量完全转移，而NPB/Alq₃量子阱结构，NPB和Alq₃之间只是部分能量转移.文中对影响能量转移的因素进行了讨论. 关键词： 有机量子阱 能量转移     

Comparison studies of infrared photodetectors with a quantum-dot and a quantum-wire base

This paper mainly presents a theoretical analysis for the characteristics of quantum dot infrared photodetectors (QDIPs) and quantum wire infrared photodetectors (QRIPs). The paper introduces a unique mathematical model of solving Poisson’s equations with the usage of Lambert W functions for infrared detectors’ structures based on quantum effects. Even though QRIPs and QDIPs have been the subject of extensive researches and development during the past decade, it is still essential to implement theoretical models allowing to estimate the ultimate performance of those detectors such as photocurrent and its figure-of-merit detectivity vs. various parameter conditions such as applied voltage, number of quantum wire layers, quantum dot layers, lateral characteristic size, doping density, operation temperature, and structural parameters of the quantum dots (QDs), and quantum wires (QRs). A comparison is made between the computed results of the implemented models and fine agreements are observed. It is concluded from the obtained results that the total detectivity of QDIPs can be significantly lower than that in the QRIPs and main features of the QRIPs such as large gap between the induced photocurrent and dark current of QRIP which allows for overcoming the problems in the QDIPs. This confirms what is evaluated before in the literature. It is evident that by increasing the QD/QR absorption volume in QDIPs/QRIPs as well as by separating the dark current and photocurrents, the specific detectivity can be improved and consequently the devices can operate at higher temperatures. It is an interesting result and it may be benefit to the development of QDIP and QRIP for infrared sensing applications.     

Long wavelength infrared (LWIR) photodetection in a modulation doped thyristor

A modulation doped thyristor concept is described for LWIR photodetection based upon intersubband bound to continuum absorption. The intersubband absorption generates photocurrent from undoped quantum wells to modulation doped layers (MDL). Due to the lower dark current compared to conventional quantum well infrared photodetectors (QWIPs), the thyristor infrared detector operates with little or no cooling and with similar or better performance than QWIPs at low temperatures. The operating characteristics of absorption coefficient, quantum efficiency, responsivity, detectivity, infrared gain, and dark current are determined as a function of thyristor voltage and input power level in the range of 1 μW/cm².     

基于石墨烯-钙钛矿量子点场效应晶体管的光电探测器

以等离子增强化学气相沉积法制备的石墨烯作为导电沟道材料,将其与无机CsPbI_3钙钛矿量子点结合,设计并制备了石墨烯-钙钛矿量子点场效应晶体管光电探测器.研究和分析了石墨烯作为场效应晶体管的电学特性及其与钙钛矿量子点结合作为光电探测器的光电特性.结果表明,石墨烯在场效应晶体管中表现出良好的电学性质,其与钙钛矿量子点的结合对波长为400 nm的光辐射具有明显的光响应,在光强为12μW时器件光生电流最大为64μA,响应率达6.4 A·W~(-1),对应的光电导增益和探测率分别为3.7×10~4,6×10~7Jones(1 Jones=1 cm·Hz~(1/2)·W~(-1)).     

量子阱共振腔增强型光电探测器的高功率特性

通过测量1.55 μm量子阱共振腔增强型光电探测器的光电流随反向电压和光功率的变化关系,以及模拟能带结构、电场分布等特性,研究了量子阱共振腔增强型光电探测器的高功率特性.分析了光电流的产生机制,测量了1.064 μm量子阱共振腔增强型光电探测器的光电响应,模拟了具有不同势垒高度的量子阱共振腔增强型光电探测器的光电响应.从实验和模拟两方面证明了量子阱的势垒高度是影响量子阱共振腔增强型光电探测器高功率特性的最主要因素.     

InGaN/GaN多量子阱LED载流子泄漏与温度关系研究

通过测量光电流,直接观察了InGaN/GaN量子阱中载流子的泄漏程度随温度升高的变化关系。当LED温度从300K升高到360K时,在相同的光照强度下,LED的光电流增大,说明在温度上升之后,载流子从量子阱中逃逸的数目更多,即载流子泄漏比例增大。同时,光电流的增大在激发密度较低的时候更为明显,而且光电流随温度的增加幅度与激发光子的能量有关。用量子阱-量子点复合模型能很好地解释所观察到的实验现象。实验结果直接证明,随着温度的升高,InGaN/GaN量子阱中的载流子泄漏将显著增加,而且在低激发密度下这一效应更为明显。温度升高导致的载流子泄漏增多是InGaN多量子阱LED发光效率随温度升高而降低的重要原因。     

Electro-optical phenomena accompanying electron and hole heating in superlattices and quantum wells GaAs/AlGaAs and Ge/GeSi

New electro-optical phenomena in quantum-well structures, i.e. modulation of the light absorption and birefringence due to carrier heating in a strong electric field, have been investigated. The effects have revealed different features in the three types of structures under investigation, namely: (1) well-dopedn-type GaAs/AlGaAs multiple quantum wells, (2) barrier-dopedn-type GaAs/AlGaAs superlattices and (3) barrier-dopedp-type Ge/GeSi multiple quantum wells. Possible mechanisms of the phenomena have been discussed.     

Localization of Surface Plasmon Waves in Hybrid Photodetectors with Subwavelength Metallic Arrays

The spectral characteristics of the hole photocurrent in plasmon photodetectors based on Ge/Si heterostructures with Ge quantum dots combined with regular arrays of subwavelength apertures of various shapes in a gold film on the semiconductor surface are investigated. Dispersion relations characterizing the propagation of surface plasmon waves along the metal–semiconductor interface are determined from the dependences of the photocurrent on the angle of incidence of light. It is established that the plasmonic enhancement of the photocurrent in rectangular aperture array is suppressed as compared to that in circular and square aperture arrays. It is found that, in hybrid structures with rectangular apertures, there exists a range of wave vectors where the energy of surface plasmons is independent of the wave vector of incident radiation. The results are explained by the excitation of dipole modes localized at rectangular apertures with a large aspect ratio by light waves.     

Thin-film capacitor M/Pb(ZrTi)O3/M as a polarization-sensitive photocell

A steady-state short-circuit photocurrent of preliminarily polarized submicron capacitors with a polycrystalline Pb(ZrTi)O₃ (PZT) film is investigated under irradiation by light with a wavelength λ > 0.4 μm. The structures with different M/PZT interfaces that differ in the leakage current by more than an order of magnitude are found to demonstrate virtually the same value of the photocurrent, which is always directed opposite to the ferroelectric polarization of the PZT film. Although the magnitude of photocurrent is determined by the degree of polarization of the film, the observed photocurrent is not a depolarization current of the ferroelectric film. Therefore, the M/PZT/M capacitor behaves like a polarization-sensitive photocell. Within the proposed theory of a heterophase medium, the dependence of the photocurrent on the magnitude of the preliminary polarization is calculated and proves to be in reasonable agreement with the experimental results.

     

TE- and TM-polarization-resolved spectroscopy on quantum wells under normal incidence

We present TE- and TM-polarization-resolved photocurrent measurements on quantum well pin diodes under normal incidence. Usually, optical experiments performed in such a geometry yield information only about transitions involving in-plane (p_x and p_y) components of the hole wave functions because of the in-plane (TE) polarization of the light. Information on transitions sensitive to the p_z components can be obtained by focussing a radially polarized laser beam through a microscope objective with high numerical aperture (NA=0.9). With our setup, the electrical field vector at the focal tail has a significant component along the optical axis (TM-polarization!) which enables excitation of transitions sensitive to p_z components also. Additionally, the existence of a degenerate (azimuthally polarized) optical mode enables switching these p_z components on and off easily.Experimental evidence of these features has been achieved by exploiting the selection rules for e–hh and e–lh transitions in a quantum well structure. We present a comparison of our recorded spectra with theoretical predictions obtained from simple geometric optics assumptions. For our quantum wells the polarization effects are small because our measurement averages the intensity distribution of the whole focal plane. We plan to extend our measurements to polarization resolved single quantum dot spectroscopy. By restricting the detection region to the spatial extent of a single dot, one can exploit the almost pure TM-polarization on the optical axis for obtaining high contrast between heavy- and light-hole exciton absorption.     

A novel tunable infrared detector based on a quantum ballistic channel

We consider the possibility of infrared signal detection using photon-induced intersubband scattering in a quantum ballistic channel (QBC). Our calculations show that light absorption in a QBC with a non-uniform narrowed end can lead to a photovoltaic effect. Using mode matching techniques, the photocurrent is obtained. Evaluations of the responsivity, detectivity and bandwidth of the proposed photovoltaic detector are favorable for practical applications.     

Exciton-polariton absorption in periodic and disordered quantum-well chains

The exciton-polariton transfer and absorption in regular and disordered structures with a finite number of quantum wells are studied theoretically. The transfer matrix method is invoked in the exciton resonance region to calculate the reflectivity, transmissivity, and absorptivity spectra, as well as the integrated absorptivity as a function of the γ/Γ₀ ratio of the parameters of nonradiative and radiative damping of quasi-two-dimensional excitons. It is shown that the integrated absorptivity as a function of γ (temperature) follows a universal pattern, more specifically, it increases monotonically from zero at γ = 0 to saturate at γ/Γ₀ ? 1. Because the exciton-polariton absorption being single mode, the integrated absorptivity in Bragg quantum-well structures is substantially lower than that in short-period structures, in which absorption involves the whole spectral multitude of modes. The intrawell disorder associated with fluctuations in the frequencies of exciton excitation in quantum wells enhances the integrated absorptivity to the level typical of light absorption with no resonance among excitons of different quantum wells. The interwell disorder originating from fluctuations in quantum-well separation likewise leads to an increase in the integrated absorptivity.     

Theory of internal photoemission in sandwich structures

The internal photoemission in sandwich structures is treated as a multi-step process. The excitation functions for electrons in the electrodes are obtained as a result of a rigorous optical analysis of the light propagation and absorption. The electron-electron and electron-phonon interactions are described in terms of energy-dependent mean free paths. The calculation of the quantum. yield and photocurrent includes the electrons which escape without scattering, after one scattering, and after two scattering processes. In calculating the probability for electrons to escape over the inner potential barrier within the structure account has been taken of both their being scattered within the barrier region and their quantum-mechanical character.The present calculations can convenietly be used for theoretical investigations of the photoemission in its dependence on various parameters of the structure. The formulae also retain their validity for the photoemission of holes when the quantities due to electrons are correspondingly replaced. An adaptation of the theory to the vacuum photoemission from thick as well as very thin samples is possible without difficulties.     

Optical spectra and Auger recombination in SiGe/Si heterostructures in 10 μm range of wavelengths

We discuss the possibility of usingp-type SiGe/Si multiple quantum well structures for infrared detection. We calculated the miniband dispersion in these structures using an empirical pseudopotential method including the effects of spin and strain. The absorption spectra of these structures is discussed and the microscopic origin of the absorption peaks identified. We present comparisons between our calculated absorption response and the experimental spectra obtained in recent experiments for both parallel and normal incidence light. We also report full-scale pseudopotential calculations of the Auger recombination in these structures and present a fresh discussion of the conditions which could help to minimize this recombination.