Temperature dependent time-resolved exciton luminescence in GaAs/AlGaAs quantum wires and dots

Transient photoluminescence of GaAs/AlGaAs quantum wires and quantum dots formed by strain confinement is studied as a function of temperature. At low temperature, luminescent decay times of the wires and dots correspond to the radiative decay times of localized excitons. The radiative decay time can be either longer or shorter than that of the host quantum well, depending on the size of the wires and dots. For small wires and dots (∼ 100 nm stressor), the exciton radiative recombination rate increases due to lateral confinement. Exciton localization due to the fluctuation of quantum well thickness plays an important role in the temperature dependence of luminescent decay time and exciton transfer in quantum wire and dot structures up to at least ∼ 80 K. Lateral exciton transfer in quantum wire and dot structures formed by laterally patterning quantum wells strongly affects the dynamics of wire and dot luminescence. The relaxation time of hot excitons increases with the depth of strain confinement, but we find no convincing evidence that it is significantly slower in quasi 1-D or 0-D systems than in quantum wells.     

Quantum wire heterostructure for optoelectronic applications

Quantum wire (QWR) heterostructures suitable for optoelectronic applications should meet a number of requirements, including defect free interfaces, large subband separation, long carrier lifetime, efficient carrier capture. The structural and opticl properties of GaAs/AlGaAs and InGaAs/GaAs quantum wire (QWR) heterostructures grown by organometallic chemical vapor deposition on nonplanr substrates, which satisfy many of these criteria, are described. These crescent-shaped QWRs are formed in situ during epitaxial growth resulting in virtually defect free interfaces. Effective wire widths as small as 10nm have been achieved, corresponding to electron subband separations greater than K_BT at room temperature. The enhanced density of states at the QWR subbands manifests itself in higher optical absorption and emission as visualized in photoluminescence (PL), PL excitation, amplified spontaneous emission and lasing spectra of these structures. Effective carrier capture into the wires via connected quantum well regions, which is important for enhancing the otherwise extremely small capture cross section of these wires, has also been observed. Room temperature operation of GaAs/AlGaAs and strained InGaAs/GaAs QWR lasers with threshold currents as low as 0.6mA has been demonstrated.     

Fabrication and optical properties of quantum dots and wires

We describe photoluminescence measurements made on mesa geometry quantum dots and wires with exposed side walls fabricated by laterally patterning undoped GaAs/AlGaAs quantum wells using electron beam lithography and dry etching. At low temperature the photoluminescence efficiency of many but not all of the GaAs quantum dot arrays scales with the volume of quantum well material down to lateral dimensions of 50nm. This behaviour contrasts with that found in wires produced at the same time where the intensity falls off rapidly with decreasing wire width for dimensions below 500nm but is recovered by overgrowth with indium tin oxide, possibly as a result of strain. Narrow overgrown wires exhibit anisotropy in polarized excitation spectra which is discussed in relation to strain and lateral confinement effects.     

Lateral and vertical ordered one-dimensional InGaAs/GaAs quantum structures

Lateral and vertical ordered one-dimensional quantum structures, i.e. InGaAs/GaAs(001) quantum dot chains and quantum wires, have been obtained using molecular beam epitaxy. It was found that the InGaAs wires or dot chains sit on two-dimensional wetting layers and run along the [-110] direction, as the result of anisotropic strain and in-plane adatom diffusion. This anisotropic nature produces a model system for studying the electronic properties of one-, two-, and three-dimensional quantum confinements and related optical responses. The strain anisotropy is of importance in determining the electronic states of the quantum structures and the surrounding strained barrier. The strain-induced effects, such as change of band-gap and splitting of heavy–light hole states, were studied experimentally and theoretically. Optical anisotropy of these quantum structures is also discussed.     

退火及铟含量对InGaAs量子点光学特性的影响

研究了退火条件和In组份对分子束外延生长的InGaAs量子点（分别 以GaAs或AlG aAs为基体）光学特性的影响。表明：量子点中In含量的增加将导致载流子的定域能增加和基态与激发态之间的能量间隔增大。采用垂直耦合的量子点及宽能带的AlGaAs基体可增 强材料的热稳定性。以AlGaAs为基体的InGaAs量子点,高温后退火工艺 (T= 830℃)可改善低温生长的AlGaAs层的质量,从而改善量子点激光器材料的质量。     

退火及铟含量对InGaAs量子点光学特性的影响

 研究了退火条件和In组份对分子束外延生长的InGaAs量子点（分别 以GaAs或AlG aAs为基体）光学特性的影响。表明：量子点中In含量的增加将导致载流子的定域能增加和基态与激发态之间的能量间隔增大。采用垂直耦合的量子点及宽能带的AlGaAs基体可增 强材料的热稳定性。以AlGaAs为基体的InGaAs量子点,高温后退火工艺 (T= 830℃)可改善低温生长的AlGaAs层的质量,从而改善量子点激光器材料的质量。     

Catastrophic degradation of pulsed lasers based on AlGaAs/InGaAs/GaAs heterostructures with electron-beam pumping

Catastrophic degradation of pulsed lasers based on InGaAs/AlGaAs/GaAs structures with different design of the active domain with transverse pumping by the electron beam at T = 300 K is studied. In lasers based on structures with a InGaAs single quantum well and with seven quantum wells, the maximal values of pulsed power are 70–90 and 10–20 W, respectively.     

Quantum structures for multiband photon detection

The work describes multiband photon detectors based on semiconductor micro-and nano-structures. The devices considered include quantum dot, homojunction, and heterojunction structures. In the quantum dot structures, transitions are from one state to another, while free carrier absorption and internal photoemission play the dominant role in homo or heterojunction detectors. Quantum dots-in-a-well (DWELL) detectors can tailor the response wavelength by varying the size of the well. A tunnelling quantum dot infrared photodetector (T-QDIP) could operate at room temperature by blocking the dark current except in the case of resonance. Photoexcited carriers are selectively collected from InGaAs quantum dots by resonant tunnelling, while the dark current is blocked by AlGaAs/InGaAs tunnelling barriers placed in the structure. A two-colour infrared detector with photoresponse peaks at ∼6 and ∼17 μm at room temperature will be discussed. A homojunction or heterojunction interfacial workfunction internal photoemission (HIWIP or HEIWIP) infrared detector, formed by a doped emitter layer, and an intrinsic layer acting as the barrier followed by another highly doped contact layer, can detect near infrared (NIR) photons due to interband transitions and mid/far infrared (MIR/FIR) radiation due to intraband transitions. The threshold wavelength of the interband response depends on the band gap of the barrier material, and the MIR/FIR response due to intraband transitions can be tailored by adjusting the band offset between the emitter and the barrier. GaAs/AlGaAs will provide NIR and MIR/FIR dual band response, and with GaN/AlGaN structures the detection capability can be extended into the ultraviolet region. These detectors are useful in numerous applications such as environmental monitoring, medical diagnosis, battlefield-imaging, space astronomy applications, mine detection, and remote-sensing. The paper presented there appears in Infrared Photoelectronics, edited by Antoni Rogalski, Eustace L. Dereniak, Fiodor F. Sizov, Proc. SPIE Vol. 5957, 59570W (2005).     

Photoluminescence of single quantum wires and quantum dots

The results of a study into the photoluminescence spectra of a set of quantum dots based on GaAs enclosed in AlGaAs nanowires are presented. The steady state and time resolved spectra of photoluminescence under optical excitation both from an array of quantum wires/dots and a single quantum wire/dot have been measured. In the photoluminescence spectra of single quantum dots, emission lines of excitons, biexcitons and tritons have been found. The binding energy of the biexciton in the studied structures was deduced to be 8 meV.     

Carrier tunneling in asymmetric coupled quantum dots

Exciton spin relaxation at low temperatures in InAlAs–InGaAs asymmetric double quantum dots embedded in AlGaAs layers has been investigated as a function of the barrier thickness by the time-resolved photoluminescence measurements. With decreasing the thickness of the AlGaAs layer between the dots, the spin relaxation time change from 3 ns to less than 500 ps. The reduction in the spin relaxation time was considered to originate from the spin-flip tunneling between the ground state in InAlAs dot and the excited states in InGaAs dot, and the resultant tunneling leads to the spin depolarization of the ground state in InGaAs dot.     

A study of GaInP/GaAs interfaces: metallurgical coupling of successive quantum wells

We present a study of GaInP/GaAs interfaces by means of photoluminescence (PL) of multi quantum wells (MQW), embedded in GaInP, or asymmetric structures having an AlGaAs barrier GaInP/GaAs/AlGaAs. The PL energies of quantum wells were compared with calculations based on the transfer matrix envelope function approximation, well suited for asymmetric structures. GaInP/GaAs/AlGaAs MQW structures (GaInP grown first) are in reasonably good agreement with calculations. Reverse ones, AlGaAs/GaAs/GaInP, present a lower PL energy than calculated. But the agreement with theory is recovered on single quantum well samples, or in MQW when the GaInP thickness is increased up to 100 nm. We interpret this phenomenon as a diffusion of arsenic atoms from the next GaAs well through the GaInP barrier. Arsenic atoms exchange with phosphorus atoms at the GaInP-on-GaAs interface of the former well, leading to a small gap strained InGaAs region responsible for the lowering of PL energies.     

弱耦合GaAs/AlGaAs/InGaAs双势阱隧穿结构的磁隧穿特性研究

研究了低温(15 K)条件下弱耦合GaAs/AlGaAs/InGaAs双势阱结构的纵向磁隧穿特性. 研究表明,器件在零偏压下处于共振状态. 通过分析不同偏压下的磁电导振荡曲线,可以得到双量子阱中的基态束缚能级随偏压的变化规律,从而可以确定隧穿电流峰对应的隧穿机制. 所得结果可为弱耦合双量子点器件的制备提供基础. 关键词： 双量子阱 隧穿结构 磁电导振荡     

Space-charge-induced interband optical nonlinearities in asymmetric coupled quantum wells

Space-charge-induced optical nonlinearities for asymmetric coupled quantum wells have been studied. Calculations for optimized AlGaAs structures show that a blue-shift of the band gap of 9 meV at a carrier density of 10¹² cm^-2 can be obtained. Experimental comparisons with a rectangular well show a large increase in room-temperature nonlinearities.     

Cracking self-assembled InAs quantum dots

We present a cross-sectional scanning tunneling microscopy (X-STM) investigation of InAs quantum dots in a GaAs matrix. The structures were grown by molecular beam epitaxy (MBE) at a low growth rate of 0.01 ML/s and consist of five layers of uncoupled quantum dot structures. Detailed STM images with atomic resolution show that the dots consist of an InGaAs alloy and that the indium content in the dot increases towards the top. The analysis of the height versus base-length relation obtained from cross-sectional images of the dots shows that the shape of the dots resembles that of a truncated pyramid and that the square base is oriented along the [010] and [100] directions. Using scanning tunneling spectroscopy (STS) we determined the onset for electron tunneling into the conduction and out of the valence band, both in the quantum dots and in the surrounding GaAs matrix. We found equal voltages for tunneling out of the valence band in GaAs or InGaAs whereas tunneling into GaAs occurred at higher voltages than in InGaAs.     

Wavelength and polarization selective multi-band tunnelling quantum dot detectors

The reduction of the dark current without reducing the photocurrent is a considerable challenge in developing far-infrared (FIR)/terahertz detectors. Since quantum dot (QD) based detectors inherently show low dark current, a QD-based structure is an appropriate choice for terahertz detectors. The work reported here discusses multi-band tunnelling quantum dot infrared photo detector (T-QDIP) structures designed for high temperature operation covering the range from mid-to far-infrared. These structures grown by molecular beam epitaxy consist of a QD (InGaAs or InAlAs) placed in a well (GaAs/AlGaAs) with a double-barrier system (AlGaAs/InGaAs/AlGaAs) adjacent to it. The photocurrent, which can be selectively collected by resonant tunnelling, is generated by a transition of carriers from the ground state in the QD to a state in the well coupled with a state in the double-barrier system. The double-barrier system blocks the majority of carriers contributing to the dark current. Several important properties of T-QDIP detectors such as the multi-colour (multi-band) nature of the photoresponse, the selectivity of the operating wavelength by the applied bias, and the polarization sensitivity of the response peaks, are also discussed.     

图形衬底量子线生长制备与荧光特性研究

报道了在V型槽图形衬底上利用分子束外延技术外延生长的GaAs/AlGaAs量子线.外延截面在扫描电子显微镜下可以看到在V型槽底部形成了弯月型量子线结构,量子线尺寸约为底边60 nm高14 nm的近三角形.低温87 K下光致发光谱测试在793.7和799.5 nm处出现峰值,验证了量子线的存在.理论近似计算结果显示,相比等宽度量子阱有8 meV的蓝移正是由于横向量子限制引起的. 关键词： V型槽图形衬底 量子线 GaAs     

Absorption and photoluminescence studies of the temperature dependence of exciton life time in lattice-matched and strained quantum well systems

We present systematic studies of the temperature dependence of linewidths and lifetimes of excitonic transitions in quantum wells grown by molecular beam epitaxy using both photoluminescence(PL) and optical absorption. The temperature ranged from 6K to room temperature. Samples under investigation were lattice-matched GaAs/AlGaAs and InGaAs/InAlAs, and strained InGaAs/GaAs and InGaAs/AlGaAs quantum wellssystems. In addition, the effects of well-size variations in GaAs/AlGaAs quantum wells were measured and analyzed. In all cases we were able to observe the excitonic transitions up to room temperature. By a careful fitting of the experimental data we separated the exciton transitions from band-to-band transitions. By deconvoluting the excitonic transitions we obtained the homogeneous and inhomogeneous linewidths. The homogeneous linewidths were used to calculate the exciton lifetimes as a function of temperature using the Heisenberg uncertainty principle. We found the lifetime decreases significantly with temperature and increases with increasing well size. These results are interpreted in terms of the exciton-phonon interaction and are expected to be very useful for the design of semiconductor optical devices operating at different temperatures.     

含有AlGaAs插入层的InAs/GaAs三色量子点红外探测器

本文报道了采用分子束外延技术制备的三色InAs/GaAs量子点红外探测器. 器件采用nin型结构, 吸收区结构是在InGaAs量子阱中生长含有AlGaAs插入层的InAs量子点, 器件在77 K下的红外光电流谱有三个峰值: 6.3, 10.2和11 μm. 文中分析了它们的跃迁机制, 并且分别进行了指认. 因为有源区采用了不对称结构, 所以器件在外加偏压正负方向不同时, 光电流谱峰值的强度存在一些差异. 不论在正偏压或者负偏压下, 当偏压达到较高值, 再进一步增大偏压时, 都出现了对应于连续态的跃迁峰强度明显下降的现象, 这是由量子点基态与阱外连续态的波函数交叠随着偏压进一步增大而迅速减小导致的.