Photoinduced infrared spectroscopy of bound-to-bound and bound-to-continuum transitions in SiGe/Si quantum wells

Infrared spectroscopy of intersubband transitions in the valence band of undoped SiGe/Si quantum wells is presented. Optical pumping of interband transitions is used to generate carriers in the wells. The spectral features of bound-to-bound and bound-to-continuum transitions are analyzed and compared to those of GaAs quantum wells. In samples with only one heavy hole bound level, a ratio of 20:1 is observed between intersubband and free carrier absorption. Room temperature photo-induced absorption is only observed in samples with high germanium content (≈50%). The feasibility of normal-incidence infrared modulators based on s-polarized intersubband absorption is also demonstrated. Resonant dispersion associated with intersubband transitions is evidenced.     

Hot electron cooling in parabolic and modulation doped quantum wells and doped superlattices

Hot electron cooling in variously structured and doped quantum wells and superlattices has been studied by low temperature steady-state photoluminescence. A parabolic quantum well realized by thickness grading of Al_0.3Ga_0.7As and GaAs epitaxial layers deposited by molecular beam epitaxy with electron level spacings of ∼25 meV did not show increased electron plasma temperatures compared to thick epitaxially deposited GaAs or square quantum wells with electron level spacings greater than the LO phonon energy of GaAs; this implies that mechanisms involving intersubband Δk ≠ 0 transitions and interfacial recombination are dominant in the parabolic structure. Investigations as a function of carrier concentration in modulation-doped quantum wells and n-type superlattices with strong miniband formation indicate that increasing the carrier concentration in either structure above ∼ 5 × 10¹⁷ cm^-3 significantly increases the electron plasma temperatures, even under low light excitation, suggesting that such structures may be suited for high efficiency hot electron photovoltaic and photoelectrochemical cells.     

Excitonic effects in the recombination radiation spectra of completely filled Landau levels of two-dimensional electrons

The recombination radiation spectra of two-dimensional electrons in an asymmetrically doped GaAs/AlGaAs quantum well are investigated at different temperatures and laser-excitation energies. At low temperatures and in high magnetic fields the recombination lines of the electrons from completely filled Landau levels are split into narrow sublevels. It is shown that this fine structure of the Landau levels is due to the presence of excitonic effects in the initial and final states of the photoexcited system. It is demonstrated that the recombination process is accompanied by the excitation of intersubband and cyclotron magnetoplasma modes. Pis’ma Zh. éksp. Teor. Fiz. 65, No. 1, 38–43 (10 January 1997)     

GaAs体材料及其量子阱的光学极化退相特性

采用飞秒时间分辨瞬态简并四波混频技术，在室温下测量了GaAs体材料及其量子阱材料GaAs/Al_0.3Ga_0.7As的光学极化超快退相时间，当激光中心波长为785nm，受激载流子浓度为10¹¹cm^-2时，它们的退相时间分别为28fs和46fs.量子阱材料的退相时间比体材料的长，这是由于量子阱中的载流子在垂直于GaAs/AlGaAs界面的运动受到限制，运动呈现二维特性，大大减小了载流子的散射概率.实验中观察到瞬态简并四波混 关键词： 时间分辨简并四波混频 飞秒激光脉冲 退相 密度矩阵     

Non-linear luminescence due to exciton - e-h plasma transformation in mixed type I - type II quantum wells

We report on the photoluminescence and its excitation spectra of mixed type I - type II GaAs/AlAs quantum wells (MTQW) under applied magnetic fields. These structures consist of alternating narrow and wide wells and are designed so that the electron density in the wide well can be optically controlled. A transition from excitonic to free carrier recombination is observed as the electron density is increased.     

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.     

InAs/GaAs quantum dot infrared photodetectors with different growth temperatures

InAs/GaAs quantum dot infrared photodetectors were fabricated with quantum dots grown at three different temperatures. Large detection wavelength shift (5–14.5 μm) was demonstrated by changing 40 degrees of the epitaxy temperature. The smaller quantum dots grown at lower temperature generate 14.5 μm responses. The detectivity of the normal incident 15 μm QDIP at 77 K is 3 × 10⁸ cm Hz^1/2/W. A three-color detector was also demonstrated with quantum dots grown at medium temperature. The three-color detection comes from two groups of different sizes of dots within one QD layer. This new type of multicolor detector shows unique temperature tuning behavior that was never reported before.     

Resonant Raman scattering of optical phonons in self-assembled quantum dots

We have investigated the carrier relaxation mechanism in InGaAs/GaAs quantum dots by photoluminescence excitation (PLE) spectroscopy. Near-field scanning optical microscope successfully shows that a PLE resonance at a relaxation energy of 36 meV can be seen in all single-dot luminescence spectra, and thus can be attributed to resonant Raman scattering by a GaAs LO phonon to the excitonic ground state. In addition, a number of sharp resonances observed in single-dot PLE spectra can be identified as resonant Raman features due to localized phonons, which are observed in the conventional Raman spectrum. The results reveal the mechanism for the efficient relaxation of carriers observed in self-assembled quantum dots: the carriers can relax within the continuum states, and make transitions to the excitonic ground state by phonon emission.     

Mechanisms for the generation of coherent longitudinal-optical phonons in GaAs /AlGaAs multiple quantum wells

We show that coherent optical phonons in GaAs multiple quantum wells are generated in a completely different way as compared to bulk GaAs. Unlike in bulk GaAs where the ultrafast screening of electric fields by photogenerated charge carriers is known to be dominant, three distinctive generation mechanisms contribute simultaneously in multiple quantum wells. The interplay between impulsive Raman scattering, forbidden Raman scattering, and screening of surface electric fields, whose relative strengths are determined by laser intensity, detuning from the exciton resonance, and the barrier width, generates a rich variety of new phenomena.     

Strong correlation of electronic and lattice excitations in GaAs/AlGaAs semiconductor quantum wells revealed by two-dimensional terahertz spectroscopy

Coulomb-mediated interactions between intersubband excitations of electrons in GaAs/AlGaAs double quantum wells and longitudinal optical phonons are studied by two-dimensional spectroscopy in the terahertz frequency range. The multitude of diagonal and off-diagonal peaks in the 2D spectrum gives evidence of strong polaronic signatures in the nonlinear response. A quantitative theoretical analysis reveals a dipole coupling of electrons to the polar lattice that is much stronger than in bulk GaAs, due to a dynamic localization of the electron wave function by scattering processes.     

Kinetics of indirect photoluminescence in GaAs/AlxGa1−x As double quantum wells in a random potential with a large amplitude

The kinetics of indirect photoluminescence of GaAs/Al_xGa_1−x As double quantum wells, characterized by a random potential with a large amplitude (the linewidth of the indirect photoluminescence is comparable to the binding energy of an indirect exciton) in magnetic fields B≤12 T at low temperatures T≥1.3 K is investigated. It is found that the indirect-recombination time increases with the magnetic field and decreases with increasing temperature. It is shown that the kinetics of indirect photoluminescence corresponds to single-exciton recombination in the presence of a random potential in the plane of the double quantum wells. The variation of the nonradiative recombination time is discussed in terms of the variation of the transport of indirect excitons to nonradiative recombination centers, and the variation of the radiative recombination time is discussed in terms of the variation of the population of optically active excitonic states and the localization radius of indirect excitons. The photoluminescence kinetics of indirect excitons, which is observed in the studied GaAs/Al_xGa_1−x As double quantum wells for which the random potential has a large amplitude, is qualitatively different from the photoluminescence kinetics of indirect excitons in AlAs/GaAs wells and GaAs/Al_xGa_1−x As double quantum wells with a random potential having a small amplitude. The temporal evolution of the photoluminescence spectra in the direct and indirect regimes is studied. It is shown that the evolution of the photoluminescence spectra corresponds to excitonic recombination in a random potential. Zh. éksp. Teor. Fiz. 115, 1890–1905 (May 1999)     

Ultrafast dynamics of intersubband relaxation in GaAs quantum wells: hot carrier and phonon populations effects

The dynamics of intersubband relaxation in GaAs quantum wells and the role of hot carriers and the phonon distributions have been investigated using two different optical techniques with femtosecond resolution: 1) time-resolved photoluminescence and 2) pump and probe experiments. The (2→1) intersubband relaxation times have been measured as functions of well widths (100Å < L_well < 220Å), under different experimental conditions (15K < T_lattice < 300K, and 1×10¹⁰ cm^-2 < excitation densities < 1×10¹² cm^-2). The electron intersubband relaxation time is deduced from the decay time of the n=2 well luminescence (or differential transmission) intensity. For thin wells (<150Å), a fast intersubband (2→1) relaxation time ≤ 3 ps has been measured. For thicker wells, the measured decay times are found to be critically dependent on the excitation conditions (vary from 5 ps to 40 ps). The well width dependence of the intersubband relaxation time does not show the strong dependence (2 orders of magnitude) predicted theoretically for electron-LO phonon scattering. Our results show that the hot phonon populations and the slow carrier cooling rate limit the observation of subpicosecond relaxation time. For thick well widths, our results also suggest that hot carriers effects play an important role in the intersubband relaxation mechanisms.     

Synthesis and characterization of binary and ternary III–V quantum dots

Quantum dots of InP, GaP, GaInP₂, and GaAs with diameters ranging from 20–80 Å can be synthesized as well-crystallized nanoparticles with bulk zinc blende structure. The synthesis is achieved by heating appropriate organometallic precursors with stabilizers in high boiling solvents for several days to produce QDs, which can then be dissolved in nonpolar organic solvents to form transparent colloidal QD dispersions. The high sample quality of the InP and Gap QDs results in excitonic features in the absorption spectra; excitonic features could not be observed for GaAs or GaInP₂ QDs. The GaP and GaInP₂ QD colloids exhibit very intense (quantum yields of 15–25%) visible photoluminescence at room temperature. The photoluminescence for InP QDs preparations show two emission bands: one band is in the visible at the band edge of the QD, and a second band appears above 800 nm. The near-IR PL is attributed to deep traps, presumably phosphorus vacancies on the QD surface. This band can be removed after controlled addition of etchant; subsequently, very intense band-edge emission (quantum yield 30%), which is tunable with particle size, is obtained. The QDs were characterized by TEM, SAXS, AFM, powder X-ray diffraction, steady-state optical absorption and photoluminescence spectroscopy, ps to ns transient photoluminescence spectroscopy, and fs to ps pump-probe absorption (i.e., hole-burning) spectroscopy.     

Electron-hole correlation singularity in optical spectra of modulation doped GaAsAlGaAs quantum wells

By studying the temperature dependence of absorption and luminescence in modulation doped GaAsAlGaAs quantum wells, we show experimentally that the lowest energy absorption peak has a behavior strikingly different from the corresponding excitonic peak in undoped quantum wells, which is alos unexpected in the frame of the single particle picture. We find good qualitative agreement with the many-body model that attributes this “correlation singularity” to the interaction between the photoexcited hole and the sea of electrons.     

Femtosecond pump and probe measurement of all-optical modulation based on intersubband transition in n-doped quantum wells

An all-optical modulation of interband-resonant light (near-infrared signal light: 800 nm) by intersubband-resonant light (mid-infrared control light: 4–7 μm) in n-doped AlGaAs/GaAs multiple quantum wells is investigated by two-color femtosecond pump–probe experiments at room temperature. The modulation of the near-infrared signal light with an ultrafast recovery as short as 1 ps is successfully observed when the quantum wells are pumped by the mid-infrared control light pulse (4 fJ/μm²). The dependence of the modulation depth on the wavelength of the control light is also measured, which is shown to be consistent with the intersubband absorption spectrum of the quantum wells. The results indicate that the utilization of the intersubband transition is promising for the ultrafast all-optical modulation and switching.     

Photoluminescence studies of GaAs/GaAlAs multiple quantum well heterostructures

The photoluminescence excited by He:Ne and Nd:YAG lasers of GaAs/Ga_0.75Al_0.25As multiple quantum well heterostructures grown by MBE was measured as a function of temperature from 4.2 K up to room temperature and for different pumping powers at constant temperature. The excitonic transitions associated with carriers confined in the quantum wells as well as other transitions associated with impurities either already present in the substrates or introduced into the samples during growth are identified in the spectra and fully characterized. From Arrhenius plots of the photoluminescence peak integrated intensities versus inverse temperature, activation energies are estimated for acceptor defects in the samples as well as for quantum well related excitonic transitions. Photoluminescence polarization experiments demonstrate a dramatic manifestation of the selection rules governing heavy hole and light hole optical transitions in quantum wells.     

基于p-n结中反常光电转换现象的新型带间跃迁量子阱红外探测器

实验发现p-n结中局域载流子具有极高抽取效率,同时伴随着吸收系数的大幅度增加.本文报道上述现象的发现和验证过程,以及基于此现象的新型带间跃迁量子阱红外探测器(interband transition quantum well infrared detector,IQWIP)原型器件的性能.采用共振激发光致发光光谱技术,在InGaN量子阱、InGaAs量子阱、InAs量子点等多个材料体系中均观察到了在p-n结电场作用下的载流子高效逃逸现象,抽取效率分别为95%,87.5%,88%.利用含有InGaAs/GaAs多量子阱的PIN二极管,实验尝试了制备新型的IQWIP原型器件.在无表面减反射膜的实验条件下,利用仅100 nm的有效吸收厚度,实现了31%的外量子效率.基于这个数值推算得到量子阱的光吸收系数达到3.7×10~4cm~(-1),该数值高于传统透射实验测量体材料和量子阱结果.此外,还利用InAsSb/GaSb量子阱材料体系进行了2μm以上波长红外探测的探索.利用上述现象,有望在提高现有器件性能的同时开发出新颖的光-电转换器件.     

CdSe/CdMnSe多量子阱的激子光学性质的研究

用分子束外延在GaAs衬底上生长了CdSe/CdMnSe多量子阱结构.利用X射线衍射(XRD)、变密度激发的PL光谱、变温度PL光谱和变密度激发的ps时间分辨光谱研究了CdSe/CdMnSe多量子阱结构和激子复合特性.讨论了随温度升高辐射线宽展宽和辐射复合效率降低的机理.发现不同激发密度下发光衰减时间不同,认为它的机理可能是无辐射复合引起的.在该材料中观测到激子激子散射发射峰,它被变密度激发和变温度PL光谱所证实. 关键词： CdSe/CdMnSe 量子阱 光学性质     

Modifying electron-phonon scattering rates in semiconductor quantum wells with thin AlAs layers

We have studied theoretically the electron-phonon scattering rates in GaAs/AlAs quantum wells which have additional thin AlAs layers in them using the dielectric continuum approach for the phonons. The confined and interface phonon modes and the intersubband electron phonon scattering rates of these structures have been calculated. The system with an additional AlAs layer is found to have intersubband electron scattering rates which are increased modestly as compared to those for the corresponding quantum well. These results show that scattering rates in general are expected to depend only weakly on the effects of system structure on the optical phonon spectra.     

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.