Optical orientation in p-doped semiconductor structures with a split valence band

The optical orientation of electron spins in heavily doped semiconductor structures with a valence band that is split as a result of size quantization or uniaxial deformation is investigated theoretically. It is shown that lowering the Fermi level by doping and by lowering the temperature should lead to sharp changes in the photon-energy-dependence of the average spin of the excited electrons in structures excited by circularly polarized light. This effect is due to an interchange of the dominant contribution of transitions from a light-hole subband and transitions from the heavy-hole subband in absorption. Pis’ma Zh. éksp. Teor. Fiz. 65, No. 12, 867–871 (25 June 1997)     

Optical orientation by linearly polarized light in intersubband quantum-well transitions

The absorption of linearly polarized light in low-dimensional semiconductor structures is investigated. It is shown that the absorption under consideration can give rise to spin orientation of free carriers. A theory of this optical orientation by linearly polarized light is developed for resonant intersubband optical transitions in n-type quantum wells. It is demonstrated that, in the vicinity of the resonance, the optical orientation undergoes spectral inversion, namely, the electron spin orientation reverses sign with increasing frequency. This behavior can be accounted for by the spin-orbit subband splitting, which is linear in the wave vector, and by the energy and quasi-momentum conservation laws.     

Valence-band structure and optical absorption inp-type GaAs–Al0.3Ga0.7As multi-quantum-well infrared photodetectors under an electric field

Hole structure of a GaAs–Al_0.3Ga_0.7Asp-type multiple quantum well (MQW) subjected to an electric field parallel to the growth axis is studied using the envelope-function approximation and taking into account the valence subband mixing. The system considered in this work consists of five GaAs wells and six thick Al_0.3Ga_0.7As barriers. The valence subband structure and the optical-absorption coefficient are calculated as functions of the electric-field strength for various doping levels. The subband structure is shown to be nonparabolic and anisotropic in the plane of the layers with a four-fold symmetry. The spin splitting due to the lack of specular symmetry of quantum wells is a growing function of the electric-field strength. The calculated optical absorption is in good agreement with the experimental spectra.     

Spin–orbit coupling effects on the in-plane optical anisotropy of semiconductor quantum wells

We theoretically study the influence of the spin–orbit coupling(SOC) on the in-plane optical anisotropy(IPOA) induced by in-plane uniaxial strain and interface asymmetry in(001) GaAs/AlGaAs quantum wells(QWs) with different well width. It is found that the SOC has more significant impact on the IPOA for the transition of the first valence subband of heavy hole to the first conduction band(1H1E) than that of 1L1E. The reason has been discussed. The IPOA of(001) InGaAs/InP QWs has been measured by reflectance difference spectroscopy, whose amplitude is about one order larger than that of GaAs/AlGaAs QWs. The anisotropic interface potential parameters of InGaAs/InP QWs are also determined. The influence of the SOC effect on the IPOA of InGaAs/InP QWs when the QWs are under tensile, compressive or zero biaxial strain are also investigated in theory. Our results demonstrate that the SOC has significant effect on the IPOA especially for semiconductor QWs with small well width, and therefore cannot be ignored.     

Strain- and twist-engineered optical absorption of few-layer black phosphorus

Density functional and many-body perturbation theories calculations were carried out to investigate fundamental and optical bandgap, exciton binding energy and optical absorption property of normal and strain- and twist-engineered few-layer black phosphorus (BP). We found that the fundamental bandgaps of few layer BP can be engineered by layer stacking and in-plane strain, with linear relationships to their associated exciton binding energies. The strain-dependent optical absorption behaviors are also anisotropic that the position of the first absorption peak monotonically blue-shifts as the strain applies to either direction for incident light polarized along the armchair direction, but this is not the case for that along the zigzag direction. Given those striking properties, we proposed two prototype devices for building potentially more balanced light absorbers and light filter passes, which promotes further applications and investigations of BP in nanoelectronics and optoelectronics.     

Hole subband non-parabolicities in strained Si/Si1 − xGexquantum wells

We study the hole subband non-parabolicities in undoped pseudomorphic Si/Si_{1 − x}Ge_xquantum wells, strained in the growth direction 100. We solve exactly the multiband effective mass equation that describes the heavy, light and split-off hole valence bands. The symmetries of the Luttinger–Kohn Hamiltonian of the system are used to decouple the degenerate subbands. We calculate the in-plane dispersion relations, investigate the importance of the inclusion of the split-off hole valence band in the Hamiltonian and comment on the resulted non-parabolicities. Resonance states are also obtained.     

Study of valence intersubband absorption in tensile strained Si/SiGe quantum wells

The hole subband structures and effective masses of tensile strained Si/Sil-yGey quantum wells are calculated by using the 6 × 6 k·p method. The results show that when the tensile strain is induced in the quantum well, the light-hole state becomes the ground state, and the light hole effective masses in the growth direction are strongly reduced while the in-plane effective masses are considerable. Quantitative calculation of the valence intersubband transition between two light hole states in a 7nm tensile strained Si/Si0.55Ge0.45 quantum well grown on a relaxed Si0.5Ge0.5 （100） substrates shows a large absorption coefficient of 8400 cm^-1.     

InAs/AlSb/GaSb量子阱中的双色光吸收

为了降低噪声对InAs/GaSb量子阱作为双色电探测器性能的影响,设计性能优良的光电探测器,在InAs/GaSb量子阱中加入AlSb夹层,以减少电子和空穴在界面处的复合,从而抑制由于电子和空穴复合引起的噪声。首先应用转移矩阵方法求解薛定谔方程得到量子阱中电子和空穴的能级和波函数,研究AlSb夹层对电子和空穴波函数的影响。应用平衡方程方法求解外加光场条件下的玻尔兹曼方程,研究所有电子和空穴跃迁通道对光吸收系数的贡献,重点研究了AlSb夹层厚度对光吸收系数的影响。结果表明:基于In As/GaSb的量子阱体系可以实现双色光吸收,加入AlSb夹层可以有效抑制电子和空穴在界面处的隧穿,从而降低复合噪声,同时AlSb夹层的加入也对吸收峰有影响。AlSb夹层的厚度达到2 nm即可有效降低电子和空穴复合噪声,双色光吸收峰在中远红外波段,为该量子阱作为性能良好的中远红外光电探测器提供理论支撑。     

Polarization dependence of two-photon transitions in quantum wells

Summary We give explicitly the polarization dependence of two-photon subband-subband transitions in semiconductor quantum wells. We consider transitions from heavy-hole subbands as well as from light-hole subbands. We study the polarization dependence in the case of absorption of one photon having an energy of the order of the band gap and one having an energy of the order of the subband separation. We show that the absorption structure depends on the polarization of the low-energy photon. We also give, in the case of equal photons with in-plane linear polarizations, the dependence of the transition rate on the angle between the polarizations.     

半导体超晶格和量子阱中量子态的光谱研究

本文讨论半导体超晶格及量子阱导带和价带的量子化亚带或子能级之间的带间光跃迁过程。所讨论的光跃迁过程或光谱研究方法有吸收光谱、光电流谱、光荧光和荧光激发谱、调制光谱以及喇曼散射光谱。关于亚带能量状态将着重讨论量子阱中的激子效应和价带亚带混和及其对带间光跃迁的影响。     

Calculation and interpretation of IR and resonant raman spectra of 5-halosubstituted uracils

We have used valence optical theory to carry out calculation and analysis of in-plane vibrations and intensities of IR spectra for isolated 5-fluorouracil and 5-bromouracil molecules. We give a complete interpretation of the corresponding bands in the absorption spectra. We have carried out a quantum mechanical calculations of the relative intensities in the resonant Raman spectra of 5-fluorouracil and 5-chlorouracil. The calculated spectra for the molecular models obtained agree quantitatively with the experimental spectra, and reproduce the observed characteristic spectral differences in the series of molecules: uracil, 5-fluorouracil, 5-chlorouracil, and 5-bromouracil. The models obtained for the molecules can be used for calculation and analysis of the spectra of other halosubstituted nucleic acid bases. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 73, No. 4, pp. 437–442, July–August, 2006.     

Theory of carrier gain dynamics in InGaAs/AlGaAs strained-layer single-quantum-well diode lasers

Calculations of the femtosecond gain dynamics in InGaAs/AlGaAs strained-layer single-quantum-well diode lasers are presented and compared to experiments which use a novel multiple-wavelength pump probe technique. We develop a detailed theoretical model for the gain dynamics in a quantum well laser diode structure to aid in the interpretation of gain dynamics induced by both interband absorption and stimulated emission of photons. In the model, transient gain and differential transmission are computed in a multiband effective mass model including biaxial strain, valence subband mixing, and scattering both within and between subbands. The transient photogeneration of electron-hole pairs by the pump pulse and subsequent relaxation of carriers by both polar optical phonon scattering and carrier-carrier scattering are calculated within a Boltzmann equation framework. A relaxation approximation for the carrier-carrier scattering is made making the coupled Boltzmann equations an effective one dimensional model which are then solved using an adaptive Runge-Kutta technique rather than a more computationally intensive Monte Carlo approach.     

Band mixing effect in semiconductor superlattices

The band mixing effect on the electronic and optical properties of semiconductor superlattices is studied within the framework of the empirical tight-binding model. It is found that the superlattice periodic potentials mix the bulk heavy hole, light hole and spin-orbit-split bands in the valence band states. As a consequence, the optical matrix elements associated with various valence-to-conduction subband transitions are very sensitive to the variation of the wavevector in directions parallel to the interface ( _t). We find that band mixing in conjunction with the exciton effect can account for the Δn≠0 forbidden transitions observed in several recent experiments.     

Subband current in resonant tunneling diode

An accumulation layer is formed on the emitter side of a biased resonant tunneling diode (RTD) leading to a similar subband structure as in the ordinary MOS-system. Electrons occupying the subbands can tunnel through the RTD-structure and give rise to a significant contribution to the diode current. We calculate the subband current from our semiclassical transport model developed earlier for the ordinary tunneling current. The model includes quantum interference and bulk scattering by utilizing an optical approximation for the coherent part of the wave function. The subband current turns out to be of the same order of magnitude as the ordinary tunneling current component. It is shifted to higher voltages and therefore it increases the valley current. In order to reduce the subband current and improve the peak-to-valley current ratio (PVCR), we propose a novel RTD-structure with a grading in front of the emitter barrier. The purpose of the grading is to suppress the formation of the accumulation layer and thereby decrease the valley current. Calculations show that PVCR increases by a factor of two using a proper design of the grading.     

Three-dimensional character of semimetallic InAs-GaSb superlattices

High-field, far-infrared magneto-absorption experiments are performed in semimetallic InAs-GaSb superlattices. The spectra exhibit extensive oscillations of cyclotron resonance and interband absorption from valence to conduction subbands. Transitions at both the center and the boundary of the superlattice zone are observed, from which the width of the ground conduction subband is obtained, demonstrating directly its three-dimensional character.     

Magnetotransport in modulation-doped In0.53Ga0.47As/In0.52Al0.48As heterojunctions: in-plane effective mass,quantum and transport mobilities of 2D electrons

Shubnikov–de Haas (SdH) and Hall effect measurements, performed in the temperature range between 3.3 and 20 K and at magnetic fields up to 2.3 T, have been used to investigate the electronic transport properties of lattice-matched In_0.53Ga_0.47As/In_0.52Al_0.48As heterojunctions. The spacer layer thickness (t_S) in modulation-doped samples was in the range between 0 and 400 Å. SdH oscillations indicate that two subbands are already occupied for all samples except for that witht_S =  400 Å. The carrier density in each subband, Fermi energy and subband separation have been determined from the periods of the SdH oscillations. The in-plane effective mass (m ^* ) and the quantum lifetime (τ_q) of 2D electrons in each subband have been obtained from the temperature and magnetic field dependences of the amplitude of SdH oscillations, respectively. The 2D carrier density (N₁) in the first subband decreases rapidly with increasing spacer thickness, while that (N₂) in the second subband, which is much smaller thanN₁ , decreases slightly with increasing spacer thickness from 0 to 200 Å. The in-plane effective mass of 2D electrons is similar to that of electrons in bulk In_0.53Ga_0.47As and show no dependence on spacer thickness. The quantum mobility of 2D electrons is essentially independent of the thickness of the spacer layer in the range between 0 and 200 Å. It is, however, markedly higher for the samples with a 400 Å thick spacer layer. The quantum mobility of 2D electrons is substantially smaller than the transport mobility which is obtained from the Hall effect measurements at low magnetic fields. The transport mobility of 2D electrons in the first subband is substantially higher than that of electrons in the second subband for all samples with double subband occupancy. The results obtained for transport-to-quantum lifetime ratios suggest that the scattering of electrons in the first subband is, on average, forward displaced in momentum space, while the electrons in the second subband undergo mainly large-angle scattering.     

Ultrafast dynamics of intersubband excitations in a quasi-two-dimensional hole gas

We present the first study of ultrafast hole dynamics after resonant intersubband excitation in a quasi-two-dimensional semiconductor. p-type Si0.5Ge 0.5/Si multiple quantum wells are studied in pump-probe experiments with 150 fs midinfrared pulses. Intersubband scattering from the second heavy-hole back to the first heavy-hole subband occurs with a time constant of 250 fs, followed by intrasubband carrier heating within 1 ps. Such processes give rise to a strong reshaping of the intersubband absorption line, which is accounted for by calculations of the subband structure, optical spectra, and hole-phonon scattering rates.     

Impurity optical absorption in parabolic quantum well

Optical absorption in GaAs parabolic quantum well in the presence of hydrogenic impurity is considered. The absorption coefficient associated with the transitions between the upper valence subband and donor ground state is calculated. The impurity ground state wave function and energy are obtained using the variational method. Dependence of the absorption spectra on impurity position in quantum well was investigated. It is shown, that along with quantum well width decrease the absorption threshold shifts to higher frequencies. Results obtained within frames of parabolic approximation are compared with results for rectangular infinite-barrier quantum well case. The acceptor state → conduction band transitions considered as well.     

Optical orientation and femtosecond relaxation of spin-polarized holes in GaAs

Optical orientation of spin-polarized heavy and light holes followed by relaxation to other valence subband states has been observed unambiguously in undoped bulk GaAs in spite of the extremely short spin relaxation time. The measured relaxation time for the heavy holes is 110 fs +/-10%. The results are relevant for applications such as interpretation of spin-polarized transport in semiconductors as well as the assessment of feasibility of hole-based spin-transport devices which relies on precise knowledge of the hole-spin relaxation time.