Photon emission during intraband tunneling through quantum well structures

Radiative transitions associated with intraband electron tunneling through DC biased quantum well structures are analyzed theoretically. Spontaneous emission and stimulated emission of photons within the quantum well structure are calculated and estimates are made of the radiative transition rate in comparison with the damping loss. The absence of an inherent long wavelength emission cutoff is in contrast with interband transition devices and suggests applications of intraband transition devices as far infrared or microwave sources.     

Polariton dispersion of periodic quantum well structures

We studied the polariton dispersion relations of a periodic quantum-well structure with a period in the vicinity of half the exciton resonance wavelength, i.e., the Bragg structure. We classified polariton modes using an approximation of a large number of quantum wells. The polariton effective masses are found to be very small and equal to 10^?3?10^?4 of the free-electron mass.     

Electric field effects on intersubband transitions in quantum well structures

We report on a Raman scattering study of the electric-field dependence of c₀ → c₁ intersubband transitions of electrons in a 264 Å GaAs- Al_0.3Ga_0.7As quantum-well structure. The measured Stark shifts are in very good agreement with theoretical predictions. The intensity of the intersubband peak increases rapidly with applied field due to parity-mixing. In contrast to the enhanced broadening shown by excitation resonances, the width of c₀ → c₁ is nearly independent of the field. This feature is attributed to effects of structural disorder.     

Excitonic transitions in Be-doped GaAs/AlAs multiple quantum well

A series of GaAs/AlAs multiple-quantum wells doped with Be is grown by molecular beam epitaxy. The photoluminescence spectra are measured at 4, 20, 40, 80, 120, and 200 K, respectively. The recombination transition emission of heavy-hole and light-hole free excitons is clearly observed and the transition energies are measured with different quantum well widths. In addition, a theoretical model of excitonic states in the quantum wells is used, in which the symmetry of the component of the exciton wave function representing the relative motion is allowed to vary between the two- and threedimensional limits. Then, within the effective mass and envelope function approximation, the recombination transition energies of the heavy- and light-hole excitons in GaAs/AlAs multiple-quantum wells are calculated each as a function of quantum well width by the shooting method and variational principle with two variational parameters. The results show that the excitons are neither 2D nor 3D like, but are in between in character and that the theoretical calculation is in good agreement with the experimental results.     

Phonon–replica transitions in InGaN/GaN quantum well structures

A side-bump feature in a photoluminescence (PL) spectrum of an InGaN compound was widely observed. With reasonable fitting to PL spectra with three Gaussian distributions, the temperature variations of the peak positions, integrated PL intensities, and peak widths of the main and first side peaks of three InGaN/GaN multiple quantum well samples with different nominal indium contents are shown and interpreted. The existence of the side peaks is attributed to phonon–replica transitions. The variations of the peak position separations and the decreasing trends of the first side peak widths beyond certain temperatures in those samples were explained with the requirement of phonon momentum condition for phonon–replica transitions. In the sample with 25% nominal indium content, the phonon–replica transition could become stronger than the direct transition of localized states.     

Ballistic effects and intersubband excitations in multiple quantum well structures

We have studied the transport properties of electrons in asymmetric quantum well structures upon far-infrared optical excitation of carriers from the lowest subband into the continuum. Here the photocurrent consists of a coherent component originating from ballistic transport upon excitation, and of an incoherent part associated with asymmetric diffusion and relaxation processes, which occur after the coherence has been lost. The signature of the coherent contribution is provided by a sign reversal of the photocurrent upon changing the excitation energy. This sign reversal arises from the energy-dependent interference between continuum states, which have a twofold degeneracy characterized by positive and negative momenta. The interference effect also allows us to estimate the coherent mean free path ( nm at 77 K). In specifically designed device structures, we use both the coherent and incoherent components in order to achieve a pronounced photovoltaic infrared response for detector applications.     

Infrared spectroscopy of intraband transitions in Ge/Si quantum dot superlattices

We report the study of infrared spectroscopy of intraband transitions in Ge/Si quantum dot superlattices. The superlattices, which were grown on (001) oriented Si substrates by a solid source molecular beam epitaxy system, are composed mainly of 20 or 30 periods of Ge dot layers and Si spacer films. The structural properties of them and of the uncapped Ge dots grown on the surfaces of some of them were tested by cross-sectional transmission electron and atomic force microscopes, respectively. It is found that the Ge quantum dots have flat lens-like shapes. Infrared absorption signals peaking in the mid-infrared range were observed using Fourier transform infrared and Raman scattering spectroscopy techniques. Experimental and theoretical analysis suggests that the mid-infrared response be attributed to intraband transitions within the valence band of the Ge quantum dots in the superlattices. The fact that the intraband absorption is strongly polarized along the growth axis of the superlattices signifies that the Ge quantum dots with flat lens-like shapes perform as Ge/Si-based quantum wells. This study demonstrates the application potential of these kinds of Ge/Si quantum dot superlattices for developing mid-infrared photodetectors.     

可用于红外探测器的自组织量子线及其带间和子带间光学跃迁

研究了自组织量子线Ga1-xInxAs的结构、应力分布及其光学性质.模拟了微应力导致的横向成序及其导引短周期超晶格形成量子线的过程，并计算出量子线在原子尺度上的微应力分布.这里考虑了价带各向异性、带间混合及局域应力分布对光学性质的作用.研究发现自组织量子线具有应用于正入射红外探测器的良好光学特性.结果显示当量子线的周期长度为15到30nm时，导带子带间跃迁波长处在10到20μm，这正是红外探测器的理想工作范围.同时，带间吸收波长在中红外范围，它提供了红外探测器的另一个窗口. 关键词： 自组织 微应力 红外探测器 量子线     

Resonant magnetopolaron effects in GaAs/AlGaAs multiple quantum well structures

A detailed experimental study of electron cyclotron resonance (CR) has been carried out at 4.2 K in three modulation-doped GaAs/Al_0.3Ga_0.7As multiple quantum well samples in fields up to 30 T. A strong avoided-level-crossing splitting of the CR energies due to resonant magnetopolaron effects is observed for all samples near the GaAs reststrahlen region. Resonant splittings in the region of AlAs-like interface phonon modes of the barriers are observed in two samples with narrower well width and smaller doping concentration. The interaction between electrons and the AlAs interface optical phonon modes has been calculated for our specific sample structures in the framework of the memory-function formalism. The calculated results are in good agreement with the experimental results, which confirms our assignment of the observed splitting near the AlAs-like phonon region is due to the resonant magnetopolaron interaction of electrons in the wells with AlAs-like interface phonons.     

Resonant modes in photonic multiple quantum well structures with single-negative materials

A type of photonic multiple quantum well (PMQWs) structure made of two different photonic crystals (PCs) with two kinds of single-negative materials is investigated. It is demonstrated by transfer matrix method that omnidirectional resonance modes are generated. The number of the resonance modes can be controlled by adjusting the periodic structure of the constituents. The resonance tunneling modes are weak dependence on incident angle and the scaling of the barrier photonic crystals. When the losses are taken into account, the effects of the losses coming from ENG media and MNG media on the resonance modes are striking difference.     

Effect of group velocity dispersion on femtosecond pulse evolution in quantum well waveguide structures

The paper deals with the theoretical investigation of ultrafast-pulse evolution in a semiconductor quantum well (QW). Semiconductor Bloch equations are used to obtain the polarization induced in the medium due to incident Gaussian electromagnetic beam. The partial differential equation with finite group velocity dispersion (GVD) is then used to analyze the effect of induced polarization on the pulse. The role of GVD on femtosecond pulse evolution in GaAs/AlGaAs waveguide is studied, giving due consideration to the intensity dependent group velocity of the medium.     

Intersubband transitions in quantum well mid-infrared photodetectors

A study of intersubband transitions in quantum well infrared detectors working at high temperatures has been reported. This study allows a greater tunability in the device designs, with the ability to control the peak wavelength, the absorption coefficient, the dark current, the quantum efficiency and the detectivity of the modeled structure operating around 3.3 μm wavelength. The detection energy and absorption coefficient dependences with an applied electric field are given. Then, the electro-optic performances of the modeled mid-infrared detector are estimated, the dark current dependence with the applied voltage and temperature as well as the quantum efficiency and the detectivity are investigated and discussed. High detectivities were found at high temperatures revealing the good performances of the designed photodetector, especially at 3.3 μm wavelength.     

Bevel fabrication for depth resolved studies of multiple quantum well structures

Beveled cross-sections of semiconductors with inclination angles down to 0.25 min of arc have been produced with a special ion beam etching process. We applied this technique to the depth resolved characterization of GaAs/GaAlAs multiple quantum well structures by photoluminescence spectroscopy. The depth dependent incorporation of impurities during the growth of the first quantum well layers is clearly revealed.     

Far-infrared two-photon intraband transitions in n-InSb

Two-photon cyclotron resonance and two-photon shallow donor transitions have been excited by a high power pulsed D₂O laser emitting λ = 119 μm and 66 μm laser lines at the same time. Transitions involving the absorption of two 119 μm photons or simultaneously one 119 μm and one 66 μm photon were observed. Two-photon selection rules are discussed by a rigorous treatment of the symmetry of the free electron Hamiltonian.     

Subband-level renormalization and absorptive optical bistability in semiconductor multiple quantum well structures

A microscopic theory of many-body effects in the lowest subbands of semiconductor multiple quantum well structures is presented. The renormalized subband levels are calculated for a broad temperature range and optical bistability due to induced absorption is predicted.     

Bound polaron in quantum dot quantum well structures

The problem of bound polarons in quantum dot quantum well (QDQW) structures is studied theoretically. The eigenfrequencies of bulk longitudinal optical (LO) and surface optical (SO) modes are derived in the framework of the dielectric continuum approximation. The electron--phonon interaction Hamiltonian for QDQW structures is obtained and the exchange interaction between impurity and LO-phonons is discussed. The binding energy and the trapping energy of the bound polaron in CdS/HgS QDQW structures are calculated. The numerical results reveal that there exist three branches of eigenfrequencies of surface optical vibration in the CdS/HgS QDQW structure. It is also shown that the binding energy and the trapping energy increase as the inner radius of the QDQW structure decreases, with the outer radius fixed, and the trapping energy takes a major part of the binding energy when the inner radius is very small.     

Resonance shifts in multiple quantum transitions

In the theory of multiple quantum transitions in alkali metals, we show that overlap effects from resonances in a different hyperfine level may profoundly affect the line shift and profile.     

Quantum dots in quantum well structures

Recent progress toward fabricating and characterizing quantum dots in III–V quantum well structures is reviewed. Quantum dots made by use of lithography and etching, including deep-etched, barrier-modulated, strain-induced and interdiffused quantum dots, are described. Quantum dots fabricated by growth, including natural quantum dots, dots on patterned substrates, and self-assembled dots, are discussed. Dot sizes and uniformity, energy-level splittings, and luminescence efficiencies that are now being achieved are discussed. The status of key issues, such as the energy relaxation in quantum dots, is mentioned.