Hole mobility and remote scattering in strained InGaSb quantum well MOSFET channels with Al2O3 oxide

Hall mobility and major scattering mechanisms in surface and buried MBE grown strained InGaSb quantum well (QW) MOSFET channels with in‐situ grown Al₂O₃ gate oxide are analyzed as a function of sheet hole density, top‐barrier thickness and temperature. Mobility dependence on Al_0.8Ga_0.2Sb top‐barrier thickness shows that the relative contribution of interface‐related scattering is as low as ～30% in the surface QW channel. An InAs top capping layer reduces the interface scattering even further; the sample with 3 nm total top‐barrier thickness demonstrates mobility of 980 cm²/Vs giving sheet resistance of 4.3 kΩ/sq, very close to the minimum QW resistance in the bulk. The mobility–temperature dependences indicate that the interface‐related scattering is dominated by remote Coulomb scattering at hole densities <1 × 10¹² cm^–2. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Approximate analytic expressions for calculation of spontaneous emission spectra for a quantum well embedded in a planar microcavity

The control of spontaneous emission is one of important characteristics of a planar microcavity. The integrals in the spherical coordinate for TE and TM modes spontaneous emission spectra of a quantum well (QW) embedded in a planar microcavity are derived with new variables dependent on wavelength and Taylor series including the two polarizations of the vacuum field. The approximate expressions of spontaneous emission in QW planar microcavities are obtained. The approximate results show that spontaneous emission spectra agree well with that in the numerical integral for planar semiconductor microcavities, in which Fermi-Dirac distribution functions of electrons and holes are considered. The main contribution to the spontaneous emission, radiated into all direction, has been found.     

不同厚度CdSe阱层的表面上自组织CdSe量子点的发光性质

利用变温和变激发功率分别研究了不同厚度CdSe阱层的自组织CdSe量子点的发光。稳态变温光谱表明:低温下CdSe量子阱有很强的发光,高温猝灭,而其表面上的量子点发光可持续到室温,原因归结于量子点的三维量子尺寸限制效应;变激发功率光谱表明:量子点激子发光是典型的自由激子发光,且在功率增加时。宽阱层表面上的CdSe量子点有明显的带填充效应。通过比较不同CdSe阱层厚度的样品的发光,发现其表面上量子点的发光差异较大,这可以归结为阱层厚度不同导致应变弛豫的程度不同,直接决定了所形成量子点的大小与空间分布^[1]。     

Magnetic field-induced charge transfer to a GaAs/(Ga,Al)As quantum well interface studied by C(e,A0i) photoluminescence

The photoluminescence (PL) from a 300 Å GaAs((Ga,Al)As quantum well (QW) has been studied for a range of excitation powers, in magnetic fields up to 16 T applied both perpendicular to and in the plane of the QW. Particular attention was paid to the intensity of the (e,A₀ⁱ) transition due to Carbon acceptors located at one interface of the QW, in the presence of in-plane fields. The low power in-plane field dependence of the PL is a competition between two effects. At fields up to 12 T charge transfer is observed to and from the interface of the QW, resulting in an increase and subsequent decrease of the acceptor PL intensity. For field values exceeding 12 T the acceptor PL intensity is found to increase again. Whereas the first effect is well described by a composite oscillator model, the latter is suggested to be due to the decreased efficiency of electronic traps, located at the QW interface, for in-plane magnetic fields. These suggestions are confirmed by the excitation power dependence of the PL intensity for in-plane fields.     

Enhanced emission of single quantum dot formed by interface fluctuations in photonic-crystal microcavities

We fabricated photonic-crystal (PhC) microcavities tuned to GaAs quantum dots (QDs) formed by interface fluctuation for the first time and observed the spontaneous emission enhancement in a weak coupling regime. A QD is a very thin GaAs quantum well (QW), and its interface steps exhibit quantum dot-like behavior. The emission intensity from the PhC cavity was stronger than that from the area where no PhC pattern was fabricated and the overall shape of the photoluminescence (PL) agreed with the cavity mode calculated with the three-dimensional (3D) finite-difference time domain (FDTD) method. The spontaneous emission enhancement factor was 10.     

Progress in Symmetric and Asymmetric Superlattice Quantum Well Infrared Photodetectors

Herein, two challenges are addressed, which quantum well infrared photodetectors (QWIPs), based on III‐V semiconductors, face, namely: photodetection within the so‐called “forbidden gap”, between 1.7 and 2.5 microns, and room temperature operation using thermal sources. First, to reach this forbidden wavelength range, a QWIP which consists of a superlattice structure with a central quantum well (QW) with a different thickness is presented. The different QW in the symmetric structure, which plays the role of a defect in the otherwise periodic structure, gives rise to localized states in the continuum. The proposed InGaAs/InAlAs superlattice QWIP detects radiation around 2.1 microns, beyond the materials bandoffset. Additionally, the wavefunction parity anomaly is explored to increase the oscillator strength of the optical transitions involving higher order states. Second, with the purpose of achieving room temperature operation, an asymmetric InGaAs/InAlAs superlattice, in which the QW with a different thickness is not in the center, is used to detect infrared radiation around 4 microns at 300 K. This structure operates in the photovoltaic mode because it gives rise to states in the continuum which are localized in one direction and extended in the other, leading to a preferential direction for current flow.     

Photoluminescence of “dark” excitons in CdMnTe quantum well, embedded in a microcavity

A diluted magnetic semiconductor (DMS) quantum well (QW) microcavity operating in the limit of the strong coupling regime is studied by magnetoptical experiments. The interest of DMS QW relies on the possibility to vary the excitonic resonance over a wide range of energies by applying an external magnetic field, typically about 30 meV for 5 T in our sample. In particular, the anticrossing between the QW exciton and the cavity mode can be tuned by the external field. We observe the anticrossing and formation of exciton polaritons in magneto-reflectivity experiments. In contrast, magneto-luminescence exhibits purely excitonic character. Under resonant excitation conditions an additional emission line is observed at the energy of the dark exciton. The creation of dark excitons is made possible due to heavy hole–light hole mixing in the QW. The emission at this energy could be due to a combined spin flip of an electron and a bright exciton recombination.     

Electric field effect on the linear and nonlinear intersubband refractive index changes in asymmetrical semiparabolic and symmetrical parabolic quantum wells

By using the displacement harmonic variant method and the compact density matrix approach, the linear and nonlinear intersubband refractive index changes (RICs) in a semiparabolic quantum well (QW) with applied electric field have been investigated in detail. The simple analytical formulae for the linear and nonlinear RICs in the system were also deduced. The symmetrical parabolic QWs with applied electric fields were taken into account for comparison. Numerical calculations on typical GaAs QWs were performed. The dependence of the linear and nonlinear RICs on the incident optical intensity, the frequencies of the confined potential of the QWs and the strength of the applied electric field were discussed. Results reveal that the RICs in the semiparabolic quantum well system sensitively depend on these factors. The calculation also shows that the semiparabolic QW is a more ideal nonlinear optical system relative to the symmetric parabolic QW systems.     

Stochastic resonance of bias signal-modulated noise in a single-mode laser＊

Stochastic resonance (SR) for bias signal modulation is studied in a single-mode laser system. By investigating a gain-noise model driven by correlated pump noise and quantum noise, we find that, whether the correlation coefficient between both the noises is positive or negative, SR always appears in the dependence of signal-to-noise ratio (SNR) upon the noise correlation time and the frequency of the modulation signal. However, only when the correlation coefficient between both noises is negative can SR occur in the dependence of SNR upon the quantum noise intensity and pump noise intensity, while when the correlation coefficient between both noises is positive, it shows monotonically.     

Electric-field control of electron–hole wave functions in a wide quantum well

The electric field dependence of the electron/hole wave function and the radiation energy of an exciton in a Be-δ-doped 80 nm quantum well (QW) is studied experimentally and compared it with variational calculation. The photoluminescence (PL) spectra show Stark shifts depending on the gate electric field and PL intensity of the exciton of the first excited state has a dip in the electric-field dependence which reflects the node of the electron wave function.     

The exciton luminescence in Zn(Cd)Se/ZnMgSSe quantum wells

We study exciton states in Zn(Cd)Se/ZnMgSSe quantum wells (QWs) with various degrees of diffusion blurring in the interfaces by the methods of optical spectroscopy. We show that at low temperatures the QW emission spectra are determined by free and neutral donor-bound excitons. Blurring of the heterointerfaces leads to the increase in the energy shift between the emission line maxima of free and bound excitons. We explain the nonlinear dependence of the steady-state photoluminescence intensity on the excitation-power density in terms of the neutralization of charged donors at the photoexcitation of heterostructures. We observed a complex long-time dynamics of the reflection coefficient, evoked by the charge-redistribution processes in the heterostructure, near the QW exciton resonances under the irradiation.     

Investigation of optical properties of InGaN-InN-InGaN/GaN quantum-well in the green spectral regime

The optical properties of the InGaN/GaN quantum well with insertion of ultrathin InN layer is investigated by using the effective mass theory taking into account the valence band mixing effects. The total spontaneous emission radiation recombination rate can be optimized by modulating the position of InN layer in the InGaN QW. Meanwhile, it is observed that the difference of the spontaneous emission rate becomes smaller with increasing the sheet carrier density. Then, the influences of intermixing effect at the interface between InN and InGaN layers on the optical gain are analyzed. It shows the emission intensity is reduced as compared to the ideal QW structure while peak wavelength is red-shifted by ∼10 nm in the investigation range of L_sn. Finally, the influence of partial strain relaxation on the lasing wavelength is discussed, which shows a blue shift of ∼27 nm in the case with residual strain of 50% in comparison to the no strain relaxation case.     

Anti-Stokes and Stokes photoluminescence in non-uniform GaAs-based quantum wells

The intensity distribution of the anti-Stokes photoluminescence (AS-PL) signal has been investigated in a quantum heterostructure containing several GaAs-based quantum wells (QWs) of different thickness. We observe four emission bands at different energies, which originate from the spatially different positions of the QWs in the heterostructure. When the photon energy of the excitation is tuned to the QW with the lowest emission energy, we detect a clear resonant enhancement of the AS-PL signal of the exciton state highest in energy, which belongs to the QW located in close proximity. This observation implies that the capture of non-equilibrium carriers within the QWs plays a very important role for determination of the AS-PL intensity distribution.     

Optical Tamm state polaritons in a quantum well microcavity with gold layers

A new type of cavity polariton,the optical Tamm state(OTS) polariton,is proposed to be realized by sandwiching a quantum well(QW) between a gold layer and a distributed Bragg reflector(DBR).It is shown that OTS polaritons can be generated from the strong couplings between the QW excitons and the free OTSs.In addition,if a second gold layer is introduced into the bottom of the DBR,two independent free OTSs can interact strongly with the QW excitons to produce extra OTS polaritons.     

Laser field induced interband absorption in a strained GaAs/GaAlAs double quantum well system

Effect of laser field intensity on exciton binding energies is investigated in a GaAs/ GaAlAs double quantum well system. Calculations have been carried out with the variational technique within the single band effective mass approximations using a two parametric trial wave function. The interband emission energy as a function of well width is calculated in the influence of laser field. The laser field induced photoionization cross-section for the exciton placed at the centre of the quantum well is computed as a function of normalized photon energy. The dependence of the photoionization cross-section on photon energy is carried out for the excitons. The resulting spectra are brought out for light polarized along and perpendicular to the growth direction. The intense laser field dependence of interband absorption coefficient is investigated. The results show that the exciton binding energy, interband emission energy, the photoionization cross-section and the interband absorption coefficient depend strongly on the well width and the laser field intensity. Our results are compared with the other existing literature available.     

Spontaneous emission coupling factors in planar micro-cavity lasers with distributed Bragg reflectors

In this article, a considerable value of spontaneous emission factor is estimated based on the spontaneous emission radiation patterns in micro-cavity lasers. After the spontaneous emission for a dipole in the micro-cavity is studied, the confinements of vacuum fields and electrons are considered together for a quantum well in the micro-cavity laser. The TE mode and TM mode spontaneous emission spectra are presented for a single quantum well embedded in a planar micro-cavity laser at 300 K. The results show that the TE mode spontaneous emission spectrum is enhanced, whereas the TM mode spontaneous emission is suppressed greatly. The maximum values obtained for the spontaneous emission coupling factors are 0.2 and 0.0055 for a dipole and a quantum well planar micro-cavity laser with distributed Bragg reflectors, respectively.     

Anisotropic strain effects on light emission characteristics of CdZnO/ZnO quantum well structures

Anisotropic strain effects by strain relaxation on TE-polarized light emission characteristics of c-plane CdZnO/ZnO quantum well (QW) structures were theoretically investigated by using the multiband effective-mass theory. The CdZnO/ZnO QW structure with anisotropic strain has much larger emission intensity than conventional CdZnO/ZnO QW structure without the strain relaxation. In the case of the strain relaxation along x(or y)-direction, the x(or y)-polarized light emission is observed to be larger than the x(or y)-polarized light emission. In particular, in the case of the strain relaxation along both x- and y-directions, the increase in the spontaneous emission peak is significant. This can be explained by the fact that the internal field is reduced owing to the decrease in the piezoelectric field by the strain relaxation.     

Photoionization of a charged exciton in a GaAs/GaAlAs corrugated quantum well

Binding energies of a charged exciton as a function of well width of a GaAs/GaAlAs corrugated quantum well are investigated. The calculations have been performed by the variational method based on a two parametric trial wave function within a single band effective mass approximation. We have also included the effect of nonparabolicity of the conduction band of GaAs. We study the spectral dependence of the charged exciton in a GaAs/GaAlAs corrugated quantum well as a function of well width. The photoionization cross section for the charged exciton placed at the center of the quantum well is computed as a function of normalized photon energy. The cross-section behavior as a function of incident energy is entirely different in the two cases of radiation being x-direction (along the growth direction) or z-direction. The interband emission energy as a function of well width is calculated and the dependence of the photoionization cross section on photon energy is carried out for the charged excitons. The resulting spectra are brought out for light polarized along and perpendicular to the growth direction. The results show that the charged exciton binding energy, interband emission energy and the photoionization cross section depend strongly on the well width. Our results are compared with the other existing literature available.     

Nonresonant radiative exciton transfer by near field between quantum wells

We experimentally observed an increase in the intensity of photoluminescence from a wider quantum well (QW) when an exciton transition was induced in the neighboring narrower QW separated from the former one by a tunneling-nontransparent AlGaAs barrier. The dependence of the efficiency of the near-field radiative transfer of excitons on the distance between QWs was studied in heterostructures without coincidence of exciton resonances in the adjacent QWs. Theoretical results were qualitatively consistent with the available experimental data.     

Super-radiant surface emission from a quasi-cavity hot electron light emitter

The Hot Electron Light Emitting and Lasing in Semiconductor Heterostructure (HELLISH-1) device is a novel surface emitter which utilises hot carrier transport parallel to the layers of a Ga1–xAlxAs p-n junction incorporating a single GaAs quantum well on the n-side of the junction plane. Non-equilibrium electrons are injected into the quantum well via tunnelling from the n-layer. In order to preserve the charge neutrality in the depletion region, the junction undergoes a self-induced internal biasing. As a result the built-in potential on the p-side is reduced and hence the injection of non- equilibrium holes into the quantum well in the active region is enhanced. The work presented here shows that a distributed Bragg reflector grown below the active region of the HELLISH device increases the emitted light intensity by two orders of magnitude and reduces the emission line-width by about a factor of 3 in comparison with the original HELLISH-1 structure. Therefore, the device can be operated as an ultrabright emitter with higher spectral purity.