Te Single Quantum Research Articles

Electron–electron scattering rate in CdTe/CdMnTe single quantum well

CdMnTe is demonstrated to be a good candidate in the X-ray and [Formula: see text]-ray detector application, however, there are few reports on theoretical analysis of electron scattering rate in CdMnTe quantum well. Within the framework of effective mass approximation and envelope function approximation, the influence of the Mn alloy composition ([Formula: see text], the well width ([Formula: see text], the electron temperature ([Formula: see text] and the electron density ([Formula: see text] on the electron–electron scattering rate (1/[Formula: see text] in the CdTe/Cd[Formula: see text]Mn[Formula: see text]Te single quantum well (SQW), are simulated by shooting method and Fermi’s Golden Rule. The results show that 1/[Formula: see text] is significant inverse proportional to [Formula: see text], but positively proportional to [Formula: see text] and [Formula: see text]. Except for a small peak at 20 K, 1/[Formula: see text] is not sensitive to [Formula: see text]. The above differential dependency of 1/[Formula: see text] on [Formula: see text] and [Formula: see text] can be interpreted by sub-band separation ([Formula: see text], which is proportional to [Formula: see text] but inversely proportional to [Formula: see text]. When [Formula: see text] decreases gradually, the electron transition becomes easier, which leads to 1/[Formula: see text] increases. The dependency of 1/[Formula: see text] on [Formula: see text] can be interpreted by kinetic energy of electrons. The larger the electron kinetic energy is, the more difficult the electron transition from first excited state to ground state is, which leads to 1/[Formula: see text] decreasing. The dependency of 1/[Formula: see text] on [Formula: see text] can be interpreted by the Coulomb interaction between electrons, i.e., the increase of electron collision probability caused by the increase of [Formula: see text].

Magneto-intersubband oscillations in two-dimensional systems with an energy spectrum split due to spin-orbit interaction

In the present paper we study magneto-intersubband oscillations (MISO) in HgTe/Hg$_{1-x}$Cd$_x$Te single quantum well with "inverted" and "normal" spectra and in conventional In$_{1-x}$Ga$_x$As/In$_{1-y}$Al$_y$As quantum wells with normal band ordering. For all the cases when two branches of the spectrum arise due to spin-orbit splitting, the mutual arrangement of the antinodes of the Shubnikov-de Haas oscillations and the maxima of MISO occurs opposite to that observed in double quantum wells and in wide quantum wells with two subbands occupied and does not agree with the theoretical predictions. A "toy" model is supposed that explain qualitatively this unusual result.

In-plane anisotropy of the hole g factor in CdTe/(Cd,Mg)Te quantum wells studied by spin-dependent photon echoes

We use the two-pulse spin-dependent photon echo technique to study the in-plane hole spin anisotropy in a 20~nm-thick CdTe/Cd$_{0.76}$Mg$_{0.24}$Te single quantum well by exciting the donor-bound exciton resonance. We take advantage of the photon echo sensitivity to the relative phase of the electron and hole spin precession and study various interactions contributing to the hole in-plane spin properties. The main contribution is found to arise from the crystal cubic symmetry described by the Luttinger parameter $q=0.095$, which is substantially larger than the one theoretically expected for CdTe or found in other quantum well structures. Another contribution is induced by the strain within the quantum well. These two contributions manifest as different harmonics of the spin precession frequencies in the photon echo experiment, when strength and orientation of the Voigt magnetic field are varied. The magnitude of the effective in-plane hole $g$ factor is found to vary in the range $|\tilde{g_h}|$=0.125--0.160 in the well plane.

Damping of Rabi oscillations in intensity-dependent photon echoes from exciton complexes in a CdTe/(Cd,Mg)Te single quantum well

We study Rabi oscillations detected in the coherent optical response from various exciton complexes in a 20~nm-thick CdTe/(Cd,Mg)Te quantum well using time-resolved photon echoes. In order to evaluate the role of exciton localization and inhomogeneous broadening we use selective excitation with spectrally narrow ps-pulses. We demonstrate that the transient profile of the photon echo from the localized trion (X$^-$) and the donor-bound exciton (D$^0$X) transitions strongly depends on the strength of the first pulse. It acquires a non-Gaussian shape and experiences significant advancement for pulse areas larger than $\pi$ due to non-negligible inhomogeneity-induced dephasing of the oscillators during the optical excitation. Next, we observe that an increase of the area of either the first (excitation) or the second (rephasing) pulse leads to a significant damping of the photon echo signal, which is strongest for the neutral excitons and less pronounced for the donor-bound exciton complex (D$^0$X). The measurements are analyzed using a theoretical model based on the optical Bloch equations which accounts for the inhomogeneity of optical transitions in order to reproduce the complex shape of the photon echo transients. In addition, the spreading of Rabi frequencies within the ensemble due to the spatial variation of the intensity of the focused Gaussian beams and excitation-induced dephasing are required to explain the fading and damping of Rabi oscillations. By analyzing the results of the simulation for the X$^-$ and the D$^0$X complexes we are able to establish a correlation between the degree of localization and the transition dipole moments determined as $\mu($X$^-$)=73~D and $\mu($D$^0$X)=58~D.

Optical detection of anisotropic g-factor and nuclear spin polarization in a single CdTe quantum well

Longitudinal and in-plane electron g-factors, and a nuclear spin polarization (NSP) have been evaluated precisely in a CdTe/Cd0.85Mg0.15Te single quantum well by using the time-resolved Kerr rotation and double lock-in detection techniques. Resident electron spin polarization (RESP) was formed via the negative trion formation and recombination, and RESP gave rise to NSP in an oblique magnetic field configuration. We observed the effective nuclear field of a few mT which was weak compared with that in III–V semiconductor nanostructures as expected, but the nuclear field can be converted to the maximal NSP of 12% in Faraday geometry.

Electron spin dephasing in n‐doped CdTe/(Cd, Mg)Te quantum wells

AbstractWe measure the spin dephasing time, T *2, of electrons in CdTe/(Cd, Mg)Te single quantum wells with various levels of n‐doping using the time‐resolved Kerr rotation technique. The formation of negatively charged trions causes strong spin polarization of the two‐dimensional electron gas (2DEG). We find the maximum spin dephasing time in the quantum well with a 2DEG density of 8 × 1010 cm−2. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

MAGNETO-PHOTOLUMINESCENCE STUDY AT A FRACTIONAL QUANTUM HALL REGIME OF CHARGED EXCITONS IN A DILUTE MAGNETIC SEMICONDUCTOR

We have performed magneto-photoluminescence measurements on modulation doped n-type Cd 1-x Mn x Te / Cd 1-y Mg y Te single quantum wells. In low magnetic fields (the electrons filling factor ν>1), we observed several photoluminescence lines associated with spin-singlet and spin-triplet charged excitons. In high magnetic fields (ν<1), we observed only one of the spin-triplet charged excitons that are called as a bright type due to characteristic spin states of electrons and holes in dilute magnetic semiconductor quantum wells, where the photoluminescence intensity increased at a fractional value of [Formula: see text]. We consider that the screening effect plays a crucial role for this behavior in high magnetic fields.

Spin sensitive dynamics of a charged exciton in magnetic fields

We have performed magneto-photoluminescence measurements on modulation doped n-type Cd 1− x Mn x Te/Cd 1− y Mg y Te single quantum wells. We observed several transitions arising from spin-singlet and spin-triplet charged excitons. The transient photoluminescence data showed characteristic decay time depending on the spin configuration of the charged excitons. The spin-triplet charged excitons of a bright type are known to realize due to characteristic spin states of electrons and holes in dilute magnetic semiconductor quantum wells. We have also found that the decay-time of charged excitons depends much on photon energies used for excitation.

Fine structure of photoluminescence spectra in a modulation-doped n-CdTe/(Cd,Mg,Mn)Te quantum well

Spin-polarized photoluminescence spectra in tilted fields were measured to 60 T at 1.5 K in a modulation-doped n-CdTe/(Cd,Mg,Mn)Te single quantum well with the thickness of 10 nm and the sheet electron density of 5.7×10 11 cm −2 . A new σ −-polarized emission peak emerged on the higher energy side of a main peak in the σ − polarization around the Landau filling factor ν of 1. Magnetic field dependence of the main peak energy exhibited a novel two-step like behavior around ν of 0.9 and that of the peak intensity was observed to take local maximum there. With increasing the field angle, these features of the main peak disappeared. Temperature dependence of the profiles of these emission peaks was also investigated to 40 T between 1.3 and 20 K . Relative intensity of the new emission peak to the main one was found to decrease with increasing temperature. Taking into consideration that a larger Zeeman gap should be unfavorable to a spin-singlet charged exciton (X −) and degradation of quantum limit by thermal excitation could be unfavorable to a spin-triplet X −, the new peak is assigned to the spin-triplet X − and the main peak to the spin-singlet X −.

Excitons diffusion in (Cd,Mn)Te-based two-dimensional electron system under magnetic fields

Spatial diffusion of excitons in n-type modulation-doped (Cd,Mn)Te/(Cd,Mg)Te single quantum wells (SQWs) was studied through time- and spatially resolved photoluminescence (PL) measurements at 4.2 K under magnetic fields up to 0.5 T . The PL spatial extent exceeded the laser spot in all samples studied here, however, the detailed manners were strongly dependent upon the intrinsic 2D carrier density. We found the PL spatial extent got larger with the increase of magnetic fields in the case of the lower carrier density sample. It is the opposite result to the case of non-magnetic SQW. These phenomena are qualitatively interpreted by considering the competition of mobile neutral exciton (X), localized negatively- charged exciton (X −) and, furthermore, the suppression of a free-exciton-magnetic-polaron formation which is one of the characteristic properties of diluted magnetic semiconductor such as (Cd,Mn)Te.

Spatial diffusion of excitons in n-type modulation-doped (Cd,Mn)Te/(Cd,Mg)Te single quantum wells under magnetic fields

Time-resolved and spatially resolved photoluminescence (PL) measurements were performed on the (Cd,Mn)Te-based two-dimensional (2D) electron gas system at 4.2 K under magnetic fields up to 0.5 T. The detailed characteristics of the PL spatial extent under these magnetic fields were strongly dependent upon the intrinsic 2D carrier density. We found that, in the lower carrier density sample, the PL spatial extent increased with an increase in the magnetic fields. This is contrary to the result for the nonmagnetic system. These phenomena are qualitatively interpreted by considering the competition of the mobile neutral exciton (X), localized negatively-charged exciton (X−) and, furthermore, the suppression of a free-exciton-magnetic-polaron formation, which is one of the characteristic properties of diluted magnetic semiconductors such as (Cd,Mn)Te.

Magneto-Photoluminescence and Transport Measurements on Modulation-Doped Cd1?xMnxTe/Cd1?yMgyTe Single Quantum Wells

Magneto-photoluminescence (PL) and transport measurements were performed simultaneously on n-type modulation-doped Cd 1-x Mn x Te/Cd 1-y Mg y Te single quantum wells with a Mn concentration x = 0.05 and 2D carrier concentrations in the range of (1.8-4.6) × 10 11 cm -2 . The simultaneous measurements made it possible to determine the field positions of the integer filling indices without any ambiguity. In magneto-PL measurements, it was found that the main PL line, corresponding to the transition between the ground Landau levels, skipped to the higher energy side at the field of the filling index v = 1. Further a kink of the PL line was observed between v = 1 and 2, which is considered to correspond to the switch of the Fermi level between the opposite spin states within the same filling index.

Spectral Anomalies of Exciton Photoluminescence at ? = 1 and 2/3 in a Modulation-Doped n-Type CdTe/(Cd, Mg, Mn)Te Single Quantum Well

Spin-polarized magnetophotoluminescence measurements have been conducted in a modulation-doped n-type CdTe/(Cd, Mg, Mn)Te single quantum well at 1.5 K under 2 s long pulsed magnetic fields up to 60 T. For the σ— polarization, a new peak was observed to emerge at ν ∼ 1 on the higher energy side of a main peak. The former peak became larger than the latter with increasing magnetic field, while the latter peak had a local maximum at ν ∼ 0.9 and attenuated. Furthermore, magnetic field dependence of the latter peak energy showed step-like behavior around ν ∼ 0.9. The intensity of the former peak was observed to increase monotonically with field, showed a kink at ν ∼ 2/3 and saturated, i.e. showed a shoulder-like structure there. With changing the angle between the direction of the sample surface normal and the magnetic field, both features at ν ∼ 1 and 2/3 exhibited tendencies to fade out, which suggests that the former is associated with spin-singlet charged exciton and the latter with fractional quantum Hall effect at ν = 2/3.

Magnetoresistance of two-dimensional electrons in modulation-doped Cd 1− xMn xTe/Cd 1− yMg yTe SQWs: the variation with carrier concentration

The magneto-transport properties of two-dimensional electron gas (2DEG) of n-type modulation-doped Cd 1− x Mn x Te/Cd 1− y Mg y Te single quantum wells were investigated under magnetic fields up to 7 T applied perpendicular to the 2D plane at 2 K . The 2D carrier concentration was varied in the range of 1.8– 4.6×10 11 cm −2 . Apart from the quantum behavior in the magneto- and Hall resistance, a positive and successively negative magnetoresistance (MR) appeared at low magnetic fields. This effect was not observed in non-magnetic 2DEG. This peculiar MR is interpreted as caused by the enhanced spin-splitting in magnetic 2DEG. The appearance of the positive and negative MR was more distinct in a sample with lower carrier concentrations. The positive MR was analyzed on the basis of the theoretical model of the quantum correction to the conductivity in the disordered system.

Inhomogeneous broadening of exciton lines in magneto-optical reflection from CdTe/CdMgTe quantum wells

An approach is proposed to determine parameters of homogeneous and inhomogeneous broadening for reflection lines of quasi-two-dimensional excitons in semiconductor quantum well structures. A phenomenological model is developed taking into account fluctuations of the exciton energy which are expected to cause inhomogeneous broadening of exciton reflectivity lines. This concept is applied to magneto-optical studies of CdTe/(Cd,Mg)Te single quantum well heterostructures containing a two-dimensional electron gas. By the analysis of excitonic reflection spectra in magnetic fields the parameters of homogeneous and inhomogeneous broadening of the exciton as well as their temperature dependencies were obtained.

Excitonic properties of Cd 1− xMn xTe quantum wells grown by molecular beam epitaxy

Time-resolved photoluminescence has been studied in molecular beam epitaxy grown Cd 1− x Mn x Te/Cd 1− y Mg y Te and CdTe/Cd 1− x Mn x Te single and multiple quantum wells. The excitonic properties were investigated at several temperatures and magnetic fields. The dynamics of the exciton magnetic polaron are greatly influenced by the quantum confinement of the excitons in the single quantum wells. The formation of a spin superlattice structure was realized in Cd 0.95Mn 0.05Te/Cd 0.95Mg 0.05Te multi-quantum wells under an external magnetic field.

Magneto-transport of two-dimensional electron system in magnetic semiconductors heterostructure Cd(Mn)Te/(Cd,Mg)Te

We have grown n-type modulation-doped Cd(Mn)Te/(Cd,Mg)Te single quantum wells (SQWs) by molecular beam epitaxy (MBE). The properties of two-dimensional electron gas in obtained samples are the following; the sheet carrier density was about 3×1011cm−2 and the Hall mobility of CdTe SQW was 6×104cm2/Vs at 2K, but that of Cd1−xMnxTe SQWs with x=0.01–0.03 was decreased by about one order of magnitude. In the magnetotransport measurements up to 14T at 2K, typical features of the integer quantum Hall effect (IQHE), that is, zeros of magnetoresistance (MR) and quantized plateaux of the Hall resistance, were observed in a CdTe SQW sample. In Cd1−xMnxTe SQWs, on the other hand, the oscillation of MR associated with the transition between spin-split levels was observed at lower fields than the non-magnetic CdTe SQW, which reflected enhanced Zeeman splitting due to the exchange interaction with Mn2+ spins. Further, large positive and negative MR at low fields was observed in Cd1−xMnxTe SQWs, both of which are also attributed to the exchange-enhanced spin splitting.

Resonant coupling of cyclotron and spin resonances in modulation-doped CdMnTe/CdMgTe quantum well structure

The far-infrared magneto-transmission measurements have been carried out on a modulation-doped Cd0.98Mn0.02Te/Cd0.89Mg0.11Te single quantum well. The well width was 100Å and the concentration of two-dimensional electrons in the well was 3×1011cm−2. At magnetic fields lower than 7T and higher than 13T we observed a single electron cyclotron resonance (CR) line. Between 7 and 13T another absorption line appears in the spectra. These two lines exhibit an anti-crossing behavior with an energy gap of 2.5cm−1 at 8T. We assign the new line to the spin-flip transitions between strongly spin-split (0,↓) and (0,↑) Landau levels. The spin-flip resonance (SFR) becomes observable in the experiment due to the resonant coupling between (0,↑) and (1,↓) Landau levels (LLs) as a result of which near the CR–SFR crossing the original magneto-dipole SFR becomes electric-dipole transition. We describe the data by a theoretical model proposed by Fal’ko taking into account the giant spin-splitting present in the CdMnTe quantum well. According to the model, the resonant coupling between the two LLs originates from the spin-orbit interaction. The observation illustrates that the spin-orbit interaction plays an important role in this 2D system.

Pressure effect on the exchange interaction in the interface region of a CdTe/CdMnTe quantum-well structure

Exchange interaction of an exciton in a quantum well with magnetic ions in the interface region and pressure effect on the interaction are investigated for a 1.9 nm thick CdTe/Cd 1− x Mn x Te ( x=0.24) single quantum well (SQW). Photoluminescence has been measured at 4 K under magnetic fields to 60 T generated by a motor-generator driven long-pulsed magnet and hydrostatic pressures to 2.45 GPa produced in a diamond anvil cell (DAC) with a plastic body. An additional red shift is observed in a Zeeman shift for the SQW above 30 T. This structure has enabled us to evaluate the above-mentioned interaction quite accurately and to find that pressure enhances it.

Semimagnetic (Cd,Mn)Te single quantum dots — technological access and optical spectroscopy

An alternative technological approach for the realization of (Cd,Mn)Te nanostructures is presented. Focused ion beam lithography and subsequent thermal annealing are used for a lateral modulation of the bandgap, allowing both, the realization of quantum wires and quantum dots with adjustable size and a well-defined lateral potential. Due to the high quantum efficiency of the buried nanostructures, photoluminescence spectroscopy even on single quantum dots becomes accessible, allowing an experimental insight on optical transitions in three-dimensionally confined semimagnetic structures.