Yoshikazu Terai

ZERO-DIMENSIONAL EXCITONIC PROPERTIES OF SELF-ORGANIZED QUANTUM DOTS OF CDTE GROWN BY MOLECULAR BEAM EPITAXY

The successful growth of self-organized quantum dots (QDs) of CdTe on ZnTe (100) surface by molecular beam epitaxy is reported. Atomic force microscope measurements on the uncapped samples revealed the formation of CdTe QDs with typical dot diameters of 20±2 nm and heights of 2.7±0.3 nm at 3.5-ML-thick CdTe deposited. The intensity of photoluminescence (PL) from the capped QDs was higher than CdTe/ZnTe single quantum wells (SQWs) by a few orders of magnitude at 4.2 K, and exhibited a thermal quenching with an activation energy of 110 meV, which is about twice as large as those in SQWs. In time-resolved PL measurements, the decay time was almost independent of temperature below 20 K. This is interpreted as due to the zero-dimensional excitonic properties in QDs.

Self-organized formation and photoluminescence of Cd1−xMnxTe quantum dots grown on ZnTe by atomic layer epitaxy

Self-organized quantum dots (QDs) of Cd1−xMnxTe were grown on the ZnTe(100) surface by atomic layer epitaxy. Atomic force microscope measurement on the surface of a 3.5-ML-thick Cd1−xMnxTe layer revealed dot formation in a high density of the order of 1010–1011 cm−2 when the Mn composition x was less than 10%. The typical dot size was given approximately by 20 nm in diameter and 2 nm in height. In photoluminescence (PL) measurements on the capped QDs, the excitonic emissions from the QDs were observed at 2.16–2.27 eV in the range of Mn composition x=0%–10%. The PL spectra from Cd1−xMnxTe QDs with x=0.6%–10.2% consisted of two lines separated by about 20 meV. The dependence of the PL energies on the Mn composition and the Zeeman shift were compared with the calculation.

Micro photoluminescence spectra of CdTe and CdMnTe self-organized quantum dots

Abstract We report on micro photoluminescence (μ-PL) experiments in CdTe and CdMnTe quantum dots (QDs). PL spectra of the CdTe QDs indicate an ensemble of discrete narrow lines, each of which arises from a single dot emission. High emission efficiency of the present QDs allows the time resolving of the μ-PL signal. Dependence of the PL decay time on the QD size shows the evidence for the quantum size effect of the radiative recombination process. For CdMnTe QDs, it is found that the discrete spectral feature disappears in the μ-PL spectrum, reflecting the fast relaxation kinetics of excitons in the manganese doped QDs.

Self-organized quantum dots of zinc-blende MnTe grown by molecular beam epitaxy

Abstract The successful growth by molecular beam epitaxy (MBE) of self-organized quantum dots (QDs) of zinc-blende (ZB) MnTe onto CdTe(1 0 0) is reported. Mn and Te fluxes were supplied either simultaneously (conventional MBE) or alternately (atomic layer epitaxy — ALE) onto the surface to form MnTe. Atomic-force microscope measurements for uncapped samples revealed that cone-shaped dots were formed in both of the two growth methods, however with different sizes. The diameter and the height are, respectively, given typically by D = 140 ± 10 nm and h = 70 ± 10 nm in QDs grown by conventional MBE, and by D = 60 ± 5 nm and h = 8 ± 1 nm in QDs grown by ALE. In photoluminescence (PL) and time-resolved PL measurements, a broad emission band due to the Mn 2+ d—d transition was observed at 2.02 eV. Its decay time was found to be of the order of several tens to hundreds of nanoseconds, which is shorter than those from ZBMnTe films by two or three orders.

Highest-order optical phonon-mediated relaxation in CdTe/ZnTe quantum dots

Abstract The highest 19th-order longitudinal optical (LO) phonon-mediated relaxation was observed in photoluminescence excitation spectra of CdTe self-assembled quantum dots grown in ZnTe. Hot excitons photoexcited highly in the ZnTe barrier layer are relaxed into the wetting-layer state by emitting multiple LO phonons of the barrier layer successively. Below the wetting-layer state, the LO phonons involved in the relaxation are transformed to those of interfacial Zn x Cd 1− x Te surrounding CdTe quantum dots. The ZnTe-like and CdTe-like LO phonons of Zn x Cd 1− x Te and lastly acoustic phonons are emitted in the relaxation into the CdTe dots. The observed main relaxation is the fast relaxation directly into CdTe quantum dots and is not the relaxation through either the wetting-layer quantum well or the band bottom of the ZnTe barrier layer. This observation shows very efficient optical phonon-mediated relaxation of hot excitons excited highly in the ZnTe conduction band through not only the ZnTe extended state but also localized state in the CdTe quantum dots reflecting strong exciton–LO phonon interaction of telluride compounds.

Self-organized quantum dots of growth on Zn(Cd)Te surface and magneto-photoluminescence

Self-organized dots of CdTe and Cd1−xMnxTe (x=0:06) were fabricated on the Zn1−yCdyTe (1 0 0) surface by MBE. The dot formation on ZnTe was not di7erent between CdTe and Cd1−xMnxTe but di7erent features appeared with the increase of y. In CdTe dots, the density was decreased continuously with y and the size was increased, in qualitative accordance with the decrease of the lattice mismatch ratio between CdTe and Zn1−yCdyTe. On the other hand, the dot formation of Cd1−xMnxTe dots was little dependent on y; the density was almost constant and the size was increased to only a little extent. In photoluminescence (PL) measurements on capped dots, the energies of the excitonic emission were red-shifted with y in both CdTe and Cd1−xMnxTe dots. The PL spectra from Cd1−xMnxTe quantum dots showed well-de=ned splitting into two lines at low temperatures less than 10 K and it was found that the Zeeman shift of the two lines was much di7erent. c

Magneto-photoluminescence from self-organized quantum dots of Cd1−xMnxTe grown by molecular beam epitaxy

Photoluminescence (PL) studies of self-organized quantum dots (QDs) of Cd 1-x Mn x Te under magnetic fields were reported. Cd 1-x Mn x Te QDs were grown successfully in the composition range of x = 0-10% on the ZnTe (100) surface by MBE. The PL spectra from capped Cd 1 -x Mn x Te QDs were composed of two lines with a separation of about 20meV at low temperatures. In the dependence on the temperature and magnetic field, the PL line in the lower-energy side exhibited peculiar behaviors, compared to that in the higher-energy side; with increasing temperature, the intensity of the lower-energy line was reduced rapidly and almost disappeared above 20 K, and Zeeman shift under magnetic fields was smaller and the degree of the circular polarization was larger and saturated at low magnetic fields. These features of the lower-energy line were discussed in relation to excitonic magnetic polaron in QDs.

Growth of self-organized dots of Cd1−xMnxTe on ZnTe by atomic layer epitaxy

Self-organized quantum dots (QDs) of Cd 1-x Mn x Te were successfully grown on the ZnTe(1 0 0) surface by atomic layer epitaxy. Atomic force microscope (AFM) measurement revealed the dot formation in a high density on the surface of 3.5-monolayer-thick Cd 1 x Mn x Te when the Mn composition x was less than 10%. The typical dot size was 20 nm in diameter and 2 nm in height. The density of dots ranged from 4 × 10 10 to 10 11 cm - 2 for x = 0-10%, but it decreased abruptly when x was more than 10%. In photoluminescence (PL) measurements for capped QDs, excitonic luminescence from QDs was observed at 2.16-2.27 eV depending on the Mn composition. One of the features of PL spectra from QDs containing Mn is that the spectra are composed of two PL lines with a separation of 20-26 meV.

Spin relaxation in CdTe/ZnTe quantum dots

We have measured photoluminescence (PL) spectra and time‐resolved PL in CdTe quantum dots (QDs) under the longitudinal magnetic field up to 10T. Circular polarization of PL increases with increasing magnetic field, while its linear polarization is absent under linearly polarized excitation. Time‐resolved PL measurements clarified that this behavior is caused by the suppression of spin relaxation induced by the longitudinal magnetic field. We believe that this behavior is related to the hyperfine interaction of electron spin with magnetic momenta of lattice nuclei.

Spin–Lattice Relaxation of Mn Ions in Nanostructures with Semiconductor Quantum Dots

We use the combination of nonequilibrium phonon and exciton luminescence techniques to study the spin dynamics in diluted magnetic semiconductor structures with (Cd,Mn)Te and (Cd,Mn)Se quantum dots (QDs). We show that the spin–lattice relaxation (SLR) of Mn ions in these structures differs strongly from the SLR in quantum wells. We explain the results by a model where SLR process in structures with QDs is modified by the spin diffusion on Mn ions from the QD to a wetting layer.