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We demonstrate a new method of measuring the exciton spin relaxation time in semiconductor nanostructures by continuous-wave photoluminescence. We find that for self-assembled CdTe quantum dots the degree of circular polarization of emission is larger when exciting polarized excitons into the lower energy spin state than in the case when the excitons are excited into the higher energy spin state. A simple rate equation model gives the exciton spin relaxation time in CdTe quantum dots equal to 4.8+/-0.3 ns, significantly longer than the quantum dot exciton recombination time 300 ps., Comment: 11 pages, 3 figures

We show that CdMnTe self-assembled quantum dots can be formed by depositing a submonolayer of Mn ions over a ZnTe surface prior to deposition of the CdTe dot layer. Single dot emission lines and strongly polarized quantum dot photoluminescence in an applied magnetic field confirm the presence of Mn in individual quantum dots. The width of PL lines of the single CdMnTe dots is 3 meV due to magnetic moment fluctuations of the Mn ions. After rapid thermal annealing, the emission lines of individual magnetic quantum dots narrow significantly to 0.25 meV showing that effect of magnetic fluctuations is strongly reduced most probably due to an increase in the average quantum dot size. These results suggest a way to tune the spin properties of magnetic quantum dots., Comment: 13 pages, 4 figures

We report on the influence of postgrowth thermal annealing on the optical properties of CdTe/ZnTe self-assembled quantum dots (SAQDs). Ultrasharp emission lines in the micro-photoluminescence spectra demonstrate the continued presence of quantum dots (QDs) after annealing. Upon annealing, the emission from CdTe QDs shifts toward higher energies with a narrowing of the photoluminescence line of QDs ensemble. In addition, the decay time of the excitonic recombination decreases from 300 ps to 200 ps. These results indicate that interdiffusion of Zn and Cd into and out of the QDs leads to an increase of the average dot size and simultaneously a decrease in the confining potentials of the QD. The absence of a wetting layer makes the II–VI CdTe SAQDs much more sensitive to the thermal treatment than the III–V InAs-based dots.

We use micro- and nano-photoluminescence to study the temperature-dependent excitonic emission from CdSe quantum dots embedded in a ZnSe matrix. By varying the spatial resolution from 200 nm to 1.7 μm, we are able to study the temperature dependence of the ultranarrow (∼200 μeV) emission from excitons confined to single quantum dots, as well as statistical ensembles of up to 200 dots. By measuring the quenching of the photoluminescence (PL) with temperature, we find compelling evidence that the PL emission from these samples results from two different kinds of states. Similar to previous work, we find that a broad PL line persists to 300 K with an activation energy of ∼40 meV. However, we find that the ultranarrow lines are quenched at about 60 K, indicating an effective activation energy of only 4.0 meV.

We have investigated the evolution of molecular beam epitaxy (MBE)-grown, CdSe self-assembled quantum dots on ZnSe surfaces using microphotoluminescence techniques. Bare CdSe dots at room temperature undergo Ostwald ripening over a time scale measured in days. At the elevated temperatures maintained for MBE growth and dot formation, ripening is expected to progress much faster. Capping the dots with a thin ZnSe layer “freezes” the ripening, allowing one to sample different stages of the dot evolution and subsequent characterization. We have grown eleven samples, each with a different time interval, or growth interruption, between dot formation and capping; the growth interruption times ranging from 0 to 300 s, during which the samples were kept at 300 °C. Using microphotoluminescence spectroscopy, we have resolved the sharp emission peaks due to individual dots in each sample and, by analyzing the ensemble characteristics, have identified a new regime in the evolution of CdSe dots.

Exciton resonances are observed in photocurrent spectra of 80 nm wurtzite InP nanowire devices at low temperatures, which correspond to transitions between the A, B, and C valence bands and the lower conduction band. Photocurrent spectra for 30 nm WZ nanowires exhibit shifts of the exciton resonances to higher energy, which are consistent with finite element calculations of wavefunctions of the confined electrons and holes for the various bands.

Recently, the first controlled n‐type doping of Zn1−xMnxSe epilayers was reported. Using time‐resolved photoluminescence we show that exciton ionization in the moderately doped samples occurs at a significantly lower temperature (50 K) than in undoped material (180 K). This behavior is consistent with Mott ionization of excitons from the increased majority carrier density at higher temperatures. The density and temperature at which the ionization occurs are estimated using calculations of the Mott density and the density‐dependent exciton binding energy. No excitons are observed down to 2 K in the samples, which were doped at the highest densities, suggesting that these samples were degenerately doped.

We show that the radiative efficiencies and lifetimes of photoexcited carriers in epitaxial GaAs may both be enhanced by three to four orders of magnitude by the preparation of thin n+, doped layers at the surface and substrate interfaces. Samples were prepared by organometallic vapor phase epitaxy with n− region (∼1015 cm−3) thicknesses of 3–10 μm and n+ regions Si doped to concentrations of 5×1018 cm−3. Time‐resolved luminescence in such structures (25–300 K), under both surface and bulk (near‐band‐ edge) excitation conditions, reveals band‐to‐band‐dominated recombination spectra, with carrier lifetimes ranging from a minimum of 150 ns at 50 K to 800 ns at room temperature. This is in sharp contrast to the subnanosecond lifetimes and poor raditive efficiencies typical in conventionally prepared, bulk or epitaxial GaAs, but is comparable to the best reported for high‐purity LPE GaAs/AlxGa1−xAs double heterostructures.

We acknowledge the financial support of the National Science Foundation through grants DMR-0806700, 0806572, 1105362, 1105121, and ECCS-1100489, and the Australian Research Council.

Photocurrent spectroscopy has been carried out on single CdS nanosheet devices in the metal-semiconductor-metal configuration with both Schottky and Ohmic contacts. Spatial imaging of the photocurrent shows that the photosensitive regions are localized at the reverse biased contact for Schottky type contacts and uniformly distributed throughout the nanosheet for Ohmic contacts. Photocurrent spectra show excitonic resonances at low temperatures corresponding to the A, B, C hole bands. Subband gap pulsed laser excitation reveals two-photon absorption dominated photocurrents consistent with a nonlinear coefficient of β=2 cm/GW for these nanosheet devices.

We use time-resolved photoluminescence spectroscopy and photoluminescence excitation spectroscopy to measure the valence band parameters of hexagonal wurtzite InP nanowires. The A exciton emission and excitation energy is observed at 1.504 eV as expected. Excitation spectra show that the B and C hole bands are 30 and 161 meV above the A hole band. From these measurements, we obtain the crystal field and spin-orbit energies of 52 meV and 139 meV, respectively.

Spatially resolved Raman scattering is used to probe the strain distribution in CdS nanosheets. We observe both significant strains and a significant strain gradient across the nanosheets. The magnitude of the strain suggests that electronic properties of the CdS nanosheets will be influenced as well. Using spatially resolved photoluminescence spectroscopy, we show that the band gap of the nanosheet experiences changes in the energy gap of 20 meV across the width of the nanosheet, which are consistent with our observations of the strain.

The InP nanowires are grown by vapor-liquid-solid growth catalyzed by 20 and 50 nm gold nanoparticles on an InP substrate at 400 C using metal-organic chemical-vapor deposition. The 50 nm nanowires were grown with a V/III ratio of 350, while the 20 nm nanowires were grown at a V/III ratio of 700. 9 Extensive cw and time-resolved PL measurements at low temperatures have shown that the 20 nm wires are predominantly WZ, while the 50 nm nanowires are predominantly ZB, with some twins and stacking faults present as determined by high-resolution transmission electron microscopy imaging. 3,4,9 To obtain individual nano

We study the magnetization dynamics in CdMnTe quantum dots using subwavelength optical microscopy imaging at B=0 T. For continuous laser illumination each dot exhibits strong and unique circular polarization despite completely unpolarized ensemble emission. This implies that after an exciton recombines, the spontaneous ferromagnetic alignment of magnetic impurities persists for over 100 μs, which is a million times longer than in CdMnTe quantum wells. The spin memory effect points toward a qualitatively different picture of magnetization dynamics in the zero-dimensional limit.

We have utilized polarized low temperature photoluminescence (PL) to probe the electronic states and structural symmetries of individual CdS nanosheets. High resolution transmission electron microscopy measurements indicate highly crystalline material with different nanosheets exhibiting significant variations of the direction of the c axis, which are consistent with polarization measurements of PL from single CdS nanosheets. The quality of the nanosheets is reflected in measurements of exciton lifetimes of ∼200ps, a value significantly longer than observed for CdS nanowires whose diameter is the same as the thickness of these nanosheets, but shorter than that observed in bulk crystals.

We have utilized spatially resolved low temperature photoluminescence to probe the electronic states and structural symmetries of individual single crystalline CdS nanosheets. Spatially resolved photoluminescence imaging of a single CdS nanosheet reveals a distinctive spectral variation across the nanosheet. We observe A-like exciton states which emit most strongly near the outer edges of the 5μm wide nanosheet, while B-like exciton states emit most strongly along the center region of the 30μm long axis of the nanosheet.

We use polarization-resolved and temperature-dependent photoluminescence of single zincblende (ZB) (cubic) and wurtzite (WZ) (hexagonal) InP nanowires to probe differences in selection rules and bandgaps between these two semiconductor nanostructures. The WZ nanowires exhibit a bandgap 80 meV higher in energy than the ZB nanowires. The temperature dependence of the PL is similar but not identical for the WZ and ZB nanowires. We find that ZB nanowires exhibit strong polarization parallel to the nanowire axis, while the WZ nanowires exhibit polarized emission perpendicular to the nanowire axis. This behavior is interpreted in terms of the different selection rules for WZ and ZB crystal structures. © 2007 American Institute of Physics.

Temperature dependent photoluminescence (PL) is used to study the electronic properties of single CdS nanowires. At low temperatures, both near-band edge (NBE) photoluminescence (PL) and spatially-localized defect-related PL are observed in many nanowires. The intensity of the defect states is a sensitive tool to judge the character and structural uniformity of nanowires. As the temperature is raised, the defect states rapidly quench at varying rates leaving the NBE PL which dominates up to room temperature. All PL lines from nanowires follow closely the temperature-dependent band edge, similar to that observed in bulk CdS., Comment: 11 pages, 4 figures

Resonant Raman scattering (RRS) was used to probe the electronic states of CdS nanowires (∼10nm diameter) grown by chemical vapor deposition. Individual Ar+ laser lines were used to vary the excitation energy while observing scattering from CdS phonons; strong 1-longitudinal optical (LO) and 2-LO Raman resonances were readily observed within the broader photoluminescence emission. The energy separation between the peaks of the 1-LO and 2-LO resonance was found to be 34 meV, in good agreement with bulk values. These results demonstrate that RRS is a powerful tool for probing the electronic and vibrational properties of semiconductor nanowires.

Resonant Raman scattering is a sensitive means to probe electronic states in semiconductor materials even when such states are not accessible either through photoluminescence or transport techniques. We show that resonant Raman scattering can be used to probe the excited state distribution of CdTe∕ZnTe quantum dots at room temperature. In particular we apply this technique to study the changes of such quantum dots after rapid thermal annealing.

We report on low temperature polarization-resolved imaging of single magnetic self-assembled CdMnTe quantum dots (QDs) in the absence of magnetic field. Using longitudinal optical phonon-assisted absorption to photoexcite spin-polarized excitons into a QD ground state, we find that the magnetic impurities within CdMnTe QDs can be aligned ferromagnetically with a single emission lines exhibiting a circular polarization as large as 65%. These results demonstrate that the magnetization of a single magnetic QD can be optically controlled with a suitably polarized laser.

We report resonant photoluminescence from CdSe/ZnSe self-assembled quantum dots. When CdSe quantum dots are resonantly excited, excitonic sharp micro-photoluminescence peaks originating from individual quantum dots are strongly enhanced in the region corresponding to optical phonon energies below the excitation. The phonons active in this process are identified as the longitudinal optical (LO) phonons from the CdSe dots, as the interface phonons, and tentatively as the LO phonons from the two-dimensional-like precursor layers. These observations suggest that exciton recombination via phonons is a major relaxation process under resonant excitation.