We present an electrically driven single quantum dot emitter that is adapted for operation at room temperature. Epitaxially grown CdSe quantum dots were embedded between ZnSSe/MgS barriers optimized with respect to both, high quantum efficiency and efficient current injection at elevated temperatures. Most important, electroluminescence from one single quantum dot is observed even at room temperature with a surprisingly low driving voltage of 2.6 V. This might be a key step for a single photon emitter operating under ambient conditions.
We have studied the emission distributions in nonpolar α-plane GaN thick films grown by HVPE using different nucleation schemes. The emission spectra show in addition to the near band edge emission band, also defect related bands due to different structural defects being enhanced/reduced to different extent in samples grown on different templates. Spatially resolved cathodoluminescence imaging reveals the in-plane distributions of the respective emission bands, which allows us to correlate the emissions with particular stacking fault structural defects independently revealed by plan-view transmission electron microscopy. In addition, emission distributions were visualized in vicinity of largescale defects like surface triangle pits, depressions and cracks attributed to prevailing defect formation and/or impurity incorporation.
Infrared Spectroscopic Ellipsometry is presented as a feasible and novel technique for contactless and nondestructive measurement of free-carrier and crystal-structure properties in the characterization of complex semiconductor heterostructures for device applications. Infrared-active lattice modes and coupling of free-carrier plasmons to longitudinal-optical lattice phonon modes strongly affect the infrared-optical response of semiconductor materials. Analysis of ellipsometry data from 2 micrometers to 100 micrometers can provide precise information on phonon mode frequencies and broadening parameters, static dielectric constants, free-carrier concentration, and free-carrier mobility at optical frequencies of III-V compound semiconductors, even for films with thicknesses only a fraction of the probing wavelengths. Alloy composition, strain, crystal quality, and free-carrier properties of constituent layers in thin-film structures, designed for optoelectronic or electronic device applications, can be derived. We demonstrate the characterization of coherent and incoherent light emitter structures based on group-III-nitride materials, where information such as concentration and mobility of free carriers in n- and p-type regions, thickness, composition, and quality of device constituents are accessible.
The optimization of the active region of ZnSe-based laser diodes for laser emission around a wavelength of 560 nm is reported. Two concepts are presented: quaternary CdZnSSe quantum wells with a high Cd content and CdSe quantum dots. With quantum well devices output powers of more than 1 W are realized. The first electrically pumped CdSe quantum dot laser is presented. The electroluminescence dynamics under current injection of these devices allow lasing operation only in pulsed-mode with pulse widths around 50 ns. Although the threshold current density of the quantum dot laser is around 7 kA/cm(superscript 2, lasing operation can be sustained up to 20 kA/cm(superscript 2 indicating a so far unexpected stability of II-VI material against current injection.
Single epitaxially grown CdSe/ZnSe quantum dots have been studied by using photoluminescence spectroscopy with a high spatial resolution. The lifting of the spin degeneracy due to exchange interaction results in a splitting of the exciton ground state, strongly dependent on the symmetry of the quantum dot. By applying a magnetic field in Faraday geometry, the energy splitting as well as the polarization properties of the exciton transition can be varied and remarkably, even at high magnetic field a spin coherence time of about 3 ns is found, which exceeds the recombination lifetime of single excitons significantly. As the biexciton state is a spin singlet, both its fine structure splitting as well as its degree of polarization are shown to be controlled by the final state of recombination, the single exciton state. Besides the discrete energy splitting of optical transitons in single quantum dots, we observe a rather statistical, but strongly correlated energy shift was well as a correlated on-off switching behavior of the exciton and the biexciton emission on a typical time constant of seconds. These effects are related to the influence of charge carriers in the nanoenvironment of the dot and to thermal or Auger-driven carrier escape into trap states in the vicinity of the dot, respectively.
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