Per-Olof Holtz

Electronic states tuning of InAs self-assembled quantum dots

We demonstrate the dimensional tuning of InAs self-assembled quantum dots (QDs) by changing the growth kinetics during the capping of InAs islands with GaAs. Modifying the growth sequence during the capping of InAs islands, allows us to tune the thickness and lateral dimensions of the QDs while keeping the wetting layer thickness constant. Using the same method but embedding the tuned InAs islands into AlAs layers allows to further blueshift the photoluminescence emission to higher energies while keeping the wetting layer thickness constant. The main process responsible for the QDs size modification is consistent with a kinetically controlled materials redistribution of the InAs islands that minimizes the energy of the epitaxial layers at the start up of the GaAs capping deposition.

Surface recombination in ZnO nanorods grown by chemical bath deposition

Verticallywell-aligned ZnO nanorods on Si substrates were prepared by atwo-step chemical bath deposition (CBD) method. The optical properties ofthe grown ZnO nanorods were investigated by time reso ...

Photoluminescence up-conversion in InAs/GaAs self-assembled quantum dots

We report up-converted photoluminescence in a structure with InAs quantum dots embedded in GaAs. An efficient emission from the GaAs barrier is observed with resonant excitation of both the dots and the wetting layer. The intensity of the up-converted luminescence is found to increase superlinearly with the excitation density. The results suggest that the observed effect is due to a two-step two-photon absorption process involving quantum dot states.

OPTICAL PROPERTIES OF DOPED INGAN/GAN MULTIQUANTUM-WELL STRUCTURES

A systematic investigation of the photoluminescence spectra from InGaN/GaN multiquantum-well structures with different levels of Si doping in the quantum well has been carried out, in order to study the screening of the strain induced piezoelectric field and potential fluctuations. It is found that the emission energy strongly depends on the carrier concentration, originating from the doping or photogeneration. The observed strong shift with Si doping can only partly be explained by the screening of the piezoelectric field. The main shift is suggested to be related to the screening of the localization potentials.

Single Excitons in InGaN Quantum Dots on GaN Pyramid Arrays

Fabrication of single InGaN quantum dots (QDs) on top of GaN micropyramids is reported. The formation of single QDs is evidenced by showing single sub-millielectronvolt emission lines in microphotoluminescence (μPL) spectra. Tunable QD emission energy by varying the growth temperature of the InGaN layers is also demonstrated. From μPL, it is evident that the QDs are located in the apexes of the pyramids. The fact that the emission lines of the QDs are linear polarized in a preferred direction implies that the apexes induce unidirected anisotropic fields to the QDs. The single emission lines remain unchanged with increasing the excitation power and/or crystal temperature. An in-plane elongated QD forming a shallow potential with an equal number of trapped electrons and holes is proposed to explain the absence of other exciton complexes.

Time‐resolved spectroscopy of Zn‐ and Cd‐doped GaN

Photoluminescence transients and time‐resolved luminescence spectra are reported for the violet‐blue emissions from epitaxial layers of Zn‐ and Cd‐doped GaN. A typical decay time of 300 ns is reported for the blue GaN:Zn emission, peaking at about 2.89 eV. For GaN:Cd a somewhat longer decay time of τ≊1 μs dominates for the broad peak centered at ≊2.72 eV. In both cases it is concluded that the simple process involving a free‐to‐bound transition of an electron to a hole bound at the ZnGa, respectively, the CdGa acceptor is the dominating recombination mechanism corresponding to these characteristic decay rates.

Radiative recombination in In 0.15 Ga 0.85 N/GaN multiple quantum well structures

We present a study of the radiative recombination in In 0.15 Ga 0.85 N/GaN multiple quantum well samples, where the conditions of growth of the InGaN quantum layers were varied in terms of growth temperature ( 18 cm −3 in the well), with excitation intensity and with delay time after pulsed excitation (also shifts up to 0.2 eV). We suggest a two-dimensional model for electron- and donor screening in this case, which is in reasonable agreement with the observed data, if rather strong localization potentials of short range (of the order 100 A) are present. The possibility that excitons as well as shallow donors are impact ionized by electrons in the rather strong lateral potential fluctuations present at this In composition is discussed

Spatially direct and indirect transitions observed for Si/Ge quantum dots

The optical properties of Ge quantum dots embedded in Si were investigated by means of photoluminescence, with temperature and excitation power density as variable parameters. Two different types of recombination processes related to the Ge quantum dots were observed. A transfer from the spatially indirect to the spatially direct recombination in the type-II band lineup was observed with increasing temperature. A blueshift of the spatially indirect Ge quantum-dot-emission energy with increasing excitation power is ascribed to band bending at the type-II Si/Ge interface for high carrier densities. Comparative studies were performed on uncapped Ge dot structures.

Origin of photocurrent in lateral quantum dots-in-a-well infrared photodetectors

Interband and intersubband transitions of lateral InAs/In0.15Ga0.85As dots-in-a-well quantum dot infrared photodetectors were studied in order to determine the origin of the photocurrent. The main intersubband transition contributing to the photocurrent (PC) was associated with the quantum dot ground state to the quantum well excited state transition. By a comparison between intersubband PC measurements and the energy level scheme of the structure, as deduced from Fourier transform photoluminescence (FTPL) and FTPL excitation spectroscopies, the main transition contributing to the PC was identified. (c) 2006 American Institute of Physics.

Temperature influence on optical charging of self-assembled InAs'GaAs semiconductor quantum dots

It is demonstrated that the photoluminescence spectra of single self-assembled InAs/GaAs quantum dots are very sensitive to excitation energy and crystal temperature. This is qualitatively explained in terms of the effective diffusivity of photogenerated particles, which affects the capture probability of the quantum dot. As a consequence, this opens the possibility of controlling the average number of excess electrons in the quantum dot by optical means. This technique may be used as a simple tool to create and study charged exciton complexes without any specially fabricated samples.