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An image-processing method combined with Fourier analysis is introduced in this work as a rapid and highly-automated method of detecting rear contact voids in passivated emitter and rear contact cells (PERC). This approach utilizes photoluminescence (PL) imaging to locate the central point of the most detrimental type of voids and associate a void fraction to each cell in a manner of seconds. Acoustic microscopy and scanning electron microscopy are also used as complementary tools to confirm the presence of voids detection using PL imaging and provide more insight into the actual physical mechanism of what is observed in PL measurements.

An alternative method to image essential variables like dark saturation current density, series resistance, and cell efficiency is introduced in this work. This approach combines the terminally connected diode model with a modified technique for extracting the spatially resolved ideality factor of the cell under non-open-circuit conditions. Following a description of the steps governing the principles of this parameter imaging approach, we explore potential benefits of this technique and how it may improve upon existing methods.

The use of proper surface preparation and cleaning methods for Si wafers prior to the deposition of passivation layers is essential to minimize surface recombination and realize high efficiencies (> 20%) in crystalline Si photovoltaic cells. In this work, the influence of wafer cleaning on the quality of surface passivation achievable for boron-doped emitters was investigated, including the use of different combinations of HCl, HF, HNO3, and ozonated deionized water (DIO3). These different surface preparations and cleaning sequences were performed on undiffused and boron diffused n-type Cz Si wafers, followed by the deposition of either silicon nitride (SiNx) or an aluminum oxide film capped with SiNx (Al2O3/SiNx stack). Additionally, both planar and anisotropically textured wafers were used. Injection-level dependent photoconductance measurements and calibrated photoluminescence imaging were performed on symmetrical boron diffused samples based on the different cleaning processes and passivation materials described above. Additionally, non-contact corona-Kelvin measurements were used to extract the total charge and interface defect density at the Si surfaces. We found that cleaning variations strongly influence carrier lifetime for SiNx passivated Si, but the effect is less pronounced in the case of Al2O3/SiNx stacks. It was further observed that DIO3-last treatment resulted in higher lifetimes for the SiNx stacks. Overall, it emerged that the (DIO3 + HF + HCl → HF → DIO3) clean is a promising and potentially low cost cleaning sequence for the photovoltaics industry.

The development of ultrafast quantum efficiency measurements has made it possible to perform spatially resolved short-circuit current mapping on large area crystalline silicon solar cells. With the inclusion of concurrent diffuse reflectance measurements, detailed loss analysis is presented that identifies the impact and spatial nonuniformity of various current loss mechanisms. We measure p-type multicrystalline aluminum back surface field and p-type monocrystalline passivated emitter and rear cells, and investigate details of the spatial variation in specific device layers such as the antireflection coating, phosphorus diffused region, bulk, and rear surface. The results are compared with traditional photoluminescence imaging, and are found to provide a complementary dataset that provides a comprehensive picture of device performance. The insight provided from these techniques is intended to allow rapid feedback for quality control in manufacturing and accelerate the pace of process development in research environment.

Diamond wires from several manufacturers were investigated in term of their impact on wafer quality and cell performance. It was identified that under identical ingot sawing conditions the diamond wire make had an impact on the resulting cell performance. Several cells exhibited defects that remained with the cell even after the saw damage etching process. These defects were investigated in terms of there impact on various solar cell performance parameters. This analysis was performed using photoluminescence imaging and spatially resolved quantum efficiency and reflectance measurements. The diamond wire marks were observed to have the largest impact on the local short-circuit current density across the cell.

Metal induced recombination (i.e., contact recombination) is a significant loss mechanism for homojunction crystalline silicon solar cells. In this work, a method is proposed to extract the metal induced recombination parameters using high- resolution photoluminescence PL imaging. By calculating the voltage distribution profile between fingers, the total current flow from the semiconductor to the finger, per area, can be obtained. The total recombination current at the semiconductor area and the metal contact area are then calculated and used to calculate the reverse saturation current for each region. The metal induced reverse saturation current $(j_{0m})$ is then obtained. This measurement allows the extraction of $j_{0m}$ on finished solar cells without the need to make special metal test structures. This is a big advantage over existing methods.

This paper describes the characteristics of damage, introduced under different conditions of diamond wire sawing, on the Si wafer surfaces. The damage occurs in the form of frozen-in dislocations, phase changes, and microcracks. The in-depth damage was determined by conventional ways such as TEM, SEM and angle-polishing/defect-etching, which only provide local information. We have also applied a new technique based on sequential measurement of the minority carrier lifetime after etching thin layers from the surfaces to determine average damage depth and its in-depth distribution. The lateral spatial damage variations, which seem to be mainly related to wire reciprocation process, were observed by photoluminescence and lifetime mapping. Our results show a strong correlation of damage depth on the diamond grit size and wire usage.

ZnO films were grown on Zn-polar ZnO substrates with 0.5° miscut toward the [1EQ \O(1,¯)00] direction by plasma-assisted molecular beam epitaxy (PAMBE). An atomically flat surface with one or two monolayer step height along the [0001] direction was achieved at low growth temperature. Wet etching of the substrates and ozone exposure reduced defects and improved photoluminescence (PL) lifetime by a factor of two. A 20 min interval between oxygen plasma ignition and growth was found necessary for excellent crystallinity and yielded high PL lifetimes of 0.179 ns. By decreasing the growth temperature in the low temperature range, the growth rate was increased from 0.02 μm/h to 0.246 μm/h, and correspondingly PL lifetime was improved by an order of magnitude, from 0.018 ns to 0.3 ns.

ZnO thin films were epitaxially grown on Zn-polar (0 0 0 1) ZnO substrates by plasma-assisted molecular beam epitaxy at a low temperature of 610 °C. The influence of Zn/O ratio on the epitaxial growth of ZnO was investigated. By optimizing the Zn/O ratio, a surface root mean square roughness of 0.277 nm with one or two monolayer height steps was achieved at a low growth temperature by growing on ZnO substrates with 0.5° miscut angle toward the [ 1 1 ¯ 0 0 ] axis. Electrostatic ion trapping of the oxygen plasma was found crucial to deflect etching oxygen species at an oxygen flow rate of two standard cubic centimeters per minute, resulting in fewer surface defects and a longer photoluminescence (PL) lifetime of 217 ps. The impact of oxygen source gas purity was studied by high resolution X-ray diffraction and time-resolved photoluminescence (TRPL) measurements of resultant epilayers and higher gas purity was found necessary to reduce non-radiative centers.

In this study, silicon wafers were marked with four different kinds of laser marks. Initial characterization of the wafers was performed to measure various parameters such as minority lifetime, residual stress, wafer thickness, Photoluminescence, and Resonance Ultrasound Imaging. The wafers were consequently processed into solar cells. EL, PL and RUV measurements were also performed on solar cells, in addition to the flash I-V measurements to determine solar cell performance parameters. Cells were laminated into custom modules and the modules were passed through static and dynamic mechanical loading tests according to IEC 61215. A procedure to quantify the performance loss due to cell cracking for each cell was developed and applied for each cell that experienced significant cracking after mechanical loading tests. Following correlations were explored: (i) Propensity of power loss due to cell cracks and residual stress at wafer level (ii) Types of laser marks and their effects on parameters measured in various wafer and cell characterization techniques.

We report on magneto-photoluminescence studies of InAs/GaAs quantum dots (QDs) of considerably different densities, from dense ensembles down to individual dots. It is found that a magnetic field applied in Faraday geometry decreases the photoluminescence (PL) intensity of QD ensembles, which is not accompanied by the corresponding increase of PL signal of the wetting layer on which QDs are grown. The model suggested to explain these data assumes considerably different strengths of suppression of electron and hole fluxes by a magnetic field. This idea has been successfully checked in experiments on individual QDs, where the PL spectra allow to directly monitor the charge state of a QD and, hence, to conclude about relative magnitudes of electron and hole fluxes toward the QD. Comparative studies of different individual QDs have revealed that the internal electric field in the sample (which was altered in the experiments in a controllable way) together with an external magnetic field will determine the charge state and emission intensity of the QDs.

A low-temperature micro-photoluminescence (μ-PL) investigation of InAs/GaAs quantum dots (QDs) exposed to a lateral external electric field is reported. It is demonstrated that the QDs PL signal could be increased several times by altering the external and/or the internal electric field. The internal field in the vicinity of the dots could be altered by means of an additional infra-red laser. We propose a model, which is based on an essentially faster lateral transport of the charge carriers achieved in an external electric field. Consequently, also the capture probability into the dots and subsequently the dot luminescence is also enhanced. The results obtained suggest that the lateral electric fields play a major role for the dot luminescence intensity measured in our experiment.

Wurtzite CdxZn1−xO epilayers with cadmium concentrations ranging from x = 0.02 to 0.30 were investigated using photoluminescence, transmission/reflection spectroscopy, and atomic force microscopy. The CdxZn1−xO photoluminescence peak was found to shift through the visible region from 421 (2.95 eV) to 619 nm (2.0 eV) as the cadmium concentration was increased from 2% to 30%. An additional broad photoluminescence peak was observed and is attributed to deep levels – the center of the broad peak was found to shift from 675 to 750 nm as the cadmium concentration was increased. RMS roughness of the epilayers increased from 1.5 nm (x = 0.02) to 9.2 nm (x = 0.30), as determined from atomic force microscopy. The demonstrated visible wavelength tunability throughout the visible range verifies the viability of using wurtzite CdxZn1−xO compounds for visible light emission in future optoelectronic devices.

In this work, the impact of different ozone-based cleaning processes on the level of surface passivation achieved is determined and compared against the RCA cleaning processes. Two different passivation materials are used in this study, including hydrogenated amorphous silicon and silicon nitride plasma enhanced chemical vapor deposition (PECVD). Photoconductance measurements and calibrated photoluminescence imaging are used to evaluate the level of passivation achieved and spatial uniformity for the different cleaning processes.

An alternative method to map essential variables like dark saturation current density, series resistance, and the ultimate cell efficiency will be introduced. This approach will combine the terminally connected diode model with the ideality factor map technique modified to a scenario where current is extracted out of the circuitry. Following the delineation of the steps governing the principles of parameter mapping, we elaborate the fidelity that could be gained via improving upon the already existing methods.

7 páginas, 5 figuras.-- PACS numbers: 78.67.Hc, 71.55.Eq, 73.50.Gr, It is demonstrated that the microphotoluminescence (μPL) spectrum of a single InAs/GaAs self-assembled quantum dot (QD) undergoes considerable changes when the primary laser excitation is complemented with an additional infrared laser. The primary laser, tuned slightly below the GaAs band gap, provides electron-hole pairs in the wetting layer (WL), as well as excess free electrons from ionized shallow acceptors in the GaAs barriers. An additional IR laser with a fixed energy well below the QD ground state transition generates excess free holes from deep levels in GaAs. The excess electron and hole will experience diffusion separately, due to the time separation between the two events of their generation, to eventually become captured into the QD. Although the generation rates of excess carries are much lower than that of the electron-hole pair generation in the WL, they considerably influence the QD emission at low temperatures. The integrated PL intensity increases by several times as compared to single-laser excitation, and the QD exciton spectrum is redistributed in favor of a more neutral charge configuration. The dependence of the observed phenomenon on the powers of the two lasers and the temperature has been studied and is consistent with the model proposed. The concept of dual excitation could be successfully applied to different low-dimensional semiconductor structures in order to manipulate their charge state and emission intensity., This work was supported by grants from the Swedish Foundation for Strategic Research (SSR) and Swedish Research Council (VR). Financial support from the Wenner-Gren Foundation and the program “Low- Dimensional Quantum Structures” of the Russian Academy of Sciences is acknowledged by E.S.M. V.D. is thankful for financial support from the Swedish Foundation for International Cooperation in Research and Higher Education (STINT) and from the Bulgarian National Science Fund.

We study optically single self-assembled quantum dots embedded within the wide quantum well of a mixed type quantum structure. We compare the steady state and pulsed photoluminescence spectra of these dots to those of previously studied "regular" dots. We unambiguously identify experimentally emission from various discrete charge state of the dots. We provide means for optically tune the charge state of the dot, both negatively and positively. Our observations are used to accurately determine the asymmetry between the quantum dots' confined electron and hole envelope wavefunctions.

The photoluminescence spectrum of individual pairs of vertically stacked InAs quantum dots is investigated as a function of an external electric field along the growth direction. Comparing samples with different inter-dot distances, we find for the case of the closest pairs clear experimental evidences of single carrier transfer from one dot to the other, with a transfer rate that is controlled by the applied voltage. We model the spectra with a theoretical calculation, which takes into account both the field-dependent carrier kinetics and the many-body interactions in the quantum dot molecule.

We report the experimental observation of photon antibunching in the photoluminescence of a chemically synthesized CdSe/ZnS quantum dot (QD) at room temperature and from a single self-assembled InAs/GaAs QD at 4 K. These results prove that a single quantum dot acts like an artificial atom, with a discrete anharmonic spectrum. Using the quantum regression theorem, we find that the normalized second-order correlation function g (2) (τ) can be described by a single exponential function in the weak-laser excitation regime. From this model we derive that the time constant of the photon antibunching signal is determined by the effective incoherent pump and recombination rates of the nonresonant optically pumped QD.

In this work, the root cause of variation in surface recombination for Si wafers after different cleaning processes and for different passivation layers is investigated using a combination of calibrated photoluminescence (PL) imaging and transmission electron microscopy (TEM). The use of a HF-last or oxide-last cleaning and/or conditioning process is shown to have a strong impact on surface recombination for SiNx passivated surfaces, but little impact for Al2O3/SiNx stacks. For a SiNx passivation layer, cross-sectional TEM imaging revealed the formation of a â1-2nm SiOx interlayer resulting from a controlled oxidation during the last cleaning/conditioning step. The presence of the SiOx layer reduces the interface defect density (Dit,midgap) by an order of magnitude and dramatically increases the effective carrier lifetime. However, for Al2O3/SiNx passivated surfaces, TEM studies revealed that a SiOx layer is formed at the interface between the c-Si and AlOx even for c leaning processes ending with HF-last treatment due to which the cleaning sequence has minimal impact on the effective carrier lifetime.

It is demonstrated that the photoluminescence spectra of single self-assembled quantum dots are very sensitive to the experimental conditions, such as excitation energy and crystal temperature. A q ...

We report experimental observation of optically-pumped continuous-wave lasing from self-assembled quantum dots (QDs) embedded in high-quality factor (Q) microdisk laser structures. For large diameter (4.5 μm) microdisks containing InAs QDs, the emission spectra show simultaneous lasing on one to five well separated modes (Q = 10000-17000) in the wavelength range between 920 and 990 nm. For small diameter (2 μm) microdisks containing (In,Ga)As QDs, lasing occurs even when the estimated average number of QDs inside the cavity-mode volume is unity.

We resolve spatially, spectroscopically and temporally the photoluminescence emission from single self-assembled In(Ga)As/GaAs quantum dots, The temporal evolution of the emission spectrum after pulsed excitation is measured for various excitation intensities at various ambient temperatures. The evolution of the spectrum with the increase in both steady state and pulse excitation intensities is measured as well. A multi-exciton model is used for calculating the temporal and excitation intensity dependence of the measured spectra. The quantitative agreement between the measured and calculated spectra provides an unambiguous determination of the radiative lifetime of a single quantum dot exciton. This lifetime is 4-6 ns long and is temperature independent. The reduced spatial coherence between the confined exciton and the radiation electromagnetic field quantitatively explains this long radiative time.

7 páginas, 5 figuras., We have succeeded in preparing excitons with a specific charge in single semiconductor quantum rings. Buried InAs quantum rings are loaded with electrons from a reservoir through a tunneling barrier and an additional electron–hole pair is generated by optical excitation. Single rings are addressed with nano-optical techniques. We observe abrupt shifts in the emission energy as electrons are added one by one. Furthermore, the experiments provide unique insights into the interaction of electrons in semiconductor nano-islands with their environment., We would like to thank J.P. Kotthaus for stimulating discussions. This work was funded by the DFG (Project SFB 348) and by QUEST, a NSF Science and Technology Center.

Using time correlated single photon counting combined with temperature dependent diffraction limited confocal photoluminescence spectroscopy we accurately determine, for the first time, the intrinsic radiative lifetime of single excitons confined within semiconductor quantum dots. Their lifetime is one (two) orders of magnitude longer than the intrinsic radiative lifetime of single excitons confined in semiconductor quantum wires (wells) of comparable confining dimensions. We quantitatively explain this long radiative time in terms of the reduced spatial coherence between the confined exciton dipole moment and the radiation electromagnetic field., Comment: 4 pages, 3 figures

We resolve spatially, spectroscopically, and temporally the photoluminescence emission from single self-assembled InAs/GaAs quantum dots. The rich photoluminescence spectrum and its evolution with time after pulse excitation and with the density of excitation is experimentally measured and analyzed using a theoretical multiexciton model. The model accounts quantitatively for the dynamics of a small number of interacting electrons and holes confined in optically excited semiconductor quantum dots.

We present a novel self-assembled quantum dot structure designed to spatially separate and store photo-generated electrons and holes in pairs of strain coupled quantum dots. The spatial separation of electron–hole pairs into quantum dots and strain-induced quantum dots has been investigated and verified by photoluminescence experiments. Results from time-resolved PL demonstrates that at low temperatures (3 K) the electron–hole pair can be stored for several seconds.

6 páginas, 4 figuras.-- PACS: 78.55.−m; 71.35.−y; 73.20.Dx; 78.66.Fd.-- Trabajo presentado en el 13th International Conference on the Electronic Properties of Two-Dimensional Systems (EP2DS), Otawa del 1 al 6 de 1999., An experimental evidence of Auger-like excitation processes in InAs/GaAs quantum dots is demonstrated. Photoluminescence spectra of resonantly excited dots exhibit a rich satellite structure below the ground-state emission band. The energy position and the intensity distribution of the satellites are analyzed and an interpretation of the satellites as due to shake-up processes of the interacting carriers in the higher quantum dot states is suggested.

ZnO thin films were epitaxially grown on Zn-polar (0001) ZnO substrates by plasma-assisted molecular beam epitaxy. Surface root mean square (rms) roughness below 0.3 nm was achieved on a large range of growth temperatures by growing on ZnO substrates with 0.5 degree miscut angle toward [11¯00] axis. Surface treatment with acid etching and ozone exposure was required to remove contamination such as silica residual and carboxyl and carbonate groups on the surface. Removal of these surface impurities reduces the likelihood of extrinsic defect migration into the epitaxial films. High growth temperature (> 640°C) and oxygen rich conditions were required for films with terrace steps, but resulted in a very low growth rate (~30nm/h) and low photoluminescence (PL) lifetimes of lower than 50 ps. With moderate growth temperature (~610°C), higher growth rate and higher PL lifetime with up to 380 ps were achieved. EIT was used for the oxygen plasma to reduce reactive oxygen species etching of the surface, resulting in a higher growth rate and fewer defects in the films. Good crystalline quality was evident in Xray rocking curves with consistent narrow full width at half maximum (FWHM) of (0002), (101¯2) and (202¯1) peaks, indicating low threading dislocations. Both room-temperature and low-temperature photoluminescence indicated high optical quality of the resultant films with few non-radiative recombination centers.

InAs quantum dots (QDs) at different levels of density have been studied by means of photoluminescence, when in addition to the main laser, a second infrared (IR) laser is employed to excite the QD. It is demonstrated that the IR laser considerably affects the QD charge state as well as the emission intensity level (an increase greater than fivefold was observed). These effects are explained in terms of separate generation of excess electrons and holes provided under dual-laser excitation. However, these effects progressively vanish with increasing QD density. The results obtained unambiguously imply that the emission intensity from the QD can be effectively enhanced by purely optical means.

We report a micro-photoluminescence study on the influence of single and multi-quantum dots (QDs) on the exposure by a low-energy laser, in addition to the principal exciting laser. At low temperatures, the presence of the low-energy laser effectively quenches the single QD luminescence. This can be explained in terms of an induced screening of a built-in electric field, which plays an important role as a carrier capture mechanism. The influence of the low-energy laser is successively decreasing when the capture efficiency is increased either by elevated crystal temperature or by increased QD densities, full consistent with the proposed model.

Photoluminescence and time-resolved photoluminescence measurements of charge tunable quantum-dot heterostructures reveal that by appropriate biasing of the device, about 90% of photogenerated holes can be stored at an interface near to the nanostructures and subsequently transferred into the nanostructures in a controlled fashion. The capture dynamics are sensitive to the form of the valence band potential in the layer that caps the Stranski–Krastanow dots. The dependence of the capture rate on applied electric field suggests that the valence band confinement potential is “soft” in the capping layer, with a spatial extent of around 14 nm.

3 páginas, 4 figuras., We report how photoluminescence from self-assembled InAs quantum dots depend on pumping power and vertical electric field. The InAs dots, which are embedded in a capacitor-like structure, act as efficient trapping centers for excitons. At a high enough electric field, however, the photoexcited electrons tunnel out of the dots fast enough to quench the emission. For samples with two adjacent layers of vertically aligned dots, we find that the threshold voltage for quenching depends very strongly on the optical pumping power. In total contrast to this, we find no comparable effect for samples grown with a single layer of dots. We explain this in terms of efficient storage of electrons and holes in the double-layer samples., The work was funded by the Deutsche Forschungsgemeinschaft SFB 348, by the BMBF (Max Plank Research Award), and by QUEST, a National Science and Technology Center Grant No. DMR 20007. S. Huant was supported by the A. vHumboldt foundation.

3 páginas, 2 figuras., 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., One of the authors (E.S.M.) gratefully acknowledges the financial support of the Swedish Institute within the Visby Programme.

Transverse spin lifetimes of spin-polarized photogenerated carriers in InAs self-assembled quantum dots are extracted from the depolarization of their photoluminescence in a magnetic field perpendicular to the spin (the Hanle effect). Hanle measurements on a series of samples reveal that the dot dimensions influence the spin lifetime and its dependence on temperature. The spin lifetime as a function of excitation intensity is qualitatively distinct for carrier spins created in the GaAs host as compared to in the InAs wetting layer.

Storage and retrieval of excitons were demonstrated with semiconductor self-assembled quantum dots (QDs). The optically generated excitons were dissociated and stored as separated electron-hole pairs in coupled QD pairs. A bias voltage restored the excitons, which recombined radiatively to provide a readout optical signal. The localization of the spatially separated electron-hole pair in QDs was responsible for the ultralong storage times, which were on the order of several seconds. The present limits of this optical storage medium are discussed.

We present an InAs self-assembled quantum dot structure designed to spatially separate and store photo-generated electron-hole pairs. The structure consists of pairs of strain-coupled quantum dots. Separation of electron-hole pairs into the quantum dots and strain-induced quantum dots has been observed using power-dependant photoluminescence and bias-dependent photoluminescence.

The effects of radiation damage exposure on InGaAs quantum wells and InAs quantum dots are compared using luminescence spectroscopy techniques. A large increase in the radiation resistance of the InAs quantum dots is observed and attributed to exciton localization in the quantum dots and a point defect strain gettering effect.

A photoluminescence study of self-assembled InAs/GaAs quantum dots under the influence of magnetic fields perpendicular and parallel to the dot layer is presented. At low temperatures, the magnetic field perpendicular to the dot layer alters the in-plane transport properties due to localization of carriers in wetting layer (WL) potential fluctuations. Decreased transport in the WL results in a reduced capture into the quantum dots and consequently a weakened dot-related emission. The effect of the magnetic field exhibits a considerable dot density dependence, which confirms the correlation to the in-plane transport properties. An interesting effect is observed at temperatures above approximately 100 K, for which magnetic fields, both perpendicular and parallel to the dot layer, induced an increment of the quantum dot photoluminescence. This effect is ascribed to the magnetic confinement of the exciton wave function, which increases the probability for carrier capture and localization in the dot, but affects also the radiative recombination with a reduced radiative lifetime in the dots under magnetic compression. (Less)

Single quantum dots (QDs), based on the InAs/GaAs material system have been characterized by micro-photoluminescence (μPL). The self-organized quantum dots studied are fabricated by the Stransky-Krastanov method, taking advantage of the strain caused by the lattice mismatch between InAs and GaAs. Well-defined narrow excitonic features from individual QDs are monitored in the μPL spectra, upon single or dual tunable laser excitation. The charge state of the quantum dot is revealed from these excitonic lines in the μPL spectra. However, by tuning the laser excitation energy, it is demonstrated that the charge state of the dot can be altered: The distribution of neutral and charged excitons is demonstrated to be extremely sensitive on the laser energy. In addition, with an additional infrared laser, striking changes are induced in the μPL spectra. The results achieved demonstrate the existence of two well-defined excitation energy regions for the main laser, in which the presence of the infrared laser will decrease or increase, respectively, the integrated dot μPL intensity. For excitation above the critical threshold energy of the main laser, the addition of the infrared laser will induce a considerable increase, by up to a factor 5, in the QD emission intensity. At laser excitation below the threshold energy, on the other hand, the QD emission intensity will decrease. This fact is due to reduced carrier capture efficiency into the dot as determined by the internal electric field driven carrier transport. In order to get further insight into the carrier capture process due to the electric field in the vicinity of the QD, the dots have also been subjected to an external electric field In most optical experiments with QDs, electrically injected or photoexcited carriers are primarily created somewhere in the sample outside the QDs, e.g. in the barriers or in the wetting layer. Consequently, excited carriers undergo a transport in the wetting layer and/or barriers prior to the capture into the QDs. This circumstance highlights the crucial role of the carrier transport and capture processes into the dot for the performance and operation of the dot based devices such as QD lasers, QD infrared detectors and QD memory devices. This transport effect on the optical response of the quantum dots has been investigated by subjecting the carriers to an external electric field in μPL measurements. This external field is formed by application of a lateral field between two top contacts. It is demonstrated that the QD PL signal intensity could be increased several times (>5 times) by optimizing the magnitude of this external field.

This paper reviews of some of the progress made in the development of ZnO-based light emitting diodes (LEDs). n-ZnO/p-AlGaN-based heterostructures have been successfully for the fabrication of UV emitting LEDs that have operated at temperatures up to 650K, suggesting an excitonic origin for the optical transitions. RF-plasma-assisted molecular beam epitaxy has been used to grow epitaxial CdxZn1-xO films on GaN/sapphire structure. These films have a single-crystal wurtzite structure as demonstrated by structural and compositional analysis. High quality CdxZn1-xO films were grown with up to x=0.78 mole fraction as determined by RBS and SIMS techniques. Optical emission ranging from purple (Cd0.05Zn0.95O) to yellow (Cd0.29Zn0.71O) was observed. Compositional fluctuations in a Cd0.16Zn0.84O films were not detected by spatially resolved CL measurements, although intensity fluctuation with features of ∼0.5 μm diameter were seen on the intensity maps. Time resolved photoluminescence shows multi-exponential decay with 21 psec. and 49±3 psec. lifetimes, suggesting that composition micro-fluctuations may be present in Cd0.16Zn0.84O film.

4 páginas, 2 figuras.-- PACS 71.55.Eq, 73.50.Gr, 78.55.Cr, 78.67.Hc, The temperature evolution of the micro-photoluminescence (PL) spectra of a single InAs/GaAs quantum dot is studied from 5 to 65 K for different excitation energies. For excitation energies within the shallow acceptor absorption region below the GaAs bandgap a strong increase of the PL intensity (more than 10 times) is observed at 45 K with respect to 5 K. Besides, with increasing the temperature the PL spectrum redistributes in favour of a more neutral exciton configuration, contrary to the tendency observed when exciting outside this region. The results are explained in terms of separate optical generation of excess electrons and thermal generation of excess holes via the shallow acceptor levels in the GaAs barriers, resulting in an efficient carrier diffusion in the wetting layer and capture into the quantum dot., Financial support from the Wenner-Gren Foundation and the Russian Academy of Sciences is acknowledged by E.S.M and from the Swedish Foundation for International Cooperation in Research and Higher Education (STINT) by V.D.

The excitonic photoluminescence spectra reveal an abrupt change of the charge-state of single In(Ga)As/GaAs quantum dots, for a certain transformation when the excitation energy below the barrier band gap. This transform energy is dependent on the thickness of the underlying wetting layer and the crucial energy is associated with an optical transition involving an energy level of the wetting layer. The effect is proposed to be used as a tool, not only to monitor charge in quantum dots in order to study related phenomena, but also to gain new insight in the electronic structure of strained In(Ga)As/GaAs layers by using the quantum dots as charge-probes.

2 páginas, 3 figuras., Pure photoluminescence spectra originating from a single InAs/GaAs quantum dot, which is small enough to possess only one single-electron level, are demonstrated. A symmetric fine structure of the exciton and the biexciton is observed.

7 páginas, 4 figuras., We have studied the low-temperature photoluminescence (PL) of the self-assembled InAs single quantum dots (QDs) using conventional micro-PL setup to detect PL from an individual QD. It is demonstrated, that at certain experimental conditions, what concerns the laser excitation energy, the laser power and the crystal temperature, several additional lines, redshifted relative to the ground state transition, appear in the PL spectra. These are interpreted in terms of charged exciton complexes which form due to the population of quantum dots with a nonequal amount of electrons and holes. The latter phenomenon is determined by the excess energies of photogenerated carriers and is proposed as an effective optical method to create and study charged exciton complexes in QDs., One of the authors (E.S.M.) gratefully acknowledges financial support of Svenska Institutet within the Visby Program and partial support of the Russian Academy of Sciences (Low-Dimensional Nanostructures, 2001).

Summary form only given. Optical studies of semiconductor quantum dots (QDs) have been a subject of very intensive recent investigations. It has been experimentally and theoretically established that the number of carriers which occupy a photoexcited QD greatly affect its photoluminescence (PL) spectrum. In spite of its neutral nature, optical spectroscopy has very recently proved to be a useful means for investigating and preparing charged QD systems. We report here on continuous wave (cw) and pulsed optical PL spectroscopy of single self-assembled QDs (SAQDs) embedded within a mixed type quantum well (QW) structure. This specific design, which facilitates charge separation by optical means, is used here to tune the charge state of the QD under study. We compare the PL emission spectra for a single In(Ga)As/GaAs SAQD occupied with an increasing number of neutral multiexcitons, and the emission of the same type of dots embedded in a GaAs/AlAs mixed type structure. Using time-resolved spectroscopy and a comparison between cw and pulse-excited PL spectra we are able to accurately determine the charge state of the single QDs at a given excitation intensity level. We determine the collective carrier state from which each spectral line originates.

11 páginas, 7 figuras.-- PACS number(s): 73.21.La, 71.55.Eq, 73.22.Dj, 71.70.Gm, We study the photoluminescence of single InAs/GaAs self-assembled quantum dots for a range of excitation powers, excitation energies and sample temperatures 4K30K), this effect vanishes due to the essential decrease of the steady-state free electron concentration in the GaAs barrier as a result of thermally excited free holes appearing in the GaAs barrier valence band which provides an effective recombination channel for the free electrons. These experimental observations could be used as an effective tool to create and study charged excitons in quantum dots., One of us (E.S.M.) gratefully acknowledges financial support from the Wenner-Gren Foundations and partial support from the Russian Academy of Sciences (Low-Dimensional Nanostructures’ 2001).

4 páginas, 3 figuras., It is demonstrated that the photoluminescence spectra of single self-assembled quantum dots are very sensitive to the excitation energy and crystal temperature. A qualitative explanation is given in terms of the effective diffusivity of the photogenerated carriers, determined by the experimental conditions, which influence the capture probability and hence also the charge state of the quantum dots (QDs). This is proposed as an effective tool to populate quantum dots with extra electrons in order to study phenomena involving charged excitons., One ofthe authors (E.S.M.) gratefully acknowledges financial support of the Swedish Institute within the Visby Programme and partial support of the Russian Academy of Sciences (Low-Dimensional Nanostructures’ 2001).

4 páginas, 3 figuras.-- PACS number(s): 78.67.Hc, 73.40.Rw, 78.66.Fd, We present results on the influence of a magnetic field on excitons in semiconductor quantum dots, concentrating on the diamagnetic curvature. We use samples with a bimodal ensemble photoluminescence (PL) and we find that for the low-energy PL branch, the diamagnetic curvature is independent of charge, yet for the high-energy branch, the diamagnetic curvature is strongly reduced with excess charge. Guided by model calculations, we interpret the two classes as typical of the strong and intermediate confinement regimes. In the light of this, we predict that in the weak confinement regime the excitonic diamagnetic shift is strongly dependent on surplus charge, corresponding to a reversal in sign of the conventional diamagnetic shift for neutral excitons., This work was supported by the DFG (Grant No. SFB348) and EPSRC. One of us, A.O.G., acknowledges financial support from the Rufus Putnam Visiting Professorship and the Volkswagen Foundation.

We study the electronic coupling between two vertically stacked InAs quantum dots, which are embedded in the center of a n-i-n structure. We use a micro-photoluminescence setup to optically isolate a single quantum dot pair and measure the time-averaged photoluminescence under an applied vertical electric field. We find that field tunable coupling between excited states of the two quantum dots leads to charge transfer from one dot to the other. We model the spectra including simultaneously the field dependent charge transfer and exciton capture rates, and the many-body spectra of the quantum dot molecule for different carrier configurations., Comment: 8 pages, 5 figures