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The optical properties of two-dimensional electron gas (2DEG) in single and multiple AlxGa1–xN/GaN heterostructures were investigated by means of photoluminescence (PL), temperature dependent PL, power dependent PL. A strong GaN band edge emission and its longitudinal optical (LO) phonon replicas were observed for all the samples. The PL peaks related to recombination between the 2DEG and photogenerated hole are observed at ∼3.45 eV for single AlxGa1–xN/GaN heterostructures and at ∼3.425 and ∼3.45 eV for multiple AlxGa1–xN/GaN heterostructures. The LO phonon replicas of the 2DEG emission were also clearly observed. The 2DEG-related emission intensity decreased with increasing temperature and disappeared at temperatures above 150 K. A possible origin of the appearance of 2DEG-related emissions in single and multiple AlGaN/GaN heterostructures has been discussed. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The effect of stacking on the optical properties of self-organized GaN/AlN quantum dots (QDs) grown by plasma-assisted molecular beam epitaxy was investigated by photoluminescence (PL), PL excitation (PLE), and cathodoluminescence (CL) spectroscopic techniques. With an increase in the number of the GaN QDs stacking layer, we observed a red shift in the main QD PL peaks. A Stokes-like shift was observed between PL emission and PLE absorption edge for all the samples. From the depth-resolved CL of a 200-period GaN QD sample, we observed that the main QD CL emission peak energy position does not change much, while the higher-energy side CL peak around ∼3.7 eV shows a variation of its peak position with the accelerating voltage probably due to the change in either distribution of QD size/shape or wetting layer properties during the growth of multiple stacking of GaN QDs. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The lateral self-alignment properties of self-assembled InAs quantum dots (QDs) on a conventional GaAs (1 0 0) substrate by molecular beam epitaxy were investigated. The shape and optical properties of QDs were investigated by atomic force microscopy, transmission electron microscope, and photoluminescence (PL). Attempts were made to grow InAs-QDs using the In-interruption growth technique, in which the In flux was periodically interrupted. QDs grown without using the In-interruption growth technique were grown randomly on all regions. On the other hand, in the case of QDs grown using the In-interruption growth technique, QDs were self-aligned at the boundary between bright and dark regions, the PL intensity was increased and the PL peak position of QDs were red-shifted to 1300 nm. This represent a new technique for growing self-aligned QDs because no extra processing such as electron-beam lithography, V-grooves and surface modification by scanning tunneling microscopy is needed, and aligned QDs can be grown in situ on conventional GaAs substrates.

Self-assembled InAs quantum dots (QDs) embedded in an InAlGaAs matrix were grown on an InP (001) using a solid-source molecular beam epitaxy and investigated using transmission electron microscopy (TEM) and photoluminescence (PL) spectroscopy. TEM images indicated that the QD formation was strongly dependent on the growth behaviors of group III elements during the deposition of InAlGaAs barriers. We achieved a lasing operation of around 1.5 µm at room temperature from uncoated QD lasers based on the InAlGaAs-InAlAs material system on the InP (001). The lasing wavelengths of the ridge-waveguide QD lasers were also dependent upon the cavity lengths due mainly to the gain required for the lasing operation.

The properties of self-assembled InAs quantum dots (QDs) grown by molecular beam epitaxy on GaAs substrates were investigated. The surface properties of samples were monitored by reflection high-energy electron diffraction to determine growth. Photoluminescence (PL) and transmission electron microscope (TEM) were then used to observe optical properties and the shapes of the InAs-QDs. Attempts were made to grow InAs-QDs using a variety of growth techniques, including insertion of the InGaAs strained-reducing layer (SRL) and the interruption of In flux during QD growth. The emission wavelength of InAs-QDs embedded in a pure GaAs matrix without interruption of In flux was about 1.21 μm and the aspect ratio was about 0.21. By the insertion InGaAs SRL and interruption of In flux, the emission wavelength of InAs-QDs was red shifted to 1.37 μm and the aspect ratio was 0.37. From the PL and TEM analysis, the properties of QDs were improved, particularly when interruption techniques were used.

Self-assembled InAs quantum dots (QDs) in an InAl(Ga)As matrix, which is lattice-matched to InP substrate, were grown under various growth conditions by a molecular beam epitaxy and their structural and optical properties were investigated by atomic force microscopy (AFM) and photoluminescence (PL) spectroscopy. From the AFM and PL results, the nucleation of an InAs QD was significantly affected by the growth conditions for the buffer layer, that is, the surface characteristics of the buffer layer. Inserting a thin GaAs layer with a thickness of 0.6–1.8 nm controlled the optical and structural properties of InAs QDs due to the different growth front, different growth behavior of group III elements at the interface between the QD and the barrier, and modulation in strain field.

The structural and optical properties of self-assembled InAs quantum dots (QDs) with various InGaAs structures were investigated by transmission electron microscopy (TEM) and photoluminescence (PL). The emission peak position of InAs QDs covered by a 6 nm In0.15Ga0.85As layer was 1.26 μm with PL linewidth of 31 meV, which is narrower than that of QDs in a GaAs matrix. By inserting a 1 nm In0.15Ga0.85As layer below the InAs QD layer with a 6 nm In0.15Ga0.85As overgrowth layer, the emission peak position was redshifted with larger energy-level spacing between the ground states and the first excited states compared to that of QDs with an In0.15Ga0.85As overgrowth layer only. By covering the InAs QDs on a 1 nm In0.15Ga0.85As layer with an 8 nm InxGa1−xAs layer having graded In composition, the emission peak position was 1.32 μm with relatively larger energy-level spacing and narrower PL linewidth compared to QDs covered by an In0.15Ga0.85As layer. The longer emission wavelength with relatively larger energy-l...

Self-assembled InAs quantum dots (QDs) covered by In0.15Ga0.85As layer with different thickness were grown by molecular beam epitaxy and their structural and optical properties were investigated by transmission electron microscopy (TEM) and photoluminescence (PL) spectroscopy. Cross-sectional TEM images showed that the shape of InAs QD, particularly the height, was controlled by changing the thickness of InGaAs overgrowth layer, thus changing the optical properties. The emission peak position of InAs QDs covered by 8.5 nm In0.15Ga0.85As layer was 1.30 μm at room temperature with the energy-level spacing between the ground states and the first excited states of 80 meV, which is about 1.5 times larger than that of InAs QDs capped by GaAs layer. The longer emission wavelength with relatively larger energy-level spacing was related to the QD aspect ratio (height/width) that was confirmed by TEM images.

In this work we report on the fabrication and characterization of a n-ZnO/p-GaN heterojunction LED. The p-GaN layer was fabricated using MOCVD on Al2O3 with Mg as the acceptor whereas the ZnO nanostructures were grown in a very simple vapor transport system without any additionally doping. Room temperature electroluminescence (EL) measurements show green deep band emission centered at 2.3 eV which is clearly visible with the naked eye when the structure is forward biased. Cathodoluminescence mapping was performed to explain the absence of the band edge emission in the EL spectrum. (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

For the fabrication of quantum dot (QD) distributed feedback (DFB) lasers, self-assembled InAs/InAlGaAs QDs were grown on the InP/InGaAs grating structures by a molecular beam epitaxy. Ridge-waveguide QD DFB lasers with a stripe width of 3 mum were fabricated. Single-mode lasing operation around the wavelength of 1.56 mum was successfully achieved under the pulsed and continuous-wave (CW) modes at room temperature. The lasing operation was also observed up to 70 degC under pulsed mode, which is the first observation on the single-mode lasing around 1.55 mum. The characteristic temperatures calculated from the temperature dependence of the threshold current density were 121.8 K from room temperature to 45 degC, which was reduced to 74.2 K from 45 degC to 70 degC.

The InAs-InAlGaAs quantum dot (QD) lasers with the InAlGaAs-InAlAs material system were fabricated on distributed feedback (DFB) grating structures on InP (001). The single-mode operation of InAs-InAlGaAs QD DFB lasers in continuous-wave mode was successfully achieved at the emission wavelength of 1.564 mum at room temperature. This is the first observation on the InP-based QD lasers operating around the emission wavelength window of 1.55 mum. The threshold current density of the InAs-InAlGaAs QD DFB laser with a cavity length of 1 mm and a ridge width of 3 mum, in which one of the cleaved facets was coated with 95% high-reflection, was 1.23 kA/cm2 (176 A/cm2 for single QD layer). The sidemode suppression ratio value of the QD DFB laser was as high as 42 dB at the driving current of 100 mA

Seven stacks of self-assembled InAs quantum dots (QDs) separated by 28nm thick InAlGaAs barriers were grown on InP (001) substrate by a solid-source molecular-beam epitaxy and were investigated by cross-sectional transmission electron microscopy and photoluminescence spectroscopy. Gain guided broad-area lasers with a stripe width of 75μm were fabricated by using the seven-stacked InAs QD layers with the InAlGaAs–InAlAs material system on InP (001). The lasing operation from InAs QDs was observed up to 260K and the characteristic temperature of the uncoated QD laser calculated from the temperature dependence of threshold current density was 377K for temperatures up to 200K, and 138K above 200K. The drastic decrease in the characteristic temperature above 200K was mainly related to the thermal behavior of carriers in QDs, and possibly the thermal coupling of the QDs to the wetting layer and the waveguide region.

Influences of a thin In0.32Ga0.68As layer on the structural and optical properties of self-assembled InAs quantum dots (QDs) embedded in an InAl(Ga)As matrix, which was lattice-matched to an InP substrate, were investigated by atomic force microscopy (AFM), transmission electron microscopy (TEM), and photoluminescence (PL) spectroscopy. AFM and TEM images showed that the size of QDs grown on a thin In0.32Ga0.68As layer was increased, with a reduction in size fluctuation compared to that of QDs on an InAl(Ga)As layer. The shape of the QD was also more isotropic, indicating that the QD would be closer to an ideal zero-dimensional system. The PL peak position of the InAs QDs grown on a 1.5 nm In0.32Ga0.68As layer was 1.55 μm, with linewidth broadening of 64 meV that was somewhat narrower than those of the QD samples without the In0.32Ga0.68As layer, which agreed well with the AFM and TEM results.

We have investigated optical properties of amorphous silicon quantum dots (a-Si QDs) embedded in a silicon nitride grown on (100) silicon substrates by plasma-enhanced chemical vapor deposition. The photoluminescence (PL) emission colors of red, orange, and blue were shown for the a-Si QDs with average dot sizes of 5.8, 2.4, and 1.3 nm, respectively. The increase in the PL peak energy with decreasing QD size was attributed to the quantum confinement effect in a-Si QDs. An anomalous temperature dependence of the PL intensity and PL peak energy was observed for a-Si QDs. The visible-light PL emission from a-Si QDs is most efficient at near room temperature, and the PL spectral shape and peak energy are almost independent of temperature, which are most favorable for light-emitting device applications. From time-resolved PL experiments, an enhancement of phonon-assisted indirect radiative recombination was proposed to explain the anomalous temperature dependence.

Influence of the size of indium clusters on optical properties of green-light-emitting InGaN quantum wells (QWs) was investigated by photoluminescence (PL), cathodoluminescence, PL excitation, and time-resolved PL techniques. Low luminescence efficiency was observed for green-light-emitting InGaN QWs with micron-sized indium clusters, in contrast to the case of InGaN QWs with submicron-sized small indium segregation. Both the thermal activation energy and the carrier lifetime dramatically decreased, while a large Stokes-like shift between absorption edge and PL peak energy was still observed for the InGaN QWs with micron-sized indium clusters. These facts indicate that the effective potential barrier between radiative and nonradiative channels (thus effective carrier localization) rapidly decreases due to the formation of micron-sized large indium clusters possessing a number of nonradiative centers, leading to significant luminescence degradation.

For the fabrication of quantum dot (QD) distributed feedback (DFB) lasers, self-assembled InAs QDs were grown on the InP/InGaAs grating structures by a molecular beam epitaxy. Ridge-waveguide QD DFB lasers with a stripe width of 3 mum were fabricated. Single-mode lasing operation around the wavelength of 1.56 mum was successfully achieved under the pulsed and continuous-wave (CW) modes at room temperature. The lasing operation was also observed up to 70degC, which is the first observation on the single-mode lasing around 1.55 mum. The characteristic temperatures calculated from the temperature dependence of the threshold current density were 121.8 K from room temperature to 45degC and 74.2 K up to 70degC, respectively.

A successful set-up of broadband convergence network is still challengeable in a point of low cost and high performance. As a key component for signal source, new laser diodes are introduced, especially, quantum dot laser diodes (QD-LDs) with high characteristic temperature will be introduced.

The optical and structural properties of Ge quantum dots (QDs) of varying size grown on Si substrates with and without a partially relaxed Si/sub 0.9/Ge/sub 0.1/ buffer layer, were investigated by means of photoluminescence and atomic force microscopy. A random, bimodal QD size distribution was observed for Ge QDs directly grown on Si substrates, while a well-aligned, unimodal size QD distribution was observed for Ge QDs with the Si/sub 0.9/Ge/sub 0.1/ buffer layer. Quantum confinement effects with dot size variation were evident from PL studies. A blue shift of the Ge QD emission energy with increasing excitation power is ascribed to the band bending at the type-II Si/Ge interface.

The emission wavelength of InAs-QDs on GaAs substrates are controlled by using five different growth techniques: (a) QD1 sample was grown in a pure GaAs matrix. (b) QD2 sample was covered with In/sub 0.13/Ga/sub 0.87/As on QD1 sample. (c) QD3 sample was grown by In interruption during QD growth in a pure GaAs matrix, (d) QD4 sample was covered with In/sub 0.13/Ga/sub 0.87/As SRL on QD3 sample and (e) QD5 sample was covered with InAs/GaAs 10 period on QD3 sample. Schematic of the five type samples are illustrated and In interruption for InAs-QDs, PL spectra of five InAs-QDs samples measured at room temperature are studied.

The introduction of a thin tensile-strained GaAs and In/sub 0.32/ Ga/sub 0.68/As right below the QD layer modulates the structural and optical properties of self-assembled InAs QDs embedded in an InAlGaAs matrix. The AFM images indicated that the average size of InAs QDs on GaAs/InAlGaAs is decreased compared to that without the thin GaAs layer, but ones on In/sub 0.32/Ga/sub 0.68/As/InAlGaAs are increased with more isotropic shape.

The light output power (LOP) of vertical-type GaN-based light emitting diodes (LED) with surface roughness (texture) can be changed by texture size, density, and thickness of GaN film or by the combined effects of texture formation and thickness of GaN film. We have investigated these changes experimentally and note that the enhancement of the LOP by a factor of 2.4 can be improved with optimum texturing parameters as compared to that without texturing. In addition, the LOP of GaN-based LEDs under the same texture density increase slightly as thickness of GaN film decreases. Base on these results, we have evidently demonstrated that the enhancement factors of LOP are related to the correlation between texture size (density) and thickness of GaN film.

This study examines the influence of the defect properties on internal quantum efficiency (IQE) in blue InGaN-based light emitting diodes (LEDs). The defect parameter is introduced for estimating defect properties among defect density, size, and defect type that are strongly correlated with IQE in InGaN LED. The value of IQE can be expressed by the value of the defect parameter, which was obtained from transmission electron microscopy and cathodoluminescence measurement.

We have investigated the photoluminescence (PL) characteristics of AlGaN/GaN heterostructure with an extremely high two-dimensional electron gas (2DEG) density. A very high 2DEG density was obtained by controlling the tensile stress during the growth of the heterostructure on silicon substrate. Strong PL emission peaks corresponding to both the band edge and the 2DEG were observed in the highly tensile-stressed heterostructure. Furthermore, the strong longitudinal optical (LO) phonon replicas (1-LO2DEG and 2-LO2DEG) of the 2DEG peak were also observed in the heterostructure.

This study examines the influence of the growth mode on defect formation and strain relaxation in GaN thin films grown on cone-shaped patterned sapphire (CSPS) substrates by metal-organic chemical vapor deposition. A dramatic reduction in the luminescence dark spot density and a red-shifted excitonic emission were found for GaN on CSPS compared to GaN on planar sapphire substrates. The results also show that both the crystal quality and optical properties are improved in GaN on CSPS with a strong spatial correlation between the dark spots and local structural properties due to the induced-lateral overgrowth on the cone-shaped patterns.

We report the optical properties of ZnO/ZnMgO multiple quantum well (MQW) structures with different well widths grown on ZnO substrates by molecular beam epitaxy. Photoluminescence (PL) spectra show MQW emissions at 3.387 and 3.369 eV for the ZnO/ZnMgO MQW samples with well widths of 2 and 5 nm, respectively, due to the quantum confinement effect. Time-resolved PL results show an efficient photogenerated carrier transfer from the barrier to the MQWs, which leads to an increased intensity ratio of the well to barrier emissions for the ZnO/ZnMgO MQW sample with the wider well width.

We have investigated optical and structural properties of three-stacked InGaAs quantum dot (QD) structure with GaAs spacer thicknesses of 22, 35, and 88 nm (denoted by QD22, QD35, and QD88, respectively) grown by migration-enhanced molecular beam epitaxy. From temperature-dependent photoluminescence (PL) analysis, it is found that thermal carrier redistribution between vertically adjacent QD layers plays an important role as the thickness of GaAs spacer is reduced from 88 to 22 nm. Although the QD sizes of upper layers are quite similar to those of the first bottom layer, the QDs of the upper layers appear to emit at higher energies probably due to different alloy compositions caused by the strain-induced intermixing effect between InGaAs QDs and GaAs barriers with stacking. Especially for QD22 sample, we observed thermally assisted carriers transfer among vertically adjacent QD layers with increasing temperature by using time-resolved PL measurements, which is in good agreement with the temperature depen...

Nominally undoped ZnO nanorods, which have been grown with intentionally incorporated large concentrations of zinc and oxygen vacancies, are studied with electron microscopy and photoluminescence spectroscopy at low temperature. Detailed photoluminescence studies reveal that the concentration of these defects depends on the growth conditions of the nanorods as well as on their annealing history. The optical signatures of these two defects at photoluminescence experiments are related with the 3.31 and 3.235 eV peaks. The activation energies of zinc and oxygen vacancies are calculated to be 123 and 199 meV, respectively.

Zinc oxide (ZnO), with its excellent luminescent properties and the ease of growth of its nanostructures, holds promise for the development of photonic devices. The recent advances in growth of ZnO nanorods are discussed. Results from both low temperature and high temperature growth approaches are presented. The techniques which are presented include metal-organic chemical vapour deposition (MOCVD), vapour phase epitaxy (VPE), pulse laser deposition (PLD), vapour-liquid-solid (VLS), aqueous chemical growth (ACG) and finally the electrodeposition technique as an example of a selective growth approach. Results from structural as well as optical properties of a variety of ZnO nanorods are shown and analysed using different techniques, including high resolution transmission electron microscopy (HR-TEM), scanning electron microscopy (SEM), photoluminescence (PL) and cathodoluminescence (CL), for both room temperature and for low temperature performance. These results indicate that the grown ZnO nanorods possess reproducible and interesting optical properties. Results on obtaining p-type doping in ZnO micro- and nanorods are also demonstrated using PLD. Three independent indications were found for p-type conducting, phosphorus-doped ZnO nanorods: first, acceptor-related CL peaks, second, opposite transfer characteristics of back-gate field effect transistors using undoped and phosphorus doped wire channels, and finally, rectifying I-V characteristics of ZnO:P nanowire/ZnO:Ga p-n junctions. Then light emitting diodes (LEDs) based on n-ZnO nanorods combined with different technologies (hybrid technologies) are suggested and the recent electrical, as well as electro-optical, characteristics of these LEDs are shown and discussed. The hybrid LEDs reviewed and discussed here are mainly presented for two groups: those based on n-ZnO nanorods and p-type crystalline substrates, and those based on n-ZnO nanorods and p-type amorphous substrates. Promising electroluminescence characteristics aimed at the development of white LEDs are demonstrated. Although some of the presented LEDs show visible emission for applied biases in excess of 10 V, optimized structures are expected to provide the same emission at much lower voltage. Finally, lasing from ZnO nanorods is briefly reviewed. An example of a recent whispering gallery mode (WGM) lasing from ZnO is demonstrated as a way to enhance the stimulated emission from small size structures.

We report low temperature cathodoluminescence spectroscopy measurements of the band edge emission from ZnO nanostructures grown by vapour phase transport on Si. A range of donor bound exciton emission lines are found and the Al-related emission at 3.3605 eV in particular shows a marked inhomogeneity in its distribution throughout the sample. Increased 3.3605 eV emission is seen at a range of locations in nanorods and nanosheets where different nanostructures cross or coalesce, suggesting aggregation of Al donors in ZnO in regions of crystal structure disruption. However, localized crystal structure disruption appears to be a necessary rather than a sufficient condition for Al aggregation, since increased 3.3605 eV emission is seen only in such regions, but not all such regions show increased emission, implying that the microscopic nature of such regions is important in determining Al aggregation. Supporting data are presented from well-aligned, non-crossing, nanorods on a-sapphire.

We investigated the spontaneous and stimulated emission properties of violet-light-emitting ultrathin In-rich InGaN/GaN multiple quantum wells (MQWs) with indium content of 60%–70%. The Stokes shift was smaller than that of In-poor InGaN MQWs, and the emission peak position at 3.196 eV was kept constant with increasing pumping power, indicating negligible quantum confined Stark effect in ultrathin In-rich InGaN MQWs despite of high indium content. Optically pumped stimulated emission performed at room temperature was observed at 3.21 eV, the high-energy side of spontaneous emission, when the pumping power density exceeds ∼31 kW/cm2.

Optical properties of p-type ZnO epilayers doped with different amounts of phosphorus by radio-frequency magnetron sputtering are investigated by x-ray diffraction, temperature dependent photoluminescence (PL), and time-resolved PL techniques. Bound exciton, free electrons-to-acceptors, donor-to-acceptor pair, and deep-level emissions are observed at about 3.356, 3.32, 3.24, and 2.4eV at 10K for p-type ZnO, respectively. The crystal quality and luminescence efficiency are improved with increasing phosphorus doping concentration. These results show that phosphorus doping plays an important role both in reducing native defects and in generating shallow acceptors in ZnO, leading to a good p-type conductivity in ZnO.

In this article, we present an in-depth study of the effects of the structural and optical properties of InAs “dots in an In0.2Ga0.8As well” (DWELL) and InAs “dots in an asymmetric In0.2Ga0.8As well” (asym. DWELL) grown by migration-enhanced molecular beam epitaxy. The energy spacing (ΔE1) between the ground-state and the first-excited-state transitions increases from 66 meV for the DWELL to 73 meV for the asym. DWELL. These results are consistent with ΔE1 measured by photoluminescence excitation and the values of activation energy fitting. The photoluminescence linewidth of the asym. DWELL (40 meV) is narrower than that of the DWELL (70 meV), which shows superior uniformity in the former over the latter. The trends of the properties of the DWELL and the asym. DWELL deduced from the structural properties are in good agreement with those from the optical properties. From the results, it is strongly supported that the asym. DWELL is more suitable for application to long wavelength optical communication than...

We have investigated optical and structural properties of various In0.5Ga0.5As quantum dot (QD) structures as a function of stacking number, grown by atomic layer molecular beam epitaxy (ALE). We found that the excitation power and temperature dependence of well-separated two emissions from 10 stacked InGaAs QD samples are different from those of other QDs with the stacking numbers of 1, 3 and 5. Although the GaAs spacer thickness is ~35 nm at which the strain field penetration can be ignored for Stranski–Krastanov mode-grown InAs QDs, our ALE-grown QDs are influenced by the strain field penetration due to the larger size of ALE QDs. From transmission electron microscopic images, we observed that the ten stacked QD sample has a complex QD size distribution predominantly due to vertical size variation with stacking and that upper stacked (5–10) QD layers are vertically realigned due to the merged strain field penetration between laterally coupled QDs.

The formation characteristics on the vertical stacks of shape-engineered InAs∕InAlGaAs quantum dots (QDs), which were formed by the alternate growth method (AGQDs), were studied in terms of the modulation in strain field and phase separation. The threshold current of the broad-area laser diodes (LDs) with five stacks of the AGQDs (AGQD-LDs) was 4.5 times smaller than that of the LDs with seven stacks of the conventionally grown QDs (CQD-LDs). The slope efficiency for the AGQD-LDs was 0.16W∕A, which was higher than that of the CQD-LDs of 0.9W∕A. These results can be attributed to better confinement of the electron wave function in QDs.

Comparative analysis of the carrier dynamics of GaN quantum dot (QD) and GaN quantum well (QW) separated confinement heterostructures (SCHs) with low-Al-content AlGaN waveguide layers is reported. A redshift (blueshift) of QD (wetting layer) emission is found with respect to QW emission, as expected from the thickness hierarchy of these objects. The influence of nonradiative processes on QD emission in QD SCH is dramatically reduced compared to the case of QW SCH. It is concluded that GaN QDs in low-Al-content AlGaN matrix are robust localization centers and that the carrier dynamics is seriously affected by the built-in internal field effect.

The authors report depth-resolved optical properties of excitonic and phonon-assisted transitions in ZnO epilayers by photoluminescence (PL) and cathodoluminescence. A weaker free exciton (FX) emission than its first longitudinal optical phonon replica (FX-1LO) is observed at elevated temperatures (T>150K) for interior area, while a stronger FX than FX-1LO is seen at all temperatures for top surface area of the sample. The authors exclude out a possible self-absorption process by PL excited at back surface of the sample. Therefore, the authors conclude that the different intensity ratios of FX and FX-1LO depending on the sample depth are strongly associated with extrinsic features of ZnO.

Zinc oxide thin films were grown in a temperature range of 70 to 180oC by means of plasma enhanced atomic layer deposition (PEALD) for the first time. The films have growth rate of 1.9Aå/cycle. The XRD patterns showed more or less random orientation of ZnO films grown by PEALD, but the reduced plasma power resulted in preferred (002) orientation of films at 150oC. The sheet resistivity of films grown with oxygen plasma was too high to measure with 4 point probe and Hall measurement system. The difference of electrical property between PEALD and H2O-based ALD films is attributed to a different O/Zn atomic ratio.

Zinc oxide films were grown in a temperature range of 75-150°C by plasma-enhanced ALD for the application of flexible transparent thin-film transistor (TFT). We have investigated the effect of precursor pulsing time, temperature, and radio frequency power on the film growth. The X-ray diffraction patterns of polycrystalline ZnO films showed rather preferred (002) orientation even at a low temperature of 150°C. We could obtain highly resistant ZnO film, being suitable for the active layer of TFT, at low temperature with the reactive oxygen plasma to result in decreased carrier density.

We have investigated the optical properties and carrier dynamics of the two-dimensional electron gas (2DEG) in Al0.4Ga0.6N∕GaN single heterostructures grown by metalorganic chemical vapor deposition by means of photoluminescence (PL), PL excitation, and time-resolved PL spectroscopy. Shubnikov-de Haas oscillations were clearly observed at 1.5K, confirming the existence of a 2DEG. An additional 2DEG PL emission appeared at about 40meV below the GaN band-edge emission and persisted up to about 100K, while this peak disappeared when the top Al0.4Ga0.6N layer was removed by reactive ion etching. We observed abrupt PLE absorption at GaN band edge energy and approximately 50‐ps delayed risetime compared to GaN and AlGaN emissions, indicating effective carrier transfer from the GaN flatband and AlGaN regions to the heterointerface. Even though the 2DEG emission is a spatially-indirect (slow) recombination, a fast decay component of ∼0.2ns is found to be dominant in 2DEG emission because of the fast exhaustion of...