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Complementary studies of atom probe tomography, secondary ion mass spectrometry, and vertical current-voltage measurements are carried out in order to unravel the influence of C-doping of GaN on the vertical leakage current of AlN/AlGaN/GaN:C heterostructures. A systematic increment of the vertical blocking voltage at a given current density is observed in the structures, when moving from the nominally undoped conditions—corresponding to a residual C-background of ~1017cm-3—to a C-content of ~1019cm-3 in the GaN layer. The value of the vertical blocking voltage saturates for C concentrations higher than ~1019cm-3. Atom probe tomography confirms the homogeneity of the GaN:C layers, demonstrating that there is no clustering at C-concentrations as high as 1020cm-3. It is inferred that the vertical blocking voltage saturation is not likely to be related to C-clustering. [ABSTRACT FROM AUTHOR]

The early stages of InGaN/GaN quantum well growth for In-reduced conditions have been investigated for varying thickness and composition of the wells. The structures were studied by monochromated scanning transmission electron microscopy-valence electron energy loss spectroscopy spectrum imaging at high spatial resolution. It is found that beyond a critical well thickness and composition, quantum dots (width >20 nm) are formed inside the well. These are buried by compositionally graded InGaN, which is formed as GaN is grown while residual In is incorporated into the growing structure. It is proposed that these dots act as carrier localization centers inside the quantum wells. [ABSTRACT FROM AUTHOR]

Capacitance-voltage [C(V)] measurements of metal-insulator-semiconductor-heterostructure capacitors are used to investigate the interface between silicon nitride passivation and AlGaN/AlN/GaN heterostructure material. AlGaN/AlN/GaN samples having different silicon nitride passivating layers, deposited using three different deposition techniques, are evaluated. Different interface state distributions result in large differences in the C(V) characteristics. A method to extract fixed charge as well as traps from the C(V) characteristics is presented. Rough estimates of the emission time constants of the traps can be extracted by careful analysis of the C(V) characteristics. The fixed charge is positive for all samples, with a density varying between 1.3×1012 and 7.1×1012 cm-2. For the traps, the peak density of interface states is varying between 16×1012 and 31×1012 cm-2 eV-1 for the three samples. It is concluded that, of the deposition methods investigated in this report, the low pressure chemical vapor deposited silicon nitride passivation shows the most promising results with regards to low densities of interface states. [ABSTRACT FROM AUTHOR]

AlGaN/AlN/GaN high electron mobility transistor heterostructures grown by metal-organic chemical vapor deposition have been studied by temperature dependent time-resolved photoluminescence. The AlGaN-related emission is found to be sensitive to the excitation power and to the built-in internal electric field. In addition, this emission shows a shift to higher energy with the reduction in the excitation density, which is rather unusual. Using a self-consistent calculation of the band potential profile, we suggest a recombination mechanism for the AlGaN-related emission involving electrons confined in the triangular AlGaN quantum well and holes weakly localized due to potential fluctuations. [ABSTRACT FROM AUTHOR]

The decoupling of epitaxial factors influencing the dynamic instabilities of AlGaN/GaN metal-insulator semiconductor high-electron-mobility transistors is investigated. Three different sets of samples have been analyzed by means of dynamic instabilities in the threshold voltage ( [ABSTRACT FROM AUTHOR]

Effects of residual C impurities and Ga vacancies on the dynamic instabilities of AlN/AlGaN/GaN metal insulator semiconductor high electron mobility transistors are investigated. Secondary ion mass spectroscopy, positron annihilation spectroscopy, and steady state and time-resolved photoluminescence (PL) measurements have been performed in conjunction with electrical characterization and current transient analyses. The correlation between yellow luminescence (YL), C- and Ga vacancy concentrations is investigated. Time-resolved PL indicating the CN ON complex as the main source of the YL, while Ga vacancies or related complexes with C seem not to play a major role. The device dynamic performance is found to be significantly dependent on the C concentration close to the channel of the transistor. Additionally, the magnitude of the YL is found to be in agreement with the threshold voltage shift and with the on-resistance degradation. Trap analysis of the GaN buffer shows an apparent activation energy of ~0.8 eV for all samples, pointing to a common dominating trapping process and that the growth parameters affect solely the density of trap centres. It is inferred that the trapping process is likely to be directly related to C based defects. [ABSTRACT FROM AUTHOR]

The tutor is important for student learning in the PBL group. The students expect the tutor to monitor and steer the group process and to support the learning process, helping students to become aware of their own learning. This study aimed at investigating the PBL tutor's role. Five PBL tutors at the Speech/Language Pathology program were interviewed regarding their view of the tutors' role and what support they need. The analysis of the transcribed interviews focused on finding patterns and variation regarding tutor-activity at different stages in the PBL work and in their views of their progress as tutors. The results indicate that being a tutor is a balancing act and that the tutor need continuous support and input from different sources. Tutors should be encouraged to reflect on their own reactions and interventions and to be explicit and confident in their thoughts about PBL. [ABSTRACT FROM AUTHOR]

Background/purpose An early diagnosis of cutaneous malignant melanoma is of high importance for good prognosis. An objective, non-invasive instrument could improve the diagnostic accuracy of melanoma and decrease unnecessary biopsies. The aim of this study was to investigate the use of Near-infrared and skin impedance spectroscopy in combination as a tool to distinguish between malignant and benign skin tumours. Methods Near-infrared and skin impedance spectra were collected in vivo on 50 naevi or suspect melanomas prior to excision. Received data were analysed using multivariate techniques and the results were compared to histopathology analyses of the tumours. A total of 12 cutaneous malignant melanomas, 19 dysplastic naevi and 19 benign naevi were included in the study. Results The observed sensitivity and specificity of the proposed method were 83% and 95%, respectively, for malignant melanoma. Conclusion The results indicate that the combination of near-infrared and skin impedance spectroscopy is a promising tool for non-invasive diagnosis of suspect cutaneous malignant melanomas. [ABSTRACT FROM AUTHOR]

Morphological variations of gallium polar (0001)-orientedhexagonalGaN pyramids grown by hot-wall metal organic chemical vapor depositionunder various growth conditions are investigated. The stability ofthe semipolar {11̅02} and nonpolar {11̅00} facets is particularlydiscussed. The presence of the {11̅02} facets near the apexof the pyramid was found to be controllable by tuning the absoluteflow rate of ammonia during the growth. Vertical nonpolar {11̅00}facets appeared in gallium-rich conditions, which automatically werecreated when the growth time was prolonged beyond pyramid completion.The result was attributed to a gallium passivation of the {11̅00}surface. [ABSTRACT FROM AUTHOR]

Currently, up to 50% of the channel temperature in AIGaN/GaN electronic devices is due to the thermal-boundary resistance (TBR) associated with the nucleation layer (NL) needed between GaN and SiC substrates for high-quality heteroepitaxy. Using 3-D time-resolved Raman thermography, it is shown that modifying the NL used for GaN on SiC epitaxy from the metal- organic chemical vapor deposition (MOCVD)-grown standard AIN-NL to a hot-wall MOCVD-grown AIN-NL reduces NL TBR by 25%, resulting in ∼10% reduction of the operating temperature of AIGaN/GaN HEMTs. Considering the exponential relationship between device lifetime and temperature, lower TBR NLs open new opportunities for improving the reliability of AIGaN/ GaN devices. [ABSTRACT FROM AUTHOR]

The emission of a charged exciton in an InGaN quantum dot (QD) on top of a GaN pyramid is identified experimentally. The intensity of the charged exciton exhibits the expected competition with that of the single exciton, as observed in temperature-dependent micro-photoluminescence measurements, performed with different excitation energies. The non-zero charge state of this complex is further supported by time resolved micro-photoluminescence measurements, which excludes neutral alternatives of biexciton. The potential fluctuations in the vicinity of the QD that localizes the charge carriers are proposed to be responsible for the unequal supply of electrons and holes into the QD. [ABSTRACT FROM AUTHOR]

Raman scattering of individual aluminum nitride (AlN) nanowires is investigated systematically. The axial direction of single nanowire can be rapidly verified by polarized Raman scattering. The angular dependencies of E2(high) mode show strongly anisotropic behavior in smaller nanowires, which results from optical antenna effect. Raman enhancement (RE) per unit volume of E2(high) increases with decreasing diameter of nanowires. Compared to the thin film, ∼200-fold increase of RE is observed in AlN nanowires having diameter less than 50 nm, which is far beyond the quantum confinement regime. Such a large RE can be attributed to the effects of resonant cavity and stimulated Raman scattering. [ABSTRACT FROM AUTHOR]

Growing InGaN quantum dots (QDs) at the apex of hexagonal GaN pyramids is an elegant approach to achieve a deterministic positioning of QDs. Despite similar synthesis procedures by metal organic chemical vapor deposition, the optical properties of the QDs reported in the literature vary drastically. The QDs tend to exhibit either narrow or broad emission lines in the micro-photoluminescence spectra. By coupled microstructural and optical investigations, the QDs giving rise to narrow emission lines were concluded to nucleate in association with a (0001) facet at the apex of the GaN pyramid. [ABSTRACT FROM AUTHOR]