Tomas L Martin

Thermal Boundary Resistance Between GaN and Substrate in AlGaN/GaN Electronic Devices

The influence of a thermal boundary resistance (TBR) on temperature distribution in ungated AlGaN/GaN field-effect devices was investigated using 3-D micro-Raman thermography. The temperature distribution in operating AlGaN/GaN devices on SiC, sapphire, and Si substrates was used to determine values for the TBR by comparing experimental results to finite-difference thermal simulations. While the measured TBR of about 3.3 x 10^-8 W^-1 ldr m² ldr K for devices on SiC and Si substrates has a sizeable effect on the self-heating in devices, the TBR of up to 1.2 x 10^-8 W^-1 ldr m² ldr K plays an insignificant role in devices on sapphire substrates due to the low thermal conductivity of the substrate. The determined effective TBR was found to increase with temperature at the GaN/SiC interface from 3.3 x 10^-8 W^-1 ldr m² ldr K at 150degC to 6.5 x 3.3 x 10^-8 W^-1 ldr m² ldr K at 275degC, respectively. The contribution of a low-thermal-conductivity GaN layer at the GaN/substrate interface toward the effective TBR in devices and its temperature dependence are also discussed.

Structural, electronic and optical properties of m-plane (In,Ga)N/GaN quantum wells: Insights from experiment and atomistic theory

In this paper we present a detailed analysis of the structural, electronic, and optical properties of an

Indium clustering in a-plane InGaN quantum wells as evidenced by atom probe tomography

m

The microstructure of non-polar a-plane (11 2¯0) InGaN quantum wells

-plane (In,Ga)N/GaN quantum well structure grown by metal organic vapor phase epitaxy. The sample has been structurally characterized by x-ray diffraction, scanning transmission electron microscopy, and 3D atom probe tomography. The optical properties of the sample have been studied by photoluminescence (PL), time-resolved PL spectroscopy, and polarized PL excitation spectroscopy. The PL spectrum consisted of a very broad PL line with a high degree of optical linear polarization. To understand the optical properties we have performed atomistic tight-binding calculations, and based on our initial atom probe tomography data, the model includes the effects of strain and built-in field variations arising from random alloy fluctuations. Furthermore, we included Coulomb effects in the calculations. Our microscopic theoretical description reveals strong hole wave function localization effects due to random alloy fluctuations, resulting in strong variations in ground state energies and consequently the corresponding transition energies. This is consistent with the experimentally observed broad PL peak. Furthermore, when including Coulomb contributions in the calculations we find strong exciton localization effects which explain the form of the PL decay transients. Additionally, the theoretical results confirm the experimentally observed high degree of optical linear polarization. Overall, the theoretical data are in very good agreement with the experimental findings, highlighting the strong impact of the microscopic alloy structure on the optoelectronic properties of these systems.

Origin of kink effect in AlGaN/GaN high electron mobility transistors: Yellow luminescence and Fe doping

Atom probe tomography (APT) has been used to characterize the distribution of In atoms within non-polar a-plane InGaN quantum wells (QWs) grown on a GaN pseudo-substrate produced using epitaxial lateral overgrowth. Application of the focused ion beam microscope enabled APT needles to be prepared from the low defect density regions of the grown sample. A complementary analysis was also undertaken on QWs having comparable In contents grown on polar c-plane sample pseudo-substrates. Both frequency distribution and modified nearest neighbor analyses indicate a statistically non-randomized In distribution in the a-plane QWs, but a random distribution in the c-plane QWs. This work not only provides insights into the structure of non-polar a-plane QWs but also shows that APT is capable of detecting as-grown nanoscale clustering in InGaN and thus validates the reliability of earlier APT analyses of the In distribution in c-plane InGaN QWs which show no such clustering.

Insights into microstructural interfaces in aerospace alloys characterised by atom probe tomography

Atom probe tomography and quantitative scanning transmission electron microscopy are used to assess the composition of non-polar a-plane (11-20) InGaN quantum wells for applications in optoelectronics. The average quantum well composition measured by atom probe tomography and quantitative scanning transmission electron microscopy quantitatively agrees with measurements by X-ray diffraction. Atom probe tomography is further applied to study the distribution of indium atoms in non-polar a-plane (11-20) InGaN quantum wells. An inhomogeneous indium distribution is observed by frequency distribution analysis of the atom probe tomography measurements. The optical properties of non-polar (11-20) InGaN quantum wells with indium compositions varying from 7.9% to 20.6% are studied. In contrast to non-polar m-plane (1-100) InGaN quantum wells, the non-polar a-plane (11-20) InGaN quantum wells emit at longer emission wavelengths at the equivalent indium composition. The non-polar a-plane (11-20) quantum wells also show broader spectral linewidths. The longer emission wavelengths and broader spectral linewidths may be related to the observed inhomogeneous indium distribution.

Continuous and discontinuous precipitation in Fe-1 at.%Cr-1 at.%Mo alloy upon nitriding; crystal structure and composition of ternary nitrides

AlGaN/GaN high electron mobility transistors with different Fe-doping density were studied using electrical and optical analysis to gain insight into the nature of traps responsible for the kink effect in electrical characteristics. Kink effect has been previously suggested to result from direct trapping of carriers in defects related to yellow luminescence (YL) centers. However, the results demonstrate that YL is suppressed by Fe doping, whereas the kink effect is not affected to the same extent. YL related defect states are therefore not exclusively responsible for the kink effect, suggesting a more complex trapping mechanism to affect device output characteristics.

The atomic structure of polar and non-polar InGaN quantum wells and the green gap problem

Atom probe tomography (APT) is becoming increasingly applied to understand the relationship between the structure and composition of new alloys at the micro- and nanoscale and their physical properties. Here, we use APT datasets from two modern aerospace alloys to highlight the detailed information available from APT analysis, along with potential pitfalls that can affect data interpretation. The interface between two phases in a Ti–6Al–4V alloy is used to illustrate the importance of parameter choice when using proximity histograms or concentration profiles to characterise interfacial chemistry. The higher number density of precipitates and large number of constituent elements in a maraging steel (F1E) present additional challenges such as peak overlaps that vary across the dataset, along with inhomogeneous interface chemistries.

Atomic-scale Studies of Uranium Oxidation and Corrosion by Water Vapour

Abstract The internal nitriding response of a ternary Fe–1 at.%Cr–1 at.%Mo alloy, which serves as a model alloy for many CrMo-based steels, was investigated. The nitrides developing upon nitriding were characterised by X-ray diffraction, scanning electron microscopy, electron probe microanalysis, transmission electron microscopy and atom probe tomography. The developed nitrides were shown to be (metastable) ternary mixed nitrides, which exhibit complex morphological, compositional and structural transformations as a function of nitriding time. Analogous to nitrided binary Fe–Cr and Fe–Mo alloys, in ternary Fe–Cr–Mo alloys initially continuous precipitation of fine, coherent, cubic, NaCl-type nitride platelets, here with the composition (Cr½,Mo½)N¾, occurs, with the broad faces of the platelets parallel to the {1 0 0}α-Fe lattice planes. These nitrides undergo a discontinuous precipitation reaction upon prolonged nitriding leading to the development of lamellae of a novel, hexagonal CrMoN2 nitride along {1 1 0}α-Fe lattice planes, and of spherical cubic, NaCl-type (Cr,Mo)Nx nitride particles within the ferrite lamellae. The observed structural and compositional changes of the ternary nitrides have been attributed to the thermodynamic and kinetic constraints for the internal precipitation of (misfitting) nitrides in the ferrite matrix.

Research data supporting "A SANS and APT study of precipitate evolution and strengthening in a maraging steel"

We have used high resolution transmission electron microscopy (HRTEM), aberration-corrected quantitative scanning transmission electron microscopy (Q-STEM), atom probe tomography (APT) and X-ray diffraction (XRD) to study the atomic structure of (0001) polar and (11-20) non-polar InGaN quantum wells (QWs). This paper provides an overview of the results. Polar (0001) InGaN in QWs is a random alloy, with In replacing Ga randomly. The InGaN QWs have atomic height interface steps, resulting in QW width fluctuations. The electrons are localised at the top QW interface by the built-in electric field and the well-width fluctuations, with a localisation energy of typically 20meV. The holes are localised near the bottom QW interface, by indium fluctuations in the random alloy, with a localisation energy of typically 60meV. On the other hand, the non-polar (11-20) InGaN QWs contain nanometre-scale indium-rich clusters which we suggest localise the carriers and produce longer wavelength (lower energy) emission than from random alloy non-polar InGaN QWs of the same average composition. The reason for the indium-rich clusters in non-polar (11-20) InGaN QWs is not yet clear, but may be connected to the lower QW growth temperature for the (11-20) InGaN QWs compared to the (0001) polar InGaN QWs.