The fabrication processes development for on III-nitride (III-N) heterojunction bipolar transistors (HBTs), heterojunction field-effect transistors (HFETs) and metal-insulator-semiconductor field-effect transistors (MISFETs) were performed. D.c, microwave and quasi-static I-V and C-V measurements were carried out to characterize the fabricated III-N transistors and diodes. The GaN/InGaN direct-growth HBTs (DG-HBTs) grown on free-standing GaN (FS-GaN) substrates demonstrated a high current gain (hfe) > 110, high current density (JC) > 141 kA/cm2, and high power density (Pdc) > 3 MW/cm2. The first III-N DG-HBT showing fT > 8 GHz and fmax > 1.3 GHz were also demonstrated on sapphire substrates. Recessed-gate AlGaN/AlN/GaN HFETs demonstrated Vth = 0 V with 0.17 V deviation across the sample. Baliga's figure of merit is 240 MW/cm2 was achieved. Current collapse was eliminated and the dynamic on-resistance was reduced by 67% after using a remote-oxygen-plasma treatment. Normally-off recessed-gate AlGaN/AlN/GaN MISFETs with Vth = 0.9 V were also fabricated with the remote-oxygen-plasma treatment. Low leakage current (< 1 pA/mm), high on-off ratio (> 2.2E11) are achieved. These achievements suggest that high-performance III-N transistors are very promising for high-power switching and microwave amplification. Findings concerning remaining process issues and implications for future research are also discussed.

Motivated by the need for a Ge surface passivation layer, chemical vapor deposition of thin (< 10 nm) films of amorphous boron carbo-nitride (BCxNy) on Ge(100) surfaces were studied to assess film continuity, interface bonding, Ge oxidation prevention, and electrical passivation. BCxNy nominally 2.5-5 nm thick continuously covers Ge(100), as determined by ion scattering spectroscopy and two angle resolved x-ray photoelectron spectroscopy (ARXPS) techniques. ARXPS analysis reveals no evidence of an interfacial layer due to elemental intermixing at the BCxNy-Ge(100) interface. High resolution transmission electron microscopy images of HfO₂ / BCxNy / Ge(100) cross-sections reveal abrupt BCxNy-Ge(100) interfaces. XPS was used to track Ge oxidation of BCxNy-covered Ge(100) upon exposure to ambient, 50 °C deionized water, and a 250 °C atomic layer deposition HfO₂ process. If the BCxNy layer is continuous ([greater-than or equal to] ~ 4 nm), the underlying Ge(100) surface is not oxidized despite incorporation of O into BCxNy. Thinner films ([less than or equal to] 3.2 nm) permitted Ge(100) oxidation in each oxidizing environment studied. Ge nanowires with a 5.7 nm BCxNy coating were resistant to oxidation for at least 5 months of ambient exposure. C-V and I-V measurements were made for metal-insulator-semiconductor (MIS) structures fabricated from n-Si(100) and n-Ge(100) wafers passivated with 4.5-5 nm BCxNy. C-rich BC0.61N0.08 films studied up to this point exhibited large amounts of hysteresis and fixed negative charge, so they were abandoned in favor of N-rich BCxNy (0.09 [less than or equal to] x [less than or equal to] 0.15, 0.38 [less than or equal to] y [less than or equal to] 0.52). N-rich BCxNy grown at 275-400 °C showed that lower deposition temperatures resulted in improved electrical characteristics, including decreased hysteresis, lower VFB shift, lower leakage current, and less C-V stretch-out. The electrical improvement is attributed to decreased bulk and interfacial defects in BCxNy deposited at lower temperatures. Even for the lowest growth temperature studied (275 °C), BCxNy-passivated Ge(100) devices had considerable hysteresis and electrical characteristics worsened after a post-metallization anneal. BCxNy-passivated Si(100) devices outperformed similar Ge(100) devices, likely due to the higher interface state densities at the BCxNy-Ge(100) interface associated with the higher relative inertness of Ge(100) to thermal nitridation. / text

Two-dimensional graphene, a single layer of graphite, has emerged as an excellent candidate for future electronic material due to its unique electronic structure and remarkably high carrier mobility. Even higher carrier mobility has been demonstrated in graphene devices using hexagonal boron nitride as an underlying dielectric support instead of silicon oxide. Interestingly, both graphene and boron nitride exhibit superior thermal properties, therefore may potentially offer a solution to the increasingly severe heat dissipation problem in nanoelectronics caused by increased power density. In this thesis, we focus on the investigation of the thermal properties of graphene and hexagonal boron nitride. First, scanning thermal microscopy based on a sub-micrometer thermocouple at the apex of a microfabricated tip was employed to image the temperature profiles in electrically biased graphene devices with ~ 100 nm scale spatial resolution. Non-uniform temperature distribution in the devices was observed, and the "hot spot" locations were correlated with the charge concentrations in the channel, which could be controlled by both gate and drain-source biases. Hybrid contact and lift mode scanning has enabled us to obtain the quantitative temperature profiles, which were compared with the profiles obtained from Raman-based thermometry. The temperature rise in the channel provided an important insight into the heat dissipation mechanism in Joule-heated graphene devices. Next, thermal conductivity of suspended single and few-layer graphene was measured using a micro-bridge device with built-in resistance thermometers. Polymer-assisted transfer technique was developed to suspend graphene layers on the pre-fabricated device. The room temperature thermal conductivity values of 1-7 layer graphene were measured to be lower than that of bulk graphite, and the value appeared to increase with increasing sample thickness. These observations can be explained by the impact of the phonon scattering by polymer residue remaining on the sample surfaces. Lastly, thermal conductivity of few-layer hexagonal boron nitride sample was measured by using the same device and technique used for suspended graphene. Measurements on samples with different suspended lengths but similar thickness allowed us to extract the intrinsic thermal conductivity of the samples as well as the contribution of contact thermal resistance to the overall thermal measurement. The room temperature thermal conductivity of 11 layer sample approaches the basal-plane value reported in the bulk sample. Lower thermal conductivity was measured in a 5 layer sample than an 11 layer sample, which again supports the polymer effect on the thermal transport in few-layer hexagonal boron nitride. / text

Mid-infrared lasers in the 3-5 µm range are important for wide variety of applications including trace gas sensing, infrared counter measures, free space optical communications, etc. GaSb-based type-I quantum well (QW) diode lasers are an attractive choice due to their relatively simple design and growth tolerances, as compared with quantum cascade lasers and interband cascade lasers. Excellent diode lasers have been demonstrated for wavelengths up to ~3.0 µm, employing GaInAsSb/AlGaAsSb QW active regions. But, device performance tends to degrade at longer wavelengths, due to Auger recombination and decreasing QW valence band offsets. In this work we look into the feasibility of using highly strained GaInAsSb/GaSb QWs as active regions for diode lasers operating at wavelengths beyond 3.0 µm. Heavy strain in the QW can improve valence band offset and also increase the splitting between the heavy and light hole bands which can help minimize Auger recombination. Through optimized molecular beam epitaxy (MBE) growth conditions we were able to incorporate up to 2.45 % compressive strain in these QWs enabling laser operation up to 3.4 µm at room temperature. An alternate path to extend the emission wavelength is to incorporate dilute quantities of nitrogen into the QW. Incorporating dilute quantities of substitutional nitrogen into traditional III-V’s strongly reduces the bandgap of the alloy. The advantage for the case of GaSb based dilute-nitrides is that the bandgap reduction is almost exclusively due to the lowering of the conduction band leaving the valence band offsets unaffected; thus providing a path to mitigating hole leakage while extending the emission wavelength. Although GaSb-based dilute-nitrides are a potentially elegant solution for extending the operating wavelength of GaSb-based type-I QW diode lasers, the luminescence efficiency of this material system has been relatively poor. This is most likely due to the presence of a high concentration of point defects, like nitrogen substitutional clusters. Through careful optimization of MBE growth conditions and post growth annealing, we demonstrate improved luminescence efficiency. With further optimization this material system can potentially extend the emission wavelength of GaSb-based type-I QW diode lasers even further into the mid-infrared spectrum. / text

Not available / text

published_or_final_version / Physics / Master / Master of Philosophy

published_or_final_version / Physics / Master / Master of Philosophy

Thermal conductivity is a crucial parameter for nuclear fuel, as it sets an upper limit on reactor operating temperature to have safety margins. Uranium mononitride (UN) is a prospective fuel for fast reactors, for which limited experimental studies have been conducted, compared to the currently dominating light-water reactor fuel, uranium dioxide. The aim of this thesis is to determine the thermal conductivity in UN and to determine its porosity dependence. This was done by manufacturing dense and porous high-purity samples of UN and examining them with laser flash analysis, which with data on specific heat and thermal expansion gives the thermal conductivity. To analyse the result, a theoretical study of the phenomenology of thermal conductivity as well as a review and comparison with previous investigations were carried out. The porosity range was 0.1–31% of theoretical density. Thermal diffusivity data from laser flash analysis, thermal expansion data and specific heat data was collected for 25–1400 C. The laser flash data had high discrepancy at higher temperatures due to thermal instability in the device and deviations due to graphite deposition on the samples, but the low temperature data should be reliable. As the specific heat data was also of poor quality, literature data was used instead. As for the thermal diffusivity data, the calculated thermal conductivity for lower temperatures are more accurate. A modified version of the porosity model by Ondracek and Schulz was used to analyse the porosity dependence of the thermal conductivity, taking into account the different impacts of open and closed porosity. / Värmeledningsförmåga är en avgörande egenskap för kärnbränslen, eftersom det begränsar den maximala drifttemperaturen i reaktorn för att ha säkerhetsmarginaler. Uranmononitrid (UN) är ett framtida bränsle för snabba reaktorer. Jämfört med det dominerande bränslet i lättvattenreaktorer, urandioxid, har endast begränsade experimentella studier gjorts av UN. Målet med detta arbete är att bestämma värmeledningsförmågan i UN och bestämma dess porositetsberoende. Detta gjordes genom att tillverka kompakta och porösa prover av UN och undersöka dem med laserblixtmetoden, vilket tillsammans med värmekapacitet och värmeutvidgning ger värmeledningsförmågan. För att analysera resultatet gjordes en teoretisk studie av värmeledning såväl som en genomgång av och jämförelse med tidigare undersökningar. Provernas porositet sträckte sig från 0.1% till 31% av teoretisk densitet. Värmediffusivitetsdata från laserblixtmetoden, värmeutvidgningsdata och värmekapacitetsdata samlades in för 25–1400 C. Värdena från laserblixtmätningen hade hög diskrepans vid höga temperaturer p.g.a. termisk instabilitet i anordningen och avvikelser p.g.a. grafitavlagring på proverna, men data för låga temperaturer borde vara tillförlitliga. Eftersom resultaten från värmekapacitetsmätningen var av dålig kvalité, användes litteraturdata istället. Som en konsekvens av bristerna i mätningen av värmediffusivitet är presenterade data för värmeledningsförmåga mest exakta för låga temperaturer. En modifierad version av Ondracek-Schulz porositetsmodell användes för att analysera värmeledningsförmågans porositetsberoende genom att ta hänsyn till olika inverkan av öppen och sluten porositet.

Cubic boron nitride (c-BN) has been in focus for several years due to its interesting properties. The possibility for large area chemical vapor deposition (CVD) is a requirement for the realization of these different properties in various applications. Unfortunately, there are at present severe problems in the CVD growth of c-BN. The purpose with this research project has been to theoretically investigate, using density functional theory (DFT) calculations, the possibility for a layer-by-layer CVD growth of c-BN.  The results, in addition with experimental work by Zhang et al.57,  indicate that plasma-enhanced atomic layer deposition (PEALD), using a BF3-H2-NH3-F2 pulse cycle and a diamond substrate, is a promising method for deposition of c-BN films. The gaseous species will decompose in the plasma and form BFx, H, NHx, and F species (x = 0, 1, 2, 3). The H and F radicals will uphold the cubic structure by completely hydrogenate, or fluorinate, the growing surface. Surface radical sites will appear during the growth process as a result of atomic H, or F, abstraction reactions. However, introduction of energy (e.g., ionic bombardment) is probably necessary to promote removal of H from the surface. The addition of NHx growth species (x = 0, 1, 2) to the B radical sites, and BFx growth species (x = 0, 1, 2) to N radical sites, will then result in a continuous growth of c-BN.

GaN RF switches are widely used in today’s communication systems. With digital communications getting more and more popular nowadays, the need for improving the performance of involved RF switches is inevitable. Designing low ON-state resistance GaN switches are exceedingly important to improve the switch insertion loss, isolation and power loss. Moreover, considerations need to be taken into account to improve the switching speed of the involved GaN HEMTs. In this dissertation, a new GaN HEMT structure called “Tunnel MOS Heterostructure FET (TMOSHFET)” is introduced which has lower ON-state resistance and faster switching speed compared to conventional AlGaN/GaN HEMTs. In the switch ON process, the channel of this device is charged up by electron tunneling from a layer underneath the channel as opposed to typical AlGaN/GaN HEMTs in which electron injection from the source is charging up the channel. The tunneling nature of this process together with the shorter travel distance of electrons in TMOSHFET provide for a faster switching speed. In order to understand the tunneling mechanisms in TMOSHFET, the fabrication of AlGaN/GaN Schottky Barrier Diodes (SBDs) with various AlGaN thicknesses is demonstrated on Si (111) substrate. The impacts of SF6 dry etching on the trap density and trap state energy of AlGaN surface are investigated using the GP/w- w method. Various tunneling mechanisms at different biases are then characterized in samples and compared with each other. To improve the source and drain resistances in TMOSHFET, a model is generated to optimize the 2DEG density and electric field in AlGaN/GaN heterostructure based on Al mole fraction, AlGaN thickness and the thickness of SiN passivation layer and it is experimentally verified by non-contact Hall 2DEG density measurements. The spontaneous and piezoelectric polarizations together with strain relaxation have been implemented into the model, taking into account the annealing effects. From the experimental data on obtained parameters, the operation and device parameterization of the TMOSHFET is outlined and design considerations to improve the device R_(ON)-V_(BR) figure of merit are discussed.