Growth and characterization of SiC and GaN

At present, focus of the SiC crystal growth development is on improving the crystalline quality without polytype inclusions, micropipes and the occurrence of extended defects. The purity of the grown material, as well as intentional doping must be well controlled and the processes understood. High-quality substrates will significantly improve device performance and yield. One of the aims of the thesis is further understanding of polytype inclusion formation as well as impurity control in SiC bulk crystals grown using PVT method also termed seeded sublimation method. Carbonization of the source was identified as a major reason behind the polytype inclusion occurrence during the growth. The aim of this work was further understanding of sublimation growth process of 4H-SiC bulk crystals in vacuum, in absence of an inert gas. For comparison growth in argon atmosphere (at 5 mbar) was performed. The effect of the ambient on the impurity incorporation was studied for different growth temperatures. For better control of the process in vacuum, tantalum as a carbon getter was utilized. The focus of the SiC part of the thesis was put on further understanding of the PVT epitaxy with an emphasis on the high growth rate and purity of grown layers. High resistivity 4H-SiC samples grown by sublimation with high growth rate were studied. The measurements show resistivity values up to high 104 cm. By correlation between the growth conditions and SIMS results, a model was applied in which it is proposed that an isolated carbon vacancy donor-like level is a possible candidate responsible for compensation of the shallow acceptors in p-type 4H-SiC. A relation between cathodoluminescence (CL) and DLTS data is taken into account to support the model. To meet the requirements for high voltage blocking devices such as high voltage Schottky diodes and MOSFETs, 4H-SiC epitaxial layers have to exhibit low doping concentration in order to block reverse voltages up to few keV and at the same time have a low on-state resistance (Ron). High Ron leads to enhanced power consumption in the operation mode of the devices. In growth of thick layers for high voltage blocking devices, the conditions to achieve good on-state characteristics become more challenging due to the low doping and pronounced thicknesses needed, preferably in short growth periods. In case of high-speed epitaxy such as the sublimation, the need to apply higher growth temperature to yield the high growth rate, results in an increased concentration of background impurities in the layers as well as an influence on the intrinsic defects. On-state resistance Ron estimated from current density-voltage characteristics of Schottky diodes on thick sublimation layers exhibits variations from tens of mΩ.cm2 to tens of Ω.cm2 for different doping levels. In order to understand the occurrence of high on-state resistance, Schottky barrier heights were first estimated for both forward and reverse bias with the application of thermionic emission theory and were in agreement with literature reported values. Decrease in mobility with increasing temperature was observed and its dependencies of T–1.3 and T–2.0 for moderately doped and low doped samples, respectively, were estimated. From deep level measurements by Minority Carrier Transient Spectroscopy (MCTS), an influence of shallow boron related levels and D-center on the on-state resistance was observed, being more pronounced in low doped samples. Similar tendency was observed in depth profiling of Ron. This suggests a major role of boron in a compensation mechanism. In the second part of the thesis growth and characterization of GaN is presented. Excellent electron transport properties with high electron saturate drift velocity make GaN an excellent candidate for electronic devices. Especially, AlGaN/GaN based high electron mobility transistors (HEMT) have received an increased attention in last years due to their attractive properties. The presence of strong spontaneous and piezoelectric polarization due to the lattice mismatch between AlGaN and GaN is responsible for high free electrons concentrations present in the vicinity of the interface. Due to the spatial separation of electrons and ionized donors or surface states, 2DEG electron gas formed near the interface of the heterostructure exhibits high sheet carrier density and high mobility of electrons. Al0.23Ga0.77N/GaN based HEMT structures with an AlN exclusion layer on 100 mm semiinsulating 4H-SiC substrates have been grown by hot-wall MOCVD. The electrical properties of the two-dimensional electron gas (2DEG) such as electron mobility, sheet carrier density and sheet resistance were obtained from Hall measurements, capacitance-voltage and contact-less eddy-current techniques. The effect of different scattering mechanisms on the mobility have been taken into account and compared to the experimental data. Hall measurements were performed in the range of 80 to 600 K. Hall electron mobility is equal to 17140 cm2(Vs)-1 at 80 K, 2310 cm2(Vs)-1 at room temperature, and as high as 800 cm2(Vs)-1 at 450 K, while the sheet carrier density is 1.04x1013 cm-2 at room temperature and does not vary very much with temperature. Estimation of different electron scattering mechanisms reveals that at temperatures higher than room temperature, experimental mobility data is mainly limited by optical phonon scattering. At relevant high power device temperature (450 K) there is still an increase of mobility due to the AlN exclusion layer. We have studied the behaviour of Ga-face GaN epilayers after in-situ thermal treatment in different gas mixtures in a hot-wall MOCVD reactor. Influence of N2, N2+NH3 and N2+NH3+H2 ambient on the morphology was investigated in this work. The most stable thermal treatment conditions were obtained in the case of N2+NH3 gas ambients. We have also studied the effect of the increased molar ratio of hydrogen in order to establish proper etching conditions for hot-wall MOCVD growth

Dislocation related droop in InGaN/GaN light emitting diodes investigated via cathodoluminescence

Todays energy saving solutions for general illumination rely on efficient white light emitting diodes (LEDs). However, the output efficiency droop experienced in InGaN based LEDs with increasing current injection is a serious limitation factor for future development of bright white LEDs. We show using cathodoluminescence (CL) spatial mapping at different electron beam currents that threading dislocations are active as nonradiative recombination centers only at high injection conditions. At low current, the dislocations are inactive in carrier recombination due to local potentials, but these potentials are screened by carriers at higher injection levels. In CL images, this corresponds to the increase of the dark contrast around dislocations with the injection (excitation) density and can be linked with droop related to the threading dislocations. Our data indicate that reduction of droop in the future efficient white LED can be achieved via a drastic reduction of the dislocation density by using, for example, bulk native substrates. (C) 2015 AIP Publishing LLC.Funding Agencies|Swedish Research Council (VR); Swedish Energy Agency</p

Improved hot-wall MOCVD growth of highly uniform AlGaN/GaN/HEMT structures

The inherent advantages of the hot-wall metal organic chemical vapor deposition (MOCVD) reactor (low temperature gradients, less bowing of the wafer during growth, efficient precursor cracking) compared to a cold-wall reactor make it easier to obtain uniform growth. However, arcing may occur in the growth chamber during growth, which deteriorates the properties of the grown material. By inserting insulating pyrolytic BN (PBN) stripes in the growth chamber we have completely eliminated this problem. Using this novel approach we have grown highly uniform, advanced high electron mobility transistor (HEMT) structures on 4 semi-insulating (SI) SiC substrates with gas-foil rotation of the substrate. The nonuniformities of sheet resistance and epilayer thickness are typically less than 3% over the wafer. The room temperature hall mobility of the 2DEG is well above 2000 cm(2)/V s and the sheet resistance about 270 Omega/sqr.Original Publication: Urban Forsberg, Anders Lundskog, A Kakanakova-Georgieva, Rafal Ciechonski and Erik Janzén, Improved hot-wall MOCVD growth of highly uniform AlGaN/GaN/HEMT structures, 2009, JOURNAL OF CRYSTAL GROWTH, (311), 10, 3007-3010. http://dx.doi.org/10.1016/j.jcrysgro.2009.01.045 Copyright: Elsevier Science B.V., Amsterdam. http://www.elsevier.com

Cognitive impairment evaluation of multiple sclerosis patients with different clinical progress

Ievads: Tēmu par kognitīvajiem traucējumiem multiplās sklerozes pacientiem es izvēlējos tāpēc, ka tā ir ļoti aktuāla problēma, jo kognitīvie traucējumi var būtiski ietekmēt darbspējas, sociālo aktivitāti un aprūpes nepieciešamību. Darba mērķis: Izvērtēt kognitīvos traucējumus multiplās sklerozes pacientiem ar dažādu klīnisko norisi; Metodes un materiāli: Pētījumā analizēju 120 dažāda vecuma pacientus, no kuriem 60 multiplās sklerozes pacienti ar dažādu klīnisko norisi, kuri ārstējas Latvijas Jūras medicīnas centra Latvijas Multiplās sklerozes centrā un 60 pacienti kontroles grupa, kam nav diagnosticēta multiplā skleroze. Šim nolūkam izmantoju standartizētus testus – SDMT, BVMT-R un CvLT II, kas ļāva izvērtēt kognitīvos traucējumus. Pētījumā analizēju arī pacientu EDSS un to saistību ar kognitīvajiem traucējumiem. Rezultāti: Multiplās sklerozes pacientiem kognitīvie traucējumi ir sastopami 46 – 78 % gadījumu. Vidējais kopējais iegūto punktu skaits CvLT II multiplās sklerozes pacientiem ir 42,98 (SD± 14,66), bet vidējais kopējais iegūto punktu skaits CvLT II kontroles grupā ir 58,20 (SD± 11,43)(p=0,01). Vidējais kopējais punktu skaits BVMT – R multiplās sklerozes pacientiem ir 19,00 (SD± 8,49), bet vidējais kopējais punktu skaits BVMT – R kontroles grupā ir 26,85 (SD± 6,65)(p=0,01). Vidējais kopējais punktu skaits SDMT multiplās sklerozes pacientiem ir 36,38 (SD± 15,10), bet vidējais kopējais punktu skaits SDMT kontroles grupā ir 47,53 (SD± 9,36). Šī sakarība ir statistiski ticama (p=0,01). Vērtējot kopumā visus BICAMS testos vidējo kopējo iegūto punktu skaitu, multiplās sklerozes pacientiem tas ir 32,78 (SD± 12,38) bet, vidējais kopējais iegūto punktu skaits kontroles grupā ir 44,19 (SD± 15,93)(p=0,01). Pētījumā izdevās pierādīt pacientu EDSS saistību ar kognitīviem traucējumiem (p=0,01), kā arī pacientu slimības formu saistību ar kognitīviem traucējumiem (p=0,05). Secinājumi: Pētījumā secināts, ka ar konkrētajiem testiem var ātri diagnosticēt kognitīvos traucējumus. Atkarībā no testa veida multiplās sklerozes pacientiem kognitīvie traucējumi ir sastopami 46 – 78 % gadījumu. Pacientiem ar augstāku EDSS, sliktāk veicas ar dažādu kognitīvo testu izpildi. Pētījumā iegūtie rezultāti, norāda, ka pacientiem ar primāri progresējošu un sekundāri progresējošu slimības formu ir izteiktāki kognitīvie traucējumi nekā multiplās sklerozes pacientiem ar recidivējoši remitējošu slimības formu.Introduction: I chose the subject of cognitive impairment evaluation of multiple sclerosis patients with different clinical progress, because it is very common problem. Cognitive impairment may have a significant impact on patients health, social activity and care needs. Aim: assessing cognitive disorders of multiple sclerosis patients with different clinical progress; Methods and materials: 120 patients of various ages were included in this study, from which 60 are multiple sclerosis patients with different clinical progress, were treated in Latvian Maritime Medical Centre, in the Multiple sclerosis centre of the Latvia and 60 patients in control group who do not have approved diagnosis of multiple sclerosis. For this purpose I used standardised tests – SDMT, BVMT–R and CvLT II, which make it possible to assess the cognitive impairment. In this study I also analysed the EDSS and its relationship to cognitive disorders. Results: 46 – 78% multiple sclerosis patients have cognitive impairment. The average total number of points obtained CvLT II in multiple sclerosis patients is 42,98 (SD 14,66), but the total number of points obtained CvLT II control group is 58,20 (SD 11,43)(p = 0,01). The average total score BVMT – R multiple sclerosis patients is 19,00 (SD 8,49) and the total number of points BVMT – R in the control group was 26,85 (SD 6,65)(p = 0,01). The average total score SDMT multiple sclerosis patients is 36,38 (SD 15,10) and the total number of points in the control group SDMT is 47,53 (SD 9,36). This relationship is statistically significant (p = 0,01). In general, all BICAMS tests assessing the total number of points acquired in multiple sclerosis patients is 32,78 (SD 12,38) and the total number of points obtained in the control group was 44,19 (SD 15,93) (p = 0,01). The study show relationship between EDSS and patients cognitive disabilities (p = 0,01) and also relationship between the form of the disease and cognitive disabilities (p = 0,05). Conclusion: the study concluded that with these tests we can quickly diagnose the cognitive impairment. 46 – 78% multiple sclerosis patients have cognitive impairment. Patients with higher EDSS have worse results in various cognitive tests. The results of the study indicate that patients with primary and secondary progressive form of the disease is more common cognitive impairment than multiple sclerosis patients with the relapsing-remitting form of the disease

Local defect-enhanced anodic oxidation of reformed GaN nanowires

Understanding formation and distribution of defects in GaN substrates and device layers is needed to improve device performance in rf and power electronics. Here we utilize conductive atomic force microscopy (c-AFM) for studying defect-related leakage paths in an unintentionally doped GaN film formed by nanowire reformation. A nanoscopic Schottky contact is formed between the c-AFM probe and the GaN surface, which, under reverse-bias conditions, reveals local leakage currents at the positions of the nanowires. Cathodoluminescence shows these areas to be dominated by yellow-band luminescence, in contrast to the surrounding GaN matrix, which mainly shows near-band-gap luminescence. These results are attributed to a high density of native and residual defects, confined to the nanowires. In addition, we use anodic oxidation to map defect-related conductive paths through locally induced growth of gallium oxide. The oxide yield, which is known to depend on the local electric field strength between the AFM tip and the sample, correlates well with the level of reverse-bias leakage current. Local irregularities in oxide height reveal extended oxidation attributed to defect-related deep-level states. Thisis confirmed by controlled dissolution of the oxide in NaOH, showing that a deeper oxide film is grown over areas where defect-related conductive paths are formed. Finally, we demonstrate how this approach can be used as a quick and easy diagnostic tool for evaluating the influence of specific growth conditions and process steps on defect-induced leakage current levels and defect distribution in GaN structures, demonstrating its potential for accelerated test of leakage degradation at critical positions in GaN-based devices

Extraordinary Renditions (Stockholm): The Cultural Negotiation of Science

Christine Borland, Fiona Crisp, Chris Dorsett, Rona Lee The Cultural Negotiation of Science is a research group based at Northumbria University, Newcastle, UK that brings together several artists and academics whose practices engage with expert cultures across a broad spectrum of science and technology. Collectively, the group is characterised by a performative approach to the production of knowledge that actively challenges the use of art as an instrumental or illustrative device to interpret science.Here at the Royal Institue of Art, four artists from the Cultural Negotiation of Science group, Christine Borland, Fiona Crisp, Chris Dorsett and Rona Lee, will talk about their work across bio-medical ethics, fundamental science, genetics & museum holdings and physical geography respectively

Fast Strain Mapping of Nanowire Light-Emitting Diodes Using Nanofocused X-ray Beams

X-ray nanobeams are unique nondestructive probes that allow direct measurements of the nanoscale strain distribution and composition inside the micrometer thick layered structures that are found in most electronic device architectures. However, the method is usually extremely time-consuming, and as a result, data sets are often constrained to a few or even single objects. Here we demonstrate that by special design of a nanofocused X-ray beam diffraction experiment we can (in a single 2D scan with no sample rotation) measure the individual strain and composition profiles of many structures in an array of upright standing nanowires. We make use of the observation that in the generic nanowire device configuration, which is found in high-speed transistors, solar cells, and light-emitting diodes, each wire exhibits very small degrees of random tilts and twists toward the substrate. Although the tilt and twist are very small, they give a new contrast mechanism between different wires. In the present case, we image complex nanowires for nanoLED fabrication and compare to theoretical simulations, demonstrating that this fast method is suitable for real nanostructured devices