<p>A research group at the Norwegian University of Science and Technology plan to integrate Capacitive Micromachined Ultrasonic Transducers (CMUTs) in a probe which can be used to image human blood vessels from the inside. With CMUT technology still being in its youth, it is important to characterize different CMUT prototypes to learn which design is best suited for the probe. In this thesis, a network analyzer and a heterodyne interferometer are used to characterize two prototypes, both electrically and optically. The heterodyne interferometer was proven to be extremely sensitive and thus a valuable tool when investigating the behaviour of single CMUT cells. Measurements revealed that the cells within both the CMUT arrays had different resonance frequencies. This effectively broadens the band widths of the prototypes, but reduces their maximum acoustical output power, given that all the cells operate at the same frequency. It was found probable that the CMUT characteristics were influenced by how charges moved within the prototypes. These charging effects are quite complex and would in practice make it hard to predict where the centre frequency of the CMUT array lies from time to time. The vibration pattern of most the cells at their respective resonance frequencies looked healthy both in air and immersed in a liquid when measuring with the interferometer. Immersed, the resonant frequency range of the CMUT become wider, but the peak was located at a lower frequency than in air. The next CMUT prototype to be tested in the future does not fit into the experimental setup. A plan to adapt the setup has been presented in this thesis, making it more practical to handle different samples. This design would also make it easier to investigate crosstalk effects between neighbouring CMUT elements within the arrays. Knowledge aquired in this thesis will hopefully help researchers figure out which areas of the CMUT design have the potential to be improved.</p>

A research group at the Norwegian University of Science and Technology plan to integrate Capacitive Micromachined Ultrasonic Transducers (CMUTs) in a probe which can be used to image human blood vessels from the inside. With CMUT technology still being in its youth, it is important to characterize different CMUT prototypes to learn which design is best suited for the probe. In this thesis, a network analyzer and a heterodyne interferometer are used to characterize two prototypes, both electrically and optically. The heterodyne interferometer was proven to be extremely sensitive and thus a valuable tool when investigating the behaviour of single CMUT cells. Measurements revealed that the cells within both the CMUT arrays had different resonance frequencies. This effectively broadens the band widths of the prototypes, but reduces their maximum acoustical output power, given that all the cells operate at the same frequency. It was found probable that the CMUT characteristics were influenced by how charges moved within the prototypes. These charging effects are quite complex and would in practice make it hard to predict where the centre frequency of the CMUT array lies from time to time. The vibration pattern of most the cells at their respective resonance frequencies looked healthy both in air and immersed in a liquid when measuring with the interferometer. Immersed, the resonant frequency range of the CMUT become wider, but the peak was located at a lower frequency than in air. The next CMUT prototype to be tested in the future does not fit into the experimental setup. A plan to adapt the setup has been presented in this thesis, making it more practical to handle different samples. This design would also make it easier to investigate crosstalk effects between neighbouring CMUT elements within the arrays. Knowledge aquired in this thesis will hopefully help researchers figure out which areas of the CMUT design have the potential to be improved.

One of the leading causes of death in the western world today is heart diseases, and atherosclerotic plaque that ruptures in the coronary arteries is believed to be one of the main causes of heart attacks.The Capacitive Micromachined Transducer has good acoustic matching with fluids, and is considered a good candidate for improving intravascular ultrasound diagnostics.Further evaluation and characterization of a CMUT operating in immersion has been the objective of this thesis, and it is a continuation of the work started in the project Characterization of Capacitive Ultrasound Transducers. The same CMUT have been studied in both, but a backing layer was added to it for the testing done during the work on this thesis.Pulse-echo measurements with the CMUT immersed in rapeseed oil have been performed, and there has also been conducted measurements to find the attenuation of the oil. The results from the attenuation measurements have then been employed to simplify the analysis of the pulse-echo measurements.Any definitive characteristics for the CMUT was not found during these tests due to disturbances in the frequency responses. This is suspected to be because of line resistances and parasitic capacitances in the system. This suspicion is supported by the findings from studying the impedance response of the system.The backing layer added to the CMUT have proved to be very effective in dampening the ringing effect on the pulses emitted by the CMUT. There have also been signs of unexpected alteration of the CMUTs frequency characteristics. One possible source of this is that the process of adding the backing layer have resulted in that the layer exerts force on the CMUT, and by this changing the tension of the membranes.

Characterize silicon wafers used in solar cell production with a NIR-LCI interferometer. The interferometer is further developed and measurements on silicon samples are carried out.

In this project, the growth parameters of Pulsed Laser Deposition are optimized in order to obtain a high quality interface between two complex oxides, LaAlO3 and SrTiO3. The prepared samples are compared by their sheet resistance, and the influence by the various growth parameters are investigated.The main finding is that reducing the laser fluence significantly lowers the sheetresistance. The lowest obtained sheet resistance was 81.2kohm. This value wasobtained with a laser fluence of 0.7 J/cm^2 and the results indicate that morecan be gained by going even lower. In previous work, a strong anisotropy of the sheet resistance has been observed. The anisotropy is further studied in this project and compared with the step-and-terrace topography at the interface. Atomic Force Microscopy is used to obtain the step directions and terrace widths, and these values are compared with resistance measurements. No correlation is found between these data. The results suggests that the anisotropy is due to other factors than the steps-and-terraces.

In the work with this master thesis, electron beam lithography (EBL) and other cleanroom methods was used to develop a reproducible process for making ohmic contactsto nanowires (NWs), with the aim of investigating different dopants&#146; influence ontheir physical properties. After optimizing the EBL process, good alignment of thecontact patterns to the NWs was achieved by doing a thorough alignment procedure.A detailed write field alignment close to the sample position prior to the 3-point alignment, along with consistent use of alignment positions, was found the be especially important to reduce misalignment.For surface treatment it is shown how a diluted HCl solution is used to removenative oxide on the NW surface, and how a solution of citric acid and hydrogen peroxidecan be used to effectively etch the AlGaAs shell of core-shell nanowires, thoughselective etching of AlGaAs on GaAs is difficult. Further, an ammonium sulphidesolution is used to passivate the surface prior to metal deposition.Metalization was done using an electron beam evaporator, however optimal thinfilms could not be achieved due to the presence of strain. This is believed to originate in the evaporation of electron beam resist (ER) during metal deposition due to high local heating of the samples. While this was remedied by, among other, depositing thinner films at high deposition rates, it could not be fully avoided. Nevertheless, satisfactory lift-off was still achieved in most cases.IV characterization of the different samples was done using a probing station. Thesamples generally display asymmetric and non-linear characteristics, indicating thatohmic contacts have not been achieved. Annealing tests was also done on a numberof samples, and while this generally resulted in more linear IV curves, the currentwas also reduced in most cases.

Intermediate band solar cells (IBSC) is a proposed new type of solar cell device that has an intermediate energy band in the band gap. One possible implementation of IBSCs is by using arrays of nano-sized semiconductor particles known as quantum dots (QDs). InAs QDs can be grown on GaAs by using molecular beam epitaxy (MBE). As a part of this growth process, an InAs wetting layer (WL) is formed on which the InAs QDs grow. One possible method of determining the WL thickness is by using x-ray diffraction (XRD), which is a non-destructive analysis technique used to extract information about the structure and composition of crystalline thin films.An InAs/GaAs bilayer sample grown by MBE was investigated. This sample was grown without rotation, meaning the thicknesses of the deposited layers are expected to exhibit a gradient across the sample. Based on the geometry of the MBE machine, the variation in deposited thicknesses across the sample was estimated to be &#177;3.4% for InAs and &#177;4.2% for GaAs. XRD rocking curve measurements were performed around the 004 reflection in thirteen different locations on this sample. LEPTOS software was used to simulate XRD curves, and these simulations were fitted to the XRD measurements by varying the thicknesses of the simulated InAs and GaAs layers. It was found that the GaAs thicknesses across the sample varied as expected, while the variation in InAs thicknesses was much larger than expected. It was also found that the measured thicknesses of the InAs layer were significantly thinner than the nominal value. This is a strong indication that there are QDs on the sample, and that only the WL of the QDs contributes to the x-ray curve.LEPTOS was also used to simulate several cases of alloying of an InAs/GaAs bilayer. It was found that if the entire InAs layer is segregated into an InAs/GaAs alloy, this causes the diffraction peaks to weaken as the InAs/GaAs interfaces become less abrupt. It was also found that intermixing causes a significant change in the XRD curve because the amount of GaAs in the InAs/GaAs bilayer structure increases.

P-type GaAs semiconducting nanowires (NWs) with NW/electrode contacts have been examined by fabricating single NW devices to investigate their electrical properties. Both NWs grown with Au-assisted and Ga-assisted vapor-liquid-solid (VLS) method have been examined, with great emphasis on Ga-assisted NWs because these show superior contact properties. The NWs were grown using molecular beam epitaxy (MBE) and electrical contacts were fabricated using electron beam litography, electron beam evaporation for metallization and annealing to enhance ohmic behaviour of the contacts. Two different types of contacts were tested. One was metal contacts where various combinations of metal layers were tested, including such metallizations as Pt/Ti/Pt/Au and Pd/Zn/Pd/Au. The second electrode type was graphene which is expected to show very interesting electric and optoelectronic properties.It is shown in this thesis that ohmic contact with linear I-V characteristics has been achieved by two different methods. The first being annealing of the NW contact after contact fabrication. The second is by increasing doping concentration during NW growth. It is also shown that Pt/Ti/Pt/Au metallization is the layer configuration that exhibits the best ohmic contact and the most consistency in electrical measurements. 4-probe measurements were also carried out to measure the intrinsic resistivity of the NW, which makes it possible to estimate its doping concentration and carrier mobility.NW/graphene contact was also tried.Mechanical exfoliation of kish graphite was performed to make high-quality graphene, which was selctively placed to make NW contact. However it is found that heating of the EBL-resist during e-beam evaporation caused the graphene to wrinkle and dissolve. CVD-grown graphene on Cu-foil was also tested. Inspection after the graphene transfer proved that the graphene that was used was incontinous and flaky making it difficult to obtain proper contacts to NWs. In addition the NWs connected to graphene probably suffered from over-etching due to multiple etching steps. Since a lot of efforts was put into obtaining a suitable measure of making NW/graphene contacts only preliminary measurements were conducted, so these results has yet to be verified.

A macro-photoluminescence study of self-catalyzed nanowires, using a specializedsetup for this purpose, has been performed. Several samples were even comparedto single nanowire micro-photoluminescence measurements, on the samesamples, done by other students in the nanowire group at NTNU. The macrophotoluminescence measurements were done by changing parameters such as temperature and excitation power. A 532 nm laser was used to excite the wires and thephotoluminescence was measured by a CCD detector attached to a spectroscope.Using macro-photoluminescence measurements in a qualitative comparison withmeasurements done on single nanowires by micro-photoluminescence spectroscopycan give the researcher a decent understanding of the quality of the sample and itscontributions to photoluminescence. Measurements done in this thesis show probablecontributions from most types of transitions expected in zinc-blende nanowires,consisting of such defects as dense zinc-blende/wurtzite heterostructures, highlytwinned regions, but also from nanowires of higher purity.As nanowires move closer to the realization of e.g. nanowire based solar cells,such macro-photoluminescence measurements will become increasingly importantin deciding the overall quality of the grown structures.

In this thesis, single zincblende GaAs nanowires grown by molecular beam epitaxy using the self-catalyzed approach were studied with m-photoluminescence spectroscopy in order to assess the optical quality of self-catalyzed GaAs nanowires grown for the first time at NTNU and to compare the optical properties of self-catalyzed GaAs nanowires with Au-assisted GaAs nanowires and bulk GaAs references. The low temperature and temperaturedependence measurements revealed type II recombinations between zincblende and wurtzite segments occurring at the nanowire tips, however it was established that radiative recombination does not take place in the zincblende GaAs nanowire core. As the thesis progressed, valuable feedback was given to the growers in order to optimize nanowire growth conditions. The thesis builds upon data from earlier work within this project andprovides a foundation for future work on self-catalyzed GaAs nanowire devices at NTNU.