GaN is a monocrystalline material that can be grown using metallo-organic chemical vapour deposition (MOCVD), and has desirable mechanical and semiconducting properties for operating as a sensor. It has a Young’s modulus of 250 to 350 GPa, which shows little decrease with respect to temperature beyond 400°C. GaN also exhibits piezoelectric and piezoresistive effects, meaning that it will generate a charge and its electrical resistance will change when the material is strained respectively. In this PhD, GaN has been used as the base material for pressure sensors that potentially can be used in excess of 400°C and at a pressure in excess of 50 bar (5 MPa), with potential applications in aerospace and oil exploration. The pressure sensor is a circular diaphragm created from a GaN/sapphire wafer, and was designed and tested in order to determine if GaN can act as a sensing material in these environments. In addition to the diaphragm sensor, GaN templates that can potentially be used for sensors were grown using an epitaxial layer overgrowth (ELOG) method. These sensors are potentially more mechanically robust than similar templates etched out of GaN/sapphire wafers because they will have less inbuilt strain due to lower dislocation densities. It was possible to release beams and cantilevers from GaN ELOG templates. Mechanical probe tests were undertaken on these devices to see if they were fully released and robust. GaN single crystal growth requires a substrate material, such as (111) silicon or (0001) sapphire, meaning that the thermal properties of the substrate are important for a device operating in excess of 400°C. GaN high electron mobility transistors are heat sensitive, experiencing a decrease in current between the drain and source terminals as the temperature increases. Therefore a GaN-based sensor needs a substrate with the highest possible thermal conductivity to act as a heat sink, which means removing as much heat as possible from the GaN sensor. Diamond has superior thermal conductivity to both sapphire and silicon, so a novel silicon/polycrystalline diamond composite substrate has been developed as a potential GaN substrate. Polycrystalline diamond (PD) can be grown on 4 inch diameter wafers using hot filament chemical vapour deposition (CVD), on (111) silicon (Si) from which single crystal GaN epitaxy can also be grown. In order for the (111) Si/PD composite substrates to be useful heat sinks, the Si layer needs to be less than 2 m. PD was initially grown on 525 to 625 m thick Si wafers that required thinning to 2 m. Achieving this Si layer thickness is difficult due to the presence of tensile stress in the Si caused by a mismatch in the coefficients of thermal expansion (CTEs) between Si and PD. This stress causes the wafer to bow significantly and has been modelled using ANSYS FE software. The models show that the bow of the wafer increases when it is thinned, which will eventually cause the Si layer to delaminate at the Si/PD interface due to poor adhesion and a build up for shear stress. When the Si layer is mechanically thinned, the Si layer can crack due to clamping. The experimental wafer bow and micro-Raman measurements validate the model for when the silicon layer is thicker than 100 m and these results show that an alternative processing route is required.

Direct heteroepitaxy of III-Vs on silicon (Si) has always been a challenge and there are various strategies to integrate these materials. This thesis deals with one such strategy known as Epitaxial lateral overgrowth (ELOG) which is extensively supported by experiments. For an application such as a multijunction solar cell, with silicon as a bottom cell, the highest efficiency can be achieved with a top cell having a bandgap of 1.7 eV and hence GaAsP as a material suits the profile. The ELOG GaAsP and GaAs samples were grown using the epitaxial growth technique known as hydride vapor phase epitaxy (HVPE). With its near equilibrium operation capacity, high quality layers were grown. To specifically focus on the crystal defects and dislocations of the atoms, GaAsP was grown on GaAs substrate. Samples with varying growth parameters are investigated using several characterization techniques such as scanning electron microscopy (SEM), Photoluminescence (PL) spectroscopy and Raman spectroscopy. Composition variations (group V elemental incorporation in GaAsP) and crystalline quality are the two major factors that are analyzed. Additionally, ELOG GaAs samples grown on GaAs substrate using HVPE are studied as a reference to observe any strain effects due to the ELOG profile and compare with the GaAsP samples. The ideal goal of this thesis is to optimize the crystalline quality of the ELOG GaAsP samples and to verify that GaAsP grown using ELOG technique has a better crystallinity than the planar growth (direct epitaxy of GaAsP on GaAs substrate) using two major optical characterization tools - PL and Raman spectroscopy. This work is a step towards the development of high efficiency multi-junction solar cells with GaAsP and Si as the respective top and bottom cells. / Direkt heteroepitaxi av III-V på kisel (Si) har alltid varit en utmaning och det finns olika strategier för att integrera dessa material. Den här avhandlingen behandlar en sådan strategi som kallas Epitaxial lateral overgrowth (ELOG) som stöds externt av experiment. För en applikation som en multi junction solcell, med kisel som bottencell, kan den högsta effektiviteten uppnås med en toppcell med ett bandgap på 1,7 eV och därmed GaAsP som ett material som passar profilen. ELOG GaAsP- och GaAs-proverna odlades med användning av den epitaxiella tillväxttekniken känd som hydriddampfasepitaxi (HVPE). Med dess nära kapacitet för jämviktsdrift odlades lager av hög kvalitet. För att specifikt fokusera på kristalldefekter och dislokationer av atomerna odlades GaAsP på GaAs substrat. Prover med varierande tillväxtparametrar undersöks med användning av flera karakteriseringstekniker såsom skanningselektronmikroskopi (SEM), Photoluminescence (PL) -spektroskopi och Raman-spektroskopi. Kompositionvariationer (grupp V elemental inkorporering i GaAsP) och kristallin kvalitet är de två huvudfaktorerna som analyseras. Dessutom studeras ELOG GaA-prover odlade på GaAs-substrat med användning av HVPE som en referens för att observera eventuella belastningseffekter på grund av ELOG-profilen och jämföra med GaAsP-proverna. Det ideala målet med denna avhandling är att optimera den kristallina kvaliteten på ELOG GaAsP-proverna och att verifiera att GaAsP som odlas med ELOG-teknik har en bättre kristallinitet än den plana tillväxten (direkt epitaxi av GaAsP på GaAs underlag) med två huvudsakliga optiska karaktäriseringar verktyg - PL- och Raman-spektroskopi. Detta arbete är ett steg mot utvecklingen av högeffektiva multi junction solceller med GaAsP och Si som respektive topp- och bottenceller.