We have developed a virtual GaN substrate on sapphire based on a two-step growth method. By optimizing the growth scheme for the virtual substrate we have improved crystal quality and reduced interface roughness. Our Al0.22Ga0.78N/GaN HEMT structure grown on the optimized semi-insulating GaN virtual substrate, exhibits Hall mobilities as high as 1720 and 7350 cm(2)/Vs and sheet carrier concentrations of 8.4 x 1012 and 10.0 x 1012 cm(-2) at 300 K and 20 K, respectively The presence of good AlGaN/GaN interface quality and surface morphology is also substantiated by X-Ray reflectivity and Atomic Force Microscopy measurements. A simplified transport model is used to fit the experimental Hall mobility.

A simple AlGaN/GaN HEMT structure without any interlayer on Fe-doped GaN (GaN:Fe) grown by LP-MOVPE in one single run on sapphire is demonstrated. The surface segregation of Fe in GaN occurring during growth is identified. Hall measurements yield 2DEG (two dimensional electron gas) mobilities of 1700 and 10700 cm²/Vs and sheet carrier concentrations of 5.1×10 ¹² and 5.7×10¹² cm^-2 at 300 K and 20 K, respectively. Good pinch-off current-voltage (I_D-V_D) characteristics is observed from AlGaN/GaN HEMTs. A maximum drain current density of 0.6 A/mm and a peak extrinsic transconductance of 200 mS/mm have been observed. The HEMT structure yields very good device isolation as indicated by an isolation current of ≀1 nA at 20 V.

Secondary ion mass spectrometry (SIMS) and transmission electron microscopy (TEM) are utilized to study precipitation and the solubility of B in 4H-SiC epitaxial layers super saturated with B. Heat treatments are performed in Ar atmosphere in an rf-heated furnace at temperatures between 1700 and 2000 degrees C. SIMS ion images, and TEM micrographs reveal the formation of two types of precipitates where the larger, more thermally stable one is suggested to be B4C. The boron solubility is determined from SIMS depth profiles and is shown to follow the Arrhenius expression: 7.1 x 10(22) exp(-1.4 eV/k(B)T) cm(-3) over the studied temperature range.

The electronic properties of Fe-doped Al0.31Ga0.69N/GaN heterostructures have been studied by Shubnikov-de Haas measurement. The lowest two subbands of the two-dimensional electron gas in the heterointerface were populated. After the low temperature illumination, the electron density increases from 11.99x10(12) cm(-2) to 13.40x10(12) cm(-2) for the first subband and from 0.66x10(12) cm(-2) to 0.94x10(12) cm(-2) for the second subband. The persistent photoconductivity effect (similar to 13% increase) is mostly attributed to the Fe-related deep-donor level in GaN layer. The second subband starts to populate when the first subband is filled at a density of 9.40x10(12) cm(-2). We obtained the energy separation between the first and second subbands to be 105 meV.

We present first results on heteroepitaxy of InP on silicon on insulator (SOI). We also demonstrate InP nanopillar fabrication by means of selective epitaxy. Selective epitaxy is also exploited to fabricate advanced photonic integrated devices for Optical Code Division Multiplex Access (OCDMA) networking applications.

Metalorganic vapour phase epitaxy (MOVPE) has proven to be a successful method for growth of structures for advanced optoelectronic semiconductor devices in III-V compounds. This thesis deals with technological and process related aspects of MOVPE from an experimental perspective. Special attention is given to three main questions: uniformity over large areas, p-type dopant diffusion and redistribution in structures for heterostructure bipolar transistors, and planar embedding of high mesas for buried heterostructure lasers.

A uniformity within a few per cents for thickness, alloy composition, and dopant incorporation over large areas is fundamental for growth of advanced device structures. We present a new horizontal reactor with a large width-to-height aspect ratio operating at reduced pressure, and with a rotating susceptor for single wafers with a diameter of up to 75 mm. The obtained uniformity over 40 mm diameter is within  1 % for thickness, composition and doping, and within   1 nm for the wavelength of quaternary InGaAsP at  = 1560 nm. The doping distribution has been used to estimate the tempera­ture gradient over the wafer to a few tenths of a degree, and the uniformity is most probably limited by gas phase diffusion and depletion of the reactants.

Abrupt doping profiles are important for some devices. An example is the high frequency performance of the n-p-n heterostructure bipolar transistor (HBT), which is improved by a very narrow and highly doped p-type base. We have compared Zn and Mg for this application. The memory effect of Mg in the reactor gives doping tails towards the surface, but with Zn we obtained an abruptness of three decades over 60 nm at a maximum doping level exceeding 21019 cm-3. However, a highly n-doped layer of AlGaAs adjacent to the Zn-doped region gives rise to a significant re-distribution of Zn into the AlGaAs layer. At a reduced growth temperature this effect is diminished and the maximum doping level is simultaneously increased.

Growth on partly masked and non-planar substrates has become an impor­tant field for realisation of devices designed in three dimensions. We have fo­cused on planar regrowth of semi-insulating InP around high mesas of buried heterostructure lasers. Unwanted growth over the masked mesa top usually occurs. These depositions are reduced at high growth temperatures and for growth around low mesas (< 2 m). Addition of CCl4 in the MOVPE process considerably improves the morphology, especially for growth around high mesas. When chlorine is provided to the growing surface nucleation is pre­vented on the phosphorus-faced {111}B lattice planes and the mask, and this effect per­mits reproducible planar regrowth not limited by the mesa height. 

The development of wide band gap semiconductors extends their applications in optoelectronics devices to the UV domain. Compact lasers and high sensitivity APD detectors in UV range are currently needed for different applications such as, purification, covert communication and real time detection of airborne pathogens. Until now, the full exploitation of these potential materials has been limited by the lack of suitable GaN substrates. Recently, a novel class of materials has been reported based on BGaN and BAlN, potentially reducing the crystal defect densities by orders of magnitude compared to existing wide band gap heterostructures. Characteristics of these new alloys are similar to those of AlGaN materials with the advantage that these can be lattice matched to AlN and SiC substrates. In addition, these materials offer the possibility of using quaternary BAlGaN alloys at Ultra Violet (UV) wavelengths and hence lead to more degrees of freedom in designing sophisticated device structures. In this paper we describe the MOVPE growth conditions used to incorporate boron in GaN and AlGaN. Detailed characterization and analysis in terms of structural and electrical properties are discussed.

We report epitaxial growth of GaN layers on z- and x-cut LiNbO₃ substrates using MOVPE. GaN layers with the thickness of 450 nm were characterized using X-ray diffraction. For both, z- and x-cut orientations of LiNbO₃ substrates, the GaN layers have c-axis orientation normal to the substrate plane and the in-plane lattice orientation of GaN layers coincides with the primary axes of LiNbO₃ substrates. Although GaN layers exhibit almost complete strain relaxation, the residual compressive strain determined with respect to a freestanding GaN is of the order of +0.37% and +0.2% for z- and x-cut substrates, respectively.