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GaMnAs can be considered as a model system for III-V diluted magnetic semiconductors, which are promising materials for future spintronics applications. In view of future applications, a proper characterization and understanding of the physical properties of GaMnAs is essential, and techniques to increase the ferromagnetic transition temperature like post-growth annealing must be explored. In this work we have studied the effect of low temperature annealing on the structural, magnetic and transport properties of GaMnAs thin films, to gain insight into the annealing mechanisms and the complex physics of diluted magnetic semiconductors. We find that a post growth annealing treatment can lead to a considerable enhancement of the hole density,the Curie temperature, the saturation magnetization and the conductivity and also to drastic changes in the magnetic anisotropy. We have successfully grown GaMnAs thin films on GaAs substrates with various Mn contents 0.02 ≤ x ≤ 0.08 using low-temperature molecular beam epitaxy. The amount of Mn that has to be incorporated to obtain ferromagnetic ordering in GaMnAs, requires relatively low growth temperatures, which inevitably leads to a high density of hole compensating As antisite and Mn interstitial (Mn_I) defects. As Mn interstitials will also couple antiferromagnetically to Mn ions at substitutional lattice sites, these defects may suppress the ferromagnetism in the GaMnAs layer. To keep the density of defects as low as possible the GaMnAs growth was performed at growth temperatures close to the Mn segregation limit, with a near stoichiometric As flux. Post-growth annealing treatments of GaMnAs at temperatures close to the growth temperature lead to a strong enhancement of the conductivity and Curie temperature Tc, which is attributed to the out-diffusion of Mn interstitials to the free surface where they are passivated. We have shown that the presence of oxygen is an important parameter for the optimization of Tc through annealing, indicating that the passivation of Mn_I indeed occurs through oxidation. Moreover, we found indications for a second annealing mechanism besides the diffusion of Mn_I which appears to diminish the amount of ferromagnetically coupled Mn ions and prevails in the absence of oxygen during the annealing treatment. We have studied the structural properties of the grown GaMnAs layers by performing high resolution X-ray diffraction measurements, which show that the GaMnAs epilayers are of high crystalline quality. The GaMnAs lattice constant is clearly reduced upon annealing, which can be attributed to the removal of Mn interstitials from the lattice. Although the compensation of donor-like defects is very small after annealing, the GaMnAs lattice constants are still significantly larger than those of GaAs, indicating that the intrinsic lattice constant of GaMnAs is significantly larger than that of GaAs, in disagreement withfirst-principles calculations. The magnetic and transport properties of a series of ~ 40 nm thick GaMnAs films with Mn contents 0.02 ≤ x ≤ 0.08 were systematically studied before and after annealing. After annealing the saturation magnetization and Curie temperature have increased considerably, with a maximum Tc = 133 K for x = 0.06. We have discussed the temperature dependence of the resistivity in zero-field and compared these results to existing transport theories. A saturation of the resistivity is observed below 1 K for all samples, indicating metallic behavior. We also found that the resistivity peak in r(T) is not always observed near Tc, but can be shifted to temperatures quite far above Tc after annealing, while the ferromagnetic transition is rather marked by a peak in d²r/dT²(T). The magnetoresistance r(H) of the GaMnAs layers was measured in magnetic fields up to 50T, which allows to test the existing transport models by applying them to fit the r(H) data in very high magnetic fields. Above Tc the magnetoresistance can be described by both spin dependent hopping and spin disorder scattering which lead to a similar magnetic field dependence. At low temperatures the negative magnetoresistance does not saturate, even in magnetic fields up to 50 T. The magnetoresistance at 4.2 K is strongly reduced after annealing and shows a good agreement with a model for weak localization effects, at least if a small metallic positive magnetoresistance is added. Despite a strong anomalous Hall contribution, we were able to determine the hole density with reasonable accuracy by performing Hall effect measurements at 4.2 K in pulsed magnetic fields up to 50 T, and find that the hole density is considerably enhanced after annealing. The measured hole densities indicate that after annealing the compensation is very small for samples with x ≤ 0.06. The low Hall mobility indicates, together with the high resistivity values, that GaMnAs behaves like a bad metal. For most samples, a decrease in the Hall mobility is observed upon annealing , which can be understood by the lowering of the Fermi energy to states with a larger effective mass. The Curie temperature of both as-grown and annealed GaMnAs samples with x ≤ 0.06 shows the same Tc µ xs ×p1/3 dependence on the hole density p and substitutional Mn content xs, as predicted by the mean field theory. Finally, we have studied the anisotropy of an as-grown and an annealed GaMnAs (x = 0.08) layer through magnetization measurements along the main crystallographic orientations. While both samples show an anisotropy which favors an in-plane orientation of the magnetization, the in-plane magnetic anisotropy itself is quite complex. In the as-grown sample we observe a temperature induced transition from a biaxial anisotropy with easy axes along the <100> crystallographic directions at low temperatures, to a uniaxial anisotropy with [1-10] easy axis at temperatures closer to Tc. After annealing the Curie temperature has increased up to Tc = 140 K and the anisotropy has become uniaxial over the entire temperature range, while a temperature induced reorientation transition of the easy axis from [1-10] to [110] is observed when the temperature is increased towards Tc. We find that the magnetic reversal process in GaMnAs can be described by the Stoner-Wohlfahrt model for coherent rotation, combined with abrupt magnetic switching via domain states. The observed anisotropy of this particular compound is to a large extent confirmed by planar Hall effect measurements and scanningHall probe microscopy. Spintronica is een nieuwe vorm van elektronica waarbij gebruik wordt gemaakt van de spin eigenschap van elektronen om informatie te verwerken en over te brengen. Dit kan in de toekomst leiden tot een technologie met kleinere, snellere en energiezuinigere componenten. De meest courante aanpak is gericht op de integratie van dit spin-concept in de materialen die traditioneel in de informatie technologie gebruikt worden: de halfgeleiders. Verdunde magnetische halfgeleiders bestaan uit een standaard halfgeleider waarin een (klein) deel van de kationen vervangen is door magnetische ionen, waardoor locale magnetische momenten zijn aangebracht in de halfgeleider. Deze materialen zijn veelbelovende materialen voor spintronics toepassingen omwille van hun compatibiliteit met de huidige halfgeleider technologieën. GaMnAs bestaat uit de halfgeleider GaAs waarin enkele procenten van de Ga atomen vervangen is door Mn, en wordt beschouwd als een belangrijk modelsysteem voor deze materiaalklasse. Met het zicht op toekomstige toepassingen is een grondige studie van de materiaaleigenschappen van GaMnAs dan ook essentieel. In deze thesis worden de structurele, magnetische en elektrische transport eigenschappen van eenreeks dunne GaMnAs films met verschillende Mn concentraties 0.02 ≤ x ≤ 0.08 bestudeerd. Deze eigenschappen blijken sterk afhankelijk van de Mn inhoud en de concentratie aan defecten zoals interstitiële Mn atomen. We vindendat de Curie temperatuur niet alleen bepaald wordt door de Mn inhoud, maar ook in sterke mate afhangt van de gatendichtheid, die we bepalen uit Hall effect metingen in gepulste magnetische velden tot 50 T. Verder tonen we aan dat een thermische nabehandeling in een zuurstof rijke omgeving leidt tot een aanzienlijke toename van de gatendichtheid, de Curie temperatuur, de saturatie magnetisatie en de conductiviteit, en dat dergelijke nabehandeling tot grote veranderingen in de magneto-kristallijne anisotropie kan leiden.