Self-interstitials, vacancies and antisites belong to an important class of defects -intrinsic defects - in compound semiconductors. They have been found to be common occurring defects in semiconductor crystals grown under non-optimized or non-equilibrium conditions or subject to high-energy particle bombardment. Intrinsic defects are also fundamental building blocks for defect complexes and in some cases nucleation sites for formation of defect clusters and extended defects. Intrinsic defects and their complexes are known to play a crucial role in determining electronic and optical properties of semiconductors. In most cases, unfortunately, they are known to be harmful to device performance by e.g. reducing carrier mobility, controlling carrier lifetime, introducing non-radiative (NR) recombination centers. Therefore, there is a great need for a good understanding of defect properties, especially concerning a positive identification of their chemical nature, so that strategies can be designed to control or eliminate them. In this thesis work,we have employed one of the most powerful experimental techniques inidentification of defects, namely optically detected magnetic resonance (ODMR), to gain detailed knowledge on important intrinsic defects in dilute nitrides and SiC. Both material systems have received great attention in recent years due to their intriguing fundamental physical properties and their potentials in high-performance device applications. Dilute nitrides such as Ga(In)NAs and (Al)GaNP have been identified as promising materials for optoelectronic and photonic devices, while SiC is known to hold potential for high power, high temperature and high frequency devices. The thesis work is presented in six papers.

Papers I and II report detailed studies of an As-antisite (As_Ga) defect in GaNAs alloys. The participation of an As atom in the defect is concluded from the experimentally resolved hyperfine (hf) structure in the ODMR spectra, i.e. a group of four lines, characteristic for the interaction between an unpaired electron spin S=1/2 and the nuclear spin 1=3/2 of the ⁷⁵As atom (100% natural abundance). The negative ODMR signals indicated that the defect is not directly involved in the monitored radiative emissions but rather participates in competing NR recombination processes. The defect is found to be preferably introduced during epitaxial growth at low temperatures and its formation was further facilitated by the presence of N. An increase in growth temperature or post-growth thermal annealing has been shown to reduce the influence of the studied NR defects, accompanied by a remarkable improvement in optical quality of the material.

The first identification of NR defects in (Al)GaNP-based alloys is presented in papers III-V. Based on the characteristic hf structure arising from the interaction between an unpaired electron spin (S=1/2) and the nuclear spin (1=3/2 for both ⁶⁹Ga and ⁷¹Ga isotopes), two Ga-interstitial (Ga_i) defects are identified as being common grown-in defects in GaNP and AlGaNP grown by molecular beam epitaxy. The observed strong and nearly isotropic hf interaction reveals an electron wave function of A₁ symmetry that is highly localized at the Ga_i and thus a deep-level nature of the defects. Based on our theoretical calculations, both defects are suggested to be complexes involving a Ga_i²⁺. By taking advantage of the freedom in altering compositions of both cations and anions of the novel alloy, compositional dependence of electron localization has been obtained that sheds light on the possible location and local surrounding of the defects in the lattice. Introduction of these defects is shown to be largely promoted by incorporation of N. In quaternary alloys, concentrations of the defects are found to critically depend on the group III atoms that replace Ga, i.e. it is largely enhanced by the presence of Al in the alloys, but is only marginally affected by In incorporation. The effect is attributed to the differences in surface adatom mobility of the group III atoms involved and their bonding strengths with N. The revealed Ga_i complexes are shown to act as efficient NR recombination centers degrading efficiency of light emission. The defects exhibit high thermal stability and can only be partially removed by post-growth thermal annealing.

In paper VI, the isolated silicon vacancy (V _si) in its neutral charge state is unambiguously identified in 4H and 6H-SiC. This is achieved by observation of ligand hf interaction with the four carbon atoms in the nearest neighbor shell and the twelve silicon atoms in the next nearest neighbor shell surrounding the vacancy, from the spin-triplet lines T_V1a and T_V2a in 4H-SiC and T_V1a, T_V2a and T_V3a in 6H-SiC.The complete hf tensors have been determined for the V_Si center residing at all inequivalent lattice sites in the two polytypes. The parameters of this interaction are very similar to the ones found for the negatively charged isolated V_Si in the respective polytypes. This finding removes the possibility of the defect being a complex defect like a close vacancy pair or a vacancy-impurity complex, and thus resolves the long-standing controversy about the chemical identity of the spin-triplet defects.