| Abstract [eng] |
III-nitrides, diamonds are extremely promising wide band gap semiconductor materials for optoelectronics and high temperature, high power electronics. Therefore, there is huge scientific interest in investigation electrical and optical properties of these materials. The light induced transient grating technique (LITG) is very suitable for exploration of carrier dynamics which governed by fundamental and defect related properties of materials. The main goals of the thesis were gain a new knowledge on carrier dynamics in wide bandgap semiconductors (namely GaN, InGaN, and diamonds) by using and developing light induced transient grating technique. The experimental studies on numerous samples, grown at different conditions, combined with extensive measurements in a wide range of carrier densities (1016-1020 cm-3) and temperature (9-300K) was targeted to identify the interplay of radiative an nonradiative recombination mechanisms, to determine carrier lifetime dependence on the excess carrier density, to explain the carrier diffusion coefficient dependence on excitation intensity, to find the optimal materials growth conditions. A novel heterodyne detection scheme for LITG technique was presented. The heterodyning was achieved by coherently mixing the picosecond pulses of diffracted and scattered light. A phase difference between theses fields was controlled by moving holographic beam splitter (HBS) along its grating vector. LITG signal decay kinetics, recorded at two HBS positions corresponding for phase difference of p, allowed to measure separately Dn2 and Dn kinetics, which were impossible in the convenient transient grating setup. This approach was employed to study a competition of coexisting free carrier and thermal optical nonlinearities in CVD grown diamond films. The LITG measurements in numerous GaN layers with different dislocation revealed clear dependence of carrier lifetime on dislocation density, thus pointing out that dislocations acts as main carrier recombination centers. The strong dependence of carrier lifetime versus threading dislocation density (TDD) (tR µ (NTD)-0.5) at large dislocation densities (NTD > 108 cm-2) is weakened by dominance of bimolecular recombination in samples with low TDD at high carrier densities. The dislocation governed carrier lifetime in these samples were deduced by low excitation measurement via below bandgap excitation or discriminated by numerical modeling. LITG measurements in GaN layers in temperature range of T = 9 – 300 K and subsequent numerical modeling of carrier dynamics allowed to determine directly the values of the bimolecular recombination coefficient (B = 2·10E11 cm3/s, at RT) and its temperature dependence (B T-3/2), to separate impacts of radiative and nonradiative recombination. The measurements of diffusion coefficient in wide temperature range allowed to identify that scattering by the acoustic phonons is dominant mechanism in all temperature range. LITG experiments provided increasing carrier diffusion coefficient values in GaN layers from D = 1.5 cm2/s at low carrier densities (N < 5·1018 cm3) to D = 4.1 cm2/s at around 5·1019 cm-3. Using numerical modeling of carrier dynamics the increase of diffusion coefficient at high carrier density was explained as a Fermi pressure at degenerate carrier plasma. The low temperature measurements confirmed the suggested model. The numerical analysis of carrier spatial-temporal redistribution allowed us to verify the origin fast time resolved photoluminescence transients measured in GaN layers as diffusion-governed carrier in-depth redistribution and reabsorption of a backward emission from the photoexcited layer. It was, therefore, concluded that determination of carrier lifetimes in high quality and thick GaN layers is more advantageous and straightforward by using LITG technique. The LITG measurements provided photoelectrical parameters for InGaN layers at high excitation case – D = 1-1.7 cm2/s, tR = 120 – 500 ps, LD = 0.2 mm. Low and In content dependent carrier diffusivity revealed In segregation induced carrier localization. At room temperature defect related recombination determined quite low carrier lifetime values and rejected the possibility of Auger recombination mechanism. The features of bimolecular recombination were observed only at low temperatures. The investigation in InGaN MQWs revealed longer carrier lifetimes up to 3 ns, the pronounced bimolecular recombination at room temperature which became dominant recombination channel at lower temperatures. The determined diffusion coefficient values increased at higher carrier densities due to degeneracy of carrier distribution. A novel all-optical way to monitor carrier lifetime, diffusion coefficient, and diffusion length in HPHT and CVD diamonds was demonstrated. The similar values of carrier diffusion coefficient D » 9.4 cm2/s and lifetime tR = 2.7 ns were deduced for high quality single crystal HPHT and CVD diamonds The correlation between the carrier lifetime and concentration of nitrogen defects pointed out that nitrogen-related defects act as the main centers of nonradiative recombination. The determined by optical mean dependence of carrier mobility on temperature showed that the scattering by acoustic phonons is a dominant mechanism at room temperature. |
