| Sažetak | 4H-SiC je široko-pojasni poluvodič s izvrsnim električkim svojstvima za upotrebu pri visoko temperaturnim, visoko naponskim i visoko frekvencijskim uvjetima. Razumijevanje defekata i fizikalnih procesa je preduvjet za kontrolu koncentracija defekata u materijalu, njihovog utjecaja na električna svojstva te daljnjeg povećanja otpornosti na zračenje i efikasnosti 4H-SiC elektroničkih komponenti. U ovom radu je iznesena karakterizacija električki aktivnih defekata metodama tranzijentne spektroskopije. Defekti su uvedeni tijekom rasta kristala ili kontroliranim ozračivanjem. Kontroliran unos defekata u 4H-SiC je ostvaren ozračivanjem epitermalnim i brzim neutronima. Istražen je negativan-U poredak akceptorskih nivoa i kinetika uhvata elektrona dominantnog rekombinacijskog centra u n-tipu 4H-SiC, oznake Z_1/2. Pripadajući Z_1(=/0) i Z_2(=/0) duboki nivoi su razlučeni te pridijeljeni vakancijama ugljika na dvama različitim položajima u kristalnoj rešetci s lokalnom heksagonalnom V_C(h) i kubičnom V_C(k) simetrijom. Z_1(=/0) i Z_2(=/0) duboki nivoi odgovaraju prijelazu između dvostruko negativnog i neutralnog stanja vakancija ugljika, dok opaženi Z_1(-/0) i Z_2(-/0) duboki nivoi su pridijeljeni prijelazima između negativnog i neutralnog stanja. Njihovi energijski položaji, udarni presjeci za uhvat elektrona i koncentracije su objašnjeni modelom vakancije ugljika, što potvrđuje njihove identifikacije. Proučeni su duboki nivoi uneseni ozračivanjem s energijama 0.4 eV i 0.7 eV ispod minimuma vodljive vrpce, uobičajenih oznaka redom EH1 i EH3. Uz poznate (-3/=) i (=/-) prijelaze vakancija silicija, opaženi su duboki nivoi metastabilnog M-centra te doprinos dodatnog defekta, najvjerojatnije kompleksnog defekta koji uključuje međupoložajne atome. Opisani su prijelazi između dvije konfiguracije M-centra, s M1 (E_C-0.43 eV) i M3 (E_C-0.72 eV) dubokim nivoima u jednoj konfiguraciji ili M2 (E_C-0.70 eV) dubokim nivoom u drugoj konfiguraciji. Razmotrili smo zamke za manjinske nosioce naboja u 4H-SiC, oznaka B i D centar, te utjecaj neutronskog ozračivanja na njih. B i D centar su pridijeljeni (-/0) prijelazu primjese bora na silicijevom BSi i ugljikovom BC mjestu u kristalnoj rešetci, tim redom. Dvije komponente D centra su razlučene i pripisane primjesi bora BC s lokalnom heksagonalnom i kubičnom simetrijom. |
| Sažetak (engleski) | 4H-SiC is a wide-bandgap semiconductor with excellent properties for high-temperature, highvoltage, and high-frequency applications. Identification of defects in 4H-SiC is a prerequisite for control of their concentrations and effect on electrical properties by material engineering. In this research work, characterization of electrically active defects is carried out by transient spectroscopy methods, which allow determination of defect parameters describing generation, recombination, and trapping of charge carriers at defects. The focus is on point defects introduced in 4H-SiC crystal during epitaxial growth and neutron irradiation. The first three chapters of this thesis give a short overview of the structure and properties of 4H-SiC semiconductor, theoretical background regarding capture and emission kinetics of charge carriers at electrically active defects, and interesting results of other authors on the subject of defect characterization relevant to study presented here. In the fourth chapter, characterization methods and materials are described. Schottky diodes produced by thermal evaporation of Ni on n-type 4H-SiC epitaxial layer were used. Controlled introduction of defects in 4H-SiC was realized by irradiation with epithermal and fast neutrons. Elastic collisions of neutrons with silicon and carbon atoms caused their displacement in 4HSiC crystal. The specified neutron fluence levels spanned several orders of magnitude, ranging from 10^9 n/cm^2 up to 10^14 n/cm^2. Quality of the Schottky diodes and parameters necessary for characterization by transient spectroscopy methods were determined by analysis of currentvoltage and capacitance-voltage characteristics. Majority carrier traps in epitaxial n-type 4HSiC were characterized by Deep Level Transient Spectroscopy (DLTS) and Laplace DLTS techniques, while minority carrier traps were characterized by Minority Carrier Transient Spectroscopy (MCTS) and Laplace MCTS techniques. The results of the characterizations, discussion, and conclusions are presented in the last two chapters. The current-voltage measurements on n-type Schottky diodes showed excellent rectifying characteristics, while capacitance-voltage measurements revealed homogenous doping and deep level concentrations. An increase in series resistance and a decrease in free carrier concentration with irradiation was observed at fluences higher than 10^12 n/cm^2 due to introduced acceptor defects. Acceptor levels of dominant recombination center in 4H-SiC, labeled as Z_1/2, were investigated. Z_1(=/0) and Z_2(=/0) deep levels contributing to Z_1/2 peak in DLTS spectra were resolved and assigned to carbon vacancies at two different lattice sites with local hexagonal V_C(h) and cubic V_C(k) symmetry, respectively. Z_1(=/0) and Z_2(=/0) deep levels were assigned to transitions between double negative and neutral charge states of carbon vacancy in accordance with its negative-U properties. Transitions between negative and neutral charge states of carbon vacancies, labeled Z_1(-/0) and Z_2(-/0), were resolved by Laplace DLTS technique and annealing procedures. Fast capture kinetics of electrons at neutral and negative carbon vacancies was examined in low temperature range (100 K-160 K). The determined activation energies, electron capture cross sections, and concentration ratios are well described by carbon vacancy model, which further confirms their identification. Irradiation induced deep levels with energy positions around 0.4 eV and 0.7 eV below the conduction band minimum, usually labeled EH1 and EH3, were examined and associated with silicon vacancy, M-center, and interstitially related defect. A higher EH3 concentration compared to EH1 concentration and observed decrease in EH3 concentration with low temperature annealing indicated a contribution of interstitially related defect to DLTS signal at the temperature of EH3 deep level. Deep levels of M-center were resolved by analyzing changes in DLTS spectra resulting from transitions between two different configurations of M-center. M1 (E_C-0.43 eV) and M3 (E_C- 0.72 eV) deep levels were observed in configuration A of M-center, while M2 (E_C-0.70 eV) deep level was observed in configuration B. Two components at the temperature of EH1 deep level were resolved by Laplace DLTS technique. In configuration B of M-center, the resolved EH1_1 and EH1_2 deep levels were assigned to (-3/=) transition of silicon vacancies with local hexagonal and cubic symmetry, respectively. In configuration A, M1 deep level contributed to one of the resolved components in Laplace DLTS spectra at emission close to EH12 deep level. The study of minority carrier traps in n-type 4H-SiC and the influence of neutron radiation on them was performed. Two minority carrier traps in n-type 4H-SiC, labeled B and D center, were assigned to (-/0) transitions of substitutional boron on silicon B_Si and carbon B_C lattice sites, respectively. Furthermore, two components of D center were resolved by Laplace MCTS technique and attributed to (-/0) transitions of B_C located at hexagonal and cubic lattice sites. Peaks in MCTS spectra related to B and D center exhibited a shift towards lower temperatures with an increase in neutron irradiation fluence, which was explained by increased strain in epitaxial 4H-SiC caused by introduced defects. |
