Structure,Optical And Electrical Properties In Hafnium Nitride And Hafnium Tantalum Nitride Films

The Group-IVB, VB and VIB transition metal nitride films(TMN, TM = Ti, Zr, Hf, V, Nb,Ta, Cr, Mo, W) have been widely used in the field of aerospace, microelectronic devices,automotive industries, geological drilling, cutting- and machining-tool industries due to their unique optical, electrical and mechanical properties. Recently, as a research highlight in the field of film materials, TMN films have attracted numerous attentions. In these applications,the optical, electrical and mechanical properties of the films are of utmost importance, as they determine the ultimate performances and service life of the devices. It is very crucial,therefore, to understand how to control their structure and properties.Although extensive researches on the optical, electrical and mechanical properties of TMN films have been carried out and numerous results have been reported, the following four questions still remain:(1) Although point defects are found to exist commonly in group-IVB transition metal nitrides, the identification of primary point defects still causes some disagreement. The formation mechanism and influence of primary point defects on optical reflectivity are not yet well explored.(2) Although the role of ion bombardment on electrical conductivity and optical reflectivity of transition metal nitrides films was reported previously, the results were controversial and the mechanism was not yet well explored.(3) A stoichiometry-driven phase transition from rocksalt to “nitrogen-rich” structure exists in group-IVB transition metal nitride films. As this phase transition is critical in controlling the film properties, it has attracted numerous studies. However, researchers are still divided withregard to the structural identity of this “nitrogen-rich” phase, not to mention detailed exploration of the phase transition mechanisms.(4) In the applications of TMN films in optical coating, the cutoff wavelength and infrared reflectance are of utmost importance, as they determine the light utilization and optical selectivity. Although the effect of chemical composition and deposition conditions on reflectivity characteristics have been studied and important results reported, how to control the cutoff wavelength and infrared reflectance, and enhance the hardness of TMN films have not yet been well explored.With the above four questions in mind, we sputtered hafnium nitride and hafnium tantalum nitride films, and carried out four aspects of studies by employing the Drude-Lorentz mode, first-principle calculations in combination of the experimental measurements, namely, ultraviolet-visible-near-infrared(UV-vis-NIR) spectrometer,Raman measurements, Hall-effect test, Nano indentation Selected Area Electron Diffraction,High Resolution Transmission Electron Microscopy, Raman, Gracing Incident X-ray Diffraction(GIXRD) and X-ray Photoelectron Spectroscopy(XPS). The main contents and results of this thesis are summarized as following:1. This work identifies the types of primary point defects in ?-Hf N_x and explores the effects of primary point defects on optical reflectivity. This study finds that the types and formation mechanism of primary point defects in rocksalt hafnium nitride(?-Hf N_x) films can be identified by a combination of first-principles calculations and experiments. It is shown that the primary point defects in sub- and over-stoichiometric ?-Hf N_x films are N and Hf vacancies, respectively, which arise preferentially because they are thermodynamically more stable than other types of point defects, such as interstitials and antisites, because they have much lower formation energy and equilibrium formation enthalpy. Furthermore, It is found that the observed tunability of optical reflectivity arises from a transition from N vacancies(VN) to Hf vacancies(VHf) in the films because this evolution from VN to VHf has important roles in changing electronic properties of the films in the following three aspects:(i) density of free electrons, wherein VN and VHf act as donor-like and acceptor-like defects, respectively;(ii) mean free path of free electrons, in which VN and VHf are the main electron scatteringsites in sub- and overstoichiometric films, respectively;(iii) interband transition absorption of bound electrons, wherein three previously unreported absorption bands originating from VN and VHf are found to occur at ~0.81, 2.27, and 3.75 e V. These point-defect-induced variations significantly affect the dielectric function of ?-Hf N_x films and thus drive the tailored evolution in reflectivity properties with x.2. We studied the effect of ion bombardment on structure, electrical conductivity and optical reflectivity of the ?-Hf N_x films. Here we show that proper ion bombardment, induced by applying the negative bias voltage(Vb), significantly improves the electrical conductivity and optical reflectivity in rocksalt hafnium nitride films regardless of level of stoichiometry(i.e., in both near-stoichiometric Hf N1.04 and over-stoichiometric Hf N1.17 films). The observed improvement arises from the increase in the concentration of free electrons and the relaxation time as a result of reduction of nitrogen and hafnium vacancies in the films. Furthermore,Hf N1.17 films have always much lower electrical conductivity and infrared reflectance than Hf N1.04 films for a given Vb, owing to more hafnium vacancies because of larger composition deviation from Hf N exact stoichiometry(N:Hf = 1:1).3. We studied the identification of the phase transition and its effect on optical and electrical properties in hafnium nitride films. We confirmed that the “nitrogen-rich” phase appearing at x = N:Hf = 4:3 is of a cubic Th3P4 structure with a space group symmetry of I-43d(220), namely c-Hf3N4. We conclude that with increasing nitrogen, phase transition takes place from rocksalt(?-Hf N) to c-Hf3N4 through three stages of structural evolution:?-Hf N(containing Hf vacancies) ? mixture of(?-Hf N + c-Hf3N4) ? c-Hf3N4. The driving force of the phase transition is energy minimization. The three stages of structural evolution are explained by comparing the EOF of the ?-Hf N and c-Hf3N4 phases. As the phase transition takes place, the hafnium nitride film morphs from a conductive and opaque metal into an insulating and transparent semiconductor.4. This work studied the effect of optical reflectivity and hardness of hafnium nitride films via tantalum alloying. Here, we find that incorporation of tantalum in hafnium nitride film induces a tunable optical reflectivity and improves the hardness. The results unambiguously prove that the reflectivity improvement arises from the formation ofHf1-x Tax N solid solutions and the resulting changes in electronic structure. The increase in infrared reflectance originates from the increase in concentration of free electrons(n) because Ta(d3s2) has one more valence electron than Hf(d2s2). The sharp blue-shift in cutoff wavelength is attributed to the increase in n and the appearance of t2 g ? eg interband absorption. These results suggest that alloying of Ta renders an effective avenue to improve simultaneously the optical and mechanical properties of Hf N films. This opens up a door in preparing high-reflectance yet hard films.