The Study Of Temperature Dependence Of Electrical Performance For Typical Semiconductor Materials

With the rapid development of micro-electronic technology and weapon equipments, high requests are brought forward in the service performance of semiconductor materials under different temperature. Hence, it is important to study the temperature-dependence of electrical and optical properties for semiconductor materials. SiC and VO2 which have exclusive temperature-dependence function features exhibit promising applications in high-temperature microwave absorption and intelligent laser protection fields. In this dissertation, the effect of different doping elements on structural stability, electrical and optical properties of SiC have been investigated. The relationships between doping elements, structural, temperature and performance were revealed. In addition, The influences of composition, crystallographic orientation, microstructure on phase transition characteristics and oxidation stability of VO2 films have been investigated. Al2O3/TiO2/VO2 multilayer films were prepared to clarify the role of TiO2 buffer in improving the transition characteristics and corresponding optic-electronic properties of VO2 films.Part 1. The electronic structure, high-temperature dielecric properties and microwave absorption of nonmagnetic elements doped 3C-SiC.The structure stability and photoelectric properties of nonmagnetic elements(Al, N) doped 3C-SiC have been investigated using the first principles calculation. It is found that the structure of Al/N codoped SiC is more stable than that of the mono Al doped SiC. The results indicate that the N incorporation facilitates the substitution of Al for Si in SiC and improves the structure stability of doped SiC. The photoconductivity of the Al/N codoped SiC is higher in the range from near infrared to ultraviolet wavelength in comparison with the undoped SiC. The calculated intrinsic absorption edge of Al/N codoped SiC shows a red-shift in ultraviolet region with respect to that of Al doped SiC. The electron transitions and compensation mechanism for mono Al doped and Al/N codoped SiC were deeply analyzed to elucidate the role of doping elements in tailoring bang gap and tuning photoconductivity of doped SiC. Al, N and Al/N doped 3C-SiC powders were synthesized by mechanical activation assisted self-propagating high-temperature synthesis. It is found that the doped elements can increase the number of point defects such as vacancies and the carrier concentration in SiC, resulting in the increase of the imaginary permittivity(ε’’) and dielectric loss of SiC. In X-band, the reflection loss and ε’’ increase with increaseing temperature(293K-623K). The increase of ε’’ of doped SiC is mainly attributed to the combined effect of both polarization relaxation and conductivity. Compared with N doped SiC, Al doped or Al/N codoped SiC has larger increase in the imaginary permittivity with the increase of temperature, which is attributed to good conductance. The imaginary permittivity of N doped SiC has two significant relaxation peaks in X-band. The multiple dielectric relaxation of N doped SiC is attributed to the polarization relaxations of dipoles originating from the doped N atom and vacancies. For Al, N doped and Al/N codoped SiC, the maximum reflection loss increases from 15.59 dB for undoped SiC to 18.46, 21.24 and 28.96 dB, and the effective absorption bandwidth [RL(dB)≧10 dB] enlarge from 1.57 GHz to 2.31, 2.74 and 2.76 GHz. It is shown nonmagnetic doping is an effectively method to improve the high-temperature microwave absorbing properties of SiC.Part 2. The electronic structure, optical properties and high-temperature dielecric properties of transition metals doped 3C-SiC.Structural stability of Ni doped 3C-SiC was studied by the DFT+U modification method. The substitution of Ni in Si sub-lattice is energetically more favorable than that in C sub-lattice. Some new impurity energy levels appear in the band gap of Ni doped SiC, which subsequently results in enhancement of electrical conductivity of SiC. The imaginary permittivity, absorption spectrum and conductivity spectrum of Ni doped SiC show three remarkable new peaks in the low frequency region, respectively. The absorption edge of Ni doped SiC red-shifts to far-infrared region. These are related to the electron transition between valence band, impurity band and conduction band. Fe, Co and Ni doped 3C-SiC powders were synthesized by mechanical activation assisted self-propagating high-temperature synthesis. The experimental results demonstrate that Fe, Co and Ni doping can improve the high-temperature dielectric properties of SiC in X-band. With the increase of temperature from 293 K to 623 K, the ε’’ of Fe, Co and Ni doped SiC present a trend of increasing. Fe doped SiC has the best dielectric properties for microwave absorption compared to Co and Ni doped SiC. The imaginary permittivities of Co and Ni doped SiC all have two relaxation peaks in X-band, but Fe doped SiC only has one relaxation peak at high frequency in X-band. The effects of doped elements and carbon vacancy on band structure and electron density difference of SiC have been investigated. It is found that the coexistence of doped elements and vacancies in the SiC supercell can introduce more impurity energies in the band gap of SiC, and attract more electrons around doped atoms or vacancies. It breaks down the balance of the neighboring electric charge distribution and induces dipoles nearby the doped atoms or vacancies. The polarization relaxation of dipoles leads to the formation of relaxation peak.Part 3. The preparation and high-temperature stability of VO2 films on differently oriented sapphire substratesThe influences of process parameters of magnetron sputtering and subsequent annealing on the composition, microstructure and phase transformation characteristics of VO2 films have been systematically investigated. The optimal technological parameters to prepare high-quality VO2 film were obtained. VO2 films prepared on m-, a-, and r-plane sapphire substrates show(402),(002) and(011) preferred orientation, respectively. All of the three samples exhibit a resistance change of more than four orders of magnitude at transition temperature. The obtained VO2 films display different microstructures depending on the different lattice-matching relationships between VO2 and oriented sapphires, i.e. mixed microstructure of striped grains and equiaxed grains on m-sapphire, big equiaxed grains on a-sapphire and fine-grained microstructure on r-sapphire. The electric resistance and infrared transmittance of the oxidized VO2 films were characterized to examine performance characteristics of VO2 films with different microstructures in oxidation environment. It is found the oxidized VO2 films on m-sapphire exhibit better electrical performance. The oxidized VO2 films on a-sapphire have the higher optical modulation efficiency in infrared region compared to other samples. The different performance characteristics of VO2 films were understood in terms of preferential orientation, grain size, grain shape, and oxygen vacancies.Part 4. Tunable phase transition temperature and laser protection performance of Al2O3/TiO2/VO2 multilayer filmsThe rutile TiO2 films were deposited in situ on m-plane sapphire substrates by magnetron sputter deposition. The effects of in situ and heat-treated TiO2 layer on the orientation, morphology and phase transition property of VO2 films were investigated. VO2 film on the in situ TiO2 layer deposited on m-sapphire is well(402)-oriented. It has a transition temperature of 59℃, at which a thermal hysteresis loop of resistance was observed with a resistance change ratio up to 4.7×104 and the hysteresis width of 2 ℃. The different thicknesses of TiO2 and VO2 films have important influence on the composition, morphology and phase transition property of VO2 films. With the increase of the thicknesses of TiO2 films, the phase transition temperature granduly shift to high temperature(40℃, 62℃ and 66℃). The increase of the thicknesses of VO2 films make grain dimension bigger and the phase transition temperature change from 40℃ to 60℃. It is because that as the thickness of VO2 film exceeds the critical thickness, mismatching stress from TiO2 buffer layer partially releases, and stress induced effect is weakened. The laser protective capacibility of VO2 film and TiO2/VO2 composite film were investigated by the 1064 nm pulsed laser, respectively. The laser damage mechanism for different kinds of VO2 films is discussed. The laser damage threshold of both VO2 films are 19.9 mJ/cm2 and 44.8mJ/cm2, respectively.