Studies On Properties Of Two Types Of Photoelectronic Functional Crystals By Optical Spectroscopies

Photoelectronic functional crystals are the key materials of modern high-tech field.Currently, as the increasing development of photoelectronics, the photo replacing theelectron is becoming the main information carrier. Because the obtaining,transmission, storage, display and process of information requiring the cooperationbetween photons and electrons depend on the photoelectronic functional crystals, theresearch and application of photoelectronic functional crystals are in full swing. Inthis dissertation, two types of photoelectronic functional crystals, the infrarednonlinear optical crystal and III-N compound crystal, are studied by the solid statespectroscopy.Nonlinear optical (NLO) materials are the foundation of laser conversion, opticalmodulation, optical parametric oscillator and telecommunications. Thus, it is of greatimportance to study the NLO materials, especially in the deep UV and IR regionswhere the NLO crystal is rare. Since the macroscopic properties of NLO crystals arerelated to the microscopic structures, NLO materials should be noncentrosymmetric(NCS). At present, using the second order Jahn-Teller (SOJT) effect, new types ofNCS materials have come into being. These materials are composed of d0transitionmetal cations and cations with nonbonded electron pairs, both types of which aresubject to the SOJT effect. Monoclinic β-BaTeMo2O9(BTM) is one of these materials,and contains cations with stereochemically active lone pair electrons (Te4+) and d0transition metal ions (Mo6+). Xutang Tao’s work on growth, thermal, electro-optic,elastic, dielectric and piezoelectric properties of BTM provided it a candidate fornear-mid-IR nonlinear optical devices.1. Using the group analysis method, the symmetry, IR-and Raman-activity ofphonons in the β-BTM have been obtained. We calculated the scattering efficiency ofRaman back-scattering with different polarization, which were also carried out byexperiments. The Raman spectra of different β-BTM planes were obtained, whichcomplemented the region below100cm-1. Comparing the experimental Ramanscattering spectra with the theoretical Raman scattering spectra from the First-Principles calculation results, it is true that the experimental and theoreticalresults are consistent with each other in despite of tolerable discrepancies. Therefor,according to the animation of lattice vibrations, the observed modes are visuallyassigned to the specific atoms’ motions in the primitive cell based on theFirst-Principles calculation results.2. The infrared dielectric property of monoclinic BaTeMo2O9single crystals isstudied by polarized IR reflectance spectra from20to4000cm-1. Based on themodified Lorentz model, the frequencies, strengths and dampings of TO modes aswell as the orientations of the dipole momenta are determined, agreeing well withRaman spectra and results from First-principles calculation. A large shift of theinternal modes of the anion groups relative to free anion coordination polyhedra isobserved, which can be used to indicate the distortions of coordination polyhedrarelated to the nonlinear optical properties. Further, the experimental results of thestrengths and dampings of the oscillators support the elimination and splitting ofdegenerate modes in free regular polyhedrons. These results offer a way to evaluatethe nonlinear optical (NLO) properties by use of traditional IR reflectivity spectra.3. By measuring the transimision spectra of the monoclinic β-BTM and theorthorhombic-BTM, the absorption spectra of both phases are obtained. Aftercomparing the differences of the absorption spectra and growth conditions betweenthe the β-BTM and the-BTM, an explaination for the primrose yellow of β-BTM isproposed. It is the F and F+color centers induced by the vacancy defect of oxygenthat are responsible for the primrose yellow of β-BTM. On the other hand, wemeasured the Photoluminescence (PL) of β-BTM at different temperatures. Fitting thePL peak by Gauss curves, the change law of the PL peak versus temperature isobtained. It reveals that the photoluminescence characteristic is determined by thevibronic, which is the coupled state involving the interaction between electronic andnuclear vibrational motions of the local MoO66-cluster.Besides, the group-III nitride semiconductors, another type of photoelectronicfunctional crystals, are developing rapidly. Much attention is paid to their applicationin the photoelectric field, such as blue-laser, LED and UV detectors. In microelectronics, these semiconductors are used to high-frequency,-speed and–power transistors, which is increasingly important. The research work about thesenitride semiconductors are as follows:1. Infrared reflectivity measurement has been made on the AlN film on the sapphiresubstrate. The frequencies of the symmetry optical phonon E1(TO) in the temperatureranging from77K to500K are reported by fitting the experimental reflectivity withthe classical multi-oscillators model. Taking the lattice thermal expansion andKlemens process of the phonon decay into account, along with the strain effectintroduced by thermal mismatch between the film and the substrate, the temperatureeffect on the frequency of the optical phonon E1(TO) is revealed. It shows that theshift of frequency is mainly attributed to the decay process while the strain effectinduced by thermal mismatch play a non-negligible role in the outcomes of thestrength and damping parameters.2. Based on the Maxwell equations, we revealed the characters of surface polaritonand its activation by ATR spectrum. As to the AlN, GaN and Sapphire semiconductorcrystals, whose dielectric functions are expressed by Lorentz model, there existsurface phonon polaritions in these crystals’ surfaces. By ATR measurements, thedependences of surface phonon polaritons on the air gap width are obtained.Combining the theoretical simulation, it confirms the characters and ATR-activationof surface phonon polaritons.