Optical Nonlinearities And Photo-excited Carrier Dynamics In Gallium Nitride Crystals

Gallium nitride(GaN) is one of the third-generation and wide band gap semiconductor materials, with high thermal conductivity, high electron mobility, high electron velocity and high breakdown voltage, etc. GaN is very favorable for fabricating high-temperature, highpower, high-frequency, blue and ultraviolet electronic devices. However, the GaN-based optoelectronic materials and devices will be involved in many scientific and technical challenges which are specific to the wide band gap semiconductors. Experimental study of optical nonlinearities and photo-excited carrier dynamics in representative wide band gap GaN material can not only understand the interaction among microscopic degrees of freedom including bound-electron, carrier and defect states, but also determine a series of photophysical parameters of GaN material, including multi-photon absorption coefficient, Kerr refractive index, carrier absorption cross-section, and lifetime. These investigations are not only helpful to better understand the nonlinear optical responses and photophysical mechanisms in wide band gap semiconductors, but also great significance for researches of optoelectronic devices based on GaN and other wide band gap semiconductors. This dissertation evalutate the optical nonlinearities and photo-excited carrier dynamics in undoped and doped GaN crystals experimentally according to the conductivity type. The main works are listed below:(1) Using Z-scan and the time-resolved pump-probe based on phase object(PO) techniques, the optical nonlinearities and carrier dynamics in an n-type undoped GaN crystal at 532 nm are investigated systematically. The nonlinear optical properties in undoped GaN under picosecond and nanosecond regime are measured qualitatively via Z-scan techniques. Sebsequently, the different optical nonlinearity mechanisms in undoped GaN are distinguished via PO pump-probe techniques. Combined with the theory of two-photon absorption-induced free-carrier nonlinearity, the photophysical parameters related to boundelectron and free-carrier of undoped GaN are determined.(2) Using Z-scan and PO pump-probe techniques in the visible(532 nm) and nearinfrared(760-1030 nm) region, the optical nonlinearities and carrier dynamics in n-type nonpolar GaN crystals are investigated. Firstly, the anisotropy of two-photon absorption, free-carrier absorption, and refraction effect in non-polar m-plane and a-plane GaN crystals are evaluated by modifying the polarization direction of incident laser. The experiments not only measure the three-photon absorption coefficient and Kerr refractive index wavelength dispersion, but also calculate and analyze the applicability of m-plane GaN material to alloptical switching and autocorrelation fields according to the ultrafast refraction dynamics.(3) Using Z-scan techniques, the effect of Fe-doping on optical nonlinearities in semiinsulating Fe-doped GaN diluted magnetic semiconductors are investigated experimentally firstly. Secondly, combined with the carrier dynamics acquired via PO pump-probe techniques, the photo-excited carriers capture effect of Fe levels in GaN is analysised, and the photophysical parameters of GaN:Fe are determined with effective three-level model. Moreover, the electron capture cross-section of Fe3+ are determined according to the relationship between effective photo-excited carrier lifetimes and Fe concentrations.(4) Using pump wavelength-tunable femtosecond transient absorption spectroscopy, the ultrafast photo-excited carrier dynamics in a p-type Mg-doped GaN wafer under different carrier distributions are systematically investigated. The photo-excited carrier lifetime parameters under both homogeneous and inhomogeneous carrier distributions are analysised. Meanwhile, the recombination mechanisms of photo-excited carriers are also discussed.