Past twenty years have observed a significant development of ultrafast terahertz (THz) technique. Opening of the THz era has made significant contribution not only to ultrafast phenomena, but to a wide variety of applications, including ultrahigh speed optoelectronics, tomographic imaging in biomedical fields. The THz radiation is roughly defined by the frequency range of 0.1 to 10 THz as shown in Fig. 1-1, which are between microwaves and visible light. These THz wave corresponds to the wavelengths of 0.003 to 1 mm, so THz waves are also called sub-millimeter wave.
At lower frequencies compared to THz wave, microwaves can be easily generated by
“electronic” devices, such as a simple dipole antenna. At higher frequencies, visible light can be generated by “optical” devices, such as a semiconductor laser diode, in which electrons jump across the energy band gap and then emit light. Until 1980’s, the spectral range of THz has been inaccessible in both electronic and optical methods. Until then, Fourier transform spectroscopy is perhaps the most common technique to study sub-THz phenomena.
103 106 109 1012 1015 1018 1021 kilo mega giga tera peta exa zetta
Visible x-ray Microwaves
Photonics Electronics
γ -ray Thz
Frequency (Hz)
1THz ~ 1ps ~ 300μ m ~ 4.1meV
Fig. 1-1. Electromagnetic spectrum
The appearance of ultrashort pulse laser of ~100 ps pulse duration made it possible to generate THz waves covering the whole THz spectral range. In 1981, Mourou and Auston first demonstrated generation and detection of pulsed THz radiation by a photoconducting switch with advantages of time resolution of picosecond and high sensitivity enhanced by phase-lock technique [1, 2]. In 1988, Grischkowsky used the photoconductor dipole antenna as the THz sensor, furthering the spectrum into the order of terahertz frequency [3]. Afterward a variety of antennas was appeared, like typical dipole antenna, large aperture photoconductor dipole antenna [4] and also another method using semiconductor surface electric field [5] to generate THz pulses by the ultrashort pulse laser. In 1996, X. C. Zhang et al. developed free-space electro-optic sampling (FS-EOS) technique to enhance signal to noise ratio (S/N ratio) up to 105 and to achieve much large dynamic range [6].
THz wave has an infinite potential in application of science. In the bioscience, the photo energy of THz is much smaller than the traditional X-Ray and the pathological changes will not be induced by THz wave in human body. Since different tissues of body have different sensitivity for THz waves, more detailed information can be obtained through the tomographic THz imaging. Photon energy of THz wave is about 4 meV for 1 THz, which approximately equals to the binding energy of the excitons in many semiconductors. Most of all, recently developed THz waves possess ultrashort duration with broad bandwidth and provide both high sensitivity and time-resolved phase information. These advantages can be used in a number of applications, such as the study of carrier dynamics of condensed matters with high temporal and spectral resolutions.
Recently, the spectroscopic technique using pulsed THz radiation, called ”terahertz time-domain spectroscopy (THz-TDS)”, has been developed, by taking advantage of short pulses of broadband THz radiation. THz-TDS has the time resolution of sub-picosecond level and the spectral resolution of 50 GHz. And THz-TDS is a non-destructive method to the
carrier concentration and mobility of doped semiconductotrs. Many researches have been performed on a variety of gases, liquids, dielectric materials and semiconductors by THz-TDS.
For example, in 1990, D. Grischkowsky et al. studied the THz-TDS with the dielectric materials, such as quartz and sapphire, and semiconductors, like silicon and GaAs. They discovered different carrier concentrations affect the absorption characteristics of the samples in the THz frequency range. The Drude Model could be used to link the frequency-dependent dielectric response to the material free-carrier dynamics properties [7].
Nowadays, with requesting of the decreased size and faster efficiency of devices, it is very important to understand the transient phenomena of the materials. As a result of the advancement of laser technology, laser pulse duration can be as short as 10-15 second. Optical measurement can achieve higher time resolution and broader frequency domain than the electric measurement. Spectroscopic study with optical-pump THz-probe system can provide additional information of materials. In this time-resolved pump-probe technique, the dynamic far-infrared optical properties of the photoexcited materials can be studied.
Hydrogenated amorphous silicon (a-Si:H) is used extensively in Thin Film Transistors (TFTs) for Flat Panel Displays (FPDs) and large area imagers, and it is also a promising photovoltaic material. The a-Si:H TFTs have low off-current and sufficient on-current for most applications. But, a-Si:H has poor carrier mobility. The poor mobility will result in the limitation on the pixel sizes for display and other imaging application. Therefore, poly-Si with higher mobility up to 300cm2/Vs has been suggested as an alternative of a-Si:H. Higher performance poly-Si device has been applied in many applications, like flat panel displays.
In order to realize such system, System on plane (SOP), we introduced the low temperature polycrystalline silicon (LTPS). Low-thermal-budget techniques, such as plasma-assisted hydrogen [8], metal [9], and laser-induced crystallization [10-14]are
employed to LTPS for the region of channel. Particularly, excimer laser annealing results in high-quality polycrystalline silicon (poly-Si) due to the efficient absorption of the ultraviolet (UV) laser radiation [12-14].
In our experiment, we used the femtosecond (fs) laser pulses to annealed the a-Si:H.
Unlike the thermal annealing using continuous-wave [15] and long pulsed lasers (tens of nanosecond range), in fs-laser annealing, nonlinear photon absorption and non-equilibrium thermodynamics are expected to dominate [16-21]. The nonlinear process provides a precise and low fluence associated with femtosecond laser ablation [18-21].
In this thesis, our samples are the poly-Si annealed by a near-infrared (λ = 800 nm) femtosecond laser with 50fs pulse duration. The determination of the grain size of annealed poly-Si is an important task for its application in TFT fabrication. Typically, Hall measurement could not measure the electrical constant due to the similar characteristic between poly-Si and silicon substrate. SEM images of the annealed samples identify the grain size, but this method has the intrinsic restrictions, such as the limited imaging area and destructive sample preparation procedure. In this thesis, we introduced optical pump-THz probe method and THz-TDS system to directly identify the annealing quality of poly-Si in a large area without any preparation process. We also use the terahertz time-domain spectroscopy (THz-TDS) to study the ultrafast carrier dynamics in the poly-Si.
In chapter 2 of this thesis, we mainly describe the theories of generation and detection of THz field. In chapter 3, the femtosecond laser system, the experimental setup of THz system and the method of analysis will be introduced. In chapter 4, we describe the preparation of the sample. In chapter 5, we show the experimental results and discussions. At last, we make conclusions in chapter 6.