1.1 - Terahertz Spectroscopy
The electromagnetic waves are widely application of anything with the progress of human’s technology, such as communication, national defense, medical science and others applications. In the 19th century, Maxwell equations proved light is a type of electromagnetic wave. Therefore, a lot of scientists study in every gap in the electromagnetic spectrum and doing the related experiment. After a long research, scientists in various frequencies for spectrum have understanding in a way. Only one frequency gap has not been extensively studied. Terahertz radiation (THz), it lies in the frequency gap between infrared and microwave as shown in Fig. 1-1 that is typically referred to as the frequencies about 100 GHz to 30 THz. 1THz is equivalent to 33.33 cm-1 in wave numbers, 4.1 meV photon energy, or 300 μm wavelength, so THz waves are also called sub-millimeter wave.
Before 1980s people don’t know much about THz because the generation and detection technologies are not well-established. Since the development of femtosecond laser, THz technology has been rapidly studies. In 1981, Auston[1]
successfully used the photoconductive switching to drive the antennas generate and detect coherent terahertz in free space. In 1995 Q-Wu and XC-Zhang[2] used ZnTe crystal to detect THz radiation by free-space electro-optic sampling (FS-EOS) which increases the detection bandwidth and signal to noise ratio (SNR).
Since the photon energy of the terahertz wave (4.1 meV) are smaller than X-ray, so the applications of THz are extended[3-4], such as THz imaging, non-destructive measurement, studies on semiconductors, security, biology and medical sciences.
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Fig.1-1 The spectrum range of electromagnetic waves (http://department.fzu.cz/lts/en/intro.html)
For semiconductors measurement, conventional four point and Hall effect measurement can measure the characteristics including mobility μ, concentration N and resistivity σ of the semiconductor materials by direct sample contact. All these electrical measurements measure only DC value of the sample. For some semiconductors with high resistivity and low concentration, the electrical properties are difficult to be measured by simple direct contact because at the metal-semiconductors interface, the Schottky barrier may disturb the measurement value. Therefore, THz-TDS (Terahertz Time-Domain Spectroscopy) with advantages of non-contact and frequency-dependent measure is desirable for semiconductors characterization. In 1990 D. Grischkowsky et al[5] successfully measured optical properties including refractive index and conductivity of GaAs wafer and the results fit well with the Drude model.
In comparison with traditional far-IR source and detection, THz-TDS is a coherent technology that can measure both amplitude and phase information at the same time.
This technology can avoid the uncertainty of Kramers-Kronig relation that simplifies the analysis process. Besides, we can analyze variety of materials, such as liquids, superconductors and nanostructure materials.
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1.2 - Issue in InN:Mg and a-plane InN semiconductors
Compared to all other Ⅲ- Nitride semiconductors, InN has the lowest effective mass, the highest mobility, highest saturation velocity and narrowest direct bandgap.
Recent research has shown that the revision of the band gap to the now almost universally accepted value of approximately 0.7 eV [6] as shown in Fig. 1-2. Since InN is a narrow band gap semiconductor, it has attracted much attention because it extends the fundamental band gap of Ⅲ- Nitride alloys can cover a wide spectral region, ranging from near infrared to ultraviolet. This characteristic has generated great interest in InN for high potential in applications such as high efficiency solar cells, near-infrared optoelectronics. But the InN film growing along the c-axis had a great effect which depends on the polarization-induced internal electric fields.[8] These electric fields are due to piezoelectric and spontaneous polarizations. The strain-dependent piezoelectric polarization along the c-axis { } of the wurtzite crystals which increases with the lattice mismatch in the nitride layers. However, the layers grown along a- { } or m-axis { } direction, on the other hand, polarization-induced electric field perpendicular to the layer interface can be minimized and the efficiency of the electronic devices can be increased.
In terahertz research, the narrow band gap and remarkably large gap between the conduction minimum and the next local minimum of InN also make it a favorable candidate for terahertz emitter.[9-10] Moreover, InN has high prospect as a material for high-frequency electronic devices.[11-12] To realize the InN-based devices, it is essential to fabricate both n-type and p-type semiconductors. However, due to its high electron affinity, as-grown InN is typically n-type and difficult to be doped p-type.[13]
Nowadays, the fabrication and the investigation of basic properties of p-type doping of InN has become a importance issue. One of the reasonably accepted methods to
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produce p-type InN is doping Magnesium.[14-15] Since Jones et al.[16] have reported indirect evidence for p-type doping in InN, many groups have verified hole conduction p-type bulk Mg-doped InN films (InN:Mg) buried under the surface layer.[17-19] However, it is still under debate whether Mg-doped InN film is p-type semiconductor.
Fig. 1-2 The energy band gaps of Group -Ⅲ Nitride semiconductors[7]
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1.3 - Organization of this Thesis
In chapter 1, an overview of THz radiation and introduction of InN and InN:Mg semiconductors. In chapter 2, we mainly describe the theories including THz generation and detection, analysis methods. In chapter 3, the basic characteristics information, such as growth method, properties of samples and Hall measurement results of semiconductor samples. In chapter 4, the experimental setups including the laser system and THz-TDS system are introduced. In Chapter 5, we will analyze data and discuss the experimental results. Finally, the experimental conclusions and future work are shown in chapter 6.
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