The Influence of Critical Electric Field、Conjugate Polymer and Saturation Effect on Terahertz Radiation from Semiconductor Microstructure

• Main Authors: Hui-Ching Lin, 林輝慶
• Other Authors: Jenn-Shyong Hwang
• Format: Others
• Language: zh-TW
• Published: 2007
• Online Access: http://ndltd.ncl.edu.tw/handle/72519901949616262480

博士 === 國立成功大學 === 物理學系碩博士班 === 95 === Terahertz radiation (THz) represents the electromagnetic waves with frequencies around 1012 Hz. In comparison with other electromagnetic waves, the characteristics of terahertz radiation have not been clearly understood and very few scientific research and practical applications have been reported. Therefore, the generation, detection, application and mechanism of terahertz radiation have become important area of studies. In this thesis we first introduce the generation and detection system and studies of the mechanism of terahertz radiation. The effects of critical electric field, conjugate polymer (DB-PPV) and saturation effect on the intensity of terahertz radiation radiated from semiconductor microstructures will also be reported. It is widely believed that, in photoconductive mode, intensity of terahertz radiation is proportional to local field (bias) by the external applied or built-in electric field. However, when GaAs surface intrinsic-N+ (SIN+) structures are used as the terahertz emitter, the larger the bias electric field, the smaller the terahertz radiation intensity is observed. There exists the so-called “the critical electric field” in semiconductors. As the local field is below the critical electric field Ec, the maximum drift velocity of free charged photo-excited carriers in a semiconductor is proportional to the electric field in the semiconductor. However, as the field rises above the critical electric field Ec, the maximum drift velocity declines slightly as the field increases. The maximum drift velocity of the free charged carriers peaks at the critical electric field, which depends on the energy difference between the Γ to L valley (intervalley threshold, L valley offset) in the semiconductor. While the intrinsic layer thickness is less than 200 nm, the electric field of GaAs is larger than the critical field thus the drift velocity is approximately constant. The amplitude of THz radiation above the critical electric field Ec is not proportional to nphEloc but proportional to nphEc. An effective electric field Eeff can be defined, which equals to the critical field Ec as the Eloc is larger than the critical field and equals to Eloc as the Eloc is smaller than the critical field. Then, the terahertz radiation intensity can be expressed by . The dependence of THz and nphEeff on the thickness of the intrinsic layer obtained experimentally are almost identical to each other implying that there is indeed a critical electric field, Ec , in the semiconductor such that ETHz is dependent on Eloc when Eloc is smaller than Ec and is independent of Eloc when Eloc exceeds Ec. As the field is lower than the critical field, the amplitude is proportional to the product of the surface field and the number of photo-excited carriers. In the high field limit where the surface field exceeds the critical field, the amplitude of THz is independent of the surface field but is proportional to the product of the critical field and the number of the photo-excited carriers. Since the critical electric field depends on the energy difference between Γ and L valley or the L valley offset in semiconductors. The L valley offset can be estimated from critical electric field determined from THz radiation. In the second part of this study, THz radiation from the surfaces of various semiconductor wafers and microstructures is investigated. Various polymer films are spin-cast on the surfaces of semiconductors and semiconductor heterostructures to enhance the terahertz radiation. The conjugate polymer (2, 3- dibutoxy-1, 4-polyphenylenevinylene (DB-PPV)) is found to effectively enhance the terahertz radiation intensity from the surface intrinsic GaAs wafer by as much as 50%. Changes in surface field and the density of interfacial states are detected associated with the enhancement of THz radiation by DB-PPV conjugate polymer surface-coating. For the semiconductors in which drift current dominates the diffusion current, THz radiation is enhanced by coating DB-PPV on their surfaces to increase their surface fields. For the semiconductors in which the diffusion current dominates the drift current, THz radiation is not enhanced by increasing in surface fields. The contactless and nondestructive modulation spectroscopy of photoreflectance is employed to determine the changes in surface field and the density of interfacial states which are closely related to the enhancement of THz radiation. Finally, we study the dependence of intensity of terahertz radiation on pump beam power in InAlAs SIN+ samples with different doping concentrations in the buffer layer. The intensity of terahertz radiation increases linearly with the pump power; reaches its saturation intensity; and declines slightly in the high power region. It is found that the intensity of the terahertz radiation is restricted by the critical electric field and the energy stored in the semiconductors, which depends on the surface field within the surface intrinsic layer. The variation in the THz intensity with pump power can be attributed to the change in the mobility of the photo-excited charged carriers.