The Study On Ge/Si Heterojunction And Its Photodetectors Characteristics

With the development of optical communication technology in rencent years, researchers have been interesting in production the photodetectors with high responsivity, high quantum efficiency, low dark current and large bandwidth at1.3μm and1.55μm wavelength in order to realize the monolithic Si-based photoelectronic integration. The industrialized Ⅲ-Ⅴ photodetectors have high cost and poor thermal conductivity and mechanical behavior. Additionally, they are not compatible with the existing mature Si-based complementary metal-oxide-semiconductor process technology (CMOS). Thin film Ge have high absorption coefficient at the wavelength of interest (1.3～1.55μm) and good transport properties, and compatibility with Si technology. Therefore, Ge photodetector fabricated on Si substrate are the ideal candidate for photodetectors used in low-cost monolithic transceivers for optical communications. But, the large dark current in Ge/Si heterojuction photodetector is the main limiting. In this paper, the poperties of Ge/Si heteroj unction and the physical mechanism of Ge/Si PIN photodetector and Sperate-Absorption-Charge-Multiplication avalanche photodetector (SACM-APD) are detailed investigated, which provide a theoretical basis for device design, manufacture, and optimization. The following are the details and research results:1. The effect of dislocations in the Ge/Si heterojunction on the characteristics of photodetecter is studied and investigated. Based on the formation mechanism and characteristics of threading dislocations in the Ge/Si heterojunction, and combined with operation mechanism and the physical effects in actual devices, the impacts of dislocation density distribution and dislocation active energy on the device performances are researched and validated by compared with experimental results. The results show that the dislocations in Ge epitaxial layer play the role of recombination centers and induce the indirect recombination of carriers, which is the main mechanism for the formation of dark current. The indirect recombination rate is influenced by the dislocation density, dislocation active energy and carrier concentration. Therefore, decreasing the dislocation density in Ge epitaxial layer, especially in Ge buffer layer, can effectively reduce the device’s dark current. The band bending due to the high bias in Ge materials decrease dislocation active energy and increases dark current, and the impact is enhanced with the Ge absorption layer thickness decreasing. Combining these two factors, as well as Ge/Si heterojunction band structure and impurities out-diffusion from heavily doped Si substrate to Ge epilayer, the calculated results of Ge/Si heterojunction PIN photodetectors and SACM-APD are presented and well consistent with the experimental data.2. The formation mechanism of strain in film Ge on Si substrate is investigated and their impact on the performance of photodetector is detailed studied. The tensile strain in Ge epitaxial layer is generated during the cooling from high growth temperature to room temperature due to the thermal-expansion coefficients mismatch between Ge and Si, which can be0.34%as maximum. The band shringkage induced by tensile strain increase the absorption coefficient of Ge material. Thereby Ge/Si photodetectors have enhanced responsivity and extended cut-off wavelength. Considering the relationship between strain and material parameters, the responsivities of photodetector varing with wavelength are calculated and consistent with experimental results.3. The influence of the geometry of the Ge/Si heterojunction photodetector on frequency response characteristics is researched. The frequency response of photodetector is mainly affected by the carrier transit time in the depletion layer, the diffusion time outside the depletion layer, and RC response time, and also limited by avalanche build-up time at large gain. The results show that the3dB bandwidth of PIN photodetector is varied the thickness of with Ge absorbing layer. The3dB bandwidth of Ge/Si SACM-APD has a enhancement behavior as the result of decrease of the multiplication time limitation due to the gain drop through the space charge effect. This effect is noteble in the Ge layer at high bias voltages or large optical input powers.These results reached in the dissertation can largely help avoid the purblindness in device design and enhance the accuracy of technological measures of manufacturing process. With a depper understanding of mutual constraint relationship between characteristics and parameters including materials, structures, and processing, this dissertation provides novel and valuable approaches to solving the problems during the application process of Ge/Si heterojunction photodetector.