| Summary: | 博士 === 國立中央大學 === 材料科學與工程研究所 === 105 === In this information explosion era, the topic of energy and communication are payed attention to many people. The Ge/Si technology provide many advantage to develop those topic, such as: quasi-direct bandgap, lattice match between GaAs and Si materials, easy monolithic integrated on Si substrate. Therefore, the development of Ge/Si device is significantly payed attention for scientists. In order to obtain high performance Ge/Si device, the Ge epilayers quality is very important. In this study, we investigated the Ge epilayers by high plasma density electron cyclotron resonance vapor deposition method and apply these films for solar cells and photodetectors development.
A suitable Ge film growth method can significantly enhance the Ge epilayer quality. The electron cyclotron resonance chemical vapor deposition provide a number of advance to grow Ge epilayers, for example: high crystallinity, low growth temperature and less ion damage to the surface. In this dissertation, the study can divide into three parts:
In the first part, we use electron cyclotron resonance chemical vapor deposition to investigate the strain behavior in Ge epilayers grown on silicon at a low temperature of 220°C. The strain in the Ge epilayers is transformed from compressive (-0.567%) to tensile (0.15%) as the process pressure decreases and main coil current increases. This tensile strain is due to intrinsic stress in the Ge epilayers at high process pressure and low main coil current. Besides, the Ge atoms have higher kinetic energy and shorter mean free path at low process pressure and high main coil current, which causes atomic bombardment effect on the Ge surfaces frequently. Thus, the intrinsic stress in Ge epilayers become compressive. The absorption coefficient of tensile and compressive strain in Ge films are measured using a UV–VIS-NIR spectrophotometer. The results show the absorption coefficients of the tensile strain Ge epilayer has a redshift condition on the absorption edge compare with compressive strain Ge epilayers. Finally, the structure information of the Ge epilayers is identified by atomic force microscopy and transmission electron microscopy. This strain control technology is modulated by film growth parameter, which can adjust the Ge bandgap for the device requirement.
In second part, we study the heavily boron-doped hydrogenated Ge epilayers are grown on Si substrates at a low growth temperature (220°C). The quality of the boron-doped epilayers is dependent on the hydrogen flow rate. The optical emission spectroscopic, X-ray diffraction and Hall measurement results demonstrate that better quality boron-doped Ge epilayers can be obtained at low hydrogen flow rates (0 sccm). This reduction in quality is due to an excess of hydrogen in the source gas, which breaks one of the Ge-Ge bonds on the Ge surface, leading to the formation of unnecessary dangling bonds. The structure of the boron doped Ge epilayers is analyzed by transmission electron microscopy and atomic force microscopy. In addition, the performance, based on the I-V characteristics, of Ge/Si photodetectors fabricated with boron doped Ge epilayers produced under different hydrogen flow rates was examined. The photodetectors with boron doped Ge epilayers produced with a low hydrogen flow rate (0 sccm) exhibited a higher responsivity of 0.144 A/W and a lower dark current of 5.33×10-7 A at a reverse bias of 1 V.
In third part, we present high quality GaAs epilayers grow on virtual substrate with 100nm Ge buffer layers. The thin Ge buffer layers were modulated by hydrogen flow rate from 60 to 90 sccm to improve crystal quality by electron cyclotron resonance chemical vapor deposition (ECR-CVD) at low growth temperature (180°C). The GaAs and Ge epilayers quality was verified by X-ray diffraction (XRD), and spectroscopy ellipsometry (SE).The full width at half maximum (FWHM) of the Ge and GaAs epilayers in XRD is 406 arcsec and 220 arcsec, respectively. In addition, the GaAs/Ge/Si interface is observed by transmission electron microscopy (TEM) to demonstrate the epitaxial growth. The defects at GaAs/Ge interface are localized within a few nanometers. It is clearly showed that the dislocation is well suppressed. The quality of the Ge buffer layer is the key of III–V /Si tandem cell, Therefore, the high quality GaAs epilayers grow on virtual substrate with 100nm Ge buffer layers is suitable to develop the low cost and high efficiency III-V/Si tandem solar cells.
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