| Summary: | 碩士 === 國立交通大學 === 電子研究所 === 106 === In this thesis, we had optimized the hetero-epitaxy of Ge on GaAs surface by systematically investigated the effects of substrate orientation and surface preparation. Hence, we proposed a new structure of GaAs MOSFET having Ge source/drain (S/D) to tackle the intrinsic issues of the low solid solubility of dopants and low density of states (DOS) in GaAs material and also possess high electron mobility and thermal velocity to boost the current drive capability, compared to conventional Si MOSFETs.
To achieve high quality Ge epitaxy of recessed S/D region, the effect of wet etching solution on GaAs surface was studied. Utilize H2SO4 solution (H2O) will induce more avable Ga sites, beneficial to the bonding of Ge adatoms on GaAs surfaces. Therefore, high quality Ge film was successfully grown on the GaAs (111)A and (100) substrates by using a ultra high vacuum chemical vapor deposition (UHVCVD); high crystallinity and smooth surface were observed for these films by x-ray diffraction (XRD), transmission electron microscopy (TEM), and atomic force microscopy (AFM).
Subsequently, We systematically studied the electrical properties of MOSCAPs on both GaAs (111)A, (100) surfaces with ALD Al2O3 as gate dielectrics. The interface state densitywas estimated tobe ca. 2E12 (cm-2eV-1) of GaAs (111)A surface in the mid-gap region. And the device work confirmed that the GaAs (111)A surface have the better electrical characteristic due to the better ability of surface potential movement. The Fermi-level on GaAs (111)A surface is unpinned through supporting data from MOSFETs, MOSCAPs and we demonstrated a real inversion-mode NMOSFET on GaAs (111)A surface.
In addition, we also found the high quality Ge film grown on the (111)A GaAs substrate has significant improvement in the surface morphology, which RMS roughness was ca. 0.2 nm, because there are more available Ga sites for Ge adatoms to bond. Finally, the fabricated GaAs nMOSFET with hetero-Ge S/D exhibited an Ion / Ioff ratio of ~2.5×102 and subthreshold swing (S.S.) ~270 mV/decade, indicating that optimization of process was required.
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