Effect of Copper-Induced Recrystallization on the Piezoresistivity of Germanium Films
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ABSTRACT Effect of copper-induced recrystallization on the piezoresistivity of germanium films is reported. SEM and XRD are used to study the crystalline structure of the films, while RBS, PIXE, and EDX are utilized to investigate their composition. Significant grain growth is observed in Ge films, upon annealing a Cu/Ge bilayer at 400'C for 30 minutes. Also a dominant orientation of Ge(l 11) is observable when amorphous precursor films are used. Hall measurements and cantilever beam bending show a considerable increase in hole mobility and longitudinal gauge factor, respectively. INTRODUCTION
It has been shown that the crystallization of germanium can be mediated by the incorporation of copper. Amorphous Ge films are recrystallized at 400°C when annealed in the vicinity of a Cu layer[l], while solid phase crystallization (SPC) of Ge occurs at temperatures beyond 500'C [2]. This is believed to be due to the formation of copper germanide, which acts as a nucleation seed for germanium. In this paper, we report the effect of metal-induced crystallization (MIC) on the piezoresistivity of Ge films. Piezoresistivity is a highly anisotropic effect, which depends strongly on the crystalline stnrcture and orientation in semiconductors. This is originated from the fact that the change in conductivity due to stress, is caused by a shift in the sub-bands relative to each other and the subsequent redistribution of carriers. Since the band structure depends greatly on the crystalline symmetries, a high anisotropic piezoresistive effect is expected [3]. EXPERIMENTAL
Germanium films with a thickness of 1000A are e-beam deposited on a 50pm thick glass substrate. Amorphous Ge samples are prepared at room temperature, while polycrystalline films are deposited at a substrate temperature of 400'C. This is followed by depositing 100A of copper on some of the samples at ambient temperature using thermal evaporation. These samples are then annealed in situ at 400'C for 30 minutes to form copper germanide. Four groups of samples are prepared in this way: as-deposited amorphous (a-Ge) and polycrystalline (p-Ge) films and MIC samples starting from amorphous (MIC a-Ge) and polycrystalline (MIC p-Ge) precursors. RBS, PIXE, and EDX are used to study the composition of the films, while SEM, XRD, and TEM are employed to investigate their crystalline structure. For the purpose of electrical measurements, copper germanide and any unreacted copper are etched away using phosphoric acid and gernanium islands are then patterned by photolithography. After sputter depositing a passivation oxide layer, windows are opened with buffered HF for electrical connections. This is followed by a bilayer Al/Cu metallization. Carrier mobility is extracted from Hall measurements, while cantilever beam bending is used to determine piezoresistance coefficients.
255 Mat. Res. Soc. Symp. Proc. Vol. 618 ©2000 Materials Research Society
PHYSICAL STUDIES
Fig. 1 shows SEM micrographs of the samples. Fig. 1(b) displays the morphology of a-Ge films without any post treat
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