硅上的低温锗薄膜

We discuss thermal evaporation of Germanium thin films as a  suitable route to realizing near-infrared detectors integrated on a Silicon  platform. We study the structural properties of samples grown at various  substrate temperatures by X-ray diffraction and transmission electron  microscopy, showing that Ge thin films are amorphous when deposited  below 225°C, mono-crystalline between 225 and 400°C, poly-crystalline  above 450°C. We further investigate their optical and electrical properties  using differential optical absorption spectroscopy, Hall and photocurrent  measurements. Finally, with the evaporated Ge thin films we demonstrate  near-infrared photodiodes with low dark current density and good  responsivity at 1.55 µm.  

In recent years, Silicon photonics has gained a leading role in applied physics and optical  engineering. To date, several photonic and optoelectronic components have been  successfully integrated on a Silicon platform exploiting several processes of the standard microelectronics industry. However, the challenge of integration is still open because of  the technological problems that prevent a viable monolithic integration of near-infrared (NIR)  sensing devices with Si electronics. In fact, while Silicon is an indirect bandgap semiconductor transparent at NIR communication wavelengths, NIR laser sources and  detectors require smaller bandgap materials, mostly III-V compounds which are essentially incompatible for integration with standard Si technologies. In this context, Germanium-onSilicon has been recognized as the best alternative towards the monolithic integration of NIR  detectors on Silicon platforms, with high performance Ge-on-Si detectors recently  demonstrated. Nonetheless, the monolithic integration of Ge photodiodes with standard Si electronics is still hindered by compatibility issues, i. e. thermal budget and crosscontamination stemming from Ge deposition. Although several Ge-on-Si deposition  techniques have been successfully implemented, most of them require high temperatures  which can hardly coexist with standard CMOS technology. Recently, we have proposed  physical vapor deposition of Ge on Si by thermal evaporation (TE) as a viable lowtemperature growth process and demonstrated optoelectronics devices by integrating Ge-on-Si photodetectors with either CMOS circuitry or Silicon photonics elements. However, a  detailed analysis and understanding of TE Germanium thin films on Si is still lacking.

In this Paper we report a comprehensive characterization of structural, optical and  electrical properties of Ge thin films thermally evaporated on Si. We show hereby that, by  simply controlling the substrate temperature, it is possible to grow Ge thin films of different  crystalline qualities, obtaining mono-crystalline epitaxial Ge at temperatures in the range 250- 400°C. Finally, we discuss this approach towards fabricating efficient NIR detectors for Si  optoelectronics.  

Ge thin films were grown on Si by thermal evaporation in a vacuum chamber with a  background pressure of 10−8  Torr and about 10−7  Torr during deposition. The material source  was high purity (99,999%) small grains of Ge in a tungsten crucible; the substrates were either  (100) Silicon or Silicon-on-Insulator (SOI) wafers, carefully cleaned in a standard RCA  solution. Prior to Ge deposition, the native silicon-oxide was removed from the substrate  surface by chemical etching with buffered hydrofluoric acid (buffered oxide etching - BOE);  this pre-TE treatment was a crucial step in obtaining crystalline films. Besides removing the  native oxide, BOE also played the important role of passivating the Si surface by terminating  the dangling bonds with hydrogen-ions; an H-passivated surface is chemically robust and  resistant to oxidization, permitting to handle the specimens in air before placing them in a vacuum. During deposition the evaporation rate was kept constant at about 2 Å/s, whereas the  substrate temperatures could be adjusted from 200 to 500°C.  

Fig1

In order to further investigate the structural properties of both mono- and poly-crystalline  TE samples we resorted to high-resolution (HR) TEM. The results are displayed in Fig. 4 with  the insets showing the Fourier transforms (FT) of the HR images. While Ge layers at 300°C  are mono-crystalline (epitaxial), those at 500°C have diffractograms typical of a  polycrystalline structure, possibly columnar with individual columns differently oriented. We  evaluated the intensity distribution of the FT spatial frequencies in order to determine which  region corresponded to a particular frequency; the results in Fig. 5 clearly indicate that the Ge  layer is structured in columns.

We measured the near-infrared (1200-1650 nm) absorption spectra of amorphous and  epitaxial Ge films on Si using differential optical absorption spectroscopy  and compared  them with reference bulk Ge, as Fig. 6 shows. The amorphous Ge films exhibit an optical  absorption slowly decreasing with wavelength without any specific features in the proximity  of the Ge bandgap. The spectra of epitaxial Ge films are comparable with those from bulk Ge  up to the cutoff wavelength, namely 1550 nm. At longer wavelengths bulk Ge shows the sharp  transition corresponding to its direct bandgap (about 0.8eV), whereas the TE thin film spectra  have a smooth variation, attributed to the high dislocation density. In particular, dislocations  introduce electronic levels in the forbidden bandgap available for energy transitions ;  when the photon energy is comparable with the gap between the valence band and the  dislocation levels, some photons are still absorbed. In addition, the measurement reliability  worsens beyond the gap as the increased transparency eases a number of multiple reflections  in the film thickness. The results demonstrate that both amorphous and crystalline evaporated  Ge films have significant NIR absorption and can be employed for light detection. However,  as we demonstrated earlier, the transport properties of the TE amorphous Ge are not well  suited for realizing optoelectronic devices.