Advances in the field of poly-Ge on Si near infrared photodetectors
Introduction
The world wide interest in optical communications has prompted the need for low cost components for optical communication systems in the near infrared second and third windows of fiber optic transmission (1.3 and 1.55 μm). The ability to integrate optical components in the mentioned range on a silicon substrate has been recognized as a key step to achieve a cost reduction via standard and large scale process fabrication. In the outlined framework, SiGe/Si heterostructures have been considered promising candidates for near infrared photodetection, thanks to the sensitivity of SiGe alloys and their compatibility with silicon technology. SiGe alloys, multi-quantum-wells and superlattice heterostructures have been employed as sensitive layers in p-i-n photodetectors, and their operation at 1.3 μm successfully demonstrated in both normal incidence [1] and waveguide configurations [2]. Photodetectors based on crystalline Ge on Si have been investigated as well [3]. Although photodetectors based on crystalline SiGe and pure Ge on Si have demonstrated good responsivity and fast response, the high temperature involved in the cleaning process for the epitaxial growth [4] places severe limitations to the integrability with silicon electronics. An approach which, being based on the evaporation of polycrystalline Ge on Si, is a viable low-temperature and low-cost solution for the fabrication of NIR photodiodes compatible with standard Si technology has recently been proposed [5]. In this work, after a first section devoted to the samples growth and characterization, recent results are presented in an effort to enhance the optoelectronic properties of the poly-Ge devices. Three items have been considered, namely the uniformity of the characteristics of the film, which allows the fabrication of arrays of photodetectors, the improvement of the speed of photoresponse obtained by a proper design of the geometry of the photodetector and the increase of the responsivity achieved by avalanche multiplication.
Section snippets
Material deposition and characterization
Ge films were deposited by thermal evaporation using a 99.999% purity commercial source and a tungsten crucible in a vacuum with a background pressure of 10−6 Torr. Samples were grown on n-type (resistivity 2–3 Ω cm) 〈100〉 silicon substrates at temperatures in the 25–400°C interval, with evaporation rates of 1.5 Å s−1 and a thickness of 2000 Å as determined with a piezoelectric crystal balance. Silicon substrates were chemically cleaned just before the introduction in the vacuum chamber by
Multi-element device
The low-cost fabrication and the reduced thermal budget of our process are well suited for both good quality layer deposition and large area wafer production, essential ingredients for integrability with silicon electronics. In order to exploit the performances of the poly-Ge/Si system a linear array of 16-elements, as sketched in Fig. 5 (inset) was designed and fabricated. Each Ge-on-Si pixel is a planar metal–semiconductor–metal (MSM) photoelement. However, due to the reduced thickness of the
Sub-nanosecond speed of photoresponse
As for a fast photodetector, we first chose a MSM configuration. This structure, while allowing an easy fabrication and a good control of the parameters, lends itself to a fast response when closely spaced interdigited electrodes are employed.
The MSM was obtained by lithographic definition of a silver layer evaporated onto the Ge. The metal was in contact with the semiconductor only in the interdigited region of the device through a properly windowed photoresist layer. Interelectrode spacing
Responsivity of avalanche detectors
The maximum responsivity of the mesa heterojunction devices grown at 300°C is 16 mA W−1 at 1.32 μm for 1.0 V reverse bias. This, corresponding to an internal Q.E. of about 20%, is the more severe limit presented by the devices.
The high conductivity of the poly-Ge film turns into an extremely narrow depletion depth of a few Ångstrom, therefore the diffusion in the quasi-neutral zone rather than the drift in the space charge region is the main photocarrier collection mechanism. The diffusion
Conclusions
In conclusion recent results have been presented on poly-Ge based NIR photodetectors integrated on silicon substrates. In particular, the suitability of the material and technology to the fabrication of arrays of photodetectors have been demonstrated; a speed of photoresponse of only 650 ps at 1.32 μm was measured, and avalanche multiplication was exploited to obtain a 4-fold increase in responsivity. The presented results prove the versatility of poly-Ge technology as a candidate for the NIR
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