Advances in the field of poly-Ge on Si near infrared photodetectors

https://doi.org/10.1016/S0921-5107(99)00289-5Get rights and content

Abstract

The fabrication and characterization of near infrared photodetectors integrated on silicon substrates are reported on where the active layer is a thermally evaporated polycrystalline germanium. Recent results are presented in the effort to enhance the optoelectronic properties of the poly-Ge film in terms of uniformity for multiple device integration, speed and responsivity. In particular we demonstrate a 16 pixel linear array, a speed of photoresponse of about 650 ps and an enhancement of responsivity by a factor of four. The fabrication process, including substrate cleaning and preparation, requires temperatures lower than 300°C being fully compatible with silicon technology.

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

References (7)

  • F.Y. Huang et al.

    Appl. Phys. Lett.

    (1995)
  • B. Schuppert et al.

    J. Lightwave Technol.

    (1996)
  • L. Colace et al.

    Appl. Phys. Lett.

    (1998)
There are more references available in the full text version of this article.

Cited by (41)

  • Barrier enhancement of Ge MSM IR photodetector with Ge layer optimization

    2015, Superlattices and Microstructures
    Citation Excerpt :

    Easy and flexible fabrication of Ge MSM photodetectors allow to use of the standard silicon process lines for signal processing. For this purpose, a Ge infrared photodetector structure based on the thermal evaporation at 300 °C was first demonstrated in the pioneering work in 2000 [25]. The scientists found that the polycrystalline Ge can be deposited at 300 °C.

  • Investigation of crystallized germanium thin films and germanium/silicon heterojunction devices for optoelectronic applications

    2015, Materials Science in Semiconductor Processing
    Citation Excerpt :

    Germanium is a relatively well-understood semiconductor material owing to several decades of research [1–5], and has recently experienced renewed interest as a mobility-enhancement channel material for advanced MOSFET devices [6–9], and as a narrow bandgap material for near-infrared wavelength photodetectors [10,11].

  • Advances in Photodetectors and Optical Receivers

    2013, Optical Fiber Telecommunications: Components and Subsystems: Sixth Edition
  • A near-infrared optoelectronic approach to detection of road conditions

    2013, Optics and Lasers in Engineering
    Citation Excerpt :

    Moreover, it should employ a simple, reliable and low-cost technology. For these reasons we resorted to measuring reflected and diffused near infrared (NIR) light, which is invisible and harmless (at low intensities) to the human eye and can be produced and detected by inexpensive elements [15]. Very few studies have been reported in literature on optical ice detection based on the measurement of reflected light.

  • Thermal evaporation of Ge on Si for near infrared detectors: Material and device characterization

    2011, Microelectronic Engineering
    Citation Excerpt :

    The technology of Germanium-on-Silicon for near-infrared (NIR) optoelectronics has emerged because of the great interest towards monolithic integration with standard CMOS electronics. Various deposition approaches have been investigated, demonstrating high quality Ge films on Si and high performance devices [1–3]. Nevertheless, systems and techniques required to keep the defect density within acceptable levels often demand for complex growth flows and/or high thermal budgets, hardly compatible with standard CMOS [4].

View all citing articles on Scopus
View full text