Ge Microcrystals Photedetectors with Enhanced Infrared Responsivity
Authors: Isella Giovanni , Falcone Virginia , Ballabio Andrea , Barzaghi Andrea , Zucchetti Carlo , Anzi Luca , Frigerio Jacopo , Sordan Roman , Biagioni Paolo 
Published: November 2022

he direct epitaxial growth of germanium on silicon (Ge-on-Si) has fostered the development of near infrared detectors for telecom and imaging applications [1]. The long wavelength responsivity of these devices is limited to approximately 1550 nm corresponding to the direct energy gap of Ge EgΓ= 0.8 eV. Indeed, the absorption coefficient at the indirect gap EgL= 0.66 eV (λ≈1800 nm) is roughly two orders of magnitudes lower than that above the direct gap threshold. A sizable absorption within the 1550-1800 nm windows would, therefore, require exceedingly thick epilayers leading to wafer bowing and crack formation. An extended infrared absorption would, however, be beneficial for imaging applications since long wavelength radiation is less affected by Rayleigh and Mie scattering limiting visibility in fog and dusty conditions.
A viable route to enhance the responsivity of Gen-on-Si photodetectors in the 1550-1800 nm region might be exploiting the micro-structuring of the absorbing layer to increase the effective volume of interaction between light and matter.
In this work we report on a new type of detector, obtained from Ge micro-crystals epitaxially grown on a patterned Si substrate [2]. The faceted morphology and relatively high aspect ratio of the microcrystals is seen to enhance the detector responsivity in the wavelength region comprised between the direct (λ≈1550 nm) and indirect (λ≈1800 nm) gap of Ge, as compared to conventional planar devices.
The epitaxial growth has been performed by means of Low-Energy Plasma-Enhanced CVD (LEPECVD). Microcrystal formation is based on the self-assembly of Ge crystals on a Si substrate, deeply patterned by optical lithography and reactive ion etching. 3D microcrystals, several micrometer tall and characterized by a limited lateral expansion, are obtained by using optimized growth parameters. Due to crystal faceting, light trapping effects are expected to take place within the microcrystals leading to increased in the fraction of absorbed light when compared to conventional epitaxial layers.
Modelling of the near-IR absorption properties of the Ge-on-Si micro-crystals, and of conventional epilayer used as a reference,has been performed by finite difference time domain (FDTD) simulations. The ratio between the fraction of absorbed power of an array of Ge-on-Si micro-crystals and a planar Ge-on-Si epilayer is estimated to be always higher than one, with a factor of two increase in the spectral region between the direct and indirect gap (λ≈1550-1800 nm).
Ge microcrystals photodetectors have been fabricated using graphene as a transparent top contact (see Fig. 1). A confocal microscope with a supercontinuum laser source (1300 - 1800 nm), has been used to obtain the photodetector responsivity. The spot size was smaller than the patterned area (100 μm x 100 μm) thus enabling the illumination of a few Ge-on-Si micro-crystals. The photocurrent measurements experimentally confirm the enhanced absorption in the 1550-1800 nm spectral region [4] with a responsivity exceeding, by a factor of seven, that of a reference conventional Ge-on-Si photodiode at 1700 nm.
These results pave the way to a new class of photodetectors, exploiting light trapping phenomena in self assembled semiconductors microstructures.

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Figure 1