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Due to recent breakthroughs, silicon (Si) photonics is now the most active discipline within the field of integrated optics and, at the same time, a present reality - with commercial products already available on the market. Silicon photodiodes are excellent detectors at visible wavelengths, but the development of Si photodetectors (PDs) operating at wavelengths of interest for telecom and datacom is not straightforward, as Si is transparent at wavelengths larger than 1.1 μm. Conventional Si-based PDs at telecom wavelengths of 1550 nm utilize integration with other material systems such as germanium (Ge). Indeed, recently pure crystalline Ge has been epitaxially grown directly on Si substrates. A limitation of Ge-based photodiodes is both their incompatibility with an unmodified CMOS process flow and the requirement for a buffer layer that causes problems in both thermal budget and planarity. These problems could be circumvented by fabricating all-Si PDs taking advantage of the internal photoemission effect (IPE). IPE is the optical excitation of electrons in the metal to energy levels above the Schottky barrier and then the transport of these electrons to the energy bands of the semiconductor (see figure where p-type Si is considered). Unfortunately, due to the metal losses, IPE-based PDs are characterized by a limited efficiency.

Energy band diagram for a metal/p-type Si junction.

 

The activity developed at the Unit of Naples of IMM concerns the design, the fabrication and the characterization of silicon photodetectors based on the internal photoemission effect (IPE) at increased efficiency.

 

Copper/p-type Si (Cu/p-Si) Schottky junctions have been successfully incorporated into a Fabry-Perot optical microcavity in order to increase the detection efficiency at near-infrared wavelengths. The device fabricated in our clean room is formed by a dielectric bottom mirror, a metallic top mirror and, in the middle, a silicon cavity spacer section as shown in the following figure:

(a) Schematic cross section and (b) top view of the Cu/pSi PD fabricated at the Unit of Naples of IMM. 

 

The dielectric bottom mirror is a distributed Bragg reflector (DBR) formed by alternating λ/4-layers of amorphous hydrogenated silicon (a-Si:H) and silicon nitride (Si3N4), while the top mirror has been realized by means of a copper (Cu) layer that worked both as an absorber and as an optical mirror, at the same time.  It has been demonstrated that, when critical coupling conditions are fulfilled, i.e., when the DBR mirror reflectivity approaches the reflectivity of the metallic top mirror, the maximum responsivity is obtained at the cavity resonance wavelengths. Critically coupled devices exhibit a maximum responsivity of 0.063 mA/W around 1550 nm and a dark current density of 28 mA/cm2 at -1V.

In order to go towards integration, waveguide IPE-based PDs have been designed, fabricated an characterized. The waveguide PD is based on an asymmetric metal-semiconductor-metal (MSM) junction integrated on to a silicon-on-insulator (SOI) substrate. The main novelty of the structure is that, with the Cu layer, being in contact with Si only on the vertical exit wall of the optical waveguide, provides shrinkage of the contact area with Si reducing the dark current. Indeed, by taking advantage of a small contact area of about 3 μm2, it is possible to increase the magnitude of the applied reverse bias voltage to as much as 21 V, while maintaining a very low dark current  of only 2.2 nA. The increase in reverse voltage reduces the Schottky barrier height (due to the image force effect) – and increases the responsivity to 4.5 mA/W, as shown in the following figure. 

 

(a) Schematic representation (b) top view of the waveguide Cu/p-Si PD fabricated at the Unit of Naples of IMM. (c) Responsivity vs. reverse voltage and (d) experimental PD bandwidth.

 

In addition an experimental bandwidth of 1 GHz has been demonstrated. It is worth noting that the device shows the potential to work at longer wavelengths than the Near-Infrared (NIR), i.e. wavelengths in the range 2 to 3 μm or longer.

 

New emerging 2D materials like graphene have recently opened new perspectives in this research activity. Indeed, graphene forms a Schottky junction with Si and pave the way to the realization of NIR Graphene/p-Si PDs. Very recently at Unit of Naples of IMM, we have performed the integration of a 200 m thick Si cavity in a Si-graphene Schottky photodetector operating at the telecom wavelength of 1550 nm. The photocurrent generation is based on IPE, where photoexcited carriers from graphene are emitted to Si over a Schottky barrier existing at the graphene-Si interface. The rectifying Schottky diode behaviour is shown from the IV curve where a negligible series resistance is demonstrated and a Schottky barrier of ~460 meV is extrapolated. The cavity mirror consists of an air/Si interface, which produces an external responsivity Rext of ~0.3 mA/W at 1V reverse bias. By integrating a gold mirror, we enhanced Rext to ~0.9 mA/W, which is a factor of 3 compared with the simple air/Si mirror. The external responsivity is further enhanced by increasing the applied reverse voltage from 1 to 10 V. We finally demonstrate that our photodetector with a gold mirror shows an external responsivity of ~20 mA/W and an internal responsivity of ~0.25 A/W with a reverse voltage of 10V. This is more than 20 times higher than state of the art metal-Si Schottky photodetectors. The interference effect from the multiple reflections within the cavity is shown from the wavelength dependent internal responsivity, which is a signature of the optical cavity. A free spectral range of 1.65 nm is obtained, which is consistent with the theoretical value.

 

(a) Schematic representation (b) top view of the waveguide Graphene/p-Si PD.

 

Our recent results prove that, due to the wideband graphene absorption, the aforementioned device is able to work at wavelengths of 2 μm, where Ge has negligible absorption. This wavelength region has an impact on the area of optical data communication because two-photon absorption, which is a limiting factor for nonlinear optical processes in the NIR, vanishes at longer wavelengths (i.e. wavelengths of 2.2 µm or longer). In addition the wavelength of 2 μm has also found use in a wide array of applications ranging from bio-agent and medical sensing to environmental monitoring. Preliminary results show that the Graphene/p-Si PD is charectized by a responsivity of 4.5 mA/W at 2 μm and zero-bias applied.

 

The ongoing activity concerns also the fabrication and the characterization of PDs whose active absorbing layer is constituted by Erbium (Er). A Radio-Frequency (RF) Sputtering technique was used for depositing the Er thin film directly on the device, from a 99,9% pure metal Er target.  The device was placed on the substrate holder and the deposition chamber was pumped down to a base pressure of 3·10–6 mbar before introducing the process gas (Ar). Er film was then deposited at r.t., with 30 W RF power, at 2.5·10–2 mbar pressure, with a constant 40 scan Ar flux and 11 min deposition time. To overcome the target surface oxidation a 30 min presputtering process at 150 W RF power was necessary before the deposition process. The advantage of sputtered Er is that it can be deposited at low temperature. We have fabricated both vertically-illuminated and waveguide PDs showing that the Er/pSi hybrid junction has the typical Schottky behaviour. Responsivity measurements show results in the mA/W range.

 

SEM image of the (a) vertically-illuminated Er/pSi PD and (b) waveguide Er/pSi PD.

 

 

Contact: Maurizio Casalino

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