High speed germanium detectors on Si

Abstract

All the common group IV semiconductors – diamond, silicon carbide, silicon, germanium‐ are indirect semiconductors which narrows their optoelectronic application width. Silicon itself has a room temperature band gap of 1.12 eV with the first direct transition at 3.4 eV delivering strong absorption in the ultraviolet and the weaker indirect absorption in the visible up to the near infrared λ = 1.1 µm. Ge has a smaller bandgap (0.66 eV) which correlates with a larger band to band absorption limit (1.9 eV) but even more important the direct transition (0.8 eV) comes down to only about 150 meV above the indirect one. The speed of photodetectors is limited by two main factors, the carrier transport within the device and the RC time constant from loading the depletion layer capacitance via the external supply. High speed operation is obtained with absorption in the depletion layer where high electric fields accelerate the carriers to saturation velocity and avoiding slow carrier diffusion. In a pin diode structure this is obtained by sharp doping profiles (orders of magnitude increase/decrease within several Nanometers) and by the misfit dislocation network of the virtual substrate blocking diffusion from below the device. Indeed within a very short time the bandwidth of Ge on Si detectors was shifted in the GHz regime. The operation bias is usually in the reverse direction, but our group demonstrated also high speed (25 GHz) with zero bias when dark currents should be minimized. These photodetectors are sensitive in a broad spectral range (from visible to 1.6 µm). The lower limit is given by the absorption in the cap layer; the upper limit is the direct absorption edge of Ge. Silicon on insulator (SOI) optical waveguide system concept is explained which uses a light signal externally coupled in the silicon circuit. For its simplicity we propose a silicon on insulator (SOI) substrate. Rectangular waveguides like rib or ridge waveguides provide strong optical confinement at low device fabrication requirements. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Room Temperature Direct Band Gap Emission from Ge p-i-n Heterojunction Photodiodes

“…As detector the photodiode is reverse biased [16,17] or favorably zero biased [18]. Under forward bias the electrons and holes are injected from their respective highly doped sides into the intrinsic depletion layer and recombine there.…”

Metal-Insulator-Semiconductor Photodetectors

“…The common types of photodetectors include p-i-n, metal-semiconductor-metal (MSM), and metal-insulator-semiconductor (MIS) structures [ 4 , 5 ]. When an intrinsic semiconductor is sandwiched between a highly doped p-type layer and a highly doped n-layer, we have the basic configuration of a p-i-n type detector.…”

Optoelectronic application of Si/Ge heterostructures

“…For on-chip signal distribution the wavelength should be large enough for transparency in Si (λ > 1.1 µm) and small enough for detection and modulation with room temperature semiconductors (λ > 1.6 µm). In our system concept [3] the laser light from the external source is coupled into the chip via a fiber positioned above the waveguide coupler. The waveguide coupler has the grating on a tapered section of the waveguide in order to facilitate fiber adjustment.…”