The Quantum Semiconductor technology platform is centered on the monolithic integration of epitaxial Group-IV films, in particular, superlattices with sophisticated compositions and doping profiles, with conventional CMOS technology.
Group-IV refers to the periodic table of the elements and includes Carbon, Silicon, Germanium, Tin and Lead. Superlattices are ordered crystals which are epitaxially grown on the silicon wafer surface. They are formed layer-by-layer using conventional epitaxial growth production tools found in CMOS foundries. For example, the band-gap engineering allowed by SiGeC superlattices enables radically-improved optoelectronic properties compared to pure Silicon or Germanium, including much higher efficiency light absorption and light emission, across a wide range of wavelengths, including wavelengths longer than those covered by Ge or InGaAs. In addition to SiGeC superlattices, Quantum Semiconductor has also investigated SiGeSnC superlattices which hold promise for imaging in MWIR and LWIR.
The improved optoelectronic properties can benefit many types of devices including:
Quantum Semiconductor’s imaging platform is applicable to all markets that use imaging, including Ultra-Violet, Visible, and Infra-Red. The approach is fundamentally different than the typical sensor manufactured today, because the photodiode uses an epitaxial thin film for the absorption of the incoming photons, instead of the conventional potential well formed by ion-implantation. The photon collection process is decoupled from CMOS junction engineering, thereby allowing these devices to track Moore’s Law with each new design generation, as well as use the most advanced substrates for state-of-the-art CMOS such as fully depleted thin-film SOI.
Because Quantum Semiconductor photodiodes are not coupled to the potential well, they can be operated in modes which are not possible for conventional CMOS photodiodes including as an Avalanche Photo-Diode (APD). In conventional photodiodes, one incoming photon results in one electron hole-pair, but in APDs, one incoming photon results in a multitude of electron-holes being formed, thereby providing an inherent magnification of the signal. This capability results in sensors with extreme sensitivity to low-light levels, and when coupled with Quantum Semiconductor patented ADC and column-parallel circuitry, results in an extremely large dynamic range.
Quantum Semiconductor has also demonstrated a new type of internal gain mechanism (inside the photodiode), which is noiseless at room temperature, similar to what has been observed in some silicon-based diodes. This noiseless gain mechanism does not result from avalanche by impact ionization but is believed to include an important contribution from Auger scattering.
The epitaxial Group-IV film, which forms the absorption layer of the photodiode, determines the cutoff wavelength and the Quantum Efficiency (QE) over the entire wavelength range of operation. A significantly higher QE across all wavelengths, enables this technology to take advantage of the full solar spectrum. Current technologies for visible and infra-red imaging have reached technological roadblocks which have stalled further improvement and no single technology exists today which can be used for multi-spectral imaging.
CMOS + SiGeC-superlattice technology improves several aspects of image sensing, namely:
1) Higher QE across the entire visible range, improves sensing with low light levels, and thus enables smaller pixels (higher resolution).
2) Photodiodes with large internal gain, coupled to suitable circuitry, enable wider dynamic range, i.e., improve imaging for scenes with large contrast in completely dark regions and super bright areas.
3) Imaging in daylight and at night with a single array of pixels.
Epitaxial SiGeC films are widely used in the manufacture of BiCMOS devices and are compatible with existing silicon manufacturing equipment and technology. These conventional films are random alloys with various doping profiles, with a small concentration of C and typically a Ge concentration less than 30%. The Quantum Semiconductor SiGeC superlattice films are formed by stacking monolayers of single elements and/or ordered alloys which can be manufactured on the same production equipment as SiGeC random alloys. The infra-red shift in light absorption and light emission depends on the film composition.
Beyond image sensors, the Quantum Semiconductor platform can be used for other applications which benefit from having an extended wavelength absorption or emission range. For example, high efficiency photovoltaic cells can take advantage of the full solar spectrum and be easily integrated with other CMOS functionality.
Some superlattices exhibit topological properties. Some of these properties can be exploited to form qubits for quantum computing and communication. Others can be used for encoding neuromorphic computing, still others for overcoming limitations in photo-voltaic conversion efficiency. A key advantage of Group-IV superlattices is their compatibility with standard CMOS which will allow the manufacture of chips which can exploit these unique light-matter interactions while maintaining manufacturability. Currently, no other 3D topological materials can be epitaxially grown on silicon substrates or can be considered compatible with standard CMOS processing.
Silicon photonics will also benefit from the unprecedented capabilities of this silicon-based technology to both absorb and emit light around the 1.55µm wavelength range. Today, photonic products are made from a myriad of different materials, many of which are incompatible with each other, and extremely expensive compared to silicon-based solutions.