InVisage ~ Quantum dots film

InVisage, a Silicon-based company, are regarded as having huge potential in photonic industry. Quantum film is the most fantasy product that InVisage first pushes it into commercial market to bring up a revolution in camera to unprecedented  image quality. In addition, Quantum film can also be used in solar cell to harness more sun's power.

Quantum Dots 

"A quantum dot is a portion of matter (e.g., semiconductor) whose excitons are confined in all three spatial dimensions." ~Wiki

Edward H. Sargent, professor of University of Toronto and CTO of Invisage, uses colloidal semiconductor nanocrystals to make quantum dots with chemical process. Because of the quantum confinement effect, quantum dot can make to operate to absorb or emit at fixed wavelength which can be tuned by simply varying its size. Increasing the diameter of particles from 1 to 10nm can shift the action from visible into infrared. 

Advantages:

• Sensitivity enhancement
  
  Higher quantum efficiency can be achieved by the effect of delocalization, hopping, and slow carrier relaxation. The normalized detectivity (unit of Jones) can be improved more than one order of magnitude compared with InGaAs which needs costly epitaxy process, especially in infrared region.  
• Noise elimination
  
  The fusing processing can decrease the dark current by the network connection of electro-nanocrystalline-layer and cross-link molecules. Besides, the specific absorption band property of QD can eliminate the noise from low energy photo absorption. Noise equivalent would be less than 10^-11 J/cm2 for visible range (400~800nm).
• Higher Fill Factor
  QD film can be paved on the top to detect photon. Moreover, QD films with different absorption spectrum can be overlaid in stacked pixel without the loss of other color.
• Gain property
  It exhibits photoconductive gain as high as over 1000A/W by multiplication of carrier, unlike conventional photodiode with no intrinsic amplification.
• Mass production and Easy integration
  Quantum dots are synthesized by solution process and can be pained or printed with spin-coating. The low temperature process is also one of the advantages.
• Tunable spectrum bandThe bandgap can be tailored under control with material and size.

Mechanism:

There are two kinds of mechanism to use quantum dots in photodetection. 

• Photoconductor
  
  The main application for QD in sensing and solar harvesting. Photoconductor conduct a single carrier type, usually hole carrier for QD. Quantum dots would trap electron to impede the extraction and to conduct the hole carrier. The longer QD traps the electron, more hole can transit to cause carrier multiplication.
• Photodiode
  
  There is a deep-work function produced a built-in potential between the metal and QD, which extract carriers of each other. In order to avoid electron-hole pair recombination, the life time of excess charge carrier must be larger than the time to carrier transport. Even though the mobility of QD is lower, its excitation lifetime is longer than 1us to enhance QE. 

Processing

The core of the colloidal quantum dot can be PbS, PbSe, InAs, InP, CdS, CdSe, Terray Semiconductor, and core-cell seiconductor. Its ligands can be Amine-terminated, Carboxyl-terminated, DNA, Oligonucleotide, and Polymer. To make the quantum dots more proximate network uses two steps to exchange the ligands attached on the outer surface of core. QDs are electrically connected to make carrier flow easily.

Annealing can fuse these colloidal nanocrystal more arranged and decrease the defect to enhance mobility.

Jess Lee, CEO of InVisage, introduced their QD film coated on TSMC CMOS chip:

Structures

Quantum film, served as optical sensitive layer (or so-called active layer) where photon conversion takes place, needs electrode and readout circuit to collect the photocurrent. In InVisage's patent, lateral electrode(Fig.39) is more common than vertical electrode arrangement(Fig.40). (sandwich type is more like the photodiode mode in Sargent's review paper) 

The layer of electrode is under the quantum film layer to deliver electric field and define the pixel region. The basic form, like a square, has positive biased electrodes in center and grounded electrode around the pixel, and then each pixel unit forms an sensor array. After electron-hole separation induced from photon absorption, carrier would be guided to the center electrode.

Stacked layer

The small quantum dot layer detects effectively the short wavelength, and the longer wavelengths are detected from larger quantum dot layer. Stacking the smaller QD layers on the larger QD layer is for dual-spectral detection. Based on each specific absorption band, stacked pixel can act like Foveon without color aliasing. (But in this patent, the biased voltage is amazing high as 100V !!)

There is dark pixel design revealed in patent. Dark pixel is located at the bottom of each optical sensitive layer (QD layer) for furthermore subtracting dark noise.

Pixel Circuitry:

Quantum dot film can be integrated with the circuit of present CMOS image sensor to serve as the function of photodiode. 4T pixle differs from 3T pixel in that it has global shutter controlled by sample and hold transistor. But remember that quantum dot film is overlaid on the circuit of these transistors and no situation of decreasing fill factor as increasing transistors.

No surprise, QD film also can use shared-pixel design which some pixel use the same amplifier and readout transistor. Even though this design can't improve fill factor in this situation, it still help for stacked pixel.

Cross-section schematics: QD film is 113 and there are planar electrode 115 to exert electric field and extract photoncurrent. Preservation layer 114 is to protect the QD film and to extent the lifetime. 

Color Filter Array (CFA)

Color filter can be put on the QD film to define the color property for each pixel. However, it is no need for color filter and use QD film's tunable spectrum property. From basic Bayer pattern to stacked in one, QD film can support any derivative of color filter array.

Concerns:

• Lifetime
  QD film need a preservation layer to protect itself. For organic material like the QD, there is some obstacle to overcome about extending lifetime.
• no-linear relationship
  The response of QD photoconductive device would vary with the intensity of illumination, but on the other way it is also a advantage to extend dynamic range.
• noise?
  According to patents and paper, InVisage shows data and schematics to proof its ability to decrease noise. But there is concern about how photoconducter beats photodiode.

Applications:

• Photodetector
• Infrared spectrum
• Solar Cell
• Flexible optoelectronics
• Printable Laser

PS. Plasmonics also inspires some integrated solution with QD film in enhancing the efficiency, no matter in photodetector or solar cell.

References:

1. Infrared Quantum dots, Advanced Materials (2005)
2. Ultrasensitive solution-cast quantum dot photodetectors, Nature (2006)
3. USPTO 7773404, Quantum dot optical devices with enhanced gain and sensitivity and methods of making same
4. USPTO 7881091, Methods of making quantum dot films
5. USPTO 7746681, Method of making quantum dot films
6. USPTO 7923801, Materials, Systems and Methods for optoelectronic devices
7. USPTO 2010/0044676, Photodetectors and Photovoltaics based on semiconductor nanocrystals
8. Nanostructured materials for photon detection, Nature Nanotechnology (2010)
9. Connecting the Quantum Dots, IEEE Spectrum (2010)
10. Paint-on Optoelectronics, OPN (2006)