The introduction of an uncooled microbolometer image sensor about a decade ago
enabled cost reduction of IR cameras. As a result, the available markets grew both in
military and civilian applications. Since then, the price of microbolometer was gradually
reduced due to introduction of devices with smaller pixel, maturity of the technology and
quantity growth. However, the requirement for a vacuum package still limits the price of
microbolometer based cameras to several thousands of dollars. Sirica's novel wavelength
conversion technology aims at breaking this paradigm by being uncooled and vacuumless,
lowering IR camera prices by an order of magnitude, opening the way to new mass
markets.
Sirica's proprietary IR-to-Visible/NIR conversion layer allows for low-cost high
performance LWIR detector with no requirement for cooling and vacuum packaging.
In the last years, the development efforts focused on development of the conversion
media. Recently, a parallel effort for the integration of the conversion layer together with
other detector components has started. Packaging of detector components, such as
conversion layer, pumping light source, dichroic filter, and their coupling with silicon
CMOS image sensor have great importance from a price-performance point of view.
According to the company's business-development roadmap, the detector prototype
should be available during the first quarter of 2010.
Thermal noise of quantum IR detectors is defined by the number of thermal carriers with energy higher or equal to the
detector's energy threshold. The energy distribution function of these carriers is of Boltzmann-type with a high energy
tail dictated solely by the device temperature. Therefore, thermal noise in such detectors can be suppressed only by
cooling the device down.
Sirica presents new technology for tunable quantum IR detector that requires no cooling. The detection is based on the
response of non-equilibrium free carriers to IR photons. Sirica's IR detector uses pumping light (NIR/Visible) to form a
steady-state non-equilibrium distribution (SNED) of free carriers with a narrow high-energy tail (i.e. low effective
temperature), which is then used to absorb and detect IR photons. Simulations of the SNED formed in the case where
the free carrier's lifetime is shorter than their energy relaxation time is presented, showing that the free carriers' effective
temperature, is significantly lower than the device temperature.
Although the total number of carriers in the SNED formed is small, IR photon absorption coefficient in Sirica's detector
is very high (equivalent to MCT). This is due to the very high effective cross-section achieved in Sirica's proprietary
detector substance. Parameters of this composite structure will be discussed.
Performance of quantum LWIR/MWIR photo-detectors is limited by dark-thermal current. Common approach is to reduce the thermal current by cooling the devices to cryogenic temperatures, preventing dark-thermal excitation of carriers disturbing the IR detection process. Sirica presents a new approach enabling quantum IR detection at room temperature. Instead of cooling the device, the free carriers are heated. Once their temperature is much higher than that of the device material lattice, heat transfer from the cold lattice to the hot free carriers is not possible due to thermodynamic laws. Heat transfer from hot carriers to the lattice is prevented by selecting a media where free carriers remain hot for long enough time (longer than their expected recombination lifetime). Thus, the device material lattice and hot free carriers are thermodynamically uncoupled and the device appears "cool" at room temperature. The hot carriers are then excited by IR photons to generate electron-hole pairs which are further converted to visible or NIR photons detectable by commercial visible CMOS/CCD sensors, a process known as "energy up-conversion". The energy required for up conversion is provided by an external low power light source. The new media required for effective light conversion is made of all silicon-based materials and offers the following benefits: (a) essentially nonequilibrium free carriers; (b) strong free carrier absorption of IR radiation; and (c) effective visible/near IR luminescence originating from the IR excited carriers. The theoretical model underlying the device and experimental results showing photo-induced free carrier IR absorption and IR-induced photoluminescence are presented.
We report on a new, simple process to fabricate planar Hg1-xCdyTe/Hg1-xCdxTe heterostructure photodiodes with p-on-n configuration. The material used for this demonstration was a double-layer p-on-n heterostructure that was grown by a liquid-phase-epitaxy technique. The p-on-n planar devices consisted of an arsenic-doped p-type cap epilayer on top of a long-wavelength IR n-type active epilayer. The ion-beam-miling p-type conversion effect was used to delineate the active device element, and to isolate the planar device. Detailed analysis of the current characteristics of these diodes as a funciton of temperatuer, show that they have high performance, and that their dark current is diffusion-limited down to 60 K. The results show that over a wide range of cut-off wavelengths, the RoA product values are close to the theoretical limit. Light Beam Induced Current technique was used to characterize the lateral and vertical dimensions of the ion beam milling induced junction. Electro-optic properties of a 2D array of small diodes with a 40-μm pitch are presented, and demonstrate the potential of the new process for implementation of 2D arrays.
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