- Uncooled IR Photodetectors Get a Performance Boost
End users can look forward to a variety of benefits from current
research, including high-performance and fast-response single-element devices.
Within the next year or so, end users can expect significant performance
improvements in medium-wavelength IR (3 to 5 μm) and long-wavelength IR (8
to14 μm) photodetectors operating without cryogenic cooling. These high-temperature
devices will gain from the use of more advanced architectures, the integration of
micro-optics and active elements, and the use of photonic crystals. The HgCdTe and
related Hg-based alloys will remain the design materials of choice.
An advanced photodetector based on HgCdTe heterostructure
monolithically integrates functions of optical concentration of IR radiation and
detection in one element.
Devices anticipated this year include HgCdTe high-temperature
photodetectors with response speeds of less than 100 ps. Background-limited operation
of medium-wavelength IR devices cooled with thermoelectric coolers (simple two-stage
or even one-stage devices), as well as improved performance of uncooled and Peltier-cooled
long-wavelength IR detectors are in the offing. In commercially available photodiodes
cooled with two-stage thermoelectric coolers, detection capabilities should exceed
1010 cm1/2Hz1/2/W at 10.6 μm.
Shown are spectral detectivities of advanced optically immersed HgCdTe devices manufactured by Vigo at the end of 2002: Line 1 shows the background limit DBLIP*(300 K, 180º); lines
2, 3, 4 and 5, the measured performance at 225 and 300 K, respectively. Maximum
detectivity of 4 x 109 cm1/2Hz1/2W has now been obtained at 10.6 μm with
two-stage thermoelectric cooling.
Uncooled and Peltier-cooled photodetectors
also will replace cryogenically cooled devices in applications such as IR gas analyzers,
laser rangefinders, warning devices, thermal imagers, fast pyrometers, infrared
spectroscopy, laser technology and metrology, lidar, medicine, fiber and free-space
high-transfer-rate optical communications, and environment protection. Detector
costs will continue to decrease, mostly because of refinement of the HgCdTe epitaxial
growth process. Designers will replace expensive CdZnTe substrates with low-cost
and large-area hybrid substrates such as GaAs, sapphire and silicon, covered with
suitable buffer layers. Device processing will see further improvements.
Engineers can expect commercial availability
of two-dimensional arrays of Auger-suppressed medium-wavelength IR photodetectors
(British) that can be successfully used for imagers with thermal resolution of
On the research front, goals for the
next two to three years will include design optimization through the use of more
complex heterojunction architecture. End users can expect improved quality of HgCdTe-based
heterostructures grown by a low-temperature epitaxial process such as molecular
beam epitaxy or metallorganic chemical vapor deposition. The main issues here will
be the development of controllable doping at both low and high levels, low-temperature
activation of acceptor dopants, reduced concentration of the Shockley-Read centers,
and sharper, better-defined interfaces. Improved surface techniques will include
heterostructural passivation obtained during epitaxial growth.
Another goal is the development of
advanced devices that integrate the optical, detection and signal processing functions
(concentration of incident radiation, effective use of infrared radiation for generation
of charge carriers and internal photoelectric gain) in detector chips. Scientists
are studying concepts such as two-color high-temperature photodetectors and low-cost,
reproducible arrays of microlenses integrated with detector arrays.
Other research developments could include:
• Significant reduction of the
low-frequency noise in uncooled Auger-suppressed, extracted long-wavelength IR photodiodes.
• Use of new narrow-gap semiconductor
materials such as III-V compounds, including nitrides.
• Use of type II and III quantum-well
devices offering Auger suppression at the equilibrium mode of operation caused by
specific band structure.
Computer simulation will be used more
in photodetector development. The simulators will account for optical, photoelectrical
and thermal phenomena in detector structures.
Meet the author
Józef Piotrowski is the development manager
of Vigo System SA in Warsaw, Poland.
- Tiny (less than 2 mm in diameter) lenses, beamsplitters and other optical components used, for example, in endoscopes or microscopes or to focus light from semiconductor lasers and optical fibers.
MORE FROM PHOTONICS MEDIA