This paper describes the fabrication and performance of our LW Hawk arrays. These are Full-TV (640x512) LW infrared
detectors at small pitch (16 μm) made from HgCdTe grown by Metal Organic Vapour Phase Epitaxy (MOVPE).
The detectors are staring, focal planes consisting of HgCdTe mesa-diode arrays bump bonded to silicon read-out
circuits. The HgCdTe structure is grown on GaAs and consists of an absorber layer sandwiched between wider band-gap
cladding layers. Device processing is wafer-scale. This is an extension of the work reported in previous years with the
innovation of dry etching for mesa isolation. The GaAs substrate is removed after bump bonding to minimise the
thermal stress on cooling.
The technology will be described. Results will be presented which show operability of 99.96% with a median NETD of
32 mK, reducing to 22 mK in binning mode. The results of various imaging trials will also be presented.
Selex Sensors and Airbourne Systems has been active in developing Very Long Wave arrays for space applications
under a contract of the European Space Agency. Arrays have been demonstrated with a 15 μm cut-off operating at 55 K.
The technology is an extension of our standard LW, described elsewhere, using MOVPE layers grown on GaAs to
provide a low cost, large area capability with state-of-the-art performance. The test vehicle for the VLW development is
a direct injection 320 x 256, 30 μm pitch ROIC with a well capacity of 20 million electrons. While it may be considered
that direct injection is not ideal for typical diode impedances expected in the VLW, and alternatives are in design, it is a
testament to our technology that the diodes have sufficient dynamic resistance to allow this approach.
Our diode design provides low diffusion currents such that at these operating temperatures the arrays are largely limited
by trap assisted tunnelling (TAT). Results of dark current as a function of voltage and temperature will be presented
along with the array electro-optical performance.
This paper describes the design, fabrication and performance of dual-band MW/LW infrared detectors made from
HgCdTe (MCT) grown by Metal Organic Vapour Phase Epitaxy (MOVPE). The detectors are staring, focal plane arrays
consisting of HgCdTe mesa-diode arrays bump bonded to silicon read-out circuits. Each mesa has one connection to the
ROIC and the bands are selected by varying the applied bias.
Arrays of 320x256 pixels on a 30 μm pitch have performed exceedingly well. For example, arrays with a cut-off
wavelength of 5 μm in the MW (mid-wave) band and 10 μm in the LW (long-wave) band have median NETDs of 10 and
17 mK and defect levels of 0.3% and 0.05%, in the MW and LW bands respectively. Interestingly the LW defect level is
often lower than the MW defect level and the defects are not correlated; i.e. a pixel that is defective in the MW band is
usually not defective in the LW band.
Arrays of 640x512 pixels on a 24 μm pitch have been developed. These use a read-out integrated circuit (ROIC) that has
two capacitors per pixel and the ability to switch bands during a frame giving quasi-simultaneous images. The
performance of these arrays has been excellent with NETDs of 14mK in the MW band and 23mK in the LW band. Dual
band-pass filters have been designed and built into a detector.
This paper describes long wavelength (LW) infra-red detectors made from HgCdTe grown by Metal Organic Vapour
Phase Epitaxy (MOVPE) and the performance in a low photon flux background compatible with a multispectral
requirement. The detectors are staring, focal plane arrays consisting of HgCdTe mesa-diode arrays bump bonded to
silicon read-out circuits. The HgCdTe structure is grown on GaAs and consists of an absorber layer sandwiched between
wider band-gap cladding layers. Device processing is wafer-scale. Wet etching is used to define the mesas and the mesa
sidewalls are passivated with inter-diffused CdTe. The GaAs substrate is removed after bump bonding to minimise the
thermal stress on cooling.
The technology is sufficiently advanced to enable production not only of LWIR detectors but also dual band
MWIR/LWIR detectors, as reported last year. Cameras for both types have been developed.
There is now increasing interest in using the technology for LWIR multispectral imaging. Due to the requirement for
narrow bandwidths, resulting in low radiant flux, the diode quality, in terms of dark current and resistance, must be
exceptionally good. This requirement has been difficult to achieve in many technologies, however MOVPE grown
MCT has consistently provided LWIR arrays with the necessary low dark current and high resistance. Performance from
arrays of size 640x512 with 24 μm pixels and having a cut-off of 10 μm will be described. These achieve diode
impedances of several GΩ's with less than 1 nA dark current at 90K.
The drive towards improved target recognition has led to an increasing interest in detection in more than one infrared band. Many groups have demonstrated two-color detection, typically by employing two back-to-back junctions, one for each color. In this paper we describe a method for introducing a third color via an absorber of intermediate wavelength placed between the two junctions. Electronic barriers are used to isolate this intermediate region. The design and location of the barriers in the structure are such that the barrier height is readily controlled by the applied bias, enabling the intermediate color to be turned on by applied bias. To provide the positional and doping control needed in the materials structure, MOVPE growth of MCT is used. Both FPA's hybridised to a read-out chips with switchable inputs, and test diodes for direct assessment, have been produced. This paper concentrates on the test diode assessment, as this provides the greater insight into the operation of the device. It is envisaged that such a device will be used with sequential framing of the different colors to provide quasi-temporal imaging.
The successful demonstration of the 3-color concept is described.