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.
There is considerable interest in sensors which are optimised for detecting infrared radiation outside the normal thermal
bands (3-12μm). This paper presents the development of photodiode arrays in Hg1-xCdxTe (MCT) that are sensitive in the
very long wave (VLW) band to 14μm or in the visible and SWIR band below 2.5μm wavelength.
The VLW arrays are heterostructure diodes fabricated from MCT grown by Metal Organic Vapour Phase Epitaxy
(MOVPE). These are staring, focal plane arrays of mesa-diodes bump bonded to silicon read-out circuits. Measurements
are presented demonstrating state-of-the-art performance over the temperature range 55-80K, for detectors with a cut-off
wavelength of up to 14μm (at 77K).
The SWIR/Visible detectors consist of an array of loophole photodiodes fabricated using MCT grown by Liquid Phase
Epitaxy (LPE). The technology is suited to imaging LIDAR, NIR/Visible imaging, spectroscopy or hyperspectral
applications. The diodes operate as avalanche photodiodes (APDs) which provides near-ideal gain in the pixel.
Measurements are presented demonstrating state-of-the-art performance in the range 80K-200K from arrays with a cut-off
Supporting technologies are also discussed. Silicon circuitry must be implemented in the SWIR and VLW bands that is
appropriate to avalanche operation or copes with the low photon flux or low photodiode impedance. Trade-offs between
conventional direct injection (DI), buffered direct injection (BDI), pixel capacitive transimpedance amplifier (CTIA) and
source-follower per detector (SFPD) are presented. Work is in progress to increase the MOVPE wafer size to 6" which
will enable large area arrays to be produced in the SW, MW, LW and VLW bands.
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. This paper describes the design, fabrication and performance of two-colour and three-colour infrared detectors made from HgCdTe 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 integrated circuits (ROICs). Each mesa diode has one connection to the ROIC and the colours are selected by varying the applied bias. Results will be presented for both two-colour and three-colour devices.
In a two-colour n-p-n design the cut-off wavelengths are defined by the compositions of the two n-type absorbers and the doping and composition of the p-type layer are chosen to prevent transistor action. The bias polarity is used to switch the output between colours. This design has been used to make MW/LW detectors with a MW band covering 3 to 5 μm and a LW band covering 5 to 10 μm.
In a three-colour n-p-n design the cut-off wavelengths are defined by the compositions of the two n-type absorbers and the p-type absorber, which has an intermediate cut-off wavelength. The absorbers are separated from each other by electronic barriers consisting of wide band-gap material. At low applied bias these barriers prevent photo-electrons generated in the p-type absorber from escaping and the device then gives an output from one of the n-type absorbers. At high applied bias the electronic barrier is pulled down and the device gives an output from both the p-type absorber and one of the n-type absorbers. Thus by varying the polarity and magnitude of the bias it is possible to obtain three-colours from a two-terminal device. This design has been used to make a SW/MW/MW detector with cut-off wavelengths of approximately 3, 4 and 6 μm.
This paper describes the fabrication and performance of affordable LW infrared focal plane arrays (IRFPAs) made from
HgCdTe (MCT) grown by Metal Organic Vapour Phase Epitaxy (MOVPE) bump bonded to silicon read-out integrated
circuits (ROICs). The growth substrate is GaAs, being readily available from several sources and suitable for wafer
scale processing. Arrays of size up to 640x512 at 24 μm pixel pitch have been produced, encapsulated, and
demonstrated in a camera system. Arrays of this size are produced in n-on-p material, that is, the common layer is p-type.
This orientation is chosen from a contact technology viewpoint. It is shown that at higher biases trap-assisted
tunnelling (TAT) can limit the performance of arrays. This becomes an issue for large arrays at high infrared flux with a
p-type common layer due to its inherent higher sheet resistance compared to n-type, this can result in debiassing of the
central elements. The key is found to be the control of the MCT structure and quality to ensure good diode performance
with minimal TAT, allowing the higher biases needed to overcome debiassing.
This paper describes the fabrication and performance of MW and LW infrared focal plane arrays (IRFPAs) made from HgCdTe (MCT) grown by Metal Organic Vapour Phase Epitaxy (MOVPE) bump bonded to silicon read-out integrated circuits (ROICs). MOVPE of HgCdTe is possible on CdTe, CdTe:Si, GaAs or GaAs:Si substrates. When choosing the substrate an important factor is the difference in thermal expansion coefficient between the array and the ROIC; if it is large the hybrid will delaminate when cooled to its operating temperature. GaAs:Si substrates provide a simple solution to the thermal stress problem so these were used initially and several hundred MW 640x512 arrays were made. The NETDs were in the range 10 to 14 mK and the defect levels could be as low as 0.1%. However, HgCdTe grown on GaAs:Si suffers to varying degrees from short-range non-uniformity in cut-off wavelength and the ability of these devices to withstand storage at elevated temperatures is also variable. Recently, the thermal stress problem for arrays on GaAs substrates has been solved and small quantities of MW and LW arrays have been made; they have excellent uniformity and bake stability. For MW 384x288 arrays with a cut-off wavelength of 4.95 μm the NETD is in the range 15 to 18 mK and the defect level can be as low as 0.05%. For LW 320x256 arrays with a cut-off wavelength of 10.0 μm the NETD is in the range 20 to 25 mK and the defect level can be as low as 1.3%. These devices will withstand temperature excursions up to 70°C and higher while in storage. The ability of the devices to withstand temperature cycling is being assessed. A 384x288 array has survived 1800 cycles between room temperature and 80 K with no change in performance. Thus GaAs is the preferred low cost substrate for MOVPE growth of HgCdTe.
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.
Conventional high performance infrared (IR) sensors need to be cooled to around 80K in order to achieve a high level of thermal sensitivity. Cooling to this temperature requires the use of Joule-Thomson coolers (with bottled gas supply) or Stirling cycle cooling engines, both of which are bulky, expensive and can have low reliability. In contrast to this, higher operating temperature (HOT) detectors are designed to give high thermal performance at an operating temperature in the range 200K to 240K. These detectors are fabricated from multi-layer mercury cadmium telluride (MCT) structures that have been designed for this application. At higher temperatures, lower cost, smaller, lighter and more reliable thermoelectric (or Peltier) devices can be used to cool the detectors. The HOTEYE thermal imaging camera, which is based on a 320x256 pixel HOT focal plane array, is described in this paper and performance measurements reported.
Infrared detectors based on Hg1-xCdxTe and grown by the MOVPE process can be designed to have very low dark currents, even for temperatures above 200K. These low dark currents are compatible with achieving background-limited performance at a temperature of 200K in f/2. However, in practice the detectors suffer from high 1/f noise. In this paper, a novel approach is explored in which most of the low frequency noise can be eliminated by operating the arrays at near zero bias. Using this technique, imaging arrays have been demonstrated at temperatures up to 220K giving a NETD of around 60mK in f/2.
Recent advances in MOVPE growth and heterostructure fabrication technology mean that infrared detector arrays based on Hg1-xCdxTe now have the potential to produce high performance imagery when operated in the temperature range 150-200 K. This has a number of system advantages including reduced cooler power consumption and increased cooler life. This paper reports the fabrication and assessment of a MW staring array with a cut-off of 4 μm at 150 K for intermediate temperature operation. Near background limited (BLIP) performance was achieved at temperatures up to 180 K with a median NETD better than 12 mK. Above this temperature, the array still operates normally however there is an exponential increase in the number of noisy pixels, and the median NETD degrades more rapidly than predicted from Shot noise. This behavior is consistent with increased low frequency or 1/f noise at the higher temperatures. This excess noise is not a fundamental limitation and if it could be eliminated, the array would remain close to BLIP up to 200 K.