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.
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.
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 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.