Photoconductive HgCdTe Research Articles

Fast rise time IR detectors for lepton colliders

Diagnostics is a fundamental issue for accelerators whose demands are continuously increasing. In particular bunch-by-bunch diagnostics is a key challenge for the latest generation of lepton colliders and storage rings. The Frascati Φ-factory, DAΦNE, colliding at 1.02 GeV in the centre of mass, hosts in the main rings few synchrotron radiation beamlines and two of them collect the synchrotron radiation infrared emission: SINBAD from the electron ring and 3+L from the positron ring. At DAΦNE each bucket is 2.7 ns long and particles are gathered in bunches emitting pulsed IR radiation, whose intensity in the long wavelength regime is directly proportional to the accumulated particles. Compact uncooled photoconductive HgCdTe detectors have been tested in both beamlines using dedicated optical layouts. Actually, the fast rise time of HgCdTe semiconductors give us the chance to test bunch-by-bunch devices for both longitudinal and transverse diagnostics. For the longitudinal case, single pixel detectors have been used, while for the transverse diagnostics, multi-pixel array detectors, with special custom design, are under test. This contribution will briefly describe the status of the research on fast IR detectors at DAΦNE, the results obtained and possible foreseen developments.

The carrier transportation of photoconductive HgCdTe detector irradiated by CW band-off laser

The response of photoconductive HgCdTe detectors to the irradiation of CW band-off laser was studied.It was found that there is a point of inflexion,T0,on the temperature dependence of the voltage response of the detector.The voltage response increases with temperature for temperature of the detector TT0,and decreases with temperature for TT0.Two time scales are found to be due to the two thermally resistive bonding layers.The inflection temperature is determined by the impurity concentration.For temperature TT0,the temperature dependence of the voltage response depends on the variation of mobility of the carrier,whereas TT0,thermally generated carriers contribute to the voltage response.

Absolute linearity measurements on a PV HgCdTe detector in the infrared

The linearity-of-response characteristics of a photovoltaic (PV) HgCdTe detector were investigated at a number of wavelengths in the infrared, using the NPL linearity of detector response characterization facility. The measurements were performed with the test detector operating under conditions identical to those in which the detectors will be used in typical infrared radiometric applications. The deviation from linearity in the generated photocurrent was shown to be strongly dependent on the area of the detector being illuminated. Plots of the linearity factor versus generated photocurrent for different illuminated wavelengths were shown to overlap. The linearity factor was shown to be a function of the photon irradiance of the illuminating beam. This behaviour was similar to that exhibited by photoconductive (PC) HgCdTe detectors, indicating that Auger recombination was the dominant source of the deviation from linearity observed in the test detector.

History of HgTe-based photodetectors in Poland

AbstractIn Poland, the HgCdTe studies began in 1960 at the Institute of Physics, Warsaw University. The material processing laboratory was created by Giriat and later by Dziuba, Gałązka, and others. Bridgman technique with sealed thick wall quartz ampoules was used to grow material suitable for research and experimental devices. Among the first papers published in 1961 and 1963 there were the Polish works devoted to preparation, doping, and electrical properties of HgCdTe.Infrared detector’s research and development efforts in Poland were concentrated mostly on uncooled market niche. At the beginning, a modified isothermal vapour phase epitaxy has been used for research and commercial fabrication of photoconductive, photoelectromagnetic and other HgCdTe devices. Bulk growth and liquid phase epitaxy were also used. Recently, the fabrication of infrared devices relies on low temperature epitaxial technique, namely metalorganic vapour phase deposition.At present stage of development, the photoconductive and photoelectromagnetic (PEM) detectors are gradually replaced with photovoltaic devices which offer inherent advantages of no electric or magnetic bias, no heat load and no flicker noise. Potentially, photodiodes offer high performance and very fast response. However, conventional photovoltaic uncooled detectors suffer from low quantum efficiency and very low junction resistance. The problems have been solved with advanced band gap engineered architecture, multiple cell heterojunction devices connected in series, and monolithic integration of the detectors with microoptics.In final part of the paper, the Polish achievements in technology and performance of HgMnTe and HgZnTe photodetectors are presented.

Absolute linearity measurements on a PbSe detector in the infrared

The non-linearity characteristics of a commercially available thin-film photoconductive PbSe detection system were experimentally investigated in the infrared using the NPL detector linearity of response characterisation facility. The non-linearity of this detector was shown to be significant, even for relatively low values of radiant power incident on the active area of the detector. The deviation from linearity was shown to be a function of the detector area being illuminated, as well as the wavelength of the incident radiation. The plot of the linearity factor as a function of photon irradiance was shown to exhibit only a weak dependence on the wavelength of radiation illuminating the detector, as well as the area of the spot being illuminated, thus confirming that the linearity factor is a function of photon irradiance. This behaviour is similar to that observed previously in thin-film photoconductive HgCdTe detectors.

On the stability of the spectral responsivity of cryogenically cooled photoconductive HgCdTe infrared detectors

The spectral responsivity of cryogenically cooled HgCdTe detectors was observed to drift slowly with time. The magnitude of the drift was shown to be strongly dependent on wavelength. The origin of the drift was investigated and was shown to arise due to a thin film of water ice depositing on the active area of the cold detector. The presence of the ice film (which is a dielectric film) interacts with the detector structure thus altering its absorbance characteristics and gives rise to the observed drifts. The drifts were temporarily eliminated by evacuating the detector dewars while baking them at 50 °C for about 48 h. This work demonstrates that HgCdTe infrared detectors should be evacuated and baked at least annually and in some cases (depending on the quality of the dewar and the measurement uncertainty required) more frequently. These observations are particularly relevant to HgCdTe detectors mounted in dewars which utilise rubber O-rings, as the ingress of moisture was found to be particularly serious in this type of dewar. This paper also identified other sources of drift present in the output of cryogenically cooled photoconductive HgCdTe detectors whose origins are currently not understood.

Absolute linearity measurements on HgCdTe detectors in the infrared region

The nonlinearity characteristics of photoconductive and photovoltaic HgCdTe detectors were experimentally investigated in the infrared wavelength region by use of the National Physical Laboratory detector linearity measurement facility. The nonlinearity of photoconductive HgCdTe detectors was shown to be a function of irradiance rather than the total radiant power incident on the detector. Photoconductive HgCdTe detectors supplied by different vendors were shown to have similar linearity characteristics for wavelengths around 10 microm. However, the nonlinearity of response of a photovoltaic HgCdTe detector was shown to be significantly lower than the corresponding value for photoconductive HgCdTe detectors at the same wavelength.

Spatial and angular responsivity measurements of photoconductive HgCdTe LWIR radiometers

Several newly developed large area photoconductive (PC) mercury cadmium telluride (HgCdTe) radiometers have been tested for spatial and angular responsivity for the purpose of determining what mode of operation (or radiometric quantity) could provide the lowest measurement uncertainty. An infrared (IR) test facility has been developed for the characterization of long wavelength IR (LWIR) detectors and radiometers for spatial response uniformity in power measurement mode and angular responsivity in both power and irradiance measurement modes. We have measured 34% to 53% spatial response non-uniformities and 1.5% to 10.5% changes in angular power reponsivity at different beam positions within the f/4 field-of-view (FOV) of the PC HgCdTe radiometers. The lowest responsivity uncertainty is achieved when these non-uniform radiometers are operated in irradiance measurement mode, where the incident uniform field of radiation averages out the detector's non-uniformity related uncertainties. The angular response deviation from the cosine function within the 16° FOV of the radiometers dominates the uncertainty budget for irradiance responsivity measurements in the 3 µm to 20 µm sensitivity range of these working standard devices.

Photocurrent Measurement of PC and PV HgCdTe Detectors.

Novel preamplifiers for working standard photoconductive (PC) and photovoltaic (PV) HgCdTe detectors have been developed to maintain the spectral responsivity scale of the National Institute of Standards and Technology (NIST) in the wavelength range of 5 μm to 20 μm. The linear PC mode preamplifier does not need any compensating source to zero the effect of the detector bias current for the preamplifier output. The impedance multiplication concept with a positive feedback buffer amplifier was analyzed and utilized in a bootstrap PV transimpedance amplifier to measure photocurrent of a 200 Ω shunt resistance photodiode with a maximum signal gain of 108 V/A. In spite of the high performance lock-in used as a second-stage signal-amplifier, the signal-to-noise ratio had to be optimized for the output of the photocurrent preamplifiers. Noise and drift were equalized for the output of the PV mode preamplifier. The signal gain errors were calculated to determine the signal frequency range where photocurrent-to-voltage conversion can be performed with very low uncertainties. For the design of both PC and PV detector preamplifiers, the most important gain equations are described. Measurement results on signal ranges and noise performance are discussed.

GOES 8–10 calibration experience

The calibration of the imagers and sounders on GOES 8–10 is reviewed, with special attention to the specialized processing added to mitigate the effects of subtle instrument problems first discovered when the satellites were in orbit. These problems include an east-west scan-position variation of the reflectance of the scan mirrors of the instruments, caused by polarization by the scan mirrors' SiO x coatings; image striping of a few tenths of a Kelvin in the imager channels that use photo-conductive HgCdTe detectors, caused primarily by high 1 f noise in the detectors; and calibration errors in the shortwave channels of up to 1K near satellite midnight, caused by radiation from solar-heated components in the instruments' scan cavities. There is also a gradual long-term decrease in the responsivity of the imagers' visible channels. Despite the lack of an on-board calibration source for these channels, we have been able to use routine observations of stars to infer that the rate of decrease is approximately 8 and 5% per year for the GOES-8 and -9 imagers, respectively.

Detector nonlinearity correction scheme for the LPMA balloonborne Fourier transform spectrometer

The Limb Profile Monitor of the Atmosphere (LPMA) instrument is a Fourier transform spectrometer designed to record stratospheric (and in some cases tropospheric) absorption spectra from a balloon gondola. This spectrometer operates with two-detector output optics (photoconductive HgCdTe and photovoltaic InSb, liquid-nitrogen cooled). The response of the HgCdTe detector becomes nonlinear for high photon fluxes, which is the case for solar occultation. We have designed a processing scheme, based on the minimization of out-of-optical-band spectral artifacts, to correct for the effect of nonlinearity in the useful spectral range. The method is explained, and sample results are presented for spectra recorded in different balloon flight conditions and with two different HgCdTe detectors.

Two-point calibration of non-linear PC HgCdTe channels

Over a radiance range such as that of terrestrial features observed in the 8–13 μm spectral region from orbital altitudes the voltage response of a photoconductive HgCdTe detector can be described accurately as a quadratic function of scene radiance. The three coefficients of the quadratic calibration equation can be determined by only two scene reference temperatures. This is possible because the quadratic and linear coefficients are functionally related. For spacecraft instruments a two-point calibration is very convenient since the on-board blackbody (at ambient temperature) and cold space are usually the only reference sources available.

A simple method for the determination of the cut-off wavelength of a near-BLIP infrared detector

A simple method for the determination of the cut-off wavelength of an IR detector is proposed, using only two responsivity measurements at two different black-body temperatures. The method as been tested for a near-BLIP photoconductive HgCdTe device operating at 77 K with a molar ratio: x = 0.27.