Epi-ready GaSb wafers with low absorption coefficients are of a special interest as substrates for molecular beam epitaxy (MBE) growth of material for IR focal plane arrays that operate under back-side illumination configuration, when the substrate is not completely removed. While low absorption coefficient across a broad IR spectral range (~2um-20um) is achievable in GaSb crystals with low Te doping, the control of the doping distribution across the wafers is especially challenging in the mass-production of optically transparent, high-resistivity Te-doped GaSb wafers. In this work, we examine data from the n-type and p-type Te-doped GaSb samples with doping concentration below 1e18 cm-3. The carrier concentration measured by the Hall and the transmission data measured by FTIR spectroscopy are correlated. We perform a rigorous analysis of the absorption coefficient based on the free-carrier absorption mechanism that is dominant for the n-type GaSb and the inter-valence band absorption due to the transitions from the light-hole to the heavy-hole band that is the dominant absorption mechanism for the p-type GaSb. Based on the correlation between the Hall and the FTIR data, carrier concentration profile can be estimated from the non-destructive FTIR transmission mapping of the wafer.
The utilization of the non-equilibrium photodiode concept for high operating temperature (HOT) FPAs is discussed, both generically, and with regard to the specific example of MWIR HgCdTe. The issues of dark current, surface passivation, and 1/f noise are considered for three different architectures, namely N+/N-/P+, N+/P-/P+, and nBn. These architectures are examined with regard to possible FPA performance limitations, and potential difficulty in reduction to practice. Performance data obtained at DRS for the N+/N-/P+ and N+/P-/P+ HgCdTe architectures will be presented.
The High Operating Temperature Auger suppressed infrared detector concept is being pursued using the high density
vertically integrated photodiode (HDVIP®) architecture and an n+-p device structure. Dark current densities as low as 2.5
mA/cm2 normalized to a 5 μm cutoff at 250K have been demonstrated on these diodes. These dark currents imply
minority carrier lifetimes in excess of 300μsec. 1/f noise in these devices arises from the tunneling of charge into the
passivation interface, giving rise to a modulation in the surface positive charge and hence to the width of the depletion
region in the p-side of the device and a modulation in the total dark current. The measured 1/f noise is in agreement with
the predictions of this model, with very low noise being observed when the lifetimes are high.
DRS uses LPE-grown SWIR, MWIR and LWIR HgCdTe material to fabricate High-Density Vertically Integrated
Photodiode (HDVIP) architecture detectors. 2.5 μm, 5.3 μm and 10.5 μm cutoff detectors have been fabricated into
linear arrays as technology demonstrations targeting remote sensing programs. This paper presents 320 x 6 array
configuration technology demonstrations' performance of HDVIP HgCdTe detectors and single detector noise data. The
single detector data are acquired from within the 320 x 6 array. Within the arrays, the detector size is 40 μm x 50 μm.
The MWIR detector array has a mean quantum efficiency of 89.2% with a standard deviation to mean ratio, σ/μ = 1.51%. The integration time for the focal plane array (FPA) measurements is 1.76 ms with a frame rate of 557.7 Hz.
Operability values exceeding 99.5% have been obtained. The LWIR arrays measured at 60 K had high operability with
only ~ 3% of the detectors having out of family response. Using the best detector select (BDS) feature in the read out
integrated circuit (ROIC), a feature that picks out the best detector in every row of six detectors, a 320 x 1 array with
100% operability is obtained. For the 320 x 1 array constituted using the BDS feature, a 100% operable LWIR array
with average NEI value of 1.94 x 1011 ph/cm 2/s at a flux of 7.0 x 1014 ph/cm2/s has been demonstrated.
Noise was measured at 60 K and 50 mV reverse bias on a column of 320 diodes from a 320 x 6 LWIR array.
Integration time for the measurement was 1.76 ms. Output voltage for the detectors was sampled every 100th frame.
32,768 frames of time series data were collected for a total record length of 98 minutes. The frame average for a
number of detectors was subtracted from each detector to correct for temperature drift and any common-mode noise.
The corrected time series data was Fourier transformed to obtain the noise spectral density as a function of frequency.
Since the total time for collecting the 32,768 time data series points is 98.0 minutes, the minimum frequency is 170 μHz.
A least squares fit of the form (A/f + B) is made to the noise spectral density data to extract coefficients A and B that
relate to the 1/f and white noise of the detector respectively. In addition noise measurements were also acquired on
columns of SWIR detectors. Measurements were made under illuminated conditions at 4 mV and 50 mV reverse bias
and under dark conditions at 50 mV reverse bias. The total collection time for the SWIR detectors was 47.7 minutes.
The detectors are white noise limited down to ~10 mHz under dark conditions and down to ~ 100 mHz under
An overview of the DRS HDVIP architecture for realization of large-area infrared focal plane arrays (IRFPAs) is given. Improvements needed to meet more stringent application requirements are discussed and modeled. Both theoretical and experimental data are presented.
An overview on DRS' approaches towards realization of HgCdTe photonic infrared detectors based on DRS's proven HDVIP technology is given. The first approach involves the use of a silicon microlens array attached to the detector array, and the second reduction of dark currents in each detector itself. Recent progress is presented.
The optical absorption coefficient of CdZnTe in the near and mid infrared spectral regions was measured at room temperature using FTIR transmission spectroscopy for several x-values in Cd1-xZnyTe grown by the horizontal Bridgman technique as well as for CdTe and ZnTe. The compositional dependence of the absorption coefficient near the band edge was used to determine the composition of bulk CdZnTe, an important material parameter in its application as a substrate for HgCdTe epitaxial growth. In the mid IR range, we find that the wavelength dependence of the absorption coefficient could be varied by adjusting the stoichiometry of the material, i.e., by annealing under various Cd overpressures. The shape of the mid IR Fourier transform infrared spectra is related to the type and the concentration of the free carriers as well as the resistivity of the material. For n-type material, the wavelength dependence of the absorption coefficient can be described by free carrier absorption.
Liquid phase epitaxy(LPE) of (Hg,Cd)Te thick films from large tellurium-rich solutions was used to produce very large quantities of high quality films used for the fabrication of focal plane arrays. The composition uniformity is found to be ix=±O.OO1 for x=O.223 over an area ranging from 43cm2 to 54cm2. The reproducibility for 2OO growth runs, <8000cm2, is x=0.226±0.0033 or equivalently Xco9.8Si44Pm. The electrical quality of the material for diodes is evaluated by growing n-type films with an indium back-doping of —5x1014cm3. The carrier concentratio:ri is reproducible and in good agreement with the indium doping level. The average carrier concentration at 77K is 5.3±2.4x10'4cm3 with a mobility of 13 1±0.31x105cm2/V sec. Th.e minority carrier lifetime for the n-type films is Auger limited with an average value of 68±2.0isec.
Mercury cadmium telluride has successfully been grown from tellurium rich solutions. Solution growth offers
a number of advantages over other thin film and bulk growth techniques, e.g. purification, thin film and bulk
growth, low dislocation density, and inexpensive growth equipment. The principal solution growth techniques
that have received the most attention for (Hg,Cd)Te so far have been liquid phase epitaxy(LPE), the traveling
heater method(THM) and the solid state recrystallization(SSR) process. These techniques have provided very
high quality material for various device structures. The solid state recrystallization process, the traveling
heater method and liquid phase epitaxy are reviewed and their similarities described.