Performance of quantum well infrared photodetector (QWIP) device parameters such as detector cutoff wavelength and the dark current density depend strongly on the quality and the control of the epitaxy material growth. In this work, we report on a methodology to precisely control these critical material parameters for long wavelength infrared (LWIR) GaAs/AlGaAs QWIP epi wafers grown by multi-wafer production Molecular beam epitaxy (MBE). Critical growth parameters such as quantum well (QW) thickness, AlGaAs composition and QW doping level are discussed.
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.
Much has been accomplished in the last few years in advancing the performance of type-II superlattice (T2SL) based
infrared photodiodes, largely by focusing on device and heterostructure design. Quantum efficiency (QE) has increased
to 50% and higher by using thicker absorbing layers and making use of internal reflections, and dark currents have been
reduced by over a factor of ten by using bandstructure engineering to suppress tunneling and generation-recombination
(G-R) currents associated with the junction. With performance levels of LWIR T2SL photodiodes now within an order of
magnitude of that of HgCdTe (MCT) based technology, however, there is renewed interest in understanding fundamental
materials issues. This is needed both to move performance toward the theoretical Auger limit, and to facilitate the task of
transitioning T2SL growth from laboratories to commercial institutions. Here we discuss recent continuing efforts at
NRL to develop new device structures for enhanced detector performance, and to further our understanding of this
material system using advanced structural and electronic probes. Results from electron beam induced current (EBIC)
imaging and analysis of point defects in T2SL photodiodes will be presented, showing differentiated behavior of bulk
defect structures. We will also describe a study comparing intended vs. as-grown T2SL photodiode structures by crosssectional
scanning microscopy (XSTM). Using parameters extracted from the XSTM images, we obtain detailed
knowledge of the composition and layer structures through simulation of the x-ray diffraction spectra.
We are developing corrugated quantum well infrared photodetector (C-QWIP) technology for long wavelength
applications. A number of large format 1024 × 1024 C-QWIP focal plane arrays (FPAs) have been demonstrated. The
measured quantum efficiency η is ranging from 15 - 37%, depending on the detector type, doping density and number
of quantum wells in the detector material. The photoconductive gain is between 0.07 and 0.19, while the spectral width
is between 1.5 and 3.5 microns. Despite the large integrated η of the C-QWIPs, the number of collected photoelectrons
can be limited in shorter cutoff, small pixel FPAs under high speed operation. In this case, the read noise will have a
large impact on the system sensitivity. In this paper, we will discuss the detector model, the measured pixel
characteristics, and the effects of read noise on the FPA performance. Our analysis shows that the tolerable read noise
improves with the cutoff wavelength. For example, to achieve a sensitivity of 20 mK at 2 msec integration time, the
respective read noise will be 1000, 2000, 3000, and 4000 e¯ at λc = 8.8, 9.4, 10.7 and 11.7 μm. This analysis will help
to determine the read noise requirement for the C-QWIP FPAs.
ARL and L3-CE have been developing corrugated quantum well infrared photodetector (C-QWIP) technology for long
wavelength applications. Several large format 1024 × 1024 C-QWIP focal plane arrays (FPAs) have been demonstrated.
In this paper, we provide a detailed analysis on the FPA performance in terms of quantum efficiency η and compare it
with a detector model. We found excellent agreement between theory and experiment when both the material
parameters and the pixel geometry are taken into account. For C-QWIPs with the bound-to-quasi-bound structure, a η of
37% is observed, albeit at a large voltage of -11V. Since this voltage is outside the operating regime of the existing
readout electronics, we investigated several more compatible structures and achieved η in the range of 15 - 26%. This
range of η, although lower than the original value, is still approximately three times higher than that of the grating
coupled QWIPs, and the coupling bandwidth is also three times wider. The C-QWIP approach thus holds significant
performance advantages over the grating approach. Combined with its economical processing steps and flexible
wavelength coverage, the C-QWIP technology has proven its advantages in infrared detection.
Recently, large format and high quantum efficiency corrugated quantum well infrared photodetector (C-QWIP) FPAs
have been demonstrated. Since the detector light coupling scheme does not alter the intrinsic absorption spectrum of the
material, the QWIPs can now be designed with different bandwidths and lineshapes to suit various applications.
Meanwhile, the internal optical field distribution of the C-QWIPs is different from that of a grating coupled detector, the
material structure thus should be designed and optimized differently with respect to quantum efficiency, conversion
efficiency and operating temperature. In this paper, we will provide a framework for the material design. Specifically,
we will present a theoretical detector performance model and discuss two specific examples, namely with 9.2 and 10.2
μm cutoff wavelengths. We found that for both λc, the photocurrent to dark current ratio is maximized at an electron
doping density ND of 0.28 × 1018 cm-3. The dark current limited detectivity meanwhile reaches a maximum at a higher
ND of 0.45 × 1018 cm-3. But the lowest noise equivalent noise temperature difference is actually obtained at an even
higher ND of 1.0 × 1018 cm-3 due to the larger quantum efficiency, if there are no limitations on the readout charge
capacity. These predictions are compared with the data of a 1024 × 1024 C-QWIP FPA hybridized to a fan-out circuit,
and the results are consistent.
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