It is not clear whether the tunneling current in QWIPs depends just on the energy corresponding to
motion perpendicular to the plane of the quantum well or on the total energy. In order to get a
quantitative assessment of the contribution of energy corresponding to motion in the plane of the
quantum well to the dark current we use the following approach. We calculate the dark current in
GaAs/AlxGa1-x s quantum well infrared detectors for both tunneling dependent only on Ez, and
tunneling dependent on the total energy, and compare the results to experimental data. Comparison
of theoretical results with experimental data at 40K shows that motion in the plane of the quantum
well plays a significant role in determining the tunneling dark current. Corrections are made to
Levine's original formula. Variation of the dark current with barrier width and doping density is
systematically studied. It is shown that increasing the barrier width and/or decreasing the doping
density in the well do not always reduce the dark current.
We present the results of the radiometric characterization of an "M" structure long wavelength infrared Type-II strained layer superlattice (SLS) infrared focal plane array (IRFPA) developed by Northwestern University (NWU). The performance of the M-structure SLS IRFPA was radiometrically characterized as a function of photon irradiance, integration time, operating temperature, and detector bias. Its performance is described using standard figures of merit: responsivity, noise, and noise equivalent irradiance. Assuming background limited performance operation at higher irradiances, the detector quantum efficiency for the SLS detector array is approximately 57%. The detector dark density at 80 K is 142 μA/cm2, which represents a factor of seven reduction from previously measured devices.
An overview of the properties of the absorption coefficient of mercury cadmium telluride
that may make this material useful for intrinsic hyperspectral detection is presented. A review of
recent work on modeling the absorption coefficient is provided, and new directions for achieving an
analytical representation with higher fidelity are suggested.
The Type-II InAs/GaSb superlattice photon detector is an attractive alternative to HgCdTe photodiodes and QWIPS. The use of p+ - π - M - N+ heterodiode allows for greater flexibility in enhancing the device performance. The utilization of the Empirical Tight Binding method gives the band structure of the InAs/GaSb superlattice and the new M- structure
(InAs/GaSb/AlSb/GaSb) superlattice allowing for the band alignment between the binary superlattice and the M- superlattice to be determined and see how it affects the optical performance. Then by modifying the doping level of the M- superlattice an optimal level can be determined to achieve high detectivity, by simultaneously improving both photo-response and reducing dark current for devices with cutoffs greater than 14.5 μm.
Recent progress made in the structure design, growth and processing of Type-II InAs/GaSb
superlattice photo-detectors lifted both the quantum efficiency and the R0A product of the detectors.
Type-II superlattice demonstrated its ability to perform imaging in the Mid-Wave Infrared (MWIR)
and Long-Wave Infrared (LWIR) ranges, becoming a potential competitor for technologies such as
Quantum Well Infrared Photo-detectors (QWIP) and Mercury Cadmium Telluride (MCT). Using an
empirical tight-binding model, we developed superlattices designs that were nearly lattice-matched to
the GaSb substrates and presented cutoff wavelengths of 5 and 11 μm. We demonstrated high quality
material growth with X-ray FWHM below 30 arcsec and an AFM rms roughness of 1.5 Å over an
area of 20x20 μm2. The detectors with a 5 μm cutoff, capable of operating at room temperature,
showed a R0A of 1.25 106 Ω.cm2 at 77K, and a quantum efficiency of 32%. In the long wavelength
infrared, we demonstrated high quantum efficiencies above 50% with high R0A products of 12 Ω.cm2
by increasing the thickness of the active region. Using the novel M-structure superlattice design, more
than one order of magnitude improvement has been observed for electrical performance of the
devices. Focal plane arrays in the middle and long infrared range, hybridized to an Indigo read out
integrated circuit, exhibited high quality imaging.
Type II superlattce photodetectors have recently experienced significant improvements
in both theoretical structure design and experimental realization. Empirical Tight Binding
Method is initiated and developed for Type II superlattice. Growth characteristics such as
group V segregation and incorporation phenomena are taken into account in the model and
shown higher precision. A new Type II structure, called M-structure, is introduced and
theoretically demonstrated high RoA, high quantum efficiency. Device design is optimized
to improve the performance. As a result, 55% quantum efficiency and 10 Ohm.cm2 RoA
are achieved for an 11.7 &mgr;m cut-off photodetector at 77K. FPA imaging at longwavelength
is demonstrated with a capability of imaging up to 171K. At 81K, the noise equivalent
temperature difference presented a peak at 0.33K.
In this work, an AlSb-containing Type II InAs/GaSb superlattice, the so-called M-structure, is presented as a candidate for mid and long wavelength infrared detection devices. The effect of inserting an AlSb barrier in the GaSb layer is discussed and predicts many promising properties relevant to practical use. A good agreement between the theoretical calculation based on Empirical Tight Binding Method framework and experimental results is observed, showing the feasibility of the structure and its properties. A band gap engineering method without material stress constraint is proposed.
The authors report on recent advances in the development of mid-, long-, and very long-wavelength infrared (MWIR, LWIR, and VLWIR) type-II InAs/GaSb superlattice infrared photodiodes. The residual carrier background of binary type-II InAs/GaSb superlattice photodiodes of cut-off wavelengths around 5 μm has been studied in the temperature range between 10 and 200 K. A four-point, capacitance-voltage technique on mid-wavelength and long-wavelength type-II InAs/GaSb superlattice infrared photodiodes reveal residual background concentrations around 5 × 1014 cm-3. Additionally, recent progress towards LWIR photodiodes for focal plane array imaging applications is presented. Single element detectors with a cut-off wavelength, λc,50%, of 10.2 μm demonstrated detectivities of approximately 1 × 1011 cmHz1/2W-1 and quantum efficiencies of 32% at the peak responsivity wavelength of around 7.9 μm. Furthermore, high-performance VLWIR single element photodiodes are discussed. The silicon dioxide passivation of VLWIR photodiodes is also presented, which resulted in an approximately 5 times increase of the sidewall resistivity. The latest developments in this material system lend further support for its use as a high-performance alternative for infrared optical systems compared to the current state-of-the-art imaging systems, especially those approaching the long-wavelength and very-long-wavelength infrared.
We present our most recent results and review our progress over the past few years regarding InAs/GaSb Type II superlattices for photovoltaic detectors and focal plane arrays. Empirical tight binding methods have been proven to be very effective and accurate in designing superlattices for various cutoff wavelengths from 3.7 μm up to 32 μm. Excellent agreement between theoretical calculations and experimental results has been obtained. High quality material growths were performed using an Intevac modular Gen II molecular beam epitaxy system. The material quality was characterized using x-ray, atomic force microscopy, transmission electron microscope and photoluminescence, etc. Detector performance confirmed high material electrical quality. Details of the demonstration of 256×256 long wavelength infrared focal plane arrays will be presented.
Leakage currents limit the operation of high performance type II InAs/GaSb superlattice photodiode technology. Surface leakage current becomes a dominant limiting factor, especially at the scale of a focal plane array pixel (< 25 μm) and must be addressed. A reduction of the surface state density, unpinning the Fermi level at the surface, and appropriate termination of the semiconductor crystal are all aims of effective passivation. Recent work in the passivation of type II InAs\GaSb superlattice photodetectors with aqueous sulfur-based solutions has resulted in increased R0A products and reduced dark current densities by reducing the surface trap density. Additionally, photoluminescence of similarly passivated type II InAs/GaSb superlattice and InAs GaSb bulk material will be discussed.
P+-v-N+ structures with HgTe/CdTe superlattice absorber regions were grown by MBE to give cut-off wavelengths in the very long wavelength infrared (14 μm and longer) at temperatures below 80 K. The superlattice period of one sample was 98.3Å according to its x-ray diffraction profile, very close the intended 98.5Å for a 48.5Å HgTe/50.Å Hg0.05Cd0.95Te superlattice. The cut-off wavelengths of another sample were approximately 14 and 18 μm at 77 and 40K, respectively, as determined by optical absorption and spectral response measurements. A first batch of devices annealed at 150°C or 180°C for 1 hr after the deposition of 50Å thick gold films showed relatively low R0A values (approximately 0.3 ohm-cm2). This was interpreted to be due to the formation of a junction near the boundary of the superlattice and a high carrier concentration region of HgCdTe alloy. A second batch of devices was annealed at 120°C for 1 hr or 5 min. to decrease the gold diffusion depth. The electrical properties showed higher R0A values (approximately 8 ohm-cm2). The detectivities of the second batch devices at 77K were in the range of 108 to 1010 cmHz½/W and showed frequency dependence because the noise had frequency dependence. We observe very low knee frequencies (below 10Hz) in their noise spectra.
InAs/InGaSb type2 strained layer superlattice (SLS) combines the advantages of III-V materials technology with the strong, broad-band absorption, and wavelength tunability of HgCdTe. In fact, the significantly reduced tunneling and Auger recombination rates in SLS compared to those in HgCdTe should enable SLS detectors to outperform HgCdTe. We report the results of our investigation of InAs/InGaSb type2 strained layer superlattices (SLS)for LWIR photovoltaic detector development. We modeled the band structure, and absorption spectrum of SLS's, and achieved good agreement with experimental data. We systematically investigated the SLS growth conditions, resulting in good uniformity, and the elimination of several defects. We designed, developed and evaluated 16x16 array of 13 micron cutoff photovoltaic detectors. Photodiodes with cutoff wavelengths of 13 and 18microns were demonstrated, which are the longest wavelengths demonstrated for this material system. Quantum efficiencies commensurate with the superlattice thickness were demonstrated and verified at AFRL. The electrical properties show excessive leakage current, most likely due to trap-assisted tunneling.
The annealing and electrical properties of extrinsic in situ doped mercury cadmium telluride epilayers grown by molecular beam epitaxy (MBE) on B CdTe/Si and CdZnTe substrates are studied. The doping is performed with an elemental arsenic source. HgCdTe epilayers of CdTe mole fraction in the range of mid-wavelength IR are grown at substrate temperatures of 175-185 degrees C. The temperature dependent Hall effect characteristics of the grown samples are measured by the van der Pauw technique. A magnetic field of up to 0.8 T is used in these measurements. The analysis of the Hall coefficient in the temperature range of 40-300 K with a fitting based on a three-band non-parabolic Kane model, a fully ionized compensating donor concentration, and tow independent discrete acceptor levels is reported. Both as-grown and annealed samples are used in this study. All of the as-grown samples showed-type characteristics whereas annealed samples showed p-type characteristics. Activation annealing at different temperatures was performed. Conversion to p-type at lower than conventional annealing temperatures was achieved. Theoretical models are utilized to understand the dependence of the activated arsenic concentration on the annealing temperature.
We report on the technology we are developing to product photovoltaic devices of HgCdTe which are sensitive in the short wave region of the solar radiation and exhibiting detectivity performance close to theoretical limits imposed by the fundamental properties of the material.
The design and characteristics of very long wavelength InAs/InGaSb strained layer superlattice photodiodes are presented. These photodiodes have cutoff wavelengths ranging from 12 to longer than 15 microns, and are among the longest wavelengths reported for photovoltaic detectors made using this material system. Structural, optical and electrical properties are reported. Measured quantum efficiencies are as high as 10% at 10 micron for a 0.7 micron thick structure at 77K. The absorption coefficients are excellent, however, the electrical properties still need improvement.
We report a set of high-quality InAs/InGaSb type-II photodetectors grown on GaSb substrates with cutoff wavelengths form 11 to 21 micrometers . The SL structural parameters were very repeatable between samples as evidenced by the consistency of the SL periods and the long wavelength photoresponse cut-off. The measured photoresponse spectra were in excellent agreement with the calculated absorption spectrum. Very low background carrier concentrations were achieved in this samples set. Based on the study, the optimum growth temperature for type-II photodetectors is between 390 to 410 C with a post growth annealing at 495 to 510 C. Thickness non-uniformity of type-II photodiodes was less than 1 percent across 2-inch wafers. We have also demonstrated photodetectors with good performance from 10 to 18 micrometers , directly grown on compliant InGaAs/GaAs substrates.
Mechanisms of incorporation of native defect and dopants in HgCdTe alloys are reviewed. Origin of the native defect related deep centers in limiting the minority carrier lifetime is explored. Primary and secondary mechanisms operative in the activation of n type and p type dopants in HgCdTe are discussed along with implications for fabrication of high performance detectors.
Hg1-xCdxTe films with x values varying from 0.2 to 0.23 have been grown and characterized. N-type carrier concentrations in the range of 1 X 1015 cm-3 to 3 X 1015 cm-3 have been obtained. Hall effect measurements before and after anneals at 250 degrees Celsius have led to the evaluation of the Hg vacancy concentration in the samples. Dislocation density less than 105 cm-2 and X-ray rocking curve width less than 25 arc- secs measured in some of the films attests to the excellent crystallinity of the material.
New formulas are derived to calculate the tunneling and thermionic dark currents in GaAs/Alx Ga1-xAs quantum well infrared detectors. Variation of the dark current (Id) with barrier width and doping density is systematically studied. It is shown that increasing the barrier width and/or decreasing the doping density in the well do not always reduce the dark current. Theoretical variation of Id with bias is compared with experimental data.
The fluence of pulsed lasers of wavelength 4 and 10.6 microns necessary to induce one and two orders of magnitude temporary degradation in the R0A values of Hg0.7Cd0.3Te p/n infrared detectors at 100 K, and Hg0.78Cd0.22Te p/n infrared detectors at 40 K have been calculated. A nonparabolic energy-momentum relationship and temperature dependent energy gap of HgCdTe were used in this calculation. The R0A values used in this calculation were obtained by simultaneously including generation-recombination, diffusion and tunneling mechanisms.
The amounts of 4 micron pulsed laser fluence necessary to induce one and two orders of magnitude degradation in the R0A value of Hg0.7Cd0.3Te at 100 K have been calculated. The R0A values used in this calculation were obtained by simultaneously including generation-recombination, diffusion and tunneling mechanisms.
The theory of laser-induced damage in dielectric films, containing highly absorbing impurities, developed previously for single-shot damage is extended to the case of multiple shots. Calculations are carried out for micro-, nano-, and pico-second pulses, with the time interval between pulses varying from equal to the pulse duration to two-orders of magnitude greater than the pulse duration. It is found that in the case of spherical impurities the size of the easiest to damage particle is in general a complicated function of the laser intensity, pulse duration, and time interval between pulses. However, for sufficiently large number of pulses, the dependence is much simpler, characterized by constant values of the following: (1) pulse duration multiplied by the number of shots divided by the square of the radius of the easiest to damage impurity; (2) the minimum intensity needed to cause damage multiplied by the radius of the easiest to damage impurity.
A simple technique based on the measurement of gate induced drain leakage current (Idl) is developed to measure the radiation induced charge in the metal oxide semiconductor transistor. After irradiation Idl of n-channel MOS transistors decreases while that of p-channel devices increases. The change of leakage current at higher tunneling fields is proportional to the increase of hole trap density in the gate oxide region. The leakage current measurement technique is an useful tool for characterizing radiation effects in MOS transistors because at high biases Idl is dependent on the increase of oxide charge while independent of the interface states. It depends on gate and drain overlap geometry and independent of the channel length. Hence Idl measurement technique is advantageous over threshold voltage technique which depends on the channel length.
Two photon absorption (TPA) coefficients in HgCdTe have been calculated for CO2 laser at 40 77 150 and 300 K using Basov formula both in the parabolic and nonparabolic band structure approximations. Temperature and composition dependent energy gaps have been included in these calculations. Reasonable agreement is obtamed with available experimental data. 1.
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