Diffusion Limited Dark Currents Research Articles

Diffusion-limited dark currents in mid-wave infrared HgCdTe-based nBn structures with Al2O3 passivation

We present the results of the study of the dark current in the MWIR nBn structure based on n-Hg1−xCdxTe grown by molecular beam epitaxy on a GaAs (0 1 3) substrate. The CdTe content in the barrier layer is 0.84, and its thickness is 210 nm. To suppress the surface current, passivation with an Al2O3 layer formed by plasma-enhanced atomic layer deposition was used. The dark current density at the temperature of 184 K and the voltage bias of −1 V is 3.1 × 10−6 A cm−2. At temperatures higher than 180 K, the total current through the nBn structure is determined by the diffusion component. In the temperature range of 180–300 K, the dark current values correspond well to the empirical model of Rule 07. These values are comparable with the dark current values for modern mid-infrared barrier detectors based on MOCVD HgCdTe and III–V materials. The studied nBn structures have lower dark currents than the previously described analogues based on MBE HgCdTe, which may be associated with high-quality passivation of the side walls of the mesa structure.

Back Surface Plasmonic Grating for Increased Quantum Efficiency of nBn Photodetectors With Ultra-Thin Metamorphic InAs0.8Sb0.2 Absorber

Using a 2D metallic grating, we demonstrate the resonant enhancement of the external quantum efficiency of an nBn structure with a 0.5 $\mu \text{m}$ -thick-absorber. A total of two structures were studied with different barrier materials: ternary AlInSb and an InAs/AlInSb superlattice (SL). The device using an SL barrier had a diffusion-limited dark current of 6.7 $\mu \text{A}$ /cm2 at 150 K, which was four times lower than the ternary-barrier device. The surface plasmon polariton (SPP) resonance wavelength for devices with six different grating periods varied as predicted by simulations. The quantum efficiency (QE) was enhanced by up to 56% by coupling to the SPP mode. A peak external QE of 39% was achieved at $4~\mu \text{m}$ with a 1100 nm grating period.

High-operating temperature InAsSb/AlSb heterostructure infrared detectors grown on GaAs substrates by molecular beam epitaxy

InAsSb/AlSb barrier detectors were grown on (100) semi-insulating GaAs substrates by a molecular beam epitaxy. We compare the performance of two detectors with different active layers denoted as p + BppBpN + and p + Bpnn + . InAs0.81Sb0.19 absorber allows to operate up to 5.3-μm cut-off wavelengths at 230 K. p + Bpnn + detector (n-type absorber) exhibits diffusion-limited dark currents above 200 K. AlSb barrier provides low dark currents and suppresses surface leakage currents. With a value of 0.13 A / cm2 at 230 K, the current is of about an order of magnitude larger than determined by the “Rule 07.” Dark currents of p + BppBpN + detector (p-type absorber) are much higher due to a contribution of Shockley–Read–Hall mechanisms. On the other hand, a device with a p-type absorber exhibits the highest value of current responsivity, up to 2.5 A / W, pointing out that there is a tradeoff between dark current performance and quantum efficiency.

Influence of shallow versus deep etching on dark current and quantum efficiency in InAs/GaSb superlattice photodetectors and focal plane arrays for long wavelength infrared detection

In this paper, a comparison of quantum efficiency and dark current densities of shallow etched and deep etched InAs/GaSb type-II superlattice photodiodes is presented. The detectors were tailored for space applications with measured cut-off wavelengths around 11.2 µm. Bias and temperature dependencies of the QE have been studied showing very low turn on bias (>−25 mV) and very small variation of the peak QE value with temperature. At 70 K, peak quantum efficiencies of shallow and deep etched devices were deduced to 34% and 30%, respectively (without antireflection coating). The difference was attributed to a larger collection area of shallow etched devices, with a collection range of approximately 22 µm outside of the defined pixel area. Diffusion limited dark currents were observed down to 60 K for both shallow and deep etched devices under low reverse bias (∼−200 mV) with dark current densities on the order of 1.7 × 10−5 A/cm2 at 60 K. A gradually increasing influence of generation-recombination (G-R) and trap-assisted tunneling (TAT) was observed at lower temperatures for high reverse bias. The lowest dark current densities were observed for deep etched photodiodes, on the order of 3 × 10−7 A/cm2 at 40 K (Vbias = −100 mV). Detector arrays were fabricated with deep etched 30 µm pitch pixels. Comparably low dark current densities were observed for the detector array pixels under low reverse bias as for the single pixel photodiodes, however a significant increase of the TAT dark current was observed under high reverse bias.

Delta Doping in HgCdTe-Based Unipolar Barrier Photodetectors

A method is described whereby the valence band (VB) discontinuity that is present in mercury cadmium telluride (HgCdTe)-based alloy-barrier nBn detectors can be minimized. It is numerically demonstrated that compositionally graded layers can provide the required transition between the wide bandgap CdTe barrier layer and the lattice-matched HgCdTe absorber and contact layers, although a large VB discontinuity is present. In addition, the incorporation of delta-doped ( $\delta $ -doped) layers in the vicinity of the barrier region can minimize the VB discontinuity, thus blocking the flow of majority carriers and allowing unimpeded flow of photogenerated minority carriers. This strategy results in diffusion-limited dark current at low reverse bias combined with efficient collection of photogenerated carriers. The nBn structure proposed in this paper can be applied to other semiconductor materials operating in various infrared wavelength bands. The method is not limited to the nBn structure and can be applied to any xBx barrier detector structure (with ${x} = \textit {n, p}$ ) configuration in order to minimize the energy band discontinuity in the corresponding minority carrier band.

Development and Production of Array Barrier Detectors at SCD

XBn or XBp barrier detectors exhibit diffusion-limited dark currents comparable with mercury cadmium telluride Rule-07 and high quantum efficiencies. In 2011, SemiConductor Devices (SCD) introduced “HOT Pelican D”, a 640 × 512/15-μm pitch InAsSb/AlSbAs XBn mid-wave infrared (MWIR) detector with a 4.2-μm cut-off and an operating temperature of ∼150 K. Its low power (∼3 W), high pixel operability (>99.5%) and long mean time to failure make HOT Pelican D a highly reliable integrated detector-cooler product with a low size, weight and power. More recently, “HOT Hercules” was launched with a 1280 × 1024/15-μm format and similar advantages. A 3-megapixel, 10-μm pitch version (“HOT Blackbird”) is currently completing development. For long-wave infrared applications, SCD’s 640 × 512/15-μm pitch “Pelican-D LW” XBp type II superlattice (T2SL) detector has a ∼9.3-μm cut-off wavelength. The detector contains InAs/GaSb and InAs/AlSb T2SLs, and is fabricated into focal plane array (FPA) detectors using standard production processes including hybridization to a digital silicon read-out integrated circuit (ROIC), glue underfill and substrate thinning. The ROIC has been designed so that the complete detector closely follows the interfaces of SCD’s MWIR Pelican-D detector family. The Pelican-D LW FPA has a quantum efficiency of ∼50%, and operates at 77 K with a pixel operability of >99% and noise equivalent temperature difference of 13 mK at 30 Hz and F/2.7.

Long Wave Infrared Type II Superlattice Focal Plane Array Detector

The XBn/XBp family of barrier detectors enables diffusion limited dark currents comparable with HgxCd1-xTe Rule-07 and high quantum efficiencies. SCD’s XBp type II superlattice (T2SL) detector contains InAs/GaSb and InAs/AlSb T2SLs, and was designed for the long wave infrared (LWIR) atmospheric window using k · p based modeling of the energy bands and photo-response. Wafers are grown by molecular beam epitaxy and are fabricated into focal plane array (FPA) detectors using standard FPA processes, including wet and dry etching, indium bump hybridisation, under-fill, and back-side polishing. The 640 × 512 pixel, 15 μm pitch, detector goes by the name of ‘Pelican-D LW’ and exhibits a quantum efficiency of ~ 50 per cent with background limited performance at an operating temperature of 77 K. It has a cut-off wave length of ~ 9.5 μm, with a pixel operability of above 99 per cent. The detector gives a very stable image with a residual non uniformity of below 0.04 per cent over its useful dynamic range. A new digital read-out integrated circuit has been designed so that the complete detector closely follows the configuration of SCD’s MWIR Pelican-D detector.

MOCVD grown HgCdTe barrier detectors for MWIR high-operating temperature operation

In the last decade, new architecture designs such as nBn devices or unipolar barrier photodiodes have been proposed to achieve high-operating temperature (HOT) detectors. This idea has been also implemented in HgCdTe ternary material systems. However, the implementation of this detector structure in an HgCdTe material system is not straightforward due to the existence of a valence band discontinuity (barrier) at the absorber-barrier interface. We report on midwavelength infrared HgCdTe barrier detectors with a zero valence band offset, grown by metal organic chemical vapor deposition on GaAs substrates. The experiments indicate the influence of the barrier on the electrical and optical performances of the p+BpnN+ device. The devices exhibit very low-dark current densities in the range of (2−3)×10−3 A/cm2 at 230 K and a high-current responsivity of about 2 A/W in the wide range of reverse bias voltage. The estimated thermal activation energy of about 0.33 eV is close to the full Hg0.64Cd0.36Te bandgap, which indicates diffusion limited dark currents.

Short-wave infrared barriode detectors using InGaAsSb absorption material lattice matched to GaSb

Short-wave infrared barriode detectors were grown by molecular beam epitaxy. An absorption layer composition of In0.28Ga0.72As0.25Sb0.75 allowed for lattice matching to GaSb and cut-off wavelengths of 2.9 μm at 250 K and 3.0 μm at room temperature. Arrhenius plots of the dark current density showed diffusion limited dark currents approaching those expected for optimized HgCdTe-based detectors. Specific detectivity figures of around 7×1010 Jones and 1×1010 Jones were calculated, for 240 K and room temperature, respectively. Significantly, these devices could support focal plane arrays working at higher operating temperatures.

Reprint of “Mid-infrared InAsSb-based nBn photodetectors with AlGaAsSb barrier layers – grown on GaAs, using an interfacial misfit array, and on native GaSb”

InAsSb-based nBn photodetectors were fabricated on GaAs, using the interfacial misfit (IMF) array growth mode, and on native GaSb. At −0.1V operating bias, 200K dark current densities of 1.4×10−5Acm−2 (on GaAs) and 4.8×10−6Acm−2 (on GaSb) were measured. At the same temperature, specific detectivity (D*) figures of 1.2×1010Jones (on GaAs) and 7.2×1010Jones (on GaSb) were calculated. Arrhenius plots of the dark current densities yielded activation energies of 0.37eV (on GaAs) and 0.42eV (on GaSb). These values are close to the 4K bandgap of the absorption layers (0.32–0.35eV) indicating diffusion limited dark currents and small valence band offsets. Significantly, these devices could be used for mid-infrared focal plane arrays operating within the temperature range of cost-effective thermoelectric coolers.

XBnn and XBpp infrared detectors

XBnn and XBpp barrier detectors grown from III–V materials on GaSb substrates have recently been shown to exhibit a low diffusion limited dark current and a high quantum efficiency. Two important examples are InAsSb/AlSbAs based XBnn devices with a cut-off wavelength of λC~4.1μm, and InAs/GaSb Type II superlattice (T2SL) based XBpp devices, with λC~9.5μm. The former exhibit background limited performance (BLIP) at F/3 up to ~175K, which is a much higher temperature than observed in standard generation-recombination limited devices, such as InSb photodiodes operating in the same Mid Wave IR atmospheric window. The Long Wave IR (LWIR) T2SL XBpp device has a BLIP temperature of ~100K at F/2. Using the k · p and optical transfer matrix methods, full spectral response curves of both detectors can be predicted from a basic knowledge of the layer thicknesses and doping. The spectral response curves of LWIR gallium free InAs/InAs1−xSbx barrier devices have also been simulated. These devices appear to have a lower quantum efficiency than the equivalent InAs/GaSb XBpp devices.

Mid-infrared InAsSb-based nBn photodetectors with AlGaAsSb barrier layers – Grown on GaAs, using an interfacial misfit array, and on native GaSb

InAsSb-based nBn photodetectors were fabricated on GaAs, using the interfacial misfit (IMF) array growth mode, and on native GaSb. At −0.1V operating bias, 200K dark current densities of 1.4×10−5Acm−2 (on GaAs) and 4.8×10−6Acm−2 (on GaSb) were measured. At the same temperature, specific detectivity (D*) figures of 1.2×1010Jones (on GaAs) and 7.2×1010Jones (on GaSb) were calculated. Arrhenius plots of the dark current densities yielded activation energies of 0.37eV (on GaAs) and 0.42eV (on GaSb). These values are close to the 4K bandgap of the absorption layers (0.32–0.35eV) indicating diffusion limited dark currents and small valence band offsets. Significantly, these devices could be used for mid-infrared focal plane arrays operating within the temperature range of cost-effective thermoelectric coolers.

Fundamental limits of MWIR HgCdTe barrier detectors operating under non-equilibrium mode

The paper presents numerical considerations of temperature-dependent performance of different mid-wave infrared HgCdTe detectors (with p- and n-type active layer) for non-equilibrium operation. Current–voltage characteristics of double heterostructure PpN photodiode, pBppN barrier photodiode, nBnn and nBnnN barrier detectors are compared to find an optimal architecture for high-operating temperature conditions. Using our model, the calculated characteristics of the devices are fitted to the experimental results for HgCdTe photodiode grown on GaAs substrate by metal organic chemical vapour deposition.The performance of photodiodes with p-type absorber are limited by the generation current associated with the Shockley–Read–Hall process, while nBnn type devices (with the n-type absorber) indicate a diffusion limited dark currents associated with Auger processes. At high values of the reverse bias (over 1V), the trap states located at dislocations lead to strong band-to-band and trap-assisted tunnelling due to high electric field within the depletion layer.

XB<italic>n</italic> barrier photodetectors based on InAsSb with high operating temperatures

We demonstrate the suppression of the bulk generation- recombination current in nBn devices based on an InAsSb active layer (AL) and a AlSbAs barrier layer (BL). This leads to much lower dark cur- rents than in conventional InAsSb photodiodes operating at the same temperature. When the BL is p-type, very high doping must be used in the AL (nBpn + ). This results in a significant shortening of the device cut- off wavelength due to the Moss-Burstein effect. For an n-type BL, low AL doping can be used (nBnn), yielding a cutoff wavelength of ∼4.1 μm and a dark current close to ∼3 × 10 −7 A/cm 2 at 150 K. Such a device with a4 -μm-thick AL will exhibit a quantum efficiency (QE) of 70% and background-limited performance operation up to 160 K at f/3. We have madenBnnfocalplane arraydetectors(FPAs)with a 320 ×256 formatand a 1.3-μm-thick AL. These FPAs have a 35% QE and a noise equivalent temperature difference of 16 mK at 150 K and f/3. The high performance of our nBnn detectors is closely related to the high quality of the molecular beam epitaxy grown InAsSb AL material. On the basis of the temperature dependence of the diffusion limited dark current, we estimate a minority carrier lifetime of ∼670 ns. C 2011 Society of Photo-Optical Instrumentation Engineers

Planar p-on-n HgCdTe heterojunction mid-wavelength infrared photodiodes formed using plasma-induced junction isolation

Planar p-on-n HgCdTe heterojunction photodiodes have been fabricated using a plasma-induced type conversion process for device junction isolation. The technique is presented as a fully planar alternative technology to the commonly used mesa isolation structure. The starting material consisted of an indium-doped n-type mid-wavelength infrared (MWIR) HgCdTe absorbing layer that was capped by a 1-µm-thick wider bandgap arsenic-doped p-type layer. Junction-isolated p-on-n diodes were formed by selectively p-to-n type converting the p-type cap layer using a plasma process. Photodiode dark current-voltage measurements were performed as a function of temperature, along with noise and responsivity. The devices have cut-off wavelengths between 4.8 µm and 5.0 µm, exhibit diffusion-limited dark currents down to 145 K, give R0A values greater than 1 × 107Ω·cm2 at 80 K and greater than 1 × 105Ω·cm2 at 120 K, and have negligible 1/f noise current at zero applied bias.

Numerical simulation of HgCdTe detector characteristics

We discuss analytic and numerical models for HgCdTe photodiodes and present examples of their application. Analytic models can account for the performance obtained by many device architectures. Numerical and analytic models agree in predicting several aspects of device performance, such as diffusion limited dark current, confirming the approximations used in deriving the analytic models. Areas are noted where improvement in the numerical models would allow application to a wider range of device simulations. Useful results are obtained from the numerical simulators that cannot be obtained from our analytic model. Flux dependent R0A products are shown to be a direct result of bias dependent quantum efficiency, a mechanism that is much more evident in heterojunction device architectures. Material compositional grading is demonstrated to lead to lower signal to noise ratio in devices designed to detect a particular infrared wavelength. We also show, particularly for high temperature operation, that heterojunction detectors can at best equal the performance of well-designed homojunction detectors; so, for photodetector design, heterojunctions do not offer any inherent performance advantages over homojunctions. Nevertheless, heterostructures, though ideally not required, may be helpful in achieving high performance in practice.