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Abstract
Mid-infrared resonant cavity-enhanced photodetectors (RCE PD) present a promising technology for targeted gas detection. We demonstrate an RCE PD incorporating an InAs/InAsSb superlattice as the detecting element, extending the resonant wavelength beyond 4 µm. AlAsSb/GaSb mirrors and a unipolar barrier active region paralleling an nBn structure are also used, and performance is compared to a conventional broadband nBn detector incorporating the same superlattice. The RCE PD exhibited a Q-factor of ∼90 and an extremely stable resonance wavelength. Peak responsivity was 3.0 A W−1 at 240 K, equalling 84% quantum efficiency, a 5.5 times increase over the reference nBn at the same wavelength. Dark current density was 3.3×10−2 A cm−2 at 240 K, falling to 2.7×10−4 A cm−2 at 180 K. The broadband BLIP limit is approached at 180 K with specific detectivity of 2.1×1011 cm Hz1/2 W−1, which presents the potential of achieving BLIP-limited operation in the thermoelectric cooling regime.
Published by The Optical Society under the terms of the Creative Commons Attribution 4.0 License. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.
1. Introduction
III-V semiconductor compounds have seen increasing interest for use in infrared detection, as an alternative to the HgCdTe material system. In particular, antimonide-based alloys and quantum structures, characterised by small effective bandgaps, have gained significance in the technologically important mid-wave infrared (MWIR) and long-wave infrared (LWIR) regions of the spectrum [1], resulting in increasing commercial implementation of photodiode detectors utilising bulk InSb or InAsSb compounds [2,3] as well as emerging quantum-structured absorbers, such as nBn, multiple quantum well and superlattice structures.
One particular application of such infrared technology is detection of gases for important environmental, safety or diagnostic purposes using their characteristic narrow-band infrared absorption fingerprints. Broadband detectors can be adapted for this purpose with additional filters or the use of dual-band responses; however, their inherent broadband sensitivity is undesirable and at times deleterious. An alternative approach arises in the form of resonant cavity-enhanced photodetectors (RCE PDs), where the active photodetector element is embedded in an optical cavity formed between two distributed Bragg reflector (DBR) mirrors, creating a structure analogous to a vertical cavity surface emitting laser. The optical mode defined by the cavity leads to strong selectivity of absorption at the resonant wavelength and high specificity of gas detection when spectrally aligned with an absorption fingerprint.
In an RCE PD, high quality mirrors enable quantum efficiencies approaching unity [4], while the thinned absorber offers proportionately reduced Auger and generation-recombination dark current magnitudes due to their dependence on absorber volume. Additionally, the broadband background-limited performance (BLIP) limit of operation does not apply to RCE PDs as absorption at non-resonant wavelengths is suppressed. The study and demonstration of RCE photodetectors has progressed since the early 1990s, with such structures demonstrated as 1.55 µm Schottky photodiodes embedded in AlGaInAs/AlInAs stacks [5], or an InGaAs p-i-n equipped with InP/InGaAsP mirrors [6]. Extension of the operating wavelength beyond 3 µm has been achieved with materials such as InAs [7], InAsSb [8], and HgCdTe [9]. Beyond bulk materials, Wu et al [10] have proposed an N-structure type-II InAs/AlSb/GaSb superlattice for methane detection, while Canedy et al [11] implemented a quantum-well based W-structure with a 4.0 µm resonant wavelength.
The aim of this work is to extend the operating wavelength of RCE PDs further into the MWIR, by introducing an InAs/InAsSb type-II strained-layer superlattice (T2SL) absorber. This extension supports important detection applications such as that of toxic CO, detectable at approximately 4.6 µm, and N2O, which is known to act as a significant greenhouse gas [12], at 4.5 µm. Indeed, with cut-off wavelengths in the VLWIR regime having been demonstrated for the InAs/InAsSb T2SL in broadband detectors [13,14], its successful incorporation as the absorber in an RCE PD defines a platform architecture which could be extended to realise RCE PDs across the LWIR spectral band, where further compounds exhibit IR absorption signatures. For example, chemical warfare agents and explosives can be detected in the 9-10 µm range [15], and health diagnostic biomarkers in the 8-12 µm range, such as acetone at 8.2 and 11.2 µm [16].
The conceived RCE PD adopts an nBn architecture for its active region (hereafter referred to as nBn RCE PD), shown in conventional broadband detectors to reduce Shockley-Read-Hall currents due to the elimination of the space charge region [17] and to supress surface current due to the self-passivation effect of the wide-bandgap barrier layer [18]. The incorporation of a T2SL into the absorber and contact regions offers further advantages: suppression of Auger currents via spatial carrier separation and miniband formation, reduced band-to-band tunnelling via increased carrier effective masses, and absorption at normal incidence which is prohibited in the quantum well structure [19–21]. Existing literature shows the achievement of the expected dark current reduction in conventional broadband T2SL nBn’s when compared to conventional photodiodes [22–24].
2. Methods
Sample growth proceeded by molecular beam epitaxy with the use of a Veeco GENXplor reactor, with group-III materials delivered by SUMO cells and valved cracker cells providing As2 and Sb2 flux. Deposition proceeded on n-type (001) GaSb substrates at 1 ML/s growth rate, and the As/Sb ratio was calibrated to achieve xSb = 0.15 composition in the InAsSb superlattice layer. For the nBn RCE PD the DBR mirror layers, AlAsSb and GaSb, were grown at thicknesses of 374 and 308 nm respectively in order to achieve the required λ/4 optical path length; the bottom and top DBR stacks feature 12 and 5 repetitions of AlAsSb/GaSb pairs respectively. The cavity consists of two 237 nm AlAsSb spacers surrounding a thin (185 nm) active layer made of 12 repetitions of the InAs(17 ML)/InAsSb(26 ML) T2SL; additionally, the upper AlAsSb spacer, which acts as the majority current blocker, is capped with 3 repetitions of the same T2SL in order to form the contact layer. The reference nBn detector features an absorber consisting of 336 repetitions of the T2SL, resulting in an active layer with a thickness of 4.18 µm. This is followed by 110 nm of the wide-bandgap Al0.90Ga0.10As0.08Sb0.92 barrier and 370 nm of the T2SL contact layer. The absorber and contact layers in both structures were n-doped using tellurium at a density of 1 × 1017 cm−3. The colour-coded structures of both detectors, with layer thicknesses presented to scale, are shown in Fig. 1(a): the volume reduction of the active layer (here in red), from the reference nBn to the nBn RCE PD is evident. This is expected to result in commensurate reduction of the dark current density.
Fig. 1. (a) Cross-sectional diagrams of both devices, with layer thicknesses presented to scale (note the reduction in thickness of the InAs/InAsSb T2SL active layer between the reference nBn and the nBn RCE PD). (b) SEM images of the entire nBn RCE PD cross-section taken at x6000 magnification (right) and a false colour x20000 magnification of the cavity (left).
Coupled high-resolution X-ray scans of the structures, presented in Fig. 2, show sharp satellite peaks arising from the InAs/InAsSb T2SL periodicity in the reference nBn, as well as an Al0.90Ga0.10As0.08Sb0.92 barrier with negligible peak separation from the substrate. An equivalent scan of the nBn RCE PD, whose absorber superlattice has a thickness of only ∼190 nm and is covered by 3.66 µm of DBR material, still reveals a set of identifiable satellite T2SL peaks, which suggests that the good crystallinity of the absorber superlattice is retained in the RCE PD structure. High interface quality in both the DBR mirrors and the cavity is further confirmed in SEM images, shown in Fig. 1(b).
Fig. 2. Coupled X-ray diffraction scans of the two detectors, with data shown in grey and the modelled spectra in colour. The model spectra are offset with respect to the data for clarity.
The samples were processed into devices using photolithography, wet etching and Ti(30 nm)/Au(200 nm) ohmic contacts. The resonantly active optical area of the nBn RCE PD was defined by wet etching of the top DBR mirror, with the top contact deposited adjacently to it on the exposed InAs/InAsSb contact layer. This was followed by an etch down to the top AlAsSb spacer and majority current-blocking barrier layer. This shallow-etch approach, which defines the electrically active device area, mimics the standard nBn architecture where the barrier acts as a self-passivating layer blocking the flow of surface current [18]. This method was also applied to the reference nBn, where mesas are defined by wet etching the top superlattice contact and where the AlGaAsSb barrier acts as a natural etch stop. No chemical passivation, thermal annealing or anti-reflective coating layers were applied to the devices.
3. Results and discussion
Temperature-dependent spectral response of both structures was obtained using a Bruker Vertex 70 FTIR spectrometer, and the normalised data in the 100–280 K is plotted in Fig. 3. The 50% cut-off wavelength (λ50%) of the reference nBn redshifts from 4.87 µm at 100 K to 5.94 µm at 280 K, as dictated by characteristic bandgap narrowing of III-V semiconductors. In contrast, the resonant response of the nBn RCE PD is almost temperature-insensitive with the peak resonant wavelength only red-shifting from 4.369 to 4.425 µm in the same temperature range. This equates to a temperature coefficient of 0.31 nm/K for the RCE PD, compared with an average gradient of 5.9 nm/K for the reference nBn’s cut-off wavelength across the same temperature range. The much weaker temperature dependence is due to the fact that resonance in the RCE PD is independent of bandgap narrowing in the absorber; instead, the residual temperature sensitivity primarily originates from a temperature dependence of the refractive index with a lesser secondary contribution from the thermal expansion of the constituent lattice constants [8]. Since 61% of the optical path length within the cavity of the RCE PD is comprised of AlAsSb, its refractive index and lattice constant temperature dependencies dominate the shift in the resonant wavelength. Note that the current device design and process flow exposes some absorber area around the periphery of the detector, which combines with an overfilled illumination to give rise to the residual off-resonance response.
Fig. 3. Normalised spectral response curves of the two detectors obtained in the 100–280 K temperature range.
A transfer matrix-based model of optical transmission through the structure [25], incorporating the temperature dependence of the refractive indices [26] and lattice constants [27] of the DBR mirror materials GaSb and AlAs0.08Sb0.92, was employed to calculate the transmission spectrum of the nBn RCE PD. Good agreement with the experimental resonant peak wavelength was found using room temperature transmission data, obtained on the sample prior to processing using Bruker Vertex 70, as seen in Fig. 4(a). The absorption coefficient αT2SL used was ∼2600 cm−1 in the vicinity of 4.4 µm. The same model was applied at temperatures between 100 and 280 K and the derived peak wavelengths, compared to those extracted from the measurement of the resonant spectral response, are shown in detail in Fig. 4(b). An offset of 35-45 nm is evident between the temperature-dependent modelled and measured resonance wavelengths. This offset is not seen in the comparison with the transmission measurement in Fig. 4(a), which was taken under conditions closely matching the model. The model assumes non-polarised illumination normally incident on the surface, however in the practical spectral response measurement the complicated beam path and alignment geometry likely results in off-normal incidence. The resonance wavelength of an RCE PD is known to blue-shift as the angle of incidence moves away from the normal [4] and hence this is proposed as a significant factor in the offset observed in Fig. 4(c).
Fig. 4. (a) Room-temperature transmission spectrum of the RCE PD (grey) showing the characteristic resonant peak centred in the cavity, and the modelled curve (red). (b) Temperature shift of the spectral response resonant peaks in the 100–280 K range. (c) Left y-axis: Spectral response peak wavelengths (solid circles) and their modelled values (hollow circles). Right y-axis: full-width-at-half-maximum of the spectral response peaks as a function of temperature.
Full-width-at-half-maximum (FWHM) of the resonance peaks, as shown in Fig. 4(c), is found to increase by ∼6 nm between 100 and 280 K. The apparent minimum in the 100–140 K range likely corresponds to thermally favourable conditions in the cavity and the mirrors, resulting in optimised resonant enhancement. However, the overall quality of the cavity as well as the wavelength of the resonant peak depend not only on optimised optical path lengths and the absorptive properties of the active layer, but also on temperature responses of these parameters, some of which are not well documented in literature yet and so require further investigation.
The corresponding Q-factors for the resonant cavity are found to decrease from 95 to 85 in the same temperature range. This compares well with values calculated for MWIR RCE PDs reported elsewhere, which range from 20∼30 [7,28] to 80∼86 [8,11].
The dark current behaviour of both devices in the 120 – 300 K temperature range was characterised using a Lake Shore TTPX cryogenic probe station with a cold shield in place. Dark current characteristics as a function of temperature and voltage (JVT), obtained at 20 K intervals, are presented in Fig. 5(a). The temperature-dependent leakage behaviour of the two detectors is further analysed in the Arrhenius plots, including a comparison with Rule 07 [29] derived from λ50% of the reference nBn, are shown in Fig. 5(b). The devices’ nominal operational biases of −0.1 and −0.5 V were chosen to correspond to the bias required for maximum specific detectivity (D*): the reference nBn device is found to peak near −0.1 V at all temperatures, while D* of the nBn RCE PD rises monotonically with voltage. In this scenario, the nBn RCE PD displays only a small reduction in dark current density (Jd) when compared to the reference nBn, which is due to the comparatively high operational bias required to overcome the photocurrent-impeding valence band offset arising at the interface of the T2SL absorber and the upper AlAsSb spacer. However, it is expected that future improvement in dark current density can be achieved by modification of the barrier layer composition or compensation-doping it, to ensure flat band structure in the vicinity of the barrier [8]. The activation energy Ea of both detectors is found using ${J_d} \propto {T^{1.5}}\exp (-{{{E_a}} \mathord{\left/ {\vphantom {-{{E_a}} {{k_B}T)}}} \right.} {{k_B}T)}}$, corresponding to the high-doped absorber regime [30]. Ea of the reference nBn is found to be ∼15 meV higher than the 77 K bandgap of 255 meV, itself derived from the spectral response curves presented in Fig. 3(a), which indicates diffusion-limited behaviour. Ea of the nBn RCE PD is fitted to be ∼264 meV, and so also pointing to diffusion as the dominant current mechanism.
Fig. 5. (a) Cold-shielded dark current densities of both detectors as functions of temperature and bias. (b) The corresponding Arrhenius plots derived at −0.5 V for the nBn RCE PD and at −0.1 V for the reference nBn. Rule 07 is presented as a solid line and the activation energies Ea as dashed/dotted lines.
Temperature dependence of responsivity (R) was obtained with the use of the Bruker Vertex 70 FTIR spectrometer and calibrated with a reference detector. Single-pass topside-illuminated responsivity for both detectors can be seen in Fig. 6(a), plotted at the temperatures and voltages corresponding to their peak R values. The resonant response peak of the nBn RCE PD sample is located at 4.41 µm, near the N2O and CO absorption fingerprints, and reaches 3.0 A/W (corresponding to quantum efficiency, or QE, of 84%) at its maximum. As the resonant peak wavelength of an RCE PD is primarily determined by accurately defined layer thicknesses, this resonant response peak can be shifted to the desired target gas fingerprint via enhanced control of deposition rates during growth, e.g. by using in-situ monitoring. The full temperature dependence of the resonant peak at −0.5 V in the range 180 – 300 K is plotted in Fig. 6(b): responsivity increases from 2.7 A/W (QE = 76%) at 180 K to its peak value of 3.0 A/W at 240 K, then drops at an accelerating rate to 0.6 A/W (QE = 17%) at room temperature. This initial positive temperature dependence and a subsequent decrease from 240 K is also observed to occur in the reference nBn, and could be due to a transition from radiative- to either Auger- or SRH-dominated carrier lifetime mechanisms in the InAs/InAsSb superlattice absorber [31–33]. This relatively high transition temperature would in turn point to a good crystalline quality of the superlattice with a low trap density.
Fig. 6. (a) A comparison of maximum responsivities of the two devices (red and blue), and the atmospheric fingerprints of three gasses in the vicinity of the nBn RCE PD’s resonant peak wavelength: carbon dioxide, nitrous oxide and carbon monoxide. (b) Responsivities of the nBn RCE PD’s resonant response as functions of temperature at −0.5 V bias.
The specific detectivity (D*) was calculated for both the nBn RCE PD and the reference nBn in the shot and Johnson noise-limited regime, and is presented in Fig. 7(a) as a function of temperature. To enable a comparison between them, the D* values for the reference nBn are calculated at wavelengths corresponding to the resonance peaks of the nBn RCE PD. Equivalent background-limited (BLIP) D* for broadband photovoltaic detectors, derived respectively for the resonance peak wavelengths for the nBn RCE PD, and for λ50% wavelengths of the reference nBn, are also overlaid as dashed lines for comparison [1]. An inherent advantage of the RCE architecture is that background photon flux outside of its resonant response is rejected by the wavelength selectivity of the top mirror. This reduces background photocurrent and commensurately increases the background-limited D* compared to equivalent broadband detectors, which is particularly pertinent at extended IR wavelengths. Here, the broadband BLIP limit is approached by the nBn RCE PD at 180 K where it achieves a value of 2.1 × 1011 cm Hz1/2 W−1, a factor of 5.5 higher than the corresponding value of 3.8 × 1010 cm Hz1/2 W−1 found for the reference nBn. The latter structure requires further cooling to 140 K to achieve its BLIP limit of 1.3 × 1011 cm Hz1/2 W−1. As noted earlier, there is potential for optimisation of the nBn RCE PD detector to realise more fully the predicted scaling down of the leakage current density with the reduced absorber volume. Hence, these results raise the potential of achieving MWIR nBn RCE PDs with D* equivalent to the comparable broadband detector BLIP limit using only 3 or 4-stage thermoelectric coolers operating at 200 K.
Fig. 7. (a) Temperature dependence of specific detectivities of both devices, calculated for the peak responsivity values of the nBn RCE PD (blue dots) and for the responsivity values of the reference nBn at wavelengths corresponding to peak values of the RCE PD (red triangles). Broadband photovoltaic BLIP limits (colour-correlated dashed lines) are calculated for the nBn RCE PD resonant peak wavelengths (blue) and for the λ50% values of the reference nBn (red). (b) Bias dependence of specific detectivities of the nBn RCE PD as functions of temperature (solid dots), and the specific detectivity of the reference nBn at 160 K (crosses). The lines are a guide to the eye.
The voltage response of D* of the nBn RCE PD is presented in Fig. 7(b) . D* of the nBn RCE PD saturates above −0.3 V, while the corresponding responsivities begin to plateau in the −0.3 to −0.4 V range. In contrast, responsivity of the reference nBn (grey crosses) is found to stabilise near −0.1 V. This increased potential barrier in the nBn RCE PD arises due to the non-negligible valence band misalignment at the absorber-barrier interface, caused by the absence of gallium from the upper AlAsSb spacer when compared to the optimised reference nBn barrier of Al0.90Ga0.10As0.08Sb0.92 . In this first demonstration of a T2SL nBn RCE PD, the gallium was omitted to maximise mirror reflectivity as determined by the difference in refractive indices. An optimised approach to eliminating this undesirable valence band offset likely lies in the addition of a gallium fraction and selective n-doping of the AlAsSb barrier, which offers the further advantage of suppressing of space charge region in the vicinity of the barrier-absorber interface and its associated dark current [8].
4. Conclusions
In conclusion, a high-performance MWIR resonant cavity-enhanced photodetector with AlAsSb/GaSb DBR mirrors and an InAs/InAsSb type-II superlattice active region has been successfully realised, to extend the detection wavelength of such devices beyond 4 µm. Direct comparison with broadband detectors has been provided through a conventional reference nBn, incorporating the same T2SL active region and fabricated and tested in parallel. The nBn RCE PD exhibited a sharply defined and extremely stable resonant response, with a full-width-at-half-maximum of ∼50 nm and a peak wavelength temperature coefficient of 0.31 nm K−1, characteristics well-suited to targeted gas detection systems. The corresponding Q-factor values were found to be in the range of 85-95, which is an improvement over existing comparable MWIR RCE PD structures. Due to the effective resonant enhancement, responsivity up to 3.0 A/W at 240 K was achieved, corresponding to a QE of ∼84%. This high responsivity, in combination with a moderate reduction in leakage current, allowed the RCE PD’s D* to approach the BLIP limit of an equivalent broadband detector at 180 K, at a value of 2.1 × 1011 cm Hz1/2 W−1. This illustrated a clear enhancement with respect to the broadband reference nBn, which required cooling to 140 K to achieve a comparable D*. Furthermore, there remains scope to optimise the device structure and realise the full potential leakage current reduction, raising the operating temperature further into the thermoelectric cooling regime.
Funding
Engineering and Physical Sciences Research Council (EP/M506369/1); Leonardo MW Ltd (EP/M506369/1); Defence Science and Technology Laboratory (DSTLX:1000116341).
Acknowledgments
Dr Sara Baldock from Lancaster University provided technical support in obtaining Scanning Electron Microscopy images. Chris Maxey from Leonardo MW Ltd supported this work with valuable technical discussion.
Disclosures
The authors declare that there are no conflicts of interest related to this article.
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