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Responsivity and lifetime of resonant-cavity-enhanced HgCdTe detectors
Solid-State Electronics 50 (2006) 1640–1648 www.elsevier.com/locate/sse
Responsivity and lifetime of resonant-cavity-enhanced HgCdTe detectors J.G.A. Wehner *, C.A. Musca, R.H. Sewell, J.M. Dell, L. Faraone School of Electrical, Electronic and Computer Engineering, The University of Western Australia, Stirling Hwy, Mail Stop M018, Crawley, WA 6009, Australia Received 13 April 2006; received in revised form 26 July 2006; accepted 28 July 2006 Available online 20 September 2006
The review of this paper was arranged by Prof. A. Zaslavsky
Abstract Resonant-cavity-enhanced HgCdTe structures have been grown by molecular beam epitaxy, and photoconductors have been modelled and fabricated based on these structures. Responsivity has been measured and shows a peak responsivity of 8 · 104 V/W for a 50 · 50 lm2 photoconductor at a temperature of 200 K. The measured responsivity shows some agreement with the modelled responsivity across the mid-wave infrared window (3–5 lm). The measured responsivity is limited by surface recombination, which limits the effective lifetime to 15 ns. The optical cut-off of the detector varies with temperature as modelled from 5.1 lm at 80 K to 4.4 lm at 250 K. There is strong agreement between modelled peak responsivity and measured peak responsivity with varying temperature from 80 to 300 K. 2006 Elsevier Ltd. All rights reserved. Keywords: Resonant-cavity-enhanced; HgCdTe; MWIR
1. Introduction Mercury cadmium telluride, Hg(1x)Cd(x)Te, has been investigated as a material for use in infrared (IR) detectors and imaging systems since 1958 [1]. The material system has found use in military and homeland defence applications, as well as medical imaging, chemical sensing, astronomy and many other applications. The wide spread use of this material system is largely due to the tunability of the energy band gap from 0.19 eV to 1.6 eV by varying the molecular composition, x, of Hg(1x)Cd(x)Te. The main regions of interest for IR sensing fall in regions of high transmission through the atmosphere, namely the midwave IR (MWIR) window (3–5 lm) and the long-wave IR (LWIR) window (8–14 lm). Molecular compositions
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of x 0.3 and x 0.22 are used for sensing in the MWIR and LWIR regions, respectively [2]. Further benefits of the Hg(1x)Cd(x)Te material system include high electron mobility and a lattice constant that does not vary greatly from HgTe (x = 0) to CdTe (x = 1). This allows for growth of hetero-structures via methods such as molecular beam epitaxy. Despite these benefits, use of Hg(1x)Cd(x)Te has not been widespread due to the low operating temperature required for background limited performance (BLIP). Furthermore, Hg is particularly unstable in the crystal lattice, and as such delicate processing methods and low processing temperatures must be employed. Resonant cavity enhancement (RCE) involves situating the absorber layer of a detector within a Fabry–Perot resonator. At the resonant wavelength, which depends on the spacing between the mirrors of the Fabry–Perot resonator, light experiences constructive interference within the cavity and an absorber layer placed within the cavity will experience multiple passes of incident light. Resonant cavity
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enhancement is a method of improving detector performance which allows the absorber layer to be thinned while maintaining quantum efficiency. The thinner absorbing layer has reduced thermal generation and recombination noise, and can be operated at higher frequencies. However, RCE only benefits a detector at the resonant wavelength, giving these devices narrow spectral bandwidth. Due to this, RCE devices have been investigated mostly for communications applications, where high electrical bandwidth and narrow spectral bandwidth are desired [3,4]. RCE detectors are not suitable for traditional broadband IR, as they are inherently narrowband, however, RCE can be of benefit for narrowband imaging devices. For the same spectral bandwidth the absorber volume can be reduced, while maintaining quantum efficiency. This benefits applications such as chemical sensors and multi- and hyperspectral sensors. There is some previous work in the field of resonant-cavity-enhanced infrared detection [5–8]. Pautrat et al. [5] studied RCE structures primarily for the purpose of RCE vertical cavity surface emitting lasers (VCSELs). However, they also noted that the structure used for the VCSELs could be operated as a photoconductor or a photodiode. The spectral photoconductivity measured by Pautrat shows strong agreement with model results, and resonant detection is observable. Arnold et. al. [6,8] also fabricated RCE detectors, however they fabricated devices using a lead-chalcogenide absorber layer coupled with a lead-chalcogenide and barium fluoride spacer and dielectric stack mirror system. Arnold reports peak quantum efficiencies of 32% at a resonant wavelength of 4.4 lm [6] with a FWHM of 0.037 lm, which equates to dkk ¼ 0:008. This is sufficient for hyperspectral imaging applications. Further devices were fabricated with resonant wavelengths at 7.3 lm with peak quantum efficiencies of above 50% [8]. Sioma and Piotrowski [7] proposes a structure for RCE detection in the LWIR window. The material system is Hg(1x)Cd(x)Te, and simulated results show unity absorption at the design wavelength. Both Sioma and Pautrat make note that the resonance condition is heavily dependant on angle of incidence. This paper investigates using RCE for HgCdTe photodetectors, particularly the ensuing benefits with respect to operating temperature. This represents the first devices fabricated in the HgCdTe material system expressly for the purpose of resonant-cavityenhanced detectors. 2. Device structure and fabrication A cross-section of the RCE device is shown in Fig. 1. Mirror 1 is a distributed Bragg reflector with 17 layers of varying thickness fabricated from Hg(0.6)Cd(0.4)Te and CdTe alternating layers. The ratio of refractive indexes, Hg(0.6)Cd(0.4)Te/CdTe, is close to one as the refractive indexes of the two materials are similar. This results in narrow spectral bandwidth reflectivity peaks for quarter wave distributed Bragg reflectors, and the requirement of many
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Fig. 1. Schematic cross-section of the fabricated structure for the resonant-cavity-enhanced HgCdTe detector, after [11].
layers to achieve high reflectance. In order to overcome the narrow spectral bandwidth, varying the individual layer thickness was investigated. Heavens and Liddell [9] proposes arithmetically and geometrically varying layer thicknesses in order to increase the bandwidth of Bragg reflectors created using a HgCdTe/CdTe material system. Design of the varying thickness mirrors is discussed in more detail elsewhere [10]. Mirror 2 is a Ge/SiO distributed Bragg reflector (DBR) added by thermal deposition after growth and photoconductor fabrication. The absorber layer is approximately 75 nm thick, and was designed for a composition of x = 0.3, and is placed at the top of mirror 1, which is an unconventional position for resonant-cavity-enhanced structures. Usually the absorber layer is situated in the center of the spacer, however, as will be discussed later, it is unsuitable to grow the absorber directly on CdTe and as an insulating spacer material must be used for photoconductors, the absorber layer must be placed on the top x = 0.4 layer of mirror 1. For photovoltaic detectors, this would not be an issue, and the absorber layer could be situated in the center of the spacer layer. The mode profile of the spacer and top two mirror layers is shown in Fig. 2. The absorber should be placed at a position of maximum energy density. For conventional single mode cavities, this is usually the center of the resonant cavity. However, for this structure, the absorber layer can become part of the mirror, and still experience a maximum in the relative energy density. It should also be noted that this RCE structure is not operating in the fundamental mode. All samples were grown on 211(B) CdZnTe substrates. The substrates were cleaned with a Br/Methanol dip etch prior to being loaded into a Riber 32 molecular beam epitaxy (MBE) system, which was calibrated to produce Hg(0.7)Cd(0.3)Te material for the absorber layer and
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Fig. 2. Model mode profile based on the as-grown structure dimensions.
x = 0.4/x = 1 material for the Bragg reflectors (see Fig. 1). Note that the CdTe layers were grown in a Hg flux (i.e. the Hg shutter was open at all times), leading to some Hg incorporation during the growth of CdTe layers. Thus the CdTe layers nominally have x P 0.95 (based on previous characterization of similar growth conditions), however the composition of these layers was not explicitly measured for these samples. The substrate temperature was maintained at approximately 180 C, which is the optimum growth temperature for the x = 0.3 absorber layer. Crystalline CdTe is grown at temperatures far in excess of the growth temperature of Hg(0.7)Cd(0.3)Te, at temperatures approaching 310 C. As a result of keeping the substrate at the lower temperature, the CdTe layers exhibit degraded crystalline quality as indicated by reflection high energy electron diffraction (RHEED), as the RHEED patterns became blurred during growth of the CdTe layers of mirror 1. However, as the CdTe layers are thin (280–350 nm thick), the layer still exhibits reasonable crystallinity when complete. It should also be noted that the RHEED patterns for the CdTe spacer indicated that by the end of the growth of the spacer, the material was completely polycrystalline. Layers of Hg(0.6)Cd(0.4)Te material grown on polycrystalline CdTe layers exhibit evidence of twinning in RHEED patterns. This twinning disappears after 50–100 nm of growth of Hg(0.6)Cd(0.4)Te material, and the RHEED pattern indicates a return to normal crystalline growth. Therefore, in order to obtain the highest possible quality absorber layer, the Hg(0.7)Cd(0.3)Te material needs to be grown on at least 100 nm of Hg(0.6)Cd(0.4)Te material. As the spacer is CdTe, the top layer of mirror 1 is the only place where the detector can be situated while still maintaining high quality Hg(1x)Cd(x)Te. The grown mirror 1 and absorber layer structure without the spacer layer was annealed in situ for 30 min under a Hg flux at the growth temperature in order to produce n-type absorber material.
Fig. 3. (a) Schematic of device cross-section showing final structure with distributed Bragg reflector (mirror 2) added ex situ. (b) 50 · 50 lm2 photoconductor after anodic oxide is grown, before indium contacts are added. Anodic oxide on CdTe (and on CdZnTe substrate) is yellow in colour (white), while anodic oxide grown on Hg(0.7)Cd(0.3)Te is purple in colour (light grey). (c) Topographical profile of 80 · 500 lm2 photoconductor after the spacer layer has been etched.
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The spacer layer was then subsequently added, once the Hg pressure had returned to the growth pressure. This differs from usual practice of annealing ex situ for approximately 24 h in a Hg atmosphere at temperatures higher than the growth temperature. The aim of the 30 min anneal is to achieve uniform n-type material with a doping density of between 1 · 1014 cm3 and 1 · 1015 cm3, without subjecting the mirror layers to high temperatures. High temperatures will cause inter-diffusion of Hg, especially at the interfaces between mirror layers. This would degrade mirror performance [10]. Initially the as grown structure consists of the mirror layers, the absorber layer, and the CdTe spacer layer. Photoconductors, with the cross-sectional structure illustrated in Fig. 3(a), are fabricated from this as-grown structure as following: Firstly, the devices were isolated by performing a mesa isolation etch in Br/HBr, followed by removal of the spacer layer from the contacts in a second Br/HBr etch. The depth of this etch was carefully controlled and monitored in order to stop the etch as close to the absorber layer as possible, and certainly within the top Hg(0.6)Cd(0.4)Te layer of the mirror. The surface of the sample was passivated with anodic oxide [12], which grows at different rates on Hg(1x)Cd(x)Te, depending on the composition, x, of the material. For thin films this leads to colour variations across the device. Fig. 3(b) illustrates the colour variations and shows the grown oxide. There are bands of purple (light grey in black and white image) which correspond with the thicker oxide grown on x = 0.3 areas. These areas are just penetrating the spacer and allow contacts to the absorber layer to be formed. The yellow (white in black and white image) areas correspond to CdTe for the spacer and CdZnTe for the substrate in the mesa isolation areas. The profile of an 80 lm long photoconductor after the spacer etch was measured using a Dektak II scanning profilometer and is displayed in Fig. 3(c). The spacer etch was performed in 2 steps to obtain the right etch depth, hence the double step. The valleys between the contact and the spacer lead to the purple rings in Fig. 3(b), clearly indicating that the absorber layer is just being exposed by the spacer layer etching process. Finally, the contacts were formed by evaporating indium in a thermal deposition system. After deposition of In contacts the devices were characterized. The Ge/SiO DBR was subsequently added by thermal deposition of Ge and SiO. 3. Modelling Resonant-cavity-enhanced photoconductors were modelled in two stages. Firstly, the devices were modelled optically (using characteristic matrix methods [13]) to determine the absorptance of the x = 0.3 absorber layer. Secondly, photoconductor models (e.g. [14,15]) are used to model electrical response. The absorptance is used as the quantum efficiency of the photoconductor, assuming that the internal quantum efficiency of Hg(1x)Cd(x)Te is
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Fig. 4. Modelled detectivity, thermal generation–recombination limited detectivity and background limited performance (BLIP) detectivity of a 10 lm non-RCE device and a 75 nm RCE device.
unity. For thick devices, which absorb all incoming photons, the quantum efficiency is taken as unity. For a RCE Hg(1x)Cd(x)Te detector the absorptance of the absorber layer is determined and at the resonant wavelength for the proposed structure this results in g = 0.96. Detectivity was modelled for Hg(1x)Cd(x)Te photoconductive devices with composition of x = 0.3 at T = 80 K, dimensions of 100 lm · 100 lm and for absorber thicknesses of 10 lm and 75 nm, with a doping density of n0 = 1 · 1014 cm3. The mobility, lifetime and hole concentrations are calculated from the composition and standard expressions [16]. The result of the model is given in Fig. 4 for surface recombination velocity S = 0 cm s1. It can be seen in Fig. 4 that both devices are background limited at low temperatures. As the temperature rises, both detectors suffer from thermal noise, however, the non-RCE device is affected first. The RCE device can operate at background limited performance at 225 K, while the non-RCE device can only sustain background limited performance up to 160 K. This gain in performance is due to the reduction in detector volume, which reduces the thermal generation and recombination noise mechanism while maintaining quantum efficiency. Fig. 5 illustrates improvement in absorptance due to RCE. A 75 nm thick absorber layer, without RCE, has an absorptance below 0.15 for the mid-wave infrared (MWIR) transmission window. Adding RCE by placing the absorber layer on a staggered dielectric mirror with a spacer (after Fig. 3(a), without the Ge/SiO mirror) boosts absorptance to 0.3 at the resonant wavelength. With the addition of a strong DBR to complete the resonant cavity (after Fig. 3(a)), absorptance reaches 0.75. It should be noted that these modelled results utilised actual measured thicknesses on an as-grown layer, which had unintentional variation of mirror layer thicknesses
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Fig. 5. Absorption of a 75 nm x = 0.3Hg(1x)Cd(x)Te layer with one DBR (after Fig. 1), two DBR reflectors (after Fig. 3(a)), and with no resonant cavity enhancement. Thickness used are as measured for an as grown sample, after [11].
from the optimised design thicknesses. Absorptance can reach unity for an optimally designed cavity. It should also be noted that the RCE device for Fig. 4 was designed for operation at 80 K. The refractive index of HgCdTe is dependant on the operating temperature, and therefore the design would need to be adjusted to ensure maximum quantum efficiency at higher operating temperatures.
Fig. 6. Responsivity of an 80 lm · 500 lm photoconductor with single mirror (after Fig. 3(a), without the Ge/SiO mirror) as a function of wavelength at bias of Eb 50 V cm1 and a temperature of 80 K. Modelled responsivity is also shown. Model parameters for the absorber layer are T = 80 K, d = 75 nm, S1,2 = 600 cm s1, x = 0.3, ND = 3.4 · 1014 cm3, Eb = 50 V cm1. Modelled responsivity of a 10 lm thick photoconductive detector is plotted on the right axis, with model parameters similar to the RCE case, except surface recombination is neglected, after [11].
Fig. 7. Normalized responsivity of a 80 lm · 500 lm photoconductor with single mirror (after Fig. 3(a), without the Ge/SiO mirror) as a function of wavelength at bias of Eb 50 V cm1 and temperatures of 80 K and 250 K. Modelled normalized responsivity is also shown. Model parameters for the absorber layer are T = 80 K and T = 250 K, d = 10 lm, S1,2 = 0 cm s1, x = 0.3, ND = 3.4 · 1014 cm3, Eb = 50 V cm1.
4. Experimental results 4.1. RCE responsivity The results of responsivity measurements performed at a temperature of 80 K on a 80 lm · 500 lm photoconductor as a function of wavelength are shown in Fig. 6. The device structure is that of Fig. 3(a) without the Ge/SiO mirror,
Fig. 8. Peak responsivity of a 50 lm · 50 lm photoconductor with single mirror (after Fig. 3(a), without the Ge/SiO mirror) as a function of temperature at a bias of Eb 36 V cm1. Modelled peak responsivity is also shown. Model parameters for the absorber layer are d = 75 nm, S1,2 = 200 cm s1, x = 0.3, ND = 1 · 1015 cm3, Eb = 36 V cm1. Extracted lifetime and model lifetime are also plotted.
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and the device is frontside illuminated at this point. Also shown in Fig. 6 is the modelled performance of a 80 lm · 500 lm photoconductor modelled for an absorber layer with d = 75 nm, surface recombination velocity of the front and back surfaces S1,2 = 600 cm s1, composition x = 0.3, and doping density ND = 3 · 1014 cm3. Layer thicknesses used for the modelled device were taken from scanning electron microscopy measurements of the fabricated structure. The general shape of both the measured and modelled data is in good agreement, and clearly shows a resonant peak at a wavelength of approximately 2.75 lm. Beyond 3.5 lm wavelength, the reflectivity of the Hg(1x)Cd(x)Te/CdTe mirror decreases, and the cavity will not reject signal for these wavelengths. It can be seen that the model does not predict the broadening that is clearly visible in the measured data, particularly broadening of the 2.75 lm resonant peak. This has been investigated and is due to broad spectral width from the monochromator used to take these measurements. Measurements taken
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using a different system with narrower spectral bandwidth (in Section 4.3) show a narrower peak. Note also the difference between the mirror cut-off of the measured data compared to the modelled curve, at approximately 3.5 lm wavelength. This is most likely due to the refractive index of the CdTe layers differing from that of the model. As the CdTe is deposited at non-ideal temperatures, it is likely that it does not behave as modelled. This also requires further investigation. It is important to note that the surface recombination velocity used to fit the curve is reasonable for MBE material. High quality MOCVD and MBE grown HgCdTe material can reach surface recombination velocities as low as S = 50 cm s1 [17]. 4.2. Temperature dependence of RCE devices By varying measurement temperature, further material properties can be investigated. Fig. 7 illustrates the normalized responsivity of a 80 lm · 500 lm photoconductor at
Fig. 9. Spatial photoresponse of a photoconductor (a) at 300 K, and (b) at 80 K.
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temperatures of 80 K and 250 K. These are compared with the modeled normalized responsivity for a 10 lm thick detector at 80 K and 250 K. There is very strong agreement in the shift in cut-off due to the changing band-gap of the Hg(1x)Cd(x)Te with temperature. This is apparent as the signal in the mirror roll-off region (3.5–5 lm) experiences a shifting cut-off. The cut-off at 250 K clearly drops to a noise floor after 4.4 lm. The cut-off of the 80 K is more difficult to determine, as a traditional half-peak responsivity [2] analysis cannot be used, and the signal does not suddenly cut out as in the 250 K case. The cut-off in the 80 K case will be in the vicinity of the model though, as the model for energy gap at low temperature is well defined, and the model fit in Fig. 6 at cut-off shows good agreement. The peak responsivity shifts in wavelength due to the change in refractive index with temperature. The effect of the change in refractive index is small compared to the change in energy gap, especially as the mirror reflectivity in the 3.5–5 lm range is low. Peak responsivity of a 50 lm · 50 lm RCE photoconductor is also measured for varying temperatures for a fixed bias field of Eb 36 V cm1, with the results shown in Fig. 8. Above 200 K, the peak responsivity is highly dependant on the intrinsic carrier concentration. Below 200 K, the model responsivity is independent of temperature as the lifetime is dominated by surface recombination. The measured and modelled responsivity show good agreement at all temperatures. Fig. 8 also shows the steady state effective lifetime extracted from the measured responsivity. Again there is reasonable agreement between the modelled lifetime and the extracted lifetime. The extracted lifetime of seff 14 ns at 80 K is very low compared to bulk n-type material, which for high quality Hg(1x)Cd(x)Te would be on the order of microseconds. This indicates that surface recombination is the dominant mechanism. Furthermore, surface recombination is modelled as being independent of temperature. Surface recombination is most likely the dominant recombination mechanism and is limiting the minority carrier lifetime. However, results of Hall measurements to determine the doping density using Van der Pauw [18] structures fabricated with the photoconductors were inconclusive (hence the fitted doping density of ND = 3.4– 10 · 1014 cm3). During measurements non-symmetric Hall voltages were observed. This indicates contact issues, or non-uniformities in the material. Spatial photoresponse measurements (shown in Fig. 9) were taken to try to determine the cause of the non-symmetries in the hall voltage. The strong signal about the contact in Fig. 9(b) suggests some issues with the doping at these areas, as the indium could be creating n+–n junctions, or alternately the annealing process failed to fully convert the x = 0.3 layer and the indium is acting to create a compensated region near the contact, and a p–n junction around the contact. This effect is not apparent in Fig. 9(a), as at room temperature the material is intrinsic. These issues with doping could be
reducing the lifetime of the material, however, given that the extracted effective lifetime is 14 ns, the most likely mechanism dominating the lifetime is surface recombination. The lifetime for compensated n-type material would be on the order of many tens to hundreds of nanoseconds, much longer than the surface recombination dominated lifetime [19]. 4.3. Performance of complete RCE structure Following initial characterisation of the device the Ge/ SiO mirror was added. The structure of the completed device is shown in Fig. 3(a), and the device was backside illuminated. Responsivity as a function of wavelength was remeasured and Fig. 10 shows the results of a 50 lm · 50 lm photoconductor measured at a temperature of 80 K, and a bias field of Eb = 50 V cm1 before and after the deposition of the Ge/SiO mirror. The same device was used for both measurements before and after the addition of the mirror. Both data sets are normalised to the responsivity at a wavelength of 2.6 lm. Below 2.5 lm there is a marked difference between the two curves. This is due to the fact that as the device is backside illuminated after deposition of the Ge/SiO mirror, incident radiation must pass through mirror 1 before entering the cavity. As the x = 0.4 material of mirror 1 is absorbing at these wavelengths, it is preventing incident light from reaching the x = 0.3 absorber layer or the top most x = 0.4 layer of mirror 1. The measured result is confirmed with modelling of the device before and after deposition of mirror 2, showing a decrease in absorptance in this region (Fig. 11). Fig. 11 also illustrates that there is no sharp resonance for wavelengths shorter than 2.8 lm due to absorption in the top x = 0.4 layer of mirror 1. This absorption is due to poor control of the layer thicknesses during MBE growth of the mirror structure resulting in the resonant wavelength being close to the x = 0.4 material cutoff wavelength. The
Fig. 10. Normalised responsivity of a 50 lm · 50 lm photoconductor at 80 K before and after the addition of the Ge/SiO mirror. Measured at a bias of Eb 50 V cm1.
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Fig. 12. Normalised responsivity of a 50 lm · 50 lm photoconductor measured with varying temperature. Measured at a bias of Eb 50 V cm1. Fig. 11. Model absorptance of the x = 0.3 absorber layer and the x = 0.4 top layer mirror 1 before and after the addition of the Ge/SiO mirror.
designed resonance was for higher wavelength, which would have produced a discrete resonant peak. For wavelengths longer than 3.6 lm, results both before and after deposition of mirror 2 agree with approximately one pass through a 75 nm absorber layer. This is logical, as mirror 1 is not highly reflective at these wavelengths. Therefore, the cavity will not reject these wavelengths or resonate. There is a resonant peak at 3.4 lm and after the Ge/SiO mirror is added, this peak becomes more well defined. However, model absorptance (Fig. 10) suggests that the peak should become more well defined and stronger. After the Ge/SiO mirror is added, wavelengths above 3.6 lm should still have some modulation due to the cavity, which does not appear in measured results. Also the model suggests a shift towards longer wavelengths of these features. The discrepancy between measured data and modelled data is possibly due to differences in the refractive indexes of the Hg(0.6)Cd(0.4)Te/CdTe and CdTe mirror layers. In particular, dispersion in the refractive index of the Hg(0.6)Cd(0.4)Te material may not be as modelled, causing wavelength shifts. More investigation is needed into the refractive index of both the Hg(0.6)Cd(0.4)Te material and the CdTe material used in mirror 1. The spectral responsivity of a 80 lm · 500 lm photoconductor with the Ge/SiO mirror added was measured under a bias field of Eb = 50 V cm1 for various operating temperatures and is shown in Fig. 12. As temperature increases the resonance at 3.4 lm decreases, in agreement with modelled results (Fig. 13). However, at a temperature of 240 K there is resonance at 3.2 lm which does not agree with modelled results. This is also possibly due to dispersion in the Hg(0.6)Cd(0.4)Te material, but further investigation is needed. Finally, the variation of the responsivity peak at approximately 2.8 lm with temperature is not
Fig. 13. Model absorptance of the x = 0.3 absorber layer with varying temperature.
monotonically shifting to shorter wavelengths, suggesting that this signal is due to resonance in the cavity being absorbed by the Hg(0.7)Cd(0.3)Te absorber layer. 5. Conclusions Resonant-cavity-enhanced Hg(1x)Cd(x)Te devices have been fabricated and demonstrated. Optical performance agrees with modelled performance, exhibiting resonant behaviour in the MWIR window. However, there are differences between modelled and measured data which are most likely due to differences between modelled and actual refractive index of the Hg(0.6)Cd(0.4)Te material and CdTe material in HgCdTe/CdTe stack. Absorber layer cut-off and peak responsivity vary with temperature as modelled. Extracted effective lifetime also agrees with model data, although extracted effective lifetime is very low at 14 ns. Both responsivity and extracted effective lifetime show that these devices are limited by surface recombination. Investi-
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gation into improving the interfaces of the MBE grown absorber layer in order to reduce surface recombination is required. Acknowledgements The authors wish to thank the Australian Research Council for financial support of this work. References [1] Lawson WD, Nielson S, Putley EH, Young AS. Preparation and properties of HgTe and mixed crystals of HgTe–CdTe. J Phys Chem Solids 1959;9:325–9. [2] Hansen GL, Schmit JL, Casselman T. Energy gap versus alloy composition and temperature in Hg(1x)Cd(x)Te. J Appl Phys 1982;53(10):7099–101. [3] Chin A, Chang TY. Multilayer reflectors by molecular-beam epitaxy for resonance enhanced absorption in thin high-speed detectors. J Vac Sci Technol B 1990;8(2):339–42. [4] Unlu MS, Kishino K, Liaw HJ, Morkoc H. A theoretical study of resonant cavity-enhanced photodetectors with Ge and Si active regions. J Appl Phys 1992;71(8):4049–58. [5] Pautrat JL, Hadji E, Bleuse J, Magnea N. Resonant-cavity infrared optoelectronic devices. J Electron Mater 1997;26(6):667–72. [6] Arnold M, Zimin D, Alchalabi K, Zogg H. Lead salt mid-IR photodetectors with narrow linewidth. J Cryst Growth 2005;278(1–4): 739–42. [7] Sioma M, Piotrowski J. Modelling and optimisation of hightemperature detectors of long wavelength infrared radiation with optically resonant cavity. Opto-Electron Rev 2004;12(1):157–60.
[8] Arnold M, Zimin D, Zogg H. Resonant-cavity-enhanced photodetectors for the mid-infrared. Appl Phys Lett 2005;87(14). [9] Heavens O, Liddell HM. Staggered broad-band reflecting multilayers. Appl Opt 1966;5(3):373–6. [10] Wehner JGA, Sewell RH, Musca CA, Dell JM, Faraone L. Mercury cadmium telluride/cadmium telluride distributed Bragg reflectors for use with resonant cavity-enhanced detectors. J Electron Mater 2005; 34(6):710–5. [11] Wehner JGA, Musca CA, Sewell RH, Dell JM, Faraone L. Mercury cadmium telluride resonant-cavity-enhanced photoconductive infrared detectors. Appl Phys Lett 2005;87(21):211104-1. [12] Catagnus PC, Baker CT. Passivation of mercury cadmium telluride semiconductor surfaces by anodic oxidation. US Patent no. 3977018, 1976. [13] Macleod HA. Thin-film optical filters. 3rd ed. Bristol: Institute of Physics; 2001. [14] Siliquini J, Musca CA, Nener BD, Faraone L. Temperature dependence of Hg(0.68)Cd(0.32)Te infrared photoconductor performance. IEEE Trans Electron Dev 1995;42(8):1441–8. [15] Willardson RK, Beer AC, editors. Semiconductors and semimetals, vol. 18. New York: Academic Press; 1981. [16] Capper P, editor. Properties of narrow gap cadmium-based compounds. INPSEC, 1994. [17] Musca CA, Siliquini JF, Fynn KA, Nener BD, Faraone L. Mocvdgrown wider-bandgap capping layers in Hg(1x)Cd(x)Te long-wavelength infrared photoconductors. Semicond Sci Technol 1996;11: 1912–22. [18] Van der Pauw LJ. A method of measuring the resistivity and hall coefficient on lamellae of arbitrary shape. Phillips Techn Rev 1958– 1959;20(8). [19] Yoo SD, Kwack KD. Analysis of carrier concentration, lifetime, and electron mobility on p-type HgCdTe. J Appl Phys 1998;83(5): 2586–92.
Responsivity and lifetime of resonant-cavity-enhanced HgCdTe detectors J.G.A. Wehner *, C.A. Musca, R.H. Sewell, J.M. Dell, L. Faraone School of Electrical, Electronic and Computer Engineering, The University of Western Australia, Stirling Hwy, Mail Stop M018, Crawley, WA 6009, Australia Received 13 April 2006; received in revised form 26 July 2006; accepted 28 July 2006 Available online 20 September 2006
The review of this paper was arranged by Prof. A. Zaslavsky
Abstract Resonant-cavity-enhanced HgCdTe structures have been grown by molecular beam epitaxy, and photoconductors have been modelled and fabricated based on these structures. Responsivity has been measured and shows a peak responsivity of 8 · 104 V/W for a 50 · 50 lm2 photoconductor at a temperature of 200 K. The measured responsivity shows some agreement with the modelled responsivity across the mid-wave infrared window (3–5 lm). The measured responsivity is limited by surface recombination, which limits the effective lifetime to 15 ns. The optical cut-off of the detector varies with temperature as modelled from 5.1 lm at 80 K to 4.4 lm at 250 K. There is strong agreement between modelled peak responsivity and measured peak responsivity with varying temperature from 80 to 300 K. 2006 Elsevier Ltd. All rights reserved. Keywords: Resonant-cavity-enhanced; HgCdTe; MWIR
1. Introduction Mercury cadmium telluride, Hg(1x)Cd(x)Te, has been investigated as a material for use in infrared (IR) detectors and imaging systems since 1958 [1]. The material system has found use in military and homeland defence applications, as well as medical imaging, chemical sensing, astronomy and many other applications. The wide spread use of this material system is largely due to the tunability of the energy band gap from 0.19 eV to 1.6 eV by varying the molecular composition, x, of Hg(1x)Cd(x)Te. The main regions of interest for IR sensing fall in regions of high transmission through the atmosphere, namely the midwave IR (MWIR) window (3–5 lm) and the long-wave IR (LWIR) window (8–14 lm). Molecular compositions
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0038-1101/$ - see front matter 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.sse.2006.07.012
of x 0.3 and x 0.22 are used for sensing in the MWIR and LWIR regions, respectively [2]. Further benefits of the Hg(1x)Cd(x)Te material system include high electron mobility and a lattice constant that does not vary greatly from HgTe (x = 0) to CdTe (x = 1). This allows for growth of hetero-structures via methods such as molecular beam epitaxy. Despite these benefits, use of Hg(1x)Cd(x)Te has not been widespread due to the low operating temperature required for background limited performance (BLIP). Furthermore, Hg is particularly unstable in the crystal lattice, and as such delicate processing methods and low processing temperatures must be employed. Resonant cavity enhancement (RCE) involves situating the absorber layer of a detector within a Fabry–Perot resonator. At the resonant wavelength, which depends on the spacing between the mirrors of the Fabry–Perot resonator, light experiences constructive interference within the cavity and an absorber layer placed within the cavity will experience multiple passes of incident light. Resonant cavity
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enhancement is a method of improving detector performance which allows the absorber layer to be thinned while maintaining quantum efficiency. The thinner absorbing layer has reduced thermal generation and recombination noise, and can be operated at higher frequencies. However, RCE only benefits a detector at the resonant wavelength, giving these devices narrow spectral bandwidth. Due to this, RCE devices have been investigated mostly for communications applications, where high electrical bandwidth and narrow spectral bandwidth are desired [3,4]. RCE detectors are not suitable for traditional broadband IR, as they are inherently narrowband, however, RCE can be of benefit for narrowband imaging devices. For the same spectral bandwidth the absorber volume can be reduced, while maintaining quantum efficiency. This benefits applications such as chemical sensors and multi- and hyperspectral sensors. There is some previous work in the field of resonant-cavity-enhanced infrared detection [5–8]. Pautrat et al. [5] studied RCE structures primarily for the purpose of RCE vertical cavity surface emitting lasers (VCSELs). However, they also noted that the structure used for the VCSELs could be operated as a photoconductor or a photodiode. The spectral photoconductivity measured by Pautrat shows strong agreement with model results, and resonant detection is observable. Arnold et. al. [6,8] also fabricated RCE detectors, however they fabricated devices using a lead-chalcogenide absorber layer coupled with a lead-chalcogenide and barium fluoride spacer and dielectric stack mirror system. Arnold reports peak quantum efficiencies of 32% at a resonant wavelength of 4.4 lm [6] with a FWHM of 0.037 lm, which equates to dkk ¼ 0:008. This is sufficient for hyperspectral imaging applications. Further devices were fabricated with resonant wavelengths at 7.3 lm with peak quantum efficiencies of above 50% [8]. Sioma and Piotrowski [7] proposes a structure for RCE detection in the LWIR window. The material system is Hg(1x)Cd(x)Te, and simulated results show unity absorption at the design wavelength. Both Sioma and Pautrat make note that the resonance condition is heavily dependant on angle of incidence. This paper investigates using RCE for HgCdTe photodetectors, particularly the ensuing benefits with respect to operating temperature. This represents the first devices fabricated in the HgCdTe material system expressly for the purpose of resonant-cavityenhanced detectors. 2. Device structure and fabrication A cross-section of the RCE device is shown in Fig. 1. Mirror 1 is a distributed Bragg reflector with 17 layers of varying thickness fabricated from Hg(0.6)Cd(0.4)Te and CdTe alternating layers. The ratio of refractive indexes, Hg(0.6)Cd(0.4)Te/CdTe, is close to one as the refractive indexes of the two materials are similar. This results in narrow spectral bandwidth reflectivity peaks for quarter wave distributed Bragg reflectors, and the requirement of many
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Fig. 1. Schematic cross-section of the fabricated structure for the resonant-cavity-enhanced HgCdTe detector, after [11].
layers to achieve high reflectance. In order to overcome the narrow spectral bandwidth, varying the individual layer thickness was investigated. Heavens and Liddell [9] proposes arithmetically and geometrically varying layer thicknesses in order to increase the bandwidth of Bragg reflectors created using a HgCdTe/CdTe material system. Design of the varying thickness mirrors is discussed in more detail elsewhere [10]. Mirror 2 is a Ge/SiO distributed Bragg reflector (DBR) added by thermal deposition after growth and photoconductor fabrication. The absorber layer is approximately 75 nm thick, and was designed for a composition of x = 0.3, and is placed at the top of mirror 1, which is an unconventional position for resonant-cavity-enhanced structures. Usually the absorber layer is situated in the center of the spacer, however, as will be discussed later, it is unsuitable to grow the absorber directly on CdTe and as an insulating spacer material must be used for photoconductors, the absorber layer must be placed on the top x = 0.4 layer of mirror 1. For photovoltaic detectors, this would not be an issue, and the absorber layer could be situated in the center of the spacer layer. The mode profile of the spacer and top two mirror layers is shown in Fig. 2. The absorber should be placed at a position of maximum energy density. For conventional single mode cavities, this is usually the center of the resonant cavity. However, for this structure, the absorber layer can become part of the mirror, and still experience a maximum in the relative energy density. It should also be noted that this RCE structure is not operating in the fundamental mode. All samples were grown on 211(B) CdZnTe substrates. The substrates were cleaned with a Br/Methanol dip etch prior to being loaded into a Riber 32 molecular beam epitaxy (MBE) system, which was calibrated to produce Hg(0.7)Cd(0.3)Te material for the absorber layer and
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Fig. 2. Model mode profile based on the as-grown structure dimensions.
x = 0.4/x = 1 material for the Bragg reflectors (see Fig. 1). Note that the CdTe layers were grown in a Hg flux (i.e. the Hg shutter was open at all times), leading to some Hg incorporation during the growth of CdTe layers. Thus the CdTe layers nominally have x P 0.95 (based on previous characterization of similar growth conditions), however the composition of these layers was not explicitly measured for these samples. The substrate temperature was maintained at approximately 180 C, which is the optimum growth temperature for the x = 0.3 absorber layer. Crystalline CdTe is grown at temperatures far in excess of the growth temperature of Hg(0.7)Cd(0.3)Te, at temperatures approaching 310 C. As a result of keeping the substrate at the lower temperature, the CdTe layers exhibit degraded crystalline quality as indicated by reflection high energy electron diffraction (RHEED), as the RHEED patterns became blurred during growth of the CdTe layers of mirror 1. However, as the CdTe layers are thin (280–350 nm thick), the layer still exhibits reasonable crystallinity when complete. It should also be noted that the RHEED patterns for the CdTe spacer indicated that by the end of the growth of the spacer, the material was completely polycrystalline. Layers of Hg(0.6)Cd(0.4)Te material grown on polycrystalline CdTe layers exhibit evidence of twinning in RHEED patterns. This twinning disappears after 50–100 nm of growth of Hg(0.6)Cd(0.4)Te material, and the RHEED pattern indicates a return to normal crystalline growth. Therefore, in order to obtain the highest possible quality absorber layer, the Hg(0.7)Cd(0.3)Te material needs to be grown on at least 100 nm of Hg(0.6)Cd(0.4)Te material. As the spacer is CdTe, the top layer of mirror 1 is the only place where the detector can be situated while still maintaining high quality Hg(1x)Cd(x)Te. The grown mirror 1 and absorber layer structure without the spacer layer was annealed in situ for 30 min under a Hg flux at the growth temperature in order to produce n-type absorber material.
Fig. 3. (a) Schematic of device cross-section showing final structure with distributed Bragg reflector (mirror 2) added ex situ. (b) 50 · 50 lm2 photoconductor after anodic oxide is grown, before indium contacts are added. Anodic oxide on CdTe (and on CdZnTe substrate) is yellow in colour (white), while anodic oxide grown on Hg(0.7)Cd(0.3)Te is purple in colour (light grey). (c) Topographical profile of 80 · 500 lm2 photoconductor after the spacer layer has been etched.
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The spacer layer was then subsequently added, once the Hg pressure had returned to the growth pressure. This differs from usual practice of annealing ex situ for approximately 24 h in a Hg atmosphere at temperatures higher than the growth temperature. The aim of the 30 min anneal is to achieve uniform n-type material with a doping density of between 1 · 1014 cm3 and 1 · 1015 cm3, without subjecting the mirror layers to high temperatures. High temperatures will cause inter-diffusion of Hg, especially at the interfaces between mirror layers. This would degrade mirror performance [10]. Initially the as grown structure consists of the mirror layers, the absorber layer, and the CdTe spacer layer. Photoconductors, with the cross-sectional structure illustrated in Fig. 3(a), are fabricated from this as-grown structure as following: Firstly, the devices were isolated by performing a mesa isolation etch in Br/HBr, followed by removal of the spacer layer from the contacts in a second Br/HBr etch. The depth of this etch was carefully controlled and monitored in order to stop the etch as close to the absorber layer as possible, and certainly within the top Hg(0.6)Cd(0.4)Te layer of the mirror. The surface of the sample was passivated with anodic oxide [12], which grows at different rates on Hg(1x)Cd(x)Te, depending on the composition, x, of the material. For thin films this leads to colour variations across the device. Fig. 3(b) illustrates the colour variations and shows the grown oxide. There are bands of purple (light grey in black and white image) which correspond with the thicker oxide grown on x = 0.3 areas. These areas are just penetrating the spacer and allow contacts to the absorber layer to be formed. The yellow (white in black and white image) areas correspond to CdTe for the spacer and CdZnTe for the substrate in the mesa isolation areas. The profile of an 80 lm long photoconductor after the spacer etch was measured using a Dektak II scanning profilometer and is displayed in Fig. 3(c). The spacer etch was performed in 2 steps to obtain the right etch depth, hence the double step. The valleys between the contact and the spacer lead to the purple rings in Fig. 3(b), clearly indicating that the absorber layer is just being exposed by the spacer layer etching process. Finally, the contacts were formed by evaporating indium in a thermal deposition system. After deposition of In contacts the devices were characterized. The Ge/SiO DBR was subsequently added by thermal deposition of Ge and SiO. 3. Modelling Resonant-cavity-enhanced photoconductors were modelled in two stages. Firstly, the devices were modelled optically (using characteristic matrix methods [13]) to determine the absorptance of the x = 0.3 absorber layer. Secondly, photoconductor models (e.g. [14,15]) are used to model electrical response. The absorptance is used as the quantum efficiency of the photoconductor, assuming that the internal quantum efficiency of Hg(1x)Cd(x)Te is
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Fig. 4. Modelled detectivity, thermal generation–recombination limited detectivity and background limited performance (BLIP) detectivity of a 10 lm non-RCE device and a 75 nm RCE device.
unity. For thick devices, which absorb all incoming photons, the quantum efficiency is taken as unity. For a RCE Hg(1x)Cd(x)Te detector the absorptance of the absorber layer is determined and at the resonant wavelength for the proposed structure this results in g = 0.96. Detectivity was modelled for Hg(1x)Cd(x)Te photoconductive devices with composition of x = 0.3 at T = 80 K, dimensions of 100 lm · 100 lm and for absorber thicknesses of 10 lm and 75 nm, with a doping density of n0 = 1 · 1014 cm3. The mobility, lifetime and hole concentrations are calculated from the composition and standard expressions [16]. The result of the model is given in Fig. 4 for surface recombination velocity S = 0 cm s1. It can be seen in Fig. 4 that both devices are background limited at low temperatures. As the temperature rises, both detectors suffer from thermal noise, however, the non-RCE device is affected first. The RCE device can operate at background limited performance at 225 K, while the non-RCE device can only sustain background limited performance up to 160 K. This gain in performance is due to the reduction in detector volume, which reduces the thermal generation and recombination noise mechanism while maintaining quantum efficiency. Fig. 5 illustrates improvement in absorptance due to RCE. A 75 nm thick absorber layer, without RCE, has an absorptance below 0.15 for the mid-wave infrared (MWIR) transmission window. Adding RCE by placing the absorber layer on a staggered dielectric mirror with a spacer (after Fig. 3(a), without the Ge/SiO mirror) boosts absorptance to 0.3 at the resonant wavelength. With the addition of a strong DBR to complete the resonant cavity (after Fig. 3(a)), absorptance reaches 0.75. It should be noted that these modelled results utilised actual measured thicknesses on an as-grown layer, which had unintentional variation of mirror layer thicknesses
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Fig. 5. Absorption of a 75 nm x = 0.3Hg(1x)Cd(x)Te layer with one DBR (after Fig. 1), two DBR reflectors (after Fig. 3(a)), and with no resonant cavity enhancement. Thickness used are as measured for an as grown sample, after [11].
from the optimised design thicknesses. Absorptance can reach unity for an optimally designed cavity. It should also be noted that the RCE device for Fig. 4 was designed for operation at 80 K. The refractive index of HgCdTe is dependant on the operating temperature, and therefore the design would need to be adjusted to ensure maximum quantum efficiency at higher operating temperatures.
Fig. 6. Responsivity of an 80 lm · 500 lm photoconductor with single mirror (after Fig. 3(a), without the Ge/SiO mirror) as a function of wavelength at bias of Eb 50 V cm1 and a temperature of 80 K. Modelled responsivity is also shown. Model parameters for the absorber layer are T = 80 K, d = 75 nm, S1,2 = 600 cm s1, x = 0.3, ND = 3.4 · 1014 cm3, Eb = 50 V cm1. Modelled responsivity of a 10 lm thick photoconductive detector is plotted on the right axis, with model parameters similar to the RCE case, except surface recombination is neglected, after [11].
Fig. 7. Normalized responsivity of a 80 lm · 500 lm photoconductor with single mirror (after Fig. 3(a), without the Ge/SiO mirror) as a function of wavelength at bias of Eb 50 V cm1 and temperatures of 80 K and 250 K. Modelled normalized responsivity is also shown. Model parameters for the absorber layer are T = 80 K and T = 250 K, d = 10 lm, S1,2 = 0 cm s1, x = 0.3, ND = 3.4 · 1014 cm3, Eb = 50 V cm1.
4. Experimental results 4.1. RCE responsivity The results of responsivity measurements performed at a temperature of 80 K on a 80 lm · 500 lm photoconductor as a function of wavelength are shown in Fig. 6. The device structure is that of Fig. 3(a) without the Ge/SiO mirror,
Fig. 8. Peak responsivity of a 50 lm · 50 lm photoconductor with single mirror (after Fig. 3(a), without the Ge/SiO mirror) as a function of temperature at a bias of Eb 36 V cm1. Modelled peak responsivity is also shown. Model parameters for the absorber layer are d = 75 nm, S1,2 = 200 cm s1, x = 0.3, ND = 1 · 1015 cm3, Eb = 36 V cm1. Extracted lifetime and model lifetime are also plotted.
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and the device is frontside illuminated at this point. Also shown in Fig. 6 is the modelled performance of a 80 lm · 500 lm photoconductor modelled for an absorber layer with d = 75 nm, surface recombination velocity of the front and back surfaces S1,2 = 600 cm s1, composition x = 0.3, and doping density ND = 3 · 1014 cm3. Layer thicknesses used for the modelled device were taken from scanning electron microscopy measurements of the fabricated structure. The general shape of both the measured and modelled data is in good agreement, and clearly shows a resonant peak at a wavelength of approximately 2.75 lm. Beyond 3.5 lm wavelength, the reflectivity of the Hg(1x)Cd(x)Te/CdTe mirror decreases, and the cavity will not reject signal for these wavelengths. It can be seen that the model does not predict the broadening that is clearly visible in the measured data, particularly broadening of the 2.75 lm resonant peak. This has been investigated and is due to broad spectral width from the monochromator used to take these measurements. Measurements taken
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using a different system with narrower spectral bandwidth (in Section 4.3) show a narrower peak. Note also the difference between the mirror cut-off of the measured data compared to the modelled curve, at approximately 3.5 lm wavelength. This is most likely due to the refractive index of the CdTe layers differing from that of the model. As the CdTe is deposited at non-ideal temperatures, it is likely that it does not behave as modelled. This also requires further investigation. It is important to note that the surface recombination velocity used to fit the curve is reasonable for MBE material. High quality MOCVD and MBE grown HgCdTe material can reach surface recombination velocities as low as S = 50 cm s1 [17]. 4.2. Temperature dependence of RCE devices By varying measurement temperature, further material properties can be investigated. Fig. 7 illustrates the normalized responsivity of a 80 lm · 500 lm photoconductor at
Fig. 9. Spatial photoresponse of a photoconductor (a) at 300 K, and (b) at 80 K.
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temperatures of 80 K and 250 K. These are compared with the modeled normalized responsivity for a 10 lm thick detector at 80 K and 250 K. There is very strong agreement in the shift in cut-off due to the changing band-gap of the Hg(1x)Cd(x)Te with temperature. This is apparent as the signal in the mirror roll-off region (3.5–5 lm) experiences a shifting cut-off. The cut-off at 250 K clearly drops to a noise floor after 4.4 lm. The cut-off of the 80 K is more difficult to determine, as a traditional half-peak responsivity [2] analysis cannot be used, and the signal does not suddenly cut out as in the 250 K case. The cut-off in the 80 K case will be in the vicinity of the model though, as the model for energy gap at low temperature is well defined, and the model fit in Fig. 6 at cut-off shows good agreement. The peak responsivity shifts in wavelength due to the change in refractive index with temperature. The effect of the change in refractive index is small compared to the change in energy gap, especially as the mirror reflectivity in the 3.5–5 lm range is low. Peak responsivity of a 50 lm · 50 lm RCE photoconductor is also measured for varying temperatures for a fixed bias field of Eb 36 V cm1, with the results shown in Fig. 8. Above 200 K, the peak responsivity is highly dependant on the intrinsic carrier concentration. Below 200 K, the model responsivity is independent of temperature as the lifetime is dominated by surface recombination. The measured and modelled responsivity show good agreement at all temperatures. Fig. 8 also shows the steady state effective lifetime extracted from the measured responsivity. Again there is reasonable agreement between the modelled lifetime and the extracted lifetime. The extracted lifetime of seff 14 ns at 80 K is very low compared to bulk n-type material, which for high quality Hg(1x)Cd(x)Te would be on the order of microseconds. This indicates that surface recombination is the dominant mechanism. Furthermore, surface recombination is modelled as being independent of temperature. Surface recombination is most likely the dominant recombination mechanism and is limiting the minority carrier lifetime. However, results of Hall measurements to determine the doping density using Van der Pauw [18] structures fabricated with the photoconductors were inconclusive (hence the fitted doping density of ND = 3.4– 10 · 1014 cm3). During measurements non-symmetric Hall voltages were observed. This indicates contact issues, or non-uniformities in the material. Spatial photoresponse measurements (shown in Fig. 9) were taken to try to determine the cause of the non-symmetries in the hall voltage. The strong signal about the contact in Fig. 9(b) suggests some issues with the doping at these areas, as the indium could be creating n+–n junctions, or alternately the annealing process failed to fully convert the x = 0.3 layer and the indium is acting to create a compensated region near the contact, and a p–n junction around the contact. This effect is not apparent in Fig. 9(a), as at room temperature the material is intrinsic. These issues with doping could be
reducing the lifetime of the material, however, given that the extracted effective lifetime is 14 ns, the most likely mechanism dominating the lifetime is surface recombination. The lifetime for compensated n-type material would be on the order of many tens to hundreds of nanoseconds, much longer than the surface recombination dominated lifetime [19]. 4.3. Performance of complete RCE structure Following initial characterisation of the device the Ge/ SiO mirror was added. The structure of the completed device is shown in Fig. 3(a), and the device was backside illuminated. Responsivity as a function of wavelength was remeasured and Fig. 10 shows the results of a 50 lm · 50 lm photoconductor measured at a temperature of 80 K, and a bias field of Eb = 50 V cm1 before and after the deposition of the Ge/SiO mirror. The same device was used for both measurements before and after the addition of the mirror. Both data sets are normalised to the responsivity at a wavelength of 2.6 lm. Below 2.5 lm there is a marked difference between the two curves. This is due to the fact that as the device is backside illuminated after deposition of the Ge/SiO mirror, incident radiation must pass through mirror 1 before entering the cavity. As the x = 0.4 material of mirror 1 is absorbing at these wavelengths, it is preventing incident light from reaching the x = 0.3 absorber layer or the top most x = 0.4 layer of mirror 1. The measured result is confirmed with modelling of the device before and after deposition of mirror 2, showing a decrease in absorptance in this region (Fig. 11). Fig. 11 also illustrates that there is no sharp resonance for wavelengths shorter than 2.8 lm due to absorption in the top x = 0.4 layer of mirror 1. This absorption is due to poor control of the layer thicknesses during MBE growth of the mirror structure resulting in the resonant wavelength being close to the x = 0.4 material cutoff wavelength. The
Fig. 10. Normalised responsivity of a 50 lm · 50 lm photoconductor at 80 K before and after the addition of the Ge/SiO mirror. Measured at a bias of Eb 50 V cm1.
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Fig. 12. Normalised responsivity of a 50 lm · 50 lm photoconductor measured with varying temperature. Measured at a bias of Eb 50 V cm1. Fig. 11. Model absorptance of the x = 0.3 absorber layer and the x = 0.4 top layer mirror 1 before and after the addition of the Ge/SiO mirror.
designed resonance was for higher wavelength, which would have produced a discrete resonant peak. For wavelengths longer than 3.6 lm, results both before and after deposition of mirror 2 agree with approximately one pass through a 75 nm absorber layer. This is logical, as mirror 1 is not highly reflective at these wavelengths. Therefore, the cavity will not reject these wavelengths or resonate. There is a resonant peak at 3.4 lm and after the Ge/SiO mirror is added, this peak becomes more well defined. However, model absorptance (Fig. 10) suggests that the peak should become more well defined and stronger. After the Ge/SiO mirror is added, wavelengths above 3.6 lm should still have some modulation due to the cavity, which does not appear in measured results. Also the model suggests a shift towards longer wavelengths of these features. The discrepancy between measured data and modelled data is possibly due to differences in the refractive indexes of the Hg(0.6)Cd(0.4)Te/CdTe and CdTe mirror layers. In particular, dispersion in the refractive index of the Hg(0.6)Cd(0.4)Te material may not be as modelled, causing wavelength shifts. More investigation is needed into the refractive index of both the Hg(0.6)Cd(0.4)Te material and the CdTe material used in mirror 1. The spectral responsivity of a 80 lm · 500 lm photoconductor with the Ge/SiO mirror added was measured under a bias field of Eb = 50 V cm1 for various operating temperatures and is shown in Fig. 12. As temperature increases the resonance at 3.4 lm decreases, in agreement with modelled results (Fig. 13). However, at a temperature of 240 K there is resonance at 3.2 lm which does not agree with modelled results. This is also possibly due to dispersion in the Hg(0.6)Cd(0.4)Te material, but further investigation is needed. Finally, the variation of the responsivity peak at approximately 2.8 lm with temperature is not
Fig. 13. Model absorptance of the x = 0.3 absorber layer with varying temperature.
monotonically shifting to shorter wavelengths, suggesting that this signal is due to resonance in the cavity being absorbed by the Hg(0.7)Cd(0.3)Te absorber layer. 5. Conclusions Resonant-cavity-enhanced Hg(1x)Cd(x)Te devices have been fabricated and demonstrated. Optical performance agrees with modelled performance, exhibiting resonant behaviour in the MWIR window. However, there are differences between modelled and measured data which are most likely due to differences between modelled and actual refractive index of the Hg(0.6)Cd(0.4)Te material and CdTe material in HgCdTe/CdTe stack. Absorber layer cut-off and peak responsivity vary with temperature as modelled. Extracted effective lifetime also agrees with model data, although extracted effective lifetime is very low at 14 ns. Both responsivity and extracted effective lifetime show that these devices are limited by surface recombination. Investi-
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gation into improving the interfaces of the MBE grown absorber layer in order to reduce surface recombination is required. Acknowledgements The authors wish to thank the Australian Research Council for financial support of this work. References [1] Lawson WD, Nielson S, Putley EH, Young AS. Preparation and properties of HgTe and mixed crystals of HgTe–CdTe. J Phys Chem Solids 1959;9:325–9. [2] Hansen GL, Schmit JL, Casselman T. Energy gap versus alloy composition and temperature in Hg(1x)Cd(x)Te. J Appl Phys 1982;53(10):7099–101. [3] Chin A, Chang TY. Multilayer reflectors by molecular-beam epitaxy for resonance enhanced absorption in thin high-speed detectors. J Vac Sci Technol B 1990;8(2):339–42. [4] Unlu MS, Kishino K, Liaw HJ, Morkoc H. A theoretical study of resonant cavity-enhanced photodetectors with Ge and Si active regions. J Appl Phys 1992;71(8):4049–58. [5] Pautrat JL, Hadji E, Bleuse J, Magnea N. Resonant-cavity infrared optoelectronic devices. J Electron Mater 1997;26(6):667–72. [6] Arnold M, Zimin D, Alchalabi K, Zogg H. Lead salt mid-IR photodetectors with narrow linewidth. J Cryst Growth 2005;278(1–4): 739–42. [7] Sioma M, Piotrowski J. Modelling and optimisation of hightemperature detectors of long wavelength infrared radiation with optically resonant cavity. Opto-Electron Rev 2004;12(1):157–60.
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