λ ≈ 3 µm InAs resonant-cavity-enhanced photodetector

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2003 Semicond. Sci. Technol. 18 964

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INSTITUTE OF PHYSICS PUBLISHING SEMICONDUCTOR SCIENCE AND TECHNOLOGY

Semicond. Sci. Technol. 18 (2003) 964–967 PII: S0268-1242(03)63628-X

λ ≈ 3 µm InAs resonant-cavity-enhancedphotodetectorAlex M Green, David G Gevaux, Christine Roberts,Paul N Stavrinou and Chris C Phillips

Experimental Solid State Group, Department of Physics, Imperial College London,London SW7 2BW, UK

Received 15 May 2003, in final form 10 July 2003Published 11 August 2003Online at stacks.iop.org/SST/18/964

AbstractThis paper presents the design, fabrication and optoelectroniccharacterization of an InAs resonant-cavity-enhanced photodetector,intended for the detection of methane gas, incorporating a 10 periodGaAs/AlAs distributed-Bragg reflector. The spectrally narrowedresponsivity was resonantly enhanced to a value of 34.7 A W−1, at an appliedvoltage of 3.0 V. This high resonantly enhanced peak in the responsivity wasat λ = 3.14 µm, which is only 5% away from the target value.

1. Introduction

Spectroscopic gas sensing is an important application foremitters and detectors in the λ = 2–12 µm range.Photodetectors offer higher signal-to-noise ratios and fasterresponse times than thermal detectors [1]. The faster responsetimes allow real-time measurement of gas concentration,whilst using lock-in detection [2].

For gas sensing, the emitter or detector, or both, are tunedto an absorption feature of the gas of interest. This criterionis commonly satisfied by using a broadband source such asa globar and a filtered broadband detector. The filter’s passband is tuned to encompass a portion of the absorption featureof the target gas. Fabry–Perot (FP) filters can be used forthis task; however, a considerable advantage is gained byplacing the detector within the FP filter, providing that the FPcavity length is comparable to the wavelength of interest. Thepassed wavelengths are absorbed more strongly than in a non-resonant device because they are resonantly enhanced by thestanding wave that is formed in the FP cavity. This is knownas resonant-cavity enhancement. The enhancement of thequantum efficiency over non-resonant detectors is analogousto the enhancement of spontaneous emission and extractionefficiency in resonant-cavity light-emitting diodes (RCLED)[3]. The resonant optical modes, in both resonant-cavity-enhanced (RCE) detectors and RCLEDs, are coupled muchmore efficiently to the optical modes outside the device.Compared to non-resonant light-emitting diodes, extractionefficiency enhancement factors of over 10 have been recorded[4]. These enhancements are particularly attractive for themid-infrared (MIR), where the alternative approaches (e.g.,

index matching or immersion lenses) for improving externalcoupling efficiency are difficult and expensive.

RCE detectors are relatively temperature insensitive andmay be highly gas selective, compared to non-resonantdetectors. The temperature stability comes from the fact thatthe wavelength of the resonance is only a function of the opticallength of the FP cavity, and the potential gas selectivity arisesfrom the resonantly narrowed spectrum.

In this paper we describe a bulk InAs absorbing layerRCE photodetector aimed at the detection of methane (CH4)gas [5] at λ = 3.3 µm. The high peak in the responsivity(34.7 A W−1) was at λ = 3.14 µm, in this non-optimizeddevice, which was encouragingly close (5%) to being suitablefor the detection of methane gas. The design considerations,fabrication techniques and optoelectronic characteristics of thedevice are detailed herein.

2. Device design and fabrication

Figure 1 shows a schematic of the layer structure of the device.The band gap of InAs at room temperature approximatelycorresponds to λ = 3.6 µm, so it is a suitable choice formethane gas sensing at λ = 3.3 µm. It was thought that the useof a bulk InAs absorbing layer was an appropriate structuralsimplification since bulk InAs p-i-n photodiodes with highquantum efficiencies are commercially available.

GaAs was the uppermost layer of the distributed-Braggreflector (DBR), which meant the high and low refractive indexlayers were in the opposite order to the way in which DBRsare normally grown. This was to protect the AlAs layer from

0268-1242/03/110964+04$30.00 © 2003 IOP Publishing Ltd Printed in the UK 964

λ ≈ 3 µm InAs resonant-cavity-enhanced photodetector

p+ InAs, λλλλ /4, t = 0.243 µµµµm

i InAs, λλλλ, t = 0.970 µµµµm

n+ GaAs, λλλλ /4, t = 0.249 µµµµm

n+ AlAs, λλλλ/4, t = 0.289 µµµµm

n+ GaAs substrate

3333λλλλ /2 cavity,t = 1.462 µµµµm

10 periodDBR

Figure 1. Schematic layer structure of the InAs resonant-cavity-enhanced photodetector.

oxidization, in the event of a long delay between growing theDBR and the InAs absorbing layer.

Because of the opposite order of the DBR, the final GaAslayer is considered part of the cavity. So, with respect tothe resonant condition L = mλ/2ncav, where L is the cavitylength, λ is the resonant wavelength and ncav is the cavityrefractive index; the cavity order (which must have an integervalue), m, is 3. From transfer matrix modelling, reflectionsfrom the InAs–GaAs interface were found not to degrade theperformance significantly.

Based on the high-quantum efficiencies achieved by otherworkers (e.g. [6]), the native air–semiconductor interface wasused as the front mirror. This made quite a low finesse cavitybut the advantage is that it made device design and fabricationsimpler.

Given that the front mirror reflectance, Rf , was fixed at≈0.3, one can see from the equation of Unlu et al for the peakquantum efficiency of a RCE photodiode [7],

η = 1 + Rb e−αd

1 − 2√

Rf Rb + Rf Rb e−2αd(1 − Rf )(1 − e−αd),

that the closer the reflectance of the back mirror, Rb, is to unitythen the higher the quantum efficiency, η. α and d are theactive layer absorption coefficient and thickness, respectively.Rb approaches unity for a GaAs/AlAs DBR, at the wavelengthof interest, at about 22 periods. However, the growth time fora 22 period DBR was too long. The 10 period DBR thatwas used had a peak reflectance of ≈0.8, incident from InAs,centred on λ = 3.3 µm.

The growth rates (GR) for GaAs (1 µm h−1) and AlAs(0.7 µm h−1) were calibrated by modelling the FP oscillationsin reflectance curves taken from samples in the λ = 1–1.7 µmwavelength range [8]. Samples for measuring the GRs ofthe two materials consisted of a layer of AlAs grown on aGaAs substrate and then a layer of GaAs. When measuringthe GR for GaAs, the thicknesses were GaAs ≈1 µm andAlAs ≈ 25 nm; the thicknesses are exchanged to measure theAlAs GR. The modelling is not sensitive to the thickness of thethinner layer but is very sensitive to the thickness of the thickerlayer; consequently the accuracy of thickness measurement is±0.5%. The near-infrared refractive index data used in themodel were taken from [9]. An 8 period DBR was grown to

-1 0 1 2 3

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-0.03

-0.04

0.00

0.01

2.0 2.5 3.0 3.50

1

2

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2.25 V

2.50 V2.75 V

3.00 VTemperature 205K

Ph

oto

curr

ent

( A

/ m

)µµ µµ

µµ µµ

Wavelength ( m)µµµµ

Cu

rren

t (A

)

Applied Voltage (V)

Figure 2. Photocurrent spectra from the RCE photodetector at205 K and various applied voltages. The T = 220 K current–voltagecurve is inset.

verify that the growth rates for GaAs and AlAs were correct,and accordingly that the DBR peak was centred correctly.

The InAs GR (1 µm h−1) was calibrated usinghomoepitaxy reflection high-energy electron diffraction(RHEED) oscillations so the layer thickness accuracy was±15%. The DBR was grown at 580 ◦C and nominally silicondoped to 5 × 1018 cm−3. The substrate was a 2 inch diametern doped (2 × 1018 cm−3) GaAs wafer, that was polished onboth sides.

The first 50 nm of InAs was grown at a lower temperature(T ≈ 370 ◦C) as a ‘dislocation layer’ [10] to improve theInAs material quality in the absorbing layer. The dislocationlayer was then annealed at T ≈ 490 ◦C for 10 min toencourage complete relaxation towards the unstrained InAslattice constant.

In the p-InAs layer, the beryllium dopant concentrationwas nominally 5 × 1018 cm−3. The wafer was processed into1 mm diameter mesa devices, with top CrAu ring contacts andwide area NiGeAu back contacts, using standard wet chemistryetching and photolithography.

3. Results and discussion

Spectra were taken on a Fourier transform spectrometer.A globar source, which approximates a blackbody at atemperature of 1000 K, was modulated with a mechanicalchopper (≈4 kHz) for all experiments. An InSb calibrateddetector was used for the reflectance and responsivityexperiments. A transimpedance preamplifier measured thephotocurrent from the sample and a source-measure unitsupplied a constant applied voltage.

Figure 2 shows photocurrent spectra from the RCEphotodetector at various applied voltages; all were taken at205 K, the temperature of the highest resonant response (below150 K no resonant peak was visible). One can see that thewavelength of the peak response at 205 K is 3.14 µm, whichis within 5% of the 3.3 µm methane absorption feature.

The strong dependence of photocurrent on applied voltageand the fact that there was no detectable photocurrent atzero applied voltage suggests that the device behaved more

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Figure 3. Photocurrent spectra from the RCE photodetector at 3.0 Vapplied voltage and various temperatures.

as a photoconductor rather than, as was originally intended,a photodiode. Evidence suggests that the n-doping in theInAs adjacent to the GaAs was too low and consequentlythe transport at the InAs–GaAs interface dominated overthe p-i-n junction. This made the device behave in ahybrid photoconductive–photovoltaic manner. The non-idealcurrent–voltage characteristics at the temperature of 220 Kcan be seen in the inset of figure 2. It should be noted that thespectra in figure 2 were taken at positive applied voltages withrespect to the current–voltage curve in the inset.

There is a very good correspondence between the fullwidth at half maximum (FWHM) of the resonant peak(0.15 µm) in figure 2, and the FWHM of the envelope ofthe λ = 3.3 µm absorption feature of methane gas (0.16 µm).The absorption centred at about λ = 2.6 µm in figure 2 is anexperimental artefact due to water vapour in the laboratory air.

Figure 3 shows photocurrent spectra from the RCEphotodetector at various temperatures. All spectra were takenat the applied voltage of 3.0 V. Aside from the gas specificityand increased sensitivity at the resonant wavelength, a majoradvantage of the RCE devices is their relative insensitivityto change in temperature compared to non-resonant devices.In non-resonant devices, the wavelength of peak response ispredominantly controlled by the band gap of the constituentmaterial; however, in RCE devices the peak wavelength iscontrolled by the optical path length of the cavity. Thetemperature dependence of the refractive index and the thermalexpansion/contraction of the cavity material tends to make aresonant peak shift less than the band edge, with changingtemperature. Figure 4 displays a comparison between thetemperature shift of the band edge of InAs and the resonantpeak. The band edge moves 0.26 µm, whereas the resonantpeak only moves 0.03 µm, in the 150–265 K temperaturerange. The RCE device is ≈9 times more thermally stablethan a non-resonant device.

It is clear that the amount of photocurrent generated bya RCE photodetector will depend on the properties of theabsorbing cavity material. As can be seen in figures 3 and 4, thepeak photocurrent rises non-monotonically with decreasingtemperature; it goes through a maximum and a minimum at205 K and 150 K, respectively. Very similar behaviour hasbeen seen in the internal efficiency of the InAs/InAsSb mid-infrared LEDs, [11, 12]. The effect was tentatively assigned to

Figure 4. Temperature dependence of the peak photocurrent,resonant peak wavelength and InAs band edge wavelength.

Figure 5. The highest recorded current responsivity curve for theRCE photodetector. It was taken at 205 K and 3.0 V applied voltage.

the changing Auger recombination rates, and therefore overallcarrier lifetime, as the band gap moves in and out of resonancewith the split-off band, with changing temperature.

Measurement of the number of electrons flowing in thecircuit per incident photon, 13.7 at peak, suggested thatphotoconductive gain was taking place. However, it was notpossible to separate the quantum efficiency from this quantum-efficiency-gain product because it was not known at whatvoltage the gain could be assumed to be unity. Therefore,the current responsivity was calculated and is displayed infigure 5 at the temperature of peak response (T = 205 K).The very large resonant peak responsivity was 34.7 A W−1 atλ = 3.14 µm.

Figure 6 shows room temperature measured (solid line)and modelled (broken line) normal incidence reflectancespectra of the RCE photodetector. The modelled spectrumof the front reflectance was done using a fully dispersivetransfer matrix approach [13], as was the FP GR calibration.The InAs refractive index and extinction coefficient data weretaken from [9]. Mid-infrared refractive index data for GaAsand AlAs were calculated using a single effective oscillator(SEO) model detailed in [14].

A reflectance spectrum from the back of the device(figure 6) allowed verification that the DBR was correctly

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λ ≈ 3 µm InAs resonant-cavity-enhanced photodetector

Figure 6. Measured (solid line) and calculated (broken line) normalincidence reflectance spectra of the RCE photodetector. The backspectrum was taken from the substrate side. The cavity resonanceappears at 3.13 µm.

centred on λ = 3.3 µm. This was possible because thedouble side polished substrate was indium-free mountedduring growth so the back surface remained shiny.

In the spectrum taken from the front of the device (figure 6)one can see the dip in reflectance, which corresponds tothe cavity resonance, occurs at λ = 3.13 µm. Thisslight difference (0.3%) between the resonance wavelengthdetermined by the reflectance measurement and by thephotocurrent measurement is due to slight layer thicknessvariations over the extent of the wafer. The sampleprocessed into devices and the sample used for the reflectancemeasurement came from different parts of the wafer. One canalso see that there is a reasonably good match between theoryand experiment. The vertical offset between the two curvestowards the right of the front spectrum in figure 6 is due tothe reflectance of the back surface of the substrate. This offsetwould be eliminated by roughening the back surface.

The parameters that can be extracted from the modelare an InAs layer thickness of 1.045 µm and a DBRcentre wavelength within 3% of the wavelength of interest.The 14% too short InAs layer caused the shift in the resonancewavelength away from the desired value. It may be possible toimprove upon the GR calibration accuracy for InAs, attainedusing RHEED oscillations, by using a method similar to the FPmethod used for GaAs and AlAs; except at longer wavelength.

4. Concluding remarks

We have designed, fabricated and carried out experiments onan InAs RCE photodetector. To our knowledge it had a longeroperating wavelength than any III–V RCE photodetectorreported in the literature. RCE photodetectors of this sortare attractive for gas sensing because of the ability to ‘pre-tune’ the enhanced sensitivity to a particular gas absorptionfeature. This will increase the potential signal-to-noise ratio(SNR) of a gas detection system, over that which would beattainable with a non-resonant photodetector.

The large lattice mismatch (≈7%) between InAs andGaAs may give rise to many dislocations threading into theabsorbing layer and acting as Shockley–Read–Hall (SRH)recombination centres. Nevertheless, GaAs and AlAs werechosen for the DBR for two reasons. Firstly, it is a relativelymature system, as opposed to, say GaSb and AlSb. Secondly,there is evidence that defects in InAs form states deep inthe conduction band so that they do not act as efficient SRHcentres [15]. In corroboration of this a high peak responsivity(34.7 A W−1) has been achieved.

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