IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 20, NO. 6, NOVEMBER/DECEMBER 2014 3805106

Isolated Electron Injection Detectors With HighGain and Record Low Dark Current

at Telecom WavelengthVala Fathipour, Omer Gokalp Memis, Member, IEEE, Sung Jun Jang, Robert L. Brown,

Iman Hassani Nia, Member, IEEE, and Hooman Mohseni, Senior Member, IEEE, Fellow, OSA

Abstract—We report on recent performance breakthroughs ina novel short-wave infrared linear-mode electron-injection-baseddetector. Detectors consist of InP material system with a type-IIband alignment and provide high internal avalanche-free amplifi-cation mechanism. Measurements on devices with 10 μm injectordiameter and 30 μm absorber diameter show internal dark currentdensity of about 0.1 nA/cm2 at 160 K. Compared with our previousreported results, dark current is reduced by two orders of magni-tude with no sign of surface leakage limitation down to the lowestmeasured temperature. Compared with the best-reported linear-mode avalanche photodetector, which is based on HgCdTe, theelectron-injection detector shows over three orders of magnitudelower internal dark current density at all measured temperatures.Using a detailed simulation with experimentally measured param-eters, dark count rate of 1 Hz at 90% photon detection efficiency at210 K is anticipated. This is a significantly higher operating tem-perature compared with superconducting detectors with a similarperformance.

Index Terms—Infrared detectors, infrared imaging, infrared im-age sensors.

I. INTRODUCTION

PHOTON number resolving (PNR) detectors have recentlyseen an upsurge in their demand due to an explosive growth

of interest in new scientific fields of research, such as quantuminformation science [1]. In particular, there are a vast numberof applications for photon counting imagers in the short-waveinfrared (SWIR) band spanning from 1 to 2.5 μm in the electro-magnetic spectrum. An example is the detection of few photonsover long integration times in astronomical applications whereextremely low dark current and sensitive detector arrays are verydesirable [2].

The strongest candidates for SWIR PNR are the transitionedge sensor (TES) superconducting detectors and the semicon-

Manuscript received January 28, 2014; revised June 28, 2014, August 15,2014, and September 7, 2014; accepted September 9, 2014. This work wassupported in part by Defense Advanced Research Projects Agency# W911NF-13-1-0485, and ARO award # W911NF-12-1-0324.

V. Fathipour, S. J. Jang, R. L. Brown, I. Hassani Nia, and H. Mohseniare with the Department of Electrical Engineering and Computer Science, North-western University, Evanston, IL 60208 USA (e-mail: valafathipour2016@u.northwestern.edu; sung.jang@northwestern.edu; RobertBrown2011@u.northwestern.edu; imanhassani2014@u.northwestern.edu; hmohseni@northwestern.edu).

O. G. Memis is with Intel, Hillsboro OR 97124 USA (e-mail: gokalpm@gmail.com).

Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/JSTQE.2014.2358077

ductor HgCdTe eAPDs [1], [3], [4]. Despite dramatic develop-ment in the operation of such detectors, there exists inherentmaterial properties that limits their utilization for applicationswith extreme demands.

TES detectors have excellent PNR capabilities with the low-est dark count rates (DCRs) ever reported. The extremely lowoperating temperatures of less than 100 mK however, makesthem expensive and prevents their practical utilization in manyapplications [3]. Also, realization of large area 2-D imagingarrays is extremely challenging in this technology.

HgCdTe ternary alloy (MCT) provides a nearly ideal infraredmaterial system with a tunable bandgap that covers SWIR tovery long wave infrared (VLWIR). Best reported linear-modeMCT eAPDs with PNR capability; operate at SWIR (λc =3 μm) [4], [5]. The avalanche process however, exerts sig-nificant limitations on the pixel size and the fill factor of thetwo-dimension APD imaging arrays.

A novel linear-mode detector with large internal gain basedon an electron injection process and operating around the tele-com wavelength was demonstrated in 2007 [6]–[10]. Electron-injection detectors present a new single photon detection methodwith high sensitivity at low operating bias voltages (for exam-ple, G � 2000 at −1.5 V) [8]. More importantly, they showa sub-Poissonian shot noise performance [9], which leads to anoiseless amplification of signal. Low-gain devices (G � 20)showed jitter values as low as 15 ps at room temperature [10].

In previous works, electron-injection detectors were not phys-ically isolated from each other. This results in a large dark cur-rent for each device (6 μA at −1.5 V bias). The reduction in thedetector dark current was reported as a function of reducing theinjector diameter from 15 to 1 μm [7].

The majority of the noise in our detector originates from theinternal dark current and thus it must be minimized. Our pre-liminary theoretical studies, intra-processing measurements, aswell as simulation results suggest that isolation of the individualdetector active regions should yield significant improvements inthe device characteristics including dark current [11], [12]. Werecently developed a process to physically isolate individualdetectors with a limited surface damage [11], [12].

In this paper, we report on the characteristics of isolated de-tectors with 10 μm injector and 30 μm absorber diameters.The detector operating principles and fabrication steps are pre-sented, and the important characteristics are compared with ourprevious reported results [8], [11]. Furthermore, we show thatthe detector internal dark current density is three orders of

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3805106 IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 20, NO. 6, NOVEMBER/DECEMBER 2014

Fig. 1. (a). Schematic cross section and (b) SEM image of an isolated electron-injection detector with 10 μm diameter injector and 30 μm diameter absorber(c) The energy band diagram as a function of depth along the central axis of thedevice (no illumination, bias voltage = −2.5 V).

magnitude lower than the best competing technology (SWIRMCT eAPD). Finally, the simulated results for the detectordark count rate at a given photon detection efficiency (PDE)is provided and compared with that of MCT eAPDs at differenttemperatures.

II. ELECTRON-INJECTION DETECTOR OPERATION

The schematic cross section as well as scanning electron mi-croscope (SEM) image of the new isolated detector design with10 μm injector and 30 μm absorber diameter are shown inFig. 1(a) and (b), respectively. Detectors are based on InP mate-rial system with a type-II band alignment [13]–[15]. The layerstructure is composed of 1000 nm n− doped (<1015 cm−3)In0.53Ga0.47As absorber, 50 nm p+ doped (5 × 1018 cm−3)GaAs0.52Sb0.48 hole trap, 50 nm undoped In0.52Al0.48As etchstop, 500 nm n+ doped (1017 cm−3) InP injector, and 50 nmn+ doped (1019 cm−3) In0.53Ga0.47As cap layer. The epitaxiallayers are grown with metal organic chemical vapor deposition(MOCVD) on 2-inch InP substrates.

The energy band diagram as a function of depth along the cen-tral axis of the device (no illumination, bias voltage = −2.5 V)is shown in Fig. 1(c). Negative bias results in thermionic emis-sion of electrons from the highly doped InP injector over theconduction band barrier to InGaAs absorber. For more nega-tive bias voltages than � −0.5 V, thermionic emission currentbecomes limited by the saturation current of the reverse biasedn-InGaAs/p+GaAsSb junction and the detector dark current, inthe absence of surface effects, follows a sub-linear relation withbias voltage. This is due to thermal generation of electron-holepairs within the depletion region.

Photon absorption results in generation of an electron-holepair in the InGaAs absorber. Under negative bias, the electronand the hole are separated and the hole gets trapped in theGaAsSb trap layer for the period of its lifetime. This leads toa change of barrier potential, and results in a large electron in-jection, and hence internal amplification in the device [6]–[8].As electrons pass over the barrier, they lower the local poten-tial and increase the barrier height, opposing the flow of moreelectrons. This negative feedback mechanism results in a stablelow noise internal amplification, and a measured Fano effect[9], [10].

III. DETECTOR DESIGN AND FABRICATION

Our intra-processing measurement results as well as three-dimensional simulations performed in SILVACO-ATLAS, showthat significant improvements in the device characteristics in-cluding substantial reduction of dark current can be achieved byphysically separating each detector active area from neighboringdevices [11], [21], [22], [23]. A systematic study of the effectof isolation and the three-dimensional geometry of device on itsdark current, gain and rise time will be provided elsewhere [12].Isolation results in charge localization dynamics to be accu-rately controlled by the predefined active area of the absorptionlayer for each detector. To account for the large surface exposureof each isolated pixel, our processing was optimized to reducesurface leakage current.

Application of a cleaning method to remove metallo-organics,minimization of contact resistance through removal of surfaceoxides, utilization of an organic free all dielectric lift-off pro-cess, improvement of the detector surface quality by improvingthe dry etching conditions, and achievement of very narrowdeep vertical dry etching conditions for detector isolation areexamples of such improvements in the processing.

Isolated electron-injection detectors are fabricated by pat-terning the wafers with e-beam lithography to form the contactmetals. Conventional metallization with an E-beam evaporatoris used to lift off multi-layer metal contacts, which act as hardmask for reactive ion etching with CH4/H2 chemistry to formthe injector pillars. Wet etching of InAlAs and GaAsSb followedby a CH4/H2 dry etching of InGaAs is then used to define theabsorber volume. The backside of the sample is then polishedto allow for backside illumination measurement.

IV. MEASUREMENT RESULTS

A. Characteristics of the Isolated 10 μm Injector and 30 μmAbsorber Diameter Electron-Injection Detector

The simulated and measured dark current characteristic foran isolated detector with 10 μm injector diameter and 30 μmabsorber diameter is shown in Fig. 2(a). Our simulation modelassumes a uniform doping profile without including the effectof any surface and interface defects and qualitatively agreeswith the measurement. In this simulation, data provided by [20]is used for the conduction band offset of GaAsSb and InAlAslayers. The smaller turn on voltage observed in the measureddata as compared with the simulation is possibly due to thesmaller actual energy barriers.

The sub-linear dependence of dark current on bias voltage isan advantage for the electron-injection detector when comparedwith eAPDs, in which the dark current raises exponentially withbias voltage [4]. As a result, when utilized in large format FPAs,the electron-injection detector gain does not vary much by thevoltage and process variations across the FPA.

Responsivity was extracted using dark and photocurrent mea-surements. For photocurrent measurements a calibrated contin-ues wave (CW) laser source with peak emission wavelength at1550 nm was used. The light was focused to a �10 μm spot size

FATHIPOUR et al.: ISOLATED ELECTRON INJECTION DETECTORS WITH HIGH GAIN AND RECORD LOW DARK CURRENT 3805106

Fig. 2. (a) Left axis: Measured dark current of an isolated electron-injectiondetector with 10 μm injector, 30 μm absorber diameter at room temperatureshows qualitative agreement with simulation result. Right axis: Measured opticalgain of the isolated electron-injection detector as a function of bias voltageshowing a stable gain of �2000 beyond bias voltage of �−1 V. (b) Logarithmicplot of dark current versus bias voltage at different temperatures.

that illuminated the detector from the backside. Laser powerwas calibrated using a calibrated PIN detector.

Similar to an APD, responsivity in an electron-injection de-tector is the composite of the internal quantum efficiency (ηint)and the internal gain (M) and it can be difficult to separate thetwo by a measurement technique. In this paper, the conserva-tive approach of equating the internal quantum efficiency to100% and thus underestimating the internal gain was taken. Themeasured optical gain (Gopt = M ∗ηint) as a function of biasvoltage is plotted on the right axis of Fig. 2(a). Optical gain in-creases with bias at low negative bias voltages and beyond biasvoltage of � −1 V a stable optical gain of �2000 is achieved.This corresponds to a responsivity of 1575. External quantumefficiency (ηext) was estimated as 78% (calculated from the un-coated surface reflectivity, the thickness of the absorbing layerand its absorption coefficient).

Fig. 2(b) is a logarithmic plot of dark current versus biasvoltage for temperatures ranging from 300 K to 160 K. For tem-peratures below 160 K, the leakage current of our measurementsetup dominates the measured current.

Fig. 3 compares the dark current as a function of temperaturefor the isolated detector at bias of −3 V with the previously re-ported un-isolated result at bias of −1.5 V [8]. The new isolateddesign provides more than two orders of magnitudes reductionof the dark current. No sign of surface leakage current, whichwould result in a significant change in the slope of the plot, wasobserved down to the instrument limited temperature of 160 K[8].

Detector rise and fall times were measured using a pulsedlaser. Laser had peak emission wavelength at 1550 nm and �3 ns

Fig. 3. Arrhenius comparison of the dark current of an isolated electron-injection detector with 10 μm injector, 30 μm absorber diameter (at −3 Vbias) with our previously reported result of un-isolated devices (at −1.5 V bias)[8]. More than 2 orders of magnitude reduction in dark current is achieved.Furthermore, no sign of temperature independent dark current was observedbelow 175 K, down to 160 K.

rise and fall times. Measured rise and fall times of the isolatedelectron-injection detector were 5 ns and 70 ns respectivelyat 40 μW optical power. The isolated detector bandwidth ismore than four orders of magnitude higher compared with theprevious results [9]. Origin of bandwidth improvement in theisolated 10 μm injector and 30 μm absorber device is discussedelsewhere [12].

B. Comparison of Dark Current Density and Optical Gainof the Isolated 10 μm Injector and 30 μm Absorber DiameterElectron-Injection Detector With the State-of-the-ArtSemiconductor Technology

As stated earlier, HgCdTe is the most important semiconduc-tor alloy system for IR detectors in the spectral range between1 and 2.5 μm [16]. To minimize the thermally generated darkcurrent at a constant temperature in MCT eAPDs, the energybandgap should be increased [4], [16], [17]. However, despitehaving lower dark current compared with the MWIR eAPDs,SWIR eAPDs suffer from problems that limit their photon de-tection ability. These include a higher excess noise factor anda lower gain compared with MWIR eAPDs [4], [17]. Thus, tobenefit from the advantages of the SWIR eAPDs in FPAs, specif-ically designed readout integrated circuits (ROICs) capable ofapplying a high reverse bias and with very low noise are critical.

Fig. 4 compares the external and internal dark current densityof the isolated electron-injection detector, biased at −3 V withthe best-reported SWIR MCT eAPD, biased at −8 V [4], [17].

For the electron-injection detector, optical gain was measuredwith the calibrated pulsed laser source down to a temperatureof 220 K and is shown on the right axis of Fig. 4. For thetemperature range of 160 K < T < 220 K, optical gain valueshave been extrapolated from the measured data points and thecorresponding internal dark current curve is shown by a dottedline. Isolated electron-injection detector has more than two or-ders of magnitude higher optical gain at any temperature whencompared with the best-reported SWIR MCT eAPD [4], [17].

3805106 IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 20, NO. 6, NOVEMBER/DECEMBER 2014

Fig. 4. Left axis : Comparison of dark current density for best-reported SWIRMCT eAPD [4], [17] with the electron-injection detector at different tempera-tures. Electron-injection detector has more than three orders of magnitude lowerinternal dark current density at any given temperature. Right axis: correspond-ing measured optical gain versus temperature. For the temperature range of160 K < T < 220 K, optical gain values for electron-injection detector havebeen extrapolated from the measured data and the corresponding internal darkcurrent curve is shown by a dotted line. Operating voltage for the MCT eAPDwas −8 V, while operating voltage for the electron-injection detector was −3 V.

The internal dark current density of electron-injection de-tector at all measured temperatures is more than 3 orders ofmagnitude lower than the SWIR MCT eAPD. This leads toa much lower DCR for the electron-injection detector at anytemperature.

C. Dark Count Rate Calculation Versus Temperature, andComparison With State-of-the-Art Semiconductor Technology

Based on the measured internal dark current and using adetailed simulation model, a plot for DCR versus temperaturewas obtained to compare the electron-injection detector withthe SWIR MCT eAPD in terms of PNR ability (see Fig. 5). Wefollow the methodology used for similar devices [18], [19]:

SNR2 =Q2

s

Q2n

Q2s = (qNMη)2

Q2n =

ε2

8

[τ

(2qM 2 〈Iint〉F +

4KT

RP

)

+ 4KTRsC2

τ+ Q2

ROIC

]. (1)

In (1), q is the electron charge, η the total quantum efficiency(η = ηint ∗ ηext), N is the number of photons, M is the detectorinternal amplification,τ is the integration time, < Iint > is theaverage internal dark current, F is the excess noise factor, C iscapacitance of the detector, QROIC is the nominal ROIC noisecharge, RP and Rs are the device parallel and series resistances.

For the plot of Fig. 5, we assume that for both detectors, theexternal quantum efficiency is increased to �95% by addingAR coating or utilizing a thicker absorbing layer. Furthermore,excess noise factor is assumed to be unity for both devices.

Fig. 5. (a) DCR versus temperature plotted for both the electron-injectiondetector and the best-reported SWIR MCT eAPD. To obtain a higher gain,operating voltage of −13.7 V was used for the MCT eAPD, while the operatingvoltage for the electron-injection detector was −3 V. (b) Statistical distributionat temperature of 220 K assuming average photon number of N̄ = 1. Thisfigure schematically shows the photon detection efficiency as well as dark countprobability for a specific detection threshold value.

Previous measurement results of electron-injection detector sup-port this assumption [9]. Our preliminary measurements of thecurrent high-speed devices using the common approach (directmeasurement of noise) also support this assumption.

For MCT eAPD, a ROIC with maximum noise of 10 e-rmsand bandwidth (BW) of 6 MHz, was used to achieve PNR at80 K, with measured DCR = 100 KHz (due to the influenceof the tunnel current) at 90% PDE [4], [17]. To get a high gain(of �70) in this experiment, the detector bias was increased to−13.7 V [17].

For the electron-injection detector, biased at −3 V, and as-suming the same ROIC noise and BW, we anticipate to achievea DCR of 1 Hz at 90% PDE at temperature of 210 K. Fig. 5(b)shows the signal statistics produced by the detection system(electron-injection detector + ROIC) for a signal with an av-erage number of photons N̄ = 1 at temperature of 220 K. Thevalue of PDE and dark count probability (DCP) are shown for aspecific detection threshold value.

FATHIPOUR et al.: ISOLATED ELECTRON INJECTION DETECTORS WITH HIGH GAIN AND RECORD LOW DARK CURRENT 3805106

V. CONCLUSION

We have achieved unprecedented reduction in electron- injec-tion detector dark current and record device performance bothat ambient and low temperature by physically isolating devices,changing their three-dimensional geometry and improving thefabrication steps.

The isolated structures with 10 μm injector and 30 μm ab-sorber diameters achieve more than 2 orders of magnitude re-duction in dark current and four orders of magnitude fasterresponse time, while preserving the high internal amplificationof previous reported results. Our measurement results have fur-ther shown that the performance of electron-injection detectorsis superior to that of state of the art SWIR MCT eAPD in termsof internal dark current density. At the same temperature, theelectron-injection structure achieves more than three orders ofmagnitude lower internal dark current density at a bias voltagethat is three times lower than the MCT eAPD.

The predicted 1Hz DCR at 90% PDE at 210 K, whichapproaches thermoelectric accessible temperatures makeselectron-injection detector a strong candidate for PNR appli-cations.

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Vala Fathipour received her B.S. and M.S. degreesin Electrical Engineering from University of Tehran,Tehran, Iran in 2009 and 2011 respectively. She iscurrently pursuing her Ph.D. degree in Electrical En-gineering, Solid-state and Photonics at NorthwesternUniversity. She has been awarded the Walter P. Mur-phy Fellowship and the Ryan Fellowship by North-western University in Fall 2011 and June 2013 re-spectively. In 2014, she was awarded URA VisitingScholars fellowship to work at Fermi National Accel-erator Laboratory on Large area ASICs for readout

of pixellated sensors. She has also received Best Student Paper Award at SPIEOptics + Photonics 2014. She has been a research assistant with Bio-inspiredSensors and Optoelectronics Lab (BISOL), at Northwestern University since2011. Her research interest includes short-wave infrared single photon detec-tors and imagers, infrared detection and semiconductor detectors. She is alsointerested in solar cells, quantum cascade lasers and plasmonic nanoantennas.

Omer Gokalp Memis received the Ph.D. degreefrom Northwestern University, Evanston, IL,USA,in 2010 and the B.Sc. and M.Sc. degrees from theBilkent University, Ankara, Turkey, in 2003 and2005, respectively. He is currently at Intel. At North-western, he was working mainly on single photondetectors, with a side interest in novel optoelectronicdesigns for biosensors, lasers, detectors.

Sung Jun Jang received the B.S. degree in electricalengineering from Chung Ang University, Korea, in2005 and the M.S. degree in information and com-munications, and the Ph.D. degree in information andmechatronics from Gwangju Institute of Science andTechnology, Gwangju, Korea, in 2006 and 2011, re-spectively. During the Ph.D. program, he had beeninvolved in research on many kind of optoelectronicdevices such as DFB laser, VCSEL, LED and solarcells. Currently, he is a Postdoctoral Research Fel-low in electrical engineering and computer science at

Northwestern University, Evanston, IL, USA. His current research interests in-clude design, fabrication and characterization of single photon infrared detectorand their camera application, and also nano-/micro-scale process developmentsfor plasmonic applications.

3805106 IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 20, NO. 6, NOVEMBER/DECEMBER 2014

Robert L. Brown was born in Utica, NY, USA. Hereceived the M.S. degree from the University of Cen-tral Florida, Orlando, FL,USA, in 2011 and the B.S.degree from the Rochester Institute of Technology,Rochester, NY, USA, in 2010. He is currently work-ing toward the Ph.D. degree at Northwestern Uni-versity, Evanston, IL, USA, in electrical engineeringworking on plasmonics, detectors, and optics. Hisresearch interests include nanophotonics, optoelec-tronics, plasmonics, and novel device fabrication. Hisprevious research involved silicon photonics, infrared

detection, and fabrication process control.

Iman Hassani Nia received the B.Sc. and M.Sc. de-grees from Shiraz University, Iran. Since 2010, hehas been working toward the Ph.D. degree at North-western University, Evanston, IL, USA. His researchinterests include opto-electronic devices, optical re-frigeration, and plasmonic application.

Hooman Mohseni received the Ph.D. degree inelectrical engineering from Northwestern University,Evanston, IL, USA, in 2001. He then joined SarnoffCorporation, where he was a Member of Techni-cal Staff leading several government, domestic, andinternational commercial projects. He joined North-western University as a faculty member in 2004. Heis the Director of Bio-Inspired Sensors and Opto-electronics Lab, and Northwestern’s Solid-state andPhotonics Initiative. He received the Young FacultyAward from Defense Advanced Project Agency in

2007. He was selected by NSF as a US delegates in U.S.-Korea Nanomanu-facturing Exchange program in 2007, and US-Japan Young Scientist ExchangeProgram on Nanotechnology in 2006. He received National Science Founda-tion’s CAREER Award in 2006. He has served as the Advisory Board, theProgram Chair, and the Cochair in several major conferences including IEEEPhotonics, SPIE Optics and Photonics, and SPIE Security and Defense. He haspublished more than 110 peer-reviewed articles in major scientific journals in-cluding Nature, Nano Letters, Small, and ACS Nano. He holds 14 issued U.S.and International patents on novel optoelectronic devices and nanoprocessing.He has presented more than 51 invited and keynote talks at different commercial,government, and educational institutes. He is a Fellow of SPIE and a Fellow ofOptical Society of America.