486 IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 58, NO. 2, FEBRUARY 2011

InAlAs Avalanche Photodiode With Type-IISuperlattice Absorber for Detection Beyond 2 μm

Daniel S. G. Ong, Jo Shien Ng, Member, IEEE, Yu Ling Goh, Student Member, IEEE,Chee Hing Tan, Member, IEEE, Shiyong Zhang, and John P. R. David, Senior Member, IEEE

Abstract—An avalanche photodiode (APD) using a InAlAs mul-tiplication region and a type-II InGaAs/GaAsSb superlattice as theabsorber (both lattice matched to InP) is reported. An optical andelectrical characterization of the photodiode is performed. TheAPD exhibited an absorption cutoff wavelength of 2.5 μm, whichis expected from the InGaAs/GaAsSb superlattice. A responsivityof 0.47 A/W (without gain) for the APD at a 2004-nm wavelengthwas demonstrated. The APD breakdown voltage showed a weaktemperature dependence of ∼40 mV/K, as a result of the excellenttemperature stability in InAlAs.

Index Terms—Avalanche photodiode (APD), infrared, quantumwell (QW), superlattice, type II.

I. INTRODUCTION

THE INTEREST for light detection within the 1.6–3-μmwavelength range is increasing due to applications such

as atmospheric carbon dioxide monitoring, free-space opticalcommunications, and medical diagnostics [1]. Devices basedon Si and In0.53Ga0.47As lattice matched to InP substrateshave only been able to cover the optical spectrum up to1.6 μm satisfactorily. Existing commercially available detectortechnologies for this wavelength range include HgCdTe andlong-wavelength InGaAs. With an appropriate composition, theHgCdTe material is able to detect light in this wavelength range,with focal plane arrays available [2]. Long-wavelength InGaAsp-i-n diodes grown on InP substrate (not lattice matched toInP) with relaxed buffer layers can extend the detection rangebeyond 1.6 μm [3], [4], albeit at the expense of detector darkcurrents.

An alternative technology for the 1.6–2.6-μm wavelengthrange is the superlattice formed by In0.53Ga0.47As andGaAs0.51Sb0.49. The two materials form a type-II staggeredband lineup, confining electrons and holes in In0.53Ga0.47Asand GaAs0.51Sb0.49, respectively. In these superlattices, spa-

Manuscript received June 17, 2010; revised October 18, 2010; acceptedOctober 25, 2010. Date of publication December 3, 2010; date of currentversion January 21, 2011. This work was supported by the European SpaceAgency under Contract 19816/06/NL/IA. The work of J. S. Ng was supportedby the University Research Fellowship of the Royal Society. The review of thispaper was arranged by Editor L. Lunardi.

D. S. G. Ong, J. S. Ng, Y. L. Goh, C. H. Tan, and J. P. R. Davidare with the Department of Electronic and Electrical Engineering, Univer-sity of Sheffield, S1 3JD Sheffield, U.K. (e-mail: elp07dso@sheffield.ac.uk;j.s.ng@sheffield.ac.uk; y.goh@sheffield.ac.uk; c.h.tan@sheffield.ac.uk; j.p.david@sheffield.ac.uk).

S. Zhang is with the Engineering and Physical Sciences Research CouncilNational Centre for III–V Technologies, S3 7HQ Sheffield, U.K. (e-mail:shiyong.zhang@sheffield.ac.uk).

Digital Object Identifier 10.1109/TED.2010.2090352

tially indirect photon absorption is possible between thevalence-band states in GaAs0.51Sb0.49 and the conduction-bandstates in In0.53Ga0.47As. This leads to an effective bandgapthat is narrower than the bandgaps of In0.53Ga0.47As andGaAs0.51Sb0.49, allowing photon absorption at wavelength> 1.6 μm while crucially maintaining lattice matching to InPsubstrates.

Sidhu et al. [5] and Inada et al. [6] reported a p-i-n diode con-sisting of these In0.53Ga0.47As/GaAs0.51Sb0.49 (5 nm/5 nm)type-II superlattices in the i-region, which are grown onInP substrates. Both achieved a room-temperature cutoffwavelength of ∼2.4 μm [5], [6]. Their room-temperaturepeak responsivities were 0.77 [5] and 0.6 A/W, respec-tively, at ∼2.23 μm [6]. An InP-based separate-absorption-multiplication (SAM) APD using the same In0.53Ga0.47As/GaAs0.51Sb0.49 type-II superlattices for the absorption regionand InP in the avalanche multiplication region was reportedin [7]. Using an APD can improve the signal-to-noise ratioof a detector system, which is desirable since many of theapplications in the 1.6–2.6-μm wavelength range must copewith a rather low signal level.

In InP, the hole ionization coefficient β is larger than the elec-tron ionization coefficient α [8]. However, both In0.52Al0.48As[9] and In0.53Ga0.47As/GaAs0.51Sb0.49 type-II superlattices[10] exhibit α > β; therefore, it is more appropriate to useIn0.52Al0.48As (lattice matched to InP) as the avalanche ma-terial in these APDs to avoid excess-noise degradation [11].Furthermore, the weaker breakdown-voltage temperature de-pendence in InAlAs, as compared with InP [12], will allowmore APD design tolerance.

This paper reports InP-based SAM APDs usingIn0.53Ga0.47As/GaAs0.51Sb0.49 type-II superlattices for ab-sorption and In0.52Al0.48As for avalanche multiplication.Reverse-dark-current and avalanche-multiplication charac-teristics as functions of the temperature of these APDs arepresented. Responsivity data are also reported.

II. WAFER STRUCTURE AND DEVICE

FABRICATION DETAILS

One p-i-n wafer and one SAM APD wafer were grownby molecular-beam epitaxy on InP substrates. The absorberfor both wafers comprised of 150 pairs of In0.53Ga0.47As/GaAs0.51Sb0.49 type-II superlattices (5-nm In0.53Ga0.47As and5-nm GaAs0.51Sb0.49). Structures of the p-i-n and SAM APDwafers are summarized in Table I. While this paper aimed to

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ONG et al.: InAlAs AVALANCHE PHOTODIODE WITH TYPE-II SUPERLATTICE ABSORBER 487

TABLE IDETAILS OF THE TYPE-II p-i-n DIODE AND SAM APD WAFERS

develop APDs, the p-i-n diode wafer would provide benchmarkdata for the APD wafer.

For the APD, the electric field is kept low across the ab-sorber through suitable doping density and thickness of theIn0.52Al0.48As charge sheet. Capacitance–voltage (C–V ) mea-surements on devices from the APD wafer indicated a dopingdensity of 2 − 3 × 1017 cm−3 in the charge-sheet layer, asintended. Data from the p-i-n diode indicated the unintentionaldoping in the type-II superlattices to be ∼1 − 2 × 1015 cm−3.

Circular and rectangular mesa devices were fabricated fromthe wafers using standard photolithography and a diluted HNO3

(1:1) etch solution. The devices were then passivated withbenzo-cyclo-butene (BCB). The spin-coated BCB was first softcured at 100 ◦C on a hotplate for 2 min and then hard cured at300 ◦C in a nitrogen furnace for 1 min. Reactive ion etching wasthen used to open up windows in the BCB for optical and elec-trical contact access to the semiconductor. Finally, Cr–Ti–Auwas deposited as p-metal contacts and bondpads, and Ti–Auwas used for the n-metal contact. The BCB, which has beenreported as a better passivation material than SiN and polyimidein [13] for an APD, was used to avoid significant dark currentsfrom the mesa sidewalls, particularly at temperatures below theroom temperature. Our device sizes range from a diameter of24 (smallest in area) to 90 × 120 μm (largest in area). Therewas no antireflection coating on the devices.

III. RESULTS

Dark current–voltage (I–V ) characteristics of devices fromthe p-i-n and the APD wafers were measured, using a Keithley236 Source-Measure Unit (SMU), at temperatures from 150to 290 K. From the Arrhenius plot of the reverse currentdensity data shown in Fig. 1, the activation energy was foundto be 0.12 eV, which is smaller than the 0.32 eV of [5]. Thisvalue indicates trap-assisted generation current as the dominantmechanism, although the exact natures of the traps differ inthese wafers.

The temperature-dependent dark-current density–voltagedata of the APD wafer are shown in Fig. 2 for a 90-μm-diameterdevice. The dark current increases rapidly with the reverse biasat ∼43 V. This was confirmed to be the punchthrough voltageVp when I–V measurements were repeated with illuminationon the devices. Another rapid increase in the dark current withvoltage can be observed at ∼60–64 V (depending on tempera-

Fig. 1. Arrhenius plot for type-II p-i-n in this work (circles, dashed line) andSidhu et al. [5] (triangles, solid line). The activation energy was found to be0.12 eV in this work. Sidhu et al. [5] obtained an activation energy of 0.32 eV.

Fig. 2. Temperature-dependent reverse-dark-current density–voltage data of a90-μm-diameter APD.

ture). This is the avalanche breakdown voltage Vbd (confirmedby avalanche multiplication measurements described later).With Vp reasonably far away from Vbd, the APD is fullydepleted with a low electric field across the type-II absorberat the breakdown voltage. The measured values of Vp and Vbd

are expected from the APD design.At biases ≥ Vp, dark currents of different-sized APD devices

scaled with the device area, indicating bulk contribution tothe dark currents. In fact, the dark-current densities of theAPD at biases ≥ Vp are similar to those of the p-i-n diode.This is expected because, once the APD is fully depleted, thedark current from the type-II superlattices (value of which wasindependently obtained from the p-i-n diode) dominates.

Based on the I–V data, the breakdown-voltage temperaturedependence was ∼40 mV/K, which is much smaller than∼88 mV/K observed in the type-II/InP separate absorption,charge, and multiplication APD of [7]. This improvementagrees with the prediction for an APD with a 1-μm-thickIn0.52Al0.48As multiplication layer and a 1.5-μm-thick ab-sorber in [12] and is due to the better In0.52Al0.48As temper-ature dependence of avalanche breakdown.

The avalanche gain M versus the voltage characteristics ofthe APDs were obtained through photomultiplication measure-ments performed at 290 and 200 K. A phase-sensitive detectiontechnique with a lock-in amplifier and mechanically choppedlight (wavelength of either 1.52 or 2.1 μm) was used in themeasurements. The 1.52-μm light was produced by a Melles

488 IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 58, NO. 2, FEBRUARY 2011

Fig. 3. (Solid line, left axis) Reverse-dark-current and (solid circles, rightaxis) avalanche-gain data of a 90-μm-diameter APD under the 2.1-μm wave-length illumination at (a) 290 and (b) 200 K. (Open circles, right axis)Avalanche gain of the APD under the 1.52-μm wavelength illumination at290 K is also shown in (a).

Griot continuous-wave He–Ne laser, whereas the 2.1-μm lightwas from a Horiba Jobin Yvon TRIAX550 monochromator(grating with a blazed wavelength of 1.5 μm) with a tungstenwhite-light source. Measurements using the 2.1-μm wavelengthlight also had a germanium filter, which blocks light withwavelengths shorter than 1.8 μm, preventing light absorptionin the top In0.53Ga0.47As layers in the APD. The dark currentand gain versus reverse bias data from these measurements areshown in Fig. 3(a) and (b), for 290 and 200 K, respectively.

In order to deduce M(V ) from the photocurrent data, itwas necessary to ascertain the value of M at −46 V, wherethe photocurrent ceased to increase rapidly with bias. Basedon the APD design and the knowledge of the dependence ofM (due to pure electron injection) with the electric field in a1-μm In0.52Al0.48As avalanche region, the APD is expected togive M ∼ 1.2 to 1.3 at −46 V. This range was experimentallyconfirmed by the ratio of photocurrents (with identical lightintensity) in the p-i-n diode at −5 V (fully depleted) and in theAPD at −46 V, when illuminated using light with wavelengthsof 1.52 and 2.004 μm.

With the phase-sensitive detection technique, we were able toreliably measure a large gain, in excess of 50 at 290 K and 150at 200 K, despite the low optical power from the 2.1-μm wave-length light. In addition, the M(V ) data obtained from the 1.52-and 2.1-μm wavelength lights were in agreement, as shown inFig. 3(a). The agreement is expected since both produced pure

Fig. 4. Spectral response of (solid black line) the p-i-n diode and (dashed grayline) the APD at room temperature.

electron injection into the In0.52Al0.48As avalanche region. Wehave measured the gain uniformity across the device active areausing the 1.52-μm wavelength illumination from a single-modefiber (core diameter of 10 μm). For a 90-μm-diameter device ata given bias, the gain varied by less than ±5% when the fiberwas positioned more than 10 μm away from the mesa edge.When the fiber was less than 10 μm away from the mesa edge,the photocurrent signal significantly falls because much of thelight did not enter the device under test (DUT).

Having assessed the avalanche-gain characteristics of theAPD, we examined the light-absorption performance of thep-i-n diode and the APD, particularly in the room-temperaturecutoff wavelength and the responsivity. Responsivity is de-fined as R = Iph/PDUT, where Iph is the photocurrent andPDUT is the incident optical power on the device. First, thespectral response of a 90-μm p-i-n and APD, as comparedin Fig. 4, were measured at room temperature using a Varian7000 Fourier transform infrared (FTIR) spectrometer with aStanford Research SR570 low-noise current preamplifier. Thedata showed that both the p-i-n diode (at −5 V) and theAPD (at −46 V) have room-temperature cutoff wavelengths of∼2.5 μm, consistent with the data in [5]–[7].

For responsivity measurements, a 2.004-μm wavelength lightfrom a fiber-coupled distributed-feedback laser diode (fromNanoplus GmbH) with a fiber collimator was used. The resul-tant photocurrent in the device was measured, again with thephase-sensitive detection method. The output-beam intensityprofile at the fiber collimator was confirmed to be Gaussian-distributed with a 1/e2 beam diameter of 3 mm, which is con-sistent with the specifications. The output optical power Ptotal

from the collimator was measured with a broadband powermeter with a 10-mm-diameter thermopile detector. PDUT wasthen given by PDUT = Ptotal × (VDUT/Vtotal), where VDUT isthe volume under the beam profile with the DUT centric withthe peak and Vtotal is the total volume under the beam profile.

We first measured the responsivity at 2.004 μm for acommercial extended-wavelength InGaAs diode (HamamatsuG8423-03) to validate our measurements. The measurementyielded ∼1.3 A/W, which is the specified responsivity (fromblackbody source-FTIR measurements).

Taking reflection losses into account, the extracted respon-sivity at 2.004 μm for the p-i-n diode was 0.52 A/W (at −5 V),corresponding to an external quantum efficiency of 32.2%. With

ONG et al.: InAlAs AVALANCHE PHOTODIODE WITH TYPE-II SUPERLATTICE ABSORBER 489

a small gain of 1.2–1.3 at −46 V, the APD gives a responsivityof 0.61 A/W.

The responsivity values of our wafers are comparable withthat in [5] and [6], although lower than those in [7], and thatof the extended-wavelength InGaAs diode. Those results wereobtained using a different definition for responsivity (black-body source-FTIR measurements), which could contribute tosome difference. Optimization on the superlattice growth andconsiderations of materials for the claddings adjacent to thesuperlattice may improve the light-absorption performance andshould lower the dark-current levels [6].

IV. CONCLUSION

A 2.5-μm cutoff wavelength type-II InGaAs/GaAsSb SAMAPD with the InAlAs multiplication region and the InGaAs/GaAsSb absorber has been demonstrated. Avalanche gain mea-surements on the APD have shown significant multiplicationfactors above 50 at 290 K and 150 at 200 K. The temperaturesensitivity of the breakdown voltage of the APD is weak andimproves on prior work using the InP avalanche layer. The APDhas yielded a responsivity of 0.51 A/W under the 2.004-μmwavelength illumination.

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Daniel S. G. Ong received the B.Eng. degree in electronic engineering from theUniversity of Sheffield, Sheffield, U.K., in 2007, where he is currently workingtoward the Ph.D. degree in electronic and electrical engineering.

His current research interests include avalanche photodiode modeling andcharacterization of Geiger-mode avalanche photodiode.

Jo Shien Ng (M’99) received the B.Eng. and Ph.D. degrees from the Universityof Sheffield, Sheffield, U.K., in 1999 and 2003, respectively, both in electricaland electronic engineering.

From 2003 to 2006, she was with the University of Sheffield and was respon-sible for characterization within the National Centre for III–V Technologies.In October 2006, she became a Royal Society University Research Fellow.She is currently with the Electronic and Electrical Engineering Department,University of Sheffield. Her research interests include avalanche photodiodes,Geiger-mode avalanche photodiodes, and material characterization.

Yu Ling Goh (S’02) received the B.Eng. degree in electronic and electricalengineering from the Imperial College, London, U.K., the M.Eng.Sc. degreefrom the Multimedia University, Cyberjaya, Malaysia, and the Ph.D. degreein electronic and electrical engineering from the University of Sheffield,Sheffield, U.K.

Her research interests include avalanche photodiodes, infrared detectors, andmaterial characterization.

Chee Hing Tan (M’95) received the B.Eng. and Ph.D. degrees in electronicengineering from the University of Sheffield, Sheffield, U.K., in 1998 and 2002,respectively.

He is currently a Senior Lecturer with the Department of Electronic andElectrical Engineering, University of Sheffield. His current research interestsinclude experimental and theoretical investigation of excess noise, breakdown,and jitter in Si and III–V APDs and single-photon avalanche diodes, de-sign of high-speed APDs and heterojunction phototransistors, and infraredphotodetectors.

Shiyong Zhang received the B.S. degree in material science from TsinghuaUniversity, Beijing, China, in 2000 and the Ph.D. degree in microelectronicsand solid-state electronics from the Chinese Academy of Sciences, Beijing,in 2006, respectively.

He is with the Engineering and Physical Sciences Research Council NationalCentre for III–V Technologies, Sheffield, U.K., and responsible for molecular-beam epitaxy growth of antimony-containing structures. His research interestsinclude quantum cascade lasers and infrared photodetectors.

John P. R. David (SM’96) received the B.Eng. and Ph.D. degrees in electronicengineering from the University of Sheffield, Sheffield, U.K.

In 1985, he was with the Central Facility for III–V semiconductors inSheffield, where he was responsible the characterization activity. In 2001, hewas with Marconi Optical Components (now Bookham Technologies). He iscurrently a Professor with the University of Sheffield. His research interestsinclude piezoelectric III–V semiconductors and impact ionization in analog andsingle-photon avalanche photodiodes.

Dr. David was an IEEE Lasers and Electro-Optics Society DistinguishedLecturer from 2002 to 2004.