Normalincidence strainedlayer superlattice Ge0.5Si0.5/Si photodiodes near 1.3 μmF. Y. Huang, X. Zhu, M. O. Tanner, and K. L. Wang Citation: Applied Physics Letters 67, 566 (1995); doi: 10.1063/1.115171 View online: http://dx.doi.org/10.1063/1.115171 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/67/4?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Normal-incidence Ge quantum-dot photodetectors at 1.5 μm based on Si substrate Appl. Phys. Lett. 80, 1189 (2002); 10.1063/1.1449525 Normalincidence epitaxial SiGeC photodetector near 1.3 μm wavelength grown on Si substrate Appl. Phys. Lett. 69, 2330 (1996); 10.1063/1.117514 Ellipsometric study of Si0.5Ge0.5/Si strainedlayer superlattices Appl. Phys. Lett. 62, 1626 (1993); 10.1063/1.108607 High photoconductive gain in Ge x Si1−x /Si strainedlayer superlattice detectors operating at λ=1.3 μm Appl. Phys. Lett. 49, 155 (1986); 10.1063/1.97209 Ge x Si1−x strainedlayer superlattice waveguide photodetectors operating near 1.3 μm Appl. Phys. Lett. 48, 963 (1986); 10.1063/1.96624

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Normal-incidence strained-layer superlattice Ge 0.5Si0.5/Si photodiodes near1.3 mm

F. Y. Huang, X. Zhu, M. O. Tanner, and K. L. WangDevice Research Laboratory, Electrical Engineering Department, University of California–Los Angeles,Los Angeles, California 90095-1594

~Received 31 March 1995; accepted for publication 10 May 1995!

Ge0.5Si0.5 strained-layerpin photodiodes, in which multiple strained layers serve as the absorptionregion, have been fabricated. These devices exhibit an optical response at wavelengths beyond 1.3mm at normal incidence. The measured external quantum efficiencies at an applied bias of 4 V are17% at 0.85mm and 1% at 1.3mm, respectively. Excellent electrical characteristics evidenced bythe avalanche breakdown at 20 V have also been demonstrated. ©1995 American Institute ofPhysics.

Although III–V compounds such as InGaAs latticematched to InP offer superior optical performance over theirSi-based counterparts in photodetectors at wavelengths near1.3 and 1.55mm for optical fiber communications, the com-patibility of Si-based materials with the state-of-the-art VLSItechnology stimulates a great number of research efforts toexploit the feasibility of photodetectors with wavelengths be-yond the limitation of the Si band gap, to 1.3mm and even to1.5mm. Recent advances in Si low-temperature epitaxy tech-niques such as molecular beam epitaxy~MBE! and ultrahighvacuum~UHV! chemical vapor deposition~UHVCVD! en-able the incorporation of Ge into Si to form high qualitySiGe alloys, and thus extend the spectral response to thiswavelength range.1–3

Because of the large lattice mismatch between Si andGe, the critical thickness for a strained SiGe alloy layer witha high Ge mole fraction~larger than 50%! is limited to lessthan 100 Å.1 This restricts the application of strained SiGematerials for photodetectors operating at normal incidence,since a sufficiently large absorption depth is required toachieve an appreciable quantum efficiency. Althoughstrained layer Ge0.4Si0.6/Si multiple-quantum well electro-optical modulators utilizing interband transitions for normalincidence operation have been demonstrated,4 so far onlywaveguide structures, with light injected laterally to increasethe light absorbing length, have been reported for photo-detectors.5,6

In order to relax the critical thickness limitation forstrained layers, a possible approach is to grow a strain reliefgraded buffer layer before growing the absorber layer. How-ever, the band gap of an unstrained bulk SiGe alloy is sub-stantially larger than that of a strained layer with the sameGe content. Therefore, a larger Ge mole fraction is necessaryto generate sufficient optical absorption at 1.3 or 1.5mmwavelengths. This will, in turn, require a thicker gradedbuffer to reduce the threading dislocation density in the re-laxed absorber layer. It has been a general practice that foreach 10% Ge mole fraction about 1mm linearly gradedbuffer is required to grow a high quality SiGe alloy with alow dislocation density.7 It is apparent that for large Ge molefractions, this approach will need a very thick buffer layerand is impractical.

On the other hand, if multiple SiGe/Si strained layers are

used for light absorption instead of a single strained layer,the total maximum absorbing layer thickness will increase tothe critical thickness of the overall multilayer with a Ge molefraction equivalent to the average composition; this will al-low for a larger absorbing thickness. Together with the straininduced band gap narrowing to increase the absorption coef-ficient at a certain wavelength, it is expected that photode-tectors with an appreciable sensitivity at normal incidenceshould be possible by using such strained layer superlattices.

In this letter, we report on the fabrication of Ge0.5Si0.5/Si photodetectors operating at normal incidence with a spec-tral response extending to 1.3mm. The active layer of thephotodetector consists of alternating layers of 100-ÅGe0.5Si0.5 and 400-Å Si on a Si substrate. These devices ex-hibit a maximum response near 0.85mm, and have a strongresponse at 1.3mm under applied bias. An external quantumefficiency of 1% at 1.3mm has been obtained with an anti-reflection coating at an applied bias of 4 V. Excellent electri-cal characteristics, as reflected by the room-temperature ava-lanche breakdown at about 20 V, have also beendemonstrated. To the best of our knowledge this is the firstreport of avalanche breakdown in SiGe/Si photodiodes withGe0.5Si0.5/Si strained layers as both the absorption and mul-tiplication regions.

The strained layer superlattice~SLS! structure used inour normal-incidence photodetectors is similar to that ofwaveguide photodetectors studied previously.5,6 The layerstructure and schematic band diagram of the device areshown in Fig. 1. The layers were grown by molecular beamepitaxy. Starting from the substrate, a 300 nmp1-Si layerwas grown as the buffer. Ten periods of Ge0.5Si0.5 on Si withthicknesses of 100 and 400 Å, respectively, were grown asthe absorption region. The 2000-Ån-Si top contact wascapped by 200 Å heavily dopedn1-Si. The growth tempera-ture for the buffer layer ranged from 550 to 650 °C, for thesuperlattice region was about 500 °C, and for the top contactregion ranged from 500 to 550 °C. Because of the superlat-tice structure with a 100-Å Ge0.5Si0.5 layer on 400-Å Si ineach period, the maximum thickness of the strained layersuperlattice without dislocations will increase to that of ahomogeneous alloy layer with a substantially lower averagedcomposition,6 i.e., 0.1 in our case.

Devices were fabricated using standard photolithography

566 Appl. Phys. Lett. 67 (4), 24 July 1995 0003-6951/95/67(4)/566/3/$6.00 © 1995 American Institute of Physics

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and dry etching. Square mesas~100 mm3100 mm! wereetched down to thep-Si buffer by SF6. Al was then evapo-rated to form the top contact. Input light can pass through awindow exterior to the top Al contact. The polished backsideof the device was then bonded to a metal holder to form theback contact.

Electrical characteristic of the device under reverse biasis shown in Fig. 2. A room-temperature avalanche process, asevidenced by the sharp breakdown at about 20 V, was iden-tified. No attempts were made to either passivate or bury theexposed sidewalls. The breakdown voltage has a positivetemperature dependence, which is typical of avalanchebreakdown. A detailed discussion on avalance breakdownand ionization multiplication will be presented elsewhere. Ata reverse bias of 7 V~one third of the breakdown voltage!,the leakage current is 6mA, corresponding to a current den-sity of 60 mA/cm22. By using proper surface passivationand circular mesas it is expected that the leakage current willbe reduced and the breakdown voltage may be increased.

The photocurrent response of the device versus wave-length is shown in Fig. 3. The spectrum was obtained byusing a lock-in amplifier. Light from a tungsten lamp passesthrough a monochromator before it is focused onto the de-vice. The device exhibits a peak response at about 0.85mm.At zero bias the response at 1.3mm diminishes. At an ap-plied bias of 5 V, the device shows a strong response at 1.3mm and a cutoff at around 1.5mm. Three peaks are clearly

resolvable in the spectra both with and without applied bi-ases. These peaks may be due to different transitions of elec-trons from the heavy- and light-hole bands to the conductionband.

The photocurrent response as a function of applied biasis shown in Fig. 4 at a fixed wavelength of 0.95mm. It isclearly seen that at a small bias the photocurrent increases asthe applied bias increases, due to the carrier trapping by po-tential barriers in the valence band of the SiGe/Si hetero-structures as shown in Fig. 1~b!. At a sufficiently large volt-age ~;4 V!, the photocurrent becomes saturated, whichindicates that all the photogenerated carriers are swept out to

FIG. 1. ~a! Schematic of the multiple strained layerpin photodiode. Theabsorption region consists of ten periods of Ge0.5Si0.5/Si with GeSi and Sithicknesses of 10 and 40 nm, respectively.~b! Schematic band diagram ofthe device under a reverse bias.

FIG. 2. Current–voltage characteristic of a strained layer superlatticeSiGe/Si photodiode. The device exhibits a room-temperature avalanchebreakdown at about 20 V.

FIG. 3. Measured photocurrent spectra of the photodiodes at various appliedvoltages with a series resistor of 15.7 kV. As the voltage increases thephotocurrent increases, indicating carrier trapping by the potential barriersin the valence band. Three peaks can be clearly resolved in the spectra.

567Appl. Phys. Lett., Vol. 67, No. 4, 24 July 1995 Huang et al.

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the electrodes. As the applied bias is further increased andbecomes larger than 10 V, the photocurrent increases sharplywith the applied bias resulting from the avalanche multipli-cation.

The quantum efficiency of the photodiode was measuredby a calibrated Ge photodetector. The device was coated withSiO2 antireflection coatings designed for 1.3mm. The mea-sured quantum efficiency is shown in Fig. 5, and is about 1%at 1.3mm. This is comparable to the theoretical estimate withan absorption coefficient of about 0.1mm21 at an overallabsorption thickness of 1000 Å.

It is worth mentioning that carrier trapping by the poten-tial barriers plays a major role in photocurrent response es-pecially at a small applied bias, while it has little effect onthe frequency response of the device. The response speed isessentially determined by the RC response time, given that

the device dimension is large so that RC time dominates overthe carrier transit time. As the device scales down, the carriertransit time becomes important.

The 1% quantum efficiency at 1.3mm corresponds to anexternal sensitivity of about 10 mA/W at this wavelength.There are several approaches which can be taken to increasethe external sensitivity. First, because of the avalanche pro-cess, as demonstrated in this device, it is possible to enhancethe sensitivity by applying high voltages close to the break-down voltage~about 20 V!. For a multiplication factor of 50,it is expected that an external sensitivity of 0.5 A/W can beachieved. Also, by using a resonant cavity scheme,8,9 an in-tegrated dielectric mirror on the bottom of the strained layerabsorber will effectively increase the absorbing length, there-fore, increasing the quantum efficiency. Besides SiGestrained layer dielectric mirrors,10 another suitable candidateis the Si/SiO2 mirror. An e-beam deposited amorphous Si/SiO2 dielectric mirror has been demonstrated.11 However,this scheme can only be used for the top mirror, since crys-talline Si is required as a bottom mirror for epitaxial growthof the device to be possible. This problem can be circum-vented by using separation by implantation of oxygen~SIMOX! wafers.

In summary, we have demonstrated the application ofGe0.5Si0.5/Si strained layer superlattices as light absorbers innormal-incidencepin photodiodes. Because of the strain-induced band-gap narrowing and increased pseudomorphicthickness for multiple strained layers, the device exhibits anoptical response at wavelengths beyond 1.3mm under nor-mal incidence. An external quantum efficiency of 1% at 1.3mm under an applied bias of 4 V has been obtained. Theavalanche breakdown at 20 V also implies potential applica-tions for avalanche photodiodes.

The authors would like to thank H. S. Li and R. Karu-nasiri for help in the photocurrent measurement. This workwas supported in part by ARPA under Contract No.DAAL01-93-K3353 through the Army Research Laboratory.Fruitful discussions with Dr. S. Z. Li and Dr. X. Y. Zheng arealso acknowledged.

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FIG. 4. Photocurrent as a function of applied voltage at 0.95mm with aseries resistor of 3.0 kV, showing the carrier trapping effect. At a small biasthe photocurrent increases as the applied voltage increases, and becomessaturated at about 4 V. The sharp increase of the photocurrent at an appliedbias above 10 V results from the avalanche multiplication.

FIG. 5. Measured external quantum efficiency of the photodiode by a cali-brated Ge detector. The quantum efficiency at 1.3mm under an applied biasof 4 V is 1% with antireflection coatings.

568 Appl. Phys. Lett., Vol. 67, No. 4, 24 July 1995 Huang et al.

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