Electroluminescence from strained n-Ge quantum well light emitting diodes grown on a silicon substrate are demonstrated at room temperature. Electroluminescence characterisation demonstrates two peaks around 1.55 μm and 1.8 μm, which correspond to recombination between the direct and indirect transitions, respectively. The emission wavelength can be tuned by around 4% through changing the current density through the device. The devices have potential applications in the fields of optical interconnects, gas sensing, and healthcare

Cecchi, S.

Chrastina, D.

Frigerio, J.

Gallacher, Kevin

Isella, G.

Paul, D.

Velha, P.

English

K. Gallacher

P. Velha

D. J. Paul

S. Cecchi

J. Frigerio

D. Chrastina

G. Isella

Crossref

Applied Physics Letters

1.55 <i>μ</i>m direct bandgap electroluminescence from strained n-Ge quantum wells grown on Si substrates

Electroluminescence from strained n-Ge quantum well light emitting diodes grown on a silicon substrate are demonstrated at room temperature. Electroluminescence characterisation demonstrates two peaks around 1.55 Î1⁄4m and 1.8 Î1⁄4m, which correspond to recombination between the direct and indirect transitions, respectively. The emission wavelength can be tuned by around 4% through changing the current density through the device. The devices have potential applications in the fields of optical interconnects, gas sensing, and healthcare. Â© 2012 American Institute of Physics

Gallacher, K

Paul, D. J.

VELHA, PHILIPPE

Archivio della ricerca della Scuola Superiore Sant'Anna

1.55 Î1⁄4m direct bandgap electroluminescence from strained n-Ge quantum wells grown on Si substrates

Enlighten

Enlighten – Research publications by members of the University of Glasgow http://eprints.gla.ac.uk        Gallacher, K., Velha, P., Paul, D., Cecchi, S., Frigerio, J., Chrastina, D., and Isella, G. (2012) 1.55 μm direct bandgap electroluminescence from strained n-Ge quantum wells grown on Si substrates. Applied Physics Letters, 101 (21). p. 211101. ISSN 0003-6951   Copyright © 2012 American Institute of Physics   A copy can be downloaded for personal non-commercial research or study, without prior permission or charge.  The content must not be changed in any way or reproduced in any format or medium without the formal permission of the copyright holder(s)  When referring to this work, full bibliographic details must be given.     http://eprints.gla.ac.uk/73084/        Deposited on: 11th December 2012   1.55μm direct bandgap electroluminescence from strained n-Ge quantumwells grown on Si substratesK. Gallacher, P. Velha, D. J. Paul, S. Cecchi, J. Frigerio et al.  Citation: Appl. Phys. Lett. 101, 211101 (2012); doi: 10.1063/1.4767138 View online: http://dx.doi.org/10.1063/1.4767138 View Table of Contents: http://apl.aip.org/resource/1/APPLAB/v101/i21 Published by the American Institute of Physics.  Related ArticlesDesign and optimization of a light-emitting diode projection micro-stereolithography three-dimensionalmanufacturing system Rev. Sci. Instrum. 83, 125001 (2012) Size-dependent efficiency and efficiency droop of blue InGaN micro-light emitting diodes Appl. Phys. Lett. 101, 231110 (2012) A bright cadmium-free, hybrid organic/quantum dot white light-emitting diode Appl. Phys. Lett. 101, 233110 (2012) Interplay of polarization fields and Auger recombination in the efficiency droop of nitride light-emitting diodes Appl. Phys. Lett. 101, 231107 (2012) Precise relationship between voltage and frequency at the appearance of negative capacitance in InGaN diodes Appl. Phys. Lett. 101, 233506 (2012)  Additional information on Appl. Phys. Lett.Journal Homepage: http://apl.aip.org/ Journal Information: http://apl.aip.org/about/about_the_journal Top downloads: http://apl.aip.org/features/most_downloaded Information for Authors: http://apl.aip.org/authors Downloaded 11 Dec 2012 to 130.209.6.41. Redistribution subject to AIP license or copyright; see http://apl.aip.org/about/rights_and_permissions1.55lm direct bandgap electroluminescence from strained n-Ge quantumwells grown on Si substratesK. Gallacher,1 P. Velha,1 D. J. Paul,1,a) S. Cecchi,2 J. Frigerio,2 D. Chrastina,2 and G. Isella21University of Glasgow, School of Engineering, Rankine Building, Oakfield Avenue, Glasgow G12 8LT,United Kingdom2L-NESS, Dipartimento di Fisica del Politecnico di Milano, Polo Territoriale di Como,Via Anzani 42, I-22100 Como, Italy(Received 15 May 2012; accepted 29 October 2012; published online 19 November 2012)Electroluminescence from strained n-Ge quantum well light emitting diodes grown on a siliconsubstrate are demonstrated at room temperature. Electroluminescence characterisationdemonstrates two peaks around 1.55lm and 1.8 lm, which correspond to recombination betweenthe direct and indirect transitions, respectively. The emission wavelength can be tuned by around4% through changing the current density through the device. The devices have potentialapplications in the fields of optical interconnects, gas sensing, and healthcare. VC 2012 AmericanInstitute of Physics. [http://dx.doi.org/10.1063/1.4767138]There is presently increased interest in using Ge as anactive material on Si substrates to realise light emittingdiodes (LEDs), lasers, and photodetectors that can either beintegrated with complementary metal-oxide semiconductorelectronics or provide added functions for More-than-Mooreapplications.1 The potential applications include fibre-to-the-home, chip-to-chip optical interconnects in addition to longerwavelength healthcare and pollution monitoring applica-tions.2 Previous works have demonstrated electrolumines-cence from Ge p-i-n structures or other configurations grownon top of standard Si wafers.3–7 Electroluminescence hasalso been demonstrated from Ge quantum well devices butso far the emission has been at wavelengths below 1.45lm(Ref. 7) which is too low for the important 1.55 lm for tele-coms. Longer wavelengths are also desirable for pollutionmonitoring and bio-sensing applications. The higher concen-tration of carriers in a two dimensional (2D) quantum wellsystem compared to bulk three dimensional devices shouldultimately allow higher efficiencies with lower thresholdsideal for all applications. Tensile-strained Ge QWs havedemonstrated direct-gap transitions.8,9 In this work, electro-luminescence from strained 10 period n-Ge multiple quan-tum well (MQW) LEDs grown directly on a silicon wafer ispresented which produces electroluminescence at the impor-tant 1.55 lm wavelength at room temperature (298K).The material was grown on a p-Si (100) substrate with a re-sistivity of 1Xcm. The substrate was degassed at 316 C for10min before loading into a low-energy plasma-enhancedchemical vapour deposition (LEPECVD) chamber.7,10 The het-erolayer design is shown in Fig. 1. A 2.1lm two step bufferwith first a 600nm relaxed layer of Si0:61Ge0:39 was grown fol-lowed by 1.5lm relaxed Si0:048Ge0:952. Next 100nm ofp-Si0:048Ge0:952 (NA ¼ 3 1018 cm3 doped from B2H6) wasgrown as required for the bottom Ohmic contact. The Ge con-centrations (and quantum well thicknesses) for all heterolayerswas measured after growth using x-ray diffraction (XRD). A30nm spacer of Si0:048Ge0:952 ðNA ¼ 5 1017 cm3Þ wasgrown before the active quantum well region. The active regionconsists of 10 periods of strained 11.2 nm n-Ge quantum wells(ND ¼ 1 1019 cm3 doped with PH3) and 8.5 nmSi0:014Ge0:986 barriers. A final cap of 10nm of Si0:048Ge0:952 fol-lowed by 3nm of Si was grown to produce a Schottky top con-tact that would allow hot electron injection into the C-valleywith the aim of producing more efficient direct recombination.Whilst the PH3 was only switched on during the growth of thequantum wells, due to segregation effects, all the heterolayersgrown after the first quantum well will be doped with significantfractions of the n-type doping level.11 For this strain relaxedbuffer Ge concentration, the calculated Matthews and Blakesleecritical thickness is 283nm.12The band structure was calculated using a self-consistent Poisson-Schr€odinger solver with the deformationpotentials from Reference 13 and the results for a singleFIG. 1. The heterolayer design for the device. Right: Optical microscopepicture of a completed LED.a)Douglas.Paul@glasgow.ac.uk.0003-6951/2012/101(21)/211101/4/$30.00 VC 2012 American Institute of Physics101, 211101-1APPLIED PHYSICS LETTERS 101, 211101 (2012)Downloaded 11 Dec 2012 to 130.209.6.41. Redistribution subject to AIP license or copyright; see http://apl.aip.org/about/rights_and_permissionsquantum well are shown in Fig. 2. An 8-band k.p model wasused for the hole bands and the C-valley whilst a 1 band toolwas used to find the L- and D-valley bands and subbandstates. The present thin buffer resulted in a tensile strain of0.13% as measured by XRD after the sample was cooled toroom temperature and the modelling of the bands and sub-band states has taken account of this strain.13 For both theL- and C-valleys, only a single subband for each valley isconfined in the quantum well. The L-valley subband state isvery weakly confined in the quantum well due to the smallconduction band discontinuity of only 8meV. At room tem-perature, confinement due to the L-valley quantum well isunlikely to be observed. The C-valley is below both D-valleybands due to the tensile strain in the substrate. The C-valleyquantum well has a discontinuity of 19meV with a singleconfined state that due to the low effective mass of 0:038m0(where m0 is the free electron mass), the subband states ineach quantum well overlap to form a miniband with widthcalculated to be 8.4meV. Modelling of the valence banddemonstrated that the ground state is the heavy-hole (HH)and the lowest subband is the HH1. The tensile strain istherefore not great enough to move the LH band above theHH band. The calculated lowest direct transition is C1 toHH1 at 0.817 eV (1.52 lm wavelength) and the indirect L1subband to HH1 is 0.663 eV (1.89 lm).Cylindrical mesas ranging from 10lm to 500 lm in di-ameter (see Fig. 1) were defined by a Vistec VB6 electronbeam lithography tool with hydrogen silsesquioxane (HSQ)resist and then etched down anisotropically using a fluorinebased chemistry in an inductively coupled plasma reactiveion etch tool. 50 nm of Ni was deposited by electron-beamevaporation for the bottom contact and this was patterned bylift-off and subsequently annealed at 340 C for 30 s in arapid thermal annealer (RTA).14 The process provides bot-tom Ohmic contacts with measured specific contact resistiv-ities of 4 108 Xcm2. An unannealed top contact of 10 nmof Ti followed by 50 nm of Al was deposited to produce aSchottky top contact which could inject carriers above theconduction band edge. The whole structure was passivatedwith Si3N4 and via holes were etched to allow interconnectsto the contacts. Bond pads of 600 nm of Al were depositedand finally, the device was wire bonded to a chip carrier inorder to connect the LED to an external power supply forcharacterisation. All results in this publication were fromdevices of 300 lm diameter measured at room temperature.The current-voltage characteristics are shown in Fig. 3.Larger currents flow when the device is forward biased withelectrons being injected into the conduction band from theSchottky top contact. The dark current is comparable tosome of the best reported from Ge on Si photodetectors.15Both the photoluminescence (PL) and electroluminescencefrom the LED were measured using a Bruker Vertex 70vFTIR system. The FTIR system uses a calcium fluoridebeamsplitter and a room temperature extended InGaAs de-tector, which can detect radiation between 0.8 and 2.5 lm.All measurements were undertaken in surface emission ge-ometry which corresponds to xy-polarization (TE polariza-tion). The selection rules for such polarization allow thefollowing transitions:13 C1 to HH1, L1 to HH1, C1 to LH1,and L1 to LH1. Fig. 4 shows the PL for 580 nm CW excita-tion. Comparison with the band structure modelling of Fig. 1indicates that the sharper peak just above 1.5 lm is theC-valley to HH1 transition and the broader 1.8 lm peak isthe L-valley to HH1 indirect transition. Above 2lm is black-body emission under CW illumination. Fig. 5 shows the elec-troluminescence as a function of current under CWconditions at room temperature. As the current is increased,the direct bandgap (C-valley to HH1) electroluminescenceincreases compared to the indirect (L-valley to HH1) transi-tions. Also the blackbody contribution demonstrates signifi-cant increases as the current is increased especially beyondthe 2 lm wavelength.Fig. 6 demonstrates the total electroluminescence-current (LI) data from the LEDs. Below 210A=cm2, the gra-dient of the LI curve is below 1 whilst above this point, thegradient increases to 1.5 and it is clear that blackbody emis-sion from Joule heating of the sample starts to become moresignificant in the measured electroluminescence power. Pre-vious publications have indicated that heating can help exciteFIG. 2. The conduction bands for a single quantum well in the device as cal-culated by self-consistent Poisson-Schr€odinger solver at 300K. The confinedsubband states for the L- and C-valleys are also plotted.FIG. 3. The current-voltage characteristics of the device at roomtemperature.211101-2 Gallacher et al. Appl. Phys. Lett. 101, 211101 (2012)Downloaded 11 Dec 2012 to 130.209.6.41. Redistribution subject to AIP license or copyright; see http://apl.aip.org/about/rights_and_permissionscarriers from the L-valley to the C-valley and increase theelectroluminescence efficiency and output power.7,16,17 Asimilar effect is observed in the present devices as the ratioof the emission from the direct bandgap compared to theindirect bandgap emission increases with increasing currentdensity but the use of the longer wavelength detector in thiswork also demonstrates the resulting strong spectral contri-bution of the blackbody emission at longer wavelengths.Fig. 7 shows the shift in the direct bandgap recombina-tion peak as a function of current density. As the current den-sity is increased to 420A=cm2, the emission wavelength isincreased by around 4% as shown in Fig. 7. The clear red-shift in the direct bandgap electroluminescence recombina-tion peak as a function of increasing current demonstratesthe Joule heating of the device with the resultant reduction inthe direct bandgap (the indirect bandgap is also reduced bythe heating). The bandgap dependence of Ge was modelledby Varshni18 and the expected variation is inverse linearlyproportional to the temperature when the semiconductor iswell above the Debye temperature of 374K for Ge. Curvefits to the blackbody part of the spectra in Fig. 5 indicate thatthe electron temperature is above 400K for current densitiesof 50A=cm2 and above thereby indicating that the bandgapis decreasing with a linear variation of temperature. As Jouleheating in semiconductors results in a near linear tempera-ture rise with current density, the linear variation of wave-length with current density in Fig. 7 therefore agrees withthe Varshni model for the bandgap for temperatures abovethe Debye temperature.18 At 300A=cm2, the direct recombi-nation electroluminescence is at the important 1.55lmwavelength for telecoms applications.To date, the majority of Ge quantum well devices haveonly demonstrated PL emission characterisation throughFIG. 7. The central position of the direct bandgap (C-valley to HH) electro-luminescent peak as a function of current density at room temperature. Theline is a linear fit to the data.FIG. 4. The photoluminescence of the device at room temperature using a580 nm pump source.FIG. 5. The electroluminescence of the device at room temperature as afunction of CW excitation currents.FIG. 6. The electroluminescence versus current density plot of the device atroom temperature.211101-3 Gallacher et al. Appl. Phys. Lett. 101, 211101 (2012)Downloaded 11 Dec 2012 to 130.209.6.41. Redistribution subject to AIP license or copyright; see http://apl.aip.org/about/rights_and_permissionsoptical pumping. Most electroluminescence has been demon-strated with bulk n-Ge apart from a single result with emis-sion below 1.45 lm wavelength.7 The present n-Ge quantumwell devices have a narrow direct recombination peak(80 nm full width half maximum) followed by the drop inelectroluminescence to near zero levels at higher energies(Fig. 5). This can be explained by the sharp two 2D densityof states which results in only the C-valley to HH1 andL-valley to HH1 recombination transitions being observedunder electroluminescence, unlike the bulk Ge devices. Such2D radiative transitions are ultimately expected to producehigher gain and lower thresholds if a laser can be produced.3The quantum well structures also provide a natural designfor confining (and guiding) the mode and therefore the ex-pectation is that this approach is interesting to produce prac-tical Ge electroluminescent LEDs and lasers on siliconsubstrates.The direct bandgap C-valley to HH1 electrolumines-cence from n-Ge quantum wells on silicon substrates hasbeen demonstrated from LEDs at the important 1.55lmwavelength with CW operation. The emission wavelength ofthe devices can be tuned by approximately 4% throughchanging the current density. At higher wavelengths, theindirect L-valley to HH1 transition is observed before black-body emission is observed. Such devices may be integrableinto Si photonic circuits, and lasing may be feasible withsuch designs when fabricated into a suitable cavity.1J. Liu, X. Sun, R. Camacho-Aguilera, L. C. Kimerling, and J. Michel, Opt.Lett. 35, 679 (2010).2D. J. Paul, Electron. Lett. 45, 582 (2009).3J. Liu, X. Sun, L. C. Kimerling, and J. Michel, Opt. Lett. 34, 1738 (2009).4X. Sun, J. Liu, L. C. Kimerling, and J. Michel, Opt. Lett. 34, 1198 (2009).5M. de Kersauson, R. Jakomin, M. E. Kurdi, G. Beaudoin, N. Zerounian, F.Aniel, S. Sauvage, I. Sagnes, and P. Boucaud, J. Appl. Phys. 108, 023105(2010).6D. Nam, D. Sukhdeo, S.-L. Cheng, A. Roy, K. C.-Y. Huang, M. Bron-gersma, Y. Nishi, and K. Saraswat, Appl. Phys. Lett. 100, 131112 (2012).7P. Chaisakul, D. Marris-Morini, G. Isella, D. Chrastina, N. Izard, X. L.Roux, S. Edmond, J.-R. Coudevylle, and L. Vivien, Appl. Phys. Lett. 99,141106 (2011).8Y. Chen, C. Li, H. Lai, and S. Chen, Nanotechnology 21, 115207(2010).9L. Carroll, F. Imbert, H. Sigg, M. S€uess, E. M€uller, M. Virgilio, G. Pizzi,P. Rossbach, D. Chrastina, and G. Isella, Appl. Phys. Lett. 99, 031907(2011).10E. Gatti, E. Grilli, M. Guzzi, D. Chrastina, G. Isella, and H. von K€anel,Appl. Phys. Lett. 98, 031106 (2011).11Z. Suet, D. J. Paul, J. Zhang, and S. G. Turner, Appl. Phys. Lett. 90,203501 (2007).12D. J. Paul, Laser Photon. Rev. 4, 610 (2010).13D. J. Paul, Phys. Rev. B 77, 155323 (2008).14K. Gallacher, P. Velha, D. J. Paul, I. MacLaren, M. Myronov, and D. R.Leadley, Appl. Phys. Lett. 100, 022113 (2012).15J. Osmond, G. Isella, D. Chrastina, R. Kaufmann, M. Acciarri, and H. vonK€anel, Appl. Phys. Lett. 94, 201106 (2009).16S.-L. Cheng, J. Lu, G. Shambat, H.-Y. Yu, K. Saraswat, J. Vuckovic, andY. Nishi, Opt. Exp. 17, 10019 (2009).17P. H. Wu, D. Dumcenco, Y. S. Huang, H. P. Hsu, C. H. Lai, T. Y. Lin, D.Chrastina, G. Isella, E. Gatti, and K. K. Tiong, Appl. Phys. Lett. 100,141905 (2012).18Y. P. Varshni, Physica 34, 149 (1967).211101-4 Gallacher et al. Appl. Phys. Lett. 101, 211101 (2012)Downloaded 11 Dec 2012 to 130.209.6.41. Redistribution subject to AIP license or copyright; see http://apl.aip.org/about/rights_and_permissions

1.55 μm direct bandgap electroluminescence from strained n-Ge quantum wells grown on Si substrates

Enlighten: Publications

Enlighten – Research publications by members of the University of Glasgow 
http://eprints.gla.ac.uk 
 
 
 
 
 
 
 
Gallacher, K., Velha, P., Paul, D., Cecchi, S., Frigerio, J., Chrastina, D., 
and Isella, G. (2012) 1.55 μm direct bandgap electroluminescence from 
strained n-Ge quantum wells grown on Si substrates. Applied Physics 
Letters, 101 (21). p. 211101. ISSN 0003-6951 
 
 
Copyright © 2012 American Institute of Physics 
 
 
A copy can be downloaded for personal non-commercial research or 
study, without prior permission or charge. 
 
The content must not be changed in any way or reproduced in any format 
or medium without the formal permission of the copyright holder(s) 
 
When referring to this work, full bibliographic details must be given. 
 
 
 
 
http://eprints.gla.ac.uk/73084/  
 
 
 
 
 
 
Deposited on: 11th December 2012 
 
 
1.55μm direct bandgap electroluminescence from strained n-Ge quantum
wells grown on Si substrates
K. Gallacher, P. Velha, D. J. Paul, S. Cecchi, J. Frigerio et al. 
 
Citation: Appl. Phys. Lett. 101, 211101 (2012); doi: 10.1063/1.4767138 
View online: http://dx.doi.org/10.1063/1.4767138 
View Table of Contents: http://apl.aip.org/resource/1/APPLAB/v101/i21 
Published by the American Institute of Physics. 
 
Related Articles
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Rev. Sci. Instrum. 83, 125001 (2012) 
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Appl. Phys. Lett. 101, 231110 (2012) 
A bright cadmium-free, hybrid organic/quantum dot white light-emitting diode 
Appl. Phys. Lett. 101, 233110 (2012) 
Interplay of polarization fields and Auger recombination in the efficiency droop of nitride light-emitting diodes 
Appl. Phys. Lett. 101, 231107 (2012) 
Precise relationship between voltage and frequency at the appearance of negative capacitance in InGaN diodes 
Appl. Phys. Lett. 101, 233506 (2012) 
 
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1.55lm direct bandgap electroluminescence from strained n-Ge quantum
wells grown on Si substrates
K. Gallacher,1 P. Velha,1 D. J. Paul,1,a) S. Cecchi,2 J. Frigerio,2 D. Chrastina,2 and G. Isella2
1University of Glasgow, School of Engineering, Rankine Building, Oakfield Avenue, Glasgow G12 8LT,
United Kingdom
2L-NESS, Dipartimento di Fisica del Politecnico di Milano, Polo Territoriale di Como,
Via Anzani 42, I-22100 Como, Italy
(Received 15 May 2012; accepted 29 October 2012; published online 19 November 2012)
Electroluminescence from strained n-Ge quantum well light emitting diodes grown on a silicon
substrate are demonstrated at room temperature. Electroluminescence characterisation
demonstrates two peaks around 1.55lm and 1.8 lm, which correspond to recombination between
the direct and indirect transitions, respectively. The emission wavelength can be tuned by around
4% through changing the current density through the device. The devices have potential
applications in the fields of optical interconnects, gas sensing, and healthcare. VC 2012 American
Institute of Physics. [http://dx.doi.org/10.1063/1.4767138]
There is presently increased interest in using Ge as an
active material on Si substrates to realise light emitting
diodes (LEDs), lasers, and photodetectors that can either be
integrated with complementary metal-oxide semiconductor
electronics or provide added functions for More-than-Moore
applications.1 The potential applications include fibre-to-the-
home, chip-to-chip optical interconnects in addition to longer
wavelength healthcare and pollution monitoring applica-
tions.2 Previous works have demonstrated electrolumines-
cence from Ge p-i-n structures or other configurations grown
on top of standard Si wafers.3–7 Electroluminescence has
also been demonstrated from Ge quantum well devices but
so far the emission has been at wavelengths below 1.45lm
(Ref. 7) which is too low for the important 1.55 lm for tele-
coms. Longer wavelengths are also desirable for pollution
monitoring and bio-sensing applications. The higher concen-
tration of carriers in a two dimensional (2D) quantum well
system compared to bulk three dimensional devices should
ultimately allow higher efficiencies with lower thresholds
ideal for all applications. Tensile-strained Ge QWs have
demonstrated direct-gap transitions.8,9 In this work, electro-
luminescence from strained 10 period n-Ge multiple quan-
tum well (MQW) LEDs grown directly on a silicon wafer is
presented which produces electroluminescence at the impor-
tant 1.55 lm wavelength at room temperature (298K).
The material was grown on a p-Si (100) substrate with a re-
sistivity of 1Xcm. The substrate was degassed at 316 C for
10min before loading into a low-energy plasma-enhanced
chemical vapour deposition (LEPECVD) chamber.7,10 The het-
erolayer design is shown in Fig. 1. A 2.1lm two step buffer
with first a 600nm relaxed layer of Si0:61Ge0:39 was grown fol-
lowed by 1.5lm relaxed Si0:048Ge0:952. Next 100nm of
p-Si0:048Ge0:952 (NA ¼ 3 1018 cm3 doped from B2H6) was
grown as required for the bottom Ohmic contact. The Ge con-
centrations (and quantum well thicknesses) for all heterolayers
was measured after growth using x-ray diffraction (XRD). A
30nm spacer of Si0:048Ge0:952 ðNA ¼ 5 1017 cm3Þ was
grown before the active quantum well region. The active region
consists of 10 periods of strained 11.2 nm n-Ge quantum wells
(ND ¼ 1 1019 cm3 doped with PH3) and 8.5 nm
Si0:014Ge0:986 barriers. A final cap of 10nm of Si0:048Ge0:952 fol-
lowed by 3nm of Si was grown to produce a Schottky top con-
tact that would allow hot electron injection into the C-valley
with the aim of producing more efficient direct recombination.
Whilst the PH3 was only switched on during the growth of the
quantum wells, due to segregation effects, all the heterolayers
grown after the first quantum well will be doped with significant
fractions of the n-type doping level.11 For this strain relaxed
buffer Ge concentration, the calculated Matthews and Blakeslee
critical thickness is 283nm.12
The band structure was calculated using a self-
consistent Poisson-Schr€odinger solver with the deformation
potentials from Reference 13 and the results for a single
FIG. 1. The heterolayer design for the device. Right: Optical microscope
picture of a completed LED.a)Douglas.Paul@glasgow.ac.uk.
0003-6951/2012/101(21)/211101/4/$30.00 VC 2012 American Institute of Physics101, 211101-1
APPLIED PHYSICS LETTERS 101, 211101 (2012)
Downloaded 11 Dec 2012 to 130.209.6.41. Redistribution subject to AIP license or copyright; see http://apl.aip.org/about/rights_and_permissions
quantum well are shown in Fig. 2. An 8-band k.p model was
used for the hole bands and the C-valley whilst a 1 band tool
was used to find the L- and D-valley bands and subband
states. The present thin buffer resulted in a tensile strain of
0.13% as measured by XRD after the sample was cooled to
room temperature and the modelling of the bands and sub-
band states has taken account of this strain.13 For both the
L- and C-valleys, only a single subband for each valley is
confined in the quantum well. The L-valley subband state is
very weakly confined in the quantum well due to the small
conduction band discontinuity of only 8meV. At room tem-
perature, confinement due to the L-valley quantum well is
unlikely to be observed. The C-valley is below both D-valley
bands due to the tensile strain in the substrate. The C-valley
quantum well has a discontinuity of 19meV with a single
confined state that due to the low effective mass of 0:038m0
(where m0 is the free electron mass), the subband states in
each quantum well overlap to form a miniband with width
calculated to be 8.4meV. Modelling of the valence band
demonstrated that the ground state is the heavy-hole (HH)
and the lowest subband is the HH1. The tensile strain is
therefore not great enough to move the LH band above the
HH band. The calculated lowest direct transition is C1 to
HH1 at 0.817 eV (1.52 lm wavelength) and the indirect L1
subband to HH1 is 0.663 eV (1.89 lm).
Cylindrical mesas ranging from 10lm to 500 lm in di-
ameter (see Fig. 1) were defined by a Vistec VB6 electron
beam lithography tool with hydrogen silsesquioxane (HSQ)
resist and then etched down anisotropically using a fluorine
based chemistry in an inductively coupled plasma reactive
ion etch tool. 50 nm of Ni was deposited by electron-beam
evaporation for the bottom contact and this was patterned by
lift-off and subsequently annealed at 340 C for 30 s in a
rapid thermal annealer (RTA).14 The process provides bot-
tom Ohmic contacts with measured specific contact resistiv-
ities of 4 108 Xcm2. An unannealed top contact of 10 nm
of Ti followed by 50 nm of Al was deposited to produce a
Schottky top contact which could inject carriers above the
conduction band edge. The whole structure was passivated
with Si3N4 and via holes were etched to allow interconnects
to the contacts. Bond pads of 600 nm of Al were deposited
and finally, the device was wire bonded to a chip carrier in
order to connect the LED to an external power supply for
characterisation. All results in this publication were from
devices of 300 lm diameter measured at room temperature.
The current-voltage characteristics are shown in Fig. 3.
Larger currents flow when the device is forward biased with
electrons being injected into the conduction band from the
Schottky top contact. The dark current is comparable to
some of the best reported from Ge on Si photodetectors.15
Both the photoluminescence (PL) and electroluminescence
from the LED were measured using a Bruker Vertex 70v
FTIR system. The FTIR system uses a calcium fluoride
beamsplitter and a room temperature extended InGaAs de-
tector, which can detect radiation between 0.8 and 2.5 lm.
All measurements were undertaken in surface emission ge-
ometry which corresponds to xy-polarization (TE polariza-
tion). The selection rules for such polarization allow the
following transitions:13 C1 to HH1, L1 to HH1, C1 to LH1,
and L1 to LH1. Fig. 4 shows the PL for 580 nm CW excita-
tion. Comparison with the band structure modelling of Fig. 1
indicates that the sharper peak just above 1.5 lm is the
C-valley to HH1 transition and the broader 1.8 lm peak is
the L-valley to HH1 indirect transition. Above 2lm is black-
body emission under CW illumination. Fig. 5 shows the elec-
troluminescence as a function of current under CW
conditions at room temperature. As the current is increased,
the direct bandgap (C-valley to HH1) electroluminescence
increases compared to the indirect (L-valley to HH1) transi-
tions. Also the blackbody contribution demonstrates signifi-
cant increases as the current is increased especially beyond
the 2 lm wavelength.
Fig. 6 demonstrates the total electroluminescence-
current (LI) data from the LEDs. Below 210A=cm2, the gra-
dient of the LI curve is below 1 whilst above this point, the
gradient increases to 1.5 and it is clear that blackbody emis-
sion from Joule heating of the sample starts to become more
significant in the measured electroluminescence power. Pre-
vious publications have indicated that heating can help excite
FIG. 2. The conduction bands for a single quantum well in the device as cal-
culated by self-consistent Poisson-Schr€odinger solver at 300K. The confined
subband states for the L- and C-valleys are also plotted.
FIG. 3. The current-voltage characteristics of the device at room
temperature.
211101-2 Gallacher et al. Appl. Phys. Lett. 101, 211101 (2012)
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carriers from the L-valley to the C-valley and increase the
electroluminescence efficiency and output power.7,16,17 A
similar effect is observed in the present devices as the ratio
of the emission from the direct bandgap compared to the
indirect bandgap emission increases with increasing current
density but the use of the longer wavelength detector in this
work also demonstrates the resulting strong spectral contri-
bution of the blackbody emission at longer wavelengths.
Fig. 7 shows the shift in the direct bandgap recombina-
tion peak as a function of current density. As the current den-
sity is increased to 420A=cm2, the emission wavelength is
increased by around 4% as shown in Fig. 7. The clear red-
shift in the direct bandgap electroluminescence recombina-
tion peak as a function of increasing current demonstrates
the Joule heating of the device with the resultant reduction in
the direct bandgap (the indirect bandgap is also reduced by
the heating). The bandgap dependence of Ge was modelled
by Varshni18 and the expected variation is inverse linearly
proportional to the temperature when the semiconductor is
well above the Debye temperature of 374K for Ge. Curve
fits to the blackbody part of the spectra in Fig. 5 indicate that
the electron temperature is above 400K for current densities
of 50A=cm2 and above thereby indicating that the bandgap
is decreasing with a linear variation of temperature. As Joule
heating in semiconductors results in a near linear tempera-
ture rise with current density, the linear variation of wave-
length with current density in Fig. 7 therefore agrees with
the Varshni model for the bandgap for temperatures above
the Debye temperature.18 At 300A=cm2, the direct recombi-
nation electroluminescence is at the important 1.55lm
wavelength for telecoms applications.
To date, the majority of Ge quantum well devices have
only demonstrated PL emission characterisation through
FIG. 7. The central position of the direct bandgap (C-valley to HH) electro-
luminescent peak as a function of current density at room temperature. The
line is a linear fit to the data.
FIG. 4. The photoluminescence of the device at room temperature using a
580 nm pump source.
FIG. 5. The electroluminescence of the device at room temperature as a
function of CW excitation currents.
FIG. 6. The electroluminescence versus current density plot of the device at
room temperature.
211101-3 Gallacher et al. Appl. Phys. Lett. 101, 211101 (2012)
Downloaded 11 Dec 2012 to 130.209.6.41. Redistribution subject to AIP license or copyright; see http://apl.aip.org/about/rights_and_permissions
optical pumping. Most electroluminescence has been demon-
strated with bulk n-Ge apart from a single result with emis-
sion below 1.45 lm wavelength.7 The present n-Ge quantum
well devices have a narrow direct recombination peak
(80 nm full width half maximum) followed by the drop in
electroluminescence to near zero levels at higher energies
(Fig. 5). This can be explained by the sharp two 2D density
of states which results in only the C-valley to HH1 and
L-valley to HH1 recombination transitions being observed
under electroluminescence, unlike the bulk Ge devices. Such
2D radiative transitions are ultimately expected to produce
higher gain and lower thresholds if a laser can be produced.3
The quantum well structures also provide a natural design
for confining (and guiding) the mode and therefore the ex-
pectation is that this approach is interesting to produce prac-
tical Ge electroluminescent LEDs and lasers on silicon
substrates.
The direct bandgap C-valley to HH1 electrolumines-
cence from n-Ge quantum wells on silicon substrates has
been demonstrated from LEDs at the important 1.55lm
wavelength with CW operation. The emission wavelength of
the devices can be tuned by approximately 4% through
changing the current density. At higher wavelengths, the
indirect L-valley to HH1 transition is observed before black-
body emission is observed. Such devices may be integrable
into Si photonic circuits, and lasing may be feasible with
such designs when fabricated into a suitable cavity.
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Electroluminescence from strained n-Ge quantum well light emitting diodes grown on a silicon substrate are demonstrated at room temperature. Electroluminescence characterisation demonstrates two peaks around 1.55 μm and 1.8 μm, which correspond to recombination between the direct and indirect transitions, respectively. The emission wavelength can be tuned by around 4% through changing the current density through the device. The devices have potential applications in the fields of optical interconnects, gas sensing, and healthcare

1.55 μm direct bandgap electroluminescence from strained n-Ge quantum wells grown on Si substrates

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