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Received 18 Sep 2013 | Accepted 26 Nov 2013 | Published 2 Jan 2014

Room-temperature sub-band gap optoelectronicresponse of hyperdoped siliconJonathan P. Mailoa1, Austin J. Akey1, Christie B. Simmons1, David Hutchinson2, Jay Mathews3,

Joseph T. Sullivan1, Daniel Recht4, Mark T. Winkler1,w, James S. Williams5, Jeffrey M. Warrender3,

Peter D. Persans2, Michael J. Aziz4 & Tonio Buonassisi1

Room-temperature infrared sub-band gap photoresponse in silicon is of interest for tele-

communications, imaging and solid-state energy conversion. Attempts to induce infrared

response in silicon largely centred on combining the modification of its electronic structure

via controlled defect formation (for example, vacancies and dislocations) with waveguide

coupling, or integration with foreign materials. Impurity-mediated sub-band gap photo-

response in silicon is an alternative to these methods but it has only been studied at low

temperature. Here we demonstrate impurity-mediated room-temperature sub-band gap

photoresponse in single-crystal silicon-based planar photodiodes. A rapid and repeatable

laser-based hyperdoping method incorporates supersaturated gold dopant concentrations on

the order of 1020 cm� 3 into a single-crystal surface layer B150 nm thin. We demonstrate

room-temperature silicon spectral response extending to wavelengths as long as 2,200 nm,

with response increasing monotonically with supersaturated gold dopant concentration. This

hyperdoping approach offers a possible path to tunable, broadband infrared imaging using

silicon at room temperature.

DOI: 10.1038/ncomms4011

1 School of Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139, USA. 2 Department of Physics,Applied Physics, and Astronomy, Rensselaer Polytechnic Institute, 110 8th Street, Troy, New York 12180, USA. 3 US Army ARDEC - Benet Laboratories,1 Buffington Street, Watervliet, New York 12189, USA. 4 Harvard School of Engineering and Applied Sciences, 29 Oxford Street, Cambridge, Massachusetts02138, USA. 5 Research School of Physics and Engineering, The Australian National University, Mills Road, Canberra 0200, Australian Capital Territory,Australia. wPresent address: IBM Thomas J. Watson Research Center, 1101 Kitchawan Road, Yorktown Heights, New York 10598, USA. Correspondence andrequests for materials should be addressed to J.P.M. (email: [email protected]) or to T.B. (email: [email protected]).

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Developing a low-cost broadband infrared detector witharray-based real-time imaging capability at room tem-perature is of interest for telecommunications, security,

energy and research and development applications1–5. Silicon-based detectors satisfy the low-cost and on-chip complementarymetal-oxide semiconductor (CMOS) compatibility criteria, buttheir infrared photoresponse is fundamentally limited by the1.12-eV band gap (l¼ 1,110 nm). Previous attempts to extend thephotoresponse of silicon-based devices into the short-wavelengthinfrared regime (l¼ 1,400–3,000 nm) at room temperaturefocused on forming heterostructures with SiGe alloys6–8 ormicrostructures of thermally absorbing material9,10, andmodifying the intrinsic band structure via intentionalintroduction of defects11–17. Growing microstructures of foreignmaterials on top of silicon results in processing complexities thatcan compromise CMOS compatibility18,19. Furthermore,integration of thermally absorbing materials can be effective forshort-wavelength infrared response but results in slow pixelresponse times intrinsically limited by the thermal time constant,which hinders applications in array-based real-time imagingsystems at room temperature10.

The direct modification of silicon’s electronic band structure isa promising approach for imaging applications, but significantadvances on room-temperature sub-band gap photoresponsein silicon to date are mostly focused on fiber-coupled single-point detector applications. Published approaches predomi-nantly involve intentionally damaging the silicon lattice12–17,which produces a measurable sub-band gap absorption coeffi-cient (a¼ 2–4 cm� 1)16 compared with untreated silicon(ao2� 10� 7 cm� 1 for l41,400 nm)20. To increase the opticalpath length and sub-band gap absorptance, these devicesare integrated with micron-sized waveguide resonators13,17. Byintentionally damaging the silicon lattice, Geis et al.16 fabricatedsingle-point, fibre-coupled waveguide photodiodes and photo-transistors with room-temperature sub-band gap (l¼ 1,550 nm)photoresponse of 20 and 50 A W� 1, respectively16,17. Theseresults correspond to impressive quantum efficiencies of 16 and40, much larger than unity because of avalanche gain16,17.Unfortunately, the waveguide architecture renders this deviceresponsive for only a narrow band of wavelengths, and therequirement for coplanar illumination impedes incorporationinto imaging arrays.

Another approach involves incorporating chalcogen dopantsinto silicon through picosecond or femtosecond laser irradiation.This approach has resulted in a limited number of reports ofinfrared photoresponse21,22. However, the origin of the sub-bandgap photoresponse is not well understood or controlled owing tothe complexity of the laser-processed microstructure involvingstructural defects and multiple chemical phases23. To elucidatethe origin of sub-band gap photoresponse in chalcogen-dopedsilicon, sulphur ions have been implanted into silicon and theresulting amorphous layer subjected to pulsed laser melting(PLM)-induced rapid solidification using a nanosecond laser,resulting in a single-crystalline silicon layer with sulphurconcentrations above 1019 cm–3 (ref. 24). This nanosecondlaser-hyperdoping process is repeatable and yields homogenousand single-crystal material, reducing the material’s complexitycompared with the microstructure with multiple phases found insilicon irradiated with picosecond or femtosecond lasers23.However, there are no reports of room-temperature sub-bandgap photoresponse for this single-crystalline material. Aschalcogen hyperdoping introduces only donor states into thesilicon band gap, the resulting material has a large background-free carrier concentration at room-temperature25,26. This largebackground-free carrier concentration overwhelms the sub-bandgap photoconductivity signal, making the observation of room-

temperature sub-band gap photoconductivity in chalgogen-hyperdoped silicon difficult27. Similar problems have beenencountered in titanium-hyperdoped silicon for which sub-bandgap photoconductivity is observed only at liquid nitrogentemperature or below28. This is similar to prior work onimpurity-diffused silicon photodetectors where the demonstrationof sub-band gap photoresponse has been limited to lowtemperatures29,30. Room-temperature sub-band gap photo-response of hyperdoped single-crystalline silicon remains elusive.

In this study, we report room-temperature sub-band gapphotoresponse from single-crystalline silicon hyperdoped withan alternative dopant, gold, to concentrations as high as5� 1020 cm–3. We present planar photodiode devices suitablefor imaging-array applications fabricated from silicon hyper-doped with gold (Si:Au). Steady-state and transient photo-response measurements demonstrate that the photodiodeoptoelectronic response extends well into the sub-band gapregime of silicon, to at least l¼ 2,200 nm. Furthermore, themagnitude of the response increases monotonically with golddopant concentration. The homogenous, single-crystalline natureof Si:Au gives insight into the likely physical mechanism ofenhanced sub-band gap photoresponse, a discrete set ofAu-induced donor and acceptor mid-gap energy levels thatgenerate free carriers in response to infrared light, yet reducebackground-free carrier concentrations via self-compensation.As PLM induces substantial heating only in the top 150 nm of thewafer, it might be added as the last step of any CMOS-compatibleoptoelectronic device fabrication process in user-defined deviceareas, limiting potential unintentional contamination effectsof Au.

ResultsGold hyperdoping of silicon. The top 150 nm of three identicalo1004 n-type silicon wafers were implanted with gold ionsusing an implantation energy of 50 keV and doses of 3� 1014,7� 1014 and 1� 1015 cm� 2. PLM was used to melt the amor-phous implanted region, which rapidly solidifies back into single-crystal silicon with high, non-equilibrium dopant concentration(Fig. 1a)24. Because of gold’s high diffusive velocity andsegregation coefficient in liquid silicon31, the high solidificationspeed outpaces the kinetics of gold segregation and resultsin a supersaturated, single-crystalline solid solution31,32. AnNd:YAG nanosecond pulse was used to melt and rapidly solidifythe gold-hyperdoped silicon (Si:Au) region, and in situ time-resolved reflectivity33 measurements showed a melt duration of25–30 ns, corresponding to a solidification speed of about 7–10 m s� 1 based on a dopant diffusion simulation in establishedliterature31. This process is represented pictorially in Fig. 1a;more detail about the PLM process can be found in theMethods section.

After PLM, secondary ion mass spectrometry (SIMS) wasperformed to measure the gold atomic concentration as afunction of depth (Fig. 1b). The solid solubility limit of gold insilicon at room temperature is estimated to be below 1015 cm� 3

(refs 34–37). Supersaturated gold concentrations were achievedup to 150 nm depth for all three gold doses, with the largest goldconcentration peak of 5� 1020 cm� 3 measured within the top10 nm of the Si:Au layer. This corresponds to 1 at.% Auconcentration, more than four orders of magnitude above theroom-temperature equilibrium solid solubility limit of goldin silicon. High-resolution transmission electron microscopicmeasurements confirmed that the entire Si:Au layer was singlecrystalline, without formation of any extended defects, secondaryphases or cellular breakdown (Fig. 1b)38. Because of the largeconcentration of mid-gap dopant levels introduced by the goldimpurities, the hyperdoped layer absorbs sub-band gap light as

ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/ncomms4011

2 NATURE COMMUNICATIONS | 5:3011 | DOI: 10.1038/ncomms4011 | www.nature.com/naturecommunications



shown in the infrared transmission image through the sampletaken using an InGaAs camera (Fig. 2a). Transmission (T) andreflection (R) were measured using ultraviolet–visible–near-infrared spectrophotometry to quantify the sub-band gapabsorptance (A¼ 1�T�R) in the hyperdoped silicon. Similarto chalcogen-hyperdoped silicon39, sub-band gap light absorp-tance increases with increasing Au concentration (Fig. 2b).Approximating the sub-band gap absorptance occurringuniformly throughout the hyperdoped region (150 nmthickness), the average sub-band gap absorption coefficient a atl¼ 1,550 nm of the Si:Au with the highest gold dose is estimatedto be about 600 cm� 1 (1% absorptance averaged over a 150-nmthick layer; see Methods), which is larger than that of germaniumat the same wavelength (B300 cm� 1)40.

Steady-state room-temperature sub-band gap photoresponse.To measure the room-temperature sub-band gap photoresponsein Si:Au, photodiodes were fabricated (see Methods). The darkI–V curves (Fig. 3b) for photodiodes fabricated on n-type siliconsubstrates exhibit rectification behaviour, as expected when themajority carriers in supersaturated Si:Au are holes41. In contrast,Si:Au on a p-type Si substrate does not show rectification(Supplementary Fig. 1). Reactive ion etching and electron-beamTi/Au contact evaporation were used to electrically isolateand contact a 1� 1 mm2 (ref. 2) Si:Au area to define the activearea of the device. An identical photodiode was fabricated on thesame substrate but with the Si:Au layer replaced by boron-dopedsilicon (Si:B) as a reference sample (Supplementary Fig. 2).

The photodiodes were then connected to a comparativeresistor, a lock-in amplifier and a voltage source (VIN¼ � 5 V)that placed the devices under reverse bias (Fig. 3a). A diode laser(optically chopped at frequency f¼ 714 Hz) was focused to a100–150-mm spot size on the photodiode surface. The comparativeresistor value of R¼ 1 kO was chosen such that the reverse biasvoltage across the photodiodes was close to 5 V (IdarkRoo|Vin|).The laser spot was then scanned across the photodiode surface.Sub-band gap optoelectronic response was observed only in the1� 1-mm2 area of the photodiode corresponding to theelectrically connected Si:Au region.

A map of sub-band gap response at l¼ 1,550 nm is shown inFig. 3d for a sample with implantation dose of 1015 Au cm� 2.

a

b

IR transmission(plane view)

Si:Au

Silicon

PatternedSi:Au

3

2

1

01,200 1,300 1,400 1,500 1,600 1,700 1,800

λ (nm)

Abs

orpt

ance

(%

)

Increaseswith dose

3×1014 cm–2

7×1014 cm–2

1015 cm–2

Figure 2 | Sub-band gap absorptance of Si:Au. (a) Transmission of sub-

band gap light through patterned Si:Au taken using an InGaAs camera. Light

transmission is reduced by absorption in Si:Au (dark areas) relative to the

crystalline silicon (bright areas). Scale bar, 1 mm. (b) Sub-band gap light

absorptance (1—transmission—reflection) of Si:Au for various gold

implantation dose relative to the sub-band gap light absorptance in the

reference plain silicon sample, measured using ultraviolet–visible–near-

infrared spectrophotometry. Sub-band gap light absorptance increases with

increasing gold dose.

a b

1018 1019 1020 1021200

150

100

50

0

Dep

th (

nm)

Au concentration(atoms cm–3)

Protective carbon

Si:Au

~150 nm

x-sect

Au– ionimplant

<100> Si <100> Si

Pulsedlaser melting

<100> Si

Rapidsolidification

Si:Au

a-Si

Figure 1 | Fabrication of Si:Au. (a) Fabrication of Si:Au: ion implantation of Au followed by PLM. Rapid solidification follows the PLM process, producing

single-crystalline Si:Au on the top 150 nm of the silicon wafer. The silicon area not melted by the PLM remains amorphous. (b) SIMS profile of Si:Au with

1� 1015 cm� 2 gold implantation dose, showing that the gold concentration in the Si:Au is more than four orders of magnitudes beyond the solid solubility

limit of gold in silicon. The transmission electron microscopic images show that for this gold implantation dose, the entire Si:Au layer is crystalline.

Scale bar, 10 nm (left) and 50 nm (right).

NATURE COMMUNICATIONS | DOI: 10.1038/ncomms4011 ARTICLE




The spatial uniformity of the photoresponse within the 1� 1-mm2

area demonstrates that internal photoemission from the metalcontact electrodes is not the dominant cause of the sub-band gapphotoresponse42. The reference Si:B photodiode control samplesdid not show any measurable sub-band gap optoelectronicresponse (Fig. 3e), further confirming that the Si:Au layer is theorigin of the room-temperature sub-band gap photoresponse.This observation is in contrast with the low-temperature sub-band gap photoresponse previously reported in titanium-hyperdoped silicon, where the sub-band gap photoresponse ofthe reference crystalline silicon is reported to be larger thanthat of the titanium-hyperdoped silicon43. The sub-band gapphotoresponse measurements on the Si:Au photodiodes under5 V reverse bias were performed with different sub-band gapwavelengths (l¼ 1,310, 1,550 and 1,650 nm); spatially optimizedexternal quantum efficiency (EQE) measurements for sub-bandgap wavelengths for all Au doses are shown in Fig. 3f. Si:Auphotodiode response increases monotonically with increasinggold concentration for all three sub-band gap wavelengths, withthe maximum EQE of 2.8� 10� 4 and 9.3� 10� 5 for l¼ 1,310and 1,550 nm, respectively. More detail about the EQE calculationcan be found in the Methods section.

Room-temperature spectral response of Si:Au photodiodes.The extent and mechanism of Si:Au room-temperature opto-electronic spectral response was further studied via transientphotoresponse measurements. Nanosecond pulses with tunablewavelengths (l¼ 1,100–2,300 nm) were derived from an opticalparametric amplifier (OPA)44 pumped by a frequency-tripled,

Q-switched Nd:YAG laser. The pulses were passed through a longpass filter and a Pellin Broca prism to eliminate any undesiredwavelengths (for example, laser harmonics) before incidence nthe photodiode. The shape of the spectral responses for the Si:Auphotodiode with 1� 1015 cm� 2 implant dose and the referenceSi:B photodiode are shown in Fig. 4a. Because of the highintensity of the sub-band gap laser pulse, a sub-band gapphotoresponse signal appeared within the reference siliconsample owing to two-photon absorption (TPA)45. This signalresponded quadratically with pulse intensity, as expected for atwo-photon process, and could be reduced by tuning the pulseintensity appropriately. The spectral response in the referencesample agrees well with the calculated value expected for TPA,I(l)b(l) where I(l) is the intensity of the pulse laser used andb(l) is the published value of two-photon absorption coefficient45

for each wavelength (see Methods for the calculation of TPA).Figure 4b shows the strength of the photoresponse signal with

respect to the applied laser pulse intensity for both above gap andsub-band gap light. The photoresponse increased linearly withincreasing laser pulse intensity except for the sub-band gapphotoresponse in the reference silicon sample, which increasedquadratically with increasing laser intensity due to TPA. Inaddition to the different scaling with photon intensity, the TPAsignal was easily distinguished from the photoconductive signalbecause the transient decay of the response was much slower;this characteristic should be expected for TPA because freecarriers were generated and had to be collected from the entiresilicon substrate. To mitigate the effect of TPA from both Si:Auand reference silicon spectral response in Fig. 4a, only the short-lived peak of the transient signal was used in the definition

x (mm)

y (m

m)

EQE x 10–5EQE x 10–5

Si reference, � = 1,550 nm

a c

e

Lock-inamplifier

Vin = –5 V

R

Choppeddiode laser

Si:(Au+B)

Ti/Aun-Si

Si:Auxy

Mapping stage

Cur

rent

(m

A)

ΔI (

μA)

Vin (V)

Increasingreverse bias

Vin (V)

f

EQ

E

Increases with dose

b

d

x (mm)

Si:Au, � = 1,550 nm

y (m

m)

10

8

6

4

2

0

–1.0 –0.5 0.0 0.5 1.0

Au dose:

3×1014 cm–27×1014 cm–21015 cm–2

2

1

0–5 –4 –3 –2 –1 0

1.0

0.5

0.0

–0.5

–1.0–1.0 –0.5 0.0 0.5 1.0

1.0

0.5

0.0

–0.5

–1.0–1.0 –0.5 0.0 0.5 1.0

9.5

7.6

5.7

3.8

1.9

0.0

9.5

7.6

5.7

3.8

1.9

0.0

10–3

10–4

10–5

10–6

1014

Au implant dose (cm–2)

2 4 6 81015

� = 1,310 nm

� = 1,550 nm

� = 1,650 nm

Figure 3 | Room-temperature sub-band gap photoresponse in Si:Au. (a) Si:Au photodiode with Si:Au layer on n-Si substrate operating at reverse bias.

The optically chopped diode laser light is focused to 100–150mm spot size and is scanned across the photodiode surface. (b) The dark I–V curve of

Si:Au on n-type Si shows rectification behaviour for gold implantation dose of 3� 1014–1� 1015 cm� 2, indicating that the majority carriers in Si:Au

are holes. (c) The difference between dark and illuminated I–V of the photodiode with 1015 cm� 2 gold dose. Increasing reverse bias across the junction

increased carrier drift velocity in the Si:Au layer, improving the collection of photo carriers excited by the sub-band gap light. (d) Mapped external quantum

efficiency (EQE) showing a 1� 1 mm2 Si:Au active area for l¼ 1,550 nm and gold implantation dose of 1015 cm� 2, confirming the Si:Au layer as the sole

source of the sub-band gap photoresponse in the photodiode. (e) No sub-band gap response is observed when the Si:Au is replaced with Si:B in the

reference silicon photodiode. (f) EQE for various sub-band gap wavelengths increases with the gold concentration in the Si:Au photodiode.





of the dimensionless figure of merit Z¼A �DIpeak/Nph, whereA¼ 4.8� 1021 A� 1 is a normalization constant, DIpeak is thephoto-generated current and Nph is the number of photons in thelaser pulse. We observe measureable spectral response of Si:Auextending to Eph¼ 0.55 eV, and a distinct change in slope atphoton energy Eph¼ 0.78 eV (Fig. 4a). This later spectral featureappears to be dopant specific, as discussed in the next section.

DiscussionWe first comment on the probable origins of the sub-bandgap photoresponse of Si:Au. Previous work by Sah et al.46

and Okuyama et al.48 utilized temperature-dependent photo-conductivity and spectrally resolved low-temperature photo-conductivity, respectively, to determine the energy level of goldimpurities in silicon diffused with gold concentration belowthe solid solubility limit. At these low dopant concentrations, theAu-induced defect states are single, discrete levels with non-interacting localized states. In that study48, the reported spectralresponse has a distinct change in slope near the photon energyEph¼ 0.78 eV, which corresponds to the substitutional gold donorenergy level in silicon. In our study, the same spectral featurewas observed in the Si:Au photodiode with a gold dose of1� 1015 cm� 2, suggesting that the sub-band gap photoresponsemechanism in these Si:Au photodiodes may be the same as that ingold-diffused silicon measured at liquid nitrogen temperature48.The higher gold concentration achieved by our PLM processapparently enables one to observe this effect at room-temperature.

Next, we address the role of ‘drift’ in carrier extraction fromthe Si:Au layer. It is known that deep-level traps such asAu in silicon reduce the electronic carrier lifetime throughnon-radiative recombination49. With the gold concentration

of Nt¼ 1020 cm� 3 and the saturated electron velocity ofvs¼ 107 cm s� 1 assumed in the depleted Si:Au layer50, anelectron trapping rate teo¼ (sn

t vsNt)� 1¼ 5 ps would be expected,given an electron capture cross-section sn

t ¼ 2� 10� 16 cm2 ofgold in silicon51. Despite the low minority-carrier lifetime inSi:Au, some free electrons excited by the sub-band gap light inthe hyperdoped region were able to escape into the n-typesubstrate. This process is aided because the Si:Au layer is fullydepleted because of the low carrier concentration in gold-dopedsilicon (see Methods and Supplementary Fig. 3). This lowcarrier concentration in the Si:Au layer is believed to be causedby the self-compensation due to the ionization of deep acceptorand donor trap levels of substitutional gold in silicon41,52,53.The resulting full depletion of the Si:Au causes the majority ofthe applied reverse bias voltage to drop across this layer,thereby facilitating carrier transport via drift.

This behaviour is in contrast with past work in sulphur-hyperdoped silicon25, where the free carrier concentration isbetween 1017 and 1020 cm–3 and the minority-carrier transport inthe hyperdoped region is driven by the comparatively weakerprocess of diffusion27. The application of 5 V reverse bias acrossthe hyperdoped layer appears sufficient to saturate thephotoresponse. This is shown in Fig. 3c, where the photo-generated current produced by l¼ 1,550 nm light saturates withincreasing reverse bias voltage. While higher gold concentration isexpected to reduce the sub-band gap photoresponse by reducingthe minority-carrier lifetime, the applied reverse bias voltageseems to be capable of overcoming the negative effect of highergold concentration. Hence, the sub-band gap optoelectronicresponse increases with the increasing sub-band gap absorptancein samples with higher gold concentrations (Fig. 3f).

Next, we discuss how the dopant type affects the infraredspectral response. In this dopant-mediated, extrinsic sub-band

b cAbove band gap,� = 1,100 nm

m = 1

m = 1

Sub-band gap,� = 1,400 nm

m = 1

m = 2

a

d

Eph > 0.78 eV

1

0.1

0.01

10–3

10–4

10–5

0.5 0.6 0.7 0.8 0.9 1.0 1.1

Eph (eV)

Noise floor

Et = 0.78 eV

Es = 42 meV

Si:AuSi RefTPA (calc.)

1,000

100

10

1

Sig

nal (

mV

)

10–4 10–3 0.01 0.1 1Photon intensity (a.u.)

Si:AuSi:Ref

Si: AuSi Ref

1,000

100

10

1

Sig

nal (

mV

)

10–4 10–30.01 0.1 1

Photon intensity (a.u.)

–1.0

–1.5

–2.0–2 –1

Log (Eph – Et)

n = 0.6

Si:Au

Log

(�)

Si:Au n-Si

–

–

–

–

–

�0n0

�0p –1

� (a

.u.)

Figure 4 | Sub-band gap photoresponse mechanism of Si:Au. (a) Si:Au spectral response Z as a function of photon energy measured using tunable

wavelength transient photoresponse measured at room temperature for the photodiode with the highest gold dose of 1015 cm� 2. A kink in the

spectral response is observed at the threshold energy Et¼0.78 eV, which corresponds to the substitutional gold donor level in silicon. For EphoEt, the

response is characterized by the Urbach absorption edge with a slope of Es¼42 meV. (b) Above-band gap and sub-band gap photoresponse of the Si:Au

and reference silicon photodiodes for varying photon intensity, confirming that the sub-band gap response in the reference silicon is caused by two-photon

absorption (TPA) in the substrate. Under standard operating conditions (B100 Wcm� 2 or lower), no measurable TPA was observed in the reference

silicon photodiode. (c) Possible sub-band gap photoresponse mechanism for substitutional gold in silicon through the donor level for Eph40.78 eV.

(d) The polynomial dependence (n¼0.6) of Z on Eph, for Eph close to Et (0.8 eVoEpho0.94 eV), suggesting that the defect-state wavefunction is

close to that of a delta function.





gap photoresponse mechanism, electrons can be excited into theconduction band through the dopant state(s). One probablemechanism is the electron excitation from the valence band to theconduction band through the donor energy level, which haspreviously been shown for gold-diffused silicon at low tempera-ture (Fig. 4c)48,54. In ref. 28, donor-assisted transition arises atthreshold energy of 0.78 eV. Inspecting Fig. 4a, similar thresholdenergy is evident in the Si:Au photodiode. For photon energy Eph

larger than the threshold energy Et¼ 0.78 eV (ref. 34), thetransitions characterized by photoionization cross-section s0

p� 1(from valence band to the donor state) and s0

n0 (from donor stateto the conduction band) are both possible. Electrons from thevalence band can then be excited into the conduction band andswept away by the strong reverse bias electric field across theSi:Au layer into the n-type substrate. They are then transported asmajority carriers in the conduction band before being extractedby the back contact. However, for EphoEt the transition from thedonor level to the conduction band characterized by s0

n0 is turnedoff; thus, electrons excited into the donor level eventuallyrecombine into the valence band without leaving the Si:Au-hyperdoped region. As the donor-level/conduction-band tran-sition is unavailable, the magnitude of the sub-band gap responsefor the energy range EphoEt becomes exponentially less likelywith decreasing excitation energy. One possible model for thisexponential decay is the dependence Z(Eph)pexp((Eph�Et)/Es)where Es is the slope of the Urbach absorption edge due todisorder in solids55. Es¼ 42 meV has been extracted from fits tothe experimental data as the slope of the Urbach edge. This islarger than kT¼ 26 meV, suggesting that there is a disorder-induced broadening in either the impurity states or theconduction band edge of Si:Au. The spectral kink atEph¼ 0.78 eV closely matches the spectral response of gold-diffused silicon at low temperature, suggesting that it should bepossible to tune the sub-band gap spectral response ofhyperdoped silicon by using alternative dopant elements thatintroduce levels at different energies48.

Further analysis can be done on the spectral response ofSi:Au for Eph40.78 eV. Various models have been proposed toexplain the dependence of the photoionization cross-sections(Eph) for deep levels in semiconductor. If the deep-levelwavefunction is a localized state with delta function like spatialdependence in the length scale of the atomic radius, for Eph closeto the threshold energy Et¼ 0.78 eV the approximate dependences(Eph)p(Eph� Et)1/2 is expected56,57. In contrast, for a statewith hydrogenic like wavefunction the approximate dependences(Eph)p(Eph� Et)3/2 is expected58. The conversion efficiencyZ is directly proportional to the photoionization cross-sectionin the Si:Au photodiode, and as such the dependenceZ(Eph)p(Eph� Et)n for Eph close to Et¼ 0.78 eV can be used toinfer the spatial behaviour of the deep-level wavefunction. Therange for the fit is set to 0.8 eVoEpho0.94 eV to reducethe influence of the Urbach edge, and the fit is shown inFig. 4d. The polynomial dependence of n¼ 0.6 is extracted,suggesting that the deep-level state wavefunction for the donorlevel of Si:Au has a spatial dependence much closer to that of adelta function56,57 than that of a hydrogenic wavefunction58.

As the first demonstration devices, there is room to improvethe absolute room-temperature sub-band gap EQE of B10� 4.Two focus areas should include improving sub-band gap lightabsorptance and incorporating best-in-class photodiode-designpractices to enhance carrier collection. This can be achieved, forexample, by using a larger gold implantation dose for the PLMprocess and applying an anti-reflection coating on the photodiodeto increase the sub-band gap absorptance of the Si:Au layerbeyond its current B1% absorptance. Increasing the thickness ofthe Si:Au layer could also increase the optical path length and

sub-band gap absorptance. Further gains may be possible viaoptimization of device design: because most of the gold dopantsare concentrated near the silicon surface (Fig. 1b), the majorityof free carriers generated by sub-band gap light are believed tobe within the top 20 nm of the Si:Au surface. As the surface-limited lifetime for W¼ 20 nm thick silicon is in the order ofts¼W/S¼ 2 ps when a non-passivated surface recombinationvelocity S¼ 106 cm s� 1 is assumed59, the sub-band gapphotoresponse in Si:Au with a bulk lifetime of B5 ps is proneto surface recombination. Incorporating surface passivationcould potentially suppress this recombination activity. Weconservatively estimate that practical application of theseimprovements may increase the room temperature sub-bandgap optoelectronic response of Si:Au photodiodes by two ordersof magnitudes to EQE¼ 10� 2, although no physical limits areknown at this time to prevent even higher EQE.

In summary, we report the room-temperature sub-band gapoptoelectronic response in silicon hyperdoped with gold using atwo-step process: ion implantation followed by PLM. The sub-band gap optoelectronic response is shown to correspond toknown gold dopant energy levels in silicon and increase with theimplanted gold concentration. This work represents a fundamen-tally new approach to achieve sub-band gap optoelectronicresponse in silicon that avoids structural defects and interfacemanagement issues associated with combining foreign materialsand silicon. While the magnitude of the room-temperature sub-band gap response demonstrated here is in the EQEB10� 4

range, further improvements are likely via device architectureoptimization. The planar, single-crystal nature of the hyperdopedsilicon layer created using the PLM process makes this anattractive material candidate for room-temperature sub-band gapphoton-imaging devices based on silicon.

MethodsNanosecond PLM. Silicon wafers (o1004, n-type, resistivity B3O cm, minority-carrier lifetime B180ms) were ion-implanted with 197Au� at an energy of 50 keV,to doses of 3� 1014, 7� 1014 or 1� 1015 atoms cm� 2. The wafers were thenpulsed-laser melted by a single pulse from an Nd:YAG laser, using the thirdharmonic wavelength of 355 nm, with a pulse duration full-width-half-maximum(FWHM) of 5 ns and a fluence of 0.7 J cm� 2. The resulting material was char-acterized by SIMS using a Physical Electronics 6650 Dynamic SIMS instrument.Operating conditions were the following: 6 keV Cs ion beam at 1 nA, with SIMScraters being 50 mm2; depth was measured ex situ by contact profilometry.Au concentrations were calibrated against known ion-implantation doses from theas-implanted regions of each sample, normalized by the 28Si signal. Transmissionelectron microscopic samples were fabricated by focused-ion beam milling andliftout; transmission electron microscopic imaging was performed on a JEOL2100 high-resolution transmission electron microscope operated at 200 keV inbright-field mode.

Approximation of sub-band gap a in Si:Au. We model the Si:Au as a uniformt¼ 150 nm thin film on top of crystalline silicon wafer. It is assumed that the realrefractive index of Si:Au is not any different from that of a normal crystallinesilicon. Furthermore, to simplify the model, it is assumed that light interference inthe thin-film Si:Au region does not play a critical role in the light propagation.As crystalline silicon does not absorb sub-band gap light, the sub-band gap lightabsorption in the Si:Au layer can then be approximated as:

A ¼ 1�Rð Þatþ 1�Rð Þat 1� atð ÞRþ 1�Rð Þat 1� atð Þ2R2 þ :::

¼ 1�Rð Þat1� 1� atð ÞR

where A is the sub-band gap absorption measured using the spectrophotometerand R is the fraction of light reflected when light propagates through the air–siliconinterface. Taking the value of RB0.36 and AB1%, a¼A(1�R)/t(1�R�AR)¼600 cm� 1 can be approximated. This is the lower bound for the value of a inSi:Au with gold implantation dose of 1015 cm� 2. In reality, 90% of the goldatoms are buried within the top 20 nm of the Si:Au layer. As the majority of thesub-band gap light absorption is believed to occur within this 20 nm thin layer,the actual value of a in the region with highest gold concentration may be severaltimes larger.





Si:Au photodiode-type determination. As we have detailed in the main text, atthe supersaturated concentrations obtained for Si:Au implanted with gold doses of3� 1014, 7� 1014 and 1� 1015 cm� 2, the majority carriers in Si:Au are holes.When Si:Au is fabricated on an n-type silicon substrate, the resulting structure is ajunction that shows the rectifying behaviour of a diode. In contrast, when Si:Au isfabricated on a p-type silicon substrate, the resulting structure does not rectify.A dark I–V curve for one such structure is shown in Supplementary Fig. 1, whereSi:Au with gold implantation dose of 1014 cm� 2 was fabricated on a p-typesubstrate.

Photodiode device fabrication. See Supplementary Fig. 2 for device dimensions.Before the PLM process, boron ions were selectively implanted into the gold-dopedsilicon area under the top contacts to improve contact resistance. The selected areafor the boron implantation was patterned using photolithography, and 11Bþ ions(3� 1014 cm� 2 dose, 2 keV implantation energy) were implanted into the top30 nm of the gold-doped area. The entire sample then underwent the PLM process(described above), creating a homogenous single-crystal Si:Au layer with a pair ofrectangular Si:(AuþB)-patterned areas positioned for subsequent contactdeposition. The depth of the boron doping is simulated to be B70 nm deep afterthe PLM process. Photolithography and SF6 reactive ion etch were then used tocreate a 1-mm-deep trench that isolated the 1� 1 mm2 rectangular Si:Au and theSi:(AuþB) contact area from the rest of the Si:Au layer. Photolithography, e-beamevaporation and lift-off were used to deposit 30/160 nm of Ti/Au contacts on top ofthe Si:(AuþB) areas. On the back of the silicon wafer, a uniform 30/160 nm Ti/Aulayer was deposited across the entire surface as the photodiode back contact. Theback contact was attached to an external electrode by conductive silver paste, whilethe top contacts were connected to electrodes by Al wire bonds. For the referencesilicon photodiode, the same boron-doping profile is used everywhere instead ofSi:Au, creating a p–n photodiode without any Au-hyperdoped silicon.

EQE calculation. A 2-V peak-to-peak square wave S(t) can be expressed asS(t)¼ 1.273 sin (ot)þ 0.4244 sin (3ot)þ 0.2546 sin (ot)þ ... where o is the angularfrequency and t is time. As the optically chopped light can be approximated as asquare wave and the lock-in amplifier output Vout is the root mean squared value ofthe first Fourier component of the signal, the mapped EQE for each coordinate onthe photodiode can be calculated using the formula EQE(x, y)¼ 2.22Vout(x, y)/Rqf(l), where Vout is the signal output from the lock-in amplifier, R is the value ofthe comparative resistor, q is the electron charge and f(l) is the number ofphotons incident on the sample per unit time measured using a calibratedgermanium photodiode. The Si:Au photodiodes were operated at � 5 V reversebias for these EQE measurements.

Modelling of fully depleted hyperdoped region in Si:Au diode. When I–Vresponses of Si:Au photodiodes were measured under large reverse biases, it isobserved that the dark current increases almost linearly with the increase of reversebias voltage. This is in contrast to that of a non-depleted diode that normallyapproach a steady dark current following the diode equation I¼ Io(eqV/kT� 1).A simple diode model to simulate this behaviour is developed in the PC1D devicesimulator. A p-type emitter layer with a thickness of 150 nm and a low carrierlifetime of 20 ps is used to represent the Si:Au layer. The base is represented with ann-type silicon with 500 mm thickness and 180 ms carrier lifetime (parameterobtained from the actual substrate). PC1D simulation shows that the hyperdopedSi:Au region is indeed fully depleted, and that the resulting dark I–V curve showsthe same linear behaviour for large reverse bias voltages (Supplementary Fig. 3).

Transient-pulse laser photoresponse. The laser pulse with 10 ns FWHM exitingthe OPA consisted of the sub-band gap signal wavelength (ls), the idler wavelength(li), as well as possible leakage of other above-band gap wavelengths (la) due to theinner working of the OPA. la was first filtered out using a razor-edge 1,064 nm longpass edge filter (Edmund Optics, NT47-510). The resulting beam was furtherpurified by dispersing it using Pellin Broca prism mounted on a rotation stage andselecting only the desired wavelength (ls) with a subsequent aperture. The rotationstage was calibrated such that across the entire spectral range considered only ls

could illuminate the sample. Infrared neutral density filters mounted on a filterwheel were then used to tune the beam intensity. The intensity was chosen suchthat the magnitude of the transient voltage was much smaller than the supplyvoltage Vin¼ � 5 V to prevent signal saturation. Furthermore, the beam intensitywas also chosen such that the signal due to two-photon absorption (TPA) in thesubstrate was minimized. An average of 200 pulses was used for the acquisition ofeach data point.

TPA model for reference photodiode sub-band gap response. One of thecharacteristics of two-photon absorption (TPA) is characterized by the sub-bandgap absorption coefficient aTPA(l)¼ I(l)b(l) where I(l) is the intensity of thepulse laser used and b(l) is the published value of two-photon absorption coeffi-cient45 for each wavelength. The value of b(l) for l¼ 1,200–2,200 nm is about1 cm GW� 1. The maximum pulse laser energy in our transient photoresponseexperiment is about 100mJ per pulse with B0.5 cm beam diameter. As the

laser pulse FWHM is about 10 ns, the maximum instantaneous intensityof the laser pulses incident on the photodiode samples are 50 kW cm� 2.In this case, maximum aTPA(l) of 5� 10� 5 cm� 1 can be approximated.This absorption coefficient is very small, and consequently for L¼ 0.5 mmthick silicon reference photodiode that we used in our experiment the totalsub-band gap light intensity absorbed due to TPA can be approximated asIABS(l)pI(l)(1� exp(� aTPA(l)2L))pI2(l)b(l)2L. The photoresponse signalobserved in the oscilloscope is directly proportional to this value, hence thequadratic dependence of the signal to the photon intensity observed in Fig. 3b. Thecorresponding value for the dimensionless figure of merit Z is obtained by dividingthe photoresponse signal with the sub-band gap photon illumination. Hence, wehave the proportionality relationship Z(l)¼AI(l)b(l) to model the two-photonabsorption in Fig. 4a, where A is the normalization constant.

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AcknowledgementsWe thank V. Suntharalingam, C. Leitz and M.W. Geis (Lincoln Laboratory) for helpfuldiscussions; Z.K. Ren (SMART) for PC1D simulation advice; S. Castellanos (MIT) for thehelp in taking infrared transmission image; and F. Frankel (MIT) for helpful suggestionsregarding graphics. Research at Harvard was supported by the US Army-ARDEC underContract number W15QKN-07-P-0092 and the US Army Research Office under Con-tract number W911NF-12-1-0196. Research at the ANU was funded by the AustralianResearch Council, and we also thank the use of facilities provided by the AustralianNational Fabrication Facility. Research at MIT was supported by National ScienceFoundation grant for Energy, Power, and Adaptive Systems under Contract numberECCS-1102050, and in part by the National Science Foundation (NSF) and theDepartment of Energy (DOE) under NSF CA number EEC-1041895. This work wasfunded in part by the MIT-KFUPM Center for Clean Water and Energy. This work madeuse of the MTL and CMSE at MIT supported by NSF award DMR-0819762. This workwas performed in part at the Center for Nanoscale Systems (CNS), a member of theNational Nanotechnology Infrastructure Network (NNIN), which is supported by theNational Science Foundation under NSF Award number ECS-0335765. CNS is a partof Harvard University.

Author contributionsJ.P.M., A.J.A., C.B.S., D.H., J.M. and J.S.W. synthesized the materials, made the devices,built the experimental apparatus and carried out the experiments. All authors analysedthe data, with critical inputs from all authors. J.P.M. and T.B. were responsible forexperimental design and wrote the manuscript, which incorporates critical input fromall authors.

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Competing financial interests: The authors declare no competing financial interests.

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How to cite this article: Mailoa, J. P. et al. Room-temperature sub-band gapoptoelectronic response of hyperdoped silicon. Nat. Commun. 5:3011 doi: 10.1038/ncomms4011 (2014).






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