Black : silicon sees the

Graduate Consortium on Energy and Environment Harvard University Cambridge, MA, 17 September 2010 Mark Winkler Renee Sher Yu-Ting Lin and also....

Eric Diebold Prof. Tonio Buonassisi (MIT) Haifei Albert Zhang Prof. Silvija Gradecak (MIT) Dr. Brian Tull Dr. Bonna Newman (MIT) Dr. Jim Carey Joe Sullivan (MIT) Prof. Tsing-Hua Her Dr. Shrenik Deliwala Prof. Augustinus Asenbaum (Vienna) Dr. Richard Finlay Dr. Michael Sheehy Dr. François Génin (LLNL) Dr. Claudia Wu Mark Wall (LLNL) Dr. Rebecca Younkin Prof. Catherine Crouch Dr. Richard Farrell (RMD) Prof. Mengyan Shen Dr. Arieh Karger (RMD) Prof. Li Zhao Dr. Richard Meyers (RMD)

Dr. John Chervinsky Dr. Pat Maloney (NVSED) Dr. Joshua Levinson Dr. Jeffrey Warrander (ARDEC) Prof. Michael Aziz Prof. Cynthia Friend Prof. Howard Stone Introduction

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outer (“valence”) determine electronic properties Introduction

pure (“intrinsic”) silicon Introduction

electrons in covalent bond are immobile Introduction

all electrons bound, so no conduction Outline

• doped

• pn-junctions

• black silicon Doped semiconductors

intrinsic silicon: no conduction Doped semiconductors

substitute phosphorous: surplus of (free) electrons Doped semiconductors

(but material as a whole still neutral!) Doped semiconductors

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apply electric field… Doped semiconductors

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…free electrons lead to conduction Doped semiconductors

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electrons in electrons out

…free electrons lead to conduction Doped semiconductors

substitute boron: deficit of electrons leaves “holes” Doped semiconductors

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apply electric field… Doped semiconductors

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…presence of holes leads to conduction Doped semiconductors

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…presence of holes leads to conduction Doped semiconductors

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…presence of holes leads to conduction Doped semiconductors

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…presence of holes leads to conduction Doped semiconductors

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…presence of holes leads to conduction Doped semiconductors

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motion of holes

holes are like positively charged particles Doped semiconductors

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holes out electrons out (electrons in) (holes in)

motion of holes

holes are like positively charged particles Doped semiconductors

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simplify representation Outline

• doped semiconductors

• pn-junctions

• black silicon pn-junctions

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bring p and n materials together… pn-junctions

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bring p and n materials together… pn-junctions

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electrons and holes diffuse across junction… pn-junctions

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…and get ‘trapped’ after they combine pn-junctions

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build-up of charge leads to electric field that stops diffusion pn-junctions

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non-conducting layer at junction pn-junctions

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apply electric field… pn-junctions

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…holes pushed to left, electrons to right… pn-junctions

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…and so depletion zone expands pn-junctions

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NO conduction pn-junctions

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reverse electric field… pn-junctions

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…depletion zone shrinks and current flows pn-junctions

so pn-junction like one-way valve for charge flow pn-junctions

diode

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current flows along arrow only (from p to n) pn-junctions

can also be used as a light detector! pn-junctions

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depletion layer can convert light into electric energy pn-junctions

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incident knocks out … pn-junctions

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…creating an electron-hole pair pn-junctions

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E-field separates eh-pair, causing current pn-junctions

how to make a miniature diode on a chip? pn-junctions

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begin with an n-doped wafer pn-junctions

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p-dope small region pn-junctions

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cover with insulating layer pn-junctions

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etch insulating layer pn-junctions

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add aluminum contacts Outline

• doped semiconductors

• pn-junctions

• black silicon Black silicon

SF6

Si irradiate with 100-fs 10 kJ/m2 pulses Black silicon Black silicon

“black silicon” Black silicon

3 µm Black silicon

10 µm Black silicon

10 µm Black silicon Black silicon Black silicon

absorptance

1.0

0.8

0.6

0.4 absorptance

0.2

0 0 1 2 3 wavelength ( µ m) Black silicon

absorptance

1.0

microstructured silicon 0.8

0.6

0.4 absorptance

0.2 crystalline silicon

0 0 1 2 3 wavelength ( µ m) Black silicon

What causes the near-unity absorptance? Black silicon Black silicon multiple reflections enhance absorption

1.0

microstructured silicon 0.8

0.6

0.4 absorptance

0.2 crystalline silicon

0 0 1 2 3 wavelength ( µ m) Black silicon multiple reflections enhance absorption

1.0

microstructured silicon 0.8

0.6

0.4 absorptance

0.2 crystalline silicon

0 0 1 2 3 wavelength ( µ m) Black silicon heavy sulfur doping causes absorption

1.0

microstructured silicon 0.8

0.6

0.4 absorptance

0.2 crystalline silicon

0 0 1 2 3 wavelength ( µ m) Black silicon Black silicon Black silicon

cross-sectional Transmission Electron Microscopy Black silicon

M. Wall, F. Génin (LLNL)

1 µm Black silicon

crystalline Si core

1 µm Black silicon sulfur-containing surface layer

1 µm Black silicon black silicon/n-type silicon junction

n-Si substrate Black silicon black silicon/n-type silicon junction

n-Si substrate Black silicon black silicon/n-type silicon junction

n-Si substrate Black silicon black silicon/n-type silicon junction

Cr/Au contacts

n-Si substrate Black silicon

black silicon/n-type silicon junction

3 mm Cr/Au contacts ~ 400 nm

260 µm n-Si substrate

100 nm

4 mm Black silicon

IV characteristics

4

2

dark 0

–2 current (mA)

–4

–6 –3 –2 –1 0 1 2 3 bias (V) Black silicon

IV characteristics

4

2

dark 0

–2 current (mA) illuminated –4

–6 –3 –2 –1 0 1 2 3 bias (V) Black silicon

responsivity

100

10

1

commercial Si PIN 0.1 responsivity (A/W) 0.01

0.001 200 600 1000 1400 1800 wavelength (nm) Black silicon

responsivity

100

10

–0.1 V bias 1

commercial Si PIN 0.1 responsivity (A/W) 0.01

0.001 200 600 1000 1400 1800 wavelength (nm) Black silicon

responsivity

100 –0.5 V bias 10

–0.1 V bias 1

commercial Si PIN 0.1 responsivity (A/W) 0.01

0.001 200 600 1000 1400 1800 wavelength (nm) Black silicon

Black silicon photo diode (at 0.5 V bias):

• 100x larger signal in visible (gain!)

• 105 larger signal in infrared Devices

http://www.sionyx.com Black silicon

New Scientist 13, 34 (2001) Black silicon

Black silicon also promising for solar cells Black silicon

What causes the large gain?

How could black silicon be useful for photovoltaics? Devices

solar spectrum

2.5 m)

μ 2 2.0

1.5

1.0

0.5 spectral irradiance (kW/m 0 0 0.5 1.0 1.5 2.0 2.5 wavelength (μ m) Devices

solar spectrum

2.5 m)

μ 2 2.0

1.5

1.0

0.5 1353 W/m2 spectral irradiance (kW/m 0 0 0.5 1.0 1.5 2.0 2.5 wavelength (μ m) Devices crystalline silicon: transparent to 23% of solar radiation

2.5 m)

μ 2 2.0

1.5

c-Si = 1.12 μm 1.0

0.5 1042 W/m2 spectral irradiance (kW/m 0 0 0.5 1.0 1.5 2.0 2.5 wavelength (μ m) Devices amorphous silicon: transparent to 53% of solar radiation

2.5

m) a-Si band gap = 0.71 μm μ 2 2.0

1.5

1.0

0.5 636 W/m2 spectral irradiance (kW/m 0 0 0.5 1.0 1.5 2.0 2.5 wavelength (μ m) Devices black silicon: potential to recover transmitted energy

2.5 m)

μ 2 2.0

1.5

1.0

0.5 1353 W/m2 spectral irradiance (kW/m 0 0 0.5 1.0 1.5 2.0 2.5 wavelength (μ m) Devices very preliminary photovoltaic cell

1 remove V source, 0 0 insert load

m

W

A measure I Devices very preliminary photovoltaic cell

5

0 dark

–5

–10 current (mA)

–15

–20 –1 0 1 bias (V) Devices very preliminary photovoltaic cell

5

0 dark

–5

–10 current (mA)

–15 illuminated

–20 –1 0 1 bias (V) Devices

1.5% efficiency, a good beginning

5

0 dark

–5 1.5 mW

–10 current (mA)

–15 illuminated

–20 –1 0 1 bias (V)

Funding:

Army Research Offi ce DARPA Department of Energy NDSEG National Science Foundation for more information and a copy of this presentation:

http://mazur-www.harvard.edu

Follow me! eric_mazur

Funding:

Army Research Offi ce DARPA Department of Energy NDSEG National Science Foundation for more information and a copy of this presentation:

http://mazur-www.harvard.edu

Follow me! eric_mazur