胡淑芬個人小檔案

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The Nanoscale

■ 10-10 m = 1 Ångstrom

■ 10-9 m = 1 Nanometer

■ 10-6 m = 1 Micrometer

■ 10-3 m = 1 Millimeter

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3

Transistor Scaling

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Moores’ Law

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Silicon Nanotechnology50 nm transistor diameter is ~ 2000x smaller than

diameter of human hair

Transistor for 90 nm Process Influenza virusSource: Intel Source: CDC

Gate dielectric thickness = 1.2 nm5/60

Nano Technology Intel 90 nm node device

1.2 nm SiO2

Gate oxide is less than 5 atomic layers 6/60

7

Transistor Nanotechnology

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Single-electron tunneling devices are promising candidates for future devices: -- low power consumption -- high integration density

Silicon – The most promising material for application to SLIs

1. Si-SETs can be used jointly with conventional CMOS circuits

2. Advanced fabrication technologies for Sub- quarter- micron CMOS LSIs can be used to make small silicon structures. 8/60

Comparison of the conventional MOSFET (left column) and the quantum dot transistor (right column) in structure, band diagram, and ID - VG characteristics.

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Q<e/2 Q>e/2

SET Operation Principle

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Stability plot for the SET transistorThe shaded regions are stable regions

Single Electron Transistor

Coulomb oscillations in a SET transistor

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• A solar cell or photovoltaic cell is a device that converts solar energy into electricity by the photovoltaic effect.

• Light shining on the solar cell produces both a current and a voltage to

generate electric power

The basic steps in the operation of a solar cell are:

• The generation of light-generated carriers; • The collection of the light-generated carries to generate a current• The generation of a large voltage across the solar cell• The dissipation of power in the load and in parasitic resistances

Solar Cell Structure

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When a photon hits a piece of silicon, one of three things can happen:

1. the photon can pass straight through the silicon — this (generally) happens for lower energy photons,

2. the photon can reflect off the surface,

3. the photon can be absorbed by the silicon which either: Generates heat, OR Generates electron-hole pairs, if the photon energy i

s higher than the silicon band gap value.

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Light Generated Current

1. The absorption of a photo creates an electron-hole pair

2. Ideally the minority carrier (in this case, a hole) make it cross the junction and

becomes a majority carrier

3. After passing through the load, the electron meets up with a hole and complete the circuit

If the light-generated minority carrier reaches the p-n junction, it is swept acrossthe junction by the electric field at the junction, where it is now a majority carrier. 14/60

Solar Cell Parameters

The IV curve of a solar cell is the superposition of the IV curve of the solar cell diode in the dark with the light-generated current

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Current-Voltage Measurements

The efficiency of a solar cell is determined as the fraction of incident power which is converted to electricity and is defined as:

Voc is the open-circuit voltage Isc is the short-circuit currentFF is the fill factorη is the efficiency.

A

B

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Quantum Efficiency The "quantum efficiency" (Q.E.) is the ratio of the number of carriers collected by the solar cell to the number of photons of a given energy incident on the solar cell

It is an accurate measurement of the device's electrical sensitivity to light If the quantum efficiency is integrated (summed) over the whole solar spectrum, one can evaluate the current that a cell will produce when exposed to white light(the short-circuit current) 17/60

Light-absorbing materials

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Solar Cells

Moving to 2nd & 3rd Generation Solar Cells

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Why Quantum Dots?

Quantum dot solar cells have the potential to increase the maximum attainable thermodynamic conversion efficiency of solar photon conversion up to about 66% by utilizing hot photogenerated carriers to produce higher photovoltages or higher photocurrents.

The former effect is based on miniband transport and collection of hot carriers in QD array photoelectrodes before they relax to the band edges through phonon emission. The latter effect is based on utilizing hot carriers in QD solar cells to generate and collect additional electron–hole pairs through enhanced impact ionization processes.

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Novel materials consisting of silicon (Si) nanocrystals embedded in a dielectric matrix have attracted considerable interest in the field of silicon optoelectronics and third generation Photovoltaics. When Si nanocrystals are made very small (< 7 nm in diameter), they ∼behave as quantum dots (QDs) due to the three-dimensional quantum confinement of Carriers.

MRS BULLETIN • VOLUME 32 • MARCH 2007 • www.mrs.org/bulletin

Indirect bandgap Quasi-directbandgap

the band gaps can be adjusted specifically to convert also longer- wave light

Enhanced electron–hole pair (exciton) multiplication in quantum dots that could lead to enhanced solar photon conversion efficiency in QD solar cells.

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Si3N4 Si3N4

Si quantum dot

Band gap

C.B.

V.B.

hν

lightC.B.

V.B.

hν < Eg

Quantum dot deviceBuck device

hν >> Eg

Thermal energy

Band Gap Diagram

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Proceedings of ECTI-CON 2008

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Quantum Dots:

E(eV)= 1.56+2.40/a2

(a is the dot size)Quantum Well:

E(eV)= 1.6+0.72/d2

(d is the well width)

1-4244-0016-3/06/$20.00 ©2006 IEEE

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APPLIED PHYSICS LETTERS 91, 163503 2007

Solar cells based on quantum dots may carry extra benefits of increasedradiation hardness and improved collection efficiency. 26/60

The cell with the 3 nm QDs had the highest efficiency, with an open-circuit voltage (Voc) of 556 mV, a short-circuit current (Jsc) of 29.8 mA cm−2

,a fill factor (FF) of

63.8%, and conversion efficiency of 10.6%.

2008

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