A brief note on porous silicon

For the discovery of
photoluminescence in porous silicon
对于发现在多孔硅的光致发光
Leigh T. Canham
Chief Scientific Officer, pSiMedica Ltd.,
Malvern, and Honorary Professor,
School of Physics and Astronomy,
University of Birmingham, Birmingham, England, UK
NOBEL PRIZE 2012 Nominee
2012 年诺贝尔物理学奖被提名人

NOBEL PRIZE 2012 Winner
突破性的实验方法，使测量和操控单个量子系统

Luminescence (cold light, annealing) : It’s ability to emit light
waves by solid states.
Generated another reason than heating.
There is a few kind of luminescence e.g. Photoluminescence (PL),
electroluminiescence (EL).
•PL – exited by photons beam.
•EL - exited by electric field

Photoluminescence ( 光致发光 )

SILICON- 硅
Is the 2nd
most abundant element in the earths crust. ( 第二最丰富的元素 )
Never occurs as a free element( 自由元素 ), it is always mixed with an other element.
When mixed with oxygen it forms silica which is usually found as sand.
Only 2 elements make up almost 3/4
of the Earth's crust
Used when turning solar energy into electricity.
Used to make concrete an/or brick.
Silica( 二氧化硅 ) is whats used to make glass.

Porous Silicon- 多孔
硅• Porous silicon was discovered by accident in 1956 by Arthur Uhlir Jr. and
Ingeborg Uhlir at the Bell Labs in the U.S .
• At the time, the Ulhirs were in the process of developing a technique for
polishing and shaping the surfaces of silicon and germanium.
• In 1980s Leigh Canham reasoned that the porous silicon may display quantum
confinement effects. The intuition was followed by successful experimental
results published in the 1990. In the published experiment, it was revealed that
silicon wafers can emit light if subjected to electrochemical and chemical
dissolution.
• Canham is suggested as a possible Nobel Prize
winner “for the discovery of photoluminescence
in porous silicon”

• Porous silicon was discovered by accident. It was produced by non-uniform etching during the
electropolishing of silicon with an electrolyte containing hydrofluoric acid.
• The etching resulted in a system of disordered pores with nanocrystals remaining in the
inter-pore regions. Porous silicon is still manufactured by Anodization ( 阳极氧化 ) or
electrochemical etching( 电化学蚀刻 )of silicon in hydrofluoric acid (HF) solutions. Aqueous
HF is unsuitable for the etching process because the silicon surface is hydrophobic.
• The porous layer can be made more structurally uniform if an ethanolic solution( 乙醇溶
液 )is used - this increases the wettability of the silicon and allows better surface penetration by
the acid. Ethanoic etch solutions also reduce the formation of hydrogen gas bubbles as ethanol
acts as a surfactant and prevents bubbles sticking to the silicon surface.

Scheme of produce PS
Platinum cathode and Silicon Anode

Porous Silicon as Explosive
多孔硅爆炸
In 2001, a team of scientists discovered that hydrogenated porous silicon
reacts explosively with oxygen at cryogenic temperatures, releasing
several times as much energy as an equivalent amount of TNT, at a
much greater speed
在 2001 年，一队科学家发现氢化多孔硅爆炸性反应在低温下与氧
气，释放出尽可能多的能量等量的 TNT 几次，在一个更大的速度
Explosion occurs because the oxygen, which is in a liquid state at the
necessary temperatures, is able to oxidize through the porous molecular
structure of the silicon extremely rapidly, causing a very quick and
efficient detonation.
发生爆炸的氧气，这是在必要的温度下以液体状态，因为能够非常
迅速地通过多孔氧化硅的分子结构，从而导致一个非常快速和高效
的爆轰。
Although hydrogenated porous silicon would probably not be effective
as a weapon, due to its functioning only at low temperatures, other uses
are being explored for its explosive properties, such as providing thrust
for satellites.
虽然氢化多孔硅可能不会有效的武器，由于其运作只能在低温，其
他用途正在探索其爆炸性能，如提供卫星推力。

Porous Silicon 多孔硅
In the most basic sense,
porous silicon is a network of
air holes within an
interconnected silicon matrix.
The size of these air holes,
called pores, can vary from a
few nanometers( 纳米 )to a few
microns( 微米 ) depending on
the conditions of formation and
the characteristics of the
silicon.
The SEM(Scaning
Electron Microscope) image
typical porous silicon sample.

Pour Size
The porosity value of silicon is a macroscopic parameter and doesn’t yield any
information regarding the microstructure of the layer. It is proposed that the
properties of a sample are more accurately predicted if the pore size and its
distribution within the sample can be obtained. Therefore, porous silicon has been
divided into three categories based on the size of its pores;
1.Macroporous 孔
2.Mesoporous 介孔
3.Microporous 微孔

Microporous
Macroporous
Mesoporous

Cross-sectional electron microscope image
of a porous silicon sample containing two distinct pore morphologies.
The morphology is controlled by the current applied during etching.
In this sample, the current was decreased suddenly during preparation,
resulting in the abrupt decrease in pore diameter observed.
Sample courtesy Manuel Orosco, University of California, San Diego.
Electron micrograph courtesy Melanie L. Oakes, Hitachi Chemical Research
Center, Irvine, CA. Inset is 590 nm in diameter.

Porosity 多孔性
Porosity or void fraction is a measure of the void (i.e., "empty") spaces
in a material, and is a fraction of the volume of voids over the total
volume, between 0–1, or as a percentage between 0–100%
孔隙率或空隙比是衡量的空隙（即，“空”）空间中的材料，是超过
总体积的空隙的体积的一小部分，在 0-1 之间，或在 0-100 ％之间
的百分比
The porosity of porous silicon may range from 4% for macroporous
layers to 95% for mesoporous layers.
It was also found that a silicon wafer with medium to low porosity
displayed more stability.

The silicon nanocrystals in
PS that emits visible light vary in
size from 10-15Å (diameter).
Raman spectroscopy( 拉曼光谱 )
gives indirect information about
the microstructure of PS and has
shown that the nanocrystals alter
the selection rules relating to the
interaction of optical phonons
with incident photons.Which
result in the MORE EFFICIENT
Photoliminance and
Electroluminance of Porour
Silicon as compared to the Non-
porous silicon.

The nanoporous structures have dimensions in the low nm-
range. If the structure size reaches a value below, say 3 nm, quantum
effects can occur and therefore nanoporous samples can exhibit
strong visible photoluminescence and electroluminscence, as can be
seen in the picture below.
Photoluminescence of a nanoporous silicon sample

SEM images and spectra of porous Si samples.
The images are examples of a low porosity 低孔隙度
(left) and high porosity (middle). The spectra (right)
indicate the fluorescence tunability of porous Si.

Photoluminescent properties of
porous silicon films
Porous silicon samples with photoluminescent peaks in
different parts of the visible range
Electroluminescent spectrum of
porous silicon film
Typical photoluminescent spectrum of
porous silicon film

Structural color from electrochemically fabricated porous silicon photonic crystals. Their
porous nanostructures reflect specific wavelengths of light. The optical spectrum is sensitive to
the refractive index of any molecules filling the pores, which allows them to be used as
chemical and biological sensors

Fabrication of porous silicon
多孔硅的制造
 Anodization 阳极氧化
 Stain etching 染色蚀刻
 Drying of porous silicon 干燥的多孔硅

Anodization 阳极氧化
1- Silicon Wafer – P-Type, 1–10Ω cm resistivity
2- Platinum Cathode( 鉑陰極 )– 99.5% pure
3- Hydrofluoric Acid( 氫氟酸 )– 50%
4- Ethanol( 乙醇 )
5- Electrolysis Cell
Electrolyte solution = HF:H2O:C2H5OH = 1:1:2
Current density = 19 mA cm-2
Electrolysis Time = 5 minutes

Different Parameters ( 不同的參數
)
Electrolyte Solution 電解液 :
HF:H2O:C2H5OH
or
HF: AGNO3
or
HF:HNO3:H2O:C2H5OH
Current = DC or AC
(DC used for more thin homogeneous porosity layer)
Electrolysis Duration( 電解時間 )= 5 Minute to 45 Minutes
By changing the different parameters, we can get different
kinds of Porosity for Silicon.

Measurement of porosity
By changing the different parameters, we can get different
kinds of Porosity for Silicon.
P = (mw-mps) / (mw-mrps)
mw= Mass of wafer before Anodization
mps = Mass of wafer after Anodization
mrps = Mass of wafer after removing part of porous silicon
layer in 1N KOH SOLUTION.

Surface modification of porous silicon 多孔矽表面改性
Freshly etched porous silicon may be unstable due to the rate of its oxidation
( 氧化 )by the atmosphere or unsuitable for cell attachment purposes.
It can be surface modified to improve stability
Thermal Oxidation: ( 熱氧化 )
The process involves heating the silicon to a temperature above 1000 C to promote
full oxidation of silicon
Thermal Oxidation can make the pSi most stable

Properties of pSi Sample
• Photoluminescence Properties(PL)
PL measurements were performed at room temperature using a Si detector.
The 442 nm line of a He–Cd laser and unfocused laser power of 24.5 mW
were used as an excitation source for photoluminescence. Standard lock-in
techniques were used to maximize the signal-to-noise ratios, and all PL
spectra were corrected for system response.

The CELL DESIGN
O-Ring
P-Silicon Wafer
G 型夹夹持夹具
PTFE CELL Platinum Wire 铂丝 Aluminum Foil 铝箔

Results
Electrolyte Solvent Used:
Hydrofluoric Acid 氢氟酸
Nitric Acid 硝酸
Acetone 丙酮
Ethanol 乙醇
Isopropyl Alcohol 异丙醇
Water 水
Voltage Range Used:
2 minutes to 60 minutes
2 Volts to 10 Volts
Time Range Used:

Results
Sample 1:
Solvent Used: (3:2:2)
Ethanol 乙醇
Water 水
Time: 5 minutes
Voltage: 5 Volts

Results
Sample 2:
Ethanol 乙醇
Water 水
Washed the sample with Nitric Acid 硝酸
after etching
Produce some White flash of Flash light for
millisecond
Time: 5 minutes
Voltage: 5 Volts

Results
Sample 3:
Solvent Used: (2:1)
Nitric Acid 硝酸
Washed the sample Acetone 丙酮
after etching
Producing Brown Fumes ( 布朗吸油烟 ) during chemical Etching
Time: 10 minutes
Voltage: 7 Volts

Results
Sample 4:
Solvent Used: (1:1)
Water 水
after etching
Time: 8 minutes
Voltage: 5-10 Volts

Results
Sample 5:
Ethanol 乙醇
Water 水
after etching
Time: 20 minutes
Voltage: 5-10 Volts

Results
Sample 7:
Solvent Used: (1:1)
Ethanol 乙醇
USE N-TYPE SILICON WAFER
after etching
Time: 20 minutes
Voltage: 5-10 Volts

Results
Sample 8:
Ethanol 乙醇
Water 水
after etching
Time: 45 minutes
Voltage: 2 Volts

Results
Sample 9:
Solvent Used: (1:1:1:1)
Nitric Acid 硝酸
after etching
Time: 10 minutes
Voltage: 10Volts

Results
Sample 10:
Solvent Used: (1:1:1:1:1)
Nitric Acid 硝酸
Acetone 丙酮
Ethanol 乙醇
Water 水
after etching
Time: 10 minutes
Voltage: 10Volts

A brief note on porous silicon

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