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. CanhamChief Scientific Officer, pSiMedica Ltd., Malvern, and Honorary Professor, School of Physics and Astronomy, University of Birmingham, Birmingham, England, UK

NOBEL PRIZE 2012 Nominee2012 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 PSPlatinum 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 SizeThe 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

http://en.wikipedia.org/wiki/Mesoporous

Microporous

Macroporous

Mesoporous

Cross-sectional electron microscope imageof 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 ResearchCenter, 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

Thanks for Attention

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




1- Silicon Wafer – P-Type, 1–10Ω cm resistivity 2- Platinum Cathode(鉑陰極 )– 99.5% pure3- Hydrofluoric Acid(氫氟酸 )– 50%4- Ethanol(乙醇 )5- Electrolysis Cell

Electrolyte solution = HF:H2O:C2H5OH = 1:1:2Current density = 19 mA cm-2 Electrolysis Time = 5 minutes

Different Parameters (不同的參數 )

Electrolyte Solution電解液 :HF:H2O:C2H5OHorHF: AGNO3

orHF: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 porosityBy changing the different parameters, we can get different kinds of Porosity for Silicon.

P = (mw-mps) / (mw-mrps)mw= Mass of wafer before Anodizationmps = Mass of wafer after Anodizationmrps = Mass of wafer after removing part of porous silicon layer in 1N KOH SOLUTION.

Electrolysis Cell

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 WaferG 型夹夹持夹具

PTFE CELL Platinum Wire 铂丝 Aluminum Foil 铝箔

The CELL DESIGN

The CELL DESIGN

ResultsElectrolyte 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:

ResultsSample 1:

Solvent Used: (3:2:2)Hydrofluoric Acid 氢氟酸Ethanol 乙醇Water 水 Time: 5 minutes

Voltage: 5 Volts

ResultsSample 2:

Solvent Used: (3:2:2)Hydrofluoric Acid 氢氟酸Ethanol 乙醇Water 水

Washed the sample with Nitric Acid 硝酸 after etchingProduce some White flash of Flash light formillisecond Time: 5 minutes

Voltage: 5 Volts

ResultsSample 3:

Solvent Used: (2:1)Hydrofluoric Acid 氢氟酸Nitric Acid 硝酸

Washed the sample Acetone 丙酮 after etchingProducing Brown Fumes (布朗吸油烟 ) during chemical Etching Time: 10 minutes

Voltage: 7 Volts

ResultsSample 4:

Solvent Used: (1:1)Hydrofluoric Acid 氢氟酸Water 水

Washed the sample Acetone 丙酮 after etching

Time: 8 minutes

Voltage: 5-10 Volts

ResultsSample 5:



Time: 20 minutes

Voltage: 5-10 Volts

ResultsSample 7:

Solvent Used: (1:1)Hydrofluoric Acid 氢氟酸Ethanol 乙醇

USE N-TYPE SILICON WAFER


Time: 20 minutes

Voltage: 5-10 Volts

ResultsSample 8:



Time: 45 minutes

Voltage: 2 Volts

ResultsSample 9:

Solvent Used: (1:1:1:1)Hydrofluoric Acid 氢氟酸Nitric Acid 硝酸


Time: 10 minutes

Voltage: 10Volts

ResultsSample 10:

Solvent Used: (1:1:1:1:1)Hydrofluoric Acid 氢氟酸Nitric Acid 硝酸Acetone 丙酮Ethanol 乙醇Water 水


Time: 10 minutes

Voltage: 10Volts

Bubbles From Holes

Bubbles From Holes

Education

A brief note on porous silicon