Chemical etching effects in porous silicon layers

SPIE. Microtechnologies for the New Millenium 2003. 19-21 May 2003.

Chemical etching effects in Chemical etching effects in porous silicon layersporous silicon layers

Daniel Navarro UrriosDaniel Navarro Urrios

Dpto. de Física Básica, University of La Laguna, SpainDpto. de Física Básica, University of La Laguna, Spain

INFM and Dipartimento di Fisica, University of Trento, ItalyINFM and Dipartimento di Fisica, University of Trento, Italy


Co-workersCo-workers

C. Pérez-Padrón, E. Lorenzo, N. E. Capuj

Dpto. de Física Básica, University of La Laguna, Avda. Astrofísico Fco. Sánchez, Dpto. de Física Básica, University of La Laguna, Avda. Astrofísico Fco. Sánchez,

La Laguna, 38071 SpainLa Laguna, 38071 Spain

Z. Gaburro, C. J. Oton and L. Pavesi INFM and Dipartimento di Fisica, University of Trento, Via Sommarive 14, INFM and Dipartimento di Fisica, University of Trento, Via Sommarive 14,

Povo, Trento 38050 ItalyPovo, Trento 38050 Italy


IndexIndex

Introduction and motivationsIntroduction and motivations

Interferometric measurementsInterferometric measurements

PL measurementsPL measurements

ConclusionsConclusions


Introduction to porous siliconIntroduction to porous silicon

What is porous silicon?What is porous silicon?– Nanostructured spongeous siliconNanostructured spongeous silicon– Electrochemically etched with HFElectrochemically etched with HF

Properties of PS:– Luminescent (quantum confinement)Luminescent (quantum confinement)– Optically homogeneousOptically homogeneous– Modulable refractive index with currentModulable refractive index with current– Promising for photonicsPromising for photonics

..

Usually etched


IntroductionIntroduction

Porous silicon formation is slightly non-homogeneous in depth

I

HF

HF chemically etches off porous silicon (porosity increases slowly)

Also luminescence properties depend on porosity(Quantum confinement)


MotivationsMotivations

Study the changes induced by chemical etching:Study the changes induced by chemical etching:

- Optical - Optical (in-situ refractive index monitoring)(in-situ refractive index monitoring)

- Structural - Structural (electronic microscopy)(electronic microscopy)

- Light emission - Light emission (PL)(PL)


IndexIndex






Photodetector

HF

PlatinumelectrodeHe-Ne laser

AttenuationFilter Mirror

c-Si

PSl

2

Experimental setupExperimental setup

Low light intensity!


2 2 22 sin ( )PS HFD l n n

Optical path difference between the two rays

During the anodization, the optical path increases

0

1( ) ( )cur

dDt t

dt

Frequency opt. path change rate

What we can seeWhat we can see

100 150 200 250 300

Dete

cte

d s

ign

al (a

.u.)

Electrochemical etch

Time (sec)

l

2


After anodization, there is still an oscillating signal!

0 1000 2000 3000 40000

20

40

anodization post-etching

Dete

cte

d s

ign

al

Time (s)

Cu

rren

t

Refractive index decrease

The phase changes sign!


(out-diffusion of chemical species, micro-bubbles…)

The signal frequency is the same before and after

It is an irreversible chemical process, not a transient

Could the post- etching oscillations be a transient?

5000 5020

0 2000 4000 60000

20

40

Det

ecte

d s

ign

al

Time (s)

Cu

rren

t

Short current pulse


During the post-etching we consider:

- constant thickness- variations in refractive index (nPS)

Signal frequency:0

2( ) PS

pednl

tdt

Frequency proportional to the sample thickness and to the refractive index change rate


ResultsResults

We studied samples with different anodization times:(increasing thickness)

400 s800 s

1200 s1600 s


0 2000 4000 6000 8000

10-3

10-2

400 s 800 s 1200 s 1600 s

PE (

s-1)

Post-etching time (s)

They overlap

The index change rate is independent on thickness

Two different exponential decays:

One fast (400 s)One slow (8200 s)

ResultsResultsFrequency vs. post-etching time

Normalizing to the thickness

0 2000 4000 6000 800010-3

10-2

Nor

mal

ized

PE (

s-1)

Post-etching time (s)

400 s (x4) 800 s (x2) 1200 s (x4/3) 1600 s (x1)


TEM micrographsTEM micrographs

Different post-etching times:

(Sample thickness: 500 nm)

50 nm 50 nm 50 nm

No post-etching 1200 sec 2400 sec

Fast post etching process Beginning of the slow process


What is happeningWhat is happening

Chemical etching enlarges the pores and reduces the

wall thickness


IndexIndex






Porous silicon luminescence is associated with quantum confinement in the nanostructures


What happens with the PL?

Post-etching reduces the nanostructure size

A blueshift is expected


We studied samples with the same anodization time (400s)(same thickness)


Different post-etching times0 s

1200 s1800 s5200 s

12000 s18000 s


500 600 700 800 900 1000

0.0

0.2

0.4

0.6

0.8

1.0

PL

inte

nsi

ty (

a.u

.)

Wavelength(nm)

0 sec 1200 sec 1800 sec 5200 sec


During post-etching, a short wavelength contribution appears, and then disappears

First minutes

It can be associated with the fast etching of a rough nanostructure on the walls of the pores


700 800 900

PL

inte

nsity

(a.

u.)

0 sec 1800 sec 5200 sec 12000 sec 18000 sec

Wavelength(nm)

Long post-etching times

The main PL peak blue-shifts


(waterfall plot)

Walls of the pores become thinner, increasing the quantum confinement


IndexIndex







1. HF chemically etches porous silicon homogeneously

- Increasing porosity

- Decreasing refractive index

2. For short times:

- Fast index decrease

- A blue component appears and disappears

3. For long times:

- Slow index decrease

- Slow blue-shift of the main PL peak


AcknowledgementsAcknowledgements

We acknowledge Spanish Ministry of Science and TechnologyProject MAT 2002-00044, the Canary Islands Government

(Project No. PI2001/093, PI2001/074), the European project EC-SINERGIA and the Italian INFM project PAIS-SMOG.

we thank Professors I. Martín and S. Gialanella






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Chemical etching effects in porous silicon layers