chemical etching effects in porous silicon layers
DESCRIPTION
Chemical etching effects in porous silicon layers. Daniel Navarro Urrios Dpto. de Física Básica, University of La Laguna, Spain INFM and Dipartimento di Fisica, University of Trento, Italy. Co-workers. C. Pérez-Padrón, E. Lorenzo, N. E. Capuj. - PowerPoint PPT PresentationTRANSCRIPT
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
SPIE. Microtechnologies for the New Millenium 2003. 19-21 May 2003.
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
SPIE. Microtechnologies for the New Millenium 2003. 19-21 May 2003.
IndexIndex
Introduction and motivationsIntroduction and motivations
Interferometric measurementsInterferometric measurements
PL measurementsPL measurements
ConclusionsConclusions
SPIE. Microtechnologies for the New Millenium 2003. 19-21 May 2003.
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
SPIE. Microtechnologies for the New Millenium 2003. 19-21 May 2003.
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)
SPIE. Microtechnologies for the New Millenium 2003. 19-21 May 2003.
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)
SPIE. Microtechnologies for the New Millenium 2003. 19-21 May 2003.
IndexIndex
Introduction and motivationsIntroduction and motivations
Interferometric measurementsInterferometric measurements
PL measurementsPL measurements
ConclusionsConclusions
SPIE. Microtechnologies for the New Millenium 2003. 19-21 May 2003.
Photodetector
HF
PlatinumelectrodeHe-Ne laser
AttenuationFilter Mirror
c-Si
PSl
2
Experimental setupExperimental setup
Low light intensity!
SPIE. Microtechnologies for the New Millenium 2003. 19-21 May 2003.
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
SPIE. Microtechnologies for the New Millenium 2003. 19-21 May 2003.
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!
SPIE. Microtechnologies for the New Millenium 2003. 19-21 May 2003.
(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
SPIE. Microtechnologies for the New Millenium 2003. 19-21 May 2003.
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
SPIE. Microtechnologies for the New Millenium 2003. 19-21 May 2003.
ResultsResults
We studied samples with different anodization times:(increasing thickness)
400 s800 s
1200 s1600 s
SPIE. Microtechnologies for the New Millenium 2003. 19-21 May 2003.
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)
SPIE. Microtechnologies for the New Millenium 2003. 19-21 May 2003.
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
SPIE. Microtechnologies for the New Millenium 2003. 19-21 May 2003.
What is happeningWhat is happening
Chemical etching enlarges the pores and reduces the
wall thickness
SPIE. Microtechnologies for the New Millenium 2003. 19-21 May 2003.
IndexIndex
Introduction and motivationsIntroduction and motivations
Interferometric measurementsInterferometric measurements
PL measurementsPL measurements
ConclusionsConclusions
SPIE. Microtechnologies for the New Millenium 2003. 19-21 May 2003.
Porous silicon luminescence is associated with quantum confinement in the nanostructures
PL measurementsPL measurements
What happens with the PL?
Post-etching reduces the nanostructure size
A blueshift is expected
SPIE. Microtechnologies for the New Millenium 2003. 19-21 May 2003.
We studied samples with the same anodization time (400s)(same thickness)
PL measurementsPL measurements
Different post-etching times0 s
1200 s1800 s5200 s
12000 s18000 s
SPIE. Microtechnologies for the New Millenium 2003. 19-21 May 2003.
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
PL measurementsPL measurements
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
SPIE. Microtechnologies for the New Millenium 2003. 19-21 May 2003.
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
PL measurementsPL measurements
(waterfall plot)
Walls of the pores become thinner, increasing the quantum confinement
SPIE. Microtechnologies for the New Millenium 2003. 19-21 May 2003.
IndexIndex
Introduction and motivationsIntroduction and motivations
Interferometric measurementsInterferometric measurements
PL measurementsPL measurements
ConclusionsConclusions
SPIE. Microtechnologies for the New Millenium 2003. 19-21 May 2003.
ConclusionsConclusions
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
SPIE. Microtechnologies for the New Millenium 2003. 19-21 May 2003.
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
SPIE. Microtechnologies for the New Millenium 2003. 19-21 May 2003.
SPIE. Microtechnologies for the New Millenium 2003. 19-21 May 2003.
SPIE. Microtechnologies for the New Millenium 2003. 19-21 May 2003.
SPIE. Microtechnologies for the New Millenium 2003. 19-21 May 2003.
SPIE. Microtechnologies for the New Millenium 2003. 19-21 May 2003.