ii workshop on chemical sensors and biosensors · ii workshop on chemical sensors and biosensors 11...

II WORKSHOP ON CHEMICAL SENSORS AND BIOSENSORS

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APPLICATIONS OF POROUS SILICON AS A GAS SENSOR

G. Di Francia1, L. Quercia1,G. Iadonisi2, V. La Ferrara2, L.

Lancellotti2, D. Ninno2,C. Baratto3,E. Comini3, G. Faglia3

and G. Sberveglieri3

1CR-ENEA Loc. Granatello, 80055 Portici(Napoli), Italy 2INFM, Dip. di Scienze Fisiche, Università di Napoli “Federico II”

Mostra d’Oltremare, Pad. 19, 80125 Napoli, Italy 3INFM Dip. di Chimica e Fisica dei Materiali, Universita' di Brescia

Via Valotti 9, 25133 Brescia, Italy

ABSTRACT: Porous Silicon has been the most investigated material over the last decade.

The main aim of this work is to describe its potential applications as gas sensor

discussing reported data and authors’ experimental findings. Fabrication methods and

material properties as well as the possible mechanisms of interaction with the

environment are also discussed.

Keywords: Sensor, Porus Silicon, Nanophase.

INTRODUCTION

In 1956 [1] it was observed that as a result of anodization in hydrofluoridric

acid solutions a brownish film formed on a crystalline silicon wafer. The

film was considered an amorphous layer produced by an

autodisproportionation reaction. Some years later, Theunissen [2]

demonstrated that it was the result of an etching process leaving

essentially a crystalline layer: thus, in proper conditions silicon

G. DI FRANCIA ET AL. “ APPLICATIONS OF POROUS SILICON AS A GAS SENSOR”

10

anodization in HF based solutions yields a porous silicon (PS) film,

crystalline in nature. In 1976 a first PS based device, a gas sensor, was

worldwide patented [3] and 8 years later it was shown that PS could be

integrated into a conventional LSI process

Since then PS has received constant attention, but only with respect to its

use in the frame of SOI technology. A strong renewed interest come at the

beginning of 1990 when two papers reported on the RT naked eye visible

photoluminescence from PS electrochemically obtained by either p and n

type Si substrates [4,5]. Both the papers strongly supported the feasibility

of the “silicon optoelectronic” opening a new era in the VLSI and as a

consequence 1990 signed the explosion of the interest in PS. Since then

and up to now more than 2200 scientific papers have been published on

this topic and the interest does not seem to substantially decrease.

Apart from a consolidated interest in the above field, a large interest has

also developed for its applications in sensor technology for reasons

discussed below. A consolidated number of patents have resulted and

scrolling the assignment list it is noticeable that important companies are

involved in this research: Schlumberger Industries, Ford Motor Company,

Siemens, IBM, Nasa, Mando Mach. Corp. ect.

MATERIAL FABRICATION &PROPERTIES

One of the most relevant advantages of PS over other porous materials is

that it can be electrochemically fabricated in a very simple and cheap

apparatus. The typical cell is schematically reported in Fig. 1. In a

conventional apparatus a voltage is applied between the anode (the

silicon wafer) and the cathode, usually an inert (platinum) electrode


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performing the etching at a constant current density using as electrolyte a

HF solution. Porous silicon produced in the above apparatus can result in

a large variety of fashions depending on substrate type and doping and on

the exact etching conditions. The porous media consists, in general, of a

highly complex network of silicon filaments or crystallites (similar to a

sponge). The pore average dimension, Wp, can be used to classify such

media into 3 classes: Macroporous: Wp>500 Å; Mesoporous:

20Å<Wp<500Å; Microporous: Wp<20Å.

Figure1. A schematic of the anodization cell used to fabricate Porous Silicon.

In TAB. 1, a rough classification of PS major features is reported with

respect to substrate type and doping. Substrates are all assumed Cz

silicon <100>oriented. It is evident the large Surface-to-Volume (S/V) ratio

and the typical dimension of the crystallite (Wc).

In the same table also some of the most important physical properties

relevant to the sensor field are reported.

+ -

PtSi HF


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Table1: Porous Silicon properties

Si-type doping

(at/cm3)

Pore

morph.

Wc(Å) S/V

(m2/cm3)

? ?

(? cm)

? ?(W/mK)

PL

p<1e16 Sponge <30 >500 <10e-10 ˜1.2 yes

p >1e17 Fil-like >100 >200 >10e-10 ˜80 yes

n>1e17 Fil-like <100 >100 =10e-10 ˜1.75 yes

n<1e16 Fil-like ~ µm >10 ˜10e-6 _ no

In Fig. 2 some examples of the various material fashions fabricated in our

PS laboratory at CR-ENEA are reported.

(a) (b)

Figure 2. Porous Silicon morphologies. Photo (a) shows a plain view of a random PS sample. Pores are in the range of a few microns. Pore walls are covered by nanostructures. Photo (b) is cross section of a regular sample obtained by means of a photolitografic step previous to the anodization process.

There are two main reasons for using PS in chemical sensor applications:

1- it is well known that the sensitivity of a given material to an external

stimulus (a gas, a liquid etc) is larger the greater is the exposed

surface. Porous silicon is in this sense almost an unique material. As


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reported in the above table, PS can be produced in a relatively simple

and economic process in samples exhibiting S/V ratio of hundreds of

m2 per cubic centimeter the exact value depending on the porosity, that

is, on the particular anodization conditions.

2- PS consists, in general, of ensembles of nanostructures. When their

dimension is in the order of 1 nm their physical properties strongly

depend on the surface atoms and, in turn, on the environment.

The above arguments have induced several research groups to test PS

response to many different molecules either in the gas or in the liquid

phase. In the following table we report an up-to-date review of those

molecules grouped by chemical family.

Table 2 Chemical species producing a response in PS. Data from Ref. 6.

Chem/ family

Phase Measurement

? dc ? ac CV Cf PL Other optical

Other non-optical.

Alcohols g. l. X X X X X X Ketons g. X X X X Alkanes g. l. X

Halogenates aliphatic

g. X

Ethers g. X Carboxylic

acid g.l. X X X

Amines l. X Cicloaliphatic g. X

Aromatic g,l. X X X Hal arom. g X

Surfactants l. X Inorganic g.l. X X X X X X Halogens g. X

Biocompound l. X X


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In general, in each of the above experiments a different PS material and/or

device has been tested in often quite peculiar conditions thus preventing

the possibility to draw general conclusions about sensitivity, stability or

selectivity. Changes in photoluminescence, reflectance, conductance and

several others physical properties have been in fact reported when PS

surface adsorbs molecules of different kinds. Nevertheless the large set of

data now available, allows some general features of PS sensing

mechanism to be highlit as far as the physical property is considered. A

general discussion of this subject can be found in ref 6. In the following we

will only report some of ours most recent findings showing the strong

potentialities that PS has in the sensor field.

RESULTS & DISCUSSION

1- Changes in photoluminescence.

As far as PL is concerned a quenching in intensity is generally reported for

different chemical groups. The quenching can be reversible or not even for

the same chemical group, depending on illumination time [7]. In N2, PS

photoluminescence is stable, both in intensity and spectra, even after a

long illumination time. In presence of Oxygen, PL quenches proportionally

to its concentration. Quenching is reversible if spectra are recorded after a

short illumination time. In Fig. 3 the peak PL quenching Io/I vs the oxygen

concentration is reported for this experimental condition. Data follow the

Stern-Volmer model and an equation of the type: Io/I=1+?C(O2), where Io is

PL intensity measured for the wavelength of the maximum intensity in inert

ambient and I is the peak intensity at different O2 concentrations, can be

written.???is related to the radiative efficiency of the PL emission and to


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how it is modified by the presence of a quenching molecule creating an

additional non radiative recombination pathway.

0,8

1

1,2

1,4

1,6

1,8

0 5 10 15 20

I N2/I

O2

CO2

(%)

Figure 3. Stern-Volmer plot for PS PL quenching. Oxygen concentrations range

from 0.1%-16%

When PS is in presence of a N2 /acetone vapors, our data show that PL

quenching is always reversible, even if samples are illuminated for long

periods. In Fig. 4 the corresponding PL modifications as function of

different concentrations of acetone vapors are reported. Measurements

have been recorded under long illumination; it is possible to note both the

PL reversibility and a blue shift when PS is in contact with acetone.

Proposed mechanisms for PL quenching are:

1-Formation of surface recombination states and/or decrease of non

radiative lifetime after an effective reaction of the molecules with PS. In

this case quenching is generally irreversible.

2-Local deformation and/or formation of surface recombination states

and/or decrease of non radiative lifetime and/or strain induced non


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radiative traps, all following a physisorbtion process as well as energy or

charge transfer to acceptor-like molecules. In those cases quenching is

generally reversible.

1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.60

2000

4000

6000

8000

10000

% acetone in N2decreasing

% acetone in N2increasing

N2 0.2% acetone 0.8% 2% 4% 8% 20% N2

PL

Inte

nsity

(arb

.un.

)

E (eV)

Figure 4. PL spectra under different concentrations of N2 /acetone vapors. It is evident a reversible quenching and 40 meV of blue shift.

2- Changes in electrical conductance.

Whatever the tested substance, a conductance increase is generally

reported. In Fig. 5 the relative change in conductance for ethanol and

methanol vapors in dry air, are reported. The strong response has been

obtained at RT, a particularly interesting experimental conditions when

flammable substances have to be monitored.


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1

10

0

500

1000

0 2000 4000 6000 8000

etanolometanolo

0

concentratione (ppm)

t(s)

? G/G=1.7

? G/G=10.3

? G/G=3

Figure 5. RT relative change in conductance for ethanol and methanol vapors in

dry air. In Fig. 6 we report the RT conductance variation at different NO2

concentrations. PS exhibits good sensibility while reversibility needs to be

improved.

Proposed mechanisms are:

1- modification of surface states (for instance, by polar molecules induced

electric field).

2- surface passivation by dangling bonds capping.

In the literature, for the same chemical group, changes are reported to be

reversible and irreversible. No data on selectivity of the response are

given. The contradictory data, exhibiting no particular correlation to any

chemical species, could, in our opinion, be also interpreted in a completely

different frame.


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10 -6

10 -5

0

5

10

15

20

0 2000 4000 6000 8000

I(A

)

NO

2 concentration (ppm)

t(s) Figure 6. PS RT conductance variation at different NO2 concentrations.

As above described, PS is a not ordered ensemble of crystalline

structures. It is well known that disorder plays a relevant role both in its

electrical and optical properties. It is possible that molecular penetration

into the pores, besides an all the same possible electrical interaction

according to one or both the above schemes, creating morphological

changes, induces electrical and optical changes resulting in the reported

electrical conductance variation. The way the electrical conductance is

modified depends on local morphological changes and as a consequence

depends on the material type, on the way the gas or liquid penetrates but

only slightly on the chemical species. In other words global electrical

conductance changes results from a sum of local environment changes.


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CONCLUSIONS.

There are two possible approaches for using PS as a base material for

sensing devices: i-using PS as active sensing material, ii-using PS as a

transducer for other recognition elements.

i- As above reported PS “sensing” ability has been reported for a large

variety of molecules and compounds and for device concepts and physical

properties which are each other even very different. This generalized

response can be considered as a drawback especially if selectivity is

taken into consideration. However we have very little concern about the

interaction nanostructure/environment. For instance large changes of the

work function [8] have been measured as a function of the nanocrystallite

dimension just for PS. Thus strong environment changes can be expected,

but a deep theoretical work is needed to model (and then control) the

above effect.

ii- This approach has collected the best results in terms of sensibility.

Sensors comparable and even much better than commercial devices

respectively for pH and for biocompounds have been fabricated. In those

few works reporting on this kind of devices, good selectivity has been also

demonstrated. If efforts are dedicated to optimize PS impregnation with

different substances and/or catalysts, the perspective of a sensor

arrangement where “different material points” recognize different

molecules and the relative signals are processed by an eventually

integrated microprocessor seems to be effectively feasible. Optical

changes are, in this respect, the most immediate candidate but a

combination with electrical response seems the most effective way.


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REFERENCES

1. A. Uhlir, Bell System Technical Journal, 3 (1956) 333.

2. M. J. J. Theunissen, Journal Electrochemical Society, 119 (1972) 351.

3. US patent assigned to IBM N°4057823 .111

4. L.T. Canham, Appl. Phys. Lett., 57 (1990) 1040.

5. V. Lehmann, U. Gosele, Appl. Phys. Lett., 58 (1991) 856.

6. G. Di Francia, V. La Ferrara, L. Quercia, F. De Filippo, L. Lancellotti, P.

Maddalena, D. Ninno, C. Baratto, E. Comini, G. Faglia in Proc. of the 3rd

Italian Conference on Sensors and Microsystems (C. Di Natale, A.

D’Amico and G. Sberveglieri Eds.), (World Scientific Publishing,

Singapore) 1998, in press.

7. G. Di Francia, V. La Ferrara, T. Fasolino, L. Quercia, L. Lancellotti, G.

Iadonisi, D. Ninno, in Proc. of the 4th Italian Conference on Sensors and

Microsystems (C. Di Natale, A. D’Amico and G. Sberveglieri Eds.), (World

Scientific Publishing, Singapore) 1999, in press.

8. T.M. Bhave and S.V. Bhoraskar, J. Vac. Sci. Technol. B.16 (1998)

2073.

ii workshop on chemical sensors and biosensors · ii workshop on chemical sensors and biosensors 11...

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