a b s t r a c t We report a new optical sensor exploiting the innovative operation principle developed for the selective registration of UV and visible radiations. The operation is based on the mutual influence of depletion regions in the photovoltaic structure composed by Schottky barrier and nÀp junction. The important feature of this structure is that the depletion regions expand over the whole base so that they contact each other. We have shown that the position of the contact point in the base is a function of voltage applied to the structure. Also absorption spectrum for each of the depletion regions and the photoresponse of the structure are functions of the applied voltage. We have revealed that the change induced by the applied voltage in the photoresponse, DI, is proportional to the change Dl induced in the spectrum, Dl$DI. This linear correlation between DI and Dl is a very important spectrophotometric property required for the precise selective registration of bio signals in dynamic and static biological processes

Ashot Khudaverdyan

Daniel L Clinciu

Janna Dokholyan

Surik Khudaverdyan

Vladimir Arustamyan

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ARTICLE IN PRESS
Nuclear Instruments and Methods in Physics Research A 610 (2009) 314–316Contents lists available at ScienceDirectNuclear Instruments and Methods in
Physics Research A0168-90
doi:10.1
$ The
 Corr
E-mjournal homepage: www.elsevier.com/locate/nimaOn the mechanism of spectral selective sensitivity of photonic biosensors$Surik Khudaverdyan a,, Janna Dokholyan a, Vladimir Arustamyan a,
Ashot Khudaverdyan a, Daniel L. Clinciu b
a State Engineering University of Armenia, SEUA, 105 Teryan Str., 0009 Yerevan, Armenia
b Department of Information Management, National Chiao Tung University, Hsinchu, Taiwana r t i c l e i n f o
Available online 23 May 2009
Keywords:
Photo detector
Optical sensor
Selective sensitivity
Spectral components02/$ - see front matter & 2009 Elsevier B.V. A
016/j.nima.2009.05.094
work is done within the framework of the EN
esponding author. Tel.: +37410 541591.
ail address: khsuren@seua.am (S. Khudaverdya b s t r a c t
We report a new optical sensor exploiting the innovative operation principle developed for the selective
registration of UV and visible radiations. The operation is based on the mutual influence of depletion
regions in the photovoltaic structure composed by Schottky barrier and np junction. The important
feature of this structure is that the depletion regions expand over the whole base so that they contact
each other. We have shown that the position of the contact point in the base is a function of voltage
applied to the structure. Also absorption spectrum for each of the depletion regions and the
photoresponse of the structure are functions of the applied voltage. We have revealed that the change
induced by the applied voltage in the photoresponse, DI, is proportional to the change Dl induced in the
spectrum, DlDI. This linear correlation between DI and Dl is a very important spectrophotometric
property required for the precise selective registration of bio signals in dynamic and static biological
processes.
& 2009 Elsevier B.V. All rights reserved.1. Introduction
Recently the role of biosensors has strongly increased due
to their application in social spheres like medical diagnostics,
medicine making, environmental protection, etc. [1–2]. Applica-
tion of biosensors in nanobiosystem studies is of special interest
as, for instance, optical spectroscopy of biological objects gives
information on electron transitions in separate organic molecules,
which can be used for molecular identification of bio-cells and
investigation of the so-called ‘‘specificity’’ of cellular interactions.
The last is critical for nano-biological applications including
adhesion, aggregation and formation of tissues as self-correlating
systems [1,3]. As the ratio between the absorbed and reflected
radiations depends on the composition and quantity of bio-objects,
the spectral line and peaks can be used for the identification of
specific impurities or their concentration [4–6].
Today silicon photo-detectors and porous silicon (PSi) films are
widely used in modern spectrophotometric biosensors [1,2,7,8].
Both theoretical and experimental studies have confirmed
that due to favorable morphological and physical properties PSi
promises to be a universal platform for chemical and biological
sensors [2,7]. However, surface passivation is required for
stabilizing PSi surface and covalent bonding of bio-probes as PSill rights reserved.
-bioen-1120 ANSEF program.
an).is highly hydrophobic [8]. Also lack of spectral selectivity limits
the application of photo-detectors in sensors.
Here we report a new spectrophotometric biosensor exploiting
the innovative operation principle developed for the selective
registration of UV and visible radiation. The sensor contains
minimum number of optical components (a micro-source of
radiation and an innovative photo-detector selectively sensitive to
the radiation spectra). We perform numerical simulation of this
innovative photo-detector.2. Model
The innovative photo-detector is composed of Si np junction
and n-doped Si Schottky barrier. The energy band diagram of this
semitransparent metal – n-doped Si – p-doped Si – ohmic contact
structure is shown in Fig. 1, where the density of doping is
Nd ¼ 3.3610
15 cm3 and d is the thickness of the base. There
are potential barriers for electrons at contacts, j1/q and j2/q
(j2j1 ¼ 0.1 eV), respectively. These barriers are oppositely
directed and their depletion regions completely cover the base
so that they create a potential well in the conduction band. The
applied voltage can shift the connection point of the depletion
regions in the base, xm, from zero to d position, along with
changing the depths of the depletion regions one at the expense of
another.
The function xm(V) can be calculated from the Poisson equation
connecting potential energy of electrons j(x) with volume density
ARTICLE IN PRESS
Idiff. 
M
d 0 
hν
Area  of driftage 
xm 2 xm
Diffusion of  
electrons
ϕ1
ϕ2
EF 
x 
Iemis. 
Idr1 Idr2 
Ln = w
d1 
Fig. 1. The energy band structure and the flow of photocurrents through the
innovative photo-detector.
mϕ
λ
,ev
,nm
0.504
0.499
0.488
0.484
0.478
4
1
2
3
300 600400 500
Fig. 2. Dependence of the minimum potential energy of electrons on the
wavelength at different voltages: 10.3 V; 20.09 V; 30 V; 40.09 V (the
specified polarity of voltage is applied to ohmic contact).
S. Khudaverdyan et al. / Nuclear Instruments and Methods in Physics Research A 610 (2009) 314–316 315of charges
d2j
dx2
¼
q2
0
Nd  nf exp
jm j
kT
 h i
(1)
Here nf and jm are the density and the potential energy of
electrons at xm, q the electron charge, e the dielectric constant and
e0 the free space permittivity.
If j1oj2+qV, then under the conditions (see Fig. 1) that the
potential energy for electrons is j1 at both x ¼ 0 and x ¼ 2xm, is
j2+qV at x ¼ d, is jm at x ¼ xm and that j0 ¼ 0 at x ¼ xm, the
Poisson Eq. (1) yields for jm and xm are given by
2 j1jmkT þ 2
nf
Nd
exp jmj1kT
 
 1
 
¼
j2j1þqV
kT
 
Ls
d2xm
 d2xmLs
h i2 (2)
Z j1jm
kT
0
dy
yþ
nf
Nd
ey  1ð Þ
h i1=2 ¼
ffi
½
p
2xm
Ls
(3)
Here y ¼ (jjm)/kT, Ls ¼
ffi
½
p
0kT=q2Nd is the Debay screening
length, and the condition is also used that due to the emission,
only few electrons are above j1. Another feature is that the
structure is thin, do1.5mm, so that electrons and holes rather
escape than recombining in the base. Therefore, the density
of electrons increase in the potential well so much that their
emission from the well If balances the generation of electrons in
the structure
If ¼ qnf vT S exp jm j1
 
=kT
 
¼ qF0S 1 exp ad1ð Þ
  (4)
where vT is the thermal velocity of carriers, F0 ¼ Popt(1R)/S hv the
total flow of incoming photons per unit area, Popt the radiation
power, h the Planck’s constant, v the frequency of radiation, S the
irradiated area, b the quantum efficiency and R the reflection
coefficient.
Eq. (4) determines the density nf that should be used in Eqs. (2)
and (3) to find out functions jm(V) and xm(V).Note illumination increases jm, however, not more than j1 if
j1oj2+qV and j2+qV if j14j2+qV.
As shown in Fig. 1, illumination also results in drift and
diffusion photocurrents, Idr1, Idr2 and Idiff, respectively, [9,10]
Idr1 ¼ qF0S½1 expðaxmÞ (5)
Idr2 ¼ qF0S½expðaxmÞ  expðadÞ (6)
Idiff ¼ qF0S
aLn
1þ aLn
expðadÞ (7)
As the additional electron emission If arises from the base (into
the contact with either Schottky barrier if j1oj2+qV), the
external photocurrent Iph can be written as
ðIPh ¼ Iemiss þ Idr1  Idr2  Idiff Þ (8)
By substituting jm and xm for functions jm(V) and xm(V) in
Eq. (4)–(8), also the photocurrent (8) can be written as a function
of V, Iph(V).
Note that the drift region of Schottky barrier is responsible for
the spectral sensitivity of the structure to the short-wavelength
radiation, while in-the-base and out-the-base regions of the np
junction are responsible for the spectral sensitivity of the
structure to long-wavelength radiation.3. Discussion and results
The detailed volt–ampere characteristics of the structure Iph(V)
can be obtained from experiments. Since the dependence of the
absorption coefficient a(l) on wavelength l is known, it is possible
to determine lm(V) that a(lm)  xm(V) ¼ 1. Under the last condition,
elimination of V from the experimental photocurrent Iph(V) will
yield Iph as a function of the wavelength lm, Iph(lm). The last
function will be used for disclosing the spectral structure of UV
and visible radiations. The simulation is performed for quantum
efficiency b ¼ 1 when reflection from the surface R ¼ 0.
Fig. 2 shows the spectral dependence of the minimum
potential energy of electrons jm at different bias voltages and at
incoming radiation intensity 1017 quantum/sm2 s, obtained from
the solution of the system of Eqs. (2)–(4).
Figure shows the two regularities of the change jm. The value
does not change with the increase of wavelength up to 450 nm,
and then it decreases, preserving such regularity until the increase
of jm occurs with the increase of the voltage of the polarity at
which the surface barrier is switched directly and the rear barrier
is switched in the opposite direction.
ARTICLE IN PRESS
B
ia
s v
ol
ta
ge
   
V
 
5
-5
0
Ph
ot
oc
ur
re
nt
 o
f i
nt
eg
ra
l r
ad
ia
tio
n 
I P
h, 
A
   
Position xm, cm  
Wavelength λ , nm 
1
1
2
2
0
x 10-3
5x 10-5
300 350 400 450 500200 250
0
Fig. 3. Dependence of the position of the minimum point of the potential energy of
the electron xm in the base on bias voltage (curve 1) (the specified polarity of
voltage is applied to ohmic contact), and dependence of integral photocurrent on
the wavelength corresponding to the absorption depth 1/a. At w ¼ 0.5mm, d
¼ 1mm.
S. Khudaverdyan et al. / Nuclear Instruments and Methods in Physics Research A 610 (2009) 314–316316The first regularity may be due to absorption depth. The more
is the absorption depth value at l4450 nm, the more quanta stay
beyond the active area. As a result the absorption efficiency
and the number of photoelectrons stored in the potential well
decrease. In the second case, jm increases with the increase of
voltage that brings to the increase of rear barrier and decrease
of surface barrier. The point of minimum potential energy of
electrons xm is linearly shifting towards the surface barrier.
For the evaluation of selective sensitivity, calculations were
made by formula (8) for solar radiation AM0 with the use of
spectral distribution of energy accurate within 1 nm [11]. In Fig. 3
curve 1 shows the dependence of the position of the minimum
point of the potential energy of the electron xm in the base on bias
voltage (Fig. 1).
Curve 2 presents the approximate dependence of integral
photocurrent on the change of the position of xm corresponding to
the absorption depth 1/a. The values of wavelengths correspond-
ing to 1/a are presented along x-axis. It is obvious, that at
Dl ¼ l2l1 the current DI ¼ I2I1 shall characterize the radiationintensity within this range of wavelengths and define the degree
of selective spectral sensitivity.
According to the results of calculations, for the described two-
barrier silicon structures the approximate selective sensitivity
with respect to current changes from 1.5105 A/nm to
5105 A/nm depending on the section of curve 2, and with
respect to voltage from 2.2 mV/nm to 6.2 mV/nm.
Thus, directing the integral flow of radiation onto the bio-
object, it is possible to selectively register the informative
radiation emergent from it without the use of filters.
The proposed principle allows to catch different regions from
the wide spectral range of electromagnetic radiations from UV
to IR by changing the structure, the initial material and its
parameters.4. Conclusion
The studies showed that in two-barrier photo receiving
structures, it is possible to change the relations between the
widths of regions exhausted by charge via changing external
voltage for DV. This brings to the redistribution of absorbed
radiation intensities in them within the range of wavelengths Dl.
The change of external photocurrent for DI will define the
radiation intensity within the range of wavelengths Dl and the
degree of selective spectral sensitivity of the structure. Scanning
DV with respect to voltage, it is possible to shift Dl along the
spectrum and receive spectral distribution of radiation intensities.
The calculations in the field of solar radiation AM0 showed
that selective sensitivity with respect to current changes from
1.5105 A/nm to 5105 A/nm, and with respect to voltage
from 2.2 mV/nm to 6.2 mV/nm. This means that at the change
of external voltage into millivolt fractions the resolution with
respect to spectrum does not exceed 0.1 nm. The possibility to
connect the amplifier to the measuring network of the photo-
detector will allow to use the sources with small radiation
intensity and high-resolution to study the spectral change of
informative radiation running from bio-objects.
References
[1] M. Bercu, et al., Biomed. Microdev. 8 (2006) 177.
[2] L. De Stefano, et al., Sensors 7 (2007) 214.
[3] S. Cakic, et al., Sensors 6 (2006) 1708.
[4] V.S.-Y. Lin, et al., Science 278 (1997) 840.
[5] N.G.A. Novel, Sensors 7 (2007) 166.
[6] D.F. da Silva, D. Acosta-Avalos, Sensors 6 (2006) 514.
[7] L. De Stefano, L. Moretti, I. Rendina, A. Rossi, Sensor Actuators 100 (2004) 168.
[8] H. Ouyang, M. Chrtistophersen, R. Viard, B. Miller, Adv. Funct. Mater. 15 (2005)
1851.
[9] S. Khudaverdyan, et al., J. Solid State Electron. 49 (2005) 634.
[10] S. Khudaverdyan, et al., Bull. SEUA ser. Modeling 9 (2006) 85.
[11] /http://rredc.nrel.gov/solar/spectra0/wehrli1985.new.htmlS.


On the mechanism of spectral selective sensitivity of photonic biosensors

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On the mechanism of spectral selective sensitivity of photonic biosensors

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