nZnO_cSi Heterojunct.pdf

(173 KB) Pobierz

Materials Science and Engineering B 110 (2004) 87–93

Electrical properties of the n-ZnO/c-Si heterojunction

prepared by chemical spray pyrolysis

R. Romero, M.C. López, D. Leinen, F. Mart´n, J.R. Ramos-Barrado

∗

Departamento de F´sica Aplicada y Departamento de Ingenier´a Qu´mica, Laboratorio de Materiales y Superﬁcie (Unidad Asociada al CSIC),

Facultad de Ciencias, Campus de Teatinos, Universidad de Málaga, E29071 Málaga, Spain

Received 20 October 2003; accepted 4 March 2004

Abstract

Electrical, structural and compositional properties of n-ZnO/c-Si heterojunctions prepared by chemical spray pyrolysis on single-crystal

n-type and p-type monocrystalline silicon(1 0 0) substrates are examined with the

C–V

method and admittance spectroscopy at temperature

ranges between 223 and 373 K. The n-ZnO/c-Si heterojunctions show a height barrier consistent with the difference in energy of the work

functions of Si and ZnO; however, the n-ZnO:Al/c-Si heterojunctions present a more complex behavior due to the defects at or near the

n-ZnO:Al/c-Si interface, causing a Fermi energy pinning.

Keywords:

Spray pyrolysis; n-ZnO/c-Si; Heterojunction; Electrical properties; Admittance spectroscopy;

C–V

method

1. Introduction

Transparent conducting oxides (TCO) are of great inter-

est in electronic and electro-optical applications. They are

used as coatings in a variety of devices as large ﬂat screens

[1],

thin ﬁlm photovoltaic cells

[2],

antireﬂective coatings

in conventional silicon solar cells

[3],

light-emitting diodes

(LEDs)

[4],

etc. ZnO is an n-type wide-band gap semicon-

ductor (3.3 eV) with a hexagonal wurtzite structure. Intrinsic

zinc oxide ﬁlms are highly resistive, but when commonly

doped with Group III elements (Ga, In or Al), they become

conducting. Thin ﬁlms of Al-doped ZnO (ZnO:Al), AZO,

are emerging as an alternative candidate to Sn-doped In

(ITO), due to features such as non-toxicity, low cost, mate-

rial abundance, and a high level of stability with hydrogen

plasma and heat cycling.

Spray pyrolysis is a useful alternative to the traditional

methods for obtaining thin ﬁlms of pure and doped ZnO. It

is of particular interest because of its simplicity, low cost

and minimal waste production. The spray pyrolysis process

allows the coating of large surfaces and it is easy to include

in an industrial production line. With spray pyrolysis, the

solution is sprayed directly onto the substrate. A stream of

∗

gas, e.g. compressed air, can be used to help the atomization

of the solution through the nozzle. Sometimes, posterior

thermal treatment is necessary to ensure the elimination of

precursor waste and to allow sintering or crystallographic

phase transformation.

The ITO/Si structure has been extensively studied during

the past two decades, having been obtained by various de-

position technologies for potential application in solar cell

technologies. The conversion efﬁciencies achieved in so-

lar cells from ITO/c-Si junctions were 10–15% when using

spray pyrolysis

[5–8].

However, little information is avail-

able in the literature on the ZnO/c-Si heterojunction

[9–13]

in spite of the interest in its potential use in solar cells and

other electro-optic devices.

In this paper, we report the electrical, structural and com-

positional properties of the junctions of both pure and Al

(1 at.%) doped thin ﬁlms of ZnO on p- and n-type silicon,

as obtained by chemical spray pyrolysis.

2. Experimental

Pure and Al doped ZnO thin ﬁlms were prepared from

−3

M of zinc acetate dihydrate dissolved in ultra pure

water. Compressed air was used to atomize the precursor

solutions. All the ﬁlms were deposited onto type p polished

Si(1 0 0) n (9

−1

cm) (Topsil, Denmark) and fused silica

Corresponding author. Tel.:

+34-9521-31922;

fax:

+34-9521-32382.

E-mail address:

barrado@uma.es (J.R. Ramos-Barrado).

doi:10.1016/j.mseb.2004.03.010

R. Romero et al. / Materials Science and Engineering B 110 (2004) 87–93

substrates. Substrate temperature was kept at 573 K with an

accuracy of

±2

K. The solution ﬂow rate and gas pressure

was kept constant at 50 ml h

−1

and 3 kg cm

−2

, respectively.

The nozzle to substrate distance was 10 cm. Aluminium dop-

ing was achieved by adding aluminium trichloride to the

starting solution. The doping level in the solution, deﬁned by

a 100

[Al]/([Al]

[Zn]) atomic ratio, was of 1%, and the

deposition time was 15 min. Film thickness was determined

ex situ by means of XRF (Siemens, SRS 3000) and the at-

tenuation of the Si K signal in ﬁlms deposited on Si, with

an accuracy of

±5

nm. After deposition, the ﬁlms were al-

lowed to cool down to room temperature and then removed

for further characterization. A more detailed description of

the method to obtain the thin ﬁlms and the characteristics

of the spray pyrolysis device used were reported in previous

papers

[14,15].

Film structure was characterized by the X-ray diffraction

method using Cu K radiation in a conventional goniome-

ter with a

θ–2θ

scanning mechanism. Surface, depth down

composition, and electronic states of the ions of pure and

doped ZnO ﬁlms were studied by X-ray photoelectron spec-

troscopy (XPS). XPS spectra were obtained using a Physical

Electronic model PHI 5700 X-ray photoelectron spectrom-

eter with Mg K radiation (1253.6 eV) and Al K radiation

(1486.6 eV) as excitation sources. The energy scale of the

spectrometer was calibrated using Cu 2p

3/2

, Ag 3d

5/2

and

Au 4f

7/2

photoelectron lines at 932.7, 368.3 and 84.0 eV, re-

spectively. A PHI ACCESS ESCA-V6.0 F software package

was used for data acquisition and analysis. Atomic concen-

trations were determined from the photoelectron peak areas

using Shirley background substraction

[16]

and sensitivity

factors provided by the instrument manufacturer. All sample

spectra were referred to the C1s line of the residual carbon

set at 284.8 eV. Depth proﬁling was carried out using 4 keV

bombardment at a current density of

∼3

A cm

−2

. A

sputter rate of 0.5 nm min

−1

was assumed, as determined by

under the same sputter conditions.

Electrical properties were determined by admittance spec-

troscopy using a Broadband Dielectric Converter (BDC)

from Novocontrol with a Solartron 1255 Frequency Re-

sponse Analyzer (FRA) for a frequency range between 1 Hz

and 1 MHz. The n-ZnO/p-Si heterojunction was placed in a

Novocontrol sample holder modiﬁed in our laboratory; the

ohmic contact with the p-Si substrate was performed with

Al and with the n-Si with In–Ga. The temperature range

was between 223 and 373 K, and was controlled by a Quatro

temperature controller from Novocontrol.

Fig. 1. X-ray diffractogram obtained from a 150 nm thick ﬁlm of pure

ZnO on p-Si(0 0 1) and ZnO:Al (Al doped 1 at.%).

observed at 2θ

34.24 and 36.26

◦

[JCPDS card 05-0664].

The XRD spectrum of pure ZnO shows a powder-like pattern

with no preferred growth orientation; the lattice parameters,

and

are estimated to be 5.24 and 3.24 Å, respectively.

Similar values are observed in the ZnO:Al doped ﬁlm. These

results are in good agreement with those from previously

published papers

[14,15].

However, when the thickness of the ﬁlms increases, e.g.

to 300 nm, we can observe that the ﬁlms grow preferentially

along the

c-axis

orientation in a (0 0 2) orientation (Fig.

2),

3. Results and discussion

3.1. Structural properties

Fig. 1

shows a typical X-ray diffractogram obtained from

a 150 nm thick ﬁlm of pure ZnO on p-Si(0 0 1) and ZnO:Al

(1 at.%). As we can see, the ZnO(0 0 2) and (1 0 1) peaks are

Fig. 2. X-ray diffractogram obtained from a 300 nm thick ﬁlm of pure

ZnO on p-Si(0 0 1).

R. Romero et al. / Materials Science and Engineering B 110 (2004) 87–93

Fig. 3. Wide scan XPS (0–1200 eV) spectra of a ZnO thin ﬁlm prepared

at 573 K and etched for 30 s with 4 keV Ar

Fig. 4. Optical transmittance spectra of pure and 1% Al-doped ZnO thin

ﬁlms as a function of the wavelength in the range 300–900 nm.

and vertically to the substrate; the lattice parameter

estimated to be 5.24 Å. For the pure ZnO, the peak at 34.2

◦

has more relative intensity than the peak at 36.25

◦

, which

is very small. The mean crystallite size of the sample is

evaluated by means of the Scherrer formula

[17];

the grain

size has been estimated to be

≈255.12

and 429 Å for 150 and

300 nm thicknesses, respectively; no signiﬁcant difference

can be observed with ZnO:Al doped ﬁlms.

3.2. Compositional and chemical surface states

Fig. 3

shows the wide scan (0–1200 eV) XPS spectra in

the binding energy range of 0–1200 eV for a ZnO thin ﬁlm

prepared at 573 K, and sputtered in the spectrometer for 30 s

with 4 keV Ar

in order to reduce the amount of adventitious

carbon.

An XPS atomic ratio of Zn/O of 1.02/1.00 (±2%) was

determined using the integrated peak area and sensitivity

factors. For the ZnO:Al (1 at.%) doped ﬁlms (not shown),

the Al 2p line consists of a highly symmetrical peak lo-

cated at 74.27 eV. This value is close to that reported for

. A more detailed discussion about the XPS results of

the Al doped ZnO thin ﬁlms will be presented in another

paper.

3.3. Optical properties

Fig. 4

shows the optical transmittance spectra of pure ZnO

and ZnO:Al doped thin ﬁlms as a function of wavelength

in the range 300–900 nm. The undoped ﬁlm transmittance

spectra exhibit an average transmittance over 95% in the

visible range of the optical spectrum. Doped ﬁlms prepared

under the same conditions but with a longer deposition time,

exhibit a slight reduction in their transmittance, to 70%,

because the thickness is higher and the homogeneity is lower

as observed from SEM pictures not shown here. Since the

ZnO is a direct transition semiconductor,

is related to the

optical energy band gap (E

) by

(h�½α)

β(h�½

−

)

(1)

where

is the optical absorption coefﬁcient,

h�½

the energy of

the incident photon and

is the edge width parameter

[18].

is determined by extrapolating the straight-line portion

of (1) to

h�½α

0. The doped ﬁlms present a larger band

gap (3.315 eV) than the pure ZnO ﬁlms (3.307 eV).

3.4. C–V characteristics: barrier height and donor carrier

density

The potential barrier at the junction can be measured by

small-signal capacitance–voltage (C–V)

[19]

characteris-

tics.

Fig. 5

presents the

C–V

characteristics in the reverse

bias region at 300 K for the n-ZnO:Al/p-Si and n-ZnO/p-Si

heterojunctions at 1 MHz. As can be seen, the capaci-

tance of the heterojunction is decreased with an increase

in the reverse bias with an approximately linear

−2

–V

bias

Fig. 5.

C–V

characteristics in the reverse bias region at 300 K for the

n-ZnO:Al/p-Si and n-ZnO/p-Si heterojunctions at 1 MHz.

R. Romero et al. / Materials Science and Engineering B 110 (2004) 87–93

Fig. 6.

−2

–V

bias

relationship for the n-ZnO:Al/p-Si and n-ZnO/p-Si

heterojunctions at 1 MHz.

Table 1

Characteristic parameters of pn junction

n-ZnO/p-Si

−3

)

(eV)

(nm)

0.66

n-ZnO:Al/p-Si

0.70

n-ZnO/n-Si

0.50

n-ZnO:Al/n-Si

0.56

relationship (Fig.

6).

This means that the depletion region

in the vicinity of the heterojunction interface is expanded

with an increase in the reverse bias. This

C–V

characteristic

is also described by the conventional heterojunction theory:

(2)

2(N

) (V

)

Here,

is the donor density in n-ZnO,

the acceptor den-

sity in p-Si,

and

are the dielectric constants of n-ZnO

and p-Si, respectively, and

is the applied voltage. Using

Eq. (2),

we can determine the effective space charge density

of the depletion region of both heterojunctions from the slope

of the

−2

) relationship. The values obtained are pre-

sented in

Table 1

for

(ZnO)

8 and

(Si)

[19].

The

values obtained for

in n-ZnO:Al 1 at.% (9

−3

)

and n-ZnO (5

−3

) are higher than for

in p-Si,

where

is estimated to be about 8

−3

from the

resistivity of p-Si (9

−1

cm). These results indicate that the

junction is an abrupt p–n

type junction. The built-in poten-

tial V

, or diffusion potential, is estimated to be 0.66 and

0.7 eV for the ZnO:Al/p-Si and ZnO/p-Si junctions, respec-

tively. This result is consistent with the energy difference

between the work functions of Si and ZnO. The Fermi level

below the vacuum level is 4.97 eV for p-Si, and 4.25 eV for

n-ZnO. The difference between them is 0.72 eV. The results

obtained for n-ZnO/n-Si are very similar (not shown here).

The built-in potential is 0.56 eV for the n-ZnO:Al junc-

tion (1 at.%)/n-Si and 0.50 for the ZnO/n-Si.

Fig. 7

shows

the band diagram for the n-ZnO/p-Si and the n-ZnO/n-Si

junctions.

Table 1

presents the most important values.

The most likely reason for a more complex behavior

(n-ZnO:Al/p-Si and n-ZnO:Al/n-Si), compared to the model

of a simple p–n

heterojunction, are the defects at or near the

n-ZnO:Al/c-Si interface, which cause a Fermi energy pin-

ning. The pinning effect makes the height of the energy bar-

rier independent of the ZnO work function. As shown in the

literature

[19],

the Fermi energy at metal/n-semiconductor

interfaces is often pinned at about

/3. In the case of Si

1.12 eV), this corresponds to 0.37 eV above the va-

lence band edge. Making this assumption, the energy bar-

rier at the interface seen by the free electrons in 9

−1

p-Si is 0.53 eV, which is quite consistent with our 1/C

–V

measurements (0.56 eV).

3.5. Admittance spectroscopy

Thermal admittance spectroscopy is a technique which

allows thermal emission rates of deep levels from the varia-

tion of capacitance and conductance of a junction as a func-

tion of temperature and frequency

[20,21]

to be determined.

These variations are due to the change in frequency of the

measuring signal with respect to the time constant of charge

and discharge processes of the deep level around the point

of the space charge region where both time constants are

Fig. 7. Band diagram for the n-ZnO/p-Si and the n-ZnO/n-Si.

R. Romero et al. / Materials Science and Engineering B 110 (2004) 87–93

equal. Under dark conditions, this point coincides with the

crossing point of the Fermi level with the deep level.

We can model the n-ZnO:Al/p-Si as a p–n

diode het-

erojunction having a single diode composed of a parallel

combination of a capacitor,

, and a resistor,

, in par-

allel with series connected capacitance and resistance ele-

ments,

and

, due to deep state responses to the in and

out-of-phase applied oscillating signal

[22];

capacitance

represents the high-frequency capacitance due to the deple-

tion region, and

describes the leakage path that often ac-

companies heterojunction devices and can physically arise

from generation-recombination current within the depletion

region, local shorting out defects across the thin ﬁlm, or

leakage due to periphery effects. If

1/R

then the parallel leakage can dominate the real part of the

complex admittance across the device and render the contri-

bution of the trap difﬁcult to determine. This problem is es-

pecially troublesome if

changes with temperature, as is

often the case, because even a change in the conductance of

the junction with temperature is difﬁcult to interpret unam-

biguously, and it is necessary to eliminate the contribution

of the parallel conductance to the admittance of the junc-

tion. We can eliminate the

conductance by determining

the elements of the equivalent circuit from a complex ﬁtting

method (LEVM from Macdonald)

[23]

and then subtracting

the effect of the parallel conductance to the experimental

data, or ﬁtting the experimental capacitance and conduc-

tance to the corresponding values of capacitance and con-

ductance of this model. The capacitance and conductance

for this model are:

(3)

(ωτ)

(τω)

(4)

Fig. 8. Characteristic time for the deep states for the n-ZnO:Al/p-Si and

n-ZnO/p-Si junction.

where

is the time characteristic (inverse emission fre-

quency) for the defect associated with the quantities

and

ds,

and

is the angular frequency of the applied oscillat-

ing signal

[18].

Both methods are in very good agreement.

Interface states can be determined from admittance spec-

troscopy

[18].

Using appropriate modeling, the measured

conductance as a function of frequency can be associated to

the density of the interface states (D

). The density of in-

terface states and the capacitance due to the interface states

are related by the equation

(5)

where

is the trap time constant.

Fig. 8

presents the

trap time constant for the n-ZnO/p-Si and

Fig. 9

shows the

−

)/ω data as a function of the logarithm of the

frequency and the ﬁtting for

Eq. (6)

at different tempera-

tures at zero bias voltage. The nature of these peaks sug-

gests that they may be associated with narrow defect energy

levels.

A characteristic frequency exists for each energy level

in the band gap. The inverse characteristic frequency is the

characteristic time associated with that energy position. This

time is, in effect, the response time for that energy level.

Therefore, if an oscillating signal is applied to the sample at

a frequency greater than that of the characteristic frequency

for a given defect level, that defect level will not be able

to respond to the applied signal. This property establishes a

demarcation energy associated with the applied signal such

that defects with energy positions below this level (having

higher response frequencies) will be able to respond to the

signal, but those defects with energy positions above this

where

is the capacitance,

the density of the interface

states and

is the electronic charge. The equation describ-

ing the measured parallel conductance versus the frequency

relationship to the

used for this junction was obtained

from

Eq. (4):

tot

ωτ

(6)

Fig. 9. (G

−

)/ω data as a function of the logarithm of the frequency

and the ﬁtting for

Eq. (5)

at different temperatures at zero bias voltage

for n-ZnO:Al/p-Si.

Plik z chomika:

tho.doduc

Inne pliki z tego folderu:

Enhanced Exciton Emission from ZnO NC Films.PDF (495 KB)
Exciton Emission ZnO SC Nanorods.PDF (161 KB)
nZnO_cSi Heterojunct.pdf (173 KB)
N_in_ZnO.PDF (518 KB)
One-dimensional protuberant optically active ZnO structure fabricated by oxidizing.pdf (618 KB)

nZnO_cSi Heterojunct.pdf

Plik z chomika:

Inne pliki z tego folderu:

Inne foldery tego chomika: