Optical and Electrical Properties of Nitrogen Ion Implanted Fluorine ...

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This is the author version of article published as:

Tesfamichael, Tuquabo and Will, Geoffrey D. and Colella, Michael and Bell, John M. (2003) Optical and Electrical Properties of Nitrogen Ion Implanted Fluorine Doped Tin Oxide Films . Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms 201(4):pp. 581-588.

Copyright 2003 Elsevier Accessed from http://eprints.qut.edu.au

T. Tesfamichael, G. Will, M. Colella, J. Bell, Optical and Electrical Properties of Nitrogen

Implanted Fluorine Doped Tin Oxide Films, Nucl. Instr. Meth. Phys. Res. B, 201 (2003) 581-

588.

Optical and Electrical Properties of Nitrogen Ion Implanted

Fluorine Doped Tin Oxide Films

Tuquabo Tesfamichaela,∗, Geoffrey Willb, Michael Colellac, John Bella

aFaculty of Built Environment and Engineering, School of Mechanical,

Manufacturing & Medical Engineering;

bFaculty of Science Centre for Instrumental and Developmental Chemistry;

Queensland University of Technology, Box 2434, Brisbane QLD 4001, Australia

cAnalytical Electron Microscopy Material Division

ANSTO, PMB 1 Menai, NSW 2234 Australia

e-mail: [email protected]; [email protected]; [email protected];

[email protected];

Abstract:

Tin oxide films were implanted with N+ at various energies between 5 to 40 keV for

different ion doses between 1014 to 1016 cm-2. The microstructure, optical and

electrical properties of the films were investigated. From Transmission Electron

Microscopy the implanted films were shown to be amorphous. The implanted

thickness for the 10 keV and 40 keV were found to be 30 nm and 110 nm,

respectively. The ion penetration depths for these films were calculated using SRIM-

2000 and found to be 35 nm (at 10 keV) and 120 nm (at 40 keV). The optical

properties of the implanted films were measured and transmittance was found to

∗ Corresponding author. e-mail: [email protected]



588.

decrease with increasing implantation energy and/or ion dose. The luminous

transmittance of the films decreased from 0.70 for the unimplanted film to about 0.57

for the highest implantation energy and largest ion dose. By annealing the films large

part of the defects have been removed and thereby increasing the transmittance of the

films. The electrical properties of the films were investigated and found an increase of

sheet resistance with increasing implantation energy and/or ion dose. The increase of

sheet resistance after ion implantation is caused by a loss of crystallinity of the tin

oxide films. After annealing the sheet resistance decreases because the crystallinity

was partially recovered.

PACS code: 68.55.L

Author Keywords: tin oxide, ion implantation, microstructure, optical properties,

electrical properties.

1. Introduction

Tin oxide is a transparent conducting semiconductor material which has a wide band

gap. Due to stable mechanical and chemical properties, the material is extensively

used in many applications such as low-E windows, heat mirrors, sensors,

electrochemical electrodes, solar absorbers and solar cells. The physical properties of

the films are influenced by the method and condition of production of the films. The

electrical and optical properties of tin oxide films can largely be modified by doping.

The upper limit of the electron density is determined by the solubility of the dopants

and if reached can form clusters in the lattice accompanied by a loss of crystallinity

[1, 2]. An increase in sheet resistance, therefore, can then be attributed due to atomic



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disorder and impurity scattering that reduces the mobility of the electrical carriers [2,

3]. However, the presence of impurities at the grain boundaries can reduce barriers

between the crystallites and consequently may increase the mobility or decrease the

resistivity of the films [4].

Tin oxide films have been doped with group III, V, VI and VII elements of the

periodic table such as In, Tl, P, As, Sb, Te, F, Cl, Br and I to modify the properties

of the films [4]. Among these elements, the most commonly used dopants is fluorine

[2]. Oxygen vacancies in the films can also be used as one of the doping methods.

Different techniques such as spray pyrolysis, sputtering, CVD and evaporation have

been employed to produced doped tin oxide films [5-15]. The most widely employed

technique is pyrolytical deposition which is now a major commercial production

method for tin oxide films doped with fluorine (SnO2:F). It is of interest to modify

commercially available SnO2:F films further by ion implantation since implantation

can cause changes in surface composition, morphology and chemical bond structure

leading to the formation of new compounds and can also be an effective method to

improve the mechanical and chemical properties [16-21]. Formation of defects and

lattice imperfections can be caused with ion implantation and thus the optical and

electrical properties of the material will be changed. It has been reported that

implanted films show a reduction in the optical band gap of the films revealing the

formation of defect levels [22-24]. The objective of this paper is to investigate the

optical and electrical properties of fluorine doped tin oxide films after ion

implantation. The goal of the project is to determine whether this can be used as an

effective method to reduce heterojunction effects at the interface between the TiO2



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layer and the SnO2:F layer in dye-sensitised solar cells. Considerable gain in

performance have been reported by the presence of thin interfacial layers on

heterojunction solar cells consisting of wide band gap semiconductor intimately

contacted to a narrow band gap semiconductor [4]. Potential barriers can be formed

due to the mismatch of energy bands of semiconductors and there may be a depletion

or accumulation of carriers at the interface [25]. Promotion of tunnelling conduction

at an interface can also be achieved by narrowing the depletion layer through

increasing carrier concentration near the interface [25].

Implantation was carried out for various ion energies and ion doses using N ion

species in the group V and the physical and electrical properties of the films were

investigated. Other ions, specifically Cl+ (group VII) and Ar+ (group VIII) were also

implanted. Transmission Electron Microscopy (TEM) was used to investigate the

microstructure of the implanted films, including its film thickness and crystalline

properties. Theoretical calculations using SRIM-2000 have been performed to

investigate the ion range. The results have been compared with the experimental

results obtained from TEM. UV-Vis spectrophotometer was used to measure the

optical properties (transmittance) of the films. The electrical properties were obtained

by the 4-point probe method and the sheet resistance was determined.

2. Experiment

2.1. Sample Preparation

Samples of SnO2:F (Libbey-Owens Ford TEC 8 – 8 ohms/square nominal resistance)

film were obtained from Libbey-Owens Ford (USA). The samples were cut in to 3x2



588.

cm size and four pieces were mounted on a rotating sample holder. The samples were

attached to the sample holder to make good electrical ground contacts and reduce

charging effects.

Implantation of the SnO2:F film was performed in a 50 cm diameter vacuum chamber.

The chamber including the ion source was evacuated by mechanical, diffusion and

cryo pumps to a pressure range between 2 x10-6 and 6 x10-6 Torr. After achieving the

required vacuum, nitrogen gas was allowed into the chamber via separate mass-flow-

controlled inlet. The nitrogen gas was ionised using Freeman ion gun to create ion

beam and the beam was monitored for different ion masses using a magnetic field ion

mass analyser. Ion current was measured using Faraday cup in the chamber and can

be adjusted before starting implantation using the gas flow. Various samples were

implanted at normal angle of incidence using ion energy between 5 to 40 keV. The

target (sample) current was maintained between 1 to 5 μA and for comparison higher

current density (≈10 μA) samples were also prepared. Uniformity of the implanted

sample was assured by sweeping the sample across the ion beam. Different samples

having ion dose range between 1014 to 1016 cm-2 were obtained.

2.2. Sample Characterization:

The microstructures of two nitrogen implanted SnO2:F films were analysed using Joel

2010F Transmission Electron Microscopy (200 keV) with an EDX system. Films

were coated with Pt prior to the cross-sectional specimen preparations to prevent

ingest of the epoxy used.



588.

The optical properties (transmittance) of the films were measured using Varian Cary

50 UV-Visible spectrophotometer in the wavelength range 300 to 1100 nm at normal

angle of incidence. As a reference, 100% baseline signals were displayed before each

measurement of transmittance.

The electrical properties of the films were determined using the four-point probe

method with spring-loaded and equally spaced pins. The probe was connected to a

Keithley voltmeter constant-current source system and direct current and voltage were

measured by slightly touching the tips of the probe on the surface of the films.

Multiple reading of current and the corresponding voltage were recorded in order to

get average values. From these measurements and the thickness of the films the

resistivity of each sample was obtained and by neglecting edge effects the sheet

resistance has been determined [26].

3. Results and Discussions

3.1 Film Microstructure

Investigation of SnO2:F films using TEM have been performed in order to study the

microstructure of the films after ion implantation. Our study refers to investigation of

N+ implanted SnO2:F films at 10 keV and 40 keV having 1x1016 cm-2 of ion dose. Fig.

1 shows the cross-sectional image of the N+ implanted SnO2:F film implanted at 10

keV. The dark layer of about 10 nm thick shows Pt film, its sole purpose is to separate

the implanted SnO2:F from the glue during specimen preparation. The tin oxide film

(SnO2:F) was found to have a thickness of about 650 nm with crystalline properties.

The implanted layer has a thickness of about 30 nm with a distinct amorphous layer of



588.

about 15 nm thick and the remainder appearing polycrystalline nature. It is probable

that the implantation enhanced diffusion would result in partial annealing of the

amorphous region leading to polycrystalline grains [27]. For the 40 keV implantation,

the implanted film thickness is found to be about 110 nm. The implanted layer

thicknesses were found to have similar values compared to the theoretical calculations

obtained using SRIM-2000. The calculated ion range for the 10 keV and 40 keV were

about 35 nm and 120 nm, respectively.

At the glass/tin oxide interface (not shown) two distinct thin layers were observed.

These layers are intrinsic-SnO2 film and SiO2 film each of about 25 nm of thickness.

The thin SnO2/SiO2 layers are used for colour suppression created due to the SnO2:F

film and also act as a barrier layer for the diffusion of sodium ions in the glass

substrate towards the film [28, 29].

3.2 Optical Properties

The transmittance of N+ implanted SnO2:F films as a function of wavelength

implanted at 10 keV and 40 keV and different ion doses between 1014 and 1016 cm-2 is

shown in Fig. 2. The transmittance in Fig. 2a (10 keV) decreases continuously with

increasing ion doses in the measured wavelength range between 300 to 1100 nm with

a major reduction appearing in the visible spectrum. For the 40 keV (Fig. 2b)

implantation energy, a small change of transmittance occurred with the different ion

doses. The spectra of the un-implanted sample is shown by the “zero” ion dose. The

sharp decrease of transmittance at the shorter wavelength is an absorption edge of the

tin oxide as a result of the on-set of inter-band transition.



588.

Similar spectra have obtained for samples implanted between 5 to 40 keV for ion dose

ranges between 1014 and 1016 cm-2 and ion beam current between 1 to 5 μA. The

decrease in transmittance of a sample implanted at higher current density (≈10 μA)

and 10 keV for 4x1015 cm-2 ion dose was found to be larger than the corresponding

film implanted at 1 μA. The large decrease of transmittance is probably due to the

high beam current introducing more damage and large defects in the films. High

current density can produce significant heating of the sample surface, which may

facilitate effective diffusive redistribution of implanted atoms owing to the higher

concentration of vacancies [19]. On the other hand, high current density at elevated

temperature can increase the sputtering rate of the surface of the coatings and can

cause surface property degradation [30].

Luminous transmittance of the N+ implanted SnO2:F films were determined by

weighting the transmittance of the films in the wavelength range 380 to 760 nm to the

intensity of the corresponding visible spectrum for air mass 1.5. Fig. 3 shows

luminous transmittance, Tlum of N+ implanted SnO2:F films as a function of N+ doses

for various implantation energy between 10 to 40 keV. The film before implantation

is fairly transparent in the measured wavelength range and has a luminous

transmittance of about 0.70. On implantation, the luminous transmittance of the films

decreases with increasing ion doses and/ or ion energy. A sharp decrease of Tlum is

observed for the small doses but the luminous transmittance changes more slowly at

higher ion doses. For the largest ion dose and energy, the luminous transmittance of

the SnO2:F film is substantially reduced to about 0.57.



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After annealing the N+ implanted SnO2:F films, the transmittance spectra in the

wavelength range 300 to 1100 nm increases. Fig. 4 shows, luminous transmittance of

N+ implanted SnO2:F films before and after annealing at 200 °C for 1 hour as a

function of implantation energy. The larger the implantation energy, the larger is the

damage created in the film and by annealing such films some of the defects induced

by the ion implantation can be removed. Consistent with this, the higher the

annealing temperature the larger was the increase in transmittance, for a given

annealing time. It is thought that larger defects can be removed at higher temperatures

than at lower ones. As shown in Fig. 3b, the luminous transmittance increased by

about 1.8% when annealed at 500 °C for one hour compared to a film heated at 200

°C for same time. A sharp increase in transmittance was observed when the annealing

temperatures were above 200 °C. When Ar+ or Cl+ ion beams were used, the

transmittance of the implanted SnO2:F films decreased, similar to that of the N+

implanted films as shown in Fig. 3b. This suggested that the change in optical

properties of the SnO2:F after implantation can be caused by the structural change

induced by the ion implantation. The rate of decrease of transmittance for a given ion

energy and ion dose was larger when the films were implanted with N+ than that of

Ar+ or Cl+ (Fig. 3b). This indicated that the penetration depth of N+ for a given ion

energy is bigger than the other two elements, leading to a thicker (both physical and

optical) layer of modified film.

3.3 Electrical Properties



588.

From the conductivity measurements, the sheet resistance of the N+ implanted SnO2:F

films have been determined. Fig. 5 shows sheet resistance of the implanted samples

as a function of ion doses for varying implanted energies. The resistance of the un-

implanted SnO2 film, which referred to the “zero” ion dose, is also included. From the

figure it can be observed that the sheet resistance increases with the increasing ion

dose and similar trend has been observed for the Cl+ and Ar+ implanted SnO2:F films.

An increase of resistance with increasing ion dose was also reported for N+ implanted

In2O3 films elsewhere [31]. The sheet resistance of the N+ implanted SnO2:F films

also increases with increasing ion energy. The increase of sheet resistance in tin oxide

films after ion implantation can be caused by a loss of crystallinity in the films.

Nitrogen may also replace oxygen vacancies in the films and could cause the decrease

of the number of carriers and hence an increase of sheet resistance [31]. By annealing

the samples, the sheet resistance decreases slightly compared with the corresponding

implanted samples as shown in Fig. 5. There was no change in electrical properties

after annealing the un-implanted SnO2:F film in air atmosphere, indicating the

existence of oxygen vacancies in the film may be low. Hence, the main cause for the

increase in sheet resistance after ion implantation can be attributed to the loss of

crystallinity in the film. The decrease of sheet resistance, Rsh, after annealing indicates

the removal of some of the defects in the film and hence the crystallinity was partially

recovered.

Fig. 6a shows the values of Tlum and Rsh as a function of ion energy for the maximum

ion dose (1x1016 cm-2) of implantation. The luminous transmittance decreases with

increasing ion energy and varies between 70% and 57%. The sheet resistance,



588.

however, increases from 7.5 ohms/square for the un-implanted sample to about 10.5

ohms/square for the highest implantation energy used in this experiment (40 keV).

The graph in Fig. 6b shows that the sheet resistance decreases with increasing

transmittance and this trend is found to be not directly linked to the inherent trade-off

between Tlum and Rsh for transparent conductors [4, 32].

The overall results in this paper showed a change in the optical and electrical

properties of the tin oxide films after ion implantation. The decrease of transmittance

and increase of sheet resistance in the films would have negative impact on the

performance of dye-sensitised solar cells. It was, however, important to investigate

further to see the effects of the interface on efficiency due to the modified

heterojunction layer between the TiO2 and the SnO2:F layers in dye-sensitised solar

cells which are planned to be done.

Conclusion

Commercial fluorine doped tin oxide films have been implanted with nitrogen using

ion energy between 5 to 40 keV for different ion doses ranging between 1014 to 1016

cm-2. Before implantation the films had crystalline properties but on implantation they

changed to amorphous nature. The luminous transmittance of the implanted films

decreased whereas the sheet resistance increased with increasing ion energy and/or

ion dose. These can be attributed due to the enhancement of defects and lattice

damage in the films with increasing ion energy or ion dose. Thus, thin layer of the

bulk SnO2:F films can be implanted to modify the surface properties of the material.



588.

Acknowledgments

The authors acknowledge Australian Nuclear Science Technology Organisation

(ANSTO) for allowing to use their facility. This work was financially supported by

Queensland University of Technology (QUT) and the Australian Institute of Nuclear

Science and Engineering (AINSE). T. Tesfamichael is indebted for the postdoctoral

fellowship position granted by QUT.

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Figure Captions

Fig. 1 Cross-sectional TEM image of N+ implanted SnO2:F films implanted at ion

energy of (a) 10 keV and (b) 40 keV. Both films have dose of 1x1016 ions/cm2.

Fig. 2 Transmittance of N+ implanted SnO2:F films implanted at (a) 10 keV and (b) 40

keV for various ion doses between 1014 and 1016 cm-2.

Fig. 3 Luminous transmittance, Tlum of (a) N+ implanted SnO2:F films as a function

of ion doses for various implantation energy between 10 to 40 keV, and (b) N+,



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Ar+, and Cl+ implanted SnO2:F films as a function of energy for 1015 cm-2 ion

dose . Lines are guide to the eye.

Fig. 4 (a) Luminous transmittance, Tlum, of N+ implanted SnO2:F films before and

after annealing 200 °C for 1 hour. The unimplanted sample is indicated by the

“zero” energy, (b) Change of Tlum for a 5 keV implanted films after annealing at

indicated temperatures.

Fig. 5 Sheet resistance of N+ implanted SnO2:F films as a function of ion dose for

indicated implantation energies. For comparison the sheet resistances of samples

implanted at 20 keV and then annealed at 500 °C for 1 hour are also shown.

Fig. 6 (a) Luminous transmittance and sheet resistance as a function of implantation

energy for ion dose of 1.09x1016 cm-2, and (b) relationship between sheet

resistance and luminous transmittance.



588.

Figures

(a)

(b)



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Fig. 1

0

10

20

30

40

50

60

70

80

300 400 500 600 700 800 900 1000 1100

02x1014

4x1015 1x1016 Tr

ansm

ittan

ce

Wavelength (nm)

Ion dose (cm-2):

Implantation Energy: 10 keV

(a)

0

10

20

30

40

50

60

70

80

300 400 500 600 700 800 900 1000 1100

02x1014

4x1015 1x1016 Tr

ansm

ittan

ce

Wavelength (nm)

Implantation Energy: 40 keV

Ion dose (cm-2):



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(b)

Fig. 2

56

58

60

62

64

66

68

70

0 2 1015 4 1015 6 1015 8 1015 1 1016 1.2 1016

10 keV20 keV30 keV40 keV

Lum

inou

s Tr

ansm

ittan

ce (%

)

Ion Dose (cm-2)

Implantation Energy:

(a)



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58

60

62

64

66

68

70

0 5 10 15 20 25 30 35 40

N+ implantation

Ar+ implantation

Cl+ implantation

Lum

inou

s Tr

ansm

ittan

ce (%

)

Implantation Energy (keV)

Ion dose: 10 15 cm-2

(b)

Fig. 3



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60

62

64

66

68

70

0 5 10 15 20

As-implantedAnnealed (200 oC)

Lum

inuo

us T

rans

mitt

ance

(%)

Implantation Energy (keV)

(a)

0

0.5

1

1.5

2

0 100 200 300 400 500

change of Tlum

T lum

(ann

eale

d) -

T lum(a

s-im

plan

ted)

(%)

Annealing Temperature (C)

Films are implanted at 5 keV



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(b)

Fig. 4

7

8

9

10

11

12

0 2 1015 4 1015 6 1015 8 1015 1 1016 1.2 10 16

10 keV20 keV30 keV40 keV20 keV (T=500C, 1 hr)

She

et R

esis

tanc

e (o

hms/

squa

re)

Dose (ion/cm2)

Implantation Energy:

Fig. 5



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45

50

55

60

65

70

7

7.5

8

8.5

9

9.5

10

10.5

0 5 10 15 20 25 30 35 40

Sheet Resistance

Luminous Transmittance

Lum

inou

s Tr

ansm

ittan

ce (%

)

Energy (keV)Sh

eet R

esis

tanc

e (o

hms/

squa

re)Ion dose=1x1016 cm-2

(a)

7

7.5

8

8.5

9

9.5

10

10.5

56 58 60 62 64 66 68 70

She

et R

esis

tanc

e (o

hms/

squa

re)

Luminous Transmittance (%)



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(b)

Fig. 6

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