University of Bucharest

Faculty of Physics Dissertation thesis

Electronic and optoelectronic devices

based on CdTe nanowire arrays

Student: Camelia-Florina FLORICA

2011

Advisors: Prof. Univ. Dr. Ştefan ANTOHE

CS I Dr. Ionuţ ENCULESCU

UNIVERSITY OF BUCHAREST

Faculty of Physics

Dissertation thesis

Electronic and optoelectronic devices

based on CdTe nanowire arrays

Student: Advisors:

Camelia - Florina FLORICA Prof. Univ. Dr. Ştefan ANTOHE

CS I Dr. Ionuţ ENCULESCU

Măgurele - Bucharest, June 2011

Acknowledgements

This thesis was carried out in the laboratory Research and Development Centre for Materials

and Electronic & Optoelectronic Devices (MDEO) within the Faculty of Physics of the

University of Bucharest and in the Multifunctional Materials and Structures laboratory of the

National Institute of Materials Physics.

This work, and with it, its author, have enjoyed a lot of encouragement and support from many

sides. I would like to thank particularly Prof. Dr. Stefan ANTOHE for supervising my training as

well as for the very good guidance during the time spent in his laboratory.

I would like also to express my gratitude to CS I Dr. Ionuţ ENCULESCU for providing the

necessary equipment and time for preparing the nanowire arrays, as well as for all the pieces of

advices which led to constantly improving my work.

Special thoughts and many thanks to Assoc. Prof. Lucian ION, Dr. Elena MATEI and PhD

students Adrian RADU and Sorina IFTIMIE, as well as to all the people in the laboratories for

the pleasant environment and for their help in practical problems.

Contents

1. Introduction ................................................................................ Error! Bookmark not defined.

1.1. Why studying electronic and optoelectronic devices? ........ Error! Bookmark not defined.

1.1.1. State of the art in photovoltaic cells ............................. Error! Bookmark not defined.

1.1.2. State of the art in sensors .............................................. Error! Bookmark not defined.

1.2. Theoretical background ....................................................... Error! Bookmark not defined.

1.2.1. Electrical behaviour of the solar cells ........................... Error! Bookmark not defined.

1.2.2. Single nanowires transistors for sensors ....................... Error! Bookmark not defined.

2. CdTe nanowire arrays for photovoltaic cells ............................. Error! Bookmark not defined.

2.1. Preparation of the CdTe nanowires ..................................... Error! Bookmark not defined.

2.1.1. Electrochemical deposition of CdTe nanowires using Au as the working electrode

................................................................................................ Error! Bookmark not defined.

2.1.2. Electrochemical deposition of CdTe nanowires using Cu as the working electrode

................................................................................................ Error! Bookmark not defined.

2.2. Preparation of the hybrid inorganic/organic photovoltaic cells based on CdTe nanowires

and ZnPc organic dye ................................................................. Error! Bookmark not defined.

2.2.1. Technological steps for producing the hybrid photovoltaic devices . Error! Bookmark

not defined.

2.2.2. Optical, photoelectrical and electrical measurements and results ..... Error! Bookmark

not defined.

2.3. Preparation of the inorganic photovoltaic cells based on CdTe nanowires and CdS thin

film ............................................................................................. Error! Bookmark not defined.

2.3.1. Technological steps for producing the inorganic photovoltaic structures ............ Error!

Bookmark not defined.8

2.3.2. Optical, photoelectrical and electrical measurements and results ..... Error! Bookmark

not defined.0

3. Ni/CdTe/Ni single nanowires properties ................................. Error! Bookmark not defined.2

3.1. Preparation of the Ni/CdTe nanowires ................................ Error! Bookmark not defined.

3.2. Preparation of the Ni/CdTe/Ni nanowires ......................... Error! Bookmark not defined.6

3.3. Aligning nanowires ........................................................... Error! Bookmark not defined.7

3.4. Contacting single nanowires with FIB/SEM ....................... Error! Bookmark not defined.

3.4.1. FIB-SEM Description ................................................. Error! Bookmark not defined.8

3.4.2. Pt contacts .................................................................................................................... 39

3.4.3. Contacting single nanowires ....................................... Error! Bookmark not defined.0

4. Conclusions and further work .................................................. Error! Bookmark not defined.4

5. References ................................................................................ Error! Bookmark not defined.5

1. Introduction

1.1. Why studying electronic and optoelectronic devices?

The arrays of semiconductor nanowires are a new class of materials which lead to larger number

of levels of functionality in building devices for electronics and optoelectronics applications.

These nanoscale devices are enabling a route of development of new technologies in key areas,

such as, communications and information processing, sensors and sensing and renewable energy.

Regarding all these applications it is nowadays usual to use them and to ask for the best results

from it. In order to do this it is obvious the fact that we have to improve them constantly,

therefore to study their properties and find the best compromise between the cost of fabrication

and the best performances.

In this thesis there are reported two types of devices with applications in the renewable energy

sector, photovoltaic cells, and sensors and sensing, single nanowire biofuntionalised transistors.

1.1.1. State of the art in photovoltaic cells

Fossil fuels which took around 400 million years to form and to be stacked away underground

are used by the people for energetic purposes in the last centuries. Besides the depletion of the

fossil fuel noticed in the last decades which is going to become deeper and deeper there is

another extremely important problem which has to be taken into consideration carbon dioxide

emissions and increased global warming.

Most politicians and policymakers agree that a massive redirection of energy policy is essential if

Planet Earth is to survive the 21st century in reasonable shape[1]. This is not simply a matter of

fuel reserves. It has become clear that, even if those reserves were unlimited, we could not

continue to burn them with impunity. The resulting carbon dioxide emissions and increased

global warming would almost certainly lead to a major environmental crisis. So the danger is

now seen as a double - edged sword: on the one side, fossil fuel depletion; on the other, the

increasing inability of the natural world to absorb emissions caused by burning what fuel

remains.

For the good of Planet Earth and future generations we have to invest heavily in renewable

technologies – including solar, wind and wave power – that produce electrical energy free of

carbon emissions.

Sun’s radiation beamed at us day by day, year by year, and century by century, its income can be

used or ignored as we wish. The challenge for the future is to harness such renewable energy

effectively, designing and creating efficient and hopefully inspiring machines to serve

humankind without disabling the planet.

Since the reduction of carbon emissions is a principal advantage of PV, wind, and wave

technologies, we should recognise that this benefit is also proclaimed by supporters of nuclear

power. It is true that all offer electricity generation without substantial carbon emissions, but in

almost every other respect they are poles apart. The renewables offer the prospect of widespread,

relatively small - scale electricity generation, but nuclear must, by its very nature, continue the

practice of building huge centralised power stations. PV, wind, and wave need no fuel and

produce no waste in operation; the nuclear industry is beset by problems of radioactive waste

disposal. On the whole renewable technologies pose no serious problems of safety or

susceptibility to terrorist attack – advantages which nuclear power can hardly claim. And finally

there is the issue of nuclear proliferation and the difficulty of isolating civil nuclear power from

nuclear weapons production.

It would however be unfair to pretend that renewable energy is the perfect answer. For a start

such renewables as PV, wind, and wave are generally diffuse and intermittent. Often, they are

rather unpredictable. And although the ‘fuel’ is free and the waste products are minimal, up -

front investment costs tend to be large. There are certainly major challenges to be faced and

overcome as we move towards a solar future.

Regarding photovoltaics this way of producing renewable energy is arguably the most elegant

and direct.

The International Energy Agency has made a scenario for the way it should be invested in the

energy sector until 2035. From their statistics it is noticed the fact that the installed power

generation capacity should increase from 4 722 GW in 2008 to about 8 600 GW by 2035.

Between 2009 and 2035, total gross capacity additions amount to 5 900 GW, with more than

40% installed by 2020.

Nuclear power and renewable energy additions respectively account for 5% and 41% of the total

between 2009 and 2020. Investment in new plants rises more quickly from 2021 to 2035, as

more capital intensive technologies are deployed and more variable resources exploited creating

a need for additional generating capacity (Figure 1.1).

Figure 1.1: World power-generation capacity additions and investment by type in the New Policies Scenario

Source: International Energy Agency study, World energy outlook

In addition to providing support as defined above, governments are engaged in substantial

continuing efforts in research and development (R&D) to bring the costs of renewable energy

technologies down and to improve their performance. Some of these technologies, such as

hydropower, onshore wind and biomass are mature or almost mature and do not require

significant additional spending on R&D, although R&D is still needed for better wind

forecasting and working variable generation into the power supply system.

Photovoltaics and concentrating solar power, though commercially available, depend for their

widespread diffusion on further supportive policy measures. For constantly improving the

efficiency of generating electrical energy investments have to be made.

Total spending on research and development reached $5.6 billion in 2009 (figure 1.2).

Figure 1.2: Global spending on research and development in renewable energy by technology, 2009

Source: International Energy Agency study, World energy outlook.

As seen in the figure above, more than half was spent on solar energies. The estimated energy

generation until 2035 is presented in figure 1.3 and it implies a huge increase of the investment

department.

Figure 1.3: Electricity generation by fuel and region in the New Policies Scenario

Coming back to the history of the field of interest for this thesis, photovoltaic cells have started

to be developed at a large scale since 1958.

Scientist and innovation Year

Becquerel discovers the photovoltaic effect 1839

Adams and Day notice photovoltaic effect in selenium 1876

Planck claims the quantum nature of light 1900

Wilson proposes Quantum theory of solids 1930

Mott and Schottky develop the theory for diodes 1940

Bardeen, Brattain and Shockley invent the transistor 1949

Charpin, Fuller and Pearson announce 6% efficient silicon solar cell 1954

Reynolds et al. highlight solar cell based on cadmium sulphide 1954

First use of solar cells on an orbiting satellite Vanguard 1 1958

All solar cells require a light absorbing material which is present within the cell structure to

absorb photons and generate free electrons via the photovoltaic effect. The photovoltaic (PV)

effect is the basis of the conversion of light to electricity in photovoltaic, for solar cells.

There are many materials that can be used for making photovoltaic cells, such as: silicon

(amorphous, crystalline), cadmium telluride and cadmium sulphide, organic and polymeric

materials. Also the way these materials are processed can be different from thick films

technologies to thin films, from using electrolyte to making nanostructured devices.

In the case of the silicon technologies it is well known that it has been the dominant one for the

supply of power modules into photovoltaic applications and the likely changes are an increasing

proportion of multi-crystalline silicon and monocrystalline silicon being used for high-efficiency

solar cells while thinner wafers and ribbon silicon technology continue to grow[2-5].

Many researchers have investigated the properties of these kind of cells, reporting about their

optical, electrical and structural properties and have searched continuously a way of making a

growth of the efficiency and lowering the price in the same time[6-10].

Amorphous (uncrystallized) silicon is the most popular thin film technology with cell

efficiencies of 5–7% and double- and triple-junction designs raising it to 8–10%. But it is prone

to degradation. Advances made in amorphous-Si PV technology led to the achievement of an

AM 1.5, 13% stable cell efficiency and set the foundation for the spectrum splitting triple-

junction structure being manufactured by the roll-to-roll continuous deposition process[11].

Crystalline silicon offers an improved efficiency when compared to amorphous silicon while still

using only a small amount of material. The commercially available multi-crystalline silicon solar

cells have efficiency around 14–19%[12-18].

Cadmium telluride (CdTe) and cadmium sulphide (CdS) are good replacement materials for the

Si which is highly expensive[19-21]. Work has to be done continuously for improving the

efficiency of this type of cells.

Organic and polymer cells have problems with the stability/ degradation methods for enhancing

the stability through the choice of better active materials, encapsulation, application of getter

materials and UVfilters are done[22-26]. Polymers have demonstrated that the bulk heterojunction

concept is a viable approach towards developing photovoltaic systems by inexpensive solution-

based fabrication technologies[27].

The most important advantage of this type of cells is the low cost of production.

Thin film technologies led to multilayered structures having the biggest known efficiency till

present of 40.7%[28].

Hybrid photovoltaic cells are representing the latest generation of solar cells that is currently

developed. A nanostructured inorganic material in combination with a polymer thin film is

expected to have higher power conversion efficiency at a lower price.

Nanostructures basis of CdTe, nanowires more precisely, will be used to develop photovoltaic

cells in this project.

1.1.2. State of the art in sensors

Sensors are the devices, which are composed of an active sensing material with a signal

transducer. Sensors can be broadly classified in to two categories as chemical sensors and

biosensors. The biosensors can be defined in terms of sensing aspects, where these sensors can

sense biochemical compounds such as biological proteins, nucleotides and even tissues[29-31].

Within these sensors, the active sensing material on the electrode should act as a catalyst and

catalyze the reaction of the biochemical chemical compounds to obtain the output signals[32, 33].

The combination of these two different ways of classifications has given rise to a new type of

sensors which are called electrochemical biosensors, where the electrochemical methods are

applied for the construction and working of a biosensor[34-36]. The selection and development of

an active material is a challenge. The recent development in the nanotechnology has paved the

way for large number of new materials and devices of desirable properties which have useful

functions for numerous electrochemical sensor and biosensor applications[37-41]. Basically by

creating nanostructure, it is possible to control the fundamental properties of materials even

without changing their chemical composition. In this way the attractive world of low

dimensional systems, together with the current tendencies on the fabrication of functional

nanostructured arrays could play a key role in the new trends of nanotechnology[42-44]. Further,

the nanostructures can be used for both efficient transport of electrons and optical excitation, and

these two factors make them critical to the function and integration of nanoscale devices[45-47]. In

fact, nanosystems are the smallest dimension structures that can be used for efficient transport of

electrons and are thus critical to the function and integration of these nanoscale devices. Because

of their high surface-to-volume ratio and tunable electron transport properties due to quantum

confinement effect, their electrical properties are strongly influenced by minor perturbations.

A good method for generating any kind of nanostructures should enable simultaneous control of

the dimensions, properties, and morphology. In general, nanostructures are synthesized by

promoting the crystallization of solid-state structures along one direction by various mechanisms

which includes, the use of templates with nanostructure morphologies to direct the formation of

nanostructures; the use of intrinsically anisotropic crystallographic structure of a solid to achieve

nanowires growth; the use of a liquid/solid interface to reduce the symmetry of a seed and the

use of appropriate capping agents to kinetically control the growth rates of various facets of a

seed[48]. Among these, highly successful methods to obtain nanowires and nanowire arrays are by

using the synthesis on templates and self assembly or self-organization processes. The first and

most important process of this strategy is the creation of a desirable surface structure on which

the nanowires growth should take place; for this goal several top-down techniques like molecular

beam epitaxy, electron beam evaporation, phase-shift optical lithography and sputtering have

been used[49-51].

Polycarbonate membranes are used to create nonporous structures which are in turn being used

as templates to grow functional nanowires of different species by mainly self-assembling process

and electroplating techniques. By means of electrodeposition processes, it could be possible to

take control on, ordering degree, i.e., the size of crystalline single domains (up to several square

micrometers); the single one dimensional structure diameter (from 15 to 200 nm) and length

(from tens to thousands of nanometers); the lattice parameter of ordered arrays (between 65 and

500 nm).

1.2. Theoretical background

1.2.1. Electrical behaviour of the solar cells

The photovoltaic effect appears when light interacts with a semiconductor material, if two

conditions are fulfilled:

- the absorbed light should generate particles that can move through the material

(electrons, holes, excitons, polarons) by direct transport inside an energetic band or by

hopping

- there should exist an electric field which have to separate the charge carriers created by

direct photogeneration or by excitons dissociation and sent them to the exterior circuit

At open circuit the structure is polarised with the voltage VOC, and therefore there exist in the

structure a dark current opposite to the current generated by the light.

The shortcircuit current has the density jSC and it is the current generated by the light

(photocurrent). In the presence of the light, through the structure there will be a current of

density j (through a resistance); the resulting power is negative which thermodynamically

corresponds to an energy generator.

In figure 1.4 the current-voltage characteristics of an ideal photovoltaic cell is shown.

Figure 1.4 The current-voltage characteristics of an ideal photovoltaic cell

The power conversion efficiency is defined by:

inc

SCOC

inc

m

P

IVFF

P

P . (1)

Where, Pm is the maximum power generated by the cell, Pinc is the incident power on the sample

and FF is the fill factor (SCOC

mm

IV

IVFF ).

The current voltage characteristic of the photovoltaic cell is described by:

L

Sh

SkT

IRVe

S IR

IRVeII

S

1 (2)

And it is obtained for the real solar cell, equivalent with the circuit from figure 1.4, where RS is

the series resistance given by the semiconductor and the contacts resistance, RSh is the shunt

resistance, which is in parallel with the p-n junction.

Figure 1.4 Equivalent circuits for a real photovoltaic cell

I

VOC

VmV

mI

SCI

0

mP

LI

SR

0R

ShR

dI

I

A detailed study of the photovoltaic effect assumes the knowledge of the mechanism, of its

sources, the generation processes and charge carries recombination, doping effects and likewise

the dependence of the photocurrent of the wavelength and of the intensity of the incident

radiation.

When discussing about heterojunctions we are actually talking about the contact between two

semiconductors which are different because of the type of conduction and of the band gaps, also

they have different effective masses and dielectric constants. In this way at the photovoltaic

effect, besides the electrostatic field there can be the contribution of an effective field of forces.

There is a large variety of heterojunctions that can be analyzed using different models. In the

case of CdTe and ZnPc, or CdTe and CdS the corresponding model is Anderson. This is for a

heterojunction p-n, where the light enters the structure through the n type semiconductor, which

has a wide band gap, with the role of the window. The band diagram is illustrated in figure 1.5.

The p type region where the photovoltaic effect is dominant is called a basis. The limited space

charge regions have the widths l1, l2.

Figure 1.5 Band diagram of the p-n heterojunction

The photons with the energy 21 gg EhE will pass through the n type region (the window)

without being absorbed in this layer getting to the p region where non equilibrium charge carriers

will be created, separated by the electric field from the barrier, leading in this way to the

photovoltaic effect. The window effect of the n type semiconductor gives the generation of the

charges in the barrier determining the lowering of losses because of recombination, while the

diffusion of the charge carriers takes place in the neutral regions until the field region.

1CE

2CE

2VE

1VE

FE

CE

VE

h

1l 2l

0x

d

n)2(

p)1(

In order to calculate the total current through the structure at illumination the current density in

the two regions has to be analysed. In this way, in region (1), where the photovoltaic effect takes

place with a percentage of 60-80% the photocurrent contains the minoritary charge carriers jl1

which are generated in the passing region and the diffusion current, jn1 of the electrons which

pass through the p type region and get to the separation field.

111 nl jjj (3)

The generation rate is calculated at the x distance from the junction plane, for semiconductor 1

being xedgxg 1

11

, where d

AeIdg 2

111

. In this case the electron current density

generated in the barrier layer is:

112

1

12 11

0

111

ld

A

l

dd

Al eeIqdxeeIqj

(4)

Or:

L

l

L

d

Al eeIqj

1

111 (5)

Where 1 the quantum efficiency of generating and d is is the thickness of each semiconductor.

The diffusion current density of the electrons generated in the neutral region of the p type

semiconductor is given by:

1

1

lx

nndx

ndqDj

(6)

The concentration of the excess electrons is obtained solving the continuity equation imposing

the right limit conditions:

0

0

0

1

2

1

22

2

n

ln

L

xg

L

n

dx

nd

n

n

n

(7)

The first condition shows that the electrons generated in the neutral region (1) are all directing to

the edge of the charge space limited region because of the field from the junction and then are

sent to the n type semiconductor (2) such as 01 pnln .

The second condition shows that the neutral region of type p is larger than the electron diffusion

length and therefore the electrons are recombining with the majoritary charge carriers (holes), in

the way to get to the back electrode.

The solution of the equation (7) is:

nL

lx

L

lx

L

l

L

d

n

A

n

n eeeeD

I

LL

LLxn

1

1

1

1

1

21

22

1

1

2

(8)

With this solution the equation (6) becomes:

1

1

2

1

11

L

l

L

d

n

Ann ee

LL

IqLj

(9)

The electronic current density in the basis is:

1

1

2

1

1

1111 1

L

l

n

L

d

Anl eLL

LeIqjjj (10)

The total current generated in the n type semiconductor has contributions from the holes current

generating in the passing region of the semiconductor, jl2 and from the holes diffusion current

generated in the neutral region, jp2 which is sent to the p type semiconductor.

222 pl jjj (11)

If the generation rate in semiconductor 2 is: 22

2

2222

L

x

Ad

A eL

IeIxg

, the current density

of the holes is the passing region (d – l2, d) is:

d

ld

L

l

L

d

A

L

x

Al eeIqdxe

L

Iqj

2

2

2

22 1

2

2

22

(12)

The diffusion current density of the holes generated in the neutral region is:

2

2

ldx

ppdx

pdqDj

(13)

The concentration of the excess holes is obtained solving the continuity equation imposing the

right limit conditions:

0

0

0

0

2

2

2

22

2

pSdx

pdD

ldp

L

xg

L

p

dx

pd

x

p

p

p

p

(14)

The first condition shows that if the illumination is not very strong, non equilibrium holes which

get to the edge of the passing region, are trapped by the field from the junction and send to

semiconductor 1 such that 02 npldp and 02 ldp .

The second condition shows that the holes generated in the neutral region of the semiconductor

1 are going towards the illuminated surface, and a total recombination phenomena is happening

at x = 0, where the superficial recombination speed is very high because the electrons

concentration in this region is very high. So region 2 has to be neutral.

With the obtained solution we get the diffusion current for holes and then the current density of

holes in the window layer:

22

2

2

2

2

2

2

2

22

2

2

2

22

1

111

L

d

p

pp

ppL

ld

p

p

A e

D

SLD

SL

L

Le

L

L

LL

LIqj (15)

where:

pp

p

p

pp

p

p

L

ldsh

D

SL

L

ldch

L

ldch

D

SL

L

ldsh

ldS22

22

2,

(16)

The total current density across the heterojunction at illumination is: 21 jjj .

Having the I – V characteristics of type:

L

AV jejj 10 (17)

Which is valid for an ideal heterojunction and from it the open circuit voltage can be determined:

1ln

1

0

21

j

jj

AVOC (18)

Knowing the absorption spectrum , and calculating j1, j2 and VOC, the spectral characteristics

of the heterojunction can be obtained. Because of the « window effect » the region of sensibility

is in a band in the incident photons energies between Eg1 and Eg2.

1.2.2. Single nanowires transistors for sensors

For NW biosensors operated as FETs[52] the sensing mechanism is the field gating effect of

charged molecules on the carrier conduction inside the NW.

The sensitivity of the nanodevices is much better than the micromaterial or bulk materials and it

is given by the reduced dimensionality and he much larger surface/volume ratio.

Therefore it is natural to expect that the highest sensitivity should be achieved when the whole

volume of the nanodevice is gated by surface charges.

The transport mechanism for such a device, when discussing about Si has been calculated last

year by Xuan P. A. Gao et all[53] and it can be clearly seen that the changes in conductance could

be evaluated and measurements on single nanowire field emission transistors based sensors:

}1)/{exp(2 2

0

TkepRedrpreG Bmaterial

R

(19)

where: G is the conductance, R is the nanowire size (radius), material is surface potential shift,

p is the hole density, e is the electron charge and µ is the mobility.

Figure 1.5 The dependence of screening length on the carrier

density. In the high carrier concentration regime NW-FET

works in the linear regime, where the conductance G varies with

gate voltage linearly. In the low carrier concentration regime the

NW-FET works in the depletion (subthreshold) regime where the

G varies with gate voltage exponentially.

Source: Nano Letters 2010 10 (2), 547-552

2. CdTe nanowire arrays for photovoltaic cells

2.1. Preparation of the CdTe nanowires

2.1.1. Electrochemical deposition of CdTe nanowires using Au as the working

electrode

Using template based electrochemical deposition, as a growth method for the CdTe nanowire

arrays, several steps have to be performed in order to get the desired nanostructured material.

Electrochemical deposition, or electrodeposition for short, has been around for a very long time

now and is exceptionally versatile, and valuable applications keep being invented. Why should

serious scientists and technologists still get excited by it? There are several answers. To begin,

electrodeposition is a fascinating phenomenon. That one can put a shiny coating of one metal on

another simply by donating electrons to ions in a solution is remarkable, and studies of the

process at an atomic level continue to yield surprises.

In order to grow nanowires using this method an electrochemical cell with 3 electrodes

configuration is needed as shown in figure 2.1.

The current flow is taking place between the working electrode (cathode) and the

counterelectrode (anode), being controlled with the help of the reference electrode.

Figure 2.1 Electrochemical cell used for growing the nanowires

The working electrode can be a metal, graphite or a semiconductor and its surface must be

constant and measurable.

Primarily, before sputtering the necessary gold working electrode, the template must be

prepared. It consists of polycarbonate foils (Makrofol N, Bayer), that have a thickness of 30 m.

In order to have pores through the membrane, it is irradiated with swift heavy ions (with a

specific energy 11.4 MeV/nucleon) at different fluencies as in figure 2.2. Varying the fluencies

of the ions in the range of 104– 10

9 ions/cm

2 is necessary for having a broad range of density of

the pores in the membranes, for different applications[54].

Figure 2.2 Polycarbonate foil irradiated with swift heavy ions of specific fluencies for a range

of the density of the pores

Once the nuclear tracks have been made in the polycarbonate foils, in order to get to a specific

diameter of the pores, these were subsequently chemically etched with aqueous solutions

containing 5M NaOH and 10% volume methanol at 50C (figure 2.3).

Figure 2.3 Etching of PC foils for having a specific diameter of the pores

A good control of the diameter of the cylindrical pores is controlled by the low etching rate of

200 nm/h.

Once the template is prepared as desired the next step is the deposition of the gold working

electrode, 50 nm thick, on one surface of the foils, by sputtering (figure 2.4).

Figure 2.4 Sputtering of Au thin film

To complete the closing of the pores and to improve the mechanical stability of the template, a

copper layer, 10 m thick, was electrochemically deposited onto the gold film (figure 2.5).

Figure 2.5 Deposition of a Cu thin for mechanical stability

The counterelectrode has a big influence upon electrodeposition. It is usually made by a material

which does not produce new species by electrolysis. For this process platinum counterelectrode

is used.

The working electrode potential is controlled by a potentiostat with the help of a commercial

saturated calomel electrode (SCE) as reference.

Deposition of CdTe nanowires (figure 2.6) was performed in a potentiostatic mode using an

acidic deposition bath (1 M CdSO4, 0.3 mM TeO2 at a pH of 1.6 adjusted with H2SO4). The

reaction mechanism leading to the formation of CdTe nanowires is described by:

HTeO2+ + 3H

++ 4e

- Te +2H2O (20)

Cd2+

+ Te + 2e- CdTe. (21)

Figure 2.6 Growing CdTe nanowires onto Au substrate

The peculiarity of CdTe forming in this case is that the deposition process is Te diffusion

limited. The polarization curves for the deposition process in a membrane containing 108

pores/cm2 of 80 nm diameter at 74C are presented in figure 2.7.

-700 -600 -500 -400 -300 -200 -100 0 100-0.30

-0.25

-0.20

-0.15

-0.10

-0.05

0.00

sweep1

sweep2

sweep3

Cu

rre

nt

(mA

)

Potential vs. SCE (mV)

Figure 2.7 Polarisation curves for CdTe nanowires deposition (sweep rate of 5 mV/s)

The peak at -500 mV corresponds to the potential where CdTe is deposited. A shift towards

more positive potential can be observed at the second and third sweep. This is most probably due

to the fact that the deposition is further favorite because of the presence of Te atoms in the

substrate. Also depositions at other voltages has been performed (-550mV, -600mV) for finding

the most stoichiometric compound.

In order to measure the properties of the nanowires, the PC foil has been removed by dissolving

it in chloroform (figure 2.8).

Figure 2.8 Exposed nanowires after the removal of PC

Structural and compositional analysis of the CdTe wires array show a stoichiometric

compound (figure 2.9) containing 50.5% Cd and 49.5% Te in the case of the sample grown at -

550 mV with a zinc-blend cubic crystalline structure (figure 2.10).

Figure 2.9 EDX spectrum of CdTe wires array grown at -550 mV, 50.5% Cd and 49.5% Te

Figure 2.10 X-ray diffraction pattern of the CdTe wires

By SEM micrograph the CdTe nanowire array can be seen, being 108/cm

2 dense (figure

2.11). In this way an image about how the future structure will look like, is formed.

0 10 20 30 40 50 60 70

0

50

100

150

200

250

300

Cu

(2

00

)

CdT

e(3

11)

Cu (111)

CdT

e(2

20)

CdT

e(1

11)

I(a

.u.)

2

Figure 2.11 SEM micrograph showing the CdTe wire array deposited on Au after dissolving the PC membrane

The Au electrode is not completely covered as it can be seen and for further processing of the

samples this should be taken into account.

The free standing nanowires are fit for being used in photovoltaic application.

2.1.2. Electrochemical deposition of CdTe nanowires using Cu as the working

electrode

In this case the procedure is similar with the one presented in sub-chapter 2.1.1. After preparing

as shown the polycarbonate foil, a copper film is sputtered on its surface (figure 2.8).

Figure 2.12 Sputtering of Cu thin film

In the same conditions as before the CdTe nanowires are grown (figure 2.9) having a polarization

curve for a membrane with the density of the pores of 109/cm

2 shown in figure 2.10.

Figure 2.13 Growing CdTe nanowires on Cu substrate

The peak around -0.5 V corresponds to the potential where CdTe is deposited, without being too

different from the deposition of Au electrode (figure 2.14).

a)

b)

Figure 2.14 a) Polarisation curves for CdTe nanowires deposition (sweep rate of 5 mV/s)

b) Deposition curves of CdTe at -550 mV

0.0 3.0x103

6.0x103

9.0x103

1.2x104

0.0

1.0x10-4

2.0x10-4

3.0x10-4

4.0x10-4

0.0 3.0x103

6.0x103

9.0x103

1.2x104

-0.60

-0.58

-0.56

-0.54

-0.52

-0.50

Cu

rre

nt (A

)

Elapsed Time (s)

Po

ten

tia

l (V

)

Elapsed Time (s)

-0.7 -0.6 -0.5 -0.4 -0.3 -0.2 -0.1

-1.8

-1.5

-1.2

-0.9

-0.6

-0.3

0.0

sweep2

sweep3

sweep1

Cu

rre

nt (m

A)

Potential (V)

Figure 2.15 Exposed CdTe nanowires on Cu substrate after the removal of the PC membrane

Reflexion spectrum of the grown nanowires were measured using a Perkin Elmer Lambda 45

UV/VIS Spectrophotometer (figure 2.15).

Figure 2.15 Reflexion spectra of the CdTe nanowires grown at -550 mV

The theory which makes possible to use reflexion spectrum was proposed by Kubelka

and Munk [55].

Figure 2.16 Kubelka-Munk function versus the photon energy for determining the energy bandgap of the CdTe

nanowires

600 700 800 900 1000

4

6

8

10

R (

%)

[nm]

1.4 1.6 1.8 2.00

4

8

12

16

Eg=1.48 eV

Photon energy (1240/) [eV]

F(R

)2

From the graphic in figure 2.16 the energy bandgap of the nanowires was determined, having a

value of 1.48 eV.

The compositional analysis of the CdTe nanowire array shows an almost stoichiometric

compound (figure 2.17) containing 47% Cd and 53% Te in the case of the sample grown at -550

mV.

Figure 2.17 EDX spectrum of CdTe wires array grown at -550 mV, 47% Cd and 53% Te

In the SEM image (figure 2.18) can be noticed a more dense CdTe nanowire array

(109pores/cm

2) with free standing nanowires having properties that make them suitable for

photovoltaic applications.

Figure 2.18 SEM image of the CdTe nanowire array deposited on Cu film after dissolving the PC membrane

2.2. Preparation of the hybrid inorganic/organic photovoltaic cells based

on CdTe nanowires and ZnPc organic dye

Using the CdTe nanowires grown on Au electrode described in sub-chapter 2.1.1 two different

photovoltaic devices were designed and measured. The photoelectrical response of the second

design which was best suited and will be used for other cells as well will be compared below, in

sub-chapter 2.2.2, with the one of the first design which had problems with collecting the charges

at the electrodes.

The difference between the two designs is the addition of a CdTe thin film of about 300 nm

which is covering the Au electrode completely and it is decreasing the charge recombination rate

at this electrode (published in [56]).

2.2.1. Technological steps for producing the hybrid photovoltaic devices

To continue the processing of the solar cell a 300 nm CdTe thin film was deposited by thermal

vacuum evaporation at a temperature of 300oC and on top of it a 400 nm ZnPc organic dye, with

the help of the same technique (figure 2.19).

a)

b)

Figure 2.19 a) Schematic representation of the of the photovoltaic device after deposition of CdTe thin film

b) Schematic representation of the of the photovoltaic device after deposition of ZnPc thin film

The SEM image of the structure at this moment of the preparation shows that the nanowires are

still standing and have a larger diameter (figure 2.20). Also at the basis of the nanowires the Au

electrode is completely covered.

Figure 2.20 SEM image after covering CdTe wires with CdTe thin film (300 nm) and ZnPc (400 nm)

As seen the morphology of the nanowires is changed. In order to finalize the structures a

transparent electrode was coated by pulsed electron deposition (schematic view in figure 2.21).

The superficial resistivity for the 300 nm ZnO thin film was 88Ω/cm2.

Figure 2.21 Supposed structure of the photovoltaic device

In the SEM image the different morphology can be noticed (figure 2.22) after this final step.

Ag contacts had been made for being able to measure the properties of the cell (one on ZnO, the

other on Au).

Figure 2.22 SEM image of CdTe nanowires covered with CdTe film, ZnPc and ZnO (300 nm)

In order to perform the optical, photoelectrical and electrical measurements on this device

contacting the electrodes is a crucial step in order to avoid having a shortcircuit (figure 2.23).

Figure 2.23 Schematic view of the sample with electrical contacts

2.2.2. Optical, photoelectrical and electrical measurements and results

The spectral dependence of the external quantum efficiency (EQE) of two photovoltaic

structures, CdTe wires/ZnPc is shown in figure 2.24, respectively CdTe wires/CdTe (300

nm)/ZnPc at ambient temperature (25°C), in figure 2.25.

200 400 600 800 1000

0.0

0.2

0.4

0.6

0.8

1.0

No

rmali

zed

Ab

sorb

an

ce

No

rmali

zed

EQ

E

Wavelength (nm)

0.0

0.2

0.4

0.6

0.8

1.0

Figure 2.24 EQE spectrum of an Au/CdTe (nws)/ZnPc/ZnO photovoltaic structure and the corresponding

absorption spectra of ZnPc

Figure 2.25 EQE spectrum of an Au/CdTe (w)/CdTe/ZnPc/ZnO photovoltaic structure. For comparison purposes,

absorption spectra of ZnPc and CdTe films deposited on optical glass in the same conditions are also given (in red

line, respectively in green line)

In the case of the structure without the thin CdTe film deposited the external quantum efficiency

follows the features in the absorption spectra of ZnPc in the investigated spectral region. Those

features correspond to Q absorption bands of ZnPc (in the range from 500 nm to 800 nm),

500 600 700 800 900 1000 1100

0.0

0.2

0.4

0.6

0.8

1.0

No

rmali

zed

Ab

sorb

an

ce

No

rmali

zed

EQ

E

Wavelength (nm)

0.0

0.2

0.4

0.6

0.8

1.0

associated to electronic excitations. Under illumination with higher energy photons (low

wavelength) EQE increases abruptly, due to B (0, 0) and B (0, 1) electronic excitations (Soret B

band, extending below 400 nm). The shoulder at 900 nm is probably due to the onset of light

absorption in the CdTe wires. The maximum value of EQE is 0.8%.

At the second sample, the external quantum efficiency of the Au/wire array CdTe/CdTe (300

nm)/ZnPc/ZnO structures was significantly increased on the measured range of wavelengths,

with respect to that of Au/wire arrays CdTe/ZnPc/ZnO structures and, as it seen in figure 2.25, it

follows the features in the absorption spectra of ZnPc, but also the absorption spectra of the

CdTe thin film in the investigated spectral region.

For all the samples the electrical characteristics were not good because of the shortcircuit which

appeared while measuring.

2.3. Preparation of the inorganic photovoltaic cells based on CdTe

nanowires and CdS thin film

The main idea for developing these structures was to improve even more the performances of the

cells presented in the previous sub-chapter. Trying to densify the array of CdTe nanowires, using

this time 109 pores/cm

2 and growing it on Cu electrode as described in sub-chapter 2.1.2 it is

expected to increase the efficiency at a lower cost.

By searching the best option and observing the number of trials for CdTe/CdS devices[57], for this

type of cells, using a nanostructured CdTe and a thin film of CdS an important difference can be

seen when comparing to the other structure responses.

The photoelectrical response of the cell having the second design (with a thin film of CdTe

deposited onto the nanowires) which was best suited has been measured.

2.3.1. Technological steps for producing the inorganic photovoltaic structures

In figure 2.26 is schematically shown the next step in processing the photovoltaic structures by

adding a 88 nm CdTe thin film with the help of thermal vacuum evaporation technique in the

following conditions: temperature of the substrate was 90oC, temperature of the source was

450oC, having a deposition rate of 0.5 nm/sec.

Figure 2.26 Schematic view of the design of the sample after the CdTe thin film deposition

In the actual processing the design was the same as the one proposed, the nanowires were

standing after the deposition of the thin film and their thickness increased due to it as seen in

figure 2.27.

Figure 2.27 SEM image of the CdTe nanowire array covered with 300 nm CdTe thin film

The morphology of the nanowires has been changed also after the next step, the deposition of the

CdTe layer by thermal vacuum evaporation. For making the deposition the temperature of the

substrate was 90oC, temperature of the source was 600

oC, having a deposition rate of 1.6 nm/sec.

The thickness of the resulting CdS layer was about 327 nm.

The SEM image (figure 2.29) shows the nanowires with a bigger diameter, having the same

shape as the projected design (figure 2.28).

Figure 2.26 Schematic view of the design of the sample after the CdTe and CdS thin films deposition

The shape of the nanowires is followed by the thin films, assuring a large number of interfaces

between CdTe and CdS.

Also the recombination rate is significantly lowered by covering the Cu electrode with the CdTe

thin film before the deposition of the CdS thin layer.

Figure 2.29 SEM image of the CdTe nanowire array covered with CdTe and CdS thin films

After processing the final step, the deposition of the top transparent electrode by magnetron

sputtering technique was made. The deposition was performed at a current of 30 mA, when the

Ar gas flow was of about 3 sccm at a pressure of 3 mtorr.

The sample has been contacted (figure 2.30) for further measurements.

Figure 2.30 Schematic view of the design of the sample after processing with electrical contacts

2.3.2. Optical, photoelectrical and electrical measurements and results

When measuring the photocurrent at each wavelength in the domain where both semiconductors

have absorption peaks, the external quantum efficiency can be calculated.

When illustrating the EQE spectrum (figure 2.31) it can be noticed that it follows the absorption

spectra of the CdTe and CdS, overlapping it in some parts. The maximum value of EQE is

1.13%.

Figure 2.31 EQE spectrum of an Cu/CdTe n(ws)/CdTe/CdS/ITO photovoltaic structure. For comparison purposes,

absorption spectra of CdS and CdTe films deposited on optical glass in the same conditions are also given (in red

line, respectively in green line

For this type of structures the electrical characteristics have been possible to measure. The

current voltage characteristics in the dark, at room temperature and current voltage characteristic

in AM 1.5 illumination conditions are shown in figure 2.32.

The supralinear behavior of the characteristics and the power generator part shows that indeed

the structure acts as a solar cell having small power conversion efficiency (figure 2.32).

Figure 2.32 I-V characteristics in the dark (black) and under AM 1.5 illumination of the

Cu/CdTe(nws)/CdTe(300nm)/CdS(400nm)/ITO cell

-1.0 -0.5 0.0 0.5 1.0-0.1

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

Cu

rre

nt (

A)

Voltage (V)

500 600 700 800 9000.0

0.2

0.4

0.6

0.8

1.0

1.2

No

rma

lize

d E

QE

(nm)

No

rma

lize

d O

ptica

l A

bso

rptio

n

EQE Cu/CdTe(nws)/CdTe(100nm)/CdS/ITO

0.0

0.2

0.4

0.6

0.8

1.0

1.2

Absorption CdS

Absorption CdTe

When enlarging the fourth quadrant and calculating the power generated by the cell (figure 2.33)

a primary idea of the performances of the cell is putted in evidence.

Figure 2.33 I-V characteristics under AM 1.5 illumination and the generated power of the

Cu/CdTe(nws)/CdTe(300nm)/CdS(400nm)/ITO cell

Taking into account the experimental determined parameters of the cell from the table below the

fill factor (FF = 27%) and the power conversion efficiency (η = 0.0064%) can be calculated.

The performances of the measured cells are not great, but an important step has been made

starting with the possibility of having electrical results.

Once this important step has been made a way of continuously improving the performances at

least at this price if not possible lower is being developed.

3. Ni/CdTe/Ni single nanowires properties

In order to make the multisegmented nanowires, first a Ni/CdTe junction has to be prepared to

find the most suitable conditions for having a stoichiometric compound of CdTe grown on Ni

(figure 3.1).

Figure 3.1 Schematic view of the metal- semiconductor junction

Isc Uoc Imax Umax Illuminated area

-2.318 x10-5

A 0.26 V -1.143 x10-5

A 0.14 V 0.2512 cm2

0.0 0.1 0.2 0.3-4.0x10

-5

-3.0x10-5

-2.0x10-5

-1.0x10-5

0.0

I (A

)

U (V)

P (

W)

-1.6x10-6

-1.2x10-6

-8.0x10-7

-4.0x10-7

0.0

3.1. Preparation of the Ni/CdTe nanowires

Preparing the electrodeposition of the nanowire with metal – semiconductor junctions was made in the same way as for the above preparation of the CdTe nanowires. First the metal was electrodeposited in the pores of the membranes at a tension of -1000mV. Deposition of nickel was performed from a Watts bath containing besides the “classical” components PVP as pore wetting additive. By adding PVP, the pore filling efficiency reaches 80%[58]. The deposition temperature was 50°±1° C. After, the deposition bath was changed and CdTe was grown from its specific above described bath. More trials were made for finding the best deposition potential. Bellow, SEM images and EDX spectrums of different processed samples at some potentials are shown. For CdTe grown at -450mV there was a Te excess in the composition shown in the EDX spectrum (figure

3.2). Ni was grown for 4 minutes and Cd Te for 4 hours.

a) c)

Figure 3.2 a) Scanning electron microscopy image of an

array of Ni – CdTe multisegment( secondary electrons

detector image) b)Scanning electron microscopy image of an

array of Ni – CdTe multisegment wires (left back scattered

detector image/ right secondary electron detector image) c)

EDX spectrum showing 44.3%Cd and 55.7% Te

b)

For CdTe grown at -500mV there was a Te excess in the composition shown in the EDX spectrum (figure

3.3). Ni was grown for 4 minutes and Cd Te for 2 hours.

a) b)

Figure 3.3 a) Scanning electron microscopy image of

an array of Ni – CdTe multisegment( secondary

electrons detector image) b)Scanning electron

microscopy image of an array of Ni – CdTe

multisegment wires (left back scattered detector

image/ right secondary electron detector image) c)

EDX spectrum showing 44.7%Cd and 55.3% Te c)

For CdTe grown at -550mV there was an almost stoichiometric compound taking into account the EDX

errors (figure 3.4). Ni was grown for 4 minutes and Cd Te for 3 hours.

a)

c)

Figure 3.4 a) Scanning electron microscopy image of an array of

Ni – CdTe multisegment( secondary electrons detector image)

b)Scanning electron microscopy image of an array of Ni – CdTe

multisegment wires (left back scattered detector image/ right

secondary electron detector image) c) EDX spectrum showing

48%Cd and 52% Te

b)

For CdTe grown at -700mV there was a Cd excess in the composition shown in the EDX spectrum (figure

3.5). Ni was grown for 4 minutes and Cd Te for 2 hours.

a) c)

Figure 3.5 a) Scanning electron microscopy image of an array of

Ni – CdTe multisegment( secondary electrons detector image)

b)Scanning electron microscopy image of an array of Ni – CdTe

multisegment wires (left back scattered detector image/ right

secondary electron detector image) c) EDX spectrum showing

56%Cd and 44% Te

b)

Two graphics (figure 3.6) were made in order to see the percentages of both Cd and Te at

different growth voltages.

Figure 3.6 Percentages of Cd and Te at different growth voltages

The right growing conditions can be now chosen for making the Ni/CdTe/Ni nanowires in order

to further try to contact them using a FIB/SEM.

The variation of the current and potential in time during electrodeposition can be seen in figure

3.7. It can be noticed that the deposition current for CdTe is higher than the one for the metal.

-700 -650 -600 -550 -500 -450

44

46

48

50

52

54

56

Cd

(%

)

U (mV)

-700 -650 -600 -550 -500 -450

44

46

48

50

52

54

56

Te

(%

)

U (mV)

Figure 3.7 Variation of the current and potential in time during electrodeposition of Ni and CdTe

3.2. Preparation of the Ni/CdTe/Ni nanowires

Knowing that is possible to grow Ni/ZnO/Ni nanowires[59], in the established conditions above,

another part of the nanowires was electrodeposited, again a Ni segment. Schematic view can be

visualized in figure 3.8.

Figure 3.8 Schematic view of the grown multisegments

Figure 3.8 Variation of the current and potential in time during electrodeposition of Ni/CdTe/Ni

0 65 130-0.005

-0.004

-0.003

-0.002

-0.001

0.000

0 6000 12000

Cu

rre

nt (A

)

Elapsed time (s)

Ni

CdTe

0 65 130

-1.4

-1.2

-1.0

-0.8

-0.6

-0.4

-0.2

0.0

0 6000 12000

Pot

entia

l (V

)

Ni

Elapsed time (s)

CdTe

0 65 130-0.005

-0.004

-0.003

-0.002

-0.001

0.000

0 6000 12000 0 600 1200 1800

Cu

rre

nt (A

)

Elapsed time (s)

Ni part 1 Ni part 2

CdTe

0 65 130

-1.4

-1.2

-1.0

-0.8

-0.6

-0.4

-0.2

0.0

0 6000 12000 0 600 1200 1800

Po

ten

tia

l (V

)

Ni part 1

Elapsed time (s)

CdTe

Ni part 2

Choosing a deposition voltage of -550 mV for CdTe the electrodeposition was made in the same

conditions as the previous samples.

After removing the template with chloroform SEM images of the nanowires were taken and also

a mapping of one of the multisegment nanowires was done (figure 3.9).

Figure 3.9 a) Scanning electron microscopy image of an array of Ni – CdTe multisegment (left back scattered

detector image/ right secondary electron detector image) b)Selecting an area of the image for mapping c) EDX

mapping showing Ni with blue, Cd with red and Te with green

These free standing nanowires can be used for making devices based on single nanowires with

the help of the FIB/SEM. One of the reasons for choosing Ni as the metal from the junction is

that it can be easily aligned using a magnetic field.

Once the copper is chemically etched, the nanowires are spread in a solution. From that solution

they can be arranged parallel on the substrate as it will be shown bellow.

3.3. Aligning nanowires

Simply by using the magnetic field between two small magnets Ni nanowires can be arranged

parallel with the field lines. One of the reasons for doing this is to avoid conglomerations of

nanowires and to be able to select single nanowires for contacting.

Immediately after putting a drop of the solvent with nanowires on the substrate, it is placed

between the two magnets which are positioned at a specific distance for allowing the nanowires

to align (figure 3.10).

a) b)

c)

Figure 3.10 Scanning electron microscopy image of aligned Ni nanowires

.

Because of the relative smaller time of growing this type of nanowires comparing with

semiconductors they will be used later for single nanowire contacting with the help of a

FIB/SEM.

3.4. Contacting single nanowires with FIB/SEM

3.4.1. FIB-SEM Description

It is a favorable combination of the electron and ion sources and optical columns attached on one

chamber. It extends imaging qualities of the scanning electron microscope with the possibility of

surface modification by a focused ion beam.

It is based on a high resolution SEM column with a Schottky field emission gun.

FIB control is fully integrated in the SEM software and simultaneous SEM imaging with FIB

etching or deposition can be done. The powerful software toolbox DrawBeam for basic shapes

creation with programmable process parameters is helping to etch or make depositions in the

desired geometries[60].

The gas injection system has ideal geometrical configuration with respect to SEM and FIB

columns and 3-axis microstage with automatic nozzles positioning (figure 3.11).

When positioning the point of interest at 55 degrees with respect to the electron gun, meaning at

the FIB- SEM intersection the design can be processed. The equipment in question has also a

nanomanipulator with which the nanowires can be moved for contacting, if necessary.

Figure 3.11 GIS nozzle for Pt deposition

3.4.2. Pt contacts

For the present experiments, a commercial “dual beam” instrument (Lyra microscope from

TESCAN) has been used. It integrates a 30 kV field-emission electron column and a Ga based 30

kV ion column placed forming 55degrees. For Pt deposition, an automatized gas-injection

system (GIS). The deposition details can be found in the table below.

Chemistry Pt deposition

Composition of deposited material (atomic fraction)4

Pt 0.2; Ga 0.1; C 0.7 (IBID)

Precursor material Trimethyl-methylcyclopentadienyl-

platinum – C9H

16Pt

Deposition rate 0.5 μm3

/nC (IBID)

0.008 μm3

/nC (EBID)

Electrical resistivity 200 μΩcm (IBID)

500000 μΩcm (EBID)

Minimum linewidth 50 nm (IBID,

15 nm (EBID)

All the rates and Pt properties can differ from contact to contact depending on the used currents,

pressure, and temperature.

3.4.3. Contacting single nanowires

When using aligned nanowires it is more easily to find one to put contacts on. Usually the

contacts are made in such a way to get to pads made with the help of photolithography or mask

assisted deposited contacts.

With the purpose of finally having a sensor based on a single nanowire transistor, Pt contacts are

deposited at the ends of a Ni nanowire.

A single nanowire is chosen (figure 3.12) and using a current of 15 pA the testing of the

deposition of the contacts begins.

Figure 3.12 Chosen nanowire for contacting

Before the actual deposition begins there are various steps to perform. The image of the

nanowire from figure 3.11 is taken at the FIB-SEM intersection, meaning at 55 degrees tilt with

respect to the e-gun direction. Once the SEM and FIB images are put in order, eliminating the

aberrations and having the beams in the best focus point the design of the contact is drawn with

the help of a program included in the apparatus software.

After setting the exact position and choosing the ion beam deposition mode the actual deposition

can start.

The design is rastered in steps and once one rectangle is filled the deposition starts for the

second. In figure 3.13 the first part of the contacts is deposited. Using relatively low currents the

surface of the nanowire should not be affected.

Figure 3.13 Pt contacts deposition with FIB – SEM with a current of ~15 pA

Because of the use of the SEM while deposition it can be noticed in figure 3.14 that not only the

pattern has Pt but it is also deposited around it.

This is pointing to a more careful use of the instrument, implying a better imaging and an

improved focusing of the images in both SEM and FIB.

Figure 3.14 Contacts depositions with artifacts

Several trials were made in order to find the best parameters for a Pt deposition which would not

affect the nanowire (figure 3.15 and figure 3.16).

Taking into account the tests made it can be concluded that for having good contacts with the

deposition only in the desired design, without touching the nanowire the smallest currents have

to be used. Also it is recommended not to use the SEM mode for viewing the insitu deposition

because it will interfere with the ion beam deposition.

In this microscope also electron beam deposition can be made, but the deposition time is highly

increased with respect to ion beam deposition.

Making a compromise between the waiting time for an electron beam deposition and the quality

it is summarize the fact that making an ion beam deposition with a low current is much more

advantageous.

For the future electrical measurements which are planned, the resistivity of the Pt layer has to be

taken into account, at different deposition rates for different conditions[61].

Figure 3.15 Pt contacts deposited at ~400 pA

Figure 3.16 Pt contacts deposited at ~200 pA

4. Conclusions and further work

Summarizing the presented experimental work it can be stated that there exist an obvious

improvement of the previously developed devices.

Taking into account the growth of the nanowires, the best conditions have been brought up for

having stoichiometric compounds with a cylindrical shape.

The structural properties of the nanowires have shown a well formed stoichiometric compound

with a zinc-blend cubic crystalline structure.

The changes made in the design of the cells got to the reduced rate of recombination at the back

electrode then increasing the holes collection.

For the Au/nanowire array CdTe/CdTe(300 nm)/ZnPc/ZnO cell the EQE follows closely the

features in the absorption spectrum of the organic dye, showing a good charge transfer at the

organic/inorganic interfaces. Also it was increased with about four orders of magnitude, with

respect to that of Au/wire arrays CdTe/ZnPc/ZnO structures.

Unfortunately electrical measurements of these samples were not made but the good

photoelectrical characteristics were pushing forward towards finding a solution for getting the

best possible results.

Once the idea of using CdTe/CdS flourished the possibility of having measurable electrical

results appeared. The quality of the nanowires and the preparation procedures were undoubtedly

the best till the present moment as a result the electrical behavior showed the characteristics of a

photovoltaic cell as predicted.

Nevertheless the efficiency was not high, but using these results as a starting point for further

development of this type of cells is the next logical step.

Further work includes varying the diameter of the nanowires, their length, making thermal

treatments of the nanowires for a better crystalline structure. Also varying the thickness of the

CdS layer and changing the top transparent electrode. All this is expected to lead to an important

increase of the power conversion efficiency.

Also the possibility to have multisegment nanowires has been properly investigated and the

success of preparing it leads to the development of many applications. Junctions made along the

wire with a perfect metal/semiconductor contact aims to less measuring errors of the properties

of this type of wires. When contacting the metallic parts of the wires it is obvious that the contact

metal/metal is much better than metal/semiconductor made by deposition. Functionalized

nanowires of this type can be used with success in fabricating sensors of different types.

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