Fabrication and Characterization of novel selfassembled nanoscale arrays and realization of nanowire

1
Fabrication and Characterization of novel
self-assembled nanoscale arrays and realization
of nanowire spintronic and nanodot field emitter
devices

• Kalyan Garre
• Advisor Dr. Marc Cahay

2
Outline

• Background
• Specific Aims
• Results
• Novel Cold Cathode
• Self-assembled Trimodal arrays for field emission
• First nanowire organic spin valve
• OLED device development
• Proposed Future Work
• Conclusions

3
Background Lanthanum Sulfide (LaS)

• Low work function (1 eV)
• High melting point (gt2000 C)
• Chemically stable
• Sheet resistance 0.1?/sq
• Synthesized by sintering La2S3 and La metal at
  1800 C. 1 C is added to remove oxysulfide
  impurity phases
• La La2S3 3LaS

J. Vac. Sci.Tech B, 23, pp 318-321 (2005) J. App.
Phys, 99, 123502 (2006)
4
LaS thin film characterization
AFM scan of LaS thin film deposited on silicon
over a 1µm2 area. The mean square roughness is
about 1.7nm over that area
X-ray diffraction pattern at three different
grazing angles of LaS thin film deposited on
silicon. Miller indices (hkl) of three peaks
of LaS are identified
5
LaS thin film characterization
Top e-beam diffraction image of the LaS film
indicating the prescence of amorphous and
nanocrystalline phases. Bottom e-beam
diffraction image of LaS on Si
HRTEM image of the LaS/Si interface
indicating the prescence of amorphous and
nanocrystalline phases
6
Process changes

• Mixing in nitrogen ambience- La metal is reactive
• Maintaining high vacuum levels 10-6 Torr
• Running empty cycles to degas contaminants from
  the walls
• XRD characterization by each sample batch and
  storage in nitrogen glove box

7
Outline

• Background
• Specific Aims
• Results
• Novel Cold Cathode
• Self-assembled Trimodal arrays for field emission
• First nanowire organic spin valve
• OLED device development
• Proposed Future Work
• Conclusions

7
8
Specific Aims

• Build a MEMS based device to measure field
  emission properties of LaS thin films
• Fabricate the LaS nanowires to build efficient
  field emitters
• Fabricate nanowire organic spin valves
• Fabricate nanoscale OLEDs and study the
  degradation in luminance with time.

9
Fabrication Sequence
Step 1 A thin film of Al is deposited through a
shadow mask leaving a circular opening
about 800µm in diameter
Step 2 Holes are etched through the Silicon
wafer using SF6/O2 plasma leaving small openings
about 400µm in diameter at the bottom of the wafer
Step 3 Aluminum is then removed by soaking the
wafer in phosphoric acid
10
Fabrication sequence
Step 4 An Array of Aluminum dots are evaporated
on the flat side of a separate Si wafer
Step 5 Aluminum is evaporated using the shadow
mask used in step 1 on the other side of the
wafer.
Step 6 The silicon wafer is etched using SF6gas.
The aluminum dot on the other side serves as a
diaphragm covering the hole on the flat side of
the wafer.
11
Fabrication sequence
12
Fabrication sequence
Step 10 Aluminum is evaporated which acts
as cathode to capture the emitted electrons in
vacuum
Step 11 A layer of gold is then evaporated to
protect aluminum during the pulsed laser
deposition of LaS
Step 12 Both the wafers are aligned and
glued together with high temperature epoxy along
the edges of the wafer.
13
Device Testing
Actual picture of the cold cathode in the
experimental set up. An array of 16 cathodes
which are individually addressed can be seen.
Step 13 A layer of LaS around 200nm thick is
then deposited by PLD.
14
Results Discussion
15
Improving the FE current

• Nanoscale features have local field enhancement
  at the tips
• Smaller the radius higher the field enhancement
  and higher the emission current
• Is there a novel way of fabricating nanoscale
  arrays of LaS ?

16
Outline

• Background
• Specific Aims
• Results
• Novel Cold Cathode
• Self-assembled Trimodal arrays for field emission
• First nanowire organic spin valve
• OLED device development
• Proposed Future Work
• Conclusions

16
17
Nanoporous alumina
Fig (a) Schematic cross-section of the porous
alumina template, (b) Top and cross-sectional
view of the porous alumina, (c) AFM image of the
bare alumina template.
Nanosynthesis using porous anodic alumina, A.
Banerjee, M.S. Thesis, 2004
18
Fabrication of nanoporous alumina

• Nano porous alumina is obtained by anodization of
  electropolished aluminum in strong acids (pH lt4)
• High purity metallic Aluminum foil is
  electropolished in an electrolyte consisting of
  1050cc ethyl alcohol, 150cc butyl cellusolve, 93
  cc perchloric acid, and 205cc distilled water.
• Electropolished aluminum is placed at the anode
  while a Pt mesh is used as cathode.

19
Fabrication of nanoporous alumina

• A dc voltage is applied across these two
  electrodes and a moderately strong acid is used
  as electrolyte (15 Sulfuric acid, 3 Oxalic acid
  or 5 Phosphoric acid)
• There is a thin barrier layer formed separating
  the porous alumina layer from the bottom aluminum
  which can be removed.
• The diameter of the pores are controlled by the
  type of acid used and the voltage applied.
• The length of the pores are controlled by the
  anodization time.

20
Advantages of nanoporous alumina

• High density of pores for nanowires (1010/cm2)
• Inexpensive ( A sheet of aluminum and an
  electrolytic cell)
• Large area fabrication
• High throughput
• Flexible

21
Results

• First Observation 3D-AFM scans indicated the
  formation of uniformly ordered nanostructures on
  the surface of the template
• Formation of Trimodal arrays
• Is the formation limited to the Pulsed Laser
  Deposition (PLD) ?
• Can the uniformly ordered structures be
  reproduced by thermal and e-beam evaporation ?

Fig. 2D AFM and FE-SEM images of LaS deposited on
nanoporous alumina
Thermal evaporated Au 334 Å
E-beam evaporated Ni 300 Å
22
Trimodal Arrays of LaS
Threshold voltage for a field emitter current of
1mA/cm2 has been reduced from 230 V/µm to 150
V/µm.
Trimodal Arrays Nanodots 1010/cm2 Nanodomes
109/cm2 Nanowires 1010/cm2
M. Cahay, K. Garre et. al J. Vac. Sci. Tech. B,
25, pp 594-603 (2007)
23
Nanoporous alumina templates before and after
PLD. The nanodome structures are more prominent
after PLD
24
Actual images of (a) bare alumina template and
(b) LaS deposited on the template
X-ray diffraction scans of bare nanoporous
alumina template (top) and LaS deposited
template (bottom)
25
Field Emission by SAFEM

• Field emission I-V characteristics of
• LaS Thin films and (b) LaS
• nanodomes and nanodots. The applied
• Voltage is decreased by a factor of 3.5

Actual image of the experimental set up of the
SAFEM probe
M. Cahay, K. Garre J. Vac. Sci. Tech. B, 25, pp
594-603 (2007)
26
Field Emission by SAFEM
M. Cahay, K. Garre J. Vac. Sci. Tech. B, 25, pp
594-603 (2007)
27
Multi-level self assembly
Avg dia 57 nm
28
Gold nanopine trees

• About 300 Å of gold is e-beam evaporated onto
  porous alumina
• templates having 50nm diameter pores
• We are trying to reproduce these structures.
  Currently studying the
• growth at a constant thickness with
  different rates of evaporation,
• different thicknesses at a constant rate of
  evaporation

29
SAFEM Results
30
Carbon nano pearl arrays
Carbon nano sphere chains are provided by Clean
Tech International Corp

• A thin film of Nickel is evaporated onto 50nm
  wide porous alumina
• templates and Carbon nano sphere chains are
  ultrasonically agitated
• Under the influence of magnets for a period of 12
  hrs

31
SAFEM results
Field emission current vs voltage curves obtained
by SAFEM technique
Fowler Nordheim plots of CNP arrays before (top)
and after (bottom) arcing
32
Potential Applications

• Field Emission displays (non high resolution)
• Electron beam sources
• High frequency traveling wave tubes
• Solid State lightning
• Thin film capacitors
• Hydrogen storage ?

33
Outline

• Background
• Specific Aims
• Results
• Novel Cold Cathode
• Self-assembled Trimodal arrays for field emission
• First nanowire organic spin valve
• OLED device development
• Proposed Future Work
• Conclusions

33
34
Organic Spin Valve

• A spin valve is a three layered device where two
  ferromagnetic materials are separated by a
  paramagnetic spacer
• One of the electrodes acts as spin injector and
  the other as spin detector
• If the spin of the electrons in the paramagnetic
  spacer is aligned to that with the detector the
  device shows a low resistance else it shows a
  high resistance
• ?-conjugated organic semiconductors have weak
  spin-orbit and weak hyperfine interaction

35
First nanowire organic spin valve

• Porous alumina templates about 50nm in diameter
  and 1µm in length are fabricated at VCU
• A layer of nickel about 500nm is filled by
  electrodeposition
• A layer of Alq3 about 33nm followed by a layer of
  cobalt to fill the pores have been evaporated
• Evaporated in high vacuum under a base pressure
  of 10-6 Torr

36
First nanowire organic spin valve

• Size of the evaporating species should be lt 50nm
  in diameter
• Evaporation flux is in the form of a sphere
  radiating out uniformly in all directions
• The mean free path of the evaporating species
  depends on the vacuum level in the chamber

Nanoporous alumina template
evaporating species
evaporating species
evaporation source
37
First nanowire organic spin valve
Alumina template placed at an oblique angle from
the source

• Placing the alumina templates at an angle from
  the evaporation source led to the deposition only
  in the beginning of the pores
• This has blocked the pores and prevented further
  deposition of the species deep inside
• The target to substrate distance is maintained
  between 6 and 12 inches

d
evaporating species
evaporation source
38
First nanowire organic spin valve

• The rate of evaporation is kept very low
  (0.1-0.5 Å/sec)
• Using energy cluster mask has improved the
  uniformity of thin films in OLEDs
• The evaporating species are more directional and
  helps in the formation of smaller clusters of the
  organic species
• Low molecular weight and having small molecules
  helped in the diffusion of Alq3 deep inside the
  pores

Philip. Draviam, M.S. Thesis, 2005
39
First Nanowire Organic Spin valve

• The spin valve peaks are obtained between the
  coercive fields of nickel and cobalt
• The coercive fields
• Nickel nanowires 800 Oe
• Cobalt nanowires 1800 Oe
• The change in the magnetoresistance
• can be increased by contacting less
• less no of wires or if possible
• individually

S. Pramanik, S. Bandyopadhyay, K. Garre and M.
Cahay Physical Review B 74 235329 (2006) S.
Pramanik et.al, Nature Nanotechnology, Vol 2, pp
216-219(2007)
40
Nanowire organic spin valve
S. Pramanik, S. Bandyopadhyay, K. Garre and M.
Cahay Physical Review B 74 235329 (2006) S.
Pramanik et.al, Nature Nanotechnology, Vol 2, pp
216-219(2007)
41
First Nanowire organic spin valve

• The TEM images confirm
• the filling of the pores
• The thicknesses of the organic
• material match with that from the
• quartz crystal oscillator
• The inset shows the I-V curves
• indicating the formation of a
• schottky barrier

42
Outline

• Background
• Specific Aims
• Results
• Novel Cold Cathode
• Self-assembled Trimodal arrays for field emission
• First nanowire organic spin valve
• OLED device development
• Proposed Future Work
• Conclusions

42
43
Organic Light Emitting Diodes

• OLED works on the principle of electroluminescence
  where light is emitted on the application of
  voltage.
• The simplest device consists of a hole transport
  layer (HTL) followed by an electron transport
  layer (ETL) sandwiched between two electrodes-
  anode and cathode.
• Electrons are injected from the cathode while
  holes are injected from the anode.
• The carriers are transported through the HTL and
  ETL and reach the interface where they recombine
  to form an exciton.
• The exciton reaches the ground state either by
  radiative or non radiative recombination

44
ITO substrate preparation

• ITO coated on glass substrates are purchased from
  Delta Technologies Inc.
• Substrates about 1 x 2 are scribed from larger
  pieces
• Substrates are patterned by masking the required
  areas with a kapton tape and the rest etched away
  by soaking in a solution of 20 HCl, 5 HNO3 and
  75 DI water heated to 55C.
• They are then ultrasonically rinsed in a
  detergent solution followed by cleaning in 20
  ethanolamine solution heated to 80C
• Substrates are heated to 80C in a vacuum oven

45
Concept of hole confinement

• Mobilities of holes in the hole transport layer
  is greater (10-100) than that of electrons in
  electron transport layer
• Holes reach the recombination interface much
  faster than the electrons
• This results in an imbalance between holes and
  electrons causing hole leakage current

46
Schematic and Band structure
47
Hole confining layer

• 2.6x improvement in peak luminance
• 2x improvement current efficiency

48
Multiple well Results
Brightness (cd/m2)
Current (mA)

• 1.4x improvement in peak luminance compared to
  single well structure
• 3.7x improvement in peak luminance compared to
  the device without the hole
• confining layer
• No significant improvement in current
  efficiency- under investigation

49
Images of working OLEDs
50
LaS cathode based OLEDs

• LaS has a melting point 2000C so high amounts
  of current is required to produce the required
  temperatures on the evaporation source
• Depositing LaS by PLD on the organics has damaged
  the surfaces due to heat and energetic
  bombardment
• So an inverted OLED structure has been employed
• The cathode (LaS) is first deposited followed by
  an electron injection layer, electron
  transport/emitting layer, hole transport layer,
  hole injecting layer and the semitransparent
  Silver (Ag) anode.

51
Results

• Cathode Au/LaS (600/500 A)
• EIL BphenLiAlq (98498 A)
• ETL Alq3 (500 A)
• HTL TPD (450 A)
• HIL kodak-x (100 A)
• Anode Ag (200 A)

Max brightness 1860 cd/m2 at 14V
52
Outline

• Background
• Specific Aims
• Results
• Novel Cold Cathode
• Self-assembled Trimodal arrays for field emission
• First nanowire organic spin valve
• OLED device development
• Proposed Future Work
• Conclusions

52
53
Proposed Nano- OLEDs
light emission
Semitransparent gold or silver
Alq3
TPD
Alumina matrix
Device 1
Device 3
Device 2
54
Literature Survey

• Fabricated by e-beam patterning
• of Si3N4.
• MEH-PPV a red emitting material
• is spin casted into the lithographically
• patterned holes
• The device behavior is compared to
• that of 1 and 0.5 µm diameter OLEDs
• The devices had approximately the same
• turn on voltage.
• A slight increase in current density and
• luminescence is reported

Nano Letters, 5, No12, 2485-2488
55
Literature Survey

• Nanoporous alumina templates about 100nm in
  diameter have been used
• Alq3, TPD and PMMA are dissolved in a solution
• of 1,2-dichloroethane
• The solution is dropped on the porous alumina
  until the solvent dries
• A layer of ITO is evaporated for the top contact
  

Jap. Jou. Appl.Phys, 43, No. 11A, pp.7552-7553
56
Advantages

• The proposed devices 1 and 2 provide a natural
  encapsulation and increase the lifetime of the
  OLEDs
• The proposed structure 3 should improve the
  carrier injection due to localized field
  enhancement. Hence should operate at a lower
  voltage or exhibit improved brightness
• The devices are flexible
• Possible reduction of the pin holes,improved
  yield.

57
Applications

• Flat panel displays
• Digital picture Frames
• Cell phones, watches, portable players
• Solid state lightning

58
Conclusions

• A novel cold cathode for field emission
  measurements has been designed and fabricated.
• Trimodal arrays of LaS have been fabricated and a
  voltage reduction of 3.5 times is observed for
  the same amount of emitted current.
• The trimodal arrays exhibit a rich diversity in
  the formation of nanoscale features and multi
  level self-assembly and hence has opened a new
  field for material scientists and electrical
  engineers
• First nanowire organic spin valve has been
  fabricated exhibiting extremely long spin
  relaxation times (upto a second) which is the
  highest reported so far
• Hole confining layer in OLEDs has shown good
  improvement in peak luminance and luminance
  efficiency

59
Research Group
Nicholas Harth Aparna Rakurti Vidhya Shankar
Arun Kumar