Gallium Oxide Ramana Library/events/2013/hbcu-ucr/Gallium... · Gallium Oxide Nanostructures for...

Gallium Oxide Nanostructures for High Temperature Sensors

C.V. Ramana (PI)Evgeny Shafirovich (Co-PI)

Mechanical Engineering, University of Texas at El PasoStudents: Ernesto Rubio (PhD); S.K. Samala (MS)A.K. Narayana Swamy (PhD); K. Abhilash (MS)

Program Manager:Richard Dunst, NETL, DOE

Project: DE-FE0007225Project Period: 10/01/2011 to 09/31/2014

106/12/2013 DOE UCR/HBCU Conference, June 11-13, 2013

2

Introduction Research Objectives Experiments

► Synthesis► Characterization

Results and Discussion► Pure Ga2O3 Thin Films► W-doped Ga2O3 Thin Films

(Physical Methods) Summary & Future Work


4

Energy Systems

High‐THigh‐T High‐PHigh‐P

High‐CHigh‐CHigh‐OHigh‐O

T,P ToleranceT,P Tolerance

Ox. and Cr. ResistanceOx. and Cr. Resistance


5

MicrostructureMicrostructure

Choice of MaterialsChoice of Materials

SensitivitySensitivity

PoisoningPoisoning

SelectivitySelectivity

ContaminationContamination

Shelf LifeShelf Life

StabilityStability


6

Fundamental Science – Phases (,,γ, and δ)

Novel Properties –Metal Doping

Surface Chemical Reactions

Chemical Sensors

Ease of Thin Film Formation

Integration into Nano‐electronics

Ga2O3 Wide band gap (>5 eV) semiconductor *High thermal and chemical stability (Tm: 1725 oC)*Due to a high melting point and stable structure, it is one of the most suitable materials for high temperature gas sensing.



Tk

EPB

A

iO exp2

Activation energy

Oxygen partial pressure Boltzmann

constantTemperature

Electrical conductivity

At T>700 oC, defects equilibrium with surrounding atmosphere n type conductivity depends on oxygen partial pressure

At T< 700 oC, Ga-oxide exhibits sensitivity to reducing gases (CO, H2)

Objective 1: To fabricate high‐quality pure and dopedGa2O3‐based materials and optimize conditions to produceunique architectures and morphology at the nano scaleObjective 2: Derive the structure‐property relationships atthe nanoscale dimensions and demonstrate enhancedhigh‐temperature oxygen sensing and stabilityObjective 3: To promote research and education in thearea of sensors and controls

Goal: Design the high temperature oxygen sensors(employing Ga2O3‐based nanostructures)


Target (for Deposition)Ga2O3 & W

Substrate(s): Si(100) Alumina

MaterialsGa-oxide

Cu-backing plate

2 inch.

1/8 inch.


11

RF magnetron sputtering Deposition Conditions

Fixed:- Base pressure ~10-6 Torr- Powers: Ga2O3100 W- Target-Substrate distance: 7 cm- Sputtering gas: Argon + O2

Variables: Sample set 1 (Intrinsic): Substrate Temperature: RT-500 ˚CSample set 2 (W-Doped): Tungsten Target Power (50 to 100W)Substrate Temperature = 500 ˚C

Fabrication – Thin Films


Sample set 3 (W-Doped): Target Powers = const.; Substrate temperature varied from 500 to 800˚C

12

Characterization


13

Characterization (cont.)

High Temperature Furnace for annealing process

UV-vis-Spectrophotometry


PVD: Sputtering


15

10 20 30 40 50 60 70 80 90

Si (2

00)

Si (2

00)

800C

(020

)(

221)

(020

)(

221)

500C

400C


Crystal Structure

300 400 500 600 70010

15

20

25

30

35 Exponential Fit

Gra

in S

ize

(nm

)

Temperature (C)

L = Lo exp (-∆E/kBT)L: Average size L0: Pre-exp. factor (film, substrate materials) ∆E: Activation energy, kB: Boltzmann constant and T: Absolute temperature.

500 °C is favorable to provide sufficient energy for Ga2O3film crystallization (β-phase)

06/12/2013 DOE UCR/HBCU Conference, June 11-13, 2013 16

Morphology


Composition - RBS

200 400 600 8000

400

800

1200

1600

2000

2400

2800

400 800 1200 1600

2 MeV He+ Ga2O3//Si(100)Ts - 600 C

Energy (KeV)

Channel Number

Bac

ksca

ttere

d Io

n Yi

eld

(Cou

nts)

experimental simulated

OGa

Si

200 400 600 8000

400

800

1200

1600

2000

2400

2800

400 800 1200 1600Energy (KeV)

2 MeV He+ Ga2O3 //Si(100)Ts - 500C

Bac

ksca

ttere

d Io

n Yi

eld

(Cou

nts)

Channel Number


OSi

Ga

200 400 600 8000

400

800

1200

1600

2000

2400

2800

400 800 1200 1600

Channel Number

Bac

ksca

ttere

d Io

n Yi

eld

(Cou

nts)

2 MeV He+ Ga2O3 //Si(100)Ts - 300 C


Energy (KeV)

O SiGa

200 400 600 8000

400

800

1200

1600

2000

2400

2800

400 800 1200 1600

2 MeV He+ Ga2O3 //Si(100)

Ts - RT

Channel Number

Energy (KeV)

Bac

ksca

ttere

d Io

n Yi

eld

(Cou

nts)


O

Si

Ga

0 100 200 300 400 500 600 700 800 900

1.2

1.5

1.8

O/G

a

Temperature (C)

O/Ga


Composition - XPS


Ga 2p peak

As Grown Sputtered

Chemical Shift in Ga 2p BE – ~1118 eV; ~1145 eVOriginal Ga 2p BE – 1117eV; 1144 eV (represented by blue lines)


Microstructure – Phase Diagram

100 200 300 400 500 600 700 800

Slightly excess oxygen

Stoichiometric T

rans

ition

Tem

pera

ture

B

egin

ing

of N

anoc

ryst

allin

e Fi

lms

Grain size 10- 40 nm

-phase

Ga2O3 Films

Amorphous

Substrate Temperature (C)


200 300 400 500 600 700 800

20

40

60

80

100

120

Tran

smitt

ance

, % T

Wavelength (nm)

RT 200 C 300 C 500C 600C 800C

0 100 200 300 400 500 600 700 800 9004.90

4.95

5.00

5.05

5.10

5.15

5.20

Ban

dgap

(eV)

Temperature (C)

Electronic Properties

(αhν) = B (hν-Eg)1/2

hν: energy of the incident photon, α: absorption coefficient, B: absorption edge width parameter, Eg:band gap.

α = [1/t]ln[T/(1-R)2]T: transmittance R: reflectance t : film thickness

PVD: Sputtering


200 ̊C

23

300 ̊C

Chemical Composition (RBS)


W‐Doped filmsW‐Power (Watts)

W‐Content (Atomic %)

Thick‐ness

0 0 38 nm

50 8.35 32 nm

75 9.58 42 nm

100 12.5 51 nm


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Crystal Structure – Power dependence

Only Gallium oxide phase is present


No secondary phase formation

Morphology (Power Dependent)

Pure Gallium Oxide Films show grains throughout the surface, and W-doped films

Pure Gallium Oxide Films show grains throughout the surface, and W-doped films avoid crystallites complete growth

Ts=500°C


27

Optical Propertiestdep= 30min


28

Band Gap (Power dependence)

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Reduction by ~0.75-1.00 eV with the inclusion of tungsten into Ga-oxide films!

E.J. Rubio and C.V. Ramana, Appl. Phys. Lett. 102, 191913 (2013).


29

Crystal Structure – (after annealing)

Deposition Temperature Ts=500 °C;Annealing TemperatureTa=700 °C for 30 min


30

Crystal Structure – (Temp. Dependent)

Only β-phase presented for all films.


Morphology – Broad View

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Why does it work?


Crystallography: MonoclinicIonic Size:

Comparable (W6+ vs. Ga3+)

Electronegative Values:Similar

W Ga

Morphology – Broad View

32

ImpactJournal Publications:1. E.J. Rubio and C.V. Ramana, Appl. Phys.

Lett. 102, 191913 (2013).2. A.K. Narayana Swamy, E. Shafirovich, and

C.V. Ramana, Ceram. Inter. 39, 7223 (2013).3. S.K. Samala, E.J. Rubio, M. Noor-A-Alam,

G. Martinez, S. Manandhar, V. Shutthanandan, S. Thevuthasan, and C.V. Ramana, J. Phys. Chem. C 117, 4194 (2013).

4. Two others (under preparation)

Conference Presentations:1. International Materials Research Congress (IMRC) – to be

presented 2. International Conference on Metallurgical Coatings and Thin

Films, April 29 – May 3, 2013, San Diego, CA3. AVS International Symposium, October 28 – November 2, 2012

Tampa, FL4. Southwest Energy Symposium, March 24, 2012, El Paso, TX

Education & Training:1. Ernesto J. Rubio: PhD

(Full)2. A.K. Narayana Swamy:

PhD (part of disseration)3. Sampath K. Samala: MS

(thesis)4. Abhilash Kongu: MS

(non-thesis)


Future Work


Materials Fabrication(Pure Ga‐Oxide) Materials

Character.& W‐Doping

Testing and Performance Evaluation

Y-1

Y-2

Y-3

Future Work


Future Work


Detailed Electrical and Sensor Characteristics (UTEP)

SUNY – Michael Carpenter (Plasmonics)

36

Summary & Conclusions

Pure and W-doped Ga‐oxide thin films were grownand characterized

Experimental conditions were optimized to obtainGa‐oxide materials with wide controlled structureand morphology in a wide range

Stability of β‐phase with controlled electronicproperties is demonstrated (with W‐incorporation)

Preliminary results obtained on the electricalproperties are encouraging

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AcknowledgementsDOE‐NETLRichard DunstEMSL/PNNL, Richland, WA

10/25/2011 UTSR Workshop, Oct. 25-27, 201106/12/2013 DOE UCR/HBCU Conference, June 11-13, 2013

THANK YOU!

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