new gallium oxide nanostructures for high temperature sensors · 2015. 4. 30. · gallium oxide...

Gallium Oxide Nanostructures for High Temperature Sensors

Ramana Chintalapalle (PI)Mechanical Engineering, University of Texas at El Paso

Students: Ernesto Rubio (PhD); S.K. Samala (MS)

Program Manager:Richard Dunst, NETL, DOE

Project: DE-FE0007225Project Period: 10/01/2011 to 12/31/2014

104/30/2015 DOE Crosscutting Technology Review Meeting


IntroductionResearch ObjectivesExperimentsResults Intrinsic Ga2O3

Tungsten (W)‐doped Ga2O3

Summary & Outlook

04/30/2015 DOE Crosscutting Technology Review Meeting 4

Power Systems

High Temperatures

Highly Corrosive Environm

entsHighly

Oxidizing Environm

entsHigh

Pressures

Challenging Environment in Power Systems

5

Structure(micro/nano)

Structure(micro/nano)

Choice of MaterialsChoice of Materials

SensitivitySensitivity

PoisoningPoisoning StabilityStability

ContaminationContamination

LifeLife SelectivitySelectivity


Cost & Manuf.Cost & Manuf.


Savings; $$$ M/year

Eliminate Pollutants

Efficiency Enhancement

7

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) One of the most suitable materials for high temperature gas sensing



Tk

EpOB

A

iexp2

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 high‐temperature oxygen sensing (faster response) 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)


Project Work Plan


Materials Fabrication(Pure Ga‐Oxide) Materials

Character.& W‐Doping

Testing and Performance Evaluation

Y-1

Y-2

Y-3

Target (for Deposition)Ga2O3 & W

Substrate(s): Si(100) Alumina

MaterialsGa-oxide

Cu-backing plate

2 inch.

1/8 inch.


2 inch

W

13

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

Sensors and Sample Matrix

• ~200 nm thick Ga-oxide

• 200 nm thick Pt interdigitedelectrodes

• Spacing: 100 µm

• Extremely thin (~50 nm) samples

• Two silver electrodes attached on the surface

• Bulk (ceramic pellets)

• Electrical Impedance


Sensor Performance ‐ Electrical

Constant current

• Oxygen introduced and partial pressures of oxygen were varied

• Evaluation at temperatures ≥ 700 °C


16

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/critical to provide sufficient energy for Ga2O3 film crystallization (β-phase)


Morphology & Composition

0 100 200 300 400 500 600 700 800 900

1.2

1.5

1.8

O/G

a

Temperature (C)

O/Ga


Microstructure & Electronic Properties

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)

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)

PVD: Sputtering


W‐Doped Ga‐OxideW‐Power (Watts)

W‐Content (Atomic %)

0 0

50 5

75 10

100 15


t = 200 nm

21

Crystal Structure – Power dependence

Only Ga-oxide phase is present


No secondary phase formation

500 C

22

Band Gap (Power dependence)

22

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).


23

Crystal Structure – (Temp. Dependent)

Only β-phase presented for all films.


Mechanical Characteristics

Hardness & Elastic Modulus increases with W-content

Can be tolerant and impact resistant


Oxygen Sensor Characteristics

Intrinsic Ga2O3 filmsTime response: 62 sec


Oxygen Sensor Characteristics

• Ga2O3 based films showed oxygen sensitivity at 700 °C, • As an n-type semiconductor the resistance of the film

increased in the presence of oxygen.• Time response was drastically improved with the

incorporation of tungsten

W (5 at%) - doped Ga2O3 films


• Increasing W-content reduces sensitivity of the sensors• Time response is increased with increasing W-content• Stability of the electrical properties is reduced with increasing

W-content


Two Probe Oxygen Sensor Characteristics


Hot Gas Exposure (600‐700 nm)

•W-doped Ga-oxide demonstrated sensing properties to oxygen at 700 °C.

10%

O2

Ar

15%

O2

Ar

20%

O2

Ar

20%

O2

Time (hrs)

Inte

nsity

(arb

. Uni

ts)


Gas Exposure plot (600nm, 800nm, 1000nm)


Intensity vs. Gas Exposure Summary

W-doped films showed being sensible to H2 and O2.

O2 sensitivity increased from 10% to 15% of O2into Ar, but decrease again at 20%

Sensitivity to gases was optimum in Ar environment, but small-to-non response in a mixture of Ar+Air



0 5000 10000 15000 20000 25000 300000

10000

20000

30000

40000

50000

60000

70000

80000

Z''

Z'

200 C

0 100 200 300 400 500 60020

40

60

80

100

120

140

160

180

Z''

Z'

500 C

Electrical Impedance Analyses

0 2000 4000 6000 8000 10000 12000 14000 160000

2000

4000

6000

8000

0 1000 2000 3000 4000 5000 60000

500

1000

1500

2000

Z''

Z'

15% -Tungsten

Z''

Z'

20% - Tungsten

Ga-oxide

W-dopedGa-oxide

W-dopedGa-oxide

15%20%

0% 0%

500 C

33

Crystal Structure – (after annealing)

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


Heat Treatment

• β-phase is stable for all films after annealing

• surface morphology suffer porous formation for W-doped films.

• Evidence of W-self diffusion

500 nm

500 nm

500 nm

500 nm

Pure Ga2O3

Pw= 50W

Pw= 100WPw= 75W


Morphology – Broad View

35

How does it work?


Crystallography: MonoclinicIonic Size:

Comparable (W6+ vs. Ga3+)

Electronegative Values:Similar

W Ga

Morphology – Broad View

36

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 (2015)2. Southwest Energy Symposium, 2015, El Paso, TX3. TMS – 2014; ICMCTF - 20154. ICMCTF-2013, San Diego, CA5. AVS International Symposium, 20126. 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)


Summary and ConclusionsOptimum conditions were established to fabricate Ga2O3 and W‐doped Ga2O3with the stable β‐phase (monoclinic)Intrinsic Ga2O3 and W‐doped Ga2O3demonstrated to be sensitive to oxygen at high temperatures, with improved time of response for W‐incorporation


Future Work


2000 3000 4000 5000 6000700

800

900

1000

1100

1200

1300

1400

25 sccm out75 sccm out

50 sccm out

R

esis

tanc

e (O

hms)

Time (seconds)

BaFeTaO3 -700C

Aerosol Jetdirect write

Engine Part

Film base Nanoink

Future Work


40

AcknowledgementsDOE‐NETLEMSL/PNNL, Richland, WAMichael Carpenter (SUNY)

10/25/2011 UTSR Workshop, Oct. 25-27, 201104/30/2015 DOE Crosscutting Technology Review Meeting

THANK YOU!

new gallium oxide nanostructures for high temperature sensors · 2015. 4. 30. · gallium oxide...

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