Surface Chemistry and Device Response on AlGaN/GaN surfaces

Jeremy Gillbanks – September 2015Supervised by

Prof. Giacinta Parish and Prof. Brett Nener

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Sensor Context

Semiconductor Doping

High Electron Mobility Transistors

Substrate Design

Field Effect Transistors

Chemical Sensors

Chemical Sensors

Field Effect Transistors

CHEMFETs ISFETs

Silicon-based devices

Heterostructure-based devices

HEMTs

AlGaN/GaN AlGaAs/GaAs

BioFETs

Other Sensors

3

Our Sensors

Semiconductor Doping

High Electron Mobility Transistors

Substrate Design

Field Effect Transistors

Chemical Sensors

Chemical Sensors

Field Effect Transistors

CHEMFETs ISFETs

Silicon-based devices

Heterostructure-based devices

HEMTs

AlGaN/GaN AlGaAs/GaAs

BioFETs

Other Sensors

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AlGaN/GaN Sensors

Advantages over traditional ISFET Sensors

– Stability– Low cost– No reference electrode

Applications– Recycled water

monitoring– Lab-on-a-chip sensor

arrays

AlGaN capped transistor Ren 2008

Sensor array designAsadnia 2015

Active area

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Research Gap

Previous research completed by the Microelectronics Research Group at UWA

Demonstrating ionic concentration, regardless of pH (2010)

Dipolar molecule orientation and sensor response

Sensor selectivity toward negative ions (2010)

GaN cap has greater affinity to Cl- ions than AlGaN (2014)

2DEG conductivity increase with positive charge build up (2014)

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Project Objectives

Aim: Molecular contact angle vs. device response

Glycine

 

Benzil (non-polar)

6-Amino-2-Naphthoic Acid

Hypothesis:• Adhesion via negatively charged

carboxyl group• Dipolar molecules will affect

device response via molecular orientation

This is the first time dipolar molecular orientation has been investigated on a GaN capped device.

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Molecule Selection

Glycine 6-Amino-2-Naphthoic Acid

NEXAFS conducted at AS

C NBackground Correction

Choose Step EdgeGaussian Peak

FittingSpectral Subtraction

Bond Angle CalculationMolecular

OrientationCompare to Device

Response

O

Benzil

Experimental Procedure

6-Amino-2-Naphthoic Acid only

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Molecule Selection

Glycine 6-Amino-2-Naphthoic Acid

NEXAFS conducted at AS

C NBackground Correction

Choose Step EdgeGaussian Peak

FittingSpectral Subtraction

Bond Angle CalculationMolecular

OrientationCompare to Device

Response

O

Benzil

Project Scope

6-Amino-2-Naphthoic Acid only

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Molecule Selection

Glycine 6-Amino-2-Naphthoic Acid

NEXAFS conducted at AS

C NBackground Correction

Choose Step EdgeGaussian Peak

FittingSpectral Subtraction

Bond Angle CalculationMolecular

OrientationCompare to Device

Response

O

Benzil

Seminar Scope

6-Amino-2-Naphthoic Acid only

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NEXAFS: How it works

• Near Edge X-ray Absorption Fine Structure

• Incident photon energy is near the edge of the ionisation potential of the scanned atom

• Ammeter allows replacement current to be recorded from photoelectron loss

• Allows measurement of individual molecular orbitals for C, N and O atoms

Experimental SetupMennell 2015

This is the first time a NEXAFS study has been conducted on a GaN substrate

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Non-linear curve fitting

6-Amino-2-Naphthoic Acid Nitrogen K-edge scan

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XPS

X-ray Photoelectron Spectroscopy

• Composition, thickness

6-Amino-2-Naphthoic AcidNitrogen XPS K-edge scan

6-Amino-2-Naphthoic AcidGallium XPS K-edge scan

Before deposition

After deposition

After deposition

Before deposition

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BenzilNitrogen K-edge scan

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Spectral Subtraction

6-Amino-2-Naphthoic Acid Curve Fitting

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Identifying the peak

Measured angle from nitrogen scan: 43.7˚ ± 10˚Measured angle from carbon scan: 46˚ ± 2˚ (Home 2015)

Naphthoic Acid Peak fit at 404 eV (corresponds to C-N σ* bond)

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Angle of naphthoic acid to surface

I can corroborate Michael Home’s finding that 6-amino-2-naphthoic acid lies at 44˚ to the device surface.

44˚

Device Surface

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Future Work• Ensure adequate coverage

• Normalise on the device surface

• Test simple alcohols/acids– Methanol– Formic acid– Benzoic acid

• Test simple amine groups– Methylamine– Aniline

• Test simple amino acids with benzene rings

– Meta-, ortho-, or para-amino benzoic acid

• Later: test larger molecules– Tyrosine

Tyrosine

Formic acid Aniline

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Key Points

• We have been the first to successfully orientate glycine and 6-amino-2-naphthoic acid on a GaN capped device– Every molecule to be sensed has a specific angle at

which it adheres to the surface– The orientation effects the device response

• Future work has been successfully identified

• Special thanks to– Prof. Giacinta Parish & Prof. Brett Nener– Farah Khir, Matt Myers, Murray Baker– Michael Home, Chris Mennell, Ben Sutton– The III-N research group

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Deleted Scenes

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Background Correction

• Remove oscillations in incident photon intensity over time and energy

• Au leaf used (300 eV – 1000 eV)

Device setup at the Australian SynchrotronCourtesy: F. Khir

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Glycine (nitrogen scan)

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Benzil

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Sources of Error & Biases (1/2)

• Noise– Using peak areas instead of peak heights decreases

effect of noise on local regression• Local regression formula inadequately smoothed

• Back scattered electrons– Reduced to insignificance due to multiple incident angles– Highly energetic photons (with adequate coverage)

shouldn’t penetrate the adsorbate• Photoelectrons generated from surrounding atoms

• Thermal motion ineffectively averaged between scans

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… (2/2)

• Monochromator’s linearly polarised light– K-shell spectra are highly polarisation-dependent– Linear polarisation simplifies the dipole matrix element

• Replacement current efficiency (resistance, etc…)

• Inconsistent incident photon intensity in excess of what is corrected for using the reference foil

• Substrate does not display three-fold or higher symmetry

• Adsorbate not a homogenous layer

• Hydrogen bonds effect spectra in a measurable way

• Ineffective spectral subtraction

• Adsorbate damaged during x-ray scan

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Limitations

• Building block model– Used when you have a new molecule that has not

been scanned using NEXAFS before• E.g. 6-amino-2-naphthoic acid or benzil

– Limitations:• Conjugated molecular orbitals are difficult to identify

during deconvolution• More of a problem for carbon K-edge NEXAFS scans

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Why not more samples?

• AS has high resolution– Resolution: 0.1 eV– Energy Range: ~40 eV– 0.25% steps

• Trends successfully identified => conclusions are valid

• Common practice is to use best spectra, not to average

• Experiment cost: ~$600k

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References1. Title Slide: Substratehttp

://pubs.rsc.org/services/images/RSCpubs.ePlatform.Service.FreeContent.ImageService.svc/ImageService/Articleimage/2006/DT/b515727g/b515727g-f5.gif

2. Title Slide: Australian Synchrotron logohttps://events.synchrotron.org.au/event/1/picture/10.jpg

3. Title Slide: Microelectronics Research Grouphttp://mrg.ee.uwa.edu.au/images/microelectonicsResearchGrou.gif

4. Slide 5: Glycinehttp://www.actgene.com/images/Glycine.jpg

5. Slide 5: 6-Amino-2-Naphthoic Acidhttp://www.sigmaaldrich.com/content/dam/sigma-aldrich/structure6/165/mfcd01861831.eps/_jcr_content/renditions/mfcd01861831-medium.png

6. Slide 5: Benzilhttp://www.sigmaaldrich.com/content/dam/sigma-aldrich/structure3/116/mfcd00003080.eps/_jcr_content/renditions/mfcd00003080-medium.png

7. Slide 15: 6-Amino-2-Naphthoic Acidhttp://pubchem.ncbi.nlm.nih.gov/image/img3d.cgi?cid=2733954

8. Slide 16: Formic Acidhttp://chem-tracking.de/onewebstatic/ed4ba8c401-Ameisensäure.jpg

9. Slide 16: Anilinehttp://chemwiki.ucdavis.edu/@api/deki/files/9113/aniline.png

10. Slide 16: Tyrosinehttp://img1.wikia.nocookie.net/__cb20140401122944/resscientiae/images/2/29/Tyrosine.jpg

All other slides are of the author’s creation unless otherwise cited.