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Characterization of Diamond Surface Termination and Electrical Properties

Georgieva, Mariela*; Weil, James; Birdwell, A. Glen; Shah, Pankaj; Crowne, Frank J., Ivanov, TonyUS Army Research Laboratory, *University of MarylandAPS-MAS October 23-25

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Motivation

Following the success of other transparent semiconductors, there has been interest in diamond for high-power electronics.

The next step is to identify those critical characterization techniques that will be most effective in establishing process control through the various steps of device fabrication.

• Mobility• Surface diamond devices are limited by large surface

roughness• Decreasing the roughness from RMS ~ 1 nm to ~

0.35 nm is likely to considerably increase the mobility, from ~ 125 cm2 N-s to ~ 400 cm2 N-s.

• High power• Microwave sources kW range• mm-wave sources 100 Watt range

• High thermal stress• Less need for cooling during operation

ARL Modeling suggests higher hole mobilities can

be found by minimizing diamond surface roughness.

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Techniques

• Atomic Force Microscopy• Roughness• root-mean-square

roughness: based on average deviation from vibration of the probe tip

• Raman Spectroscopy• compositional & structural

data • Identifies intensities,

positions and widths of different inverse wavelength peaks

• Kelvin Probe Microscopy• electrical properties• work function: based on the contact potential

difference between the Au tip and diamond sample

Au

Kelvin Probe Sample Mount and Tip

Negative Electron Affinity

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Goals

• To develop a preliminary procedure for fabricating diamond FETs

• To develop diamond surfaces that exhibit better electrical properties for high-power devices

• To compare the performance of diamond substrates from two different suppliers

• To compare the performance of diamond substrates with different terminations

Top: Band scheme and electron affinity for the bare, oxidized and hydrogenated diamond (100) surface. Bottom: The atomic arrangement of the bare, oxidized and hydrogenated (100) diamond surface.

Negative Electron Affinity

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Characterization Procedure

Starting Materials: Diamond wafers were polished at the vendor, then shipped to ARL where they are characterized.

CVD-grown, single-crystal diamond (001)Sample 1: Vendor A, N-content (<25ppb)Sample 2: Vendor B, N-content (<3ppb)

Post-Polish Characterization|

Surface Preparation(Organic Cleans and Chemical Oxidation)

|Post Oxide Characterization

|Hydrogenation

|Post Hydrogenation Characterization

|Device Fabrication

Substrate Fabrication Process

CVD Growth Laser Cutting Polishing Substrates

ARL’s In-House Characterization

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Post-Polish: Raman

Sample 1

Sample 2

Characteristic Diamond Peaks:o 1332cm-1: 1st Order Peak, Characteristic of diamond quality.

- Natural Diamond FWHM: 2-2.5cm-1

- High Quality, CVD Diamond FWHM: <10cm-1

o 1350cm-1, 3120cm-1: Reveal N-V- centers

N-V-

1st Order Peak

Sample 1st Order(cm-1)

FWHM(cm-1)

N-V- Notes:

1 1333.6 4.8 Yes Other peaks indicate bulk/surface inclusions

2 1333.4 5.0 No Very clean wafer

S1 Inclusions S2 Variation in 1st Order Peak Position

2nd Order Peaks

Nitrogen below Detection Limit

= TBD

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AFM Data Summary

Sample 1

Sample 2

Polish Oxygenation Hydrogenation

AFM - RMS Surface Roughness (nm)Polish Oxygenation Hydrogenation

Sample 1 0.46 1.278 0.315Sample 2 0.46 1.14 0.616

Outgassing from Cassette(cleaned prior to processing)

Chemical Contamination?

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Kelvin Probe Data Summary

Post-Polish Post-Oxygenation Post-Hydrogenation

Sample 2 Sister Sample

Expected Values:

~4.8 – 5.2 eV

(Hydrogenation)

~5.8 – 6.2 eV

(Oxygenation)

Chemical Contamination?

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Findings

• There is a significant difference between the measurements taken using samples from the two manufacturers as-received

• Large variation in work function values indicate likely chemical contamination issues during the oxidation step• Supported by AFM data

• Hydrogen plasma treatment may “decorate” inclusion sites and other surface defects

• Need to be careful with sample handling with respect to metal tweezers, etc…metals can diffuse into diamond during high-temperature processes

Laser confocal microscopy image of hydrogenated diamond surface

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Conclusions

• Consequences of the wafer manufacturing and device processing must be addressed• Example: Better cleaning between process steps

• Need to study additional samples from each manufacturer

• Overall, data is generally in-line with predictions based on literature• Sample 1 shows the most correspondence to expected

behavior

• Techniques utilized appear adequate to evaluate the quality of diamond

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Future Plans

• Environmental chamber for Kelvin probe measurements

– Temperature and environmental control

– UHV-Scanning capbility– Illumination

• Polishing study– Intended to review different

polishing processes’ effects on the quality of the material surface

• Test device manufacture– The development of Hall bar

structures to measure the electrical performance for different polishes

UHV-Kelvin Probe

Photons

Electrons

Photoemission

Temperature Ramping (77-875 K)

In-house Capability BuildUHV-Scanning Kelvin Probe and

Photoemission Spectroscopy

UHV-Optical Chamber

w/ x,y,z scanning!

UV Light Source

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Sample CPD (mV)

CPD Stdev (mV) WF (eV) Comments

S1 111.8 5.8 5.2518N2 ambient (20 SCFH overnight, 5 SCFH during measurement);

Temp=26.1; RH=2%; no illumination

S1 112.2 6.3 5.2522N2 ambient (20 SCFH overnight, 5 SCFH during measurement);

Temp=25.1; RH=2%; no illuminationAl

Standard

-1060.0 2.3

WF (tip) = 5.140

N2 ambient (20 SCFH overnight, 5 SCFH during measurement); Temp=25.6; RH=2%; no illumination

S2 683.0 3.9 5.7531N2 ambient (20 SCFH overnight, 5 SCFH during measurement); no

illumination; after cleaningAl

Standard 990.1 4.6

WF (tip) = 5.07 normal room ambient

NOTES Used 4.080eV as the WF for Al; Tip size = 2mm; Gradient = ~300.

Post-Polish: Kelvin Probe

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Post-Oxygenation: Kelvin Probe

Sample CPD (mV)

CPD Stdev (mV) WF (eV) Comments

S2 -429.1 118.46 4.675N2 ambient (20 SCFH overnight, 5 SCFH during

measurement); no illumination

Al Standard -1024.1 3.6 WF (tip) = 5.104 normal room ambient

S1 1022.2 3.3 6.142N2 ambient (20 SCFH overnight, 5 SCFH during

measurement); no illumination

Al Standard -1039.8 3.8WF (tip) =

5.1198 normal room ambient

NOTES Used 4.080eV as the WF for Al; Tip size = 2mm; Gradient =

~300.

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Post-Hydrogenation: Kelvin Probe

SampleCPD (mV)

CPD Stdev (mV) WF (eV) Comments

S1 -193.7 3.05 4.8504N2 ambient (20 SCFH overnight, 5 SCFH during measurement);

no illumination

Al Std -964.1 4.21 WF (tip) = 5.0441N2 ambient (20 SCFH overnight, 5 SCFH during measurement);

no illumination

S2 7.098 2.7 5.0577N2 ambient (20 SCFH overnight, 5 SCFH during measurement);

no illumination

Al Std -970.6 4.08 WF (tip) = 5.0506N2 ambient (20 SCFH overnight, 5 SCFH during measurement);

no illumination