Younes Sina, Ion implantation and thermal annealing of α-Al2O3 single crystals

H. Naramoto, C.W. White, J.M. Williams, C.J. McHargue, O.W. Holland., M.M. Abraham, and B.R. Appleton

Solid State Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee

Presentation by: Younes Sina

Ion implantation and thermal annealing of α-Al2O3 single

crystals

Experimental

Single crystal of Al2O3 of high purity (100 ppm total) with low dislocation density (103-104 cm-2) from Union Carbide Corp., and Crystal System, Inc.

Sample preparation

Disc specimens were cut perpendicular (to within ± 2°) to the <0001> (c axis) and <1-210> (a axis) from single crystalline rods using a diamond saw.

ExperimentalSample preparation

These specimens were polished to a mirrorlike surface finish with a fine diamond paste(< 1 m mesh) and annealed at 1200 °C in air for 120 h to remove the surface damage induced by mechanical polishing.

280 or 300 keV

52 Cr+

1016-1017 ions/cm2

7° off

Current density : < 2×10-6 amp/cm2

Estimated temperature during implantation due to beam heating : 150°C

Implanted region

Unimplanted region(virgin)

musk

Experimental

Thermal annealing in air

1 hr800°C to 1600 °C

RBSIon scattering /channeling

Using 2 MeV 4He+

Experimental

Determine the depth profile of the implanted speciesDepth distribution of damage in the latticeLattice location of the impurity

Using RBS to

Experimental

Experimental

Some details about RBS

There are no strong nuclear reaction to complicate the backscattering analysis using 2 MeV 4He+.Random spectra were obtained while continuously rotating the crystal to average over all crystallographic directions.The specimens were covered with a stainless- steel plate with a small open aperture for analysis to minimize the charge buildup.The probing beam current was held to 10 nA ( 1 mm diameter).The scattered ion detector was cooled with Freon to 22°C, which improved the energy resolution to 14 keV.A scattering angle of 160˚was used for analysis.

Lattice location measurements were carried out using both aligned axial channeling spectra as well as detailed angular scans across the following major axes and planes:<0001> , <1-210> , <10-10> , {0001} , {1-210} , and {10-10}

Experimental

Angular scan measurements were taken only after annealing to T=1300° and 1500°.After these temperatures, substantial recovery of displacement damage in both the Al and O sublattices occurs.

Experimental

The valence state of the implanted impurity after thermal annealing was determined using standard Electron Paramagnetic Resonance (EPR) absorption measurements.

Experimental

EPR absorption measurement were made using a Kα- band microwave spectrometer (35 GHz, 1.2 cm-1) with the magnetic field applied perpendicular to the <0001> axis of the crystal.

Experimental

Changes in the hardness were measured by the use of the Knoop microhardness technique.

A force of 0.147 N was used in order to confine the impression depth to the near- surface region (0.3 which is corresponds roughly to the full width of a typical Gaussian distribution of the implanted impurity ).

Results and DiscussionImplantation damage

2- MeV He+ backscattering spectra from 52Cr(280 keV, 3×1016/cm2) implanted α-Al2O3.

Random<0001> aligned

virgin

Results and DiscussionImplantation damage

2- MeV He+ backscattering spectra from 52Cr(280 keV, 3×1016/cm2) implanted α-Al2O3.


virgin

Al surface peak

O surface peak

(Random) Yield

(Aligned) Yieldχ min

% 1.2(Al)χ min

% 6.0(O)χ min

Results and Discussion

The near-surface region was not turned amorphous by implantation (the aligned yield after implantation dose not reach the random value).

We have not observed a completely disordered surface region up to dose of 1×1017/cm2. This is in contrast to the case of semiconductors such as Si, where dose of 1014-1015/cm2 would be sufficient to turn the near- surface region completely amorphous.


virginThe implanted Cr shows a small channeling effect (the aligned yield is 85% of the random yield)


The fact that Al2O3 is not turned amorphous at these implantation energies and doses is inconsistent with the existence of a reordering process during implantation.The implanted Cr shows a small channeling effect (the aligned yield is 85% of the random yield), again suggesting a reordering process during implantation.

Sample temperatures during implantation are estimated to less than 150°C, and if the ion beam current is reduced by an order of magnitude, there is no significant change in the damage distribution.

Results and DiscussionEffect of integrated dose

Effect of integrated dose on the damage distribution produced as a result of 300- keV implantation.

Random

<0001>Align virgin

<0001>Align (1×1016/cm2)

<0001>Align (1×1017/cm2)

The near-surface region is relatively damage free.


The main effect of increasing dose is to broaden the damage profile to greater depth with little or no increase in the magnitude of the damage level.

Higher surface peak

1×1017/cm2)

1×1016


Plot of the dose dependence of χmin measured in the Al substrate at a depth corresponding to the peak in the implanted Cr distribution.

These result shows that χmin (Al) is essentially independent of implantation dose, indicating a saturation of damage along all three crystallographic direction.


Thermal annealing behavior of 52Cr(300 keV, 1×1017/cm2) implanted α-Al2O3

Thermal annealing behavior No change in the damage distribution in the O or Cr

Damage recovery for Cr &O

Results and DiscussionThermal annealing behavior

Thermal annealing behavior of 52Cr(280 keV, 3×1016/cm2) implanted α-Al2O3

Change in the damage distribution in the Al & O & Cr

Damage recovery for Cr &O


From the results presented so far, it is impossible to determine whether Cr becomes substituonal in the Al or O sublattice, but the angular scan results clearly show that Cr is substitutional in the Al sublattice.


52Cr(300 keV, 1×1017/cm2)

52Cr(280 keV, 3×1016/cm2)


virgin


virgin

Aligned yield for Al and O is very close to the virgin yield.

Aligned yield for Al is very close to the virgin yield.

The dechanneling rate in the near-surface region is greater for the high-dose crystal compared to the lower dose case.


This increased dechanneling in the near-surface region, which is a function of the dose (or concentration) of the impurity, may be due to either residual defects or to lattice strain resulting from the incorporation of large concentrations of Cr into the Al sublattice.

Results and DiscussionThermal annealing

behavior

Comparison of total and substitutional concentration for 52Cr(300 keV,1×1017/cm2) in α-Al2O3 after annealing at 1500°C

%98(Al)]χ[1

(Cr)]χ[1(%)Fraction onalsubstituti

min

min


behavior

Thermal annealing behavior for 52Cr (300 keV, 1×1016/cm2) in α-Al2O3

Random

<0001> aligned

virgin

Random

<0001> aligned

virgin

Random

<0001> aligned

virgin

Substantial redistribution of the dopant occurs in the range of 1500-1600 °C.


Thermal annealing behavior

Concentration profile for 52Cr(300 keV, 1×1017/cm2) in α-Al2O3 after annealing at 1500°C and 1600°C compared to as-implanted profile.

Substantial redistribution of the dopant occurs in the range of 1500-1600 °C.

After annealing at 1600°C, Cr is observed to be redistribution both toward the surface and into the crystal.

Cr diffuses by a substitutional diffusion mechanism


Results:Damage recovery begins selectively in the Al sublattice at a temperature of 800°C.Damage recovery begins in the O sublattice at 1000°C.Incorporation of Cr into substitutional lattice sites occurs predominantly in the temperature range 1200-1500°C. After 1500°C annealing, Cr is 95% substitutional in the lattice.The onset of substitutional Cr diffusion occurs in the temperature range 1500-1600°C.


The features of Cr incorporation can be better distinguished by separating the Cr profile into three different segments:(1)0.05 m(2)0.05-0.15 m(3)0.15-0.3 m

Results:


Results:

(1)0.05 m(2)0.05-0.15

m(3)0.15-0.3

mWhere damage is the least in the as-implanted conditionThe χmin(Cr) value increases slightly with annealing temperature up to 1200°C even though χmin(Al) decreases, indicating no further incorporation of Cr at this depth into substitutional lattice sites in this temperature range.

In region (1):


Results: (1)0.05 m(2)0.05-0.15

m(3)0.15-0.3

mSurface side of the damage distribution, χmin(Cr) change very little with annealing to 1200°C

In region (2):


Results: (1)0.05 m(2)0.05-0.15

m(3)0.15-0.3

mSaturation of damage occurred in the as-implanted state, χmin(Cr) decreased with annealing up to 1200°C.

In region (3):


behaviorResults:

(1)0.05 m(2)0.05-0.15

m(3)0.15-0.3

mThese results suggest that up to 1200°C, damage recovery in Al sublattice competes with Cr incorporation. With annealing to 1500°C, χmin(Cr) decreases substantially in region (2) and (3), while the aligned yield in the oxygen sublattice increases slightly. These results suggest that Cr incorporation in region (2) and (3) may be accompanied by oxygen indiffusion from the surface during annealing at the higher temperatures.


Results:

Thermal annealing behavior

Summary of thermal annealing result for 52Cr(300 keV, 1×1017/cm2) in α-Al2O3

Results and DiscussionLattice location of implanted 52Cr in α–Al2O3 after thermal

annealing

4a

4c

4b

4d 7b

7c7a

5a5b

5c

Results presented in previous Figs. suggest that implanted Cr is substitutional in α-Al2O3 after thermal annealing to temperatures in the range of 1300-1500 °C, because the implanted Cr exhibits a pronounced channeling effect. However these measurements alone are not sufficient to determine weather Cr is substitutional in the Al or O sublattice.


annealing

To determine whether Cr is substitutional in the Al or O sublattice, angular scans across the major axis and planes are necessary.


annealing

Axial angular scans for 2-MeV He+ incident on virgin α-Al2O3(depth range=0.05-0.35)

Yield of particles scattered from Al and O atoms in depth interval 0.05-0.35 m normalized to the random value plotted as a function of tilt angle away from the major axis or plane

2 ψ1/2: full width at half maximum of the channeling dip

Results and DiscussionLattice location of implanted 52Cr in α–Al2O3 after thermal annealing

Planar angular scans for 2-MeV He+ incident on virgin α-Al2O3(depth range=0.05-0.35)

2 ψ1/2: full width at half maximum of the channeling dip


annealing

1021cutx 0001cut z


annealing

Calculated and measured planar channeling critical half angles (ψ1/2) for 2-MeV He+ scattering from Al, O, and Cr atoms in virgin and Cr-implanted α-Al2O3.

Uncertainties in the experimental critical half angles are estimated to be 10% of the measured value.


annealing

Calculated and measured axial channeling critical half angles (ψ1/2) for 2-MeV He+ scattering from Al, O, and Cr atoms in virgin and Cr-implanted α-Al2O3.

Uncertainties in the experimental critical half angles are estimated to be 10% of the measured value.


annealing

Critical angles for both axis and planar were calculated using Barrett method:

2/112/1 )]/)([ EmVk

Adjustable parametersk=0.76, m=1.6 (for planar critical angles)k=0.83, m=1.2 (for axial critical angles) Mean one- dimensional vibrational amplitude( for planes) Mean two- dimensional vibrational amplitude( for

axis)


annealing

The potential was calculated using a model given by :

0VVVil ji

Contribution to the continuum potential due to the jth atomic species in the ith plane

A constant to make the minimum potential energy equal to zero

Such a model assumes that mixed atomic sheets such as the Al2 + O sheet in the {10-10} planar channel can be treated as a superposition of atomic sheets each with a unique atomic species.


annealing

Thermal vibrational amplitudes were determined using a Debye model of the solid with a Debye temperature of 1034°K.


annealing

Static continuum potential for the various major planes in Al2O3. The atomic constituent is indicated for each plane in a given configuration by the atomic symbol, and a superscript which indicates relative atomic abundance.


annealing

O3

Al


annealing

(Al green, O red)


annealing

There is a good agreement between experiment and theory data of channeling critical half angles. Therefore all assumptions during calculated angles can be justified.

Results and DiscussionLattice location of implanted 52Cr in α–Al2O3 after thermal annealing

Angular scans on implanted crystals were obtained using crystals implanted to dose of 1 and 3×1016/cm2 after thermal annealing at temperatures of 1300 and 1500 °C.

Angular scan across the <0001> axis for 52Cr (300 keV, 1×1016/cm2) in α–Al2O3 after 1300°C annealing.

Critical angles for scattering from Al and Cr have approximately the same width.


annealing

Critical angles for scattering from Al and Cr have approximately the same width but different from O.

Most of Cr atoms are substitutional in the Al sublattice

There are some Cr and O atoms in interstitial lattice sites after annealing at 1300°C. Interstitial Cr can trap O atoms and diffuses in from surface during annealing.


annealing

Al & O critical angle after Cr implantation and annealing

Al O

1300°C

Al & O critical angle for the virgin sample

Al critical angle is considerably wider on the implanted crystal compared to the virgin, indicating that damage recovery is not complete after annealing at this temperature.


Lattice location of implanted 52Cr in α–Al2O3 after thermal annealing

Axial angular scan for 52Cr(280keV, 3×1016/cm2) in α-Al2O3

1500 °C thermal annealing


Lattice location of implanted 52Cr in –Al2O3 after thermal annealing

Planar angular scan for 52Cr(280keV, 3×1016/cm2) in α-Al2O3

1500 °C thermal annealing



Axial angular scan for 52Cr(280keV,

3×1016/cm2) in α-Al2O31500 °C thermal annealing

Al & O critical angle for the virgin sample

Axial angular scan for 52Cr(300keV,

3×1016/cm2) in α-Al2O31300 °C thermal annealing

Comparison of axial/planar angular scans for different cases shows that critical angle in higher annealing temperature is closer to the virgin case.



Conclusion: Near-surface region is not turn completely amorphous with Cr implantation on sapphire at doses less than 1017/cm2. Upon annealing, damage recovery begins selectively in the Al sublattice at T~800 °C. Recovery in the oxygen sublattice begins at T~1000 °C for Cr. After Cr implantation followed by thermal annealing at ~1500 °C, the implanted impurity is observed to be >95% substitutional in the Al sublattice.

Valence state of implanted 52Cr

The valence state of the implanted impurity can be determined using Electron Paramagnetic Resonance absorption (EPR) techniques. The EPR spectrum of substitutional trivalent chromium ions (Cr3+) in Al2O3 may be described by the following spin Hamiltonian:

]3/)1([)( 2 SSSDSHSHgSHgH zyyxxBzzB

1cm 0.382D 1.987,g 1.984,g 3/2,S

EPR spectroscopy is the measurement and interpretation of the energy differences between the atomic or molecular states.These measurements are obtained because the relationship between the energy differences and the absorption of electro-magnetic radiation. To acquire a spectrum, the frequency of the electromagnetic radiation is changed and the amount of radiation which passes through the sample with a detector is measured to observe the spectroscopic absorptions.

EPR Spectroscopy

http://upload.wikimedia.org/wikipedia/commons/a/a1/Corundum.GIF

EPR

•Like a proton, an electron has a spin, which gives it a magnetic property known as a magnetic moment.•When an external magnetic field is supplied, the paramagnetic electrons can either orient in a direction parallel or antiparallel to the direction of the magnetic field . •This creates two distinct energy levels for the unpaired electrons and measurements are taken as they are driven between the two levels.

α-Al2O3 with trace Cr3+ impurity

Valence state of implanted 52Cr

52Cr(300keV, 1×1016/cm2) in α-Al2O3

EPR line shape of high field Cr3+

absorption line (Ms=-1/2↔Ms=-3/2)

Microhardness change of Al2O3 with 52Cr implantation followed by thermal annealing

HARDNESS CHANGES DUE TO ANNEALING For implanted Cr(1017/cm2) and Zr (4×1019/cm2) in α- Al2O3

Annealing temperature(˚C)

Kurdish rug with hexagonal grid

Thank you

EPR spectroscopy is the measurement and interpretation of the energy differences between the atomic or molecular states.These measurements are obtained because the relationship between the energy differences and the absorption of electro-magnetic radiation. To acquire a spectrum, the frequency of the electromagnetic radiation is changed and the amount of radiation which passes through the sample with a detector is measured to observe the spectroscopic absorptions.

EPR Spectroscopy

http://upload.wikimedia.org/wikipedia/commons/a/a1/Corundum.GIF

EPR

•Like a proton, an electron has a spin, which gives it a magnetic property known as a magnetic moment.•When an external magnetic field is supplied, the paramagnetic electrons can either orient in a direction parallel or antiparallel to the direction of the magnetic field . •This creates two distinct energy levels for the unpaired electrons and measurements are taken as they are driven between the two levels.

Younes Sina, Ion implantation and thermal annealing of α-Al2O3 single crystals

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