•LASER – Light Amplification by Stimulated Emission of Radiation•Coherent waveforms emitted in powerful, highly focused beam•Stimulation occurs from electron transitions in the split energy band•Requires inverted population: >50% electrons in excited state•Lasers used in precision measuring, data retrieval/storage, and medical equipment

•Czochralski growth (CZ) is a common method for growing single crystals•Material for the crystal is melted and the temperature is adjusted so the solid/liquid interface is at equilibrium•A seed crystal is placed at the interface and the melt cooled while the seed is drawn out to form the crystal•Crystal size and shape are dictated by drawing rate of seed crystal – color is controlled by dopant atoms such as neodymium (Nd) and cerium (Ce)

•Defects in a crystal lattice that cause the shifting of a plane of atoms•Edge dislocation – a ½ plane of atoms is forced between two other planes, distorting the crystal lattice•Places stresses on the atoms around the dislocation•Dislocations arise during solidification and move along slip planes of the crystal

•Determine what defects are present in YAG laser crystals that could act as scatter centers and how they are dispersed within the crystal•Scatter center – defect in a crystal that scatters the coherent waveform and reduces laser output efficiency•Scatter centers may include: dislocations, inclusions, secondary phases, and voids•Dislocations – determine dislocation density and map how they move through the crystal•Secondary phases/inclusions – determine if they are present in solidified melt samples and correlate with resulting crystal quality

Steps in lasing process:• Pumping – ground state electrons

are excited by UV radiation • Excited electrons spontaneously

relax to a lower energy level, giving off infrared radiation in the form of heat

• Electrons populate lower energy level, and one relaxes to the ground state, emitting a photon

• Ends of laser rod are polished (one side partially) so the waveform reflects along the rod axis

•Doping causes energy band splitting necessary for lasing•In yttrium aluminum garnet (YAG) crystals, dopant atoms substitute for the Y atoms in the unit cell•YAG is an excellent laser core material because of high melting point, resistance to thermal stress, and good mechanical properties•Allows it to be used in high-end applications that require powerful lasers

Neodymium-doped YAG laser rod

A single YAG (Y3Al5O12) unit cell contains 160 atoms

Schematic of a CZ furnace and examples of single crystal YAG boules –Nd doped (top) and Ce doped (bottom)

Etch pit morphologies: a) Triangular-(1 1 1) plane b) Elliptical-(1 1 0) plane c) Comet tail d) Etch pit clusters around a void e) Etch pits along a possible dislocation source

1µm 1µm1µm

1µm 1µm

a b c

d e

Change in etch pit density and size through the horizontal cross-section of a Nd-YAG crystal - cross-section is 6 cm in diameter

1µm 1µm

1µm 1µm

1µm1µm

•Samples were taken from the horizontal cross-section of a Nd-YAG boule to determine how the density of dislocations changed along this plane•Changes in the density of etch pits could indicate if there was dislocation motion along slip planes in the crystal•Slip planes allow dislocations to move easily along their axis•Dislocations could pile up in the crystal at the end of a slip plane•The dislocation density at the edge of the crystal was larger than at the center, which was virtually dislocation-free

•Samples were taken vertically along the path of the seed crystal during solidification to determine if there is a change in the density of dislocations•Different etch morphology results from the etch pits being viewed in the (1 1 0) plane versus the (1 1 1) plane of the horizontal section•Dislocation density increased slightly in parts of the crystal closer to the seed crystal

Schematic of an edge dislocationSource J. Weertman & J. R. Weertman, Elementary Dislocation Theory. 1992.

•Cerium doped YAG demonstrated similar etch pit morphologies, but with much more frequent occurrences of pit clusters•Clusters were also much more densely populated with pits than those observed in the Nd-YAG boule•The clusters consisted of large numbers of dislocations piled on each other•Nd-YAG pit clusters did not exhibit such intense stacking

1µm

1µm

1µm

Change in etch pit density through the vertical cross-section of a Nd-YAG boule up to the seed crystal (top) – cross-section is 3 cm tall

Seed Crystal

•Two samples from different melts of Nd-YAG used in growing single crystals were analyzed using Energy Dispersive Spectroscopy (EDS) analysis techniques on a scanning electron microscope•The samples were solidified, polycrystalline remains from a melt that produced a good crystal and a melt that produced a defective crystal

1µm 1µm

Etch pits present in a Ce-YAG crystal: a) Triangular (1 1 1) plane pits b) Densely populated clusters of etch pits (dark areas) c) Etch pit stacking in clusters in Ce-YAG compared to d) clusters in Nd-YAG

0.2µm0.2µm

a b

c d

a b2 cm 1 cm

Solidified polycrystalline Nd-YAG specimens: a) Core from melt that produced a usable crystal – inset is close up of fiber-like grain structureb) Sample from a melt that produced a defective crystal

Financial support was provided by the National Science Foundation's Division of Materials Research REU program under grant number 0139125.

Invaluable Support: Dr. Norton, Joel LeBret, Dan Eakins, Carmelo Delgado, Scott Cornelius, NSF, WSU, and the TEM.

a b

c

Scanning electron micrographs of the surface morphology of polycrystalline Nd-YAG: Fiber-like grains from the solidified piece of a usable melt a) looking at the ends of the grains and b) along the length of the grains compared to the c) faceted crystal structure of the solidified piece of the failed melt.

•The EDS mode on an SEM was used to determine elemental concentrations on different portions of the sample surface•Changes in composition could indicate different phases present as well as the possibility of inclusions in the melt•Inclusions and different phases could contribute to a decrease in the efficiency of a laser core taken from these melts by acting as scatter centers

EDS point analysis of usable Nd-YAG melt. Values are atomic % of each element present at a location. Two readings were taken at each point.

•EDS results indicate that there are local variations in surface chemistry that could be due to particles or secondary phases•Preliminary analysis using an electron microprobe and the Wavelength Dispersive Spectroscopy (WDS) mode demonstrates the different phases present in both samples•WDS can be used to determine the variations in surface chemistry over a bulk sample to see how secondary phases are distributed

•Dislocations in Nd-YAG crystals appear to increase in density from the center to the edge and from the center to the top of the boule, indicative of some dislocation movement in the crystal•Dislocation morphology corresponded to the (1 1 1) and (1 1 0) planes, similar to those reported by Yang et. al. [1]•Ce-YAG crystals showed a high density of dislocations, especially in the form of large, stacked dislocation clusters•EDS point analysis demonstrated some differences in local surface composition of polycrystalline samples from a useful and failed melt of Nd-YAG – could indicate some secondary phases or inclusions present•Backscatter imaging shows the presence of several different phases in the failed melt, which could be responsible for growth of a poor crystals

1. Y. Peizhi, D. Peizhen, Y. Zhiwen, T. Yulian, Journal of Crystal Growth, Vol. 218, p 87-92, (2000).

2. D. E. Eakins, Electron Microscopy of Compound Oxide Laser Crystals. MS Thesis Washington State University, p. 1-51, (2003).

3. J. Weertman, J. R. Weertman, Elementary Dislocation Theory. Oxford University Press, New York, 1992, p 4-5.

Electron backscatter images of the a) failed and b) usable Nd-YAG melt. The orange area is Al and Y, with the yellowish areas containing more Nd. The orange area in the usable melt has more Al, less Y, and less Nd than similar areas in the failed melt.

ab

Al and Y with major Nd

Al and Y

Al, Y, and Nd

Al with alittle Nd (dark area)

1

4

3

2

EDS point analysis of failed Nd-YAG melt. Values are atomic % of each element present at a location. Two readings were taken at each point.

1

2

43

1.2215.793.356.564

1.2714.23.486.014

1.5532.089.385.243

1.7438.111.134.363

1.5124.7510.074.932

1.8328.088.885.072

2.4815.0512.274.741

1.9131.148.573.031

NdYAlOID

NdYAlOID

4.3624.1910.974.354

4.7119.739.354.514

5.8422.6716.0325.383

5.5322.3815.8826.023

2.5732.3117.4611.612

2.7432.9517.5411.232

3.1534.9118.6514.381

3.7733.4717.7912.461

Incr

easi

ng E

nerg

y

Ground State

Ground State

Incr

easi

ng E

nerg

y

Photon

Popu

latio

nIn

vers

ion

Incr

easi

ng E

nerg

y

Ground State

Heat

Pumping

• Another electron relaxes to GS and releases a photon that is in phase with the first

• Each relaxing electron adds to the intensity of the coherent waveform until it is powerful enough to leave the system

• Process starts over from GS

•Dislocations can be identified by etch pitting•Samples are diamond polished and etched in an H3PO4 acid bath at 250 oC•Dislocations have higher energy than the surrounding crystal lattice so they etch preferentially in the acid, leaving a pit on the surface•Pits have different morphologies depending on the plane being observed•Pits on the (1 1 1) plane have a triangular shape while pits on the (1 1 0) plane have an elliptical shape [1]•Pit clusters appear around voids or inclusions while lines of pits tend to indicate the presence of dislocations sources

Background on Solid State Lasers

Growth of Single Crystals

Dislocations

Focus of Study

Etch Pitting

Polycrystalline Nd-YAG Analysis

EDS Point Analysis

Conclusions

References

Acknowledgements

0.5 cm

3 cm

An Analysis of Defects in Single Crystal Yttrium Aluminum Garnet Laser CrystalsResearch Experience for Undergraduates – Washington State University

M. A. Anderson M. G. Norton