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Incident Beam Characterization: AFRL/WPAFB Panalytical MRD

http://www.flickr.com/photos/85210325@N04/9430767893/

This characterization allows us to quantify the effect of the INVARIANT Incident Beam Shape Factor and deconvolute its effect from the topographic XRD 2D data.

http://www.flickr.com/photos/85210325@N04/9430767893/

We will be displaying topographic maps of the Full Width at 12.5% (1/8) Maximum (in comparison with the traditional FWHM - Full Width at Half Maximum) to study the deviation from "theoretical profile" as a function of pixel spatial location:

http://www.flickr.com/photos/85210325@N04/9430820747/

We will eventually use the area under the region indicated as "deviation" (in yellow above) from theoretical profile and map this parameter as a function of spatial location on the XRD topograph.

http://www.flickr.com/photos/85210325@N04/9430820747/

Normalized Relative Intensity: One very reasonable assumption we could make is the that the penetration depth in GaAs would be uniform throughout the sampling surface of the wafer due to the high degree of incident beam monochromation and collimation. Hence, the diffracting volume will be constant. Therefore, the integrated intensity under each RCP (Rocking Curve Profile) for individual pixel must be conserved. We've used this simple assumption to normalize the relative intensities laterally across the topographic image.

Examples: http://www.flickr.com/photos/85210325@N04/9315391455/in/photostream/

YouTube Video of "raw data" and XRD Rocking Curve Analyses for GaAs (004) Symmetric Reflection: http://www.youtube.com/watch?v=KFQOkOFLIlg&list=PL7032E2DAF1F3941F

Most recently updated YouTube Video for GaAs (004) data August-17-2013:

Version 1. With music: http://www.youtube.com/watch?v=9SHH2GJ8IYE

Version 2. Without music: http://www.youtube.com/watch?v=0Yz80o7lmPE

Version 2 is fully annotated and should have no Google Ads playing either.

Please review and critique when convenient. Need your advise regarding annotations/illustrations and/or voice-over to explain details both to XRD experts as well as laymen.

Thanks to all participants, in anticipation!

Questions: (may need rephrasing?)

A. What is the Burger's Vector for the GaAs (004) plane?

B. What are the active TWIN & STACKING FAULT directions (if any) in GaAs?

The reason I'm interested in this is due to the fact that there seem to be preferred orientation of Bragg FWHM & Bragg Peak Shift contours along the directions in the XRD Rocking Curve Micrographs. In our procedure of "optimizing" we are aligning a direction to be parallel to the XRD "rocking axis", along the "vertical" direction in the diffraction image.

Thanks for your help!

Interesting reading! Electrical Properties of Dislocations in Semiconductors, R. Labusch and W. Schroter. From Dislocations in Solids, Editor F. R. N. Nabarro, Volume 5, 1980.

pp. 162.

"8. Dislocations in III-V compounds

In III-V compounds the two face centered sublattices of the diamond structure are occupied by different kinds of atoms. The (111) slip planes are alternately occupied by atoms of group III and atoms of group V. The core of an edge type dislocation therefore may consist of atoms of group III or group V, depending on the sign of the dislocation. As first pointed out by Hansen [87], the dynamical and electrical properties of the two types should be different on account of their different cores.

This qualitative statement is fully confirmed by velocity measurements of single dislocation half loops in InSb [39, 88-90] and GaAs [90]. So far, the electrical properties of dislocations have been studied in detail only for InSb [6, 91-97].................."

In my opinion, somewhere herein lies a solution to the original query! Help!

pp. 183.

"Core configuration of dislocations in semiconductors

The diamond structure has a cubic face-centered space lattice with a basis of two atoms. The stacking of {111}-planes in Si, Ge and many III-V compounds is that of the face-centered cubic structure, except that in the diamond structure each layer consists of two atomic planes. The spacings between adjacent {111}-planes are alternately narrow and wide in the ratio 1:3. Consequently there are two sets of glide planes and two sets of dislocations. If the glide occurs between two widely spaced planes, the dislocations belong to the shuffle set, if between two narrowly spaced planes, it belongs to the glide set. Complete dislocations in the shuffle set are believed to be stable, while those of the glide set are unstable with respect to a dissociation into two Shockley partials with a stacking fault between them; the reaction is of the usual type:

1/2a[110]----->1/6a[211] + 1/6a[121]............................"

BTW come join us and learn: Friday August 30th, 2013 10AM EST! TAKE ADVANTAGE! "The 3rd Dimension & Beyond in XRD 2013" by Weiguang Guan of McMaster University! http://www.chemistry.mcmaster.ca/facilities/xray/221-max3d

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The Bragg XRD Rocking Curve Profile (RCP) is a measure of the coherence lengths for a specific VOXEL (76x140x15.5um^3). So in this case the "excess dislocations" (deviation compared to theoretically expected values - "yellow shaded area" http://www.flickr.com/photos/85210325@N04/9430820747/) is predominantly on the lower angle side of the Bragg Peak (dominant coherence length) in the Voxel and below the FWHM for the individual pixel/voxel RCP. This would imply a larger coherence among the more distant contributing planes which in this case would be those containing As not Ga. (since As with an atomic number of 33 which is larger than Ga with atomic number 31). Therefore, more dislocations on the "glide set" (narrower spacing - missing Ga atoms), "unstable with respect to a dissociation into two Shockley partials with a stacking fault between them". Thus fitting the observed data. And allowing the inference that the "yellow area" can be used as a measure of these "stacking faults" caused by missing Ga atoms.

Additional reference: http://www.polyse.de/pdf/1990/dislocations.pdf

More excitement! Still needs theoretical rocking curve for symmetric SiC 6H (0006) with Bartel(220).

Check out this SiC 6H (0006) data from the exact same instrument and incident beam using a 3" SiC 6H production grade wafer:

http://www.youtube.com/watch?v=BceEgqdCyZA&list=PL7032E2DAF1F3941F

Skeptics! Open your minds! These are not instrument artifacts!

Notice the following characteristics:

a). Asymmetry (in RCP) is on the high angle side in comparison with the GaAs (004). The difference between the GaAs & SiC 6H wafer is the growth method, CZ versus CVD. I'm reviewing Nabarro & Guinier for possible answers. It is in there, I just have to figure it out. The metallurgy and defect characterization of these materials is fairly well studied & understood. Not new!

b). The contribution of the background signal to the integrated intensity in low SNR RCP's is significant.

c). Clear multiple peaks due to sub-grain/twining(?) observable in 0D RCP's.

d). SNR is significantly enhanced by using the 0D simulation approach even with 5x11 pixels in this case. Due to the geometry/optics as illustrated in this Figure 1 (http://www.flickr.com/photos/85210325@N04/9311681125/), there would be a 326% horizontal compression. I may need to re-word this later. See Figure 2 (http://www.flickr.com/photos/85210325@N04/9314468316/) and Figure 3 (http://www.flickr.com/photos/85210325@N04/9311681113/). Therefore, the 5x11 pixel 0D simulation would be 1.24mmX2.72mm. If we use a 6x10 pixel 0D simulation for SiC (0006), that would be approximately a 2mmX2mm 0D simulation. @ 76um/pixel.

Figure 4: SiC 6H (0006) 0D RCP's simulation 5x11 pixel sampling area on the topograph -

http://www.flickr.com/photos/85210325@N04/9713460973/

http://www.flickr.com/photos/85210325@N04/9713460973/

Figure 5: Further enhancement of the "normalization" method for relative intensity values in order to directly compare relative intensity values across the sample topographically: http://www.flickr.com/photos/85210325@N04/9864016255/

Quite contrary to GaAs the CVD grown SiC 6H wafer exhibits Bragg Profile Asymmetry on the high angle side of the Bragg Peak. Thus implying an abundance of stacking fault due to missing Si rows not C.

Reference: "Silicon Carbide: The Return of an Old Friend" Nicholas G. Wright and Alton B. Horsfall. Materials Matters Vol 4, No.2. [email protected]

http://www.flickr.com/photos/85210325@N04/9864016255/

http://www.sigmaaldrich.com/technical-documents/articles/material-matters/silicon-carbide-the.html

Reiterating, in the case of GaAs, Ga-31 & As-33, the larger distance planes would be those containing As and not Ga. Just the reverse. So, the appearance of asymmetry on the low angle side indicates an excess of stacking faults arising from missing Ga atoms rather than the larger As atoms. Hence, an excess of the larger d-spacing than the "Predominant" d-spacing (Bragg Condition). This will result in excess intensity in reciprocal space (diffraction space) at lower Bragg angles.

The Bragg XRD Rocking Curve Profile (RCP) is a measure of the coherence lengths for a specific VOXEL (76x140x15.5um^3). So in this case the "excess dislocations" (deviation compared to theoretically expected values - "yellow shaded area"): http://www.flickr.com/photos/85210325@N04/9430820747/

OK! The veritable "Pandora's Box! More SiC 6H (0006) Symmetric Reflection!

I'll discuss this image in greater depth, next. Here we go for SiC 6H (0006) symmetric reflection! Many modifications to the postulations regarding asymmetry in the Bragg RCP observed using Real Time Bragg XRD Microscopy:

http://www.flickr.com/photos/85210325@N04/10008196446/

Observation 1: The GaAs Sample seems to be much more consistent in its asymmetry in that it is predominantly on the lower Omega side of the individual RCP. http://www.flickr.com/photos/85210325@N04/9315391455/in/photostream/ This is more likely due to the "nearer equilibrium" CZ growth of GaAs in comparison with CVD for SiC 6H. Observing the topographic map of individual VOXEL's reciprocal space volume (Peak Position, FWHM & Normalized Peak Intensity) and the Omega-Y as well as Omega-

X maps we may conclude the following: The grown-in Nano structure of the GaAs wafer predominantly consists of excess defects arising from missing Ga atoms rather than the larger As atoms.

Observation 2: The CVD grown SiC 6H wafer is a different specimen altogether. The reciprocal space in this case has a much higher volume than the GaAs sample. In addition, the symmetry of the RCP was strongly a function of the spatial location of the VOXEL examined (mapped). The RCP shapes ranged from:

a. Skew to the lower Omega angle. (Pixel locations "B")

b. Skew towards the higher omega angle. (Pixel locations "A & B")

c. "Saw tooth" profile indicating potential "micro-twinning" or such other strain perturbation within the subject VOXEL. (Pixel location "C")

d. "Flat top"! (Pixel locations "A") ???

Observation 3: SNR is significantly enhanced with spatial Pixel Integration while normalization w.r.t. the total integrated intensity below the Bragg Profile (RCP) for individual Pixel allows us to compare relative intensity besides Bragg Peak Position & Bragg FWHM on the Micrograph effectively.

The correct theoretical profile for SiC 6H (0006) with a Hybrid Bartel (220) beam conditioner would be the key to the final solution!

http://www.flickr.com/photos/85210325@N04/10008196446/

Updated with dimensions and crystallographic orientation:

http://www.flickr.com/photos/85210325@N04/10019182954/

http://www.flickr.com/photos/85210325@N04/10019182954/

Additional asymmetry and profile shapes of RCP's for SiC 6H (0006): http://www.flickr.com/photos/85210325@N04/10025917043/

This adds a whole new dimension to the SiC 6H wafer sample. Due to the high entropy state CVD growth process the Nano structure is understandably less homogenous than the CZ grown GaAs wafer. The presence of distinctly different Bragg RCP for each VOXEL is testament to the dynamic CVD growth process. It is fascinating to observe the "as-grown" Nano structure in the wafer sample non-destructively, without the need for vacuum or clean room, up to depths ranging from 1-10's of Micrometers. Such Nano structural data would normally require tedious destructive specimen preparation and subsequent careful TEM examination to acquire. A correlation of data from XRD Microscopy and EPD/TEM/SEM/PL/RHEED/RBS/etc. is quintessential in order to instill the confidence in the scientific community for using this real time XRD method routinely.

http://www.flickr.com/photos/85210325@N04/10025917043/

Can't wait to show you the GaSb data! Get excited!

GaSb (004) symmetric reflection:

http://www.flickr.com/photos/85210325@N04/10318285463/

Still missing is the theoretical RCP for SiC 6H & GaSb using the Leptos Bruker Software. If someone can help compute that, it would be awesome to compare with the experimental data. The GaAs RCP is a pretty good fit with the experimental data for GaSb. The difference is in the "deviation" from ideal theoretical RCP. More analyses to come. Amazing potential! NDE!

http://www.flickr.com/photos/85210325@N04/10318285463/

The asymmetry beyond the expected theoretical asymmetry could be used to examine greater Nano structural details for each real space sample VOXEL. The fit of the experimental data to the theoretical shape is exceptional for GaAs & GaSb. Further TEM work will help those that are faint of faith and expertise in XRD. The results are unequivocal regarding the ability to monitor materials Nano structure in real time & in situ using 2D Bragg XRD Microscopy.

Some definitions to facilitate shape analyses of the RCP (Rocking Curve Profile) or RSP (Reciprocal Space Profile). See attached image:

Still trying to figure out Ga or Sb missing due to defect and its consequence on the "shoulder" shape. Help would be welcome. Jump in!

Real Time YouTube Video for GaSb (004) reflection & more when you are motivated:

https://www.youtube.com/watch?v=KJA5cLNi8Gk&list=PL7032E2DAF1F3941F

http://www.flickr.com/photos/85210325@N04/10318285463/

http://www.flickr.com/photos/85210325@N04/10329683013/

http://www.flickr.com/photos/85210325@N04/10329683013/

Super Conductors! LSAO LaSrAlO4 - Substrate (004) Symmetric Reflection - BNL Condensed Matter Group, Panalytical X'Pert, Bartel (220) Soller Slit (Hybrid):

http://www.flickr.com/photos/85210325@N04/10411736916/

LSAO (004) RCP compared with GaAs (004) theoretical RCP generated by Bruker LEPTOS. Reasonable fit for shape. It would be very helpful if anyone with access to the Bruker LEPTOS software could help create the theoretical RCP for LaSrAlO4. Thanks!

http://www.flickr.com/photos/85210325@N04/10411736916/

Summary:

1. GaAs (004) RCP, Theory=Experiment+-Small Asymmetric Deviation (one sided of Bragg Peak below FWHM):

http://www.flickr.com/photos/85210325@N04/9430820747/in/photostream/

2. GaSb (004) RCP Shape=GaAs Theoretical+-Small Deviations (on both sides of the Bragg Peak below FWHM):

http://www.flickr.com/photos/85210325@N04/10329683013/

Updated Image 11-2-13 with additional scale data:

http://www.flickr.com/photos/85210325@N04/10633782105/

3. SiC 6H (0006) indicates large variability in RCP shape based on location on the topograph:

http://www.flickr.com/photos/85210325@N04/10019182954/

4. LSAO (LaSrAlO4) (004) RCP shape is nearly identical with GaAs theoretical RCP with small deviations:

http://www.flickr.com/photos/85210325@N04/10411736916/

http://www.flickr.com/photos/85210325@N04/10221065324/

did you ever tried to analyse a polar plane like {111}? The used 004 for Gaxx is not polar so that I wouldn't expect any anisotropy coming from the material. However, i cannot say that I am the absolute expert. I was analysing III-V semiconductors with Kossel diffraction and there asymmetric effects are the result of the polarity. SiC 6H is polar as well (at least the 0006) but LSAO isn't, right? Twins - at least caused by m - contain an inversion operation so the possible weak polar effect should reduced but not increased. You see, I am more speculating....:-(

Great to see you here Gert! I'm not yet the "expert" I need to be. Just having fun getting there.

How about the possibility of excess missing rows (due to stacking faults, shuffle set, glide set) of one or the other constituent? Will that cause "polar" behavior?

I need to confirm that we can quantitatively measure (deconvolute) minute changes in the Nano structure of any VOXEL by determining the integrated area under the "deviation" from theoretical values on either side of the Bragg Peak. Is this my clue below? Help!

Electrical Properties of Dislocations in Semiconductors, R. Labusch and W. Schroter. From Dislocations in Solids, Editor F. R. N. Nabarro, Volume 5, 1980.

pp. 162.

"8. Dislocations in III-V compounds:

In III-V compounds the two face centered sublattices of the diamond structure are occupied by different kinds of atoms. The (111) slip planes are alternately occupied by atoms of group III and atoms of group V. The core of an edge type dislocation therefore may consist of atoms of group III or group V, depending on the sign of the dislocation. As first pointed out by Hansen [87], the dynamical and electrical properties of the two types should be different on account of their different cores.

This qualitative statement is fully confirmed by velocity measurements of single dislocation half loops in InSb [39, 88-90] and GaAs [90]. So far, the electrical properties of dislocations have been studied in detail only for InSb [6, 91-97].................."

pp. 183.

"Core configuration of dislocations in semiconductors:

The diamond structure has a cubic face-centered space lattice with a basis of two atoms. The stacking of {111}-planes in Si, Ge and many III-V compounds is that of the face-centered cubic structure, except that in the diamond structure each layer consists of two atomic planes. The spacings between adjacent {111}-planes are alternately narrow and wide in the ratio 1:3. Consequently there are two sets of glide planes and two sets of dislocations. If the glide occurs between two widely spaced planes, the dislocations belong to the shuffle set, if between two narrowly spaced planes, it belongs to the glide set. Complete dislocations in the shuffle set are believed to be stable, while those of the glide set are unstable with respect to a dissociation into two Shockley partials with a stacking fault between them; the reaction is of the usual type:

1/2a[110]----->1/6a[211] + 1/6a[121]............................"

http://www.youtube.com/watch?v=9SHH2GJ8IYE&list=PL7032E2DAF1F3941F

Gert! What does "Polar" mean? Why is it polar?

So, I'm not sure of the answer to your question. I'll dig up some of the literature about the LSAO & LSCO that I was reviewing earlier. I'll post it. Maybe we could figure this out?

I've tried the (333) symmetric reflection for a (111) oriented Si sample. Here's the data analysis including the raw data:

http://www.youtube.com/watch?v=0tO7onKXXzk&list=PL7032E2DAF1F3941F

GaAs (004) & Si (333) compared: http://www.flickr.com/photos/85210325@N04/7809789732/in/set-72157632728949610

http://www.flickr.com/photos/85210325@N04/10242907544/

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A common cause of asymmetrical Bragg peaks is the axial divergence of the beam. The is most noticable at low Bragg angles. Soler slits can help, but often are not enough and a collamating optics in needed. You've probably already determined if this is a problem from peak profile standards, but thought I'd mention it for others who see asymmetry in low angle peaks. It can be modeled in most Reitveld refinement programs, but of course it is better to lower the axial divergence so you don't miss real sample effects which would also cause this.

Larry! "A common cause of asymmetric Bragg peaks is the axial divergence of the beam".

Is there any correlation with any specific direction to the axial divergence?

BTW the beam conditioning system in this case included a soller slit and a hybrid Goebel(?) mirror. 

Is there a clever way to identify the cause as material Nano structure or beam optics?

The fact that this RCP shape depends on individual sample irrespective of instrument, strengthens the notion that it is Nano structure related. We made some interesting "divergence" measurements with the NIST 2000 SRM (113)- reflection. See below:

https://www.researchgate.net/post/What_is_the_best_way_to_interpret_REAL_TIME_3D_X_Y_o-2TH_o_ph_ch_reciprocal_space_relative_intensity_data_from_HRXRD_rocking_curve_analyses

https://www.flickr.com/photos/85210325@N04/17238558731/in/datetaken/

https://www.youtube.com/watch?v=lwVPe25wGJI&t=1619s

please anyone can explain me about Asymmetric and Symmetric X-Ray difraction?