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Nuclear Instruments and Methods in Physics Research B 266 (2008) 1897–1902

NIMBBeam Interactions

with Materials & Atoms

Nitrogen sublattice analysis in GaN by non-RutherfordHe-ion scattering

A. Turos a,b,*, R. Ratajczak b, K. Pagowska b, A. Stonert b, L. Nowicki b, P. Caban a

a Institute of Electronic Materials Technology, 01-919 Warsaw, ul. Wolczynska 133, Polandb Soltan Institute for Nuclear Studies, 05-400 Swierk/Otwock, Poland

Received 21 September 2007; received in revised form 18 January 2008Available online 26 January 2008

Abstract

Non-Rutherford 14N(4He,4He)14N scattering using the cross-section resonance at 3.7 MeV has been applied for GaN templates anal-ysis. Although this resonance provides the cross-section enhancement by a factor of 9 only the N-scattering yield is high enough to beeasily detected for GaN layers of thickness below 4 lm grown on sapphire. In such a case, the N signal appears on the low background ofthe Al-portion of the substrate spectrum.

The characteristic parameters of the method and its limitations are discussed. Special attention was paid to the analysis of ingrowndefects located at the GaN/sapphire interface. Channeling critical angles and mean values of atom thermal vibrations for both sublatticeshave been estimated.� 2008 Elsevier B.V. All rights reserved.

PACS: 61.82.Fk; 61.85.+p; 68.55.Lk; 68.35.Dv

Keywords: GaN templates; Non-Rutherford backscattering; Ion channeling; N-sublattice analysis

1. Introduction

Progress in epitaxial growth techniques of GaN and itsalloys made them possible to serve as the basis of new gen-eration of opto- and microelectronic devices. Since GaNsingle crystals are hardly available the most commonlyused substrates for GaN epitaxy are sapphire (Al2O3) andSiC. Because of the great lattice mismatch, the structureof grown layers is far from being perfect. As grown defectsare typically misfit dislocations originated at the interfacewith the density decreasing with the layer thickness. Devicestructures (transistors and lasers) are typicaly grown onseveral lm thick GaN epilayers called templates.

0168-583X/$ - see front matter � 2008 Elsevier B.V. All rights reserved.

doi:10.1016/j.nimb.2008.01.034

* Corresponding author. Address: Soltan Institute of Nuclear Studies,05-400 Swierk/Otwock, Poland.

E-mail address: Andrzej.Turos@itme.edu.pl (A. Turos).

For improving the functionality of GaN-based devices,templates are still to be optimized primarily regarding thereduction of dislocation density and grain boundaries.Consequently, strong demand persists for further develop-ment of analytical techniques. Ion channeling has beenapplied for the purpose since more than two decades [1,2]primarily for the analysis of ion implantation produceddefects [3]. However, because of the large mass differencebetween the constituents of alloy only the Ga sublatticecould be analyzed. One of the possible solutions is theapplication of non-Rutherford scattering for N detectionas demonstrated by Jiang et al. [4]. As shown in the insetin Fig. 1, the 14N(4He,4He)14N elastic scattering cross-sec-tion exhibits several resonances at energies above3.5 MeV [5]. In this paper we report on the use of thenon-Rutherford scattering using the cross-section reso-nance at 3.70 MeV. In general, the resonance cross-sectionenhancement of factor of 9 over the Rutherford one is too

Energy (keV)

500 1000 1500 2000 2500 3000

Co

un

ts

0

5000

10000

15000

20000

aligned randomMC simulation for aligned SIMNRA simulation for random

Ga

N resonanceat 3.70 MeV

Al

O resonanceat 3.36 MeV

14N(α,α)14Nθ=165O

Energy (keV)

3200 3400 3600 3800 4000 4200 4400σ/

σ R

0

2

4

6

8

10

12

Fig. 1. Random and [0001] aligned EBS spectra for 1160 nm thick GaN film measured at 3.77 MeV incident 4He-ion energy. Portion of aligned spectrumdue to the scattering by Ga atoms reveals the damage peak due misfit defects at the GaN/sapphire interface. Excitation curve for 14N(4He,4He)14N elasticscattering is shown in the inset.

1898 A. Turos et al. / Nucl. Instr. and Meth. in Phys. Res. B 266 (2008) 1897–1902

small for N-scattering yield to be effectively analyzed whenappearing on the Ga-scattering background. The situationchanges favorably for epitaxial layers of thickness below4 lm grown on sapphire substrate. In such a case, the Nsignal appears on the much lower background of Al.

Since such spectra contain portions which are producedby non-Rutherford scattering (N- and O-parts) and theremaining part that is the conventional RBS spectrum, fol-lowing terminology of [6], they are called Elastic Backscat-tering (EBS) spectra.

2. Experimental

Samples were prepared using MOVPE technique in theAixtron AX100HTRD reactor at Institute of ElectronicMaterials Technology, Warsaw. The films were depositedonto 2-in. diameter (0001) sapphire wafers. A standardtwo-step deposition process was employed [7]. The sub-strate was first heated before the growth in H2 ambientat high temperature then after cooling down to the temper-ature of 500 �C, AlN nucleation layer of approximately20 nm thickness was deposited. Subsequently, the waferwas heated to 1050 �C for the deposition of 300–8000 nmthick GaN films. Trimethylgalium TMGa and ammonia

NH3 were used as precursors with hydrogen as the carriergas. The produced films were Ga-polar.

Virgin and annealed samples were analyzed with RBS/channeling using 2.0–4.2 MeV 4He-ions from the Tande-tron accelerator at the Institute for Ion Beam Physicsand Materials Research Forschungszentrum Dresden, Ger-many. To reduce analyzing beam effects, an annular detec-tor covering the detection solid angle of 35 mstrd was used.The mean detection angle was 174� and the detector reso-lution was 20 keV. The modified Monte Carlo simulationcode McChasy was applied for spectra analysis [8,9].

Complementary structural analysis was performed bythe use of transmission electron microscopy (TEM) usingJeol JEM 3010 electron microscope operating at 300 keV.Cross-section TEM specimens were prepared by a standardmethod of mechanical pre-thinning followed by Ar ionmilling.

3. Results and discussion

3.1. EBS spectra

Fig. 1 shows random and aligned EBS spectra for1160 nm thick GaN film measured at 3.77 MeV incident

A. Turos et al. / Nucl. Instr. and Meth. in Phys. Res. B 266 (2008) 1897–1902 1899

4He-ion energy. The solid lines delineate the simulationperformed with McChasy code. In the random spectrum,well distinguished peak at high energies is due to the scat-tering by Ga atoms. The resonant scattering on N atomsproduces the peak around 1000 keV, it covers the depthinterval 50–200 nm. The peak around 550 keV is due the16O(4He,4He)16O resonance at 3.36 MeV in the sapphiresubstrate [10]. The overlap of N and O signals is the factorlimiting the probing depth for N-profiling. It takes place

Ene

2200 2400

Co

un

ts

0

200

400

600

800

1000

α=0.55

α=0

Fig. 2. Adjustment of channeled ions stopping power for correct location of mvalues of a using the McChasy code.

Ene

3700 3750 3800

Co

un

ts

0

2000

4000

6000

8000

ΔEs=64ke(90nm)

900nm

E0=4.00 MeV

Fig. 3. Energy distributions of channeled ions at diff

for the incident energy of 4.2 MeV, for which the N-reso-nance appears at the depth of 1200 nm. One notes thatquite a good fit of the random spectrum can be obtainedusing the SIMNRA code [11].

Channeling spectra along [0001] axis measured for vir-gin samples revealed a very low channeling minimum yield,vmin 6 2%. The large peak in aligned spectrum around2400 keV is due to defect agglomeration, mostly misfit dis-locations, in the vicinity of interface produced by the large

GaN/Al2O3

rgy (keV)

2600 2800 3000

experiment

MC simulations

for different α

α=1

isfit defects peak. Ga-portion of aligned spectra was calculated for different

rgy (keV)

3850 3900 3950

ΔEs=25keV(36nm)

V

ΔEs=44keV(62nm)

600nm

300nm

erent depths. Incident ion energy was 4.0 MeV.

1900 A. Turos et al. / Nucl. Instr. and Meth. in Phys. Res. B 266 (2008) 1897–1902

lattice mismatch between GaN layer and sapphire sub-strate. Because of the very low vmin, the N-signal cannotbe distinguished from the Al-portion of the spectrum.One notes that because of the structural mismatch betweenGaN and sapphire substrate vmin for Al is appreciablyhigher than that for Ga or N. The N-detection limit inchanneling direction is limited by the height of the Al-spec-trum and has been estimated to be of the order of 3 at %.

3.2. Defect depth distribution

Assuming that the defects are randomly displaced Gaand N atoms, the defect profile can be obtained by fitting

Ang

-2 -1

No

rmal

ized

yie

ld

0.0

0.2

0.4

0.6

Ang

-2 -1

No

rmal

ized

yie

ld

0.0

0.2

0.4

0.6

Fig. 4. Axial dips for (0001) axis in (1120) plane for Ga- and N-sublattice.covered by the N-resonance.

the Ga-portion of aligned spectrum using the McChasycode. The quality of such a fit can be seen in Fig. 1. Calcu-lations have been performed assuming that defects are ran-domly displaced lattice atoms in equally in both sublattices.This assumption is justified by the TEM observationsrevealing a irregular tangle of dislocations, loops, stackingfaults and other defects in the vicinity of GaN/sapphireinterface. The best fit to experimental spectrum has beenobtained for defect distribution consisting of a rectangularpeak containing 25 at % of displaced atoms covering thedepth interval 1020–1160 nm. It is worth pointing out that,according to TEM analysis, threading dislocations origi-nated in this region are predominantly of screw or mixed

le (deg)

0 1 2

Ga

le (deg)

0 1 2

simulationexperiment

N

The incident energy was 3.77 MeV. The depth interval of 50–150 nm was

A. Turos et al. / Nucl. Instr. and Meth. in Phys. Res. B 266 (2008) 1897–1902 1901

type with the Burgers vector along the (00 01) direction.Such defects do not obstruct ion motion and are practically‘‘invisible” for channeling along the (000 1) axis.

3.3. Stopping power of channeled ions

Presence of defect group at a well-defined depth pro-vides a very useful marker for the adjustment of channeledions stopping power, Sch. In the McChasy code, it isdefined as

Sch ¼ aSl þ ð1� aÞSr ð1Þwhere Sr is the random stopping power, Sl is the local stop-ping power (cf. [9] for calculation details), a is a free param-eter (0 6 a 6 1).

Fig. 2 shows the calculated aligned spectra for differentvalues of a. The correct location of misfit damage peak hasbeen obtained for a = 0.55.

3.4. Depth resolution of N-sublattice analysis

In the case of resonant scattering, the depth resolution,Dx, is limited by the width of a resonance C = 35 keV,beam straggling at the given depth on in- and out-wardpaths, X1 and X2, respectively, and detector resolution,DEd, and is given by

Dx ¼ ½k2ðC2 þ X21 þ X2

2 þ DE2d�

1=2=½S� ð2Þ

where k is kinematical factor and [S] is the effective stop-ping power.

For channeled ions, the variety of possible trajectoriesleads to different energy loss at a given depth, thus produc-ing much larger energy straggling X1 than that for the ran-dom path. Fig. 3 shows calculated energy distributions atdifferent depths of channeled 4He-ions incident a GaNcrystal with 4 MeV. Resulting depth resolution amountsat the depth of 300–46 nm for Ga and 40 nm for N andat the depth of 900–88 nm for Ga and 54 nm for N.

3.5. Thermal vibration amplitudes of lattice atoms

Measurement of angular scans in the well-defined crys-tallographic direction allows not only determination ofcritical angles for channeling but also estimation of theaverage thermal vibration amplitudes for both the alloycomponents separately. Axial dips for [00 01] axis in(0211) plane for Ga- and N-sublattice are shown inFig. 4. The incident energy was 3.77 MeV. The depth inter-val of 50–200 nm was covered by the N-resonance. Thewidth of Ga dip at half depth is 1.1� whereas that for Nis 0.95�. Monte Carlo simulation of angular dips is verysensitive to the amplitudes of lattice atoms thermal vibra-tions, aT. The best fit has been obtained for aN

T = 6.15 pmand aG

T a = 5.70 pm for N- and G-sublattice, respectively.There are at least two reports in the literature concerningthis issue. The channeling experiment and Monte Carlosimulations of Seppala et al. [12] provided the same aG

T a

value of 5.7 pm, whereas Obata et al.[13] using XRDobserved much larger values of 7.4 pm for Ga-sublatticeand 8.1 pm for N-sublattice.

4. Conclusions

Our results have demonstrated that non-Rutherfordscattering of 4He-ions on nitrogen can be successfullyapplied for the analysis of GaN epitaxial layers. In thestudied case of GaN on sapphire, the cross-sectionenhancement of a factor of 9 over the Rutherford one issufficient for precise analysis of the N-sublattice. Usingthe appropriate simulation codes like SIMNRA or McCh-asy, the experimental spectra can be accurately reproduced.Useful incident beam energy range of 3.7–4.0 MeV is acces-sible for small tandems and even for single-end 2 MeV Vande Graaff accelerators, provided the double charged He-ionbeam can be obtained from the ion source.

The major problem of EBS analysis is to have accuratescattering cross-sections for light elements. Majority ofdata was obtained in the beginning of 1990s and somerefinements, both experimental and theoretical, are desir-able [14–16].

Besides of the possibility of the N-sublattice analysis,He-ion EBS at energies around 4 MeV can be applied forthick epilayer analysis with an accuracy and sensitivitynot significantly inferior than those for conventional RBSanalysis at 2 MeV.

Acknowledgments

We thank J. Borysiuk for TEM analysis. This work wassupported by the Polish Ministry of Science and HigherEducation, Grant No. N 51502931/1104.

RBS/channeling analysis has been carried out in theframework of the EU Program: Access to Large ScaleFacilities AIM at the Institute for Ion Beam Physics andMaterials Research (IIM), Forschungszentrum Dresden(Germany). Helpful assistance of D. Grambole is warmlyacknowledged.

References

[1] A. Turos, L. Nowicki, A. Stonert, M. Leszczynski, I. Grzegory, S.Porowski, Mater. Sci.Forum. 248–249 (1997) 419.

[2] B. Hollander, S. Mantl, M. Mayer, C. Kirchner, A. Pelzman, M.Kamp, S. Christiansen, M. Albrecht, H.P. Strunk, Nucl. Instr. andMeth. B 136–138 (1998) 1248.

[3] S.O. Kucheyev, J.S. Williams, S.J. Pearton, Mater. Sci. Eng. 33 (2001)51.

[4] W. Jiang, W.J. Weber, S. Thevuthasan, V. Shutthanandan, Nucl.Instr. and Meth. B 191 (2002) 509.

[5] Y. Feng, Z. Zhou, G. Zhou, F. Yang, Nucl. Instr. and Meth. B 94(1994) 11.

[6] A.F. Gurbich, N. Barradas, C. Jeynes, E. Wendler, Nucl. Instr. andMeth. B 190 (2002) 237.

[7] S.C. Jain, M. Willander, J. Narayan, R. Van Overstraeten, J. Appl.Phys. 87 (2000) 965.

[8] A. Dygo, A. Turos, Phys. Rev. B40 (1989) 7704.

1902 A. Turos et al. / Nucl. Instr. and Meth. in Phys. Res. B 266 (2008) 1897–1902

[9] L. Nowicki, A. Turos, R. Ratajczak, A. Stonert, F. Garrido, Nucl.Instr. and Meth. B 240 (2005) 277.

[10] J.A. Leavitt Jr., L.C. McIntyre, M.D. Ashbaugh, J.G. Oder, Z. Lin,B. Dezfouly-Arjomandy, Nucl. Instr. and Meth. B 44 (1990) 260.

[11] M. Mayer, Nucl. Instr. and Meth. B 44 (1990) 260.[12] A. Seppala, E. Rauhala, R. Grotzschel, J. Phys. D: Appl. Phys. 34

(2001) 269.

[13] T. Obata, N. Matsumura, K. Ogiwara, H. Hirayama, Y. Aoyagi, K.Ishibashi, Phys. Status Solidi C 3 (2006) 1775.

[14] Z.S. Zheng, J.R. Liu, X.T. Cui, W.K. Chu, Nucl. Instr. and Meth. B118 (1996) 214.

[15] M. Bozoian, Nucl. Instr. and Meth. B 82 (1993) 602.[16] A.F. Gurbich, Nucl. Instr. and Meth. B 261 (2007)

401.