X-RAY PHOTOELECTRON DIFFRACTION MEASUREMENTS OF HEXAGONALGaN(0001) THIN FILMS

R. DENECKE 2 , J. MORAIS', C. WETZEL', J. LIESEGANG*, E. E. HALLER"3,C. S. FADLEY1.2'Materials Sciences Division, Lawrence Berkeley National Laboratory,Perkeley, CA 94720, USA3Department of Physics, University of California, Davis, Davis, CA 95616, USADepartment of Materials Science, University of California, Berkeley, Berkeley, CA 94720 USA

Lawrence Berkeley National Laboratory, Materials Science Division, Berkeley, CA 94720, USA

ABSTRACT

We report on the first scanned-angle x-ray photoelectron diffraction measurements onGaN(0001) in the wurtzite structure, as grown on sapphire substrates using MOCVD. These as-grown samples reveal forward scattering peaks in agreement with a theoretical calculation using asingle scattering cluster calculation. The surface contamination by 0 and C does not exhibit anyclear structure. From the combination of experiment and theoretical calculation and from a simpleintensity ratio argument the surface termination for these samples could be determined to be N.The data also indicate that C is on average closer to the GaN surface than 0.

INTRODUCTION

GaN is a promising material for the fabrication of blue light-emitting diodes (LEDs) andlasers due to its large and direct bandgap. The electronic properties depend, however, strongly onthe geometric structure and the quality of the samples. There exist two different phases of GaN: ahexagonal wurtzite structure which is the stable structure (called ct-GaN), and a zinc-blendestructure which can only be achieved by epitaxial growth (the O3-GaN phase). Since the normallyused technique of X-ray diffraction (XRD) is more a bulk probe and is furthermore not element-specific, it can only determine the overall structure of a given epitaxial sample, and is not able todetermine the actual positions of the atoms with respect to the surface. Therefore the use ofelement-specific x-ray photoelectron diffraction (XPD) promises to give a more detailed view ofthe near-surface structure, including the nature of the surface termination, which can be via Ga orN, or some mixture of these two growth orientations. This is especially important since surfacemorphology investigations have revealed a columnar growth for a wide range of growth condi-tions [1]. So an additional question is whether or not contaminants like 0 or C are preferentiallyincorporated in the films in the interstitial regions between the columns.

EXPERIMENT

The samples we used have been grown using metal-organic chemical-vapor deposition(MOCVD). The substrate for the wurtzite structure films was the c-plane (0001) of sapphire, onwhich layers of about 2-3 Vim thickness have been grown without a buffer layer. With XRD theoverall quality of the layers has been checked and the presence of a (0001) oriented ct-GaN couldbe confirmed [2]. Looking at the sample surface with a light microscope showed the aforemen-tioned hexagonal columns.

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Mat. Res. Soc. Symp. Proc. Vol. 468 0 1997 Materials Research Society

The photoemission measurements have been performed using a standard X-ray source (AlK(,, hv =1486.6 eV, Mg Ka,, hv =1253.6 eV ) and a VG ESCALAB electron analyzer, which has

been modified so as to permit automated XPD measurements [3]. This system is equipped with atwo-axis goniometer, enabling us to rotate the sample on two perpendicular axes so as to coveressentially the full 21r solid angle of emission directions above the surface. The sample for thedata shown here was as-grown, meaning that it was not treated in the UHV chamber. Prior tointroducing it to the chamber the sample was cleaned using a standard chemical cleaningprocedure [4]. XPD measurements were then begun directly after a system bakeout and theattainment of a base pressure of-I x 10"10 Torr range. We have measured the Ga 3p and N Iscore levels at kinetic energies of 1382 eV and 856 eV, respectively, together with the 0 1 s and CI s levels at 722 eV and 1203 eV, respectively, to monitor the surface contamination. Thediffraction patterns have been measured with starting angle steps of 3' for both azimuthal (4) andpolar (0) angles. However, the azimuthal angle step was adjusted throughout the measurements toensure an equal sampling of solid angle. In order to cut the measuring time we used the expectedthree-fold symmetry of the GaN (0001) surface. Overall measurement times were several days,but the relative intensities of all component (Ga, N, 0, and C) were found to be stable from startto finish.

RESULTS

Fig. 1 shows full 27t photoelectron diffraction patterns for all four core levels measured.Shown is the intensity for each core level as a function of emission angle (4,0), as obtained via alinear background subtraction and an integration over the width of a given peak in an energydistribution curve. Furthermore, an isotropic function Io due to unscattered intensity has beensubtracted from each diffraction pattern to obtain the so-called X-function.

One clearly observes strong diffraction peaks for Ga 3p and N Is emission in a six-foldpattern. Most of these maxima result from scattering along high symmetry directions of the crystaland are mainly brought about by the highly forward peaked nature of the scattering factors at suchhigh kinetic energies. Therefore, these peaks are referred to as forward scattering maxima. Com-paring this with the 0 Is and C Is diffraction patterns shows immediately that C and 0 do nothave any ordered scatterers in between them and the detector, as they have basically featurelesspatterns with no forward scattering maxima. The weak three-fold symmetry in the C Is pattern isprobably artifactual, and arises from the three-fold symmetry operation used to obtain the fulldiffraction pattern. Just from a simple analysis of the angle of the forward scattering peaks onecan thus confirm the overall hexagonal structure of the GaN epilayer.

A more quantitative understanding of the structure can be obtained by comparing thesepatterns with single-scattering or multiple-scattering diffraction calculations. Using the unit-cellcrystal structure as reported in the literature and a cluster with five atomic layers and about 100atoms we performed preliminary single-scattering calculations based on a Rehr-Albers approach[5]. The results are shown in Fig. 2. We show the diffraction patterns for both Ga 3p and N Isemission and for a N terminated surface on the cluster.

Both patterns here are three-fold symmetric, which is the symmetry of the GaN(OOO1) clusteras seen in the surface-sensitive XPD experiment. As the experiment is six-fold symmetric, thissuggests the presence of two domain types rotated by 60' with respect to one another in differenthexagonal columns. With this in mind, the agreement between experiment and theory is fairlygood.

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Ga 3p emission N Is emission90 75 90 75

00

15 45 75 75 45 15 15 45 75 75 45 15

theta theta

0 1 s emission C i s emission90 7590 7phi 60

:30 3

1515

thet theta~*

~ a 0

15 45 75 75 45 15theta

75 75theta

Fig. 1: Experimental photoelectron diffraction patterns (x-functions) for the four corelevels Ga 3p, N Is, 0 Is and C Is, as excited with Al or Mg Ka radiation, respec-tively. Light colors correspond to high intensity, dark colors to low intensity. Data ob-tained over 120' in azimuth have been three-fold symmetrized to yield the full pattern.

There is also an ongoing discussion about whether Ga or N is the terminating layer of thissurface [6]. Although not shown here, a comparison of experiment with theory for the Ga-termi-nated surface yields much less agreement, strongly suggesting a N termination of the sampleunder study.

In order to further reveal the relative positions of the Ga and N, as well as the contaminant 0and C atoms, one can use a rather simple analysis of the diffraction patterns. Since the photo-electron sampling depth varies with polar takeoff angle, 0, due to the finite electron escapedepths, Ae, according to AesinO, plotting azimuthally-averaged intensity ratios of the differentcore levels as a function of polar angle is a way to get such information. Therefore we have

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Fig.2: Theoretical photoelectron diffraction patterns for a N-terminated GaN clusterusing a single scattering cluster calculation scheme. Shown are X-functions for Ga 3pand N Is emission at the same kinetic energies as in Fig. 1. Again light colors markhigh intensity and dark colors low intensity.

plotted in Fig. 3 the ratio of these azimuthally-averaged intensities for various combinations of themeasured core levels. First of all, the baseline of the ratio Ga 3p/N Is is very flat, in agreementwith the nearly uniform distribution of these atoms in the overall crystal. At first sight, it is a littlesurprising that there is no enhancement of the N Is relative intensity at low takeoff angles, in viewof our conclusion of N termination based on the XPD patterns. However, this can be explainedby the slightly bigger escape depths for the Ga 3p photoelectrons, which have a higher kineticenergy as compared to the N Is photoelectrons. So this data is not in contradiction to the abovefinding of N termination of the surface.

Considering now the contaminant peaks, we find that the 0 1s/Ga 3p (or 0 is/N Is) and CIs/Ga 3p (or C Is/N Is) ratios show a dramatic increase for low takeoff angles, indicating that 0and C are primarily surface contaminants. The lack of any increase in this ratio for near-normalemission also suggests that not much 0 or C is present in the interstitial regions between columns,although this might be a reasonable initial conjecture. Finally, the increase in the 0 1 s/C Is ratiofor low takeoff angles suggests that 0 is present in the outermost regions of the contaminant layerand thatC is on-average closer to the GaN surface (e.g. as adsorbed CO).

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Ga 3p emission N Is emission90 75 90 75

0 ,0

15 45 75 75 45 15 15 45 75 75 45 15

theta theta

'1 . I " I . a . . " . . Fig. 3: Ratios of azimuthal-

3.0 - Ga 3p / N is averaged intensities from the

0 Is / Ga 3p diffraction patterns of Fig. Iplotted versus polar angle.

2.5 I ............ O ls / C ls Shown are the ratios Ga3p/Nls2(solid line), 01 s/Ga3p (dotted

line), and 0 1 s/C l s (dashed line).

o 2.0 Ratios have not been correctedfor the different escape depthsof the photoelectrons and the

.•1.5 - different photoemission cross

/ D" sections.

- 1.0 'I *s

....... .. . o.

0.5 , ..:

0.0 , , p • , ' • i-90 -60 -30 0 30 60 90

Polar Angle (deg)

CONCLUSION

We have performed the first x-ray photoelectron diffraction (XPD) measurements on an as-grown wurtzite GaN(OOO 1) sample. We obtained diffraction patterns which show the expectedcrystal structure for the bulk material. A comparison of experiment with single scattering clustercalculations further shows the best overall agreement for a N-terminated surface. An analysis ofvarious azimuthally-averaged peak intensity ratios also permits concluding that both 0 and C arepresent as surface impurities, without being significantly incorporated into the interstitial regionsbetween hexagonal columns, and that C is on average closer to the surface than 0. These resultsillustrate the potential of XPD for a more detailed study of the different annealing and cleaningprocedures and their effect on the surface structure, and such work is now in progress.

ACKNOWLEDGMENTS

We would like to thank R. X. Ynzunza for helping with the experiments, S. Ruebush forusage of azimuthal-averaging and smoothing routine, and Y. Chen for the use of his program forcalculating diffraction patterns. Work has been supported by ONR (Contract N00014-94-1-0162),DOE, BES, Mat. Sci. Div. (Contract DE-AC03-76SF00098), CNPq (Brazil), and DFG(Germany).

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REFERENCES

* On leave from La Trobe University, Dept. of Physics, Bundoora 3083, Australia

I. T. Sasaki, J. Crystal Growth 129, 81 (1993)2. Th. Metzger, H. Angerer, 0. Ambacher, M. Stutzmann, E. Born, phys. stat. sol. (b) 193,

391 (1996)3. J. Osterwalder, M. Sagurton, P. J. Orders, C. S. Fadley, B. D. Hermsmeier, D. J.

Friedman, J. Electr. Spectr. Relat. Phenom. 48, 55 (1989); Y.J. Kim, Ph.D. thesis(University of Hawaii, 1995).

4. V. M. Bermudez, J. Appl. Phys. 80, 1190 (1996)5. J.J. Rehr and R. C. Albers, Phys. Rev. B 41, 2974 (1993).6. M. AsifKhan, J. N. Kuznia, D. T. Olson, R. Kaplan, J. Appl. Phys. 73, 3108 (1993)

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