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

Transcription

1 X-RAY PHOTOELECTRON DIFFRACTION MEASUREMENTS OF HEXAGONAL GaN(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, USA 3Department of Physics, University of California, Davis, Davis, CA 95616, USA Department of Materials Science, University of California, Berkeley, Berkeley, CA 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 on GaN(0001) in the wurtzite structure, as grown on sapphire substrates using MOCVD. These asgrown samples reveal forward scattering peaks in agreement with a theoretical calculation using a single scattering cluster calculation. The surface contamination by 0 and C does not exhibit any clear structure. From the combination of experiment and theoretical calculation and from a simple intensity 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) and lasers due to its large and direct bandgap. The electronic properties depend, however, strongly on the geometric structure and the quality of the samples. There exist two different phases of GaN: a hexagonal wurtzite structure which is the stable structure (called ct-gan), and a zinc-blende structure which can only be achieved by epitaxial growth (the O3-GaN phase). Since the normally used technique of X-ray diffraction (XRD) is more a bulk probe and is furthermore not elementspecific, it can only determine the overall structure of a given epitaxial sample, and is not able to determine the actual positions of the atoms with respect to the surface. Therefore the use of element-specific x-ray photoelectron diffraction (XPD) promises to give a more detailed view of the near-surface structure, including the nature of the surface termination, which can be via Ga or N, or some mixture of these two growth orientations. This is especially important since surface morphology investigations have revealed a columnar growth for a wide range of growth conditions [1]. So an additional question is whether or not contaminants like 0 or C are preferentially incorporated 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, on which layers of about 2-3 Vim thickness have been grown without a buffer layer. With XRD the overall quality of the layers has been checked and the presence of a (0001) oriented ct-gan could be confirmed [2]. Looking at the sample surface with a light microscope showed the aforementioned hexagonal columns. 263 Mat. Res. Soc. Symp. Proc. Vol Materials Research Society

2 The photoemission measurements have been performed using a standard X-ray source (Al K(,, hv = ev, Mg Ka,, hv = ev ) and a VG ESCALAB electron analyzer, which has been modified so as to permit automated XPD measurements [3]. This system is equipped with a two-axis goniometer, enabling us to rotate the sample on two perpendicular axes so as to cover essentially the full 21r solid angle of emission directions above the surface. The sample for the data shown here was as-grown, meaning that it was not treated in the UHV chamber. Prior to introducing it to the chamber the sample was cleaned using a standard chemical cleaning procedure [4]. XPD measurements were then begun directly after a system bakeout and the attainment of a base pressure of-i x 10"10 Torr range. We have measured the Ga 3p and N Is core levels at kinetic energies of 1382 ev and 856 ev, respectively, together with the 0 1 s and C I s levels at 722 ev and 1203 ev, respectively, to monitor the surface contamination. The diffraction patterns have been measured with starting angle steps of 3' for both azimuthal (4) and polar (0) angles. However, the azimuthal angle step was adjusted throughout the measurements to ensure an equal sampling of solid angle. In order to cut the measuring time we used the expected three-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 start to 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 a linear background subtraction and an integration over the width of a given peak in an energy distribution curve. Furthermore, an isotropic function Io due to unscattered intensity has been subtracted 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-fold pattern. Most of these maxima result from scattering along high symmetry directions of the crystal and are mainly brought about by the highly forward peaked nature of the scattering factors at such high kinetic energies. Therefore, these peaks are referred to as forward scattering maxima. Comparing this with the 0 Is and C Is diffraction patterns shows immediately that C and 0 do not have any ordered scatterers in between them and the detector, as they have basically featureless patterns with no forward scattering maxima. The weak three-fold symmetry in the C Is pattern is probably artifactual, and arises from the three-fold symmetry operation used to obtain the full diffraction pattern. Just from a simple analysis of the angle of the forward scattering peaks one can thus confirm the overall hexagonal structure of the GaN epilayer. A more quantitative understanding of the structure can be obtained by comparing these patterns with single-scattering or multiple-scattering diffraction calculations. Using the unit-cell crystal structure as reported in the literature and a cluster with five atomic layers and about 100 atoms 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 Is emission and for a N terminated surface on the cluster. Both patterns here are three-fold symmetric, which is the symmetry of the GaN(OOO1) cluster as seen in the surface-sensitive XPD experiment. As the experiment is six-fold symmetric, this suggests the presence of two domain types rotated by 60' with respect to one another in different hexagonal columns. With this in mind, the agreement between experiment and theory is fairly good. 264

3 Ga 3p emission N Is emission : s emission C i s emission phi 7 60 thet ~* ~ a Fig. 1: Experimental photoelectron diffraction patterns (x-functions) for the four core levels Ga 3p, N Is, 0 Is and C Is, as excited with Al or Mg Ka radiation, respectively. Light colors correspond to high intensity, dark colors to low intensity. Data obtained 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 this surface [6]. Although not shown here, a comparison of experiment with theory for the Ga-terminated surface yields much less agreement, strongly suggesting a N termination of the sample under study. In order to further reveal the relative positions of the Ga and N, as well as the contaminant 0 and C atoms, one can use a rather simple analysis of the diffraction patterns. Since the photoelectron sampling depth varies with polar takeoff angle, 0, due to the finite electron escape depths, Ae, according to AesinO, plotting azimuthally-averaged intensity ratios of the different core levels as a function of polar angle is a way to get such information. Therefore we have 265

4 Ga 3p emission N Is emission , Fig.2: Theoretical photoelectron diffraction patterns for a N-terminated GaN cluster using a single scattering cluster calculation scheme. Shown are X-functions for Ga 3p and N Is emission at the same kinetic energies as in Fig. 1. Again light colors mark high intensity and dark colors low intensity. plotted in Fig. 3 the ratio of these azimuthally-averaged intensities for various combinations of the measured core levels. First of all, the baseline of the ratio Ga 3p/N Is is very flat, in agreement with the nearly uniform distribution of these atoms in the overall crystal. At first sight, it is a little surprising that there is no enhancement of the N Is relative intensity at low takeoff angles, in view of our conclusion of N termination based on the XPD patterns. However, this can be explained by the slightly bigger escape depths for the Ga 3p photoelectrons, which have a higher kinetic energy as compared to the N Is photoelectrons. So this data is not in contradiction to the above finding 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 C Is/Ga 3p (or C Is/N Is) ratios show a dramatic increase for low takeoff angles, indicating that 0 and C are primarily surface contaminants. The lack of any increase in this ratio for near-normal emission 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 ratio for low takeoff angles suggests that 0 is present in the outermost regions of the contaminant layer and thatc is on-average closer to the GaN surface (e.g. as adsorbed CO). 266

5 '1. I " I. a.. ".. Fig. 3: Ratios of azimuthal Ga 3p / N is averaged intensities from the 0 Is / Ga 3p diffraction patterns of Fig. I plotted versus polar angle. 2.5 I... O ls / C ls Shown are the ratios Ga3p/Nls 2(solid line), 01 s/ga3p (dotted line), and 0 1 s/c l s (dashed line). o 2.0 Ratios have not been corrected for the different escape depths of the photoelectrons and the different photoemission cross / D" sections ' I *s o. 0.5,..: 0.0,, p, ' i Polar Angle (deg) CONCLUSION We have performed the first x-ray photoelectron diffraction (XPD) measurements on an asgrown wurtzite GaN(OOO 1) sample. We obtained diffraction patterns which show the expected crystal structure for the bulk material. A comparison of experiment with single scattering cluster calculations further shows the best overall agreement for a N-terminated surface. An analysis of various azimuthally-averaged peak intensity ratios also permits concluding that both 0 and C are present as surface impurities, without being significantly incorporated into the interstitial regions between hexagonal columns, and that C is on average closer to the surface than 0. These results illustrate the potential of XPD for a more detailed study of the different annealing and cleaning procedures 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 for usage of azimuthal-averaging and smoothing routine, and Y. Chen for the use of his program for calculating diffraction patterns. Work has been supported by ONR (Contract N ), DOE, BES, Mat. Sci. Div. (Contract DE-AC03-76SF00098), CNPq (Brazil), and DFG (Germany). 267

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