APPLIED PHYSICS LETTERS

VOLUME 82, NUMBER 4

27 JANUARY 2003

Pyramidal-plane ordering in AlGaN alloys M. Benamaraa) Universita¨t Erlangen-Nu¨rnberg, Institut fu¨r Werkstoffwissenschaften-Mikrocharakterisierung, Cauerstrasse 6, D-91058 Erlangen, Germany

L. Kirste Freiburger Materialforschungszentrum, Universita¨t Freiburg, Stefan-Meier-Str. 21, D-79104 Freiburg, Germany

M. Albrecht Universita¨t Erlangen-Nu¨rnberg, Institut fu¨r Werkstoffwissenschaften-Mikrocharakterisierung, Cauerstrasse 6, D-91058 Erlangen, Germany

K. W. Benz Kristallographisches Institut, Universita¨t Freiburg, Germany

H. P. Strunk Universita¨t Erlangen-Nu¨rnberg, Institut fu¨r Werkstoffwissenschaften-Mikrocharakterisierung, Cauerstrasse 6, D-91058 Erlangen, Germany

共Received 24 September 2002; accepted 6 December 2002兲 This letter reports the identification of long-range ordering in AlGaN compounds along the pyramidal planes by transmission electron microscopy. This ordering consists of the alternate ¯ 01其 planes and is evidenced by the comparison of stacking of GaN and AlN layers on 兵 11 ¯ 0 兴 and 关 112 ¯ 0 兴 with calculated patterns. A formation experimental diffraction patterns along 关 011 model of this ordering is presented. It is based upon Ga incorporation on reduced-N coordination ¯ 01其 pit facets. © 2003 American Institute of Physics. sites that are located at step edges on 兵 11 关DOI: 10.1063/1.1541093兴

Structural and optical studies of ternary III-nitrides alloys 共e.g., AlGaN, InGaN兲 have been mainly focused on alloy fluctuations and phase separation and their relation to optical and electronic properties.1,2 While widely studied in III–V compounds,3,4 ordering was only recently found in III nitrides. A number of authors have demonstrated that the spontaneous formation of a 关0001兴 superstructure occurs for certain growth conditions.5–11 Parameters like the III/V flux ratio, the growth rate, or the surface orientation were found to affect the degree of ordering. In most cases ordering is not stable in the bulk since it is a growth phenomenon. Ordering domains form at the surface during growth and are subsequently incorporated into the bulk as growth proceeds. Ordering lowers the crystal symmetry and gives rise to superstructure reflections in diffraction patterns, which can be used for ordering analysis. In this way, in AlGaN alloys, 共1:1兲, 共3:1兲, 共6:1兲, 共11:1兲, 共10:2兲 chemically ordered structures have been deduced from the detection of superstructure reflections.6 – 8 All these stackings 共i,j兲 correspond to a periodicity of ‘‘i’’ GaN layers followed by ‘‘j’’ AlN layers along the 关0001兴 direction. This letter reports the identification of long-range ordering in AlGaN compounds along the pyramidal normals by selective area diffraction 共SAD兲 and high-resolution transmission electron microscopy 共TEM兲. We develop a formation model according to which Ga incorporation on ¯ 01其 reduced-N coordination sites located at step edges on 兵 11 pit facets leads to this type of ordering. The analyzed AlGaN a兲

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layers have been grown by molecular beam epitaxy on sapphire 共0001兲 substrates in a Riber P32 machine equipped with gallium and aluminum effusion cells. Nitrogen radicals were produced by the OAR CARS25 rf-plasma source with a nitrogen flow rate of 1.4 sccm and a rf power of 440 W. The concentration of Al present in the grown films was monitored by Rutherford backscattering and was equal to 78%.12 Crosssectional samples have been prepared by mechanical thinning followed by ion milling using Ar ions to achieve electron transparency. The TEM experiments were conducted on a Philips CM 300 transmission electron microscope with a resolution of 0.17 nm. In order to determine the symmetry of the supercell formed by ordering, SAD has been carried out ¯ 0 兴 , 关 112 ¯ 0 兴 , and 关0001兴 zone axes respectively. along 关 011 Experimental diffraction patterns were compared to patterns calculated with the EMS software.13 The growth surface of all layers is covered by hexagonal growth hillocks extended over 100–200 nm. The side faces exhibit an inclination angle of between 1° and 3° to the basal plane, i.e., represent vicinal growth surfaces. Each hillock is separated from the adjacent ones by 20 nm deep troughs. The ¯ 0 其 facets. SAD in 关 011 ¯ 0兴 walls of these troughs are 兵 011 zone axis underneath the slightly tilted zones of the growth hillocks show 0001 spots to be present. These spots are indicative of 共1:1兲 关0001兴 ordering and have been reported in both AlGaN and InGaN. In the vicinity of the troughs, a different type of ordering was found to coexist in the layer. Figure 1共a兲 is a typical ¯ 0 兴 and taken electron diffraction pattern obtained along 关 112 ¯ close to the 兵 11 01其 facet that bounds the trough. In addition to the fundamental reflections, it exhibits satellite reflections

0003-6951/2003/82(4)/547/3/$20.00 547 © 2003 American Institute of Physics Downloaded 04 Feb 2003 to 131.188.99.118. Redistribution subject to AIP license or copyright, see http://ojps.aip.org/aplo/aplcr.jsp

548

Appl. Phys. Lett., Vol. 82, No. 4, 27 January 2003

Benamara et al.

¯ 0 兴 projection of ordered supercell of AlGaN. FIG. 2. 关 112

¯ 0 兴 zone axis (g1 FIG. 1. Cross-sectional SAD patterns 共a兲 along 关 112 ¯ 0 兴 zone axis (g1⫽ 关 0002兴 ,g2 ⫽ 关 0001兴 ,g2⫽ 关 ¯1 100兴 ) and 共b兲 along 关 011 ¯ ¯1 0 兴 ). ⫽ 关 21

¯ 01兴 , determined by the reciprocal vectors k⫽1/2关 11 ¯ ¯ ¯ ⫺1/2关 11 01兴 , 1/2关 1 101兴 , and ⫺1/2关 1 101兴 . One may notice that these reflections do not have equal intensity. Intensity ¯ 01兴 and ⫺1/2关 11 ¯ 01兴 . maxima are found at positions 1/2关 11 ¯ The brightest reflections point towards the 兵 11 01其 facet of the trough, which indicates that ordering on planes parallel to this facet may be involved. The superlattice reflections closest to the origin correspond to a periodicity in real space of ¯ 01其 planes. Supercells have twice the distance between 兵 11 been built and simulations of their diffraction patterns have been performed. As expected, several ordered structures were found to be consistent with the present pattern, all of them consisting of a type of first-order pyramidal plane ordering arrangement. In order to distinguish between them, additional observations along different directions were conducted. The electron diffraction 关Fig. 1共b兲兴 pattern obtained ¯ 0 兴 also exhibits satellite reflections the position of along 关 011 which allowed us to unambiguously determine the three dimensional structure. The proposed structure, shown in Fig. 2, ¯ 01其 planes consist is a (2⫻1⫻2) ordered structure, the 兵 11 of the alternate stacking of GaN and AlN layers. The corre¯ 0 兴 and sponding simulated diffraction patterns along 关 112 ¯ 关 011 0 兴 are represented in Figs. 3共a兲 and 3共b兲 and are both in good agreement with the experimental observation. There are some additional features in these experimental diffraction pattern we have to comment on. The 0001 reflections do not vanish in the experimental pattern because even the smallest size of the used SAD apertures permitted intensities scattered from the nearby areas in between troughs to contribute to the pattern. Hence, reflections due to the 共1:1兲 关0001兴 AlN:GaN ordering are always present. Diffuse scattering rods seen to lie along the 关0001兴 direction indicate ordered domains of limited size in a disordered matrix. Statistical parameters of their distribution are presently under investigation since knowledge of size and shape of these domains could lead to

better understanding of their influence on the fine structure of the streaks.14,15 ¯ 01其 chemical A mechanism to explain the analyzed 兵 11 ordering in AlGaN is finally presented and is based upon an analogy of the energetic considerations that were previously used to explain 共1:1兲 关0001兴 ordering in InGaN.16 The observed type of ordering occurs only in the vicinity of the troughs. Consequently, one can assume a corresponding regular incorporation of atoms at steps moving on the ¯ 01其 facets during growth. Figure 4 represents a step mov兵 11 ¯ 01其 surface. Analogously to Northrup ing on such a 兵 11

¯ 0 兴 zone axis (g1 FIG. 3. Calculated SAD patterns 共a兲 along 关 112 ¯ 0 兴 zone axis (g1 ⫽ 关 0001兴 ,g2⫽1/2关 ¯1 100兴 ) and 共b兲 along 关 011 ¯ ¯1 0 兴 ). Note superstructure reflections 共S兲. 共To be ⫽ 关 0001兴 ,g2⫽⫺1/2关 21 compared with Fig. 1.兲 Downloaded 04 Feb 2003 to 131.188.99.118. Redistribution subject to AIP license or copyright, see http://ojps.aip.org/aplo/aplcr.jsp

Benamara et al.

Appl. Phys. Lett., Vol. 82, No. 4, 27 January 2003

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Work at Erlangen-Nu¨rnberg University was supported by the European Union within the framework of the IPAM research and training network. Part of electron microscopy investigation took place in the Central Facility for HighResolution Electron Microscopy.

1

¯ 01其 surface. It is favorable FIG. 4. Representation of a step moving on a 兵 11 for Al to occupy B1 sites and Ga to occupy either B2 or T1 sites.

et al., we assume that the occupation statistics of Al and Ga atoms on the group III sublattice is governed by the local energy balance at the growth surface. Since the Al–N bond is stronger than the Ga–N bond, the system can minimize its energy by incorporating Al atoms on sites where most N bonds can be saturated. In Fig. 4, the atoms in B1 sites are bonded to three N atoms in the layer below while the atoms in T1 and B2 positions are bonded to one and two N atoms, respectively. During growth, a succession of B1 sites will ¯ 01典 direction as the step is advancing appear along this 具 11 ¯ on the 兵 11 01其 facet. The step edge will consist alternately of T1 or B2 sites. These sites are one and twofold coordinated and hence it will always be favorable for Al to occupy B1 sites and Ga to occupy either B2 or T1 sites. In conclusion, our study shows that first-order ¯ 01其 growth facets. pyramidal-plane ordering occurs on 兵 11 We ascribe its formation to preferential incorporation of Ga ¯ 01其 step edges. atoms at 兵 11

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