Raman scattering in single-variant spontaneously ordered GaInP 2 F. Alsina Departament de Fisica, Universitat Autonoma de Barcelona, 08193 Bellaterra, Spain N. Mestres Institut de Ciencia de Materials de Barcelona, Consejo Superior de Investigaciones Cientificas, Campus de Bellaterra, 08193 Bellaterra, Spain J. Pascual Departament de Fisica, Universitat Autonoma de Barcelona and Institut de Ciencia de Materials de Barcelona, Consejo Superior de Investigaciones Cientificas, Campus de Bellaterra, 08193 Bellaterra, Spain C. Geng, P. Ernst, and F. Scholz 4 Physikalisches Institut, Universitat Stuttgart, Pfaffenwaldring 57, D-70550, Stuttgart, Germany ~Received 30 November 1995! Raman scattering is used to study the phenomenon of spontaneous long-range ordering in GaInP 2 epilayers. The measurements are performed in a series of single-variant ordered samples with different order parameter values. The samples have been grown by metalorganic vapor phase epitaxy on GaAs ~001! substrates misori- ented 6° off towards the @ 111# B direction, and the ordering degree has been varied by changing the growth temperature. The Raman selection rules of the ordered phase are derived, using group theory. Polarized Raman scattering measurements allow the identification of modes associated with the trigonal atomic arrangement. I. INTRODUCTION The phenomenon of spontaneous long-range ordering has been observed in many common-cation III-V alloy semicon- ductors, when grown by metalorganic vapor phase epitaxy ~MOVPE! under the appropriate growth conditions. 1 Order- ing induced changes to the band structure allows device de- signers well documented flexibility in controlling specific electronic and optical properties. The GaInP 2 system, lattice matched to GaAs, is of particular interest, due to its applica- tions in semiconductor lasers, 2 visible light emitting diodes, 3 and high performance tandem solar cells. 4 A rather striking difference between the CuPt-like ordered and disordered phases of GaInP 2 lies in their different band gap. The band gap of the ordered phase can be as much as 100 meV lower in energy than that of the disordered one. This opens the possibility of growing unicompositional disorder-order- disorder quantum well structures, in which the quantum well and barrier layers are composed of ordered and disordered GaInP 2 respectively. 5 From the theoretical side, Wei and Zunger 6 presented a theory in which the ^ 111& ordered monolayer superlattice ~GaP! 1 /~InP! 1 , makes band folding from the L point to the G point and alters dramatically the Brillouin zone electronic states. In particular, recent calculations 7 predict that the band gap reduction in fully or- dered GaInP 2 should attain 490 meV. Previous studies indicate that kinetic processes control the degree of order produced during epitaxial growth of GaInP alloys. Thus, variations in growth parameters such as growth rate, 8 III-V ratio, 9 temperature, 10 and substrate misorientation 11–13 can determine the degree of order in the alloy. Epitaxial growth on nominally ~001!-oriented sub- strates, normally yields the CuPt structure with ^ 111& order- ing planes. Out of the four possible variants, only the CuPt B variant, with two degenerate subvariants @ 1 ¯ 11# and @ 11 ¯ 1 # , is ever observed. Experiments on misoriented sub- strates, show that a tilt by a few degrees towards @ 111# B enhances the ordering, so that only one of the two variants is observed. 13–15 Although much effort has been made in the study of the electronic properties of ordered GaInP 2 through different techniques, such as modulated electroreflectance, 16–18 photoluminescence, 16,19–22 photoluminescence excitation spectroscopy, 21,23 photocurrent spectroscopy, 24 and ellipsometry, 25 less information is found in characteristics associated with the lattice dynamics. In the ordered phase, the crystal symmetry is reduced from the cubic zinc-blende structure with point group T d 54 ¯ 3 m , to a trigonal structure with point group symmetry C 3 v 53 m , where the trigonal C 3 axis is oriented along the @ 111# axis of the ordered vari- ant. The new crystal symmetry should lead to phonon spec- trum differences, due to the different atomic arrangement in the ordered alloy, and to modified selection rules for first order Raman scattering. Two modes, G 1 1G 3 associated with the folding of the acoustic L ~LA! and L ~TA! zinc-blende phonon branches, and the splitting of the optic phonon ener- gies are expected. 26 So far, however, the phonon spectra ap- peared not to be significantly influenced by ordering effects, due to the fact that samples are only partially ordered under nowadays growth techniques. 20 The Raman spectra of Ga 1 2x In x P alloys display a two mode type behavior, i.e., two longitudinal ~LO! and two transverse ~TO! optic modes associated with the respective PHYSICAL REVIEW B 15 MAY 1996-I VOLUME 53, NUMBER 19 53 0163-1829/96/53~19!/12994~8!/$10.00 12 994 © 1996 The American Physical Society