Many III-V A_{x}B_{1-x}C semiconductor alloys
exhibit spontaneous CuPt ordering when grown from the vapor
phase. The ordered phase consists of alternate cation monolayer
planes A__{x+h/2}B_{1-x-}_{h}_{/2}
and A_{x-}_{h}_{/2}B_{1-x+}_{h}_{/2} stacked along the
[111] (or equivalent) directions, where 0 < h < 1 is the long range order (LRO)
parameter. Spontaneous ordering in semiconductor alloys leads to
significant changes in alloy physical properties. Thus, by
controlling the ordering parameter h
(which depends on growth temperature, growth rates, III/V ratio,
substrate misorientation and doping), one can adjust the physical
properties of a semiconductor alloy to fit a specific device
application.

The Solid State Theory Group at NREL has pioneered the
theoretical understanding of ordering induced phenomena in
semiconductor alloys. Using first-principles self-consistent
electronic structure theory, the researchers at the Theory Group
have predicted a serious of changes in material properties of
ordered alloy, including (i) new X-ray structure factors that
appear at **G**_{ZB} + (1/2,1/2,1/2), where **G**_{ZB}
are zinc-blende reciprocal lattice vectors, (ii) a reduction in
the band gap E_{g}, (iii) a valence band splitting DE_{12} at the valence band maximum
(VBM), (iv) an increase of spin-orbit splitting D_{SO},
(v) polarization dependence of optical transition intensities,
(vi) enhancement of the degree of spin polarization of
photoelectrons emitted from the VBM, (vii) modified NMR chemical
shifts, (viii) anisotropy of effective masses, (ix) reduced
direct gap pressure deformation potentials, (x) new optical
transition of high-energy folded-in states, and (xi) new phonon
modes. They provided a general theory for describing these
changes as a function of the degree h
of LRO, so the predictions can be compared directly to
experiments with partially ordered samples.

This figure is taken from the
front cover of the
July 1997 issue of the MRS Bulletin. These figures show the predicted
lowest-energy configurations of a (001) film of Ga_{0.5}In_{0.5}P
(Ga = Blue, In = Red, P = Yellow) on a (001) GaAs
substrate (As = Green), illustrating the relation between
(a) surface reconstruction, (b) surface segregation, and
(c) subsurface ordering.

The top figure shows that when the reconstruction is
Beta2(2x4), there is an In segregation (under the dimers)
and CuPt-B ordering (alternating <111> planes of In
and Ga). In the latter case, there is Ga *under*
the dimer (site "A") and In *between*
dimer rows (site "D"). If we swap these Ga and
In atoms, a large strain field results, raising the total
energy by 0.15 eV/pair. This shows that the CuPt-B
arrangement is more stable.

The bottom figure shows that when the reconstruction is c(4x4) with the surface dimers 90 degrees rotated with respect to those of Beta2(2x4), the In segregation disappears and the subsurface ordering is not CuPt-B but rather CuPt-A.

S. B. Zhang (e-mail: szhang@nrel.gov) and Alex Zunger (e-mail: azunger@nrel.gov)

A. Zunger, and S. Mahajan, "Atomic Ordering and Phase Separation in Epitaxial III-V Alloys," in

*Handbook of Semiconductors*, Vol 3, second edition, Elsevier, Amsterdam, pp. 1399-1513 (1994).G. P. Srivastava, J. L. Martins, and A. Zunger, ``Atomic Structure and Ordering in Semiconductor Alloys'', Phys. Rev. B

**31**, 2561 (1985).S. Froyen and A. Zunger, ``Surface-Induced Ordering in GaInP'', Phys. Rev. Lett.

**66**, 2132 (1991).S.-H. Wei, D. B. Laks, and A. Zunger, ``Dependence of the Optical Properties of Semiconductor Alloys on the Degree of Long-Range Order'', Appl. Phys. Lett.

**62**, 1937 (1993).S. H.-Wei, A. Franceschetti, and A. Zunger, ``E1, E2 and E'0 Transitions in Ordered GaInP, Phys. Rev. B.

**51**, 13097 (1995).A. Franceschetti, S. H. Wei and A. Zunger, ``Effects of Ordering on the Electron effective mass and strain deformation potential in GaInP2: deficiencies of the k.p model'', Phys. Rev. B.

**52**, 13,992 (1995).A. Zunger, ``Spontaneous Atomic Ordering in Semiconductor Alloys: Causes, Carriers, and Consequences'', MRS Bull.

**22**, 20 (1997).V. Ozolins and A. Zunger, ``First-principles Theory of the Evolution of Vibrational Properties with Long-range Order in GaInP

_{2}'', Phys. Rev. B**57**, R9404 (1998).S.-H. Wei and A. Zunger, "Fingerprints of CuPt ordering in III-V Semiconductor Alloys: Valence Band Splitting, Band Gap Reduction, and X-Ray Structure Factors,"

*Phys. Rev. B.***57**, 8983 (1998).T. Mattila, L. Bellaiche, L.W. Wang and A. Zunger, "Electronic Structure induced by lateral composition modulation in InGaAs alloys," Appl. Phys. Lett.

**72**, 2144 (1998).T. Mattila, S.H. Wei and A. Zunger, "Electronic structure of lateral composition modulation in semiconductor alloys,"

*Phys. Rev. B.***59**, 15,270 (1999).T. Mattila, S.H. Wei and A. Zunger, "Electronic structure of sequence mutations in ordered GaInP

_{2}," Physical Review Letters**83**, 2010-2013 (1999).

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