2. 1088 Peiris et al.: Optical properties of molecular-beam-epitaxy-grown InGaMnAs 1088
While the Mn concentrations were obtained using elec-
tron probe microanalysis and x-ray photoelectron spectros-
copy, the In and Ga concentrations were determined using
the lattice parameters obtained by x-ray diffraction experi-
ments and comparing them to previously calibrated samples.
The thickness of the quaternary film was estimated from
RHEED oscillations and verified by x-ray reflectivity and
ellipsometry. Ellipsometric spectra were obtained using two
ellipsometers; a rotating analyzer ellipsometer operating be-
tween 200 and 1800 nm and a rotating compensator ellip-
someter operating between 2 and 30 m. For a given
sample, once the spectra were taken separately in each of
these instruments, they were merged together for the analy-
sis. Additionally, for each sample, room temperature ellipso-
metric data were obtained for at least two different incident
angles.
III. RESULTS AND DISCUSSION FIG. 1. Real part ͑⑀1͒ of the complex dielectric function of four different
Spectroscopic ellipsometry generally measures two pa- samples of ͑In0.5Ga0.5͒1−xMnxAs.
rameters, ⌿ and ⌬, at each wavelength that are related to the
ratio of reflection coefficients by
nonzero values for ⑀2͒ seems to blueshift as a function of the
Rp Mn concentration. In order to fully recognize the dependence
= = tan͑⌿͒ei⌬ ,
Rs of the critical point energies with respect to the Mn concen-
where R p is the complex reflection coefficient for light po- tration of the quaternary system, ⑀ for each sample was rep-
larized parallel to the plane of incidence, and Rs is the coef- resented using a parametric semiconductor model.19 In this
ficient for light polarized perpendicular to the plane of inci- method, ⑀ is expressed as a summation of energy-bounded,
dence. One must note that both ⌿ and ⌬ obtained from Gaussian-broadened continuous functions, accounting for
ellipsometry depend on the optical properties of the entire absorption effects that occur outside the model region.
structure, and since the technique is an inverse problem, a The ⑀ for all of the films were modeled according to the
suitable model has to be formulated to arrive at a reliable above stated scheme. This allowed us to determine two of
solution.17,18 the critical points associated with the electronic transition in
The ͑In0.5Ga0.5͒1−xMnx samples used in this study were the ͑In0.5Ga0.5͒1−xMnxAs quaternary system. In Fig. 3, E0 and
represented by a four layer model ͑i.e., InP substrate, E1 critical point energies are plotted as a function of Mn
In0.5Ga0.5As buffer, quaternary layer, and a surface oxide concentration. It is important to note that since the excitonic
layer͒. Using the sample in which the quaternary layer was effects dominate near the E1 critical point, the measurement
absent, ⑀ of the In0.5Ga0.5As buffer layer was first deter-
mined. The results obtained for the buffer layer were consis-
tent with the literature values for this particular alloy.19 For
the samples with the quaternary alloy, the thicknesses and ⑀
of the ͑In0.5Ga0.5͒1−xMnx layer were adjusted to match the
experimental data. This was achieved in two steps. First,
focusing only on the ⌿ and ⌬ spectra obtained in the trans-
parent region, ⑀ in the transparent region ͑i.e., below the
fundamental E0 band gap͒ as well as the thicknesses of the
quaternary system were determined. The thicknesses ob-
tained from this method fell within 10% of the values re-
corded by RHEED and x-ray reflectivity. After the layer
thickness and the transparent region optical properties were
determined, the next step was to simulate the above band gap
optical properties of this layer.15
The components of the complex dielectric function, ⑀1
and ⑀2, determined from the above procedure are plotted in
Figs. 1 and 2, respectively. In both figures, ⑀ of In0.5Ga0.5As
is shown as solid lines. As is evident from both Fig. 1 and 2,
the incorporation of Mn into the lattice alters ⑀, particularly FIG. 2. Imaginary part ͑⑀2͒ of the complex dielectric function of four differ-
as noted in Fig. 2, the onset of the initial absorption ͑i.e., ent samples of ͑In0.5Ga0.5͒1−xMnxAs.
J. Vac. Sci. Technol. B, Vol. 25, No. 3, May/Jun 2007
3. 1089 Peiris et al.: Optical properties of molecular-beam-epitaxy-grown InGaMnAs 1089
these dielectric functions were represented by a parametric
semiconductor model which accounts for absorption effects
outside the model region. Our analysis indicates that in
͑In0.5Ga0.5͒1−xMnxAs, while the critical point associated with
the fundamental gap, E0, blueshifts as a function of Mn con-
centration, the E1 critical point shows a redshift with respect
to the Mn concentration.
ACKNOWLEDGMENTS
The work at Kenyon was supported by grants from Re-
search Cooperation ͑CC-6027͒, American Chemical Society
͑PRF-41803B͒, and National Science Foundation ͑DMR-
0521147͒. The work at Penn State was supported by the Na-
tional Science Foundation.
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14
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JVST B - Microelectronics and Nanometer Structures