Thin Solid Films 364 (2000) 244±248
www.elsevier.com/locate/tsf
Optical and structural properties of InGaP heterostructures
G.R. Moriarty a, M. Kildemo a,*, J.T. Beechinor a, M. Murtagh a, P.V. Kelly a,
G.M. Crean a, S.W. Bland b
a
NMRC, Lee Maltings, Prospect Row, Cork, Ireland
Epitaxial Products International Ltd., Pascal Close, Cypress Drive, St. Mellons, Cardiff, South Glamorgan CF3 OEG, Wales, UK
b
Abstract
In this study, optical and structural properties of InGaP/GaAs epitaxial heterostructures are investigated as a function of growth temperature and substrate orientation (2 and 108). Both ordered and disordered InGaP layers were grown by metal organic vapour phase epitaxy
(MOVPE). The complex refractive index both above and below the fundamental band gap has been determined using spectroscopic
ellipsometry (SE). In particular an InGaP/GaAs intermixing layer, in addition to the InGaP oxide overlayer was identi®ed and introduced
in order to improve the accuracy of the extracted refractive index. Variations in Raman TOm mode and photoluminescence (PL) emission
ef®ciencies show different degrees of ordering, which are correlated to different PL peak energies or bandgaps extracted from SE data. Mis®t
strain and lattice mismatch are also investigated. q 2000 Elsevier Science S.A. All rights reserved.
Keywords: InGaP/GaAs; Ordering; Intermixing; Ellipsometry; Raman; Strain
1. Introduction
The InGaP/GaAs heterostructure is an attractive material
system for various optical and high performance electronic
devices such as heterojunction bipolar transistors (HBTs).
InGaP alloys have bandgaps comparable with those of
AlGaAs and offer several advantages from the point of
view of optical, electrical and chemical properties [1] for
many optoelectronic and microelectronic applications.
Among these advantages are improved carrier injection
into the base because of higher InGaP band-gap, a more
favourable conduction band alignment at the heterojunction
(#0.2 eV), the absence of DX centres as well as lower
surface recombination velocity. However, sublattice cationic ordering effects and related anomalous optical properties
seem to occur more frequently in InGaP than in AlGaAs,
therefore experimental investigations of ordering in InGaP
are essential to the full understanding in the optical properties of InGaP [2].
The ordering has ®rst been observed in epitaxial layers
grown by MOVPE by a shift of the near band edge PL to
lower energies, depending on growth parameters such as
growth temperature, III/V ratio, growth rate and substrate
orientation. Different degrees of ordering can be correlated
to different PL peak energies [3]. Furthermore, a combina* Corresponding author. Tel.: 1 353-21-904-269; fax: 1 353-21-270271.
E-mail address: mkildemo@nmrc.ucc.ie (M. Kildemo)
tion of PL and Raman makes it possible to differentiate the
ordering induced band gap changes from the compositional
induced changes [4]. The ordering in¯uences further physical properties, e.g. carrier lifetimes, and consequently is
suspected to change the optical properties. The non-contact
optical diagnostic technique of spectroscopic ellipsometry
(SE) was employed to extract refractive index data for the
ternary alloy layers with the dielectric response for the
different levels of InGaP sub-lattice atomic ordering investigated. For the evaluation of these data, the growth of the
native oxide was determined and taken into account as well
as an InGaAs intermixing layer between the GaAs and
InGaP interface which has proven to improve the ®t. A
combination of the Adachi dispersion model [5] and the
Forouhi±Bloomer [6] model was employed in order to
determine the layer thicknesses and refractive index both
above and below E0.
Finally, the study of the strain and lattice mismatch of
InGaP is discussed which is essential for the full utilisation
of the device properties.
2. Experimental
The InGaP/GaAs based HBT samples in this study were
grown on (001) 2 and 108 off-axis toward (110) GaAs
substrates, using (MOVPE) at both high and low temperatures. The samples were undoped with a layer thickness of
0040-6090/00/$ - see front matter q 2000 Elsevier Science S.A. All rights reserved.
PII: S00 40-6090(99)0089 4-9
G.R. Moriarty et al. / Thin Solid Films 364 (2000) 244±248
245
1.5 mm. The following notation will be used throughout the
text
A
B
C
Low temperature growth
28 off-axis toward (110) GaAs substrate
Low temperature growth
108 off-axis toward (111) GaAs substrate
High temperature growth
108 off-axis toward (111) GaAs substrate
SE was performed using a Jobin±Yvon UVISEL phase
modulated spectroscopic ellipsometer over the energy range
from 1.5 to 4.8 eV at 0.02 eV intervals at an angle of incidence 708 with a spot size of 5 £ 2 mm. Raman analysis was
performed using a backscattering geometry described by the
z(x,x 1 y)z 0 arrangement. An incident Ar 1 laser line having
a wavelength of 514.5 nm and a power of 30 mW was
focused to a 1 mm diameter spot. The scattered light was
focused onto an optical multi-channel charge coupled detector. Photoluminescence (PL) measurements were made
using the Raman experimental arrangement with reduced
laser intensity.
3. Results and discussion
The optical properties of InGaP was extracted using the
procedure described by Schubert et al [7]. First, the oxide in
the regression analysis was modelled using an appropriately
weighted Bruggeman effective medium approximation
(EMA) of (50%) GaP and 50% (InP) oxide, following the
method of Zollner to ®nd the reference spectra for GaP
oxide and InP oxide over a wide spectral range [8]. A dispersion model in the wavelength range 1.5±2.5 eV was used to
describe the dielectric function of InGaP. It was found that a
dispersion model based on a combination of the Adachi
model and the Forouhi±Bloomer model produced good ®ts
to the data in the given energy range. The optical model
consisted of an InGaP oxide overlayer, the bulk InGaP
layer, and an intermixing layer on top of GaAs substrate.
The parameters of the dispersion model were simultaneously ®tted together with the various layer thicknesses.
The addition of a thin interface region between the InGaP
layer and GaAs substrate improves the unbiased estimator,
(x 2), and the shape of the dielectric response curve. Such
intermixing is known to occur at the termination of growth
of an arsenic containing layer in MOVPE reactors due to the
time lag in ¯ushing the arsine precursor from the reactor
Fig. 1. The extracted optical properties of samples A, B and C indicated by
the solid lines, dashed lines and and semi-dashed lines, respectively.
tube. As summarised in Table 1, the intermixing layer
included in the model improves the ®t, for A and B,
where A0 is A without the intermixing layer (dIM), while
dInGaP and dox are the thicknesses of the InGaP layer and
the oxide layer, respectively. Finally, the resulting thicknesses were ®xed in order to extract the complex dielectric
function in the entire energy range 1.5±4.8 eV. Fig. 1 shows
the extracted optical properties of A, B and C. It can be seen
from Fig. 1 that a number of changes in the optical properties occur with the different process conditions, notably the
energy gap positions of critical points (CPs), and amplitude
changes. Fig. 2 shows the comparison between the extracted
data and the dispersion model, which shows very good
agreement with each other. X-ray-diffraction (XRD) also
indicated an interfacial region between the InGaP layer
and the GaAs substrate which appeared as an irregular
peak between the InGaP layer and substrate.
The ®nal results of the ellipsometric analysis are
presented in Table 2, where A0 is the amplitude and g 0 is
the broadening parameter entering into the Adachi model
for the dielectric function originally derived from a 3D M0
in the joint density of states [5]. The band gaps of the ternaries are a function of composition. In addition, the increasing E0 gap has been reported to be a function of CuPt-type
Table 1
Ellipsometric results with and without the intermixing layer for A and B
Sample
Ê)
dIM (A
Ê)
dox (A
Ê)
dInGaP (A
x2
A
A0
B
B0
27
0
48
0
28
28
25
26
10230
10255
10388
10441
0.19
0.27
0.16
0.42
Fig. 2. Comparison of extracted dielectric function (A) and the generated
Adachi±Forouhi±Bloomer model dielectric function (±).
246
G.R. Moriarty et al. / Thin Solid Films 364 (2000) 244±248
Table 2
Summary of results for ellipsometry, PL and Raman
Sample
A
B
C
Spectroscopic ellipsometry
Photoluminescence spectroscopy
Raman spectroscopy
E0 (eV)
E1 (eV)
A0
g0
PL Peak (eV)
FWHM (meV)
norm. PL amplit.
ITOm/ILO
Strain
1.892
1.904
1.960
3.293
3.293
3.299
10.90
11.43
15.34
0.02
0.02
0.06
1.90
1.91
1.93
59.4
56.3
55.5
0.18
0.35
1.00
0.29
0.70
1.13
2 3.49 £ 10 23
2 2.95 £ 10 23
2 2.93 £ 10 23
ordering in InGaP [9], and it is now widely known that as
the E0 gap decreases the ordering increases. Assuming
ordering induced band gap changes, then the SE results
shows that sample A is the most ordered having the lowest
energy with C the most disordered. The E1 gap, treated as a
2D M0 [1], was determined from lineshape analysis of the
second derivative spectrum of the dielectric function
measured by SE. No considerable changes in the lineshape
of the E1 gap as a function of ordering was observed from
these samples.
A combination of PL and Raman spectroscopies were
employed in order to differentiate the ordering induced
band gap changes from the compositional induced changes.
From Raman this is obtained through the variation of
TOm mode intensity band (ITOm). The variation of the ITOm/
ILO is in this paper proposed to verify the structural changes
from the zincblende (Td) symmetry to C3v symmetry similar
to Ref. 4. The latter is also founded on the argument that, in
PL the change in the emission amplitude at the E0 energy
peak, has been correlated to ordering induced changes [3].
The Raman spectrum shown in Fig. 3 has been decomposed into four Lorentzian peaks representing the InP-like,
and GaP-like LO modes positioned according to the literature
[10], around 360 and 381 cm 21, respectively. It further shows
two TOm modes positioned at 330 and 368 cm 21, respectively. The latter is weak and may be neglected. Only LO
scattering is nominally allowed in the zincblende structure
for backscattering from the [001] plane. As the sample
normal is tilted towards the [110] plane, the TO mode is
partly allowed. Furthermore, the micro Raman backscattering geometry relaxes the selection rules due to the nonnormal incidence created through focusing by the lens.
Fig. 4 and Table 2 summarises the main ®ndings from the
PL and Raman experiments. The top of Fig. 4 shows the
intensity of the TOm mode as a function of band gap energy
determined from PL. The top of Fig. 4 also shows the amplitude of the measured emission energy at the bandgap
energy, as obtained from the PL experiment. It is observed
from Fig. 4 that both the emission amplitude and the ratio
ITOm/ILO changes with band gap. The latter indicating a
structural change, which may be from zincblende or disordered structure at higher E0 energy, and the appearance of
some C3v structure (ordered) at lower E0 energies. The
Raman signature may in the current scattering geometry
only prove that C is more disordered than B (in both cases
grown 108 off axis).
Fig. 3. Fit of the Raman spectra for sample A, measured and modelled values indicated by the triangles and full lines, respectively.
G.R. Moriarty et al. / Thin Solid Films 364 (2000) 244±248
247
Fig. 4. The ratio of the Raman intensity bands ITOm/ILO (left axis) and the amplitude of the PL emission energies at E0, as a function of the the fundamental band
gap E0 (top). E0 determined from SE (left axis), and E1 determined from SE (right axis) (bottom).
The PL measurements show thus that sample C, has the
highest radiative/non-radiative recombination intensity
ratio, with full width half maximum (FWHM) of 55.5
meV, and B has a broader PL peak and is 21 meV lower
in energy than C. The combination of Raman and PL in Fig.
4, backs up the previous ®ndings from reference [3], that
low (high) emission energy of A (C) is indicative of highly
ordered (disordered) material. It has been reported [3] that
ordered samples have more non-radiative loss mechanisms.
We measured 30 meV difference in PL peak position in
ordered and disordered material with highest energies of
1.93 eV for C grown at 108 at high temperature (disordered)
and the lowest energy of 1.90 eV observed for A grown at 28
at low temperature.
We observe a disagreement between the band gap determined from SE and PL, in case of the most disordered
248
G.R. Moriarty et al. / Thin Solid Films 364 (2000) 244±248
sample C. SE ®nds a larger broadening of the E0 gap, and the
®t is found to be poorer than in case of samples A and B.
Ê ) is bigger
Since the lattice constant of InGaP (5.670 A
Ê
than that of GaAs (5.653 A), mis®t between the layer and
the substrate is accommodated by a tetragonal compression
of the InGaP layer. The layer is strained such that the inplane lattice constant is the same as that of the substrate. The
strain is homogenous and is known as mis®t strain. Strain
induced shifts in InP and GaP-like phonon positions, were
measured using the method of Lee et al. [11]. They used Eq.
(1) for the calculation, where v LO is the phonon frequency
of the strained layer, v LO the phonon frequency of the
unstrained layer, x the molar fraction and d is the mis®t
strain which is obtained using Eq. (2)
v LO ˆvLO …1 1 0:77d†
vLO…InP† ˆ 394:59 2 80:36x130:26x2
1†
cm21 †
2†
The strain results for the InP LO phonon are as follows;
23:49 £ 1023 , 22:95 £ 1023 and 22:93 £ 1023 for A, B
and C, respectively. From this observation it may be indicated that there are low dislocation densities in B and C and
a higher dislocation density in A.
These results agree with our cross hatching investigation.
Cross hatching was observed using a micrograph which
revealed cross-hatched lines normal to the [001] directions
due to stacking faults or dislocations on the sample surface
which results in internal strain in the epilayer. Sample A has
a lot of dislocations which would account for the larger
strain while samples B and C show very few dislocation
marks. X-ray diffraction (XRD) using the (004) plane was
performed which showed that all three sample are lattice
matched, moreover, the analysis revealed good uniformity
of the indium composition for all three samples.
The strain measurements, dislocation observations, and
ITOm/ILO ratios seem to correlate in that both C and B have
the least amount of dislocations, the least amount of strain,
and are the most disordered structures. In contrast, sample A
has more dislocations, larger strain and have here been
shown to have a more ordered structure.
4. Conclusions
In summary, the optical properties of In0.5Ga0.5P have
been extracted using spectroscopic ellipsometry. The optical model included an intermixing layer, determined from
the improved quality of ®t. Order to disorder changes in
optical spectra has been decoupled from compositional
variations, through the combination of PL and Raman.
This opens up for an unambiguous method for studying
the lineshape parameters characterising the optical gaps,
as a function of ordering.
Acknowledgements
This work has been partly funded by the European
Commission under Brite-EuRam project HEROS contract
number BRPR-CT98-0789.
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