Full text of DOI:10.1007/BF02665866

High-Pressure Bridgman Grown
Journal of ELECTRONIC MATERIALS, Vol. 30, No. 6, 2001
Special Issue P7a4pe3r
CdZnTe for Electro-Optic Applications
High-Pressure Bridgman Grown CdZnTe for
Electro-Optic Applications
A. ZAPPETTINI,1,2 L. CERATI,1 A. MILANI,1 S.M. PIETRALUNGA,1 P.
BOFFI,1 and M. MARTINELLI1
1.—Corecom, via Ampère, 30 20131 Milano, Italy. 2.—e-mail: zappettini@corecom.it
The linear electro-optic response of high pressure Bridgman growth semi-
insulating Cd0.9Zn0.1Te bulk samples has been characterized. Measurements
havebeenperformedinthethirdopticalwindowfortelecommunicationsaround
l = 1.5 mm. The dependence of the Pockels figure of merit on the modulation
frequency and on the intensity of the optical probe beam is presented and
discussed. Despite the residual optical absorption, Cd0.9Zn0.1Te is not affected by
photo-generated auto-inhibition of the Pockels effect. This can be attributed to
the action of an efficient intragap recombination center. It is, therefore, a
suitable basic material for electro-optic switching of near-infrared beams at low
frequency and quasi-continuous wave (cw) regimes. The implementation of a
Cd0.9Zn0.1Te-based cross-bar switch for optical communication applications is
also presented, which reaches –30 dB of extinction ratio and sub millisecond
response time.
Key words: Cadmium telluride, telluride-based compounds, Bridgman
growth, electro-optic modulators, electro-optic switching
INTRODUCTION
tion, the photo-induced shielding results in auto-
inhibition of the electro-optic effect at frequencies
lower than the inverse of its characteristic time con-
stant. For this reason, GaAs and indium-doped CdTe
are not suitable for low frequency and quasi-continu-
ous wave (cw) electro-optical modulation in the near-
infrared and, in particular, in the third window for
optical communications around l = 1.5 mm.
In this work we present the experimental electro-
optic characterization of the II-VI ternary alloy
CdZnTe, grown from the melt by the high-pressure
Bridgman method. Measurements have been focused
to evaluate the Pockels efficiency at l = 1.5 mm.
In the next section we describe the material and
brieflysummarizethetheoreticalaspectsoftheelectro-
optic characterization. In the subsequent section the
description of the experimental apparatus and the
presentation of the results are given. CdZnTe single
crystals have also been used as basic material to
implement a 2¥2 electro-optic switch, operating on
free-space propagating signals, to be considered as an
elementary cross-bar building block of larger optical
switching matrices. The experimental performances
of this device are also described in this section. An
explanationofthemeasurementsresultsintheframe
of available theories is given in the final section.
The linear electro-optic Pockels effect is one of the
most effective mechanisms for the modulation and
processing of optical signals and beams.
Among electro-optic materials, crystals of cubic
symmetry are particularly attractive because they
are optically isotropic in the absence of an externally
applied electric field. Cubic highly resistive semicon-
ductors like III-V GaAs and II-VI CdTe:In have been
known for decades as electro-optic intracavity modu-
lators for infrared Q-switched laser sources.1 A prom-
ising electro-optic material must show, at the same
time, high Pockels figure of merit and suitable photo-
conductive behavior. Actually, free charge carriers,
photoexcited by the unavoidable absorption of light
from the incoming beam, result in a volume space-
charge distribution that locally shields the externally
applied electric field. This mechanism lowers the
effective Pockels effect and may result in its complete
inhibition at suitable incident optical intensities. In
principle, the higher is the absorption, the stronger is
the shielding effect. In the past, this has been also
exploited to implement all-optical processing of sig-
nals.2–4 However, when dealing with Pockels modula-
(Received November 21, 2000; accepted March 1, 2001)
743

744
Zappettini, Cerati, Milani, Pietralunga, Boffi, and Martinelli
Fig. 1. Optical absorption spectrum of Cd0.9Zn0.1Te. A value of
Eg = 1.55 eV for the bandgap energy can be estimated.
THE MATERIAL
Cubic single crystals of II-VI semiconductor ter-
naryalloyCd1–xZnxTebelongtothezincblendepoint
symmetrygroup
.Wehaveexperimentallychar-
43m
acterized the linear electro-optic (EO) response of
material with 10% zinc ratio Cd0.9Zn0.1Te, working
with commercially available samples. The material
has been grown from the melt by the high-pressure
Bridgman method. This modified version of the vertical
Bridgman method enables the production of crystals of
large dimensions, high purity, and required semi-insu-
lating characteristics.5 The effective performances in
termsofPockelseffectarestronglyrelatedtotheoverall
optical and electrical features of the semiconductor that
still constitute an open field of research.
Fig.2.SpectraldependenceoftheactualfigureofmeritforthePockels
effect; (a) undoped Cd0.9Zn0.1Te modulator rod with the dimensions of
2 ¥ 2 ¥ 10 mm3; (b) semi-insulating CdTe:In modulator rod with the
dimensions of 2.5 ¥ 2.5 ¥ 10 mm3.
The optical absorption spectrum of the material
under test is shown in Fig. 1. From quantitative
reflection and transmission measurements the ab-
sorption coefficient at l = 1.5 mm has been evaluated,
resulting in a = 0.23 cm–1. In this spectral range,
therefore,theternaryalloyismoreabsorptivethanthe
binary CdTe:In, for which a value of a = 0.09 cm–1 has
been measured. The nominal 10% zinc content has
beenconfirmedbycomparingexperimentaldatumfor
theabsorptionedgewiththeexpressionforthedepen-
dence of the bandgap energy on the alloy composition
at room temperature:6
years. In particular, the nature of intragap defect
centers in defining the semi-insulating character has
been investigated.8 Samples tested in the present
study were declared to have a resistivity of 1011 Wcm.
High resistivity enables the crystals to sustain elec-
tricbiasingfieldswithnegligiblepowerconsumption,
which represents a necessary condition for EO modu-
lation.
EXPERIMENTAL RESULTS
The linear EO response of CdZnTe was tested in a
Sénarmontcompensator-likeconfigurationasdetailed
in Ref. 9. The probe beam from a semiconductor laser
source, tunable between l = 1460 nm and
l = 1600 nm,waslinearlypolarizedandcollimatedin
freespacepropagationto200 mmofwaist. Thecrystal
samples were 2 ¥ 2 ¥ 10 mm3 modulators rods, am-
plitude modulation (AM)-cut, for maximizing EO
amplitude modulation effect. A linear polarization
analyzer converted the Pockels induced retardation,
G(t), into optical intensity modulation. The transmis-
sion at the photodiode followed the well known rela-
tion:
Egap (x,300K) = 1.4637
(1)
2
+0.49613x + 0.2289x )[eV]
where x represents the zinc fraction.
The dispersion of the refractive index has been
modeled using the Sellmeier relation n2(l) = A + Bl /
2
2
(l – C), where interpolation of results by Adachi and
Kimura7 have been used for the parameters A, B, and
C. Values no = 2.736 and no = 2.732 are, respectively,
derivedforbinaryCdTeandCd0.9Zn0.1Tatl =1.55mm.
Due to its importance as a basic material for the
implementation of x- and g-ray detectors, the photo-
conductive and electronic transport properties of
CdZnTe have been objects of intense studies in recent
T = Pout/Pin = sin2(G(t)/2)
(2)

High-Pressure Bridgman Grown
CdZnTe for Electro-Optic Applications
745
Table I. Comparison Between Measured Values for Clamped
and Unclamped Pockels Coefficients in Cd0.9Zn0.1Te and CdTe:In
3
n r
3
n r
(pm/V)
r41 (pm/V)
unclamped
(pm/V)
r41 (pm/V)
clamped
no
0
41
0 41
Material
unclamped
clamped
(l = 1550 nm)
Cd0.9Zn0.1Te
CdTe:In
110
110
5.4
5.4
88
82
4.3
4.0
2.732
2.736
voltage applied to the crystals. Following its defini-
tion, M expresses the Pockels response due to the
effective electric field within the volume of the modu-
lator rod, as sensed by the probe beam.
By spanning the modulating frequency from 1 Hz
up to 2.2 MHz, M frequency behavior has been ob-
tained, shown in Fig. 2a. In Fig. 2b the analogous
resultforasemi-insulatingCdTe:Inrodofd = 2.5 mm
and L = 10 mm is shown. In the high frequency spec-
tral region, the amplitude of M is affected by piezo-
electric resonances.10 As the modulating frequency
decreases, M takes on a relatively constant value. In
this spectral region, the “unclamped” figure of merit
and related EO coefficient, r41, can be determined. At
frequencies higher than the first piezoelectric reso-
nance, the “clamped” values can be evaluated. The
results for the CdZnTe and CdTe:In samples are
reported in Table I.
Both II-VI semiconductors show comparable
unclamped and clamped values. However, from com-
parison of Fig. 2a and b, the superior EO performance
of Cd0.9Zn0.1Te becomes evident at low modulating
frequencies. At f < 100 Hz the M value for CdTe:In
dramatically decreases. On the contrary, the Pockels
performance of the ternary Cd0.9Zn0.1Te does not sub-
stantially change. For better characterization of the
field shielding effect in the ternary alloy, the depen-
dence of M on the intensity of the optical beam at low
modulating frequencies has been measured. Mea-
surements have been performed at 20 V peak voltage.
Results at f = 1 Hz are shown in Fig. 3a, where a
linear interpolation is also indicated. The actual fig-
ure of merit decreases with increasing beam power,
reaching a 25% reduction at Pin = 1 mW. The depen-
dence of M on the peak amplitude of the applied
voltage, at constant modulating frequency and opti-
cal beam intensity, is shown in Fig. 3b at f = 1 Hz and
Pin = 4 mW. M increases with applied voltage and
te3ndstosaturatetotheunclampedPockelscoefficient
saturation region happens at peak voltage values
considerablylowerthantheestimatedhalf-wavevolt-
age, Vp = 1460 V, of the material, therefore suggest-
ing a good efficiency of modulation on a wide range of
retardation values. In particular, EO switching per-
formance of the Cd0.9Zn0.1Te rods has been tested at
different optical beam powers. For this, the applied
voltage has been switched from Va= 0 V to
Va= Vp = 1460 V, resulting in a 90∞ rotation of the
linear polarization of the input beam and correspond-
ing on-off intensity modulation after the polarization
Fig. 3. Characterization of the Pockels figure of merit for Cd0.9Zn0.1Te
at f = 1 Hz. (a) dependence on the probe beam intensity; (b) depen-
dence on the peak value of applied voltage Va. The probe beam power
is fixed to P = 4 mW.
3
where G(t) = (2p/l)(L/d) r41V(t)G(t), l is the optical
n0
41. It is interesting to remark that the onset of this
n0r
wavelength in vacuum, L is the length of the crystal
rod, d is the interelectrode spacing , r41 is the Pockels
coefficient and V(t) is the modulating voltage.
The EO performance of the crystals has been char-
acterized by evaluating the actual figure of merit:
Ê
ˆÊ
ˆ
l
2p
d
DV0
M =
◊
(3)
Á
˜Á
˜
L
V0DV
Ë
¯Ë
¯
DVÆ0
where Vo is the output voltage at the photodiode and
DV is the peak-to-peak amplitude of the modulating

746
Zappettini, Cerati, Milani, Pietralunga, Boffi, and Martinelli
allyreferredtoa"BAR"state, whereinputports1and
2 are, respectively, connected to output ports 1 and 2.
IfVa =Vp isappliedtothecrystal,thelinearSOPis90∞
rotated, resulting in the exchange of the output port,
after recombination. The switching from the bar to
the cross state is, therefore, performed. By defining
the extinction ratio (ER) between the two channels as
theratiooftheinputopticalpowerPin tothepowerPout
emergingfromthenon-connectedoutput, ER > 27dB
forVa =Vp andER = 30dBforVa = 0. Theinsensitivity
tothestateofpolarizationoftheinputsignalhasbeen
experimentally evaluated to be –30 dB, the limitation
beingattributedtothepolarizingopticsandnottothe
crystal itself. The switching time has also been mea-
sured to be less than one millisecond, therefore suit-
able for circuit switching applications.
DISCUSSION
Fig. 4. Schematic representation of the 2¥2 Cd0.9Zn0.1Te-based cross-
bar switch. (a) In absence of an applied field the switch is in the BAR
state. (b) At Va = Vp, the crystal acts as a half-waveplate, resulting in
spatial switching of the optical beams at the two inputs and CROSS
function.
The experimental tests described above demon-
strate that, despite absorption at l = 1.5 mm for
undopedCd0.9Zn0.1TebeingstrongerthanforCdTe:In,
the ternary compound exhibits better EO perfor-
mance.Inparticular,unclampedandclampedPockels
figure of merit of the two materials can be considered
asequivalent, but, asthefrequencyofthemodulating
electric field is reduced, the EO effect of CdTe:In is
auto-inhibited by a photo-generated local counter-
field. On the other hand, the impact of this phenom-
enon on the ternary alloy can be made negligible.
The role of free-charge carriers in determining the
effective local electric field in the bulk of a polarized
EO crystal and the related frequency response of the
Pockels effect has been object of studies in the past.11
Thedistributionofopticallygeneratedcarriersevolves
according to the dielectric relaxation time of the
material td = (s/e)–1, where s is the material conduc-
tivity and e the dielectric constant. The EO effect due
tospectralcomponentsoftheexternallyappliedmodu-
lating field, which are lower than the dielectric relax-
ation frequency n = (s/e), is depressed by the
reconfigurationofthespacechargeregionswithinthe
volume of the crystal. The electric dipoles associated
with the space charge region orient themselves under
the influence of the external field, resulting in a
shielding of the effective local field. This translates
into lowering of the Pockels figure of merit at the
consideredfrequency.Thebehaviorofthefieldshield-
ingeffectatdifferentopticalpowersoftheprobebeam
can be explained if the following expression for the
conductivity is used:9
analyzer,followingEq. 2.Increasingtheopticalpower
up to P = 11 mW did not result in any performance
degradation.
A measurement of the dependence of the Pockels
effect on probe beam wavelength has also been per-
formed. The wavelength spanned the range from
1480 nm to 1600 nm, and amplitude and frequency of
themodulatingvoltagewerekeptfixed.Opticalpower
levels lower than 500 mW were used. As expected, the
value of M remained relatively constant in the wave-
length range considered, slightly decreasing in mov-
ing to longer wavelengths because of the spectral
dependenceoftherefractiveindex.Thiscorresponded
to a 7% maximum increase in the half-wave voltage.
The polarization switching mechanism has been
applied to the implementation of a 2 ¥ 2 cross-bar
switch for optical communication signals. The cross-
bar constitutes the elementary building block for
more complex architectures of switching matrices,
either leading to the optical cross-connect structures
or generally to become part of the network node
structures, performing circuit switching functions in
the optical domain. The scheme of the EO cross-bar
elementary switch is shown in Fig. 4. A polarization-
diversity architecture has been adopted, so that the
performance is independent on the state of polariza-
tion (SOP) of the input optical signals. Two polarizing
beam splitters (PBS) are used at the input ports to
project the SOP of each of the incoming beams on the
linear s and p states, at 45∞ with the EO-induced
birefringence axes of the crystal. With the aid of two
right-angle prisms (RAP), two separate light paths
are produced within the modulator rod, that eventu-
ally recombine at the output ports, always using the
combination of RAP and PBS. In the absence of
external voltage, the linear SOP of the beams within
the crystal is unperturbed. This configuration is usu-
(4)
s
= s (T) + Ds(P,T)
TOT
0
In Eq. 4, s0 = enmn + epmp and Ds(P) = eDn(P)mn +
eDp(P)mp are, respectively, the dark value and the
photoexcited increase in conductivity, e is the elec-
troniccharge, nandparedarkfree-carrierconcentra-
tions, mn and mp are mobilities, respectively, for elec-
tron and holes. Photo-conductivity can be expressed
as:
Ds(P) = Gtc µ aPtc
(5)

High-Pressure Bridgman Grown
CdZnTe for Electro-Optic Applications
747
where G is the photoexcitation rate, tc is the effective
carrier lifetime, a is the absorption coefficient, and P
is the optical power.12 Experimental results of Figs. 2
and 3 can be explained by using the expression for the
relaxation frequency, n = (s/e), together with Eqs. 4
and 5. In general, the higher the photo-conductivity
fromintragaplevelsthestrongertheshieldingeffect.9
From Eq. 5, this corresponds to increasing the beam
power and optical absorption. In principle, then, one
could expect the auto-inhibition in Cd0.9Zn0.9Te at
l = 1.5 mm to be stronger than in CdTe:In, due to
higher absorption coefficient. Actually, the opposite
occurs. This can be explained, by taking into account
the role of the carrier recombination time, tc. This, in
turn, is affected by the concentration and nature of
the intragap defect centers. In particular, the differ-
ent shielding effects in the two materials can be
explained by admittingtc in Cd0.9Zn0.9Te to be reduced
by the presence of a fast recombination center.4,13
Nevertheless, at quasi-cw regimes the penalty in
Pockelsfigureofmeritbecomesconsiderable,asshown
in Fig. 2a, and worsen with beam power. However, as
shown in Fig. 2b, by increasing the modulating peak
voltage, the effect of the counter-field is progressively
reduced, until M eventually saturates at the nominal
inhibition effect, due to the photo-generation of carri-
ers consequent to near-infrared absorption, is re-
duced with respect to the binary CdTe:In resulting in
lower actual half-wave voltage at low modulating
frequencies. For these reasons, the ternary alloy is
particularly suitable for implementing EO devices to
be used as basic building blocks of spatial switching
matrices in optical communication network architec-
tures.
ACKNOWLEDGEMENT
The authors thank Dr. A. Baraldi and Prof. R.
Capelletti, both of Istituto Nazionale Fisica della
Materia (INFM) and Physics Dept. of University of
Parma (Italy), for IR absorption spectrum measure-
ments.
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3
n0r41
unclamped value
. The saturating voltage is
lowerthanVp,sothatfullPockelsyieldcanbereached
when operating the modulator as a half-waveplate,
even at low frequency regimes and high
photoexcitation values. The phenomenon can be ex-
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is increased, the carrier drift time is reduced and
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CONCLUSION
Semi-insulating high-pressure Bridgman growth
Cd0.9Zn0.1Te bulk crystals have proven to be a reliable
basic materials to perform EO processing of signal
beams in the near-infrared range typical of optical
communications. In particular, Cd0.9Zn0.1Te-based
switches show constant performance on the whole
thirdcommunicationwindowaroundl = 1.5 mm,both
at high and low modulating frequencies. The auto-