J. Vac. Sci. Technol. B 19„4…, Jul/Aug 2001

1. High reflectivity symmetrically strained ZnxCdyMg1ÀxÀySe-based
distributed Bragg reflectors for current injection devices
O. Maksimov,a)
S. P. Guo, F. Fernandez, and M. C. Tamargo
Department of Chemistry, City College and Graduate Center of CUNY, New York, New York 10031
F. C. Peiris and J. K. Furdyna
Department of Physics, University of Notre Dame, Notre Dame, Indiana 46556
͑Received 2 January 2001; accepted 12 March 2001͒
Distributed Bragg reflectors ͑DBRs͒ with different numbers of periods were grown by molecular
beam epitaxy from ZnxCdyMg1ϪxϪySe-based materials on InP substrates. The alternating ZnCdSe/
ZnCdMgSe layers were symmetrically strained to the InP substrate greatly simplifying the growth
process and increasing the uniformity. High crystalline quality was also achieved in these structures.
A maximum reflectivity of 99% was obtained for a DBR with 24 periods. Chlorine doped ͑n-type͒
DBRs were grown and their electrical and optical properties were investigated. Electrochemical
capacitance–voltage profiling indicated that the doping concentrations of the ZnCdSe and
ZnCdMgSe layers were 4ϫ1018
and 2ϫ1018
cmϪ3
, respectively. The reflectivity of the doped DBR
structures was comparable to that of the undoped ones. © 2001 American Vacuum Society.
͓DOI: 10.1116/1.1374625͔
I. INTRODUCTION
There has recently been considerable effort to obtain
highly reflective semiconductor distributed Bragg reflectors
͑DBRs͒. This is due to the growing interest in a number of
semiconductor microstructures, such as vertical cavity sur-
face emitting lasers ͑VCSELs͒ that use highly reflective mir-
rors, usually in the form of DBRs. These DBRs form the
Fabry–Pe´rot ͑FP͒ cavities necessary for operation of the de-
vices. Although significant success has been achieved in the
development of III–V systems ͑GaAs/AlAs on GaAs
substrates1
and AlAsSb/GaAsSb on InP substrates2
͒, at-
tempts to fabricate similar structures from wide-gap II–VI
compounds have been limited. One of the reasons is that the
difference in the indices of refraction ͑⌬n͒ of many of these
materials is relatively small, making it hard to achieve high
reflectivity by the growth of an adequate number of periods.
Nevertheless, reflectivity in excess of 90% has been reported
for ZnSe/ZnTe, ZnMgSe/ZnCdSe, and ZnSe/ZnMgS systems
on GaAs substrates.3–5
However, reflectivity in excess of
99% is necessary for actual device applications. Further-
more, at least one electrically conductive semiconductor
DBR is required for current injection devices.
The growth of ZnxCdyMg1ϪxϪySe alloys has been exten-
sively studied due to their applicability for the fabrication of
red–green–blue light emitting diodes6
and high crystalline
quality of the materials has been demonstrated.7
Due to the
relatively large difference between the indices of refraction
of ZnCdSe (Egϭ2.18 eV͒ and ZnCdMgSe ͑Egϭ2.93 eV͒
materials, ⌬n/n is on the order of 12% at ␭ϭ633 nm.8
The-
oretical calculations predict that reflectivity above 99% can
be achieved after the growth of 20–26 periods.9
We have fabricated several DBR structures from
ZnxCdyMg1ϪxϪySe materials with different numbers of pe-
riods. In order to facilitate the growth process and to increase
the reproducibility in the layer composition, alternating sym-
metrically strained layers were used. Although a significant
amount of strain was present, layers and structures with high
crystalline quality were achieved. Since high n-type doping
levels, above 1018
cmϪ3
, were previously achieved in
ZnxCdyMg1ϪxϪySe epilayers doped with chlorine,10
doped
DBRs were also grown. In this article, we report a very high
reflectivity ͑99%͒ for undoped II–VI based DBRs as well as
promising optical and electrical characteristics for an n-type
doped DBR structure.
II. EXPERIMENTAL TECHNIQUES
Growth was performed in a Riber 2300 molecular beam
epitaxy ͑MBE͒ system. This system consists of two growth
chambers connected by an ultrahigh vacuum transfer chan-
nel. One growth chamber is used for the growth of As-based
III–V materials and the other is for wide band gap II–VI
materials. All the undoped samples were grown on epi-ready
semi-insulating ͑Fe-doped͒ InP ͑001͒ substrates. The sub-
strates were de-oxidized in the III–V chamber by heating to
480 °C with As flux impinging on the surface. A lattice
matched InGaAs buffer layer ͑100 nm͒ was grown after de-
oxidation. Then, the samples were transferred to the II–VI
chamber for growth of the ZnxCdyMg1ϪxϪySe layers.
Growth was initiated at 170 °C by Zn irradiation7
followed
by growth of a 9 nm thick ZnCdSe interfacial layer. The
temperature was then increased to the optimum growth tem-
perature of 270 °C, after which an additional 50 nm of
ZnCdSe was deposited. Then, alternating symmetrically
strained ZnCdMgSe/ZnCdSe layers were grown. The use of
symmetrically strained layers allowed us to keep the cell
temperatures constant during growth. The Zn to Cd beam
equivalent pressure ͑BEP͒ ratio was fixed at 0.6. The Cd, Zn,
and Se shutters were kept open while the Mg shutter was
alternatively opened and closed, producing ZnCdMgSe and
ZnCdSe epilayers. Under these conditions we fabricated
a͒
Author to whom correspondence should be addressed; electronic mail:
maksimov@netzero.net
1479 1479J. Vac. Sci. Technol. B 19„4…, JulÕAug 2001 1071-1023Õ2001Õ19„4…Õ1479Õ4Õ$18.00 ©2001 American Vacuum Society

2. three DBR structures having 12, 18, and 24 periods, all de-
signed to have nominally the same optical layer thicknesses.
Chlorine doped DBRs that consisted of 12 periods and
had nominally the same layer thickness as the undoped
DBRs were grown under the same conditions as the undoped
DBRs described previously. Semi-insulating and n-type ͑S-
doped͒ InP ͑001͒ substrates were used. Chlorine was pro-
vided by a solid ZnCl2 source heated in a Knudsen effusion
cell. The ZnCl2 cell temperature was kept constant during
growth.
Prior to growth of the DBR structures, a series of calibra-
tion layers of ternary ZnCdSe and quaternary ZnCdMgSe
alloys were grown to determine the composition, indices of
refraction, and growth rates of the alloys used. The compo-
sition of the ternary ZnCdSe alloys was calculated assuming
a linear dependence of the lattice constant on the alloy com-
position ͑Vegard’s law͒. The lattice constant was estimated
using double crystal x-ray diffraction ͑DCXRD͒ measure-
ments. Since the calibration layers could be partially
strained, ͑115͒ a and b asymmetrical XRD measurements
were made to obtain the perpendicular and parallel lattice
constants: aЌ and aʈ. The bulk lattice constant (a0) was then
calculated from the following equation:
a0ϭaЌ͕1Ϫ͓2␯/͑1ϩ␯͔͓͒͑aЌ /aʈ ͒/aЌ͔͖. ͑1͒
A value of 0.28 was used for ␯, which is the Poisson ratio.
The composition of the quaternary ZnCdMgSe alloys was
determined by combining the bulk lattice constant and band
gap energy data as described elsewhere.9
The refractive in-
dices of the epilayers grown were estimated by extrapolation
from the dispersion curve8
that was previously obtained for
ZnCdMgSe layers with different band gap energies using
the prism coupler technique.11
Electrical characteristics of n-type doped DBRs were
studied by Hall effect measurements, current–voltage ͑I–V͒
measurements, and electrochemical capacitance–voltage
͑ECV͒ profiling. For the Hall effect measurements, 0.5ϫ0.5
cm squares ͑van der Pauw geometry͒ were cut from the DBR
structure grown on a semi-insulating substrate. For I–V
measurements, gold dots ͑0.3 mm2
͒ were deposited on the
top surface of the DBR structure grown on a nϩ
-InP sub-
strate. Gold wires were attached to the back of the InP sub-
strate, which was covered with In and to the gold dots on the
top. ECV profiling was performed using a BioRad Pn 4300
ECV profiler. A solution of 1 M NaOH and 1.25 M Na2SO3
was used as an electrolyte. This solution was previously de-
veloped for electrochemical etching of ZnxCdyMg1ϪxϪySe
alloys.12
The room-temperature reflectance of the DBRs was mea-
sured using a Cary 500 ultraviolet ͑UV͒-visible spectropho-
tometer with a variable angle specular reflectance accessory.
The data were calibrated using an Ag-coated mirror of
known reflectivity as reference.
III. RESULTS AND DISCUSSIONS
The calibration layers were designed to be symmetrically
strained to the InP substrate. Although the calibration layers
were relatively thick ͑640 nm for ZnCdSe and 980 nm for
ZnCdMgSe͒, the x-ray analysis indicates that the layers were
nearly pseudomorphic. The ZnCdSe layers were under biax-
ial tension and the ZnCdMgSe layers were under biaxial
compression. The ZnCdSe and ZnCdMgSe lattice constants
and their lattice mismatch to the InP substrates are given in
Table I. The composition and the lattice mismatch were de-
signed to provide a relatively large difference in the indices
of refraction between the constituent layers ͑⌬n/nϭ12%͒
and to make the mean ͑perpendicular͒ lattice constant equal
to that of the InP substrate. The mean lattice constant was
calculated from the perpendicular lattice constants (aЌ) of
the calibration layers. The mean lattice mismatch of the DBR
to InP, (⌬a/a)Ќ , was designed to be less than 0.01%. Using
the dispersion relations, the desired thicknesses of the indi-
vidual layers ͑i.e., ␭/4n͒ were calculated to be 59 and 67 nm,
respectively, for ZnCdSe and ZnCdMgSe at the design
wavelength of 633 nm.
The DCXRD rocking curve obtained from an undoped
DBR structure with 12 periods is shown in Fig. 1. The solid
line represents the experimental spectrum. The most intense
FIG. 1. Double crystal x-ray rocking curve obtained from a symmetrically
strained ZnCdSe/ZnCdMgSe DBR structure with 12 periods grown on an
InP substrate.
TABLE I. Parameters of the ZnCdSe and ZnCdMgSe calibration layers used for the DBR structure design.
Layer
aЌ
͑Å͒
⌬(a/as)Ќ
͑%͒
aʈ
͑Å͒
⌬(a/as)ʈ
͑%͒
a0
͑Å͒
⌬(a/as)0
͑%͒
Zn0.58Cd0.42Se 5.828 ϩ0.69 5.864 ϩ0.08 5.846 ϩ0.39
Zn0.22Cd0.24Mg0.54Se 5.903 Ϫ0.58 5.870 Ϫ 0.02 5.886 Ϫ0.29
1480 Maksimov et al.: ZnxCdyMg1ÀxÀySe DBRs 1480
J. Vac. Sci. Technol. B, Vol. 19, No. 4, JulÕAug 2001

3. narrow peak in the center represents the ͑004͒ reflection from
the InP substrate and the zero order peak from the superlat-
tice is seen as a shoulder on the InP peak. More than six
satellite peaks on each side of the zero order peak ͑which
corresponds to the mean lattice constant of the superlattice͒
are visible in the DCXRD rocking curve, indicative of a high
quality periodic structure. The observed satellite peak posi-
tions are plotted in the form of sin␪ in Fig. 2. The linear
relation to the order of the peaks gives evidence that the
observed peaks are diffraction peaks from the superlattice
structure. From the slope, the thickness of the period ͑L͒ is
estimated to be 124.5 nm, very close to the designed thick-
ness of 126 nm. The dotted line in Fig. 1 represents a theo-
retical simulation based on the period calculated above. The
position and the intensity of the satellite peaks are in good
agreement with the experimental data.
The reflectivity spectrum for the sample of Fig. 1 is
shown in Fig. 3͑a͒. A maximum reflectivity of 88% was
obtained at around 617 nm. The peak reflectance of the DBR
is somewhat blueshifted relative to the intended value. This
shift is in agreement with the difference between the de-
signed and measured thickness. The reflectivity spectra of
DBR structures that consisted of 18 and 24 periods are
shown in Figs. 3͑b͒ and 3͑c͒, respectively. Increasing the
number of periods to 18 and 24 periods increases the reflec-
tivity to 98% and 99%, respectively. The reflectivity spec-
trum of a chlorine-doped DBR that consisted of 12 periods is
shown in Fig. 3͑d͒. The maximum reflectivity of that struc-
ture is 89% at around 597 nm.
Using the thickness for each layer calculated from the
positions of the x-ray satellite peaks and the indices of re-
fraction at the specific stop-band centers obtained from the
dispersion curves, we calculated the optical thickness of in-
dividual layers in the four DBR structures. These are sum-
marized in Table II. The optical thicknesses are very close to
␭/4 for the four DBR structures grown.
The electrical characteristics of the n-type DBR structure
were studied. Room-temperature Hall effect measurement of
the DBR structure gave a free electron density of 3.1
ϫ1018
cmϪ3
and a parallel mobility of 72 cm2
/V s. This
electron mobility was lower than that measured for bulk
ZnCdSe and ZnCdMgSe epilayers at this doping concentra-
tion ͑Ϸ200 cm2
/V s),9
possibly due to the interface scatter-
ing in the DBR structure. ECV profiling was performed to
determine the net carrier concentration (ndϪna) in the indi-
vidual layers. The ECV profile plot is shown in Fig. 4. Since
the electrochemical etching rate in these materials was not
known precisely, etching was continued until the substrate
was reached. At this point the measured carrier concentration
changed abruptly. The etching depth was then estimated
from the known overall thickness of the DBR structure. Net
electron concentrations of 4.4ϫ1018
cmϪ3
for ZnCdSe and
1.9ϫ1018
cmϪ3
for ZnCdMgSe were measured by this tech-
nique, in general agreement with the Hall measurements and
the expected values.
Electron transport perpendicular to the layers is of pri-
mary importance for device applications of these structures.
FIG. 2. Position ͑sin ␪͒ of satellite peaks plotted vs satellite peak order for a
symmetrically strained ZnCdSe/ZnCdMgSe DBR structure with 12 periods.
FIG. 3. Reflectivity spectra of four DBR structures with ͑a͒ 12, ͑b͒ 18, and
͑c͒ 24 periods, and ͑d͒ n-type doped with 12 periods.
1481 Maksimov et al.: ZnxCdyMg1ÀxÀySe DBRs 1481
JVST B - Microelectronics and Nanometer Structures

4. Therefore, I–V measurements were performed on the struc-
ture. The I–V characteristics of the n-type DBR stack mea-
sured at room temperature is shown in the inset of Fig. 4. A
rectifying behavior, most probably due to the band offsets in
the conduction band, was observed. The rectifying behavior
is expected to be minimized by the use of step graded or
digitally graded layers13
as well as by the modulation doping
of the graded interfaces.14
IV. CONCLUSION
We have fabricated distributed Bragg reflectors from
symmetrically strained ZnCdSe/ZnCdMgSe layers grown by
molecular beam epitaxy. High crystalline quality was dem-
onstrated by DCXRD measurements. A 99% reflectivity was
obtained from a 24 period DBR structure. The optical and
electrical properties of chlorine-doped n-type ZnCdSe/
ZnCdMgSe DBR structures were also investigated. Reflec-
tivity spectra, similar to those of undoped DBR structures
with the same number of periods, were observed from the
doped structures. High carrier concentration in the constitu-
ent layers was achieved. These results demonstrate that
ZnxCdyMg1ϪxϪySe is a promising material system for the
design of highly reflective, conductive DBRs for application
in VCSELs.
ACKNOWLEDGMENTS
The group from the City College of New York acknowl-
edge the National Science Foundation, Grant No. ECS-
9707213, and the Army Research Laboratory, Grant No.
DAAD17-99-C-0072, for support provided for this research.
This work was performed under the auspices of the New
York State Center for Advanced Technology on Ultrafast
Photonics and the Center for Analysis of Structures and In-
terfaces. The group from the University of Notre Dame ac-
knowledges the support provided by the National Science
Foundation through Grant No. DMR-0072897.
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TABLE II. Parameters of the DBR structures grown.
DBR Composition
di
͑nm͒ n N
␭max
͑nm͒
R
͑%͒
Optical thickness
͑␭͒
Nominal Zn0.58Cd0.42Se 59 2.68 633 0.25
Zn0.22Cd0.24Mg0.54Se 67 2.36 0.25
a Zn0.58Cd0.42Se 58.3 2.72 12 617 88 0.26
Zn0.22Cd0.24Mg0.54Se 66.2 2.37 0.25
b Zn0.58Cd0.42Se 63 2.67 18 646 98 0.26
Zn0.22Cd0.24Mg0.54Se 71.6 2.35 0.26
c Zn0.58Cd0.42Se 65.9 2.65 24 661 99 0.26
Zn0.22Cd0.24Mg0.54Se 74.9 2.33 0.25
d Zn0.58Cd0.42Se:Cl 53.7 2.75 12 597 89 0.25
Zn0.22Cd0.24Mg0.54Se:Cl 61 2.38 0.24
FIG. 4. Elecrochemical C–V profile for a ZnCdSe/ZnCdMgSe n-type DBR
structure with 12 periods, showing the net electron concentration as a func-
tion of depth. The inset shows the I–V characteristic of the same structure
measured at room temperature.
1482 Maksimov et al.: ZnxCdyMg1ÀxÀySe DBRs 1482
J. Vac. Sci. Technol. B, Vol. 19, No. 4, JulÕAug 2001