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\begin{document}
\title{Optical Properties of ZnCdSe/ZnSe Structures}
\author{ Tak Gurung\\
     \\
         Department of Physics\\
         University of Cincinnati \\
         Cincinnati, Ohio 45221\\[1em]}

\date{March 9, 2001 }
\maketitle
\begin{abstract}
\vspace{0.1in}
The investigation of optical properties has
revealed that the exciton confinement in $Zn_{1-x}Cd_{x}Se/ZnSe$
QWs can be tuned independently by varying the composition and the
width of the well and the position of PL peak can be controlled by
varying the composition of the materials. The investigation also
proves that it has a very short PL decay time. The device
degradation and the short device lifetimes have been the major
problem with this material for the commercial production.
    \\[1em]
\end{abstract}

\pagebreak

\def\baselinestretch{1.1}



INTRODUCTION:

\vspace{0.1in}

II-VI compound semiconductors have been attracting much interest
because of wide-band gap property. ZnSe-based semiconductors are
the most promising candidates of light emitting and laser diods in
the blue-green spectral region. Till now, the active material in
these lasers are $Zn_{x}Cd_{1-x}Se$ quantum wells \cite{ref 1}.
Thus, this material has been studied extensively in different
forms like bulk, thin layers, quantum wells, quantum wires,
quantum dots. A brief overview of some of optical properties of
this material is given in this paper.

\vspace{0.1in}

THE REFRACTIVE INDEX:

\vspace{0.1in}

The refractive index is a very important physical parameter
related to the microscopic atomic interactions. From the
theoretical stand point, there are basically two different
approaches in viewing this subject: On the one hand, considering
the crystal as a collection of  individual entities polarizable
under the action of a electric field, the refractive index will be
related to the density and the local polarizability of these
entities. On the other hand, considering the crystalline structure
represented by a delocalized picture, the refractive index will be
closely related to the energy band structure of material, through
the dielectric constant.

Harve' and Vandamme \cite{ref 2} have found the following
empirical relation between the refractive index and energy gap:

$n=\sqrt{1+\left( \frac{A}{E_{g}+B} \right)^{2}}$ ;

A=13.6 eV and B=3.4 eV.

A.H. Ammar\cite{ref 3} has studied refractive index of
$Zn_{x}Cd_{1-x}Se$ alloys in the form of thin films. He has found
the spectral behaviour of the refractive index showing abnormal
dispersion at different wavelengths depending on film composition.

For x=0.6, or 0.8 the peak lies at $\lambda \approx 650 nm$, but
for x=0.2 and 0.4 the peak shifts towards shorter
wavelength($\lambda\approx 600nm$). For wavelengths above 650 nm,
the refractive index shows normal dispersion for all compositions.
The dispersion curves of Fig.1 indicate that the refractive index
increases with increasing Zn content.

\begin{figure}[htb]
\centerline{\psfig{figure=Fig11.eps}}

\caption{The refractive index (n) as a function of
wavelength($\lambda$) with different film compositions(x).\cite{ref 2}}
\end{figure}

\vspace{0.1in}

PHOTOLUMINESCENCE:

\vspace{0.1in}

Photoluminescence is one of many optical tools used to
characterize semiconductors. An electron in the valence band of a
semiconductor can be promoted into the conduction band by thermal
excitation, electrical excitation, optical excitation etc . This
leaves a vacancy in the semiconductor called a hole. The electron
and hole can recombine either by emitting light (photon), called
radiative recombination, or by emitting heat (phonons), called
non-radiative recombination, or in a three-body process, called
Auger recombination. When the rediative recombination of an
electron-hole pair with emission of photons is the result of
light, e.g. from a laser, then the phenomenon is called
photoluminescence. Photoluminescence is a characteristic of the
band and interface properties of the semiconductor. A measure of
the photoluminescence as a function of time, referred to as
time-resolved photoluminescence, yields information on the
dynamics of the electron-hole pair.

The PL spectra of $Zn_{0.85}Cd_{0.15}Se/ZnSe$ SQW at 19 K has been
shown in the Fig. 2\cite{ref 4}. This shows that the peak of the
PL intensity occurs on the lower wavelength side. For this
particular case, the two samples with and without growth
interruption are also compared. The spectrum of the sample with
growth interruption has less well defined peak than that of the
sample without the growth interruption has. But both of the
emissions are attributed to the excitons which are composed of n=1
conduction electrons and n=1 heavy holes. The presence of poor
optical properties arise from the rough interface and bad crystal
quality caused by the Se-rich surface after growth interruption.

\begin{figure}[htb]
\centerline{\psfig{figure=Fig21.eps}}

\caption{PL spectra of $Zn_{0.85}Cd_{0.15}Se/ZnSe$ SQWs with (\#1)
and without (\#2) the growth interruption.\cite{ref 4}}
\end{figure}

K. Mogi et al\cite{ref 5} has photoluminescence spectrum in
$Zn_{0.82}Cd_{0.18}Se/ZnSe$ MQW at room temperature as shown in
the Fig. 3. There is a simple peak at 484 nm (2.562), which is
consistent with the fact that the PL peak in ZnCdSe/ZnSe single QW
appears on the lower energy side of the excitonic absorption peak.
There is no structure at longer wavelengths. The PL intensity at
RT decreases when the pump light power exceeded 20 mW, which
corresponds to the intensity of 710 $W/cm^{2}$. This change is
reversible and the spectrum shape remains unchanged.

\begin{figure}[htb]
\centerline{\psfig{figure=Fig31.eps}}

\caption{PL spectrum at RT for the 2 nm well.\cite{ref 5}}
\end{figure}

The PL peak shifts to lower photon energy with temperature in
accordance with the temperature dependence of bandgap of
$Zn_{0.85}Cd_{0.15}Se$ \cite{ref 4}. PL intensity as a function of
reciprocal temperature is plotted in the Fig. 4. The intensity
decreases by two orders of magnitude with the temperature increase
from 20 K to 300 K. PL spectra start to reduce in intensity above
50 K, and then quench rapidly above about 100 K. The one possible
reason of the thermal quenching of the PL intensity is due to the
overflow of the thermally excited carriers from the well layer.
The other plausible mechanism for the quenching is an increase in
the non-radiative recombination process through strong coupling
with the LO phonon scattering.

\begin{figure}[htb]
\centerline{\psfig{figure=Fig41.eps}}

\caption{PL intensity as a function of reciprocal temperature.\cite{ref 4}}
\end{figure}

The fast  luminescence decay in the time-resolved PL for
ZnCdSe/ZnSe SQW is shown in the Fig. 5\cite{ref 5}. The decay time
for this is relatively shorter than that of III-V semiconductors.
The decay time does not exhibit a strong dependence on wavelength
of pumping light as well as on PL wavelength. The fast decay times
in II-V MQW are probably related to non-radiative recombinations.
If band-filling effect were dominant like it is in III-V
semiconductors, the decay time would depend on the wavelength. The
decay time is strongly temperature dependence. This also supports
the non-radiative recombination. The non-radiative process is
thought to become more extensive at room temperature because the
excited carriers can move more easily with the help of thermal
energy.

\begin{figure}[htb]
\centerline{\psfig{figure=Fig4-12.eps}}

\caption{A temporal profile of the TR-PL at RT.\cite{ref 5}}
\end{figure}

\vspace{0.1in}

EXCITON SPECTROCOPY:

\vspace{0.1in}

Exciton absorption is one of the primary aspects of light
propagation in crystals. When the photon energy is near the
excitonic resonance, the attenuation of light is determined by the
scattering of phonons, impurities and other deffects. If there is
no scattering process in an infinite crystal, absorption does not
occur.The scattering process in polar crystals is primarily
determined by the LO-phonon. At very low temperatures the
absorption of light decreases due to reduced phonon scattering.

In II-VI semiconductors the exciton-phonon interaction,
$\Gamma_{ep}$ is usually stronger than in III-V materials. Because
the exciton-phonon interaction is dependent on well width, this
phenomenon will vary with different well widths. The absorption
intensity of excitons in most ZnCdSe/ZnSe QWs decreases at lower
temperatures. This phenomenon becomes more prevalent with the
wider wells and wells with larger Cd compositions. These results
are explained qualitatively by a polarisation model accounting for
the interaction between excitons and phonons. The larger Cd
composition creates a higher confinement potential to the excitons
as well as to the phonons, and the QW system is closer to the
two-dimensional limit; therefore, as the well width is increased
the exciton-phonon interaction strengthens rapidly, compared with
the QWs with a smaller Cd composition.\cite{ref 6}


The spectrum of the exciton absorption at low temperature is shown
in the Fig. 6.\cite{ref 7} The excitonic feature around 2.8 eV is
due to the ZnSe barrier and buffer layer absorption. At lower
energies all samples exhibit distinct excitonic peaks superimposed
to the QW continuum. The blueshift of the absorption spectrum with
decreasing well width clearly shows the quantum-size effect.
Further the samples with higher Cd content show absorption spectra
extended toward the green range, reflecting the increased depth of
the quantum well. The narrow well exhibits a single confined
subband. The absorption spectra become more structured in the
20-nm-wide MQWs, where at least three electron subbands may be
confined in the well. The photoluminescence spectra ( the dashed
lines in Fig. 6) exhibit clear excitonic bands with shift ranging
between 2 meV for wide wells with little Cd content, and 20 meV
for the narrow wells with high Cd content. This indicates that the
exciton localization at compositional or thickness fluctuations is
more important in deep quantum wells. Deep quantum wells with the
higher Cd content have strongly confined excitons, with large
binding energies and clear exciton features observed up to room
temperature. On the contrary, shallow quantum wells with lower Cd
content do not exhibit room-temperature excitonic absorption.

\begin{figure}[htb]
\centerline{\psfig{figure=Fig61.eps}}

\caption{Absorption spectra of different $Zn_{1-x}Cd_{x}Se/ZnSe$
MQWs at 10 K. For (a) $L_{w}=3 nm$ , x=0.11; (b) $L_{w}= 3 nm$ ,
x=0.16; (c) $L_{w}=7 nm$ , x=0.11; (d) $L_{w}=3 nm$ , x=0.23. The
dashed lines are the PL spectra recorded under low-power(10mW)
continuous wave excitation.\cite{ref 7}}
\end{figure}

The systematic analysis of the low-temperature absorption spectra
can give the values of the ground level exciton binding energies.
The spectra of exciton binding energy vs. well width show that the
binding energy increases as the well width decreases up to a
certain limit. Further decrease in the well width decreases the
binding energy again, due to the increased penetration of the
exciton wave function in the barriers.

Thus, the exciton confinement in these $Zn_{1-x}Cd_{x}Se/ZnSe$
MQWs can be tuned independently by varying the well composition or
thickness of the wells. For example, an increase in exciton
binding energy by a factor of two is obtained by decreasing
$L_{w}$ from 20 to 3 nm. On the other hand, for a given well width
of 3 nm, an increase in the Cd content from x=0.11 to 0.26, i.e.,
an increase in well depth, results in a 15 percent enhancement of
the exciton binding energy that reflects the enhanced localization
of the carrier wave functions in the well.


\vspace{0.1in}


RAMAN SCATTERING:

\vspace{0.1in}

The RS spectra of $Zn_{1-x}Cd_{x}Se/ZnSe$ MQWs is shown in Fig.
7.\cite{ref 8} There is a small shift of the band frequencies as
the content of the Cd increases. The small shifts of band
frequencies can be explained by internal strains in structure
caused by mismatch of crystalline lattice periods of ZnSe barriers
and $Zn_{1-x}Cd_{x}Se$ quantum wells. So it is possible to
determine barrier internal mechanical strains from Raman spectra.

\begin{figure}[htb]
\centerline{\psfig{figure=Fig71.eps}}

\caption{RS spectra $Zn_{1-x}Cd_{x}Se/ZnSe$ MQWs at excitation by
488 nm laser line, where the sample 178 is for $L_{w}=6.6 nm,
x=0.4, No. of wells (n)=100, Barrier width (b_{w})= 10 nm;$ 182 is
for $L_{w}=8.0 nm, x=0.5,n=30, b_{w}=50 nm;$ 183 is for $L_{w}=5.5
nm, x=0.27, n=15, b_{w}=100 nm$ and 184 is for $L_{w}=5.1 nm,
x=0.30, n=4, b_{w}=500 nm.$ \cite{ref 8}}
\end{figure}



CONCLUSION:

\vspace{0.1in}

The PL spectra has though found significant at low temperature,
but not found significant at high temperature. This kind of
thermal quenching could be the overflow of thermally excited
carriers from the barrier or non-radiative recombination. TR-PL
spectra shows this compound has the shorter decay time than that
of III-V semiconductors. This could be because of non-radiative
recombination. The spectra of exciton absorption show that the
exciton confinement in ZnCdSe/ZnSe MQWs can be tuned independently
by varying the well composition or width of the well.

The major problems associated with ZnCdSe/ZnSe MQWs are device
degradation and short device lifetimes. The electric power
dissipated in the device is still high and causes rapid
degradation. Much improvement must be realized in this field in
order to have wide practical application.\cite{ref 9}











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\end{document}
