Spectroscopic evidence for the excitonic lasing mechanism in ultraviolet
ZnS/ZnCdS multiple quantum well lasers
K. B. Ozanyan,
a)
J. E. Nicholls, M. O’Neill, L. May, J. H. C. Hogg, and W. E. Hagston
Department of Applied Physics, University of Hull, Hull HU6 7RX, United Kingdom
B. Lunn and D. E. Ashenford
Department of Engineering Design and Manufacture, University of Hull, Hull HU6 7RX, United Kingdom
͑Received 24 May 1996; accepted for publication 30 October 1996͒
We demonstrate optically pumped lasing at room temperature from ZnS/ZnCdS quantum-well
structures grown on ͑100͒ GaP substrates by molecular beam epitaxy. In the structures with the
lowest Cd composition, optically pumped lasing at wavelengths as low as 333 nm ͑at8K͒is
observed. We present spectroscopic evidence which suggests that the stimulated emission involves
states in the low-energy tail of an inhomogeneously broadened excitonic resonance. While lasing is
excitonic at low thresholds, a transition to an electron-hole plasma mechanism may occur if the
pump power approaches 100 kW cm
Ϫ2
.©1996 American Institute of Physics.
͓S0003-6951͑96͒04553-6͔
UV lasing in ZnS/ZNCdS QW structures grown on
GaAs by the MOCVD technique has been demonstrated by
Yamada et al.
1
under the conditions of optical pumping
2
and
current injection.
3
In these works the gain mechanism in-
volved has been discussed mainly in the light of phase-space
filling in the low-energy side of an inhomogeneously broad-
ened exciton resonance in analogy with work on ZnSe-based
structures,
4
where convincing evidence for this mechanism
was presented. Thus, to date, the elucidation of the gain
mechanism in the ZnS/ZnCdS lasers has been experimentally
supported only by nanosecond pump-probe measurements.
2
In our current work we supply spectroscopic evidence
for the excitonic lasing mechanism under the conditions of
optical excitation. We have studied UV lasing from low Cd-
composition (0.03рxр0.20) ZnS/Zn
1Ϫx
Cd
x
S QW struc-
tures grown on GaP by MBE. In the case of the lowest Cd
composition we observe lasing at wavelengths as low as 333
nm at 8 K, which, to our knowledge, is the shortest lasing
wavelength yet reported in a semiconductor heterostructure.
Details concerning the growth and preparation of the laser
structures, as well as about the experimental setup have been
published elsewhere.
5
The low-temperature emission spectra from structures
with three different quantum-well depths, with Cd composi-
tions of 0.03 ͑s60R͒, 0.18 ͑s58R͒, and 0.20 ͑s61R͒, are
shown in Fig. 1. The laser emission ͑labeled L͒ is red-shifted
with respect to the PL emissions taken in front-face geom-
etry ͑labeled FL and FH͒. The PL bands under high-power
pulsed excitation conditions ͑FH͒ continuously evolve from
those taken under low-power cw conditions ͑FL͒, as evi-
denced by additional spectra taken at intermediate excitation
densities ͑not shown in the figure͒, without any detectable
shift comparable with the band width.
Figures 2 and 3 show the emission from the lasers at 8
and 300 K with Cd compositions 0.20 and 0.18. For the x
ϭ0.20 structures the thresholds are measured
5
as 80 and 6.5
kW.cm
Ϫ2
at temperatures of 300 and 8 K, respectively,
while for the xϭ0.18 structures the thresholds increase to
250 kW cm
Ϫ2
and9kWcm
Ϫ2
. The spontaneous emission
spectra in front-face geometry were taken with pumping
close to the lasing threshold.
Figure 4 shows the low-temperature stimulated emission
excitation ͑SEE͒ spectrum of sample S61 across the barrier
band-gap region. The experimental points were calculated
for each pump photon energy by integrating the recorded
stimulated emission band, and have been joined by spline
interpolation as a guide to the eye. The increasing errors at
the lowest energies result from the falling pump laser inten-
sity. The energy positions of the free exciton ͑FX͒ and the
band gap (E
g
) of unstrained ZnS are shown with arrows.
Barrier excitons have been reported to be prominent in
the SEE spectra of ZnSe-based lasers with excitonic gain.
6
Figure 4 shows that this applies to ZnS/ZnCdS as well and
strongly emphasizes the role of the excitons in the lasing
process. Analysis of relative heights is however hindered by
the fact that they depend on mediatory processes like trap-
a͒
Electronic mail: k.ozanyan@sheffield.ac.uk
FIG. 1. Low-temperature ͑8K͒emissions from laser structures with x
ϭ0.03 ͑s60͒, 0.18 ͑s58͒, and 0.2 ͑s61͒. L: stimulated emission, FH: front-
face emission at pumping close to threshold, FL: front-face PL at cw exci-
tation ͑ϳ10 mW cm
Ϫ2
).
4230 Appl. Phys. Lett. 69 (27), 30 December 1996 0003-6951/96/69(27)/4230/3/$10.00 © 1996 American Institute of Physics