Changes in luminescence emission induced by proton irradiation: InGaAs/GaAs
quantum wells and quantum dots
R. Leon, G. M. Swift, B. Magness, W. A. Taylor, Y. S. Tang, K. L. Wang, P. Dowd, and Y. H. Zhang
Citation: Applied Physics Letters 76, 2074 (2000); doi: 10.1063/1.126259
View online: http://dx.doi.org/10.1063/1.126259
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APPLIED PHYSICS LETTERS

VOLUME 76, NUMBER 15

10 APRIL 2000

Changes in luminescence emission induced by proton irradiation:
InGaAsÕGaAs quantum wells and quantum dots
R. Leon and G. M. Swift
Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena,
California 91109

B. Magness and W. A. Taylor
Department of Physics and Astronomy, California State University, Los Angeles, California 90032

Y. S. Tanga) and K. L. Wang
Department of Electrical Engineering, University of California, Los Angeles, California 90095

P. Dowdb) and Y. H. Zhang
Department of Electrical Engineering and Center for Solid State Electronics Research,
Arizona State University, Tempe, Arizona 85287

共Received 14 October 1999; accepted for publication 14 February 2000兲
The photoluminescence 共PL兲 emission from InGaAs/GaAs quantum-well and quantum-dot 共QD兲
structures are compared after controlled irradiation with 1.5 MeV proton fluxes. Results presented
here show a significant enhancement in radiation tolerance with three-dimensional quantum
confinement. Some additional radiation-induced changes in photocarrier recombination from QDs,
which include a slight increase in PL emission with low and intermediate proton doses, are also
examined. © 2000 American Institute of Physics. 关S0003-6951共00兲01615-6兴

Semiconductor quantum dot 共QD兲 lasers with low
threshold currents and high gain,1,2 and QD infrared
photodetectors3 capable of incident photon absorption are
showing successful implementations of the unique optical
properties of self-forming semiconductor QDs. Future device
applications include the use of coupled QDs as the basic
structures in the fabrication of cellular automata in novel
computing architectures4 and frequency domain optical
storage5 based on self-assembled QDs.
Minimizing the impact of radiation induced degradation
in optoelectronic devices is important for several applications. In space, protons pose a particularly severe threat to
both planetary and Earth-orbiting spacecraft because they
produce damage effects by several mechanisms. Due to their
mass, protons can cause significant displacement damage in
the semiconductor lattice, which is the primary cause of performance degradation and failure in several types of semiconductor devices. The effects of proton irradiation are also
of interest in the use of ion beam modification or ‘‘defect
engineering’’ in electronic materials. Proton implantation is
often used for device isolation in compound
semiconductors,6 and can also be used to induce interfacial
compositional disordering in both quantum wells7 and quantum dots,8 which in turn, results in blue-shifted photoluminescence emission from both types of quantum structures.9
Some of the fundamental properties of QDs suggest that optoelectronic devices incorporating QDs could tolerate more
displacement damage than other heterostructures. One of
them is based on a simple geometrical argument: the total
volume percentage of the active region is very small 共in selfa兲

Present address: Research and Development Laboratories, Culver City,
CA 90230.
b兲
Present address: Nanyang Technological University, School of Electrical
and Electronic Engineering, Singapore 639798.

forming InGaAs/GaAs QDs surface coverage range from 5%
to 25%, depending on growth conditions10兲. Therefore exciton localization in the QDs due to three-dimensional confinement 共the InGaAs dots used here average 5 nm height and 25
nm diameter兲 will reduce the probability of carrier nonradiative recombination at radiation induced defect centers outside the QDs. Here we compare the optical emission from
InGaAs quantum well 共QW兲 and QD structures after controlled irradiation with 1.5 MeV protons.
Details of the growth conditions of InGaAs/GaAs QDs
by metal organic chemical vapor deposition have been described in previous work.10 After deposition of GaAs buffer
layers at 650 °C, the temperature was lowered to 550 °C and
nanometer sized InGaAs islands were grown by depositing
⬃5 ML of In0.6Ga0.4As. QW samples were obtained by stopping the growth of InGaAs before the onset of the Stranski–
Krastanow transformation, giving thin 共1 nm兲 QWs. Ternary
compositions between the samples were identical, and so
was the capping layer thickness 共100 nm for both QDs and
QWs兲, therefore these results are not dependent on material
or proton energy loss differences. Atomic force microscopy
and transmission electron microscopy10–13 have given structural information on island sizes and surface densities in
capped and uncapped InGaAs QDs. Samples were irradiated
at room temperature using a Van De Graaff accelerator with
1.5 MeV protons at doses ranging from 7⫻1011 to 2
⫻1015 cm2 and a dose rate of 6⫻1012 proton/s. Dose uniformity was monitored using radiochromic film at low doses.
Variable temperature photoluminescence 共PL兲 measurements
共from 4 K兲 were done using the 514 nm line of an Argon ion
laser for excitation and a cooled Ge detector with lock-in
techniques for signal detection.
Figure 1 shows the effects of different proton fluences
on the measured PL emission from both types of samples,

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2074
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Appl. Phys. Lett., Vol. 76, No. 15, 10 April 2000

2075

FIG. 2. Comparison of initial 共solid line兲 and postirradiation 共dotted line兲
PL spectra 共measured at 5 K兲 at a proton dose of 2.7⫻1012 cm2 from QD
structures with low QD density (3.5⫻108 dots per cm2). The spectra were
obtained at constant excitation and show simultaneous emission from QD
and wetting layer states.

than QWs. This increase in radiation hardness is significant,
because QW based devices already represent a vast improvement in radiation tolerance over bulk devices like optocouplers, which show significant degradation with proton
irradiation;16 and light emitting diodes 共LEDs兲 based on
QWs have shown an order of magnitude greater tolerance to
proton induced damage when compared to LEDs based on
p – n junction geometries.17 These results show that QDs can
be used in radiation hard optoelectronic devices. This is confirmed by recent data showing effects of phosphorus ion irradiation on QD laser diodes and detectors.18
Figure 2 shows some of the effects of proton irradiation
in QD structures with a low QD density. These structures
show a strong PL peak from the wetting layer 共WL兲. The
FIG. 1. 共a兲 Comparison of PL spectra 共measured at 5 K兲 from InGaAs/GaAs
quantum wells and from quantum dots in high surface densities (2.4
WL is a very thin QW which forms prior to the dots in
⫻1010 dots per cm2) after irradiation with selected proton fluxes. The solid
Stranski–Krastanow growth. If the average QD separation is
lines show spectra before irradiation. The dotted lines show spectra after 1.5
greater than the two-dimensional 共2D兲 diffusion lengths in
MeV proton irradiation in doses 共per cm2兲 of 共1兲 7⫻1012, 共2兲 6⫻1013, 共3兲
15
12
13
14
the WL, recombination from WL states will occur for pho2⫻10 , 共4兲 3⫻10 , 共5兲 6⫻10 , and 共6兲 2⫻10 . 共b兲 Integrated PL emission normalized to the as-grown samples for QW and QDs as a function of
tocarriers generated in the WL12–14 and PL peaks will be
proton dose.
observed from both structures. Figure 2 shows that proton
irradiation has different effects on the WL peak 共at 1.3 eV兲
than on the QD peaks 共1.7 eV for the ground state—excited
InGaAs/GaAs QDs and QWs. The differences in the nonstates emission is seen here兲.
irradiated 共as-grown兲 PL emission are apparent and have
Figure 1 共and Fig. 2兲 show a slight increase in PL signal
been discussed in previous work9,12–14 Due to increased excitonic oscillator strength in the structures with three共from ⬃10% to 70%兲 after low to intermediate proton doses
dimensional confinement,15 the integrated emission intensity
共from 7⫻1011 to 7⫻1012 cm2兲. Since no such increase is
is greater, even though only a fraction of the area is covered
observed in the QWs we attribute this PL enhancement to
by QDs. Figure 1共a兲 also shows that the luminescence from
effects from three-dimensional quantum confinement. Rethe QDs is broader. This inhomogeneous broadening origiduction of the phonon bottleneck by defect assisted phonon
nates from slight size nonuniformities and from the effects of
emission has been proposed19 as a mechanism to explain the
11
varying lateral strain in disordered dense dot ensembles.
bright PL emission in QDs. Introduction of deep level deThe PL emission from the QW is at a higher energy than the
fects as those originated from displacement damage might
QDs because very thin QWs 共1 nm兲 are used to obtain
provide additional relaxation paths20 for thermalization of
dislocation-free In0.6Ga0.4As QWs. Figure 1共a兲 also shows
carriers and therefore increase the luminescence emission.
that proton irradiation did not shift the emission wavelength
The mechanisms responsible for the small degradation
in either QD or QW structures. Figure 1共b兲 compares the
observed in the optical emission from QD structures 共with
measured integrated PL intensities from QWs and QDs 共norproton fluences above 1013 cm2兲 also remain to be fully inmalized to the nonirradiated values兲 as a function of proton
vestigated. Carrier generation, capture, transfer, and recomThis article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions.
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Leon et al.

Appl. Phys. Lett., Vol. 76, No. 15, 10 April 2000

degradation of the hole mobility in the GaAs barrier and
InGaAs wetting layer,13 which peaks at ⬃80–100 K for nonirradiated structures. Mobility degradation due to proton
damage in the barrier and WL would then affect carrier capture and transfer into the dots. Figure 3共b兲 shows a more
pronounced decrease in the inhomogenous PL broadening
with temperature after radiation damage. This decrease in the
full width at half maximum 共FWHM兲 of the PL band has
been attributed to carrier thermal emission from the smaller
dots in the ensemble.13 With radiation damage, the onset of
thermionic emission will also be acompanied by defect assisted nonradiative recombination, making this effect even
stronger, which might explain the stronger decrease in inhomogeneous PL broadening seen in Fig. 3共b兲.
In summary, results presented here show that the luminescence from QDs structures is inherently radiation tolerant
due to the effects of three-dimensional quantum confinement. An increase in radiation hardness of as much as two
orders of magnitude has been obtained by comparisons with
similar quantum wells. Additionally, we show that a slight
increase in PL emission from InGaAs/GaAs QDs can be observed with low to moderate proton doses.
Part of this work was carried out by the Jet Propulsion
Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration.
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1
2

FIG. 3. Radiation induced changes 共with 1.5 MeV protons at a dose of
3.5⫻1013 cm2兲 in the QD PL temperature dependence. 共a兲 Total integrated
PL emission from QD structures, filled circles show signal before proton
irradiation, hollow squares indicate signal after irradiation. 共b兲 Temperature
dependence of the inhomogeneous broadening of the PL emission from QDs
before 共filled circles兲 and after irradiation 共hollow squares兲.

ated carrier diffusion lengths in the barrier and wetting layer
materials. These will be affected by radiation induced damage and will contribute to degradation in QD PL emission,
by limiting carrier capture into the dots. The rate of carrier
transfer to the QDs is limited by the rate of lateral transport
in the InGaAs WL, which for photogenerated carriers is governed by hole diffusion. Reduction in diffusion lengths or
mobilities in the barrier material 共GaAs兲 and in the InGaAs
WL is the main cause for the first PL degradation observed
in QDs with very high proton doses.
Figure 3 shows some subtle effects of proton radiation
on the temperature dependence of the QD luminescence signal 共these are normalized over the degraded signal measured
at 5 K兲. In the absence of midgap levels 共nonradiative recombination兲 the temperature dependence of the integrated PL
signal from dense QD ensembles is closely related to their
confining potential21 just as in QWs.22 Defect induced recombination could lower the values for this activation energy. This could explain the slightly lower activation energy
shown in Fig. 3共a兲 after radiation damage. The lower normalized PL at temperatures ⬃100 K can be explained from the

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