Blue-light emission from GaN self-assembled quantum dots due to giant piezoelectric effect
F. Widmann, J. Simon, B. Daudin, G. Feuillet, J. L. Rouvie
`
re, and N. T. Pelekanos
De ´partement de Recherche Fondamentale sur la Matie `re Condense ´e, CEA/Grenoble, SPMM,* 17 rue des Martyrs,
38054 Grenoble Cedex 9, France
G. Fishman
Laboratoire de Spectrome ´trie Physique, UMR C5588, Universite ´ Joseph Fourier, Grenoble 1, CNRS, Boı ˆte Postale 87,
38402 St. Martin d’He `res Cedex, France
Received 27 July 1998; revised manuscript received 25 September 1998
It is shown that the optical properties of GaN quantum dots with the wurtzite structure result from a balance
between confinement and piezoelectric effects. In ‘‘large’’ quantum dots with an average height and diameter
of 4.1 and 17 nm, respectively, the photoluminescence peak is centered at 2.95 eV, nearly 0.5 eV below the
bulk GaN band gap. We attribute this enormous redshift to a giant 5.5 MV/cm piezoelectric field present in the
dots, in agreement with theoretical calculations. S0163-18299851248-9
The current interest in low-dimensional heterostructures
mainly relies on the possibility of achieving high-quality de-
vices, due to the optical and electronic properties which are
expected to result from one-dimensional 1Dquantum
wires or 0D quantum dots carrier confinement. In particu-
lar, it has been theoretically predicted that the realization of
light-emitting diodes LEDs or laser diodes LDs with
quantum dots QDs in the active layer would lead to im-
proved optical characteristics, such as low threshold current
and weak temperature dependence of the threshold current.
1
However, the practical observation of 0D confinement ef-
fects requires the use of objects with typical sizes in the 10
nm range. The first pioneering works in the field used almost
exclusively lithographic patterning of QDs. Nevertheless,
this approach is practically abandoned today because of the
damage that lithographic processing causes on the lateral QD
walls, seriously degrading the optical properties when the
QD dimension becomes of the order of 10 nm. By contrast,
self-organization resulting from the Stranski-Krastanov
growth mode has proven to be very successful in achieving
nanostructures with excellent 0D optical properties. In this
growth mode, deposition of a strained 2D wetting layer is
followed by elastic relaxation through 3D islanding which
results in free surface formation.
2
It has been observed for
various materials grown under compressive stress, such as
InAs on GaAs,
3
InP on Ga
x
In
1 -x
P,
4
or SiGe on Si,
5
opening
the way for achievement of lasers based on self-organized
In
x
Ga
1 -x
As/GaAs Refs. 6 and 7 and InAs/GaAs Refs. 8
and 9 QDs.
Concerning III-V nitrides, the present day blue LEDs and
LDs consist of stacking of 2D layers with appropriate
composition.
10
Despite the successful operation of such
LEDs and LDs it is still a current challenge to improve char-
acteristics such as device life time or current threshold. In
particular, the insertion of GaN QDs in the active layer ap-
pears particularly promising, following the experimental
demonstration that nitrides grown under compression by mo-
lecular beam epitaxy MBE exhibit a Stranski-Krastanov
growth mode
11
and that the size of the 3D islands is suffi-
ciently small to allow them to behave as QDs.
12,13
Alterna-
tively, it has also been demonstrated that 3D growth of GaN
could be induced in metalorganic chemical vapor deposition
MOCVD by using Si as an antisurfactant.
14,15
Whatever the growth technique, i.e., MBE or MOCVD,
the QDs exhibit the wurtzite structure, with the 0001 axis
parallel to the growth direction. Thus, as a consequence of
the noncentrosymmetry of the wurtzite structure, piezoelec-
tric effects are expected to be present and to govern the op-
tical properties of the dots to a certain extent, due to the huge
piezoelectric constant values which are one of the most fas-
cinating aspects of nitrides.
Actually, for fully strained GaN on AlN =2.5% piezo-
electric fields as high as several MV/cm are expected.
16–18
These values are more than one order of magnitude larger
than the piezoelectric fields that can be found in zincblende
semiconductors for the same amount of strain.
19
Neverthe-
less, the role of these giant piezoelectric fields on the optical
properties of nitride nanostructures has only recently started
to be assessed. For example, large piezoelectric field-induced
redshifts in photoluminescence PL spectra of
GaN/Al
x
Ga
1 -x
N quantum wells have been reported
lately.
20,21
However, the role of dislocations, interdiffusion, and re-
sidual impurities in compensating the piezoelectric field in
those systems needs to be further investigated. From this
point of view, GaN quantum dots are of special interest as
they are fully strained in the AlN matrix and exhibit almost
no interdiffusion. Although it has been observed
12
that the
threading dislocations existing in the AlN buffer and origi-
nating from the sapphire/AlN interface act often as nucle-
ation centers for the QDs, their far inferior density
10
10
cm
-2
compared to the QDs which are in the
10
11
cm
-2
range explains how the vast majority of the QDs
are dislocation-free. This is further supported by the obser-
vation of intense and temperature-independent QD
photoluminescence.
12
The samples used in the present study were grown on
0001 sapphire substrate. After the nitridation step of the
sapphire, a low-temperature AlN layer, about 15 nm thick,
was deposited followed by the growth of a 1.5-m-thick
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PHYSICAL REVIEW B 15 DECEMBER 1998-II VOLUME 58, NUMBER 24
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