Three - University of Arkansas Physics Department

APPLIED PtlYSICS LETTERS VOLUME 78. NUMBER 15
'Thermal annealing effect on the intersublevel transitions
in lnAs quantum dots
Y. Berhane and M. 0. ~anasreh~)
Depurtment qf'Electric~rl and Computer Engineering, Universir)' of'iVew Mexico, Albuguerque.
.Vew Mexico 8 71 3 1
H. Yang and G. J. Salamo
Deportment of Phvsics, Uni~rersify of'Arkansus. Fu)rt/eville, Arkc~roos 727111
(Received 4 January 200 1 ; accepted for publication 14 February 200 1)
Isochronal thermal annealing effect on the photolurnmescence (PL) spectra of lntersublevel
transltlons in InAs self-assembled quantum dots was investigated. Several peaks due to intersublevel
transitions in the quantum dots were observed in the PL spectra of two samples consisting of 10
stacks of InAs quantum dots and InP barriers. Isochronal hrnace annealing in the temperature range
of 500-800 "C was conducted on thc two san~plcs. The results show that the intensity of the PL
peaks was dramatically reduced, and a new peak attributed to the wetting layer was observed after
thc samples were thermally annealed above 550 "C. A small blue shift of thc PL peaks duc to
intermixing was observed. O 2001 Arnericail Institute of'Physic\.. [DOI: 10 1063/1.1363693]
Recent progress on the epitaxial growth of selfassen~bled
quantum dots, which are also called atomic designers,
open the door for many basic and applied investigations.
This is due to the fact that the growth of these
structures has progressed to the point where a good degree of
control on thc size and shape of the quantum dots can be
achieved. With this growth control, one can design a structure
with a specific interband or intersubband transitions that
could be utilized for a practical application. Recent applications
of quantum dots include lasing1-' and infrared
detectors.'-' Thermal annealing and intermixing effects on
the emission energies of quantum dots have been
reported."-" In this letter, we report on some results of recent
investigation of isochronal hrnace thermal annealing
effect on the photoluminescence (PL) spectra, obtained for
self-assembled InAs quantum dot structures. The PL spectra
exhibit several peaks due to intersublevel transitions in the
quantunl dot structures, as predicted by a model proposed by
Raymond et al. '"he observed PL peaks are not at a fixedenergy
spacing. However, several additional peaks with
fixed-encrgy spacing appcarcd in one samplc after annealing
at 500 OC. The emission from the wetting layer (WL) was
obscrved after thc samplcs were thermally annealed at or
above 500 OC. The intensities of the PL pcaks werc dramatically
reduced as the annealing temperature is increased. A
blueshift of the PL peaks energy positions, due to intermixing,
is observed to be significant at annealing temperatures
above 600 "C.
The InAs quantum dots were grown in a solid source
molecular beam epitaxy system in conjunction with a scanning
tunneling microscope. The substrate temperature was
maintained at 480 OC during growth with an accuracy of
+2 OC." The growth rate was -0.3 monolayer/s. Two
samples were chosen for the present study and their characteristics
are shown on Table I. The PL spectra were mea-
"'Electron~c mail: manasreh~eece.unm.rdu
sured using a BOMEM Fourier-transform interferolnetsl.
with a PL attachment in conjunction with a contjnuous Row
cryostat. The temperature was controlled within z l .(J K and
the spectra were measured at 77 K. Furnace isocl,~.onal thsrma1
annealing was performed in a continuous flow of nittbgen
gas in thc annealing temperature (T,) rangc of 500-
800 "C. The samples were sandwiched betu.:.cn semiinsulating
GaAs wafers during annealing ;: . -J;,.t the Icss
of As. The annealing timc at each temperature w,rs I- ,I]; I-
utes.
Selected PL spectra of intersublevel transitit)rls in ,ihe
two InAs quantum dot samples beforc and aftel. :i;errnal ail;
nealing at different temperatures are plotted in F~gs. 1 and 2.
In both samples, the PL peakfrom the WL is ob>~rved only
aftcr annealing at 550 "C (sample A) or 500 "C (sa:lplc B) at
around 1.4 eV. The PL spectra show a series of pcqks labeled
1 to 6 in sample A and 1 to 5 in sample B. These peaks are
due to intersublevel transitions within the quantum dots. The
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energy spacing between these. peaks is not fixed. but it is
ranging between 32 and 56 nieV. This is in disagreenlent
with previous which indicates that thc cner:gy '.:
spacing between the peaks is fixed. This discrepa~icv !nay P@
due to the fact that the thickness and the size of tl;: quaiinlrn
dots used in the present study are not quite unifhnl~. Mmeovcr,
the prcscnt sa~nplcs are composed of 10 stack of quiiii-':.
tum dots, while the previous measurements (Refs S and 15)
were made on single layer of quantul;l i[~ii,:c c:e
would expect to see more variation in the enrig) I;~!-L:I,:
betwccn peaks in sanlples with lnultistacks as comparcd toaii
. '
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single layer of quantum dots.
Fixed-energy spacing between intcrsublevcls i:) ql~a~~ihn
dots was proposed in a model16 to explain excited.-state.r;ldiativc
lifetimes in sclf assembled quantum dots Howevil.
fixed-energy spacing is not common in conventional qnin-
turn wells.1R20 On the other hand, we have obssr~cd peaks
in sample B with tixed-energy spacing that may s~ipport Ray:
inond et 01. inodel.'"~ spectra with additiotlal sebsral peak
between 0.6-0.8 eV were observed after annealing s3nlplq'B,
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0003-6951/2001/78(15)/2196/3/$18.00 21 96 Q 2001 American lnstrtute of Physics