Self-assembled GaIn(N)As quantum dots are fabricated on GaAs by atmospheric pressuremetalorganic vapor-phase epitaxy using dimethylhydrazine (DMHy) precursor as a nitrogen source. The incorporation of nitrogen into the islands is observed to be negligible. However, the areal density of the islands is increased by up to one order of magnitude compared to that of the respective GaInAs islands. The GaIn(N)As island size can also be controlled by varying the DMHy flow. An enhancement of the room-temperature luminescence at 1.3 μm is observed in the GaIn(N)As samples grown with DMHy.Peer reviewe

Hakkarainen, T.

Lipsanen, Harri

Sopanen, Markku

Toivonen, J.

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This is an electronic reprint of the original article.This reprint may differ from the original in pagination and typographic detail.Author(s): Hakkarainen, T. & Toivonen, J. & Sopanen, M. & Lipsanen, HarriTitle: Self-assembled GaIn(N)As quantum dots: Enhanced luminescence at1.3 µmYear: 2001Version: Final published versionPlease cite the original version:Hakkarainen, T. & Toivonen, J. & Sopanen, M. & Lipsanen, Harri. 2001. Self-assembledGaIn(N)As quantum dots: Enhanced luminescence at 1.3 µm. Applied Physics Letters.Volume 79, Issue 24. P. 3932-3934. ISSN 0003-6951 (printed). DOI: 10.1063/1.1425082.Rights: © 2001 AIP Publishing. This article may be downloaded for personal use only. Any other use requires priorpermission of the author and the American Institute of Physics. http://scitation.aip.org/content/aip/journal/japAll material supplied via Aaltodoc is protected by copyright and other intellectual property rights, andduplication or sale of all or part of any of the repository collections is not permitted, except that material maybe duplicated by you for your research use or educational purposes in electronic or print form. You mustobtain permission for any other use. Electronic or print copies may not be offered, whether for sale orotherwise to anyone who is not an authorised user.Powered by TCPDF (www.tcpdf.org)Self-assembled GaIn(N)As quantum dots: Enhanced luminescence at 1.3 μmT. Hakkarainen, J. Toivonen, M. Sopanen, and H. Lipsanen  Citation: Applied Physics Letters 79, 3932 (2001); doi: 10.1063/1.1425082 View online: http://dx.doi.org/10.1063/1.1425082 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/79/24?ver=pdfcov Published by the AIP Publishing  Articles you may be interested in Structural and optical properties of self-assembled InAs quantum dot molecules on GaAs substrates J. Vac. Sci. Technol. B 28, 1271 (2010); 10.1116/1.3516010  The role of arsine in the self-assembled growth of In As ∕ Ga As quantum dots by metal organic chemical vapordeposition J. Appl. Phys. 99, 044908 (2006); 10.1063/1.2173687  Controlling alloy composition of InAsP self-assembled quantum dots embedded in GaAs J. Appl. Phys. 94, 3051 (2003); 10.1063/1.1597762  Optical and structural properties of InAsP ternary self-assembled quantum dots embedded in GaAs Appl. Phys. Lett. 81, 2953 (2002); 10.1063/1.1513215  Self-assembled GaInNAs quantum dots for 1.3 and 1.55 μm emission on GaAs Appl. Phys. Lett. 76, 994 (2000); 10.1063/1.125917    This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:130.233.216.27 On: Mon, 04 May 2015 10:50:40Self-assembled GaInNAs quantum dots: Enhanced luminescenceat 1.3 mmT. Hakkarainen,a) J. Toivonen, M. Sopanen, and H. LipsanenOptoelectronics Laboratory, Helsinki University of Technology, P.O. Box 3000, FIN-02015 HUT, Finland~Received 31 July 2001; accepted for publication 1 October 2001!Self-assembled GaIn~N!As quantum dots are fabricated on GaAs by atmospheric pressuremetalorganic vapor-phase epitaxy using dimethylhydrazine ~DMHy! precursor as a nitrogen source.The incorporation of nitrogen into the islands is observed to be negligible. However, the arealdensity of the islands is increased by up to one order of magnitude compared to that of the respectiveGaInAs islands. The GaIn~N!As island size can also be controlled by varying the DMHy flow. Anenhancement of the room-temperature luminescence at 1.3 mm is observed in the GaIn~N!Assamples grown with DMHy. © 2001 American Institute of Physics. @DOI: 10.1063/1.1425082#In the last few years there has been considerable atten-tion to material and device research for 1.3 and 1.55 mmlaser structures grown on GaAs substrates. One method toachieve 1.3 mm laser emission on GaAs is to use self-assembled GaInAs quantum dots ~QDs! in the activeregion.1,2 The same emission wavelength can also beachieved with InAs QDs embedded in GaAs3 or inGaInAs.4,5 The GaInNAs alloy has recently been proposed asa possible material for long wavelength lasers,6 andGaInNAs/GaAs quantum well ~QW! lasers with emissionwavelengths around 1.3 mm have been fabricated by molecu-lar beam epitaxy ~MBE!7–9 and by metalorganic vapor-phaseepitaxy ~MOVPE!.10,11 Solid-source MBE has been used tofabricate GaInNAs/GaAs QW laser diodes operating at 1.52mm.12 Room-temperature photoluminescence at 1.52 mm hasrecently been reported from GaInNAs QDs grown by gas-source MBE.13 GaInNAs QDs have also been grown bychemical beam epitaxy ~CBE!,14 but until now there has beenno reports of MOVPE-grown GaInNAs QDs. In this workwe have grown self-assembled GaIn~N!As QDs by atmo-spheric pressure MOVPE and investigated the effect of ni-trogen on formation and optical properties of the GaIn~N!Asislands.The samples were grown on semi-insulating GaAs ~100!substrates in a horizontal MOVPE reactor at atmosphericpressure. Trimethylgallium ~TMGa!, trimethylindium~TMIn!, tertiarybutylarsine ~TBAs!, and dimethylhydrazine~DMHy! were used as sources for gallium, indium, arsenic,and nitrogen, respectively. The sample structure consists of a100-nm-thick GaAs buffer layer grown at 650 °C, a layer ofGa12xInx~N!As islands with the In composition of x50.45or x50.6 and a GaAs cap layer in some of the samples. TheGaIn~N!As layer and the cap layer were grown at the sametemperature, varied in the range of 450–570 °C. All the tem-peratures mentioned here are thermocouple readings. Thenominal growth rate and the nominal thickness were variedin the range of 0.2–4 ML/s and 2.5–8 ML, respectively. Theuncovered islands on the sample surface were imaged byatomic force microscopy ~AFM! using SiN tips. The opticalproperties of the covered islands were investigated by lowtemperature ~LT!, i.e., 10 K, and room-temperature ~RT!photoluminescence ~PL! measurements. An argon ion laserwas used for excitation and the luminescence was detectedby a liquid-nitrogen-cooled Ge detector.To investigate how the nominal coverage affects the is-land formation, uncovered islands with the In composition ofx50.6 were grown at 530 °C with the DMHy/TBAs ratio of16 and the nominal growth rate of 1.3 ML/s. This growthrate was chosen because the island density was observed tobe highest within a growth rate range of 1.3–2 ML/s. Figure1~a! shows an AFM image from a 3 ML Ga0.4In0.6~N!Assample with an island density of 33108 cm22 and an aver-age island height of 8 nm. The coverage of 3 ML is close tothe threshold thickness for island formation since no islandswere observed when the coverage was decreased to 2.5 ML.Figures 1~b! and 1~c! show AFM images from 3.5 and 4.5ML Ga0.4In0.6~N!As samples, respectively. The areal densityand the average height of the islands in the 4.5 ML sampleare 531010 cm22 and 5 nm, respectively. An AFM imagea!Author to whom correspondence should be addressed; electronic mail:thakkara@cc.hut.fiFIG. 1. AFM images taken from QD samples grown at 530 °C with ~a! 3ML Ga0.4In0.6~N!As, ~b! 3.5 ML Ga0.4In0.6~N!As, ~c! 4.5 MLGa0.4In0.6~N!As, and ~d! 4.5 ML Ga0.4In0.6As. The scan size is 0.530.5 mm2.APPLIED PHYSICS LETTERS VOLUME 79, NUMBER 24 10 DECEMBER 200139320003-6951/2001/79(24)/3932/3/$18.00 © 2001 American Institute of Physics This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:130.233.216.27 On: Mon, 04 May 2015 10:50:40from a 4.5 ML Ga0.4In0.6As sample grown without DMHyflow is shown in Fig. 1~d!. The island density in this N-freesample is smaller and the islands are higher on average com-pared to the sample grown with DMHy flow. This is in con-trast to the results that were observed for MBE growth.13Figures 2~a! and 2~b! show the island density and the aver-age island height, respectively, as a function of coverage.The maximum island density and the minimum island heightare achieved with the coverage of 4 ML. When the coverageis increased, the island density remains fairly constant in therange of 3 – 431010 cm22 and the average height of the is-lands increases to 9 nm. Also larger, relaxed islands with anareal density of 107 – 109 cm22 are observed in the samples.The areal density of these relaxed islands increases with in-creasing coverage.To investigate the optical properties of the islands, theislands were covered with a 50-nm-thick layer of GaAs. Fig-ure 3 shows LT-PL spectra from three QD samples: ~a! 3.5ML Ga0.4In0.6~N!As, ~b! 4.5 ML Ga0.4In0.6~N!As, and ~c! 4.5ML Ga0.4In0.6As. The major peak in each spectrum origi-nates from the covered islands acting as QDs. As the cover-age is increased from 3.5 to 4.5 ML the PL intensity de-creases since the density of large, incoherent islandsincreases. The PL peak of the Ga0.4In0.6As reference sampleis at smaller energy compared to the sample grown withDMHy flow. This indicates that a negligible amount of nitro-gen is incorporated into the islands. The energy difference isexplained by the fact that the average height of theGa0.4In0.6As islands is larger than that of the Ga0.4In0.6~N!Asislands. The full width at half maximum of the PL peak isabout the same for samples ~b! and ~c!. The reason for thenegligible incorporation of nitrogen lies behind the large Incomposition of the islands. It has been observed that theincorporation of nitrogen is seriously hindered by the pres-ence of In during MOVPE growth of GaInNAs.15 Themechanism behind this behavior is not completely under-stood yet, although it has been speculated that it could becaused by surface segregation of In.16 An attempt was madeto increase the incorporation of nitrogen by decreasing thegrowth temperature but this only resulted in degradation ofthe optical quality.To study whether more nitrogen could be incorporatedinto GaInNAs with lower In composition, samples with an Incomposition of x50.45 were grown. Since the wetting layermay become thicker due to the decreasing lattice mismatchwith decreasing In composition, the nominal coverage waschosen to be 8 ML to make sure that islands are formed.Figure 4 shows the dependence of the island density and theaverage island height on the DMHy/TBAs ratio. Also shownare the data for a Ga0.55In0.45As reference sample. The islanddensities in the Ga0.55In0.45~N!As QD samples are signifi-cantly larger compared to the reference sample for all theused DMHy/TBAs ratios. When the DMHy/TBAs ratio isincreased from 24 to 49 the average island height increasesfrom 7 to 13 nm. The height of the Ga0.55In0.45As islands is10 nm on average. Thus, by using DMHy during the islandgrowth the island density can be increased by one order ofmagnitude. On the other hand, the size of the islands can bedetermined by choosing the appropriate DMHy/TBAs ratio.Figures 5~a! and 5~b! show the RT-PL spectra fromGa0.55In0.45~N!As and Ga0.55In0.45As QD samples, respec-tively. The Ga0.55In0.45~N!As sample was grown with aDMHy/TBAs ratio of 24 and it has about one order of mag-nitude larger island density than the Ga0.55In0.45As referencesample. The spectrum of the reference sample exhibits a QDground state transition at 0.93 eV ~1.33 mm! and a dominat-ing wetting layer transition at 1.18 eV ~1.05 mm!. One canalso see an excited state transition 76 meV higher in energycompared to the ground state transition. On the contrary, theFIG. 2. ~a! Island density and ~b! average island height measured by AFMas a function of coverage for Ga0.4In0.6~N!As QD samples grown at 530 °C.The triangles show the respective data for the reference Ga0.4In0.6As QDsample.FIG. 3. LT-PL spectra from ~a! 3.5 ML Ga0.4In0.6~N!As, ~b! 4.5 MLGa0.4In0.6~N!As, and ~c! 4.5 ML Ga0.4In0.6As QD samples grown at 530 °C.FIG. 4. Island density and average island height as a function of DMHy/TBAs ratio for 8 ML Ga0.55In0.45~N!As QD samples grown at 520 °C. Thetriangles show the respective data for the reference Ga0.55In0.45As QDsample.3933Appl. Phys. Lett., Vol. 79, No. 24, 10 December 2001 Hakkarainen et al. This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:130.233.216.27 On: Mon, 04 May 2015 10:50:40Ga0.55In0.45~N!As sample shows no wetting layer transitionindicating efficient capture of the carriers into the QDs. Onecan see both a well pronounced ground state transition at0.98 eV ~1.26 mm! and an excited state transition 130 meVhigher in energy. The integrated PL intensity from theGa0.55In0.45~N!As sample is about 3 times larger than fromthe Ga0.55In0.45As QDs in Fig. 5~b!. The difference in theground state peak emission wavelength between the twosamples can again be explained by the larger size of theGa0.55In0.45As QDs. LT-PL measurements of theGa0.55In0.45~N!As samples grown with different DMHy/TBAs ratios are consistent with the assumption that the in-corporation of nitrogen is negligible. This shows that theincorporation of nitrogen into GaInNAs QDs grown by at-mospheric pressure MOVPE cannot be increased simply byincreasing the DMHy/TBAs ratio. However, the use ofDMHy flow during the growth of GaIn~N!As islands im-proves the areal density of the islands and enhances the PLintensity in the covered samples.In summary, self-assembled GaIn~N!As QDs weregrown by atmospheric pressure MOVPE. The areal densityof the GaIn~N!As islands is higher than that of the GaInAsislands grown with similar parameters. The island height ofthe GaIn~N!As islands can be varied to some extent bychanging the DMHy/TBAs ratio. PL studies of overgrownislands indicate that the incorporation of nitrogen is negli-gible. However, an enhancement of the 1.3 mm RT lumines-cence is observed. Thus, by introducing DMHy during theisland growth the optical properties of the GaIn~N!As islandscan be improved.1 D. L. Huffaker, G. Park, Z. Zou, O. B. Shchekin, and D. G. Deppe, Appl.Phys. Lett. 73, 2564 ~1998!.2 G. Park, O. B. Shchekin, S. Csutak, D. L. Huffaker, and D. G. Deppe,Appl. Phys. Lett. 75, 3267 ~1999!.3 I. Mukhametzhanov, R. Heitz, J. Zeng, P. Chen, and A. Madhukar, Appl.Phys. Lett. 73, 1841 ~1998!.4 V. M. Ustinov, N. A. Maleev, A. E. Zhukov, A. R. Kovsh, A. Yu. Egorov,A. V. Lunev, B. V. Volovik, I. L. Krestnikov, Yu. G. Musikhin, N. A. Bert,P. S. Kop’ev, Zh. I. Alferov, N. N. Ledentsov, and D. Bimberg, Appl.Phys. Lett. 74, 2815 ~1999!.5 K. Nishi, H. Saito, S. Sugou, and J.-S. Lee, Appl. Phys. Lett. 74, 1111~1999!.6 M. Kondow, K. Uomi, A. Niwa, T. Kitatani, S. Watahiki, and Y. Yazawa,Jpn. J. Appl. Phys., Part 1 35, 1273 ~1996!.7 K. Nakahara, M. Kondow, T. Kitatani, M. Larson, and K. Uomi, IEEEPhotonics Technol. Lett. 10, 487 ~1998!.8 A. Yu. Egorov, D. Bernklau, D. Livshits, V. Ustinov, Zh. I. Alferov, and H.Riechert, Electron. Lett. 35, 1643 ~1999!.9 M. Reinhardt, M. Fischer, M. Kamp, J. Hofmann, and A. Forchel, IEEEPhotonics Technol. Lett. 12, 239 ~2000!.10 F. Ho¨hnsdorf, J. Koch, S. Leu, W. Stolz, B. Borchert, and M. Druminski,Electron. Lett. 35, 571 ~1999!.11 C. Ellmers, F. Ho¨hnsdorf, J. Koch, C. Agert, S. Leu, D. Karaiskaj, M.Hofmann, W. Stolz, and W. W. Ru¨hle, Appl. Phys. Lett. 74, 2271 ~1999!.12 M. Fischer, M. Reinhardt, and A. Forchel, Electron. Lett. 36, 1208 ~2000!.13 M. Sopanen, H. P. Xin, and C. W. Tu, Appl. Phys. Lett. 76, 994 ~2000!.14 S. Makino, T. Miyamoto, K. Nageyama, F. Koyama, and K. Iga, J. Cryst.Growth 221, 561 ~2000!.15 R. Bhat, C. Caneau, L. Salamanca-Riba, W. Bi, and C. Tu, J. Cryst.Growth 195, 427 ~1998!.16 D. J. Friedman, J. F. Geisz, S. R. Kurtz, J. M. Olson, and R. Reedy, J.Cryst. Growth 195, 438 ~1998!.FIG. 5. RT-PL spectra from 8 ML ~a! Ga0.55In0.45~N!As and ~b!Ga0.55In0.45As QD samples grown at 520 °C.3934 Appl. Phys. Lett., Vol. 79, No. 24, 10 December 2001 Hakkarainen et al. This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:130.233.216.27 On: Mon, 04 May 2015 10:50:40

Self-assembled GaIn(N)As quantum dots: Enhanced luminescence at 1.3 µm

Aaltodoc

This is an electronic reprint of the original article.
This reprint may differ from the original in pagination and typographic detail.
Author(s): Hakkarainen, T. & Toivonen, J. & Sopanen, M. & Lipsanen, Harri
Title: Self-assembled GaIn(N)As quantum dots: Enhanced luminescence at
1.3 µm
Year: 2001
Version: Final published version
Please cite the original version:
Hakkarainen, T. & Toivonen, J. & Sopanen, M. & Lipsanen, Harri. 2001. Self-assembled
GaIn(N)As quantum dots: Enhanced luminescence at 1.3 µm. Applied Physics Letters.
Volume 79, Issue 24. P. 3932-3934. ISSN 0003-6951 (printed). DOI: 10.1063/1.1425082.
Rights: © 2001 AIP Publishing. This article may be downloaded for personal use only. Any other use requires prior
permission of the author and the American Institute of Physics. http://scitation.aip.org/content/aip/journal/jap
All material supplied via Aaltodoc is protected by copyright and other intellectual property rights, and
duplication or sale of all or part of any of the repository collections is not permitted, except that material may
be duplicated by you for your research use or educational purposes in electronic or print form. You must
obtain permission for any other use. Electronic or print copies may not be offered, whether for sale or
otherwise to anyone who is not an authorised user.
Powered by TCPDF (www.tcpdf.org)
Self-assembled GaIn(N)As quantum dots: Enhanced luminescence at 1.3 μm
T. Hakkarainen, J. Toivonen, M. Sopanen, and H. Lipsanen 
 
Citation: Applied Physics Letters 79, 3932 (2001); doi: 10.1063/1.1425082 
View online: http://dx.doi.org/10.1063/1.1425082 
View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/79/24?ver=pdfcov 
Published by the AIP Publishing 
 
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 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:
130.233.216.27 On: Mon, 04 May 2015 10:50:40
Self-assembled GaInNAs quantum dots: Enhanced luminescence
at 1.3 mm
T. Hakkarainen,a) J. Toivonen, M. Sopanen, and H. Lipsanen
Optoelectronics Laboratory, Helsinki University of Technology, P.O. Box 3000, FIN-02015 HUT, Finland
~Received 31 July 2001; accepted for publication 1 October 2001!
Self-assembled GaIn~N!As quantum dots are fabricated on GaAs by atmospheric pressure
metalorganic vapor-phase epitaxy using dimethylhydrazine ~DMHy! precursor as a nitrogen source.
The incorporation of nitrogen into the islands is observed to be negligible. However, the areal
density of the islands is increased by up to one order of magnitude compared to that of the respective
GaInAs islands. The GaIn~N!As island size can also be controlled by varying the DMHy flow. An
enhancement of the room-temperature luminescence at 1.3 mm is observed in the GaIn~N!As
samples grown with DMHy. © 2001 American Institute of Physics. @DOI: 10.1063/1.1425082#
In the last few years there has been considerable atten-
tion to material and device research for 1.3 and 1.55 mm
laser structures grown on GaAs substrates. One method to
achieve 1.3 mm laser emission on GaAs is to use self-
assembled GaInAs quantum dots ~QDs! in the active
region.1,2 The same emission wavelength can also be
achieved with InAs QDs embedded in GaAs3 or in
GaInAs.4,5 The GaInNAs alloy has recently been proposed as
a possible material for long wavelength lasers,6 and
GaInNAs/GaAs quantum well ~QW! lasers with emission
wavelengths around 1.3 mm have been fabricated by molecu-
lar beam epitaxy ~MBE!7–9 and by metalorganic vapor-phase
epitaxy ~MOVPE!.10,11 Solid-source MBE has been used to
fabricate GaInNAs/GaAs QW laser diodes operating at 1.52
mm.12 Room-temperature photoluminescence at 1.52 mm has
recently been reported from GaInNAs QDs grown by gas-
source MBE.13 GaInNAs QDs have also been grown by
chemical beam epitaxy ~CBE!,14 but until now there has been
no reports of MOVPE-grown GaInNAs QDs. In this work
we have grown self-assembled GaIn~N!As QDs by atmo-
spheric pressure MOVPE and investigated the effect of ni-
trogen on formation and optical properties of the GaIn~N!As
islands.
The samples were grown on semi-insulating GaAs ~100!
substrates in a horizontal MOVPE reactor at atmospheric
pressure. Trimethylgallium ~TMGa!, trimethylindium
~TMIn!, tertiarybutylarsine ~TBAs!, and dimethylhydrazine
~DMHy! were used as sources for gallium, indium, arsenic,
and nitrogen, respectively. The sample structure consists of a
100-nm-thick GaAs buffer layer grown at 650 °C, a layer of
Ga12xInx~N!As islands with the In composition of x50.45
or x50.6 and a GaAs cap layer in some of the samples. The
GaIn~N!As layer and the cap layer were grown at the same
temperature, varied in the range of 450–570 °C. All the tem-
peratures mentioned here are thermocouple readings. The
nominal growth rate and the nominal thickness were varied
in the range of 0.2–4 ML/s and 2.5–8 ML, respectively. The
uncovered islands on the sample surface were imaged by
atomic force microscopy ~AFM! using SiN tips. The optical
properties of the covered islands were investigated by low
temperature ~LT!, i.e., 10 K, and room-temperature ~RT!
photoluminescence ~PL! measurements. An argon ion laser
was used for excitation and the luminescence was detected
by a liquid-nitrogen-cooled Ge detector.
To investigate how the nominal coverage affects the is-
land formation, uncovered islands with the In composition of
x50.6 were grown at 530 °C with the DMHy/TBAs ratio of
16 and the nominal growth rate of 1.3 ML/s. This growth
rate was chosen because the island density was observed to
be highest within a growth rate range of 1.3–2 ML/s. Figure
1~a! shows an AFM image from a 3 ML Ga0.4In0.6~N!As
sample with an island density of 33108 cm22 and an aver-
age island height of 8 nm. The coverage of 3 ML is close to
the threshold thickness for island formation since no islands
were observed when the coverage was decreased to 2.5 ML.
Figures 1~b! and 1~c! show AFM images from 3.5 and 4.5
ML Ga0.4In0.6~N!As samples, respectively. The areal density
and the average height of the islands in the 4.5 ML sample
are 531010 cm22 and 5 nm, respectively. An AFM image
a!Author to whom correspondence should be addressed; electronic mail:
thakkara@cc.hut.fi
FIG. 1. AFM images taken from QD samples grown at 530 °C with ~a! 3
ML Ga0.4In0.6~N!As, ~b! 3.5 ML Ga0.4In0.6~N!As, ~c! 4.5 ML
Ga0.4In0.6~N!As, and ~d! 4.5 ML Ga0.4In0.6As. The scan size is 0.5
30.5 mm2.
APPLIED PHYSICS LETTERS VOLUME 79, NUMBER 24 10 DECEMBER 2001
39320003-6951/2001/79(24)/3932/3/$18.00 © 2001 American Institute of Physics This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:
130.233.216.27 On: Mon, 04 May 2015 10:50:40
from a 4.5 ML Ga0.4In0.6As sample grown without DMHy
flow is shown in Fig. 1~d!. The island density in this N-free
sample is smaller and the islands are higher on average com-
pared to the sample grown with DMHy flow. This is in con-
trast to the results that were observed for MBE growth.13
Figures 2~a! and 2~b! show the island density and the aver-
age island height, respectively, as a function of coverage.
The maximum island density and the minimum island height
are achieved with the coverage of 4 ML. When the coverage
is increased, the island density remains fairly constant in the
range of 3 – 431010 cm22 and the average height of the is-
lands increases to 9 nm. Also larger, relaxed islands with an
areal density of 107 – 109 cm22 are observed in the samples.
The areal density of these relaxed islands increases with in-
creasing coverage.
To investigate the optical properties of the islands, the
islands were covered with a 50-nm-thick layer of GaAs. Fig-
ure 3 shows LT-PL spectra from three QD samples: ~a! 3.5
ML Ga0.4In0.6~N!As, ~b! 4.5 ML Ga0.4In0.6~N!As, and ~c! 4.5
ML Ga0.4In0.6As. The major peak in each spectrum origi-
nates from the covered islands acting as QDs. As the cover-
age is increased from 3.5 to 4.5 ML the PL intensity de-
creases since the density of large, incoherent islands
increases. The PL peak of the Ga0.4In0.6As reference sample
is at smaller energy compared to the sample grown with
DMHy flow. This indicates that a negligible amount of nitro-
gen is incorporated into the islands. The energy difference is
explained by the fact that the average height of the
Ga0.4In0.6As islands is larger than that of the Ga0.4In0.6~N!As
islands. The full width at half maximum of the PL peak is
about the same for samples ~b! and ~c!. The reason for the
negligible incorporation of nitrogen lies behind the large In
composition of the islands. It has been observed that the
incorporation of nitrogen is seriously hindered by the pres-
ence of In during MOVPE growth of GaInNAs.15 The
mechanism behind this behavior is not completely under-
stood yet, although it has been speculated that it could be
caused by surface segregation of In.16 An attempt was made
to increase the incorporation of nitrogen by decreasing the
growth temperature but this only resulted in degradation of
the optical quality.
To study whether more nitrogen could be incorporated
into GaInNAs with lower In composition, samples with an In
composition of x50.45 were grown. Since the wetting layer
may become thicker due to the decreasing lattice mismatch
with decreasing In composition, the nominal coverage was
chosen to be 8 ML to make sure that islands are formed.
Figure 4 shows the dependence of the island density and the
average island height on the DMHy/TBAs ratio. Also shown
are the data for a Ga0.55In0.45As reference sample. The island
densities in the Ga0.55In0.45~N!As QD samples are signifi-
cantly larger compared to the reference sample for all the
used DMHy/TBAs ratios. When the DMHy/TBAs ratio is
increased from 24 to 49 the average island height increases
from 7 to 13 nm. The height of the Ga0.55In0.45As islands is
10 nm on average. Thus, by using DMHy during the island
growth the island density can be increased by one order of
magnitude. On the other hand, the size of the islands can be
determined by choosing the appropriate DMHy/TBAs ratio.
Figures 5~a! and 5~b! show the RT-PL spectra from
Ga0.55In0.45~N!As and Ga0.55In0.45As QD samples, respec-
tively. The Ga0.55In0.45~N!As sample was grown with a
DMHy/TBAs ratio of 24 and it has about one order of mag-
nitude larger island density than the Ga0.55In0.45As reference
sample. The spectrum of the reference sample exhibits a QD
ground state transition at 0.93 eV ~1.33 mm! and a dominat-
ing wetting layer transition at 1.18 eV ~1.05 mm!. One can
also see an excited state transition 76 meV higher in energy
compared to the ground state transition. On the contrary, the
FIG. 2. ~a! Island density and ~b! average island height measured by AFM
as a function of coverage for Ga0.4In0.6~N!As QD samples grown at 530 °C.
The triangles show the respective data for the reference Ga0.4In0.6As QD
sample.
FIG. 3. LT-PL spectra from ~a! 3.5 ML Ga0.4In0.6~N!As, ~b! 4.5 ML
Ga0.4In0.6~N!As, and ~c! 4.5 ML Ga0.4In0.6As QD samples grown at 530 °C.
FIG. 4. Island density and average island height as a function of DMHy/
TBAs ratio for 8 ML Ga0.55In0.45~N!As QD samples grown at 520 °C. The
triangles show the respective data for the reference Ga0.55In0.45As QD
sample.
3933Appl. Phys. Lett., Vol. 79, No. 24, 10 December 2001 Hakkarainen et al.
 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:
130.233.216.27 On: Mon, 04 May 2015 10:50:40
Ga0.55In0.45~N!As sample shows no wetting layer transition
indicating efficient capture of the carriers into the QDs. One
can see both a well pronounced ground state transition at
0.98 eV ~1.26 mm! and an excited state transition 130 meV
higher in energy. The integrated PL intensity from the
Ga0.55In0.45~N!As sample is about 3 times larger than from
the Ga0.55In0.45As QDs in Fig. 5~b!. The difference in the
ground state peak emission wavelength between the two
samples can again be explained by the larger size of the
Ga0.55In0.45As QDs. LT-PL measurements of the
Ga0.55In0.45~N!As samples grown with different DMHy/
TBAs ratios are consistent with the assumption that the in-
corporation of nitrogen is negligible. This shows that the
incorporation of nitrogen into GaInNAs QDs grown by at-
mospheric pressure MOVPE cannot be increased simply by
increasing the DMHy/TBAs ratio. However, the use of
DMHy flow during the growth of GaIn~N!As islands im-
proves the areal density of the islands and enhances the PL
intensity in the covered samples.
In summary, self-assembled GaIn~N!As QDs were
grown by atmospheric pressure MOVPE. The areal density
of the GaIn~N!As islands is higher than that of the GaInAs
islands grown with similar parameters. The island height of
the GaIn~N!As islands can be varied to some extent by
changing the DMHy/TBAs ratio. PL studies of overgrown
islands indicate that the incorporation of nitrogen is negli-
gible. However, an enhancement of the 1.3 mm RT lumines-
cence is observed. Thus, by introducing DMHy during the
island growth the optical properties of the GaIn~N!As islands
can be improved.
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FIG. 5. RT-PL spectra from 8 ML ~a! Ga0.55In0.45~N!As and ~b!
Ga0.55In0.45As QD samples grown at 520 °C.
3934 Appl. Phys. Lett., Vol. 79, No. 24, 10 December 2001 Hakkarainen et al.
 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:
130.233.216.27 On: Mon, 04 May 2015 10:50:40


English

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Self-assembled GaIn(N)As quantum dots: Enhanced luminescence at 1.3 µm

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