Tuning of conduction intersublevel absorption wavelengths in (In,Ga)As/GaAsquantum-dot nanostructuresDong Pan, Elias Towe, Steve Kennerly, and Mei-Ying Kong Citation: Applied Physics Letters 76, 3537 (2000); doi: 10.1063/1.126699 View online: http://dx.doi.org/10.1063/1.126699 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/76/24?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Electric field control of a quantum dot molecule through optical excitation Appl. Phys. Lett. 96, 211115 (2010); 10.1063/1.3430506 Selective area wavelength tuning of InAs/GaAs quantum dots obtained by TiO 2 and SiO 2 layer patterning Appl. Phys. Lett. 94, 161906 (2009); 10.1063/1.3120229 Effects of alloy intermixing on the lateral confinement potential in In As ∕ Ga As self-assembled quantum dotsprobed by intersublevel absorption spectroscopy Appl. Phys. Lett. 90, 163107 (2007); 10.1063/1.2724893 Lithographic tuning of photonic-crystal unit-cell resonators with In Ga As ∕ Ga As quantum dots emitting at 1.2 μm J. Vac. Sci. Technol. B 23, 252 (2005); 10.1116/1.1852464 Tuning InAs/GaAs quantum dot properties under Stranski-Krastanov growth mode for 1.3 μm applications J. Appl. Phys. 91, 6710 (2002); 10.1063/1.1476069

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Tuning of conduction intersublevel absorption wavelengthsin „In,Ga…As ÕGaAs quantum-dot nanostructures

Dong Pana) and Elias Toweb)

Laboratory for Optics and Quantum Electronics, University of Virginia, Charlottesville,Virginia 22903-2442

Steve KennerlyUS Army Research Laboratory, Adelphi, Maryland 20783

Mei-Ying KongSemiconductor Institute, Chinese Academy of Science, Beijing, 100083, China

~Received 8 October 1999; accepted for publication 18 April 2000!

We demonstrate that by increasing the amount of~In, Ga!As deposit in a quantum dot layer, theintersublevel absorption wavelength for~In, Ga!As/GaAs quantum-dot infrared photodetectors canbe blue-shifted from 15 to 10mm while the photoluminescence peak is red-shifted. We directlycompare the measured energy spacing between intersublevels obtained from infrared absorptionspectroscopy with those obtained from photoluminescence spectroscopy. We find that theintersublevel energy spacing determined from absorption measurements is much larger than thatobtained from the photoluminescence measurements. ©2000 American Institute of Physics.@S0003-6951~00!04524-1#

It has been suggested that the first decade of the 21st

century will witness the development of a new generation ofelectronic and optoelectronic devices based on nanoscienceand nanoengineering principles. The first generation of suchdevices is likely to make extensive use of quantum dot nano-structures. In fact, there is already intensive research focusedon the study of intersublevel transitions in~In, Ga!As/GaAsquantum dots for applications in one such potential device: along wavelength infrared photodetector.1–7 Novel normal-incidence quantum-dot infrared photodetectors have been re-alized using either ~In, Ga!As/GaAs1–4 or ~In, Ga!As/~In, Ga!P5 heterostructures. For infrared sensing applications,the control of the intersublevel absorption wavelength in thedots is essential. Quantum dot nanostructures are currentlysynthesized via what is known as the Stranski–Krastanowgrowth mode during molecular beam epitaxy or metal-organic chemical vapor deposition.8 The essential physics ofa single dot is embodied in its size, shape, and the strainassociated with it. These parameters can be used to adjust theintersublevel energy spacing in both the conduction and va-lence bands of the dot. In past experiments, the manipulationof the energy levels of quantum dots has been achieved byadjusting the growth conditions; these directly control theshape and size of the dots; the level separation can also bemanipulated byex situ rapid thermal annealing. In one ex-periment, Fafardet al.9,10 have found that the intersublevelenergy spacing can be decreased by increasing the InAs cov-erage; this is most likely a consequence of an increase in thesize of the dot. This approach to controlling the dot param-eters, however, is not universal. A device such as the quan-tum dot infrared photodetector requires a high dot density foracceptable signal responsivity and high quantum efficiency.For structures with high dot densities~e.g.,.1010cm2!, thedot size is almost impossible to increase beyond a certain

critical size before the onset of dot coalescence. It is there-fore highly desirable to develop an alternative approach forchanging or tuning the intersublevel absorption energy~wavelength! of the~In, Ga!As/GaAs quantum dot nanostruc-tures.

The confined energy level locations and separations inquantum dot structures have been widely studied by low-temperature photoluminescence spectroscopy. In this type ofmeasurement, the multiple peaks observed in the low-temperature spectra give a very rough estimate of the trueenergy separations. The measured estimates are typically ob-tained by noting the energy differences between adjacentpeaks. However, an analysis such as this, obtained from mea-surements which essentially giveinterband transition ener-gies, have never been directly compared to measurementswhich give energy information about conductionintersub-level transitions; for example, intersublevel infrared absorp-tion measurements.

In this letter we show that with increasing~In, Ga!Asdeposit in the quantum-dot layer, the intersublevel absorp-tion wavelength can be blue-shifted from 15 to 10mm. Wedirectly compare the intersublevel energy spacing obtainedfrom infrared absorption spectra with that obtained fromlow-temperature photoluminescence measurements. It isfound that the intersublevel energy spacing, as determinedfrom the photoluminescence spectra, is much smaller thanthat obtained from the absorption measurements.

Our samples were grown by solid-source molecularbeam epitaxy. The prototypical structure of our samples issimilar to that in our previous work on quantum dots.1,3,6 Itconsists of a 1.0mm n1-doped GaAs layer, followed by an~In, Ga!As/GaAs quantum dot superlattice, which is cappedby a 1.0 mm n1-doped GaAs layer. The nominal indiumcomposition in the~In, Ga!As superlattice layer is about30%. The GaAs barrier layer in the superlattice is sufficientlylarge~30–50 nm! that the vertical quantum mechanical cou-pling of the dots can be ignored. The major difference among

a!Electronic mail: dp7u@virginia.edub!Electronic mail: towe@virginia.edu

APPLIED PHYSICS LETTERS VOLUME 76, NUMBER 24 12 JUNE 2000

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the samples is the amount of~In, Ga!As deposit in thequantum-dot layers for each sample. The nominal thicknessof the ~In, Ga!As layer is 46, 62, 70, and 80 Å, respectivelyfor samples A–D. The dot density in sample A is estimatedto be around 131010cm2 from transmission electron micros-copy ~TEM! observations. The dot density for samples B–Dis 331010, 431010, and 731010cm2, respectively. Thesevalues are determined from atomic force microscopy mea-surements performed on uncapped samples grown underidentical conditions.

The optical characterization measurements were carriedout as discussed in the following. The absorption measure-ments were performed under normal-incidence conditions; aroom-temperature absorption spectrum was measured forsample A with a Fourier transform infrared spectrometer.Samples B–D were fabricated into devices for low-temperature photocurrent measurements. Figure 1 shows theroom-temperature spectrum of sample A, and the photocur-rent spectra for the other three samples. It is evident that withan increase of the~In, Ga!As nominal thickness, the intersub-level absorption wavelength is blue-shifted from 14.5~forsample A! to 10mm ~for sample D!. Although the absorptionspectrum for sample A was taken at room temperature, theabsorption peak at low temperature can be extrapolated. InRef. 7, we showed that the absorption wavelength of a quan-tum dot nanostructure~sample B! at room temperature isblue-shifted by about 0.5mm from its low-temperaturevalue. By inference therefore, the absorption peak, at lowtemperature, for sample A should be located roughly at 15mm.

There are a number of reasons why one would observesuch a blue-shift. It could be due to a change in the dot size,a change in the indium composition of the dots, or a changein the shape of the dots. Another possibility is coupling be-tween the dots. The following experiment suggests that theunderlying mechanism for the blue-shift could be a result ofenhanced coupling between the dots. We have measured thelow-temperature ~77 K! photoluminescence spectra ofsamples A–D; Fig. 2 shows the spectra. Note that increasingthe ~In, Ga!As deposit in the dot layers results in a red-shiftof the photoluminescence peak. This red-shift is accompa-nied by a corresponding spectral narrowing. Observe, for ex-

ample, that the peak for sample D, which is located at 1.18eV, has a spectral width of 25 meV; the peak for sample A,which has a spectral width of 53 meV, is located at 1.25 eV.The corresponding dot density for sample A is 131010cm2, and it is 731010cm2 for sample D.

The information from the photoluminescence spectrasuggests that enhanced lateral coupling between the dotsmight be responsible for the observed wavelength tuning. Ina previous letter, Solomonet al. have reported on verticallyaligned, quantum mechanically coupled dots whose photolu-minescence spectra exhibit behavior similar to ourobservations.11 We believe that the mechanism operative intheir vertical dot is identical to that in our lateral dots. Wespeculate that when the areal dot density is sufficiently high,an increase in~In, Ga!As deposit primarily leads to an in-crease in dot density—and not dot size; this in turn leads toan enhanced lateral coupling between the dots.

It is known that self-assembled quantum-dots synthe-sized under the Stranski–Krastanow growth mode exhibit alarge size distribution, and hence a complex overall energyspectrum. If the dots are widely separated~sparse!, then thereis little quantum mechanical coupling between them, so con-fined electrons will have a very small probability of tunnel-ing between the dots. When the dot nanostructures are pho-toexcited with light whose photon energy is larger than theaverage quantum dot band gap, light of different wave-lengths will be emitted. This results in a broad compositeluminescence spectrum. The typical spectral linewidth of aself-formed quantum dot nanostructure is around 50–70meV; our sample A, for example, has a spectral width of 53meV. As the lateral coupling between the dots is increased,the electronic energy structure of the dots forms a miniband.In the simplified picture, this miniband is formed from thesum of the bonding and antibonding states from the overlapof the individual quantum dot states. The photoexcited elec-trons with higher energy will transfer to the lowest energylevel within the miniband to emit photons. The photolumi-nescence peak from such a miniband is red-shifted, with a

FIG. 1. Room-temperature absorption spectrum for sample A, and the low-temperature~40 K! photocurrent spectra of samples B–D.

FIG. 2. Low-temperature~77 K! photoluminescence spectra for samplesA–D.

3538 Appl. Phys. Lett., Vol. 76, No. 24, 12 June 2000 Pan et al.

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corresponding reduction in the spectral width, as seen in Fig.2. The spectral narrowing is due to the fact that the emissionis from the bottom of the conduction miniband instead ofseveral different energy levels. The excited state levels of thedots in the conduction band, on the other hand, are intrinsi-cally insensitive to changes in the degree of quantum me-chanical coupling. This is because the excited state energylevel is itself either a continuum state or a quasicontinumquantum well-like state due to the existence of wetting layersin the dots. In the final analysis, the intersubband energyspacing increases with increasing dot density. Note that thisresult is in contrast to the results reported by Fafardet al.9,10

Figure 3 shows the change of intersubband transition energy~as a result of manipulating the dot areal density! plotted as afunction of photoluminescence emission peak location. Wenote that an energy band gap shift of 62 meV~as determinedby photoluminescence spectroscopy! results in a conductionintersubband energy change of 42 meV~from 15 to 10mm!in the absorption signature. These results suggest that lateralcoupling may be a more effective method for changing theintersubband energy spacing in the conduction band than inthe valence band. This is the first time that the shift in energyin the photoluminescence band gap of quantum dots has beencorrelated with a change in the intersubband energy spacingin the conduction band of such nanostructures. This kind ofcorrelation is useful because it can be used to rapidly esti-mate the absorption wavelength of quantum dot nanostruc-tures from the shift of the photoluminescence emission peak.

It is interesting to compare, as we noted earlier, the en-ergy spacing between intersubband transitions determinedfrom infrared absorption measurements to that obtained fromseparations between adjacent peaks in photoluminescencespectra. Figure 4 shows the low-temperature photolumines-cence spectra obtained when the excitation power of thepump laser is increased to about 100 times that used in theexperiment that yielded the spectra in Fig. 1. The spectra inFig. 4 are for samples B~a 13mm photodetector! and D ~a10 mm photodetector!. The emergence of a second peak isevident in both spectra; this is due to filling of the excitedstates from which recombination radiation is now possible.The energy separation between the observed two peaks is 47meV for sample B and it is 51 meV for sample D. Theseenergy spacings are very different from what is obtainedfrom the infrared absorption measurements. For instance, our10 mm photodetector has an intersubband energy spacing of

124 meV. Since infrared absorption spectroscopy is a moredirect method for measuring the intersubband energy spac-ing, it gives a more complete picture of the confined energylevels in the conduction band of a quantum dot. Because ofthe complexity of the valence band structure~owing to theexistence of a light- and heavy-hole sublevels! and opticaltransition selection rules, photoluminescence spectra are notstraight-forward to interpret vis-a`-vis the energy spacing be-tween confined levels in the conduction and valence bands ofa quantum dot.

In summary, we have demonstrated a practical approachto tuning the conduction intersublevel transition wavelengthsfor ~In, Ga!As/GaAs quantum dot nanostructures. By in-creasing the areal density of the dots, they become electroni-cally coupled; this results in a tuning of the absorption wave-length. Since there is no clear reproducible method forcontrolling the size and shape of dots at this time, the tech-nique of changing the density offers a much easier way tunethe wavelength.

This work is supported by the Army Research Office andthe U.S. Army Research Laboratory in Adelphi, Maryland.The portion of the work by M. Y. Kong is supported by theChinese Academy of Science.

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FIG. 3. Primary conduction intersubband energy spacing as a function ofobserved photoluminescence peak energy.

FIG. 4. Low-temperature~77 K! photoluminescence spectra for samples Band D. The excitation power used in this experiment is 100 times higherthan that used in the experiments in Fig. 2.

3539Appl. Phys. Lett., Vol. 76, No. 24, 12 June 2000 Pan et al.

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