Local Control of energy structure of semiconductor quantum dots
using phase change material
Motoki Takahashi, Nobuhiro Tsumori, and Toshiharu Saiki
Graduate School of Science and Technology, Keio University

Abstract
We proposed a new method to precisely control the emission energy of semiconductor quantum dot(QD) by applying a local strain due to a volume expansion of phase change GeSbTe uponamorphizatoin. We experimentally demonstrated the method through PL spectroscopy of InAs QDs
on which a GeSbTe thin film is deposited. Significant red shift of PL peak energy uponamorphization and its recovery due to recrystallization by laser annealing were observed.

1. Introduction
Semiconductor quantum dots (QDs) have unique characteristics such as discrete energy
levels and strong electron-hole interaction due to carrier confinement in a nanoscale volume. We cantake advantage of these properties in the application of quantum communication and quantum
information processing. One of the main challenges to be addressed in these applications is precise
control of energy structure of electrons confined in a QD by applying a magnetic field [1], electricfield [2], or mechanical strain [3]. For the energy level tuning with a magnetic field, an elaborate
apparatus is needed to generate a strong field. In the case of electric field application, a strong field
causes a decrease and broadening of photoluminescence (PL) emission line due to the reduction inelectron-hole wavefunction overlap. Besides these drawbacks, in the current techniques, the external
field cannot be applied to an individual QD.
In this study we propose a new method to locally apply mechanical strain that allows
precise energy control by using a volume expansion of phase change material upon amorphization.

2. Principle
Figure 1 is a conceptual schematic of the proposed method. A crystalline GeSbTe thin film
was formed on a QD sample. Nanoscale amoprhization was induced by irradiation of femtosecondor subnanosecond laser pulse, which enables nonthermal amorphization below the melting point andtherefore thermal damage on the sample to be avoided. Since approximately 10% volume expansion
accompanies the transformation from crystalline to amorphous phase, amorphization of the GeSbTefilm, which is confined by a SiO2 layer, can give rise to significant strain on the QD. It is expectedthat compressive strain leads to a blue shift of the PL peak energy of QD while tensile strain causes ared shift. We can control the magnitude of peak shift because the amorphized volume is proportional
to the number of irradiation pulse.
It should be noted that the GeSbTe film also works as an optical mask for spatiallyselective observation of the strain induced QD. At the wavelength of visible excitation light, since
the extinction coefficient of amorphous phase is much smaller than that of crystalline phase, QDs
just below the amorphized region (amorphous aperture) is selectively illuminated and photoexcited.
For near-infrared PL emission, on the other hand, the GeSbTe film is almost transparent, which
ensures high collection efficiency of PL signal.

Fig. 1. Principle of local strain application to a QD using a phasechange thin film deposited on a QD sample.

3. Experimental and results
To experimentally demonstrate the proposed method we performed

anInAs/InP self-assembled QD sample covered with a GeSbTe thin film (Fig. 2). InAs QDs weregrown on an InP substrate by solid-source molecular beam epitaxy. After the growth of a 100-nm InPcap layer on the QDs, a 40-nm Ge10Sb2Te13 mask layer and a 50-nm SiO2 layer for GeSbTe
confinement were deposited by sputtering.

Ge10Sb2Te13 40 nm

SiO2 50 nm
Amorphous
Φ～1 μmInAs Quantum dot

InP cap layer 100 nm
Fig. 2. Sample structure.

Local amorphization was performed by focusing femtosecond Ti: Sapphire laser pulse (nm) or Subnanosecond Qthethrough a microscope objective. In

PL measurement the QDs were excited by He-Ne laser light and PL signal was detected by aspectrometer and a liquid nitrogen cooled InGaAs diode array. All the measurement was performed
at 7K in a cryostat.
Figure 3 demonstrates the optical mask effect of the GeSbTe film. Compared to a
non-masked region (blue), the number of QDs observed through the amorphous aperture (red) is
significantly reduced. Figure 4 shows PL peak shifts accompanying amorphization and successive
recrystallization of the GeSbTe film. Upon amorphization the PL peak exhibited a red shift and it
recovered to original peak position when the amorphized region is recrystallized by laser annealing.
In Fig. 5, the maximum red shift as large as 1.2 meV was obtained.

Fig. 3. PL spectra of InAs QDs obtained at non-masked region(blue) and through the amorphous aperture(red).

Fig. 4. PL spectra of InAs QDs measured before (green) and after (red)amorphization. The PL peak energy recovered to the original position after
recrystallization by laser annealing (blue).

Fig. 5. The maximum PL peak shift we obtained upon amorphization.

4. Acknowledgement
We would like to thank R. Kubota and M. Gendry for providing us with an InAs QD
sample and K. Yusu for forming a GeSbTe film on the QD sample.

5. References[1] R. M. Stevenson, R. J. Young, P. See, D. G. Gevaux1, K. Cooper, P. Atkinson, I. Farrer, D. A.
Ritchie, and A. J. Shields, Phys. Rev. B 73, 033306 (2006).
[2] K. Kowalik, O. Krebs, A. Lemaître, S. Laurent, P. Senellart, P. Voisin, and J. A. Gaj, Appl. Phys.
Lett. 86, 041907 (2005).
[3] H. D. Robinson, M. G. Müller, B. B. Goldberg, and J. L. Merz, Appl. Phys. Lett. 72, 2081
(1998).
[4] David Strand, David V. Tsu, Robert Miller, Michael Hennessey, and David Jablonski, E/PCOS(2006).
[5] Nobuhiro Tsumori, Motoki Takahashi, Yoshiki Sakuma, and Toshiharu Saiki, Appl. Opt. 50,5710 (2011).