Electromodulated reflectance study of selfassembled Ge/Si quantum dots
 Andrew Yakimov^{1}Email author,
 Aleksandr Nikiforov^{1},
 Aleksei Bloshkin^{1} and
 Anatolii Dvurechenskii^{1}
DOI: 10.1186/1556276X6208
© Yakimov et al; licensee Springer. 2011
Received: 13 August 2010
Accepted: 9 March 2011
Published: 9 March 2011
Abstract
We perform an electroreflectance spectroscopy of Ge/Si selfassembled quantum dots in the nearinfrared and in the midinfrared spectral range. Up to three optical transitions are observed. The lowenergy resonance is proposed to correspond to a bandtocontinuum hole transition in the Ge valence band. The other two modulation signals are attributed to the spatially direct transitions between the electrons confined in the L and Δ(4) valleys of the Ge conduction band, and the localized hole states at the Γ point.
Introduction
In order to realize Sibased optoelectronics, Ge quantum dots (QDs) in Si matrices have attracted a large interest during the past years. Detailed knowledge on the electronic band structure and related optical transitions is very important when using selfassembled Ge/Si QDs in Sibased photonic devices. To date, most work on the optical properties of Ge/Si QDs is based on the photoluminescence (PL) spectroscopy [1–4]. However, as a rule, PL measurements provide information on the groundstate transitions only. To study highenergy excited states, it is more useful to perform absorption [5] or reflectance [6, 7] experiments. In this study, the optical transitions in layers of Ge/Si QDs are investigated by electroreflectance (ER) spectroscopy as a function of applied electric field.
Experimental details
The Si intrinsic region is not intentionally doped, nevertheless we estimated a residual MBE background doping at about 10^{16} cm^{3} of ptype. From scanning tunneling microscopy experiments, we observe the Ge nanoislands to be approximately 15 nm in lateral size and about 1.5 nm in height. They have the form of hut clusters bounded by {105} facets. The density of the dots is about 10^{11} cm^{2}. A typical PL spectrum of the dot sample (not shown here) is dominated by a broadband emission peaked around 820 meV.
The ER measurements were performed at room temperature using a stepscan VERTEX70 Fouriertransform infrared spectrometer. The incident light from a tungsten halogen lamp was unpolarized and the devices were under normal incidence. A 2kHz sinewave voltage with a peaktopeak amplitude of 200 mV served as an ac modulation source. Various values of reverse dc bias voltage U _{b} was employed. The modulation reflectance of the samples was filtered with a lockin amplifier before the Fourier transform. The phase correction recorded for background spectrum without the sample was used.
Results and discussion
We consider a realistic situation when both Si matrix and Ge nanoclusters are inhomogeneously strained due to the lattice mismatch between Si and Ge. The QD is assumed to have a pyramid shape with the base oriented along the [100] and [010] directions. The pyramid base is 15 nm and the height is 1.5 nm. The pyramid lies on a 4 ML Ge WL. First, the finite element calculations of threedimensional spatial distribution of strain components are performed. The strain modifies the band structure through the deformation potentials. A further numerical analysis of the band structure is based on a sixband k·p approach for the valence band and a singleband effectivemass approximation for the conduction band (CB) [9]. Coulomb interaction between the electron and hole is included into the problem.
From the comparison between the calculated transition energies (Figure 5) and the experimental ER spectrum (Figure 3) we may conclude that the lowenergy resonance corresponds to a bandtocontinuum hole transition in the Ge valence band. The other two modulation signals are attributed to the spatially direct transitions between the electrons confined in the L and Δ(4) valleys of the Ge CB, and the localized hole states at the Γ point.
According to the studies by Larsson et al. [3, 4] and by Adnane et al. [15], it is possible to observe the spatially direct recombination processes in the Ge/Si dot systems by using the PL measurements in specific experimental conditions which are elevated temperatures, higher excitation power [3, 4] or employment of PL excitation spectroscopy [15]. Unfortunately, all these conditions are inaccessible in our experimental setup, so we did not observe direct transitions mentioned above in our PL experiments.
Abbreviations
 CB:

conduction band
 ER:

electroreflectance
 MBE:

molecular beam epitaxy
 PL:

photoluminescence
 QDs:

quantum dots
 WLs:

wetting layers.
Declarations
Acknowledgements
The authors like to thank V.A. Volodin for Raman measurements. This study has been supported by the Russian Foundation for Basic Research (Grant No. 090212393).
Authors’ Affiliations
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