Improved infrared photoluminescence characteristics from circularly ordered self-assembled Ge islands
© Das et al; licensee Springer. 2011
Received: 28 October 2010
Accepted: 9 June 2011
Published: 9 June 2011
The formation of circularly ordered Ge-islands on Si(001) has been achieved because of nonuniform strain field around the periphery of the holes patterned by focused ion beam in combination with a self-assembled growth using molecular beam epitaxy. The photoluminescence (PL) spectra obtained from patterned areas (i.e., ordered islands) show a significant signal enhancement, which sustained till 200 K, without any vertical stacking of islands. The origin of two activation energies in temperature-dependent PL spectra of the ordered islands has been explained in detail.
The confinement of charge carriers in low-dimensional Ge/Si heterostructures allows one to increase the efficiency of the radiative recombination, making the indirect gap group-IV semiconductors attractive for optical devices. Owing to the type-II band alignment , Ge dots form a potential well only for holes, whereas the electrons are weakly confined in the vicinity of the Ge dots, i.e., by the tensile strain field in the Si cap induced by Ge quantum dots (QDs) [2, 3]. The resulting recombination energy depends strongly on size, shape, strain, and composition of the QDs leading to a wide emission energy spectrum. Therefore, intensive effort is currently undertaken to prepare arrays of "identical" QDs, which emit in a resonant mode . Infrared (IR) photoluminescence (PL) at room temperature has been reported by vertical ordering of Ge islands in three-dimensional stack of 10-20 periods [5, 6].
To improve the lateral ordering of QDs, one of the strategies is to convert the stochastic nucleation process into a deterministic one by directing nucleation on the predefined surface sites, using a combination of self-assembly and surface pre-patterning [7–10]. In general, the 2D Ge dot arrays reported so far have considerably larger inter-dot distance, thus lateral coupling is quite weak. The IR PL emission from randomly distributed islands is reported to be quenched at a relatively low temperature [2, 11], because of thermal dissociation of excitons. In this article, we report the superior IR PL characteristics, which exist up to a temperature as high as 200 K, owing to lateral coupling in circularly ordered Ge islands on pre-patterned Si (001) substrates.
Ge QDs were grown by solid source molecular beam epitaxy (MBE) on focused ion beam (FIB) patterned (FEI HELIOS 600 dual beam system) substrates. The Si (001) substrate surface was patterned with two-dimensional periodic hole arrays using an FIB with Ga+ ion energy of 30 keV and a beam current of 21 pA. Arrays of about 50 × 50 holes of diameter in the range of 100-200 nm and depth varying from 20 to 50 nm were fabricated at a fixed volume per dose (0.15 μm3/nC). The hole spacing and pitch were varied from 50 to nearly 200 nm and 50 to 600 nm, respectively. After removing Ga contamination from the surface, Ge QDs were grown using solid source MBE (Riber Supra 32) system using an electron gun for the deposition of thin buffer layer (approx. 5 nm) of Si with a growth rate of 0.4 Å/s, and a Knudsen cell for Ge deposition followed by a 2-nm Si cap layer. The Ge growth rate was kept constant at 0.5 Å/s at a substrate temperature of 580°C. PL spectra were recorded under excitation from a 325-nm He-Cd laser line with an output power of 1.3 W/cm2 using a standard lock-in technique and a liquid N2 cooled InGaAs detector with a spectral range of 0.9-2.1 μm. The laser beam with a spot size of less than 500 μm was used for the selective probing of the sample in the patterned region.
Results and discussion
The transition from 2D layer to 3D island mode for Ge growth occurs randomly on unpatterned substrates, whereas the same occurs preferentially in a circular organization on patterned substrates. It is known that the surface energy of a virgin surface can be increased significantly by ion bombardment. The difference between the chemical potential of a patterned surface and that planar one is described by the change of the surface energy with the surface curvature and the change of the local strain energy induced by the holes . Therefore, when the effect of the stress dominates the surface energy component, the nucleation of dots takes place preferentially on the edges of the holes resulting in circularly ordered islands. The formation of islands in between holes from the residual Ge available on the substrate can be reduced by reducing the pitch of the array, since the mean free path for Ge diffusion is limited .
Summary of different PL peak energies and their origins for both unpatterned and patterned samples
Peak energy (meV)
65 ± 7
7 ± 1
95 ± 8
18 ± 2
Patterned (500 nm pitch)
75 ± 5
17 ± 1
In conclusion, we have grown the circularly ordered Ge islands by MBE on FIB patterned Si(001) surfaces. The PL spectra obtained from the ordered islands show the existence of the signal up to a temperature as high as 200 K, as compared to 45 K for the control sample. The improvement in PL characteristics in 2D array is attributed to lateral coupling between Ge QDs in the circularly ordered islands. The observed two thermal-quenching activation energies are explained by the competition between phonon-assisted hole tunneling and hole thermoionic emission over the valence band energy barriers at the heterointerfaces.
atomic force microscope
focused ion beam
high-resolution transmission electron microscopy
molecular beam epitaxy
scanning electron microscopy.
The research by K. Das and A. K. Raychaudhuri is supported by the Department of Science and Technology, Government of India as a Centre for Nanotechnology. The research at IIT Kharagpur is supported by DST MBE and DRDO FIR project grant, Government of India.
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