MBE growth of Ge/Si quantum dots upon low-energy pulsed ion irradiation

doi:10.1016/j.tsf.2008.08.166

Thin Solid Films

Volume 517, Issue 1, 3 November 2008, Pages 309-312

https://doi.org/10.1016/j.tsf.2008.08.166 Get rights and content

Abstract

Photoluminescence (PL) and Raman spectra of Ge/Si quantum dot (QD) structures grown using conventional molecular beam epitaxy, MBE, (sample M) and pulsed ion beam-assisted MBE (sample MI) were measured before and after vacuum annealing and hydrogen passivation. Two broad bands are observed in the PL spectra, one ascribed to the QDs and another one to radiation defects. Sample MI exhibits a higher overall PL intensity. The post-growth vacuum annealing partially suppresses the defect-related emission, enhances the QD-related PL in sample M, but almost does not affect the latter emission in sample MI. After the atomic hydrogen treatment that passivates the defect-related radiative and nonradiative recombination centers, a high-energy shift by 20–25 meV of the QD PL peak in sample MI appears when compared to sample M. The analysis shows that the shift is mainly due to a reduction of the QD size, which occurs in the case of the ion-assisted MBE. We conclude that the radiation defects created at the QD growth temperatures do not introduce nonradiative recombination channels, on the contrary to the defects caused by the nonequilibrium character of the MBE growth. In situ high-temperature ion bombardment during the QD growth pushes the epitaxial layer system towards equilibrium, thus enhancing the QD PL intensity.

Introduction

Self-assembled semiconductor quantum dots (QDs) formed during initial stages of the Stranski–Krastanov growth are under investigation as an alternative platform for traditional quantum-well based optoelectronic components. To be used in optoelectronics devices, QDs should have small sizes and high density. However, it is a difficult task to establish a technique achieving sufficiently uniform island sizes with regular spatial distribution and high density. A new method of the creation of high density arrays of small-size Ge/Si QDs based on the use of pulsed irradiation with low-energy Ge⁺ ions during the Ge/Si heteroepitaxy has been proposed in [1]. It has been shown that such a treatment stimulates the nucleation of 3D Ge islands, reduces the critical thickness at which the 2D–3D transition occurs, increases the 3D island density and narrows the island size distribution, as compared with conventional MBE experiments. However, nothing is known about the effect of in situ ion irradiation on the defect creation and the composition of nanoislands. It is only known that proton irradiation after the deposition induces an intermixing of the materials due to the generation of point defects that stimulate interdiffusion in QDs [2]. Increasing the temperature during irradiation can, on the one hand, lead to a dynamic annealing of point defects and enhancement of the PL intensity; on the other hand, it can provoke a coalescence of defects into large clusters or loops that act as nonradiative recombination centers reducing the QD-related PL intensity. To reveal the effect of radiation defects produced upon ion irradiation on the formation of QDs, we carried out PL and Raman scattering measurements of Ge/Si QD structures grown by conventional and ion beam-assisted MBE.

Section snippets

Experimental

The samples were grown by molecular beam epitaxy (MBE) on p-Si(100) wafers. Two growth modes were implemented: 1) conventional MBE of Ge on Si (sample M); 2) MBE with pulsed Ge⁺ ion beam action during the epitaxial deposition (sample MI). A system of ionization and acceleration of Ge⁺ ions provided the degree of ionization of Ge molecular beam ranging from 0.1 to 0.5%. A pulsed accelerating voltage supply generated ion-current pulses with a duration of 0.5 s and an ion energy of 100 eV. The

Results and discussion

The PL spectra of the as-grown (M, MI) and annealed (MAN and MIAN) samples are shown in Fig. 1a. All spectra were normalized to the spectral response of the Ge detector. Two broad peaks centered at 0.75 eV and 0.84 eV are observed for all samples. We see that the pulsed ion bombardment enhances the intensity of both peaks, though to a slightly different extent (it should be noted that for convenience the intensity of the PL spectrum of sample M in Fig. 1a was multiplied by 2.5). On the

Conclusions

In summary, a comparative study of quantum dot structures grown by conventional and ion beam-assisted MBE (samples M and MI, respectively) has been conducted. The low-energy ion bombardment during the growth pushes the Ge/Si system towards the equilibrium, thus enhancing the photoluminescence (PL) intensity. Hydrogen passivation leads to an increase of the overall PL intensity for both kinds of samples, grown with and without in situ ion irradiation. The activation energy of the thermal PL

Acknowledgments

The work has been supported by INTAS (Project No. 03-51-5015), European Network of Excellence SANDiE and Russian Foundation for Basic Research (Grant No. 06-02-08077).

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Citation Excerpt :
One of such systems are Ge–Si heterosystems, with Ge nanoislands acting as quantum dots. Such systems can be built by using, for instance, the molecular beam epitaxy (MBE) technique [2–4]. Ge nanoislands with diameters between 10 and 100 nm were observed on dislocation-free Si (0 0 1) surface after it was fully covered with Ge film [5,6].
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MBE growth of Ge/Si quantum dots upon low-energy pulsed ion irradiation

Abstract

Introduction

Section snippets

Experimental

Results and discussion

Conclusions

Acknowledgments

Int. J. Nanosci.

Appl. Phys. Lett.

Physics

Phys. Status Solidi (c)