Elsevier

Applied Surface Science

Volume 469, 1 March 2019, Pages 870-878
Applied Surface Science

Full Length Article
Enhanced photoconductivity of SiGe nanocrystals in SiO2 driven by mild annealing

https://doi.org/10.1016/j.apsusc.2018.11.061Get rights and content

Highlights

  • Short-term furnace annealing facilitates formation of SiGe NCs in SiO2 matrix.

  • An uncommon result; i.e. NCs size gets smaller with increasing annealing temperature.

  • Photocurrent spectra with bimodal feature.

  • Strain field and NCs govern the intensity of bimodal feature individually.

  • Annealing results in over an order of magnitude higher photosensitivity.

Abstract

Photosensitive films based on finely dispersed semiconductor nanocrystals (NCs) in dielectric films have great potential for sensor applications. Here we report on preparation and characterization of photosensitive Si1-xGex NCs sandwiched between SiO2 matrix. A radio-frequency magnetron sputtering was applied to obtain a multilayer-structures (MLs) by depositing SiO2/SiGe/SiO2 films on Si (0 0 1) substrate. The Si1-xGex NCs were formed by a post-deposition annealing at 100–700 °C for 1–5 min. The effect of annealing temperature and time on MLs morphology and NCs size and density was studied using grazing incidence X-ray diffraction, transmission electron microscopy, X-ray photoelectron spectroscopy, energy-dispersive X-ray spectroscopy and measurements of spectral distribution of photocurrent. It is demonstrated how the photoconductive properties of the MLs can be enhanced and tailored by controlling the NCs formation conditions and the presence of stress field in MLs and defects acting as traps and recombination centers. All these features can be adjusted/controlled by altering the annealing conditions (temperature and time). The MLs photosensitivity was increased of more than an order of magnitude by the annealing process. A mechanism, where a competition between crystallization process (NCs formation and evolution i.e. size and shapes) and stress field appearance determines the peak position in the photocurrent spectra, was identified.

Introduction

One of the challenges faced by modern day technology is the fabrication of high quality films incorporating nanoparticles with improved optoelectronic properties for sensitive functional devices. For this purpose, silicon-germanium (SiGe) quantum dots (QDs) have attracted the interest of numerous researchers [1], [2], [3], [4], [5] due to the compatibility of Ge with Si and their self-assembled structures. One of the objectives is to increase the efficiency of photoconductive devices by widening the spectral sensitivity interval from visible to near infrared with the aim towards applications in photovoltaics [6], optoelectronics [7], [8], photodetectors [9], and optical-telecommunications [8]. The misfit between the Si and Ge lattices (4.2% lattice mismatch) was considered an obstacle in the fabrication of such structures. However, more recently, SiGe systems turned out to be quite useful for electronic and optoelectronic devices due to the possibility of optimizing the band gap of the device [4], [10]. Being an indirect bandgap semiconductor, Si has relatively poor light absorption. Mixing it with Ge results in reduced bandgap and increased absorption coefficient at longer wavelengths. The bandgap can be further reduced by introducing strain in the Si1-xGex system [4]. Ge is also an indirect bandgap material but with much smaller energy difference between the indirect and direct bandgaps than in the case of Si (136 meV for Ge vs 2.4 eV for Si) [11]. Introducing a tensile strain in the crystal lattice additionally reduces the energy difference between the direct and indirect bandgaps because the direct bandgap shrinks faster than the indirect one. Thus, the bandgap of the SiGe nanocrystals (NCs) can be adjusted to enhance energy conversion efficiency and optical properties by utilizing strain engineering [12], [13].

In previous studies, a considerable interest has been paid to Si and Ge nanocrystals embedded in oxide matrices such as SiO2 [1], [14], [15], [16], high-dielectric Al2O3 [17], [18], and HfO2 [18]. Of these materials, SiO2 has been shown to be one of the most interesting since it remains amorphous up to high annealing temperatures while providing lateral confinement to the nanoparticles [19], [20], and also due to compatibility with the well-established Si technology [21]. Among methods used to fabricate SiGe NCs embedded in a SiO2 matrix are sol-gel [22], molecular beam epitaxy [23], [18], ion implantation [24], and radio frequency (rf) magnetron sputtering [1], [16], [22], [25], the last one being the most common. It is well understood that the NCs size determines the bandgap energy, and hence the electronic and optical properties of the device that are governed by quantum confinement effects [16], [26].

The present work addresses the effect of mild annealing in furnace (i.e. from temperature as low as 100–600 °C for a minuscule period of 1 min) on the morphology of multilayer-structure (MLs) with SiGe nanocrystals sandwiched between in SiO2 matrix and its photoconductive properties. The sample is exposed to heat treatment for short period after the furnace reaches desired annealing temperature. The aim here is to attain high photosensitivity with their application in silicon based devices which requires low processing temperature to preserve the functionality of devices.

Section snippets

Experiment

Multilayers of SiO2 and SiGe were deposited on 12 × 12 mm2 Si (0 0 1) substrates by rf magnetron sputtering using CESAR© 136 rf power generator (13.56 MHz). Prior to deposition, the substrates were etched with 2 M HF for 120 s to remove native oxide. Firstly, the deposition chamber was throttled to 5 × 10−6 Pa and then Argon (Ar) of 99.999% (5 N) purity was used as a working gas. For the SiO2 deposition, the working gas was mixed with 5 N O2 in order to prevent oxygen deficiency of SiO2 films.

Result and discussion

Two sample series were fabricated, one in which the annealing temperature was kept constant at 600 °C and annealing time was varied (1, 3 and 5 min) and other one in which the annealing time was kept constant (1 min) and the annealing temperature was varied from 100 to 600 °C. A list of the samples is shown in Table 1.

The diffractograms of as-deposited (asd) and 600 °C annealed samples for 1 min, (Si-wafer/buffer SiO2/SiO2/SiGe/top SiO2), are presented in Fig. 2. For the asd sample, only broad

Conclusion

Si wafer/ buffer SiO2 (230 nm)/SiGe (195 nm)/top SiO2 (35 nm) ML with SiGe NCs were prepared by using rf magnetron sputtering followed by short annealing in furnace with the aim to enhance its photoconductive properties in the infrared region. By using short thermal annealing time (1 min), we hamper deterioration of the structure and consequent degradation of the photosensitivity. An average NCs size of roughly 8 nm was formed by the annealing process. An increased sensitivity in the infrared

Acknowledgements

This work is funded through M-ERA.NET project PhotoNanoP UEFISCDI Contract no. 33/2016, PCE project UEFISCDI Contract no. 122/2017 and by Romanian Ministry of Research and Innovation through NIMP Core Program PN16-480102 and by the Technology Development Fund of the Icelandic Centre for Research, grant no. 159006-0611. The atomistic simulation were performed via high performance computer cluster, garpur.ihpc.hi.is, which is operated by the University of Iceland and Reykjavik University.

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