• Dr. Magdalena Lidia Ciurea - Project Director
• Dr. Ionel Stavarache
• Dr. Toma Stoica
• Dr. Sorina Lazanu
• Dr. Valentin Serban Teodorescu
• Dr. Ana-Maria Lepadatu
• Dr. Adrian Slav
• Dr. Catalin Palade
• Vacant position for PhD student

• December 10th, 2020 - Ioana Dascalescu - public defense of PhD Thesis “Study of electrical properties of some nanostructures based on SiGeSn”, PhD Adviser Dr. Magdalena Lidia Ciurea (PCE 122 Project Director).
• INTERNATIONAL SEMICONDUCTOR CONFERENCE IEEE CAS 2018 BEST PAPER AWARD: "GeSi nanocrystals in SiO₂ matrix with extended photoresponse in near infrared", I. Stavarache, L. Nedelcu, V.S. Teodorescu, V.A. Maraloiu, I. Dascalescu, M.L. Ciurea

Summary

The aim of the project NcSiGeSnOPELD is the fabrication of two optoelectronic devices – optical sensor and photovoltaic device, based on SiGeSn nanocrystals (NCs) embedded in oxide matrix for NIR-VIS range.

For achieving this, the following specific objectives of the project were realised:

• O1) fabrication of test samples based on SiGeSn nanocrystals (NCs) embedded in SiO₂ for optical and photovoltaic devices (MS deposition, thermal treatment, electrical contacts);
• O2) complex characterisation of test samples for structure, composition, crystallinity, nanocrystal morphology, electrical and optical properties and their correlation;
• O3) simulation of optical sensor and photovoltaic device;
• O4) fabrication and characterisation of optical sensor;
• O5) fabrication and characterisation of photovoltaic device;

The time development of the tasks followed closely the project work plan. All the films were fabricated by magnetron sputtering deposition (MS) and rapid thermal annealing (RTA).

In Stage I we realised and characterised films formed from SiGe and GeSn NCs respectively, embedded in SiO₂ or TiO₂ matrices. SiGe-SiO₂ films with Si:Ge:SiO₂ concentrations of 25:25:50 after RTA between 650 ^oC and 1000 ^oC show similar morphologies, being formed of SiGe NCs embedded in SiO₂, having diameters of 5 – 10 nm and 10 – 20 nm respectively, for RTA 650 and 1000 ^oC. The absorption coefficient was obtained from transmittance and reflectance measurements, for photon energies in the range 0.5 – 4.5 eV in as-deposited and RTA 800 ^oC films with Si:Ge:SiO₂ concentrations of 7.5:42.5:50 and 12.5:37.5:50 respectively. α^1/2 –E curves for as-deposited samples are monotonic, and the absorption coefficient has higher values for the samples with lower Ge content. The curves corresponding to RTA 800 ^oC samples present a non-monotonic behaviour, showing NCs formation. Experimental results are in good agreement with those obtained from simulations. The morphology of SiGe-TiO₂ films depends mainly on the relative concentration Si:Ge. Thus, for Si:Ge:TiO₂ concentration of 35:40:25 and for RTA temperatures of 775, 800 si 825 ^oC the formation of reach Ge SiGe NCs, together with TiO₂ NCs was found. For Si:Ge:TiO₂ of 25:25:50, after RTA at 800 ^oC the films contain only TiO₂ nanocrystallised as anatase and rutile (no SiGe NCs), while for the composition 45:35:20 with the same RTA treatment, SiGe NCs with cubic structure are evidenced, together with NCs of Ti suboxides or GeTiO structures. α^1/2 –E  curves in the range 1.2-3.2 eV were obtained from transmittance and reflectance optical measurements performed on as deposited and RTA 800 ^oC samples. These curves are monotonic for as-deposited samples, and non-monotonic for RTA treated samples, showing the formation of NCs, similarly with SiGe-SiO₂ samples. The best GeSn-SiO₂ films, with 10% – 15% Sn and 10% – 15%  SiO₂ are those obtained after RTA at 400 ^oC. They present GeSn NCs with high Sn concentration of 14% and 5-10 nm diameter, without b-Sn segregation, but with GeSn nanocrystalisation.

In Stage II we fabricated SiGeSn ternary alloys, multilayers of 5 and 20 times SiGeSn/SiO₂ nanostructures, as well as SiGeSn-HfO₂ samples. The concentrations x and y of the alloy
(Si_yGe_1-y)_1-xSn_x were varied by using SiGe targets of different compositions, and by controlling the conditions in the sputtering plasma for SiGe and Sn codeposition. Test samples based on nanocrystallised SiGeSn and SiGeSn/SiO₂ are photosensitive in a large range of wavelengths, from 0.5 to 1.35 µm. An even more extended sensitivity, up to 1.75 µm was obtained for SiGeSn-HfO₂ films. The simulations performed in this stage show the IR extension of the absorption for SiGeSn alloy in respect to SiGe, due to Sn addition.

In Stage III we continued to investigate the fabrication, optimisation and characterisation of films and structures based on (Ge_1-xSi_x)_1-ySn_y NCs embedded in oxide matrix, for the realization of optical sensor and photovoltaic device. We defined the demonstrators corresponding to the optical sensor and photovoltaic device by the operating parameters responsivity R and specific detectivity D*. The demonstrators were fabricated using the technological parameters corresponding to the best structures.

The webpage of the project was created and updated at the address: http://www.infim.ro/ro/projects/dispozitive-optoelectrice-pe-baza-de-nanocristale-de-sigesn-matrice-oxidica.

The results expected in the work plan for the whole project were surpassed: (i) 12 papers were published (6 expected), from which 8 in ISI quoted journals; (ii) 9 communications presented at international conferences (6 expected); (iii) 1 patent application; (iv) 2 demonstrator devices were realised: 1 optical sensor and 1 photovoltaic device; (v) young people were involved in project activities (1 PhD Thesis in final stage redaction, 1 young people admitted to PhD studies, 1 Master Thesis, all in project topic).

In conclusion, the objectives and the activities expected for Contract PCE Nr. 122 /2017 Optoelectric devices based on SiGeSn nanocrystals in oxide matrix NcSiGeSnOPELD were fully realised.

Summary of Stage III

The aim of the project NcSiGeSnOPELD is the fabrication of two optoelectronic devices – optical sensor and photovoltaic device, based on SiGeSn nanocrystals (NCs) embedded in oxide matrix for NIR-VIS range.

In Stage III we were interested in extending in IR the sensitivity of the structures based on
(Ge_1-xSi_x)_1-ySn_y NC embedded in SiO₂ matrix. For this, we increased Sn concentration, and decreased the Si concentration in respect to Ge one. We prepared layers with x = 5 % and y = 14-19 %. The films were deposited by MS, and we investigated both SiGe-Sn-SiO₂ and SiGeSn/SiO₂ multilayers, and then annealed by RTA. Codeposited layers are preferred mainly to to the fact that nanocrystallisation is more uniform on the layer thickness. RTA temperatures in the range 380 – 550 ^oC were used. 3 types of samples were investigated in deep: with moderate Sn concentration (~14 %), with high Sn concentration (~19 %), both directly deposited on Si substrate, and with moderate Sn concentration (14%) deposited on a SiGe buffer.

In the samples with moderate Sn concentration deposited directly on Si substrate, a good nanocrystallisation is obtained after RTA at 520 ^oC. By increasing Sn concentration to 19 %, a much lower nanocrystallisation temperature, of 430 ^oC is found, accompanied by Sn segregation. A decrease of nanocrystallisation temperature was observed also in the sample where the active layer is deposited on the SiGe buffer. This phenomenon could be explained by the clean interface SiGeSn/SiGe (without oxide): SiGeSn nuclei could form here, and in their turn they could facilitate the nanocrystallisation of SiGeSn-SiO2 layer at lower temepratures.

Photoelectric measurements were performed on samples with ITO electrodes. Based on the results obtained, the best structures for photovoltaic device and optical sensor were selected.

Optimised photovoltaic devices and optical sensors were obtained, using both moderate and high Sn concentrations layers. Each of these layers is performs better in specified conditions related to working type (photovoltaic or sensor with external electrical polarisation) and illumination (monochromatic or integral light). Both types of layers have the same Si (5 %) and SiO₂ (15 %) concentrations.

In what regards the photovoltaic device, the responsivity and the spectral dependence of open-circuit are best for layers with moderate Sn content and RTA 520 ^oC, with R = 14 mA/W in VIS-NIR, with a large maximum extending up to 1350 nm, and sensitive up to 2000 nm. In integral and more intense light (10 mW/cm²), the photovoltaic devices based on high Sn content perform better, short circuit responsivity being of 10 mA/W at room temperature.

For optical sensors, the responsivity and specific detectivity are higher for layers with moderate Sn content and RTA 520 ^oC: responsivity R = 65 mA/W and specific detectivity D* = 10¹¹ cm Hz^1/2 W^-1 at room temperature and 3 V polarisation. Under integral ligh, a higher responsivity is obtained from high Sn concentration layers, up to 100 mA/W at room temperature and 1 V polarisation, and a specific detectivity of 2.0 10¹⁰ cm Hz^1/2 W^-1.

The web page of the project was up-dated at the address: http://www.infim.ro/ro/projects/dispozitive-optoelectrice-pe-baza-de-nanocristale-de-sigesn-matrice-oxidica.

In Stage III we published 9 papers, from which 7 in ISI quoted journals and 2 in ISI indexed proceedings, 5 communications at conferences. A patent application was deposited to OSIM.

In conclusion, the objectives and the activities expected for Stage III of the Contract PCE Nr. 122 /2017 NcSiGeSnOPELD were fully realised.

Summary of Stage II

The aim of the project NcSiGeSnOPELD is the fabrication of two optoelectronic devices – optical sensor and photovoltaic device, based on SiGeSn nanocrystals (NCs) embedded in oxide matrix for NIR-VIS range.

In Stage II we fabricated films based on SiGeSn ternary alloys, multilayers of 5 and 20 times repeated (SiGeSn/SiO₂) nanostructures, as well as SiGeSn-HfO₂ samples. We started from the results obtained during Stage I for films based on NCs of binary compounds SiGe and GeSn embedded in SiO₂ matrix. All films were deposited, as in Stage I, by MS and nanostructured by RTA, in controlled conditions of temperature, time duration and gas flux.

The concentrations x and y of the alloy (Si_yGe_1-y)_1-xSn_x were varied by using SiGe targets of different compositions, and by controlling the conditions in the sputtering plasma for SiGe and Sn codeposition. SiGeSn-HfO₂ samples were obtained by codeposition of SiGe, Sn and HfO₂. Two different concentrations of the Si_yGe_1-y alloy, y= 10% and 50%, were used in (Si_yGe_1-y)_1-xSn_x Sn concentrations x of 3, 6 and 9 % were obtained, by controlling the power in the plasma.

Test samples were then prepared, by performing RTA for nanocrystallisation. The crystallisation process was investigated by HRTEM, EDX and XPS measurements. It is known that by alloying amorphous Ge with Sn, the crystallisation temperature decreases. On the other hand, the presence of Si in SiGeSn alloy increases the crystallisation temperature. The crystallisation temperature of GeSn is in the range 300-450 ^oC, depending on Sn concentration, as obtained in the Stage I of the project; in SiGeSn we found the corresponding temperature in the range 500-550 ^oC. The nanocrystallisation is accompanied by Sn segregation in SiGeSn clusters, rich in Sn. Thus, this segregation helps NCs formation. We can explain this way why SiGeSn NCs can have higher Sn concentrations than the average concentration in the layer – as found by XRD and HRTEM measurements. The formation of SiGeSn NCs is also accompanied by Sn diffusion toward the surface, as found from XPS. The diffusion is favoured by the higher temperature necessary for SiGeSn for crystallisation. Therefore, the temperature range for RTA for SiGeSn nanocrystallisation is restricted in respect to the corresponding one for GeSn.

In SiGeSn/SiO₂ and in SiGeSn-HfO₂ samples the crystallisation processes have the same characteristics as in SiGeSn layers, the range of thermal treatment temperatures being also 500-550 ^oC for SiGeSn NCs in oxide.

Diode test structures were obtained by depositing ITO electrodes on the surface of the active layer, and Al ones on the back of the Si substrate. Rectifying diodes based on the heterojunction between the layer containing SiGeSn NCs, SiGeSn/SiO₂ multilayers or codeposited SiGeSn-HfO2 and the Si substrate are obtained. The rectifying character was revealed by I-V measurements at different temperatures, and also by the spectral photocurrent.

SiGeSn and SiGeSn/SiO₂ test samples are photosensitive in a large range of wavelengths, from 0.5 to 1.35 µm. An even more extended sensitivity, up to 1.75 µm was obtained for SiGeSn-HfO₂ films. The simulations performed in this stage show the IR extension of the absorption for SiGeSn alloy in respect to SiGe, due to Sn addition. Sn addition in samples with high Si concentration could significantly increase energy absorption from a solar spectrum, and thus such layers are of interest for IR devices.

The webpage of the project at the address http://www.infim.ro/ro/projects/dispozitive-optoelectrice-pe-baza-de-nanocristale-de-sigesn-matrice-oxidica was updated

The results expected for Stage II (#Test samples for optical sensor, characterised for structural, electrical and optical properties; #2 ISI papers; #1 communication) were fully realised, and partially surpassed (4 articles in ISI papers, 2 oral presentations at international conferences).

In conclusion, the objectives and the activities expected for Stage II of the Project NcSiGeSnOPELD were fully realised.

Summary of Stage I

The aim of the project NcSiGeSnOPELD is the fabrication of two optoelectronic devices – optical sensor and photovoltaic device, based on SiGeSn nanocrystals (NCs) embedded in oxide matrix for NIR-VIS range.

In Stage I we aimed to fabricate films based on binary compounds from the SiGeSn system, namely films formed by SiGe and GeSn NCs embedded in SiO₂ and TiO₂ matrices. All the films have been deposited by magnetron sputtering (MS) and have been nanostructured by rapid thermal treatments (RTA) in controlled conditions of temperature, time duration and gas flux.

For this: ● we performed: studies to find MS deposition and RTA parameters for films of different compositions; ● we iteratively tested the films in order to modify the preparation conditions according to the results obtained from testing their characteristics; ● we deposited electrical contacts by thermal evaporation and in electron beams; ● we simulated optical parameters (refraction index and extinction coefficient), and obtained the absorption coefficient for the materials to be used in the fabrication of the two devices; ● we estimated the short-circuit current as a function of the concentrations of SiGe and GeSn in the oxide.

SiGe-SiO₂ films were deposited by MS on n-type Si (100) wafers, in Ar atmosphere, at work pressure of 4 mTorr. Films with 3 compositions of Si:Ge:SiO₂ were deposited, i.e. 25:25:50, 12.5:37.5:50 and 7.5:42.5:50 respectively. The films were nanocrystallised by RTA at 650, 800 and 1000 ^oC. HRTEM investigations performed on the sample with composition 25:25:50, after 650 ^oC RTA, reveal that these films contain SiGe NCs of 5-10 nm diameter embedded in SiO₂, while for RTA at 1000 ^oC SiGe NCs are much greater (10-20 nm diameter).

Optical measurements of transmittance and reflectance performed on as deposited and RTA 800 ^oC samples with Si:Ge:SiO₂ concentrations of 7.5:42.5:50 and 12.5:37.5:50 respectively have been used to derive the energy dependence of the absorption coefficient in the range 0.5- 4.5 eV (Tauc representation: α^1/2 –E). The curves for as-deposited samples are monotonic, and the absorption coefficient has higher values for the samples with lower Ge content. The curves corresponding to RTA samples present a non-monotonic behaviour, showing NCs formation. Experimental results are in good agreement with those obtained from simulations.

SiGe-TiO₂ films have been deposited by MS on p-type Si wafers with an RTO buffer layer, in Ar atmosphere (work pressure 4 mTorr). 3 compositions for Si:Ge:TiO₂ were used: 25:25:50, 35:40:25 and 45:35:20 respectively. RTA at 775, 800 si 825 ^oC in Ar were performed for nanostructuring. All RTA films with composition 35:40:25 contain SiGe NCs with high Ge content and with cubic structure, and TiO₂ rutile NCs (as XRD and HRTEM analyses reveals). The best results are obtained on RTA 800 ^oC samples. The samples with composition 25:25:50 and RTA 800 ^oC contain TiO₂ NCs of anatase and rutile (no SiGe NCs), while for the composition 45:35:20 with the same RTA treatment, SiGe NCs with cubic structure are evidenced, together with NCs of Ti suboxides or GeTiO structures.

The α^1/2 –E  curves in the range 1.2-3.2 eV were obtained from transmittance and reflectance optical measurements performed on as-deposited and RTA 800 ^oC samples. These curves are monotonic for as-deposited samples, and non-monotonic for RTA treated samples, showing the formation of NCs, similarly with SiGe-SiO₂ samples.

GeSn-SiO₂ films have been deposited by MS on p-type Si(100) wafers, in Ar atmosphere (work pressure 4 mTorr). The aim was to fabricate samples with high Sn concentration (10-15 %), and with SiO₂ concentration of 10-15 %. Amorphous as-deposited films were nanocrystallised by RTA treatments at 250, 350, 400 and 450 ^oC. The optimum RTA temperature for films with 10-12 % Sn and ~10 % SiO₂ is 400 ^oC. These films contain GeSn NCs with 14 % Sn, higher than the 12% average Sn concentration in the film (EDX, HRTEM, XRD). The absorption coefficient was derived from optical measurements (transmittance and reflectance), and the optical thresholds of 0.44 eV and 0.37 eV were obtained for 14 % and 16 % Sn concentration respectively, corresponding to 2.82 and 3.35 µm.

Simulations for obtaining optical parameters (refractive index and extinction coefficient) as a function of photon energy were performed, and the absorption coefficient for films formed from GeSi NCs embedded in SiO₂ or TiO₂, and GeSn NCs embedded in SiO₂ was obtained for different compositions. For this, Bruggeman effective medium approximation was used. The short-circuit current was estimated as a function of SiGe and GeSn concentrations in oxides respectively.

The web page of the project was created at the address http://infim.ro/project/dispozitive-optoelectrice-pe-baza-de-nanocristale-de-sigesn-in-matrice-oxidica

Thus, the results for the Stage I of the project, as expected from the Work Plan (# Nanostructured oxide films with NCs from the SiGeSn system with preliminary tests; # 1 communication at international conference) were fully realised, some provisions being exceeded (# 1 paper submitted for publication and 2 more papers presented at conference).

In conclusion, the objectives and tasks for Stage I / 2017 were fully realised.

Attracting young people and their formation in the project topic

• Ioana Dascalescu -December 10th, 2020 - public defense of PhD Thesis entitled “Study of electrical properties of some nanostructures based on SiGeSn”, PhD Adviser Dr. Magdalena Lidia Ciurea (PCE 122 Project Director).
• Ioana Dascalescu - final stage of writing PhD Thesis  “Study of electrical properties of some nanostructures based on SiGeSn”, PhD Adviser Dr. M.L. Ciurea.
• Ovidiu Cojocaru presented the Dissertation Thesis (Facultaty of Physics, University of Bucharest, July 2019) entitled „ Study of Ge nanocrystals embedded in TiO₂ films with photosensitive properties in vis–nir”, Supervisors Dr. Magdalena Lidia Ciurea (PCE 122 Project Director) and Dr. Alexandru Nemnes.
• Ovidiu Cojocaru was accepted in September 2019 to Doctoral School of Physics, University of Bucharest, PhD Adviser Dr. Magdalena Lidia Ciurea (PCE 122 Project Director); thesis subject: “Photoelectrical properties of nanostructures based on SiGeSn in correlation to energy structure”.

• Ovidiu Cojocaru – participation to GraFOx Summer School on Oxide Semiconductors for Smart Electronic Devices, 3-9 iunie 2019, Loveno di Menaggio (CO) – Italy, with poster entitled “Ge nanocrystals in TiO₂ films for near infrared optical sensors" and co-authored by O. Cojocaru, I. Dascalescu, C. Palade, A. Slav.