Proposta di ricerca:
The Project will deal with:
1) study of the effects of hydrogenation in amorphous Si/Ge nanostructures deposited by sputtering,
for solar cells and photonics applications;
2) optimisation of the MOVPE deposition of cubic SiC (3C-SiC) nanostructures, for sensoristics;
3) MOVPE growth and study of epistructures based on Ge/GaAs, for space and thermophotovoltaic
solar cells;
4) Light generation and carrier recombination in InGaAsP/InP heterostructures.
1– Hydrogenated amorphous Si/a-Ge nanostructures, for solar cells and photonics, prepared
by sputtering
Hydrogenated amorphous SiGe alloy has application in the photovoltaic conversion of the solar
energy as it provides the lower band gap cell in multi-junction solar cells in combination with a
high band gap material like a-Si. It is also used in photonic devices. The a-SiGe alloy can be
obtained from a sequence of a-Si/a-Ge nanolayers (3 nm thick) and annealing them so as to intermix
Si and Ge by diffusion. Hydrogenation is necessary to improve the electro-optical properties as H
saturates the Si and Ge dangling bonds. The instability of H upon annealing and its effect on the
structure of the whole assembly of the a-Si/a-Ge nanolayers is an important issue of these materials.
The research activity will concern the study of the relationship between H bonding configurations
(mono- and poly-hydrides) and the structure of the a-Si/a-Ge:H nanostructures, in particular the
formation of surface blisters, as a function of the H content and annealing conditions. Single layers
of a-Si and a-Ge, prepared as the a-Si/a-Ge:H nanostructures, will also be studied to better
understand the H behaviour. All samples will be prepared by RF sputtering. Investigations will be
made by Atomic Force Microscopy (AFM), Infrared (IR) spectroscopy and Transmission Electron
Microscopy (TEM). MFA tasks: growth of the hydrogenated a-Si/a-Ge nanostructures, a-Si and aGe single layers by RF sputtering; IR spectroscopy. IMEM tasks: AFM and TEM analysis of the
nanostructures.
2- Nanostructures of cubic silicon carbide (3C-SiC) for sensoristics
Cubic silicon carbide (3C-SiC) epilayers and nanostructures have several technological
applications, e.g. in microelectromechanical structures (MEMS), like accelerometers as the μgravimeters used in the satellites. Such sensoristic applications rely on the use of SiC membranes
and cantilevers. It is thus important to control the SiC samples warpness and, more generally, the
release of the strain naturally born in the SiC layers because of the high lattice and thermal
mismatch of SiC to the Si substrate.
The project activity will deal with the growth of 3C-SiC by MOVPE (Metal Organic Vapor Phase
Epitaxy) with special focus on the optimization of the growth parameters, such as C/Si ratio and
temperature T, with the final aim to obtain stress-free layers and minimal wafer warp. A new
growth process at temperature higher than normally used is being developed at IMEM. The SiC
samples prepared by this procedure will be characterized so as to compare the sample properties
between the processes at high and low T. Makyoh Topography (MT) will be used to assess their
surface properties and deformations caused by internal stresses. Information on the crystal defects
will be gained by TEM in order to understand the strain release mechanims while XPS (X-ray
Photoelectron Spectroscopy) is used to check the stoichiometry. IMEM tasks: MOVPE growth of
3C-SiC; TEM, scanning electron microscopy (SEM) and X-ray diffraction of 3C-SiC. MFA tasks:
characterization by MT, XPS and TEM.
3- Epistructures based on Ge/GaAs for space solar cell and thermophotovoltaic applications
GaAs on Ge heteroepitaxy is nowadays commonly used for solar cells, mostly in satellites, and
electronic devices. Infrared (IR) cells based on Ge are also gaining interest for thermophotovoltaics.
The Ga and As diffusion into Ge is a known effect which is used to dope the Ge substrate to realize
photovoltaic converters for IR light in triple junction solar cells such as InGaP/GaAs/Ge. Also,
despite the fact that GaAs layers on Ge are used in devices, the MOVPE Ge epitaxy on GaAs is
much less studied since till recently it was difficult to find suitable Ge precursors. Epitaxial
techology for Ge deposition will also be utilized for realizing Ge/Si structures, which could be of
interest for infrared detector and photovoltaic cells.
The joint MFA-IMEM activity will regard the optimization of the growth conditions for the
deposition of the Ge/GaAs junctions and of the InGaP/GaAs/Ge system as well as the study of the
Ge, Ga and As diffusion across the junctions. The latter will be studied by SIMS/SNMS (Secondary
Ion Mass Spectrosocopy/Secondary Neutral Mass Spectrometry), RBS (Rutherford Backscattered
Spectrosocopy) and TEM-EDS (Energy Dispersive Spectrosocopy) in Ge/GaAs samples grown at
low temperatures (380 to 500 °C), where lower diffusion is expected. As to the samples grown at
high T (700 °C) they will contain a Ge buffer grown at lower temperature to prevent the high
diffusion expected at 700 °C. IMEM tasks: MOVPE growth of Ge/GaAs etc; SEM, TEM, AFM and
X-ray diffraction of the samples. MFA tasks: characterization by RBS and SIMS/SNMS.
4- Light generation and carrier recombination in InGaAsP/InP heterostructures
InGaAsP/InP hetrostructures are applied to obtain LEDs and lasers. The research will focus on the
study of the reasons why some LEDs show no or weak light emission even if the absorption
spectroscopy confirmed the existence of the active InGaAsP layer. Optical, electro-optical and TEM
studies are planned to be used simultaneously to understand this phenomenon. In particular, the
presence and structure of the anti-meltback layer and the presence of precipitates possibly due to the
Zn dopant will be investigated to check whether they can yield non-radiative carrier recombination.
MFA tasks: preparation of the InGaAsP LEDs and optical measurements. IMEM tasks: TEM study
of the LEDs.
Obiettivi:
This is a joint CNR-MTA (Hungary) 3-year research project between the Institutes CNR-IMEM of
Parma and MTA-MFA (Technical Physics and Materials Science) of Budapest, Konkoly-Thege út
29-33. Objective of the joint project is the optimization of the growth procedures and properties of
some types of semiconductor nanostructures of mutual interest to MTA-MFA and CNR-IMEM that
can find applications in devices for photovoltaics and thermophotovoltaics, sensoristics and
photonics. The materials to be studied are SiC nanostructures (sensoristics), Ge/GaAs based
epistructures (photovoltaics), amorphous a-Si/a-Ge, a-Si, a-Ge nanostructures (photovoltaics and
photonics) and InGaAsP/InP junctions (photonics). The proposed Project will take advantage of the
complementarity of the expertises and facilities available in the two Institutes. The former two
material systems, in fact, are prepared in CNR-IMEM whilst the latter two are grown in MTAMFA. On the other hand, the material properties will be investigated by using several techniques
available in the two Institutes.