Periodic Reporting for period 1 - microSPIRE (micro-crystals Single Photon InfraREd detectors)
Reporting period: 2017-11-01 to 2018-10-31
Our purpose is to go beyond the state-of-the-art, by employing a novel deposition approach, which we termed vertical heteroepitaxy (VHE). VHE exploits the patterning of conventional Si substrates, in combination with epitaxial deposition, to attain the self-assembly of arrays of epitaxial micro-crystals elongated in the vertical direction. VHE is particularly suited for the deposition of epitaxial heterostructures. Indeed, besides Si-only devices, we will develop Ge/Si and GaAs/Si heterostructures, with structural and electronic properties unparalleled by “conventional” epitaxial growth. Such VHE micro-crystals are the elementary microcells for novel single-photon detectors with performance far beyond those of current state-of-the-art SPADs.
I. Designing and fabricating single photon avalanche diodes (SPAD) based on epitaxial Si micro-crystals.
The microSPAD (μSPAD) devices targeted by microSPIRE rely on the avalanche multiplication taking place within the Si microcrystals. During the first year, efforts have been dedicated to the demonstration of avalanche multiplication in Si microcrystals. This required a close collaboration between the different competences gathered by the microSPIRE’s consortium: substrate patterning, material growth and characterization, epitaxial growth modelling and electronic design.
II. Modelling the 3D morphological evolution of epitaxial microcrystals.
A 2D model, based on rate equations describing the facet evolution during growth, was exploited to get an effective description of the growth profile for the design of Si μSPADs. However, this is insufficient to achieve a detailed control and understanding of the physical mechanisms determining the morphology of microcrystals. A 3D model including all relevant physical processes (adatom flux, diffusion and incorpora-tion, surface energy etc…) is indeed required for the completion of microSPIRE’s objectives.
A relevant step forward in this direction has been done by implementing a 2D model based on the phase-field approach. In the remaining part of the project the model will be extended to cope with the 3D geometry of microcrystals.
III. Reducing, and possibly fully eliminating in Ge and GaAs microcrystals, defects typical of hetero-epitaxy such as dislocations and antiphase domains.
By relying on the elastic deformation of etched Si pillars with sub-micrometre size, it is possible to inhibit dislocation nucleation within a SiGe microcrystals grown on a patterned Si substrate. For this approach to be effective, Si pillars with typical sides of a few hundred nanometres are required. This poses two main challenges. The first one is the substrate patterning itself, which requires the fabrication of high aspect-ratio pillars with sub-micrometres sides and spacing. The second one is the identification of growth condition leading to the formation of vertical microcrystals on such sub-micrometre-size pillars. Indeed, the onset of vertical epitaxial growth requires deposition temperatures and growth rates resulting in an adatoms diffusion length not exceeding the typical pillars size, however an excessive reduction of the adatom diffusion length could lead to a breakdown of epitaxy.
A set of deposition run has been performed on substrates featuring Si pillars as small as 250 nm and spacing between adjacent pillars of 500 nm.
IV. Optical properties of microcrystals.
The optical properties of single microcrystals and microcrystal arrays have been investigated by means of photoluminescence (PL), time resolved PL, micro PL and reflectivity measurements. These techniques, besides giving relevant information on the quality of epitaxial layers and microcrystals highlighted an interesting effect of the microcrystal morphology and pattern periodicity on light absorption. The combination of reflectivity measurements and finite difference time domain calculations indicates that in microcrystal arrays light absorption is increased, as compared to flat epilayers, particularly in the spectral region around the absorption edge.
In µSPIRE, by exploiting VHE we aim at radically changing how photon-counting detectors are designed and fabricated. The combination of a novel fabrication platform and the innovative design of detectors made of many microcrystals will give birth to a non-incremental and revolutionary approach for designing active devices with the introduction of other (Ge, GaAs, GaAlAs, GaN, SiC) materials.