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micro-crystals Single Photon InfraREd detectors

Periodic Reporting for period 1 - microSPIRE (micro-crystals Single Photon InfraREd detectors)

Reporting period: 2017-11-01 to 2018-10-31

µSPIRE aims at developing a key enabling technology and processing that will empower the integration on silicon of active devices, using many different semiconductor heterostructures based on materials such as Ge, GaAs and AlGaAs. In fact, many electronic and optoelectronic devices will benefit from µSPIRE fabrication technology, which provides a high degree of flexibility in the integration of dissimilar materials to the well-established silicon technology. µSPIRE pursues this goal through the development of a novel set of photon counting detectors integrated on a Si standard substrate. Single-photon detectors are crucial in several fields such as quantum information, medical physics, security, metrology, aerospace, electronics and biotechnology. In practical applications, solid-state devices, such as single-photon avalanche diodes (SPADs), offer unparalleled advantages, e.g. in terms of compactness, robustness, reliability and ease of operation, over photomultiplier tubes or superconductive detectors. Moreover, silicon SPADs can be micro-fabricated in arrays (called SiPM) in order to achieve photon-number resolving (PNR) capability that is essential for medical applications (like PET scanners), LIDAR, quantum computing, and others. SiPMs demonstrated so far are all silicon based (thus limiting their sensitivity to wavelengths shorter than 1.1 µm) and their planar fabrication technology sets a trade-off between fill factor (i.e. the ratio between photon-sensitive area and overall area) and number of micro-cells.
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.
The work carried out during the first year of microSPIRE can be associated to three main activities which are summarised here below.
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.
µSPIRE ambition is to radically change how epitaxial heterostructures are integrated on Si. The novelty of µSPIRE's approach resides in the control of out-of-equilibrium epitaxial growth under geometrical constraints imposed by the substrate patterning. It is therefore a growth mode that is strongly kinetically controlled and thus allows for extreme flexibility in material properties engineering, from morphology to composition.
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.
Ge microcrystals deposited on a Si patterned substrate.