Continuous
development of Si photonics requires ecological and cost-effective
materials. In this work, SiGe nanocrystals (NCs) embedded in TiO2 are investigated as a photosensitive material for visible
(VIS) to short-wave infrared (SWIR) broad-range detection. The TiO2 matrix has the advantage of a lower band gap than SiO2, facilitating transport of photogenerated carriers in NCs.
The advantage of SiGe NCs over Ge NCs is emphasized by elucidating
the mechanisms involved in rapid thermal annealing (RTA)-induced nanocrystallization.
An efficiently increased NC stabilization is achieved by avoiding
the detrimental fast Ge diffusion. For this, the structure, morphology,
and composition were carefully characterized by high-resolution transmission
electron microscopy, energy-dispersive X-ray spectroscopy, X-ray diffraction,
and Raman spectroscopy. Two types of structures were investigated,
a film of SiGe–TiO2 alloy and a multilayer of a
stack of six SiGe/TiO2 pairs. The layers have been deposited
on Si wafers using magnetron sputtering of Si, Ge, and TiO2 followed by RTA in an inert atmosphere. The stabilization of SiGe
NCs is achieved by the formation during RTA of protective SiO2 thin layers through Si oxidation at the SiGe NC surface,
acting as a barrier for Ge diffusion. Thus, embedded Ge-rich SiGe
NCs are obtained, resulting in the SWIR extension of the spectral
photocurrent up to 1700 nm for films and 1600 nm for multilayers.
This study has shown that in multilayers, the local anisotropy of
crystallization is compensated by the stress field developed in the
SiGe lattice, highly visible in the bottom part. Also, SiGe crystallizes
faster than TiO2 in the rutile phase, and therefore, TiO2 remains mainly amorphous.
Detection in short-wave
infrared (SWIR) has become a very stringent
technology requirement for developing fields like hyperspectral imaging
or climate changes. In a market dominated by III–V materials,
GeSn, a Si compatible semiconductor, has the advantage of cost efficiency
and inerrability by using the mature Si technology. Despite the recent
progress in material growth, the easy fabrication of crystalline GeSn
still remains a major challenge, and different methods are under investigation.
We present the formation of GeSn nanocrystals (NCs) embedded in oxide
matrix and their SWIR characterization. The simple and cost-effective
fabrication method is based on thermal treatment of amorphous (Ge1–x
Sn
x
)
y
(SiO2)1–y
layers deposited by magnetron sputtering. The nanocrystallization
for Ge1–x
Sn
x
with 9–22 at. % Sn composition in SiO2 matrix
with 9% to 15% mole percent was studied under low thermal budget annealing
in the 350–450 °C temperature range. While the Sn at.%
content is the main parameter influencing the band-structure of the
NCs, the SWIR sensitivity can be optimized by SiO2 content
and H2 gas component in the deposition atmosphere. Their
role is not only changing the crystallization parameters but also
to reduce the carrier recombination by passivation of NCs defects.
The experiments indicate a limited composition dependent temperature
range for GeSn NCs formation before β-Sn phase segregation occurs.
NCs with an average size of 6 nm are uniformly distributed in the
film, except the surface region where larger GeSn NCs are formed.
Spectral photovoltaic current measured on SiO2 embedded
GeSn NCs deposited on p-Si substrate shows extended SWIR sensitivity
up to 2.4 μm for 15 at. % Sn in GeSn NCs. The large extension
of the SWIR detection is a result of many factors related to the growth
parameters and also to the in situ or ex
situ annealing procedures that influence the uniformity and
size distribution of NCs.
GeSn alloys have
the potential of extending the Si photonics functionality
in shortwave infrared (SWIR) light emission and detection. Epitaxial
GeSn layers were deposited on a relaxed Ge buffer on Si(100) wafer
by using high power impulse magnetron sputtering (HiPI-MS). Detailed
X-ray reciprocal space mapping and HRTEM investigations indicate higher
crystalline quality of GeSn epitaxial layers deposited by Ge HiPI-MS
compared to commonly used radio frequency magnetron sputtering (RF-MS).
To obtain a rectifying heterostructure for SWIR light detection, a
layer of GeSn nanocrystals (NCs) embedded in oxide was deposited on
the epitaxial GeSn one. Embedded GeSn NCs are obtained by cosputtering
deposition of (Ge1–x
Sn
x
)1–y
(SiO2)
y
layers and subsequent rapid thermal
annealing at a low temperature of 400 °C. Intrinsic GeSn structural
defects give p-type behavior, while the presence of oxygen leads to
the n-character of the embedded GeSn NCs. Such an embedded NCs/epitaxial
GeSn p–n heterostructure shows superior photoelectrical response
up to 3 orders of magnitude increase in the 1.2–2.5 μm
range, as compared to performances of diode based only on embedded
NCs.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.