High-performance,
multispectral, and large-format infrared focal
plane arrays are the long-demanded third-generation infrared technique
for hyperspectral imaging, infrared spectroscopy, and target identification.
A promising solution is to monolithically integrate infrared photodetectors
on a silicon platform, which offers not only low-cost but high-resolution
focal plane arrays by taking advantage of the well-established Si-based
readout integrated circuits. Here, we report the first InAs/GaAs quantum
dot (QD) infrared photodetectors monolithically integrated on silicon
substrates by molecular beam epitaxy. The III–V photodetectors
are directly grown on silicon substrates by using a GaAs buffer, which
reduces the threading dislocation density to ∼106 cm–2. The high-quality QDs grown on Si substrates
have led to long photocarrier relaxation time and low dark current
density. Mid-infrared photodetection up to ∼8 μm is also
achieved at 80 K. This work demonstrates that III–V photodetectors
can directly be integrated with silicon readout circuitry for realizing
large-format focal plane arrays as well as mid-infrared photonics
in silicon.
An InGaAs quantum dot (QD) photodetector is directly grown on a silicon substrate. GaAs-on-Si virtual substrates with a defect density in the order of 10 6 cm −2 are fabricated by using strainedlayer superlattice as dislocation filters. As a result of the high quality virtual substrate, fabrication of QD layer with good structural properties has been achieved, as evidenced by transmission electron microscopy and x-ray diffraction measurements. The InGaAs QD infrared photodetector is then fabricated on the GaAs-on-Si wafer substrate. Dual-band photoresponse is observed at 80 K with two response peaks around 6 and 15 μm.
Photoluminescence (PL) is investigated as a function of the excitation intensity and temperature for lattice-matched InGaAs/InAlAs quantum well (QW) structures with well thicknesses of 7 and 15 nm, respectively. At low temperature, interface fluctuations result in the 7-nm QW PL exhibiting a blueshift of 15 meV, a narrowing of the linewidth (full width at half maximum, FWHM) from 20.3 to 10 meV, and a clear transition of the spectral profile with the laser excitation intensity increasing four orders in magnitude. The 7-nm QW PL also has a larger blueshift and FWHM variation than the 15-nm QW as the temperature increases from 10 to ~50 K. Finally, simulations of this system which correlate with the experimental observations indicate that a thin QW must be more affected by interface fluctuations and their resulting potential fluctuations than a thick QW. This work provides useful information on guiding the growth to achieve optimized InGaAs/InAlAs QWs for applications with different QW thicknesses.
We report on the effect of strain on the optical and structural properties of 5-, 10-, and 20-period GaN/AlN superlattices (SLs) deposited by plasma-assisted molecular beam epitaxy. The deformation state in SLs has been studied by high resolution transmission electron microscopy (HRTEM), X-ray diffraction, and micro-Raman, Fourier transform infrared (FTIR), and photoluminescence spectroscopy. HRTEM images showed that the structural quality of the SL layers is significantly improved and the interfaces become very sharp on the atomic level with an increase of the SL periods. A combined analysis through XRD, Raman, and FTIR reflectance spectroscopy found that with increasing number of SL periods, the strain in the GaN quantum wells (QWs) increases and the AlN barrier is relaxed. Based on the dependence of the frequency shift of the E2High and E1TO Raman and IR modes on the deformation in the layers, the values of the biaxial stress coefficients as well as the phonon deformation potentials of these modes in both GaN and AlN were determined. With increasing number of SL periods, the QW emission considerably redshifted in the range lower than the GaN band gap due to the quantum confined Stark effect. The influence of strain obtained by the XRD, Raman, and FTIR spectra on the structural parameters and QW emission of GaN/AlN SLs with different numbers of periods is discussed.
Crystalline zinc
blende GaAs has been grown on a trigonal c-plane
sapphire substrate by molecular beam epitaxy. The initial stage of
GaAs thin film growth has been investigated extensively in this paper.
When grown on c-plane sapphire, it takes (111) crystal orientation
with twinning as a major problem. Direct growth of GaAs on sapphire
results in three-dimensional GaAs islands, almost 50% twin volume,
and a weak in-plane correlation with the substrate. Introducing a
thin AlAs nucleation layer results in complete wetting of the substrate,
better in-plane correlation with the substrate, and reduced twinning
to 16%. Further, we investigated the effect of growth temperature,
pregrowth sapphire substrate surface treatment, and in-situ annealing
on the quality of the GaAs epilayer. We have been able to reduce the
twin volume below 2% and an X-ray diffraction rocking curve line width
to 223 arcsec. A good quality GaAs on sapphire can result in the implementation
of microwave photonic functionality on a photonic chip.
Plastic strain relaxation
in epitaxial layers is one of the crucial
factors that limits the performance of III-nitride-based heterostructures.
In this work, we report on strain relaxation and crystalline defects
in heterostructures consisting of compositionally graded AlGaN epitaxial
layers tensile-strained between a GaN-buffer and a GaN-cap. We demonstrate
the effects of Al concentration and the shape of the concentration-depth
profile in the buried graded layers on the accumulated elastic strain
energy and how this influences the critical thickness for crack generation
or fracture. It is shown that this fracture leads to the formation
of partially relaxed regions with their degree of strain relaxation
directly related to the density of cracks. Nevertheless, even though
the in-plane coherency between the AlGaN layer and the GaN-buffer
is broken, the in-plane coherency within the AlGaN layer is preserved
for all regions. Furthermore, the tensile strain released in the buried
graded AlGaN layers is consistent with compressive strain induced
in the GaN-cap layers. Finally, the localized stress and the densities
of threading dislocations are correlated with the features of the
resulting fractured heterostructures. These results are important
toward the control of complex plastic strain relaxation and further
facilitate the growth of high quality compositionally graded AlGaN-based
devices.
A 2D-to-3D transition from nanostructured films to multifaceted InN nanocrystals for growth on GaN(0001) is accompanied by a 30-fold enhancement of InN photoluminescence emission.
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