We investigate and compare the performance of 30 layers strain-coupled quantum dot (SCQD) infrared photodetectors capped with one of two different layers: a quaternary (In 0.21 Al 0.21 Ga 0.58 As) or ternary (In 0.15 Ga 0.85 As) alloy of 30 Å and a GaAs layer with a thickness of 120-150 Å . Measurements of optical properties, spectral responsivity, and cross-sectional transmission electron microscopy were conducted. Results showed that quaternary capping yielded more superior multilayer QD infrared photodetectors than ternary capping. Quaternary capping resulted in enhanced dot size, order, and uniformity of the QD array. The presence of Al in the capped layer helped in the reduction in dark current density and spectral linewidth as well as led to higher electron confinement of the QDs and enhanced device detectivity. The vertically ordered SCQD system with quaternary capping exhibited higher peak detectivity (*10 10 cm Hz 1/2 /W) than that with ternary capping (*10 7 cm Hz 1/2 /W). In addition, a very low noise current density of *10 -16 A/cm 2 Hz 1/2 at 77 K was achieved with quaternary-capped QDs.
The optical, electrical, and spectral properties of a strain coupled InAs quantum dot detector with a fixed quaternary capping of InAlGaAs and variable GaAs barrier thickness were investigated along with an equivalent uncoupled structure. Self-assembled quantum dots with a multimodal dot size distribution were achieved owing to vertical strain coupling. Strain and electronic coupling were utilized to improve the optical and electrical performance of the fabricated quantum dot infrared photodetector. The peak spectral response was tuned by varying barrier thickness, and a blue shift (almost 1 μm) was observed by increasing the capping thickness from sample A (90 Å capping) to E (500 Å capping). High responsivity and detectivity (∼1010 cm Hz1/2/W) were observed for all coupled samples as compared to the uncoupled sample. All coupled samples showed high thermal stability in the photoluminescence peak with high-temperature annealing.
This
study describes the effect of a thin GaAs spacer of 4.5 nm
thickness in a bilayer-coupled InAs quantum dot (QD) heterostructure.
Here, we report the first demonstration of InAs/GaAs QDs capped by
self-assembled In
x
Ga1–x
As layers. Self-assembled In
x
Ga1–x
As layers were introduced
into each intermediate layer across the interface of InAs QDs and
the GaAs layer in a vertical-coupled bilayer QD (VCBQD) heterostructure
to prevent indium desorption from the QDs. A change in the indium
content in the seed-layer InAs QDs changes the self-assembly position
and modifies the In
x
Ga1–x
As layer thickness. A theoretical approach was presented
to study the formation of self-assembled In
x
Ga1–x
As layers at each strain-free
layer. We showed that the strain energy at the second intermediate
(ε
zz2) layer is greater than that
at the first intermediate (ε
zz1)
layer; ε
zz2 depends on the vertical
strain channel length. The impact of the In
x
Ga1–x
As layer thickness
on the strain energy was studied using high-resolution transmission
electron microscopy; a shorter strain channel length was found to
facilitate the formation of a more relaxed and larger-sized self-assembled
In
x
Ga1–x
As layer in the active layer. This In
x
Ga1–x
As layer formed at the intermediate
layer acts as a capping layer or a protective shield for the indium
adatoms, preventing their desorption from the InAs QDs. Furthermore,
we studied the thermal stability of the self-assembled In
x
Ga1–x
As layer
by annealing the VCBQD samples at 700 and 800 °C. This aspect
has been investigated for the first time ever in a study of the coupling
efficiency between the InAs QDs and In
x
Ga1–x
As capping layer. A high-resolution
in-plane (2θχ/ϕ) reciprocal space mapping (RSM)
technique provided the connection between the in-plane reciprocal
lattice point of the InAs QDs and In
x
Ga1–x
As layers and revealed the strain
and coupling between them. InAs QDs fully covered with the self-assembled
In
x
Ga1–x
As layer enhanced the photoluminescence intensity by 77% and had
an activation energy of 467 meV.
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