C/L-band emission of InAs QDs monolithically grown on Ge substrate

Abstract and Figures

In recent years, the growing demand for silicon based light sources has boosted the research field of III-V/IV hybrid lasers. Here, the C/L-band light emission (1.53 μm-1.63 μm) of InAs/In0.25Ga0.75As quantum dots (QDs) epitaxially grown on Ge substrate by solid-source molecular beam epitaxy (MBE) is reported. By hybrid III-V/IV epitaxial growth, ultra-thin and anti-phase domains (APD) free III-V materials are achieved on Ge substrate. Step-graded InGaAs metamorphic buffer layers are applied to reduce the strain in InAs QDs in order to extend the emission wavelength. At last, a high quality InAs/In0.25Ga0.75As QD structure on Ge(001) substrate is obtained, which has a strong C/L-band emission centered at the wavelength of 1.6 μm with a full-width-half-maximum (FWHM) of 57 meV at room temperature.
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C/L-band emission of InAs QDs monolithically
grown on Ge substrate
1Institute of Physics, Chinese Academy of Sciences, Beijing, 100190, China
2School of Physics and Technology, Wuhan University, Wuhan, 430072, China
Abstract: In recent years, the growing demand for silicon based light sources has boosted the
research field of III-V/IV hybrid lasers. Here, the C/L-band light emission (1.53 μm-1.63 μm)
of InAs/In0.25Ga0.75As quantum dots (QDs) epitaxially grown on Ge substrate by solid-source
molecular beam epitaxy (MBE) is reported. By hybrid III-V/IV epitaxial growth, ultra-thin and
anti-phase domains (APD) free III-V materials are achieved on Ge substrate. Step-graded
InGaAs metamorphic buffer layers are applied to reduce the strain in InAs QDs in order to
extend the emission wavelength. At last, a high quality InAs/In0.25Ga0.75As QD structure on
Ge(001) substrate is obtained, which has a strong C/L-band emission centered at the
wavelength of 1.6 μm with a full-width-half-maximum (FWHM) of 57 meV at room
©2017 Optical Society of America
OCIS codes: (140.3380) Laser materials; (160.4760) Optical properties; (230.5590) Quantum-well, -wire and -dot
devices; (160.3130) Integrated optics materials.
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Received 1 May 2017; revised 7 Jun 2017; accepted 7 Jun 2017; published 21 Jul 2017
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1. Introduction
Over the past few years, silicon based III-V photonic materials and devices have drawn strong
attention in the silicon photonic research fields [1–3]. As known, for the large-scale integration
of complex opto-electronic circuits, the major challenge is the lack of reliable and silicon based
light sources [4]. By ccombining the existing silicon photonic techniques with the outstanding
optical properties of III-V materials, the hybridization of group III-V and IV materials will be
the key to boost the performances of photonic integration [5–7].
The direct epitaxial growth of III-V materials on Si has been recently reported, including
the fabrication of 1.3 μm InAs/GaAs QD lasers on Si substrate [8–11]. However, most of the
previous experimental results are referring to the 1.3 μm wavelength at the O-band telecom
window. For the C-band or L-band telecom window, there is no work having been reported yet.
Since most of the Si photonic passive and active devices are based on C-band applications,
long-wavelength (1.55 μm) III-V light sources on Si are becoming strongly demanded.
Especially, for long-haul transmission, Si-based high-gain III-V semiconductor optical
amplifiers (SOAs) at C/L-band [12] are also essential components as an on-chip replacement
for erbium-doped fiber amplifier (EDFA) in the case of future all-Si photonic integration
In this work, it is first time to realize room-temperature C/L-band light emission of
InAs/In0.25Ga0.75As QDs epitaxially grown on Ge(001) substrate. Since the strain-relaxed Ge on
Si growth technique is well established, this reported result on Ge substrate is a strong
indication that it could be further implemented onto the Ge/Si virtual substrate for device
applications, such as C/L-band lasers and SOAs. In our approach, a unique Ge epi-layer with
double-atomic steps [13] is implemented to prevent the emergence of antiphase domains
Vol. 7, No. 8 | 1 Aug 2017 | OPTICAL MATERIALS EXPRESS 2956
(APDs) between group III-V and IV materials. To achieve C/L-band emission of InAs QDs,
step-graded metamorphic InGaAs buffer layers are applied [14–16]. Finally, a broadband light
emission that covers the wavelength ranging from 1.53 μm to 1.63 μm has been obtained.
2. Experimental methods
The InAs/In0.25Ga0.75As QD structure was grown on Ge(001) substrate with 2° offcut towards
[110] orientation by Solid-Source Molecular Beam Epitaxy (MBE). In order to prevent the
formation of APDs while growing polar III-V materials on a non-polar germanium substrate, it
is a prerequisite to form a double atomic layer at the Ge surface. The schematic diagram of the
structure is shown in Fig. 1. Firstly, the Ge substrate is de-oxidized at 450 °C for 15 minutes
before the epitaxial growth. Then an ultra-thin layer of Ge buffer (60 nm) is deposited on the Ge
substrate at 300 °C, following by an in situ annealing at 540 °C for 90 minutes to create the
double atomic layer of Ge. The GaAs nucleation layer is first deposited by migration enhanced
epitaxy (MEE) at 360 °C, following by an ultra-thin two-step GaAs growth of 20 nm and 230
nm at 450 °C and 560 °C, respectively. In comparison with the previous report of 1.3 μm InAs
QDs on Ge [5], which requires approximately 1.5 μm thick GaAs buffer structures, in this
work, the double-atomic formation technique on Ge substrate effectively avoids the formation
of APDs. Therefore, it requires only an ultra-thin GaAs buffer layer which is APD free.
Furthermore, with step-graded epitaxial growth method, InGaAs metamorphic buffer layer
with thickness of 700 nm is grown on top of the GaAs buffer. Here the InGaAs metamorphic
buffer consists of two layers: a 200 nm step-graded InGaAs layer from In0.09Ga0.91As to
In0.13Ga0.87As, followed by a 200 nm In0.13Ga0.87As layer both grown at 380 °C and a 200 nm
step-graded InGaAs layer from In0.13Ga0.87As to In0.25Ga0.75As, followed by a 100 nm
In0.25Ga0.75As layer at 380 °C and 500 °C, respectively. The low growth temperature (380 °C)
of InGaAs metamorphic layer can greatly suppress the propagation of misfit dislocation [17].
After each 200 nm InGaAs buffer layer, a 30 minutes annealing at 500 °C is implemented
adjacently in order to further reduce the defect densities [18]. To notice, before each annealing
process a thin AlAs layer with thickness of 20 Å is deposited at 380 °C as a protective layer to
avoid the indium desorption at high temperature. The active region including 3 periods of InAs
QD layer is grown on the high quality and flat top In0.25Ga0.75As buffer layer. Each InAs QD
layer consists of 2.8 monolayer of InAs capped by a 4 nm In0.25Al0.75As layer, which are both
grown at 465 °C. The InAs QD layers are separated by 45 nm In0.25Ga0.75As spacer layers,
which are grown at an optimum temperature of 500 °C. At last, surface InAs QDs are deposited
with the same growth condition as the buried InAs QD layer, for AFM characterization.
Fig. 1. Schematic of self-organized InAs/InGaAs QDs on Ge(001) substrate.
3. Results and discussion
In order to extend the InAs QDs emission wavelength to C/L-band, a multiple-step-graded
In0.25Ga0.75As metamorphic buffer is used to form larger QDs in sizes, which are due to the
strain reduction of QDs on the InGaAs virtual layer. In Fig. 2(a), a flat In0.25Ga0.75As virtual
Vol. 7, No. 8 | 1 Aug 2017 | OPTICAL MATERIALS EXPRESS 2957
buffer layer is achieved on Ge substrate with root-mean-square (RMS) roughness of 0.45 nm in
a 5 x 5 um2 region. The X-Ray Diffraction (XRD) spectrum of the metamorphic buffer is
showed in Fig. 2(b), which indicates a high quality epitaxial growth of In0.25Ga0.75As buffer on
Ge substrate. The peak of GaAs buffer in the XRD spectrum is overlapped with that of Ge
substrate due to their similar lattice constant.
Fig. 2. (a) A 5 x 5 μm 2 AFM image of In0.25Ga0.75As buffer layer epitaxial growth on Ge
substrate. (b) XRD result of InGaAs metamorphic buffer on Ge(001) substrate.
The epitaxial structure is also characterized using scanning transmission electron
microscopy (STEM) on a focused ion beam (FIB) fabricated cross-sectional lamella as shown
in Fig. 3. It is observed in Fig. 3(a) that there is no apparent defect propagation from the
GaAs/Ge interface and InGaAs metamorphic buffer to the active layer. Figure 3(b) has shown
the bright-field TEM image of GaAs/Ge interface, where the low-density defects are mostly
localized at the interface region. A high-magnification STEM image of InAs QDs is shown in
Fig. 3(c), which indicates the active layers are defect-free. Due to intermixing during the
growth of InAlAs capping layer as observed in Ge/Si system [19], the InAs QDs are truncated
as shown in Fig. 3(c).
Fig. 3. (a) The STEM image of the epitaxial layers. The white arrow shows the growth direction
of the sample. (b) Bright-field TEM image of GaAs and Ge interface. (c) High-magnification
STEM image of InAs QDs. The red-marked region represents the cross section of a top-flattened
InAs QD. The white arrow shows growth direction. All images are taken along [110] direction.
Vol. 7, No. 8 | 1 Aug 2017 | OPTICAL MATERIALS EXPRESS 2958
Normalized photoluminescence (PL) spectra of the InAs/InGaAs QDs on both Ge and
GaAs substrates are measured here as shown in Fig. 4(a). It shows that the room-temperature
PL intensity of InAs QDs grown on Ge substrate is more than 85% of that of QDs on GaAs
substrate, with a C/L-band emission wavelength of ~1.6 μm. The inset picture of Fig. 4(a)
shows a 1 × 1 μm2 AFM image of uncapped surface InAs/InGaAs QDs on Ge substrate with a
density of 2.55 × 1010 /cm2. With the InGaAs metamorphic buffer, the InAs QDs here have a
relatively larger size of approximately 50 nm in diameter and 6.5 nm in height, in comparison
with conventional 1.3 μm InAs/GaAs QDs [20]. As observed in Fig. 4(a), due to the
non-uniformity of the QD sizes, there are two PL peaks appeared in the plot for QDs on Ge
substrate, where the peaks at 1.45 μm and 1.6 μm correspond to the ensembles of small and
large sized QDs, respectively. This broadband emission at C/L band enables the potential
applications as a gain medium in saturable absorber, detectors and SOAs [12]. Additionally, the
influence of the thickness of In0.25Al0.75As capping layer on the room-temperature PL peak
intensity has been also investigated as shown in Fig. 4(b). From the comparison of different
thickness of In0.25Al0.75As capping layer, the sample with a 4 nm In0.25Al0.75As capping layer
shows a strongest room-temperature PL intensity.
Fig. 4. (a) Room-temperature photoluminescence spectra of InAs/InGaAs QDs grown on Ge
substrate and GaAs substrate, respectively. Inset: AFM image of surface InAs QDs on Ge
substrate. (b) Peak intensity of room-temperature PL spectra with different thickness of
In0.25Al0.75As capping layer.
Temperature-dependent PL measurements are also performed here as shown in Fig. 5(a).
As the temperature decreases from 300 K to 5 K, the PL peaks blue-shift to shorter wavelengths
with enhanced PL intensity. Figure 5(b) shows the variations in peak wavelength against the
temperature. The PL peak at 1.45 μm disappears with the decrements of temperature in Fig.
5(a), therefore, it can be ensured that the emission peak at shorter wavelength is not induced by
excited state emission of InAs QDs. The temperature dependent plot of the
full-width-half-maximum (FWHM) of the PL spectra is shown in Fig. 5(c). For temperature
lower than 50 K, the FWHM reaches the maximum value of approximate 75 meV, and the
minimum value of 52 meV appears at 200 K. This variation can be explained considering the
distribution of carriers in InAs QDs at different temperature [21, 22]. For low temperature
regime, the carriers in InAs QDs are not in a near-equilibrium state described by quasi-Fermi
levels, thus, allowing dots with different sizes and shapes occupied by carriers. Therefore, those
InAs QDs which are not showing PL emission at room temperature, would be randomly
populated by carriers and will contribute to the cryogenic PL emission [23, 24]. As a
consequence, the FWHM would broaden with decreasing temperature as shown in Fig. 5(c). In
the temperature regime above 200K, the PL spectrum also broadens mainly due to the thermal
excitation of carriers towards the higher excited states [10, 25].
Vol. 7, No. 8 | 1 Aug 2017 | OPTICAL MATERIALS EXPRESS 2959
Fig. 5. (a) Temperature-dependent PL spectra analysis. (b) The variations in the peak
wavelengths against temperature. (c) FWHM of PL spectra as a function of temperature. (d)
Arrhenius plots of temperature-dependentt IPLI. The data have been normalized in the plot. The
red solid line is the fitting result.
Figure 5(d) shows the Arrhenius plot of integrated PL intensity (IPLI) of the sample against
inverse temperature (1000/T), where the red curve indicates the Arrhenius fitting of the
experimental data. The IPLI can be described inverse proportional to exp(Ea/kT) [13, 24, 26],
where Ea is the thermal activation energy. For the high temperature linear regime (>100 K),
namely strong thermal quenching regime, dissociated excitons (electron-hole pairs) will escape
from QDs into the adjacent barrier layers, thus, the IPLI will decrease linearly with the rising
temperature beyond the quenching point. Consequently, the corresponding Ea can be extracted
by measuring the gradient of the slope. By calculation, the Ea can be deduced with a value of
approximate 103.9 meV for the InAs/In0.25Ga0.75As QDs. Considering the probability of
dissociated excitons escaping into In0.25Al0.75As capping layers and excited states of QDs, the
thermal activation energy Ea is within a comparable range to those of 1.3 μm InAs/GaAs QDs
reported before [27]. Clearly, the In0.25Ga0.75As barrier has less confinement than GaAs, leading
to a relatively weaker carrier confinement, which explains reduction in the PL intensity of
InAs/In0.25Ga0.75As QDs at room temperature.
Referring to the PL measurements on our reference sample (1.3 μm InAs/GaAs QDs), the
room-temperature PL intensity of InAs/InGaAs QDs on Ge is approximately 1/10 of the
reference sample, with an effective wavelength extension toward C/L-band wavelengths (1.53
μm - 1.63 μm).
4. Conclusion
In conclusion, we have achieved the first room-temperature C/L-band emission of
InAs/In0.25Ga0.75As QDs epitaxially grown on Ge substrate. By growing an ultra-thin 60 nm Ge
buffer layer, following by a surface annealing at 540 °C for 90 mins, double atomic layers are
Vol. 7, No. 8 | 1 Aug 2017 | OPTICAL MATERIALS EXPRESS 2960
produced to prevent the formation of APDs between group III-V and IV materials.
Additionally, step-graded In0.25Ga0.75As metamorphic structure has been epitaxially grown with
a surface roughness less than 0.5 nm. With this high-quality In0.25Ga0.75As/Ge buffer structure,
the top InAs/In0.25Ga0.75As QDs exhibit a strong broadband light emission at a peak wavelength
of 1.6 μm, where the FWHM is measured to be ~57 meV. The experimental results provide a
promising approach to realize C/L-band light sources and SOAs for silicon photonic
integration. With further optimization of the growth of structures, Ge/Si based electrically
pumped InAs/In0.25Ga0.75As QD lasers and SOAs are to be expected in the near future.
National Natural Science Foundation of China (Grants 11504415, 11434041, 11574356 and
161635011); the Ministry of Science and Technology (MOST) of Peoples’ Republic of China
(2016YFA0300600 and 2016YFA0301700); and the Key Research Program of Frontier
Sciences, CAS (Grant No. QYZDB-SSW-JSC009).
Vol. 7, No. 8 | 1 Aug 2017 | OPTICAL MATERIALS EXPRESS 2961
... Recently, we achieved monolithic growth of III-V on Ge miscut substrate by only using a 250-nm-thick GaAs buffer layer through the formation of a double atomic layer which could reduce the TDs [68]. In addition, as previously stated, most of the work so far has referred to the 1.3 µm wavelength at the O-band telecom window. ...
... APD free surface can be obtained then by migration-enhanced epitaxy (MEE) of the GaAs nucleation layer and two-step GaAs growth of 20 nm and 230 nm at 450 • C and 560 • C, respectively. Therefore, for photonics integration, only an ultra-thin GaAs buffer layer of 250 nm thick which is APD free was deposited, in comparison with the work stated previously, in which at least a 1-µm-thick buffer layer for III-V QD structure growth is required [68]. Furthermore, as reported in our previous work [68,69], with the step-graded epitaxial growth method, the InGaAs metamorphic buffer layer with the thickness of 700 nm is grown on top of the GaAs buffer layer. ...
... Therefore, for photonics integration, only an ultra-thin GaAs buffer layer of 250 nm thick which is APD free was deposited, in comparison with the work stated previously, in which at least a 1-µm-thick buffer layer for III-V QD structure growth is required [68]. Furthermore, as reported in our previous work [68,69], with the step-graded epitaxial growth method, the InGaAs metamorphic buffer layer with the thickness of 700 nm is grown on top of the GaAs buffer layer. To suppress the propagation of misfit dislocations, the low growth temperature of 380 • C was used for the InGaAs metamorphic layer. ...
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... To achieve long wavelength emission of InAs QDs under the phosphorus-free material system, step-graded InGaAs metamorphic buffer layers are used here [23,24]. Figure 1(a) shows the schematic diagram of five-layer InAs/In 0.35 Ga 0.65 As QDs grown on a GaAs/SOI platform with a 900 nm step-graded InGaAs metamorphic buffer layer. ...
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III-V semiconductor lasers epitaxially grown on silicon, especially on a silicon-on-insulator (SOI) platform, have been considered one of the most promising approaches to realize an integrated light source for silicon photonics. Although notable achievements have been reported on InP-based 1.5 µm III-V semiconductor lasers directly grown on silicon substrates, phosphorus-free 1.5 µm InAs quantum dot (QD) lasers on both silicon and SOI platforms are still uncharted territory. In this work, we demonstrate, to the best of our knowledge, the first phosphorus-free InAs QD microdisk laser epitaxially grown on SOI substrate emitting at the telecommunications S-band by growing metamorphic InAs/InGaAs QDs on (111)-faceted SOI hollow structures. The lasing threshold power for a seven-layer InAs QD microdisk laser with a diameter of 4 µm is measured as 234 μW at 200 K. For comparison, identical microdisk lasers grown on GaAs substrate are also characterized. The results obtained pave the way for an on-chip 1.5 µm light source for long-haul telecommunications.
... Since GaAs are both lattice and thermal matched to Ge, the major challenge to grow high quality GaAs film on Ge substrate is the polarity mismatch which will cause APDs. Techniques such as off-cut Ge or Si substrates can effectively reduce the APDs, as the high density atomic steps along specific directions can annihilate APDs at the interface [15,18] or the bi-atomic steps are able to avoid the formation of APDs [14,25] . The Ge {113} can be considered as a large off-cut substrate towards [110] direction. ...
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The direct epitaxial growth of GaAs on Si suffers from their nature of lattice mismatch, thermal mismatch and polarity difference induced anti-phase domains (APDs). Here, we report the high quality and thin GaAs film grown on (113)-faceted Ge/Si (001) hollow substrate by in-situ hybrid molecular beam epitaxy. By directly growth of Ge on U-shape patterned Si (001), a strain-relaxed high-quality Ge sawtooth hollow structure with {113} facets is obtained. With additional 400 nm GaAs deposition, an APD-free surface with a root-mean-square roughness of merely 0.67 nm is obtained on such Ge (113) /Si (001) substrate. The lattice mismatch dislocation between Ge and Si is found to terminate mostly at the sidewalls of the hollow structures. The (113)-faceted Ge surface is acting as an equivalent to the miscut substrate, which annihilates the APDs at the GaAs/Ge interface. High resolution X-ray diffraction characterization reveals that the hollow structures can effectively reduce the thermal strain, leading to a crack-free GaAs film up to 7 µm. Five-layer InAs/GaAs quantum dots (QDs) on such virtual GaAs/Ge (113) /Si (001) substrate without any dislocation filter layers exhibits almost the same photoluminescence (PL) intensity as that on the GaAs substrate, providing a promising method for integrating III-V QD lasers with silicon photonic platform.
... The thickness of III-V epi-layer and the change of wafers' temperature (ΔT) both have an important influence on the formation of thermal micro-cracks. Critical thickness of the crack formation on GaAs epi-layer on Si to be experimentally observed is approximately 7 μm for a ΔT of 575 °C, 5.1 μm for a ΔT of 675 °C, and 4.9 μm for a ΔT of 725 °C [24,40] . ...
The realization of monolithic integration of a stable III-V laser on a standard silicon-on-insulator (SOI) substrate has been regarded as a challenging technology for silicon-based photonic integration circuits (PICs). Here, we successfully demonstrated the electrically pumped P-doped 1300 nm InAs/GaAs quantum dot (QD) laser epitaxially grown on {111}-faceted SOI hollow substrates. These III-V QD lasers, which are epitaxially grown on an SOI substrate, generally exhibit strong thermal accumulation due to the oxide layer underneath. By applying a double-side heat dissipation design, the maximum operation temperature of the SOI-based InAs/GaAs QD laser under a continuous-wave (CW) operation mode is ramped up to 35°C from 20°C. Moreover, the thermal profile simulation of three different structures has also been carried out to show the effectiveness of the top heat sink design in order to improve laser performance. An integrated thermal shunt design is proposed to improve heat dissipation without using the external top heat sink. The successful realization of room-temperature SOI-based InAs/GaAs QD lasers pave a viable way for integrating light sources in PICs.
GaAs/Ge heterostructures have been employed in various semiconductor devices such as solar cells, high-performance CMOS transistors, and III–V/IV heterogeneous optoelectronic devices. The performance of these devices is directly dependent on the material quality of the GaAs/Ge heterostructure, while the material quality of the epitaxial GaAs layer on the Ge is limited by issues such as the antiphase domain (APD), and stacking-fault pyramids (SFP). We investigate the epitaxial growth of high-quality GaAs on a Ge (001) mesa array, via molecular beam epitaxy. Following a systematic study of the Ge terrace via an in situ scanning tunneling microscope, an atomically step-free terrace on the Ge mesa measuring up to 5 × 5 μm ² is obtained, under optimized growth conditions. The step-free terrace has a single-phase c (4 × 2) surface reconstruction. The deposition of a high-quality GaAs layer with no APD and SFP is then achieved on this step-free Ge terrace. High-resolution transmission electron microscopy and electron channel contrast image characterizations reveal the defect-free growth of the GaAs layer on the step-free Ge mesa. Furthermore, InAs quantum dots on this GaAs/Ge mesa reveal photoluminescent intensity comparable to that achieved on a GaAs substrate, which further confirms the high quality of the GaAs layer on Ge.
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The integration of III-V on silicon is still a hot topic as it will open up a way to co-integrate Si CMOS logic with photonic devices. To reach this aim, several hurdles should be solved, and more particularly the generation of antiphase boundaries (APBs) at the III-V/Si(001) interface. Density functional theory (DFT) has been used to demonstrate the existence of a double-layer steps on nominal Si(001) which is formed during annealing under proper hydrogen chemical potential. This phenomenon could be explained by the formation of dimer vacancy lines which could be responsible for the preferential and selective etching of one type of step leading to the double step surface creation. To check this hypothesis, different experiments have been carried in an industrial 300 mm metalorganic chemical vapor deposition where the total pressure during the annealing step of Si(001) surface has been varied. Under optimized conditions, an APBs-free GaAs layer was grown on a nominal Si(001) surface paving the way for III-V integration on silicon industrial platform.
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The first operation of an electrically pumped 1.3-μm InAs/GaAs quantum-dot laser was previously reported epitaxially grown on Si (100) substrate. Here the direct epitaxial growth condition of 1.3-μm InAs/GaAs quantum on a Si substrate is further investigated using atomic force microscopy, etch pit density and temperature-dependent photoluminescence (PL) measurements. The PL for Si-based InAs/GaAs quantum dots appears to be very sensitive to the initial GaAs nucleation temperature and thickness with strongest room-temperature emission at 400°C (170nm nucleation layer thickness), due to the lower density of defects generated under this growth condition, and stronger carrier confinement within the quantum dots.
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Reliable, efficient electrically pumped silicon-based lasers would enable full integration of photonic and electronic circuits, but have previously only been realized by wafer bonding. Here, we demonstrate continuous-wave InAs/GaAs quantum dot lasers directly grown on silicon substrates with a low threshold current density of 62.5 €...A € -2, a room-temperature output power exceeding 105 €...mW and operation up to 120 €...°C. Over 3,100 €...h of continuous-wave operating data have been collected, giving an extrapolated mean time to failure of over 100,158 €...h. The realization of high-performance quantum dot lasers on silicon is due to the achievement of a low density of threading dislocations on the order of 10 5 € 2 in the III-V epilayers by combining a nucleation layer and dislocation filter layers with in situ thermal annealing. These results are a major advance towards reliable and cost-effective silicon-based photonic-electronic integration.
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In this article, the recent progress of III-V quantum dot lasers on silicon substrates for silicon photonic integration is reviewed. By introducing various epitaxial techniques, room-temperature 1.3-μm InAs/GaAs quantum dot laser on Si, Ge and SiGe substrates have been achieved respectively. Quantum dot lasers on Ge substrate has an ultra-low threshold current density of 55.2 A/cm2 at room temperature, which can operate over 60℃ in continuous-wave mode. Futhermore, by using the SiGe virtual substrate, at 30℃ and an output power of 16.6 mW, a laser lifetime of 4600 h has been reached, which indicates a bright future for the large-scale photonic integration.
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Serving as the electrical to optical converter, the on-chip silicon light source is an indispensable component of silicon photonic technologies and has long been pursued. Here, we briefly review the history and recent progress of a few promising contenders for on-chip light sources in terms of operating wavelength, pump condition, power consumption, and fabrication process. Additionally, the performance of each contender is also assessed with respect to thermal stability, which is a crucial parameter to consider in complex optoelectronic integrated circuits (OEICs) and optical interconnections. Currently, III-V-based silicon (Si) lasers formed via bonding techniques demonstrate the best performance and display the best opportunity for commercial usage in the near future. However, in the long term, direct hetero-epitaxial growth of III–V materials on Si seems more promising for low-cost, high-yield fabrication. The demonstration of high-performance quantum dot (QD) lasers monolithically grown on Si strongly forecasts its feasibility and enormous potential for on-chip lasers. The superior temperature-insensitive characteristics of the QD laser promote this design in large-scale high-density OEICs. The Germanium (Ge)-on-Si laser is also competitive for large-scale monolithic integration in the future. Compared with a III-V-based Si laser, the biggest potential advantage of a Ge-on-Si laser lies in its material and processing compatibility with Si technology. Additionally, the versatility of Ge facilitates photon emission, modulation, and detection simultaneously with a simple process complexity and low cost.
We report on the first electrically pumped continuous-wave (cw) InAs/GaAs quantum dot (QD) lasers monolithically grown on on-axis Si (001) substrates without any intermediate buffer layers. A 400 nm antiphase boundary (APB) free epitaxial GaAs film with a small root-mean-square (RMS) surface roughness of 0.86 nm was first deposited on a 300 mm standard industry-compatible on-axis Si (001) substrate by metal-organic chemical vapor deposition (MOCVD). The QD laser structure was then grown on this APB-free GaAs/Si (001) virtual substrate by molecular beam epitaxy (MBE). Room-temperature cw lasing at ∼1.3 μm has been achieved with a threshold current density of 425 A/cm² and single facet output power of 43 mW. Under pulsed operation, lasing operation up to 102°C has been realized, with a threshold current density of 250 A/cm² and single facet output power exceeding 130 mW at room temperature.
The epitaxial integration of III–V optoelectronic devices on silicon will be the enabling technology for full-scale deployment of silicon photonics and the key to improving communication systems. Silicon photonics also offer new opportunities for the realization of ultracompact and fully integrated sensing systems operating in the mid-infrared (MIR) regime of the spectrum. In this article, we review recent developments, through several approaches, in the direct metamorphic epitaxial growth of various III–V materials-based lasers on silicon substrates. We show that GaAs-based 1.3-μm III–V quantum dot lasers and GaSb-based MIR quantum-well lasers grown on silicon substrates can operate with low threshold current density and high operating temperature, which hold promise for the future.
Silicon photonics is the optical analogue of silicon microelectronics. It promises to use photons to detect, process and transmit information more efficiently than electrical signals, and yet have low manufacturing costs as a result of using conventional silicon-integrated-circuit processes.
Using rapid thermal annealing, we fabricated a series of InAs/GaAs quantum dot samples with ground-state emission ranging from 1.05 eV to 1.35 eV. This set of annealed samples, all having the same density, allows us to study the influence of the barrier height on the temperature dependence of the photoluminescence (PL). The integrated PL follows an Arrhenius-type behavior, with activation energies matching the barrier heights. However, the quenching occurs at lower temperatures as the barrier height decreases. The modeling of these data enables us to understand the important mechanisms determining the critical temperature where the quenching occurs. We also present a detailed investigation into the excitation density dependence of the photoluminescence at different temperatures. Under relatively low excitation, this dependence is linear at 10 K, and becomes increasingly superlinear and eventually quadratic as the temperature is increased and carriers escape from the dots. However, under high excitation, the dependence remains linear even at high temperatures and the activation energy for quenching is different. We show that all these results can be understood by considering the independent capture and escape of electrons and holes in the dots.
The effect of the InAs deposition rate on the properties of InAs/GaAs quantum dots (QD’s) grown on GaAs(001) substrates by molecular-beam epitaxy has been studied by scanning tunneling microscopy (STM) and photoluminescence (PL). PL studies performed on GaAs capped QD samples show that the emission wavelength increases with decreasing growth rate, reaching a maximum around 1.3 μm, with the linewidth decreasing from 44 to 27 meV. STM studies on uncapped dots show that the number density, total QD volume, and size fluctuation all decrease significantly as the growth rate is reduced. We deduce that the composition of the dots is also dependent on the growth rate, the indium fraction being highest at the lowest growth rates. The shifts in the emission wavelength and linewidth correlate with changes in the QD size, size distribution, and composition.