![(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.](https://www.researchgate.net/profile/Ting-Wang-276/publication/318614652/figure/fig3/AS:639931080781825@1529582867262/a-The-STEM-image-of-the-epitaxial-layers-The-white-arrow-shows-the-growth-direction-of_Q320.jpg){kind=icon}
C/L-band emission of InAs QDs monolithically
grown on Ge substrate
WEN-QI WEI,1,2 JIAN-HUAN WANG,1 YUE GONG,1 JIN-AN SHI,1 LIN GU,1
HONG-XING XU,2 TING WANG,1,3 AND JIAN-JUN ZHANG1,4
1Institute of Physics, Chinese Academy of Sciences, Beijing, 100190, China
2School of Physics and Technology, Wuhan University, Wuhan, 430072, China
3wangting@iphy.ac.cn
4jjzhang@iphy.ac.cn
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
temperature.
©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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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
systems.
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.
Funding
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).
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