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/In_0.25Ga_0.75As 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 (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/In_0.25Ga_0.75As 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 In_0.09Ga_0.91As to In_0.13Ga_0.87As, followed by a 200 nm In_0.13Ga_0.87As layer both grown at 380 °C and a 200 nm step-graded InGaAs layer from In_0.13Ga_0.87As to In_0.25Ga_0.75As, followed by a 100 nm In_0.25Ga_0.75As 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 In_0.25Ga_0.75As buffer layer. Each InAs QD layer consists of 2.8 monolayer of InAs capped by a 4 nm In_0.25Al_0.75As layer, which are both grown at 465 °C. The InAs QD layers are separated by 45 nm In_0.25Ga_0.75As 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.

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3. Results and discussion

In order to extend the InAs QDs emission wavelength to C/L-band, a multiple-step-graded In_0.25Ga_0.75As 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 In_0.25Ga_0.75As virtual 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 In_0.25Ga_0.75As 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 In_0.25Ga_0.75As buffer layer epitaxial growth on Ge substrate. (b) XRD result of InGaAs metamorphic buffer on Ge(001) substrate.

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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.

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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 In_0.25Al_0.75As 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 In_0.25Al_0.75As capping layer, the sample with a 4 nm In_0.25Al_0.75As 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 In_0.25Al_0.75As capping layer.

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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].

 

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.

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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(E_a/kT) [13, 24, 26], where E_a 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 E_a can be extracted by measuring the gradient of the slope. By calculation, the E_a can be deduced with a value of approximate 103.9 meV for the InAs/In_0.25Ga_0.75As QDs. Considering the probability of dissociated excitons escaping into In_0.25Al_0.75As capping layers and excited states of QDs, the thermal activation energy E_a is within a comparable range to those of 1.3 μm InAs/GaAs QDs reported before [27]. Clearly, the In_0.25Ga_0.75As barrier has less confinement than GaAs, leading to a relatively weaker carrier confinement, which explains reduction in the PL intensity of InAs/In_0.25Ga_0.75As 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/In_0.25Ga_0.75As 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 produced to prevent the formation of APDs between group III-V and IV materials. Additionally, step-graded In_0.25Ga_0.75As metamorphic structure has been epitaxially grown with a surface roughness less than 0.5 nm. With this high-quality In_0.25Ga_0.75As/Ge buffer structure, the top InAs/In_0.25Ga_0.75As 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/In_0.25Ga_0.75As QD lasers and SOAs are to be expected in the near future.