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We compare InAlAs/GaAs and InGaAs/GaAs strained-layer superlattices (SLSs) as dislocation filter layers for 1.3-μm InAs/GaAs quantum-dot laser structures directly grown on Si substrates. InAlAs/GaAs SLSs are found to be more effective than InGaAs/GaAs SLSs in blocking the propagation of threading dislocations generated at the interface between the GaAs buffer layer and the Si substrate. Room-temperature lasing at ~1.27 μm with a threshold current density of 194 A/cm2 and output power of ~77 mW has been demonstrated for broad-area lasers grown on Si substrates using InAlAs/GaAs dislocation filter layers.
© 2014 Optical Society of America
1. Introduction
The need to develop practical laser sources on a Si platform becomes increasingly urgent for silicon photonics [1]. Unfortunately, Si is an indirect bandgap semiconductor and thus an inefficient light-emitting material. Recently, much research effort in both industry and academia has been devoted to searching for the last missing element of Si photonics - an efficient, electrically pumped laser grown directly on a Si substrate. Significant progress in gaining light from Si has been made in the last decade. Novel approaches, such as heterogeneous/monolithic integration of III-V/Si, stimulated Raman scattering, nanostructured Si, and rare-earth-doped Si, have been demonstrated as alternative means to extract light from Si. These efforts have led to successful demonstrations for room-temperature lasing, including Si Raman lasers, hybrid Si lasers, and III-V and Ge lasers epitaxially grown on silicon [2, 3], that can potentially address the light source issue on Si. However, there are still numerous challenges and practical issues facing these techniques. The application of Raman lasers is constrained by optically pumped operation [4]. The feasibility of using low-dimensional Si materials, such as nanoporous Si and Si nanocrystals, as efficient emitters has not yet been demonstrated [5]. The potential of band-engineered Ge-on-Si lasers is also undermined by high optical loss and a high material gain is required for electrically pumped lasers [6].
In the last a few years, III-V on Si (III-V/Si) photonics via monolithic or heterogeneous integration has attracted much attention. Heterogeneous III-V/Si integration through wafer bonding technology has demonstrated impressive lasers with milliwatt power output and continuous-wave operation to temperatures over 100 °C [7, 8]. However, the yield and reliability for heterogeneous integration has yet to be proved [9, 10]. Monolithic growth of III-V materials on Si is considered as the most desirable approach for III–V/Si integration, but the high dislocation density caused by large lattice mismatch and the difference in thermal expansion coefficient between III-V epilayers and Si substrates make the monolithic III–V/Si integration challenging [11, 12]. Recently, III-V quantum dots (QDs) have been emerging as a promising technique for practical III-V/Si photonics due to their attractive properties, in particular the improved tolerance to defects and delta-function density of states [13, 14]. As a result, high performance QD lasers at optical communications wavelengths with low threshold currents, high power output, and high operation temperature have been demonstrated on Ge and Ge-on-Si substrates [15–20]. For direct epitaxial growth of III-V materials on Si substrates, a buffer between Si and III-V active regions plays a critical role in the performance of laser devices due to the large lattice mismatch [3]. In this paper, the effect of strained-layer superlattices (SLSs) as dislocation filter layers (DFLs) on the density of threading dislocations has been investigated. The density of threading dislocations can be effectively reduced down to ~106 cm−2 by using InAlAs/GaAs SLSs. A QD laser directly grown on a Si substrate with InAlAs/GaAs SLSs by molecular beam epitaxy is demonstrated with a low threshold current density of 194 A/cm2, a peak lasing wavelength at ~1.27 μm, and output power of ~77 mW at room temperature. Operation up to 85 °C has been measured for the as-cleaved broad-area lasers.
2. Effects of SLS on the quality of III-V materials directly grown on Si substrates
InAs/GaAs QD samples were grown on n-doped Si (100) substrates with 4° offcut to the [011] plane using a solid-source III-V molecular beam epitaxy system. Oxide desorption of Si substrates was performed at 900 °C for 10 minutes. The substrates were then cooled down to 400 °C for the growth of a GaAs nucleation layer. The nucleation layer consists of an optimized two-step growth scheme [3, 14]. Three repeats of SLS DFLs separated by 400-nm GaAs spacing layers were grown on the top of a 1000-nm GaAs buffer layer. Two types of SLSs, five-period of 10-nm In0.15Ga0.85As/10-nm GaAs and five-period of 10-nm In0.15Al0.85As/10-nm GaAs were investigated in this study. After another 400-nm GaAs spacing layer, a typical ðve-layer InAs/GaAs dot-in-a-well (DWELL) structure was grown at ~510 °C similar to that optimized on GaAs substrates [21–23]. The DWELLs were embedded between two 100-nm GaAs layers grown at 580 °C and 50-nm AlGaAs layers grown at 610 °C. Each DWELL layer consisted of 3-monolayer InAs QD layer sandwiched by 2-nm In0.15Ga0.85As and 6-nm In0.15Ga0.85As. Undoped GaAs spacer layers of 45 nm were used to separate the InAs/InGaAs DWELLs.
Atomic force microscopy (AFM) and cross-sectional transmission electron microscopy (TEM) measurements are used to characterize the structural properties of QDs grown on Si substrates with different DFLs. The AFM scans were performed with a Nanoscope Dimension 3100 SPM AFM system in ambient conditions using a tapping mode. The TEM imaging was carried out by using a FEI Titan 80-300 high-resolution TEM. Despite the use of different types of SLSs as DFLs, the InAs/GaAs QDs grown on the Si substrates share similar structural properties. Typical AFM and TEM images of uncapped InAs QDs are shown in Figs. 1(a) and 1(b), respectively. From the AFM and TEM, the dot density is estimated to be ~4 × 1010 cm−2 and typical dot size is about 25 nm in diameter and ~7-8 nm in height. Figure 1(c) shows a cross-sectional TEM image of five-layer DWELL structure grown on Si substrates. No dislocation is observed in a number of similar images, which suggests that, by using the growth techniques we developed to suppress antiphase domains and threading dislocations [15, 18, 24], the active regions with low defect density can be obtained.
Fig. 1 (a) AFM image (1 × 1 μm2) of InAs/GaAs QDs grown on a Si substrate. (b) Cross-sectional TEM image of an uncapped InAs QD. The scale bar is 10 nm. (c) Cross-sectional TEM bright field image of five layers of DWELL structure grown on Si substrate. The scale bar is 100 nm.
In order to study the effects of InGaAs/GaAs and InAlAs/GaAs SLSs serving as threading dislocation filters, TEM images with low magnification are compared as shown in Figs. 2(a) and 2(b). For both samples, a high density of dislocations is generated at the GaAs/Si interface as a result of the large lattice mismatch. Most of the defects are confined in the first ~200 nm region thanks to the two-step low temperature growth but still a quite high density (~109 cm−2) of threading dislocation is propagating towards the active region. As shown in Figs. 2(a) and 2(b), the SLSs can effectively suppress the propagation of the threading dislocations by bending the threading dislocations into the growth plane [24, 25]. The TEM images in Figs. 2(a) and 2(b) shows that GaAs layers are visually dislocation free after two sets of InAlAs/GaAs SLSs with a few dislocations after the InGaAs/GaAs SLSs. The reduction of dislocation density induced by SLSs is estimated by TEM, as shown in Fig. 2(c). After the first set of SLSs, the dislocation density is reduced over one order of magnitude (<108 cm−2) for both samples. At such a level of dislocation density, TEM measurements become unreliable for quantitative comparison. Nevertheless, the TEM results plotted in Fig. 2(c) qualitatively indicate that both InGaAs/GaAs and InAlAs/GaAs SLSs have similar effects on the reduction of threading dislocations due to similar strain field or misfit to GaAs. In order to distinguish the effectiveness of both SLSs in blocking the propagation of threading dislocations, other techniques that are able to probe defect density in large area are favored in this case. Etch pit defects (EPD) were counted for both samples. After three sets of SLSs, the sample with InAlAs/GaAs SLSs shows an average defect density of about 2.0 × 106 cm−2 while the one with InGaAs/GaAs SLSs has about 5.0 × 106 cm−2, as shown in Fig. 2(c).
Fig. 2 Cross-sectional TEM dark field multi-beam images showing defect reduction induced by (a) InGaAs/GaAs SLS and (b) InAlAs/GaAs SLS. The scale bars are 1 µm. (c) Reduction of dislocation induced by the SLS layers measured at different position. The defect density is measured in ~20 microns area. After the third SLS, EPD technique is also used to estimate the defect density.
In addition, photoluminescence (PL) was used to compare the optical properties of samples using InGaAs/GaAs and InAlAs/GaAs SLSs. PL measurements were performed under 532 nm excitation from a diode-pumped solid-state laser. As shown in Fig. 3(a), the room-temperature emission from the sample with InAlAs/GaAs SLSs is about two times stronger than that with InGaAs/GaAs SLSs. To gain further insight into the effects of DFLs on the optical properties of the overgrown InAs/GaAs QDs, the integrated PL intensity (IPLI) as a function of temperature was studied for both samples, as shown in Fig. 3(b). The PL quenching at high temperatures is fitted with the Arrhenius equation, giving thermal activation energies of about 216 meV and 232 meV for the samples with InGaAs/GaAs SLSs and InAlAs/GaAs SLSs, respectively. The photoluminescence quenching is attributed to the thermal escape of carriers from QD ground states into the continuum followed by non-radiative recombination in the barriers [24, 26]. As the emission peak and thermal activation energy remains nearly the same, similar band structures of the QDs are expected for the two samples [24]. As a result, the emission intensity difference between the two samples shown in Fig. 3(a) is mainly attributed to the defect density in the materials. Both the EPD and PL measurements suggest that the sample with InAlAs/GaAs SLSs has less defects within the active region. It has been well established that the parameters of the strained layers play a critical role in bending the threading dislocations [25]. A strain field large enough to interact with the threading dislocations is required but must not exceed a critical value which leads to the formation of additional threading dislocations [25]. Although the misfit of InGaAs and InAlAs to GaAs is about the same, the stress in the epilayer has to be different. The stress in the epilayer can be expressed in terms of misfit parameters, shear modulus, and Poisson’s ratio [27]. Due to the larger shear modulus of InAlAs, it is expected that the critical misfit for generating new threading dislocations in InAlAs/GaAs SLSs is larger than that of InGaAs/GaAs SLSs. In addition, the misfit force is also directly proportional to shear modulus and interfacial strain [28], and the threshold misfit force to bend threading dislocations can be more easily reached by using InAlAs/GaAs SLSs. Therefore, InAlAs/GaAs SLSs are found to be more effective in blocking the propagation of threading dislocations than InGaAs/GaAs SLSs under the similar growth conditions.
Fig. 3 (a) Room-temperature PL spectra and (b) integrated PL intensities as a function of temperature for the DWELL structure grown on Si substrates with InGaAs/GaAs and InAlAs/GaAs SLSs.
3. Crystal growth and device fabrication details
InAs/GaAs QD laser devices grown on Si substrates were studied with the InAlAs/GaAs SLSs as DFLs. The buffer and active regions of the laser device were grown with the same conditions as the sample shown in Fig. 2(b). P-doped and n-doped Al0.4Ga0.6As p-doped cladding layers of 1.5 µm were used to confine the five-layer DWELL structures. A 300-nm p-doped GaAs contact layer was deposited to finish the laser structure. Broad-area lasers were fabricated as shown schematically in Fig. 4.Standard lithography and wet etching techniques were used to define a 50-µm wide ridge waveguide. The ridges were etched down to 200 nm below the active region for an improved carrier confinement. Ti/Pt/Au and InGe/Au contact layers were deposited on the p-GaAs contacting layer and the exposed n-GaAs buffer layer, respectively. Devices of 3-mm length were mounted and wire bonded on ceramic tiles to enable testing. No facet coating is applied.
4. Results and discussion
Figure 5(a) shows the single facet output power against current density for the Si-based InAs/GaAs QD laser with InAlAs DFLs. Device measurements were taken with the laser mounted epi-side up with a sub-mount temperature of 20 °C with no active cooling under pulsed conditions of 1% duty-cycle and 1µs pulse-width. A threshold current density of 194 A/cm2 is achieved from this device. This is significantly lower than our previous reported value of 725 A/cm2 for Si-based InAs/GaAs QD laser with InGaAs DFLs [3]. In addition, the measured single facet output power is 77 mW at an injection current density of 1.2 kA/cm2 for the InAs/GaAs QD laser on Si substrates with InAlAs DFLs, with no evidence of power saturation up to this current density. This value is also much higher than 26 mW obtained from Si-based InAs/GaAs QD laser with InGaAs DFLs under the same pumping conditions [3]. It also should be noted that this device is processed with as-cleaved facets; the combined use of facet-polishing and high reflection (HR) coating on facets can further improve the output power and decrease the threshold current density [19]. Room temperature emission spectra for InAs/GaAs QD lasers with InAlAs DFLs operating below and above threshold are shown in Fig. 5(b). At the low pumping level of 67 A/cm2, the device emits broad spontaneous emission with the emission peak at ~1.3 µm. With increased injection current density to 180 A/cm2, a shoulder emerges at shorter wavelength, and eventually laser oscillation is clearly seen at 1.27 µm at a current density of 194 A/cm2. Further increasing the current density to 267 A/cm2, a multimode lasing spectrum appears, as shown in the inset of Fig. 5(b).
Fig. 5 (a) Single facet output power against current density for 3mm-long InAs/GaAs QD laser grown on Si with InAlAs DFLs under pulsed mode (1% duty-cycle and 1µs pulse-width) at room temperature. The inset shows the threshold kink at 194A/cm2. (b) Lasing spectrum of Si based InAs/GaAs QD laser with InAlAs DFLs at an injection current density of 194A/cm2. The inset shows the emission spectra for different drive current densities below and above threshold.
Figure 6 shows the L-I characteristic for a 3 mm-long Si-based InAs/GaAs QD laser with InAlAs DFLs as a function of substrate temperature at a fixed duty-cycle and pulse-width (1%, 1µs). This device has an 85 °C maximum heatsink temperature for ground-state lasing, with a characteristic temperature T0 of ~46 K between 20 °C and 85 °C. This T0 value is in line with previous reported values, in the range of 35-60 K for normal 1.3-µm GaAs-based InAs/GaAs QD laser diodes above room temperature [29, 30]. The poor T0 observed here is mainly due to increased nonradiative recombination and carrier escape from the heterostructures with increasing temperature [31, 32]. This strong effect of temperature on L-I characteristics could be reduced in future work either by using p-type modulation doping of the QDs [33] or by using an epi-down mounting process [34].
Fig. 6 Light output power against current density for Si-based InAs/GaAs QD laser with InAlAs DFLs at various substrate temperatures under pulsed mode.
5. Conclusions
We have compared the effectiveness of InAlAs/GaAs as dislocation filter layers with that of InGaAs/GaAs SLSs for epitaxial growth of InAs/GaAs QD laser structures on Si substrates. InAlAs/GaAs SLSs are found to be more effective than InGaAs/GaAs SLSs in blocking the propagation of threading dislocations and lead to lower etch pit density and higher PL intensity. By using InAlAs/GaAs SLSs, 1.3-μm InAs/GaAs QD lasers directly grown on Si substrates have been demonstrated with low threshold current density and high output power. As-cleaved InAs/GaAs QD lasers can be operate at temperatures up to 85 °C under pulsed mode. This study could provide an essential step to improve further the performance of InAs/GaAs QD lasers monolithically grown on a Si platform for silicon photonics.
Acknowledgments
The authors acknowledge financial support from UK EPSRC under Grant No. EP/J012904/1. H. Liu would like to thank The Royal Society for funding his University Research Fellowship.
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