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We report the first InAs/GaAs quantum-dot (QD) superluminescent diode (SLD) monolithically grown on a Ge substrate by molecular beam epitaxy. The QD SLD exhibits a 3dB emission bandwidth of ~60 nm centered at 1252 nm with output power of 27 mW at room temperature. The 3dB bandwidth is very stable over the temperature range from 20 °C to 100 °C, which highlights the potential for integration with high performance ICs.
© 2014 Optical Society of America
1. Introduction
The realisation of efficient and reliable light sources monolithically integrated on a silicon platform will enable fast chip-to-chip and system-to-system communications by using complex photonic integrated circuits [1–3]. It is highly anticipated that the bandwidth demand will jump to 100 Gb/s or higher in the near future [4]. At the same time, the heat generation on-chip needs to be kept to a minimum. Optical interconnects on-chip and on-board have been suggested as the most promising method to achieve this. Wavelength division multiplexing (WDM) systems integrated into current large-scale electronic integrated circuits are one of the most straightforward methods to obtain high bandwidth. Traditionally an array of laser diodes emitting at different wavelength for WDM are needed. However, the alternative is to integrate one broadband light source and use spectrum-slicing techniques [5] to cover all the channels. This enables the number of active components on the chip to be kept to a minimum for easier thermal management [6, 7]. Broadband light sources on Ge and/or Si substrates are also of interest for applications in Lab-on-Chip biosensors [8, 9] and optical coherence tomography [10, 11]. However, as Si and Ge are indirect band-gap materials, the radiative recombination rate is extremely low and it is difficult to realize suitable optical sources. Integration of direct bandgap III-V materials on Ge or Si substrates would be preferable as they have ideal photonic properties [12–15]. The major difficulty growing III-Vs on Si substrate is the high density of threading dislocations (TD) created at the interface due to the large lattice mismatch [16–18]. One solution is to use Ge since the lattice constant is very closely lattice matched to GaAs (0.08% mismatch). The growth of Ge on Si is an established technology and Ge-on-Si substrates are now commercially available with low defect density (<105 cm−2). Furthermore, since the hole mobility in Ge is much higher than in Si, the use of Ge epilayers has been suggested as an alternative to Si in the microelectronic industry [19–22].
For conventional bulk or quantum well (QW) devices grown on Ge-on-Si, any TD which propagates through the active region will act as a non-radiative centre. This will drastically reduce the device performance. A better alternative is to use quantum dots (QDs), because QDs are more mechanically robust and less sensitive to defects [23]. In QD devices, each TD will only ‘kill’ one or a few isolated dots, which will not significantly affect device performance. Further advantages of III-V QD devices include the ability to access telecommunication wavelengths [24] and low threshold current densities [25–28]. Recently the first 1.3-μm III-V quantum dot lasers grown on Ge substrates with performance comparable to devices grown on native III-V substrates have been demonstrated [29]. Using a similar approch results have also been reported with III-V quantum dot light sources monolithically grown on Ge-on-Si substrates [30–36]. Self-assembled QDs, such as an InAs/GaAs QD systems, are also very attractive for the realisation of broadband light sources, due to their naturally broad emission spectra arising from the inhomogeneity of QD sizes. Superluminescent diodes (SLDs) using self-assembled QDs also benefit from single-Gaussian shaped emission spectra without any spectral dip, which would impact the optical system. In addition, p-type modulation doping has been used to improve the thermal stability in QD lasers [37–42], which should be promising to increase the operation temperature for QD SLDs. In this paper, we report on the fabrication of InAs/GaAs QD SLD monolithically grown on a Ge substrate with p-type modulation. Under pulsed operation, a single facet output power of 27 mW with a bandwidth of ~60 nm centered at ~1250 nm is obtained at room temperature. Device operation up to 100 °C is also reported.
2. Wafer growth
The sample was grown by solid source MBE on a p + Ge(100) substrate with 6° offcut towards the [111] plane. Figure 1(a) shows a schematic of the QD SLD structure. The Ga prelayer technique was used to achieve a single-domain GaAs buffer layer [29,42]. The nucleation of GaAs was initialised using migration enhanced epitaxy which is designed to promote two-dimensional growth. Figure 1(b) shows the TEM image of the Ge/GaAs buffer layer interface where very few defects/dislocations can be identified [29]. The etch pit density measurement confirmed the density of TD is 1 ± 0.2 × 106 cm−2. The active region consists of 5 layers of InAs/InGaAs dot-in-well (DWELL) structure, which is shown schematically in Fig. 1(c). The QD layers were formed by the deposition of 3 ML InAs and then capped with 6 nm InGaAs capping layers. Spacer layers of 45-nm GaAs separate each QD layer. Within the spacer layers, 6 nm thick Be-doped regions were placed 9 nm below each QD layer to provide about 20 holes per dot. Figure 1(d) shows a cross sectional TEM image of the DWELL active region, which is very similar to those grown on GaAs substrates [43]. The active region was sandwiched between 1500-nm n- and p-doped Al0.4Ga0.6As cladding layers, and capped with a heavily n-doped 300-nm GaAs contacting layer. Photoluminescence (PL) spectra were measured at room temperature using an Accent 2000 PL mapper system and 532 nm diode pumped solid-state laser excitation source.
Fig. 1 (a) Schematic structure of QD SLD grown on a p + Ge substrate; (b) TEM image of Ge/GaAs interface; (c) Single repeat of schematic of modulation Be-doped DWELL structure; (d) TEM image of InAs/InGaAs DWELL active region.
Figure 2 shows the photoluminescence spectrum measured from the wafer. An intense peak can be observed at 1252 nm with FWHM of 50 nm corresponding to electron-hole recombination from the QD ground state. Two other peaks at 1180 nm and 1120 nm can be identified with Gaussian peak fitting, which correspond to emission from the first QD excited states and InGaAs layer, respectively. This indicates that the quality of QDs grown on Ge substrate with p-type modulation is similar to those grown without p-type modulation [29, 42]. The inset of Fig. 2 shows an atomic force microscope (AFM) image of InAs/GaAs grown on Ge substrates, where the dot density is approximately 3.72 × 1010cm−2. The AFM also shows a broad size distribution of QDs. As expected, there is no observation of APDs in the III-V materials.
Fig. 2 Photoluminescence characteristics of InAs/GaAs QDs grown on Ge substrate at room temperature. The inset shows 1μm × 1μm atomic force microscope image of InAs quantum dots.
3. QD superluminescent diode on a Ge substrate
The wafer was processed to 20-µm wide ridge SLDs by standard photolithography, where the ridge was aligned at 6° to the facet normal to suppress lasing. Figure 3 shows a schematic of the InAs/GaAs QD SLDs grown on Ge substrates. The ridge etch was stopped 200 nm above the DWELL active region, and the p-type contact window etch stopped just after p-Al0.4Ga0.6As cladding layer. Ni/AuGe/Ni/Au and AuZn alloy ohmic contacts were thermally evaporated on the top of the wafer for n- and p-type contacts, respectively. Top contacts are used to avoid driving current through the high defect density GaAs/Ge interface. The wafer was then thinned to 100 µm using a Buehler polisher. A 3 mm long bar was cleaved without any facet coating. QD SLD devices were mounted on a gold plated copper tile using InAgPb solder and were tested in pulsed mode using 5 µs pulses and a duty cycle of 1% in the temperature range from 20 °C to 100 °C.
Figure 4(a) shows a typical light current relationship measured from the QD SLD device at room temperature. There is no obvious kink to indicate lasing. At a current of 3000 mA, an output power of ~27 mW was achieved. The spectral characteristics at various injection currents are presented in the inset to Fig. 4(a), which suggest that lasing has been fully suppressed by the tiled ridge. The peak of spectra is at ~1255 nm, similar to the pervious PL measurement. A FWHM of ~60 nm was obtained at 3000 mA which is wider than the PL results. Figure 4(b) shows the 3dB bandwidth coverage at injection currents from 500 to 3000 mA. The small decrease of the 3dB bandwidth with increasing injection current is due to amplified spontaneous emission.
Fig. 4 (a), Light-current characteristics of Be-doped QDSLED operating under pulsed condition at room temperature from 0 to 3 A; Inset, Spectral characteristics correspondent to the LI curve; (b), Full width half maximum bandwidth and peak wavelength of QDSLED from 500mA to 3000mA, respectively.
Figure 5 shows the light current relationships at various heatsink temperatures from 20 °C to 100 °C. By increasing the heatsink temperature, it can be clearly observed that the quantum efficiency of the device decreases and the maximum power output is reduced. Above 80 °C, the ‘roll-over’ behavior is shown after 2500 mA. At 100 °C, an optical output power as high as ~2 mW was achieved. Under high injection current, increased Auger recombination is a significant factor reducing the efficiency. In spite of this, to the best of our knowledge, this is the first time a QD SLD has been demonstrated to work at 100 °C. This may be related to the effect of p-type modulation. This technique has been widely utilized to improve the operation of QD-based devices at elevated temperatures, such as lasers, due to the increased hole occupancy in the valence band. Therefore, the QD SLDs could potentially work with silicon ICs operating at elevated temperature. The inset to Fig. 5 shows the emission spectra at a fixed current of 2000 mA from 20 °C to 100 °C. The peak wavelength red-shifted from 1255 nm to 1312 nm with increasing temperature. The FWHM increases from 60 nm at 20 °C to 75 nm at 100 °C. Compared to the best available InAs/GaAs QD SLDs grown on GaAs substrates which has a gain span of ~300 nm, the FWHM reported here is relatively narrow. Wider bandwidth could be achieved by using multi-section devices or intermixing [44–47].
Fig. 5 Temperature dependent L-I curves measured from 20 °C to 100 °C of 3-mm cavity devices for QD SLD. The inset shows the spectral characteristics correspondent to the different temperature at injection of 2000 mA.
4. Conclusion
In summary, we have demonstrated the first InAs/GaAs QD SLD monolithically grown on a Ge substrate. A standard 6° tilted wet etched ridge QD SLD structure was used to suppress lasing. Up to 27 mW ex-facet optical power was obtained with FWHM of >60 nm at room temperature. The InAs/GaAs QD SLD achieves ~2 mW at 100 °C with no significant reduction in the FWHM. These results show the potential for future integration on high performance ICs.
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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