1. INTRODUCTION

Recent years have witnessed significant progress in making ever smaller and more efficient lasers ^{[1–9" target="_self" style="display: inline;">–9]}. Lasing in quantum dot (QD) microcavity structures has opened a new paradigm that allows exceptional lasing performance, even for lattice-mismatched material systems ^[10,11]. The discrete number of modes and small volume of the microlasers allow ultralow threshold lasing down to milliampere values ^[12]. The carrier localization enabled by QDs provides a notable advantage in scaling to small dimensions through reduced sidewall recombination ^[13]. The use of QDs over quantum wells as a gain medium for silicon (Si) photonics is further motivated by the quest for reduced sensitivity to dislocations generated by heteroepitaxy ^{[1416" target="_self" style="display: inline;">–16]}. Such epitaxial approaches not only provide a lower barrier to entry for Si photonics devices through larger substrate growth, but also benefit from the inherent high temperature stability and low threshold lasing characteristics of QD devices, compared to quantum well counterparts ^{[1719" target="_self" style="display: inline;">–19]}. Great progress has been made in the field of heteroepitaxial growth of QD-on-Si lasers with reliable, continuous-wave (CW) operation ^{[20–27" target="_self" style="display: inline;">–27]}. By employing an on-axis (001) GaP/Si substrate using molecular beam epitaxy (MBE), record setting Fabry–Perot lasers have been achieved with low threshold current densities of 132A/cm2, and single-side output powers of 175 mW at room temperature under CW operation ^[27]. However, these edge-emitting laser structures have large footprints of hundreds of square micrometers and power consumptions of several picojoules per bit. Such sizes are orders of magnitude too large to be viable for integration with electronic integrated circuits. Proportional to the active volume of the lasers, the power consumption of such devices is also orders of magnitude higher than that required for on-chip optical interconnects, where the complete energy budget for an optical link should be only about 10 fJ per bit ^[28].

In this paper, we report whispering gallery mode (WGM) cavity microring lasers on (001) GaP/Si substrate for easily manufacturable on-chip Si light sources with dense integration and low power consumption. A p-doped active region was adopted to improve temperature stability ^[29] and to suppress gain saturation ^[30]. A maximum lasing temperature of 80°C under CW injection has been achieved while maintaining a low threshold of 3 mA. This vastly surpasses the performance of prior lasers with undoped active regions ^[31,32]. The hole concentration of 5×1017cm−3 from p-doping in the barriers also suppresses hole depletion and improves the carrier transport to the dot active layers ^[30,33]. Small signal modulation responses were measured for the first time for the microrings directly grown on Si, and a 3 dB bandwidth of 6.5 GHz is reported here.

2. EXPERIMENTS AND RESULTS

The complete epitaxial structure is shown in Fig. 1. The detailed growth procedure on the crystalline (001) GaP/Si substrate was reported in Ref. ^[34]. Two GaAs/AlxGa1−xAs graded index separate confinement heterostructure (GRINSCH) lasers with seven layers of InAs/InGaAs QD-in-a-well (DWELL) active layers were grown on such substrates. A dot density of 5.2×1010cm−2 was measured using atomic force microscopy (AFM) and a strong luminescence at 1285 nm with a full width at half-maximum of 28 meV was obtained from photoluminescence measurements. In one laser, the GaAs barriers separating the QDs were partially p-modulation-doped with beryllium (The p-doping density is 5×1017cm−3. The hole sheet density is 5×1011cm−2 considering the p-GaAs layer thickness of 10 nm. The hole density per QD is 10) ^[35]. Otherwise, the two structures are nominally the same.

Fig. 1. Schematic of the epilayer structure. Inset, AFM morphology of the uncapped dots.

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Ring resonators with varying radii and ring waveguide widths were fabricated from the as-grown materials ^[22]. A schematic of the device structure is presented in Fig. 2(a). We used an i-line (365 nm) step-and-repeat exposure tool for lithographic patterning to provide high resolution and critical alignment for definition of the electrode metallization in the microsized cavity. The laser cavity was patterned into deeply etched rings to provide strong index guiding of the optical mode as well as suppression of current spreading. A scanning electron microscope (SEM) image of the fabricated device is presented in Fig. 2(b). The devices were then placed and probed on a copper heat sink. An infrared image of a microring laser operated above threshold is shown in Fig. 2(c). All laser measurements presented here were conducted in CW mode.

Fig. 2. (a) Schematic illustration and (b) tilted SEM image of one fabricated microring laser; (c) top view of the probed microring under infrared imaging.

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2.5 A. Static Characteristics

We first injected a direct current (dc) into the device and measured the laser characteristics at room temperature. The light-current-voltage (L-I-V) curve of an undoped microring laser with an outer ring radius (R) of 15 μm and a ring waveguide width (W) of 4 μm is shown in Fig. 3. CW thresholds as low as 3 mA were measured, as shown in the inset in Fig. 3.

Fig. 3. Measured L-I-V curve of a microring laser with intrinsic active region. The device features an outer ring radius of 15 μm and a ring waveguide width of 4 μm. Inset, zoomed-in view of the L-I curve in the low-injection region.

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Figure 4 shows the lasing spectra of this ring resonator with increasing injection current. Bright and clearly defined cavity mode peaks, well above the background QD emission in the O-band, were observed. An excellent extinction ratio over 30 dB for primary lasing mode and good side-mode suppression ratio of 16 dB can be achieved at certain injection levels (20–25 mA in this case). More consistent single-wavelength lasing was observed in smaller diameter devices with larger free spectral range (FSR).

Fig. 4. Emission spectra for the same device in Fig. 3 at various injection currents under CW operation at room temperature. Inset, emission spectrum around lasing threshold.

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The same microring laser was tested at various heat sink temperatures and its L-I characteristic was analyzed in Fig. 5(a). CW lasing was observed up to 40°C, and a characteristic temperature T0 was extracted to be around 22 K through linear fitting [blue dotted lines in Fig. 5(c)]. The relatively low value of T0 was partially attributed to self-heating effects and carrier leakage of the undoped active region under CW excitation. Similar L-I-V characteristics are obtained for the same nominal laser structure grown on a separate wafer but with modulation p-doped active regions. Measured L-I curves as a function of the heat sink temperature were plotted in Fig. 5(b). The red dots and line in Fig. 5(c) show the threshold versus temperature curve and fitting. Compared with the undoped device in Fig. 3(a), CW lasing temperature was elevated up to 80°C. An excellent T0 was extracted to be ∼103K near room temperature (20°C–40°C), and ∼35K between 40°C and 80°C under CW excitation. The dramatically improved high temperature performance with p-doped active region stems from the compensation for the thermal excitation of holes in the lasing mode at higher temperatures ^[29].

Fig. 5. Measured L-I curves as a function of the heat sink temperature for two microring lasers with (a) an intrinsic active region and (b) a modulation p-doped active region. Both devices have an outer ring radius of 15 μm and a ring waveguide width of 4 μm. (c) Temperature-dependent threshold current versus heat sink temperature for the two microring lasers, where the dashed lines represent the linear fit to the experimental data.

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Figure 6 summarizes the threshold currents obtained from a series of microring lasers with different outer-ring radii and a constant-ring waveguide width of 4 μm. In Fig. 6(a), devices with intrinsic active region (black) possess average thresholds in the range of 1.8–4.8 mA, roughly 2 times lower than from devices with a modulation p-doped active region (blue), where the average thresholds fall into the range of 2.7–8.9 mA. The threshold current was monotonically decreased with reduction of the ring diameter in both kinds of devices. This further features the carrier localization capability of QDs that enables device miniaturization without imposing a heavy penalty on the threshold current of laser devices ^[14]. In Fig. 6(b), devices grown on GaP/Si template and native GaAs substrates were compared to evaluate the effect of dislocations on the device performance. Both devices have an intrinsic active region. Around 2–3 times higher threshold current for devices with the same dimension was observed on the GaP/Si templates compared to that on the native substrate, which originates from nonradiative recombination. Better intrinsic performance of the heteroepitaxially grown lasers could be achieved by reducing nonradiative losses via enhancing the quality of the GaAs films on Si.

Fig. 6. Threshold currents as a function of outer ring radius for microring lasers (a) with an intrinsic active region and a modulation p-doped active region on the GaP/Si, and (b) on GaP/Si substrate and native GaAs substrate with an intrinsic active region.

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2.6 B. Dynamic Characteristics

A low fiber coupling output power prevents us from performing the signal response of the small size of lasers at this time, due to the large coupling loss between the device and the fiber. The small-signal response, S21, was measured with a larger device with an outer ring radius of 50 μm and a ring waveguide width of 4 μm, using a 20 GHz light wave component analyzer (LCA, HP8703A).

The devices were directly probed using a signal/ground (SG) radio frequency (RF) probe. No temperature control was applied. Figure 7 shows small-signal modulation response S21 of the device, and the inset in Fig. 7 shows the corresponding L-I-V characteristics. The injected currents were varied from 21 to 86 mA. These responses are normalized at low frequency, and a 3 dB bandwidth of 6.5 GHz was attained at a bias current of 86 mA. The responses were further fit using a three-pole fitting function H(f) ^[36]. The fitted curves are presented in the dashed lines in Fig. 7.

Fig. 7. Small-signal modulation responses of the QD ring laser biased from 21 to 86 mA. The fitting curves are drawn using a three-pole fitting function H(f). Inset, L-I-V characteristics from the same device.

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The damping rate γ and relaxation oscillation frequency fr at each bias current were extracted. The extracted fr, together with the measured f3dB are plotted in Fig. 8 as a function of the square root of bias current above threshold. The modulation efficiencies of 0.38GHz/mA1/2 for f3dB and 0.34GHz/mA1/2 for fr were extracted by linear fitting, using the data points below (Ib−Ith)1/2=8mA1/2. By linear fitting the damping rate γ versus squared fr, shown in the inset in Fig. 8, the K-factor is 0.91 ns, and a maximum K-factor limited f3dB,max, was calculated to be 9.7 GHz using the equation ^[37] γ=K·fr2+γ0,(1)where γ0 represents the damping offset.

Fig. 8. 3 dB bandwidth f3dB and relaxation oscillation frequency fr versus square root of the bias current above threshold. Inset, damping rate γ versus squared relaxation oscillation frequency fr. The maximum 3 dB bandwidth limited by K-factor f3dB,max is 9.7 GHz.

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Since the maximum f3dB,max is calculated to be 9.7 GHz, smaller than the measured f3dB of 6.5 GHz from the small-signal modulation, the first assumption is due to the large pad capacitance of the electrodes, which are not optimized for high-frequency operation. To better understand how the extrinsic electrical properties of the QD microring lasers affect their modulation bandwidth, we then measured and analyzed the reflection coefficient S11 to extract the impedance and estimate the resistor–capacitor (RC) cutoff frequency. The equivalent circuit model ^[38] used for the fitting is shown in Fig. 9(a). The model consists of an inductance L, a total capacitance C (parasitic capacitance between the contact pads and junction capacitance of the laser diode), a device resistance R, a characteristic impedance of the transmission line (50Ω)Z0, and a voltage source Vs. Figure 9(b) shows 50-Ω normalized Smith chart of S11 characteristics of the QD ring laser. Reverse (−3V) and forward (50 mA) biased conditions were applied, respectively. The S11 data were fit to the circuit model to obtain the circuit capacitances and resistances. The total capacitance was extracted to be 0.74 pF. The pad capacitance was estimated to be 0.33 pF by approximating the p-probe pad area with 80-μm width and 120-μm length over a 1-μm-thick SiO2 layer (RF dielectric constant ϵ=3.9). The inductance L was negligibly small, as the electrical contact was performed by an RF probe without any wire bonding. Using the obtained circuit parameters (C=0.74pF, R=20.9Ω), the RC cutoff frequency is calculated to be 10.3 GHz. To further improve the bandwidth, the pad capacitance can be reduced by depositing the metals on a several micron-thick benzocyclobutene (BCB, ϵ=2.6) layer. Moreover, the dot density can be increased to enhance the differential gain ^[39], and a graded p-modulation doping could improve the carrier transport further ^[40].

Fig. 9. (a) Impedance measurement of QD microring laser on Si; (b) equivalent circuit model used for the fitting. Measured and fitted curves of reflection S11 characteristics for reverse (−3V) and forward (50 mA) biased condition from 0.14 to 5 GHz.

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3. CONCLUSION

In conclusion, we characterize the lasing performance for QD microlasers grown on exact (001) GaP/Si under electrical injection. CW thresholds as low as 3 mA and high temperature operation up to 90°C were simultaneously achieved for small footprint ring resonators with an outer ring radius of 15 μm. We further performed a dynamic characterization of the laser and realized a 6.5 GHz direct modulation without heat sinking. Integration of the directly modulated ring lasers on Si substrates with low threshold and high temperature operation looks promising.

4 Acknowledgment

Acknowledgment. The authors would like to thank Prof. Kei May Lau, Di Liang, Chong Zhang, Minh Tran, Chao Xiang, Xinru Wu, and the UCSB nanofabrication clean room staff for helpful discussions and assistance.