An optically pumped 2.5 μm GeSn laser on Si operating at 110 K

The development of optoelectronic integration with a complementary metal-oxide-semiconductor (CMOS) has long been limited by the unavailability of monolithic Si-based laser sources.^1,2 Tremendous efforts have been made to overcome this technical barrier. An all-Si Raman laser was reported in 2005,³ which relies on stimulated Raman scattering rather than band-to-band transition. The development of Ge techniques has led to the optically and electrically pumped Ge lasers.^4,5 However, Γ compensates the energy difference of ∼140 meV between the direct Γ-valley and the indirect L-valley; heavily n-type doping or tensile strain was employed, which results in either a high threshold or technical fabrication difficulties.^6,7 The hybrid integration of III–V lasers on Si has been investigated extensively.^8–10 The III–V materials feature efficient light emission, and the recent hybrid integration based results show significantly improved material quality.^11,12 This route requires either wafer-bonding process or direct growth of III–V on Si techniques. The studies on group-IV GeSn alloys open a venue for the development of gain medium monolithically integrated on Si for laser applications.^13–17 Theoretical calculations revealed that the incorporation of Sn into the Ge lattice reduces the energies between the Γ- and L-valley and eventually could convert GeSn into a direct bandgap material.^18–20 A direct bandgap GeSn alloy with Sn composition of 10% has been experimentally demonstrated in 2014.²¹ A recent breakthrough on an optically pumped GeSn laser indicated a major progress towards a fully integrated solution on the Si photonics platform.²² The reported edge-emitting devices show lasing operation up to 90 K and have a threshold intensity of 325 kW/cm² at 20 K, all pumped with a 1064 nm nano-second pulsed laser. In 2016, the same team obtained the optically pumped GeSn microdisk laser²³ with improved performance such as a reduced threshold of 220 kW/cm² at 50 K. In this paper, we report the demonstration of optically pumped GeSn edge-emitting lasers based on the Ge/GeSn/Ge double heterostructure (DHS) grown on Si. The significantly lower lasing threshold of 68 kW/cm² at 10 K (pumped with a 1060 nm nano-second pulsed laser) is attributed to the intrinsically relaxed high quality GeSn alloys grown in a unique chemical vapor deposition (CVD) process using GeH₄ rather than other high order Ge based hydrides as the precursor. The device showed lasing operation up to 110 K, and the laser characteristic temperature (T₀) was extracted to be 65 K.

The device studied in this project was grown using an ASM Epsilon^® 2000 plus reduced pressure chemical vapor deposition (RPCVD) reactor. The low cost commercially available GeH₄ and SnCl₄ were used as Ge and Sn precursors, respectively. The growth started with a nominal 700 nm-thick Ge buffer layer using a standard two-step growth method followed by an in-situ annealing to reduce the defects.²⁴ Then, a pre-calibrated 9% GeSn growth recipe was used to grow a thick layer with a nominal thickness of 1 μm. Finally, a 10 nm-thick Ge cap layer was grown. A detailed growth process is reported elsewhere.²⁴ 

After growth, the material quality, the layer thickness, the Sn composition, and the strain of the sample were carefully analyzed using transmission electron microscopy (TEM) and high-resolution X-ray diffraction (XRD) techniques. Figure 1(a) shows the TEM image of the sample. For the Ge buffer layer, the majority of defects were localized at the Ge/Si interface, indicating the high material quality. For the GeSn alloy, two distinct layers can be clearly observed: (i) a 210 nm-thick bottom GeSn layer over the Ge buffer. This layer is defective due to the high density of threading dislocations, which arose mainly from the lattice mismatch between the Ge buffer and the GeSn alloy. The Sn composition in this layer was measured as 8.95%; (ii) a 760 nm-thick high quality top GeSn layer above the bottom GeSn layer. Since the threading dislocation loop is formed in the bottom GeSn layer and does not propagate to the top GeSn layer, the top GeSn layer exhibits extremely high material quality. The threading dislocation densities (TDDs) of 3 × 10⁶ cm⁻² for the top GeSn layer and 2 × 10⁷ cm⁻² for Ge buffer layer (in a separate control sample with the same Ge thickness) were obtained based on the etch pit density measurement. The Sn composition in the top GeSn layer was measured as 10.90%, which is ∼22% higher than that of the bottom GeSn layer. The higher Sn composition in the top GeSn layer might be due to the ease of Sn incorporation when the underneath layer is relaxed.²⁵ The reciprocal space map (RSM) of the sample is shown in Fig. 1(b). The broadened contour plot of GeSn indicates the existence of two layers. The part annotated by dashed ellipse corresponds to the bottom GeSn layer, whereas the solid ellipse part featuring full relaxation is associated with the top GeSn layer. The XRD rocking curve (data not shown here) shows a clear peak at 64.6° and a shoulder at 64.9°, corresponding to the top and bottom GeSn layers, respectively. The broadened GeSn peak (full width half maximum (FWHM) of 0.21°) compared to that of Ge peak (0.15°) is due to the superposition of two GeSn layers with different Sn compositions.

The temperature-dependent photoluminescence (PL) characterization was conducted to investigate the material quality. The PL measurements were performed using a standard off-axis configuration with a 532 nm continuous wave (CW) laser as an excitation source. The detailed PL setup description can be found in Ref. 19. The PL spectra at the temperatures from 300 to 10 K showed that as the temperature decreases, the PL peak intensity significantly increases (Fig. 2 inset, top right), revealing a typical characteristic of a direct bandgap material.²⁶ The PL emission wavelength of 2510 nm was observed at 300 K (no other emission was observed below 1800 nm), which features a longer wavelength compared to our previous PL study on a thin film sample with the same Sn composition. This is mainly due to the relaxation of the GeSn resulting in a narrower bandgap. As the temperature decreases, the PL peak blue-shift was observed, as shown in the Fig. 2 inset (bottom, only selected temperatures were shown for clarity). The full width at half maximum (FWHM) of each peak was extracted using Gaussian fitting, which monotonically decreases as the temperature decreases. At 10 K, the extracted PL spectrum FWHM is 133 nm (28 meV) and is one fourth of that of 300 K.

The sample was fabricated into a ridge waveguide with 5 μm-width for the optical pumping characterization. A low temperature wet chemical etching process was developed in this study. By using the mixture of HCl:H₂O₂:H₂O = 1:1:10 at 0 °C, smooth sidewalls were achieved as shown in Fig. 2 inset (top left). The average etching rate is ∼20 nm/min. The etching depth was measured as 800 nm. Due to the lateral etch, the waveguide width at the top was measured as 3 μm, while at the bottom it was measured as 5 μm. An extensive etching study showed that the etching rate is almost a constant of 20 nm/min regardless the Sn composition. In addition, based on our observation, the wet chemical etching process does not result in any surface passivation. However, it does provide a smooth side wall that could slightly reduce optical scattering loss for the waveguide structure. Therefore, the wet etching process developed in this study offers a robust recipe for the fabrication of GeSn-based devices. After etching, the sample was lapped down to ∼70 μm followed by cleaving to form the mirror-like facets. Devices with the cavity lengths of 300, 600, and 1100 μm were investigated in the following optical pumping experiment.

The optical pumping characterization was performed using a pulsed laser operating at 1060 nm with 45 kHz repetition rate and 6 ns pulse width. The laser beam was collimated to a narrow stripe (∼20 μm width and 0.3 cm length) via a cylindrical lens to pump the GeSn waveguide structure. Since the spatial intensity profile of the laser beam features Gaussian distribution, the knife-edge technique was applied to determine the pumping power density.^27,28 The device was first mounted on a Si chip carrier and then placed into a continuous flow cryostat for low temperature measurement. The emission from the facet was collected by a spectrometer and then sent to a thermoelectric-cooled lead sulfide (PbS) detector with the cut-off at 3.0 μm. The integrated emission intensity was measured by setting the grating at zero order.

Figure 2 shows the laser-output versus pumping-laser-input (L-L) curves of the 600 μm-long device at 10 and 90 K. The threshold characteristic was clearly observed. The threshold values were measured as 68 and 166 kW/cm² at 10 and 90 K, respectively. The low resolution (1 nm) directional emission spectrum measured at 3 times of the threshold at 90 K is plotted in the Fig. 2 inset (red peak). The FWHM of the peak is 26 nm (5.1 meV). Compared to the FWHM of the PL peak at 10 K (28 meV), the dramatically reduced line-width further confirms the lasing characteristic. The emission spectrum measured at 10 K (not shown) also revealed a similar FWHM of 28 nm (5.6 meV), which is comparable with that of the previously reported GeSn lasers.²² The laser operating wavelengths were determined as 2476 and 2503 nm at 10 and 90 K, respectively (supplementary material).

In order to further investigate the lasing mode characteristics, the device with a cavity length of 300 μm was studied. The L-L curve of device at 10 K was plotted in Fig. 3 showing a threshold of 106 kW/cm². The high-resolution spectrum (0.1 nm, spectrometer limit) measurement was performed for the device operating at 2 and 5 times of threshold (Fig. 3 inset). Due to the relative large area of the cavity facet, the device spectra show a typical multimode lasing characteristic. The spectrum taken under 2 times of threshold shows multi-peaks that are located between 2400 and 2500 nm. As the pumping power increased to 5 times of the threshold, most peaks grow and the overall intensity increases.

The 1100 μm-long device was selected for detailed temperature dependent characteristic study since the longer length cavity reduces the mirror loss and therefore enables the exhibition of the intrinsic characteristics of the material. The L-L curve shows that the lasing operation could reach as high as 110 K, as shown in Fig. 4. The measured thresholds range from 87 to 396 kW/cm² at the temperatures from 10 to 110 K. The characteristic temperature T₀ was further studied. By fitting the temperature-dependent lasing threshold, the T₀ was extracted as 65 K, as shown in Fig. 4 inset. The T₀ was also extracted from L-L curves of 300 and 600 μm-long devices as 78 and 90 K, respectively. Since many factors such as facet quality, wave guide quality, and cavity length could cause the variation of T₀, we conservatively choose 65 K as the feature T₀ for the GeSn laser in this study. Characteristic temperature measured at low temperature range provides useful information to benchmark the material with the early phase development of III–V material for laser applications. For example, according to the earlier studied III–V optically pumped lasers, the reported values of T₀ were 100 and 129 K for InP/InGaAsP/InP and GaAs/AlGaAs/GaAs DHS devices, respectively.^29,30

We have analyzed the merits of material growth and the device structures leading to the lasing achievement, which are summarized as follows: (i) the use of GeH₄ in GeSn growth provides a favorable relaxation of material, which not only improves the material quality but also makes the direct bandgap GeSn achieved with lower Sn composition; (ii) based on our ellipsometry study, the refractive index of GeSn is slightly higher than that of Ge; therefore, the relatively thicker GeSn (∼1 μm) and thinner Ge buffer (700 nm) configuration, rather than thin GeSn active layer and thick Ge buffer layer structure, could offer a better optical field confinement in GeSn layer, which increases modal gain; (iii) the Ge/GeSn/Ge DHS structure offers an improved carrier confinement according to the band structure analysis. Note that since the bottom GeSn layer features lower Sn composition compared to the top GeSn layer, at lower temperature, its wide-bandgap confines the carriers in the top GeSn layer and therefore prevents them from recombination in the defective bottom GeSn layer, resulting in more effectively injected carriers and enhanced radiative recombination.

In summary, the Ge/GeSn/Ge DHS laser sample was grown using an industry standard CVD reactor and low cost commercially available precursors in a single run epitaxy process. The use of GeH₄ provides a favorable relaxation for the GeSn material. The TEM image showed a two-layer GeSn film, where a defect-free top GeSn layer was obtained. A wet chemical etching process was developed to fabricate the ridge waveguide with smooth sidewalls achieved. Temperature-dependent characteristics of laser-output versus pumping-laser-input were investigated. The unambiguous lasing operation was observed up to 110 K. The laser mode was analyzed via high-resolution PL spectra, which revealed the multimode operation of the laser. The lasing threshold and operation wavelength were measured as 68 kW/cm² and 2476 nm at 10 K, respectively. Based on the temperature-dependent threshold, a characteristic temperature of 65 K was extracted. According to the band structure calculation and the lasing mode profile analysis, the optimizing solution for laser structure was proposed, which could reduce the lasing threshold and increase the operating temperature. The capability of producing the GeSn laser in a “manufacture ready” process (industry reactor, low cost precursor, and single run epitaxy process) indicates the great potential of GeSn to be easily adopted by future foundry for integrated photonics applications when the material is mature.

See supplementary material for the complete band structure calculation, mode pattern calculation, and discussion of laser performance.

The work in UA is supported by the Air Force Office of Scientific Research (AFOSR) under FA9550-14-1-0205, the National Science Foundation (NSF) under DMR-1149605. Randy Quinde acknowledges the support of the NSF Research Experiences for Undergraduates (REU) Program under Grant No. EEC-1359306. Dr. Soref and Dr. Sun appreciate support from AFOSR under FA9550-14-1-0196 and from the Asian Office of Aerospace, Research and Development (AOARD) under FA2386-16-1-4069.