Strong extended SWIR cavity resonances in a single GeSn nanowire

doi:10.21203/rs.3.rs-1946870/v1

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Strong extended SWIR cavity resonances in a single GeSn nanowire

https://doi.org/10.21203/rs.3.rs-1946870/v1

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published 20 Jul, 2023

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Nanowires are promising platforms for realizing ultra-compact light sources for photonic integrated circuits. In contrast to impressive progress on light confinement and stimulated emission in III-V and II-VI semiconductor nanowires, there has been no experimental demonstration showing the potential to achieve strong cavity effects in a bottom-up grown single group-IV nanowire, which is a prerequisite for realizing silicon-compatible infrared nanolasers. Herein, we address this limitation and present the first experimental observation of cavity-enhanced strong photoluminescence from a single Ge/GeSn core/shell nanowire. A sufficiently large Sn content (~ 10 at%) in the GeSn shell leads to a direct bandgap gain medium, allowing a strong reduction in a material loss upon optical pumping. Efficient optical confinement in a single nanowire enables many round trips of emitted photons between two facets of a nanowire, achieving a narrow width of 3.3 nm. Our demonstration opens new possibilities for ultrasmall on-chip light sources towards realizing photonic-integrated circuits in the underexplored range of extended SWIR.

Nanowires grown by bottom-up synthesis hold great promise for building ultrasmall and power-efficient light sources for photonic integrated circuits (PICs)¹. In the past two decades, several research groups reported various types of nanowire light sources and lasers using a variety of III-V and II-VI semiconductor materials such as GaAs, InAs, and CdS^2–9. Despite the superior optical properties of those direct bandgap materials, their inherent incompatibility with mainstream complementary metal-oxide-semiconductor (CMOS) technology prevents the III-V and II-VI nanowire lasers from being widely adopted for PICs. Recently, CMOS-compatible group-IV materials such as Ge and GeSn have attracted much attention as a gain medium for on-chip lasers operating at wavelengths deep in the infrared relevant to free-space communication and sensing applications^10–15. Particularly, group-IV bottom-up nanowires have the potential to produce the smallest, CMOS-compatible on-chip light sources, thus motivating many researchers to grow high-quality Ge and GeSn^16–25. In the last decade, the photoluminescence characteristics of Ge and GeSn nanowires have been unveiled^16–23. Temperature-dependent photoluminescence from an ensemble of vertically grown nanowires revealed the direct bandgap nature of GeSn nanowires with a high Sn contents up to ~ 19 at%^19,20. However, possibly due to the mode leaking from nanowires into the substrate and because of the inability to precisely measure the emission characteristic of a single nanowire, it has thus far remained elusive whether it is feasible to achieve a strong cavity resonance in a single group-IV nanowire. A very recent study reported clear photoluminescence emission from a single Ge/GeSn core/shell nanowire transferred onto an Au and Si substrate, signs of cavity resonances were not detected²³.

Herein, we present the first experimental observation of strong cavity resonances from a single Ge/GeSn core/shell nanowire at a Sn content of ~ 10 at.%. Strongly enhanced photoluminescence at low temperatures indicates the realization of direct bandgap GeSn light-emitting media. In stark contrast to a broadband spontaneous emission characteristic from as-grown vertical nanowires, a single nanowire transferred onto a SiO₂ layer on a Si substrate shows very strong cavity resonances that are in excellent agreement with finite-difference time-domain (FDTD) simulations. The linewidths of the cavity resonances initially reduce significantly with increasing pump power due to the reduced material loss, but the resonant peaks broaden rapidly and disappear at higher pump powers. Theoretical calculations based on k⋅p method combined with numerical analyses indicate that GeSn nanowire reaches the transparency condition very closely, but the material loss dominates the material gain at increased pump powers, hindering the observation of lasing. Via FDTD simulations and theoretical calculations, we propose a viable pathway towards achieving lasing by reducing the optical losses in a cavity via a simple lithography step. This experimental demonstration provides a strong motivation for delving into group-IV nanowire light emitters to achieve on-chip CMOS-compatible nanowire extended SWIR lasers.

Nanowires growth and characterization

The Ge/Ge_0.90Sn_0.10 core/shell nanowires were grown using 20 nm Au colloids dispersed on a Ge(111) substrate using a chemical vapor deposition (CVD) reactor (see methods for more details). The scanning electron micrograph (SEM) in Fig. 1a shows as-grown Ge/Ge_0.90Sn_0.10 core/shell nanowires at lengths exceeding 10 µm and diameters larger than 400 nm. A small inverse tapering is visible, as a result of the enhanced precursor decomposition in the proximity of the Au droplet¹⁹. Scanning Transmission Electron Microscopy (STEM) and Atom Probe Tomography (APT) measurements were performed to unveil the structural properties of the nanowire heterostructure. Cross-sectional high angle annular dark field (HAADF) scanning-TEM (STEM) images and energy-dispersive X-ray spectroscopy (EDX) compositional maps are shown in Fig. 1b. The typical sunburst-like morphology of the Ge_0.90Sn_0.10 shell, with Sn-rich {112} wide facets and Ge-rich {110} narrow facets^19,26, is preserved across the entire 250 nm thickness of the shell. The composition of the GeSn shell was characterized using APT measurements, as displayed in Fig. 1c. Since it is challenging to perform APT measurements on nanowires with diameters larger than 200 nm, a second nanowire sample was grown with identical conditions and a thinner ~ 30 nm shell. The compositional profile estimated along the < 112 > radial direction of the shell from EDX and APT measurements are plotted in Fig. 1d. Due to the limited field-of-view of the APT instrument only ~ 60% of the cross-section of the nanowires is visible, however the composition of the group-IV nanowires can be reliably measured^27,28. The EDX curve, which is affected by a background signal, was vertically translated to overlap with the APT curve. A steep transition (~ 5 nm-wide) to ~ 6 at.% Sn is visible along the < 112 > direction, while the incorporation of Sn gradually increases toward the outer region of the shell, eventually reaching a uniform Sn ~ 10 at.% at more than 100 nm away from the Ge core. Along the < 110 > direction of the shell a rather constant composition of Sn ~ 3 at.% was obtained. We note that the nanowires offer excellent strain minimization due to their one-dimensional geometry (i.e. highly compliant substrate), and the residual compressive strain is progressively reduced with increasing shell thickness²⁹, until an equilibrium configuration with uniform composition is reached.

Direct bandgap emission from as-grown Ge/GeSn core/shell nanowires

To study whether the bandgap of the light-emitting medium in the nanowires is intrinsically direct or indirect, we conducted temperature-dependent photoluminescence (PL) measurements on as-grown nanowires (see Methods for the detailed description of photoluminescence spectroscopy). Figure 2a shows a schematic illustration of our experiments on the as-grown nanowire sample, showing several nanowires being optically excited simultaneously. The schematic here is to illustrate that there exist many nanowires to be simultaneously pumped since the optical pumping area is large even with a tightly focused pump laser spot. It should be noted that the experimentally grown nanowire array (Fig. 2b) is not as perfectly periodic and vertical as in the schematic. Figure 2b presents the scanning electron microscopy (SEM) image of the as-grown nanowire sample. It is evident from the SEM image that vertically standing as-grown nanowires are densely packed on a Ge layer with distances between nanowires of 1–5 µm. Since the diameter of the optical pump in our setup is ~ 10 µm, a few nanowires are excited during the measurements. Figure 2c shows the direct bandgap emission spectra from the as-grown nanowire sample at 200 K (yellow), 100 K (pink), and 4 K (red). A fixed pump power density of 301.8 kW cm^− 2 was used at all temperatures. The inset to Fig. 2c compares two spectra measured at 4 K (red) and 100 K (pink). The spectrum for 100 K is multiplied by a factor of 6 for clarity. A broadband spontaneous emission peak at ~ 2275 nm was clearly observed at 4 K. The measured peak position is consistent with other reports using GeSn with a similar Sn content^13,30. It is worth noting that emission intensity rapidly increases at lower temperatures, which is typical behavior of direct bandgap materials³¹. At lower temperatures, the carrier density in the lowest direct Γ-valley in a direct bandgap material can be significantly increased³², resulting in stronger photoluminescence. Also, non-radiative recombination processes can also be suppressed at lower temperatures, further increasing the emission intensity¹¹. Figure 2d presents the emission spectra from the as-grown nanowire sample at different pump power densities at 4 K. The emission peak position remains the same as the pump power density is increased, confirming the negligible pump-induced heating effect in the sample at all pump power levels used in this study. The inset to Fig. 2d shows that the integrated emission intensity linearly increases as a function of pump power density, further confirming the absence of any heating effect. Unlike other reports showing strong cavity effects in vertically standing III-V and II-VI nanowires^2,33,34, we were unable to observe any cavity modes in as-grown samples. From our FDTD simulations, we confirmed that the small refractive index contrast between the nanowires and the Ge substrate causes a significant leakage of optical fields from the nanowires to the substrate (see Supplementary Note 3 for more details on the FDTD simulations). We would like to emphasize that even if the optical mode can be well confined in the nanowires, it remains challenging to probe the potential to confine light in a single nanowire because a relatively large pump beam spot size prohibits studying the emission characteristics of a single nanowire, as shown in Fig. 2a.

Strong cavity resonances from a single Ge/GeSn core/shell nanowire

To investigate the possibility of confining light in a single Ge/GeSn nanowire, we transferred as-grown nanowires onto an SiO₂ layer on an Si substrate (see Supplementary Note 1 and 2 for more details on the procedure to transfer the nanowires) and performed PL measurements on the transferred nanowires (See Methods for the detailed description on photoluminescence spectroscopy). Figure 3a shows a schematic illustration of these experiments. It is worth noting that it is possible to optically pump only one single nanowire in our optical setup that can focus the pumping laser into a ~ 10 µm diameter spot. Figure 3b presents the SEM image of a single transferred nanowire with a length of ~ 18 µm. A simulated cross-sectional optical mode profile of the transferred nanowire is shown in Fig. 3c. We confirmed via FDTD simulations that a single transferred nanowire can achieve a high Q factor of up to ~ 200 since the large refractive index contrast between the Ge/GeSn nanowire and the SiO₂ layer prevents the optical field from leaking into the substrate (see Supplementary Note 3 for more details on the FDTD simulation). Figure 3d presents the emission spectra from a single nanowire shown in Fig. 3b. All measurements were performed at 4 K. At a low pump power density of 96.6 kW cm^− 2, the spectrum exhibits a broadband spontaneous emission without any clear cavity resonances. At an increased pump power density of 120.7 kW cm^− 2, two very strong cavity resonances are observed at ~ 2251 and ~ 2275 nm, respectively, which contrasts with only a broad spontaneous emission spectrum from the as-grown nanowires at a similar pumping level. At higher pump power densities of 181.1 and 301.8 kW cm^− 2, the two cavity modes become smaller in intensity and disappear, respectively. Figure 3e shows a zoomed-in spectrum in a black dashed rectangle in Fig. 3d, which highlights the two strong cavity resonances with a free spectral range (FSR) of ~ 24 nm and a narrow full-width at half-maximum (FWHM) of ~ 3.3 nm for a peak at 2275 nm. We confirmed via FDTD simulations that the two cavity resonances are from the longitudinal modes in Ge/GeSn nanowire cavity (see Supplementary Note 4 for more details on the comparison study between simulated and experimental cavity resonances). We also studied the cavity linewidth change as a function of the pump power density (Inset to Fig. 3e). Since the cavity linewidth is directly related to the Q-factor in the nanowire governed by the material’s loss and gain³⁵, investigation of the pump-dependent linewidth provides a deeper understanding towards achieving lasing. At low pump power densities of 96.6 and 120.7 kW cm^− 2, the linewidth of the cavity mode at 2275 nm reduces from ~ 5.8 nm to ~ 3.3 nm, indicating an increased net gain as the pump power is increased. As the pump power density is further increased to 181.1 kW cm^− 2, the linewidth broadens to ~ 3.8 nm, showing the reduction in the net gain at the higher pump power. This change in optical net gain as a function of the pump power density will also be explained in the following net gain modeling section.

Theoretical modeling for gain and loss in single Ge/GeSn core/shell nanowire

For a comprehensive understanding of the changes in the intensity and linewidth of cavity modes, we performed theoretical modeling for the gain and loss dynamics in the GeSn nanowire (see Supplementary Note 5 for more details on the theoretical modeling). Figure 4a shows the calculated material gain plotted with reverse sign (solid line) and loss (dashed line) as a function of injected carrier density at 4 K. The material gain increases initially and becomes saturated at an injection density of ~ 2.2 × 10¹⁷ cm^− 3, while the material loss increases steadily at the injection density range from 1 to 6 × 10¹⁷ cm^− 3. The material loss considers all possible loss mechanisms including free-carrier absorption (FCA) and inter-valence band absorption (IVBA) (see Supplementary Note 5 for more details). Figure 4b shows the net gain that is computed by subtracting the loss from the gain. The net gain increases initially and starts decreasing at the injection density of ~ 2.2 × 10¹⁷ cm^− 3 because the gain saturates at that injection density while the loss keeps increasing. It is worth mentioning that the injection density of ~ 2.2 × 10¹⁷ cm^− 3 showing the peak net gain corresponds to the pump power density of ~ 120 kW cm^− 2, at which the experimentally measured cavity resonances are the strongest and narrowest (blue curve of Fig. 3d). This net gain trend as a function of the injection density shown in Fig. 4b clearly explains the experimental observation of the cavity resonances at different pump powers. When the net gain is the largest at the injection density of ~ 2.2 × 10¹⁷ cm^− 3, the measured single nanowire reaches close to the transparency regime, thus resulting in the emergence of strong cavity resonances.

To realize lasing in a single Ge/GeSn nanowire, the net gain should overcome the cavity loss¹². This can be achieved by introducing tensile strain that can improve the net gain³⁶ and also by improving the optical cavity. To investigate the possibility of improving the optical cavity of a single Ge/GeSn nanowire, we performed comprehensive FDTD simulations to study optical quality (Q) factors of various nanowires. Figure 4c shows the calculated Q factors as a function of the nanowire diameter for structures mimicking an as-grown nanowire (black dots) and a transferred single nanowire (unoptimized, red dots).

The Q factors of the transferred nanowire are > 2 times higher than those of as-grown nanowires at all simulated diameters. These simulation results clearly present the superior optical confinement of the nanowire transferred onto an SiO₂ layer compared to that of as-grown nanowires, thus explaining the experimental observation of cavity modes only from the transferred nanowire. In addition, it is evident from the simulation that it is important to have a large diameter of the nanowire for achieving high Q factors. By increasing the nanowire diameter the effective refractive index increases⁷, thus the refractive index contrast between the nanowire and the substrate can be substantially increased. This reduces the optical field leaking from the nanowire to the substrate, thereby increasing the optical confinement. The diameter of the single transferred nanowire that showed strong cavity resonances is ~ 500 nm, which can achieve good optical confinement according to the simulation.

We also designed an optimized nanowire structure as shown in the inset of Fig. 4c and calculated its Q factors (optimized, blue dots) to provide insight toward realizing a Ge/GeSn nanowire laser. The unoptimized structure has a flat facet on one side and a non-flat facet with a gold seed attached to it on another facet¹⁹, which scatters the photons and reduces the Q factor. We designed the optimized structure to have flat facets on both sides without a gold seed. This optimized structure can be achieved through a one-step aligned photolithography step. The optimized structure shows ~ 30% improved Q factors compared to the unoptimized nanowire at all simulated diameters because the optimized structure does not have the scattering site at a non-flat facet. We believe that it may be feasible to achieve lasing in the optimized structure, possibly also by harnessing tensile strain to improve the material gain since the observed cavity resonances in our measurements already show a very narrow FWHM of ~ 3 nm that is is generally seen just before the lasing occurs^13,14,37.

In summary, we have presented the first experimental observation of strong cavity resonances in a single Ge/GeSn core/shell nanowire grown by bottom-up synthesis. The single nanowire transferred onto an oxidized Si substrate secures a large refractive index contrast between the nanowire and the SiO₂ layer, which played a major role in enabling the observation of strong cavity modes in the nanowire. The experimentally measured spectra showing two strong cavity peaks are in excellent agreement with the FDTD simulation results, which confirm that the cavity resonances are due to the longitudinal modes confined in the Ge/GeSn nanowire. Theoretical modeling shows that the net gain increases initially and decreases at higher pump powers, which explains the behavior of the strong cavity resonances in our material system. We have also shown the possibility of improving the nanowire optical cavity that can help achieve lasing in the Ge/GeSn nanowire. The deposition of an external stressor layer such as silicon nitride can induce tensile strain in the nanowire³⁸ and improve the net gain³⁶, thus leading to the realization of the first group-IV bottom-up nanowire laser. This first experimental observation of strong cavity resonances in a single Ge/GeSn nanowire paves the way towards the realization of a group-IV nanowire laser, opening new possibilities for ultrasmall on-chip light sources in the extended SWIR range and beyond.

Nanowire growth. The Ge/Ge_0.90Sn_0.10 core/shell nanowires were grown in a chemical vapor deposition (CVD) reactor using germane (GeH₄) and tin-tetrachloride (SnCl₄) as precursor gases. Prior to the growth, the Ge(111) substrate was cleaned with a 2% HF solution and then the 20 nm Au colloids were deposited on the wafer, dried with Nitrogen flow, and then loaded in the CVD reactor. The vapor-liquid-solid (VLS) growth of the 20 nm Ge nanowires was performed for 20 min at 340 ℃, while the Ge_0.90Sn_0.10 shell was grown for 150 min at 310 ℃ (Ge/Sn ratio in gas phase of 2200). The sample for APT measurements was grown with identical parameters, but with a reduced shell growth time of 30 min.

Scanning Transmission Electron Microscopy. In order to carry out STEM, GeSn nanowires were swiped over a silicon piece to transfer them. The transferred wires were embedded using electron beam deposited Carbon and Platinum as well as ion beam induced platinum and then embedded in a TEM lamella using Focused Ion beam milling as described in [https://doi.org/10.1557/mrs2007.63]. Focused Ion beam preparation was carried out in a FEI Helios Nanolab 660 DualBeam using a Ga ion beam at voltages between 30 and 5kV. TEM imaging and EDX analysis was carried out in a Thermo Scientific Talos F200X G2 with ChemiSTEM technology at a voltage of 200kV. The EDX data were analyzed using the Velox Software package.

Atom Probe Tomography. Samples for APT were prepared by transferring single nanowires onto isolated tips in a FEI Helios Nanolab 660 DualBeam as described in [https://doi.org/10.1021/acs.nanolett.6b03109]. APT was carried out in a Cameca LEAP 5000XS at a base temperature of 30K, a detection rate of 4% and a laser pulse energy of 5pJ. The APT data were reconstructed and analyzed using the IVAS software package.

Photoluminescence spectroscopy. The samples were mounted into a closed-cycle helium cryostat (Cryostation s50, Montana Instrument) and measured at 4 K. A 1550 nm laser with a pulse width of 10 ns and repetition rate of 1 MHz was focused on the samples using a 15× reflective objective lens. The spot size was set to ~ 10 µm. For the as-grown nanowire sample, about 10 nanowires were optically pumped within the pumping spot. For the transferred nanowire sample, only a single nanowire with a length of ~ 18 µm was illuminated within the pumping beam area. The emitted signal from the samples was collected by the same reflective objective lens. The collected signal was then coupled into Fourier transform infrared (FTIR) interferometer and detected by InGaAs and InSb detectors.

Acknowledgements

The authors thank J. Bouchard for the technical support with the CVD system, O.M. acknowledges support from NSERC Canada, Canada Research Chairs, Canada Foundation for Innovation, Mitacs, PRIMA Québec, and Defence Canada (Innovation for De fence Excellence and Security, IDEaS). The research of the project was in part supported by Ministry of Education, Singapore, under Grant No. AcRF TIER 1 (RG 115/21). The research of the project was also supported by Ministry of Education, Singapore, under Grant No. AcRF TIER 2 [MOE2018-T2-2-011 (S)]. This work was also supported by the National Research Foundation of Singapore through the Competitive Research Program (No. NRF-CRP19-2017-01). This work was also supported by the National Research Foundation of Singapore through the NRF-ANR Joint Grant (No. NRF2018-NRF-ANR009 TIGER). This work was also supported by the iGrant of Singapore A*STAR AME IRG (No. A2083c0053). The authors would like to acknowledge and thank the Nanyang NanoFabrication Centre (N2FC).

Author contributions

Y.K., S.A., D.N. and O.M. conceived the initial idea of the project. Under the guidance of O.M., S.A. grew the nanowires. S.A., S.K. and L.L carried out material characterization and analyses of the as-grown nanowires. Under the guidance of D.N., Y.K., M.C., X.S., and H.-J.J. performed photoluminescence measurements and analyses. Y.K. performed optical simulations. Y.K. and S.A carried out Raman measurements. Z.I. performed theoretical net gain simulation and modeling. Under the guidance of O.M. and D.N., Y.K. and S.A. performed data analyses. H.-J.J. and D.B. helped with low-temperature measurements. Y.K., S.A., D.N., and O.M. wrote the manuscript. All authors revised the manuscript.

Competing financial interests

The authors declare no competing financial interests.

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Strong extended SWIR cavity resonances in a single GeSn nanowire

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Abstract

Figures

Introduction

Results

Nanowires growth and characterization

Direct bandgap emission from as-grown Ge/GeSn core/shell nanowires

Strong cavity resonances from a single Ge/GeSn core/shell nanowire

Theoretical modeling for gain and loss in single Ge/GeSn core/shell nanowire

Discussion

Methods

Declarations

References

Additional Declarations

Supplementary Files

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