In-Situ n-Type Doped Carrier-Injection Layers in GeSn Direct Bandgap LEDs for Methane Sensing
Authors: Cardoux Clement , Casiez Lara , Chretien Jeremie , Goulain Paul , Frauenrath Marvin , Pauc Nicolas , Calvo Vincent , Coudurier Nicolas , Rodriguez Philippe , Koelling Sebastian , Moutanabbir Oussama , Gravrand Olivier , Hartmann Jean-Michel , Tchelnokov Alexei , Reboud Vincent 
Published: December 2024

GeSn-based group-IV alloys are attracting great attention in the Si photonics community, as they are considered to be compatible with Complementary Metal Oxide Semiconductor (CMOS) technology. Alloying germanium with more than 8% of tin (Sn) results in direct bandgap semiconductors, with some optical gain in devices such as lasers. Recently, room temperature optically pumped lasing was achieved thanks to high Sn content stacks. GeSn alloys can be used for near, short and mid wavelength infra-red spectral range operation, notably for gas detection. Current efforts are focused on electro-optical GeSn IR devices such as light-emitting devices (LEDs), electrically pumped lasers and photodetectors.
In this paper, we evaluate the impact of various types of in situ n-type doped carrier injection layers on top of GeSn direct bandgap LEDs. More precisely, we compare the performances of GeSn:P and Ge:P –capped mid IR LEDs. Using reduced pressure chemical vapor deposition and metastable growth conditions (e.g. fast growth rates at low temperature), high crystalline quality GeSn layers were grown on 200 mm diameter Ge-buffered Si(001) wafers (Figure a). In situ n-type doped Ge layers (sample A) and GeSn layers (sample B) were used to inject carriers into direct band gap Ge0.87Sn0.13 active layers beneath (Figure b). I(V) curves (Figure c) and Electro-Luminescence spectra (Figure d) were compared for sample B (GeSn:P) and sample A (Ge:P). Very similar I(V) behaviors were observed under forward bias for both samples. However, a higher dark current was measured under reverse bias for sample B than for sample A. This could be due to a higher number of defects in the thicker capping layer of sample B (240 nm) compared to that of sample A (50 nm). However, a 2-fold increase in the EL signal was obtained for sample B than sample A (Figure d). Temperature dependence electroluminescence measurements and atom probe tomography data will be used to explain the electroluminescence behavior differences between Samples A and B.
Finally, even higher Sn content LEDs were fabricated to emit light at 3.3 µm (Figure e). A direct band gap Ge0.846Sn0.154LED (Sample C) with an in situ n-type doped Ge cap was placed in a gas cell filled with diluted methane. Its emission overlapped well with the absorption of methane (Figure f). The methane detection limit of our setup was evaluated (data not shown). To further increase the emitted power of GeSn LEDs, current spreading was studied for different top contact geometries. Emission maps collected by an InSb camera at room temperature showed a definite dependence of light emission on electrical contact geometry (Figure g). Investigations are underway to further increase light extraction from LEDs and improve their performances for use in environmental sensors.

Acknowledgement: This work was supported by the European Union’s Horizon 2020 LASTSTEP Project under the grant agreement ID: 101070208, the EDEN Carnot project and the CEA DRF-DRT Phare project. We gratefully acknowledge the clean room staff from LETI and IRIG for their technical support.

Figure 1