Doping of Direct Gap Ge1-ySny Alloys to Attain Electroluminescence and Enhanced Photoluminescence

Alloying of elemental Ge with Sn permits tuning of the band gap, as well as a transition towards direct gap behavior. Epitaxial deposition of direct gap Ge_1-ySn_y alloys (y≥0.09) requires ultra-low temperatures of ~300°C.^1–3 In order to fully realize the electroluminescence and photoluminescence (PL) potential of this class of materials, strategies for p- and n-type doping that are compatible with this temperature range are needed.

In our previous work, combination of both traditional and novel dopant sources were utilized for doping of materials ranging from pure Ge to Ge_1-ySn_y alloy at the direct to indirect crossover composition of y=0.09.^4,5 Presented here are the extensions of these methods to include alloys beyond the crossover composition. The n-type doped alloys were grown on virtual Ge substrates in a hot wall UHV-CVD reactor using trigermane (Ge₃H₈) and deuterostannane (SnD₄) sources to provide the respective elements. Trisilylphosphine (P(SiH₃)₃) was used as a P source for n-type doping alloys in the composition range y=0.09-0.13, which required growth temperatures of 300°C-275°C. Choice of this doping agent was due to its greater stability (which facilitates long term storage) and higher vapor pressure (which allows straightforward use in mixture preparation) when compared to trigermylphosphine (P(GeH₃)₃), another agent capable of delivering donor P atoms at low temperatures. The use of these precursor combinations has enabled the deposition of direct gap Ge_1-ySn_y alloy films with 300-500 nm thicknesses with dopant concentrations
5x10¹⁸-3x10¹⁹ cm^-3, as determined by IR spectroscopic ellipsometry. RBS channeling spectra and (004) rocking curves show these materials to have excellent crystallinity. Furthermore, analysis using SIMS shows that the incorporated donor atoms are fully activated, and the amount of Si incorporated into the materials from using P(SiH₃)₃ as the doping precursor is at impurity levels which does not affect the band gap properties. These observations are in agreement with previous experiments in which P(SiH₃)₃ was used as a doping agent, albeit at higher temperatures.^4,5

Using identical protocols but removing P(SiH₃)₃ from the precursor mixtures, it was possible to obtain intrinsic
Ge_1-ySn_y alloys with concentrations up to y=0.14. By choosing n-type doped virtual Ge substrates for these depositions, it was possible to investigate electroluminescence, after growth of appropriate p-type layers to form n-i-p diodes, as described below.

The p-type doped alloys were grown on either intrinsic or n-type doped Ge­_1-ySn_y alloys with y=0.09-0.14 in order to fabricate n-p or n-i-p device stacks. The depositions were carried out in a hot wall UHV-CVD reactor with a temperature profile that enables the pre-activation of the precursor molecules prior to reaching the growth surface. The precursors used for the depositions were digermane (Ge₂H₆), SnD₄ and diborane (B₂H₆). The films obtained were 100-300 nm thick, with active dopant concentrations in the 2-3×10¹⁹ cm^-3 range, and compositions of y=0.08-0.12.

As shown in Fig.1, n-type doping of the Ge_1-ySn_y alloys enhances PL in comparison to intrinsic counterparts. Furthermore, prototype LEDs produced with the aid of above doping strategies exhibit strong electroluminescence,² paving the way for obtaining emission for a broad spectral range, from pure Ge at 1550 nm up to 2700 nm and beyond.

References

(1)         Gallagher, J. D.; Senaratne, C. L.; Kouvetakis, J.; Menéndez, J. Appl. Phys. Lett. 2014, 105, 142102.

(2)         Gallagher, J. D.; Senaratne, C. L.; Sims, P.; Aoki, T.; Menéndez, J.; Kouvetakis, J. Appl. Phys. Lett. 2015, 106, 091103.

(3)         Senaratne, C. L.; Gallagher, J. D.; Jiang, L.; Aoki, T.; Menéndez, J.; Kouvetakis, J. J. Appl. Phys. 2014, 116 (13), 133509.

(4)         Senaratne, C. L.; Gallagher, J. D.; Aoki, T.; Kouvetakis, J.; Menéndez, J. Chem. Mater. 2014, 26 (20), 6033–6041.

(5)         Xu, C.; Gallagher, J. D.; Sims, P.; Smith, D. J.; Menéndez, J.; Kouvetakis, J. Semicond. Sci. Technol. 2015, 30 (4), 045007.

Fig. 1 – PL spectra obtained from a liquid nitrogen cooled InGaAs detector for n-type doped (solid line) and intrinsic (dashed line) Ge_1-ySn_y samples with y=0.09. Both samples show strong luminescence as expected for direct gap materials. However the doped sample, which has an active carrier concentration of 1×10¹⁹ cm^-3, shows clear enhancement of PL compared to the intrinsic sample. The inset shows the same spectra obtained from a thermoelectrically cooled PbS detector with extended spectral range beyond the cutoff point of the InGaAs detector, albeit at expense of poorer signal-to-noise ratio.