Direct gap Ge1-ySny alloys: Fabrication and design of mid-IR photodiodes

Abstract

Chemical vapor deposition methods were developed, using stoichiometric reactions of specialty Ge3H8 and SnD4 hydrides, to fabricate Ge1-ySny photodiodes with very high Sn concentrations in the 12%–16% range. A unique aspect of this approach is the compatible reactivity of the compounds at ultra-low temperatures, allowing efficient control and systematic tuning of the alloy composition beyond the direct gap threshold. This crucial property allows the formation of thick supersaturated layers with device-quality material properties. Diodes with composition up to 14% Sn were initially produced on Ge-buffered Si(100) featuring previously optimized n-Ge/i-Ge1-ySny/p-Ge1-zSnz type structures with a single defected interface. The devices exhibited sizable electroluminescence and good rectifying behavior as evidenced by the low dark currents in the I-V measurements. The formation of working diodes with higher Sn content up to 16% Sn was implemented by using more advanced n-Ge1-xSnx/i-Ge1-ySny/p-Ge1-zSnz architectures incorporating Ge1-xSnx intermediate layers (x ∼ 12% Sn) that served to mitigate the lattice mismatch with the Ge platform. This yielded fully coherent diode interfaces devoid of strain relaxation defects. The electrical measurements in this case revealed a sharp increase in reverse-bias dark currents by almost two orders of magnitude, in spite of the comparable crystallinity of the active layers. This observation is attributed to the enhancement of band-to-band tunneling when all the diode layers consist of direct gap materials and thus has implications for the design of light emitting diodes and lasers operating at desirable mid-IR wavelengths. Possible ways to engineer these diode characteristics and improve carrier confinement involve the incorporation of new barrier materials, in particular, ternary Ge1-x-ySixSny alloys. The possibility of achieving type-I structures using binary and ternary alloy combinations is discussed in detail, taking into account the latest experimental and theoretical work on band offsets involving such materials.

Mid-infrared (3–8 μm) Ge1−ySny alloys (0.15 < y < 0.30): Synthesis, structural, and optical properties

“…1 Attempts to circumvent the quality issues to achieve alloys with y >> 0.15 are based on lowering the growth temperature to about 150 ℃, [2][3][4][5] or using complex buffer layers with intermediate compositions. [6][7][8] However, very few reports have been published on band gaps from such samples, 4,8,9 and the few results available are difficult to compare due to unknown or large strains present, compositional uncertainties, different methodologies for extracting the band gaps, and sample complexity. In this letter, we report on the structural and optical characterization of a series of Ge1-ySny alloys that meet three criteria that reduce the band gap uncertainty and simplify the analysis of optical experiments: first, a smooth and monotonic compositional dependence of the structural properties that is consistent with previous measurements of low-Sn alloys with proven quality; second, small levels of strain that minimize errors associated with deformation potentials and elastic parameters 10 (and their unknown compositional dependence); and third, elimination of buffer layers that make it difficult to extract the optical properties of the Ge1-ySny layer of interest.…”

Gold-Hyperdoped Germanium with Room-Temperature Sub-Band-Gap Optoelectronic Response

“…Such detectors, therefore, only operate under cooled conditions, typically between 4 and 78 K. Here, we show that hyperdoped Ge:Au has fundamentally different properties from those of conventional Ge:Au, namely, a much larger sub-band-gap absorption coefficient comparable to that of direct-band-gap semiconductors and room temperature sub-band-gap SWIR photodetection [38]. We note that significant progress has been made on Ge-Sn alloys [39,40], which have recently demonstrated room-temperature SWIR photodetection [41][42][43][44] and lasing [21,45], but which operate by a fundamentally different sub-bandgap absorption mechanism than that of the Au-dopant mediation investigated here. Ge-Sn alloying (∼+9 % Sn) creates high compressive strain, which can induce a redshifted direct-band-gap transition [39,40].…”

Towards a direct band gap group IV Ge-based material

“…Mathews et al demonstrated the first photodetectors made from Ge-Sn alloys, and detectors with 2% Sn were shown to cover the entire telecommunications spectrum [49]. Since then, photodetectors with Sn content up to 16% have been achieved using this method [50,51]. The CVD approach using SnD 4 also lead to the first observable luminescence from Ge-Sn alloys.…”