Ge1-ySny alloys with compositions in the 0.15 < y < 0.30 range have been grown directly on Si substrates using a chemical vapor deposition approach that allows for growth temperatures as high as 290 o C. The films show structural properties that are consistent with results from earlier materials with much lower Sn concentrations. These include the lattice parameter and the Ge-Ge Raman frequency, which are found to depend linearly on composition. The simplicity of the structures, directly grown on Si, makes it possible to carry out detailed optical studies. Sharp absorption edges are found, reaching 8 μm near y =0.3. The compositional dependence of edge energies shows a cubic deviation from the standard quadratic alloy expression. The cubic term may dramatically impact the ability of the alloys to cover the long-wavelength (8-12 μm) mid-IR atmospheric window. * chixu@asu.edu
A unique class of crystalline Ge 1−y Sn y alloys with ultrahigh Sn concentrations reaching 33% as well as Si-doped analogues have been synthesized directly on large-area Si wafers by chemical vapor deposition (CVD). The band gaps of these materials are measured to be as low as 0.15 eV, which allows them to cover a significant portion of the coveted mid-IR optical range past 8 μm, making them a potential alternative to Hg 1−x Cd x Te for long-wavelength applications. The film synthesis, based on stoichiometric CVD reactions of polygermanes and stannanes, is carried out at temperatures between 240 and 290 °C, indicating that the alloys are stable at typical device-operating temperatures and at complementary metal oxide semiconductorcompatible conditions. The main difference between Ge 1−y Sn y (y ≈ 0.33) and the more diluted alloys (y < ∼0.2) featured in prior studies is the predicted presence of a significant fraction of Sn−Sn bonds, which could have a profound impact on the structural and electronic properties in view of the large size mismatch between Ge and Sn. However, X-ray measurements of the lattice parameter over the entire 0−33% Sn compositional range show a linear dependence of the cubic lattice parameter consistent with Vegard's law. Theoretical ab initio simulations assuming random alloys show a monotonic compositional dependence of the alloy energy of formation and confirm the experimental lattice parameter. The crystalline structure and the interface defects that accommodate the large lattice mismatch with the substrate are elucidated using transmission electron microscopy studies. Optical studies indicate that the fundamental direct band gap and other critical point features in the electronic structure also display a smooth compositional dependence, which should facilitate device engineering based on these materials.
A chemical vapor deposition (CVD) strategy is presented for the synthesis of Ge1–y Sn y alloys on Si wafers with band gaps in the short-wave infrared (SWIR) range of 1.8–2.6 μm, as well as in the long-wave infrared (LWIR) at 12 μm and beyond. This broad compositional versatility is achieved by CVD reactions of Ge3H8 and SnH4 as Ge and Sn precursors, respectively. The use of conventional SnH4 instead of the previously used SnD4 is found to be critical in synthesizing alloys with Sn contents between 30 and 36%, suggesting that at very low temperatures required to grow these supersaturated alloys, the incorporation of Sn is reaction-rate-dependent. The SnD4 precursor was historically preferred to SnH4 due to its longer lifetime at room temperature. However, the difference is found not to be significant from a practical perspective, and the fact that SnH4 can be synthesized from cheap and readily available starting materials represents a significant cost reduction and portends feasibility for commercial applications. The subtle differences between SnD4 and SnH4 that may explain why alloys with Sn incorporation higher than 30% can only be achieved with the latter are discussed in detail. Optical experiments indicate that such concentrations may be sufficient to cover much of the LWIR range. Device structures containing SWIR Ge1–y Sn y alloys were grown with the SnH4 precursor to demonstrate its suitability as a viable Sn source. The fabricated heterostructure pin diodes display excellent structural properties and a clear rectifying behavior, providing evidence that SnH4 is a practical and versatile CVD source of Sn at all concentrations of practical interest.
We report a versatile chemical vapor deposition (CVD) method to dope Ge films with Ga atoms in situ over a wide concentration range spanning from 3 × 10 18 to 2.7 × 10 20 cm −3 . The method introduces a stable and volatile Ga hydride [D 2 GaN(CH 3 ) 2 ] 2 that reacts readily with Ge 4 H 10 to deliver Ga dopants controllably and systematically at complementary metal-oxide-semiconductor compatible ultralow temperatures of ∼360 °C. Thick and monocrystalline layers (1.3 μm) are produced on Si substrates at growth rates approaching 50 nm/min. The doped crystals are fully epitaxial and devoid of misfit defects and Ga precipitates as evidenced by Rutherford backscattering spectrometry, X-ray diffraction, and cross-sectional transmission electron microscopy. The Ga contents measured by secondary ion mass spectrometry and the active carrier concentrations determined by spectroscopic ellipsometry (as well as Hall effect measurements in several cases) are in close agreement, indicating near full activation. Photoluminescence spectra show a strong emission peak at 0.79 eV corresponding to the direct gap E 0 transition, evidence of the indirect transition, and additional structures characteristic of ptype Ge. Electroluminescence and I−V curves measured from p(Ga)−i−n photodiodes are found to be at par with those from boron-based reference devices. These results are promising and demonstrate that a single-source CVD approach allows independent control of Ga doping level and junction depth, producing flat dopant profiles, high activation ratios, uniform distributions, and sharp interfaces. This method potentially represents a viable alternative to state-of-the-art boron-based p-type doping and activation of Ge-like materials.
This paper describes the fabrication of Ge1−ySny layers with 2%–13% Sn, utilizing a unique method that combines high-order Ge4H10 and Ge5H12 hydrides and gas source molecular epitaxy techniques. The latter operate at very low working pressures of 10−6–10−7 Torr leading to molecular flow regime conditions, promoting layer-by-layer epitaxy of crystalline materials at ultralow-temperatures (250–160 °C) that cannot be achieved by conventional thermal CVD. In both cases, a “direct injection” approach is employed, using the pure vapor of Ge4H10 and Ge5H12 as the source of the Ge flux, which is then reacted on the substrate surface with SnD4 in the absence of gaseous carriers. Ge4H10 reactions were conducted at 215–190 °C, producing 6%–12% Sn samples. These were grown on both conductive, resistive, single-side, and double-side polished Si(100) with n-type Ge1−xSix buffer layers (x = 2%–3%) to explore conditions and substrate formats that facilitate back-side illumination, enabling transparency and enhanced responsivity at 1550 nm in prototype p-i-n devices. Exploratory reactions of Ge5H12 with SnD4 produced Ge1−ySny with 2%–13% Sn at 250–160 °C for the first time. All samples were characterized by XRD, RBS, IR-ellipsometry, AFM, and TEM to investigate the structure, composition, strain state, and morphology. The samples grow partially relaxed (T > 180 °C) and their compressive strains gradually diminish in situ with increasing film thickness (up to 700 nm) without epitaxial breakdown and Sn segregation. Residual strains are further reduced by RTA processing. The experiments described here demonstrate the practicality of our chemistry-based method as an alternative to thermal CVD for the fabrication of high crystal quality samples on larger area wafers for potential applications in IR devices.
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