V. R. D’Costa

Perfectly tetragonal, tensile-strained Ge on Ge1−ySny buffered Si(100)

High-quality, tensile-strained Ge layers with variable thickness (>30nm) have been deposited at low temperature (350–380°C) on Si(100) via fully relaxed Ge1−ySny buffers. The precise strain state of the epilayers is controlled by varying the Sn content of the buffer, yielding tunable tensile strains up to 0.25% for y=0.025. Combined Raman analysis and high resolution x-ray diffraction using multiple off-axis reflections reveal unequivocally that the symmetry of tensile Ge is perfectly tetragonal, while the strain state of the buffer (∼200nm thick) remains essentially unchanged. A downshift of the direct gap consistent with tensile strain has been observed.

Versatile buffer layer architectures based on Ge1−xSnx alloys

We describe methodologies for integration of compound semiconductors with Si via buffer layers and templates based on the GeSn system. These layers exhibit atomically flat surface morphologies, low defect densities, tunable thermal expansion coefficients, and unique ductile properties, which enable them to readily absorb differential stresses produced by mismatched overlayers. They also provide a continuous selection of lattice parameters higher than that of Ge, which allows lattice matching with technologically useful III-V compounds. Using this approach we have demonstrated growth of GaAs, GeSiSn, and pure Ge layers at low temperatures on Si(100). These materials display extremely high-quality structural, morphological, and optical properties opening the possibility of versatile integration schemes directly on silicon.

Low temperature chemical vapor deposition of Si-based compounds via SiH3SiH2SiH3 : Metastable SiSn/GeSn/Si(100) heteroepitaxial structures

Growth of Si1−xSnx alloys on Ge1−ySny-buffered Si(100) was achieved via reactions of SnD4 and SiH3SiH2SiH3 at 275°C. Kinetic studies indicate that unprecedented low growth temperatures are made possible by the highly reactive SiH2 groups. The authors obtain supersaturated metastable compositions (y∼25%) near the indirect to direct band gap crossover predicted by first principles simulations. Extensive characterizations of composition, structure, and morphology show that the SiSn∕GeSn films grow lattice matched via a “compositional pinning” mechanism. The initial Raman observations of Si–Sn bond vibrations in a condensed phase are discussed in the context of simulated bond distributions in the alloys.

Direct integration of active Ge1−x(Si4Sn)x semiconductors on Si(100)

Doped and intrinsic Ge1−x−ySixSny alloys are synthesized directly on Si(100) using simple deposition chemistries and their optical and electrical properties are determined. Tuning the Si/Sn ratio at ∼4 yields strain-free films with Ge-like cell dimensions, while variation of the ratio around this value produces compressively strained, tetragonal structures with an in-plane lattice constant “pinned” to a value close to that of pure Ge (5.658 A). First-principles calculations show that mixing entropy thermodynamically stabilizes SiGeSn in contrast to GeSn analogs with the same Sn content. GeSn and SiGeSn are predicted to become metastable for 2% and 12% Sn, respectively, in good agreement with experiment.

Ge1−ySny photoconductor structures at 1.55μm: From advanced materials to prototype devices

Prototype detector structures were fabricated on Si substrates using Ge1−ySny as active material for the first time. This alloy system covers the entire near-IR telecommunication spectrum and grows at a low temperature of 350°C, compatible with complementary metal-oxide-semiconductor (CMOS) Si technology. Processing protocols were developed for photolithography-based patterning and subsequent etching, CMOS compatible metallization, and for the formation of low-resistivity Ohmic contacts. A first generation of devices based on as-grown Ge1−ySny layers was followed by a second generation incorporating ex situ rapid thermal annealing for defect reduction, as well as additional growth and processing improvements, leading to enhanced mobilities and simultaneous reduction in intrinsic carrier concentrations. While both device generations show a significant photoconductive response at 1.55μm, the thicker second-generation samples yield improved performance due to better confinement of deleterious defects near th...

Compliant tin-based buffers for the growth of defect-free strained heterostructures on silicon

We describe the compliant behavior of Ge1−ySny buffer layers grown strain-free on Si(100). Deposition of lattice-mismatched epilayers on these buffers introduces significant strains in both systems. Ge1−x−y′SixSny′ and Ge1−xSix alloys are deposited on these buffers via reactions of designer hydrides to quantify these strains in detail. X-ray analysis reveals that Ge1−x−y′SixSny′∕Ge1−ySny and Ge1−xSix∕Ge1−ySny bilayers adopt strain states which minimize their combined elastic energy, as if the films were decoupled from the substrate. Compliant Ge1−ySny buffers thereby enable growth of highly mismatched Ge-rich semiconductors on Si and thus facilitate the long-sought on-chip integration of micro- and optoelectronic functions.

Above-bandgap optical properties of biaxially strained GeSn alloys grown by molecular beam epitaxy

The complex dielectric function of biaxially strained Ge1−xSnx (0 ≤ x ≤ 0.17) alloys grown on Ge (100) has been determined by spectroscopic ellipsometry from 1.2 to 4.7 eV. The effect of substitutional Sn incorporation and the epitaxial strain on the energy transitions E1, E1 + Δ1, E0′, and E2 of GeSn alloys is investigated. Our results indicate that the strained GeSn alloys show Ge-like electronic bandstructure with all the transitions shifted downward due to the alloying of Sn. The strain dependence of E1 and E1 + Δ1 transitions is explained using the deformation potential theory, and values of −5.4 ± 0.4 eV and 3.8 ± 0.5 eV are obtained for the hydrostatic and shear deformation potentials, respectively.

Practical B and P doping via SixSnyGe1−x−y−zMz quaternaries lattice matched to Ge: Structural, electrical, and strain behavior

We describe the fabrication of B and P doped SiGeSn ternaries, lattice-matched to Ge, with compositions adjusted to independently tune the band gap. These are deposited at 320–350 °C with superior crystallinity and morphology via in situ reactions of diborane (p-type) and designer P(SiH3)3 and P(GeH3)3 precursors (n-type). Device-level carrier concentrations in the 1019–1020/cm3 range are produced yielding film resistivities and carrier mobilities comparable to those of Ge indicating negligible alloy scattering. High boron levels induce a significant and systematic contraction of the host lattice, which is compensated by an adjustment of the Sn/Si ratio in accord with a simple model based on Vegard’s law, the mismatch of covalent radii of the constituents, and the absolute hydrostatic deformation potentials for the band edges.

Ge1−ySny∕Si(100) composite substrates for growth of InxGa1−xAs and GaAs1−xSbx alloys

We describe unique methodologies for integration of InxGa1−xAs and GaAs1−xSbx semiconductor alloys with Si involving an innovative buffer layer approach based on lattice-engineered Ge1−ySny alloys. These are grown strain-free on Si(100) via formation of Lomer edge dislocations and exhibit a continuous selection of lattice parameters higher than that of Ge. This allows close lattice matching with the InxGa1−xAs and GaAs1−xSbx compounds, thereby providing a manifestly different approach to the integration of mismatched III-V semiconductors with silicon. A series of compositions across the entire alloy range were grown for both systems using metal organic chemical vapor deposition at low temperatures between 500–550°C. The materials displayed high quality morphological, structural, and optical properties as evidenced by Rutherford backscattering spectroscopy, ion channeling, cross sectional transmission electron microscopy, atomic force microscopy, and photoluminescence characterizations. High resolution x-r...

Practical routes to (SiH3)3P: Applications in group IV semiconductor activation and in group III–V molecular synthesis

The (SiH₃)₃P hydride is introduced as a practical source for n-doping of group IV semiconductors and as a highly-reactive delivery agent of -(SiH₃)₂P functionalities in exploratory synthesis. In contrast to earlier methods, the compound is produced here in high purity quantitative yields via a new single-step method based on reactions of SiH₃Br and (Me₃Sn)₃P, circumventing the need for toxic and unstable starting materials. As an initial demonstration of its utility we synthesized monosubstituted Me₂M-P(SiH₃)₂ (M = Al, Ga, In) derivatives of Me₃M containing the (SiH₃)₂P ligand for the first time, in analogy to the known Me₂M-P(SiMe₃)₂ counterparts. A dimeric structure of Me₂M-P(SiH₃)₂ is proposed on the basis of spectroscopic characterizations and quantum chemical simulations. Next, in the context of materials synthesis, the (SiH₃)₃P compound was used to dope germanium for the first time by building a prototype p(++)Si(100)/i-Ge/n-Ge photodiode structure. The resultant n-type Ge layers contained active carrier concentrations of 3-4 × 10¹⁹ atoms cm⁻³ as determined by spectroscopic ellipsometry and confirmed by SIMS. Strain analysis using high resolution XRD yielded a Si content of 4 × 10²⁰ atoms cm⁻³ in agreement with SIMS and within the range expected for incorporating Si₃P type units into the diamond cubic Ge matrix. Extensive characterizations for structure, morphology and crystallinity indicate that the Si co-dopant plays essentially a passive role and does not compromise the device quality of the host material nor does it fundamentally alter its optical properties.