Gordon Grzybowski

Next generation of Ge1−ySny (y = 0.01-0.09) alloys grown on Si(100) via Ge3H8 and SnD4: Reaction kinetics and tunable emission

Film growth and reaction kinetics studies have shown that trigermane (Ge3H8) is a superior Ge source for the epitaxial synthesis of Ge1−ySny/Si(100) alloys using ultra-high vacuum chemical vapor deposition. The Ge3H8/SnD4 combination yields 3-4 times higher growth rates than the traditional Ge2H6/SnD4 approach, with film Sn/Ge ratios reflecting the corresponding gas-phase stoichiometries much more closely. These advances have led to optical quality Ge1−ySny layers with Sn concentrations up to at least 9% and thicknesses approaching 1 μm. These thick films are found to be crucial for the observation of a strong, tunable photoluminescence signal near the threshold of the predicted direct-indirect bandgap crossover.

Direct versus indirect optical recombination in Ge films grown on Si substrates

The optical emission spectra from Ge films on Si are markedly different from their bulk Ge counterparts. Whereas bulk Ge emission is dominated by the materials indirect gap, the photoluminescence signal from Ge films is mainly associated with its direct band gap. Using a new class of Ge-on-Si films grown by a recently introduced chemical vapor deposition approach, we study the direct and indirect photoluminescence from intrinsic and doped samples and we conclude that the origin of the discrepancy is the lack of self-absorption in thin Ge films combined with a deviation from quasi-equilibrium conditions in the conduction band of undoped films. The latter is confirmed by a simple model suggesting that the deviation from quasi-equilibrium is caused by the much shorter recombination lifetime in the films relative to bulk Ge.

Compositional dependence of the absorption edge and dark currents in Ge1−x−ySixSny/Ge(100) photodetectors grown via ultra-low-temperature epitaxy of Ge4H10, Si4H10, and SnD4

Lattice-matched Ge1−x−ySixSny (x ≤ 0.2, y ≤ 0.05) alloys were deposited defect-free on Ge(001) substrates via low-temperature (330–290 °C) reactions of Ge4H10, Si4H10 and SnD4 hydrides, and used to fabricate pin photodetectors. The growth is carried out under gas-source molecular beam epitaxy conditions in a specially designed single-wafer reactor. Optical responsivity measurements reveal absorption edges between 0.88 eV and 0.98 eV, which are used to determine the compositional dependence of the direct band gap. A study of the I-V characteristics of the diodes shows that the dark current is very weakly correlated with the number of Si-Sn bonds in the alloy.

Photoluminescence from heavily doped GeSn:P materials grown on Si(100)

Photoluminescence has been observed at room temperature in phosphorus-dopedGe1−ySny/Si(100) alloys containing carrier densities in the 1-6 × 1019 cm−3 range. The emission intensity is one order of magnitude stronger than observed in similar undoped films, and the enhancement is consistent with theoretical predictions for doped-Ge like materials. The ratio Idir/Iind of direct over indirect gap emission is found to increase for high-Sn concentrations as a result of the reduced Γ-L valley separation in Ge1−ySny alloys. These results confirm that alloying with Sn is a viable alternative to tensile strain as a tool to enhance direct-gap emission in Ge-like semiconductors.

New strategies for Ge-on-Si materials and devices using non-conventional hydride chemistries: the tetragermane case

We introduce a practical chemical vapor deposition strategy for next-generation Ge-on-Si epitaxy utilizing recently introduced Ge4H10 hydride sources that confer unprecedented deposition efficiencies at very low-temperatures (<400 °C). The corresponding high growth rates produce thick bulk-like Ge films with structural and electrical properties significantly improved relative to state-of-the-art results obtained using conventional approaches. The use of a pure, single-source compound facilitates the control of residual doping, and enables p-i-n devices whose dark currents are not entirely determined by defects and whose zero-bias optical collection efficiencies are higher than obtained from samples fabricated using alternative Ge-on-Si approaches. The reaction pathways leading to the high-yield synthesis of Ge4H10 are identified on the basis of quantum thermochemistry simulations. The results suggest a simple approach to routine synthesis of tetragermane as the main product in quantities sufficient to be deployed as a commercial source.

Nanosynthesis routes to new tetrahedral crystalline solids: Silicon-like Si3AlP

We introduce a synthetic strategy to access functional semiconductors with general formula A(3)XY (A = IV, X-Y = III-V) representing a new class within the long-sought family of group IV/III-V hybrid compounds. The method is based on molecular precursors that combine purposely designed polar/nonpolar bonding at the nanoscale, potentially allowing precise engineering of structural and optical properties, including lattice dimensions and band structure. In this Article, we demonstrate the feasibility of the proposed strategy by growing a new monocrystalline AlPSi(3) phase on Si substrates via tailored interactions of P(SiH(3))(3) and Al atoms using gas source (GS) MBE. In this case, the high affinity of Al for the P ligands leads to Si(3)AlP bonding arrangements, which then confer their structure and composition to form the corresponding Si(3)AlP target solid via complete elimination of H(2) at ∼500 °C. First principle simulations at the molecular and solid-state level confirm that the Si(3)AlP building blocks can readily interlink with minimal distortion to produce diamond-like structures in which the P atoms are arranged on a common sublattice as third-nearest neighbors in a manner that excludes the formation of unfavorable Al-Al bonds. High-resolution XRD, XTEM, and RBS indicate that all films grown on Si(100) are tetragonally strained and fully coherent with the substrate and possess near-cubic symmetry. The Raman spectra are consistent with a growth mechanism that proceeds via full incorporation of preformed Si(3)AlP tetrahedra with residual orientational disorder. Collectively, the characterization data show that the structuro-chemical compatibility between the epilayer and substrate leads to flawless integration, as expected for pseudohomoepitaxy of an Si-like material grown on a bulk Si platform.