Approaches to low-cost infrared sensing.

The Air Force Research Laboratory's Sensors Directorate has multiple missions, including the development of next generation infrared sensors. These sensors reflect advancements in both academic and research communities, as well as requirements flow-down from operators. There has been a multitude of developments over the past decade in each community. However, there has also been consilience that low-cost infrared sensing will be necessary for the Air Force. This paradigm stands in contrast to the current generation of high performance infrared sensors, i.e., cryogenically cooled, hybridized HgCdTe, InSb, and III/V strained layer superlattices. The Sensors Directorate currently has a multi-pronged approach to low-cost infrared sensing to meet this paradigm shift, including research in silicides, SiGeSn, and lead salts. Each of these approaches highlights our integration of materials, devices, and characterization.

Two-dimensional BN buffer for plasma enhanced atomic layer deposition of Al2O3 gate dielectrics on graphene field effect transistors.

Here, we investigate the use of few-layer metal organic chemical vapor deposition (MOCVD) grown BN as a two-dimensional buffer layer for plasma enhanced atomic layer deposition (PE-ALD) of Al2O3 on graphene for top gated field effect transistors (FETs). The reactive nature of PE-ALD enables deposition of thin (2 nm) dielectrics directly on graphene and other two-dimensional materials without the need for a seed or functionalization layer; however, this also leads to significant oxidation of the graphene layer as observed by Raman. In FETs, we find this oxidation destroys conductivity in the graphene channel. By transferring thin (1.6 nm) MOCVD BN layers on top of graphene channels prior to PE-ALD, the graphene is protected from oxidation enabling BN/Al2O3 layers as thin as 4 nm. Raman and X-ray photoelectron spectroscopy on BN films show no significant oxidation caused by PE-ALD of Al2O3. Inserting the BN layer creates an atomically abrupt interface significantly reducing interface charges between the graphene and Al2O3 as compared to use of a 2 nm Al buffer layer. This results in a much smaller Dirac voltage (- 1 V) and hysteresis (0.9 V) when compared to FETs with the Al layer (VDirac = - 6.1 V and hysteresis = 2.9 V).

Molecular synthesis of high-performance near-IR photodetectors with independently tunable structural and optical properties based on Si-Ge-Sn.

This Article describes the development of an optimized chemistry-based synthesis method, supported by a purpose-built reactor technology, to produce the next generation of Ge(1-x-y)Si(x)Sn(y) materials on conventional Si(100) and Ge(100) platforms at gas-source molecular epitaxy conditions. Technologically relevant alloy compositions (1-5% Sn, 4-20% Si) are grown at ultralow temperatures (330-290 °C) using highly reactive tetragermane (Ge(4)H(10)), tetrasilane (Si(4)H(10)), and stannane (SnD(4)) hydride precursors, allowing the simultaneous increase of Si and Sn content (at a fixed Si/Sn ratio near 4) for the purpose of tuning the bandgap while maintaining lattice-matching to Ge. First principles thermochemistry studies were used to explain stability and reactivity differences between the Si/Ge hydride sources in terms of a complex interplay among the isomeric species, and provide guidance for optimizing process conditions. Collectively, this approach leads to unprecedented control over the substitutional incorporation of Sn into Si-Ge and yields materials with superior quality suitable for transitioning to the device arena. We demonstrate that both intrinsic and doped Ge(1-x-y)Si(x)Sn(y) layers can now be routinely produced with defect-free microstructure and viable thickness, allowing the fabrication of high-performance photodetectors on Ge(100). Highlights of these new devices include precisely adjustable absorption edges between 0.87 and 1.03 eV, low ideality factors close to unity, and state-of-the-art dark current densities for Ge-based materials. Our unequivocal realization of the "molecules to device" concept implies that GeSiSn alloys represent technologically viable semiconductors that now merit inclusion in the class of ubiquitous Si, Ge, and SiGe group IV systems.

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.