Epitaxial synthesis of Ga(As1–xPx)Ge3 alloys on Si(100) substrates is demonstrated using chemical vapor deposition reactions of [D2GaN(CH3)2]2 with P(GeH3)3 and As(GeH3)3 precursors. These compounds are chosen to promote the formation of GaAsGe3 and GaPGe3 building blocks which interlink to produce the desired crystalline product. Ge-rich (GaP)yGe5–2y analogues have also been grown with tunable Ge contents up to 90% by reactions of P(GeH3)3 with [D2GaN(CH3)2]2 under similar deposition protocols. In both cases, the crystal growth utilized Ge1–xSix buffer layers whose lattice constants were specifically tuned as a function of composition to allow perfect lattice matching with the target epilayers. This approach yielded single-phase materials with excellent crystallinity devoid of mismatch-induced dislocations. The lattice parameters of Ga(As1–xPx)Ge3 interpolated among the Ge, GaAs, and GaP end members, corroborating the Rutherford backscattering measurements of the P/As ratio. A small deviation from the Vegard’s law that depends on the As/P ratio was observed and corroborated by ab initio calculations. Raman scattering shows evidence for the existence of Ga–As and Ga–P bonds in the Ge matrix. The As-rich samples exhibited photoluminescence with wavelengths similar to those observed for pure GaAsGe3, indicating that the emission profile does not change in any measurable manner by replacing As by P over a broad range up to x = 0.2. Furthermore, the photoluminescence (PL) data suggested a large negative bowing of the band gap as expected on account of a strong valence band localization on the As atoms. Spectroscopic ellipsometry measurements of the dielectric function revealed a distinct direct gap transition that closely matches the PL emission energy. These measurements also showed that the absorption coefficients can be systematically tuned as a function of composition, indicating possible applications of the new materials in optoelectronics, including photovoltaics.Keywords: gallium arsenide; gallium phosphide; Ge1−xSix buffer layers; III−V−IV alloys; photoluminescence;

Hybrid (III–V)–(IV) alloys described using the general formula (GaP)y(Si)5–2y have been synthesized as epitaxial layers on Si(100) using specially designed chemical vapor deposition methods. Reactions of [D2GaN(CH3)2]2 with P(SiH3)3 between 525 and 540 °C gave GaPSi3 (y = 1) with a fixed Si content of 60%, while analogous reactions of [H2GaN(CH3)2]2 at >590 °C produced Si-rich derivatives with tunable Si contents in the range of 75–95% (y < 1). In both cases, diffraction studies and optical characterizations demonstrate single-phase, monocrystalline structures with an average diamond cubic lattice akin to Si. Raman scattering supports the presence of a tetrahedral structure containing isolated Ga–P pairs randomly embedded within the parent Si matrix. This outcome is consistent with theoretical simulations of a crystal growth mechanism involving interlinking GaPSi3 tetrahedra in a manner that precludes the formation of energetically unfavorable Ga–Ga bonds. Ellipsometry measurements of the dielectric function reveal systematic tuning of the absorption coefficient as a function of composition and demonstrate an enhanced optical response in the visible range relative to crystalline Si. The seamless integration with Si wafers and the intriguing optical response suggest these materials are promising candidates for optoelectronic applications.

Crystalline Al1–xBxPSi3 alloys (x = 0.04–0.06) are grown lattice-matched on Si(100) substrates by reactions of P(SiH3)3 and Al(BH4)3 using low pressure CVD. The materials have been characterized for structure, composition, phase purity, and optical response by spectroscopic ellipsometry, high-resolution X-ray diffraction, high-resolution transmission electron microscopy, electron energy loss spectroscopy, and energy dispersive spectroscopy, which indicate the formation of single phase monocrystalline layers with tetrahedral structures based on AlPSi3 parent phase. The latter comprises interlinked AlPSi3 tetrahedra forming a cubic lattice in which the Al–P pairs are imbedded within a diamond-structured Si matrix as isolated units. Raman scattering of the Al1–xBxPSi3 films supports the presence of substitutional B in place of Al and provides strong evidence that the boron is bonded to P in the form of isolated pairs, as expected on the basis of the AlPSi3 prototype. The substitution of small size B atoms is facilitated by the stabilizing effect of the parent lattice, and it is highly desirable for promoting full lattice matching with Si as required for Si-based solar cell designs. The substitution of B also increases the bond-length disorder leading to a significantly enhanced absorption relative to crystalline Si and AlPSi3 at E < 3.3 eV which may be beneficial for PV applications. Analogous reactions of As(SiH3)3 with Al(BH4)3 produce Al1–xBxAsSi3 crystals in which the B incorporation is limited to doping concentrations at 1020 atoms/cm3. In both cases the classical Al(BH4)3 acts as an efficient delivery source of elemental Al to create crystalline group IV–III–V hybrid materials comprising light, earth abundant elements with possible application in the fields of Si-based technologies and light-element refractory solids.

The dielectric functions of GeSn and GeSiSn alloys were measured in the 1–6 eV energy range using spectroscopic ellipsometry. The contributions from the E1, E1 + Δ1, E0′, E2, and E1′ critical points in the joint density of electronic states were enhanced by computing numerical second derivatives of the measured dielectric function, and the resulting lineshapes were fitted with model expressions from which the critical point energies, amplitudes, broadenings, and phases were determined. A detailed analysis of the compositional dependence of the different transition energies is presented. By describing this dependence in terms of quadratic polynomials, the bowing parameter (quadratic coefficient) for each transition is determined. It is shown that the bowing parameters in the ternary alloy follow a distinct chemical trend, in which the ternary is well described in terms of bowing parameters for the underlying binary alloys, and these bowing parameters increase as a function of the size and electronegativity mismatch of the alloy constituents.

Recent theoretical and experimental studies of a new class of diamond-like group III–V/group IV alloys composed of neighboring Earth-abundant Al, Si, and P elements, with a common stoichiometry of AlPSi3, indicate that these materials are promising candidates for high-performance top junctions in tandem solar cells integrated on low-cost Si platforms. These materials are grown lattice-matched on Si(100) via self-assembly of AlPSi3-like tetrahedral units generated by reaction of molecular P(SiH3)3 and Al atoms to form silicon crystal analogues, which are distinguished by intact Al–P bonding units distributed within a Si matrix. In this paper, aberration-corrected annular-dark-field imaging and atomic-column elemental mapping, are applied to characterize bonding configurations and elemental distributions in this intriguing family of monocrystalline solids. The detailed arrangements and chemical environment of Al–P and Si components have been identified for the first time and are found to be correlated with bulk optical behavior by directly comparing quantum simulations with experimental maps of the crystal structure along common crystallographic projections. The AlPSi3 alloys exhibit uniform atomic-scale composition but variations in local bonding motifs, observed by element-selective imaging and corroborated by Raman scattering. From an electronic structure perspective, the materials show extended optical coverage in the visible range compared to Si, with minor variations dependent on specific ordering of Al–P and Si within the alloy network, as confirmed by ab initio simulation of the dielectric properties. This study lays the groundwork for a systematic approach to correlating fundamental properties to atomic structure and processing conditions, which should facilitate the development of the group III–IV–V family materials with applications in Si-based PV technologies.

Intrinsic and n-type Ge1–ySny alloys with y = 0.003–0.11 have been grown on Ge-buffered Si via reactions of Ge3H8 and SnD4 hydrides using UHV-CVD techniques. The films exhibit large thicknesses (t > 600 nm), low dislocation densities (107/cm2), planar surfaces (AFM RMS ≈ 2 for intrinsic films) and mostly relaxed microstructures, making them suitable for subsequent characterization of the emission properties using photoluminescence (PL) spectroscopy. The PL spectra are acquired at room temperature and show tunable and distinct direct and indirect gap emission peaks versus composition. The peak intensity in a given sample is found to increase by exposing the layers to hydrogen plasma, indicating that surface passivation plays an important role in eliminating carrier recombination traps. The PL intensity is further increased by n-type doping with P/As atoms at levels 0.8–7 × 1019 cm–3using P(GeH3)3, P(SiH3)3, and As(SiH3)3 precursors, indicating that desirable direct gap conditions can be approached even at relatively modest 6–8% Sn contents. The indirect and direct gap energies of the samples are then used to determine the direct gap cross over point at ∼9% Sn. Collectively the results in this paper show that strong light emission can be generated in this class of narrow gap alloys by adjusting the Sn content, subjecting the samples to post growth passivation treatments or doping the system n-type. The influence of precursor chemistry on the activation properties and optical behavior of the materials is explored with the objective to optimize the PL response near the indirect–direct gap threshold. New methods embodying environmentally safe conditions are designed to produce the dopant compounds in high purity for application in future generation working devices requiring enhanced IR optical performance.

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 Ge1–x–ySixSny 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 (Ge4H10), tetrasilane (Si4H10), and stannane (SnD4) 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 Ge1–x–ySixSny 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.

We introduce a synthetic strategy to access functional semiconductors with general formula A3XY (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 AlPSi3 phase on Si substrates via tailored interactions of P(SiH3)3 and Al atoms using gas source (GS) MBE. In this case, the high affinity of Al for the P ligands leads to Si3AlP bonding arrangements, which then confer their structure and composition to form the corresponding Si3AlP target solid via complete elimination of H2 at ∼500 °C. First principle simulations at the molecular and solid-state level confirm that the Si3AlP 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 Si3AlP 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.

We report the fabrication of a new class of Sn/P-doped Ge-like materials with high quality optical, structural, and device properties. A flux of Ge2H6 with trace amounts of SnD4 is used to deposit thick Sn-doped Ge films via chemical vapor deposition (CVD) at low temperatures (∼390–370 °C) directly on Si(100) substrates. The presence of Sn in the gas mixture alters the standard Ge growth mechanism (Stranski–Krastanov) to yield atomically smooth layers with minimal threading defects at growth rates as high as 15–30 nm/min. The films, dubbed “quasi-Ge”, contain ∼1019 cm–3 Sn “impurities”, which do not produce any measurable shift in the lattice constant or emission wavelength. This new method represents a low-cost, high-performance alternative to the standard CVD approaches to grow high-quality Ge-on-Si for optoelectronic applications. In this regard, the optical quality of the materials is corroborated by studying photoluminescence (PL) of both intrinsic samples and n-type analogues doped well above 1019 atoms cm–3, using the single-source P(GeH3)3. Heavy n-doping significantly enhances the PL intensity, allowing the observation of distinct indirect and direct gap peaks. The device quality of the material was evaluated by fabricating prototype heterostructure photodetectors in n-i-p geometry. These are found to exhibit significantly higher responsivities than pure Ge p-i-n analogues and dark current densities comparable to the state of the art.Keywords: Ge; GeSn alloys; IR optoelectonics; photodetectors; Sn-doped Ge;

This paper reports a comprehensive experimental and theoretical account of synthesis, optical response, transport properties, and thermodynamic stability for a new family of Ge1−x−ySixSny semiconductor alloys based entirely on group IV elements. Device quality layers are grown directly on both Ge(100) and Si(100) wafers using low-temperature chemical vapor deposition (CVD) of commercially available sources such as trisilane, digermane, and stannane, thereby making the process suitable for direct industrial scale up and applications. This soft chemistry process is extended to demonstrate fabrication of p- and n-type layers on Si and determine their transport properties by both contactless optical methods and conventional Hall experiments. Spectroscopic analyses by UV-IR ellipsometry and Raman scattering show that the alloys possess fundamental optical and bonding properties identical to those of the materials previously grown on Ge−Sn buffers. Transmission electron microscopy (XTEM), Rutherford backscattering (RBS), and high resolution X-ray diffraction (HRXRD) characterizations demonstrated that precise tuning of the composition to achieve a Si/Sn ratio of ∼3.7 yields strain-free films with Ge-like unit cell dimensions. In the case of growth on Ge(100) the films exhibit the expected flawless registry afforded by the perfect chemical and structural matching with the underlying platform. When grown on Si(100) the lattice misfit with the substrate is compensated by periodic edge-type dislocations at the interface. Independent variation of the Si/Sn ratio from ∼1.5−4 produces a range of tetragonally distorted films on Si(100) with significant compressive strains (<0.60%) and in-plane lattice constants that are found to be “pinned” near the Ge value of 5.658 Å. The composition/temperature phase diagrams for SiGeSn and GeSn systems are obtained from first principles by calculating the Gibbs free energy via density functional theory. The stability fields of the alloys are used to predict accessible compositions, and these are compared in detail with the results from experimental studies. The principle outcome is that the mixing entropy stabilizes the ternary alloys with respect to the binaries at the same Sn content, in agreement with experimental observations.

A hybrid substrate technology based on nearly lattice matched GaN/ZrB2-buffered Si(111) was utilized to grow AlxGa1−xN heterostructures via a new method involving displacement reactions of D2GaN3 vapors and Al atomic beams at unprecedented low temperatures of 650–700 °C, compatible with Si-processing conditions. Homogeneous films exhibited strong cathodoluminescence with narrow peak widths comparable to those observed in MOCVD samples grown at 1100 °C. The formation of the enabling GaN/ZrB2buffer is investigated theoretically using first principle simulations. As an alternative to the GaN/ZrB2buffer technology we also developed novel HfxZr1−xB2 heterostructures (x = 0–1) possessing adjustable in-plane strain, which accommodates direct growth of lattice matched AlxGa1−xN on Si(111). Spectroscopic ellipsometry indicated that the boride films possess tunable band structure evolving smoothly from ZrB2 to HfB2, in the spirit of the Virtual Crystal Approximation model. This paves the way for the fabrication of optimized hybrid substrates that enable large scale nitride device integration with Si technologies via simultaneous optical and strain engineering.

We describe the systematic epitaxial engineering of device-quality elemental structures in the Ge/Si system. By introducing small concentrations of (GeH3)2CH2 or GeH3CH3 organometallic additives into conventional Ge2H6, we have developed several new low-temperature CVD growth strategies that permit heteroepitaxy of highly dissimilar materials and provide unprecedented control of film microstructure, morphology, composition, and tuning of optical properties. Optimized molecular mixtures of these compounds have enabled layer-by-layer growth via facile elimination of extremely stable CH4 and H2 byproducts, consistent with calculated chemisorption energies and surface reactivities. Collectively, our experiments indicate that the additives confer unique pseudosurfactant behavior that profoundly alters the classic Stranski–Krastanov growth mechanism of epitaxial Ge on Si surfaces. Using this approach, we have produced atomically smooth, carbon-free Ge layers directly on Si with dislocations densities less than 1 × 105 cm−2 (significantly less than those attainable from the best competing processes) at unprecedented low temperatures (350–420 °C) compatible with selective area growth applications. Full relaxation of the film is readily achieved via formation of Lomer dislocations confined to the Ge/Si interface, which should, in principle, allow film dimensions approaching bulk values to be achieved on a Si substrate. Here, films with thicknesses up to several micrometers have been grown for use as passive/active heterostructure components. The practical utility of the approach is demonstrated for the first time by growing pure Ge seamlessly, conformally, and selectively in the “source/drain” regions of prototypical device structures. This innovation represents an ultimate extension of uniaxial strain techniques using group IV materials and is likely to have applications in the integration of microelectronics with optical components (photodiodes) into a single chip. As an additional example for high-mobility device template application, we have grown tensile Si films on the Ge buffers via decomposition of SiH3SiH2SiH3. The new Ge growth processes also provide a unique route to extend the utility of elemental Ge into the wider IR optoelectronic domain by tuning its fundamental optical properties using tensile strain as a main parameter. In this study, we use the metal–organic additives to circumvent traditional surface-energy limitations and produce for the first time high-quality, thermally stable, tensile strained Ge layers at low temperature (350–380 °C) on Ge1–ySny-buffered Si(100). The precise strain state of the epilayers is controlled by varying the Sn content of the buffer, yielding tunable record-high tensile strains as high as 0.43%. This strain-tuning strategy may offer the prospect of producing direct optical gaps in elemental Ge.

Metal cyanide framework materials with stoichiometries M(CN)2 and M′(CN)3 represent an intriguing family of inclusion compounds with technological potential in materials science and energy storage applications. In this paper we develop fundamental new insights by comparing, experimentally and theoretically, the structure and bonding trends in several molecular and solid-state main group compounds containing the same basic M—C≡N “building blocks”. In particular we describe for the first time the synthesis and structural characterization of molecular analogues of the Be(CN)2 and Ga(CN)3 frameworks such as Be(CN)2(NC5H5)2 and Ga(CN)3(NC5H5)2, which represent prototypical examples of simple binary cyanides of the main group element class. We also describe the formation of closely related analogues of boron such as B(CN)3·NC5H5 and B(CN)4·HNC5H5 and report their molecular crystal structures. Complementary density functional theory simulations are then used to elucidate: (i) the origin of the structural differences between the Ga(CN)3(NC5H5)2 and the corresponding Ga(CN)3 framework solid, (ii) bonding and energetic trends in the M(CN)3(NC5H5)2 series of molecules (M = Al, Ga, In), (iii) deviations from idealized structure in the M—CN—M units within the framework solids, and (iv) bond distributions in orientationally disordered framework solids.

Ge/Sn-based group IV semiconductors with tunable band gaps across the wide IR range were synthesized using designer hydrides with tailored Si, Ge and Sn stoichiometries and structures. GeSn, SiGeSn, SiSn and SiGeSn/Ge heterostructures undergo indirect to direct band gap transitions via strain engineering and alloy composition tuning, providing the basis for integration of microelectronics with optical components into a single chip. SiGeSn systems also enable buffer layer technologies with unprecedented lattice and thermal matching capabilities for applications in monolithic integration of III–V semiconductors with Si electronics.

Epitaxial SiCAlN films with single-phase wurtzite structures were grown by molecular beam epitaxy via reactions of a specifically designed molecular precursor H3SiCN and Al atoms at 750 °C, considerably below the miscibility gap of SiC and AlN at 1900 °C. The film growth was conducted directly on Si(111) despite the 19% lattice mismatch between the two materials. Commensurate heteroepitaxy was facilitated by the conversion of native and thermally grown SiO2 layers into crystalline Si–Al–N–O interfaces in registry with the Si(111) surface. This crystalline interface acted as a template for nucleation and growth of SiCAlN. Integration of wide bandgap semiconductors including AlN and GaN with Si was achieved by this process. Perfectly epitaxial SiCAlN was also grown on 6H-SiC(0001) substrates and exhibited novel crystallographic and physical properties such as hexagonal structures with 2H/2H and 4H/2H SiC/AlN stacking, metastable cubic structures, wide bandgaps in the UV, and extreme mechanical hardness. These properties have been measured by a wide range of characterization techniques and ab initio density functional theory simulations have been used to elucidate the structural and spectroscopic behavior.Graphic

The molecular structure of CH(GeBr3)3 has been determined by gas electron diffraction (GED) and ab initio calculation at the HF/6-31G∗ level. The calculations indicate that the equilibrium structure has C3 symmetry. The most important bond distances are (GED/HF-MO); Ge–C=199.5(10)/193.6 pm, Ge–Br (mean)=228.1(2)/228.0 pm, and valence angles