Vijay R. D'Costa

Molecular-Based Synthetic Approach to New Group IV Materials for High-Efficiency, Low-Cost Solar Cells and Si-Based Optoelectronics

Ge(1-x-y)Si(x)Sn(y) alloys have emerged as a new class of highly versatile IR semiconductors offering the potential for independent variation of band structure and lattice dimension, making them the first practical group IV ternary system fully compatible with Si CMOS processing. In this paper we develop and apply new synthetic protocols based on designer molecular hydrides of Si, Ge, and Sn to demonstrate this concept from a synthesis perspective. Variation of the Si/Sn ratio in the ternary leads to an entirely new family of semiconductors exhibiting tunable direct band gaps (E(o)) ranging from 0.8 to 1.2 eV at a fixed lattice constant identical to that of Ge, as required for the design of high-efficiency multijunction solar cells based on group IV/III-V hybrids. As a proof-of-concept demonstration, we fabricated lattice-matched Si(100)/Ge/SiGeSn/InGaAs architectures on low-cost Si(100) substrates for the first time. These exhibit the required optical, structural, and thermal properties, thus representing a viable starting point en route to a complete four-junction photovoltaic device. In the context of Si-Ge-Sn optoelectronic applications, we show that Ge(1-x-y)Si(x)Sn(y) alloys serve as higher-gap barrier layers for the formation of light emitting structures based on Ge(1-y)Sn(y) quantum wells grown on Si.

Sn-alloying as a means of increasing the optical absorption of Ge at the C- and L-telecommunication bands

The optical properties of Ge1−ySny alloys (y ~ 0.02) grown by chemical vapor deposition on Si substrates have been studied using spectroscopic ellipsometry and photocurrent spectroscopy. The system shows a 10-fold increase in optical absorption, relative to pure Ge, at wavelengths corresponding to the C-telecommunication band (1550 nm) and a 20-fold increase at wavelengths corresponding to the L-band (1620 nm). Measurements on a series of samples with different thicknesses reveal nearly identical dielectric functions, from which the composition reproducibility of the growth method is estimated to be as good as 0.1%. It is shown that a model that includes excitonic effects reproduces the measured onset of absorption using the direct band gap E0 as essentially the only adjustable parameter of the fit.

Epitaxial semimetallic HfxZr1−xB2 templates for optoelectronic integration on silicon

High quality heteroepitaxial HfxZr1−xB2 (x=0–1) buffers were grown directly on Si(111). The compositional dependence of the film structure and ab initio elastic constants were used to show that hexagonal HfxZr1−xB2 possess tensile in-plane strain (0.5%) as grown. High quality HfB2 films were also grown on strain compensating ZrB2-buffered Si(111). Initial reflectivity measurements of thick ZrB2 films agree with first principles calculations which predict that the reflectivity of HfB2 increases by 20% relative to ZrB2 in the 2–8eV range. These tunable structural, thermoelastic, and optical properties suggest that HfxZr1−xB2 templates should be suitable for broad integration of III nitrides with Si.

Sn-based group-IV semiconductors on Si: New infrared materials and new templates for mismatched epitaxy

We report growth and properties of GeSn and SiGeSn alloys on Si (100). These materials are prepared using a novel CVD approach based on reactions of Si-Ge hydrides and SnD4. High quality GeSn films with Sn contents up to 20%, and strain free microstructures have been obtained. The lattice mismatch between the films and Si is relieved by Lomer edge dislocations located at the interface. This material is of interest due to the predicted cross-over to a direct gap semiconductor for moderate Sn concentrations. We find that the direct band gap, and, consequently, the main absorption edge, shifts monotonically to lower energies as the Sn concentration is increased. The compositional dependence of the direct band gap shows a strong bowing, such that the direct band gap is reduced to 0.4 eV (from 0.8 eV for pure Ge) for a concentration of 14% Sn. The ternary SiGeSn alloy has been grown for the first time on GeSn buffer layers. This material opens up entirely new opportunities for strain and band gap engineering using group-IV materials via decoupling of strain and composition. Our SiGeSn layers have lattice constants above and below that of pure Ge, and depending on the thickness and composition of the underlying buffer layer they can be grown relaxed, with compressive, or with tensile strain. In addition to acting as a buffer layer for the growth of SiGeSn, we have found that GeSn can act as a template for the subsequent growth of a variety of materials, including III-V semiconductors.

Infrared spectroscopic ellipsometry study of sulfur-doped In0.53Ga0.47As ultra-shallow junctions

Sulfur mono-layer doped In0.53Ga0.47As films were investigated by infrared spectroscopic ellipsometry. The complex dielectric function of doped layers shows free carrier response which can be described by a single Drude oscillator. Electrical resistivities, carrier relaxation times, and active carrier depths are obtained for the shallow n-In0.53Ga0.47As films. Our results indicate that sub-10 nm sulfur-doped layers with active carrier concentration as high as 1.7 × 1019 cm−3 were achieved. Sheet resistances estimated from infrared spectroscopic ellipsometry are in good agreement with those obtained by electrical methods.

Advances in Si-Ge-Sn materials science and technology

SiGeSn-based optical materials are synthesized on silicon and designed to undergo indirect-to-direct bandgap transitions via strain engineering and composition tuning across the IR range. These provide enabling buffer-layer technologies for integration of semiconductors with Si.

Synthesis of (Hf, Zr)B2-based heterostructures: hybrid substrate systems for low temperature Al–Ga–N integration with Si

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.