Nick Bennett

Large thermoelectric power factors and impact of texturing on the thermal conductivity in polycrystalline SnSe

Single crystals of SnSe have been reported to have very high thermoelectric efficiencies with a maximum figure merit zT = 2.5. This outstanding performance is due to ultralow thermal conductivities. We report on the synthesis of highly textured polycrystalline SnSe ingots with large single-crystal magnitude power factors, S2/ρ = 0.2–0.4 mW m−1 K−2 between 300–600 K, increasing to 0.9 mW m−1 K−2 at 800 K, and bulk thermal conductivity values κ300K = 1.5 W m−1 K−1. However, small SnSe ingots, which were measured in their entirety, were found to have a substantially reduced κ300K = 0.6 W m−1 K−1. Microscopy and diffraction revealed two distinct types of texturing within the hot-pressed ingots. In the interior, large coherent domains of SnSe platelets with a ∼45° orientation with respect to the pressing direction are found, while the platelets are preferentially oriented at 90° to the pressing direction at the top and bottom of the ingots. Fitting the κ(T) data suggests an increase in defect scattering for the smaller ingots, which is in keeping with the presence of regions of structural disorder due to the change in texturing. Combining the measured S2/ρ with the bulk ingot κ values yields zT = 1.1 at 873 K.

Extended Point Defects in Crystalline Materials: Ge and Si

B diffusion measurements are used to probe the basic nature of self-interstitial point defects in Ge. We find two distinct self-interstitial forms--a simple one with low entropy and a complex one with entropy ∼30  k at the migration saddle point. The latter dominates diffusion at high temperature. We propose that its structure is similar to that of an amorphous pocket--we name it a morph. Computational modeling suggests that morphs exist in both self-interstitial and vacancylike forms, and are crucial for diffusion and defect dynamics in Ge, Si, and probably many other crystalline solids.

Highly conductive Sb-doped layers in strained Si

The ability to create stable, highly conductive ultrashallow doped regions is a key requirement for future silicon-based devices. It is shown that biaxial tensile strain reduces the sheet resistance of highly doped n-type layers created by Sb or As implantation. The improvement is stronger with Sb, leading to a reversal in the relative doping efficiency of these n-type impurities. For Sb, the primary effect is a strong enhancement of activation as a function of tensile strain. At low processing temperatures, 0.7% strain more than doubles Sb activation, while enabling the formation of stable, ∼10-nm-deep junctions. This makes Sb an interesting alternative to As for ultrashallow junctions in strain-engineered complementary metal-oxide-semiconductor devices.

Constraints on micro-Raman strain metrology for highly doped strained Si materials

Ultraviolet (UV), low penetration depth, micro-Raman spectroscopy, and high-resolution x-ray diffraction (HRXRD) are utilized as complementary, independent stress characterization tools for a range of strained Si samples doped by low energy (2keV) Sb ion implantation. Following dopant implantation, good agreement is found between the magnitudes of strain measured by the two techniques. However, following dopant activation by annealing, strain relaxation is detected by HRXRD but not by micro-Raman. This discrepancy mainly arises from an anomalous redshift in the Si Raman peak position originating from the high levels of doping achieved in the samples. This has serious implications for the use of micro-Raman spectroscopy for strain characterization of highly doped strained Si complementary metal-oxide semiconductor devices and structures therein. We find a direct correlation between the Si Raman shift and peak carrier concentration measured by the differential Hall technique, which indicates that UV micro-R...

Antimony for n-type metal oxide semiconductor ultrashallow junctions in strained Si: A superior dopant to arsenic?

The creation of stable, highly conductive ultrashallow junctions in strained Si is a key requirement for future Si based devices. It is shown that in the presence of tensile strain, Sb becomes a strong contender to replace As as the dopant of choice due to advantages in junction depth, junction steepness, and crucially, sheet resistance. While 0.7% strain reduces resistance for both As and Sb, a result of enhanced electron mobility, the reduction is significantly larger for Sb due to an increase in donor activation. Differential Hall and secondary-ion mass spectroscopy measurements suggest this to be a consequence of a strain-induced Sb solubility enhancement following epitaxial regrowth, increasing Sb solubility in Si to levels approaching 1021cm−3. Advantages in junction depth, junction steepness, and dopant activation make Sb an interesting alternative to As for ultrashallow doping in strain-engineered complementary metal-oxide semiconductor devices.

The luminescent properties of CuAlO2

In this work we examine the room temperature photoluminescence, Raman and low temperature photoluminescence properties of CuAlO2 prepared using different precursors. At room temperatures the luminescence associated with bulk CuAlO2 occurs at 355 nm and is associated with strong resonant Raman effects. At low temperatures we find that the UV emission is dominated by strong electron–phonon coupling leading to a Franck–Condon type emission band. A second strongly phonon coupled band is also observed in the blue spectral region. In addition we also show that at low temperatures the luminescent properties of CuAlO2 are meta-stable, with anomalous temperature dependence. The possible origins of the blue band, the meta-stability and anomalous temperature dependence are discussed.

Model for electron mobility as a function of carrier concentration and strain in heavily doped strained silicon

Strain engineering plays a pivotal role in modern devices due to the advantages it offers in enhancing carrier mobility, μ. In addition to strain, e, carrier concentration, N, also determines mobility and an understanding of the functional dependence μ(N,e) at various levels of strain is vital. Although well established for low and moderate doping, currently little is known about μ(N) for high carrier concentrations (>1019 cm−3) in strained Si. We present experimental data to fill this void, allowing an extension of the current model for μ(N) [Masetti et al., IEEE Trans. Electron DevicesIETDAI0018-9383 30, 764 (1983)] to account for strain. We also consider the influence of strain induced from dopant atoms. Experiments show the effects of tensile strain as a mobility enhancer are reduced but still significant at high doping concentrations. The model reproduces this effect and accounts for μ(N,e) across the full range of doping concentrations.

Structural investigation of MOVPE-grown GaAs on Ge by x-ray techniques

The selection of appropriate characterization methodologies is vital for analyzing and comprehending the sources of defects and their influence on the properties of heteroepitaxially grown III?V layers. In this work, we investigate the structural properties of GaAs layers grown by metal-organic vapour phase epitaxy on Ge substrates?(1?0?0) with 6? offset towards ?1?1?1??under various growth conditions. Synchrotron x-ray topography is employed to investigate the nature of extended linear defects formed in GaAs epilayers. Other x-ray techniques, such as reciprocal space mapping and triple axis ?-scans of (0?0?l)-reflections (l = 2, 4, 6), are used to quantify the degree of relaxation and presence of antiphase domains (APDs) in the GaAs crystals. The surface roughness is found to be closely related to the size of APDs formed at the GaAs/Ge heterointerface, as confirmed by x-ray diffraction (XRD), as well as atomic force microscopy and transmission electron microscopy.

Comprehensive investigation of Ge–Si bonded interfaces using oxygen radical activation

In this work, we investigate the directly bonded germanium-silicon interfaces to facilitate the development of high quality germanium silicon hetero integration at the wafer scale. X-ray photoelectron spectroscopy data is presented which provides the chemical composition of the germanium surfaces as a function of the hydrophilic bonding reaction at the interface. The bonding process induced long range deformation is detected by synchrotron x-ray topography. The hetero-interface is characterized by measuring forward and reverse current, and by high resolution transmission electron microscopy.

V2O5 as an inexpensive counter electrode for dye sensitized solar cells

In pursuit of an abundant, inexpensive and stable counter electrode as an alternative to platinum for dye-sensitized solar cells (DSSCs), we report a new, low-cost substitute material. Here for the first time, we demonstrate that V2O5 can be used as a counter electrode material in DSSCs. We note that the efficiency of DSSCs with commercial V2O5 and hydrothermal treated V2O5 are upto 1.2% and 1.6%, respectively. The results indicate that, with optimization, V2O5 can be a promising choice to replace platinum from a cost perspective. The innovation of new economical counter electrodes offers a potential way to cut down the industrial costs which is crucial for large-scale production and commercial applications of DSSCs.