Search

Despite more than seven decades of active research and development in thermoelectricity, the accurate measurement of the thermoelectric (TE) properties of bulk materials has remained a challenge, mainly because of the strong interrelation between thermal and electrical phenomena. This work highlights practical advancements in methods and instrumentation dedicated to the simultaneous measurements of TE properties such as the Seebeck coefficient (S), the thermal (κ), and electrical (σ) conductivities and the dimensionless TE figure of merit ZT = S2σT/κ. The accuracy of a Harman based approach, as implemented by the ZT-Scanner (TEMTE Inc.), applicable to the simultaneous measurement of the above TE properties, has been made possible by a self-contained calibration procedure, which is based on the availability of two samples of the same homogeneous material having different shape factors. It is of practical importance that this approach provides a simple procedure to obtain the calibration for the figure of merit ZT and the thermal conductivity in the temperature interval from 300 to 720 K. In addition, we show that a simplified Harman setup with no thermocouples attached to the sample can also be used for self-contained calibrated ZT measurements. It is concluded that the implemented steady-state approach decreases the relative error down to 1%–2% for ZT measurements and can be recommended for most applications not involving dynamical behavior. In particular, it is proposed that self-generated calibration samples can critically increase the quality and ease of comparison of TE measurements if they are adopted by the TE community. [ABSTRACT FROM AUTHOR]

The effect of strain on controlled optical modes of strained InAs/InP short-period strained-layer super lattices is studied using Raman scattering and x-ray diffraction of InAs/InP. Both InAs and InP layers have longitudinal-optical (LO) confined modes. Competition between strain and confined LO phonon frequencies of individual layers is indicated by scattering of confined phonons. The strain in each component layer is determined by the confined effect.

The metal organic vapor deposition of MnP films on GaP (100) substrates is shown to have a substantial endotaxial component. A study of the growth time evolution of the endotaxial depths of MnP grains reveals a diffusion-controlled growth with a relatively large diffusion coefficient of Mn in GaP. The value (2.2±1.5)×10-15 (cm2/s) obtained at 650 °C is at least two orders of magnitude larger than the reported Mn diffusion in bulk GaP. GaP surface mounds provide further indirect evidence that this large diffusion coefficient is concurrent with the out-diffusion of Ga atoms at the growing MnP/GaP interface. No trace of dislocations could be observed at or near this interface, which strongly suggests that Mn diffusion occurs through vacant sites generated by the difference between the crystallographic structures of MnP and GaP. [ABSTRACT FROM AUTHOR]

MnP nanostructures embedded in GaP epilayers, and MnP polycrystalline films, grown from the vapor phase on GaP(001) substrates using metalorganic precursors are compared with bulk MnP. We observe a large increase of the low transition temperature from the ferromagnetic to the antiferromagnetic screw phase, from TN = 47 K for bulk to 82 K for nanocrystals in MnP:GaP films, while the Curie temperature TC, close to room temperature, varies only slightly. A net magnetic moment is measured in the nanocrystals and films at 5 K, as well as large coercive fields, contrary to bulk MnP. X-ray absorption spectroscopy and diffraction show that epilayers and films contain MnP grains in the nanometric range with average Mn-P bond lengths very close to those of bulk MnP. The MnP film lattice parameters are almost identical to bulk values (within 0.5%) and the main crystallographic preferential orientations are those also present in the epilayers but with different relative populations. Overall the local structures of all MnP forms are very similar, except for indications of more disorder in the nanocrystals. Such combined changes of TN and TC are in apparent contradiction with the known response of bulk MnP to strains induced by hydrostatic, uniaxial or chemical pressure. We conclude that the differences in the low temperature magnetic behavior are most probably originated by local structural disorder at the surface of the nanostructures and by finite size effects. [ABSTRACT FROM AUTHOR]

Hybrid ferromagnetic-semiconductor GaP:MnP thin films were grown at 650 °C by metal-organic vapor phase epitaxy on GaP(001) using trimethylgallium, tertiarybutylphosphine, and methyl cyclopentadienyl manganese tricarbonyl (MCTMn). Overall Mn concentrations in the hybrid films, determined by Rutherford backscattering spectrometry, were found to be nearly proportional to the MCTMn precursor gas flow rate and ranged from 2 to 3.5 at. %. Cross-sectional transmission electron microscopy (TEM) analyses revealed the presence of a homogeneous distribution of 15–30 nm wide nanoclusters in a dislocation-free GaP matrix that is fully coherent with the substrate. The nanocluster facets are predominantly aligned along the (220) planes of the GaP matrix and selected-area electron diffraction patterns in TEM indicate that the nanoclusters are coherent (or semicoherent) with the single-crystal GaP matrix. The Mn:P composition ratio in the nanoclusters was determined to be 1.00±0.05 from parallel electron energy loss spectroscopy analyses. Increasing the MCTMn flow rate during film growth resulted in an increased concentration of MnP nanoclusters in the epilayer while their dimensions remained virtually unchanged. Magnetometric characterization indicates a ferromagnetic order, with a Curie temperature of about 294 K, originating from the MnP clusters. [ABSTRACT FROM AUTHOR]

We have measured the absorption coefficient at the fundamental band edge in Ga1-yInyAs1-xNx and GaAs1-xNx alloys. In contrast to what is observed in most III-V semiconductors, the absorption coefficient of Ga(In)AsN increases when the band gap energy decreases. In the Ga1-yInyAs1-xNx alloys, the absorption coefficient at threshold saturates near x=0.009, whereas the absorption coefficient of the GaAs1-xNx layers shows no sign of saturation for 01-yInyAs1-xNx layers than for indium-free GaAs1-xN films with the same nitrogen content, a result opposite to what is observed for the nitrogen-free GaInAs alloy. The comparison of our results with calculations performed using the k·p formalism suggests that the proper description of the optical properties of Ga(In)AsN alloys must go beyond the effective mass approximation. [ABSTRACT FROM AUTHOR]

We have investigated the optical emission from GaAs1-xNx epilayers with 4.3×10-4

High-resolution Brillouin spectroscopy experiments were carried out to determine the velocity of surface acoustic waves (SAW) as a function of composition in GaAs1-xNx/GaAs(100) epilayers (0

We establish, using a tetrahedral model, the bonding properties of amorphous silicon oxynitride (a-SiOxNy) films deposited at low temperatures (LTs) by electron-cyclotron resonance chemical-vapor deposition (ECRCVD) on several substrates and under various conditions of gas flows and total gas pressure in a dilute mixture of SiH4+N2 in Ar. The atomic percentage of each tetrahedral unit incorporated in the film network is calculated from the deconvolution of the high-resolution x-ray photoelectron spectroscopy (XPS) spectra in the Si 2p3/2 region and corroborated by the results obtained from both survey scans and the high-resolution XPS spectra in the N 1s region. The Si3N4 phase is the most important one and the only bonding unit which is incorporated in all our LT ECRCVD SiOxNy films. The incorporation of all the other component tetrahedrons depends strongly on growth conditions. The threshold values of the N/Si atomic ratio for which intrinsic defects, such as Si–Si bonds, are not incorporated in the network depend on the O/Si ratio incorporated in the films, mainly due to the competition between oxygen and nitrogen atoms in their reaction with silicon dangling bonds. The effect of the total gas pressure on the atomic percentages of the oxidation states present in the LT ECRCVD SiOxNy films is qualitatively similar to the effect of the ion bombarding energy or the plasma density. O–N bonds are present only in samples having high amount of oxygen and nitrogen in their networks. For these films, our results show unambiguously the presence of the N–Si2O tetrahedron and suggest that N–Si3-νOν tetrahedrons with ν>=2 are not incorporated in their networks. A correlation is observed between the N–Si2O and the Si–O3(ON) tetrahedrons whose component peak is localized at (104.0±0.2) eV in the Si 2p3/2 region of the XPS data, which suggests that both bonding units coexist in these films as some sort of complex bonding configuration. [ABSTRACT FROM AUTHOR]

A tetrahedral model is presented to explain the bonding properties of nonstoichiometric amorphous silicon oxynitride (a-SiOxNy) alloys, grown under highly nonequilibrium conditions, whose structures obey neither the random bonding model nor the random mixture model. Based on our approach, a numerical procedure is proposed to obtain the relative atomic percentages of each component structural phase from the deconvolution of the high-resolution x-ray photoelectron spectroscopy (XPS) spectra in the Si 2p3/2 region. The tetrahedral model is then used to study the bonding properties of a-SiOxNy films grown by electron-cyclotron resonance plasma-enhanced chemical-vapor deposition, having relatively low values of the O/Si atomic ratio (<=0.37) incorporated in their networks. The experimental results show that five tetrahedral phases (tetrahedrons Si–Si4, Si–Si2ON, Si–N4, Si–Si3O, and Si–O4) are present in a-SiOxNy films with low N/Si atomic ratios (<=0.93), while only three phases (Si–SiON2, Si–N4, and Si–O2N2) are present in samples with higher N/Si atomic ratios (>=1.12). The Si3N4 phase is the most important bonding unit and it is the only phase present in all our samples. These results are corroborated by survey scans and by comparison with the high-resolution XPS spectra in the N 1s region. They support the validity of the model proposed for a-SiOxNy alloys and the XPS analysis, correlated with growth conditions, presented in this work. [ABSTRACT FROM AUTHOR]

A concept for the implementation of a compliant substrate using a buried layer of nanocavities is presented. The purpose of this nanocavity layer is to mechanically decouple a thin substrate layer from the rest of the substrate in order to relax stress in mismatched epilayers. The nanocavities were created by helium implantation in InP(001) followed by thermal annealing under a phosphorous rich atmosphere. Metalorganic vapor phase epitaxy of InAsP/InP heterostructures grown simultaneously on substrates with nanocavities and on conventional substrates were characterized by high-resolution x-ray diffraction, transmission electron microscopy, and optical absorption. It is found that strain relaxation is enhanced for heterostructures grown on substrates with nanocavities and that the dislocations propagate partly in the compliant layer instead of the epilayer. The critical thickness of heterostructures grown on conventional substrates is roughly double that of structures grown on substrates containing nanocavities. [ABSTRACT FROM AUTHOR]

The formation of nanocavities in InP(001) by room-temperature He implantation and subsequent thermal annealing was studied using a combination of high-resolution x-ray diffraction (HRXRD) and cross-sectional transmission electron microscopy (XTEM) analyses. The nanocavities size and depth distributions were measured as a function of He ion dose [lowercase_phi_synonym][sub He] (1×10[sup 16] to 9×10[sup 16] cm[sup -2]) and ion energy E (25 to 70 keV), as well as annealing temperature T[sub a] (600 to 750 °C) and time t[sub a] (5 to 25 min). HRXRD scans from annealed samples indicate an expansion of the InP lattice, contrary to what is usually observed following heavy-ion implantation. The critical [lowercase_phi_synonym][sub He] and T[sub a] values for the formation of nanocavities were found by XTEM analysis to be between 1 and 2×10[sup 16] cm[sup -2] and between 600 and 620 °C, respectively. Cavities of diameter 4–50 nm with {110}, {101}, and {001} facets were obtained. Increasing T[sub a] and t[sub a] resulted in larger cavities and increasing [lowercase_phi_synonym][sub He] produced a larger number of cavities. Furthermore we find that nanocavities are metastable as their size first increases with annealing temperature and time but then decreases until they disappear for t[sub a]>25 min at T[sub a]=640 °C or t[sub a]>10 min at T[sub a]=750 °C. Results are compared with similar work carried out on He-implanted silicon and differences between the two materials are explained in terms of defect diffusivity and surface energy, higher diffusivity enhancing cavity collapse and lower surface energy enhancing cavity growth. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We present the electrical transport properties of thermoelectric n-type Bi[sub 2](Te[sub 1-χ] Se[sub χ])[sub 3] (χ ≤ 0.125) polycrystalline alloys obtained by mechanical alloying and extrusion. It was determined that the charge carrier mobility decreases in the temperature range 90-300 K as the content (χ) of Bi[sub 2]Se[sub 3] is increased. The observed decrease is larger than what could be predicted by the introduction of an alloy scattering mechanism due to the electronegativity difference between Se and Te atoms. It can be explained by the introduction of additional defects induced by the Se incorporation, whose concentration is reduced by thermal annealing following extrusion. In the extrinsic regime, the observed temperature dependence of the mobility [µ = µ[sub 0](T/T[sub 0])[sup r]] of as extruded alloys apparently indicates that charge carrier scattering is mostly limited by acoustic phonons (temperature coefficient r near -1.5). Annealing after extrusion changes the temperature coefficient to values near -1.7 which is closer to what has been reported for pure crystalline Bi[sub 2]Te[sub 3]. [ABSTRACT FROM AUTHOR]

We establish the role of oxygen atoms on the structural, chemical, and mechanical properties of SiO[sub X]N[sub Y] films grown on Si and InP substrates by electron-cyclotron resonance chemical-vapor deposition (ECR CVD) using a diluted SiH[sub 4] and N[sub 2] mixture in Ar, under controlled conditions. The mechanical and chemical properties of ECR-CVD SiN[sub X] films depend on the oxygen contamination even when this element is present in low concentrations. The compressive stress of SiN[sub X] films deposited with a low (and constant) content of oxygen (less than 12%) is shown to be in qualitative agreement with a model of repulsive Coulomb forces related mainly to polar [sup -]N-H[sup +] units in the SiN[sub X] network. We observe a decrease of the film compressive stress when the N[sub 2]/SiH[sub 4] flow ratio increases, which is due to the increase of Si-N bonds in detriment of N-H bonds. Films deposited with high oxygen content in the plasma show a decrease of nitrogen incorporation. Oxygen radicals species compete with those of nitrogen in their reaction with silicon dangling bonds, which has as a consequence a decrease in the incorporation of nitrogen. Additional creation of oxygen radicals, with no hydrogen dilution, is more effective in decreasing the number of N-H bonds, or the compressive stress in the SiN[sub X] films, than the corresponding creation of nitrogen radicals. The mechanical properties of SiN[sub X] films contaminated with oxygen are controlled, in general, by the total number of both nitrogen plus oxygen atoms relative to silicon. The buffered HF (BHF) film etch rate is enhanced and thus is mainly controlled by the oxygen content. Low values of the compressive stress do not necessarily imply low values of BHF etch rate or a high N/Si ratio. We also present a discussion of the origin of the unintentional incorporation of oxygen in a ECR-CVD system designed for industrial production. [ABSTRACT FROM AUTHOR]

Fully-coherent Si[sub 0.7]Ge[sub 0.3] layers were deposited on Si(001) by gas-source molecular beam epitaxy (GS-MBE) from Ge[sub 2]H[sub 6]/Si[sub 2]H[sub 6] mixtures in order to probe the effect of steady-state hydrogen coverages θ[sub H] on surface morphological evolution during the growth of compressively strained films. The layers are grown as a function of thickness t at temperatures, T[sub s]=450–550 °C, for which strain-induced roughening is observed during solid-source MBE (SS-MBE) and deposition from hyperthermal beams. With GS-MBE, we obtain three-dimensional (3D) strain-induced growth mounds in samples deposited at T[sub s]=550 °C for which θ[sub H] is small, 0.11 monolayer (ML). However, mound formation is dramatically suppressed at 500 °C (θ[sub H]=0.26 ML) and completely eliminated at 450 °C (θ[sub H]=0.52 ML). We attribute these large differences in surface morphological evolution primarily to θ[sub H](T[sub s])-induced effects on film growth rates R, adatom diffusion rates D[sub s], and ascending step-crossing probabilities. GS-MBE Si[sub 0.7]Ge[sub 0.3](001) growth at 450 °C remains two dimensional, with a surface width <0.15 nm, at all film thicknesses t=11–80 nm, since both R and the rate of mass transport across ascending steps are low. Raising T[sub s] to 500 °C increases R faster than D[sub s] leading to shorter mean surface diffusion lengths and the formation of extremely shallow, rounded growth mounds for which remains essentially constant at =0.2 nm while the in-plane coherence length increases from =70 nm at t=14 nm to 162 nm with t=75 nm. The low ascending step crossing probability at 500 °C results in mounds that spread laterally, rather than vertically, due to preferential attachment at the mound edges. At T[sub s]=550 °C, the ascending step crossing probability increases due to both higher thermal activation and lower hydrogen coverages. ... [ABSTRACT FROM AUTHOR]

Research in copper selenide thermoelectric (TE) alloys has raised the possibility of a significant enhancement of the TE figure-of-merit ZT when the Seebeck coefficient is affected by a concurrent phase transformation. This ZT increase has also been related to a radical reduction of the thermal conductivity evaluated by transient laser flash thermal diffusivity measurements. In contrast, steady-state Harman-based measurements do not support a significant ZT increase only a modest one, because the thermal conductivity instead of decreasing goes through a sharp maximum as it approaches the critical phase transformation temperature of 407 K. The nature of this sharp increase of heat transfer has not been related to the well-known electronic or phononic contributions. Below the critical temperature, when the alloy is exposed to a steady-state temperature gradient, an additional heat transfer phenomenon takes place, induced by the ongoing gradual phase transition. We show that the enthalpy associated to the α to β and β to α phase transformations can lead to heat flow in the direction of the temperature gradient above and beyond conventional heat conduction. This unconventional heat transfer mechanism disappears when the temperature rises above the critical temperature where only a stable β phase remains. We propose a model of such a heat transport which leads to the sharp maximum of the related thermal conductivity. Numerical results obtained from the model compare favorably to the experimentally measured thermal conductivity. [ABSTRACT FROM AUTHOR]

Examines the temperature dependence of photoluminescence spectra of InAsP/indium phosphide strained multiple quantum wells. Experimental details; Calculation results; Conclusions.

Reports the growth and structural properties of InP, indium arsenide (InAs)/InP strained single quantum wells and short-period InAs/InP strained layer superlattices by atomic layer epitaxy. Significance of high resolution x-ray diffraction; Growth rate of InP for different growth conditions; Biaxial in-plane strain in a fully strained InAs layer.

Discusses a study which detailed the reverse I-V and C-V characteristics of Schottky barrier type diodes on zinc doped InP epilayers grown by metalorganic vapor phase epitaxy. Properties that were fabricated by the evaporation of gold onto a zinc doped epitaxial layer of InP; Role played by the interface layer thermionic emission theory; Devices in which InP is considered a promising material.

Focuses on a study which investigated the structural and optoelectronic properties of thick gallium indium phosphide epilayers on sulfur-doped indium phosphide substrates. Experimental procedures; Discussion on x-ray diffraction of the epilayers; Optical absorption and photoluminescence of indium phosphides.

Investigates the effect of small changes in the substrate lattice constant on the photoluminescence (PL) properties of high-purity nominally undoped InP epilayers. Effect of high concentrations of substitutional dopants; Comparison of low-temperature exciton PL spectra for two high-purity epilayers during the same growth run but on different semi-insulating substrates; Assessment of substrate lattice parameter variation due to substrate dopants.

Studies the effect of doping concentration on the characteristic parameters of surface states in thick epitaxial layers in In[sub0.21]Ga[sub0.79]As doped with zinc and grown by metalorganic vapor phase epitaxy. Potential applications of ternary strained-layer quantum wells; Description of the epitaxial layers used in the experiment; Analysis of capacitance-frequency measurements.

Examines the defect energy levels in metalorganic chemical vapor deposition grown gallium-indium-phosphorus/indium-phosphorus:iron and gallium-indium-phosphorus/indium-phosphorus:sulfur epilayers by photoluminescence (PL) and photoconductivity measurements. Origin of the observed deep levels; Temperature dependence of the intensity and half-width of the dominant deep-level PL peaks; Difference of the dominant deep-level peaks of the samples.

Focuses on a study which examined the growth and structural properties of epitaxial gallium-indium-phosphorus on indium phosphide. Methodology of the study; Findings of the high-resolution x-ray diffraction; Results of photoluminescence measurements; Comparison between the calculated value and prediction from elasticity theory.