R. A. Masut

Growth and characterization of InP on silicon by MOCVD

Abstract The heteroepitaxy of InP on Si (8% mismatch) is investigated using low pressure MOCVD. The two-step growth method is employed to grow InP directly on Si substrates. The effects of growth temperature, V/III ratio and thickness of the first layer on the quality of the second layer are examined. By optimising the growth conditions the full width at half maximum (FWHM) of the (004) X-ray rocking curve for the 2 μm thick layer is reduced to less than 450 arc sec, and the FWHM of the photoluminescence exciton peak at 8 K is 4.4 meV; this indicates a good quality of InP grown directly on Si, comparable to layers obtained using GaAs or a superlattice as the intermediate layer. The tetragonal distortion of the InP unit cells of the layers grown on Si(001) was measured and the coexistence of weakly and strongly strained domains of the layers grown on Si(001) 3° off towards [110] was observed by high resolution X-ray diffraction.

Influence of Se on the electron mobility in extruded Bi2(Te1−xSex)3 (x⩽0.125) thermoelectric alloys

We present the electrical transport properties of thermoelectric n-type Bi2(Te1−xSex)3 (x⩽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 (x) of Bi2Se3 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=μ0(T/T0)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 fo...

Numerical Simulation of Performance and Thermomechanical Behavior of Thermoelectric Modules with Segmented Bismuth-Telluride-Based Legs

The approach of using segmented legs to build thermoelectric (TE) modules can enhance the performance of TE generators. This approach is based on the selection of materials for different segments that are optimized in terms of their TE properties with respect to the temperature range to which they are exposed during module operation. For this purpose, by carefully controlling the chemical composition of ternary and quaternary bismuth-telluride-based alloys, we have optimized the figure of merit ZT of p-type and n-type alloys implemented by a powder technology approach. The alloys were prepared by mechanical alloying followed by hot extrusion, and their mechanical and TE properties were fully characterized as a function of temperature, which gave us a solid database for simulation of modules containing these materials. Finite-element numerical simulation was applied to evaluate the impact of TE materials properties on the level of mechanical stresses generated by thermal gradients in modules made of segmented legs. Keeping the same total length of two-segment p- and n-type legs, the relative length of each segment was varied to obtain an 8% relative increase of generated electrical power compared with homogeneous legs of the same total length. Under these conditions, the presence of solder interface between the two segments and between the segments and the copper conductors of the module concentrates plastic strain, leading to a significant reduction of the stress level in the TE materials compared with that resulting from using nonsegmented legs. Leg segmentation not only leads to improved TE performance but could also significantly modify the maximum values and distribution of thermomechanical stresses in the modules, depending on how it is realized. The study presents how this numerical simulation tool can be used to optimize the design of segmented modules.

Growth and characterization of InAs microclusters in InP and InAsP/InP heterostructures by low pressure MOCVD using tertiarybutylarsine

Abstract InAs microclusters embedded in an InP matrix and InAs x P 1– x /InP (0.1 ≤ x ≤0.3) strained multiple quantum wells (SMQW) have been prepared by LP-MOCVD using tertiarybutylarsine (TBAs) as a substitute for AsH 3 . The relatively large cracking efficiency of TBAs resulted in low As/In ratios (from 4.3 to 8.8) for InAs/InP structures and low As/P ratios (from 1×10 -3 to 3.5×10 -3 for InAs x P 1– x /InP (0.1 ≤ x ≤0.3). High-resolution X-ray diffraction (HRXRD) and photoluminescence (PL) show that the TBAs-grown material is of high quality. The HRXRD patterns of InAs microclusters embedded in the InP matrix demonstrate that the InAs unit cell is tetragonally distorted in accordance with the elasticity theory. For InAs microclusters embedded in the InP matrix, the 7 K emission PL peak shifts from 1.35 to 1.39 eV when the InAs coverage is reduced from 0.8 to 0.2 monolayers, a clear indication of a lateral size effect. Picosecond time resolved PL shows a weak coupling of excitons confined by these InAs microclusters. We report also the results of a study of the interface roughness of InAs x P 1– x /InP heterostructures. HRXRD, PL, absorption and photoconductivity (PC) measurements for InAsP/InP SMQW reveal that the InAsP/InP interface is very sensitive to growth procedure. For nonoptimal growth procedures a large density of interface states is created, probably as a consequence of compositional modifications within the interface region. The optical measurements clearly resolve the fundamental intersubband transitions in the InAsP/InP SMQW, including heavy hole-light splittings, even at room temperature.

Persistent room‐temperature relaxation of InP amorphized and compacted by MeV ion beams

Ion beam induced deformation and compaction has been observed in InP, amorphized by MeV Se ion implantation. The initial density of amorphous InP is 0.55%±0.05% larger than that of crystalline InP. During a period of two months, most of the excess density is lost in a spontaneous, room‐temperature relaxation. This relaxation can be described by two time constants: τ1≊8±2 h and τ2≊14±1 days.

Interfaces of InAs/InP multiple quantum wells grown by metalorganic vapor phase epitaxy

We present results of the growth of InAsxP1−x/InP strained heterostructures by low pressure metalorganic vapor phase epitaxy. A large incorporation of arsenic into the InAsP ternary was observed using tertiarylbutylarsine as precursor. High resolution x-ray diffraction, photoluminescence, and optical absorption measurements for InAsP/InP strained multiple quantum wells reveal that the InAsP/InP interface is very sensitive to growth interruption. A systematic study of a growth in terruption sequence designed to improve the InAs/InP interface was carried out. For nonoptimal growth interruption procedures a large density of interface states is created, probably as a consequence of compositional modifications within the interface region. We find that the absorption spectrum may reveal a significant density of interface states. Thus, photoluminescence on its own is insufficient to characterize the interface roughness even for structures showing narrow low-temperature photoluminescence peaks. We also observe an enhancement of the As content for structures grown on InP (001) relative to those simultaneously grown on InP(001) two degrees off toward [100], which suggests that the composition of As in the ternary is limited by its surface diffusion.

Hydrogen-mediated quenching of strain-induced surface roughening during gas-source molecular beam epitaxy of fully-coherent Si0.7Ge0.3 layers on Si(001)

Fully-coherent Si0.7Ge0.3 layers were deposited on Si(001) by gas-source molecular beam epitaxy (GS-MBE) from Ge2H6/Si2H6 mixtures in order to probe the effect of steady-state hydrogen coverages θH on surface morphological evolution during the growth of compressively strained films. The layers are grown as a function of thickness t at temperatures, Ts=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 Ts=550 °C for which θH is small, 0.11 monolayer (ML). However, mound formation is dramatically suppressed at 500 °C (θH=0.26 ML) and completely eliminated at 450 °C (θH=0.52 ML). We attribute these large differences in surface morphological evolution primarily to θH(Ts)-induced effects on film growth rates R, adatom diffusion rates Ds, and ascending step-crossing probabilities. GS-MBE Si0.7Ge0.3(001) growth at 450 °C remains two d...

Effect of strain on confined optic phonons of highly strained InAs/InP superlattices

We have measured Raman scattering and high‐resolution x‐ray diffraction from highly strained [(InAs)4(InP)4]N short‐period superlattices grown on InP substrates by atomic layer epitaxy at 355 °C. The InAs and InP confined phonons are observed in these highly strained short‐period superlattices. The energy of the InAs confined longitudinal‐optical phonon (LO) modes of a fully strained superlattice (with N=8) is blue shifted by about 10 cm−1 compared to the LO phonon of bulk InAs. This effect is explained by the large biaxial strain existing in the InAs layers. The observed frequency shift agrees with the lattice‐mismatch strain given by elasticity theory and independently measured by high‐resolution x‐ray diffraction. No evidence of a frequency shift of the InP confined LO modes in the N=8 fully strained superlattice is observed, indicating that the strain is confined to the InAs layers. We show that in a partially relaxed superlattice (with N=20), the InAs layers are in compression, while the InP layers a...

MnP nanoclusters embedded in GaP epitaxial films grown by organometallic vapor-phase epitaxy: A reciprocal space mapping and transmission electron microscopy study

Full three dimensional x-ray diffraction reciprocal space maps combined with transmission electron microscopy measurements provide a systematic determination of the texture of GaP epilayers containing embedded MnP nanoclusters grown on GaP(001) by organometallic vapor phase epitaxy. This approach reveals that the texture of the MnP clusters depends on the growth surface morphology and bonding configuration and on the lattice mismatch at the cluster/matrix interfaces during growth. It demonstrates that the orthorhombic MnP nanoclusters are oriented along specific GaP crystallographic directions, forming six well defined families, whose population is influenced by the growth temperature and the film thickness. The clusters principally grow on GaP(001) and GaP{111} facets with a small fraction of clusters nucleating on higher-index GaP{hhl} facets. Most epitaxial alignments share a similar component: the MnP(001) plane (c-axis plane) is parallel to the GaP{110} plane family. Axiotaxial ordering between the MnP clusters and the GaP matrix is also observed. Furthermore, with this systematic approach, all phases present in these heterogeneous films can be identified. In particular, traces of hexagonal Mn2P precipitates have been observed while their formation can be avoided by lowering the growth temperature. Comparing the structural results presented here with magnetic measurement carried out on similar samples confirms that the effective magnetic properties of the heterogeneous layer can be tuned by controlling the texture of the ferromagnetic nanoclusters.

Evidence of valence band perturbations in GaAsN/GaAs(001): Combined variable-angle spectroscopic ellipsometry and modulated photoreflectance investigation

The contribution of the fundamental gap