Search Results (23 total)

We study the vibrational spectrum of AlN grown on Si(111). The AlN was deposited using gas-source molecular beam epitaxy. Raman backscattering along the growth c axis and from a cleaved surface perpendicular to the wurtzite c direction allows us to determine the E₂¹, E₂², A₁(TO), A₁(LO), and E₁(TO) phonon energies. For a 0.8-μm-thick AlN layer under a biaxial tensile stress of 0.6 GPa, these are 249.0, 653.6, 607.3, 884.5, and 666.5 cm^-1, respectively. By combining the Raman and x-ray diffraction studies, the Raman stress factor of AlN is found to be -6.3±1.4 cm^-1/GPa for the E₂² phonon. This factor depends on published values of the elastic constants of AlN, as discussed in the text. The zero-stress E₂² energy is determined to be 657.4±0.2 cm^-1. Fourier-transform infrared reflectance and absorption techniques allow us to measure the E₁(TO) and A₁(LO) phonon energies. The film thickness (from 0.06 to 1.0 μm) results in great differences in the reflectance spectra, which are well described by a model using damped Lorentzian oscillators taking into account the crystal anisotropy and the film thickness.

We report a Raman-scattering and infrared (IR) reflectivity investigation of chalcopyrite (ordered) and sphalerite (disordered) ZnSnP₂ grown by gas-source molecular-beam epitaxy on (001) GaAs substrates. Variation in the ZnSnP₂ structure was obtained by controlling the Sn/Zn flux ratio during growth. Experimental results are compared with extensive calculations of the vibrational structure for the ordered ZnSnP₂. The rigid-ion model is used to obtain lattice-dynamical properties, and the bond polarizability model was used for calculation of Raman tensor components. The model predicts a strong dependence of Raman intensity of Γ₁ and Γ₃ modes on tetrahedral distortion of chalcopyrite structure. For the ordered ZnSnP₂ films (≈-0.01% lattice mismatch with the substrate) we observed several narrow (2–5 cm^-1) Raman and IR lines in the 290–320 cm^-1 range. Using Raman and IR selection rules for c orientation of the growth axis and lattice-dynamical calculations, we identify Γ₁, Γ₃, 2Γ_4L, and 3Γ_5T,L modes. For the disordered sphalerite ZnSnP₂ films (≈+0.1% lattice mismatch with the substrate) we observed much broader features (10–25 cm^-1) consisting of two IR-active polar Γ₁₅ modes and four nonpolar Γ₁ modes. We assign the nonpolar modes to allowed (Γ₁,Γ₃) and forbidden (2Γ₂) vibrations of chalcopyrite-type phase (nanocrystals) in a disordered matrix. Using spatial correlation model the average nanocrystal size is estimated to be ≈70 nm. The spectra of the films of intermediate lattice mismatch are well described by a superposition of the spectra of the ordered and disordered ZnSnP₂.

We report the results of a pressure study of strained In_xGa_1-xAs/GaAs thin single quantum wells (QW’s) grown along [100] and [111] directions. The 〈100〉-grown QW exhibits the expected two-slope behavior with a pressure coefficient of ∂E/∂P=10.3±0.1 meV/kbar for the direct emission. The observation of a type-II crossover yields a valence-band offset for the [100] of 0.060 eV. In contrast, the 〈111〉-grown quantum well exhibits an unusual three-slope pressure behavior. We find a pressure coefficient of ∂E₁₁₁/∂P=10.1±0.3 meV/kbar for the direct emission. Excitation intensity studies assist in the identification of above crossover emissions which imply a large valence-band discontinuity for the 〈111〉 QW of ≈100 meV.

We have used excitation intensity-dependent photoluminescence (PL) spectroscopy to study strained In_xGa_1-xAs/GaAs (x≈0.23) single quantum wells grown along the 〈001〉 and 〈111〉A directions. PL intensities from these two quantum-well samples exhibit a linear dependence on incident power density over the excitation intensity range examined. The linewidth of the emission from the 〈100〉 well is well described by band-filling effects as I_exc is increased. By careful analysis of the excitation intensity dependence of the 〈111〉 linewidth and the corresponding blueshift of the PL peak energy, the strain-induced piezoelectric field is estimated to be 70 kV/cm.

We report a Raman study of the effects pressure has on the vibrational structure of Ge_nSi_m multiple quantum wells (MQW’s) with n<~4 and m<~7. Three primary phonon bands are studied: Ge-Ge within the germanium layers, Si-Si within the silicon layers, and the Ge-Si interface mode. Pressure shifts each of these bands consistent with a mode-Grüneisen constant of unity for all samples and laser excitations used. We observe resonance effects with the confined Ge-like E₁ transition for the Ge₄Si₅ sample. The transition is near 2.4 eV at ambient pressure and blueshifts at ≈4±1 meV/kbar. This pressure coefficient is smaller than the corresponding quantity in bulk germanium. This is attributed to the fact that silicon dictates the in-plane contraction of the Ge layer that is at a smaller rate than the corresponding quantity in bulk germanium. We see no evidence of resonance enhancement in samples with thinner Ge layers in each MQW period. This implies that at least four Ge atoms are necessary to form the states producing the E₁ transition, consistent with previous studies. An additional feature seen in the spectra near 310 cm^-1 is identified by the pressure study to be 2TA Raman scattering from silicon.

We have studied a series of sharp photoluminescence emission lines between 1.65 and 1.80 eV in synthetic diamond films. The series of lines is decomposed into a set of parent lines plus vibrational sidebands spaced 24 meV apart. This energy does not correspond to any phonons with high density of states in the diamond crystal. The relative intensities of the main lines exhibit no temperature dependence (between 20 and 160 K), implying ground-state splitting. The narrow linewidth and temperature-independent emission energy imply only weak interaction with the diamond host. The temperature-dependence of the linewidth is well described by thermal broadening. We attribute the emission to optical centers as a consequence of tungsten incorporation into the diamond film. The tungsten originates from the electrode during deposition. © 1996 The American Physical Society.

We have conducted an extensive Raman scattering study of the effects hydrostatic pressure has on Al_xGa_1-xAs alloy phonons for 0⩽x⩽0.70. The mode-Grüneisen parameter γ is found to depend on x. The variation is monotonic in dilution and increases by 30% over the range of x studied. We find that γ for GaAs-like and AlAs-like longitudinal-optic (LO) phonons correlates with Born’s transverse dynamic effective charge on the respective alloy component. We suggest that this phenomenon is specific to alloys, and interpret it as a consequence of charge transfer on the cation sublattice. Pressure induced resonance Raman scattering is examined for x=0.40. We observe strong enhancement for both LO phonons when in resonance with the direct energy gap. © 1996 The American Physical Society.

We report results of a detailed temperature dependence study of photoluminescence lifetime and continuous emission properties in silicon-doped GaAs. The primary focus is on a defect-related emission at 1.269 eV (T=20 K). GaAs crystals were grown using molecular-beam epitaxy with most of the experiments conducted on a sample having a carrier concentration of 4.9×10¹⁸ cm^-3. The intensity is seen to decrease above 100 K, with no corresponding decrease in the measured lifetime of 9.63±0.25 ns. The intensity decrease implies an activation energy of 19±2 meV, which is approximately one order of magnitude smaller than what was previously obtained for similar defects in Czochralski-grown GaAs with other dopants. We interpret our results in terms of a configuration coordinate model and obtain a more complete picture of the energy-level structure. The experiments indicate that the upper level in the recombination process is about 20 meV below the conduction-band continuum, with the lower state approximately 300 meV above the valence band. Our results are consistent with the identification of the corresponding defect complex microstructure as being a siliconat-gallium substitution, weakly interacting with a gallium vacancy second-nearest neighbor, known as the Si-Y defect complex.

We have measured the temperature-dependent photoluminescence from the [Cr(en)₃]³⁺ ion intercalated layered aluminosilicate compounds. The temperature dependence of the lifetime and the intensity of the ²E→⁴A₂ transition have been consistently interpreted in terms of a three-level model that involves ⁴T₂, ²E, and ⁴A₂ states of the [Cr(en)₃]³⁺ ion. We show that the observed temperature-dependences are largely due to variations in the nonradiative transition processes. The observed composition-dependent line-shape broadening from the intercalation of a solid solution of [Cr(en)₃]³⁺ and [Co(en)₃]³⁺ ions has been attributed to the strain distribution in the host layers due to fluctuations in the gallery height.

We report cryogenic high-pressure measurements of a defect-related emission at 1.25 eV in silicon-doped GaAs. The pressure measurements prove that the 1.25-eV photon energy is relative to the conduction band, implying a deep defect level 0.30 eV above the valence band and an electron-capture process from the conduction band into the defect. The defect level moves up in the band gap at a rate of 23±3 meV/GPa. These results are consistent with a vacancy-related defect level, possibly stemming from a gallium-vacancy–silicon-at-gallium (second-nearest-neighbor) defect complex.

We have used hydrostatic pressure as a means for studying a resonant Raman mode observed at 47 cm^-1 in highly disordered, ion implanted, unannealed GaAs. The mode shifts weakly (-0.07±0.15 cm^-1/GPa), supporting an identification of this band-resonant vibration as stemming from the breathing mode of the gallium vacancies, which are expected to be in high concentration. We measure a pressure coefficient of the longitudinal-optic phonon in these (5.5 nm) nanocrystals of GaAs to be 3.6±0.1 cm^-1/GPa. The good agreement between our value and the pressure shift of this phonon in bulk GaAs implies that the bulk modulus is independent of size at least down to this size crystallite.

We have used Raman scattering to investigate the effects of intense laser pulses on the structure of resolidified graphite. Graphite was irradiated with 0.325–3.25-J/cm², 620-nm, 90-fs single-laser pulses causing it to melt and rapidly resolidify. Raman studies of the resolidified carbon in the crater show that the rapid annealing process (by pulses with energy fluences ≥0.82 J/cm²) causes a breakdown in the ordered layers of hexagonal carbon rings and disorder in the intraplanar spacing upon resolidification into a nanocrystalline material. The thickness of the nanocrystalline-graphite near-surface layer increases with increasing fluence. Residual planar structure of the resulting material is observed for the various pulse-energy values by comparing the narrow graphitic 1581-cm^-1 and the broad 1360-cm^-1 and 1600-cm^-1 vibrational bands. The interplanar structure of our nanocrystalline graphite is also studied quantitatively via the low-frequency shear mode at 42 cm^-1. The Raman spectrum of our glassy carbon is found to be well described by planar ordering approximately 2 to 3 layers in extent using a simple correlation function approach. Our results indicate a layered morphology is present in our nanocrystalline graphite, confirming a strong sp² bonding character.

We have conducted extensive high-pressure Raman-scattering studies in order to determine the effects of structure on the vibrational properties of layered aluminosilicate materials. The basal oxygen torsional mode is found to shift up with pressure at a rate that agrees reasonably with that expected from a simple van der Waals model. We identify a mode at 50 cm^-1 in margarite as the interlayer shear mode. The Raman band near 700 cm^-1 in all clays is linked to an oxygen-layer breathing mode. The pressure coefficient of the hydroxyl stretching mode near 3600 cm^-1 is found to have a structure-dependent sign; it is positive for trioctahedral structures and negative for dioctahedral silicates. This is found to be associated with hydroxyl orientation and Coulombic repulsion between the hydroxyl proton and the interlayer cation.

We report on hydrostatic-pressure Raman measurements (to 2 GPa) of a superlattice composed of pure GaAs and pure AlAs grown in the (012) direction. We observe Raman and resonance-Raman scattering, where the resonance is achieved by pressure tuning the direct superlattice transition to match the excitation photon energy. The GaAs confined longitudinal and transverse optic modes are seen to resonate via the Fröhlich electron-phonon interaction. This is possible for the transverse mode due to mixing with the GaAs longitudinal-optic vibrations. Growth in this direction permits the observation of folded quasitransverse-acoustic phonons. The observation of these modes in first-order Raman scattering is supported by the weak pressure softening at a relative rate of -0.025±0.005 GPa^-1, implying a mode-Grüneisen parameter of -1.9±0.4 for the transverse-acoustic phonon.

We have measured photoluminescence and Raman spectra of liquid-phase-epitaxy-grown Al_xGa_1-xAs under hydrostatic pressure (0<P<6 GPa) at low temperatures (T<10 K). The X-point exciton shifts with pressure to lower energies, for an aluminum concentration of x=0.70 at a rate of -16.2(5) meV/GPa, for x=0.92 at -16.5(5) meV/GPa. Taking into account the pressure dependence of the exciton binding energy, we determine the pressure shift of the indirect gap in these two alloys. For x=1 (pure AlAs), an extrapolation leads to a rate of -15.3(5) meV/GPa for the indirect gap. Using this value, data on the pressure shift of the valence-band offset in GaAs/AlAs superlattices are reexamined. This shift approaches a value of 4.7 meV/GPa for large superlattice periods (>20 monolayers).

We have conducted a high-pressure study of asymmetric-period GaAs/AlAs superlattices in order to observe the effects of inter-GaAs-well electronic coupling on the superlattice optical luminescence properties with varying AlAs layer thickness and approximately constant GaAs thickness (∼20 Å). When the AlAs layers are thick (60 Å), confinement shifts the GaAs Γ-like state well above the AlAs X-like state in the conduction band, producing a type-II band alignment. For decreasing AlAs thickness, the X-like state shows increasing confinement energy while the Γ-like state drops in energy due to wave-function delocalization across the barriers. Simultaneously the pressure coefficient of the GaAs-AlAs valence-band energy difference approaches zero for thinner barriers. We interpret this as evidence that the superlattice valence band loses its square-well nature in the thin-barrier limit.

We have measured low-temperature photoluminescence and photoluminescence excitation spectra of (GaAs)_m/(AlAs)_n symmetric superlattices (m=n) near the type-I-to-type-II crossover under high hydrostatic pressures. For an (m,n)=(15,15) superlattice we observe a type-I band structure which becomes type II at a pressure of ∼0.22 GPa. Based on pressure coefficients, the zero-pressure splitting is extrapolated as 27(4) meV. When (m,n)=(12,12), the superlattice is type II at zero pressure, showing two luminescence bands separated by 46(4) meV which linearly diverge in energy with increasing pressure. This implies the crossover occurs when 12<m,n<15. From intensity measurements on the (m,n)=(15,15) sample, the type-I recombination rate is estimated to be 1000 times slower than scattering into the lowest-energy X-like conduction-band state. We also find the valence-band offset to be dependent on pressure. These measurements then permit us to determine the pressure coefficients of all the lowest-energy confined states relative to the AlAs-like valence band. Near crossover we see no evidence of interaction between the type-I and type-II conduction-band states.

We have measured the pressure dependence (0–18 GPa) of folded-acoustic-phonon and confined-optic-phonon frequencies in thin-layer GaAs/AlAs superlattices using Raman scattering. The longitudinal-optic phonons of the superlattice increase in frequency with pressure at a rate comparable to that of zone-center optic phonons in bulk materials. For folded-longitudinal-acoustic (LA) phonons we find an increase in frequency, which is consistent with the expected bulk-material behavior under pressure. For superlattices with periods of 41.8 and 20.8 Å the mode Grüneisen parameters are 1.2±0.1 for the folded-LA and confined-optic phonons. These results are discussed relative to bulk and superlattice properties.

We have measured Raman spectra of the quasi-one-dimensional (1D) halogen-bridged mixed-valence gold complexes AuCl₂(DBS) and AuBr₂(DBS) (DBS, dibenzylsulfide) under hydrostatic pressure up to 7 GPa. The symmetric stretching vibration of the bridging halogens about the gold (III) ion (the Peierls mode) is found to decrease in frequency in the pressure range from zero to 3–5 GPa. This negative frequency shift is attributed to a reduction of the Peierls distortion under pressure. Results for the gold complexes are discussed in light of the high-pressure behavior of related 1D platinum complexes.

We report observations of resonant Raman scattering by confined-optical and interface-vibrational overtones and their combinations in thin-layer GaAs/AlAs superlattices. The superlattice electronic-band energies are brought into resonance with the excitation photon energy via pressure tuning of the band gap. Resonance is determined to be with E_1h, the lowest-energy Γ-like valence-to-conduction quantum-well subband (n=1 heavy hole), for outgoing scattering via the Fröhlich electron-phonon interaction. This technique permits measurement of the E_1h energy well above the pressure at which the superlattices become indirect-band-gap materials. Experimental results are compared with theoretical predictions for the phonon and energy-band symmetries for resonance due to Fröhlich electron–optic-phonon and electron-interface vibration scattering. The two-phonon resonance is seen to diminish in strength with decreasing superlattice period, despite the fact that the density of internal surfaces is increasing. This is attributed to weak broadening of the superlattice E_1h band gap.

We have observed a new, strong feature at 47 cm^-1 in the first-order Raman spectrum of ion-implanted GaAs, prior to any anneal. It is not present in the Raman spectrum of either amorphous or single-crystal GaAs. The peak is strong between excitation photon energies ∼1.5 and 2.2 eV. Above 2.2 eV it is masked by the Raman spectrum of the amorphous GaAs component of the mixed microcrystalline-amorphous system. Its frequency and line shape are not dependent upon implant species or energy. The photon-energy dependence of the intensity of the amorphous GaAs component of the Raman spectrum is found to be completely accounted for by the photon-energy dependence of the optical penetration depth over the full range studied (1.55–2.71 eV). This then serves as an internal intensity standard for our measurements, permitting us to separate scattering efficiencies from mere scattering volume effects. The longitudinal-optical phonon of the microcrystalline remnant resonates near the E₁ interband transition peak in the electronic density of states, consistent with a feature corresponding to a Raman-active crystal phonon. However, the new feature at 47 cm^-1 is observed to resonate strongly at an energy near the E₀ and E₀+Δ₀ electronic transition energies and not near E₁. We propose that this feature is an acoustic vibration of microcrystalline GaAs, observed via defect-assisted scattering between an electron or hole and the crystalline-amorphous interface regions characteristic of ion-implanted GaAs. The additional scattering breaks the k=0 infinite-crystal selection rule, and double-resonance effects result in intense scattering for phonons in special regions of the Brillouin zone. Electronic wave functions with sufficiently large wavelengths (on the scale of the crystallite size) are strongly affected by the disorder. A phenomenological model accounts for the resonance behavior reasonably well.

We have carried out an extensive Raman-scattering investigation of the structure of beryllium-implanted gallium arsenide. Single-crystal GaAs was bombarded with 45-keV Be⁺ ions, and backscattering Raman measurements were made, prior to any anneal, as a function of ion fluence, laser photon energy, and depth (via chemical-etch removal of surface layers). Line-shape and intensity analyses of the observed first-order Raman spectra, especially of the longitudinal-optical- (LO) phonon line (which is superimposed on the broad spectral signature of amorphous GaAs), support a structural model of the implantation-induced damage layer as a fine-scale mixture of amorphous and crystalline GaAs. The etch studies yield a structural depth profile in terms of the depth dependence of the amorphous volume fraction (derived from measured scattering intensities) and of the characteristic crystallite size. The first 1500 Å is a high-damage layer having nearly constant structure; this is followed by a structurally graded transition region in which the crystalline volume fraction and the crystallite size smoothly increase until the bulk crystal is reached at about 4000 Å. For a fluence of 5×10¹⁴ ions/cm², the near-surface high-damage plateau is characterized by an amorphous volume fraction of 0.25 and a crystallite size of 60 Å. This plateau begins at the surface; there is no evidence of the near-surface decrease in disorder which appears in some commonly used theoretical simulations. Varying the laser photon energy from 1.55 to 2.71 eV reveals that the LO intensity (arising from the crystalline component) increases at both ends of this spectral range. The intensity increase at low photon energies reflects the increasing optical penetration depth (i.e., effective scattering volume), but the increase at high photon energies signifies a real rise in the scattering efficiency. We interpret this as a resonance-Raman effect associated with the approach toward the E₁ interband transition. This resonance is partially quenched as the crystallite size is decreased for heavily implanted samples.

We have observed a new, strong, low-frequency peak (at 47 cm^-1) in the Raman spectrum of ion-implanted GaAs having a mixed amorphous-microcrystalline microstructure. It is strongly resonant near 1.7 eV, just above the band gap, in contrast to the longitudinal-optic phonon line of the microcrystals (which resonates differently) and the bands of the amorphous component (which do not resonate). We tentatively interpret this peak in terms of acoustic phonons made Raman active by the presence of microcrystal-amorphous interface regions, and discuss several models.