E. Bedel

OPTICAL DETERMINATION OF STRAINS IN HETEROSTRUCTURES - GAAS/SI AS AN EXAMPLE

Raman and photoluminescence spectroscopies are used to characterize crystalline quality and interfacial strain in heterostructures. The effect of a biaxial stress on electronic and vibronic energies is reviewed and then applied to the case of a GaAs layer. Measurements on GaAs grown on Si(100) by molecular‐beam epitaxy are made over a wide temperature range (4→700 K). The evolution of the strain is deduced from the shift of both the energy‐band gaps and the long‐wavelength transverse and longitudinal‐optical‐phonon frequencies. The sensitivity of the Raman probe is dramatically enhanced by excitation under resonant conditions at the E1 edge of GaAs. The measurements confirm the anisotropy of the strain and demonstrate that both its sign and value at room temperature result from a balance between two reverse phenomena: the thermal expansion and the lack of complete relaxation of the lattice mismatch during growth.

Raman study of longitudinal optical phonon‐plasmon coupling and disorder effects in heavily Be‐doped GaAs

Raman spectroscopy measurements have been performed on GaAs:Be samples with high crystalline quality and exceptional heavy doping level ranging from 1019 to 1.4×1021 cm−3. The recorded spectra show a structure we assigned to a coupled LO phonon‐damped plasmon mode. A theoretical expression for the Raman scattering rate by this mode has been derived from a dielectric model and compared to the experimental data. Using a fitting procedure the doping level of the samples has been estimated in agreement with Hall measurements. Moreover, the study of the Raman intensity evolution of both unscreened‐LO and coupled phonon‐plasmon structures, provided a convenient and rapid method to determine the activated carrier density in p‐doped polar semiconductors. Disorder effects due to the dopant impurities have been also observed and analyzed using a spatial correlation model description.

RAMAN CHARACTERIZATION OF TWINNING IN HETEROEPITAXIAL SEMICONDUCTOR LAYERS - GAAS/(CA,SR)F2

Detailed analysis of Raman spectra recorded from (100)‐oriented GaAs layers grown by molecular‐beam epitaxy on the lattice‐matched insulator (Ca,Sr)F2 gives evidence of internal misorientation effects (twins). This analysis accounts for the various phenomena (doping, disorder, electron‐phonon coupling) likely to modify the scattering efficiency. Calculations are performed in order to obtain quantitative evaluations of the misoriented volume amount.

Two-dimensional-like nucleation of GaAs on Si by room-temperature deposition

Nucleation of GaAs on Si in molecular‐beam epitaxy is studied by Auger electron spectrometry and reflection high‐energy electron diffraction. It is shown that, if growth is initiated at room temperature and if a GaAs equivalent thickness of ∼15 A is deposited on Si, an amorphous, nonstoichiometric layer is obtained which covers completely the substrate surface. Stoichiometry and monocrystallinity can be restored by thermal annealing at 350 °C by a solid‐phase epitaxy mechanism. Under such conditions the initial stages of GaAs/Si growth can then proceed via two‐dimensional nucleation, instead of the three‐dimensional mode observed at higher growth temperatures.

Interaction Strength between the Highly Localised Nitrogen States and the Extended Semiconductor Matrix States in GaInNAs

We have investigated the temperature dependence of photoluminescence (PL) emission from sequentially grown Ga 0.8 In 0.2 As and Ga 0.8 In 0.2 N 0.015 As 0.985 quantum wells between 2 K and room temperature. A significant reduction in the temperature dependence of the GaInNAs bandgap compared to nitrogen-free GaInAs is observed. The results are analysed using the band-anticrossing model, which accurately predicts the temperature dependence of the GaInNAs energy gap from the behaviour of the GaInAs energy gap. We also compare the band-anticrossing interaction parameter C NM used to fit our data with other published values for GaNAs and GaInNAs. The results suggest that C NM may not be independent of indium fraction.

S-shaped behaviour of the temperature-dependent energy band gap in dilute nitrides

We have investigated the temperature dependence of the band gap energy in GaInNAs, GaNAs and InGaAs quantum wells. In the structures containing nitrogen the well-known S-shaped characteristic was observed at low temperatures. We explain this anomalous temperature behaviour by strong carrier localization in potential fluctuations at low temperatures. In the nitrogen free samples, there was no S-shaped behaviour and the empirical Varshni dependence was followed.

Atomic scale saw by dislocation slipping: A new method to generate one‐dimensional structure

A new method to generate one‐dimensional (1D) semiconductor structures or controllable steps on a surface is proposed. This method, which is not material specific, is based on intrinsic dislocation slip properties and heterogeneity of straining. Dislocations can be used as an atomic scale saw to cut two‐dimensional structures (2D) in order to obtain 1D wires. Atomic force microscope observations of GaAs surfaces and transmission electronic microscopy cross sections of GaAs/GaAlAs single quantum wells are presented to demonstrate the feasibility of the method. Although being based on dislocation slipping this method is shown to preserve the optical and crystalline properties of the starting 2D structure as confirmed by photoluminescence spectra.

RAMAN-STUDY UNDER RESONANT CONDITIONS OF DEFECTS NEAR THE INTERFACE IN A GAAS-SI HETEROSTRUCTURE

A Raman study has been performed, under resonant conditions, on a GaAs bevelled‐edge layer grown on a Si substrate to characterize the optical and crystalline properties of the epilayer near the interface. According to the geometrical characteristics of the sample, a theoretical expression for the Raman intensities profile has been established and compared to the experimental data. This fitting procedure enables us to investigate the absorption coefficient of the GaAs layer due to the disorder‐induced softening of the E1 edge. A quantitative analysis of the lattice disorder has been carried out on both longitudinal and transverse optical modes by studying the Raman line‐shape evolution versus the laser spot position on the bevel edge. From this study, we have followed the recovery of the crystalline quality of the epilayer while going away from the interface, and evaluated the ‘‘Raman thickness’’ of the dislocated layer. Using the spatial correlation model as a relationship between the disorder amount and...

Diameter of As clusters in LT-GaAs by Raman spectroscopy

Raman scattering measurements on low temperature GaAs layers are presented. Phonons in both GaAs and As are studied. The transition from diluted As in the GaAs matrix to the small As clusters formed after annealing is analyzed. This is performed by observing the reduction of the GaAs bound charge, i.e., the longitudinal optical GaAs frequency downshift, to the appearance of the crystalline As vibrational mode. From the phonon shifts of crystalline As, the diameter and the strain are derived, accordingly to transmission electron microscopy measurements.

Si‐substrate preparation for GaAs/Si molecular‐beam epitaxy at low temperature under a Si flux

A technique for preparing Si substrates in molecular‐beam epitaxy (MBE) systems devoted to III–V compound growth is described. A C‐ and O‐free Si surface is obtained on previously oxidized substrates using a HF solution etch. A (2×1) reconstructed surface with steps of monoatomic height is achieved with an atomic Si flux provided by a standard effusion cell, while the substrate is maintained at 600 °C for ∼10 min. The experimental results presented were obtained using Auger electron spectrometry and reflection high‐energy electron diffraction.