A. Muñoz‐Yagüe

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.

Epitaxial growth of lattice‐matched CaxSr1−xF2 on (100) and (110) GaAs substrates

Epitaxial growth of monocrystalline layers of mixed fluorides (CaxSr1−xF2) using molecular beams on GaAs substrates is reported for the first time. Good morphology and crystalline quality of the layers have been obtained. Some possible applications of these structures are considered, and preliminary results are reported concerning the electrical behavior of metal‐insulator‐semiconductor devices and the use of CaxSr1−xF2 as encapsulant or as masking material for GaAs.

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.

Nd3+ incorporation in CaF2 layers grown by molecular beam epitaxy

Molecular beam epitaxy of Nd3+‐doped CaF2 monocrystalline layers on CaF2 substrates is demonstrated. Nd concentration is controlled by the temperature of an evaporation cell containing NdF3. Photoluminescence spectra of the samples show emissions from Nd3+ centers in tetragonal symmetry sites as a consequence of the charge compensation mechanism provided by interstitial F− ions.

Er3+ doping of CaF2 layers grown by molecular beam epitaxy

Molecular beam epitaxy of CaF2 monocrystalline layers Er3+ doped up to a concentration of 5 wt % is demonstrated on CaF2 substrates. Separated effusion cells containing CaF2 and ErF3 were used. The photoluminescence spectra of the samples show emissions from centers of different symmetry identified by reference to published results obtained on CaF2:Er3+ bulk crystals. No influence of the substrate orientation—(100) or (111)—on the luminescence characteristics was observed.

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...

Raman determination of the composition in semiconductor ternary solid solutions

The use of Raman spectrometry for the precise determination of the mole fraction in semiconductor solid solutions is presented. It is shown that if the frequency difference between the modes related to the two binaries involved is used, the scattering of the results due to various experimental conditions (temperature, spectrometer calibration, ...) is avoided. In addition, the method is rapid, nondestructive, reliable, and allows to probe the composition at different depths into the layer.

1.54 μm wavelength emission of highly Er‐doped CaF2 layers grown by molecular‐beam epitaxy

CaF2:Er layers have been grown by molecular‐beam epitaxy on (100)‐oriented CaF2 substrates; the Er concentration ranges from 1% to 50% (mole fraction). The 1.54 μm emission observed under excitation around 800 nm was studied by photoluminescence. Up to 35% Er concentration the integrated emission increases monotonously, quenching appearing for higher doping levels. Photoluminescence results are discussed within the framework of previous studies of Er3+ emission in the near‐infrared range (830–860 nm) in order to gain insight into the Er centers involved in the 1.54 μm emission.