T. Farrell

Reflection anisotropy spectroscopy

Reflection anisotropy spectroscopy (RAS) is a non-destructive optical probe of surfaces that is capable of operation within a wide range of environments. In this review we trace the development of RAS from its origins in the 1980s as a probe of semiconductor surfaces and semiconductor growth through to the present where it is emerging as a powerful addition to the wide range of existing ultra-high vacuum (UHV) surface science techniques. The principles, instrumentation and theoretical considerations of RAS are discussed. The recent progress in the application of RAS to investigate phenomena at metal surfaces is reviewed, and applications in fields including electrochemistry, molecular assembly, liquid crystal device fabrication and remote stress sensing are discussed. We show that the experimental study of relatively simple surfaces combined with continuing progress in the theoretical description of surface optics promises to unlock the full potential of RAS. This provides a firm foundation for the application of the technique to the challenging fields of ambient, high pressure and liquid environments. It is in these environments that RAS has a clear advantage over UHV-based probes for investigating surface phenomena, and its surface sensitivity, ability to monitor macroscopic areas and rapidity of response make it an ideal complement to scanning probe techniques which can also operate in such environments.

Dynamic optical reflectivity to monitor the real‐time metalorganic molecular beam epitaxial growth of AlGaAs layers

Oscillations in the reflectivity of AlxGa1−xAs, grown on GaAs at 870 K have been measured at 632.8 nm over the range 0≤x≤1. The oscillations are fitted to the standard theory of reflection from a bilayer with the complex refractive index (n+ik) of substrate and film as fitting parameters. For GaAs the values of n and k are measured as 3.9+i0.23 at 870 K. Assuming n varies linearly with x for AlxGa1−xAs between 3.1(A1As) and 3.9(GaAs) then the period of the oscillations gives an accurate measurement of growth rate, while the amplitude allows the film composition to be monitored simultaneously. All layers were grown by metalorganic molecular beam epitaxy (MOMBE).

A rapid reflectance anisotropy spectrometer

A 16 channel reflectance anisotropy (RA) spectrometer capable of simultaneously acquiring reflectance spectra and real and imaginary RA spectra on the 0.1 s time-scale is reported. Its performance was evaluated by monitoring the electrolytic deposition/growth and removal of copper on a gold(110) surface. The slow deposition enabled very low noise spectra to be obtained, which served as the yardstick with which the spectra obtained during the rapid removal could be compared. The spectra obtained over the 15 s removal mirrored those obtained during the 500 s deposition period. The spectra are discussed in terms of the copper growth being in the form of aligned islands.

Monitoring real-time CBE growth of GaAs and AlGaAs using dynamic optical reflectivity

Abstract Dynamic optical reflectivity (DOR) uses the interference oscillations arising from the multiple reflections, of a normally incident CW laser beam, between the surface of a growing film and the film-substrate interface. The oscillations have a period determined by the refractive index of the film and the laser wavelength. DOR measurements have been made, in real time, during the CBE growth of Al x Ga 1− x As layers on a GaAs(100) substrate. The results show that the growth rate and the aluminum composition x can be monitored.

Monolayer resolved monitoring of AlAs growth with metalorganic molecular beam epitaxy by reflectance anisotropy spectroscopy

Metalorganic molecular beam epitaxy (MOMBE) growth is studied here for the first time in situ with reflectance anisotropy spectroscopy (RAS). Growth of AlAs on GaAs(001) and optical monitoring was performed in a standard MOMBE system. Triisopropylgallium, dimethyl‐ ethylaminoalane, and precracked arsine were used as precursors. RAS spectra obtained are similar to the ones observed under molecular beam epitaxy or metalorganic vapor phase epitaxy conditions and correspond to the GaAs(001) and AlAs(001) c(4×4) surface reconstructions. Initiating AlAs growth from an arsenic stabilized c(4×4) GaAs(001) or AlAs (001) surface, the RAS signal shows oscillations with a period corresponding to the growth of one AlAs monolayer, as verified by thickness determination on thicker layers. As opposed to growth on an AlAs surface, when growing on GaAs the growth rate was not found to be constant right from the start, but was increasing slightly until it stabilized after several monolayers had been deposited.

Optical evaluation of an AlAs/AlGaAs visible Bragg reflector grown by chemical beam epitaxy

A 21 layer AlAs/Al0.4Ga0.6As multilayer structure, designed as a Bragg reflector centered at 670 nm, has been grown by chemical beam epitaxy. The growth was monitored in real time by dynamic optical reflectivity (DOR) using a 670 nm semiconductor diode laser. The resultant DOR trace was compared to a computer simulation for the growth structure and good agreement is obtained using layer thicknesses measured by transmission electron microscopy. The wavelength dependent reflectivity of the Bragg reflector was measured using a grating spectrometer and good agreement is obtained to a computer simulation once the dispersive complex refractive index is taken into account.

The influence of monolayer coverages of Sb on the optical anisotropy of vicinal Si(001)

Abstract The influence of monolayer coverages of antimony (Sb) on vicinal silicon (001) is studied by reflectance anisotropy spectroscopy (RAS). Adsorption of Sb at room temperature, which is known to break the Si dimer bond and to cause step reconstruction, changes the shape of the RAS spectrum completely, even before annealing. An interesting feature appears in the region of 3.7 eV, which sharpens when the system is annealed to 300°C to form the Si(001)-(1 × 1)-Sb structure. Our results support the view that Si(001)-(1 × 1)-Sb is a distinct phase and we associate the 3.7 eV peak with optical transitions involving SiSb backbonds. Further annealing to 550°C forms Si(001)-(2 × 1)-Sb, where the Sb atoms are now dimerised. This heat treatment decreases the vicinal surface domain imbalance by splitting up the double-height steps, and the RAS signal is reduced in size. Annealing to 750°C causes Sb desorption and the formation of the 0.25 ML Si(001)-c(4 × 4)-Sb structure, with an increase in optical anisotropy, which we attribute to double-height steps reforming as the Sb dimer-induced strain in the system is alleviated. Complete desorption of Sb restores the Si(001)-(1 × 2) structure, and the original RAS signal.

Optical monitoring of the growth of heavily doped GaAs by chemical beam epitaxy and of the in situ etching of GaAs using CBr4

We report the in situ optical monitoring of the growth of heavily doped GaAs by chemical beam epitaxy. The normal incidence reflectance of a 670 nm semiconductor laser was monitored in real time using dynamic optical reflectivity (DOR). Oscillations in the reflectance of the growing film arising from small changes in the refractive index due to doping were observed for carbon doping in the range 2×1019–6×1020 cm−3. No oscillations were obtained for samples with carbon or sulphur doping levels in the range 1018–1019 cm−3. A reduction in growth rate was observed for carbon concentrations above 1020 cm−3 and this was attributed to etching by the CBr4 dopant source. In situ etching of GaAs layers by CBr4 prior to growth was also monitored using DOR.

Effect of adlayer dimer orientation on the optical anisotropy of single domain Si(001)

The effect of adlayer dimer formation on the optical anisotropy of single domain Si(001) is reported. Reflection anisotropy spectroscopy (RAS) from Ga and Sb adsorbed on the Si(001) surface reveals complex behavior which depends strongly on the atomic species and the RAS signal cannot be simply related to dimer orientation. Dimer formation in the same direction on the surface by Si and Ga is shown to produce RAS signals, below the direct optical gap of Si, of the opposite sign. This contrasts with the simpler behavior observed for Ga and As adsorption on GaAs(001) surfaces.

The adsorption of bipyridine molecules on Au(110) as measured by reflection anisotropy spectroscopy

The reflection anisotropy spectra of 2,-bipyridine and 4,-bipyridine adsorbed onto an Au(110) surface in an electrochemical cell demonstrate that both systems form ordered structures. It is shown that reflection anisotropy spectroscopy can be used to distinguish between structural isomers adsorbed on the Au(110) surface.