John V. Armstrong

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).

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.

XeCl excimer laser assisted CBE growth of GaAs

Abstract The effect of 12 ns, 308 nm (XeCl) excimer laser pulses on the CBE growth rate of GaAs, at temperatures below the maximum non-laser assisted growth rate, G max , has been studied as a function of laser fluence and repetition frequency. There is a threshold fluence for growth rate enhancement, above which the growth rate is dependent on repetition frequency, being restored to G max at 20 Hz. The growth rate in the laser spot is measured by dynamic optical reflectivity (DOR).

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.

MICROSTRUCTURE OF GAAS GROWN BY EXCIMER LASER-ASSISTED CHEMICAL BEAM EPITAXY

Selective-area CBE growth of GaAs on (100) GaAs substrates has been achieved using focused XeCl (308 nm) excimer laser assistance. The highest selectivity, at the growth rates used ( approximately 1 mu m h-1), occurred when the substrate temperature was <400 degrees C and the laser fluence was approximately 90 mJ cm-2. GaAs grown by laser-assisted CBE was found to contain dislocation tangles and to have an associated regular ripple structure on a scale of 300 nm.

Optical monitoring of the growth of a GaAs/AlGaAs superlattice

Dynamic optical reflectivity (DOR) is used to monitor, in real time, the chemical beam epitaxial growth of a 6 nm GaAs/AlGaAs superlattice. Good agreement with a theoretical simulation, assuming infinitely sharp interfaces, is achieved with monolayer sensitivity. The applicability of the DOR technique to real-time monitoring of superlattice growth is compared with ellipsometry and it is shown that the two techniques yield essentially identical information.

In-situ laser reflectometry of the epitaxial growth of thin semiconductor films

Abstract The usefulness of laser reflectometry as a tool for the real-time monitoring of the epitaxial growth of thin semiconductor films is examined. Laser reflectometry is most suitable for monitoring films having a thickness ≥ λ/4, where λ is the monitoring wavelength: it therefore provides an excellent means of monitoring the growth of Bragg reflectors and an example of such a reflector grown with the aid of laser reflectometry is given. In addition it enables both growth rate and chemical composition of alloy semiconductors to be obtained in real time for films of the above thickness and this is illustrated by reference to the AlGaAs system. By monitoring the growth of AlGaAs/GaAs superlattices it is shown that this relatively simple technique has monolayer sensitivity. Results are presented which demonstrate that laser reflectometry is ideal for the monitoring of Ar + laser assisted chemical beam epitaxial growth since the reflected signal contains growth information relating exactly to the laser footprint; this “auto-monitoring” is providing valuable insight into the mechanisms involved in the laser assistance.

Orientations and morphology of Al layers grown on GaAs by chemical beam epitaxy

Abstract Aluminium layers were grown on GaAs(001) surfaces by chemical beam epitaxy (CBE). The growth was monitored in real time using dynamic optical reflectivity. A new orientation relationship between Al and GaAs has been found in addition to the three orientation relationships which are seen in molecular beam epitaxy (MBE) grown Al/GaAs samples. All Al layers were discontinuous; the small islands have four orientations in roughly equal proportions, whilst most of the large islands have the single orientation Al(011) a . This occured for all surface reconstructions present on the GaAs surface. A model is proposed for the evolution of morphology and orientation of these islands.

Real-time monitoring of the growth of AlGaAs layers by dynamic optical reflectivity (DOR)☆

Abstract A simple optical in-situ method of characterizing the growth of AlGaAs layers is presented. The AlGaAs layers are grown by chemical beam epitaxy (CBE). The sensitivity and versatility of the dynamic optical reflectivity (DOR) method is shown and it is argued that it would be a useful addition to the growth community.