Abstract: Physical vapor deposition of GaxIn1−xSb

Abstract

Films of GaxIn1−xSb have been deposited by the electron-beam evaporation of three-component billets. The films have potential use in low-cost electroabsorptive devices such as switches and modulators. The band gap of the ternary solid solution can be varied from 0.2 to 0.7 eV by changing the film composition. Modulation of this band gap with an electric field will then provide the desired control over the transmittance of the films in the infrared. A typical deposit was made in a vacuum of 10−3 Pa, using 1 kW of electron beam power to cause evaporation of the single billet. The films were usually 10–30 μm thick and were formed on alumina and sapphire substrates. X-ray-diffraction analysis was used for both film and billet samples to identify the phases present and to determine homogeneity. Values of x for the ternary phases were assigned by computing lattice parameters from the x-ray peak positions and then applying Vegard’s law. The values of x ranged from 0.05 to 0.47 for the films consisting of a single phase of ternary solid solution. This range of x was observed because of the use of billets with different compositions. It was further observed that the x values of the deposits were approximately one-half the x values of the billets used to produce them. This relationship can be used to control the x values of deposits. The analysis of the phases present in the films further showed that single-phase deposits of ternary solid solutions were only formed for a particular range of substrate temperatures. Deposits formed at substrate temperatures of 486 °C or less contained free Sb and GaSb in addition to a ternary solid solution. At substrate temperatures of 567° to 655 °C, the films consisted of a single-phase ternary solid solution. The deposits contained free In as well as the ternary solid solution at the higher substrate temperatures of 735° and 820 °C. The films then consisted solely of the ternary solid solution only when the substrate temperatures were in the range T1<T<T2, where 486<T1?567 °C and 655°?T2<735 °C. The compositional homogeneity of the films was checked over two scales of distance. The homogeneity of the deposits from substrate-to-substrate (distance scale ∠5 cm) was confirmed by agreement of the diffraction patterns from three substrates of a given run. Smaller scale homogeneity was evaluated using the width of the 422 x-ray peak. The results showed that the x variation of the ternary solid solution is not more than ±0.015 over the interaction volume (∠0.25 cm2×7×10−4 cm) of the x-ray beam. The x-ray analysis of the billets showed that they were composed of free In and ternary solid solutions. The compositional homogeneity of the billets was confirmed by the agreement of the diffraction patterns from material at different positions in a particular billet. The films and billets were analyzed chemically using atomic absorption spectroscopy and arc spectroscopy. The values of x determined chemically for films of ternary solid solutions agreed to within ±0.05 with the x values determined from the use of Vegard’s law. This agreement is within that expected from considerations of the sources of error, so Vegard’s law is considered to be applicable to the films. In addition, the chemical analysis revealed that two of the six single-phase ternary solutions essentially achieved the stoichiometry represented by the formula GaxIn1−xSb (to within 2%). The remaining four ternary solid solutions contained more than 50 at.% Sb. The analysis also showed that the films were contaminated by impurities from the alumina substrates. The transmission properties of four deposits on sapphire substrates were studied. Static reflection measurements were made in the 2.5–10-μm range at room temperature. The observed absorption edges were in agreement with band-gap data for the two deposits stoichiometric in Sb. The observed edges were at shorter wavelengths than predicted for the two deposits with excess Sb. The maximum transmission observed in this work was 40%. Electroreflectance measurements on the four films at liquid-nitrogen temperature showed that one of the films (a stoichiometric film) could be modulated by an electric field. The above results are encouraging for the synthesis of electroabsorptive materials by this method of physical vapor deposition. Ternary solutions of a given band gap can be deposited and modulation with an electric field is feasible. In order to make good quality devices more work needs to be done in controlling the impurity content and percentage of Sb in the films.