Importance of initial nucleation step on low temperature photoassisted MOVPE growth of ZnSe

Abstract

In this work, low temperature photoassisted metal-organic vapor phase epitaxy (MOVPE) growth of ZnSe on GaAs was carried out using dimethylzinc (DMZn) as the Zn precursor and dimethylselnium (DMSe) or diethylselnium (DESe) as the Se precursor at atmospheric pressure under H2 carrier gas, using the 458 nm wavelength from an Ar+ laser. In situ laser interferometry showed that the growth rate for thin films is non-linear and was explained in terms of the efficiency of coupling of the photo-irradiation with the growing film. The growth rate (GR) can be described by GR = P η F (1 − exp (−α d)) where η is the photo-chemical quantum efficiency, d is the film thickness, F is a structural factor, P is the laser intensity and α is the absorption coefficient. In situ laser interferometry has been essential in making these measurements to observe the initial non-linearity and ensure that characteristic growth rates are taken from steady state growth. Growth using DMSe as the Se precursor was very photosensitive due to its higher photochemical quantum efficiency arising from the additional source of CH3. radicals. These layers were mostly rough due to the slight variation in laser intensity across a region giving different degree of enhancement which is magnified by the growth rate non-linearity leading to non-uniform, and roughened growth. In the case of growth using DESe, it was possible to grow an initial nucleation layer of ZnSe pyrolyticaly prior to photo-assisted growth which ensured that there was sufficient thickness, d, of ZnSe such that the exponential term in the above equation became negligible and the growth rate was dependent only on the linear terms. The slight variation in laser intensity across the surface does not then cause a very significant variation in growth rate, thus maintaining smooth morphology and uniform growth rate.