Structure and energy gap of Cd1?xCaxTe and Cd1?yCaySe as a function of Ca2+ incorporation
Single crystals of Cd1?xCaxTe and Cd1?yCaySe ternaries, grown by the Bridgman technique and characterized by microprobe and x-ray techniques, showed the upper limit of x and y to be 0.05. The excitonic energy gap, determined from the derivative signature of free excitons in wavelength modulated reflectivity, shows a linear increase with increasing Ca2+ concentration in both ternary alloys. The near-band gap emissions of donor-bound and of acceptor-bound excitons observed in photoluminescence, investigated as a function of alloy composition, reveal blue shifts of donor-bound excitonic signatures, in good agreement with those observed for free excitons. The Raman spectra of Cd1?xCaxTe reveal the clear presence of CaTe-like LO phonons, confirming that the Ca incorporation has indeed resulted in a ternary Cd1?xCaxTe with a characteristic two-mode behaviour.