So far, light emission characteristics of porous III-V materials have been restricted mainly to photoluminescence (PL). PL of porous GaP, GaAs and InP at energies above the band gap of the bulk materials has been attributed to quantum size effects [45, 49, 68, 58]. However, less attention has been paid to the investigation of the direct correlation between luminescence properties and morphology features of porous III-V materials. In what follows, the morphology and cathodoluminescence (CL) characteristics of porous layers obtained by electrochemical dissolution of n-GaP substrates in H2SO4 aqueous based electrolytes will be discussed in details. Additional information concerning CL basics are presented in Appendix.
CL experiments were performed by means of a Scanning Electron Microscope ( SEM) equipped with a cathodoluminescence imaging and spectral analysis system and a cryogenic specimen stage. For a direct comparison CL and SEM images have to be taken from the same sample areas. CL is normally excited by a continuous electron beam at normal incidence and measured using a retractable parabolic mirror collector. CL spectra were collected over the wavelength range from 250 to 900 nm. The used beam energies (Eb=15-30 keV) and correspondingly beam currents (Ib=0.25-100 nA) were as low as to reduce electron beam induced effects. The spectra were usually converted from wavelength to energy space and corrected for monochromator dispersion and total instrument response.
A CL system has two operating modes: panchromatic and monochromatic. In panchromatic mode all the light is directed to the detector. This allows the combined intensity of all CL wavelengths within the response of the detector to be imaged. The monochromatic mode is used for recording photon counting spectra in serial mode and thus for imaging with just one wavelength bandpass.