Practicable Method for Estimating Thermal Depths from Phonon‐Broadened Photoexcitation Cross‐Section Bands. II. Analytical Representation for Neutralized Centres

The zero‐phonon threshold behaviour of thermally broadened absorption cross‐sections σ ( hv, T ) is examined for processes of photoexcitation of electrons and holes from deep acceptor and donor levels into unperturbed Bloch states of nearly parabolic conduction or valence band edge regions, respectively. In terms of parabolic cylinder functions, analytical descriptions are given for alternative photon energy dependences of the transformed excitation curves ϱ ( hv, T ) ≡ exp (− hv /2 kT ) σ ( hv, T ). Their vertex positions hv ′ ( T ) are found to be shifted with respect to the thermal depths J ( T ) in question by amounts of about (1 + f ) 2 kT , where f = 0 or 1, for allowed or forbidden transitions. A remaining uncertainty up to ∓ kT , at maximum, is due to possible variations of the electron–phonon coupling from an extremely weak (vanishing) up to an extremely strong one. From our assessment of optical electron excitation data given by Wang et al. for the B level in GaAs and by Kullendorff et al. for the Cu B level in InP we conclude that 1. both transitions are allowed ( f = 0) in dipole approximation, 2. both levels are located at low temperatures by about 0.1 eV below the middles of the respective forbidden gaps, and 3. the actual distances of both levels from the respective conduction band edges decrease by about (0.08 ± 0.01) eV when the temperature increases from very low up to room temperature. This “depth shrinkage effect” is pointed out to have important consequences for theoretical interpretations of thermal electron capture properties on the basis of the nonradiative multiphonon (NMP) capture mechanism. The complementary processes of optical hole excitation at both centres appear to be markedly affected by Rydberg‐like states extending up to several dozens of meV above the respective valence band edges.