Influence of Surface Electric Field on Exciton Photoluminescence of CdS. Self‐Reversal of Spectral Lines

Abstract

The structure of luminescence spectra of free excitons in CdS single crystals at T = 77 K is investigated. The dip formation is found in the top part of free A and B exciton resonant peaks of photoluminescence (PL) spectra after heating of the CdS crystals followed by fast cooling down to 77 K. Similar effects are observed when an external electric field is applied. It is shown that the structure of exciton PL spectra is due to self-absorption of resonance radiation in the near-surface layer with low exciton concentration. Our calculations demonstrate that the formation of an exciton concentration gradient may be caused by the influence of a surface electric field on the binding of electron-hole pairs to excitons. It is assumed that the increase of surface electric fields after heat treatment is caused by the thermostimulated adsorption of oxygen, which results in the formation of electron trap levels.

Free-exciton spectra in heteroepitaxial ZnSe/GaAs layers

“…This effect was already investigated in more detail in CdS. 19 The near-surface field can lead ͑i͒ to the ionization of FE states, ͑ii͒ to the drift of carriers, and ͑iii͒ to a decrease of an exciton binding parameter. The last two effects can lead to the decrease of FE concentration in the near-surface layer, and to the appearance of a gradient of concentration causing the self-reabsorption or self-reversal effect.…”

“…The last two effects can lead to the decrease of FE concentration in the near-surface layer, and to the appearance of a gradient of concentration causing the self-reabsorption or self-reversal effect. 19 They cannot lead to the appearance of a reflecting dead layer completely devoid of excitonic states at values of the electric field strength less than the FE ionization field F cr . Therefore, the main mechanism in the formation of a dead layer is the ionization of the free excitons by the electric field.…”

“…A similar fine structure was previously observed in the FE PL cathodoluminescence and reflection spectra of cubic bulk crystals ZnSe, ZnTe, and CdSe, as well as in spectra of A excitons in CdS and CdSe. [9][10][11][12][13][14][15][16][17][18][19][20] This structure was explained both by polaritonic effects, 5,[14][15][16][17] and by absorption reversal effects. 9,11,19,20 However, despite the large amount of work performed, a satisfactory agreement between the experimental results and theoretical description was not achieved within the framework of these models.…”

“…[9][10][11][12][13][14][15][16][17][18][19][20] This structure was explained both by polaritonic effects, 5,[14][15][16][17] and by absorption reversal effects. 9,11,19,20 However, despite the large amount of work performed, a satisfactory agreement between the experimental results and theoretical description was not achieved within the framework of these models. The absorption reversal effects and their influence on the shape of FE PL line were ignored in most papers devoted to the investigation of PL in ZnSe epilayers.…”