Trap states near the surface in (Cd,Mn)Te crystals

Laser-induced transient current technique (L-TCT) and photocurrent-voltage characteristics (PC-V) are commonly used in many laboratories to measure the values of μeτe and τe. For both techniques (PC‑V and L-TCT), we observed a significant impact of the surface preparation method for the contacts on the obtained results. Different contact shapes were used on the anode to increase the interaction of light-generated charge at the cathode with near‑surface trap states and near-surface recombination, while a planar cathode was point-illuminated. In one experiment, the charge (electrons) generated at the cathode flowed directly to the pixel located vis a vis on the anode, while in another experiment, the charge had to flow first through the near-surface layer at the cathode and then to the lateral pixel on the anode. In the PC-V method, LEDs with photon energy ħω≤Eg or ħω>Eg (Eg is the bandgap energy) were used as the light source with adjustable light intensity. In the L‑TCT method, both contacts (on the cathode and anode) were in the form of pixels. In the L-TCT method, the laser radiation energy ħω was about 2Eg. In some experiments the cathode was additionally illuminated by using LED, with ħω>Eg or ħω≤Eg, in other not. By using the PC-V method for the well prepared surface we obtained μeτe ~ 10⁻³ cm²V⁻¹ from fitting with the Hecht formula. For a poorly prepared surface, the results were fitted with the Ridzonova formula. The electron mobility μe was obtained from the L-TCT method. The observed results for current waveforms in the L-TCT measurements for unilluminated and LED-illuminated cathodes with different electric fields and light intensity of LEDs were compared with the PC-V results, particularly for ħω of LED≳Eg. In our interpretation we consider two processes: ionization of traps by the electric field, and deionization of traps by recombination with electrons excited by the LED light illuminating the cathode.