Ionized donor pairs and microwave and far-infrared absorption in semiconductingCdF2

Abstract

The dielectric permittivity ϭ 1 Ϫi 2 of semiconducting CdF 2 :In, CdF 2 :Ga, and CdF 2 :Y crystals was studied over the frequency range from 34.0 to 37.5 GHz at temperatures from 1.8 to 100 K. The photoinduced transition from a semi-insulating to a conducting state in photochromic CdF 2 :In and CdF 2 :Ga crystals results in a significant increase of both the dielectric constant ͑by ⌬ 1 ϭ0.5 to 1.4͒ and the dielectric-loss factor ͑by about an order of magnitude͒. The low-field dielectric-loss factor in these photodecolored crystals and in CdF 2 :Y ͑ 2 ϭ0.1 to 0.3͒ may be decreased by approximately an order of magnitude with an increase in the microwave-field power at 1.8 K. However, 2 ceases to depend on the field at TϾ4 K. These features are explained by the theory of Tanaka et al. for resonant-saturated absorption of ionized donor pairs. We have modified this theory to cover the far-IR range of the absorption spectrum of semiconductors with various degree of compensation. Results following from the modified theory were compared with those obtained by three other groups, namely, Blinowski and Mycielski, Efros and Shklovskii, and Baranovskii and Uzakov. We have shown that the ionized donor pairs are responsible for the far-IR absorption in semiconducting CdF 2 studied experimentally by Eisenberger, Pershan, and Bosomworth. The role of impurity clusters in storage of an ''excess'' impurity and as a source of interstitial F Ϫ ions during thermochemical treatment of as-grown crystals ͑which determines the ultimate semiconductive properties of CdF 2 ͒ is also discussed.

Photoionization processes of rare-earth dopant ions in ionic crystals

“…Previously, microwave studies of photoconductivity were performed under valence band to conduction band excitation for AgCl powders (forbidden band gap ¼ 3 eV) [54], with nanocrystalline semiconductors [55] and with impurity doped crystals possessing semiconducting properties [56][57][58][59]. For wide-band-gap crystalline materials however, observable microwave signals are supposed to be considerably smaller.…”

10.1007/s11449-008-3006-5

“…This resulted in total destruction of the two-electron DX -states with the transfer of electrons to donor states [10]. The limiting concentration of electrons in the studied samples was ~5 × 10 17 , 2 × 10 18 , and 5 × 10 18 cm -3 for the crystals doped with Ga, In, and Y, respectively.…”

“…The limiting concentration of electrons in the studied samples was ~5 × 10 17 , 2 × 10 18 , and 5 × 10 18 cm -3 for the crystals doped with Ga, In, and Y, respectively. Additional information about the samples under study can be found in [10,11].…”

Semiconductor CdF2:Ga and CdF2:In Crystals as Media for Real-Time Holography

“…It was proposed that this absorption is due to the existence of so-called “ionized donor pairs” in the crystals, near-by disposed couples of donors having one electron (the analog of the molecular hydrogen ion) [35]. It is absent for CdF 2 :Ga and CdF 2 :In crystals cooled in the dark but appears on their illumination in the UV-VIS band.…”