Nonequilibrium Charge Carrier Generation - Recombination Mechanisms in CdMnTe Thin Films

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Nonequilibrium Charge Carrier Generation − Recombination Mechanisms in CdMnTe Thin Films Iuliana M. Caraman, Sergiu A. Vatavu1, Valentina Z. Nicorici1 and Petru A. Gasin1 Solid State Physics Department, Faculty of Physics, Al.I.Cuza University, Bd. Carol I, No 11, Iasi, 700506, Romania 1 Faculty of Physics, Moldova State University, 60 A.Mateevici str., Chisinau, MD 2009, Republic of Moldova ABSTRACT The absorption, photoconductivity and photoluminescence spectra of Cd0.5Mn0.5Te thin films, deposited by “flash” evaporation and vapor phase deposition have been analyzed. The energy gap and the energetic position of the recombination levels in CdMnTe thin films for different substrate temperatures have been determined. The investigation results have been compared with the same parameters for CdMnTe single crystals used as evaporation material.

INTRODUCTION Multicomponent semiconductor compounds have the tendency to dissociate in stable components during the evaporation process. The presence of several types of atoms in a compound leads to the formation of a large spectrum of compositional and structural defects and respectively to multiple energetic levels in the energy gap. One can expect, that the thin films deposited from Cd0.5Mn0.5Te single crystals should have a compositional structure different from the initial compound and as a result − specific optical and photoelectrical properties determined by the electronic states energetic diagram. The localized states energetic diagram can be revealed by complex studies of the generation-recombination mechanisms of the nonequilibrium charge carriers. The generationrecombination mechanisms manifest themselves directly in optical, photoelectrical and luminescent properties of the samples. The lattice parameter and the coefficient of thermal dilatation of the semiconductor materials are among the parameters that determine their practical perspectives. As one can see [1], the lattice parameter for cubic CdTe and Cd0.5Mn0.5Te differs by 8%. This fact classifies the Cd1-xMnxTe solid solutions as perspective semiconductors for superlattices and quantum wells manufacturing [2]. The optical, photoelectrical and luminescent properties of the Cd1-xMnxTe diluted magnetic semiconductors are presented in this paper. The Cd1-xMnxTe basic parameters and the generation-recombination mechanisms of the nonequilibrium charge carriers have been determined.

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EXPERIMENTAL DETAILS Cd0.5Mn0.5Te single crystals have been used as material for evaporation. Cd0.5Mn0.5Te has been synthesized from elementary elements by Bridgmann method using the two zone furnace. CdMnTe thin films have been deposited by two methods: “flash” evaporation of the Cd0.5Mn0.5Te powder, the granule dimensions ≤15 µm and the deposition from vapor phase. The substrate temperatures have been varied in order to deposit monocrystalline layers or conglomerate of monocrystalline blocks. Mica was used as a substrate. The W-crucible heated up to ∼1700-1750K was used for “flash” evaporation of Cd0.5Mn0.5Te. The mic