Influence of Au diffusion on structural, electrical and optical characteristics of CdTe thin films

Abstract

Diffusion of Au and its effects on structural, electrical and optical properties of CdTe films fabricated by the close-spaced sublimation technique have been investigated. Diffusion of Au was studied in the range 400–550 °C using energy dispersive x-ray fluorescence analysis. Au-doped and un-doped CdTe films were characterized by x-ray diffraction (XRD), electrical and optical absorption measurements. The temperature dependence of the diffusion coefficient of Au in CdTe films is described as D = 4.4 × 10−7exp(−0.54 eV/kT). The mechanism of Au diffusion in polycrystalline CdTe films is attributed to the fast migration of Au along grain boundaries with simultaneous penetration into grains and settling on Cd-vacancies. It is supposed that the weak influence of Au diffusion on XRD patterns of CdTe films can be explained by dispersal of Au atoms preferentially on Cd-vacancies owing to proximity of the covalent radius of Au and Cd. Au atoms, placed on Cd-vacancies (AuCd) during fast cooling from diffusion temperature to room temperature, show an acceptor behaviour with an energy level about of Ev + 0.2 eV. The nature of this level is discussed.

Physics and chemistry of CdTe/CdS thin film heterojunction photovoltaic devices: fundamental and critical aspects

“…Thus, the GBs with a high density of vacancies facilitate the rapid diffusion process. 136 It is suggested that grain boundary diffusion is a very likely mechanism for the impurity transport into the device and dopant diffusion coefficient are enhanced at GBs. 127,131 GBs in CdTe have downward bending, while those in CdTe 1Àx S x have upward bending.…”

“…116 For Au as back contact, Au diffusion in CdTe can be described by the Arrhenius expression D ¼ 4.4 Â 10 À7 exp(À0.54 eV)/kT with a relatively low activation energy (0.54 eV). 136 The diffusion of Au into grains proceeds by the dissociative mechanismthe migrating impurity quickly moves by way of interstitial sites and settles in vacancies. The mechanism of Au diffusion in CdTe takes place in two ways: (i) by fast migration along the GBs; (ii) by relatively slow migration into grains which are characterized by low and high activation energies, respectively.…”

“…The weak inuence of Au diffusion on the structural properties of CdTe was due to dispersal of Au atoms preferentially on V Cd owing to the covalent radius proximity of Au and Cd. 136 Other stress-related changes include the partial conversion of Cu 2Àx Te to Cu x Te with varied copper content that is responsible for the back contact and device degradation. 233 The high diffusivity of copper was found to be assisted by grain boundaries as well as the polycrystalline nature of CdTe.…”

Empirical Correlations Between the Arrhenius’ Parameters of Impurities’ Diffusion Coefficients in CdTe Crystals

“…Though three temperature dependencies of In, Ga and Tl diffusivity intersect in the same point at T = 1390 K (i.e. in CdTe molten state), it can't be a solid base for a conclusion about the data fit to the NMR [37,38] Hg [39,40] *Zn [35] Zn [36,59] ln(D 0 , cm 2 /s) [24,26,27] Cu [25,28,29] *Ag [30] Ag [28,31] Au [34] ln(D Li [22] Na [23] Al [22] *Ga [41,42] In [23,43,48] *In [44][45][46][47] *Tl [49] *Ge [50] *Sn [51] ln(D T 0 (a) (b) Fig. 4 (a) The plot of the ln D 0 versus DE a for the diffusivity of the Groups I, III and IV impurities in CdTe crystals and thin films.…”

Three Dimensionally Structured CdTe Thin-Film Photovoltaic Devices with Self-Aligned Back-Contacts: Electrodeposition on Interdigitated Electrodes

“…For the homojunction devices fabricated here the benefits of microstructural changes obtained through annealing 29,30 are offset by type conversion of part or all of the deposit 23,24 as well as diffusion from the gold contacts, where applicable. 31 The use of different contact materials for the two electrodes might enable improvement through annealing by differential doping around the metallizations as well as improve the electrical properties of the contacts. One might also cap the metallization͑s͒ with conducting oxide or nitride ͑or grow it͒ to decrease recombination and act as a diffusion barrier if it does not degrade the CdTe deposition process.…”