A New Model for Interpreting the Electric Conduction Phenomena in Cu2O Single Crystals

Noguet, C.; Tapiero, M.; Zielinger, J. P.

doi:10.1002/pssa.2210240224

Cited by 26 publications

(12 citation statements)

References 15 publications

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“…Judging from the activation energy and the pre-exponential factor in the Arrhenius plots, trap B is the same as that reported, which is assigned as Cu vacancy. [9][10][11][12][13] The assignment is completely consistent with that obtained theoretically. 14 The average activation energy of trap A was calculated to be 0.25 eV.…”

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confidence: 89%

Defects in Cu2O studied by deep level transient spectroscopy

Paul

Nawa

Sato

et al. 2006

Applied Physics Letters

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Hole traps in p-type Cu 2 O were studied by means of deep level transient spectroscopy in the heterostructure of p-Cu 2 O/i-ZnO/ n-ZnO. In addition to the trap level at about 0.45 eV from the valance band edge, which is already reported as being due to Cu vacancy, we found a new trap level at about 0.25 eV. The new trap is tentatively assigned as Cu-di-vacancy from the trap concentration dependence on oxygen flow rate and substrate temperature. © 2006 American Institute of Physics. ͓DOI: 10.1063/1.2175492͔ Cuprous oxide ͑Cu 2 O͒, a direct band gap semiconductor with a band gap energy of 2.0 eV, has been regarded as one of the most promising materials for application to photovoltaic cells, 1,2 especially for the top cell in a tandem structure. The attractiveness of Cu 2 O as a photovoltaic material lies in the fact that the constituent materials are nontoxic, low cost and abundantly available. Cu 2 O / ZnO heterojunction has been fabricated by radio frequency ͑rf͒ magnetron sputtering and showed the photovoltaic effects, but did not demonstrate good performance. 3 Knowledge of defect energies as well as their densities is an important input for further improvement of the performance of Cu 2 O thin film polycrystalline solar cells. There are several reports on the deep trap in Cu 2 O by measuring deep level transient spectroscopy ͑DLTS͒. 4-6 A hole trap with the activation energy of about 0.45 eV from the valence band edge has been observed and assigned as Cu vacancy using Schottky diodes. However, the temperature range in the DLTS measurements was too narrow. It is necessary to measure the DLTS in the expanded temperature range in order to understand the origin of the defect in Cu 2 O. In this work, the DLTS spectra of Cu 2 O with the junction of n-ZnO/ i-ZnO/ p-Cu 2 O structure in the temperature range between 100 and 350 K is reported. We observed a new trap with the activation energy of 0.25 eV from the valence band edge in addition to the 0.45 eV trap, and the origin of the 0.25 eV trap is discussed based on the sample preparation conditions.Polycrystalline p-Cu 2 O/n-ZnO heterostructure was grown by means of rf magnetron sputtering on Corning 7059 glass substrate using a Cu target of 99.99% purity, ZnO target and Ar as sputtering gas. Oxygen was introduced during the growth of Cu 2 O through a nozzle whose end was placed near the substrate. We prepared two sets of samples for Cu 2

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supporting

confidence: 89%

Defects in Cu2O studied by deep level transient spectroscopy

Paul

Nawa

Sato

et al. 2006

Applied Physics Letters

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“…An unintentional doping level of 10 18 cm À3 uncompensated acceptors in Cu 2 O might be possible [44], but appears to be rather high. In the literature [43,45] more authors agree on values in the range 10 13 cm À3 N A À 10 16 cm À3 . We calculate now the width of the space charge layer and the maximum field strength using N A À ¼ 10 16…”

Section: Field Ionization and The Schottky Barriermentioning

confidence: 86%

Some Considerations Concerning the Detection of Excitons by Field Ionization in a Schottky Barrier

Klingshirn

Fleck

Jörger

2002

phys. stat. sol. (b)

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Dedicated to Professor Dr. Roland Zimmermann on the occasion of his 60th birthdayThe possibility of Bose-Einstein condensation of excitons in semiconductors and/or their superfluid transport are long-standing, still controversially discussed problems. In a recent series of experiments by E. Fortin and coworkers [1-3] [phys. stat. sol. (b) 191, 345 (1995); Phys. Rev. Lett. 77, 896 (1996); and Proc. Internat. Conf. Exciton Processes in Condensed Matter, World Scientific, Singapore 2000, respectively], (para-)excitons in Cu 2 O have been detected electrically by their field ionization in a Schottky barrier. We analyze the electric field in the Schottky barrier and its screening by the holes remaining after field ionization and find serious screening and saturation effects of this Schottky-barrier exciton detector under the excitation conditions of the experiment. These findings make a reinterpretation of the data very likely. IntroductionExcitons are the quanta of the collective excitation of the electron system in semiconductors and insulators. This excitation can be described to a very good approximation as a two-particle state, consisting of an electron in the conduction band and of the hole left behind in the valence band plus Coulomb and exchange interaction between these two particles. In so far the problem is closely related to the hydrogen or the positronium atom (see, e.g., [4,5], and references therein). Obviously the exciton is an electrically neutral quasiparticle with integer spin, but it is composed of two fermions, namely electron and hole. As a consequence of this fact, the commutation relations between exciton creation and annihilation operators are those of bosons at low densities, but with a deviation from ideal boson character via a term which increases with the density of excitons [4].The interplay of bosonic character and fermionic constituents triggered a lot of research over several decades about what happens with excitons at the highest densities. In the following we summarize shortly the present situation for bulk semiconductors. It became clear that at the highest densities a transition to a metallic electron-hole plasma (EHP) occurs [4][5][6]. Depending on the material parameters and excitation conditions such as carrier lifetime, excitation density, and carrier temperature, the plasma may even undergo a first-order phase transition to a liquid-like state, the analogon to molten metallic hydrogen. This is even true in CuCl [7] in spite of its large exciton binding energy and small Bohr radius, shifting the density for the occurrence of an EHP (also called Mott density) from values below 10 17 cm À3 , e.g., for Ge to very high values around 10 20 cm À3 . Experimental investigations and the theoretical modelling of the electron-hole plasma have been pushed by many research groups world-wide as can be seen, e.g., from the references in reviews and textbooks like [4,5]. These refer-

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“…Specifically, there is a linear relationship between the activation energy and the logarithm of the annealing temperature, which is a manifestation of the Meyer-Neldel rule . To explain this behavior, an electronic model was proposed with a band of acceptor levels in the gap as opposed to a single acceptor level . By annealing the sample at a specific temperature, a unique internal arrangement of defects is fixed in the crystal, in addition to a specific compensation ratio, resulting in a distinct activation energy at each annealing temperature.…”

Section: Introductionmentioning

confidence: 99%

Electronic Structure of Pure and Doped Cuprous Oxide with Copper Vacancies: Suppression of Trap States

2013

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Cuprous oxide (Cu2O) is an attractive material for solar energy applications, but its photoconductivity is limited by minority carrier recombination caused by native defect trap states. We examine the creation of trap states by cation vacancies, using first principles calculations based on density functional theory (DFT) to analyze the electronic structure and calculate formation energies. With several DFT-based methods, a simple vacancy is predicted to be consistently more stable than a split vacancy by 0.21 ± 0.03 eV. Hybrid DFT is used to analyze the density of states and charge density distribution, predicting a delocalized hole for the simple vacancy and a localized hole for the split vacancy, in contrast to previously reported results. The differing character of the two defects indicates that they contribute to conduction via different mechanisms, with the split vacancy as the origin of the acceptor states that trap minority carriers. We explore methods of improving photoconductivity by doping Cu2O with Li, Mg, Mn, and Zn, analyzing their impact on vacancy formation energies and electronic structures. Results suggest that the Li dopant has the greatest potential to improve the photoconductivity of the oxide by inhibiting the creation of trap states.

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A New Model for Interpreting the Electric Conduction Phenomena in Cu2O Single Crystals

Cited by 26 publications

References 15 publications

Defects in Cu2O studied by deep level transient spectroscopy

Defects in Cu2O studied by deep level transient spectroscopy

Some Considerations Concerning the Detection of Excitons by Field Ionization in a Schottky Barrier

Electronic Structure of Pure and Doped Cuprous Oxide with Copper Vacancies: Suppression of Trap States

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