Nickel‐related defects in ZnO – A deep‐level transient spectroscopy and photo‐capacitance study

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The deep level E3 is a commonly observed defect in bulk crystals and thin films of ZnO independently of the method of growth. Its chemical origin is still not finally understood. In this paper, previous research on the E3 level is reviewed and recent experimental results acquired by optical deep level transient spectroscopic (DLTS) methods are presented for the temperature range from 4 to 350 K, for photon energies between 0.25 and 4 eV, and for emission rates ranging from 10−3 to 104 s−1. The capture cross section of the optical transition of E3‐trapped electrons into the conduction band is described. Additionally, a second optical transition occurs for photon energies higher than 1 eV, which accounts for the lowering of the E3 DLTS signal at low temperatures under illumination. Further, it is unambiguously shown that the E3 defect can bind up to two electrons and that it has the properties of a negative‐U center. Photoluminescence measurements reveal that the E3 defect is directly connected to a radiative recombination channel at 2.09 eV. Further, electron paramagnetic resonance (EPR) measurements were performed with optical excitation in dependence on the temperature and photon energy. Interestingly, the temperature‐dependent EPR signal of normalVnormalO+ is similar to that of the photo‐ionization cross‐section of the E3 defect. These experimental findings are discussed in comparison with published EPR results and results obtained by first‐principle calculations of native point defects in ZnO.

Section: Introductionsupporting

confidence: 79%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

“…First we will summarize the discussion of E3 in the literature. E3 is characterized by a thermal activation energy

E_{c} - E_{t}

of from 260 to 310 meV and an apparent electron capture cross‐section between 10 −14 and 10 −17 cm 2 . The E3 concentration takes values up to 20% of the net doping concentration.…”

Section: Introductionmentioning

confidence: 99%

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Negative‐U Properties of the Deep Level E3 in ZnO

Pickenhain

Schmidt

Wenckstern

et al. 2018

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“…The knowledge on acceptor states in the vicinity of the valence band edge is scarce, see for example [13][14][15][16]. Junction capacitance techniques like deep-level transient spectroscopy (DLTS) [7,11,[15][16][17][18][19][20][21][22], deep-level optical spectroscopy (DLOS) [15], optical DLTS (ODLTS) [23] and photo-capacitance [16,22] have been used to study the properties of deep-levels in the zinc oxide band-gap. In doing so, knowledge on deep-levels was gained especially in the upper third of the zinc oxide band-gap.…”

mentioning

confidence: 99%

On the T2 trap in zinc oxide thin films

Schmidt

Ellguth

Karsthof

et al. 2011

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We investigated the electronic properties of the T2 deep-level in zinc oxide thin films. It was found that T2 preferentially forms under zinc-rich conditions and can be generated by either annealing the samples at reduced oxygen partial pressures (p O 2 < 1 bar) or implanting zinc or copper ions, respectively. A strong dependence of its activation energy and high temperature limit of its cross-section for electron capture on the T2 concentration in the sample is reported. Double DLTS measurements showed that the T2 activation energy decreases with increasing electric field due to phonon assisted tunnelling. Furthermore T2 can be photo-ionised with a threshold photon energy of about 700 meV. Depth-resolved concentration profiles of the T2 level in the samples were measured by optical capacitance-voltage spectroscopy.

Nickel Oxide–Based Heterostructures with Large Band Offsets

Karsthof

Wenckstern

Zúñiga‐Pérez³

et al. 2019

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We present research results on the electronic transport in heterostructures based on p-type nickel oxide (NiO) with the n-type oxide semiconductors zinc oxide (ZnO) and cadmium oxide (CdO). NiO is a desirable candidate for application in (opto-)electronic devices. However, because of its small electron affinity, heterojunctions with most n-type oxide semiconductors exhibit conduction and valence band offsets at the heterointerface in excess of 1 eV. ZnO/NiO junctions exhibit a so called type-II band alignment, making electron-hole recombination the only process by which a current can vertically flow through the structure. These heterojunctions are nevertheless shown to be of practical use in efficient optoelectronic devices, as exemplified here by our UVconverting transparent solar cells. These devices, although exhibiting high conversion efficiencies, suffer from two light-activated recombination channels connected to the type-II interface, one of which we identify and analyse in more detail here. Furthermore, CdO/NiO contacts were studied a heterostructure with even larger band offsets such that a type-III band alignment is achieved. This situation theoretically enables the development of a 2-dimensional electronic system consisting of topologically protected states. We present experiments demonstrating that the CdO/NiO heterostructure indeed hosts a conductive layer absent in both materials when studied separately.