The source of n -type conductivity in undoped transparent conducting oxides has been a topic of debate for several decades. The point defect of most interest in this respect is the oxygen vacancy, but there are many conflicting reports on the shallow versus deep nature of its related electronic states. Here, using a hybrid quantum mechanical/molecular mechanical embedded cluster approach, we have computed formation and ionization energies of oxygen vacancies in three representative transparent conducting oxides: In 2 O 3 , SnO 2 , and ZnO. We find that, in all three systems, oxygen vacancies form well-localized, compact donors. We demonstrate, however, that such compactness does not preclude the possibility of these states being shallow in nature, by considering the energetic balance between the vacancy binding electrons that are in localized orbitals or in effective-mass-like diffuse orbitals. Our results show that, thermodynamically, oxygen vacancies in bulk In 2 O 3 introduce states above the conduction band minimum that contribute significantly to the observed conductivity properties of undoped samples. For ZnO and SnO 2 , the states are deep, and our calculated ionization energies agree well with thermochemical and optical experiments. Our computed equilibrium defect and carrier concentrations, however, demonstrate that these deep states may nevertheless lead to significant intrinsic n -type conductivity under reducing conditions at elevated temperatures. Our study indicates the importance of oxygen vacancies in relation to intrinsic carrier concentrations not only in In 2 O 3 , but also in SnO 2 and ZnO.

https://link.aps.org/pdf/10.1103/PhysRevMaterials.2.054604

Deep vs shallow nature of oxygen vacancies and consequent n -type carrier concentrations in transparent conducting oxides

Spatially-resolved cathodoluminescence spectroscopy of ZnO defects

This chapter aims to review analytical techniques for the detection of electrically active defects in semiconductor materials. In all cases, the operation principles, the strengths, and the weaknesses will be outlined and illustrated for state-of-the-art examples. Based on the impact of deep level defects on the main semiconductor parameters (resistivity, carrier lifetime, fixed space charge, etc.,) one can define different analysis methods: from simple resistivity measurements to more spectroscopic-like techniques, relying on capacitance or current transients obtained after applying bias or optical pulses to a diode structure. While in the pioneering days, Hall effect versus temperature was the technique of reference for deep level studies in silicon and germanium, nowadays, deep level transient spectroscopy is the standard, with high sensitivity for small relative concentrations of defects. In some cases, complementary information can be gathered from admittance spectroscopy, revealing also details on shallow levels in the band gap. However, it turns out that in many practical cases, the carrier lifetime and related device characteristics (generation and recombination current) are more sensitive than the spectroscopic techniques. The possibility for applying these techniques to nanometric structures will be discussed, eventually resulting in what can be considered as single-defect spectroscopies.

Analytical techniques for electrically active defect detection

Many linked processes occur concurrently in strongly excited semiconductors, such as interband and intraband absorption, scattering of electrons and holes by the heated lattice, Pauli blocking, bandgap renormalization and the formation of Mahan excitons. In this work, we disentangle their dynamics and contributions to the optical response of a ZnO thin film. Using broadband pump-probe ellipsometry, we can directly and unambiguously obtain the real and imaginary part of the transient dielectric function which we compare with first-principles simulations. We find interband and excitonic absorption partially blocked and screened by the photo-excited electron occupation of the conduction band and hole occupation of the valence band (absorption bleaching). Exciton absorption turns spectrally narrower upon pumping and sustains the Mott transition, indicating Mahan excitons. Simultaneously, intra-valence-band transitions occur at sub-picosecond time scales after holes scatter to the edge of the Brillouin zone. Our results pave new ways for the understanding of non-equilibrium charge-carrier dynamics in materials by reliably distinguishing between changes in absorption coefficient and refractive index, thereby separating competing processes. This information will help to overcome the limitations of materials for high-power optical devices that owe their properties from dynamics in the ultrafast regime.

https://iopscience.iop.org/article/10.1088/1367-2630/aba7f3/pdf

Ultrafast dynamics of hot charge carriers in an oxide semiconductor probed by femtosecond spectroscopic ellipsometry

Native and hydrogen-plasma induced shallow traps in hydrothermally grown ZnO crystals have been investigated by charge-based deep level transient spectroscopy, photoluminescence and cathodoluminescence microanalysis. The as-grown ZnO exhibits a trap state at 23 meV, while H-doped ZnO produced by plasma doping shows two levels at 22 meV and 11 meV below the conduction band. As-grown ZnO displays the expected thermal decay of bound excitons with increasing temperature from 7 K, while we observed an anomalous behaviour of the excitonic emission in H-doped ZnO, in which its intensity increases with increasing temperature in the range 140?300 K. Based on a multitude of optical results, a qualitative model is developed which explains the Y line structural defects, which act as an electron trap with an activation energy of 11 meV, being responsible for the anomalous temperature-dependent cathodoluminescence of H-doped ZnO.

/pdf/shallow-carrier-traps-in-hydrothermal-zno-crystals-2kdxy76czj.pdf

Shallow carrier traps in hydrothermal ZnO crystals

Electrical properties of a ZnO microwire grown by carbo-thermal evaporation, a ZnO thin film grown by pulsed-laser deposition and a hydrothermally grown ZnO bulk crystal are compared. Deep defects were investigated by means of deep-level transient spectroscopy. The defect level E3 was observed in all samples investigated. Additionally, a defect labelled T2 that preferentially forms under Zn-rich condition was detected in the microwire, the thin film and the bulk sample. Our results indicate that V Zn is likely involved in this defect.

H-S Kim

Papers

Comparative study of deep defects in ZnO microwires, thin films and bulk single crystals