Doping limits in p-type oxide semiconductors

Abstract

The ability to dope a semiconductor depends on whether the Fermi level can be moved into its valence or conduction bands, on an energy scale referred to the vacuum level. For oxides, there are various suitable n-type oxide semiconductors, but there is a marked absence of similarly suitable p-type oxides. This problem is of interest not only for thin-film transistors for displays, or solar cell electrodes, but also for back-end-of-line devices for the semiconductor industry. This has led to a wide-ranging search for p-type oxides using high-throughput calculations. We note that some proposed p-type metal oxides have cation s-like lone pair states. The defect energies of some of these oxides were calculated in detail. The example SnTa2O6 is of interest, but others have structures more closely based on perovskite structure and are found to have more n-type than p-type character. Graphic abstract

Smart Design of Fermi Level Pinning in HfO2‐Based Ferroelectric Memories

“…Also given are the calculated CNLs for a defect-free interface. [25,48] further reduction of the VBO. At this point, the so-called doping limit according to the allowed doping range becomes important: In a previous study of La-doped HfO 2 , [66] the nearly independent position of the Fermi level from the La doping concentration was attributed to the doping limit.…”

“…This is the so-called doping limit, well known for semiconductors and insulators. [25,48] For Si and III-V as well as II-VI semiconductors, the possible doping range energetically covers or even exceeds the full band gap, and in these cases, there is no practical limitation on doping. [49] For HfO 2 , the situation is completely different: According to Robertson et al, [25] the doping range amounts to only 1.3 eV within the 5.8 eV wide band gap.…”

“…The observed VBO of 3.0 eV is nearly in line with the FL pinning model, assuming a defect-free CNL (see Table 3). Here, the doping range shifted toward the VBM by oxygen [25,48] The energy scale assumes a band gap of 5.4 eV and an electron affinity of 2.2 eV for HZO. The Ir and IrO 2 VBOs are experimentally observed.…”

Control of Hole Density in Russellite Bi2WO6 via Intentional Chemical Doping

“…Because of the metastable nature of Sn 2+ , which has an electronic configuration of [Kr]­4 d 10 5 s 2 , some Sn 2+ -based oxides have only marginal phase stabilities, , which may induce difficulties in intentional chemical doping. , In contrast, Bi 3+ -based oxides are rather stable because of the stability of Bi 3+ , which has an electronic configuration of [Xe]­5 d 10 6 s 2 . Another grouping is based on the dimensionality of the crystal structure, namely, three-dimensional (3D) structures, such as pyrochlore and double-perovskite structures, − or two-dimensional (2D) layered structures. − It has been determined experimentally and theoretically , that oxygen vacancies ( V O ), which generate electrons and electrically compensate for holes, tend to form more easily in 3D oxides than in 2D oxides. Consequently, the hole generation efficiencies (η), which is defined as the ratio between the appeared hole carrier density and acceptor density, of 2D Sn 2+ -oxides (1.4–1.7%) are greater than those of 3D Sn 2+ -oxides (0.03–0.005%), , whereas those of the acceptor-ion-doped 3D Bi 3+ -oxides are extremely low (0.0003–0.001%). ,, …”

“…Another grouping is based on the dimensionality of the crystal structure, namely, three-dimensional (3D) structures, such as pyrochlore and doubleperovskite structures, 16−19 or two-dimensional (2D) layered structures. 20−22 It has been determined experimentally 23 and theoretically 24,25 that oxygen vacancies (V O ), which generate electrons and electrically compensate for holes, tend to form more easily in 3D oxides than in 2D oxides. Consequently, the hole generation efficiencies (η), which is defined as the ratio between the appeared hole carrier density and acceptor density, of 2D Sn 2+ -oxides (1.4−1.7%) are greater than those of 3D Sn 2+ -oxides (0.03−0.005%), 21,23 whereas those of the acceptor-ion-doped 3D Bi 3+ -oxides are extremely low (0.0003−0.001%).…”