Index of content:
Volume 119, Issue 18, 14 May 2016
- SPECIAL TOPIC: DEFECTS IN SEMICONDUCTORS
119(2016); http://dx.doi.org/10.1063/1.4948236View Description Hide Description
Transparent conductors play an increasingly important role in a number of semiconductor technologies. This paper reports on the defects and properties of Cadmium Oxide, a transparent conducting oxide which can be potentially used for full spectrum photovoltaics. We carried out a systematic investigation on the effects of defects in CdO thin films undoped and intentionally doped with In and Ga under different deposition and annealing conditions. We found that at low growth temperatures (<200 °C), sputter deposition tends to trap both oxygen vacancies and compensating defects in the CdO film resulting in materials with high electron concentration of ∼2 × 1020/cm3 and mobility in the range of 40–100 cm2/V s. Thermal annealing experiments in different ambients revealed that the dominating defects in sputtered CdO films are oxygen vacancies. Oxygen rich CdO films grown by sputtering with increasing O2 partial pressure in the sputter gas mixture results in films with resistivity from ∼4 × 10−4 to >1 Ω cm due to incorporation of excess O in the form of O-related acceptor defects, likely to be O interstitials. Intentional doping with In and Ga donors leads to an increase of both the electron concentration and the mobility. With proper doping CdO films with electron concentration of more than 1021 cm−3 and electron mobility higher than 120 cm2/V s can be achieved. Thermal annealing of doped CdO films in N2 ambient can further improve the electrical properties by removing native acceptors and improving film crystallinity. Furthermore, the unique doping behavior and electrical properties of CdO were explored via simulations based on the amphoteric defect model. A comparison of the calculations and experimental results show that the formation energy of native donors and acceptors at the Fermi stabilization energy is ∼1 eV and that the mobility of sputtereddeposited CdO is limited by a background acceptor concentration of ∼5–6 × 1020/cm3. The calculations offer an insight into understanding of the effects of defects on electrical properties of undoped and doped CdO and offer a potential to use similar methods to analyze doping and defect properties of other semiconductor materials.
Investigation of dopant clustering and segregation to defects in semiconductors using atom probe tomography119(2016); http://dx.doi.org/10.1063/1.4948238View Description Hide Description
The role of atom probe tomography in the investigation of clustering and segregation of dopants to lattice defects in semiconductors is highlighted on the basis of some selected salient illustrations obtained at the Groupe de Physique des Matériaux of Rouen (France). The instrument is shown to be able to map out the 3D distribution of chemical species in the three dimensions of space at the ultimate scale. Results related to clustering, segregation of dopants (As, B, and P) to grain boundaries, dislocation loops, and extended defects in silicon are discussed.
119(2016); http://dx.doi.org/10.1063/1.4948239View Description Hide Description
The role of defects in the chemical activity of the rutile TiO2(110) surface remains a rich topic of research, despite the rutile (110) being one of the most studied surfaces of transition-metal oxides. Here, we present results from hybrid functional calculations that reconcile apparently disparate views on the impact of donor defects, such as oxygen vacancies and hydrogen impurities, on the electronic structure of the (110) rutile surface. We find that the bridging oxygen vacancy and adsorbed or substitutional hydrogen are actually shallow donors, which do not induce gap states. The excess electrons from these donor centers tend to localize in the form of small polarons, which are the factual cause of the deep states ∼1 eV below the conduction band, often observed in photoelectron spectroscopy measurements. Our results offer a new framework for understanding the surface electronic structure of TiO2 and related oxides.
119(2016); http://dx.doi.org/10.1063/1.4948240View Description Hide Description
The threshold current density of n-type, tensile-strained Ge lasers strongly depends on the electron density. Although optical net gain analyses indicate that the optimum electron density should be on the order of 1 × 1020 cm−3 to get the lowest threshold, it is not a simple task to increase the electron density beyond the mid range of 1019 cm−3. The present paper analyzes the phenomenon where electron density is not proportional to phosphorus donor density, i.e., “saturation” phenomenon, by applying the so-called amphoteric defect model. The analyses indicate that the saturation phenomenon can be well explained by the charge compensation between the phosphorus donors (P+) and doubly negative charged Gevacancies (V2−).