Six years after their birth, terahertz quantum-cascade lasers can now deliver milliwatts or more of continuous-wave coherent radiation throughout the terahertz range — the spectral regime between millimetre and infrared wavelengths, which has long resisted development. This paper reviews the state-of-the-art and future prospects for these lasers, including efforts to increase their operating temperatures, deliver higher output powers and emit longer wavelengths.

Terahertz quantum-cascade lasers

This paper reviews the present knowledge on tantalum pentoxide (Ta 2 O 5 ) thin films and their applications in the field of microelectronics and integrated microtechnologies. Different methods used to produce tantalum oxide layers are described, emphazing elaboration mechanisms and key parameters for each technique. We also review recent advances in the deposition of Ta 2 O 5 in the particular field of microelectronics where high quality layers are required from the structural and electrical points of view. The physical, structural, optical, chemical and electrical properties of tantalum oxide thin films on semiconductors are then presented and essential film parameters, such as optical index, film density or dielectric permittivity, are discussed. After a reminder of the basic mechanisms that control the bulk electrical conduction in insulating films, we carefully examine the origin of leakage currents in Ta 2 O 5 and present the state-of-the-art concerning the insulating behaviour of tantalum oxide layers. Finally, applications of tantalum oxide thin films are presented in the last part of this paper. We show how Ta 2 O 5 has been employed as an antireflection coating, insulating layer, gate oxide, corrosion resistant material, and sensitive layer in a wide variety of components, circuits and sensors.

Tantalum pentoxide (Ta2O5) thin films for advanced dielectric applications

Resistive switching effect in transition metal oxide (TMO) based material is often associated with the valence change mechanism (VCM). Typical modeling of valence change resistive switching memory consists of three closely related phenomena, i.e., conductive filament (CF) geometry evolution, conduction mechanism and temperature dynamic evolution. It is widely agreed that the electrochemical reduction-oxidation (redox) process and oxygen vacancies migration plays an essential role in the CF forming and rupture process. However, the conduction mechanism of resistive switching memory varies considerably depending on the material used in the dielectric layer and selection of electrodes. Among the popular observations are the Poole-Frenkel emission, Schottky emission, space-charge-limited conduction (SCLC), trap-assisted tunneling (TAT) and hopping conduction. In this article, we will conduct a survey on several published valence change resistive switching memories with a particular interest in the I-V characteristic and the corresponding conduction mechanism.

Conduction Mechanism of Valence Change Resistive Switching Memory: A Survey

Frequency down-conversion of solid-state laser sources to the mid-infrared spectral range using non-oxide nonlinear crystals

Combining scanning electron microscopy and electron-beam-induced current imaging with transport measurements, it is shown that the current flowing across a two-terminal oxide-based capacitor-like structure is preferentially confined in areas localized at defects. As the thin-film device switches between two different resistance states, the distribution and intensity of the current paths, appearing as bright spots, change. This implies that switching and memory effects are mainly determined by the conducting properties along such paths. A model based on the storage and release of charge carriers within the insulator seems adequate to explain the observed memory effect.

Electrical current distribution across a metal–insulator–metal structure during bistable switching

Introduction to infrared spectroscopy the properties of infared transparent substrates the measurement of oxygen and carbon in silicon the calculation of epitexial layer thickness the characterization of silicon dioxide and silicon nitride thin films the characterization of PSG and BPSG the characterization of amorphous silicon and related materials miscellaneous applications of infrared spectroscopy in microelectronics.

Infrared Characterization for Microelectronics

Defect state D (0.8 eV) was experimentally detected in Ta2O5 capacitors with ultrathin (physical thickness <10 nm) Ta2O5 films using zero-bias thermally stimulated current spectroscopy and correlated with leakage current. Defect state D can be more efficiently suppressed by using N2O rapid thermal annealing (RTA) instead of using O2 RTA for postdeposition annealing and by using TiN instead of Al for top electrode. We believe that defect D is probably the first ionization level of the oxygen vacancy deep double donor. Other important defects are Si/O-vacancy complex single donors and C/O-vacancy complex single donors.

Detection of oxygen vacancy defect states in capacitors with ultrathin Ta2O5 films by zero-bias thermally stimulated current spectroscopy

Cross-sectional transmission electron microscopy (XTEM), secondary ion mass spectrometry (SIMS) and capacitance measurements were used to study the effect of post-deposition annealing on Ta2O5/Si structures. A significantly thicker SiOx interfacial layer was formed at the Ta2O5/Si interface, if N2O was used instead of O2 for post-deposition annealing. This indicates that N2O is a stronger oxidizing agent than O2. It is known that the leakage current of Ta2O5 capacitors is greatly reduced if N2O is used instead of O2 for post-deposition annealing. This may also be partially explained by postulating that N2O annealing is more effective in the suppression of oxygen vacancies. Furthermore, the suppression of Si diffusion from the Si substrate into Ta2O5 due to the thicker SiOx interfacial layer can be another factor. The basic reason for the superiority of N2O is that the energy required to produce free O atoms is lower than that for O2. From this point of view, we can also predict that the use of NO will be worse than that of O2 because the energy required to produce free O atoms is higher than that of O2.

https://iopscience.iop.org/article/10.1143/JJAP.36.661/pdf

Evidence that N2O is a Stronger Oxidizing Agent than O2 for the Post-Deposition Annealing of Ta2O5 on Si Capacitors

Defect states responsible for leakage current in ultrathin (physical thickness <10 nm) tantalum pentoxide (Ta2O5) films were measured with a novel zero-bias thermally stimulated current technique. It was found that defect states A, whose activation energy was estimated to be about 0.2 eV, can be more efficiently suppressed by using N2O rapid thermal annealing (RTA) instead of using O2 RTA for postdeposition annealing. The leakage current was also smaller for samples with N2O RTA than those with O2 RTA for postdeposition annealing. Hence, defect states A are quite likely to be important in causing leakage current.

Detection of defect states responsible for leakage current in ultrathin tantalum pentoxide (Ta2O5) films by zero-bias thermally stimulated current spectroscopy

In this letter, the authors will point out that defect states related to oxygen vacancies in tantalum oxide capacitors can be suppressed by titanium doping, resulting in significant leakage current reduction. The theory is that titanium forms an acceptor which can move at high temperature and neutralize other donors. However, defect states which cannot be suppressed by titanium doping were detected. These are explained by H2O-related contamination occurring at low temperature (<400°C) during the cooling down period.

Wai Shing Lau

Papers

Infrared Characterization for Microelectronics

Detection of oxygen vacancy defect states in capacitors with ultrathin Ta2O5 films by zero-bias thermally stimulated current spectroscopy

Evidence that N2O is a Stronger Oxidizing Agent than O2 for the Post-Deposition Annealing of Ta2O5 on Si Capacitors

Detection of defect states responsible for leakage current in ultrathin tantalum pentoxide (Ta2O5) films by zero-bias thermally stimulated current spectroscopy

Mechanism of leakage current reduction of tantalum oxide capacitors by titanium doping