Al-Moatasem El-Sayed

On the volatility of oxide defects: Activation, deactivation, and transformation

Recent studies have clearly shown that oxide defects are more complicated than typically assumed in simple two-state models, which only consider a neutral and a charged state. In particular, oxide defects can be volatile, meaning that they can be deactivated and re-activated at the same site with the same properties. In addition, these defects can transform and change their properties. The details of all these processes are presently unknown and poorly characterized. Here we employ time-dependent defect spectroscopy (TDDS) to more closely study the changes occurring at the defect sites. Our findings suggest that these changes are ubiquitous and must be an essential aspect of our understanding of oxide defects. Using density-functional-theory (DFT) calculations, we propose hydrogen-defect interactions consistent with our observations. Our results suggest that standard defect characterization methods, such as the analysis of random telegraph noise (RTN), will typically only provide a snapshot of the defect landscape which is subject to change anytime during device operation.

Gate-sided hydrogen release as the origin of "permanent" NBTI degradation: From single defects to lifetimes

The negative bias temperature instability (NBTI) in pMOS transistors is typically assumed to consist of a recoverable (R) and a so-called permanent (P) component. While R has been studied in great detail, the investigation of P is much more difficult due to the large time constants involved and the fact that P is almost always obscured by R. As such, it is not really clear how to measure P and whether it will in the end dominate device lifetime. We address these questions by introducing a pragmatic definition of P, which allows us to collect long-term data on both large and nanoscale devices. Our results suggest that (i) P is considerably smaller than R, (ii) that P is dominated by oxide rather than interface traps and therefore (iii) shows a very similar bias dependence as R, and finally (iv) that P is unlikely to dominate device lifetime. We argue that a hydrogen-release mechanism from the gate-side of the oxide, which has been suspected to cause reliability problems for a long time [1-6], is consistent with our data. Based on these results as well as our density-functional-theory (DFT) calculations we suggest a microscopic model to project the results to operating conditions.

The “permanent” component of NBTI revisited: Saturation, degradation-reversal, and annealing

While the defects constituting the recoverable component R of NBTI have been very well analyzed recently, the slower defects forming the more “permanent” component P are much less understood. Using a pragmatic definition for P, we study the evolution of P at elevated temperatures in the range 200°C to 350°C to accelerate these very slow processes. We demonstrate for the first time that P not only clearly saturates, with the saturation value depending on the gate bias, but also that the degradation at constant gate bias can also slowly reverse. Furthermore, at temperatures higher than about 300° C, a significant amount of additional defects is created, which are primarily uncharged around Vth but contribute strongly to P at higher VG. Our new data are consistent with our recently suggested hydrogen release model which will be studied in detail using newly acquired long-term data.

Role of hydrogen in volatile behaviour of defects in SiO2-based electronic devices.

Charge capture and emission by point defects in gate oxides of metal–oxide–semiconductor field-effect transistors (MOSFETs) strongly affect reliability and performance of electronic devices. Recent advances in experimental techniques used for probing defect properties have led to new insights into their characteristics. In particular, these experimental data show a repeated dis- and reappearance (the so-called volatility) of the defect-related signals. We use multiscale modelling to explain the charge capture and emission as well as defect volatility in amorphous SiO2 gate dielectrics. We first briefly discuss the recent experimental results and use a multiphonon charge capture model to describe the charge-trapping behaviour of defects in silicon-based MOSFETs. We then link this model to ab initio calculations that investigate the three most promising defect candidates. Statistical distributions of defect characteristics obtained from ab initio calculations in amorphous SiO2 are compared with the experimentally measured statistical properties of charge traps. This allows us to suggest an atomistic mechanism to explain the experimentally observed volatile behaviour of defects. We conclude that the hydroxyl-E′ centre is a promising candidate to explain all the observed features, including defect volatility.

A mechanism for Frenkel defect creation in amorphous SiO2 facilitated by electron injection

Using density functional theory (DFT) calculations we demonstrate how electron injection can facilitate the creation of Frenkel defects in amorphous (a)-SiO2. The precursor sites composed of wide O-Si-O bond angles in amorphous SiO2 act as deep electron traps and can accommodate up to two extra electrons. Trapping of two electrons at these intrinsic sites results in weakening of a Si-O bond and creates an efficient bond breaking pathway for producing neutral O vacancies and [Formula: see text] interstitial ions characterized by low transition barriers. The low barriers for the migration of [Formula: see text] ions of about 0.2 eV facilitate the separation of created defects. This mechanism may have important implications for our understanding of dielectric breakdown and resistance switching in a-SiO2 based electronic and memory devices.

Optical signatures of intrinsic electron localization in amorphous SiO2

We measure and analyse the optical absorption spectra of three silica glass samples irradiated with 1 MeV electrons at 80 K, where self-trapped holes are stable, and use ab initio calculations to demonstrate that these spectra contain a signature of intrinsic electron traps created as counterparts to the holes. In particular, we argue that optical absorption bands peaking at 3.7, 4.7, and 6.4 eV belong to strongly localised electrons trapped at precursor sites in amorphous structure characterized by strained Si-O bonds and O-Si-O angles greater than 132°. These results are important for our understanding of the properties of silica glass and other silicates as well as the reliability of electronic and optical devices and for luminescence dating.

Atomistic Modeling of Defects Implicated in the Bias Temperature Instability

Capture and emission of carriers by point defects in gate dielectrics, such as SiO2 and HfO2, and at their interfaces with the substrate are thought to be responsible for performance and reliability issues in MOS devices, particularly dielectric degradation and the bias temperature instability (BTI). Ultra-thin silicon dioxide films are present at the interface between Si and high-κ oxides; thus it is hoped that understanding the defects in silica which contribute to BTI will also aid the reliability of devices containing high-κ oxides. This chapter reviews the state of the art of modeling oxygen deficiency defects implicated in both electron and hole trapping in amorphous silica (a-SiO2).