Formation Of Interface States Research Articles

Post-irradiation cracking of H2 and formation of interface states in irradiated metal-oxide-semiconductor field-effect transistors

Molecular hydrogen is alternately introduced into and removed from the gate oxide of irradiated metal-oxide-semiconductor field-effect transistors at room temperature by changing the ambient between forming gas (10/90% H2/N2) and nitrogen. Using charge pumping, it is observed that H2 causes a simultaneous buildup of interface states and decrease of trapped positive charge. The results are explained by a reaction sequence in which H2 is cracked to form mobile H+, which under positive bias drifts to the Si/SiO2 interface, and reacts to produce a dangling-bond defect. The rate limiting step over most of the time domain studied is the cracking process. Two types of cracking sites are modeled by molecular orbital calculations: oxygen vacancies (E′ centers) and broken bond hole traps (BBHTs). Initial- and final-state energies, as well as the activation energies, are calculated. The calculations indicate that the latter is the more likely H2 cracking site. The combined experimental and theoretical results suggest that at least 15% of the trapped positive charge is at sites similar to the BBHT sites. Implications of the model and similarities between interface-state formation by cracked H2 and irradiation are discussed.

Hot-Hole-Induced Interface State Generation in p-Channel MOSFETs with Thin Gate Oxide

Interface state generation in p-channel metal oxide semiconductor field effect transistors (MOSFETs) due to uniform hot-hole injection into the thin gate oxide and postinjection behavior of the generated interface states are investigated. The absence of a significant oxide electric field dependence of interface state generation supports the existence of a single generation mechanism. The experimental results suggest a mechanism associated with the interaction of neutral hydrogen for interface state generation. The number of generated interface states is linearly proportional to the number of trapped holes regardless of oxide thickness, which indicates active involvement of hole trapping in the formation of interface states. In contrast to the reports describing time-delayed formation of interface states during relaxation, we observed spontaneous postinjection annihilation of the generated interface states which was also linearly related to the number of detrapped holes. The experimental results suggest the presence of a reversible atomic structure at the Si/SiO2 interface with respect to trapping and subsequent detrapping of holes. The annealing efficiency of the generated interface states depends on the postinjection oxide field polarity.

Study of device parameters for analog IC design in a 1.2 mu m CMOS-SOI technology after 10 Mrad (for hadron colliders)

Radiation-hardened SOI-CMOS is a candidate technology for mixed analog-digital signal processing electronics in experiments at future high luminosity hadron colliders. In view of this application, the analog parameters of transistors realized in the Thomson TMS HSO13-HD technology were studied before and after exposure to total doses similar to those expected in the CERN Large Hadron Collider (LHC). The devices were irradiated up to 10 Mrad with /sup 60/Co and 3*10/sup 14/ neutrons/cm/sup 2/. The changes in the threshold voltage V/sub th/ of front and back gates, leakage current, and transconductance are studied. Subthreshold slope and charge pumping techniques were used to estimate the interface state formation. Noise measurements in the 0.1-kHz to 3-MHz bandwidth range were performed. The noise is comparable to that of a bulk CMOS process. Postirradiation effects were regularly monitored, and threshold voltage rebound for n-channel transistors was observed.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Hot carriers effects in polycrystalline silicon thin-film transistors

The application of bias-stress with high source-drain voltage and negative gate voltage (transistor in off-status) produces a marked reduction in the off-current as well as a transconductance degradation. These effects have been explained in terms of hot-holes injection into the gate insulator and formation of interface states near the drain.

Dynamic effects in hot-carrier degradation relevant for CMOS operation

We describe different dynamic degradation effects in n- and p-MOSFETs as they are clearly proven and generally accepted to date. It turns out that they are connected with time constants in the oxide and at the interface and that time constants related to the device operation are too short to be relevant in this context. The effects are detrapping of fixed charges, the slow movement of holes in the oxide, an enhanced-degradation effect caused by alternating voltage conditions, during dynamic stress, and a post-stress interface state formation effect in nitride passivated n-MOSFETs. Furthermore, we discuss the relevance of those effects, under different operation conditions, finding that the fast non-stationary effects are of little significance. Only the “slow” effects, with time constants of seconds and above, play a role in reliability issues of MOSFETs.

Model for Si–SiO2 interface state formation during irradiation and during post-irradiation exposure to hydrogen environment

When the oxide of a previously x-ray irradiated metal–oxide–semiconductor field-effect transistor is exposed to a hydrogen environment at room temperature the number of interface traps is observed to double, and the number of fixed oxide traps to decrease by a similar amount. From an analysis of this post-irradiation behavior we derive a model for the formation of interface states during and immediately after irradiation. In this model holes formed in the oxide during the irradiation split ≡SiH bonds in the oxide to form atomic hydrogen and ≡Si+. At room temperature the atomic hydrogen quickly dimerize to form H2. The H2 is then cracked by the ≡Si+ to form H+. Each H+ is transported to the interface, where it forms an interface state. This model also explains the results of a recent experiment which had seemed to indicate that no H+ was formed in the oxide as a result of irradiation.

The effect of liquid-phase sintering on the properties of Pr6O11-based ZnO varistors

Conventionally batched varistors in the ZnO–Co3O4–Pr6O11 system that were sintered at or above 1280 °C exhibited a sharp increase in the average breakdown voltage per grain boundary. In these samples, barrier heights and donor concentration values, derived from capacitance-voltage analysis, exhibited sharp changes at the same soaking temperature. At or above 1280 °C, grain growth in this system proceeds with the assistance of an eutectic liquid. The improvement of the distribution of Pr6O11, by precipitation of Pr(OH)3 during batching, resulted in varistors markedly more insensitive to firing temperature than conventionally batched ones. The presence of a liquid phase, at least during grain growth, results in an efficient distribution of grain boundary materials and dopants in general. Suitable chemisorbed gases along the grain boundaries are believed to be responsible for the formation of interface states. Segregated materials appeared to provide for enhanced transport of gaseous species along the grain boundaries. The need for the addition of oxides with large ionic-radius cations, i.e., varistor-forming oxides such as bismuth, barium, praseodymium, or lanthanum oxide, to varistor compositions is explained within this context.

Si-SiO/sub 2/ interface state generation during X-ray irradiation and during post-irradiation exposure to a hydrogen ambient (MOSFET)

The changes in the number of interface traps and oxide traps which result when a previously X-ray irradiated MOSFET is exposed to a hydrogen ambient were measured. At room temperature, a doubling in the number of positively charged oxide traps was observed. The increase in the number of interface states is approximately the same as the decrease in the number of oxide traps. These changes could result from the cracking of H/sub 2/ by positively charged radiation-induced defects in the oxide. This cracking reaction produces H/sup +/ ions which are then transported to the interface (under positive gate bias), where they react to form interface states. The necessity of positively charged radiation-induced defects in the oxide to explain the postirradiation buildup of interface states leads to the proposal of a model for the buildup of interface states during and immediately after irradiation. This model supports recent models of interface state formation as resulting from H/sup +/ transport and clarifies the origin of the H/sup +/.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Post-irradiation formation of Si-SiO2 interface states in a hydrogen atmosphere at room temperature

Si MOSFETs were irradiated with x-rays and then exposed to various partial pressures of H2 at either room temperature or 125 °C. The number of interface traps and the net positive oxide trapped charged were measured during the hydrogen exposure using spectroscopic charge pumping techniques. During the hydrogen exposure the gate electrode was held at a positive bias to maintain a field of 0.65 MV/cm across the gate oxide. It was found that during the room temperature hydrogen exposure the number of interface traps increased by a factor of about two. The change in the oxide trapped charge during hydrogen exposure indicated that the decrease in the number of positively charged oxide traps was approximately the same as the increase in the number of interface traps. The time evolution and bias dependence of these changes are explained by a model that we previously proposed. In this model positively charged radiation induced defects in the oxide crack the H2 to form H+. Under positive gate bias the H+ then drifts to the Si-SiO2 interface where it forms an interface state, while at the same time removing positive charge from the oxide.

A new self-consistent modeling approach to investigating MOSFET degradation

A modeling tool is presented that allows a complete analysis of a DC stress experiment without assuming the location and amount of trapped oxide charges and interface states. To describe the buildup of oxide damage, a semiempirical rate equation approach is outlined. A completely self-consistent calculation is presented of the time dependence of the DC stress experiment. This calculation monitors the amount and location of charges built up in the 2-D oxide region during the stress line. The model includes competing trap mechanisms such as the formation of interface states and fixed oxide traps. This permits consideration of n- and p-channel MOSFETs with the same model. The calculations are compared to DC stress measurements on n- and p-channel devices with gate lengths of 0.65 mu m that are typical for 16-Mb DRAMs.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Relationship between trapped holes, positive ions, and interface states in irradiated Si-SiO2 structures

We show that trapped holes at the Si-SiO2 interface account for all of the interface states generated by gate-positive irradiation of metal-oxide-silicon structures. The field-induced conversion of trapped holes to interface states is found to be the rate-limiting step in interface state buildup. Interface-state generation by hole trapping at the Si-SiO2 interface also plays a role for gate-negative irradiation. However, our experiments demonstrate an additional avenue for interface-state formation under these conditions. Holes created in the SiO2 layer are swept to the Al-SiO2 interface where they release positive ions. The transport of these ions to the Si-SiO2 interface under gate-positive field results in new interface states. Our data do not support models involving liberation of protons in the bulk of the SiO2 layer by hole transport through the oxide.

Interface state generation by negative gate-bias irradiation of MOS structures

Summary form only given. Irradiation of MOS structures under a negative gate-bias results in trapped positive charge at both the Al-SiO/sub 2/ and Si-SiO/sub 2/ interfaces. It is shown that the trapped charge at the Si-SiO/sub 2/ interface can be converted to interface states in a manner consistent with trapped hole models. This finding is significant because it appears to contradict other models which implicate H/sup +/ ions (in contrast to holes) as being primarily responsible for radiation-induced interface state generation. It is also demonstrated that trapped positive charge at the Al-SiO/sub 2/ interface can play a role in the interface state formation. >

Dynamic stress experiments for understanding hot-carrier degradation phenomena

The results of inhomogeneous hot-carrier injection experiments in which static and dynamic stresses are applied to n-MOSFETs are presented. A qualitative model in which holes play a key role for the final formation of interface states is developed. The holes are injected and trapped within the strained oxide region. The hole-injection process is controlled by hole traps in the oxide, close to the interface. With this model, a large number of dynamic and static hot-carrier stress experiments are consistently explained. Finally, a simple method by which the lifetime of a device under real operation can be predicted from dynamic stress experiments is given.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Total dose radiation hardness of MOS devices in hermetic ceramic packages

The total-dose radiation hardness of hardened MOS transistors is experimentally shown to be degraded by hydrogen gas commonly trapped in hermetic-ceramic-package cavities. This introduces another variable to be considered in the manufacture and hardness assurance of total-dose-hardened integrated circuits. The observation also extends the link between hydrogen and interface-state formation to the understanding of hydrogen's role in ionizing radiation effects. It is concluded from experiments in which hydrogen is diffused into irradiated oxides after Co-60 exposure that the diffusing hydrogen interacts with independently created radiation damage to form interface states. >

Dynamic channel hot-carrier degradation of NMOS transistors by enhanced electron-hole injection into the oxide

Hot-carrier stressing has been carried out on NMOS transistors under dynamic stressing conditions in which the drain voltage is kept constant and a square wave is applied to the gate such that electrons are injected into the oxide during the upper part of the cycle, while the lower part is varied so as to vary the oxide hole current. It is found that maximum degradation occurs when the hole current is maximum, indicating that the injection of both holes and electrons into the oxide is necessary for enhanced surface state generation, and supporting the model by which the trapping of holes and the subsequent neutralization by injected electrons leads to the formation of interface states.

Photoluminescence and Photocurrent Studies of Interfacial Defects at the InP / 1m KCl Electrolyte Junction

not Available.

The origin of a highly resistive layer at a growth-interrupted interface of GaAs grown by molecular-beam epitaxy

The origin of the formation of a highly resistive layer around a growth-interrupted interface in GaAs has been studied by capacitance-voltage carrier profiling and deep-level transient spectroscopy. It is concluded that the origin of the highly resistive layer is attributable to the formation of interface states created by exposure of the GaAs surface to air. The Fermi level at the interface is pinned in n-type GaAs, but is not in p-type GaAs.

Optical emission properties of interface states for metals on III-V semiconductor compounds.

We report the first study of optical-emission properties associated with interface-state formation for metals on III-V semiconductor surfaces. Cathodoluminescence spectroscopy reveals discrete levels distributed over a wide energy range and localized at the microscopic interface. Our results demonstrate the influence of the metal, the semiconductor, and its surface morphology on the energy distributions. Evolution of spectral features with interface formation, particularly above monolayer metal coverage, is correlated with Fermi-level movements and Schottky-barrier heights.

Evaluation of hot carrier degradation of N-channel MOSFET's with the charge pumping technique

Hot carrier degradation in n-channel MOSFET transistors has been evaluated using the charge pumping technique in addition to the conventional I_{ds}-V_{gs} measurements. It is shown that the generation of large amounts of interface states is the primary result of moderate hot carrier stress. The simultaneous injection and recombination of injected electrons and holes is suggested to be responsible for this formation of interface states. The net charge in the oxide and interface states after any hot carrier stress is shown to be positive.

Strain Modification of Alloy Fluctuations in CdTe/(Cd,MnTe) Superlattice Systems

AbstractWe have calculated the effect of strains on the valence band offset and bandgap in the CdTe/(Cd,Mn)Te superlattice system for various growth directions and alloy compositions. We find that strain can modify the effect of alloy fluctuations on band offset and bandgap. The consequences of this for the formation of interface states will be discussed.