Recent developments in rare-earth doped materials for optoelectronics

The origin of visible luminescencefrom “porous silicon”: A new interpretation

Degradation of silicon dioxide films is shown to occur primarily near interfaces with contacting metals or semiconductors. This deterioration is shown to be accountable through two mechanisms triggered by electron heating in the oxide conduction band. These mechanisms are trap creation and band‐gap ionization by carriers with energies exceeding 2 and 9 eV with respect to the bottom of the oxide conduction band, respectively. The relationship of band‐gap ionization to defect production and subsequent degradation is emphasized. The dependence of the generated sites on electric field, oxide thickness, temperature, voltage polarity, and processing for each mechanism is discussed. A procedure for separating and studying these two generation modes is also discussed. A unified model from simple kinetic relationships is developed and compared to the experimental results. Destructive breakdown of the oxide is shown to be correlated with ‘‘effective’’ interface softening due to the total defect generation caused by both mechanisms.

Impact ionization, trap creation, degradation, and breakdown in silicon dioxide films on silicon

Trap creation in both the bulk of silicon dioxide films and at its interfaces with silicon and metallic contacting electrodes is shown to depend on the presence of hot electrons in the oxide. For thick oxides (≥100 A), little trap creation is observed in the near‐thermal transport regime at electric field magnitudes less than 1.5 MV/cm. At these low fields, electrons travel in a streaming fashion close to the bottom of the oxide conduction band at energies less than that of the dominant optical phonon mode at 0.153 eV. At higher electric fields, the rate of bulk trap creation is proportional to the average energy of the hot electrons, which move in a dispersive manner and can reach energies as large as 4 eV. For thin oxides (<100 A) where electrons can travel ballistically (i.e., without scattering), traps are not produced unless injected electrons acquire more than 2 eV of kinetic energy from the applied electric field, regardless of the magnitude of this field. All data on both thin and thick oxides are shown to give a threshold for trap creation of about 2.3 eV by the hot electrons in the oxide conduction band. Also, trap creation is shown to be suppressed by lowering the lattice temperature below ≊150 K. Our results are discussed in terms of a model involving hydrogen‐related‐species release from defect sites near the anode by the hot electrons and the subsequent motion of these molecules to regions near the cathode where they can interact with the lattice and form the trapping sites which are measured.

Trap creation in silicon dioxide produced by hot electrons

Leakage currents introduced in the low‐field, direct‐tunneling regime of thin oxides during high‐field stress are related to defects produced by hot‐electron transport in the oxide layer. From these studies, it is concluded that the ‘‘generation’’ of neutral electron traps in thin oxides is the dominant cause of this phenomenon. Other mechanisms due to anode hole injection or oxide nonuniformities are shown to be unrealistic for producing these currents. Exposure of thin oxides to atomic hydrogen from a remote plasma is shown to cause leakage currents similar to those observed after high‐field stress, supporting the conclusion that these currents are related to hydrogen‐induced defects.

Mechanism for stress-induced leakage currents in thin silicon dioxide films

Electroluminescence from metal‐insulator‐semiconductor structures with silicon dioxide (SiO2) layers containing varying amounts of excess silicon (Si) in the form of tiny Si precipitates have been studied in detail. Bulk insulator emission from the Si islands is shown to dominate over emission from either the SiO2 matrix material or the metallic gate material by studies of oxide or metal gate material, voltage polarity, and insulator thickness dependencies. Several distinct spectral peaks are observed in the energy range from 1.5 to 5 eV which cannot be attributed to optical interference effects. The higher‐energy peaks show a strong dependence on electric field relative to that at the lowest energy (1.7–2 eV). The entire spectral amplitude shows a strong dependence on high‐temperature annealing and excess Si content, decreasing drastically with increasing Si or decreasing annealing temperature. These results are shown to be consistent with light emission during electronic transitions between discrete energy levels associated with Si islands and/or their interface with the SiO2 host matrix material. Quantum size effects, similar to those observed in semiconductor superlattices, are proposed as one possible explanation.

Electroluminescence studies in silicon dioxide films containing tiny silicon islands

Electron heating in silicon dioxide (SiO2) at electric fields ≲5 MV/cm is demonstrated using three different experimental techniques: carrier separation, electroluminescence, and vacuum emission. Gradual heating of the electronic carrier distribution is demonstrated for fields from 5 to 12 MV/cm with the average excess energy of the distribution reaching ≳4 eV with respect to the bottom of the SiO2 conduction band edge. Off‐stoichiometric SiO2 (OS‐SiO2) layers are shown to behave similarly to very thin SiO2(≲70 A in thickness) with a transition occurring from ‘‘cool’’ to ‘‘hot’’ electrons as the conduction mechanism changes from direct tunneling between silicon (Si) islands in the SiO2 matrix of the OS‐SiO2 material to Fowler‐Nordheim emission into the conduction band of the SiO2 regions. The relationship of electron heating to electron trapping, positive charge generation, interface state creation, and dielectric breakdown is treated. The importance of various scattering mechanisms for stabilizing the el...

Electron heating in silicon dioxide and off‐stoichiometric silicon dioxide films

The electrical characteristics of off‐stoichiometric silicon dioxide films have been investigated. The off‐stoichiometric oxide films studied had an excess atomic silicon (Si) content in the range of 1%–6%. Raman spectroscopy and photoconductivity measurements indicate that the excess Si is present as amorphous Si islands or small crystallites embedded in silicon dioxide (SiO2) forming a two‐phase material. These films differ in structure from previously reported films where dual dielectric layers of stoichiometric SiO2 and Si‐rich SiO2 with ≥13% excess atomic Si were used. These dual dielectric films were observed to produce electron injection from contacting electrodes via the Si‐rich SiO2 layer into the SiO2 at lower average electric fields. This injection mechanism was believed to be due to localized electric field enhancement near the SiO2–Si‐rich SiO2 interface caused by the curvature of the tiny Si islands in the SiO2 matrix. The current versus voltage characteristics of the off‐stoichiometric oxid...

Charge transport and trapping phenomena in off‐stoichiometric silicon dioxide films

The energy distribution of hot electrons in high‐field stressed amorphous silicon dioxide (SiO2) films have been measured using a vacuum emission technique. Electrons having average energies ≳2 eV and an energy relaxation length of λ≊32 A are observed at all fields studied (≳ 2 MV/cm). However, contrary to previous theoretical expectations, the majority of carriers in the distribution remains stable at all fields. The results are in agreement with other recent experiments (electroluminescence and carrier separation) which only measure the average energy of hot electrons in SiO2 and with recent Monte Carlo transport calculations which include scattering by both optical and acoustic phonon modes. Results for varying SiO2 thickness, metal gate thickness, oxide composition, and metal gate composition will be discussed.

Direct measurement of the energy distribution of hot electrons in silicon dioxide

An electrically alterable read‐only memory using silicon dioxide and silicon‐rich silicon dioxide layers capable of being cycled ≳107 times by minimizing electron charge trapping in the SiO2 layers of the device by incorporation of small amounts of silicon is discussed in detail. Charge transfer to and from a floating polycrystalline silicon layer from a control gate electrode is accomplished by means of a modified dual‐electron‐injector‐structure stack. This modified stack has the intervening silicon dioxide layer, which is sandwiched between silicon‐rich silicon dioxide injectors, replaced by a slightly off‐stoichiometric oxide containing between 1 and 6% excess atomic silicon above the normal 33% found in silicon dioxide. The operation of the electrically alterable device structures in terms of write/erase voltages, cyclability, breakdown, and retention is related to current‐voltage characteristics obtained from capacitors. A physical model based on direct tunneling between Si islands in the off‐stoichiometric oxide layer is proposed to account for the observed increase in the moderate electric field conductance and decrease in charge trapping in these oxide layers incorporated into devices and capacitors. This model and the observed current‐voltage characteristics are used to predict device operation for a variety of conditions.

F. L. Pesavento

Papers

Electroluminescence studies in silicon dioxide films containing tiny silicon islands

Electron heating in silicon dioxide and off‐stoichiometric silicon dioxide films

Charge transport and trapping phenomena in off‐stoichiometric silicon dioxide films

Direct measurement of the energy distribution of hot electrons in silicon dioxide

Enhanced conduction and minimized charge trapping in electrically alterable read‐only memories using off‐stoichiometric silicon dioxide films