T. W. Hickmott

Thermally stimulated ionic conductivity of sodium in thermal SiO2

The effect of Na+ concentration, oxide thickness, applied field, and metal electrode on thermally stimulated ionic conductivity (TSIC) measurements of positive‐ion motion in SiO2 grown on single‐crystal silicon has been studied. A surface trapping model, assuming blocking electrodes, has been used to analyze TSIC curves. Using a hyperbolic hearing rate (1/T ∝ time), for which analytic expressions for normalized first‐order TSIC curves are obtained, both energies E and preexponential factors s appropriate to the detrapping processes are obtained. Two distinct positive‐charge peaks are observed in TSIC curves. The magnitude of the low‐temperature peak, the α peak, is proportional to the amount of Na+ introduced into the sample. At high fields, the magnitude of the high‐temperature peak, the β peak, is independent of the evaporated Na+ concentration; its origin is uncertain but it may be due to mobile hydrogen. The temperature for the maxima in the TSIC curves, Tm, decreases with field for both the α and β p...

Annealing of surface states in polycrystalline‐silicon–gate capacitors

Charge injection at the polycrystalline‐silicon–SiO2 interface of phosphorus‐doped, polycrystalline‐silicon–gate MOS capacitors can cause excess currents during quasistatic C‐V measurements and hysteresis in high‐frequency C‐V curves. The effect of annealing in nitrogen and forming gas at temperatures between 300 and 850 °C on these instabilities as well as on surface states Nss and surface charge Qss has been studied. To distinguish effects occurring at the substrate Si‐SiO2 interface from effects occurring at the polycrystalline‐silicon–SiO2 interface, measurements are also reported of the effect of annealing Si‐SiO2 structures without gate electrodes, in the same temperature range and atmospheres. There is a minimum in Nss after anneal of samples without electrodes in forming gas at 400 °C. Reaction with hydrogen is essential to anneal surface states; thermal anneal is not sufficient. Some reaction, possibly formation of SiO, occurs above 450 °C that increases Nss and may increase Qss. Anneal of polycr...

Dipole layers at the metal‐SiO2 interface

The existence of dipole layers at the metal‐insulator interface or dipole layers in the bulk of the insulator can play an important role in determining electrical conduction and dielectric loss in insulators, and the nature of the barrier to electron injection into the insulator. The conventional analysis of metal‐SiO2‐semiconductor (MOS) structures is extended to include the effect of dipoles on capacitance‐voltage (C‐V) characteristics. The occurrence of both dipole layers and trapped charge can be established by measuring the flat‐band voltage of MOS capacitors as a function of oxide thickness. Such measurements are suitable for measuring changes in the work function at the metal‐insulator interface due to metal‐insulator reaction. C‐V measurements of Au‐SiO2‐Si capacitors are combined with thermally stimulated ionic conductivity (TSIC) measurements of Na+ in SiO2 to show that annealing of the Au‐SiO2 interface between 150° and 250 °C produces a positive dipole at the Au‐SiO2 contact while annealing be...

Production and Annealing of Color Centers in rf Sputtered SiO2 Films

Radio‐frequency sputtered SiO2 films can be made that contain a variety of optical absorption bands and electron spin resonance centers. In addition to absorption bands at 5.1, 5.8, and 7.6 eV that are typical of irradiated fused SiO2, two new absorption bands have been found. The G band at 5.4 eV and the H band at 6.55 eV are present in certain sputtered SiO2 films and develop further when they are annealed in forming gas or N2. The occurrence of these bands and the density of paramagnetic centers produced in SiO2 films when they are sputtered under different deposition conditions has been studied. The effect of annealing to 950°C in N2, O2, and forming gas on optical absorption bands and ESR centers has been determined. Absorption bands appear to be associated with oxygen deficiency in the sputtered SiO2 films since they will anneal completely at 930°C in oxygen. Hydrogen plays an important role in annealing of the B1 band at 5.1 eV, the E1′ band at 5.8 eV, and all the ESR centers. They anneal at 200°C ...

Barrier heights at the polycrystalline silicon‐SiO2 interface

Capacitance‐voltage measurements of the dependence of flat‐band voltage of polycrystalline‐silicon (polysilicon) gate metal‐oxide‐silicon capacitors on oxide thickness are used to measure the position of the Fermi level in degenerately doped polysilicon as a function of the doping of the polysilicon. For boron‐doped polysilicon, the experimental difference between the Fermi level in polysilicon and the intrinsic level in the substrate silicon, φ m s , is ∼0.54 V. The Fermi level is pinned within 20 meV of the valence band edge. For arsenic‐doped and phosphorus‐doped polysilicon, evaluation of φ m s is complicated by the occurrence of positive charge at the polysilicon‐SiO2 interface which is generated by reaction of doped polysilicon with SiO2. If such positive charge is close to the polysilicon‐SiO2 interface it acts as a dipole layer and lowers the values of φ m s . If positive charge is slightly deeper in the SiO2 it contributes to negative bias instability due to electron injection into polysilicon capacitors. If electron exchange of positive charge with electrons in polysilicon does not occur, positive charge generated at the polysilicon‐SiO2 interface appears as fixed positive charge. For As‐doped polysilicon, φ m s ∼−0.46 V; for P‐doped polysilicon, φ m s ∼−0.52 V. These experimental values of φ m s are influenced by positive charge at the polysilicon‐SiO2 interface. We conclude that the Fermi level for degenerately doped n‐type polysilicon is pinned close to but below the conduction band edge and does not depend on doping for carrier concentrations between 3×1019 and 4×1020 cm−3.

Stoichiometry and atomic defects in rf‐sputtered SiO2

Electron microprobe and helium ion backscattering are used to measure the stoichiometry of rf‐sputtered SiO2 films at a precision of ?1%. Both oxygen‐excess and oxygen‐deficient films occur. Optical absorption and electron spin resonance characterize atomic defects in the films. The correlation between stoichiometry and atomic defects is poor. The reproducibility of composition and of atomic defects from run to run, when sputtering conditions are held constant, is good.

Thermoluminescence and Color Centers in rf‐Sputtered SiO2 Films

Thermoluminescent (TL) glow curves of rf‐sputtered SiO2 films deposited on silicon have been measured and correlated with optical absorption of the films. As‐sputtered SiO2 films are thermoluminescent without further exposure to exciting radiation. Films with a strong optical‐absorption band at 7.6 eV (E band) give first‐order glow curves. They have a TL center with energy ∼ 0.66 eV and a low‐frequency factor s ∼ 104 sec−1, which suggests that trapping centers and recombination centers are physically separated. Irradiation of sputtered SiO2 films with x rays produces complex TL glow curves which have been analyzed by thermal cleaning techniques. Trapping centers with energies of 0.66, 0.84, and 1.04 eV are found. The complexity of TL glow curves in sputtered SiO2 films after x‐ray irradiation arises primarily from variations in s, rather than from a broad spectrum of trapping energies. The effect on TL glow curves of annealing SiO2 films in oxygen and forming gas has been studied. When samples with a stro...

Defect centers in oxygen‐deficient rf‐sputtered SiO2 films. I. Electron spin resonance

Two new defect centers have been observed by electron spin resonance (ESR) in rf‐sputtered SiO2 films with low metallic impurity content. At 77°K, as‐sputtered films have three ESR centers, two HX centers, and an HY center. The HY center, a hole center, has g = 2.008–2.009. The HX center has two components, the HXa center with axial symmetry and with ga∥ = 2.022 and ga⊥ = 2.037; the HXb center with gb = 1.984 which is split by hyperfine interaction with hydrogen. The hydrogen hyperfine interaction has axial symmetry, Ab∥ = 30.2 G and Ab⊥ = 63.6 G. Both the HX and HY centers show hyperfine splitting due to interaction with Si29. Annealing of as‐sputtered films in hydrogen‐containing atmospheres at 350°C eliminates the HY center, leaving the HX center. Annealing in N2 or O2 at 550°C also eliminates the HY center; the resulting HX resonance is identical to that obtained after hydrogen anneal. The activation energy for the appearance of the HX center is 0.036 eV when samples are cooled to 77°K. From similarit...

Defect centers in oxygen‐deficient rf‐sputtered SiO2 films. II. Thermoluminescence

Thermoluminescent (TL) glow curves of rf‐sputtered SiO2 films deposited on silicon have been measured and correlated with two new defect centers detected by electron spin resonance (ESR) of the films, the HX and HY centers. As‐sputtered films have low optical absorption and ESR absorption is dominated by the HY center. TL intensity is low after x irradiation. Annealing at 400°C in hydrogen, or at 550°C in nitrogen or oxygen, converts the HY center to the HX centers. TL intensity increases with annealing and goes through a maximum after annealing at 550°C in hydrogen or 650°C in nitrogen or oxygen. A characteristic TL glow curve pattern is associated with the HX centers. Trapping centers with energies of 0.74, 0.96, 1.04, and 1.20 eV are found. The complexity of TL glow curves is due primarily to variations in s, the frequency factor, rather than to a range of trap energies. It is proposed that trapping centers are associated with oxygen vacancies, and with Si–H bonds, in oxygen‐deficient SiO2 films, while...

Charge injection from polycrystalline silicon into SiO2 at low fields

Electron injection from polycrystalline silicon into thermal SiO2 at low fields is observed in polycrystalline‐silicon–SiO2–Si capacitors. C‐V, pulsed‐charge injection, and charge‐relaxation measurements show that the injected electrons are captured by centers in the SiO2. These trapping centers appear to be located at about 30 A from the polycrystalline‐silicon–SiO2 interface and are characterized by an energy level approximately 0.3 eV above the Fermi level of the degenerate n‐type polycrystalline silicon. Annealing of the samples in nitrogen or forming gas strongly affects the charge injection.