Osamu Maida

Nitric acid oxidation of Si to form ultrathin silicon dioxide layers with a low leakage current density

Ultrathin silicon dioxide (SiO2) layers with excellent electrical characteristics can be formed using the nitric acid oxidation of Si (NAOS) method, i.e., by immersion of Si in nitric acid (HNO3) solutions. The SiO2 layer formed with 61 wt % HNO3 at its boiling temperature of 113 °C has a 1.3 nm thickness with a considerably high density leakage current. When the SiO2 layer is formed in 68 wt % HNO3 (i.e., azeotropic mixture with water), on the other hand, the leakage current density (e.g., 1.5 A/cm2 at the forward gate bias, VG, of 1 V) becomes as low as that of thermally grown SiO2 layers, in spite of the nearly identical SiO2 thickness of 1.4 nm. Due to the relatively low leakage current density of the NAOS oxide layer, capacitance–voltage (C–V) curves can be measured in spite of the ultrathin oxide thickness. However, a hump is present in the C–V curve, indicating the presence of high-density interface states. Fourier transformed infrared absorption measurements show that the atomic density of the SiO...

Ultrathin silicon dioxide layers with a low leakage current density formed by chemical oxidation of Si

Chemical oxidation of Si by use of azeotrope of nitric acid and water can form 1.4-nm-thick silicon dioxide layers with a leakage current density as low as those of thermally grown SiO2 layers. The capacitance–voltage (C–V) curves for these ultrathin chemical SiO2 layers have been measured due to the low leakage current density. The leakage current density is further decreased to ∼1/5 (cf. 0.4 A/cm2 at the forward gate bias of 1 V) by post-metallization annealing at 200 °C in hydrogen. Photoelectron spectroscopy and C–V measurements show that this decrease results from (i) increase in the energy discontinuity at the Si/SiO2 interface, and (ii) elimination of Si/SiO2 interface states and SiO2 gap states.

Decrease in the leakage current density of Si-based metal–oxide–semiconductor diodes by cyanide treatment

Crown-ether cyanide treatment, which includes the immersion of Si in KCN solutions containing 18-crown-6 molecules, is found to greatly decrease the leakage current density of Si-based metal–oxide–semiconductor (MOS) diodes. The decrease by one order of magnitude for the single crystalline Si-based MOS diodes is attributable to the elimination of Si/SiO2 interface states by reaction with cyanide ions and formation of Si–CN bonds. The reduction in the leakage current density by two orders of magnitude is caused for polycrystalline Si-based MOS diodes, and this decrease is attributed to the passivation of trap states in poly-Si as well as the interface states.

Effects of postmetallization annealing on ultrathin SiO2 layer properties

Observation of both longitudinal optical and transverse optical phonons of ∼1.3 nm ultrathin silicon dioxide (SiO2) layers formed by immersion in nitric acid shows that the SiO2 density increases by 16% after postoxidation annealing (POA) at 900 °C. For the SiO2 layers without POA, postmetalization annealing (PMA) greatly decreases the SiO2 thickness from 1.3 to 0.2 nm, the effect of which is attributable to the reaction of aluminum with SiO2 to form a metallic mixture of aluminum oxide and Si. For SiO2 layers with POA, PMA decreases the SiO2 thickness to a lesser extent (from 1.4 to 0.9 nm), because of the suppression of aluminum diffusion into SiO2 due to its dense structure. PMA is found to decrease the interface state density but increase the leakage current density.

Postoxidation Annealing Treatments to Improve Si/Ultrathin SiO2 Characteristics Formed by Nitric Acid Oxidation

We have succeeded in the fabrication of Si-based metal-oxide-semiconductor diodes with an ultrathin (i.e., ∼1.4 nm thick) silicon dioxide (SiO 2 ) layer whose leakage current density is lower than those for thermally grown SiO 2 layers with the same thickness. The leakage current density of the as-grown SiO 2 layers formed in concentrated nitric acid (HNO 3 ) solutions (i.e., ∼3 A/cm 2 at the forward gate bias, V G , of 1 V) is as high as those for thermal SiO 2 layers. The leakage current density is decreased to ∼1.5 and ∼1 A/cm 2 at V G = 1 V by postoxidation annealing (POA) at 400 and 500°C (in hydrogen), respectively, and the decreases are mainly attributed to the elimination of interface states and slow states, respectively. The vibrational frequency of longitudinal optical (LO) phonons of Si-O asymmetric stretching vibration increases with the POA temperature, while that of the transverse optical (TO) phonons remains constant. From the analysis of these vibrational frequencies, the atomic density of the as-grown SiO 2 layer and that after POA at 500°C are estimated to be 2.34 and 2.56 × 10 22 cm 3 , respectively, i.e., higher than that of crystalline SiO 2 bulk of 2.28 × 10 22 cm 3 . Measurements of the valence band spectra for the Si/SiO 2 structure show that the valence band discontinuity energy at the Si/SiO 2 interface is increased from 4.3 to 4.6 eV by POA at 500°C. Therefore, the other reason for the decrease in the leakage current density by POA is a reduction in the tunneling probability through SiO 2 caused by the increase in the band discontinuity energy. Postmetallization annealing treatment at 200°C in hydrogen performed after POA further decreases the leakage current density, showing that atomic hydrogen passivates defect states more effectively than molecular hydrogen.

Characterization of diamond ultraviolet detectors fabricated with high-quality single-crystalline chemical vapor deposition films

We have fabricated high-performance ultraviolet (UV) detectors with high-quality undoped and B-doped homoepitaxial diamond layers which were sequentially grown on a high-pressure/high-temperature-synthesized (HPHT) type-Ib (100) substrate by means of a high-power microwave-plasma chemical vapor deposition method. The detector performance measured had large quantum efficiencies due to an effective built-in current amplification function, fast temporal responses, and high UV/visible sensing ratios although the HPHT substrate used had considerable amounts of various defects inducing visible light absorptions and slow detector responses. The usefulness of the bilayer detector structure employed is discussed.

Preparation and Characterization of High-k Praseodymium and Lanthanoid Oxide Thin Films Prepared by Pulsed Laser Deposition

Several lanthanoid oxide thin films such as those of PrOx, Sm2O3, Tb4O7, Er2O3 and Yb2O3 have been prepared on Si(100) wafers by the pulsed laser deposition method (PLD). PrOx film shows thin SiO2-equivalent oxide thickness (EOT) and low leakage current simultaneously. On the other hand, SmOx thin film does not show good properties. It is revealed by XPS spectra of the PrOx film that the deposition in O2 ambient of 0.2 Torr produces an interfacial SiO2 or silicate layer. The sample deposited in a high vacuum at RT has only an ultra-thin interfacial layer, but hysteresis in the C–V characteristic and leakage current are large. Other techniques have been carried out to reduce the energy of ablated particles in order to prevent the growth of an interfacial layer. In the deposition method using a shadow mask, very flat thin films were obtained. However, the deposition rate was very low, and growth of the interfacial layer could not be prevented. By enlarging the distance between substrate and target, the smallest EOT with the PrOx film in our study has been obtained by the reduction of the energy of ablated particles.

Interface states for HfO2∕Si structure observed by x-ray photoelectron spectroscopy measurements under bias

A 1.0nm silicon nitride (SiN) layer can prevent reaction between HfO2 and Si completely. In this case, the interface state spectra obtained from x-ray photoelectron spectroscopy measurements under bias have two peaks above and below the midgap, attributable to Si dangling bonds interacting weakly with an atom in SiN, indicating a high atomic density of the SiN layer. When a HfO2 layer is deposited on a 1.0nm SiO2 layer, the SiO2 thickness increases to 1.6nm. For this structure, one interface state peak is present near the midgap, attributable to isolated Si dangling bonds, indicating a low atomic density.

Low temperature formation of SiO2∕Si structure by nitric acid vapor

Si can be oxidized at temperatures between 350 and 500°C by use of nitric acid (HNO3) vapor, resulting in 5–10nm SiO2∕Si structure. The oxidation kinetics follows a parabolic law, indicating that diffusion of oxidizing species (i.e., oxygen atoms generated by decomposition of HNO3 molecules) through SiO2 is the rate-determining step. The leakage current density flowing through the SiO2 layer formed at 350°C follows the Poole-Frenkel mechanism, indicating the presence of trap states in the SiO2 band gap, and the trap energy is estimated to be 0.57eV below the SiO2 conduction band. On the other hand, the leakage current density for the SiO2 layer formed at 500°C follows the Fowler-Nordheim mechanism, showing the absence of trap states.

Evaporation and Expansion of Poly-tetra-fluoro-ethylene Induced by Irradiation of Soft X-Rays from a Figure-8 Undulator

The effects of soft X-ray irradiation of poly-tetra-fluoro-ethylene (PTFE) have been investigated using a figure-8 undulator. In the case of high-intensity irradiation, the surface temperature of the irradiated region increased and PTFE near the surface was evaporated effectively. In contrast, the PTFE surface swelled by the under low-intensity irradiation with the insertion of Al filters more than 9 µm thick. This reaction was found to be strongly dependent on the flux. The surface profile is largely determined by the relationship between fragment desorption and porous structure generation, and an increase in the surface temperature enhances the fragment desorption and has considerable influence on the surface profile. Al filter insertion decreases the flux and suppresses low-energy photons which are absorbed near the surface. Hence irradiated photons are composed of high-energy photons which penetrate deeply, and the surface temperature is difficult to increase by Al-filter-inserted irradiation.