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

Abstract

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 SiO2 layers increases by 7% with an increase in the HNO3 concentration from 61 to 68 wt %. Measurements of valence band spectra clarify that this concentration increase causes the enhancement of the valence band discontinuity at the Si/SiO2 interface from 4.1 to 4.3 eV. When postmetallization annealing (PMA) treatment is performed at 400 °C in hydrogen on 〈aluminum (Al)/chemical SiO2/Si(100)〉 metal–oxide–semiconductor diodes, the leakage current density markedly increases, and this increase is attributed to a reaction between the Al electrode and the chemical SiO2 layer, resulting in a decrease in the SiO2 thickness. With PMA at 200 °C in hydrogen, on the other hand, the SiO2 thickness decreases only slightly to 1.3 nm. In this case, the leakage current density greatly decreases (e.g., 0.4 A/cm2 at VG=1 V and 5×10−3 A/cm2 at VG=−1 V), and consequently it becomes 1/3–1/10 of those for thermally grown SiO2 layers with the same thickness. The hump in the C–V curves disappears after PMA at 200 °C, indicating the elimination of interface states, and the interface state passivation is attributed to one of the reasons for the decrease in the leakage current density. Measurements of the valence band spectra show that another reason for the decrease in the leakage current density by PMA are an increase in the band discontinuity at the Si/SiO2 interface, and the elimination of SiO2 gap states.

Acoustic spectroscopy and electrical characterization of SiO2/Si structures with ultrathin SiO2 layers formed by nitric acid oxidation

“…Using the chemical SiO 2 layers formed in HNO 3 , the formation of ultrathin * E-mail: bury@fel.uniza.sk SiO 2 layers with a leakage current density much lower than those of thermally grown SiO 2 layers, or layers prepared by a different chemical oxidation procedure with the same thickness [2], can be performed. Although the density of interface states present at SiO 2 /Si interfaces is usually low, it can seriously affect the electrical characteristics of metal-oxide-semiconductor (MOS) devices.…”

“…Recently, the nitric acid (HNO 3 ) oxidation method of Si (NAOS), which can be performed at relatively low temperatures (∼120°C), was developed [1][2][3]. Using the chemical SiO 2 layers formed in HNO 3 , the formation of ultrathin * E-mail: bury@fel.uniza.sk SiO 2 layers with a leakage current density much lower than those of thermally grown SiO 2 layers, or layers prepared by a different chemical oxidation procedure with the same thickness [2], can be performed.…”

Acoustic Spectroscopy of Nanostructures in Condensed Matter

“…A silicon-silicon dioxide (Si/SiO 2 ) structure mostly prepared by thermal oxidation at above 800°C in oxidizing atmospheres is widely used for MOS devices [2]. Because the high temperature oxidations results in high interfacial stress producing the interface defect states, the low temperature direct oxidation methods such as plasma oxidation or method of nitric acid oxidation of Si were developed [3]. Although due to the many improvements in preparation technology the density of interface states, especially at SiO 2 /Si interfaces is usually very low, it can still seriously affect electrical properties of MOS devices formed from these structures.…”

Fabrication of Al2O3/Al structure by nitric acid oxidation at room temperature

“…Immersion of Si wafers in azeotropic HNO 3 (i.e., 68wt% HNO 3 aqueous solutions at 120°C) forms an ultrathin (1.2∼1.4 nm) SiO 2 layer with a leakage current density lower than those of thermally grown SiO 2 with the same thickness [6][7][8]. Si oxidation by use of 98wt% HNO 3 aqueous solutions can produce an ultrathin SiO 2 layer with a lower leakage current density, i.e., lower than that of a silicon oxynitride layer with the same equivalent oxide thickness [9].…”

“…A low temperature oxidation method using HNO 3 , applicable to Si [6][7][8][9][10][11] and SiC devices [12][13][14][15], has been developed by us. Immersion of Si wafers in azeotropic HNO 3 (i.e., 68wt% HNO 3 aqueous solutions at 120°C) forms an ultrathin (1.2∼1.4 nm) SiO 2 layer with a leakage current density lower than those of thermally grown SiO 2 with the same thickness [6][7][8].…”