Nitrogen Plasma Exposure Research Articles

(Invited) Impact of Stacked AlON/SiO2 Gate Dielectrics for SiC Power Devices

We propose the use of an aluminum oxynitride (AlON) gate insulator for SiC-based MOS power devices. Although direct deposition of AlON on 4H-SiC substrate causes electrical degradation, the fabricated MOS capacitor with AlON/SiO2 stacked gate dielectric shows no flatband voltage (VFB) shift and negligible capacitance-voltage (C-V) hysteresis. Owing to the high dielectric constant of AlON, as compared to single SiO2 insulator, significant gate leakage reduction was achieved even at high-temperatures, especially in high electric field condition. Moreover, in order to improve electrical properties of thermally grown SiO2/SiC interfaces, the impact of a combination treatment of nitrogen plasma exposure and forming gas annealing (FGA) was investigated. We found that channel mobility enhancement of SiC-MOSFETs was consistent with the reduction in interface state density (Dit) depending on the process conditions of the combination treatment, and obtained 50% mobility enhancement, while maintaining low gate leakage current.

Improved Electrical Properties of SiC-MOS Interfaces by Thermal Oxidation of Plasma Nitrided 4H-SiC(0001) Surfaces

We propose a treatment of nitrogen radical irradiation to 4H-SiC surfaces for improving thermally grown SiO2/SiC interfaces. X-ray photoelectron spectroscopy (XPS) analyses revealed that a 1.7-nm-thick nitride film was formed by nitrogen radical exposure for 30 min and that Si-N bonds were retained after subsequent 10 min oxidation. It was also confirmed by secondary ion mass spectrometry (SIMS) that nitrogen atoms were piled up at the SiO2/SiC interface for the samples fabricated by thermal oxidation for 3 min with nitrogen plasma exposure. The metal-oxide-semiconductor (MOS) capacitors with a thin oxynitride layer formed by nitrogen radical exposure to the SiC surface and subsequent thermal oxidation exhibited excellent capacitance-voltage (C-V) characteristics. The interface state density (Dit) was significantly reduced by nitrogen radical exposure even at the shallow energy level near the conduction band edge. A minimum Dit value of 1.4 × 1011 cm-2eV-1 at Ec – E = 0.44 eV was achieved. Therefore, we can conclude that the treatment of nitrogen radical irradiation to the SiC surface prior to thermal oxidation is a promising method for improving SiC-MOS characteristics.

Improved Characteristics of 4H-SiC MISFET with AlON/Nitrided SiO<sub>2</sub> Stacked Gate Dielectrics

We investigated the impact of a combination treatment of nitrogen plasma exposure and forming gas annealing (FGA) for a thermally grown SiO2 layer on channel electron mobility in 4H-SiC metal-insulator-semiconductor field-effect-transistors (MISFETs) with and without deposited aluminum oxynitride (AlON) overlayers. This treatment was effective for improving the interface properties of nitrided SiO2/SiC structures formed by thermal oxidation in NOx ambient as well as pure SiO2/SiC structures. A channel mobility enhancement was perfectly consistent with a reduction in interface state density depending on the process conditions of the combination treatment, and a peak mobility of 26.9 cm2/Vs was achieved for the MISFETs with the nitrided SiO2 single dielectric layer. Comparable channel mobility was obtained with a gate insulator consisting of the AlON stacked on a thin nitrided SiO2 interlayer, indicating that both the combination treatment and the AlON/SiO2 stacked dielectrics can be integrated into the SiC MISFET fabrication processes.

Impact of a Treatment Combining Nitrogen Plasma Exposure and Forming Gas Annealing on Defect Passivation of SiO<sub>2</sub>/SiC Interfaces

We propose a treatment combining nitrogen plasma exposure and forming gas annealing (FGA) to improve the electrical properties of SiO2/SiC interfaces. Although conventional FGA at 450°C alone is not effective for reducing interface traps and fixed charges, our combination treatment effectively reduces both even at moderate temperatures. We achieved further improvement by applying our treatment at higher (over 900°C) FGA temperatures, including lower interface state density (Dit) values for both deep and shallow energy levels (1 - 4 x 1011 cm-2eV-1). Considering that nitrogen incorporation promotes hydrogen passivation of interface defects, a possible mechanism for the improved electrical properties is that interface nitridation eliminates carbon clusters or Si-O-C bonds, which leads to the formation of simple Si and C dangling bonds that can be readily terminated by hydrogen. We therefore believe that our treatment is a promising method for improving the performance of SiC-based MOS devices.

Synthesis of GaN bulk crystals and melt growth of GaN layers under nitrogen plasma

Abstract Gallium nitrides of wurtzite structure were synthesized by reacting gallium metal with atomic nitrogen in a microwave plasma operating at 200–400 Pa with the power of 420 W and the frequency of 2.45 GHz to avoid high equilibrium pressure. Polycrystalline GaN can be synthesized at low equilibrium pressures of 200–400 Pa and low temperature of 610–700 °C. The yield of polycrystalline GaN was increased by increasing the nitrogen plasma exposure time and reached 96% at 3 h. The growth of GaN layers on (0 0 0 1) sapphire substrate was carried out using the same technique. When the sapphire substrate was placed in the bottom of the BN crucible, GaN layer of about 3 μm was grown on the substrate with the growth rate of 0.5 μm/h. The layer was nearly oriented to (0 0 0 1)GaN∥(0 0 0 1) sapphire. These results indicate that the direct conversion of gallium metal into GaN layer under nitrogen plasma is a useful method.

Structural and photoluminescence studies of InAsN quantum dots grown on GaAs by MBE

The growth of self-organized InAs(N) quantum dots on GaAs by molecular beam epitaxy has been investigated. Photoluminescence (PL) and transmission electron microscopy (TEM) experiments have been carried out to determine the influence of the growth temperature and nitrogen plasma exposure on the optical and structural properties of the dots. At low growth temperature, in the 410–450 °C range, some nitrogen is found to be incorporated in the quantum dots. This incorporation does not significantly affect their shape but causes a red-shift of the PL emission. At higher temperature, 510 °C, a bimodal distribution rapidly appears with nitrogen exposure. Cross-sectional TEM images reveal a first population of coherent dots whose shape is similar to that of InAs dots, and a second population of large, flat and occasionally relaxed dots; in that case the associated PL spectrum is typical of low density arrays of N-free InAs quantum dots. Discussion focuses on the different mechanisms involved in the dots growth. It is proposed that the occurrence of the second population of large dots is due to the defects and composition fluctuations induced by nitrogen at 510 °C.

Oxynitridation of silicon with nitrogen plasma for flash memory applications characterized by high frequency capacitance–voltage measurements

Si(100) wafers were oxynitrided by nitrogen plasma exposure under different conditions and thermally oxidized in dry O2 without the use of toxic or global warming gases. The atomic concentration ratio of N/Si was 0.075 at a plasma discharge power of 0.5 kW and an exposure time of 1 min and 0.176 at a plasma power of 2.0 kW and an exposure time of 3 min, as determined from X-ray photoelectron spectroscopy measurements. A progressive reduction in the oxidation rate with increasing N concentration, corresponding to an increase in N plasma power, was observed. The uniformity of film thickness over a 4-inch wafer was improved after nitrogen plasma exposure and rapid thermal oxidation, with a thickness variation of less than 2.6% compared with 13.2% for the N2O thermal oxidation process. The advantages of exposing Si to N plasma for application in silicon devices was investigated by high-frequency capacitance–voltage measurements. A distortion in the capacitance–voltage curve was observed for samples that did not undergo nitridation, a distortion that was not observed in the nitrided samples. Capacitance–voltage curve distortion was found to be significantly reduced by Si nitridation under low plasma power.

Influence of electron cyclotron resonance nitrogen plasma exposure on the electrical characteristics of SiNx:H/InP structures

We have studied the influence of nitrogen plasma exposure of the InP surface on the electrical characteristics of electron cyclotron resonance deposited Al/SiNx:H/InP devices. Nitrogen plasma exposure was performed just before the SiNx:H deposition without vacuum breaking. A 30 s plasma treatment at low microwave power (60 W) sharply reduces the minimum of the interface trap density, from 4×1012 to 1.6×1012 eV−1 cm−2 (obtained by the high–low frequency capacitance method). We explain this reduction due to the formation of P–N and/or In–P–N complexes at the InP surface during the plasma-exposure step. These bonds are broken thermally after a rapid thermal annealing of the device. The minimum of the interface trap density, obtained at the optimum plasma exposure conditions, is among the lowest ones reported in the literature and similar to those obtained with more complicated processing technologies of the SiNx:H/InP structure. This is a clear indication that plasma exposure in N2 atmosphere seems to be a valuable and simple surface conditioning method.

C–V, DLTS and conductance transient characterization of SiNx:H/InP interface improved by N2 remote plasma cleaning of the InP surface

Electrical characterization of Al/SiNx:H/InP structures shows that ECR nitrogen plasma cleaning of InP surfaces gives rise to a noticeable improvement in the interface quality, whereas insulator and semiconductor bulk properties are maintained at a level sufficient to be used as the gate dielectric in MIS devices. Nitrogen plasma exposure was carried out just before the SiNx plasma deposition without vacuum breaking. To obtain interface state density and to detect deep levels in the semiconductor bulk, deep level transient spectroscopy (DLTS) measurements were carried out. We have also evaluated the insulator damage density, the so-called disorder-induced gap states (DIGS), by means of conductance transient analysis. Our results show that the plasma exposure in N2 atmospheres is a valuable and simple surface conditioning method.

Contribution of radicals and ions in atomic-order plasma nitridation of Si

Atomic-order nitridation of Si(100) by an ultraclean electron-cyclotron-resonance nitrogen plasma and contribution of radicals and ions to the nitridation have been investigated. The N atom concentration on Si(100) increases with the plasma exposure time and tends to saturate to a value corresponding to a few atomic layers. In the initial stage, the N atom concentration is normalized by the product of the relative radical density with the nitrogen plasma exposure time and the number of the incident ions is much smaller than the nitridation amount, which means the radical reaction is dominant. Assuming Langmuir-type kinetics neglecting desorption, an excellent agreement is observed by fitting the experimental data. In the saturation region, the N atom concentration is normalized by the number of incident ions and becomes higher than that corresponding to the double atomic layers. Therefore, it is suggested that nitridation of the deeper atoms below the surface is induced by the ion incidence.

Excellent rectifying characteristics in Au/n-CdTe diodes upon exposure to rf nitrogen plasma

Nitrogen plasma exposure (NPE) effects on indium doped bulk n-CdTe are reported here. Excellent rectifying characteristics of Au/n-CdTe Schottky diodes, with an increase in the barrier height, and large reverse breakdown voltages are observed after the plasma exposure. Surface damage is found to be absent in the plasma exposed samples. The breakdown mechanism of the heavily doped Schottky diodes is found to shift from the Zener to avalanche after the nitrogen plasma exposure, pointing to a change in the doping close to the surface which was also verified by C-V measurements. The thermal stability of the plasma exposure process is seen up to a temperature of 350 °C, thereby enabling the high temperature processing of the samples for device fabrication. The characteristics of the NPE diodes are stable over a year implying excellent diode quality. A plausible model based on Fermi level pinning by acceptor-like states created by plasma exposure is proposed to explain the observations.

Si nanostructures fabricated by nanoscale local oxidation using an ECR plasma nitride nanomask

A new method for Si nanofabrication was developed by exploiting nanoscale local oxidation using an ultrathin ECR plasma oxynitride nanomask (NANOLOX). ECR nitrogen plasma exposure with resist mask patterns directly forms a nanomask on Si surface. This oxynitride is 2-3-nm thick and acts as an excellent oxidation mask. Using this method, 10-nm-scale Si nanostructures, such as wires, dots, and single electron device structures, can be fabricated. This the first report of lithographically controlled Si nanostructures fabricated using nanoscale local oxidation.

Nanometer-scale studies of nitride/arsenide heterostructures produced by nitrogen plasma exposure of GaAs

We have investigated the nanometer-scale structure and electronic properties of nitride/arsenide superlattices produced by nitridation of a molecular beam epitaxially grown GaAs surface. Using cross-sectional scanning tunneling microscopy and spectroscopy, we find that the nitrided layers are not continuous films, but consist of groups of atomic-scale defects and larger clusters. We identify the defects and clusters as NAs and GaN with dilute As concentration, respectively. Thus, the nitrided regions consist of alloys from both sides of the miscibility gap predicted for the GaAsN system. In addition, spectroscopy on the clusters reveals an upward shift of the band edges and band gap narrowing, with significant change in the conduction band structure. We estimate the contribution of strain to band gap narrowing in terms of an elasticity calculation for a coherently strained spherical GaN cluster embedded in GaAs.

Nitridation Effects in n-CdTe

Nitrogen plasma exposure (NPE) effects in indium doped bulk n-CdTe as studied by electrical measurements are reported here, Excellent rectifying characteristics of schottky diodes, with an increase in the barrier height is observed after the plasma exposure. This effect is attributed to the fermi level pinning by acceptor-like surface states created by plasma exposure. Surface damage in the plasma exposed samples is however, found to be absent as seen from the SEM. Capacitance-voltage (C-V) depth profile shows the active donor concentration at the surface to decrease by as much as 88% in the highly doped samples. This decrease in the donor concentration is explained by the compensation process

Magnetic properties of Fe/Fe-N and Fe/Cu-N multilayered films having intermediate layers containing nitrogen

The structures and magnetic properties of Fe/Fe-N and Fe/Cu-N multilayered films having intermediate layers containing nitrogen were investigated. Fe/Fe-N films formed by alternate Fe sputtering and electron cyclotron resonance (ECR) nitrogen plasma exposure had Fe layers containing approximately 3 at% nitrogen, the /spl alpha/-Fe(110) lattice spacing being larger than that of pure iron. This expansion of the /spl alpha/-Fe(110) lattice spacing was observed after Fe deposition. The Fe layers containing a little nitrogen improved the soft magnetic properties. Fe/Cu-N multilayered films formed by the alternation of Fe sputtering and Cu-N nitrogen reactive sputtering also showed expansion of the /spl alpha/-Fe(110) lattice spacing and reduction of the Fe crystallite size with increasing partial N/sub 2/ gas pressure (PN/sub 2/) during Cu-N reactive sputtering. The Fe/Cu-N multilayered films achieved minimum coercivity at PN/sub 2/=7%. >

Anion Exchange Reactions and initial Gan Epitaxial Layer Formation Under Nitrogen Plasma Exposure of a GaAs Surface

AbstractInitial nitridation of molecular beam epitaxially (MBE) grown GaAs surfaces by means of an electron cyclotron resonance (ECR) microwave plasma source is investigated in situ through time-resolved reflection high energy electron diffraction (RHEED), and ex situ high-resolution X-ray diffraction (HRXRD) techniques. Brief (< 8-10 s) plasma exposure of GaAs (100) surfaces results in surface N-for-As anion exchange and a new, specular, commensurate (3x3) RHEED pattern which appears to correspond to up to 1 monolayer of coherently strained GaN on GaAs. anion exchange kinetics is studied through the time-dependence of the onset and decay of the (3x3) RHEED pattern as a function of substrate temperature. For the first time, coherently strained GaNyAs1_y /GaAs heterostructures are grown and characterized. Direct evidence for thermally activated processes from both RHEED and HRXRD observations is presented, and N desorption and surface-segregation phenomena are proposed to explain the experimental results.

Soft Magnetic Properties of Fe/Fe-N Multilayered Films Prepared at Various Sputtering Gas Pressures

Fe/Fe-N multilayered films were prepared by alternation of Fe sputtering at various Ar gas pressures and ECR nitrogen plasma exposure, and the structure and magnetic properties of the resulting films were investigated. Growth of columnar structures was observed in Fe film prepared at high Ar gas pressure. Fe/Fe-N multilayered films with an Fe layer prepared at low Ar gas pressure have a low H <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</inf> and high B <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">s</inf> . X-ray diffraction suggests that the lattice spacing of Fe (110) and the compressive stress in these films increase with decreasing Ar gas pressure. These structural changes are caused by nitrogen diffusion from the Fe-N layer into the Fe layer. Nitrogen exists in solid solution in Fe layers prepared at low Ar gas pressure, and in the form of nitrides in Fe layers prepared at high Ar gas pressure.

Mössbauer spectra and structures of Fe/Fe-N multilayered films formed by Fe sputtering and ECR nitrogen plasma exposure

Fe/Fe-N multilayered films were prepared by an alternation of Fe sputtering and Electron Cyclotron Resonance (ECR) nitrogen plasma exposure, and the films were investigated by 57Fe Conversion Electron Mössbauer Spectroscopy (CEMS). ECR nitrogen plasma exposure of the sputtered Fe layers generated fine crystal ϵ-Fe2-3N and Fe2N grains, rather than thermally stable γ′-Fe4N. The Fe/Fe-N multilayered films has a periodic structure of [α-Fe]/[ϵ-Fe2-3N and Fe2N], and showed good soft magnetic properties.

スパッタガス圧を変えて作製したＦｅ／Ｆｅ‐Ｎ積層膜の軟磁気特性

Fe/Fe-N multilayered films were made by alternation of Fe sputtering at various Ar gas pressures and ECR nitrogen plasma exposure, and the structure and magnetic properties of these films were investigated. Growth of a columnar structure was observed in an Fe film prepared at a high Ar gas pressure. Fe/Fe-N multilayered films with an Fe layer prepared at a low Ar gas pressure have a low Hc and high Bs. X-ray diffraction suggests that the lattice spacing of Fe(110) and the compressive stress in these films increase with a decrease in Ar gas pressure. These structural changes are caused by the nitrogen diffusion from the Fe-N layer into the Fe layer. Nitrogen exists as a solid solution in an Fe layer prepared at a low Ar gas pressure, but as a nitride in an Fe layer prepared at a high Ar gas pressure.