Properties of non‐stoichiometric nitrogen doped LPCVD silicon thin films

The influence of nitrogen on the internal structure and so on the electrical properties of silicon thin films obtained by low‐pressure chemical vapor deposition (LPCVD) was studied using several investigation methods. We found by using Raman spectroscopy and SEM observations that a strong relationship exists between the structural order of the silicon matrix and the nitrogen ratio in film before and after thermal treatment. As a result of the high disorder caused by nitrogen on silicon network during the deposit phase of films, the crystallization phenomena in term of nucleation and crystalline growth were found to depend upon the nitrogen content. Resistivity measurements results show that electrical properties of NIDOS films depend significantly on structural properties. It was appeared that for high nitrogen content, the films tend to acquire an insulator behavior. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

ESR study of the hydrogenated nanocrystalline silicon thin films

We studied the stability and light-induced paramagnetic centers in hydrogenated nanocrystalline silicon thin films (nc-Si:H) by electron-spin-resonance (ESR) and photothermal-deflection-spectroscopy (PDS). There is no measurable change in defect density upon illumination with white light with a light intensity of 300 mW cm^?2 for 300 h. At low temperatures, upon illumination with sub-bandgap light, a light-induced ESR signal appears. This signal is similar to that in hydrogenated micro-crystalline silicon (μc-Si:H).     

Radiation-induced trapping centers in thin silicon dioxide films

In this paper the technological and scientific aspects of radiation-related charge trapping in thin SiO₂ films are reviewed. These films are amorphous in nature and are thermally grown on single crystal silicon substrates serving as the insulating layer in metal-oxide-semiconductor (MOS) capacitors and transistors. The structure and operation of these devices are reviewed with special emphasis on the effect of charges trapped in the oxide. The technical importance of understanding the interaction of ionizing radiation with thin SiO₂ films is illustrated with two practical examples. The first involves the operation of MOS transistors in environments where ionizing radiation is present, leading to an accumulation of positive space charge in the oxide. The second deals with process-induced defects generated by radiation encountered during the fabrication of devices by processes such as electron beam lithography or electron gun metallization. Unannealed traps of this type capture hot electrons producedin the substrate during the operation of the MOS transistor. In both these examples, the charging of the oxide results in instabilities which degrade operation.

Its sensitivity to charge trapped in the insulator makes the MOS system an ideal vehicle for scientific study of these phenomena. The basic techniques for characterizing the density, capture cross-sections, and location are briefly discussed and applied to the problem of radiation-induced defects in thin SiO₂ films. Ionizing radiation is shown to interact with the SiO₂ in two modes. In the first it supplies carriers to fill pre-existing hole traps at the interfaces. In the second it creates electron and hole traps in the bulk of the thin film. These latter defects are in a neutral state after irradiation and are detectable only when either electrons or holes are subsequently injected into the oxide. The capture cross-sections, trap densities and location of these centers in the film are presented. The annealing treatments required to remove these traps from aluminium and polysilicon gate devices are also discussed. The number traps produced by an incident 25 KV electron beam is found to depend weakly on the dosage. A dipolar defect, produced by the ionizing radiation, seems to explain the behavior of the neutral centers.     

Electron spin resonance study of hydrogenated microcrystalline silicon–germanium alloy thin films

Paramagnetic defects of undoped hydrogenated microcrystalline silicon–germanium alloys (μc-Si_1?xGe_x:H) grown by low temperature (200 °C) plasma-enhanced chemical vapor desposition (PECVD) have been measured by electron spin resonance (ESR) and compared with those of hydrogenated amorphous silicon–germanium (a-Si_1?xGe_x:H). The spin density of μc-Si_1?xGe_x:H increases with Ge content and shows a broad maximum of ～10¹⁷ cm^?3 at x ～ 0.5, which reasonably accounts for the decreased photoconductivity. While the Ge dangling bond defects prevail in a-Si_1?xGe_x:H for Ge-rich compositions, we detected no ESR signal in μc-Si_1?xGe_x:H for x > 0.75 where an electrical change occurs from weak n- to strong p-type conduction. These results indicate that dangling bonds are charged in large densities due to the presence of the acceptor-like states in undoped μc-Si_1?xGe_x:H.     

The effect of dopants on the microstructure of polycrystalline silicon thin film grown by MILC method

The effect of dopants on the crystal growth and the microstructure of poly-crystalline silicon (poly-Si) thin film grown by metal induced lateral crystallization (MILC) method was intensively investigated. PH₃ and B₂H₆ were used as source gases in ion mass doping (IMD) process to make n-type and p-type semiconductor respectively. It was revealed that the microstructure of MILC region varies significantly as the doping type of the samples varied from intrinsic to n-type and p-type, which was investigated by field emission (FE)-SEM. The microstructure of MILC region of the intrinsic was bi-directional needle network structure whose crystal structure has a (1 1 0) preferred orientation. For p-type doped sample, the microstructure of MILC region was revealed to become unidirectional parallel growth structure more and more as MILC growth proceed, which was led by unidirectional division of needlelike grain at the front of MILC region. And for n-type doped sample, the microstructure was random-directional needlelike growth structure. These phenomena can be explained by an original model of Ni ion and Ni vacancy hopping in the NiSi₂ phase and its interface at the front of MILC region.     

SIMS and Raman characterizations of ZnO:N thin films grown by MOCVD

Nitrogen was incorporated into ZnO films grown by metalorganic chemical vapour deposition (MOCVD) on ZnO substrates using DMZn-TEN, tert-butanol and diallylamine, respectively, as zinc, oxygen and doping sources. The carrier gas was either hydrogen or nitrogen and the partial pressure ratio (R_VI/II) was varied in order to favor the nitrogen incorporation and/or reduce carbon related defects. The ZnO films have been characterized by Micro-Raman scattering and SIMS measurements. SIMS measurements confirm the nitrogen incorporation with concentrations extending from ∼10¹⁹ cm⁻³ to ∼4×10²⁰ cm⁻³. Raman spectra show nitrogen local vibration modes in films grown at low R_VI/II ratio and using H₂ as carrier gas. However, a vibration band attributed to carbon clusters dominates the Raman spectra for films grown with N₂ carrier. The contribution of N complexes was discussed. The strain was calculated for the as-grown and annealed films and it changes from tensile to compressive after annealing.     

Mapping of mechanical stress in silicon thin films on silicon cantilevers by Raman microspectroscopy

We have used plasma enhanced chemical vapor deposition (PECVD) to deposit silicon thin films (～0.2–1 μm) with different crystallinity fractions on Nanosensors PtIr5 coated atomic force microscopy (AFM) cantilevers (450 × 50 × 2 μm³). Microscopic measurements of Raman scattering were used to map both internal stress and extrinsic stress induced in the films by bending the cantilevers using a nanomanipulator (Kleindiek Nanotechnik MM3A). Thanks to the excellent elasticity of the cantilevers, the films could be bent to curvature radii down to 300 μm. We observed the stress induced shift of the TO–LO phonon Raman band of more than 3 cm^?1 for fully microcrystalline film corresponding to the stress ～0.8 GPa. The shift of the similar film with amorphous structure was ～2.5 cm^?1.     

Laser crystallization of compensated hydrogenated amorphous silicon thin films

Raman backscattering and hydrogen effusion measurements were performed on compensated, highly P- and B-doped laser crystallized polycrystalline silicon. From hydrogen effusion spectra the hydrogen chemical potential, μ_H, is determined as a function of hydrogen concentration, which can be related to the hydrogen density-of-states distribution. Interestingly, hydrogen bonding is affected by doping of the amorphous starting material. Below the hydrogen transport states, four peaks are observed in the hydrogen density-of-states at 2.0, 2.2, 2.5 and 2.8 eV. The latest peak is not observed in B-doped samples. The hydrogen effusion results will be correlated with the results obtained from Raman backscattering measurements.     

Crystallinity of the mixed phase silicon thin films by Raman spectroscopy

Raman spectra of the mixed phase silicon films were studied for a sample with transition from amorphous to fully microcrystalline structure using four excitation wavelengths (325, 514.5, 632.8 and 785 nm). Factor analysis showed the presence of two and only two spectrally independent components in the spectra within the range from 250 to 750 cm^?1 for all four excitation wavelengths. The 785 nm excitation was found optimal for crystallinity evaluation and by comparison with surface crystallinity obtained by atomic force microscopy, we have estimated the ratio of integrated Raman cross-sections of microcrystalline and amorphous silicon at this wavelength as y = 0.88 ± 0.05.     

A simple quality factor for characterization of thin silicon films

Four series of intrinsic thin Si films were prepared by plasma enhanced chemical vapor deposition at standard and high growth rate conditions. We suggest a simple ‘μc-Si:H layer quality factor’ based on the ratio of subgap optical absorption coefficient values: α(1.4 eV)/α(1 eV). This ratio minimizes the light scattering effects for rough films and can serve as a reliable detection of the amorphous/microcrystalline structure transition and also as a figure of merit for the microcrystalline layer. The quality factor is evaluated for series of our samples with well known structure and also compared with samples from other laboratories with different deposition and measurement techniques.     

Photoluminescence in silicon rich oxide thin films under different thermal treatments

Photoluminescence (PL) was studied in silicon rich oxide (with the atomic percentage ranges of Si from 35% to 75%) thin film samples, fabricated by the plasma assisted CVD technique. A broad PL peak, blue-shifted from the bulk silicon band edge of ～1.1 eV, was observed. In one typical sample, the PL peak intensity shows a non-monotonic temperature dependence. This non-monotonic dependence was also observed in previous work by others and attributed to an energy splitting between the excitonic singlet and triplet levels in silicon nanocrystals, a consequence of quantum confinement effect. Finally, in more than 20 samples under different thermal treatments (with the annealing temperature range from 800 °C to 1100 °C), the wavelength of PL peak was observed to be pinned between ～900 and ～1000 nm, independent of thermal budget. This pinning effect, we believe, is probably due to the formation of oxygen-related interface states.     

Lateral photovoltage measurements in hydrogenated amorphous silicon and silicon-oxygen thin films

Hydrogenated amorphous silicon (a-Si:H) and hydrogenated amorphous silicon-oxide alloy films (a-SiO_x:H) were investigated by temperature dependence of lateral photovoltage (LPV) measurements. The suboxide sample with [O] = 27 at.%, was found to exhibit larger LPV compared to the unalloyed sample. It is difficult to simply correlate LPV measurements to related diffusion length measurements, only. On the other hand, the observed magnitude of LPV in a-Si:H and its decrease with temperature, could be explained based on an internal electric field induced by diffusion electron and hole currents, and multiple trapping of the photocarriers.     

Growth and characterization of zirconium oxide thin films on silicon substrate

The growth and characterization of zirconium oxide (ZrO₂) thin films prepared by thermal oxidation of a deposited Zr metal layer on SiO₂/Si were investigated. Uniform ZrO₂ thin film with smooth surface morphology was obtained. The thermal ZrO₂ films showed a polycrystalline structure. The dielectric constant of the ZrO₂ film has been shown to be 23, and the equivalent oxide thickness (EOT) of the ZrO₂ stacked oxide is in the range of 3.38–5.43 nm. MOS capacitors with ZrO₂ dielectric stack show extremely low leakage current density, less than 10^?6 A/cm² at ?4 V. Consequently, using this method, high-quality ZrO₂ films could be fabricated at oxidation temperature as low as 600 °C.     

Infrared semiconductor laser irradiation used for crystallization of silicon thin films

We report the rapid thermal crystallization of silicon films using infrared semiconductor laser. Carbon films were used on silicon films to absorb the laser light. Uniform crystalline regions were achieved by a line shape laser beam with a length of 20 μm. Polycrystalline silicon thin film transistors were fabricated in crystallized regions. The effective electron carrier mobility and threshold voltage were achieved to be 130 cm²/Vs and 0.4 V, respectively, when the crystalline volume ratio of the silicon films was 0.95.     

Characterization of hydrogenated amorphous silicon thin films prepared by magnetron sputtering

Magnetron sputtered hydrogenated amorphous silicon (a-Si:H) thin films have been characterized. Hydrogen (H₂) with argon (Ar) was introduced into the sputtering chamber to create the plasma. A sudden increase in the deposition rate occurred when the hydrogen was added. The maximum hydrogen content of 16 atomic percent (at.%) was achieved and a bandgap of about 2.07 eV was determined from the spectral investigations of the hydrogenated films. The effect of radio frequency (RF) power on the deposition rate, as well as on the hydrogen content was investigated. To change the hydrogen content in the films, the hydrogen flow rate was varied while keeping the argon flow rate constant. The hydrogen content in the films increased with increasing hydrogen flow rate up to the maximum content of 16 at.% and then decreased for further increases in hydrogen flow.     

Characterization of chromium silicide thin layer formed on amorphous silicon films

A detailed investigation of the compositional, optical and electrical properties of a chromium silicide layer grown at room temperature on top of doped amorphous silicon films is presented. The formation of the layer is promoted only when phosphorous atoms are present in the film. The deposition of a very thin n-type doped layer (around 5 nm) on top of a p-type doped film has allowed us to achieve the chromium silicide formation also on p-type material without changing its doping properties. Angle resolved X-ray photoelectron spectroscopy measurements demonstrate the presence of chromium-oxide, chromium silicide and metallic chromium in similar percentages for both p- and n-type doped layers. From the ellipsometric analysis, the refractive index spectra have been extracted, and the layer thickness has been estimated to be 5 nm for both p- and n-type doped layers. From planar conductivity measurements, we have found that the chromium silicide promotes an activation energy reduction from 0.24 eV down to 0.017 eV for the n-type layer and from 0.36 eV down to 0.14 eV for the p-type film.     

Experimental evidence for extended hydrogen diffusion in silicon thin films during light-soaking

We have used mass spectrometry to detect hydrogen effusing from silicon thin films exposed to light. Our results indicate a long range diffusion of hydrogen through the whole film, which ends with its release into the vacuum system. The changes in the film properties are characterized by dark and photoconductivity and hydrogen exodiffusion measurements. From the evolution of dark conductivity measurements after turning off the light, we show that this long range motion of hydrogen is not due to the heating of the sample. A comparison of hydrogen exodiffusion spectra of as-deposited and light-soaked samples shows that the weakly-bonded hydrogen content decreases by 30% for a-Si:H films and that the tightly-bonded hydrogen migrates to grain boundaries of crystalline regions in the case of pm-Si:H films. These results clearly demonstrate the long range motion of hydrogen during light soaking.