I. Mártil

Optical properties and structure of HfO2 thin films grown by high pressure reactive sputtering

Thin films of hafnium oxide (HfO2) have been grown by high pressure reactive sputtering on transparent quartz substrates (UV-grade silica) and silicon wafers. Deposition conditions were adjusted to obtain polycrystalline as well as amorphous films. Optical properties of the films deposited on the silica substrates were investigated by transmittance and reflectance spectroscopy in the ultraviolet, visible and near infrared range. A numerical analysis method that takes into account the different surface roughness of the polycrystalline and amorphous films was applied to calculate the optical constants (refractive index and absorption coefficient). Amorphous films were found to have a higher refractive index and a lower transparency than polycrystalline films. This is attributed to a higher density of the amorphous samples, which was confirmed by atomic density measurements performed by heavy-ion elastic recoil detection analysis. The absorption coefficient gave an excellent fit to the Tauc law (indirect gap), which allowed a band gap value of 5.54 eV to be obtained. The structure of the films (amorphous or polycrystalline) was found to have no significant influence on the nature of the band gap. The Tauc plots also give information about the structure of the films, because the slope of the plot (the Tauc parameter) is related to the degree of order in the bond network. The amorphous samples had a larger value of the Tauc parameter, i.e. more order than the polycrystalline samples. This is indicative of a uniform bond network with percolation of the bond chains, in contrast to the randomly oriented polycrystalline grains separated by grain boundaries.

Lifetime recovery in ultrahighly titanium-doped silicon for the implementation of an intermediate band material

The doping of conventional semiconductors with deep level (DL) centers has been proposed to synthesize intermediate band materials. A recent fundamental study of the nonradiative recombination (NRR) mechanisms predicts the suppression of the NRR for ultrahigh DL dilutions as a result of the delocalization of the impurity electron wave functions. Carrier lifetime measurements on Si wafers doped with Ti in the 1020–1021 cm−3 concentration range show an increase in the lifetime, in agreement with the NRR suppression predicted and contrary to the classic understanding of DL action.

Titanium doped silicon layers with very high concentration

Ion implantation of Ti into Si at high doses has been performed. After laser annealing the maximum average of substitutional Ti atoms is about 1018 cm−3. Hall effect measurements show n-type samples with mobility values of about 400 cm2/V s at room temperature. These results clearly indicate that Ti solid solubility limit in Si has been exceeded by far without the formation of a titanium silicide layer. This is a promising result toward obtaining of an intermediate band into Si that allows the design of a new generation of high efficiency solar cell using Ti implanted Si wafers.

A comparative study of the electrical properties of TiO2 films grown by high-pressure reactive sputtering and atomic layer deposition

Oxide–semiconductor interface quality of high-pressure reactive sputtered (HPRS) TiO2 films annealed in O2 at temperatures ranging from 600 to 900 °C, and atomic layer deposited (ALD) TiO2 films grown at 225 or 275 °C from TiCl4 or Ti(OC2H5)4, and annealed at 750 °C in O2, has been studied on silicon substrates. Our attention has been focused on the interfacial state and disordered-induced gap state densities. From our results, HPRS films annealed at 900 °C in oxygen atmosphere exhibit the best characteristics, with Dit density being the lowest value measured in this work (5–6 × 1011 cm−2 eV−1), and undetectable conductance transients within our experimental limits. This result can be due to two contributions: the increase of the SiO2 film thickness and the crystallinity, since in the films annealed at 900 °C rutile is the dominant crystalline phase, as revealed by transmission electron microscopy and infrared spectroscopy. In the case of annealing in the range of 600–800 °C, anatase and rutile phases coexist. Disorder-induced gap state (DIGS) density is greater for 700 °C annealed HPRS films than for 750 °C annealed ALD TiO2 films, whereas 800 °C annealing offers DIGS density values similar to ALD cases. For ALD films, the studies clearly reveal the dependence of trap densities on the chemical route used.

Influence of defects on the electrical and optical characteristics of blue light-emitting diodes based on III–V nitrides

We have measured the electrical and optical properties of blue light-emitting diodes (LEDs) based on III–V nitrides. The current–voltage characteristic is described by means of the relation I=I0 exp(αV). In this equation α is temperature independent, suggesting a process of conduction by tunneling, as was recently reported also for blue-green LEDs based on III–V nitrides [Appl. Phys. Lett. 68, 2867 (1996)]. We explain the differences between blue and blue-green devices taking into account the tunneling process across semiconductor interfaces, in which a great number of defects is present. The light output intensity of the LED as a function of junction–voltage data reveals a dependence on the junction–voltage of the type L=L0 exp(qV/1.4 KT), indicating that the radiative recombination path is via deep levels located at the forbidden gap. Furthermore, we find that the light output–current characteristic follows a power law like L∝Ip. From the analysis of data it appears that, contrary to expectations, the n...

Deposition of SiNx : H thin films by the electron cyclotron resonance and its application to Al/SiNx : H/Si structures

We have analyzed the electrical properties and bonding characteristics of SiNx:H thin films deposited at 200 °C by the electron cyclotron resonance plasma method. The films show the presence of hydrogen bonded to silicon (at the films with the ratio N/Si<1.33) or to nitrogen (for films where the ratio N/Si is higher than 1.33). In the films with the N/Si ratio of 1.38, the hydrogen content is 6 at. %. For compositions which are comprised of between N/Si=1.1 and 1.4, hydrogen concentration remains below 10 at. %. The films with N/Si=1.38 exhibited the better values of the electrical properties (resistivity, 6×1013 Ω cm; and electric breakdown field, 3 MV/cm). We have used these films to make metal-insulator-semiconductor (MIS) devices on n-type silicon wafers. C–V measurements accomplished on the structures indicate that the interface trap density is kept in the range (3–5)×1011 cm−2 eV−1 for films with the N/Si ratio below 1.38. For films where the N/Si ratio is higher than 1.4, the trap density suddenly ...

Isotopic study of the nitrogen-related modes in N+-implanted ZnO

Micro-Raman measurements were performed to study the nitrogen-related modes in ZnO samples implanted with N+. The two stable N isotopes, N14 and N15, were implanted. Distinct peaks at 277 and 512cm−1 are observed irrespective of the implanted isotope, both before and after rapid thermal annealing. The insensitivity of the mode frequencies to the implanted isotope rules out the explanation of these modes as local vibrational modes involving N motion. These modes were not detected in ZnO samples implanted with Zn+, O+, or P+, which suggests that they may be associated with distortions∕defects favored by the presence of N.

Full composition range silicon oxynitride films deposited by ECR-PECVD at room temperature

Abstract Silicon oxynitride films were deposited at room temperature using the ECR-PECVD technique. Precursor gases were O 2 , N 2 and SiH 4 . The composition of the films can be controlled by regulating the gases flow ratio. R = (O 2 + N 2 )/SiH 4 and R′ = O 2 /SiH 4 have proved to be the key deposition parameters. FTIR spectroscopy, AES and ellipsometric measurements were performed in order to characterise the films. A single Si O/Si N stretching band is observed in the FTIR spectrum for all compositions, indicating single-phase homogeneous SiO x N y films. FWHM of the stretching band shows a maximum for the composition corresponding to the same number of Si O and Si N bonds. Samples cover the whole composition range from silicon nitride to silicon oxide including nitrogen-rich films, even though the gas flow ratio R″ = N 2 /O 2 during deposition was small (from r″ = 1.0 for SiO 1.9 N 0.04 to R″ = 6.7 for SiO 0.26 N 1.2 ). Silicon oxide composition samples (SiO 2.0 ) show essentially the same IR features as the thermal oxide: Si O stretching band located at 1072 cm −1 , with a FWHM of 96 cm −1 and a shoulder/peak ratio of 0.30, while nitrogen-rich samples (SiO 0.26 N 1.2 ) show a total bonded hydrogen content below 2 × 10 22 cm −3 .

Influence of rapid thermal annealing processes on the properties of SiNx:H films deposited by the electron cyclotron resonance method

We have analyzed the effects of rapid thermal annealing on the composition and on the bonding and optical properties of amorphous hydrogenated silicon nitride (a-Si-x:H) thin films deposited at room temperature by the electron cyclotron resonance plasma method. Films with three different as-grown compositions have been studied, namely x = 0.97, 1.43 and 1.55, Annealing effects were related to film composition. In films with the presence of both Si-H and N-H bonds (as-grown compositions x = 0.97 and 1.43), we found that a reorganization of bonds takes place at temperatures less than or equal to 500 degrees C, where the well-known cross linking reaction Si-Si + N-H --> Si-H + Si-N occurs without detectable release of hydrogen. In the same range of temperatures, an increase of the band gap was observed and attributed to Si-Si bond substitution for Si-H, but no changes in composition were detected. At higher temperatures (T greater than or equal to 600 degrees C), the optical gap decreases and both Si-H and N-H bonds are lost along with a release of hydrogen and nitrogen. For the films with an as-gown composition x = 1.55, we observe that the release of hydrogen only occurs at temperatures above 900 degrees C, but it is not accompanied by any loss of nitrogen. An increase of the optical gap until the release of hydrogen begins and a decrease thereafter is observed.

Optical analysis of absorbing thin films: application to ternary chalcopyrite semiconductors

The refractive index n and the absorption coefficient a of radio frequency sputtered CuGaSe(2) and CuInSe(2) thin films were obtained by means of transmissivity (T) and reflectivity (R) measurements at normal incidence. The optical properties were determined from the rigorous expressions for the transmission and the reflection in an air/film/(glass)substrate/air multilayer system. The solutions to this system of equations are not unique, and the physically meaningful solution is identified by trying different thicknesses in the numerical approach. Usually, nonacceptable n dispersion curves are found for all thicknesses. To be able to obtain a good n dispersion curve and, therefore, a correct absorption coefficient, we propose a simple modification of the equations for R and T through a factor called the coherence factor (CF). Because of the surface roughness and the nonuniformity of n and alpha, the light rays that reflect internally in the interface between the substrate and the film have a random difference in opt cal path. The CF accounts for this effect. This modification leads to an unambiguous and accurate determination of the optical properties and thickness of thin films for all wavelengths where transmission is not negligible. The CF is shown to be greatly dependent on the thickness of the film. This method can be used even when the R and T spectra do not have interference fringes. This method is applied successfully to the optical analyses, in the 0.4-2.5-mum wavelength range, of CuInSe(2) and CuGaSe(2) ternary chalcopyrite thin films deposited onto glass substrates by radio-frequency sputtering.