Silicon Interface Research Articles

Observation of Normally Distributed Energies for Interface Trap Recovery After Hot-Carrier Degradation

We investigate the temperature accelerated recovery from hot-carrier (HC) damage with the help of local polycrystalline heating structures in n-MOSFETs designed for power applications. These devices have a rather thick gate oxide and long channel, which assures that mainly interface traps are created through the HC stress. We further verify with frequency-dependent charge pumping that in our devices border traps are of vanishing importance compared to interface traps. We analyze the time and temperature dependence of the recovery of the interface traps after HC stress using models from the literature. The data are fairly consistent with the assumption of interfacial silicon dangling bonds that become passivated by hydrogen. The forward passivation energy is found to be normally distributed because of the distribution of atomic defect configurations. The distribution parameters are independent of the overall degradation level which shows that the passivation process is limited by the bond association kinetics rather than hydrogen supply. Our results are of importance for HC research as well as for the ongoing discussion regarding the quasi-permanent component of bias temperature instability.

Plasma hydrogenated, reactively sputtered aluminium oxide for silicon surface passivation

AbstractThe intentional addition of hydrogen during reactive sputtering of AlOx films has led to a dramatic improvement of the surface passivation of crystalline silicon wafers achieved with this technique. The 5 ms effective minority carrier lifetime measured on 1.5 Ω cm n‐type CZ silicon wafers is close to the 6 ms of a control wafer coated by atomic layer deposition (ALD) of AlOx. Hydrogen‐sputtered films also provide excellent passivation of 1 Ω cm p‐type silicon, as demonstrated by an effective lifetime of 1.1 ms. It is likely that the improved passivation is related to the formation of an interfacial silicon oxide layer, as indicated by FTIR measurements. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Effect of Doping Density on the Charge Rearrangement and Interface Dipole at the Molecule–Silicon Interface

The interface level alignment of alkyl and alkenyl monolayers, covalently bound to oxide-free Si substrates of various doping levels, is studied using X-ray photoelectron spectroscopy. Using shifts in the C 1s and Si 2p photoelectron peaks as a sensitive probe, we find that charge distribution around the covalent Si–C bond dipole changes according to the initial position of the Fermi level within the Si substrate. This shows that the interface dipole is not fixed but rather changes with the doping level. These results set limits to the applicability of simple models to describe level alignment at interfaces and show that the interface bond and dipole may change according to the electrostatic potential at the interface.

Impact of Iron Surface Contamination on the Lifetime Degradation of Samples Passivated by Fired Al $_{\bf 2}$O$_{\bf 3}$/SiN$_{\bm x}$ Stacks

We present results on the impact of iron surface contamination on the firing stability of the bulk lifetime of silicon samples passivated by Al2O3/SiNx stacks. The effect of blistering, hydrogen passivation, and formation and transformation of the interfacial silicon oxide layer, respectively, has been widely discussed. This paper focuses on an up-to-now largely ignored effect that may dominate the observed degradation in frequent cases: bulk degradation by in-diffusion of impurities from the interface between silicon and atomic layer deposition layer during firing. In order to enhance the apparent passivation quality of thin Al2O3 layers, an appropriate predeposition cleaning plays an important role. By experimental data and theoretical calculation, we could prove that the short firing step is sufficient to promote the diffusion of iron from the surface into the bulk. Even low iron surface contamination levels of less than 1.4 × 1011 at/cm2 can have a detrimental impact on bulk recombination. The detected enhancement of the global Shockley-Read-Hall recombination was attributed essentially to interstitial iron contamination of the bulk. This indicates that a close control of iron surface contamination is a key factor to guarantee for firing stable Al2O3/SiNx stack systems.

Microstructure analysis of the interface situation and adhesion of thermally formed nickel silicide for plated nickel–copper contacts on silicon solar cells

For low cost high efficiency fully plated NiCu contacts on silicon solar cells, a thermal formation of nickel silicide is commonly performed for adhesion promotion. Since the results of this strategy have been found to be ambiguous, and as the interface situation is not understood, a microstructural analysis of the metal–silicon interface after thermal annealing of plated nickel layers on silicon solar cells has been carried out.The point of failure upon performing solder and peel testing on sintered contacts has been identified to be at the silicide–nickel interface. Microstructural analysis shows that void formation can be observed at this interface. The mechanisms of silicide and void formation from plated layers on solar cells are studied taking into account different electrolytes and pretreatments. The cavities can be removed by selective nickel etching and replating nickel onto the formed silicide after silicide pretreatment. Excellent adhesion of up to 2.5N/mm has been measured, which represents the force of silicon wafer breakage rather than true metal adhesive force.

Ultralow Contact Resistivity for a Metal/p-Type Silicon Interface by High-Concentration Germanium and Boron Doping Combined with Low-Temperature Annealing

A contact resistivity of 6.9×10-9 Ω·cm2 has been obtained in an AlSi (1 wt %)–Cu (0.5 wt %) alloy/silicon system by using heavy-dose ion implantations of germanium and boron combined with low-temperature annealing. The analysis of the combined state showed that B12 cluster was incorporated and the supersaturation activation layer was formed into the region where germanium separated. Separated germanium is expected to have high interface state density. It is considered that this interface state density also has a Fermi level, and in order to reduce the difference from the Fermi level of the substrate, the charge moves to interface state density from the substrate. As a result, it is not based on a metallic material but a work function becomes small because pinning by which a Fermi level is fixed to interface state density occurs owing to the substrate/metal interface. It is considered to be attributable to the existence of a Ge-rich layer formed by low-temperature annealing, and a supersaturation activation layer that lowers contact resistance was formed.

Amorphous/crystalline silicon interface defects induced by hydrogen plasma treatments

Excellent amorphous/crystalline silicon interface passivation is of extreme importance for high-efficiency silicon heterojunction solar cells. This can be obtained by inserting hydrogen-plasma treatments during deposition of the amorphous silicon passivation layers. Prolonged hydrogen-plasmas lead to film etching. We report on the defect creation induced by such treatments: A severe drop in interface-passivation quality is observed when films are etched to a thickness of less than 8 nm. Detailed characterization shows that this decay is due to persistent defects created at the crystalline silicon surface. Pristine interfaces are preserved when the post-etching film thickness exceeds 8 nm, yielding high quality interface passivation.

Gadolinium scandate by high-pressure sputtering for future generations of high-κ dielectrics

Gd-rich gadolinium scandate (Gd2–xScxO3) was deposited by high-pressure sputtering on (1 0 0) silicon by alternating the deposition of <0.5 nm thick films of its binary components: Sc2O3 and Gd2O3. The formation of the ternary oxide was observed after the thermal treatments, with a high increase in the effective permittivity of the dielectric (up to 21). The silicon diffuses into the Gd2–xScxO3 films, which show an amorphous character. After the annealing no interfacial silicon oxide is present. CHF–VG curves indicated low hysteresis (55 mV) and a density of interfacial defects of 6 × 1011 eV–1 cm–2.

Antireflection effects at nanostructured material interfaces and the suppression of thin-film interference

Thin-film interference is a well-known effect, and it is commonly observed in the colored appearance of many natural phenomena. Caused by the interference of light reflected from the interfaces of thin material layers, such interference effects can lead to wavelength and angle-selective behavior in thin-film devices. In this work, we describe the use of interfacial nanostructures to eliminate interference effects in thin films. Using the same principle inspired by moth-eye structures, this approach creates an effective medium where the index is gradually varying between the neighboring materials. We present the fabrication process for such nanostructures at a polymer–silicon interface, and experimentally demonstrate its effectiveness in suppressing thin-film interference. The principle demonstrated in this work can lead to enhanced efficiency and reduce wavelength/angle sensitivity in multilayer optoelectronic devices.

The effect of grain boundary characteristics on the morphology of the crystal/melt interface of multicrystalline silicon

The effect of grain boundary characteristics on the crystal/melt interface morphology during the unidirectional solidification of multicrystalline Si was studied by in situ observations. It was shown that sharp and smooth grooves were formed at the crystal/melt interfaces at Σ27 and random grain boundaries, whereas no grooves were formed at Σ3 {1 1 1} twin boundaries. We explain the reasons why the grooves form at grain boundaries and discuss the impurity segregation at the grain boundary grooves on the crystal/melt interface.

Doslidzhennya metodamy yadernoho mikroanalizu poverkhnevykh nanoshariv struktury zoloto−kremniy

The Rutherford backscattering and particle-induced X-ray emission methods are used to study the surface layers in gold–silicon structures, the parameters of which govern the operational characteristics of electron devices constructed on their basis. The measurements are performed on a high-precision microanalytical unit “Nuclear scanning probe” recently put into operation at the “Spectrum” laboratory. The thicknesses of a gold layer sputtered onto the specimen surface were about 17 and 20 nm for two different specimens. The layer non-uniformity was less than 1.6 nm and did not exceed the experimental error. A substantial amount of the fluorine impurity was revealed under the gold layer in the gold–silicon interface layer. Probably, it may be remnants of fluorine remaining after the etching of the silicon surface in a mixture of acids that included the hydrofluoric one (HF). The amount of fluorine detected in a series of measurements was found to strongly correlate with the current of alpha-particles at the target surface during the spectrum measurement. Since the local heating of the target depends on the current, it is evident that the local diffusion rate of fluorine atoms varied over the target surface.

(Invited) Nanostructure Modified Porous Interfaces for Enhanced Sensing and Directed Microcatalysis

Gaseous analyte interactions on a metal oxide decorated n-type porous silicon interface show a dynamic electron transduction to and from the interface depending upon the relative strength of the gas and metal oxides. The dynamic interaction of NO with TiO2, SnO2, NiO CuxO, and AuxO, in order of decreasing acidity, demonstrates this effect. Surface interactions are also controlled by the in-situ nitridation of the oxide nanoparticles, enhancing the basicity of the decorated interface. This process changes the interaction of the interface with the analyte. The observed change to the more basic oxynitrides does not represent a simple increase in surface basicity but appears to involve a change in molecular electronic structure which is well explained by the IHSAB model.

Structural defects and magnetic properties of gadolinium silicide nanoparticles synthesized by laser ablation technique in liquid

Magnetic nanoscale compounds and alloys of gadolinium are promising materials for technological and medical applications. In this paper the influence of structural defects on the properties of the binary GdSi nanopowders formed in laser ablation and post-irradiation processes have been investigated. To obtain compound GdSi nanoparticles (NPs) we used laser ablation of a combined target consisting of Gd and Si plates tightly pressed to each other with focusing a laser beam onto the gadolinium–silicon interface. The crystalline structure, phase composition of the NPs formed were examined by SEM and X-ray diffraction (XRD). Magnetic measurements showed that the resulting NPs did not exhibit hysteresis at low temperature (6 K), which is typical to nanosized magnetic materials demonstrating superparamagnetic behavior. Magnetization increased after laser processing of the samples due to ordering in the grain structure and in the magnetic moments at the surface.

Evaluation of the influence of an embedded porous silicon layer on the bulk lifetime of epitaxial layers and the interface recombination at the epitaxial layer/porous silicon interface

ABSTRACTPorous silicon plays an important role in the concept of wafer‐equivalent epitaxial thin‐film solar cells. Although porous silicon is beneficial in terms of long‐wavelength optical confinement and gettering of metals, it could adversely affect the quality of the epitaxial silicon layer grown on top of it by introducing additional crystal defects such as stacking faults and dislocations. Furthermore, the epitaxial layer/porous silicon interface is highly recombinative because it has a large internal surface area that is not accessible for passivation. In this work, photoluminescence is used to extract the bulk lifetime of boron‐doped (1016/cm3) epitaxial layers grown on reorganised porous silicon as well as on pristine mono‐crystalline, Czochralski, p+ silicon. Surprisingly, the bulk lifetime of epitaxial layers on top of reorganised porous silicon is found to be higher (~100–115 µs) than that of layers on top of bare p+ substrate (32–50 µs). It is believed that proper surface closure prior to epitaxial growth and metal gettering effects of porous silicon play a role in ensuring a higher lifetime. Furthermore, the epitaxial layer/porous silicon interface was found to be ~250 times more recombinative than an epitaxial layer/p+ substrate interface (S ≅ 103 cm/s). However, the inclusion of an epitaxially grown back surface field on top of the porous silicon effectively shields minority carriers from this highly recombinative interface. Copyright © 2013 John Wiley &amp; Sons, Ltd.

Laser irradiation of nano-metric Al/Ti multilayers

Multilayered 5×(Al/Ti) structures, deposited on a Si substrate to a total thickness of 130 nm, were treated by unfocused Nd:YAG laser pulses (150 ps) with energies of 85 and 65 mJ. Irradiations were performed in air with 10, 50 and 100 successive laser pulses. Characterizations were done by using Rutherford backscattering spectrometry (RBS), elastic recoil detection analysis (ERDA), x-ray diffraction (XRD) and transmission electron microscopy (TEM). The results obtained show that laser irradiation, at either energy, induced almost full intermixing of deposited layers and formation of intermetallic compounds, but this was more pronounced for the applied laser pulses with a higher energy. The intermixed layer–silicon interface remains intact for all numbers of applied laser pulses and for both energies. The formation of an oxide layer on the sample surfaces was also observed, the thickness of which is greater for the higher laser beam energy.

Bioimpedance system for monitoring muscle and cardiovascular activity in the stump of lower-limb amputees

A bioimpedance system for the continuous measurement of non-invasive physiological parameters in lower-limb amputees is presented. The aim of the system is to monitor as many physiological parameters as possible from a single bioimpedance electrode configuration. In this way, a simple, low-cost and low-size autonomous system is developed that is able to continuously monitor the amputee in different environments (home, work, etc). The system measures both electrical impedance myography and electrical impedance plethysmography in the stump with electrodes placed in the inside face of a silicone interface. Such a system allows for the monitoring of a patient's muscle activity, and heart and breath rate, thus enabling the study and continuous monitoring of prosthesis adaptation and improvement of patient's gait to reduce physiological stress. Additionally, it can prevent cardiovascular problems due to the effort involved in the use of prostheses, which can decrease the life expectancy of amputees with previous vascular diseases. Experimental results obtained from different amputees' test validate the purpose of the system.

ITO/p+nc-Si:H contact barrier effects on n-i-p′-p silicon solar cell performances

Abstract The computer program AMPS-1D (analysis of microelectronic and photonic structures) has been used to explore the effect of front contact barrier heights for electrons ( φ b 0 ) or holes ( φ h ) on the performances of n-i-p′-p amorphous/nanocrystalline silicon based solar cell, with a p type hydrogenated nanocrystalline silicon double layer and with no back reflector. φ b 0 is the result of band bending at the indium tin oxide (ITO)/p + doped hydrogenated nanocrystalline silicon (p nc-Si:H) interface. This paper presents results for a n-i-p′-p device, when the p nc-Si:H layer is used as a window and the p′-nc-Si:H layer as a buffer. Band diagram at thermodynamic equilibrium and current–voltage characteristics ( J – V ), under dark and illumination conditions, for the considered solar cell structure, are calculated. The modeling showed that the reverse bias currents do not depend on the front contact barrier heights. However, in the forward direction, this contact barrier influences strongly the J – V characteristic in the dark and under illumination. As a result, when φ b 0 increases, output cell parameters, like open circuit voltage ( V OC ), fill factor (FF) and efficiency ( E ff ) increase. The best values obtained are 0.893 V, 0.757 and 8.04%, respectively. These values correspond to a front contact barrier height ( φ b 0 ) equal to 1.65 eV. Such a value of φ b 0 can be realized experimentally by using an indium tin oxide (ITO) front contact electrode, with a work function value about 5.35 eV.

Nanostructure-directed chemical sensing: The IHSAB principle and the dynamics of acid/base-interface interaction

Nanostructure-decorated n-type semiconductor interfaces are studied in order to develop chemical sensing with nanostructured materials. We couple the tenets of acid/base chemistry with the majority charge carriers of an extrinsic semiconductor. Nanostructured islands are deposited in a process that does not require self-assembly in order to direct a dominant electron-transduction process that forms the basis for reversible chemical sensing in the absence of chemical-bond formation. Gaseous analyte interactions on a metal-oxide-decorated n-type porous silicon interface show a dynamic electron transduction to and from the interface depending upon the relative strength of the gas and metal oxides. The dynamic interaction of NO with TiO2, SnO2, NiO, CuxO, and AuxO (x >> 1), in order of decreasing acidity, demonstrates this effect. Interactions with the metal-oxide-decorated interface can be modified by the in situ nitridation of the oxide nanoparticles, enhancing the basicity of the decorated interface. This process changes the interaction of the interface with the analyte. The observed change to the more basic oxinitrides does not represent a simple increase in surface basicity but appears to involve a change in molecular electronic structure, which is well explained by using the recently developed IHSAB model. The optical pumping of a TiO2 and TiO2−xNx decorated interface demonstrates a significant enhancement in the ability to sense NH3 and NO2. Comparisons to traditional metal-oxide sensors are also discussed.

Comparative measurements on atomic layer deposited Al2O3 thin films using ex situ table top and mapping ellipsometry, as well as X-ray and VUV reflectometry

In this study we compare the thicknesses and optical properties of atomic layer deposited (ALD) Al2O3 films measured using table top and mapping ellipsometry as well as X-ray and optical reflectometry. The thickness of the films is varied in the range of 1–50nm. ALD samples are used as references with well-controlled composition and thickness, as well as with a good lateral homogeneity. The homogeneity is checked using mapping ellipsometry. Optical models of increasing complexity were developed to take into account both the top (surface roughness on the nanometer scale) and bottom interfaces (buried silicon oxide and interface roughness). The best ellipsometric model was the one using a single interface roughness layer. Since the techniques applied in this work do not measure in vacuum, organic surface contamination even in the sub-nanometer thickness range may cause an offset in the measured layer thicknesses that result in significant systematic errors. The amount of surface contamination is estimated by in situ reflectometry measurement during removal by UV radiation. Taking into account the surface contamination the total thicknesses determined by the different methods were consistent. The linearity of the total thickness with the number of atomic layer deposition cycles was good, with an offset of 1.5nm, which is in good agreement with the sum of thicknesses of the interface layer, surface nanoroughness, and contamination layer.

All in-situ GaSb MOS structures on GaAs (001): Growth, passivation and high-k oxides

The GaSb-on-GaAs growth was optimized for the fabrication of metal-oxide-semiconductor (MOS) capacitors (Caps) with low interface state trap density (Dit) using in-situ deposited amorphous silicon (a-Si) interface passivation layer (IPL) and high-k oxides. The best top surface with the average roughness Ra=0.37nm and with spiral type “step-flow” growth mode was observed in the GaSb structure with the initial 0.5μm grown at 410°C and the top 0.5μm grown at 485°C. N- and p-type GaSb MOSCaps with reasonable capacitance–voltage(C–V) characteristics at room temperature (RT) were demonstrated using all in-situ 0.5nm a-Si IPL and 10nm Al2O3+HfO2 or Al2O3. A-Si IPL was found essential for n-MOSCaps but not in the case of p-MOSCaps where comparable C–V characteristics with a similarly low Dit=1–2×1012cm−2eV−1were demonstrated without IPL.