Low-temperature Passivation Research Articles

Low temperature back-surface-field contacts deposited by hot-wire CVD for heterojunction solar cells

The growing interest in using thinner wafers (< 200 μm) requires the development of low temperature passivation strategies for the back contact of heterojunction solar cells. In this work, we investigate low temperature deposited back contacts based on boron-doped amorphous silicon films obtained by Hot-Wire CVD. The influence of the deposition parameters and the use of an intrinsic buffer layer have been considered. The microstructure of the deposited thin films has been comprehensively studied by Spectroscopic Ellipsometry in the UV–visible range. The effective recombination velocity at the back surface has been measured by the Quasi-Steady-State Photoconductance technique. Complete double-side heterojunction solar cells (1 cm 2) have been fabricated and characterized by External Quantum Efficiency and current–voltage measurements. Total-area conversion efficiencies up to 14.5% were achieved in a fully low temperature process (< 200 °C).

Selective area metalorganic vapor-phase epitaxy of gallium arsenide on silicon

The selective area, metalorganic vapor-phase epitaxy of gallium arsenide on silicon substrates was investigated. Low-temperature arsenic passivation of the silicon surface was realized at 650 °C. A two-step growth method was used to deposit the GaAs films with an optimum nucleation temperature of 400 °C. Layers nucleated at 350 °C or below were found to be polycrystalline whereas those nucleated at 400 °C and above were single crystal. The X-ray diffraction full-width-at-half-maximum decreased from 890 to 775 arcsec for a 1.0-μm-thick GaAs layer grown on unpatterned versus patterned Si (1 0 0) wafers. The 77 K photoluminescence peak position was 1.489 and 1.498 eV for a 1.0-μm-thick GaAs layer on unpatterned and patterned substrates, respectively. Based on the photoluminescence peak shift, we find that selective area growth of GaAs on Si improves film crystallinity while reducing the in plane strain in the film by 35%.

Evaluation of Surface Passivation Layers for Bulk Lifetime Estimation of High Resistivity Silicon for Radiation Detectors

In order to identify an appropriate low-temperature surface passivation that could be used for bulk lifetime estimation of high resistivity (HR) (&gt; 1 k·cm) silicon for radiation detectors, different passivating layers were evaluated on n-type and p-type standard Czochralski (CZ), HR magnetic CZ and HR float zone (FZ) substrates. Minority carrier lifetime measurements were performed by means of a μW-PCD set-up. The results show that SiNx PECVD layers deposited at low temperatures (≤ 250°C) may be used to evaluate the impact of different processing steps and treatments on the substrate characteristics for radiation detectors. First results are obtained about a preliminary thermal treatment experiment to evaluate the thermal stability of the passivating layers, as well as the potential impact of the generation of thermal donors on minority carrier lifetime.

Passivation issues in active pixel CMOS image sensors

Most of the integrated circuit industry follows a final passivation process which consists of a low temperature passivation layer deposition and a thermal anneal. This two step process is particularly relevant in CMOS imagers where the dark current is a major issue. This work shows that passivation material plays an important role in the device performance. We measured H diffusion through the final silicon nitride layer and we compare these results with the material properties and passivation efficiency.

The low-temperature a-SiNx films with high impermeability and high optical gap with application to organic light-emitting diode

A low-temperature passivation layer (≦100°C) with high impermeability and a high optical gap is required in top-emitting organic light-emitting diodes. Hydrogenated amorphous silicon nitride (a-SiNx:H) films fabricated by the plasma-enhanced chemical-vapor deposition of SiH4+NH3+N2 at 100°C fulfill this requirement. Infrared-absorption spectroscopy was used to monitor the permeability of the a-SiNx:H film. The optimum a-SiNx:H film with an optical gap of 3.1eV was fabricated at a gas ratio of SiH4:NH3:N2=10:3:197SCCM. The main absorption peaks of the a-SiNx:H films deposited at 70°C did not change at all, even following a 3000-h-long storage test.

Innovative nitride passivated pseudomorphicGaAs HEMTs

A novel low-temperature nitride passivation technique using high-density inductively coupled plasma chemical vapor deposition (HD-ICP-CVD) with SiH/sub 4//N/sub 2/ chemistries to passivate 0.15 /spl mu/m pseudomorphic GaAs HEMTs has been developed for the first time. HD-ICP-CVD nitrides have higher density and lower hydrogen concentration than those of nitrides deposited by plasma-enhanced CVD (PECVD). Furthermore, HD-ICP-CVD passivated devices exhibit better performance in reverse breakdown voltage, transconductance, and cut-off frequency than those of PECVD passivated devices. The results achieved here warrant the applications of HD-ICP-CVD for next-generation nitride passivation in compound semiconductor technologies.

To the Low-Temperature Passivity of Iron at the Gaseous Oxidation

The low-temperature passivation of the oxide growth on iron is studied with the use of digital ellipsometry at a temperature of 300°C and an oxygen pressure from 10–3 to 1 mmHg. The maximum increment in the oxide thickness as a function of the oxygen pressure P a–p is observed in an hour of exposure, which indicates the active–passive transition. This passivity of iron and other metals can be caused by the multilayer and multiphase structure of the oxide film formed. As the oxygen pressure is increased, an external protective hematite layer appears on iron, and the inner quickly growing magnetite layer has no time to reach its limiting thickness. Shortening of the period of a rapid growth of magnetite, which is observed upon the increase in the oxygen pressure at the same exposure, results in the maximum of the summary thickness of the layer at a certain pressure P a–p. Hematite over the magnetite layer is usually formed as laterally spreading islets, the coalescence of which sharply decelerates the oxidation. In the time–pressure–oxide thickness plot, the areas of the low-temperature passivation of iron can be distinguished in wide ranges of temperature and pressure. The electrophysical treatment in the range of the active–passive transition sharply intensifies the oxidation of iron-based alloys and leads to the formation of layers with a substantial thickness and protective ability.

Low temperature passivation of silicon surfaces by polymer films

A novel surface passivation method for silicon carrier lifetime measurements and solar cells using a polymer film is introduced. It is easy to apply, no special pre-treatment, e.g. no hydrofluoric acid (HF)-treatment, is necessary. The surfaces to be passivated are covered with the polymer solution, dried at 90°C and encapsulated. Surface recombination velocities ( S) as low as S=30 cm/s for various doping concentrations have been observed, nearly independent of the bulk injection level. The passivation is stable for at least 6 h. For a polymer-passivated rear contact solar cell the same open circuit voltage is achieved as for a cell with thermally grown oxide.

Infrared multispectral detection using Si/SixGe1−x quantum well infrared photodetectors

A modified p-type Si/SiGe quantum well infrared photodetector for multispectral infrared imaging applications is demonstrated. In order to improve the detector’s performances we have used a SiGe emitter and a low-temperature wet passivation process that give rise to a reduced dark current, even at relatively high bias voltages. Multispectral photoresponse at the long, mid and short wavelength infrared atmospheric windows was observed. The response peaks are assigned to the various classes of intervalence band transitions in the quantum wells and in the SiGe emitter layers.

Field effect surface passivation of SiO 2/Si interfaces by heat treatment with high-pressure H 2O vapor

We investigated a simple field effect passivation of the silicon surfaces using the high-pressure H 2O vapor heating. Heat treatment with 2.1×10 6 Pa H 2O vapor at 260°C for 3 h reduced the surface recombination velocity from 405 cm/s (before the heat treatment) to 38 cm/s for the thermally evaporated SiO x film/Si. Additional deposition of 140 nm-SiO x films ( x<2) with a high density of fixed positive charges on the SiO 2/Si samples further decreased the surface recombination velocity to 22 cm/s. We also demonstrated the field effect passivation for n-type silicon wafer coated with thermally grown SiO 2. Additional deposition of 210 nm SiO x films on both the front and rear surfaces increased the effective lifetime from 1.4 to 4.6 ms. Combination of thermal evaporation of SiO x film and the heat treatment with high-pressure H 2O vapor is effective for low-temperature passivation of the silicon surface.

Characterization of uranium corrosion products involved in a uranium hydride pyrophoric event

Uranium metal corrosion products involved in a recent pyrophoric event were characterized using thermo-gravimetric analysis, X-ray diffraction, and BET gas sorption techniques to determine the effects of passivation treatment and long-term storage on chemical reactivity. Characterization was performed on corrosion products in three different conditions: immediately after separation from the source metal, after low-temperature passivation, and after passivation and extended vault storage. The hydride fraction and ignition temperature of the corrosion products were found to be strongly dependent on the corrosion extent of the source metal. There was little change in corrosion product properties resulting from low-temperature passivation or vault storage. The results indicate that the energy source for the pyrophoric event was a considerable quantity of uranium hydride present in the corrosion products, but the specific ignition mechanism could not be identified.

High-efficiency polycrystalline silicon film solar cells

High-efficiency polycrystalline silicon film solar cells are fabricated using gettering and low-temperature plasma-enhanced chemical vapor deposition (PECVD) passivation schemes. Thin layers (∼10 nm) of PECVD SiO 2 are used to passivate the emitter of the solar cell, while direct hydrogen rf plasma and PECVD silicon nitride (Si 3N 4) are implemented to provide emitter and bulk passivation of the cells. The combination of Al and POCl 3 gettering is found to be very effective in improving the minority carrier diffusion length of the polycrystalline silicon film wafers and consequently, the efficiency of the solar cells. It is shown in this work that hydrogen rf plasma treatments can significantly improve the solar cell's blue and long wavelength responses when performed through a thin layer of PECVD Si 3N 4. A very high efficiency for Si-supported film of 15.2% on 1 cm 2 area has been achieved using a simple solar cell process with Al/POCl 3 gettering and low-temperature passivation.

Express characterization of indirect semiconductor surfaces by in situ photoluminescence during chemical and electrochemical treatments

The interband photoluminescence (PL) of indirect semiconductors is controlled by the non-radiative bulk and surface recombination processes and in this way the PL signal is a measure for surface passivation. We demonstrate a stroboscopic PL probe as a powerful tool for the express in situ control of indirect semiconductor surfaces during low temperature passivation processes. Samples of Group IV and III–V indirect-gap semiconductors (c-Si, c-Ge, c-SiGe and epitaxial layers and c-GaP) are investigated during chemical and electrochemical treatments. The surface passivation of Si and Ge depends differently on the pH.

Group IVB refractory metal crystals as lattice-matched substrates for growth of the group III nitrides by plasma-source molecular beam epitaxy

Several possible methods for the synthesis of usefully large and perfect Hf crystals have been identified. Two of these methods, the iodide process and strain annealing, have been carried to the demonstration stage and yielded centimeter-sized substrates suitable for a GaN molecular beam epitaxial growth process. X-ray rocking curve full-widths of the Hf [0 0 0 2] peak are as low as 98 arcsec. Methods for preparing the substrate material and cleaning it in situ were developed, including sputtering of the native oxide using an electron-cyclotron-resonance Ar ion beam and an electrostatically biased substrate stage. The nucleation of GaN on Hf appears to require a low-temperature (< 500°C) passivation in order to avoid unwanted reaction products. X-ray diffraction and electron microscopy data show that the resulting films are smooth, single crystal, epitaxial, and in registry with the substrate, even away from the perfect lattice-matching condition that occurs on the basal plane. Low-temperature photoluminescence measurements show a near-band-edge peak that is consistent with donor-bound exciton recombination in strain-free GaN, and a set of peaks associated with the Mg acceptor.

Novel Low-Temperature Double Passivation Layer in Hydrogenated Amorphous Silicon Thin Film Transistors

Hydrogenated amorphous silicon nitride ( a-SiNx :H) films fabricated by plasma-enhanced chemical vapor deposition (PECVD) at low temperatures (<150° C) tend to be oxidized within one month because of their porous structure which permits the percolation of H2O (moisture) molecules from air. The a-SiNx :H film deposited at a low temperature of 100° C oxidizes more quickly (within 1 day) due to its greater porosity. However, it is found that a very thin coating ( >6.4 nm) of PECVD a-Si:H can prevent the percolation of H2O molecules. Hence, for a-Si:H thin film transistor (TFT) with a 100° C-deposited 470-nm-thick a-SiNx :H passivation layer, the ON/OFF current ratio will be degraded by ∼2 orders of magnitude after 7 days of exposure to air. On the other hand, for the 100° C-deposited a-SiNx :H passivation layer with an additional 100° C-deposited 50-nm-thick a-Si:H coating, the measured ON/OFF current ratio does not show any sings of degradation for up to one week. Therefore, a novel low-temperature (<150° C) water-resistant a-Si:H coating on a a-SiNx :H passivation layer deposited at the same temperature, which can be consecutively processed using the same PECVD system without breaking the vacuum, is proposed.

Progress in Low-temperature Surface Passivation of Silicon Solar Cells using Remote-plasma Silicon Nitride

Using a remote-plasma technique as opposed to the conventional direct-plasma technique, significant progress has been obtained at ISFH in the area of low-temperature surface passivation of p-type crystalline silicon solar cells by means of silicon nitride (SiN) films fabricated at 350–400°C in a plasma-enhanced chemical vapour deposition system. If applied to the rear surface of the low-resistivity p-type substrates, the remote-plasma SiN films provide outstanding surface recombination velocities (SRVs) as low as 4 cm s−1, which is by a clear margin the lowest value ever obtained on a low-resistivity p-Si wafer passivated by a solid film, including highest quality thermal oxides. Compared to direct-plasma SiN films or thermally grown oxides, the remote-plasma films not only provide significantly better SRVs on low-resistivity p-silicon wafers, but also an enormously improved stability against ultraviolet (UV) light. The potential of these remote-plasma silicon nitride films for silicon solar cell applications is further increased by the fact that they provide a surface passivation on phosphorus-diffused emitters which is comparable to high-quality thermal oxides. Furthermore, if combined with a thermal oxide and a caesium treatment, the films induce a UV-stable inversion-layer emitter of outstanding electronic quality. Due to the low deposition temperature and the high refraction index, these remote-plasma SiN films act as highly efficient surface-passivating antireflection coatings. Application of these films to cost-effective silicon solar cell designs presently under development at ISFH turned out to be most successful, as demonstrated by diffused p-n junction cells with efficiencies above 19%, by bifacial p-n junction cells with front and rear efficiencies above 18%, by mask-free evaporated p-n junction cells with efficiencies above 18% and by MIS inversion-layer cells with a new record efficiency of above 17%. All cells are found to be stable during a UV test corresponding to more than 4 years of glass-encapsulated outdoor operation. © 1997 John Wiley & Sons, Ltd.

Low-temperature passivation of copper by doping with Al or Mg

The doping of copper with Al and Mg is discussed as a method of passivating the exposed surface of copper films proposed for use as a conductor in microelectronics. Doping by co-deposition and by diffusion from Cu/M/SiO 2 bilayers, where M = Al or Mg, is discussed. Annealing such doped films in an ambient containing small amounts of oxygen produces a thin surface barrier layer of dopant oxide which stops further oxidation. In good cases, the conductivity of these films approaches that of films of pure sputtered copper. Such doping also improves adhesion to SiO 2 and stability on SiO 2. Improvements in stability include both decreased roughening due to grain growth and decreased diffusion transport into SiO 2.

Low temperature passivation of Si1−xGex alloys by dry high pressure oxidation

Thermal passivation of Si1−xGex using high pressure (70 MPa) oxidation was studied for potential use in MOS-device applications. Alloys of CVD-grown Si1−xGex (x = 10 and 15 at.%, 200 and 150 nm thick, respectively), were oxidized at 500 and 550°C using high purity dry oxygen at a pressure of 70 MPa. For comparative purposes, a second set of alloys were oxidized using conventional wet atmospheric pressure oxidation at 800°C. X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, transmission electron microscopy (TEM), and metal-oxide semiconductor capacitance-voltage (C-V) measurements were used to characterize the as-grown oxides. Chemical analysis by XPS confirmed that under high pressure conditions compositionally congruent oxides are grown from these alloys. High resolution TEM and Raman spectroscopy show that the as-grown oxide/semiconductor interface is planar and free of Ge enrichment on a scale of 1-2 monolayers. A midgap interface state density for both the 10 and 15 at.% samples of 1 × 1012 cm−2 eV−1 was estimated based on 1 MHz C-V measurement.

Anodic oxidation as a method for low temperature passivation of silicon radiation detectors

After cleaning and annealing treatment at 400 and 450°C the electrophysical properties of anodically and thermally grown silicon oxide films are practically identical, although the SiOSi bond angle and the H depth profile are different. For several years such anodic oxide films have been applied for the passivation of electron detectors with very high resistivity Si starting material (10–30 kω cm).

ECR Plasma Synthesis of Silicon Nitride Films ON GaAs and InSb

ABSTRACTThe growth of high-quality dielectric films from Electron Cyclotron Resonance (ECR) plasmas provides for low-temperature surface passivation of compound semiconductors. Silicon nitride (SiNx) films were grown at temperatures from 30°C to 250°C on GaAs substrates. The stress in the films was measured as a function of bias applied during growth (varied from 0 to 200 V), and as a function of sample annealing treatments. Composition profiles of the samples were measured using ion beam analysis. The GaAs photoluminescence (PL) signal after SiNx growth without an applied bias (ion energy = 30 eV) was twice as large as the PL signal from the cleaned GaAs substrate. The PL signal from samples biased at -50 and -100 V indicated that damage degraded the passivation quality, while atomic force microscopy of these samples showed a three fold increase in rms surface roughness relative to unbiased samples. The sample grown with a bias of-200 V showed the largest reduction in film stress but also the smallest PL signal.