Plasma-deposited Amorphous Silicon Research Articles

Heterojunction Silicon Wafer Solar Cells using Amorphous Silicon Suboxides for Interface Passivation

Current industrial monocrystalline silicon wafer solar cells based on screen-printing technology for contact formation and selective emitter have an efficiency potential of around 20%. Heterojunction silicon wafer solar cells combine plasma-deposited amorphous silicon thin films and n-type crystalline silicon wafers to industrially viable high-efficiency solar cell devices. The excellent surface passivation provided by an intrinsic amorphous silicon interlayer leads to a very high open-circuit voltage, enabling industrial heterojunction cells with efficiencies of over 23%, as demonstrated by the Japanese company Sanyo. The key point of these structures is the removal of the highly recombination-active solar cell contacts from the crystalline surface, by insertion of a thin film with a wide bandgap. To reach the full device potential, the defect density at the hetero-interface must be minimised. Commonly used hydrogenated amorphous silicon films of only a few nanometer thickness are appealing candidates for this. Their bandgap (∼1.7eV) is much larger than that of c-Si and, when intrinsic, such films can reduce the c-Si surface state density by hydrogenation. Additional oxygen built into the amorphous network results in amorphous silicon suboxides, which in turn leads to an improved surface passivation. With this approach we are able to achieve implied voltages of 740mV and an optical bandgap of about 2eV, which improves the cell's blue response. In addition, microcrystalline silicon emitter and back-surface-field layers provide enhanced doping efficiency, enabling the fabrication of contacts with low saturation current density values. This new approach has already demonstrated promising cell efficiencies of about 21%. This paper summarises the recent progress in heterojunction silicon wafer solar cell research using amorphous silicon suboxides.

Surface Reactions and Growth Kinetics of Plasma-deposited Amorphous and Microcrystalline Silicon Thin Films

The growth kinetics of amorphous silicon (a-Si) and microcrystalline silicon (μc-Si) thin films fabricated by plasma-enhanced chemical vapor deposition (PECVD) is discussed for developing low-cost and high-efficiency solar cells. Surface reactions are reviewed associated with the a-Si growth and the nucleation of μc-Si. With respect to the film growth of μc-Si after the nucleation, roughness evolutions were evaluated using an atomic force microscope, and scaling analyses were carried out on fractal structures of growing surfaces, because scaling exponents give a great insight into the growth kinetics. On the basis of results of scaling analyses in conjunction with Monte Carlo simulations, the growth kinetics of microcrystalline silicon thin films, actually used for the high-efficiency solar cells, is discussed.

Atomistic analysis of the mechanism of hydrogen diffusion in plasma-deposited amorphous silicon thin films

We report the mechanism and activation barrier of H diffusion on the surface and in the bulk of plasma-deposited hydrogenated amorphous silicon (a-Si:H) films during postdeposition exposure of the films to H atoms from a H2 plasma. Our study is based on molecular-dynamics simulations of repeated impingement of H atoms on surfaces of smooth a-Si:H films over the temperature range 475–800K. The H diffusion mechanism is identical both on the a-Si:H surface and in the bulk a-Si:H film. Specifically, the H atom diffuses rapidly through a floating-bond-mediated migration process; this floating bond accompanies the H atom as it hops from one Si atom to another. The Si atoms between which the H hops during its diffusion are typically either very weakly bonded or not bonded to each other. The calculated activation barrier for H diffusion is only 0.10eV.

Comparison of stress in single and multiple layer depositions of plasma-deposited amorphous silicon dioxide

The stress in a single-layer continuous deposition of amorphous silicon dioxide (SiO2) film is compared with the stress within multiple-layer intermittent or ‘stop–start’ depositions. The films were deposited by helicon activated reactive evaporation (plasma assisted deposition with electron beam evaporation source) to a 1 µm total film thickness. The relationships for stress as a function of film thickness for single, two, four and eight layer depositions have been obtained by employing the substrate curvature technique on a post-deposition etch-back of the SiO2 film. At film thicknesses of less than 300 nm, the stress–thickness relationships clearly show an increase in stress in the multiple-layer samples compared with the relationship for the single-layer film. By comparison, there is little variation in the film stress between the samples when it is measured at 1 µm film thickness. Localized variations in stress were not observed in the regions where the ‘stop–start’ depositions occurred. The experimental results are interpreted as a possible indication of the presence of unstable, strained Si–O–Si bonds in the amorphous SiO2 film. It is proposed that the subsequent introduction of a ‘stop–start’ deposition process places additional strain on these bonds to affect the film structure. The experimental stress–thickness relationships were reproduced independently by assuming a linear relationship between the measured bow and film thickness. The constants of the linear model are interpreted as an indication of the density of the amorphous film structure.

Simultaneous recrystallization, phosphorous diffusion and antireflection coating of silicon films using laser treatment

Laser technique application to polycrystalline silicon thin-film solar cell fabrication on glass substrates has received appreciable attention. In this paper, a laser-doping technique is developed for plasma-deposited amorphous silicon film. A process involving recrystallization, phosphorous diffusion and antireflection coating can be achieved simultaneously using the laser annealing process. The doping precursor, a phosphorous-doped titanium dioxide (TiO 2) solution, is synthesized using a sol–gel method and spin-coated onto the sample. After laser irradiation, the polycrystalline silicon grain size was about 0.5∼1.0 μm with a carrier concentration of 2 × 10 19 cm − 3 and electron mobility of 92.6 cm 2/V s. The average polycrystalline silicon reflectance can be reduced to a value of 4.65% at wavelengths between 400 and 700 nm, indicating the upper TiO 2 film of antireflection coating.

Laser Doping and Recrystallization for Amorphous Silicon Films by Plasma-Enhanced Chemical Vapor Deposition

One of the most challenging problems to develop polycrystalline silicon thin-film solar cells is the growth of crystalline silicon on foreign, low-cost and low-temperature substrates. In this paper, a laser doping technique was developed for the plasma-deposited amorphous silicon film. A process combination of recrystallization and dopant diffusion (phosphorous or boron) was achieved simultaneously by the laser annealing process. The doping precursor was synthesized by a sol-gel method and was spin-coated on the sample. After laser irradiation, the grain size of the doped polycrystalline silicon was examined to be about 0.5~1.0 µm. The concentrations of 2×1019 and 5× 1018 cm-3 with Hall mobilities of 92.6 and 37.5 cm²/V-s were achieved for the laser-diffused phosphorous- and boron-type polysilicon films, respectively.

Use of real-time ion-mass spectrometry and discharge voltage trending for analysis of a-Si DC plasma CVD processes

DC discharge voltage monitoring is a convenient non-intrusive tool for in-line monitoring of plasma-deposited amorphous silicon deposition processes. We discuss the observed variations of the discharge voltage due to (i) changing hydrogen dilution of silane, (ii) etch back of silicon by a hydrogen plasma, (iii) admixing of germane and methane and correlate the results with in situ ion-mass spectroscopy. We suggest that negative ion formation by electron attachment to silane and/or higher-silane species causes the changes of discharge voltage. Time dependences following discharge initiation are discussed in terms of the relative concentrations of silane versus higher-silane species.

Donor formation in plasma-deposited amorphous silicon (a-Si:H) by erbium incorporation

Erbium-doped amorphous silicon films have been prepared by plasma decomposition of silane doped with the volatile erbium compound tris(2,4-pentanedionato)-Er(III), which has a more favourable erbium–carbon ratio than previously used material. The films exhibit a rather high n-type dark conductivity ( σ d ≈3×10 −4 S/ cm at T=300 K), a high photoconductivity and a relatively low defect density (N D ≈3×10 17 cm −3) . These properties are similar to those of films doped with approximately 10 ppm phosphine. By co-doping with diborane we obtain compensated (near-intrinsic) a-Si:H(Er) films and p-type samples. These films thus may be suitable for the fabrication of light emitting p–i–n diodes.

Investigations on doping of amorphous and nanocrystalline silicon films deposited by catalytic chemical vapour deposition

Hydrogenated amorphous and nanocrystalline silicon, deposited by catalytic chemical vapour deposition, have been doped during deposition by the addition of diborane and phosphine in the feed gas, with concentrations in the region of 1%. The crystalline fraction, dopant concentration and electrical properties of the films are studied. The nanocrystalline films exhibited a high doping efficiency, both for n and p doping, and electrical characteristics similar to those of plasma-deposited films. The doping efficiency of n-type amorphous silicon is similar to that obtained for plasma-deposited electronic-grade amorphous silicon, whereas p-type layers show a doping efficiency of one order of magnitude lower. A higher deposition temperature of 450°C was required to achieve p-type films with electrical characteristics similar to those of plasma-deposited films.

Hydrogenated amorphous and polycrystalline silicon TFTs by hot-wire CVD

We studied the incorporation of hydrogenated amorphous silicon and polycrystalline silicon in thin-film transistors. These semiconductor layers are deposited with hot-wire chemical vapour deposition. The saturation mobility of the amorphous silicon transistors is 0.62 cm 2/V s and the activation energy of the OFF-current and ON-current 0.77 eV and 0.12 eV, respectively. The polycrystalline silicon transistor shows `amorphous' behaviour, with properties similar to the amorphous silicon transistor, except for the activation energy of the OFF-current (0.45 eV) which is only slightly less than that of bulk polycrystalline silicon. We conclude that the initial stage of the polycrystalline silicon growth is actually amorphous, which was confirmed by Raman spectroscopy. This amorphous incubation layer completely determines the ON-state transistor properties. We show that with hot-wire deposition, amorphous silicon thin-film transistors can be made under different deposition conditions. These transistors are stable upon gate bias stress, unlike conventional plasma-deposited amorphous silicon transistors.

The Inverted Meyer-Neldel Rule in the Conductance of Nanostructured Silicon Field-Effect Devices

AbstractThin film transistors (TFTs) offer the possibility to study the electronic transport properties of an intrinsic semiconductor as a function of the Fermi level position without the introduction of dopants and/or doping related defects. Recently, we reported on the first TFTs incorporating nanostructured silicon deposited with the Hot-Wire Chemical Vapor Deposition technique. These structures offer significant advantages over conventional plasma-deposited amorphous silicon TFTs. First of all, the HW deposited nanocrystalline silicon (nc-Si:H) TFTs do not show any threshold voltage shift upon prolonged gate voltage stress. Therefore, it is now possible to study the transport characteristics at a relatively large gate voltage in a controlled fashion, unhampered by any drift of the characteristics due to the creation of metastable electronic defect states and/or charge trapping. Second, the result of the field effect is that the Fermi energy moves into the conduction band of the virtually defect-free nanocrystalline domains in the channel region of the TFT. As the effective mobility gap of the surrounding amorphous phase is higher than that of the silicon crystallites, the Fermi energy is driven deep into the band-tail distribution of the amorphous phase, a situation that could never be achieved in purely amorphous silicon TFTs nor by heavily doping an amorphous semiconductor. Thus, the nanostructured nature of the silicon thin film near the gate insulator allows to shift the Fermi level far into the tail states region of the amorphous phase. This situation reveals for the first time the inverted Meyer-Neldel relationship in an intrinsic semiconductor.

Light-Induced Changes Among Fast-Relaxing Hydrogens in Plasma-Deposited Hydrogenated Amorphous Silicon

ABSTRACTLight-induced rearrangements in a-Si:H,D during and after illumination have been studied by deuteron magnetic resonance (DMR). A film sample was deposited on a 230°C Al substrate by plasma decomposition of S1H4 and D2. The high quality film contained 4 at.% H and 4 at. % D. DMR spectra of fast-relaxing components were examined at 700 msec or less after radio frequency saturation. At 30 K these showed a broad central signal (primarily nanovoid-trapped HD and D2), part of which shifted paramagnetically during illumination. No comparable shifts were shown by spectral components from Si-bonded hydrogen or from microvoid-contained dense fluid hydrogen. The observed post-illumination shifts vanished upon 150°C dark anneal and reflected light-induced metastable magnetic changes in the surroundings of hydrogen trapped in nanovoids.

Enhanced negative ion formation in ultraviolet-laser irradiated silane: Implications for plasma deposition of amorphous silicon

Observation of enhanced electron attachment to ArF-excimer-laser irradiated silane is reported. Evidence is presented that highly excited electronic states of silane or its photofragments are responsible for the observed enhanced electron attachment. Since such electronically excited states may be produced in silane plasmas (by direct electron impact or by excitation transfer via metastable states of rare gases that are commonly used in silane discharges), the possible significance of this electron attachment process for negative ion formation in silane plasmas is indicated.

In Situ Study of the Interfaces Between Plasma-Deposited Amorphous Silicon and Silicon Dioxide by UV-IR Spectroellipsometry

An in situ ellipsometry study of the interfaces between plasma-deposited amorphous silicon (a-Si:H) and silicon dioxide ( a-SiO2) is presented. In order to achieve a more detailed description of the interface formation, measurements obtained by UV-visible and infrared (IR) spectroellipsometry are combined. Intermixing layers, about 30–40 Å thick, are clearly revealed at the interface independent of the deposition sequence used. In particular, the presence of SiO and (O)SiHn bonds in the first few monolayers of the growth of the “top” a-Si:H is identified. Likewise, hydrogen accumulation, revealed by SiH vibrations, is observed during the early stage of the growth of a-SiO2 on top of a-Si:H. The behaviours of the a-Si:H-a-SiO2 and a-Si:H-a-SiNx interfaces are compared.

Chemical equilibration of plasma-deposited amorphous silicon with thermally generated atomic hydrogen.

Hydrogenated amorphous silicon (a-Si:H) thin films prepared by plasma-enhanced chemical vapor deposition (PECVD) from ${\mathrm{SiH}}_{4}$ have been further hydrogenated in situ by exposure to atomic H generated by a filament heated in ${\mathrm{H}}_{2}$ gas. Upon equilibration of the network with gas-phase H, as many as \ensuremath{\sim}2\ifmmode\times\else\texttimes\fi{}${10}^{21}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$ additional Si-H bonds form within the top 200 \AA{} of the film without significant etching, surface roughening, or coordination defect generation. Real-time spectroscopic ellipsometry is applied to study the kinetics of near-surface Si-H bond formation at 250 \ifmmode^\circ\else\textdegree\fi{}C in order to improve our understanding of the effects of excess atomic H in the a-Si:H growth environment. Atomic H entering the film surface exhibits an effective diffusion coefficient >3\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}15}$ ${\mathrm{cm}}^{2}$/s and is trapped within the top 200 \AA{} of the film at a rate of \ensuremath{\sim}${10}^{\mathrm{\ensuremath{-}}3}$ ${\mathrm{s}}^{\mathrm{\ensuremath{-}}1}$. Most of this H is trapped irreversibly on the time scale of deposition with emission rates 2\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}7}$ ${\mathrm{s}}^{\mathrm{\ensuremath{-}}1}$. We also find that monolayer levels of surface oxide are an effective diffusion barrier to H, preventing chemical equilibration between the gas and solid phases.

In situ investigation of amorphous silicon/silicon nitride interfaces by infrared ellipsometry

A detailed in situ study by infrared phase-modulated ellipsometry of interfaces between plasma-deposited amorphous silicon (a-Si:H) and silicon nitride (a-SiNx) is presented. The structure of the interface is affected by the deposition sequence. A behavior compatible with a sharp interface is observed when a-SiNx is deposited on top of a-Si:H, the underlayer material being very weakly influenced by the deposition of the overlayer. In contrast, a graded transition is observed when a-SiNx is deposited first. In the latter case, the infrared measurements directly reveal a nitrogen tail incorporated in the first monolayers of a-Si:H (10–20 Å thick). The formation mechanisms of the interfaces are discussed.

In situ infrared measurements of film and gas properties during the plasma deposition of amorphous hydrogenated silicon

This research has performed preliminary in situ Fourier transform infrared (FTIR) measurements during the plasma deposition of amorphous silicon (a-Si:H). Experiments demonstrate both gas phase and film measurements within a simple SiH4 plasma reactor using a specially modified FTIR spectrometer. Films are deposited on substrates of either gold (mirror finish) or stainless steel (matte finish). In particular, in situ emission/reflection FTIR of the film yields information about surface temperature, film thickness, and film composition. We have measured surface temperature to ±5 K and detected the onset of poor film growth at a thickness of 500–1000 Å using the 2080 cm−1 absorption feature. A simple model for the reflectance of a film on a metal is employed to determine the thickness of the films. In situ emission/transmission FTIR of the plasma determines the gas composition and average gas temperature. Measurements show that the silane conversion is ∼11% within the plasma region for a typical deposition at 250 °C and roughly doubles for a deposition at room temperature. The FTIR spectra show that most of this converted silane reappears as disilane (Si2H6). Before starting the plasma, the silane gas is ∼30 K cooler than the nominal substrate temperature of 250 °C; starting the plasma raises the average temperature another 20 °C.

Fast solid-phase crystallization of amorphous silicon films on glass using low-temperature multi-step rapid thermal annealing

Effects of multi-step rapid thermal annealing of plasma-deposited amorphous silicon films on Corning 7059 glass are investigated. A three-step rapid thermal annealing for 10 s/step at 730° C after film deposition reduces the activation energy of electrical conductivity for silicon films from 0.64 to 0.51 eV and causes (111) grain growth with a size of 1500 A, which is determined using scanning electron microscopy, Raman spectroscopy and X-ray diffractometry.

Composition Analysis of RF Plasma-Deposited Amorphous Silicon Oxynitride Thin Films by Spectroscopic Phase-Modulated Ellipsometry

ABSTRACTThin films of highly transparent amorphous silicon oxynitride (a-SiOx Ny:H) were prepared by rf glow discharge decomposition of SiH4,NH3 and N2O gases, in a PECVD reactor provided with an in situ phase-modulated ellipsometer. The ratio [N2O]/ ([N2O] + [NH3]) was varied from 0 to 1 in a dilution with 5% of SiH4, keeping constant the total gas flow rate. From spectroscopic ellipsometric measurements, performed in the range 1.5–5.0 eV, the refractive index of the films was calculated applying the Bruggeman effective medium theory. The structural model used considers a three-component mixture layer, composed of void, silicon dioxide (glass) and silicon nitride (pyrolitic), on a c-Si substrate. The x and y composition parameters were calculated from the fitted relative volume fraction of each constituent, ranging from a-SiO2 N0:H to a-SiO0N1.33:H, which are in fair agreement with those obtained from XPS measurements. These results, along with data of FTIR spectra of the films show a clear correlation between the void fraction and the bonded hydrogen content of the films.

Hydrogen bonding in amorphous silicon with use of the low-pressure chemical-vapor-deposition technique.

Infrared spectroscopy, elastic-recoil detection analysis (ERDA), and hydrogen evolution have been used to carefully determine the hydrogen content of low-pressure chemically-vapor-deposited a-Si:H films. The amount of hydrogen and the bonding character can be easily varied by changing the deposition conditions. The stretching peaks at \ensuremath{\sim}2000 and \ensuremath{\sim}2100 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ have been deconvoluted and the integrals calibrated by means of ERDA. The results provide the exact calibration constants for both peaks, and the values for the dipole effective charge obtained so far are consistent with the theory. Hydrogen-effusion measurements show that it is fairly certain that the material presents a small amount of voids, if compared with plasma-deposited amorphous silicon, despite the fact that the \ensuremath{\sim}2100-${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ peak can be detected in vibrational spectra. This result seems to confirm that the character of hydrogen bonding is not related to the amount of voids in the material. On the other hand, no presence of microcrystallites is detectable in Raman spectra and the sample quality, tested by means of photothermal-deflection spectroscopy, does not seem to depend on the relative strengths of the two stretching modes.