Phosphorus-doped Polycrystalline Silicon Films Research Articles

In situ phosphorus-doped polycrystalline silicon films by low pressure chemical vapor deposition for contact passivation of silicon solar cells

In situ phosphorus (P)-doped polycrystalline silicon (poly-Si) films by low pressure chemical vapor deposition (LPCVD) were studied in this work for the fabrication of poly-Si passivating contacts. In situ doping was targeted for enabling the full potential of the high-throughput LPCVD technique, as it could allow leaner fabrication of industrial solar cells featuring poly-Si passivating contacts than the more common ex situ doping routes. By careful optimization of the deposition temperature and the flows of the carrier gas (H2) and the dopant precursor (PH3), high doping in the poly-Si layers was achieved with active P concentrations up to 1.3⋅1020 cm−3. While reduction in the deposition rate (rdep) and thus in the throughput is a known problem when growing in situ P-doped films by LPCVD, this reduction could be limited, and the resulting rdep was equal to 0.078 nm/s. The developed poly-Si films were characterized both structurally and in terms of their passivation potential in poly-Si contacts. The latter yielded recombination current densities down to 1.5 fA/cm2 in passivated (J0,p) and 25.6 fA/cm2 in screen-printing metallized (J0,m) regions on saw-damage removed (SDR) Cz-Si surfaces, accompanied by a contact resistivity (ρc,m) of 4.9 mΩ⋅cm2. On textured Cz-Si surfaces, the corresponding values were J0,p = 3.5 fA/cm2, J0,m = 56.7 fA/cm2, and ρc,m = 1.8 mΩ⋅cm2. Optical impact of the developed poly-Si films was also assessed and a short circuit density loss of 0.41 mA/cm2 is predicted per each 100 nm of poly-Si applied at the rear side of solar cells.

Hydrogenation of Phosphorus-Doped Polycrystalline Silicon Films for Passivating Contact Solar Cells.

We characterize and discuss the impact of hydrogenation on the performance of phosphorus-doped polycrystalline silicon (poly-Si) films for passivating contact solar cells. Combining various characterization techniques including transmission electron microscopy, energy-dispersive X-ray spectroscopy, low-temperature photoluminescence spectroscopy, quasi-steady-state photoconductance, and Fourier-transform infrared spectroscopy, we demonstrate that the hydrogen content inside the doped poly-Si layers can be manipulated to improve the quality of the passivating contact structures. After the hydrogenation process of poly-Si layers fabricated under different conditions, the effective lifetime and the implied open circuit voltage are improved for all investigated samples (up to 4.75 ms and 728 mV on 1 Ω cm n-type Si substrates). Notably, samples with very low initial passivation qualities show a dramatic improvement from 350 μs to 2.7 ms and from 668 to 722 mV.

Optical properties of evaporated poly-Si thin-films on glass

The optical properties of boron- and phosphorus-doped polycrystalline silicon films with light (~ 1 × 10 16 cm −3), moderate (~ 5 × 10 17 cm −3) and heavy doping (~ 1 × 10 19 cm −3) were investigated in this work. The films were prepared by solid-phase crystallization of evaporated amorphous silicon films on borosilicate glass. Tauc–Lorentz models with one or two oscillators were used to model both reflection and transmission data collected by a spectrophotometer over the wavelength range of 400 nm–2000 nm. The results indicate that the crystal quality of the films is improved by phosphorus doping, while boron has a negligible impact on the crystal quality. The poly-Si films exhibit greater absorption than c-Si for visible wavelengths. This enhanced absorption is believed to be associated with defected a-Si material at the grain boundaries and intra-grain defects.

Electron transport and band structure in phosphorus-doped polycrystalline silicon films

We study transport mechanisms, effective mass, and band structure by measuring the resistivity, Hall, and Seebeck and Nernst coefficients in heavily phosphorus-doped polycrystalline silicon films made by thermal crystallization of amorphous silicon. We observe a change in transport mechanism which results in an increase in electron mobility from 10% to 80% of the single-crystal silicon mobility as the carrier concentration increases from 1019 to 1020 cm−3. Our measurements of effective mass at the Fermi level indicate that as the carrier concentration increases, there is a shift from impurity-band transport to conduction-band transport, and that the electron effective mass is lower in the impurity band than in the conduction band of Si. The shift to conduction-band transport improves electron mobility with carrier density by improving intragrain carrier mean free path lengths and relaxation times.

Recrystallization behaviour and electrical properties of germanium ion implanted polycrystalline silicon films

The recrystallization behaviour of undoped and phosphorus-doped polycrystalline silicon films amorphized by germanium ion implantation at doses ranging from 1 × 1015 to 1 × 1016cm-2 are investigated, and the electrical properties of phosphorus-doped films after recrystallization are studied. The phosphorus doping concentration ranges from 3 × 1018 to 1 × 1020cm-3. It is found that the nucleation rate decreases for undoped films and increases for phosphorus-doped films with increasing germanium dose; the growth rates decrease for both doped and undoped films. The decrease in nucleation rate is caused by the increase in implantation damage. The decrease in growth rate is considered to be due to the increase in lattice strain. The grain size increases with germanium dose for undoped films, but decreases for phosphorus-doped films. The dependence of the electrical properties of the recrystallized films as a function of phosphorus doping concentration with different germanium doses can be explained in terms of the grain size, crystallinity and grain boundary barrier height.

The structure, morphology and resistivity of in situ phosphorus doped polysilicon films

The structural and morphological properties of in situ phosphorus doped polycrystalline silicon films have been investigated using transmission electron microscopy. The effect of deposition parameters (temperature, pressure in a reactor chamber and phosphine-silane flow ratio in the gaseous phase) on the structure, grain size and microroughness of P-doped films have been studied. The peculiarities of recrystallization processes occuring in P-doped films during activating anneal has been investigated. The assumed production condition dependences of the P-doped film growth and recrystallization as well as the correlation between resistivity of the films and their microstructure are discussed on the basis of analysis of the obtained experimental results.

Scanning tunneling microscopy investigation of phosphorus-doped polycrystalline silicon films

Scanning tunneling microscopy (STM) in air has been used to investigate the surface of highly phosphorus-doped polysilicon films produced by low pressure chemical vapour deposition (LPCVD). It has been found that, even when working in air, the technique gives detailed information about microroughness and mean grain size. Distinguishing between two different length scales, values for inter — and intra-grain roughness as a function of phosphorus doses are reported.

Morphological instability and Si diffusion in nanoscale cobalt silicide films formed on heavily phosphorus doped polycrystalline silicon

In this letter we examine the interdiffusion and reaction of deposited cobalt layers during the formation of nanoscale (<35 nm) silicide films on heavily phosphorus doped polycrystalline silicon films. The onset of morphological instability is indicated by an increase of layer resistivity. Cross-section transmission electron micrographs of high resistance films, which were formed by rapid thermal annealing at 700 °C for 30 s, show a series of disconnected ‘‘islands’’ of CoSi2 with or without a highly disordered (amorphous) Si-rich top surface layer. A continuous band of voids, attributed to the Kirkendall effect and to phase transformation induced volume changes, initially appear at the CoSi/CoSi2 interface. In the highly agglomerated films, these voids separate the buried CoSi2 islands and buried polycrystalline Si from the disordered surface layer. The sequence of events is analyzed in terms of grain boundary diffusion, grain boundary grooving, and the impact of phosphorus concentration (∼1020 cm−3) on Si diffusivity.

Formation of different types of polysilicon film structures and their grain growth under annealing

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Anisotropic etching of n+ polycrystalline silicon with high selectivity using a chlorine and nitrogen plasma in an ultraclean electron cyclotron resonance etcher

Heavily phosphorus-doped polycrystalline silicon films (n+ poly-Si) were etched in a pure chlorine plasma using an ultraclean electron cyclotron resonance etcher. Compared against undoped polycrystalline etching, horizontal etch rates were too high to allow anisotropic etching of n+ poly-Si. With the addition of more than about 10% N2, highly anisotropic etches of n+ poly-Si can be obtained simultaneously with selectivities as high as 160 to SiO2 in a 4 mTorr plasma. These results are significant to lower submicron fabrication. X-ray photoelectron spectroscopy studies show that Si—N bonds are formed on the n+ poly-Si surface during etching and it is proposed that this layer protects the sidewall against Cl radicals in a N2/Cl2 plasma. The suppression of SiO2 etching by O2 addition to a N2/Cl2 plasma has also been demonstrated.

Reactive‐Ion Etching (RIE) of TaSi2 / n + Polysilicon Bilayers

Dry etching of a binary layer of cosputtered 2900Å thick on 3900Å thick phosphorus doped polycrystalline silicon films in has been studied. The influence of oxygen concentration, RF power, and sample temperature, on etch rate was investigated. The surface of the etched silicide was examined using field emission scanning electron microscope (FESEM) and scanning Auger electron spectroscopy (AES). It is found that the etch rates of (i) increased with increasing oxygen percentage initially, reached a maximum value near 4% , then started to decrease with increasing oxygen in gas at applied power at 275W, sample temperature of 60°C, total gas flow rate of 20 sccm, and chamber pressure of 40 mtorr, and (ii) increased nonlinearly with the RF power in gas at chamber pressure of 40 mtorr and total gas flow rate of 20 sccm. Etch rates followed an Arrhenius‐type temperature dependence with activation energy of 0.08 eV. The etching in has been related to the large differences in the metal fluoride and silicon fluoride vapor pressures. Also, based on Auger electron spectroscopy data from the etched surface it is suggested that the tantalum enrichment on the surface during silicide etching is responsible for the reduced steady‐state etch rate.

Interpretation of 1/f noise in phosphorus-doped polycrystalline silicon films: Hooge’s model of lattice scattering

The empirical relation by Asbrock and Brand between the 1/f noise power, i2n[A2/Hz], and relevant parameters in phosphorus-doped polycrystalline silicon films at cryogenic temperatures is shown to follow Hooge’s 1/f noise model of lattice scattering, i2n/Idc∼2 =α(μ/μ1)2/(Nf), where α≂2×10−3 is a dimensionless universal constant, N is the number of carriers in the sample, and Idc is the dc current. It is assumed here that the temperature dependence of lattice mobility is μ1∼T−1.5 and that of electronic mobility is μ∼T, in polycrystalline Si:P+ films. The former relation is because of acoustic phonon interaction in covalent silicon and the latter is attributable to the decreasing barrier resistance at grain boundaries as the sample warms up.

Interpretation of dc characteristics of phosphorus-doped polycrystalline silicon films: Conduction across low-barrier grain boundaries

Current–voltage curves of phosphorus-doped polycrystalline silicon films of Lee and Cheng are reanalyzed with the same trapping model of thermionic and thermionic-field emission across symmetrical Schottky barriers between grains. The analysis yields unexpectedly low values of the effective number of grains Neff in the conducting path. Values of Neff/Ngeo decreases from 0.5 at 20 K to 0.1 at 300 K, where Ngeo =(film length)/(typical grain size). A dc conduction model is proposed. Carriers in phosphorus-doped polysilicon films travel extensively across low-barrier grain boundaries, where electrons from segregated phosphorus are abundant enough to fill most of the electron traps. The model can also explain an observed current–temperature relation i∝T3 at low bias voltages and yield values of barrier resistance comparable to reported data on polycrystalline silicon neutron-transmutation doped with phosphorus measured by a small-area traveling potential probe.

Electrical properties of phosphorus-doped polycrystalline silicon films contaminated with oxygen

Resistivity and Hall measurements were made on polycrystalline silicon films implanted with both oxygen and phosphorus ions. The increase in resistivity due to oxygen contamination was studied as a function of the oxygen concentration and it was found that the resistivity starts to increase for an oxygem density of about 10 20 cm -3. It is also shown that the presence of the oxygen reduces both the carrier concentration and the carrier mobility by inhibiting grain growth but it does not directly affect either the dopant segregation or the grain boundary scattering. Some results indicating that chlorine contamination also gives rise to an increase in the resistivity of phosphorus-doped polycrystalline silicon films are reported.

A model for conduction in polycrystalline silicon—Part II: Comparison of theory and experiment

In the preceding paper [1], a new phenomenological model for the electrical conduction in polycrystalline silicon was developed. Electrical conduction in polycrystalline silicon was shown to be controlled by dopant segregation, carrier trapping, and carrier tunneling through the grain boundaries. In this paper, the theoretical model is compared to experiment. The electrical behavior of polycrystalline silicon is shown to be influenced by the properties of the grain boundaries. In arsenic and phosphorus-doped polycrystalline-silicon films the grain boundaries are best modeled by rectangular barriers with a height of 0.66 eV and an approximate width of 7 A. The width of the grain-boundary barriers and the density of carrier trapping states are found to be weak functions of the dopant species and sample processing. The resistivity is found to be a strong function of dopant concentration, dopant species, and processing history at low and intermediate dopant concentrations, and the model can be used to predict this behavior.

Tantalum silicide/polycrystalline silicon—High conductivity gates for CMOS LSI applications

High conductivity gate electrode structures of sheet resistance ≲3 Ω/⧠ have been fabricated using a 2500 A (nominal) TaSi2 layer on 4500 A thick phosphorus doped polycrystalline silicon films. The TaSi2 layer was formed by cosputter depositing Ta and Si, using magnetron sources, and sintering in H2 at 900°C for 30 min. The composite structure was patterned using a CF4–O2 plasma etch process. The use of this structure has been compared directly with polycrystalline silicon gates in experiments using CMOS and NMOS devices. Device parameters, with the exception of a slight perturbation of VT for the CMOS devices, have been virtually identical for both types of gate electrode. Device yields are not affected by incorporating the TaSi2 layer. Device parameters show a tighter distribution probably due to the more uniform sheet resistance of the composite TaSi2/polycrystalline silicon layer.