Doped-Chamber Deposition of Intrinsic Microcrystalline Silicon Thin Films and Its Application in Solar Cells

A series of microcrystalline silicon thin films were fabricated by very high frequency plasma enhanced chemical vapor deposition （VHF-PECVD） at different silane concentrations in a P chamber. Through analysis of the structural and electrical properties of these materials,we conclude that the photosensitivity slightly decreased then increased as the silane concentration increased,while the crystalline volume fraction indicates the opposite change. Results of XRD indicate that thin films have a （220） preferable orientation under certain conditions. Microcrystalline silicon solar cells with conversion efficiency 4. 7% and micromorph tandem solar cells 8.5% were fabricated by VHF-PECVD （p layer and i layer of microcrystalline silicon solar cells were deposited in P chamber）, respectively.

Large Scale Homogeneous Growth Mechanics of Microcrystalline Silicon Films on Rough Surface

Large scale homogenous growth of microcrystalline silicon （μ.c-Si：H） on cheap substrates by inductively coupled plasma （ICP） of Ar diluted Sill4 has been studied. From XRD and Raman spectrum, we find that substrates can greatly affect the crystalline orientation, and the μc-Si：H films are comprised of small particles. Thickness detection by surface profilometry shows that the thin μc-Si：H films are homogenous in large scale. Distributions of both ion density and electron temperature are found to be uniform in the vicinity of substrate by means of diagnosis of Langmuir probe. Based on these experimental results, it can be proposed that rough surfaces play important roles in the crystalline network formation and Ar can affect the reaction process and improve the characteristics of μc-Si：H films. Also, ICP reactor can deposit the thin film in large scale.

Formation mechanism of incubation layers in the initial stage of microcrystalline silicon growth by PECVD

The incubation layers in microcrystalline silicon films （μc-Si：H） are studied in detail. The incubation layers in μc- Si：H films are investigated by biracial Raman spectra, and the results indicate that either decreasing silane concentration （SC） or increasing plasma power can reduce the thickness of incubation layer. The analysis of the in-situ diagnosis by plasma optical emission spectrum （OES） shows that the emission intensities of the SiH＊（412 nm） and Hα （656 nm） lines are time-dependent, thus SiH＊/Hα ratio is of temporal evolution. The variation of SiH＊/Hα ratio can indicate the variation in relative concentration of precursor and atomic hydrogen in the plasma. And the atomic hydrogen plays a crucial role in the formation of μc-Si：H; thus, with the plasma excited, the temporal-evolution SiH＊/Hα ratio has a great influence on the formation of an incubation layer in the initial growth stage. The fact that decreasing the SC or increasing the plasma power can decrease the SIH＊/Hα ratio is used to explain why the thickness of incubation layer can reduce with decreasing the SC or increasing the plasma power.

Effect of substrate temperature and pressure on properties of microcrystalline silicon films

In this paper intrinsic microcrystalline silicon films have been prepared by very high frequency plasma enhanced chemical vapour deposition （VHF-PECVD） with different substrate temperature and pressure. The film properties were investigated by using Raman spectra, x-ray diffraction, scanning electron microscope （SEM）, and optical transmittance measurements, as well as dark conductivity. Raman results indicate that increase of substrate temperature improves the microcrystallinity of the film. The crystallinity is improved when the pressure increases from 50Pa to 80Pa and the structure transits from microcrystalline to amorphous silicon for pressure higher than 80Pa. SEM reveals the effect of substrate temperature and pressure on surface morphology.

The effect of initial discharge conditions on the properties of microcrystalline silicon thin films and solar cells

This paper studies the effects of silane back diffusion in the initial plasma ignition stage on the properties of microcrystalline silicon （μc-Si：H） films by Raman spectroscopy and spectroscopic ellipsometry, through delaying the injection of SiH4 gas to the reactor before plasma ignition. Compared with standard discharge condition, delayed SiH4 gas condition could prevent the back diffusion of Sill4 from the reactor to the deposition region effectively, which induced the formation of a thick amorphous incubation layer in the interface between bulk film and glass substrate. Applying this method, it obtains the improvement of spectral response in the middle and long wavelength region by combining this method with solar cell fabrication. Finally, results are explained by modifying zero-order analytical model, and a good agreement is found between the model and experiments concerning the optimum delayed injection time.

The study of amorphous incubation layers during the growth of microcrystalline silicon films under different deposition conditions

The structural un-uniformity of microcrystalline silicon, thin film, amorphous incubation layerc-Si：H films prepared using very high frequency plasma-enhanced chemical vapour deposition method has been investigated by Raman spectroscopy, spectroscopic ellipsometer and atomic force mi- croscopy. It was found that the formation of amorphous incubation layer was caused by the back diffusion of SiH4 and the amorphous induction of glass surface during the initial ignition process, and growth of the incubation layer can be suppressed and uniform μc-Si：H phase is generated by the application of delayed initial SiH4 density and silane profiling methods.

Effects of ratio of hydrogen flow on microstructure of hydrogenated microcrystalline silicon films deposited by magnetron sputtering at 100 ℃

Hydrogenated microcrystalline silicon(μc-Si:H)films were prepared on glass and silicon substrates by radio frequency magnetron sputtering at 100°C using a mixture of argon(Ar)and hydrogen(H2)gasses as precursor gas.The effects of the ratio of hydrogen flow(H2/(Ar+H2)%)on the microstructure were evaluated.Results show that the microstructure,bonding structure,and surface morphology of theμc-Si:H films can be tailored based on the ratio of hydrogen flow.An amorphous to crystalline phase transition occurred when the ratio of hydrogen flow increased up to 50%.The crystallinity increased and tended to stabilize with the increase in ratio of hydrogen flow from 40%to 70%.The surface roughness of thin films increased,and total hydrogen content decreased as the ratio of hydrogen flow increased.Allμc-Si:H films have a preferred(111)orientation,independent of the ratio of hydrogen flow.And theμc-Si:H films had a dense structure,which shows their excellent resistance to post-oxidation.

Research on the boron contamination at the p/i interface of microcrystalline silicon solar cells deposited in a single PECVD chamber

This paper studies boron contamination at the interface between the p and i layers of μc-Si：H solar cells deposited in a single-chamber PECVD system. The boron depth profile in the i layer was measured by Secondary Ion Mass Spectroscopy. It is found that the mixed-phase μc-Si：H materials with 40% crystalline volume fraction is easy to be affected by the residual boron in the reactor. The experimental results showed that a 500-nm thick μc-Si：H covering layer or a 30-seconds of hydrogen plasma treatment can effectively reduce the boron contamination at the p/i interface. However, from viewpoint of cost reduction, the hydrogen plasma treatment is desirable for solar cell manufacture because the substrate is not moved during the hydrogen plasma treatment.

Reduction of the phosphorus contamination for plasma deposition of p-i-n microcrystalline silicon solar cells in a single chamber

This paper investigates several pretreatment techniques used to reduce the phosphorus contamination between solar cells. They include hydrogen plasma pretreatment, deposition of a p-type doped layer, i-a-Si：H or μc-Si：H covering layer between solar cells. Their effectiveness for the pretreatment is evaluated by means of phosphorus concentration in films, the dark conductivity of p-layer properties and cell performance.

Influence of the total gas flow rate on high rate growth microcrystalline silicon films and solar cells

This paper reports that high-rate-deposition of microcrystalline silicon solar cells was performed by very-highfrequency plasma-enhanced chemical vapor deposition. These solar cells, whose intrinsic μc-Si：H layers were prepared by using a different total gas flow rate （Ftotal）, behave much differently in performance, although their intrinsic layers have similar crystalline volume fraction, opto-electronic properties and a deposition rate of - 1.0 nm/s. The influence of Ftotal on the micro-structural properties was analyzed by Raman and Fourier transformed infrared measurements. The results showed that the vertical uniformity and the compact degree of μc-Si：H thin films were improved with increasing Ftotal. The variation of the microstructure was regarded as the main reason for the difference of the J V parameters. Combined with optical emission spectroscopy, we found that the gas temperature plays an important role in determining the microstructure of thin films. With Ftotal of 300 sccm, a conversion efficiency of 8.11% has been obtained for the intrinsic layer deposited at 8.5 A/s （1 A=0.1 nm）.

High Growth Rate of Microcrystalline Silicon Films Prepared by ICP-CVD with Internal Low Inductance Antennas

The plasma parameters in ICP-CVD system with internal low inductance antennas（LIA） were diagnosed by Langmuir probe.The ions density（Ni） reached 1011-1012 cm-3,and the electron temperature（Te） was below ca.2 eV,which was slightly decreased with applied power.A p-type hydrogenated microcrystalline silicon（μc-Si：H） film was prepared on glass substrate.After optimization of the processing parameters in flow ratio of SiH4：B2H6：H2,a high quality μc-Si：H film with deposition rate above 1.0 nm/s was achieved in this work.

Optical emission spectroscopy study on deposition process of microcrystalline silicon

This paper reports that the optical emission spectroscopy （OES） is used to monitor the plasma during the deposition process of hydrogenated microcrystalline silicon films in a very high frequency plasma enhanced chemical vapour deposition system. The OES intensities （Sill^＊, H^＊ and H^＊β） are investigated by varying the deposition parameters. The result shows that the discharge power, silane concentrations and substrate temperature affect the OES intensities. When the discharge power at silane concentration of 4% increases, the OES intensities increase first and then are constant, the intensities increase with the discharge power monotonously at silane concentration of 6%. The SiH^＊ intensity increases with silane concentration, while the intensities of H^＊α and H^＊β increase first and then decrease. When the substrate temperature increases, the SiH^＊ intensity decreases and the intensities of H^＊α and H^＊β are constant. The correlation between the intensity ratio of IH^＊α/ISiH^＊ and the crystalline volume fraction （Xc） of films is confirmed.

Electronic structure and defect states of transition films from amorphous to microcrystalline silicon studied by surface photovoltage spectroscopy

In this paper, surface photovoltage spectroscopy （SPS） is used to determine the electronic structure of the hydrogenated transition Si films. All samples are prepared by using helicon wave plasma-enhanced chemical vapour deposition technique, the films exhibit a transition from the amorphous phase to the microcrystalline phase with increasing temperature. The film deposited at lower substrate temperature has the amorphous-like electronic structure with two types of dominant defect states corresponding to the occupied Si dangling bond states （D^0/D^-） and the empty Si dangling states （D＋）. At higher substrate temperature, the crystallinity of the deposited films increases, while their band gap energy decreases. Meanwhile, two types of additional defect states is incorporate into the films as compared with the amorphous counterpart, which is attributed to the interface defect states between the microcrystalline Si grains and the amorphous matrix. The relative SPS intensity of these two kinds of defect states in samples deposited above 300℃ increases first and decreases afterwards, which may be interpreted as a result of the competition between hydrogen release and crystalline grain size increment with increasing substrate temperature.

STRUCTURAL PROPERTIES INVESTIGATION ON MICROCRYSTALLINE SILICON FILMS DEPOSITED WITH VHF-PECVD TECHNIQUE

Raman scattering spectroscopy and scanning electron microscopy (SEM) techniques were used to determine the structural properties of two typical series of microc rystalline silicon (μc-Si:H) films deposited at different VHF plasma power and different working gas pressure by very high frequency plasma enhanced chemical v apor deposition (VHF-PECVD) technique. Raman spectra measurements show that both crystalline volume fraction Xc and average grain size d of μc-Si : H films ar e strongly affected by the two deposition conditions and are more sensitive to w orking gas pressure than VHF plasma power. SEM characterizations have further co nfirmed that VHF plasma power and working gas pressure could clearly enhance the surface roughness of μc-Si : H films ascribing to polymerization reactions, w hich is also more sensitive to working gas pressure than VHF plasma power.

Influence of Boron doping on microcrystalline silicon growth

Microcrystalline silicon （μc-Si：H） thin films with and without boron doping are deposited using the radio-frequency plasma-enhanced chemical vapour deposition method. The surface roughness evolutions of the silicon thin films are investigated using ex situ spectroscopic ellipsometry and an atomic force microscope. It is shown that the growth exponentβ and the roughness exponent cχ are about 0.369 and 0.95 for the undoped thin film, respectively. Whereas, for the boron-doped μc-Si：H thin film, t3 increases to 0.534 and cχ decreases to 0.46 due to the shadowing effect.

Micromorph tandem solar cells: optimization of the microcrystalline silicon bottom cell in a single chamber system

We report on the development of single chamber deposition of microcrystalline and micromorph tandem solar cells directly onto low-cost glass substrates. The cells have pin single-junction or pin/pin double-junction structures on glass substrates coated with a transparent conductive oxide layer such as SnO2 or ZnO. By controlling boron and phosphorus contaminations, a single-junction microcrystalline silicon cell with a conversion efficiency of 7.47% is achieved with an i-layer thickness of 1.2 μm. In tandem devices, by thickness optimization of the microcrystalline silicon bottom solar cell, we obtained an initial conversion efficiency of 9.91% with an aluminum （Al） back reflector without a dielectric layer. In order to enhance the performance of the tandem solar cells, an improved light trapping structure with a ZnO/Al back reflector is used. As a result, a tandem solar cell with 11.04% of initial conversion efficiency has been obtained.

Analysis of heating effect on the process of high deposition rate microcrystalline silicon

A possible heating effect on the process of high deposition rate microcrystalline silicon has been studied. It includes the discharge time-accumulating heating effect, discharge power, inter-electrode distance, and total gas flow rate induced heating effect. It is found that the heating effects mentioned above are in some ways quite similar to and in other ways very different from each other. However, all of them will directly or indirectly cause the increase of the substrate surface temperature during the process of depositing microcrystalline silicon thin films, which will affect the properties of the materials with increasing time. This phenomenon is very serious for the high deposition rate of microcrystalline silicon thin films because of the high input power and the relatively small inter-electrode distance needed. Through analysis of the heating effects occurring in the process of depositing microcrystalline silicon, it is proposed that the discharge power and the heating temperature should be as low as possible, and the total gas flow rate and the inter-electrode distance should be suitable so that device-grade high quality deposition rate microcrystalline silicon thin films can be fabricated.

SUBSTRATE EFFECT ON HYDROGENATED MICROCRYSTALLINE SILICON FILMS DEPOSITED WITH VHF-PECVD TECHNIQUE

Raman spectra and scanning electron microscope （SEM） techniques were used to determine the structural properties of microcrb＇stalline silicon （μc-Si：H） films deposited on different substrates with the very high frequency plasma-enhanced chemical vapor deposition （VHF-PECVD） technique. Using the Raman spectra, the values of crystalline volume fraction Xc and average grain size d are 86%, 12.3nm; 65%, 5.45nm; and 38%, 4.05nm, for single crystalline silicon wafer, coming 7059 glass, and general optical glass substrates, respectively. The SEM images further demonstrate the substrate effect on the film surface roughness. For the single crystalline silicon wafer and Coming 7059 glass, the surfaces of the μc-Si：H films are fairly smooth because of the homogenous growth or h＇ttle lattice mismatch. But for general optical glass, the surface of the μ-Si： H film is very rough, thus the growing surface roughness affects the crystallization process and determines the average grain size of the deposited material. Moreover, with the measurements of thickness, photo and dark conductivity, photosensitivity and activation energy, the substrate effect on the deposition rate, optical and electrical properties of the μc-Si：H thin films have also been investigated. On the basis of the above results, it can be concluded that the substrates affect the initial growing layers acting as a seed for the formation of a crystalline-like material and then the deposition rates, optical and electrical properties are also strongly influenced, hence, deposition parameter optimization is the key method that can be used to obtain a good initial growing layer, to realize the deposition of μc-Si：H films with device-grade quality on cheap substrates such as general glass.

Microcrystalline Silicon Materials and Solar Cells Prepared by VHF-PECVD

A series of samples deposited by VHF-PECVD at different pressures were studied.The measurement results of photosensitivity (photo conductivity/dark conductivity) and activation energy indicated near the same rule with the change of the pressure.The results measured by Raman scattering spectra,X-ray diffraction and FTIR all proved the evident crystallization of the materials.Treating the p/i interface by hydrogen has a great improving effect on the performance of the microcrystalline silicon (μc-Si) p-i-n solar cells if the treatment time was appropriate.An efficiency of 4.24% for μc-Si p-i-n solar cells deposited by VHF-PECVD was firstly obtained.

The Microstructure Evolution of Intrinsic Microcrystalline Silicon Films and Its Influence on the Photovoltaic Performance of Thin-Film Silicon Solar Cells

Hydrogenated microcrystalline silicon （μc-Si：H） intrinsic films and solar cells are prepared by plasma enhanced chemical vapor deposition （PECVD） with various hydrogen dilution ratios. The influence of hydrogen dilution ratios on electrical characteristics is investigated to study the phase transition from amorphous to microcrystalline silicon. During the deposition process,the optical emission spectroscopy （OES） from plasma is recorded and compared with the Raman spectra of the films,by which the microstructure evolution of different 1-12 dilution ratios and its influence on the performance of μc-Si： H n-i-p solar cells is investigated.