R. Brenot

Stable microcrystalline silicon thin-film transistors produced by the layer-by-layer technique

Microcrystalline siliconthin films prepared by the layer-by-layer technique in a standard radio-frequency glow discharge reactor were used as the active layer of top-gate thin-film transistors(TFTs). Crystalline fractions above 90% were achieved for silicon films as thin as 40 nm and resulted in TFTs with smaller threshold voltages than amorphous siliconTFTs, but similar field effect mobilities of around 0.6 cm2/V s. The most striking property of these microcrystalline silicontransistors was their high electrical stability when submitted to bias-stress tests. We suggest that the excellent stability of these TFTs, prepared in a conventional plasma reactor, is due to the stability of the μc-Si:H films. These TFTs can be used in applications that require high stability for which a-Si:HTFTs cannot be used, such as multiplexed row and column drivers in flat-panel display applications, and active matrix addressing of polymer light-emitting diodes.

Real-time measurement of the evolution of carrier mobility in thin-film semiconductors during growth

A method which allows real-time microwave mobility measurements in thin-film semiconductors is described. Carriers mobility is determined during growth by combining two diagnostics: time-resolved microwave conductivity (TRMC) and spectroscopic ellipsometry (SE). TRMC provides the product of the number of free carriers, generated by a laser pulse, by their microwave mobility. The number of photogenerated carriers is calculated from real-time SE measurements. Therefore, the mobility of excess carriers in the growing layer can be deduced from TRMC measurements. In order to illustrate this technique, a TRMC setup has been implemented in situ together with real-time SE to analyze the growth of microcrystalline silicon (μc-Si) by radio frequency glow discharge. An increase of the average carrier mobility as a function of the film thickness is observed, which is compared with the increase of the crystalline fraction evidenced by SE.

Transport mechanisms in hydrogenated microcrystalline silicon

Transport properties of microcrystalline (μc-Si/H) and polycrystalline (p-Si) silicon films are analyzed by time resolved microwave conductivity (TRMC), diffusion-induced TRMC (DTRMC), and Hall measurements. The comparison of carrier mobilities in microcrystalline silicon determined by TRMC as well as DTRMC shows that trapping in the disordered part of these films is not the main limiting parameter for transport in microcrystalline silicon. Besides, it is demonstrated that TRMC measurements are not sensitive to barriers between the crystallites. Our measurements reveal that, contrary to the case of p-Si, the influence of barriers in μc-Si/H can be neglected. Transport in μc-Si/H is consequently mainly limited by defects inside the crystallites.

Microcrystalline silicon films deposited by hot-wire CVD for solar cells on low-temperature substrate

Abstract The structural and electronic properties of undoped microcrystalline silicon ( μ c-Si:H) thin films prepared by hot-wire chemical vapor deposition (HWCVD) at various hydrogen dilutions have been studied. UV–visible ellipsometry was used to quantify the crystalline, amorphous and void fractions, and to determine the presence, or otherwise, of an amorphous incubation layer. Diffusion-induced time-resolved microwave conductivity measurements showed that the electronic transport along the growth direction is notably improved for samples prepared by a double-dilution process, where the H 2 dilution is decreased as a function of the deposition time. These results should be useful for further HWCVD μ c-Si:H solar cells.

Measurement of transversal ambipolar diffusion coefficient in microcrystalline silicon

Abstract Transport properties of microcrystalline silicon (μc-Si) are analyzed by diffusion-induced time resolved microwave conductivity (DTRMC), a new contactless method. Computer simulations show that with this method the ambipolar diffusion coefficient of carriers in the transversal direction, even for thin layers (typically 100 nm), can be determined. Ex situ measurements are made on several samples with various μc-Si thicknesses deposited in the same conditions.The evolution of transport parameters with layer thickness is correlated with the inhomogeneity of the layer structure detected by ellipsometry. In particular, we demonstrate that the presence of an amorphous interface between the substrate and μc-Si can be the main limiting factor of transversal transport.

Deposition of microcrystalline silicon in an integrated distributed electron cyclotron resonance PECVD reactor

Abstract The deposition of μc-Si in a low pressure high density plasma reactor is studied. Films were deposited either from pure silane or from the mixture of SiH4 and H2 onto glass substrates and deposition kinetics followed with kinetic phase modulated ellipsometry Growth rates of up to 0.8 nm/s were achieved with good quality material. Crystalline fraction shows a strong dependence on process pressure and exceeds 80% for samples grown at optimal conditions. It is found that hydrogen dilution is not needed for integrated distributed electron cyclotron resonance (IDECR) discharge to produce crystallized material. The grain size measured with X-ray diffraction was found to be between 10 and 15 nm and of single (111) orientation. Both ellipsometric data and Raman analysis show a strong dependence of crystallinity or hydrogen residence time in the reactor.

Contactless electronic transport analysis of microcrystalline silicon

Abstract Electronic transport of microcrystalline silicon is analyzed by diffusion-induced time resolved microwave conductivity (DTRMC), a new contactless method based on time resolved microwave conductivity (TRMC) and related to the carrier diffusion in the analyzed sample. This method, associated with TRMC and with Hall measurement, is used to investigate the transport in microcrystalline silicon. The techniques are used to compare the mean longitudinal, the mean transversal and the local transport. Comparison of various samples illustrates the influence of film structure on the electronic transport.

In situ characterization of microcrystalline silicon by time resolved microwave conductivity

Abstract Time resolved microwave conductivity (TRMC) provides the product of the number of free carriers, generated by a laser pulse, by their mobility. A TRMC set-up is implemented in situ together with ultraviolet (UV) visible spectroscopic ellipsometry to analyze and optimize the growth of microcrystalline silicon ( μ c-Si) by conventional radio frequency discharges. The modelling of TRMC experiment is presented, including numerical simulations of microwave reflectivity, and carrier generation and recombination kinetics. Various materials are analyzed. For the material obtained from SiF 4 –H 2 mixtures, the recombination lifetime varies with the power −0.5 of the carrier density, and the best effective mobility is μ eff =25(±5) cm 2 V −1 s −1 . The set-up allows quantitative comparisons with other materials. For μ c-Si films deposited by the integrated distributed electron cyclotron resonance (IDECR) technique, we observe a bimolecular recombination at high laser fluence, followed by a monomolecular one. The effective mobility is μ eff =6(±2) cm 2 V −1 s −1 .

Time resolved microwave conductivity measurements for the characterization of transport properties in thin film micro-crystalline silicon

Abstract The time resolved microwave conductivity (TRMC) technique measures the transient change in microwave reflectivity that carriers generated by a laser pulse induce in a sample. The reflectivity variation with time reflects, combined with the mobility, the carrier generation or recombination, either in the bulk or at the interfaces. From the measurements, one may deduce the carrier lifetime, and the carrier mobility, either in the bulk or near the interface. Computer simulations, for thin films deposited on glass, has been performed so as to modelize the TRMC transients. The transport properties of various micro-crystalline silicon thin films, deposited either with laser annealing technique, or with layer by layer technique, are obtained from TRMC. The results are compared with other characterizations. The TRMC mobility is smaller than field effect mobility in micro-crystalline silicon. The TRMC diagnostic can be used for material optimization.

Structure and transport properties of integrated distributed electron cyclotron resonance grown micro-crystalline silicon

A new reactor utilizing electron cyclotron resonance at 2.45 GHz is used for the deposition of microcrystalline silicon. Due to its planar geometry, such deposition process can be scaled for deposition onto large surfaces. Ultraviolet–visible ellipsometry, activation energy measurements and time resolved microwave conductivity (TRMC) measurements were made on the samples. The crystalline volume fraction obtained varied between 50% and 80%, as found by ellipsometry, and the growth rate is around 0.3 nm/s. The effective mobility is found to be between 1 and 6 cm2 V−1 s−1. The effect of the preparation conditions, such as substrate temperature, pressure, H2 dilution and microwave power was investigated. This technology seems to be promising approach for the deposition of μc-Si based thin film transistors.