J.P. Kleider

Midgap density of states in hydrogenated polymorphous silicon

When silicon thin films are deposited by plasma enhanced chemical vapor deposition in a plasma regime close to that of the formation of powder, a new type of material, named polymorphous silicon (pm-Si:H) is obtained. pmSi:H exhibits enhanced transport properties as compared to state-of-the-art hydrogenated amorphous silicon (a-Si:H). The study of space-charge-limited current in n(+)-i-n(+) structures along with the use of the modulated photocurrent technique, of the constant photocurrent method and of steady-state photoconductivity and dark conductivity measurements allows us to shed some light on the origin of these improved properties. It is shown that the midgap density of states in the samples studied here is at least ten times lower than in a-Si:H, and the electron capture cross section of deep gap states is also expected to be lower by a factor of 3-4 to account for photoconductivity results. An interesting field of theoretical research is now open in order to link these low densities of states and capture cross sections to the peculiar structure of this new material

Properties of a new a-Si:H-like material : hydrogenated polymorphous silicon

Abstract A new a-Si:H-like material has been obtained in a radio frequency-powered plasma-enhanced chemical vapor deposition system (RF-PECVD). This material prepared with dilution of silane into He or H2, under high total pressure (≈132 Pa) and high RF power exhibits enhanced electronic transport properties. The room temperature electronic mobility-lifetime product is increased by a factor up to 200 compared to hydrogenated amorphous silicon (a-Si:H) prepared under standard deposition conditions (lower pressure, lower RF power). The density of states measured by modulated photocurrent and the deep defect density measured by the constant photocurrent method are both less than that of standard a-Si:H. These transport properties are linked to the structure of this new material deposited under conditions close to those for powder formation. This structure seems to result in a decrease of the deep defect density and capture cross sections.

Very low densities of localized states at the Fermi level in hydrogenated polymorphous silicon from capacitance and space-charge-limited current measurements

The density of states at the Fermi level N(E-F) has been measured on hydrogenated polymorphous (pm-Si:H) silicon samples using both capacitance measurements on Schottky barriers and space-charge-limited current measurements on n(+)/i/n(+) structures. From both techniques, N(E-F) values of 7-8 x 10(14) cm(-3) eV(-1) have been obtained, which is significantly lower than reported in the literature for hydrogenated amorphous silicon (a-Si:H). Such values demonstrate that pm-Si:H is a very low defect density material which should be able to replace a-Si:H in the field of applications like photovoltaics

Electronic and topographic properties of amorphous and microcrystalline silicon thin films

Electronic properties of microcrystalline silicon (μc-Si) thin films prepared by different techniques are presented and compared to that of device-grade, undoped, hydrogenated amorphous silicon (a-Si:H). It is found that whatever the preparation technique, the conductivity of μc-Si is significantly larger and the mobility-lifetime products and ambipolar diffusion lengths of optimised layers can be higher than in a-Si:H. In addition, no light-induced degradation of electronic properties is observed. Local topographic and electrical probing results on μc-Si films are also shown. The surface roughness of μc-Si samples depends on the preparation technique, but a common aging phenomenon in the local electrical probing is found and described.

Some electronic and metastability properties of a new nanostructured material: hydrogenated polymorphous silicon

When silicon thin films are deposited by plasma enhanced chemical vapour deposition in a plasma regime close to that of the formation of powder, a new type of material, called polymorphous silicon (pm-Si), is obtained. We present here the optoelectronic and stability properties of pm-Si films deposited from a mixture of silane diluted with hydrogen at total gas pressures in the range 800-1600 mTorr. A comparison with the properties of standard hydrogenated amorphous silicon (a-Si:H) is made. While some properties of both materials are similar, many others differ in a striking manner. Characterizations of as-deposited pm-Si films show that the best samples exhibit enhanced transport properties, such as the fact that the quantum efficiency-mobility-lifetime product eta mu tau is increased by a factor of 200-700 compared with that measured on a-Si:H under the same conditions. This correlates with a lower density of deep states. The kinetics of creation of defects, performed under 670 mW cm(-2) white light illumination and at a high temperature (100 degrees C) in order to attain a final steady state, have been studied, pm-Si samples exhibit faster kinetics of creation as well as of annealing of metastable defects than do a-Si:H samples. In their light-soaked state the best pm-Si samples exhibit eta mu tau products of the same order as those measured on device-grade a-Si:H in the annealed state. These enhanced transport properties, new properties and better stability are linked to the peculiar structure of pm-Si, namely ordered silicon nanoparticles embedded in an amorphous matrix.

A new treatment of Schottky barrier capacitance-voltage characteristics: Discussion of usual assumptions and determination of the deep gap states density in a-Si1−xGex:H alloys

Abstract A new treatment of the bias dependence of Schottky diodes is presented. It allows the determination of the DOS integral between around midgap and the Fermi-level, when dealing with film thicknesses of a few times the Debye length associated to the localized states at the Fermi-level. Applying this treatment to a set of a-SiGe:H alloys, we have found an increase of about one order of magnitude of the integrated DOS when the Ge content increases from 0 to 24 at.%.

Transport properties of hot-wire CVD μc-Si:H layers for solar cells

Abstract Transport properties of microcrystalline silicon (μc-Si:H) films prepared by hot-wire/catalytic chemical vapor deposition (HWCVD) have been investigated by steady-state photocarrier grating (SSPG) and conductivity measurements. Improved diffusion length has been obtained when the hydrogen dilution approaches the value corresponding to the amorphous–microcrystalline phase transition, i.e. 0.92±0.01. An efficiency of 5.1% has been obtained for μc-Si:H n–i–p superstrate type solar cells on a glass substrate, with a 3.5 μm thick i-layer deposited by HWCVD, using a variable hydrogen dilution (VHD) process with the final dilution value close to 0.92. Future use of a n–i–p substrate structure should improve the performance of HWCVD μc-Si:H solar cells.

Source and Drain Parasitic Resistances of Amorphous Silicon Transistors : Comparison between Top Nitride and Bottom Nitride Configurations

The source and drain parasitic resistances of amorphous silicon based thin film transistors (aSi:H TFT) are investigated using a very simple TFT model including a parameter extraction method. We show that this method provides an accurate measurement of these resistances and clearly explains their influence on the apparent field effect mobility µ a of the TFTs. We compare the parasitic resistances of TFTs for the top nitride (TN) and bottom nitride (BN) configurations and we show that the usual different performances observed on the two configurations can be mainly attributed to the differences in the parasitic resistances.

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.

Polymorphous silicon: Transport properties and solar cell applications

Transport properties of hydrogenated polymorphous silicon layers (pm-Si:H) deposited at 150 C under various pressures in the range 80--293 Pa in sandwich (Schottky and p-i-n diodes) and coplanar structures have been compared to those of hydrogenated amorphous silicon (a-Si:H) samples deposited at the same temperature in standard conditions. The layers have been studied as-deposited, annealed and after light-soaking. With increasing pressure up to 240 Pa: (1) the density of states above the Fermi level decreases as determined by means of the modulated photocurrent technique, (2) the mobility-lifetime products of electrons and holes measured by means of steady-state photoconductivity and photocarrier grating techniques both increase. The highest values for the diffusion length of minority carriers exceed 200 nm. Capacitance measurements as a function of frequency and temperature show that the density of states at the Fermi level is lower in the pm-Si:H than in the a-Si:H films. After light-soaking the diffusion length of minority carriers in a-Si:H is reduced by a factor of two whereas it is less reduced or not affected in the pm-Si:H layers. Solar cells including this new material present an excellent stability.