A. Hadjadj

EXPERIMENTAL EVIDENCE FOR NANOPARTICLE DEPOSITION IN CONTINUOUS ARGON-SILANE PLASMAS : EFFECTS OF SILICON NANOPARTICLES ON FILM PROPERTIES

These last few years a great effort has been made to understand the mechanisms of powder formation in silane discharges. It is now well established that powders are negatively charged and thus confined in the plasma. Therefore, one does not expect powders to contribute to the deposition, unless the plasma is switched off. We present here experimental evidence for the deposition of nanoparticles, even in the case of a continuous discharge. Experimental conditions for nanoparticle formation while avoiding powder formation have been determined from light‐scattering and transmission electron microscopy measurements. Nanoparticle deposition has been studied by in situ ellipsometry in silane–argon discharges. From a comparison of the growth kinetics and the optical properties of films obtained under continuous and modulated discharges we conclude that nanoparticle deposition can take place even when the discharge is on. The implications of these discoveries on the properties of hydrogenated amorphous silicon ar...

New features of the layer‐by‐layer deposition of microcrystalline silicon films revealed by spectroscopic ellipsometry and high resolution transmission electron microscopy

Spectroscopic ellipsometry and high resolution transmission electron microscopy have been used to characterize microcrystalline silicon films. We obtain an excellent agreement between the multilayer model used in the analysis of the optical data and the microscopy measurements. Moreover, thanks to the high resolution achieved in the microscopy measurements and to the improved optical models, two new features of the layer‐by‐layer deposition of microcrystalline silicon have been detected: (i) the microcrystalline films present large crystals extending from the a‐Si:H substrate to the film surface, despite the sequential process in the layer‐by‐layer deposition; and (ii) a porous layer exists between the amorphous silicon substrate and the microcrystalline silicon film.

Ellipsometry investigation of the amorphous-to-microcrystalline transition in a-Si:H under hydrogen-plasma treatment

We have investigated by ellipsometry the structural evolution of intrinsic and doped hydrogenated amorphous silicon (a-Si:H) thin films during their exposure to a hydrogen plasma in conditions of chemical transport. The real-time kinetic ellipsometry trajectory at 4.2 eV allowed a precise determination of the amorphous-to-microcrystalline (a→μc) phase transition time. In situ spectroscopic ellipsometry spectra were collected at both sides of the transition to identify the hydrogen induced changes in the film structure and its time evolution under hydrogen-plasma exposure. The whole ellipsometry diagnostics reveal that, while intrinsic and phosphorus-doped a-Si:H present a similar trend during the plasma treatment, boron-doped a-Si:H differs by the following special features: (i) a rapid formation of the hydrogen-rich subsurface layer with a high hydrogen excess. (ii) An early amorphous-to-microcrystalline phase transition. (iii) A less porous and small-grains formed transition layer. Such a particular beh...

Analytical compensation of stray capacitance effect in Kelvin probe measurements

The Kelvin probe is a very sensitive technique for surface studies. The contact potential (CP) value measured by this method is related to the work function of the material under study. In order to use this technique in a plasma enhanced chemical vapor deposition reactor to investigate in situ the electronic properties of semiconductors and devices during their growth, we need to take into account the variations of the apparent values of the CP, with the mean probe‐to‐sample distance, due to stray capacitance. In this article, we describe a simple method to compensate analytically the stray capacitance contribution in order to obtain the true values of the CP. We have used a simple model based on the assumption of a constant stray capacitance contribution, which allows the formulation of an analytical expression of the difference between the apparent and true values of the CP. We show that, in the configuration in which the vibrating plate of the Kelvin capacitor is grounded and in the case of low modulat...

OPTIMUM DOPING LEVEL IN A-SI:H AND A-SIC:H MATERIALS

The changes in the optical and electrical properties of thick a-Si:H and a-SiC:H films doped with diborane are investigated. In situ spectroscopic ellipsometry measurements reveal that, at a ratio of diborane to silane Cg=[B2H6]/[SiH4]<10−3, the optical properties of both materials are not strongly modified by boron doping. However, in the case of a-Si:H films, an improvement of the morphological and optical properties is observed at Cg=0.45×10−3. The existence of an optimum doping level at Cg<10−3 in the case of an a-Si:H p layer is confirmed by the dependence of the open-circuit voltage of a-Si:H based solar cells on the doping level of the p layer.

Role of the surface roughness in laser induced crystallization of nanostructured silicon films

The crystallization of hydrogenated nanostructured silicon (ns-Si:H) films deposited from Ar-silane mixture in a low-pressure pulsed radio-frequency glow discharge has been studied in relation with their structural and morphological properties. Different techniques of characterization converge to the fact that both the porosity and the surface roughness of the film increase with the plasma duration (Ton) used for the deposition. The correlation between the film structure and the crystallization threshold has been investigated. The modifications of the bulk structure of the film with Ton partly explain the decrease of the crystallization threshold (Ecryst). The role of the surface roughness in the lowering of the crystallization threshold is emphasized. Its contribution is interpreted by the enhancement of the electromagnetic field at the ns-Si:H film surface.The crystallization of hydrogenated nanostructured silicon (ns-Si:H) films deposited from Ar-silane mixture in a low-pressure pulsed radio-frequency glow discharge has been studied in relation with their structural and morphological properties. Different techniques of characterization converge to the fact that both the porosity and the surface roughness of the film increase with the plasma duration (Ton) used for the deposition. The correlation between the film structure and the crystallization threshold has been investigated. The modifications of the bulk structure of the film with Ton partly explain the decrease of the crystallization threshold (Ecryst). The role of the surface roughness in the lowering of the crystallization threshold is emphasized. Its contribution is interpreted by the enhancement of the electromagnetic field at the ns-Si:H film surface.

Effect of doping on the amorphous to microcrystalline transition in a hydrogenated amorphous silicon under hydrogen plasma treatment

Just after their deposition, we have exposed intrinsic and doped hydrogenated amorphous silicon (a-Si:H) films to a hydrogen plasma. We have investigated, by in situ spectroscopic ellipsometry measurements, the structural evolution of the film during the amorphous to microcrystalline phase transition. To take into account the complex morphological development of microcrystalline silicon (μc-Si:H), the grown film is modeled by a graded layer with a linear index variation along the growth direction. By comparing the time evolution of the structural parameters of the deposited μc-Si:H layer, we have observed a particular behavior in the case of boron-doped a-Si:H. Despite a faster and deeper hydrogen diffusion from the beginning of hydrogen plasma exposure, and a later nucleation, a compact μc-Si:H layer is obtained. In the case of intrinsic and n-type a-Si:H the nucleation is faster and leads to a thin and very porous μc-Si:H layer. The particular behavior of the p-type material is attributed to the effects...

Crystallization of nanostructured silicon films deposited under a low-pressure argon–silane pulsed-glow discharge: Correlation with the plasma duration

In this work, we compare the thermal crystallization and the laser crystallization of hydrogenated nanostructured silicon (ns-Si:H) films in relation with their deposition conditions. The samples are grown in a low-pressure pulsed radio-frequency glow discharge of an argon–silane mixture. The laser crystallization shows a decrease of the crystallization threshold (Ecryst) and an increase of the induced crystalline fraction when the plasma duration (Ton) used for the deposition increases. No correlation with Ton is observed in the case of the thermal annealing, indicating that the modifications of the bulk structure of the film with Ton are not the main parameter in the determination of Ecryst. The role of the surface roughness in the lowering of the laser crystallization threshold is then emphasized.

Mobility-edge shift during diborane doping in hydrogenated amorphous silicon and hydrogenated amorphous silicon carbide

Abstract Kelvin probe, ellipsometry and dark-conductivity measurements were made on diborane-doped hydrogenated amorphous silicon (a-Si:H) and hydrogenated amorphous silicon carbide (a-SiC:H) films. Combining the results of the contact potential V k with the activation energies E a and the optical gap Eg measurements we reproduced the absolute Fermi level shift and the mobility edges evolution with doping in these two materials. The quasi-symmetrical shift of the band edges towards midgap is attributed to the different possible boron bonds with the amorphous network. In addition to the optical gap narrowing, this mobility edge movement makes it possible to interpret the disparity in the variation of E a and V k with doping.

Self-bias voltage diagnostics for the amorphous-to-microcrystalline transition in a-Si:H under a hydrogen-plasma treatment

The authors demonstrate the possibility of using self-bias voltage on the radio-frequency electrode of a capacitively coupled deposition system as a diagnostic tool to detect the amorphous-to-microcrystalline silicon transition during the exposure of a-Si:H thin films to a hydrogen plasma. This is achieved by combining self-bias voltage (Vdc) and kinetic-ellipsometry measurements, which provide real-time information on the film properties. On intrinsic and n-type a-Si:H films, the hydrogen-plasma exposure results in the formation of a hydrogen-modified layer, which is accompanied with a decrease in the absolute values of Vdc, until a plateau corresponding to the nucleation and the growth of the microcrystalline layer occurs. On p-type a-Si:H, the amorphous-to-microcrystalline transition is characterized by a rapid increase in the absolute values of Vdc. This particular trend is ascribed to the effects of boron on both the solid and plasma phases.