B. Equer

A high-resolution scanning Kelvin probe microscope for contact potential measurements on the 100 nm scale

The article describes the principles and current performance of a scanning Kelvin probe microscope for contact potential measurements with a lateral resolution on the 100 nm scale and a sensitivity in the millivolt range. Preliminary results are presented regarding the variation of the surface potential across charged grain boundaries in polycrystalline silicon.

Light induced defects in a-Si:H : Saturation or equilibrium?

A model for the light induced creation of excess defects in hydrogenated amorphous silicon is proposed. The model fits well a set of experiments performed on standard a-Si:H samples under high intensity, white light illumination. The main conclusion from this the study is that a light induced reverse reaction contributes to recover weak bonds and explains the observed variable density of defects for different levels of illumination, as an equilibrium.

High electric‐field amorphous silicon p‐i‐n diodes: Effect of the p‐layer thickness

We present amorphous silicon p‐i‐n diodes able to sustain a reverse bias corresponding to 106 V/cm with a reasonably low leakage current. The influence of the p‐layer thickness on the reverse bias current and the breakdown voltage is investigated. The high‐voltage reverse current at room temperature is attributed to two different mechanisms: field enhanced thermal generation in the p‐i interface region and, at the highest bias, electron injection through the p layer. Variable range hopping is also contributing to the low‐temperature reverse current. Charge collection measurements after pulsed photogeneration were also performed up to the maximum voltage. No evidence for signal amplification is found, which sets a lower limit of 106 V/cm for impact ionization and avalanche phenomena.

Effect of primary ionization in amorphous silicon detectors

Abstract The characteristics of the signal produced in amorphous silicon detectors have been studied with alphas and protons of energy in the range 400 keV to 5.8 MeV. The detectors are p-i-n structures fabricated by plasma decomposition of silane. They are operated under inverse polarization from 0 to 100 V. The collected charge is studied as a function of the energy deposited in the detectors. The mechanisms involved in the charge collection are analyzed and the consequences over future detector development are discussed.

Response of amorphous silicon p-i-n detectors to ionizing particles

Abstract Prototypes of thin film p-i-n amorphous silicon detectors, having intrinsic layer thicknesses ranging from 3 to 18 μm, were designed and fabricated, and their response to protons of 1.3 to 11.6 MeV, and to α particles of 1.6 to 14.6 MeV was studied. The collection efficiency of the charges generated by a particle is found to decrease sharply with increasing dE dx , for dE dx values greater than 10 keV μm . However, there exists a domain of oblique angles of incidence, relatively high applied biases and low dE dx values over which the signal amplitude varies linearly with energy loss. Pulse shape studies show the signal to consist of a steep leading edge, attributed to prompt electron collection, and a slow component showing evidence of multiple-trapping transport of holes, which is a possible cause for incomplete collection of the generated charge at the maximum attainable field of 60 V μm . Finally, all detectors give signal amplitudes compatible with a mean electron-hole pair creation energy of 3.4 to 4.4 eV in a-Si: H.

High reverse voltage amorphous silicon p‐i‐n diodes

New types of a‐Si:H p‐i‐n diodes, capable of sustaining high reverse electric fields up to 6×105 V/cm with very low leakage currents, have been designed specifically for charged particle detection. Because of the low hole mobility and lifetime in this type of material, very high applied fields are required to collect a large fraction of the charge carriers generated by an ionizing particle. Since the leakage current is shown to be essentially due to electron tunneling through the p‐doped layer, the new design includes a thicker p layer (300 nm) at the expense of an increased surface leakage current. An appropriate etching of the top doped layer around the electrode is then required to eliminate such currents. A model, taking into account the doped layer conductivities, is presented to roughly compare the reverse current‐voltage characteristics of the samples before and after etching.

Characterization of new a-Si:H detectors fabricated from amorphous silicon deposited at high rate by helium enhanced PECVD

This paper is concerned with the characterization of new detectors fabricated from a-Si:H films deposited at high rates through the dilution of SiH/sub 4/ in Helium by PECVD (plasma enhanced chemical vapor deposition) technique. Rates up to ten times (5.5 /spl mu/m/h) that of the standard technique are obtained. We have investigated the electrical characteristics -depletion voltage, residual space charge density- of the helium diluted material and compared them to that of the standard material. Finally, the response of detectors, fabricated from both materials, to 5.5 MeV alpha particles are compared. >

Experimental study and modeling of reverse-bias dark currents in PIN structures using amorphous and polymorphous silicon

Polymorphous silicon (pm-Si:H) is a nanostructured silicon thin film, with a lower defect density of states and better electronic properties than standard amorphous silicon. We have studied the reverse-bias dark current in PIN structures using this material as the intrinsic layer and compared the results to amorphous silicon PIN devices. All the structures were grown using a standard plasma enhanced chemical vapor deposition process. For thick pm-Si:H devices, we have achieved reverse-bias dark current densities about ten times lower than those obtained using amorphous silicon as the intrinsic layer. This is consistent with the lower defect density of states in polymorphous silicon, which is about 7×1014 cm−3 against 5×1015 cm−3 for amorphous silicon. For a 2.5-μm-thick pm-Si:H diode, the current density obtained is as low as 3 pA cm−2 at −3 V. However, for thinner structures (∼0.5 μm), polymorphous and amorphous silicon show nearly the same reverse-bias leakage current. The experimental dark as well as i...

Observation of single minimum ionising particles with amorphous silicon diodes

The detection of minimum ionising particles using a junction made out of hydrogenated amorphous silicon has been considered. The response of a 21 μm thick layer of a-Si:H to electrons emitted by a 106Ru source with a β spectrum with an end point at 3.5 MeV has been studied. Electrons have been observed with the amorphous silicon using 1 mm2 pads in coincidence with a crystalline silicon detector. The observed signal shows the characteristic Landau spectrum with the most probable value at 770 e− leading to a collected charge of 37 e− μm−1 on average over the full thickness of the detector. The signal-to-noise ratio was 2.5 for this measurement. New samples are being developed with thicknesses larger than 50 μm. This should lead, together with an improved electronics, to potentially attractive devices usable for minimum ionising particle detection and calorimetric measurements in high energy physics where large areas are required.

The photoyield of CsI, amorphous silicon and organometallic reflective photocathode materials

Abstract We investigated the photoyield from CsI, p and n doped and undoped amorphous silicon and some organometallic (decamethyl-ferrocene,1,1′,dimethylferrocene and ruthenocene) reflective photocathodes, in the spectral range 140–220 nm. The measurements were made in a current collection mode, in vacuum and in methane. Powder and crystal forms of CsI were used for evaporation of bulk and porous photocathodes. The radiation resistance of CsI was measured at doses reaching 4500 Gy. The effect on the quantum efficiency of various n and p doping levels of amorphous silicon was measured.