Nathalie Coulon

Optical amplification of Pr3+ -doped ZBLA channel waveguides for visible Laser emission.

We report on the first observation of optical signal amplification in the visible range into praseodymium doped ZBLA glass channel waveguides obtained by ion exchange. Up to 30% signal amplification was obtained at 639 nm. This result shows the potential of rare earth doped fluoride glasses in the form of channel waveguides for integrated solid state visible laser sources.

Polycrystalline silicon deposited on glass by subatmospheric-pressure chemical vapour deposition at a high rate

Abstract Amorphous silicon films have been deposited on glass by subatmosphericpressure chemical vapour deposition and then crystallized by solid-phase crystallization. The structural and electrical properties of these polycrystalline silicon films are presented in this work. Good crystalline quality at a deposition pressure of about 400 mbar has been achieved as well as values of the mobilitylifetime product above 10−5cm2V−1 and an ambipolar diffusion length near 200 nm. Depending upon the deposition temperature and pressure, growth rates of up to 20 μm h−1 can be obtained. In-situ doping with arsenic and boron has been studied using Hall effect measurements. High mobilities around 45cm2V−1s−1 have been attained for highly n-type doped samples and mobilities in the range from 20 to 30 cm2 V−1 s−1 for boron-doped samples.

Highly Flexible Microcrystalline Silicon n-Type TFT on PEN Bent to a Curvature Radius of 0.75 mm

Electrical and mechanical performances of microcrystalline silicon top-gate thin-film transistors (TFTs) on flexible substrate under high bending is presented. These devices are directly fabricated on 25-μm-thick Polyethylene naphthalate (PEN) at a maximum temperature of 180 °C. Tensile and compressive bending are performed and revealed that the TFTs can hold curvature radii of 1.5 mm without losing their performance and 0.75 mm with lower electrical performances. The limiting factor on the flexibility is shown to be the mechanical behavior of silicon nitride film used as a gate insulator. These extremely high curvatures demonstrate the possibility to use silicon technology in foldable electronics. TFTs are also shown to fully recover their characteristics when reflattened after such very low curvature radii. It opens the way to fold in half an electronic circuit to be stored and reused when reflattened.

Influence of precursors gases on LPCVD TFT's characteristics

Abstract In this work, undoped amorphous silicon films were deposited on Corning glass substrates by low pressure chemical vapour deposition using pure disilane as source gas. The deposition temperature and pressure varied from 450 to 525 °C and 20 to 70 Pa, respectively. Silicon films were annealed by solid phase crystallisation at 600 °C and were compared with silicon films deposited from silane at 550 °C and 90 Pa (standard deposition conditions). Results show that layers obtained using disilane gas exhibit higher quality than those from silane. Especially, Hall effect mobility increased from 20 to 55 cm2/V s as a consequence of a higher nucleation time for optimised deposition temperature and pressure. So, two different types of unhydrogenated thin film transistors were made. A 150 nm thick active layer made from silane at 90 Pa and 550 °C permits obtaining an average mobility of 45 cm2/Vs and a threshold voltage of 6 V (with a gate oxide thickness of 40 nm) for N-type. The use of disilane as the precursor gas produces an increase in the mobility of 25% and reduces the threshold voltage to 4 V. Identical CMOS-TFTs devices as inverters were made from silane and disilane for the N- and P-type, respectively. These structures present a good behaviour mainly due to the reduction of the threshold voltage difference.

High performance polysilicon air-gap thin film transistor on low temperature substrates

Three-dimensional polysilicon microstructures, as cantilevers or bridges, give new opportunities to increase the application field of polycrystalline silicon. Polysilicon devices with new functions can be imagined. Air-gap thin film transistors can be built from polysilicon bridges for example. They use a highly doped polysilicon bridge as gate; the gate insulator is the air or any other ambience. Then, these devices can be used as ambience sensors including chemical or biological sensors. They are very interesting as they are deposited on any substrate. Moreover, their main interest is their full compatibility with the microelectronics technology on glass performed at temperature lower than 600°C and developed first for the flat panel displays addressing. Fully integrated sensing systems are then imaginable. Here we present a low temperature process (< 600 °C) to built air-gap-polysilicon-thin-film-transistor with high mechanical and electrical performance. The transistor-active-layer is made of Low Pressure Chemical Vapor Deposition (LPCVD) undoped polysilicon while the suspended gate is made of LPCVD highly doped polysilicon. Deposition and annealing conditions of the doped polysilicon has first optimized to obtain high mechanical performance for the polysilicon micro-bridges. Hence, an optimal tensile stress to obtain longer bridges has been found. The mechanical stress has been determined from micro-Raman analysis. Then, air-gap transistors with 500 nm thick air-gap as gate insulator have been designed and fabricated. The transistor channel is 7-μm long and 7-μm wide, with a 15-μm long and 10-μm wide bridge. Their subthreshold slope, threshold voltage, and field effect mobility are 0.22 V/dec, 2.4 V and 130 cm2/V.sec respectively. These parameters are comparable to that of the best polysilicon thin film transistors with thin silicon dioxide (100 nm) as gate insulator. Then, these transistors are interesting as electronic devices firstly. Moreover they can be involved in ambience sensors. Indeed, they are checked as oxygen sensor in a preliminary experiment. Transistor characteristics are measured in air ambience, then in oxygen and finally in air. The electrical parameters have shown a considerable dependence on the ambience with an excellent recovering after removing oxygen.

High rectifying behavior in Al/Si nanocrystal-embedded SiOxNy/p-Si heterojunctions

We examine the electrical properties of MIS devices made of Al/Si nanocrystal-SiOxNy/p Si. The J V characteristics of the devices present a high rectifying behavior. Temperature measurements show that the forward current is thermally activated following the thermal diffusion model of carriers. At low reverse bias, the current is governed by thermal emission amplified by Poole-Frenkel effect of carriers from defects located in the silicon nanocrystals/SiOxNy interfaces, whereas tunnel conduction in silicon oxynitride matrix dominates at high reverse bias. Devices exhibit a rectification ratio >104 for the current measured at V=  1V. Study reveals that thermal annealing in forming gas (H2/N2) improves electrical properties of the devices due to the passivation of defects.

13.56 MHz rectifier based on a microcrystalline silicon Schottky diodes for RFID application

Nowadays there is a lot of interest for the development of smart and low cost radio frequency identification (RFID) tags to replace barcodes. To achieve this goal of the reduction of production cost the components of RFID tag should be built on flexible substrates. These substrates require the use of materials deposited at low temperature. Our microcrystalline silicon with his deposition temperature (T<165°C) and his structural and electrical performance [1 2] is appropriate to the manufacturing of components dedicated to radio-frequency identification application. In the RFID circuitry one of the main components that should operate at 13.56 MHz is the rectifying device. One way to achieve the high frequency rectification is to fabricate Schottky diode devices [3] We fabricated on corning glass substrate Schottky diodes based on an interface between gold metal and microcrystalline silicon films deposited at low temperature (T<165°C) (Fig.l). The process starts by an isopropanol, acetone and de-ionized water cleaning of the glass substrate to delete any surface impurities. After that three films are deposited, aluminium for cathode contact, Highly N-doped microcrystalline silicon and silicon nitride for to reduce electric field stress at edge terminations and to minimize leakage current of diodes. Then by doing a first photolithographic step silicon nitride is etched to define ohmic bottom contact. Lightly N-doped microcrystalline silicon and gold metal are deposited immediately to form a Shottky contact. Finally a second photolithographic step was done to define the gold contact and to access aluminium bottom contact. Microcrystalline silicon and silicon nitride layers are deposited by using a classical capacitive 13.56 MHz PECVD reactor. Current — voltage measurement were performed at room temperature. Figure 2 shows the I-V characteristics of Au/μc-Si/Al Schottky diodes. For a voltage of −2V and +2V, current is 1.22 μA and 322 μA respectively. The rectifying factor is 264 and he indicates a good rectifying behaviour. The microcrystalline silicon Schottky diodes were then characterized in a rectifier circuit. For an incoming a.c. signal this Schottky diode allowed recovery of rectified voltage at 13.56 MHz. In conclusion Schottky diode based on microcrystalline silicon deposited at T<165 °C was fabricated and will be integrate RFID application on flexible substrates.

(Invited) Similarities between [micro sign]c-Si TFT with Very Thin Active Layer and FD-SOI FETs

The subthreshold slope of microcrystalline silicon Thin Film Transistor is shown to be highly improved when the thickness of the active layer is enough decreased. In the same manner, the threshold voltage of these TFTs with very thin active layer is shown to be controlled dynamically with high efficiency by a second gate in front of the first one. These behaviours are explained considering the similarities between these TFTs and fully depleted SOI-MOSFETs as well as with dual-gate SOI-MOSFETS.

(Invited) Similarities between µc-Si TFT with Very Thin Active Layer and FD-SOI FETs

The subthreshold slope of microcrystalline silicon Thin Film Transistor is shown to be highly improved when the thickness of the active layer is enough decreased. In the same manner, the threshold voltage of these TFTs with very thin active layer is shown to be controlled dynamically with high efficiency by a second gate in front of the first one. These behaviours are explained considering the similarities between these TFTs and fully depleted SOI-MOSFETs as well as with dual-gate SOI-MOSFETS.

Similarities between µc-Si TFT with Very Thin Active Layer and FD-SOI FETs

The subthreshold slope of microcrystalline silicon Thin Film Transistor is shown to be highly improved when the thickness of the active layer is enough decreased. In the same manner, the threshold voltage of these TFTs with very thin active layer is shown to be controlled dynamically with high efficiency by a second gate in front of the first one. These behaviours are explained considering the similarities between these TFTs and fully depleted SOI-MOSFETs as well as with dual-gate SOI-MOSFETS.