Bernd Stannowski

Amorphous-silicon thin-film transistors deposited by VHF-PECVD and hot-wire CVD

We investigate the impact of new growth techniques on the mobility and stability of amorphous silicon (a-Si:H) thin film transistors (TFTs). It was suggested that the key parameter controlling the field-effect mobility and stability is the intrinsic mechanical stress in the a-Si:H layer. We study a series of bottom-gate TFTs incorporating a-Si:H deposited by VHF PECVD and hot-wire CVD. All TFTs exhibit good characteristics with mobilities of 0.6–0: 7c m 2 =V s. The mean activation energy EA and the slope of the barrier-height distribution kBT0 for defect creation in the a-Si:H are deter

Growth process and properties of silicon nitride deposited by hot-wire chemical vapor deposition

Hot-wire chemical vapor deposition (HWCVD) is a promising technique for the deposition of silicon nitride layers (a-SiNx:H) at low temperatures. In contrast to the commonly used plasma-enhanced chemical vapor deposition, no ion bombardment is present in HWCVD, which makes it particularly attractive for the deposition of passivation layers on structures that are sensitive to the impact of energetic ions. We deposit hot-wire a-SiNx:H from a mixture of silane and ammonia at substrate temperatures in the range of 300–500u200a°C. Layers deposited with an ammonia/silane gas-flow ratio of R=30 are close to stoichiometry (N/Si=1.33) with a hydrogen content around 10 at.u200a%. Such films have been implemented in hot-wire a-Si:H thin-film transistors. Deposition with R>30 did not result in an increase of the N content, but led to more porous films. Infrared spectroscopy revealed that moisture penetrates these layers and that oxygen is incorporated in the network under air exposure. Cross-sectional transmission electron mi...

New challenges in thin film transistor (TFT) research

Abstract This paper addresses the current trends in research and development for: (1) Thin film transistors (TFTs) on plastic substrates, (2) low-temperature poly-silicon (LTPS) for the pixel TFTs and for row and column drivers on glass, (3) addressing of organic light emitting diodes by silicon TFTs. For these advanced applications of TFTs the relevant issues are: (i) higher electron mobility, (ii) stability, and (iii) defect free, uniform deposition of thin silicon films and gate dielectrics at a high deposition rate (reduced cost). At Utrecht University, we are investigating hot wire (catalytic) chemical vapor deposition (CVD) as a deposition technique for novel TFTs that have a high potential to meet the above mentioned requirements. Bottom gate, inverted staggered TFTs with hot wire CVD (HWCVD) silicon films have been made with an electron mobility of 1.5 cm 2 / V s , and with field effect characteristics that are completely stable under operating conditions. Top gate, coplanar TFTs with polycrystalline silicon (poly-Si) films have been made, which showed a mobility of 4.7 cm 2 / V s . This has been obtained without any post treatment, and the hot wire technology can thus avoid expensive, time-consuming steps such as laser recrystallization as currently used in the production of the latest poly-Si lap top displays. HWCVD is also suitable for the deposition of SiNx:H gate dielectrics. TFTs with a hot wire silicon nitride gate dielectric have been deposited.

Thin-film transistors deposited by hot-wire chemical vapor deposition

Abstract In the past few years hot-wire chemical vapor deposition (HWCVD) has become a popular technique for the deposition of silicon-based thin-film transistors (TFTs). Several groups have been using hot-wire deposited amorphous and microcrystalline silicon as the active layers in TFTs. In such devices either thermal SiO 2 or plasma-deposited silicon nitride was the gate insulator. Recently ‘All-Hot-Wire TFTs’ have been realized, with also the silicon nitride deposited by HWCVD. This paper reviews the characteristics of hot-wire TFTs with amorphous and microcrystalline silicon using plasma- or hot-wire deposited silicon nitride as the gate insulator. It has been shown that hot-wire TFTs have a higher stability upon gate-bias stress as compared to their plasma-deposited counterparts. We present an overview of the stability of hot-wire TFTs deposited at a range of substrate temperatures. The higher stability of hot-wire TFTs that have been deposited at temperatures of 400–500 °C is ascribed to an enhanced structural order, i.e. a higher degree of medium-range order of the silicon network.

Hot-wire silicon nitride for thin-film transistors

Abstract We present silicon nitride layers deposited by the hot-wire chemical vapor deposition technique at low substrate temperatures of 300–475°C. We show that materials with a range of compositions from silicon rich to nitrogen rich can be made. Post-deposition oxidation and moisture penetration of layers deposited in a defined parameter regime is studied. Stoichiometric, dense layers with a hydrogen content of approximately 9 at.% were deposited and incorporated in TFTs, with both the silicon and silicon nitride deposited by HWCVD. Good device properties with mobilities of 0.3–0.6 cm2/V s were reached.

Application of hot-wire chemical vapor-deposited Si:H films in thin film transistors and solar cells

Abstract This paper shows a wide range of applications of the hot-wire chemical vapor deposition (HWCVD) method to either deposit photosensitive hydrogenated amorphous silicon (a-Si:H), polysilicon (poly-Si) and heterogeneous Si materials, dielectric hydrogenated amorphous silicon nitride (a-SiNx:H) of device quality, or to carry out hydrogen passivation treatments. Various choices of deposition parameters, i.e. process temperature, wire temperature and filament materials, have been investigated to decide the growth rate, substrate material and device configuration. The main feature of our poly-Si films is the compact nature, which manifests itself in anti-ferromagnetically-coupled defect dimer formation and hydrogen diffusion through compact sites. Very thin grain-boundary defects, purely intrinsic nature and very low oxygen incorporation, both during and post-deposition, have resulted in poly-Si films of device quality. Thin film transistors (TFTs) and solar cells have been fabricated using the above-mentioned HWCVD-deposited materials. Highly stable TFTs in the inverted staggered configuration, using heterogeneous Si, showed a field effect mobility of 1.5 cm2 V−1 s−1, whereas top-gate TFTs using poly-Si on glass showed a mobility of 4.7 cm2 V−1 s−1. The paper demonstrates the application of HWCVD a-SiNx as a dielectric material in bottom-gate TFTs, and complete TFTs have been successfully fabricated by the HWCVD process (except the contact layers). Solar cells in an n–i–p structure on plain stainless steel (SS) substrate showed efficiencies of 7.2% for a-Si:H and 4.4% for poly-Si i-layers. An a-Si/poly–Si tandem cell, with all-HWCVD i-layers, on a SS substrate without a back reflector and texturization yielded an efficiency of 8.1%. This was achieved at a deposition rate of 10 A s−1 for poly-Si.

Hot-wire amorphous silicon thin-film transistors on glass

Abstract We present amorphous silicon thin-film transistors on glass substrates deposited by hot-wire chemical vapor deposition with a high deposition rate of 17 A/s. The TFTs have a field-effect mobility of 0.4 cm2/Vs, and a high stability upon gate–voltage stress. Hot-wire silicon nitrides were developed. First TFTs incorporating this material as gate dielectric have a mobility of 0.6 cm2/Vs and a threshold voltage of 2.9 V.

High energy-barrier for defect creation in thin-film transistors based on hot-wire amorphous silicon

Thin-film transistors based on amorphous silicon deposited by hot-wire chemical-vapor deposition (CVD) exhibited a high mean barrier height of 1.074 eV for defect creation after gate-voltage stress. This is 77 meV higher than for glow-discharge devices. Transistors with a SiO2 or a-SiNx:H gate dielectric showed good performance with a field-effect mobility up to 0.7 cm2/Vu200as. Thus, good thin-film transistors with a superior stability can be deposited by hot-wire CVD at high deposition rates of 1.7 nm/s. We demonstrate that a reduced defect creation in the silicon and not the hot-wire-specific absence of interface ion bombardment is responsible for this higher stability.

Influence of Chemical Composition and Structure in Silicon Dielectric Materials on Passivation of Thin Crystalline Silicon on Glass

In this study, various silicon dielectric films, namely, a-SiOx:H, a-SiNx:H, and a-SiOxNy:H, grown by plasma enhanced chemical vapor deposition (PECVD) were evaluated for use as interlayers (ILs) between crystalline silicon and glass. Chemical bonding analysis using Fourier transform infrared spectroscopy showed that high values of oxidant gases (CO2 and/or N2), added to SiH4 during PECVD, reduced the Si-H and N-H bond density in the silicon dielectrics. Various three layer stacks combining the silicon dielectric materials were designed to minimize optical losses between silicon and glass in rear side contacted heterojunction pn test cells. The PECVD grown silicon dielectrics retained their functionality despite being subjected to harsh subsequent processing such as crystallization of the silicon at 1414 °C or above. High values of short circuit current density (Jsc; without additional hydrogen passivation) required a high density of Si-H bonds and for the nitrogen containing films, additionally, a high N-H bond density. Concurrently high values of both Jsc and open circuit voltage Voc were only observed when [Si-H] was equal to or exceeded [N-H]. Generally, Voc correlated with a high density of [Si-H] bonds in the silicon dielectric; otherwise, additional hydrogen passivation using an active plasma process was required. The highest Voc ∼ 560 mV, for a silicon acceptor concentration of about 10(16) cm(-3), was observed for stacks where an a-SiOxNy:H film was adjacent to the silicon. Regardless of the cell absorber thickness, field effect passivation of the buried silicon surface by the silicon dielectric was mandatory for efficient collection of carriers generated from short wavelength light (in the vicinity of the glass-Si interface). However, additional hydrogen passivation was obligatory for an increased diffusion length of the photogenerated carriers and thus Jsc in solar cells with thicker absorbers.

Low Temperature Poly-Si Layers Deposited by Hot Wire CVD Yielding a Mobility of 4.0 cm 2 V −1 s −1 in Top Gate Thin Film Transistors

ABSTRACT Direct deposition of polycrystalline silicon (poly-Si) thin films by the Hot Wire CVD method has been used for the first time for the fabrication of poly-Si top gate Thin Film Transistors (TFTs). The TFTs have a high electron mobility in saturation of up to 4 cm 2 V -1 s -1 as well as a remarkably large ON/OFF ratio of up to 6 x 10 5 . INTRODUCTION To increase the size of active-matrix liquid crystal displays (AMLCDs) and to relax the design rules, the trend in this field is the replacement of a-Si:H pixel TFTs by polysilicon TFTs. Poly-Si TFTs have a higher field-effect mobility and allow larger drive currents, so that the pixel aperture ratio can be increased and bright, low power LCDs can be obtained even when they have a large size. By supporting both n-channel and p-channel operation, poly-Si TFTs also enable CMOS circuits for display drivers. High quality poly-Si films can be obtained, e.g., by CVD at high temperature, but high temperatures must be avoided as the cost of LCDs would increase due to the required use of large-area temperature-resistant quartz substrates. Therefore, low temperature approaches have to be developed to create poly-Si thin films on glass. By excimer-laser crystallization, excellent polycrystalline films can be obtained using a-Si:H layers as the starting material [1-3]. Laser crystallization, however, is by definition an extra step and the throughput may be limited. In addition, if the starting material is deposited by Plasma Enhanced CVD (PECVD), the material has to be dehydrogenated carefully in order to avoid explosive evolution of hydrogen. While Hot Wire CVD (or catalytic CVD) has already demonstrated that a-Si:H films with a hydrogen content lower than 1 at.-% can be obtained [4,5], which would make fast single-pulse crystallization possible, the same CVD method has now shown that poly-Si can also be