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Deposition of Zn3n2 Thin Films and Application in TFT Structures Deposition of Zn3N2 thin films and application in TFT structures Maria Luísa Lobo Moutinho Soares de Melo Physics Department of IST, Lisbon, Portugal Under supervision of Dr. Reinhard Schwarz Abstract Transistors are the key component of everyday electronics, so the search for new materials and production methods becomes indispensable. The present work has two main goals: the study of the optimal deposition conditions for the production of Zinc Nitride films by Pulsed Laser Deposition (PLD) and its later use to produce a Thin Film Transistor (TFT). We have focused our study on two main deposition parameters: laser wavelength and substrate temperature. The film properties, particularly its morphology, showed a strong dependence on laser wavelength. The green line of a Nd:YAG laser (532 nm) proved to be the best compromise. The substrate temperature also influences the film properties leading to the conclusion that Zinc Nitride might only be PLD produced at temperatures beyond 350ºC. The films showed crystalline structure, the estimated band gap was on average 3.22 eV and resistivity had a range of 10-2-100 Ω.cm. A high percentage of Oxygen was found on the film surface, as predicted by the literature. Three MIS structures were analysed using I-V and C-V measurements. We have decided to use two substrates for the TFT test structures: AlN on p-Si and ATO on ITO. Transfer characteristics and output characteristics measurements were undertaken. Although some optimization is necessary, we demonstrated that Zinc Nitride is a good candidate to be used as channel layer in TFT production. The AlN-based TFTs showed the best performance with threshold voltage of 1.1 V, sub-threshold voltage swing of 1.1 V/decade, an on/off ratio of 104 and 0.6 cm2/Vs saturation mobility. 1. Introduction perform the I-V measurements needed in this work. The TFT is a field effect transistor (FET). Its Several other characterization methods were used: structure and operation principles are similar to transmission, SEM, C-V, XRD and XPS. those of the metal oxide field effect transistor This experimental work was done at the Physics (MOSFET). However, there are some important Department of Instituto Superior Técnico, Lisbon, differences to consider: TFT frequently uses an Portugal. The work was supervised by Professor insulation substrate, the production temperatures Reinhard Schwarz and Dr. Rachid Ayouchi. PLD are usually lower and the absence of p-n junctions in depositions, I-V, C-V and transmission the Drain and Source contacts, which leads to a measurements were obtained using the research slightly different mode of operation. group infrastructure available at the Physics In the last decade, a new impulse has been given Department. XRD and SEM were done with a to this research field especially in areas such as the diffractometer of the ICEMS group. XPS analysis transparent and organic TFTs. was carried out at the Physics Department of Some of the possible TFT applications include Universidade Nova de Lisboa by the “Ciência de integrated displays, photovoltaics, sensing devices, Superfícies e Tecnologia de Vácuo” group. smart labels, lightning, and integrated logic devices. Part of the experimental results of this work will Zinc Nitride (Zn3N2) is a group II-V compound be presented at a poster presentation at the EMRS semiconductor. This is still a poorly understood 2013 conference, in Strasbourg, France with the material with reported shifting properties, depending title: Reactive Pulsed Laser Deposition of Zinc on the production method. It has recently drawn the Nitride thin films. attention of a number of research groups, including ours. Several production methods have been used. 2. Literature Review It is a hot topic in materials science, with several 2.1 Pulsed Laser Deposition (PLD) articles published in the last couple of years. Pulsed Laser Deposition (PLD) is a physical Properties like the optical band gap are yet to be vapour deposition technique. There are three main established by the scientific community with new types of PLDs: conventional, inert gas and reactive. results cropping up recently. In this thesis the Reactive PLD was used. Zinc Nitride is a promising semiconductor to use An optical system focuses a high power laser and in electronic and optoelectronic applications enters the deposition chamber through a viewport. It (photovoltaic, sensors, TFTs and so on) owing to then travels through the chamber and hits the target, low cost and ecological friendliness. The high evaporating part of it. The evaporated material availability of its composing materials is also subsequently interacts with the environmental gas appealing. and deposits on the substrate. The deposition This work focuses on the Pulsed Laser process may be split into four main phases: laser- Deposition of Zinc Nitride thin films and its possible target interaction, gas phase transportation, use as the semiconductor layer of a Thin Film deposition and nucleation and film growth. Transistor (TFT). An auxiliary programme was The presence of particles on the film and the specifically built on the Labview 8.6 platform to unintentional sputtering of the film and the substrate 1 are two critical problems of this production method. method. The mostly reported peaks are the 2θ [1-4] angles: 34.4º that corresponds to the (321) Miller plane and 36.8º that corresponds to (400) plane. 2.2 Zinc Nitride: Properties and Production Both of these peaks exist in the antibixyte crystal Methods structure [26]. However, the (400) peak is also Zinc Nitride (Zn3N2) is a group II-V compound observed in the metallic Zinc diffraction pattern [27]. semiconductor with cubic antibixyte structure, the The electrical properties of this material have lattice constant of which is a = 9.78(1) Å. The crystal also been extensively studied. Resistivity values are structure is shown in Figure 1. reported in the range of 10-3 - 10-1 Ω.cm with a strong dependence on the substrate temperature [13,15]. The carrier mobility of zinc nitride films was reported to have a range of 10-1-100 cm2 V−1 s−1 [13,14,16.18,19,22]. Hall measurements reported n- type films [13,14,18,19,32]. Metallic contacts with Al, Au and Cu showed good ohmic behaviour. [8] Oxygen contamination has been reported as a Figure 1 - Zn3N2 crystal structure [11] common Zinc Nitride production issue. Zinc is a hydrophilic material [18], which means it has the Zinc Nitride has in the recent years drawn the tendency to absorb the water vapour present in the attention of several research groups. The results, atmosphere. Also, Oxygen is more reactive than however, have been very scarce so far. In particular, Nitrogen because of their different electronic reports differ strongly concerning the compound properties. These two facts make very probable the band gap, which has shown dependence on the contamination of Oxygen in Zinc Nitrides films. The production method. This fact makes the Zinc Nitride Oxygen may thus combine with Zinc atoms forming a still poorly understood material that requires Zinc Oxide (ZnO), Zinc Oxynitride(ZnON) or Zn(OH)2 further research. structures and altering several film properties [9,13,18,23]. 2.2.1 Zn3N2 Production by PLD The deposition of Zinc Nitride by PLD has not yet 2.3 Thin Film Transistor (TFT) been extensively studied, to our knowledge, it has Thin Film Transistors (TFTs) are FETs and have only been produced by S. Simi et al. [5] and our a very similar mode of operation to the MOSFETs. group either RF-assisted [6,7] or not [8]. However, MOSFETs work in inversion mode and The optical band gap was reported to be direct TFTs in accumulation mode. The staggered bottom and its value varied from 3.2 to 3.5 eV [5-8]. The X- gate configuration will be used in this work. The Ray peaks seen were observed at 34.2° and 36.8° following analysis is done for an n-type TFT. Two [6-8]. The morphology reported sub-micrometer assumptions are made: carrier mobility and crystallites distributed discontinuously over the film capacitance are constant along the channel length. surface. 2.3.1 Mode of Operation 2.2.1 Zn3N2 Production by other methods Off-state: When a negative voltage is applied to the Several other methods were used to produce this gate (VG<0V), it induces a depletion of the electrons material such as Molten Salt Electrochemical from the semiconductor-dielectric interface, which Process [9], Molecular Beam Epitaxy (MBE) [10], prevents the current flow from drain to source. Filtered Cathodic Vacuum Arc Deposition [11], Plasma-assisted Metal Organic Chemical Vapour Linear Region: When a positive voltage (VG>0V), Deposition [12] and, predominately, RF Assisted higher than the threshold voltage, is applied to the Magnetron Sputtering [13-24]. gate electrode, it produces a downward band Concerning the film morphology, there is no bending and an accumulation of electrons in the agreement in the literature whether the growth semiconductor-insulator interface. This creates a conditions influence the film morphology and grain conducting channel between the drain and the size. Either no dependence was detected [13] or it source contacts. When a positive voltage is applied was reported that the grain size increased with to the drain contact, with < − , the induced increasing temperatures [14]. electrons flow from the source to the drain through Regarding the optical properties, there is a strong the channel and the current has the opposite discrepancy between the results reported. The direction. The higher the voltage applied to the drain, material is generally said to be a direct the higher the current flow. The device is in the semiconductor. The optical band gap has a high linear region and its drain current is given by: inconsistency between the obtained values with two W 1 groups of values: 1.01-1.47 eV [9,13,14,16-18] and = µ C [( − ) − ] 3.2 eV [21].
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