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Synthesis of zinc tin nitride thin films and characterization methods
In this chapter, we describe the main techniques used during this work. The ZnSnN2 thin films have been grown by reactive magnetron co-sputtering. The structure of the films has been studied by X-ray diffraction and their microstructure has been studied by transmission electron microscopy (TEM) and scanning electron microscopy (SEM). The oxidation states for the tin element have been studied using conversion electron Mossbauer spectroscopy and X-ray photoemission spectroscopy. The optical properties have been studied by UV-visible and by ellipsometry spectroscopies. The electrical properties have been determined using the four-point probe method and Hall effect measurements.
Zinc tin nitride thin film growth
Among the various physical vapor deposition processes, the reactive magnetron sputtering one can be used to grow oxides, nitrides, carbides, … using a metallic target and a mixture of inert (usually Ar) and reactive (oxygen, nitrogen, methane, …) gases [1]–[3]. In this work, zinc tin nitride thin films have been elaborated by reactive magnetron co-sputtering, the use of two targets is very powerful to control the film stoichiometry compared to the use of an alloyed target. In general, magnetron sputtering process can be described as the interaction between a target and ions produced in a cold plasma. Figure 2.1 shows a schematic view of the magnetron sputtering process. The sputtered atoms are transferred into the gas phase and condensate at the substrate surface to form the film. In our work, nitrogen (N2) was used as a reactive gas to grow ZnSnN2 on silicon or glass substrates.
The sputtering process is based on the use of a metallic target that acts as the source for the element constituting the growing film. A plasma is created close to the target by applying either a radiofrequency bias or a DC one. The ions (Ar+, N2+) formed into the plasma are accelerated due to this voltage and bombard the target surface. After momentum transfer to the target atoms, some of them can be sputtered depending on the sputtering yield of the considered element. Since the 70’s or the 80’s, the use of concentric magnets on the target back side allows to increase the deposition rate of the films due to the so-called magnetron effect. Magnetron sputtering allows the deposition of thin films at low total pressure, i.e. few tenths of Pa, that present higher density than films previously deposited using the diode sputtering process.
In the magnetron sputtering system the target can be considered as the cathode and the substrate as the anode. There are several important parameters, which have great effects on film properties in reactive magnetron sputtering processes:
• The reactive gas partial pressure,
• The total pressure,
• The electric parameters applied to the target,
• The substrate temperature,
• The distance between the target and the substrates.
These parameters are able to change the physical and chemical properties of the films.
Structure zone diagram (SZD)
A structure zone diagram (SZD) is a helpful tool to described the expected microstructure of the films as a function of the deposition parameters: pressure, temperature, energy… In 1974, Thornton [4] applied the structure zone model for the description of thin film morphologies obtained with sputtering process, Anders [5] improved this diagram by taken into account the energy for the films deposited by HIPIMS (fig. 2.2).
Magnetron sputtering is based on the exchange of kinetic energy between the ions and the metallic target. One of the advantage of sputtering is that it could lead to metastable phases by changing this kinetic energy or, in general, by changing the thermodynamic condition (pressure, temperature, mean free path), as well as this parameter can change the microstructure as shown in (fig.2.2). The table 2.1 describes each zone.
Elaboration setup
Zinc tin nitride thin films were elaborated by using two metallic targets of zinc and tin. The diameter of the targets was 50 mm and their thickness was 3 mm. The experimental sputtering device was a 30 L sputtering chamber equipped with a rotating substrate-holder parallel to the target. The fig. 2.3 shows a schematic view of the deposition chamber. A base vacuum of approx. 10-4 Pa was ensured by a primary and a secondary pumps. Although we have tested the use of a reactive Ar-N2 mixture to deposit zinc tin nitride films, most of the ZnSnN2 films presented in this manuscript were grown using a pure nitrogen as gas phase. The flow rates were controlled using mass flow controllers. The nitrogen flow rate in this work was varied between 30 standard cubic centimeters per minute (sccm) and 90 sccm. In addition, the total pressure was controlled using a throttle valve.
Figure 2.3 Schematic view of the reactive sputtering chamber used for the growth of zinc tin nitride thin films.
The distance between the substrates (glass or silicon) and the targets was 60 mm. Thin films were deposited at different total pressures from 0.5 to 3 Pa. A pulsed-DC supply was used to sputter the zinc target, the frequency of 100 kHz and 2 µs off time were fixed, while the tin target was sputter using a DC supply. During the deposition, no intentional heating was applied to the substrates and the substrate temperature was close to 50 °C.
X-ray diffraction (XRD)
The X-ray diffraction is one of the most important methods to characterize the structure of thin films. Information of crystalline materials correspond to the phase nature, the lattice constant, their preferred orientation or texture. This method can also be used to get information on the grain size (apparent coherence length), stress, … This method which is widely used in the material science community is of primary importance to get information about the nature of as-deposited thin films.
X-rays are electromagnetic waves with wavelength corresponding to the distances between the atoms constituting the analyzed materials. When propagating through a crystal, the X-ray radiations interact with ordered atoms (lattice) and are diffracted due to Bragg law (fig.2.4).
Here, d is the distance between diffracting planes, the indices h, k, and l are called Miller indices that represent the specific crystallographic planes. In the equation, q is the diffraction angle, l is the wavelength and n is a positive integer.
In X-ray diffraction method, it is possible to obtain different information about the material depending on the geometrical configuration used. In this work, the measurements were done using two diffractometers. The first one corresponds to Brucker D8 Advance with Cu Kal wavelength (l=0.15406 nm) in Bragg-Brentano geometry configuration (fig. 2.5). In this geometry, the sample is fixed and the X-ray source and the detector are rotated around the sample with an angle q and 2q respectively.
In the Bragg-Brentano (q/2q) configuration, due to the Bragg diffraction law 2 = , only crystallographic planes in condition of diffraction and parallel to the sample surface are probed. The detector measures the intensity of the diffracted beam as a function of angle, and we obtain a diffractogram (the evolution of the intensity (number of counts) as a function of angle). JCPDS database and EVA software provided by Brucker were used for the structure determination.
X-ray energy dispersive spectroscopy (EDS) and scanning electron microscopy (SEM)
Energy dispersive spectroscopy of X-rays (EDS) is a method for chemical measurements that is usually encountered on scanning electron microscopy (SEM). However, such method is also present on a transmission electronic microscope. EDS method allows qualitative and semi-quantitative chemical analysis. In SEM (fig. 2.6) an electron gun is the source of the electron beam that impacts the surface sample generating different types of interactions. Secondary electron emission intensity is related with the surface morphology and this information is used to produce images.
The chemical composition of as-deposited ZnSnN2 thin films has been estimated using the EDS method. Since we do not use calibrated samples and since the EDS method is not accurate to estimate the nitrogen concentration, only the atomic ratio Zn/Sn was determined using EDS. Our EDS equipment was coupled with a scanning electron microscope (SEM): Philips XL30-S. This analysis technique is based on the interaction between the atoms of the film and an accelerated electron beam scanning the surface. Different types of interactions can occur, and lead to different material responses (Fig. 2.6) :
• Emission of secondary electrons
• Emission of backscattered electrons
• X-ray emission
After acceleration of the incident electrons, their interaction with the atoms of the film leads a transfer of energy, slowing down the incident electrons and ejecting an electron from an atom of the film when the incident energy is sufficient. This mechanism leads to the emission of an electron, called secondary electron. Its energy depends largely on the angle between the incident beam and the surface. As a result, the secondary electrons, if they are collected during the sweeping the sample, will create characteristic images of the topography of the surface (observation in SE mode: secondary electrons). Following the beam bombardment, some electrons can interact almost elastically with the elements of the film. Then, they are re-emitted in a direction similar to the direction of incidence, but with different energy depending on the element involved. These « backscattered » electrons therefore largely depend on the weight of the elements, and allow to create images with contrasts between phases rich in heavy elements and those containing lighter elements (observation in BSE mode: back scattered electrons)
However, in EDS, it is necessary to have an electron beam with an energy high enough to excite inner core electrons from a specific element. The ejection of an electron leaves an empty level. Afterwards, a relaxation process occurs and electrons from upper levels fill the empty level, generating X-ray emission lines corresponding to specific transitions between electronic levels with a cross section depending on the electron incident energy. Each element leads to a unique emission spectrum related with K, L, M or N electronic levels.
In this work related to ZnSnN2 films, EDS was used to evaluate the atomic ratio (FGHIGFG) between the Zn and Sn atomic concentrations by neglecting contributions from N and Si (substrate) elements.
Conversion electron Mössbauer spectroscopy
Mössbauer spectroscopy is based on the use of nuclear radiation for chemical characterizations. This spectroscopic method brings information about the chemical environment of the atoms which have a radioactive isotope active in Mössbauer, mainly iron and tin. Mossbauer effect is the recoilless emission and the recoilless resonant absorption of g photons by the nucleus [6], [7].
Table of contents :
Chapter 1. Zinc tin nitride: a new material with large applications for photovoltaics
1.1 Introduction
1.1.1 Context, Positioning and Objectives of the Work: The Need of New Material Developments
1.1.2 The Question of Realization Complexity and Indium Supply Shortage vs. Industrial Needs
1.1.3 Photovoltaics concerns
1.1.4 PV technologies
1.2 Potentialities of New ZnSnN2 Material Physical Properties
1.2.1 ZnSnN2 historical aspects: an overview
1.2.2 Focus on the issue of the crystal structure.
1.2.3 The issue of the electronic structure, defects and band gap
1.3 Conclusion
References
Chapter 2. Synthesis of zinc tin nitride thin films and characterization methods
2.1 Introduction
2.2 Zinc tin nitride thin film growth
2.3 Structure zone diagram (SZD)
2.4 Elaboration setup
2.5 X-ray diffraction (XRD)
2.6 X-ray energy dispersive spectroscopy (EDS) and scanning electron microscopy (SEM)
2.7 Conversion electron Mossbauer spectroscopy
2.8 Transmission Electron Microscopy (TEM).
2.9 Four points probe method
2.10 Hall effect measurements
2.11 UV-visible spectroscopy
2.12 Ellipsometry
2.13 X-ray photoelectron spectroscopy (XPS)
2.14 Conclusion
References
Chapter 3. ZnSnN2 Thin Films: Synthesis and characterization
3.1 First attempt to grow zinc tin nitride thin films by PVD
3.1.1 Growth of ZnSnN2 by controlling the discharge current
3.1.2 Growth of ZnSnN2 by fixing the discharge voltage
3.1.3 Growth of ZnSnN2 by controlling the discharge voltage in a pure nitrogen plasma.
3.1.4 Effect of the substrate nature
3.2 Crystalline ZnSnN2 thin films structure, microstructure, physical and chemical properties
3.2.1 Introduction
3.2.2 Chemical composition
3.2.3 Structure of ZnSnN2 films
3.2.4 Microstructure and morphology of ZnSnN2 thin films
3.2.5 X-ray photoelectron spectroscopy analysis of ZnSnN2 thin films
3.2.6 Mossbauer spectrometry of ZnSnN2 thin films
3.2.7 Electrical and optical properties of ZnSnN2
3.3 Effect of base pressure before the film growth
3.4 Conclusion
Reference
Chapter 4 Microstructure and tuning the structure of ZnSnN2 thin films
4.1 Focus on the structure of ZnSnN2
4.1.1 Orthorhombic, monoclinic or hexagonal structures
4.1.2 Monoclinic vs orthorhombic
4.1.3 Monoclinic versus hexagonal phases
4.2 Tuning the structure of ZnSnN2
4.2.1 Tuning the structure with the stoichiometry
4.2.2 Tuning the structure by changing the pressure
4.2.2.1 Tuning the structure with the working pressure
4.2.2.2 Tuning the structure by changing the pressure by controlling the stoichiometry
4.3 Conclusion
References
