Electrical measurements

Four probes measurements

The nickel oxide thin films deposited on insulating substrate were electrically characterized with a four probe setup (also called Four Point Probe Resistivity Measurements) when the thin films conductivity was high enough. As the name indicates, the setup is made of four probes, aligned and equally spaced, where the two outer probes are used as current source and the two inners probe are used for voltage measurements (Figure 3.15, left). In this way, the outer probes measure a potential drop generated by the current source in the thin films. The relationship between the thin film resistivity(ρ), the thin film thickness (t), the voltage (V ) and the current (I) is:

ρ = π ln(2)  V I  t (3.10)

Transmission line measurements

When the NiO thin films were not enough conductive enough, the transmission line measurement (TLM) was instead adopted. The TLM method consists in depositing, e.g.: by sputtering, parallel metallic stripes on the NiO thin films where the space between each stripes increases but the dimensions of the stripes are identical (Figure 3.15, right). By measuring the total resistance (RT) for different spacing between stripes (d), it is possible to determine the

contribution of the contact resistance (Rc), between the metallic contacts and

the thin film, from the thin film resistivity (ρ) following [132]: RT = 2 Rc+

ρ

W td (3.11)

with t the thickness of the thin film and W the length of one stripe. Plotting RT = f (d) provides a linear curve where the intercept with the Y-axis

corresponds to 2 Rc and the slope is equal to ρ/(W t). This method enables to

measure thin film resistivity where the four probes measurement method fails. The reason could be that the surface of the contact between the probes and the thin film is higher for the TLM method as the contact is defined by the area of the metallic stripes, whereas for the four probes measurement, the contact is reduced to a point.

Figure 3.15: Schematic of the setup for the four probes measurements (Left) and the transmission line measurements (Right).

Temperature dependent conductivity measurements

Temperature dependent conductivity measurements have been conducted on NiO thin films at TU-Darmstadt. The setup can measure conductivity in a controlled atmosphere in a furnace regulated by an Eurotherm temperature controller [11] (Figure 3.16). Beforehand, platinum contacts are deposited at

the corners of the sample in the Van der Pauw geometry. Once the sample is mounted on the sample holder specially designed for the measurements and inserted in the furnace, a programmed thermal cycle is ran and conductivity is measured in the span time. For the NiO samples produced along this thesis, the temperature is ramped up to 500 °C at 1 °/min, then the temperature is held at 500 °C for 1 h before that the furnace is cooled down at 1 °/min. Two atmospheres have been used: pure Argon and 10 % oxygen (1:9, O2/Ar).

A more detailed description of a similar setup for conductivity temperature dependence experiments is presented in the work of Hohmann et al. [11].

Figure 3.16: Schematic representation of the conductivity temperature dependence setup. The schematic has been adapted from the work of Hohmann et al. [11].

Electrical impedance spectroscopy and IV curves

Electrical impedance spectroscopy and IV measurements were performed on the Schottky junction which back-contacting by phosphor diffusion beforehand. Both measurements were conducted in the same probe station for electrical characterizations (Figure 3.17).

The measurements were realized in the dark with an Agilent 4294A impedance analyser from 40 Hz to 10 MHz at different polarization potential in four probes configuration terminal. Fitting of the impedance spectra is made with simple Randles elements with, after files conversion, the Echem Analyst Software. From the electrical impedance spectroscopy, the contact resistance (Rc), the equivalent capacitive element Cmeas and the equivalent resistance at

with a simple R+R//C circuit as presented in Figure 2.20 of the Section 2.4.2. Fitting provided error value representing less than 0.1% of the absolute values, which meant that the fits were reliable.

The IV curves were obtained with an Agilent 4156C semiconductor parameter analyser in a two probes configuration and the potential was swept from -1 V to 0.5 V. Although the connectors were not absolutely the same, it has been assumed that the value of the contact resistance (Rc) estimated by impedance

spectroscopy could be used for iR correction of the IV curves.

Figure 3.17: Picture of the electrical setup for the electrical impedance spectroscopy (IS) and the IV measurements. The samples are measured in the electromagnetic- shielded probe station.

CHAPTER

4

Defects and charge compensation in nickel oxide

Summary

Let’s start to introduce the results obtained during this thesis by a general approach of the NiO thin films surface properties. In this chapter the numerous in-situ UPS/XPS measurements acquired on NiO thin films reactively deposited by DC-sputtering are compiled. The study endeavour at unveiling relationship between the workfunction, the Fermi energy, and electronic features in the O 1s region and the Ni 2p region. Defective states, named O 1s(Def.), have been observed which would arise because of the introduction of defects. The defective state O 1s(Def.) can be eliminated at high temperature and low oxygen concentration (10 %) in the deposition chamber but it is prominent at room temperature and high oxygen concentration (20-25 %). The weight of the O 1s(Def.) in the O 1s region would define the surface electronic properties of NiO:

ˆ at low O 1s(Def.) weight the measured workfunction is about 4.5 eV and the Fermi energy is about 1.1 eV above the valence band maximum.

ˆ with increasing O 1s(Def.) weight the Fermi energy is reduced and the workfunction increases to ∼ 5.2 eV.

ˆ above a certain O 1s(Def.) weight the Fermi energy is pinned to ∼ 0.6 eV and the workfunction is constant (∼ 5.2 eV)

Eventually, a charge compensation of the dopants (nickel vacancies and maybe oxygen interstitials) has been proposed and can be summarized as following:

ˆ at low dopant concentration, charge compensation is obtained by free holes and positive charges localized on oxygen atoms.

ˆ when the dopant concentration is large, the Fermi level position is pinned to 0.6 eV and the insertion of dopant is compensated by the formation of peroxy species O− _{and probably to a lesser extent, by}

4.1

Introduction

NiO is a transition metal oxide (TMO) having a wide optical band gap (3.6-4.3 eV) for which the resistivity, in a defect-free structure, can reach up to 1013_{Ω cm [50] and 10}7_{Ω cm at 1 kHz [73]. However, when sputtered at low}

temperature (. 200°C), NiO thin film conductivity in the 0.1-1 Ω cm range can be measured [87, 95, 163].

NiO can be p-doped by holes in a metal deficient structure (Ni1−δO),

which can be obtained in an oxygen-rich environment. In such a case, NiO can be intrinsically doped by nickel vacancies (V00Ni) or by oxygen interstitials

(O00i) [82, 83]. Moreover, for charge neutrality, intrinsic doping of NiO has

to be compensated by positive charges, e.g. Ni3+_{, O}−_{, holes [83], or oxygen}

vacancies. Taking Kr¨oger-Vink notation, this can be formalized as following: 2 [V00_Ni] + 2 [O00_i] = [h ] + 2 [V_O] + [Ni_Ni] + [O_O] (4.1) where ’[ ]’ correspond to the concentration of the species per volume. Equation 4.1 is only a general view of the defect chemistry, which can develop in nickel oxide. The left-hand side of equation (4.1) are electronic defects (the dopants), which can be introduced in nickel oxide under oxygen-rich condition while the right-hand side represents the charge compensating species of the dopants. Regarding the left-hand side (the dopants), Lany et al. have shown that in NiO crystal, under oxygen rich condition, nickel vacancies V_{N i}00 and not oxygen interstitials O_i00, are the defects leading to nonstoichiometry [82]. It is generally accepted that p-type conductivity in NiO originates from nickel vacancies V_{N i}00 [83, 164, 165]. However, in most cases, the NiO thin films produce grain boundaries and to the best of our knowledge, the possibility of stabilizing oxygen interstitials at the grain boundaries of a nickel oxide thin film has not yet been discussed in literature. Therefore, a non-stoichoimetry associated to oxygen interstial at the grain boundaries cannot be excluded as will be discussed in the Chapter 5.

Regarding the right-hand side of equation (5.1), associated to the compensating species, in reality the stability of each compound is not the same as charge compensation is driven by thermodynamic rules, which would promote the formation of charge compensation having the lowest formation energy. The question is therefore, what is the most stable charge compensation mechanism of intrinsic doping in nickel oxide?

The answer to this question in the literature might be elusive as the studies are often carried out on only one type of sample or in a limited range of conditions [83, 87]. However, it can be assumed that the formation of oxygen vacancies V_O•• in oxygen rich conditions, is very unlikely. Moreover, in a NiO thin film prepared at room temperature, it has been proposed that delocalized holes (free holes) would be interacting with oxygen atoms and localized holes might be found on both nickel and oxygen atoms [83]. These

results would suggest that the dopants in NiO (left hand side of equation (5.1)) can be compensated by free (delocalized) holes (h•) interacting with oxygen atoms, and by localized holes on oxygen (O_O•) and nickel (N i•_{N i}) atoms. Although NiO can be adopted in various applications, clarification needs to be brought about the intrinsic charge compensation mechanism in nickel oxide. This chapter presents the compilation of results obtained by in-situ XPS/UPS measurements, realized in the DAISY-MAT system, with various NiO thin films prepared by reactive sputtering. Therefore, this chapter endeavours at emphasizing the impact of the temperature and the oxygen concentration during sample preparation on the measured surface electronic properties. The aim is to explore how the condition of deposition affects the NiO surface electronic properties measured by in-situ UPS/XPS. Eventually, the work provided enough clues to propose a charge compensation mechanism of nickel vacancies in NiO.