Production and characterization of indium oxide and indium nitride

Abstract

Thin films of indium oxide 200 nm thick and indium nitride 150 nm thick were produced by reactive sputtering deposition onto soda lime substrates. The Indium cathode was kept under vacuum attached to a high voltage dc. The In xOy films were obtained in argon-oxygen mixture with total pressure between 2 and 7 Pa, current density between 0.04 and 0.56 mA/cm² and substrate temperature of 300 K. The In xNy films were obtained in argon-nitrogen mixture with total pressure between 1 and 5 Pa, current density between 0.18 and 14 mA/cm² and substrate temperature of 320 K. Results revealed that thin films give rise to high conductivity, are transparent and have a carrier density of 10(19) cm- 3 for In xOy and 10(17) cm- 3 for In xNy. These results were obtained mainly from temperature dependent alpha and sigma measurements. The reduced chemical potential was calculated considering three different scattering mechanisms i.e. interaction with optically polarized phonons, interaction with ionized impurities and interaction with grains, in order to identify the main scattering mechanism, which resulted to be due to impurities.

Indium oxide; Indium nitride; Scattering mechanisms; Thermoelectric power

SURFACES, INTERFACES, AND THIN FILMS

Production and characterization of indium oxide and indium nitride

Luis C. Jimenez B.; Henry A. Méndez P.; Beynor A. Páez S.; María E. Ramírez O.; Hernán Rodríguez H.

Grupo de Películas Delgadas, Depto. de Física Pontificia Universidad Javeriana, Bogota D.C., Colombia

ABSTRACT

Thin films of indium oxide 200 nm thick and indium nitride 150 nm thick were produced by reactive sputtering deposition onto soda lime substrates. The Indium cathode was kept under vacuum attached to a high voltage dc. The In_xO_y films were obtained in argon-oxygen mixture with total pressure between 2 and 7 Pa, current density between 0.04 and 0.56 mA/cm² and substrate temperature of 300 K. The In_xN_y films were obtained in argon-nitrogen mixture with total pressure between 1 and 5 Pa, current density between 0.18 and 14 mA/cm² and substrate temperature of 320 K. Results revealed that thin films give rise to high conductivity, are transparent and have a carrier density of 10¹⁹ cm ^{- 3} for In_xO_y and 10¹⁷ cm ^{- 3} for In_xN_y. These results were obtained mainly from temperature dependent a and s measurements. The reduced chemical potential was calculated considering three different scattering mechanisms i.e. interaction with optically polarized phonons, interaction with ionized impurities and interaction with grains, in order to identify the main scattering mechanism, which resulted to be due to impurities.

Keywords: Indium oxide; Indium nitride; Scattering mechanisms; Thermoelectric power

I. INTRODUCTION

A research program on production and characterization of semiconductor oxides with high conductivity and high transmittance in the IR near region as required for optoelectronic devices is being carried out. Semiconducting oxides namely, ZnO, SnO₂, SnO₂:F, and particularly InO thin films prepared by DC reactive sputtering show high conductivity and high transmittance in the near IR region. Because of these characteristics and other important properties, InO thin films are suitable for technological applications where high transparency in the visible region and high conductivity are required [1], [2], [3]. Indium oxide is a wide-band-gap semiconductor ( ~ 3.7 eV), which exhibits an isolating behavior in the stoichiometric form In₂O₃. Nevertheless, when it is prepared in an oxygen-deficiency way, it can reach a high n-type doping level due to the intrinsic defects such as oxygen vacancies. In_xN_ythin films possess interesting properties such as high conductivity and transmittance in the visible and near IR regions. Due to this special property amongst others, great attention has been put to the study of optical and electrical properties aimed to their use in technological applications. [4], [5].

II. EXPERIMENTAL

Reactive sputtering of indium was carried out at a pressure of 2.8 Pa. The base pressure inside the chamber was 5 mPa, and the average cathode-anode voltage was 2.2 kV. In_xO_y thin films were grown on well-cleaned soda lime substrates in a pure atmosphere of O₂/Ar, (PO/PAr = 0.09). More details about the reactor can be found elsewhere. [6]. Reactive sputtering of indium was carried out in Ar/N₂ at a pressure of 2.6 Pa. The cathode-anode average voltage was 1.8 kV. In_xN_y thin films were grown on well-cleaned soda lime substrates in a pure atmosphere of N₂/Ar (PN/PAr = 0.3).

The films were investigated by means of experimental measurements of transmittance (400 to 1000 nm), dc electrical conductivity s, Hall Effect and thermoelectric power (a) in order to determine the electrical transport properties. The thermoelectric power and electrical conductivity measurements were carried out in vacuum, using equipment as that shown in Fig. 1. The description of the assembly can be found elsewhere [7]. The growth conditions (current density, anode-cathode potential, substrate temperature) as well as the gas mixture were systematically changed until the best compromises between transparency and conductivity were obtained.

III. RESULTS AND DISCUSSION

Typical curves of carrier density and thermoelectric power measurements versus temperature in In_xO_y thin films are shown in Fig. 2, and were used to calculate the reduced chemical potential. Both curves have been plotted together for a better comparison. A particular sample grown at a pressure of 2.7 Pa and in an O₂/Ar (PO/PAr = 0.09) atmosphere, with an anode-cathode potential of 2.2 kV, and current density of 0.7 mA/cm², revealed the best quality and therefore was selected out of the sample set grown under different conditions. Additionally, transmittance values were 86%, which were determined in the [350 nm, 1000 nm] region corresponding to the far UV and near IR regions. The values of the thermoelectric power correspond to degenerated semiconductors [8] and the theoretical expression after solving the Boltzmann equation is given by [8]

being k the Boltzmann constant, e the charge-electron magnitude, h the reduced chemical potential, F is Fermi's integral and r a quantity which involves the scattering mechanisms i.e. Interaction with optically polarized phonons (r = 1/2), interaction with ionized impurities (r = -1/2) and interaction with grains (r = 3/2).

Fermi's integral for this sort of semiconductors is given by [9]:

in this case the thermoelectric power yields to

The advantage of equation (3) is that the chemical potential can be readily evaluated thus allowing the carrier concentration, n, to be found. Figure 3 shows the dependence of the reduced chemical potential on temperature for all the three scattering mechanisms mentioned above. The importance of this calculation is based on the clear-cut differences among the three different scattering mechanisms that the charge carriers can likely suffer under the transport process.

Figure 3

The main scattering mechanism with In_xO_y thin films is due to impurities. The other two are not considered, since for a high doping level the charge carrier-phonon interaction becomes weaker due to the induced intrinsic defects, as it has been proved by other techniques [10]. The grain potential is not high enough to induce a main effect in the charge carrier transport because the net charge density per charge can easily overpass the barrier potential on the grain boundaries [1].

Figure 4 shows the temperature-dependent behavior of the electrical conductivity measurements and mobility. Mobility was evaluated by means of the relation s = enµ. Mobility values are strongly affected by changes in temperature. This means that despite having a main scattering mechanism in all the measurement range, changes in mobility can be due to changes in carrier concentration as it can be seen in Fig. 2, since for low concentrations the mean free path is higher than for high concentrations, supporting the argument that the main scattering mechanism in In_xO_y thin films prepared by DC reactive sputtering is due to impurities.

The typical curve of thermoelectric power measurements against temperature in In_xN_y thin films is shown in Figure 5. A particular sample grown at a pressure of 2.8 Pa, in an O₂/Ar (PN/PAr = 0.3) atmosphere, with an anode-cathode potential of 2.2 kV, and current density of 14 mA/cm², showed the best quality and therefore was selected out of the sample set grown under different conditions. Additionally, transmittance values were 92%, which were determined in the [350 nm, 1000 nm] region and correspond to the far UV and near IR regions. The values of the thermoelectric power correspond to non degenerated semiconductors and the theoretical expression after solving the Boltzmann equation is given by equation (1).

Figure 6 shows the behavior of the temperature-dependent electrical conductivity measurements. From thermoelectric power a and conductivity s measurements, a carrier density of 10¹⁷cm ^{- 3} for In_xN_y, with m*=0.3m_o, r = 1/2, was obtained in order to identify the main scattering mechanism, which was due to an interaction between free carriers and ionized impurities.

Figure 7 shows temperature-dependent behavior of concentration n and mobility µ. Mobility was evaluated by means of the relation s = enµ. The n grows and µ decreases. They are approximately constant for temperatures more than 200 K, possibly, since when increasing temperature, the density of free carriers as result of increase in the density of ionized impurities N_Iis increased and saturation of the ionization of impurities appears for temperatures more than 200K. This one leads to saturation in the density of free carrier. The reduction of µ could be by the increase N_I in when the mechanics of dominant scattering are ionized impurities.

IV. CONCLUSION

In_xO_y thin films prepared by DC reactive sputtering reveal suitable electrical and optical characteristics when grown at a pressure of 2.8 Pa and in an O₂/Ar atmosphere with an anode-cathode potential of 2.2 kV and current density of 0.7 mA/cm². Transmittance values were 86%, which were determined in the [350nm, 1000 nm] region corresponding to the far UV and near IR regions. A carrier density in the order of 10¹⁹ cm ^{- 3} was determined from the thermoelectric power measurements, indicating that In_xO_y thin films grown under the established conditions are degenerated. Additionally, it was found from calculations of the reduced chemical potential that the main scattering mechanism is that due to impurities. It was determined that for high temperatures the mobility is high, what basically means that mobility values are well mediated by carrier concentration.

In_xN_y thin films prepared by DC reactive sputtering possess suitable electrical and optical characteristics when grown at a pressure of 2.6 Pa and in an N₂/Ar atmosphere with an anode-cathode potential of 2 kV and current density of 14 mA/cm². Transmittance values were 92 %. A carrier density in the order of 10¹⁷ cm ^{- 3} was determined from thermoelectric power measurements, indicating that they are non degenerated semiconductors. It was found from calculations of the reduced chemical potential that the main scattering mechanism is that due to ionized impurities.

We report a simple method to produce conductive, transparent foils. The films are non-stoichiometric and their good conductivity and transmittance may be explained by the relatively large crystallite size. The main scattering mechanism is due to impurities. We describe promising findings for further research on the optoelectronics material fields.

Received on 8 December, 2006

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Publication Dates

• Publication in this collection
  29 Nov 2006
• Date of issue
  Sept 2006

History

• Received
  08 Dec 2006