Analysis of Zinc Nitride Resistive Indicators under Different Relative

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Analysis of zinc nitride resistive indicators under different relative humidity conditions Mayte Gómez-Castaño, Andrés Redondo-Cubero, Luis Vazquez, and Jose Luis Pau ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b09805 • Publication Date (Web): 05 Oct 2016 Downloaded from http://pubs.acs.org on October 12, 2016

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ACS Applied Materials & Interfaces

Analysis of Zinc Nitride Resistive Indicators under Different Relative Humidity Conditions

M. Gómez-Castaño 1, A. Redondo-Cubero 1*, L. Vázquez 2, J.L. Pau 1 1

Electronics and Semiconductors Group, Departamento de Física Aplicada, Universidad Autónoma de Madrid, 28049 Madrid, Spain

2

Instituto de Ciencia de Materiales, Consejo Superior de Investigaciones Científicas, E-28049 Madrid, Spain

Abstract Zinc nitride (Zn3N2) is a metastable material in ambient conditions due to its high reactivity with water molecules. In this work we perform a systematic analysis of the oxidation of Zn3N2 layers grown by RF magnetron sputtering at room temperature. The aging and transformation of the layers towards a ZnO film is explored by means of spectroscopic ellipsometry and scanning electron microscopy for conditions with different relative humidity (RH). Accurate depth profiling by means of elastic recoil detection analysis with a time-of-flight telescope demonstrated the substitutional reaction between O and N and the important effect of the RH in this process. Due to this metastability the resistivity of the layers changes several orders of magnitude. Taking advantage of this principle, we develop electronic indicators and characterize the transformation of their electrical properties as a function of the ambient RH, finding a good correlation between the transformation time and the RH level. Keywords: zinc nitride, relative humidity, indicator, sputtering, oxidation

* Corresponding author. E-mail: [email protected]

1 Environment ACS Paragon Plus


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1. Introduction Zinc nitride (Zn3N2) is a n-type semiconductor with a low resistivity (10-2-10-4 Ω·cm), a high carrier concentration (1019-1020 cm-3) and a high mobility (30-156 cm2/V·s)1-3. The variability of these values typically arises from the diverse growth conditions and used methods, but some important properties of Zn3N2 are still controversial and poorly understood. In particular, very different values have been obtained for the band gap of this material, varying from 1.23 eV1 up to 3.4 eV4. This surprising inconsistency has been recently explained because of the, frequently overlooked, oxidation of Zn3N2 under ambient conditions3,5. This process has attracted more interest in the last years due to the potential fabrication of ZnO:N layers by this method6,7, since the achievement of stable p-type ZnO layers has become a bottleneck for the production of light emitting devices based on this wide band gap semiconductor8. As it was early recognized1, Zn3N2 can react with water producing ammonia according to different reactions: Zn3N2 + 6 H2O → 3 Zn(OH)2 + 2 NH3 Zn3N2 + 3 H2O → 3 ZnO + 2 NH3

7,9

10

.

Therefore, Zn3N2 can be easily oxidized in air due to the interaction with water molecules and, indeed, Zn-OH and Zn-O fingerprints have been observed by Auger and X-ray photoelectron spectroscopy, indicating that these processes do take place1,7. Consequently, the relative humidity (RH) is an important factor contributing to the aging of Zn3N2 and its progressive oxidation. As an example, Figure 1 shows the transformation of a 630 nm thick Zn3N2 grown on glass with time in air. As-grown layers exhibit a black opaque surface, but this color changes after a long exposure to air and, finally, leads to a completely transparent film.


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The oxidation rate and the control of these aging mechanisms are far from being wellunderstood yet and need to be further studied. This circumstance has limited the potential applications of Zn3N2 such as thin film transistors (TFTs)11 or as negative electrode in Li-ion batteries12, demanding more careful experiments when dealing with this material. In this work we analyze the effect of RH by taking advantage of the change in the structural, optical and electrical properties of Zn3N2. For that, we previously perform a series of experiments in order to understand the oxidation of this material under different environments, including direct depth profiling of the layers by ion beam analysis and optical characterization. Then, we determine the effect of this process on the electrical properties of the material, i.e. the resistivity, and develop a device that can operate as an indicator of the maximum RH.

2. Experimental Zn3N2 layers were grown at room temperature using an Alcatel A450 RF (13.56 MHz) magnetron sputtering system. Both Si(100) and glass substrates were used, being the later devoted to the development RH indicators on an insulating platform. The plasma discharge was induced between a 4” circular Zn target (99.995% purity) and the substrate, using 30 sccm flux of N2 gas (99.999%) and a power of 100 W. The base and working pressure are 10-5 mbar and 10-2 mbar, respectively. The thickness of the layers was determined by profilometry, and the resulting growth rate was 42(2) nm/min. The morphology of the layers was analyzed by scanning electron microscopy (SEM), using a FEI XL30-SFEG system, operating with 10 keV electron beam with a nominal lateral resolution of 4 nm, being the secondary electrons detected with a through-lens



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detector. Roughness of the films was additionally measured by atomic force microscopy (AFM) with an Agilent PicoPlus 5500 system operating in the dynamic mode. Silicon cantilevers with a nominal radius of 8 nm were employed for the topographical measurements. The progressive oxidation of the layers was monitored by means of spectroscopic ellipsometry (SE). SE characterization was carried out in a Horiba-Jobin-Yvon (Uvisel model) system, operating with a Xe arc lamp covering the 1.5 - 4.5 eV range. The incident light is linearly polarized and modulated in phase with 50 kHz cycles. The elliptically polarized light reflected from the sample is collimated and transmitted through an optical fiber to the monochromator, with a diffraction grating of 1200 grooves/mm and a spectral range of 240-850 nm. Depth profiling of the samples was obtained by means of elastic recoil detection analysis with a time-of-flight telescope (ERDA-TOF). Due to the independent measurement of the time and the kinetic energy of the recoils, this technique acts as a spectrometer, allowing a complete mass separation of the elements of the sample (Zn, N, and O). The experiments were carried out at the Center of Micro-Analysis of Materials, using a 5 MV Cockroft-Walton ion accelerator. The sample, mounted in a 3axes goniometer and tilted 20º in the standard symmetrical geometry, was analyzed with a collimated 30 MeV I5+ beam, ensuring a large penetration depth of at least 400 nm. The telescope is placed at 40º scattering angle and equipped with two carbon foil stations separated 420 mm. For the time measurements the signal from the first station is delayed 200 ns and then used as the stop signal of the electronic clock (started with the second station signal). The particle energy was detected using a Si detector with an


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area of 300 mm2 placed 964 mm away from the sample. The beam current was monitored in real time thanks to a transmission Faraday cup. The resistivity of the samples was assessed with a conventional Dumas four-point probe head, a Keithley 220 current source, and a Keithley 619 multimeter. Further electrical characterization of the devices was carried out in a probe station using a HP4145B parameter analyzer. In-situ measurements at different RH values were carried out in a closed chamber with a commercial humidifier and a real-time power control dependent on the RH monitored with a Honeywell HIH-4000 detector. The resistance of the Zn3N2 films was measured by comparison with a commercial reference using a voltage divider.

3. Results and discussion

To analyze the stability of the Zn3N2 layers we performed two kind of experiments. Firstly, we followed the aging in air for a time of 1 month, monitoring the optical, morphological and compositional changes of a Zn3N2 layer with a thickness of 630 nm. Secondly, we analyzed the variations for samples exposed to different RH conditions during a fixed time of 1 day. All the experiments were carried out under normal temperature and pressure conditions. Figure 2 shows the ellipsometric functions (ψ,∆) of a Zn3N2 exposed to air during 1 month. These functions represent the amplitude and the phase difference for the polarized light, reflecting changes in the refractive index and the extinction coefficient13. The as-grown layer shows a quite constant value of both ψ and ∆ over the whole range, in agreement with previous reports5. Slight changes are observed for the



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sample after 3 days but, as the time evolves, the optical properties change and the ellipsometric parameters develop abrupt oscillations (for 7 days and longer times). As explained in the model (Fig. 2c) these oscillations respond to the interference at the interface between the oxide layer (formed during air exposure) and the remaining nitride film, due to the different band gap of both materials. This fact is supported by the absence of oscillations above ZnO band gap (~3.5 eV) and further simulations explained in ref. 5. The number of oscillations increases with the thickness of the oxide layer, what suggests that there is a substitutional reaction of N. This transformation into ZnO was previously proved by X-ray diffraction14 and, in this work, we also confirm it unambiguously by ERDA-TOF measurements. Figure 3 shows the change in ψ and ∆ for an as-grown Zn3N2 exposed to air at different RH: ambient (

Analysis of Zinc Nitride Resistive Indicators under Different Relative

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