MODIFICATION OF THE OPTICAL AND ELECTRICAL PROPERTIES OF NIO FILMS BY THERMAL ANNEALING Текст научной статьи по специальности «Физика»

i l St. Petersburg Polytechnic University Journal. Physics and Mathematics. 2022 Vol. 15, No. 3.3 Научно-технические ведомости СПбГПУ. Физико-математические науки. 15 (3.3) 2022

Conference materials

UDC 621.317.39.084.2

DOI: https://doi.org/10.18721/JPM.153.356

Modification of the optical and electrical properties of NiO films by thermal annealing

Ya. B. Enns 1 2H, A. N. Kazakin 1 2, I. M. Komarevtsev 1 2, E. A. Vyacheslavova А. S. Kondrateva M. V. Mishin 1

1 Alferov University, St. Petersburg, Russia;

2 Peter the Great St.Petersburg Polytechnic University, St. Petersburg, Russia

H [email protected]

Abstract. This paper presents the results of studying the effect of thermal annealing on the optical and electrical characteristics of NiO films. NiO layers were synthesized using DC magnetron sputtering from a Ni target. The films were deposited in an Ar/O2 gas mixture with a ratio of 70%/30%, respectively. The deposition power was 100 W. The resulting films had low transparency and high conductivity, which is associated with a high content of oxygen vacancies in the NiO structural layer. The influence of thermal annealing on the characteristics of NiO films was studied on films obtained by magnetron sputtering. Annealing was carried out in an oxygen-containing environment at temperatures from 200 °C to 550 °C and an annealing duration from 5 to 120 minutes. The results of optical studies have shown that annealing at temperatures up to 550 °C leads to an increase in transparency from 5% to 80% at a wavelength of 700 nm. In this case, an increase in the temperature and duration of the process is accompanied by an increase in the optical band gap. A similar trend was observed in the study of film conductivity, where an increase in the annealing temperature leads to an increase in resistivity from 0.2 Q cm to 1460 Q cm.

Keywords: nickel oxide, transparency, thermal annealing, resistivity, optical band gap

Funding: The work was done as a part of the state assignment (No. FSRM-2020-0011) of the Ministry of Education.

Citation: Enns Ya. B., Kazakin A. N., Komarevtsev I. M., Vyacheslavova E. A., Kondrateva A. S., Mishin M. V., Modification of the optical and electrical properties of NiO films by thermal annealing. St. Petersburg State Polytechnical University Journal. Physics and Mathematics, 15 (3.3) (2022) 285-289. DOI: https://doi.org/10.18721/JPM.153.356

This is an open access article under the CC BY-NC 4.0 license (https://creativecommons. org/licenses/by-nc/4.0/)

Материалы конференции

УДК 621.317.39.084.2

DOI: https://doi.org/10.18721/JPM.153.356

Модификация оптических и электрических свойств пленок NiO с примененим термического отжига

Я. Б. Эннс 1 2Н, А. Н. Казакин 1 2, И. М. Комаревцев 1 2, Е. А. Вячеславова \ А. С. Кондратьева \ М. В. Мишин 1

1 Санкт-Петербургский национальный исследовательский Академический университет имени Ж. И. Алферова, Санкт-Петербург, Россия; 2 Санкт-Петербургский государственный университет, Санкт-Петербург, Россия

н [email protected]

Аннотация. В данной работе представлены результаты исследования влияния термического отжига на оптические и электрические характеристики пленок NiO. Слои NiO были синтезированы с использованием DC магнетронного напыления из мишени Ni. Напыление пленок осуществлялось в газовой смеси Ar/O2 соотношением 70%/30%,

© Enns Ya. B., Kazakin A. N., Komarevtsev I. M., Vyacheslavova E. A., Kondrateva A. S., Mishin M. V., 2022. Published by Peter the Great St.Petersburg Polytechnic University.

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St. Petersburg Polytechnic University Journal. Physics and Mathematics. 2022 Vol. 15, No. 3.3

соответсвенно. Мощность напыления составила 100 W. Полученные пленки имели низкую прозрачность и высокую проводимость, что связано с высоким содержанием кислородных вакансий в структурном слое NiO. Влияние термического отжига на характеристики пленок NiO изучалось на пленках, полученных методом магнетронного напыления. Отжиг осуществлялся в кислородосодержащей среде при температурах от 200 °С до 550 °С и длительностью отжига от 5 до 120 минут. Результаты оптических исследований показали, что отжиг при температурах до 550 °С приводит к увеличению прозрачности с 5 % до 80 % на длине волны 700 нм. При этом увеличение температуры и длительности процесса сопровождается с увеличением оптической ширены запрещенной зоны. Подобная тенденция наблюдалась при исследовании проводимости пленок, где повышение температуры отжига приводит к увеличению удельного сопротивления с 0.2 Ом-см до 1460 Ом-см.

Ключевые слова: оксид никеля, прозрачность, термический отжиг, удельное сопротивление, оптическая запрещенная зона

Финансирование: Работа выполнена в рамках Государственного задания (№ FSRM-2020-0011).

Ссылка при цитировании: Эннс Я. Б., Казакин А. Н., Комаревцев И. М., Вячеславова Е. А., Кондратьева А. С., Мишин М. В. Модификация оптических и электрических свойств пленок NiO с примененим термического отжига // Научно-технические ведомости СПбГПУ. Физико-математические науки. 2022. Т. 15. № 3.3. C. 285-289. DOI: https://doi.org/10.18721/JPM.153.356

Статья открытого доступа, распространяемая по лицензии CC BY-NC 4.0 (https:// creativecommons.org/licenses/by-nc/4.0/)

Introduction

There is an increase in attention to transition metal oxides such as TiO2, MoO3, WO3, CuO, ZnO, NiO and others. These materials can be used both for manufacturing power diode structures [1] and for optoelectronic applications [2]. Nickel oxide (NiO) is the most prominent representative of this class of substances. At the moment, there are several models for the conductivity of semiconductor nickel oxide, but all of them are based on the fact that NiO is a direct-gap material, has its own hole conductivity, and has a band gap of 3.6-4.0 eV. Thus, this oxide semiconductor has a high prospect in optoelectronic devices.

NiO films can be synthesized by various methods, among which the method of magnetron sputtering is highly promising, due to the high quality of the film, low cost, and high speed of coating formation. Films are deposited using both RF magnetrons and targets made of high-purity Ni or stoichiometric NiO [3] and DC magnetrons with Ni targets [4]. In both cases, deposition occurs by adding O2 to the working gas mixture. A high concentration of acceptors can be obtained due to spontaneously formed Ni vacancies [5], which is achieved by choosing the reactive sputtering technological parameters. At the same time, it is rather difficult to achieve a high quality of NiO films after magnetron sputtering, where films can mainly have low crystallinity, low mobility, and low transparency. This is due to the high concentration of point defects inside the film due to the high oxygen content [6]. The simplest method for modifying the resulting film coatings is high-temperature annealing. Annealing can lead to an increase in the structural perfection of the material and, as a consequence, to a decrease in the concentration of point defects inside the film, an increase in the width of the optical band gap, and an increase in resistivity.

In this work, we study in detail the effect of high-temperature annealing on the optical and electrical characteristics of magnetron sputtered NiO films.

Materials and Methods

In the study, NiO films were formed on a glass substrate (Corning 0211) using DC magnetron sputtering from a Ni (99.95%) nickel target. Magnetron sputtering was carried out in a mixture of working gases 30% O2 / 70% Ar at a working pressure of 3 mTorr. The gas flow rate was 6 sccm and 14 sccm for O2 and Ar, respectively. The magnetron sputtering power was 100W.

© Эннс Я. Б., Казакин А. Н., Комаревцев И. М., Вячеславова Е. А., Кондратьева А. С., Мишин М. В., 2022. Издатель: Санкт-Петербургский политехнический университет Петра Великого.

After deposition, the glass substrates were divided into 10* 10 mm chips using a circular saw and NiO films were subjected to thermal annealing in an oxygen-containing controlled environment. A series of annealings were carried out in the temperature range of 200-550 °C with a duration of 5-120 min.

Film thicknesses were determined using an Ambios XP-1 contact profilometer. The optical and electrical parameters of the resulting films were measured.The optical transmission and reflectance spectra were measured applying an AvaSpec-ULS2048XL-EVO-RS spectrometer and a xenon lamp in the wavelength range of 300-800 nm. Electrical measurements were carried out using a probe station and semiconductor device analyzer (Keysight B1500A) to determine the current-voltage characteristics.

Results and Discussion

The thickness of the NiO layer was about 150 nm , which corresponds to a deposition rate of 1.6 nm/min. The resulting NiO films had high conductivity and low transparency. The results obtained after thermal annealing showed an increase in the transparency of the films with an increase in the temperature and duration of the annealing of the films. Fig. 1, a shows examples of optical spectral characteristics of NiO films with different annealing conditions. The trend of increasing the transparency of films can be traced in Fig. 1, b. Thus, after annealing at 550 °C, the transmission of the films increased from 5% to 80% at a wavelength of 700 nm.

Wjolangm [nm| Tims [irin|

Fig. 1. Optical transmission, reflection and absorption spectra of samples obtained by annealing at different temperatures (a); Dependence of the transmittance (700 nm) on the annealing time (b)

The increase in the transparency of the films to a greater extent may be due to a decrease in optical absorption, a decrease in the concentration of carriers, and an increase in the crystallinity of the film.

The optical bandgap (E) of each sample was extracted from the absorption edge. According to the theory of optical absorption, the absorption coefficient is calculated as follows:

(ahv)k = A(hv - E)

where A is a constant, a is the absorption coefficient, and hv is the photon energy. Since NiO is a direct band gap material, the exponent k is set to 2. The Tauc plot was used to determine the optical band gap and is shown in Fig. 2, a. Eg of NiO films can be obtained by extrapolating the linear part (ahv)2 to the hv axis. It was found that an increase in the temperature and duration of annealing is accompanied by an increase in the width of the optical band gap. Fig. 2, b shows the trend in optical band gap versus annealing time for different process temperatures. The oxygen interstices in NiO films can either act as a lattice defect, scattering and/or absorbing incident light, or it can introduce some energy levels of impurities into the band gap and reduce the optical band gap [3]. Annealing of NiO films can promote the diffusion of interstitial oxygen atoms from NiO films, which appear itself in an increase in the optical band gap.

In addition, it has been found that an increase in the transparency of the films is accompanied by a significant increase in resistivity from 0.2 Q cm to 1460 Q cm. Fig. 3 shows the trend in the

^St. Petersburg Polytechnic University Journal. Physics and Mathematics. 2022 Vol. 15, No. 3.3

Fig. 2. The plot of (ahv)2 versus photon energy hv (a); dependence of the optical band gap on the

annealing time for different process temperatures (b)

Fig. 3. Dependence of the resistivity of NiO films on the annealing time for various temperature

processes

change in the resistivity of NiO films as a function of the annealing time for various temperature processes. This can also be a consequence of the relaxation of interstitial oxygen atoms as a result of annealing.

Conclusion

The NiO layer was synthesized using DC magnetron sputtering from a Ni metal target, which is a simple, scalable and inexpensive method. The effect of annealing the obtained films on the optical and electrical characteristics was studied. Conductivity measurements show that the resistivity of the NiO films increases significantly with the annealing temperature from 0.2 Q cm (initial) to 1460 Q cm (annealing at 550 °C). In addition, an increase in resistivity is accompanied by an increase in the transparency of the films and an increase in the optical band gap after annealing of the films. This may be due to a decrease in the density of point defects inside NiO after annealing due to the diffusion of oxygen interstices.

REFERENCES

1. Mistrya B. V., Bhattb P., Bhavsara K. H., Trivedia S. J., Trivedib U. N., Joshi U. S., Growth and properties of transparent p-NiO/n-ITO (In2O3:Sn) p—n junction thin film diode, Thin Solid Films. 519 (11) (2011) 3840-3843.

2. Warasawa M., Watanabe Y., Ishida J., Murata Y., Chichibu S. F., Sugiyama M., Fabrication of Visible-Light-Transparent Solar Cells Using p-Type NiO Films by Low Oxygen Fraction Reactive RF Sputtering Deposition, Japanese Journal of Applied Physics. 52 (2R) (2013) 021102.

3. Ren Y., Li L., Liu N., Zhang K., Li Ch., Chen Zh., Zhang B., Quasi-vertical GaN heterojunction diodes with p-NiO anodes deposited by sputtering and post-annealing, Vacuum. 182 (2020) 109784.

4. Li L., Wang X., Liu Y., Ao J-P., NiO/GaN heterojunction diode deposited through magnetron reactive sputtering, Journal of Vacuum Science & Technology A. 34 (2016) 02D104.

5. Lany S., Osorio-Guillfen J., Zunger A., Origins of the doping asymmetry in oxides: hole doping in NiO versus electron doping in ZnO, Phys. Rev. B Condens. Matter. 75 (2007) 1—4.

6. Kim S.-K., Yang H., Kim Y.-S., Effects of double heat treatment of NiO hole transport layer on the performance of QLEDs, Journal of Materials Science. 55 (2020) 17046-17060.

THE AUTHORS

ENNS Yakov B.

[email protected] ORCID: 0000-0003-4396-2368

VYACHESLAVOVA Ekaterina A.

[email protected] ORCID: 0000-0001-6869-1213

KAZAKIN Alexey N.

[email protected] ORCID: 0000-0001-8762-1587

KONDRATEVA Anastasia S.

[email protected] ORCID: 0000-0003-3915-9329

KOMAREVTSEV Ivan M.

[email protected] ORCID: 0000-0002-6800-9218

MISHIN Maxim V.

[email protected]

ORCID: 0000-0003-3250-1515

Received 25.07.2022. Approved after reviewing 26.07.2022. Accepted 27.07.2022.

© Peter the Great St. Petersburg Polytechnic University, 2022