Research Article www.acsami.org
Cite This: ACS Appl. Mater. Interfaces 2018, 10, 38159−38165
Tunable Electrical and Optical Properties of Nickel Oxide (NiOx) Thin Films for Fully Transparent NiOx−Ga2O3 p−n Junction Diodes Maria Isabel Pintor-Monroy,† Diego Barrera,† Bayron L. Murillo-Borjas,† Francisco Javier Ochoa-Estrella,‡ Julia W. P. Hsu,† and Manuel A. Quevedo-Lopez*,† †
Downloaded via YORK UNIV on December 20, 2018 at 23:53:51 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
Department of Materials Science and Engineering, The University of Texas at Dallas, 800W. Campbell Road, Richardson, Texas 75080, United States ‡ Departamento de Investigación en Física, Universidad de Sonora, Rosales y Luis Encinas, Hermosillo, Sonora 83000, Mexico S Supporting Information *
ABSTRACT: One of the major limitations of oxide semiconductors technology is the lack of proper p-type materials to enable devices such as pn junctions, light-emitting diodes, and photodetectors. This limitation has resulted in an increased research focus on these materials. In this work, ptype NiOx thin films with tunable optical and electrical properties as well as its dependence with oxygen pressure during pulsed laser deposition are demonstrated. The control of NiOx films resistivity ranged from ∼109 to ∼102 Ω cm, showing a p-type behavior with Eg tuning from 3.4 to 3.9 eV. Chemical composition and the resulting band diagrams are also discussed. The all-oxide NiOx−Ga2O3 pn junction showed very low leakage current, an ideality factor of ∼2, 105 on/off ratio, and 0.6 V built-in potential. Its J−V temperature dependence is also analyzed. C−V measurements demonstrate diodes with a carrier concentration of 1015 cm−3 for the Ga2O3 layer, which is fully depleted. These results show a stable, promising diode, attractive for future photoelectronic devices. KEYWORDS: oxide semiconductors, pn junction, pulsed laser deposition, nickel oxide, gallium oxide tion.2,5,11−16 Among these deposition methods, PLD offers the advantages of excellent thickness control, low porosity, less defects, smooth surfaces, and, in general, more homogeneity throughout the film. In addition, PLD deposition can be carried out at low temperatures, ideal for their implementation in organic photovoltaic devices (OPVs).12 Some of the parameters that affect PLD deposition include laser wavelength, laser energy and energy density, atmosphere, pressure, temperature, distance substrate−target, and repetition rate. Although PLD deposition of NiO has been previously studied,4−6,12 in this paper, we demonstrate the tuning of the electrical and optical properties of NiOx films deposited at room temperature, without any further heat treatment, by controlling the oxygen pressure, and study its transport behavior. Finally, the films are used to demonstrate working devices including a semitransparent NiOx−Ga2O3 pn junction which proves to being stable even at higher temperatures. Although thin-film oxide pn junctions deposited by PLD have been demonstrated before, at least one of its contacts is not transparent. In addition, previous reports use ZnO as the ntype layer and leakage current is higher than that reported here
1. INTRODUCTION Although n-type transparent conductive oxide (TCO) has been extensively studied and improved in recent years, high performance p-type oxides are still a challenge, even when using high-temperature processes.1 Nickel oxide (NiO) is a 3d transition-metal oxide part of the MO family, where M = Co, Ni, Fe, and Cu.2 NiO is TCO, with a cubic, rock salt structure, wide band gap (Eg) (3.15−4.3 eV), and p-type conductivity.1,3−7 As a semiconductor, NiO is an attractive material with an extensive range of applications, for example, it has been used in antiferromagnetic devices, electrochemical electrodes, active optical fibers, fuel cell electrodes, hole transport layer (HTL) in solar cells, organic light emitting devices, chemical sensors, battery systems, CO oxidation catalysts,1,2,5,8 and as active channels in thin film transistors.9 NiO is especially important for p-type conducting films in optical windows for devices where hole injection/transport is required, while being transparent in the ultraviolet, visible, and near infrared range is desired.3 An additional advantage of NiO is that it is abundant and stable, which makes it ideal for commercial applications.10 Thin films of NiO have been deposited by several techniques including sol−gel, spin coating, radio frequency sputtering, pulsed laser deposition (PLD), thermal evaporation, electrochemical evaporation, spray pyrolysis, chemical deposition, and metalorganic chemical vapor deposi© 2018 American Chemical Society
Received: May 16, 2018 Accepted: October 15, 2018 Published: October 15, 2018 38159
DOI: 10.1021/acsami.8b08095 ACS Appl. Mater. Interfaces 2018, 10, 38159−38165
Research Article
ACS Applied Materials & Interfaces
Figure 1. (a) Resistivity dependence of NiOx films as a function of oxygen pressure, as measured by CTLM. Lowest resistivity corresponds to 10 mTorr oxygen pressure. (b) Resistivity, carrier concentration, and mobility for 10 mTorr as-deposited films as a function of working temperature. Mobility probes to be dependent on lattice scattering rather than impurity scattering. (c) Carrier concentration vs temperature plot to calculate activation energy. (d) Experimental band diagrams for NiOx films as a function of oxygen pressure. Clearly, all films are p-type regardless of the oxygen pressure during deposition.
(10−8 A/mm2).17−19 We present a novel NiOx/amorphous Ga2O3 pn junction deposited at room temperature, which has not been reported before. The only report found in the literature is for a Ga2O3 single crystal with Li−NiO deposited by sol−gel spin coating on top by Kokubun et al. and using metals as contacts.20 Films are also used to fabricate conventional poly(3-hexylthiophene):phenyl-C60-butyric acid methyl ester and poly(5-bromo-4-(2-octyldodecyl)selenophen2-yl)-5,6-difluorobenzothiadiazole-5,5′-bis(trimethylstannyl)2,2′-bithiophene (PFBT2Se2Th):phenyl-C71-butyric acid methyl ester OPVs. In addition to being p-type and highly transparent and having high work function (Φ), an HTL material must be highly conductive. 21,22 Albeit these challenges, OPVs using NiOx as HTL have been demonstrated before.4,23−25 Herein, we correlate the resistivity of the asdeposited NiOx films with OPVs performance, optimize the NiO electrical properties, and demonstrate the application of these NiOx films deposited by PLD as HTL in OPVs with two different active layer systems (Supporting Information).
different Ni/O ratio, of 2:3, or Ni2O3. Ni2O3 has a wider Eg and, in general, is less stable.27−29 Resistivity of the films for different oxygen partial pressures agrees with values for films deposited by other methods discussed in the literature,5,7,11,13,30−32 ranging from insulating (stoichiometric NiO at low oxygen pressures) to conductive films (at 10 mTorr oxygen pressure).33 The conductivity of NiOx films is determined by cation vacancies and compensated by electronic holes with low formation energies;34 therefore, deviation from stoichiometry is directly related with the electrical conductivity of the films. It has been previously demonstrated by Molaei et al. that the concentration of charge carriers depends on the temperature and the oxygen pressure during deposition for p-type NiO samples deposited by PLD.35 To maintain neutrality in the crystal, Ni2+ cations are converted to Ni3+ cations, trapping positive holes. At high enough oxygen pressures, the number of Ni3+ cations and oxygen excess leads to the formation of Ni2O3 instead of increasing the number of carriers. Because of the high resistivity for films deposited at low and high pressures, Hall measurements could not be performed. However, Hall measurements for 10 mTorr samples showed carrier concentration (p-type) in the order of ∼1017 cm−3, carrier mobility around 0.52 cm2/V s, and a resistivity of 127 (∼102) Ω cm. These values agree with values reported by Chen et al. for films deposited by sputtering at room temperature and 200 W.14 Hall measurements as a function of temperature (150−300 K) for 10 mTorr samples were also carried out to study its transport properties. Resistivity, mobility, and carrier concentration values for different temperatures are shown in Figure 1b; resistivity increases as the temperature decreases, as expected for a semiconducting material, from 105 Ω cm at 150 K to 102 Ω cm at room
2. ELECTRICAL PROPERTIES OF NIOX FILMS NiOx thin films were deposited by PLD at room temperature, without any further heat treatment, on glass substrates to study their electrical and morphological properties as well as their composition. Figure 1a shows the dependence of the resistivity for as-deposited 30 nm thick films as a function of oxygen pressure as measured by the circular transmission line method (CTLM).26 The resistivity shows a minimum at ∼10 mTorr likely due to more nickel vacancies and higher carrier concentration. Higher resistivity for P < 10 mTorr is because of more stoichiometric NiO films (NiO is the insulator when ratio is =1:1). The behavior for >10 mTorr is because of a 38160
DOI: 10.1021/acsami.8b08095 ACS Appl. Mater. Interfaces 2018, 10, 38159−38165
Research Article
ACS Applied Materials & Interfaces
3. OXYGEN PRESSURE EFFECTS Figure 2a shows the transmittance spectra and Eg of the films in the inset. Eg was calculated from a standard plot of (αhν)2
temperature. This behavior is because of carrier excitation/ generation as the temperature increases. The carrier concentration increases from ∼1013 cm−3 at 150 K to ∼1017 cm−3 at room temperature. Mobility, on the other hand, decreases as temperature increases from 1.78 (150 K) to 0.52 cm2/V s (300 K). The temperature effect on mobility is lower compared with the effect on carrier concentration. The resistivity behavior at low temperatures resembles the values measured by Morin for an oxygen-rich NiO sample obtained at 1473 K by decomposition of Ni(NO3)2, with a resistivity of 107 Ω cm at 150 K and 2 × 102 Ω cm at 300 K.36 The mobility values have a linear dependence on T−3/2, which is related to lattice scattering, rather than to impurity scattering, which dominates at lower temperatures (10 mTorr), the valence and the conduction band vary because of an increase in Eg and an increase in the IE of the films. This is explained as the increase in oxygen leads to more VNi and Ni3+ to maintain the neutrality of the films which, at higher oxygen pressures, results in the formation of more Ni2O3.
Figure 2. (a) Transmittance and Eg (inset) measurements. Both transmittance and Eg increase as the oxygen pressure increases. (b) GIXRD spectra, (c) SEM images, (d) RBS spectra, and (e) Ni/O ratio depth profile for NiOx films deposited at different oxygen pressures. GIXRD spectra and SEM images support the low crystallinity of the films. Ni2O3 is formed at 100 mTorr oxygen pressure, generating cracks on the surface of the films. Stoichiometry for the most conductive sample is around 0.83 Ni/O ratio. As the ratio increases or decreases, the resistivity of the films increases.
versus hν using the absorbance spectra obtained from UV−vis measurements. Samples deposited at lower oxygen pressures show a fairly constant Eg value of ∼3.42 eV, however Eg tends to increase as the oxygen pressure is increased during the deposition (>10 mTorr). This is likely due to increased nickel vacancies and more Ni2O3. The presence of Ni2O3 is further demonstrated by the X-ray diffraction (XRD) analysis discussed below. The grazing incidence X-ray diffraction (GIXRD) patterns for films deposited at different oxygen pressures at room temperature are shown in Figure 2b. Films deposited at oxygen pressure
