Controlling Carrier Type and Concentration in NiO Films To Enable in

Subscriber access provided by UNIV OF SOUTHERN INDIANA

Functional Inorganic Materials and Devices

Controlling carrier type and concentration in NiO films to enable in-situ PN homojunctions Maria Isabel Pintor Monroy, Bayron L Murillo-Borjas, Massimo Catalano, and Manuel A. Quevedo-López ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b04380 • Publication Date (Web): 08 Jul 2019 Downloaded from pubs.acs.org on July 17, 2019

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Materials & Interfaces

Controlling Carrier Type and Concentration in NiO Films to Enable in-situ PN Homojunctions

Maria Isabel Pintor-Monroy†, Bayron L. Murillo-Borjas†, Massimo Catalano†a and Manuel A. Quevedo-Lopez†* †Department

of Materials Science and Engineering, The University of Texas at Dallas

800 W. Campbell Road, Richardson, Texas, 75080, USA aInstitute

for Microelectronics and Microsystems, CNR-IMM

Via Monteroni, 73100 Lecce, Italy Keywords: oxide semiconductors, nickel oxide, magnetron sputtering, reactive sputtering, TEM

Abstract: In this work, the oxygen partial pressure during NiO deposition in reactive sputtering of a Ni target is used to control its carrier type and concentration, obtaining both n- and p- type films. Carrier concentration can be controlled, ranging from 1019 cm-3 to 1014 cm-3. Films deposition is carried out at 200 °C, a relatively low temperature that enables the use of glass as substrate. Experimental band diagrams for n- type NiO are obtained for the

1 ACS Paragon Plus Environment

ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 31

first time. Finally, A NiO homojunction is demonstrated by introducing a low carrier concentration layer in between n- and p+-type NiO layers. Layers are deposited in-situ, preventing contamination and improving the interface quality, as observed by TEM. The Ni:O ratio for each layer was also obtained by analytical TEM measurements, demonstrating the impact of the oxygen partial pressure on the films stoichiometry and the simplicity of our process to control carrier type and carrier concentration in oxide semiconductors.

1. Introduction

A demanding need in oxide-based semiconductors technology is the design and fabrication of high quality p-n junctions. This requires oxides with either n- or p-type conduction, which is an issue in current oxide-based electronics. Ideally, such p- and n-type conduction should be achieved with the same material by simply changing the dopants, similarly to what happens in Si-based technology. Furthermore, if the n- and p- type materials are deposited in-situ, then cleaner interfaces should result in better device performance. One of the oxide that can be either p- or n- type is nickel oxide (NiO). Most NiO films are commonly oxygen rich, leading to nickel vacancies and p-type character. N-type NiO is also possible to achieve, but controlling its carrier concentration is challenging.

Semiconductor oxides, such as NiO, are of great interest to enable both unipolar and bipolar devices.1 Bipolar structures require p-n junctions similarly as those needed in junction field-effect transistors (JFETs), solar cells, and photodiodes, among others. However the


Page 3 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


vast majority of oxide semiconductors are unipolar (either p- or n-type), limiting the type of devices that can be demonstrated. This limitation has resulted in device research that focuses mostly in heterojunctions.2 However, if both, p- and n- conduction can be achieved with the same material, then technologies similar to Si-based technology could be explored. Another challenge for homojunctions is the difficulty to form the p-n junction in-situ to enable a cleaner and sharper interface between the two semiconductors. Having materials with similar lattice constants and coordination numbers should also result in interfaces with reduced defects. Most of the reported oxide-based homojunctions have been fabricated using ZnO, and usually high temperatures are required, limiting the substrates available and the homojunction applications.2 Nevertheless, controlling both carrier type and carrier concentration in oxides is difficult to achieve.

Several p-type oxides have been studied, including SnO, MoOx and Cu2O.3 However, NiO is normally favored for applications such as hole transport layer (HTL) in solar cells and organic light emitting devices (OLEDs), due to its wide band gap (Eg), ranging from 3.15 to 4.0 eV.4,5 More important, NiO exhibits a small electron affinity (~1.7 eV), offering a good option to be used in heterojunctions with several n-type oxides.2,

6-8

NiO has also been

demonstrated to act as active channel in thin films and in chemical sensors.9-12

NiO thin films have been deposited by both chemical and physical methods such as chemical bath deposition, spin coating, magnetron sputtering, pulsed laser deposition (PLD), thermal evaporation and several anodic electrochemical deposition techniques.5, 8, 10, 12-16

Most of the resulting NiO films exhibit p-type character, due to the nickel vacancies (



Page 4 of 31

) created during the film deposition. This is especially true for physical methods that use a nickel oxide target. Among the physical methods, films deposited with magnetron sputtering show good thickness control over large areas. Further, magnetron sputtering allows to deposit films in several ambient: inert (usually argon), reactive (as oxygen) or mixed (argon-oxygen) atmospheres.9, 10, 13, 17 Typical reactive sputtering usually involves a metallic target (Ni) and the NiOx films are obtained by controlling the pressure, power, temperature or oxidizing ambient (O2, O2/Ar, O2/N2, etc.). On the other hand, NiO films deposited in inert atmospheres require a target with the desired composition (NiO, in this case). The main advantage of using reactive sputtering is the ability to closely control the oxidation of the resulting NiO films, and with that to obtain either oxygen or nickel deficient films. This is very important because it defines the carrier type and concentration of the films.

In this paper, p- and n-type NiOx films are demonstrated, using reactive magnetron sputtering in different argon/oxygen mixtures from a Ni target. The method demonstrated enables control carrier type and carrier concentration. P-type or n-type behavior is achieved by simply controlling the partial oxygen pressure of the chamber. In addition, all other process conditions such as pressure, power and temperature are held constant, It is worth mentioning that all deposition were carried out at low temperature (200 °C). The low temperature allows better oxidation control in the NiOx films while also enabling the use of flexible substrates given the reduced thermal budget. Up to the authors knowledge, this is the first time that a single variable is used to control majority carriers type and carrier concentration in NiO films. Moreover, the obtained band diagrams of the films clearly


Page 5 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


establish a relation between electrical and optical properties and the films composition. Finally a p+pn junction was fabricated in-situ to demonstrate its potential application in bipolar devices and the ability to obtain clean interfaces.17 Transmission electron microscopy (TEM) analysis of the device was carried out to investigate the interface between the NiOx layers and its composition, by mean of Energy-dispersive X-ray spectroscopy (EDS).

2. Results and discussion

The NiOx deposition rate as function of oxygen percentage in argon/oxygen (Ar/O2) atmosphere is shown in Figure 1. As the percentage of oxygen increases the deposition rate decreases, ranging from ~22 nm/min (5 % O2) to ~2 nm/min (50 % O2). This agrees with previous effect of oxygen percentage during DC reactive sputtering of a metallic target for zinc oxides films, as reported by Kappertz et al.18 The decrease in the deposition rate is mostly due to “poisoning” of the target surface, that is, the target surface gets oxidized during reactive sputtering. The formation of a thin oxide film on the surface increases the resistance of the target and the oxygen partial pressure during deposition as there is no more metal left to oxidize and a replenishment of oxygen at the surface occurs.18, 19, 20

The

films change in appearance is shown in Figure 2a, where it can be seen that the films deposited using the lowest and highest oxygen percentage are darker than those in the middle region. Hall measurements (Figure 2b) for films deposited at low oxygen percentages (≤ 25 % O2) showed n-type behavior, while films deposited in atmospheres with O2 >25 % are p-type. The n-type behavior of the films deposited at low oxygen



Page 6 of 31

percentages (≤ 25 % O2) is due to the resulting higher concentration of oxygen vacancies ( , which are responsible for n-type conductivity in NiOx. For films deposited with 5 % O2, oxygen is not enough to completely oxidize the Ni. The excess of Ni leads to dark films with resistivity of ~102 Ω·cm and electron concentration of ~1018 cm-3. Films deposited with 10< O2% 25%), the films conductivity changes from n to p-type.

Carrier (holes) concentration in films deposited with O2 > 25% show an exponential dependence on the oxygen percentage, resulting in an exponential reduction of the film resistivity while mobility is mostly unchanged. This dependence allows to control carrier concentration in the p-type films from ~1015 cm-3 (30 % O2) to ~1019 cm-3 (50 % O2) by simply varying the oxygen partial pressure. The maximum reported carrier concentration and mobility for p-type NiOx films deposited using a NiO target and an RF source at 200 °C in pure oxygen atmosphere are ~2×1018 cm-3 and ~ 3.5 cm2/V·s, respectively.21 The maximum carrier concentration of the p-type films reported here is at least one order of magnitude higher.

NiOx control from n-type to p-type was reported by PLD and sputtering; however, this has been demonstrated at either higher deposition temperatures,22 or with films annealed at


Page 7 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


temperatures as high as 500 °C.23 Laser radiation anneals have also been used to induce this transition.24 Stamataki et al. reported n- to p- transition for NiO deposited by PLD by varying the oxygen pressure, but with no concentration control nor device demonstrations.25 In this paper, both n- and p- type films are obtained at a lower deposition temperature (200 °C) without further heat treatment and offering carrier concentration control; this enables the use of glass substrates and the possibility to use this process for flexible substrates.

The temperature dependent resistivity measured using the van der Pauw four-point probe configuration in a Hall effect system, in the temperature range from 200 to 350 K for a ptype film deposited with 50% O2 is shown in Figure 3a (black line). The resistivity is inversely proportional to temperature ranging from 40 Ω·cm (350 K) to 4×104 Ω·cm (200 K); the higher resistivity at lower temperatures is due to carrier freeze-out (non-ionized carriers).26,

27

A similar behavior has been reported before for NiOx films deposited by

PLD.8 This trend is expected in semiconductor materials due to the lack of ionized carriers at low temperature.28 The corresponding Arrhenius plot (Figure 3b, black) was used to calculate the activation energy, that is, the difference between the edge of the valence band and the Fermi level for the p-type samples. The activation energy calculated was 0.26 eV, corresponding to a carrier concentration in the films of ~1019 cm-3 at room temperature. Similar measurements performed in n-type NiOx films deposited in an oxygen flow of 5 % (Figure 3a, red) show that as the temperature decreases, the resistivity increases from 70 Ω·cm (350 K) to ~4×104 Ω·cm (225 K). The n-type behavior of the films is then corroborated by the resistivity dependence on temperature: the films become more resistive as the temperature decreases, as is typical for a semiconductor; instead, the resistivity of a 7 ACS Paragon Plus Environment


Page 8 of 31

metal decreases as temperature decreases. The corresponding Arrhenius plot (Figure 3b, red) was also obtained to calculate the activation energy (difference between the edge of the conduction band and the Fermi level) and was calculated to be 0.35 eV, corresponding to a carrier concentration of ~1018 cm-3 at room temperature. This is consistent with the temperature dependence of resistivity by Hall measurements reported for NiOx.8,

29

Both

films, deposited at 50 and 5 % O2 were chosen as they are the ones exhibiting higher carrier concentration and lower resistivity for each p- type or n- type behavior respectively. This makes easier to perform the temperature dependent resistivity measurements, yielding to more reliable measurements. More resistive films would be more diffucult to measure as the films resistivy increases as the temperature decreases, and the resistivity would be higher than the Hall system limit.

Figure 4a and Figure 4b show the SEM images and XRD spectra for films deposited at different oxygen percentages. Films deposited using lower and higher oxygen percentage SEM results show a more homogeneous surface, while isolated round structures can be seen in films deposited using medium (15 – 30 %) oxygen percentages with decreasing size as the oxygen flow increases (Figure 4a). All films are polycrystalline with each peak corresponding to NiO structure according to XRD; furthermore, no peaks corresponding to metallic Ni were detected. The XRD peak intensity decreases for oxygen ≥ 40 %, as films become more nanostructured.5, 9, 10, 21 This also generates a slight shifting of the peaks at high oxygen percentage. Figure 4c and 4d show the transmittance spectra and band gap (Eg) for the same set of films. The low transmittance and Eg (3.2 eV) for films deposited with 5 % oxygen flow is due to its highly metallic character resulting from the poor oxidation due


Page 9 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


to the low oxygen content in the deposition ambient. This also results in high electron concentration.

As the oxygen flow increases (10-35 % O2), both the transmittance and Eg (3.6 eV) increase, and the carrier concentration decreases. This is a consequence of more stoichiometric NiO films. Clearly, the composition of the NiO films is affected by the increased oxygen content during the deposition, generating

and switching to p-type

behavior (Eg = 3.4 eV). A similar trend for Eg was reported by Reddy et al., in reactive sputtered NiO using a nickel target, with Eg of 3.0 eV for low oxygen partial pressure, and 3.6 eV and then decreasing to 3.1 eV as the oxygen pressure increases.10

The band diagrams constructed from the experimental Eg, ionization energy (IE, valence band edge) measured by photoemission spectroscopy in air (PESA) and Φ (Fermi level) measured by scanning Kelvin probe (SKP) values in the three p-type films with lower resistivity (40, 45 and 50 % O2) are shown in Figure 4e. As the carrier concentration of the samples increases, the Fermi level moves closer to the valence band edge, as expected. For the films deposited using 50 % O2, the difference is 0.21 eV, in agreement with the 0.26 eV obtained from the conductivity vs temperature plot shown in Figure 3b. In general, the band diagrams calculated for p-type NiOx are similar to those obtained for films deposited by PLD, solution methods or RF sputtering using a NiO target.8, 30, 31 Figure 4e also shows the band diagram for n-type films deposited using 5 % O2. In this case Φ was obtained from the activation energy (Figure 3b, red) and further supports the fact that an n-type NiOx has been obtained, as the Φ is close to the conduction band and much lower than that of metallic Ni.



Page 10 of 31

The SKP system was unable to measure the very low Φ values for these films. Authors have not found any reports for n-type NiOx band diagrams at the moment of submitting this work.

XPS spectra for films deposited using different oxygen partial pressures are shown in Figure 5a and Figure 5b for the Ni 2p3/2 and O 1s regions, respectively. The energies are calibrated using the C 1s peak (284.6 eV).32, 33 According to the XPS spectra, the films are composed of a mixture of both NiO and NiO2O3 with the presence of metallic Ni. The metallic nickel is mainly observed for films deposited with 5 % O2 and it decreases as the oxygen percentage increases. A small shift to higher energies for films deposited with higher oxygen flow (15 – 35 % O2) can be associated to a change in the chemical environment for the NiOx films due to a decrease in metallic Ni as oxygen flow increases. The deconvolution of the Ni 2p3/2 region is usually complicated due to shake-up satellites and the overlapping of the Ni2+ and Ni3+ signals, making difficult to assign the peaks either to NiO or Ni2O3.34-36 Therefore, only the Ni:O ratio was extracted from the XPS results. As expected from the Hall measurements and transmittance results, films deposited using 5 % O2 have a Ni:O ratio of higher than 1 (Figure 5c), which means the NiO films are Ni rich due to oxygen deficiency. This is supported by the larger metallic Ni peak in the Ni 2p3/2 region. Hence,

and n-type behavior. As oxygen flow increases, the Ni:O ratio decreases,

reaching values close to stoichiometric (0.98) for films deposited around 20 % O2, which results in films with the highest resistivity and lowest carrier concentration. Stoichiometry is lost as oxygen flow continues to increase to a minimum of ~0.96 Ni:O for films deposited at 50 % O2, that is, films become nickel deficient as the oxygen percentage


Page 11 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


increases, acquiring the p-type behavior and higher hole carrier concentration. It is worth mentioning that even when the Ni:O ratio of the film deposited using 15 % O2 is somehow lower than expected, the peak corresponding to metallic nickel is higher when compared with films deposited using higher oxygen percentage. The amount of metallic nickel from the Ni 2p3/2 region in the films is 13.7, 11.6 and only 8.6% for films deposited at 5, 15 and 50 % O2 respectively. However, a fraction of the detected metallic nickel might be due to NiOx reduction on the surface after the films are cleaned using Ar clusters, which makes extracting an exact stoichiometry value difficult.31 Similar values for NiOx films deposited using a metallic Ni target and a mixture of oxygen and argon as atmosphere with DC source were obtained by Hotovy et al. using RBS and XPS analysis and varying the oxygen content. In their case, for an oxygen content of 15 %, the Ni:O ratio is 1.02, indicating as the one reported here; similarly, with an oxygen content of 40 %, the Ni:O is 0.96. Both values are very close to the values reported here. However, they use a much higher power (600 W) and they do not evaluate the effect of the composition in the films carrier type and concentration.37

Finally, the p- and n- type character of NiOx films was demonstrated by fabricating a diode. Previously, pn heterojunctions have been demonstrated using NiO as the p-type material and ZnO, IGZO or Ga2O3 as the n-type material.6, 8, 38-40 Also just a few reports of NiO homojunctions exist. For example, Tyagi et al. fabricated a NiO-based homojunction by sputtering and achieved two orders of rectification; however the diodes were fabricated at temperatures as high as 500 °C and have very high electron concentration (~ 1020 cm-3) for the n-type layer, resulting in field-dependent leakage current. No carrier concentration



Page 12 of 31

control for either layer was observed.41 Sun et al. also fabricated a NiO homojunction by sputtering, using a NiO target and introducing Al by co-deposition to obtain the n-type films; however, the devices obtained did not show diode behavior.42 Stamataki et al. fabricated a NiO homojunction by PLD at 200 °C, obtaining poor ideality factor and rectifier behavior when using p-Si as substrate. The leakage current drops and rectifier behavior improves to around two orders of magnitude by changing the substrate to n-Si. This suggests an impact of the ohmic contact between substrate and the p-type NiO film in the diode behavior: n-Si has a low work function, resulting in a Schottky contact with the ptype NiO film.25

To demonstrate the potential use of the p- and n- type films demonstrated in this work in bipolar devices, a p+pn homojunction was fabricated in-situ, to maintain clean interfaces between layers. The proposed structure is shown in Figure 6a, using commercial ITO (140 nm) as the bottom contact, and NiOx (180 nm) film with a carrier concentration of ~1017 cm-3 and  of 4.8 eV as the p+. Next a more resistive NiOx (180 nm) with a carrier concentration of ~1015 cm-3 is used as the p- layer. Finally NiOx (160 nm) with a carrier concentration of ~1018 cm-3 and  of ~2.1 eV was used as the n- layer. The homojunction obtained band diagram is shown in Figure 6b. In this case,  of the slightly p-NiOx does not affect the band diagram as the depletion region exists within the layer. Finally, Mg (20 nm) was selected as the top contact for the n-NiOx, depositing an Au (150 nm) layer on top to prevent oxidation of the contacts. The depletion region falls in the slightly (almost intrinsic) p-NiOx layer, obtaining a built-in potential of 2.7 eV. Figure 6c shows the


Page 13 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


SEM cross section of the device without top contacts; a clean interface between layers is observed, due to the in-situ deposition, with vertical growth and low crystallinity, as expected. Transmittance spectra for the ITO bottom contact and the homojunction deposited on ITO without the top contacts are shown in Figure 6d inset. The transmittance is relatively low, mostly due to the n-type NiOx layer, which has the lower transmittance among all the layers (Figure 4c). The homojunction JV plot is also shown in Figure 6. An on/off ratio of ~102 orders of magnitude was obtained, with field-dependent leakage current increasing as the negative voltage increases. It has to be mentioned that  of Mg is 3.6 eV, which might not be ideal for the n-type NiOx, however Mg was selected over other metals such as Ca or Na ( of 2.8 and 2.3 eV respectively), which have a lower work function but that are more unstable. Although pn homojunctions have been reported before by Tyagi et al., temperatures as high as 500 °C to obtain the p-type films, using Pt contacts for both p- and n-type NiO were needed. In that case, a similar I-V curve shape with lower rectification and a close built-in potential of 2.5 V were obtained.41 Nevertheless, our work demonstrates that pn homojunctions can be obtained using 13 ACS Paragon Plus Environment


Page 14 of 31

more accessible contacts and lower temperature. These results can be improved with better contacts. Another device was fabricated with the structure ITO/p+-NiOx/pNiOx/Mg, in order to evaluate the effect of the n-type NiOx layer on the homojunction performance. The J-V curve for this device is shown in Figure S1. As it can be seen, even then the leakage current for both devices is similar, the forward current is affected and a rectification of ~1 order of magnitude was obtained.

Transmission electron microscopy (TEM) and Scanning transmission electron microscopy (STEM) analyses were carried out to study the homojunction. Figure 7a and b show the homojunction cross section images obtained by TEM bright field and STEM high angle angular dark field (HAADF). Interfaces between layers are clean and difficult to distinguish, especially for the case between n- and p-type layers. From HAADF images the polycrystalline nature of the films can be observed, with grains in the nanometer scale: < 10 nm diameter for p+-NiOx (bottom) and ~ 50 nm diameter for n-NiOx (top).

To further confirm the

nanostructured nature of each layer, the diffraction pattern of the homojunction was obtained (Figure 7c), which confirms that no layer is amorphous, in agreement with


Page 15 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


XRD patterns shown in Figure 4. Energy Dispersive X-Ray Spectroscopy (EDS) scan line non-quantitative analysis was obtained to demonstrate the cleanness of the interfaces, with no other elements detected besides nickel and oxygen (Figure 7d). The Ni:O ratio (Figure 7e) was obtained to confirm the XPS results for each film: p+-NiOx has a lower Ni:O ratio, due to higher

, and this ratio increases from

bottom to top as the films go from p+- to p- to n- type, indicating

for the n-type

layer. EDS scan line analysis also allows to separate between p- and n- NiOx, something that is not possible just by the TEM cross section image, further proving that the interface is clean and the transition between layers is smooth.

3. Conclusions

In conclusion, simple magnetron sputtering of a metallic nickel target and a reactive atmosphere were demonstrated to control carrier concentration and carrier type of NiOx films. The reported method results in p- and n-type NiOx by simply controlling the oxidation during deposition. This method allows intrinsic films with very low carrier concentration (high resistivity) and higher Eg for the region between 20-30 % O2, and more conductive films, with carrier concentration as high as ~1018 or ~1019 cm-3 for n- or p-type respectively. The n-type behavior of the films is proven due to the resistivity dependence 15 ACS Paragon Plus Environment


Page 16 of 31

on temperature, as the resistivity increases while temperature decreases, linked to semiconductor behavior. The transition from n- to p- type behavior of the films occurs in the region between 25 – 30 % O2. The mobility of the films, regardless of their composition or oxygen flow during deposition, remains low due to their low crystallinity. Carrier concentration is in agreement with the band diagrams obtained for p-type films and the Ni:O ratio calculated from the XPS spectra. N-type films show an oxygen deficiency with Ni:O ratio as high as 1.05, while p-type films show

and a ratio as low as 0.96. Being

able to obtain both p- and n-type NiOx films enables their use in bipolar devices like pn homojunctions, deposited in-situ. A p+pn homojunction with an on/off ratio of 2 orders of magnitude was demonstrated, that further indicates devices worthy p- and n- NiOx films. TEM and STEM analysis were performed, showing nanostructured polycrystalline films with clean interfaces which are difficult to distinguish just through TEM cross section images. EDS scan line analysis supports XPS results showing that the p+- and p- layers are oxygen rich while the n- layer is more nickel rich. This indicates that the transition between p- and n- behavior is smooth and controllable while in-situ depositions provide that no further contamination from other elements is detected.

4. Experimental section

Thin film deposition: Thin films of NiOx (~ 140 nm) were deposited on glass and ITO substrates by magnetron sputtering in an AJA Orion 8 system, using a dc source and a 99.99% Ni metallic target acquired from Kurt J. Lesker. Prior to deposition, the substrates were cleaned in ultrasonic bath using acetone, isopropanol and deionized water in that


Page 17 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


order, and dried out using nitrogen. The sputtering chamber was evacuated to a background pressure of 4.0 eV). IE was measured by PESA using a Riken Keiki AC-2 Photoelectron Spectrometer with 100 nW deuterium lamp power with a step of 0.05 eV. TEM cross-sectional samples of the NiOx homojunction were prepared on a FEI Nova 200 dual-beam FIB/SEM, by using the lift-out method. To protect the region of interest during focused ion beam milling, SiO2 and Pt layers were deposited on top of the sample. 18 ACS Paragon Plus Environment

Page 19 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Atomic STEM-high angle annular dark field (HAADF) images were obtained in a JEOL ARM200F microscope equipped with a spherical aberration (Cs) corrector (CEOS GmbH, Heidelberg, Germany) and operated at 200 kV. The corrector was carefully tuned using the Zemlin-tableau method with Cs¼0.5 lm, and the resolution was demonstrated to be around 1A °. Line scan experiments were performed by scanning the sub nanometer electron probe at 0.5-1.0 nm steps, with an acquisition time of 1.0 sec/pixel. Elemental profiles were obtained by Energy Dispersive X-Ray Spectroscopy (EDS), which was performed using an Aztec Energy Advanced Microanalysis System with an X-MaxN 100N TLE Windowless 100mm2 analytical silicon drift detector. The p+pn homojunction devices current density versus voltage (J-V) curves were obtained using a probe station Cascade SUMMIT 11741B-HT, Keithley 4200 and HP 4280A.



Page 20 of 31

Figure 1. Deposition rate for NiOx films as function of oxygen percentage (% O2). As the oxygen percentage increases the deposition rate decreases.


Page 21 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 2. a) Image of films appearance. Films with higher resistivity are clear, while more conductive films are darker. b) Resistivity, carrier concentration and mobility values for NiOx films deposited with different O2 percentages. Films deposited with 25% or less O2 flow are n-type while films deposited with 30 % or more O2 are p-type.



Page 22 of 31

Figure 3. a) Resistivity vs temperature plot from 200 to 350 K and b) Arrhenius plot for ptype NiOx film deposited with 50 % O2 flow and n-type NiOx film deposited with 5 % O2 flow. Resistivity increases as temperature decreases, mainly due to carriers freezing out at low temperatures. Activation energy was calculated as 0.26 eV and 0.35 eV for p- and ntype films respectively.


Page 23 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 4. a) SEM images, b) XRD, c) transmittance spectra and d) Eg values for NiOx films deposited at 200 °C, varying the percentage of oxygen during deposition. From XRD patterns and SEM images, it can be seen that less oxygen leads to more crystalline films; however, isolated structures can be seen at lower magnifications for films deposited between 15 and 30 % of O2. Film deposited with 5 % O2 shows lower transmittance and Eg due to high metallic behavior and high oxygen vacancies. e) Band diagram for p-type samples with low resistivity, showing the movement of the Fermi level closer to the valence band as the carrier concentration increases.



Page 24 of 31

Figure 5. a) Ni 2p3/2 region and b) O 1s region for XPS spectra of films deposited using different oxygen flow and c) their Ni:O ratio obtained from the XPS. Films deposited using low oxygen percentage have a Ni:O ratio of 1.05, which is related to oxygen vacancies and n-type behavior.


Page 25 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 6. a) Proposed structure and b) band diagram for fabricated NiOx homojunction. c) SEM cross section for the device without top contacts; the interfaces between layers are clean and difficult to differentiate. d) NiOx homojunction J-V plot. Devices have an on/off ratio of ~2 orders of magnitude. Transmittance for ITO layer and full device without top contacts (inset), the low transmittance is mostly due to the n-type NiOx layer.



Page 26 of 31

Figure 7. a) TEM and b) HAADF cross section images of the NiOx homojunction. Interfaces between layers are clean and difficult to differentiate, especially between n- and p-type layers (top two). Grains are small, from < 10 nm on the bottom (p+- NiOx) to ~ 50 nm on top (n- NiOx). The polycrystalline nature of the films is supported by c) the homojunction diffraction pattern. d) EDS scan line from the top of the homojunction to the bottom. As can be seen, there is no other elements detected between layers. e) A nonquantitative Ni:O ratio was calculated from the EDS spectra to observe easier the homojunction three layers and the effect of the oxygen partial pressure on the films when


Page 27 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


deposited in-situ. As expected the amount of nickel respect to oxygen decreases as the layers go from n-type (top) to p+-type (bottom), in agreement with XPS results. ASSOCIATED CONTENT

Supporting Information. The following files are available free of charge.

J-V curve for ITO/p+-NiOx/p-NiOx/Mg structure to corroborate the effect of the ntype NiOx layer on the homojunction device performance.

AUTHOR INFORMATION

Corresponding Author

Manuel A. Quevedo-Lopez†* †Department

of Materials Science and Engineering, The University of Texas at

Dallas 800 W. Campbell Road, Richardson, Texas, 75080, USA E-mail: [email protected]

Author Contributions



Page 28 of 31

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Funding Sources This work was partially supported by AFOSR grants FA9550-18-1-0019 and NSF/PFI:AIR TT 1701192. MIPM is grateful for a CONACyT PhD fellowship.

ACKNOWLEDGMENT We thank Professor Julia Hsu for letting us use the SKP 5050, KP Technology and the Riken Keiki AC-2 Photoelectron Spectrometer. We thank Gonzalo Velazquez for his help with XPS analysis.

REFERENCES (1) Hata, M.; Tanaka, K; Watanabe, T.; Noma, M.; Tagawa, A.; Yoshida, K.; Yamamoto, K.; Nagata, K.; Miyazaki, S.; Kitagawa, K. Development of Middle Size Full In‐Cell LCD Module for PC with IGZO. J. Soc. Inf. Disp. 2018, 26, 229-236. (2) Grundmann, M.; Klüpfel, F.; Karsthof, R.; Schlupp, P.; Schein, F.-L.; Splith, D.; Yang, C.; Bitter S.; von Wenckstern, H. Oxide Bipolar Electronics: Materials, Devices and Circuits. J. Phys. D: Appl. Phys. 2016, 49, 213001-213025. (3) Guzmán-Caballero, D. E; Quevedo-López, M. A.; De la Cruz, W.; Ramírez-Bon, R. Fully Patterned p-Channel SnO TFTs Using Transparent Al2O3 Gate Insulator and ITO as Source and Drain Contacts. Semicond. Sci. Technol. 2018, 33, 035010-035017. (4) Xu, X.; Liu, Z.; Zuo, Z.; Zhang, M.; Zhao, Z.; Shen, Y.; Zhou, H.; Chen, Q.; Yang, Y.; Wang, M. Hole Selective NiO Contact for Efficient Perovskite Solar Cells with Carbon Electrode. Nano Lett. 2015, 15, 2402-2408. 28 ACS Paragon Plus Environment

Page 29 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(5) Park, J. H.; Seo, J.; Park, S.; Shin, S. S.; Kim, Y. C.; Jeon, N. J.; Shin, H.-W.; Ahn, T. K.; Noh, J. H.; Yoon, S. C.; Hwang, C. S.; Seok, S. I. Efficient CH3NH3PbI3 Perovskite Solar Cells Employing Nanostructured p‐Type NiO Electrode Formed by a Pulsed Laser Deposition. Adv. Mater. 2015, 27, 4013-4019. (6) Gupta, R.; Ghosh, K.; Kahol, P. Fabrication and Characterization of NiO/ZnO p–n Junctions by Pulsed Laser Deposition. Phys. E. 2009, 41, 617-620. (7) Ohta, H.; Kamiya, M.; Kamiya, T.; Hirano, M.; Hosono, H. UV-detector Based on pnHeterojunction Diode Composed of Transparent Oxide Semiconductors, p-NiO/n-ZnO. Thin Solid Films. 2003, 445, 317-321. (8) Pintor-Monroy, M. I.; Barrera, D.; Murillo-Borjas, B. L.; Ochoa-Estrella, F. J.; Hsu, J. W.; Quevedo-Lopez, M. A. Tunable Electrical and Optical Properties of Nickel Oxide (NiOx) Thin Films for Fully Transparent NiOx–Ga2O3 p–n Junction Diodes. ACS Appl. Mater. Interfaces. 2018, 10, 38159-38165. (9) Chen, H. L.; Lu, Y. M.; Hwang, W. S. Characterization of Sputtered NiO Thin Films. Surf. Coat. Technol. 2005, 198, 138-142. (10) Reddy, Y.; Reddy, A. M.; Reddy, A. S.; Reddy, P. S. Preparation and Characterization of NiO Thin Films by DC Reactive Magnetron Sputtering. J. Nano- Electron. Phys. 2012, 4, 04002-04005. (11) Li, M. H.; Shen, P. S.; Wang, K.C., Guo, T. F., Chen, P. Inorganic p-Type Contact Materials for Perovskite-Based Solar Cells. J. Mater. Chem. A. 2015, 3, 9011-9019. (12) Liu, A.; Liu, G.; Zhu, H.; Shin, B.; Fortunato, E.; Martins, R.; Shan, F. Hole Mobility Modulation of Solution-Processed Nickel Oxide Thin-Film Transistor Based on High-k Dielectric. Appl. Phys. Lett. 2016, 108, 233506-233510. (13) Sato, H.; Minami, T.; Takata, S.; Yamada, T. Transparent Conducting p-Type NiO Thin Films Prepared by Magnetron Sputtering. Thin Solid Films. 1993, 236, 27-31. (14) Martínez-Gil, M.; Pintor-Monroy, M.I.; Cota-Leal, M.; Cabrera-German, D.; GarzonFontecha, A.; Quevedo-López, M.A.; Sotelo-Lerma, M. Influence of Annealing Temperature on Nickel Oxide Thin Films Grown by Chemical Bath Deposition. Mater. Sci. Semicond. Process, 2017, 72, 37-45. (15) Lin, T.; Li, X.; Jang, J. P-Channel Oxide Thin Film Transistors Using Sol-Gel Solution Processed Nickel Oxide. AM-FPD. 2015, 129-130. (16) Wu, M. S.; Yang, C. H.; Wang, M. J. Morphological and Structural Studies of Nanoporous Nickel Oxide Films Fabricated by Anodic Electrochemical Deposition Techniques. Electrochim. Acta. 2008, 54, 155-161. (17) Hotový, I.; Buc, D.; Haščík, Š.; Nennewitz, O. Characterization of NiO Thin Films Deposited by Reactive Sputtering. Vacuum. 1998, 50, 41-44. (18) Kappertz, O.; Drese, R.; Wuttig, M. Correlation Between Structure, Stress and Deposition Parameters in Direct Current Sputterd Zinc Oxide Films. J. Vac. Sci. Technol. A. 2002, 20, 2084-2095. (19) Howson, R. P. The Reactive Sputtering of Oxides and Nitrides. Pure Appl. Chem. 1994, 66, 1311-1318. (20) Mushtaq, S.; Pickering, J. C.; Steers, E. B.; Horvath, P.; Whitby J.A.; Michler, J. The Role of Oxygen in Analytical Glow Discharges: GD-OES and GD-ToF-MS Studies. J. Anal. At. Spectrom. 2011, 26, 1746-1755. (21) Chen, H. L.; Yang, Y. S. Effect of Crystallographic Orientations on Electrical Properties of Sputter-Deposited Nickel Oxide Thin Films. Thin Solid Films. 2008, 516, 5590-5596. 29 ACS Paragon Plus Environment


Page 30 of 31

(22) Molaei, R.; Bayati, R.; Narayan, J. Crystallographic Characteristics and p-Type to nType Transition in Epitaxial NiO Thin Film. Cryst. Growth Des. 2013, 13, 5459-5465. (23) Tyagi, M.; Tomar, M.; Gupta, V. Postdeposition Annealing of NiOx Thin Films: A Transition from n-Type to p-Type Conductivity for Short Wave Length Optoelectronic Devices. J. Mater. Res. 2013, 28, 723-732. (24) Gupta, P.; Dutta, T.; Mal, S.; Narayan, J. Controlled p-Type to n-Type Conductivity Transformation in NiO Thin Films by Ultraviolet-Laser Irradiation. J. Appl. Phys. 2012, 111, 013706-013712. (25) Stamataki, M.; Tsamakis, D.; Brilis, N.; Fasaki, I.; Giannoudakos, A.; Kompitsas. M. Hydrogen Gas Sensors Based on PLD Grown NiO Thin Film Structures. Phys. Status Solidi A. 2008, 205, 2064-2068. (26) Slack, G. A.; Schowalter, L. J.; Morelli, D.; Freitas Jr, J. A. Some Effects of Oxygen Impurities on AlN and GaN. J. Cryst. Growth. 2002, 246, 287-298. (27) Weber, D. Oxygen Transport in Thin Oxide Films at High Field Strength. Forschungszentrum Jülich. 2014. (28) Kasap, S. O. Principles of Electronic Materials and Devices Vol. 2. McGraw-Hill New York. 2006. (29) Zhai, P.; Yi, Q.; Jian, J.; Wang, H.; Song, P.; Dong, C.; Lu, X.; Sun, Y.; Zhao, J.; Dai, X.; Lou, Y.; Yang, H.; Zou, G. Transparent p-Type Epitaxial Thin Films of Nickel Oxide. Chem. Commun. 2014, 50, 1854-1856. (30) Widjonarko, N. E.; Ratcliff, E. L.; Perkins, C. L.; Sigdel, A. K.; Zakutayev, A.; Ndione, P. F.; Gillaspie, D. T.; Ginley, D. S.; Olson, D. C.; Berry, J. J. Sputtered Nickel Oxide Thin Film for Efficient Hole Transport Layer in Polymer–Fullerene BulkHeterojunction Organic Solar Cell. Thin Solid Films. 2012, 520, 3813-3818. (31) Steirer, K. X.; Chesin, J. P.; Widjonarko, N. E.; Berry, J. J.; Miedaner, A.; Ginley, D. S.; Olson, D. C. Solution Deposited NiO Thin-Films as Hole Transport Layers in Organic Photovoltaics. Org. Electron. 2010, 11, 1414-1418. (32) Mansour, A. Characterization of NiO by XPS. Surf. Sci. Spectra. 1994, 3, 231-238. (33) Xiao, Z.; Liu, Y.; Zhang, J.; Zhao, D.; Lu, Y.; Shen, D.; Fan, X. Electrical and Structural Properties of p-Type ZnO:N Thin Films Prepared by Plasma Enhanced Chemical Vapour Deposition. Semicond. Sci. Technol. 2005, 20, 796-800. (34) Oswald, S.; Brückner, W. XPS Depth Profile Analysis of Non‐Stoichiometric NiO Films. Surf. Interface Anal. 2004, 36, 17-22. (35) Grilli, M.; Menchini, F.; Dikonimos, T.; Nunziante, P.; Pilloni, L.; Yilmaz, M.; Piegari, A.; Mittiga, A. Effect of Growth Parameters on the Properties of RF-Sputtered Highly Conductive and Transparent p-Type NiOx Films. Semicond. Sci. Technol. 2016. 31, 055016-055025. (36) Grosvenor, A. P.; Biesinger, M. C.; Smart, R. S. C.; McIntyre, N. S. New Interpretations of XPS Spectra of Nickel Metal and Oxides. Surf. Sci. 2006, 600, 17711779. (37) Hotovy, I.; Huran, J.; Siciliano, P.; Capone, S.; Spiess, L.; Rehacek, V. The Influences of Preparation Parameters on NiO Thin Film Properties for Gas-Sensing Application. Sens. Actuators, B. 2001, 78, 126-132. (38) Li, H.; Chen, T.; Hu, S.; Li, X.; Liu, Y.; Lee, P.; Wang, X. P.; Li, H. Y.; Lo, G. Q. Highly Spectrum-Selective Ultraviolet Photodetector Based on p-NiO/n-IGZO Thin Film Heterojunction Structure. Opt. Express. 2015, 23, 27683-27689. 30 ACS Paragon Plus Environment

Page 31 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(39) Ohta, H.; Hirano, M.; Nakahara, K.; Maruta, H.; Tanabe, T.; Kamiya, M.; Kamiya, T.; Hosono, H. Fabrication and Photoresponse of a pn-Heterojunction Diode Composed of Transparent Oxide Semiconductors, p-NiO and n-ZnO. Appl. Phys. Lett. 2003, 83, 10291031. (40) Kokubun, Y.; Kubo, S.; Nakagomi, S. All-Oxide p–n Heterojunction Diodes Comprising p-Type NiO and n-Type β-Ga2O3. Appl. Phys. 2016, 9, 091101-091103. (41) Tyagi, M.; Tomar, M.; Gupta, V. PN Junction of NiO Thin Film for Photonic Devices. IEEE Electron Device Lett. 2013, 34, 81-83. (42) Sun, H.; Liao, M. H.; Chen, S. C.; Li, Z. Y.; Lin, P. C.; Song, S. M. Synthesis and Characterization of n-Type NiO: Al Thin Films for Fabrication of pn NiO Homojunctions. J. Phys. D: Appl. Phys. 2018, 51, 105109-1051114. For Table of Contents only


Controlling Carrier Type and Concentration in NiO Films To Enable in

Recommend Documents