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Investigation of the Changes in Electronic Properties of Nickel Oxide (NiO) due to UV/Ozone Treatment x

Raisul Islam, Gang Chen, Pranav Ramesh, Junkyo Suh, Nobi Fuchigami, Donovan Lee, Karl A. Littau, Kurt Weiner, Reuben T. Collins, and Krishna C. Saraswat ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 27 Apr 2017 Downloaded from http://pubs.acs.org on April 28, 2017

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ACS Applied Materials & Interfaces

Investigation of the Changes in Electronic Properties of Nickel Oxide (NiOx ) due to UV/Ozone Treatment

Raisul Islam,∗ † Gang Chen,‡ Pranav Ramesh,† Junkyo Suh,† Nobi Fuchigami,¶ ,

Donovan Lee,¶ Karl A. Littau,¶ Kurt Weiner,¶ Reuben T. Collins,‡ and Krishna C. Saraswat∗ † ,

†Department

of Electrical Engineering, Stanford University, 420 Via Palou Mall, Stanford, CA 94305, USA ‡Department of Physics, Colorado School of Mines, Meyer Hall 466, Golden, CO 80401, USA ¶Intermolecular Inc., 3011 N 1st St., San Jose, CA 95134, USA E-mail: [email protected]; [email protected]

Phone: +1(408)-708-6007

)>IJH=?J Drastic reduction in nickel oxide (NiOx ) lm resistivity and ionization potential is observed when subjected to ultraviolet (UV)/ ozone (O3 ) treatment. X-ray photoemission spectroscopy suggests that UV/O3 treatment changes the lm stoichiometry by introducing Ni vacancy defects. Oxygen-rich NiOx having Ni vacancy defects behaves as a p type semiconductor. Therefore, in this work, a simple and eective technique to introduce doping in NiOx is shown. Angle resolved XPS reveals that the 1

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eect of UV/O3 treatment does not only alter the lm surface property, but also introduces oxygen-rich stoichiometry throughout the depth of the lm. Finally, simple metal/interlayer/semiconductor (MIS) contacts are fabricated on p type Si using NiOx as the interlayer and dierent metals. Signicant barrier height reduction is observed with respect to the control sample following UV/O3 treatment, which is in agreement with the observed reduction in lm resistivity. From an energy band diagram point of view, the introduction of the UV/O3 treatment changes the defect state distribution resulting in a change in the pinning of the Fermi level. Therefore, this work also shows that the Fermi level pinning property of NiOx can be controlled using UV/O3 treatment.

Keywords Nickel Oxide, UV/Ozone Treatment, Barrier Height, XPS, Ionization Potential

1 Introduction Binary metal oxides oer versatile applications in exible electronics, solar photovoltaics, & sensors, ' display technology and memory technology. Transition metal oxides are desirable in these applications because of their transparency, robustness, ability to be deposited on a variety of substrates and varied semiconducting (both p and n) properties. Most of these oxides (e.g. TiO , ZnO, SnO , Cu O, NiO) show semi-insulating properties in perfect stoichiometry. However, a slight deviation from the stoichiometry introduces native defects (e.g. vacancies or interstitials) contributing electrons or holes to the lattice system. For example, formation of oxygen vacancies is energetically favorable in oxides like TiO and ZnO. An oxygen-decient stoichiometry is believed to be the reason for n type semiconducting properties in these oxides. ! On the other hand, NiO and Cu O exhibit the opposite behavior, i.e. these oxides tend to exhibit p type behavior as a result of an oxygen-rich sto2


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ichiometry.

13

In NiO, oxygen-rich stoichiometry results in the formation of nickel vacancies,

not the oxygen interstitials. site, two Ni

2+

14

In order to maintain charge neutrality near the Ni vacancy

ions lose an extra electron each to form Ni

2+

. Thus a quasi-localized pair of

holes is generated for each Ni vacancy causing p type behavior.

15,16

Controlling Ni vacancy defects in NiO lattice would enable us to control its resistivity and transport properties, which can be useful in various applications.

For example, in

resistive-RAM (RRAM), this can improve the switching characteristics of the device.

17

Also,

the formation of Ni vacancy defects can introduce localized states near the valance band and lower the hopping transport barrier for holes resulting in an increase in hole mobility - a phenomenon desired in exible substrate thin lm transistors (pMOS). Nickel oxide is also a potential candidate for p type carrier selective contacts with Si. The high conduction band oset, low valence band oset, and depinning of the Fermi level with Si, which have been demonstrated, tron transport.

18

can provide a contact that allows hole transport while blocking elec-

Recently, reports of band engineered metal oxides as selective contacts to

heterojunction Si solar cell have created an eective way to design future generation solar cells at lower cost and lower thermal budget.

28

In this application, controlling the resistivity

of the oxide selective contact is very important to optimize the device design.

It has been

shown by simulation that higher doping density in the oxides in heterojunction solar cell can eectively compensate for negative eects of poor interface passivation. ozone (

O3 )

19

Ultraviolet (UV)/

treatment has been used to modify the surface properties of thin lms.

cially Wang

et al.

demonstrated that UV/

O3

2022

Spe-

treatment to nickel acetate anode buer layer

can signicantly improve the open circuit voltage in polymer solar cells.

20

In this work, we have demonstrated that post deposition UV/ozone treatment can lower the resistivity of solution deposited NiO x . We have also observed the reduction in resistivity for NiOx lms deposited in dierent methods like atomic layer deposition (ALD), sputtering, and evaporation. Besides resistivity, ionization potential increases, which makes it more suitable as a hole selective contact to Si.

Detailed study using X-ray photoemission spec-

3



0.0377


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2 Experimental Section 2.1 Film Deposition

NiON lm was deposited using spin coating technique where a solution of Nickel Formate Dihydrate (150 mg), Ethylene Glycol (3 ml) and Ethylene Diamene (150 μl) was made and stirred at 50 ◦C - 60 ◦C overnight. After that the solution was ltered with 0.2 μm Nylon lter before spin casting it at 4000 rpm for 60 s on the substrate. Glass substrates were used for resistivity characterization. They were sonicated in acetone and isopropyl alcohol (IPA) for 15 mins each followed by a UV/ozone cleaning for 15 mins. Finally the sample was annealed at 300 ◦C for 60 min. Thicker layers were built up by repeating the spin coating procedure following the anneal. Each coating procedure increased the NiO N thickness by roughly 10 nm. All the steps were done in ambient condition. Surface morphology of the lm before and after UV/ozone treatment was obtained using atomic force microscopy (AFM) (see supplementary section gure S-5). The AFM images show that the surface morphology does not change signicantly with UV/ozone treatment. The rms roughness of the lm before and after UV/ozone treatment is 1.216 nm and 1.366 nm respectively. 2.2 Resistivity Measurement

A Van-Der-Pauw structure was made for 4 point probe resistivity measurement. In this technique, a small square sample was prepared by spin coating 40 nm NiO N on glass substrate with 100 nm gold contacts evaporated at each corner of the square sample using a shadow mask. 2.3 UV/Ozone Treatment 4 UV/ozone treatment was performed using a Jelight Inc. UVO-Cleaner system (model 42). This tool is capable of performing a photosensitized oxidation process, where atomic oxygen and ozone is generated when molecular oxygen is dissociated by the UV light (184.9 nm

5



and 253.7 nm wavelength) generated from a mercury vapor lamp. The 184.9 nm UV line decomposes oxygen molecules and synthesizes ozone according to the following reaction, 184.9 nm

2O2 −−−−−→ O3 + O∗ The 253.7 nm UV line decomposes ozone and produces high energy O∗ (activated oxygen) according to the following reaction, 253.7 nm

O3 −−−−−→ O2 + O∗ The end result of this process is to create highly activated atomic oxygen that reacts with NiOx to create Ni vacancy in the lm. Additionally, UV/ozone treatment removes the organic contaminants from the surface of the lm. Detailed mechanism of this process can be found in literature. 24

2.4 MIS Contact Fabrication Figure 1 shows the schematic of the MIS device used to extract the barrier height for holes at the Si/NiOx interface. A lightly doped p-type Si wafer (0.1-0.9 Ω-cm resistivity) was RCA cleaned with a nal dilute (2%) HF dip to remove any oxide produced during cleaning. It was then placed into the oxidation furnace to grow 300 nm SiO 2 using a wet oxidation process. This provides passivation of the Si surface. An open area in the SiO 2 was then dened using photolithography and a wet etch in a 2% HF solution. After removing the photoresist in acetone and plasma ashing, a very thin layer of NiO x (5 nm) was deposited by atomic layer deposition (ALD) technique. ALD deposition of NiO x was used in this experiment in order to control the thickness of the material precisely, since barrier height is sensitive to interlayer thickness. This technique used alternating pulses of Ni amidinate (Bis(N,N'-di-t-butylacetamidinato) nickel(II)) precursor and ozone in an Intermolecular Inc. Tempus A-30 ALD system to deposit very conformal and uniform (non-uniformity of < 6


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2% over a 300 mm wafer) lms. The precursor temperature was 120 ◦ C and the substrate temperature was 200 ◦ C. In this recipe, the deposition rate was 0.75 - 0.85 Å/cycle. Grazing Incidence X-ray Diraction result suggests that the lm is poly-crystalline with a cubic lattice (See supplementary information (Figure S3)). A 30 nm metal contact (Al, Cu or Pd) followed by a 50 nm Pt capping layer was deposited next using e-beam evaporation. The main function of the Pt capping layer is to protect the contacting metal layer from the environmental degradation. Since the I-V characteristics from the three devices containing metal contact having dierent workfunction is being compared, it is important that the top surface of the metal contact is protected and the surface remains pristine. We deposited the Pt capping layer in the same chamber as the contacting metal without breaking the vacuum. This ensures that the workfunction of the contacting metal remains protected from the environmental degradation. Finally, a second photolithography-patterning step followed by dry etching of metals and underlying NiO N using an ion-milling process isolated each MIS structure by removing the metals and NiO N from the SiO passivation layer. The UV/ozone treatment (30 min) was done immediately after the NiO N deposition and before the metal deposition step. We also fabricated control samples where no UV/ozone treatment was performed.

2.5 Characterization A Hall measurement system from MMR Technologies Inc. (model H50) was used in the Van-Der-Pauw mode to determine the room temperature conductivity and mobility. Photoelectron Spectroscopy in Air (PESA) was used to measure the ionization potential of the NiON lms. PESA is a spectroscopic (ultraviolet photoemission spectroscopy (UPS)) technique, which counts the photoemission electrons in air instead of ultra high vacuum. can reveal the ionization potential of the material and is very surface sensitive.

$

#

It

XPS was

performed on a PHI Versaprobe system, which uses Al (K α) radiation (1486 eV). A cascade microtech probe station was used at dierent temperatures ranging from -40 ◦ C to 120 7


◦

C


12

Resistivity ( -cm)

10

5.30

102 101

5.25 5.20 0 10 20 30 40 50 60

As Deposited UV/O3 10 min UV/O3 60 min

10

Yield1/3 (cps1/3)

3

Ionization Potential (eV)

5.35

8

5.25 eV

6

5.20 eV

4 2

5.33 eV

0 4.2 4.5 4.8 5.1 5.4 5.7 Energy (eV)

UV/O3 Treatment Duration (min) =

>

Figure 2: (a) Film resistivity and ionization potential as a function of UV/ozone treatment duration. 0 min signies the as-deposited lm. (b) PESA measurement showing the valence band spectra of NiOx , which was used to extract the ionization potential.

855.8 eV

1.0 0.8 0.6 865

860

1.6

855

Binding Energy (eV)

0.8 850

540

535

As Deposited UV/O

10 min

UV/O

60 min

3

530

Binding Energy (eV)

=

1.4

525

1.2

3

1.0 0.8 0.6 0.4

292

288

C1s

284.8 eV

2.4

O1s

As Deposited UV/O3 10 min UV/O3 60 min

286 eV

60 min

3

3.2

288.5 eV

UV/O

3/2

4

10 min

3

arb. unit (x 10 )

UV/O

arb. unit (x 104)

1.2

Ni 2p

As Deposited

854.5 eV

4

1.4

531.7 eV 529.9 eV

1.6 arb. unit (x 10 )

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 8 of 23

284

280

Binding Energy (eV)

>

?

Figure 3: X-ray photoelectron spectroscopy elemental scan of NiO x for (a) Ni 2p3/2 (b) O 1s and (c) C 1s for dierent UV/ozone treatment condition. controlled by the temptronic temperature controller to measure the I-V characteristics of the MIS devices, which were used to extract the barrier height for hole transport between Si and the contacts.

3 Results and discussion The resistivity of a solution synthesized, as-deposited NiO x lm is around

103 Ω-cm

(Fig-

ure 2a). When the lm is treated with UV/ozone for 10 minutes, the resistivity drops more 8


0.6

531.7 eV 529.9 eV

0.4 0.2 0.0 545

540

535

530

Binding Energy (eV) =

525

0.8

30 degree 45 degree 60 degree

1.0

0.6 0.4 0.2 0.0 545

540

535

530

Binding Energy (eV)

525

>

0.8


0.6 0.4

531.7 eV 529.9 eV

1.0

A. U. (Normalized)

0.8


A. U. (Normalized)

1.0

A.U. (Normalized)

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


531.7 eV 529.9 eV

Page 9 of 23

0.2 0.0 545

540

535

530

Binding Energy (eV)

525

?

Figure 4: Angle resolved XPS spectra of O 1s for (a) as-deposited sample (b) 10 min UV/ozone treated sample and (c) 60 min UV/ozone treated sample for three dierent angles. than one order of magnitude. For a longer UV/ozone treatment of 60 minutes, the resistivity decreases by two orders of magnitude to values near 10 Ω-cm. Figure 2a also shows the ionization potential of NiO x using photoelectron spectroscopy in air (PESA) measurements (Figure 2b). With UV/ozone treatment, the ionization potential of NiO x increases. Higher ionization potential signies better hole injection capability of the material when a junction is formed. Liu et al. also reported improvement in hole injection capability of NiO x with UV/ozone treatment. 23 In this paper, 23 the improvement in hole injection capability is explained by an increase in NiO(OH) species on the surface which is dipolar in nature. Eventually the surface dipole on NiO x is modied resulting in a vacuum level shift at the interface, which improves the hole injection capability. However, the paper concludes that more than 5 min of UV/ozone treatment does not impact the surface of NiO x anymore and longer UV/ozone treatment does not improve the hole injection eciency any further. Our results show that longer than 5 min of UV/ozone treatment has signicant impact on the lm properties (both resistivity and ionization potential). In order to explain our results, we compared the XPS spectra of Ni 2p 3/2 , O 1s and C 1s for three dierent conditions of UV/ozone treatment (0, 10 min, 60 min) in Figure 3. Dierent oxidation states of Ni manifest themselves in both the Ni 2p 3/2 and O 1s spectra. Therefore both the spectra are used to identify Ni 2+ and Ni3+ states. Figures 3a and 3b both reveal that with increasing UV/ozone treatment duration the peaks associated with 9



Page 10 of 23

the Ni3+ oxidation state (855.8 eV for Ni 2p 3/2 and 531.7 eV for O 1s) increase with respect to peaks associated with the Ni 2+ (854.5 eV for Ni 2p 3/2 and 529.9 eV for O 1s) oxidation state. 2729 It is known that the Ni 3+ oxidation state arises either from NiO(OH) species or from Ni vacancies inside the lattice. 2729 Therefore, with UV/ozone treatment, the growth in the relative strength of the Ni 3+ state with respect to Ni 2+ state indicates a shift in the thermodynamic equilibrium toward Ni vacancy formation. 30 After the surface of NiO x is saturated with NiO(OH) species, the oxygen rich environment causes the atomic oxygen to change the stoichiometry by introducing Ni vacancies. The relative peak intensity for dierent states of C 1s (284.8 eV for C, 286 eV for C-O and 288.5 eV for C=O) does not show any signicant dierence with UV/ozone treatment (Figure 3c). This carbon probably originates from a brief exposure of the sample to air between UV/ozone treatment and XPS. Sputter depth prole of the lm indicates (see supplementary information (Figure S4)) that there is no carbon residue inside the lm from the organic precursors used in the deposition process. Table 1: Percentage of area under the tted curve of O 1s spectra for NiOx having dierent UV/Ozone treatments at dierent angles of X-ray incidence. 30◦ UV/Ozone Condition

As Deposited

UV/Ozone 10 min

UV/Ozone 60 min

45◦

Binding Energy

60◦

Binding % Area

(eV)

Energy

Binding % Area

(eV)

Energy

% Area

(eV)

Peak Identication

529.28

46.1

529.22

52.79

529.21

56.72

Ni2+

531.11

43.46

531.02

29.74

531.07

27.1

Ni3+

532.67

10.44

532.1

17.46

532.16

16.18

C=O/Si=O

529.13

38.36

529.07

43.54

529.21

48.19

Ni2+

530.93

45.21

530.92

43.73

531.07

38.82

Ni3+

532.05

16.44

532.19

12.72

532.28

12.99

C=O/Si=O

529.57

38.8

529.67

45.47

529.57

43.81

Ni2+

531.25

33.81

531.38

32.04

531.1

30.07

Ni3+

532.85

27.4

533.01

22.49

532.63

26.12

C=O/Si=O

10


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In order to determine if the stoichiometry change was limited to the surface or occurred throughout the lm thickness, we performed angle resolved XPS (AR-XPS) on the same sample for three dierent angles of photon emission with respect to the sample surface. As angle increases, photoelectrons generated within the sample must pass through more material to escape and be collected. Hence, at high angles the XPS signal contains the most information from the depth of the lm and as angle decreases the XPS signal becomes more surface sensitive. Figure 4 shows the AR-XPS spectra of O 1s for three dierent UV/ozone treatment conditions. Since the XPS signal strength varies signicantly for dierent angles, we present the data by normalizing each spectrum to the corresponding highest peak. In the as-deposited sample (Figure 4a) the O 1s peak associated with the presence of Ni (531.7 eV) decreases in intensity with respect to the Ni increases indicating a higher concentration of Ni treatment (Figure 4b), the Ni

3+

3+

2+

3+

related peak (529.9 eV) as angle

near the surface. For a 10 min UV/ozone

related peak intensity remains close to the Ni

2+

related

peak irrespective of the angle. This signies two points. First, UV/ozone treatment does not only aect the surface of the lm but it also introduces Ni vacancies throughout the lm. Secondly, the relative height of Ni

3+

compared to the Ni

2+

is larger at each angle after 10

minutes of UV/ozone treatment compared to the as-deposited sample. However, when the lm is exposed to UV/ozone for even longer time (60 min) (Figure 4c), we observe that the Ni

3+

related peak appears to be reducing with respect to the peak corresponding to Ni

2+

as

the angle increases. In order to resolve the peaks properly, we t the O 1s peaks for dierent angles with Gaussian curves and calculate the underlying area for each peak. The result is tabulated in Table 1. The plot of the tted peaks are shown in the supplementary section (Figure S1). From this table, we observe that besides the usual Ni

2+

and Ni

3+

peaks, properly tting the

spectra requires including a relatively weaker peak in between 532-533 eV. This peak can be either due to the oxygen bonded with carbon or the oxygen bonded to Si at the interface (Note that the XPS samples were all spin-coated directly on Si substrate instead of glass).

11



292

288

284

Binding Energy (eV)

0.2 0.0

280

292

288

284

Binding Energy (eV)

=

0.6 0.4 0.2 0.0

280

288.5 eV

A. U. (Normalized)

0.4

0.8


292

288

286 eV 284.8 eV

0.0

0.6

1.0

286 eV 284.8 eV

0.2

0.8

288.5 eV

0.4

A. U. (Normalized)

0.6


1.0

286 eV 284.8 eV

0.8

30 degree 45 degree 60 degree 288.5 eV

A. U. (Normalized)

1.0

284

Binding Energy (eV)

>

280

?

Figure 5: Angle resolved XPS spectra of C 1s for (a) as-deposited sample (b) 10 min UV/ozone treated sample and (c) 60 min UV/ozone treated sample for three dierent angles.

855.8 eV 854.5 eV

0.4 0.2 0.0

865

860

855

Binding Energy (eV) =

850

1.0

0.6 0.4 0.2 0.0

865

860

855

Binding Energy (eV)

850

>

0.8


0.6 0.4 0.2 0.0

865

860

855.8 eV 854.5 eV

0.6

0.8


A. U. (Normalized)

1.0

855.8 eV 854.5 eV

0.8


A. U. (Normalized)

1.0

A. U. (Normalized)

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 12 of 23

855

Binding Energy (eV)

850

?

Figure 6: Angle resolved XPS spectra of Ni 2p 3/2 for (a) as-deposited sample (b) 10 min UV/ozone treated sample and (c) 60 min UV/ozone treated sample for three dierent angles. The interesting point here is that after a relatively long UV/ozone treatment (60 min), this peak (between 532-533 eV) area increases signicantly compared to both the as-deposited and 10 min UV/ozone treated samples. However, if we compare the C 1s spectra of these samples in Figures 5a, 5b and 5c, it is clear that the C=O peak (288.5 eV) intensity reduces with respect to the C-C peak (284.8 eV). This is reasonable since the UV/ozone treatment is expected to clean the surface of the sample by removing the organic contaminants. 24 This also suggests that the peak between 532-533 eV in the O 1s spectra is due to the oxygen bonded to Si at the NiOx /Si interface. Nevertheless, our t suggests that Ni 3+ increased related to Ni2+ even if we take 532-533 eV peak into account. Therefore, UV/ozone treatment done for suciently longer duration could alter the thermodynamic equilibrium of the bulk lm creating Ni vacancy. 12


Page 13 of 23

Figure 6 shows the AR-XPS of Ni 2p 3/2 for dierent UV/ozone treatment conditions. If we compare Figures 6a, 6b and 6c together, we nd that irrespective of the angle, Ni 3+ state becomes stronger than Ni 2+ state due to UV/ozone treatment. Also, the spectrum at 30 degree seems to be shifted towards higher binding energy (obtained after peak shift correction with respect to C 1s peak). We conclude that the surface is more oxidized (due to adsorbed species, vacancy defects, higher O/Ni ratio etc.) than deeper inside the lm. This apparent spectral shift reduces from the as-deposited sample to the UV/ozone treated samples, which suggests that UV/ozone treatment starts impacting the surface rst and gradually moves to change the stoichiometry inside the lm.

1.8 1.6

O/Ni Ratio

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


1.4 1.2

UV/O3 60 min UV/O3 10 min

1.0 As Deposited 30 35 40 45 50 55 60

Angle (degree)

Figure 7: NiOx lm stoichiometry extracted from XPS spectra as a function of the angle of XPS. Three dierent UV/ozone treatment conditions are shown. The dotted lines are a guide to the eye. From Figures 4, 5 and 6 we can extract the lm stoichiometry (O/Ni ratio) as a function of angle for dierent UV/ozone treatment conditions (See supplementary information for details of lm stoichiometry extraction (Figure S2)). The lm stoichiometry is shown in Figure 7. The as-deposited lm is close to stoichiometric. The higher relative oxygen content at 30 degree is because of the defects at the surface. After UV/ozone treatment of 10 min, 13



30 min UV/O

-12

3

-3

10

ln(J0/T2)

-16 2.0 V

-4

10

-20

1.5 V 1.0 V

-5

10

0.5 V

0

@ 20 C

-2

0.06 V

-1 0 1 Voltage (V)

2

-24

(c)

-2

10

-3

10

-5

10

2.5 3.0 3.5 4.0 4.5

@ 20 C 0

-2

-1

1000/T (K )

-1

0

1

Voltage (V)

2

-12

-20

10

Pd/NiO /Si

10

2

0

-12

3

-3

10

-16 2.0 V 1.5 V 1.0 V

-4

10

0.5 V 0.06 V

0

@ 20 C

-2

1000/T (K-1)

-24

(h)

30 min UV/O

-5

2.5 3.0 3.5 4.0 4.5

-20

0.5 V No Anneal 0.06 V x

-2

10

-24

2.0 V 1.5 V 1.0 V

ln(J /T )

2

2

-24

-16

2.0 V 1.5 V 1.0 V 0.5 V (f) 0.06 V

-4

10

-20

0

10

-16

3

-1

-1 0 1 Voltage (V)

2

2

No Anneal 0.06 V

-24

(b)

30 min UV/O

0

0.5 V

3

3

(g)

ln(J /T )

10

1.0 V

10

-12

No UV/O

No UV/O 0

-16

Current Density (A/cm )

x

2.0 V 1.5 V 1.0 Cu/NiOx/Si 0.5 VV No Anneal 0.06 V (e) 30 min UV/O3

-1

-12

ln(J /T )

Al/NiO /Si -2

No UV/O3

No UV/O3 30 min UV/O3

(d)

2

-20

1.5 V

10

2

2.0 V

Current Density (A/cm )

ln(J0/T2)

3

-1

0

10

-16

0

30 min UV/O

ln(J /T )

3

3

(a)

-12

No UV/O

No UV/O 0

10

Current Density (A/cm2)

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 14 of 23

-20 -24

(i)

2.5 3.0 3.5 4.0 4.5 -1

1000/T (K )

Figure 8: Current-voltage characteristics of metal/NiO N /p-Si MIS contact (schematic shown in Fig. 1) for (a) Al (d) Cu and (g) Pd. Richardson plots extracted from the I-V characteristics at dierent temperatures are shown at dierent reverse bias voltages for as-deposited NiON [(b) - Al, (e) - Cu, (h) - Pd] and for 30 min UV/ozone treated NiO N [(c) - Al, (f) - Cu, (i) - Pd]. we see a slight increase in O content relative to Ni especially at 45 degree. This change in stoichiometry is consistent with the formation of Ni vacancies. After a 60 min UV/ozone treatment, the O/Ni ratio increases signicantly for all acceptance angles. The overall trend of the curve looks similar to the as deposited one. This conrms that the reduced stoichiometry (and high Ni vacancy concentration) relative to the as-deposited sample extends across the thickness of the lm. Figure 8 shows the current-voltage (I-V) characteristics of the MIS devices (details in experimental section) for dierent metal contacts ((a) Al, (b) Cu and (c) Pd) comparing the I-V between as-deposited and 30 min UV/ozone treated NiO N . Each of these gures provides two additional Richardson plots shown in the smaller panels extracted from the associated I-V plot. & If we assume the eective barrier height for holes to be

φBp ,

then the Schottky

contact equation for a metal-semiconductor junction is, J = A∗ T 2 e −

where

J

is the current density,

A∗

qφBp kT

qV

e− kT − 1

is the Richardson constant,

(1) T

is the temperature in

Kelvin, q is the electronic charge, k is Boltzmannâs constant and V is the applied voltage. The reverse saturation current of the aforementioned junction ( J0 ) can be approximated as, 14


510

(b) Cu

(a) Al

(c) Pd

B

Effective Barrier Height,

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


(meV)

Page 15 of 23

480 No UV/O

3

450 420

UV/O

3

30 min

390 0

20

1

1

20

1

Reverse Bias Voltage (V)

2

Figure 9: Eective barrier height extracted from the slope of the Richardson plots in Figure 8 as a function of reverse bias voltage for dierent metal contacts (a) Al (b) Cu and (c) Pd comparing the as-deposited and 30 min UV/ozone treated NiO N .

J 0 ≈ A∗ T 2 e −

qφBp kT

(2)

From rearranging this equation we get, ln

Therefore, if ln

J 0

T2

J0 T2

≈ ln (−A∗ ) −

qφBp kT

(3)

is plotted as a function of T1 a straight line is generated which is called

the Richardson plot. The slope of this curve gives the eective barrier height. It is referred to as an eective barrier height because this technique of extracting barrier height cannot distinguish between the thermionic emission over the barrier and tunneling through the peak of the barrier. Since the barrier shape and hence tunneling probability changes with bias, the eective barrier height varies with reverse bias voltage. The I-V curves shown in Figure 8 were obtained at 20 ◦ C. Corresponding Richardson plots for 5 dierent reverse bias voltages are shown with each I-V curve for both the as-deposited and 30 min UV/ozone treated NiO N . 15



Figure 10: Energy band diagram of Metal/NiO N /p-Si for (a) as deposited and (b) UV/Ozone treated NiON . Three dierent metals' Fermi level pinning position is shown. UV/Ozone treatment reduces the pinning and increases vacancy defects (not drawn to scale). Al, Cu, Pd metal layers were chosen because they span a wide range of metal workfunctions (Al = 4.05 - 4.26 eV, Cu = 4.53 - 5.10 eV, Pd = 5.22 - 5.6 eV). We can see that with an Al metal contact the I-V curve does not change signicantly after UV/ozone treatment. This dierence between the untreated and the treated I-V curves gradually increases from Al to Cu to Pd. In particular, as the metal workfunction increases so does the reverse saturation current. All the Richardson plots show excellent ts to straight lines, which lends condence that this model is an appropriate way to extract barrier heights. We have extracted barrier heights from the Richardson plots and presented them as a function of the reverse bias voltage in Figure 9. Starting from the zero bias, as voltage increases, the band bending increases and the current becomes completely thermionic emission dominated. ! Therefore initially the barrier height increases sharply. However, increasing reverse bias results in higher tunneling (both trap assisted and direct tunneling). Therefore, the barrier height starts decreasing at higher reverse bias. While UV/ozone treatment has 16


Page 16 of 23

Page 17 of 23 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


almost no eect on devices with Al metal, barrier height decreases with treatment for the other metals, and the greater the workfunction of the metal, the larger the magnitude of the decrease. The results shown in Figure 9 can be explained by taking two factors into consideration. First, UV/ozone treatment aects the surface dipole of NiO N ,

!

which shifts the vacuum level

at the metal/NiON interface resulting in a change in the pinning properties. Similar eects have also been observed for oxygen plasma treatment. ! Secondly, lowering resistivity by creating Ni vacancies introduces defect states near the valence band, # which increases trap assisted tunneling. In order to visualize the eect of these two phenomena, we look into the schematic energy-band diagram of the MIS contact in Figure 10. This gure summarizes all the changes that occur in NiO N due to UV/ozone treatment. We can explain the results in Figure 9c if we consider the increase in trap assisted tunneling and assume the reduction in pinning when NiON is UV/ozone treated for high workfunction metal like Pd. Since Al has much lower workfunction than Pd, the reduction in pinning due to the UV/ozone treatment does not change the eective barrier height signicantly. The defect states due to the Ni vacancies appear near the valence band. # Therefore, for a lower workfunction metal (Al), trap assisted tunneling does not increase signicantly with UV/ozone treatment as well. The as-deposited Cu shows a lower barrier height than both Al and Pd, despite high amount of pinning. This might be because of the formation of an interfacial copper oxide, which increases trap assisted tunneling resulting in a lower eective barrier height.

4 Conclusion We have investigated the eect of UV/ozone treatment on NiO N . Our experimental results suggest that UV/ozone treatment creates non-stoichiometry in NiO N resulting in Ni vacancy defects, which lowers the lm resistivity. Metal/NiO N /p-Si devices show lower Schottky barrier height for holes due to this increase in vacancy defects and a reduction in pinning

17



caused by the change in surface dipole created from the treatment. This study shows an eective way to engineer p type contacts to NiO N and to control the electrical properties of the lm.

Acknowledgement This research is based upon work supported by the Solar Energy Research Institute for India and the U.S. (SERIIUS) funded jointly by the U.S. Department of Energy subcontract DE AC36-08G028308 (Oce of Science, Oce of Basic Energy Sciences, and Energy Efciency and Renewable Energy, Solar Energy Technology Program, with support from the Oce of International Aairs) and the Government of India subcontract IUSSTF/JCERDCSERIIUS/2012 dated 22nd Nov. 2012. RI acknowledges the support from Stanford Graduate Fellowship (SGF). RI, PR and KCS acknowledge use of Stanford Nanofabrication Facility. GC and RTC also acknowledge use of facilities of the Renewable Energy Materials Research Science and Engineering Center at the Colorado School of Mines. The authors also acknowledge discussions and technical assistance of P. C. Taylor, Idemudia Airuoyo, and Ryan Clarke.

Supporting Information Available The following les are available free of charge.

• Islam-supporting-info.pdf: This le includes lm characterization (XRD, XPS sputter depth prole, AFM) of NiO N , XPS data tting and the method of extracting O/Ni ratio from XPS spectra.

18 ACS Paragon Plus Environment

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Chem. Mater. , 23,

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