n‒4H–SiC Schottky contacts

Journal Pre-proof Electron and gamma irradiation effects on Al/n‒4H‒SiC Schottky contacts Indudhar Panduranga Vali, Pramoda Kumara Shetty, M.G. Mahesha, V.C. Petwal, Jishnu Dwivedi, D.M. Phase, R.J. Choudhary PII:

S0042-207X(19)32322-X

DOI:

https://doi.org/10.1016/j.vacuum.2019.109068

Reference:

VAC 109068

To appear in:

Vacuum

Received Date: 11 October 2019 Revised Date:

7 November 2019

Accepted Date: 8 November 2019

Please cite this article as: Vali IP, Shetty PK, Mahesha MG, Petwal VC, Dwivedi J, Phase DM, Choudhary RJ, Electron and gamma irradiation effects on Al/n‒4H‒SiC Schottky contacts, Vacuum, https://doi.org/10.1016/j.vacuum.2019.109068. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Elsevier Ltd. All rights reserved.

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Electron and gamma irradiation effects on Al/n‒4H‒SiC Schottky contacts Indudhar Panduranga Vali, 2

1

Pramoda Kumara Shetty,

2

3

1, a)

Petwal, Jishnu Dwivedi, D. M. Phase and R. J. Choudhary

M. G. Mahesha,

1

V. C.

3

1

Department of Physics, Manipal Institute of Technology, Manipal Academy of Higher Education, Manipal 576104, India

2

Industrial accelerator section, Raja Ramanna Centre for Advanced Technology, Indore 452012, India

3

UGC‒DAE Consortium for Scientific Research, University Campus, Khandwa Road, Indore 452017, India

1, a)

Author to whom correspondence should be addressed: [email protected]

Abstract The Al/n‒4H‒SiC Schottky contacts were prepared and studied their contact properties by current‒ voltage ( ‒ ) characteristics and X‒ray photoelectron spectroscopy (XPS) technique. The results have shown localized Fermi‒level pinning (FLP) due to grown‒in defects in n‒4H‒SiC. The electron and gamma irradiation on Schottky contacts have shown zero‒bias offset in the ‒

characteristics.

The XPS studies revealed that the observed behaviour was mainly attributed to irradiation‒induced defects in n‒4H‒SiC bulk and their role in tunneling mechanism rather than contribution from the surface or interface chemical features alone.

Key words: Silicon carbide; Schottky barrier height; Fermi level pinning; irradiation; tunneling

1

1. Introduction Over the last three decades an extensive research on the single crystal growth of SiC have contributed several electronic device applications in the field of high‒power, high‒frequency and high‒temperature environments [1, 2]. Despite significant progress, the quality of SiC wafer is still an outstanding issue due to the presence of grown‒in defects such as stacking faults, different kinds of dislocations, micropipes, point defects and extended defect states [1, 2]. The processing techniques like metallization, heat treatments, irradiation etc. could magnify the influence of these defects and therefore affects the properties and reliability of electronic devices [3]. The Schottky contacts are the primitive devices that are sensitive to the surface, interface and bulk electronic properties of a material [3, 4]. In the past it has been reported the linear relationship between Schottky barrier height (Φ ) and metal work function (Φ ) with slope parameter ( = Φ /Φ ) varying in the range of 0.2 to 0.7 [5]. The variation of

was found to depend on the

polytype and face of SiC (Si‒ or C‒face). Several efforts were made to prepare ideal SiC Schottky contacts as well as to make reliable measurements on Φ by using different techniques. However, most practical contacts have shown deviation from unity ideality factor (η) and Schottky‒Mott relationship, due to which a wide‒range of Φ values have been reported in the early review articles [5‒7]. The primary reasons for such deviations are attributed to presence of surface states [8], partial ionic character of SiC [2] and influence of point or complicated defects near the metal/SiC junction. Among the different polytypes of SiC, 4H‒SiC is most popular due to its practical stability, wide bandgap and minimum anisotropic electron mobility [2]. Although, no strong Fermi level pinning (FLP) was noticed for 4H‒SiC (Si‒face), the deviation from unit slope parameter is still an indicative of partial FLP [5‒7, 9‒11] and/or inhomogeneous nature of the Schottky barrier. An account of influence of defects in the bulk or interface and their modification under processing as well as irradiation environments are crucial in the realisation of Schottky contact devices. The

2

process‒ or irradiation‒induced defects, or modification in the properties of pre‒existing defects near the metal/4H‒SiC interface, their dependence on interface chemical features, and the correlation of their energies with that of Fermi‒level (E ) would play significant role in altering the junction properties [3]. In the past, the electron [12‒14], proton [15‒17] and neutron [16‒19] irradiation effects on different metal/4H‒SiC Schottky contacts have been reported. But irradiation studies on Al/n‒4H‒SiC Schottky contacts are limited. Previously, Harrell et al. [20] have reported the effect of surface treatment on Φ of Al/n‒ 4H‒SiC contacts. Kohlscheen et al. [21] have provided in situ description on the formation of Schottky barrier in Al/n‒4H‒SiC contacts by photoemission spectroscopy (PES) technique. Gorji and Cheong [22] have reported Φ modifications in Al/n‒4H‒SiC Schottky diodes by embedding gold nanoparticles at the interface. Kim and Koo [23] have studied the influence of Al4C3 layer on the electrical properties of Al/n‒4H‒SiC Schottky diode. More recently Li et al. [24] have reported in situ ion and electron irradiation effects on Al/n‒4H‒SiC Schottky contacts. The irradiation studies have shown carrier compensation effects due to the formation of complex defect centres in the material. But in the present study, the effect of high energy electron and gamma irradiation on Al/n‒ 4H‒SiC Schottky contact properties has been reported by analysing

‒

characteristics and

photoelectron spectra. Also, a wide discrepancies in the previously reported Φ values has been discussed by considering several factors. 2. Experimental details The N‒doped 4H‒SiC (n‒4H‒SiC) wafer having <0001> orientation was procured from Semiconductor Wafer, Inc. (Taiwan). The thickness of the wafer was 330 ± 25 was in the range of 0.012 − 0.03 Ω. less than 30

"$

(

≈ 5 × 10 !

"#

and its resistivity

[25]). The density of micropipes was

. The wafer was diced into square pieces of dimension ~1 × 1

and cleaned

according to the procedure as described in Ref. [26]. The Al Schottky contacts on cleaned n‒4H‒SiC

3

were formed by thermal evaporation through a shadow mask of small diameters of 2

. The

deposition was carried out at a rate of 3 Å/' and at a base pressure of ∼ 6 × 10"* +,--. The thickness of the Al Schottky contact was kept at ~10 . monitor (DTM). The ‒

by monitoring through digital thickness

characterisation of the Schottky contacts were carried out by using

Keithley 2450 source meter. The Al pressure contacts were used as back ohmic contacts during the measurements. The electron beam irradiation (EBI) was carried out using electron linear accelerator (LINAC) 10 MeV LINAC located at Raja Ramanna Centre for Advanced Technology (RRCAT), India. The beam energy and beam diameter were kept at 7.5 /0

and 30

respectively. These

irradiation parameters ensure the complete penetration of electrons and approximately uniformity in the irradiation‒induced damage on the sample surface, interface and bulk. The samples were exposed to an irradiation dose of 1500 123 at the dose rate of 6.5 123/'. The similar irradiation conditions were used in our previous EBI studies on Al/n‒Si Schottky contacts [27, 28]. The gamma irradiation (GI) on Schottky contacts was carried out by using 60Co gamma chamber GC‒5000 located at Indira Gandhi Centre for Advanced Research (IGCAR), Kalpakkam India. The

60

Co gamma source emits

gamma photons of two different energies of 1.17 and 1.33 /0 . The strength of the

60

Co source

during irradiation was approximately 2.8 123/ℎ-. The Schottky contacts were exposed to a cumulative radiation dose of 1500 123. The GI experimental details were similar to our previous GI studies on Al/n‒Si Schottky contacts [26] and structural studies on n‒4H‒SiC [29]. X‒ray photoelectron spectroscopy (XPS) technique was used in this study with the aim of analysing irradiation‒induced modification in the surface/interface chemical features of Al/n‒4H‒ SiC Schottky contacts. The spectra were collected by using Omicron energy analyser (EA‒125) with Al‒Kα (1486.6 0 ) as a source of X‒rays operated at 120 7. The pass energy was set to 30 0 for the measurements. The vacuum in the experimental chamber during measurements was 1.3 ×

4

10"8 +,-- (with X‒rays on). The individual C 1s, O 1s and Al 2p core‒level spectra were collected with a step size of 0.02 0 . The analysis area of the sample was of 10

in diameter. The

observed C 1s peak was due to Si‒C bond of the sample present at binding energy of 283.4 0 . The adventitious carbon has not been used for charge referencing [30, 31]. Prior to XPS data acquisition, the samples were cleaned by sputtering 600 0

Ar+ ions for about three minutes to remove any

contamination at the surface. The Ar+ ion beam diameter was 12

. All the spectra were compared

before and after irradiation. To describe different chemical species on the sample surface, the individual peaks in C 1s, O 1s and Al 2p core‒level spectra were fitted by Gaussian peak deconvolution technique. Shirley background corrections were made prior to all core-level spectral analysis. All the given details ensure reliable description of the XPS data as reported by Greczynski and Hultman [31]. 3. Theory and calculations According to thermionic emission (TE) theory, the expression for forward current ( ) through the Schottky barrier (Φ ) when the applied voltage ( ) is given by [4] =

9

:exp >

where,

9

?@

ABC

D − 1E

= AA∗∗ + $ exp >−

(1) ?HI BC

D

(2)

is the reverse saturation current, A is effective area of the diode, A∗∗ is the Richardson constant (for .‒ 4J‒ KL, A∗∗ = 146 M ∙

"$

O "$ ), P is charge of the electron, Φ is Schottky barrier height, 1

is Boltzmann constant and + is absolute temperature. The parameter η in Eq. (1) is known as ideality factor, which accounts for non‒ideal behaviour of a Schottky contact. For

> 31+/P, the term −

9

in Eq. (1) can be neglected. One obtains a linear polynomial equation

of the form:

5

ln I =

?@

ABC

+ ln

(3)

V

Thus by plotting ln vs.

one can obtain Φ and η from the intercept and slope respectively. These

parameters can be evaluated by using the following expressions: Φ = η=

BC ?

]

ln > `@

YY∗∗ C Z [\

D

(4)



(5)

^_ `abc [d

The above procedure for determining Φ and η was developed without considering an account of series resistance R f of the junction. This sometimes leads to erroneous determination. In consideration of R f , TE model Eq. (1) for =

9

:exp >

?a@"[gh ABC

> 31+/P takes the form as [32]:

DE

(6)

Substituting for If from Eq. (2) into Eq. (6) and then differentiating with respect to resulting in the simplified form as: i@

iabc [d

= Rf +

ABC

(7)

?

Therefore the by plotting d ⁄d aln d against

one can determine R f and η from the slope and

intercept respectively. On the other hand, Φ in consideration of R f can be evaluated by plotting following expression: Ha d = R f + ηΦ

(8)

where, Ha d =

−

ABC ?

[

ln >YY∗∗CZ D

Thus by plotting Ha d against

(9) one obtains R f and Φ from the slope and intercept respectively. In

the evaluation of Ha d one must consider η value obtained from d ⁄d aln d vs. plot (Eq. (7)). The 6

R f obtained from the Eqs. (7) and (8) (known as Cheung functions) can be verified to check the consistency in the obtained value of R f [30]. Table 1: Schottky barrier height values of Al/n‒4H‒SiC Schottky contacts (Nn =dopant concentration; Φ =Schottky barrier height). ‒#

Nn (

) [Ref.]

Measurement technique of Φ (eV)

Surface

Vacuum

I‒V

C‒V

PES

preparation

pressure

16

0.60, 0.67

0.39, 0.65

‒

Etching

5×10 Torr

16

0.97, 0.99, 1.01

1.65, 1.72, 1.92

‒

Etching

4×10 Torr

18

0.79

‒

‒

Etching

Not given

18

‒

‒

0.98

No Etching

5×10 mbar

18

0.91

‒

‒

Etching

8×10 mbar

0.89

‒

‒

Etching

4×10 Torr

0.98

0.98

‒

Etching

5×10 Torr

5×10 [20] 5×10 [24] 3×10 [11] 5×10 [21] 5×10 [Present] Not given [22] (ρ=0.021 Ω.cm) 15

1×10 [23]

‒6 ‒5

‒9 ‒6

‒5

‒3

4. Results and discussion 4. 1. Discrepancy in op values of Al/n‒4H‒SiC Schottky contacts Table 1 shows list of Φ values reported by various researchers for Al/n‒4H‒SiC Schottky contacts. In the first two studies [20, 24], despite similar Nn of 10

*

"#

, processing and measurement

techniques, the obtained Φ differs by 0.4 0 . While the greater discrepancy of Φ evaluated from C‒V characteristics suggests different levels of Φ Empirically, for Nn ≈ 10

*

"#

inhomogeneity across the junction [33].

Φ is given by, Φ = 0.70Φ − 1.95 [5]. For Φ = 4.2 0 [28],

one obtains a Φ of 0.99 0 . The similar Φ in the range of 0.97 − 1.01 0 has been reported by Li et al. [24] and Kim and Koo (despite Nn = 10

r

"#

) [23] (Table 1). But a lower Φ of 0.60 or

0.67 0 obtained by Harrell et al. [20] could be caused due to the presence of defects such as Z 8H‒SiC stacking faults in n‒4H‒SiC [34]. For Nn ≈ 5 × 10

!

"#

or

, no empirical relations are

available in literature. But in situ PES studies [20] showed a Φ of 0.98 0 7

/$

for Al thickness of

10 . . In the present study for the same Nn and Al thickness, we obtained a Φ of 0.91 0 . But for Al thickness of 250 .

it has been reported a Φ of 0.89 0 [21]. Small discrepancies among these

values can be attributed to surface treatments, vacuum pressure and Al thickness. But another important feature which go unnoticed for Nn of 5 × 10

!

"#

is the presence of defect level at

~2.35 0 in the energy bandgap of n‒4H‒SiC [29]. These defects are attributed to complex defects involving N‒dopant and a structural defect [35] or 3C‒SiC stacking faults in 4H‒SiC [34, 36]. Subtracting the defect energy level 2.35 0 from the energy bandgap Et of 3.26 0 gives a value of 0.91 0 . This suggests that the Fermi‒level of the n‒4H‒SiC wafer of Nn = 5 × 10 pinned at 0.91 0

!

"#

was

prior to the deposition of Al. Irrespective of different Nn , such defect level has

been noticed in most of the reported studies [34‒36]. The processing treatments, thickness and vacuum conditions can contribute small variation in Φ of ~0.1 0 (Table 1). Thus, in the present study and in other studies one can envisage localized FLP in Al/n‒4H‒SiC Schottky contacts due to the presence of defect level at ~2.35 0 in Et of n‒4H‒SiC. 4. 2. u‒ v characteristics of Al/n‒4H‒SiC Schottky contacts Fig. 1 shows the semi log ‒

characteristics of Al/n‒4H‒SiC Schottky contacts before and after

EBI and GI at the irradiation dose of 1500 123. In the irradiated Schottky contacts, an increase in the forward and reverse currents are the consequences of ionization damage induced by electrons and gamma radiations [37]. An important feature observed in the irradiated Schottky contacts is the zero‒ bias offset and the resemblance of double switch on feature. Previously, the offset behaviour has been observed in Ru and Al/6H‒SiC [38] and Au‒ and Au‒Ge/6H‒SiC Schottky contacts [39]. Both the studies have attributed offset behaviour to the existence of complex defects at the junction. In the present study, the zero‒bias offset is caused due to the influence of irradiation‒induced defects in n‒ 4H‒SiC bulk [29] but not due to the interface chemistry as suggested from the XPS studies (Fig. 2). In the present study such an offset < 0.4

(or asymmetrical barriers) is attributed to tunneling

8

mechanism [40]. Similar bias offset due to tunneling mechanism have been also found in the wide bandgap heterojunctions like nanocrystalline diamond/4H‒SiC [41].

Fig. 1. Semi log ‒

characteristics of Al/n‒4H‒SiC Schottky contacts before and after electron and

gamma irradiation. Inset plot shows the linear voltage region where TE model was applied to determine contact parameters. From TE model (Eq. (7)), the semi log forward ‒

characteristics should be linear.

However, most practical Schottky contacts exhibit non‒linearity behaviour at higher voltages due to inhomogeneous nature of the junction, participation different transport mechanisms and voltage dependence of Φ [4, 33]. Ewing et al. [34] have reported two barrier heights working in parallel to account for double switch on feature in the inhomogeneous Schottky junctions pinned by Fermi‒ level. In this model, the authors have assumed minimum interaction between low and high barrier 9

heights and low barriers are fixed by defects. Such an explanation would be reasonable when there exist sharp switch on feature between low and high barrier heights as well as their ‒

relationships

are governed by TE mechanism. But in the present study, a sharp double switch on feature has not been observed. This suggests the existence of multiple local patches which are interacting with each other due to the influence of localized irradiation‒induced defects. Thus low barriers in the present study cannot be accounted by TE model. Due to greater

or greater electric field or defect levels

encountered in the wide band gap semiconductors [2], one can predict tunneling mechanism at low barriers via defect levels. This is in fact be evident from zero‒bias offset observed in the ‒ characteristics. The phonons could also assist in the tunneling mechanism [42]. On the other hand high barriers are governed by TE process. Therefore, we estimated junction parameters by fitting the linear part of the forward –

characteristics as shown in the inset of Fig. 1. The obtained values are

given in the inset table of Fig. 1. In the irradiated Schottky contacts a decrease in both Φ and y was observed due to irradiation‒induced modifications in the bulk by generating defects [29]. A greater decrease observed in the gamma irradiated Schottky contact could be caused due to prolonged exposure to the gamma radiation as well as the greater oxidation of Al (Fig. 2). 4.3. XPS analysis of Al/n‒4H‒SiC Schottky contacts Fig. 2 (a), (b) and (c) shows the Al 2p, C 1s and O 1s core‒level XPS spectra of Al/n‒4H‒SiC Schottky contacts before and after EBI and GI respectively. The spectra shown in the figures have been corrected by subtraction of a Shirley background. The Gaussian peak deconvolution showed different chemical species on the surface of Schottky contacts. Before irradiation, the Al 2p spectra have shown metallic Al and Al‒O (Al2O3) species at the binding energies 72.9 and 75.6‒ 76.0 0 respectively. C 1s spectra have shown C‒Si (SiC), C‒C and C‒O species at the binding energies of 283.4, 285.0 and 286.5 0 respectively. O 1s spectra have shown Al‒O, oxygen contamination and C‒O peaks at the binding energies of 531.5, 532.6 and 534.2 0 respectively. As noticed, negligible

10

modifications were found in all the core‒level XPS spectra of the electron beam irradiated samples, while that of the gamma irradiated ones have shown chemical shift as well as peak broadening effects. These modifications were attributed to the irradiation effects as well as prolonged exposure of the samples (~22.5 days) to the atmosphere during irradiation. This has resulted in the increase of C‒C, C‒O and Al‒O bonds in the core‒level spectra of the samples. While the observed chemical shift has been attributed to presence of Al‒O‒C (AlxOyCz) states at the binding energies of 74.2 [43], 281.5 and 530.1 0

in the Al 2p, C 1s and O 1s core‒level spectra respectively. Such chemical

species were not observed in the electron beam irradiated samples. However, both electron and gamma irradiated Schottky contacts have shown zero‒bias offset in the ‒ characteristics. This suggests that irradiation‒induced defects in the bulk have significant role in the bias offset behaviour and their participation in tunneling mechanism rather than modification in the surface/interface chemistry.

11

Fig. 2. (a) Al 2p, (b) C 1s and (c) O 1s core‒level XPS spectra of Al/n‒4H‒SiC Schottky contacts before and electron beam and gamma irradiation. 5. Conclusion The prepared Al/n‒4H‒SiC Schottky contacts showed inhomogeneous nature of Φ

as well as

localized FLP due to grown‒in defects in n‒4H‒SiC at ~2.35 0 . But a wide discrepancy in the previously reported Φ for different Nn was mainly attributed to the presence of different grown‒in defect states in n‒4H‒SiC bulk rather than processing techniques. In the electron and gamma irradiated Schottky contacts, zero‒bias offset in the ‒

characteristics or decrease in the junction

parameters has been attributed to the role of irradiation‒induced defects in n‒4H‒SiC bulk and their participation in tunneling mechanism. The XPS studies have revealed no direct influence of surface

12

and interface chemical states on the zero‒bias bias offset behaviour in irradiated Al/n‒4H‒SiC Schottky contacts. Acknowledgments This work has been carried out under UGC DAE CSR Indore, India collaborative research scheme (CSR‒IC‒BL‒48/CRS‒145‒2014‒15/1241). The authors would like to acknowledge Dr. M. T. Jose, Harikrishnan and Shailesh Joshi, RSD, IGCAR Kalpakkam India for extending their support in carrying out gamma irradiation on the samples. Rashmitha Keshav, MAHE, Manipal for assisting in the thermal deposition of Al. Avinash Wadikar and Sharad Karwal, RRCAT Indore, India for carrying out XPS characterisation of the samples. References [1]

R. Madar, Silicon carbide in contention, Nature. 430 (2004) 974–975.

[2]

T. Kimoto, J. A. Cooper, Fundamentals of Silicon Carbide Technology: Growth, Characterization, Devices and Applications, Wiley (2014).

[3]

L.J. Brillson, Contacts for Compound Semiconductors : Schottky Barrier Type, Reference Module in Materials Science and Materials Engineering (2016) 1‒8.

[4]

E. H. Rhoderick and R. H. Williams, Metal‒Semiconductor Contacts, Second Edition, Oxford Science Publication (1988).

[5]

A. Itoh, H. Matsunami, Analysis of Schottky barrier heights of metal/SiC contacts and its possible application to high‒voltage rectifying devices, Phys. Status Solidi Appl. Res. 162 (1997) 389–408.

[6]

L.M. Porter, R.F. Davis, A critical review of ohmic and rectifying contacts for silicon carbide, Mater. Sci. Eng. B. 34 (1995) 83–105.

[7]

V. Saxena, A.J. Steckl, Chapter 3 Building Blocks for SiC Devices: Ohmic Contacts, Schottky Contacts, and p‒n Junctions, 1998; doi:10.1016/S0080‒8784(08)62845‒8.

[8]

M. Wiets, M. Weinelt andT. Fauster, Electronic structure of SiC (0001) surfaces studied by 13

two‒photon photoemission, Phys. Rev. B 68 (2003) 125321. [9]

R. Yakimova, C. Hemmingsson, M.F. Macmillan, T. Yakimov and E. Janzén, Barrier Height Determination for n‒Type 4H‒SiC Schottky Contacts Made Using Various Metals, Journal of Electronic Materials. 27 (1998).

[10] S. Lee, C. Zetterling, M. Östling, Schottky Barrier Height Dependence on the Metal Work Function for p‒type 4H‒Silicon Carbide, 30 (2001) 242–246. [11] S.Y. Han, J.L. Lee, Interpretation of Fermi level pinning on 4H‒SiC using synchrotron photoemission spectroscopy, Appl. Phys. Lett. 84 (2004) 538–540. [12] H. Ohyama, K. Takakura, T. Watanabe, K. Nishiyama, K. Shigaki, T. Kudou, M. Nakabayashi, S. Kuboyama, S. Matsuda, C. Kamezawa, E. Simoen, C. Claey, Radiation Damage of SiC Schottky Diodes by Electron Irradiation, J. Mater. Sci. ‒ Mater. Electron. 16 (2005) 455– 458. [13] K. Çinar, C. Coşkun, E. Gür, Ş. Aydoǧan, Radiation effects on ohmic and Schottky contacts based on 4H and 6H‒SiC, Nucl. Instruments Methods Phys. Res. Sect. B Beam Interact. with Mater. Atoms. 267 (2009) 87–90. [14] V. V. Kozlovski, A. A. Lebedev, M. E. Levinshtein, S. L. Rumyantsev, and J. W. Palmour, Impact of high energy electron irradiation on high voltage Ni/4H‒SiC Schottky diodes, Appl. Phys. Lett. 110, (2017) 083503. [15] V. Kažukauskas, R. Jasiulionis, V. Kalendra, and J.‒V. Vaitkus, Effect of High‒Energy Protons on 4H‒SiC Radiation Detectors, Lith. J. Phys. 45 (2005) 487‒495. [16] S. Sciortinoa, F. Hartjes, S. Lagomarsino, F. Nava, M. Brianzi, V. Cindro, C. Lanzieri, M. Moll, P. Vanni, Effect of heavy proton and neutron irradiations on epitaxial 4H‒SiC Schottky diodes, Nucl. Inst. Meth. Phys. Res. A 552 (2005) 138–145. [17] J. A. Kulisek and T. E. Blue, Neutron and Proton Radiation Damage and Isothermal Annealing of Irradiated SiC Schottky Power Diodes, AIP Conf. Proc. 1103 (2009) 478.

14

[18] J. Wu, Y. Jiang, J. Lei, X. Fan, Y. Chen, M. Li, D. Zou, B. Liu, Effect of neutron irradiation on charge collection efficiency in 4H‒SiC Schottky diode, Nucl. Instr. Meth. Phys. Res. A 735 (2014) 218‒222. [19] L. Liu, A. Liu, S. Bai, L. Lv, P. Jin and X. Ouyang, Radiation Resistance of Silicon Carbide Schottky Diode Detectors in D‒T Fusion Neutron Detection, Sci. Rep. 7 (2017) 13376. [20] W.R. Harrell, J. Zhang, K.F. Poole, Aluminum schottky contacts to n‒type 4H‒SiC, J. Electron. Mater. 31 (2002) 1090–1095. [21] J. Kohlscheen, Y. N. Emirov, M. M. Beerbom, J. T. Wolan, S. E. Saddow, G. Chung, M. F. MacMillan, and R. Schlaf, Band line‒up determination at p‒ and n‒type Al/4H‒SiC Schottky interfaces using photoemission spectroscopy, J. Appl. Phys. 94 (2003) 3931. [22] M. S. Gorji, K. Y. Cheong, Au nanoparticles embedded at the interface of Al/4H‒SiC Schottky contacts for current density enhancement, Appl. Phys. A 118 (2015) 315–325. [23] So‒Mang Kim, Sang‒Mo Koo, Electrical properties of Al/Al4C3/4H‒SiC diodes, Materials Science in Semiconductor Processing 74 (2018) 170–174. [24] H. Li, C. Liu, Y. Zhang, C. Qi, Y. Wei, J. Zhou, T. Wang, G. Ma, Z. Wang, S. Dong, M. Huo, Irradiation Effect of Primary Knock‒on Atoms on Conductivity Compensation in N‒type 4H‒ SiC Schottky Diode Under Various Irradiations, Semiconductor Science and Technology; https://doi.org/10.1088/1361‒6641/ab33c4. [25] Y. Cui, X. Hu, K. Yang, X. Yang, X. Xie, L. Xiao, X. Xu, Influence of Nitrogen Concentrations on the Lattice Constants and Resistivities of n ‒Type 4H‒SiC Single Crystals, Cryst. Growth Des. 15 (2015) 3131–3136. [26] I.P. Vali, P.K. Shetty, M.G. Mahesha, R. Keshav, V.G. Sathe, D.M. Phase, R.J. Choudhary, Gamma irradiation effects on Al/n‒Si Schottky junction properties, Nucl. Instruments Methods Phys. Res. Sect. B Beam Interact. with Mater. Atoms. 436 (2018) 191–197. [27] I.P. Vali, P.K. Shetty, M.G. Mahesha, V.C. Petwal, J. Dwivedi, R.J. Choudhary, Tuning of

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Schottky barrier height of Al/n‒Si by electron beam irradiation, Appl. Surf. Sci. 407 (2017) 171–176. [28] I.P. Vali, P.K. Shetty, M.G. Mahesha, V.C. Petwal, J. Dwivedi, D.M. Phase, R.J. Choudhary, Implications of electron beam irradiation on Al/n‒Si Schottky junction properties, Microelectron. Reliab. 91 (2018) 179–184. [29] I.P. Vali, P.K. Shetty, M.G. Mahesha, V.G. Sathe, D.M. Phase, R.J. Choudhary, Structural and optical studies of gamma irradiated N‒doped 4H‒SiC, Nuclear Inst. and Methods in Physics Research B 440 (2019) 101–106. [30] G. Greczynski, L. Hultman, C 1s Peak of Adventitious Carbon Aligns to the Vacuum Level: Dire Consequences for Material's Bonding Assignment by Photoelectron Spectroscopy, ChemPhysChem 18 (2017) 1507 – 1512. [31] G. Greczynski, L. Hultman, X‒ray photoelectron spectroscopy: Towards reliable binding energy referencing, Progress in Materials Science 107 (2020) 100591. [32] S.K. Cheung, N.W. Cheung, Extraction of Schottky diode parameters from forward current‒ voltage characteristics, Appl. Phys. Lett. 49 (1986) 85–87. [33] R.T. Tung, The physics and chemistry of Schottky barrier height, Appl. Phys. Rev. 1 (2014) 011304. [34] D. J. Ewing, L. M. Porter, Q. Wahab, X. Ma, T. S. Sudharshan, S. Tumakha, M. Gao, and L. J. Brillson, Inhomogeneities in Ni ⁄ 4 H ‒ Si C Schottky barriers: Localized Fermi‒level pinning by defect states, J. Appl. Phys. 101 (2007) 114514. [35] A. A. Lebedev, B. Ya. Ber, N. V. Seredova, D. Yu. Kazantsev and V. V. Kozlovski, Radiation‒stimulated photoluminescence in electron irradiated 4H‒SiC, J. Phys. D: Appl. Phys. 48 (2015) 485106. [36] L. J. Brillson, S. Tumakha, R. S. Okojie, M. Zhang and P. Pirouz, Electron‒excited luminescence of SiC surfaces and interfaces, J. Phys.: Condens. Matter 16 (2004) S1733–

16

S1754. [37] A. Y. C. Wu and E. H. Snow, Radiation effects on silicon schottky barriers, IEEE Trans. Nucl. Sci. 16 (1969) 220–226. [38] A. Venter, M.E. Samiji, A.W.R. Leitch, Formation of surface states during Schottky barrier fabrication on Al‒doped p‒type 6H–SiC, Diamond and Related Materials 13 (2004) 1166– 1170. [39] E. van Wyk, A.W.R. Leitch, Oxygen passivation and reactivation of interface states introduced during Schottky diode fabrication on bulk n‒type 6H–SiC, Applied Surface Science 221 (2004) 415–420. [40] W. F. Brinkman, R. C. Dynes, and J. M. Rowell, Tunneling Conductance of Asymmetrical Barriers, J. Appl. Phys. 41 (1970) 1915. [41] M. J. Tadjer, K. D. Hobart, T. J. Anderson, T. I. Feygelson, R. L. Myers‒Ward, A. D. Koehler, F. Calle, C. R. Eddy, D. K. Gaskill, B. B. Pate and F. J. Kub, Thermionic‒Field Emission Barrier Between Nanocrystalline Diamond and Epitaxial 4H‒SiC, IEEE Electron Device Letters 35 (2014). [42] A. O. Evwaraye, Phonon‒Assisted Tunneling from Z1/Z2 in 4H‒SiC, J. Electron. Mater. 39, (2010) 751‒755. [43] B. Putz, G. Milassin, Y. Butenko, B. Völker, C. Gammer, C. Semprimoschnig, M. J. Cordill, Interfacial mutations in the Al‐polyimide system, Surf. Interface. Anal. 50 (2018) 579‒586.

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Supplementary figures:

Fig. 3. Room temperature PL spectra of n‒4H‒SiC before and after electron and gamma irradiation.

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Fig. 4. Cheung plots of Mz/.‒ 4J‒ KL Schottky contacts before and after electron and gamma irradiation.

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Highlights •

As prepared Al/n‒4H‒SiC Schottky contacts showed inhomogeneous nature of Φ as well as localized FLP due to grown‒in defects in n‒4H‒SiC at ~2.35

•

.

A wide discrepancy in the Φ for different N has been attributed to the presence of different grown‒in defect states in n‒4H‒SiC bulk rather than processing techniques.

•

The zero‒bias offset in the I‒V characteristics of irradiated Schottky contacts is attributed to irradiation‒induced defects in n‒4H‒SiC bulk and their participation in tunneling mechanism rather than interface chemistry.