Current Flow Mechanism in Ohmic Contact to n 4H SiC - Springer Link

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ANALYSIS OF PUBLISHED DATA. Silicon carbide, especially its 4H SiC polytype, presently serves as the basis for high temperature high power semiconductor ...
ISSN 10637826, Semiconductors, 2010, Vol. 44, No. 4, pp. 463–466. © Pleiades Publishing, Ltd., 2010. Original Russian Text © T.V. Blank, Yu.A. Goldberg, E.A. Posse, F.Yu. Soldatenkov, 2010, published in Fizika i Tekhnika Poluprovodnikov, 2010, Vol. 44, No. 4, pp. 482–485.

SEMICONDUCTOR STRUCTURES, INTERFACES, AND SURFACES

Current Flow Mechanism in Ohmic Contact to n4HSiC T. V. Blank^, Yu. A. Goldberg, E. A. Posse, and F. Yu. Soldatenkov Ioffe Physical–Technical Institute, Russian Academy of Sciences, St. Petersburg, 194021 Russia ^email: [email protected] Submitted August 18, 2009; accepted for publication September 7, 2009

Abstract—Current flow in an In–n4HSiC ohmic contact (n ≈ 3 × 1017 cm–3) has been studied by analyzing the temperature dependence of the perunitarea contact resistance. It was found that the thermionic emis sion across an ~0.1eV barrier is the main current flow mechanism and the effective Richardson constant is ~2 × 10–2 A cm–2 K–1. DOI: 10.1134/S1063782610040093

1. ANALYSIS OF PUBLISHED DATA Silicon carbide, especially its 4HSiC polytype, presently serves as the basis for hightemperature highpower semiconductor electronics because of the following properties: (I) large energy gap (3.23 eV); (II) high thermal conductivity (3.7 W cm–1 K–1); (III) high breakdown field [(3–5) × 106 V cm–1]; and (IV) high electron saturation velocity (2 × 107 cm s–1) (all the parameters are given for 300 K [1]). 4HSiC has a sufficiently low density of surface states: (2.63–4.67) × 1012 cm–2 eV–1 for the Si plane and (4.93–6.58) × 1012 cm–2 eV–1 for the C plane [2]. Therefore, in contrast to the semiconductors widely used in electronics (Si, GaAs, GaP, etc.), the height ϕB of a metal–4HSiC barrier strongly depends on the electron work function Φm of the contacting metal. In particular, a linear dependence of ϕB on Φm was obtained in [3, 4], as it should be according to the Schottky theory. Recent data on ntype 4HSiC for the most part confirm this model. As can be seen in Table 1 [3, 5–11], ϕB increases with Φm, but this rise is weaker than that suggested by the ideal Schottky theory. It is noteworthy that parameters of the Schottky barrier were found from I–V and C–V characteristics by using expressions of the thermionic emission theory. According to the Schottky theory, a barrier contact to ntype semiconductors is formed when the electron work function of a metal exceeds the electron affinity χs of a semiconductor, i.e., Φm > χs, and an ohmic con tact is produced at Φm < χs. Since Φm exceeds the elec tron affinity of 4HSiC (χs = 4.05 eV) for most metals, ohmic contacts were fabricated using the same metals (or alloys) that give a barrier contact (Φm > χs), but with an appropriate thermal treatment leading to for mation of new compounds. For example, contacts to n4HSiC have been fabricated from Ni [12–14],

Ti/Al and Ti/Ni/Ag [6], and Au/Ti/Al [15], and those to p4HSiC from Ni/Al and NiTiAl [16]. In the case of n4HSiC, a special thermal treatment produced Ni2Si and Ni3Si2 layers on the surface, with donorlike vacancies C appearing in the surface region of the semiconductor. For p4HSiC, NiAl3 and TiSiC2 lay ers were formed on the surface. As a result, a lowresis tance ohmic contact was fabricated. It is known [17] that the principal current flow mechanisms in an ohmic contact are the following. (I) Thermionic emission; in this case, the specific (the perunitarea) contact resistance Rc of the ohmic contact decreases with increasing temperature T and becomes higher as the metal–semiconductor barrier height ϕB grows: qϕ k R c = ⎛ ⎞ exp ⎛ B⎞ . ⎝ qA*T⎠ ⎝ kT ⎠

(1)

Here, k is the Boltzmann contact, q is the elementary charge, and A* is the effective Richardson constant. Table 1. Height of the metal–4HSiC Schottky barrier

463

Metal

Electron work function of a metal, Φm, eV

Al Ti

4.18–4.28 3.83–4.33

Mo Cr Ni

4.6 4.4–4.6 5.1–5.2

Au Pt

5.15 5.43–5.65

Barrier height ϕB, eV 0.58–0.69 0.85–1.21 1.21–1.25 0.91–1.21 1.06–1.12 1.45–1.72 1.1–1.45 1.68 1.48–1.58 1.4–2.1

[5] [6] [7] [6] [8] [6] [9] [3] [10] [11]

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Table 2. Current flow mechanism in an ohmic contact to p4HSiC Metal

Current flow mechanism

Al/Ti/Pt/Ni Thermionic emission Ti/Al, Ni/Ti/Al Thermofield emission Al/Ti The same

Barrier height ϕB, eV

Nd Nd – 15 E 00 = ប    .  = 18.5 × 10 2 ε s ε 0 m* ε sr m r

0.097 0.4 0.82

[18] [19] [20]

(II) Field emission; Rc is independent of T and increases with the metal–semiconductor barrier height ϕB: 2 ε s ε 0 m*⎞ ⎛ ϕ B ⎞ R c ∝ exp ⎛   .   ⎝ ⎠ ⎝ N 1/2⎠ ប d

(2)

Here, εs is the relative permittivity, ε0 is the permittivity of free space, m* is the effective electron mass in the semi conductor, ប is the Planck constant, and Nd is the con centration of ionized impurities in the semiconductor. (III) Thermofield emission; resistance Rc weakly decreases as temperature is raised and increases with the metal–semiconductor barrier height ϕB: ϕB (3) R c ∝ exp ⎛  ⎞ , ⎝ E coth ( qE /jT )⎠ 00

with the Padovani–Stratton parameter E00 given for an ntype semiconductor by

00

Rmeas, Ω 1 300

(4)

Here, mr = m*/m0 is the relative effective electron mass in the semiconductor and εsr is the relative per mittivity of the semiconductor. (IV) Current flow via metallic shunts; resistance Rc increases with temperature T. The current flow mechanism in ohmic contacts to semiconductors with high density of surface states (Si, GaAs, GaP), to which the Bardeen model is applicable, has been extensively studied and almost all possible mechanisms have been identified under vari ous conditions. The current flow mechanism in an ohmic contact to 4HSiC with low density of surface states has been only studied for ptype crystals and was identified as thermionic of thermofield emission [18–20]. As can be seen in Table 2, the metal–semiconductor barrier heights widely vary in different reports. The current flow mechanism in ohmic contacts to n4HSiC has not been analyzed in the available publications. The goal of our study was to examine the current flow mechanism in ohmic contacts to moderately doped n4HSiC. When an ohmic contact to a mate rial of this kind is formed, the electron work function of the metal is highly important (Schottky model). Therefore, we chose In, a metal with a low electron work function (3.97 eV), which is even lower than the electron affinity of 4HSiC (4.05 eV).

2 3 4

200

5 6

100

0

0.2

0.4

0.6 d, cm

Fig. 1. Resistance of In–4HSiC–In structures with two ohmic contacts vs. the distance between the contacts at temperatures T = (1) 250, (2) 260, (3) 270, (4) 280, (5) 300, and (6) 350 K.

2. EXPERIMENTAL Singlecrystal 4HSiC with an electron density n ≈ 3 × 1017 cm–3 and electron mobility μn ≈ 600 cm2 V–1 s–1 (300 K) served as the starting material. A number of In contacts were alloyed into n4HSiC wafers in a flow of purified hydrogen at 1200°C. The area of the contacts was S ≈ 10–4 cm2, and the total length of the wafer was 1.5 cm. After the contact alloying and cooling the samples to room temperature, their current–voltage (I–V) characteristics were measured in the temperature range 77–420 K between the first and all the rest of the contacts. The temperature in the thermostat was maintained constant to within 1 K. All the structures we fabricated had linear I–V characteristics. To determine the contact resistance Rc, the depen dence of the experimentally measured resistance Rmeas on the distance d between the contacts was determined (Fig. 1): d R meas = 2R c + R bulk = 2R c +  . qNμ n S SEMICONDUCTORS

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CURRENT FLOW MECHANISM IN OHMIC CONTACT TO n4HSiC

Here, Rbulk is the semiconductor bulk resistance. The intercept of the linear dependence Rmeas(d) on the ver tical axis corresponds to the doubled contact resis tance, and its slope ratio is proportional to the bulk 1 resistivity of the semiconductor, ρ =  . qNμ n 3. CURRENT FLOW MECHANISM IN THE In–4HSiC OHMIC CONTACT For all wafers with In–4HSiC–In contacts, we measured the resistance Rmeas. The results obtained were the following. (i) At low temperatures T = 77–200 K, the mea sured resistance Rmeas sharply decreases with increas ing temperature; the semiconductor bulk resistivity ρ also steeply falls, which is presumably due to freezing out of impurities. (ii) At temperatures T = 200–350 K, the specific contact resistance Rc of the ohmic contacts decreases as temperature is raised. The dependence of Rc on 1/T is linear on the semi log scale (Fig. 2), in agreement with the thermionic emission theory, [Eq. (1)]. According to this theory, the slope ratio of the dependence Rc = f(1/T) plotted on the semilog scale must be proportional to the potential barrier height ϕB. The potential barrier height ϕB was found to be 0.1 eV by comparing the the ory and the experiment (Fig. 2). Although the electron work function of In (3.97 eV) does not exceed the electron affinity of 4HSiC (4.05 eV), a small potential barrier remains at the contact between In and 4HSiC, which may be due to formation of vacancies in contact alloying or to presence of an intermediate oxide layer that could not be completely removed in thermal treatment. It is noteworthy that the In–4HSiC ohmic contact can also be formed at room temperature, but its resistance is very high because of the presence of a thick oxide layer. The effective Richardson constant was found to be ~2 × 10–2 A cm–2 K–1 from the intercept of the Rc = f(T) dependence (Fig. 2). If the effective conduction electron mass in 4HSiC is taken to be 0.36 eV [1], the effective Richardson constant must be 42 A cm–2 K–1, which exceeds by approximately three orders of mag nitude the value found from the experiment. The Richardson constant for the ohmic contact to n4HSiC has not been determined before, but the effective Richardson constants found for Schottky diodes in other studies were even smaller: 1.39 × 10–3 A cm–2 K–1 for Ni–4HSiC, 3.83 × 10–3 A cm–2 K–1 for Pt–4HSiC [21], and 4 × 10–4 A cm–2 K–1 for Al–4HSiC [22]. The authors of [21, 22] attributed the deviation of the Richardson constant for Schottky diodes from the theoretical values to a quantum mechanical reflection of electrons from the metal– SEMICONDUCTORS

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Rc, Ω cm2

10−2

10−3 0.0028

0.0032

0.0036

0.0040 1/T, K−1

Fig. 2. Specific contact resistance of the ohmic contact In–4HSiC vs. inverse temperature.

semiconductor interface, tunneling across the inter face, and structural changes in the metal–4HSiC contact. In our case of the ohmic contact of In to 4HSiC, the difference between the experimental effective Richardson constant and its theoretical value is not so pronounced as that in other studies and may be due to a quantummechanical reflection of elec trons from the metal–semiconductor interface. To conclude, the current flow mechanism in an ohmic contact of In to moderately doped n4HSiC is that by thermionic emission. ACKNOWLEDGMENTS The study was supported by the Russian Foundation for Basic Research, project no. 080213552ofi_ts. REFERENCES 1. Properties of Advanced Semiconductor Materials, Ed. by M. Levinshtein, S. Rumyantsev, and M. Shur (Wiley, New York, 2001). 2. Shaweta Khanna, Arti Noor, Man Singh Tyagi, and Sonnathi Neeleshwar, Mater. Sci. Forum 615–617, 427 (2009). 3. A. Itoh and H. Matsunami, Phys. Stat. Solidi A 162, 389 (1997). 4. A. Itoh, T. Kimoto, and H. Matsunami, IEEE Trans. Electron. Dev. Lett. 16, 280 (1995).

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Translated by M. Tagirdzhanov

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