ELEVATED TEMPERATURE ELECTRONICS

Figure 1. Formation of Schottky barrier of metal to n-type semiconductor.

the metal into the semiconductor conduction band is

qj_Bn = q(j_m - c). (1)

For contact between a metal and a p-type semiconductor with j_m < j_s, the barrier height qj_Bp is given by

qj_Bp = E_g - q(j_m - c), (2)

where E_g is the bandgap. Figure 2 gives a Schottky barrier formed by contacting an metal with an p-type semiconductor when j_m < j_s in an ideal case The two other cases of ideal metal-semiconductor contacts (j_m < j_s for n-type and j_m > j_s for p-type semiconductors) result in ohmic contacts characterized with an linear current-voltage curves. Conclusions derived from the ideal metal-semiconductor contacts are referred to Schottky-Mott rule.

Ideally, the barrier height depends only on the metal work function and on the semiconductor bandgap and electron affinity. In practice, however, it is

(a) Band diagrams for the metal and (b) Equilibrium band diagram for

the semiconductor before joining the junction

Figure 2. Formation of Schottky barrier of metal to p-type semiconductor.

difficult to alter the barrier height by using metals of the varying work function. It is experimentally observed that the barrier height for the common semiconductor materials Ge, Si, GaAs, and other III-V materials is relatively independent of the work function of the metal. A Schottky contact is generally formed on both n-type and p-type semiconductors with j_B » E_g/3 for both cases. The relative constancy of the barrier height with work function of metals is sometimes called Fermi level pinning, referring to the fact that the Fermi level in the semiconductor is pinned at some energy in the band gap to create a Schottky contact^3,4. J. Pelletier et al.⁵ reported Fermi level pinning in 6H-SiC attributed to intrinsic surface states, suggesting little dependence of barrier height on the work function of the metal. L.M. Porter et al.⁶ found that the barrier height differences of Ti, Pt, and Hf contacts to n-type (0001) 6H-SiC were all within a few tenths of 1 eV, giving evidence that the Fermi level

(a) Barrier height vs. work function for 3C-SiC

(b) Barrier height vs. work function for 6H-SiC

Figure 3. Schottky barrier height j_B of various metal contacts to SiC versus metal work function j_m (after J.R. Waldrop et al.^7,8)

is pinned in the bandgap. On the other hand, J.R. Waldrop et al.^7,8 reported a strong dependence of barrier height for metal contacts to 3C- and 6H-SiC on the work function of the metal. Figure 3 (a) and (b) show the Schottky barrier height j_B plotted against the respective metal work function j_m for 3C- and 6H-SiC, respectively. The details of Schottky barrier formation are not yet fully understood. It appears, however, that interfacial electronic states due to defects, metal induced gap states, and interfacial chemistry play important roles during contact formation.

4. Carrier Transport processes

The carrier transport mechanisms through the metal-semiconductor interface are strongly influenced by the donor concentration in the semiconductor and the temperature. Three typical cases are schematically shown in Figure 4 for a n-type semiconductor. Figure 4(a) depicts the situation where a semiconductor is lightly doped (N_d < 10¹⁷/cm³). In this case, the depletion width W is wide and the electrons cannot tunnel through the interface. The only way for the electron to transport between the metal and the semiconductor is by thermionic emission (TE) over the potential barrier j_Bn. Figure 4(b) shows the band diagram of a metal contacting a semiconductor doped at an intermediate level (N_d = 10¹⁷ to 10¹⁸/cm³). In this case, the electrons can partially tunnel through the interface and both thermionic and tunneling process are important, which is referred to thermionic-field-emission (TFE). When the semiconductor is extremely heavily doped ( N_d > 10¹⁸/cm³), the electrons can tunnel through from the Fermi level in the metal into the semiconductor. This process is called field-emission (FE), which is shown in Figure 4(c).

(a) Low N_D; (b) Intermediate N_D; (c) High N_D.

Figure 4. Conduction mechanisms through metal/n-type semiconductor interface with different doping levels.

A useful parameter indicating the electron tunneling probability is kT/E₀₀, where E₀₀ is a characteristic energy defined by

E₀₀ = (3)

where h is Planck’s constant, h, divided by 2p, m* is the effective mass of the tunneling electron, e is the dielectric constant of the semiconductor. With increasing doping concentration Nd, the width of the depletion region W decreases, making it easier for carriers to tunnel through. This indicates that when E₀₀ is high relative to thermal energy kT, the probability of electron transport by tunneling increases. Therefore, the ratio kT/E₀₀ is a useful measure of the relative importance of the thermionic process to the tunneling process. For lightly doped semiconductors, kT/E₀₀ >> 1 and thermionic emission is the dominant current flow mechanism. For kT/E₀₀ » 1, both thermionic and tunneling mechanisms are dominant, and for kT/E₀₀ << 1, the tunneling mechanism dominants the carrier transport mechanism⁹.

5 SiC metallization process and characterization

The metallization procedures for SiC are basically the same as those used for Si and GaAs techniques. Some highlights of metallization process and characterization are briefly given as follows.

Wafer surface preparation The preparation of the SiC wafer surface prior to metal deposition is a very important step for metallization. Surface contamination can reduce adhesion of metals to SiC and increase contact resistance. Any contaminants and native oxide layer on the wafer surface must be removed before metal deposition. Solvents, such as trichloroethylene (TCE), trichloroethane (TCA), acetone, methanol, and propanol are used to degrease SiC wafer. In order to obtain a fresh SiC surface free of oxide, A. Evwaraye at al.¹⁰ and M.I. Chaudhry et al.¹¹ suggested to grow thermal oxide and subsequently remove it by acid etching. This procedure also removed defects on SiC surface caused by mechanical polishing. Various acids and bases are used to etch SiC wafer surface, such as HF, H₂SO₄, HCl, NH₄OH:H₂O₂:H₂O, HCl:H₂O₂:H₂O, H₂SO₄:H₂O₂, HNO₃:H₂SO₄:H₂O, K₂CO₃, and KOH melt. Prior to metal deposition, in-situ surface cleaning is also utilized in which sputter etching or irradiation of high-energy laser beam is commonly performed.

Metal deposition techniques The most common techniques used for metal deposition include sputtering, evaporation, chemical vapor deposition (CVD), and molecular-bean epitaxy (MBE). Sputtering is a well-established deposition technique for metallization. It is based on the bombardment of a target with energetic ions, which knock off surface atoms by energy transfer. These released atoms land on the wafers to become part of the film coating.

In evaporation techniques, the deposition material is vaporized in a vacuum from its liquid phase, and the vapor is then transported and deposited onto the wafer. The vacuum used for evaporation is higher than 10^-3 torr. At this low pressure, the residual gas molecules have a mean free path of the order of 1 m. Therefore the evaporated vapor suffers no collision from the residual gas and is able to achieve a straight-line travel from the target to the surface of the wafer. The condensation of the vapor on the wafer is achieved through a nucleation and growth process.

Chemical vapor deposition (CVD) has been used as a deposition technique for many years and for a wide variety of end users. Its use in electronics is widespread as well. CVD uses volatile compounds containing the deposition species as transport agents, with these agents being chemically reacted on the semiconductor surface, creating the desired deposit and other chemical species as the reaction product. Excellent step coverage and selectivity are two major advantages that CVD has and make it unique and very attractive as a deposition technique.

A new approach to metal deposition is provided by the molecular-beam epitaxy techniques (MBE). In this method, a beam made of atoms of the material to be deposited is directed at the semiconductor substrates. The incidence rate on the surface is usually low in order to allow the atoms to rearrange themselves on the surface in structured layers according to their particular lattice structure in the bulk form. For the same reason, the substrate is usually kept at an elevated temperature to assist migration of the impinging atoms. A successful MBE growth requires an absolutely clean substrate surface. This means that an extremely high vacuum is required to minimize surface contamination that would interfere in the formation of a smooth transfer of the crystalline structure at the interface.^12,13

Annealing With a few exceptions, metal contacts on SiC usually illustrate rectifying characteristics. Annealing is often required to form ohmic contacts with low contact resistivity and good thermal stability. Since the annealing time required for contacts to SiC is generally longer than contacts to, for example, GaAs, traditional annealing is more often used than rapidly thermal annealing (RTA). Annealing is usually performed in an atmosphere of Ar, vacuum or forming gas (3%H₂ in N₂). Depending on contact systems, the annealing temperatures are ranging from 300° to 1200°C, and the annealing time varies from a few seconds to a few hours. During annealing, silicides and/or carbides are usually formed that may play a role of decreasing the Schottky barrier height and hence the contact resistance.

Characterization In metal-semiconductor contacts a critical quantity which describes the relationship between the two materials is the Schottky barrier height, j_B. In general, Schottky barrier height is also a property which best indicates the electrical characteristics of the contact. Schottky barrier height can be determined by several different techniques, such as current-voltage measurements, capacitance-voltage measurements, and x-ray photoelectron spectroscopy.^14,15 The most important parameter describing an ohmic contact is its specific contact resistance or contact resistivity r_c. There are different methods to measure contact resistivity. Among them, the transmission line measurement (TLM)^16,17 is a popular technique. In TLM, the contact resistivity is given by

r_c. = R_c² z² / r_s = z L_t (4)

where R_c is the contact resistance, R₀ is the total resistance at zero distance and R_c = R₀/2, z is the width of the TLM pad, r_s is the semiconductor sheet resistance, and L_t is the transfer length. Other common methods used for contact resistivity determination includes the circular transmission line method¹⁸ and the four point probe method^19,20.

In order to study the interface reaction and to identify compounds formed during annealing, Auger electron spectroscopy (AES), transmission electron microscopy, Rutherford backscattering spectrometry (RBS), and x-ray diffraction (XRD) are techniques often employed. A concise description on these analyzing techniques can be found in ref. 21.

6. Schottky contacts on SiC

Many metals form Schottky contacts on SiC in the as-deposited condition. In order to obtain good Schottky contacts, suitable metals must possess large barrier height j_B with SiC. If Schottky-Mott rule applies, this means the metals must have high work function for contacts to n-type SiC or low work function for contacts to p-type SiC. Another important consideration is the high temperature stability of the contacts. At elevated temperatures, metals react with SiC to form silicides and/or carbides, which may increase or decrease the Schottky barrier height.

The formation of Schottky barrier contacts to n-type 3C-SiC (100) for Pd, Au, Co, Ti, Ag, Tb, Al was investigated by J.R. Waldrop and et al.⁷ These metal contacts exhibited a wide range of j_B, 0.16-0.95 eV; within this range an individual contact j_B, value depended strongly on the metal work function in general accord with the Schottky-Mott limit. J.R. Waldrop et al.^8,22 also systematically studied the formation of Schottky barrier contacts to n-type 6H-SiC for Pd, Au, Ag, Tb, Er, Mn, Al, and Mg. The j_B values for these metal were found to extend over a wide 1.3 eV range. It was also discovered that to a varying degree j_B depended on the 6H-SiC crystal faces and the interface chemical reactivity and changes in j_B that occur for annealed metal contacts were also crystal face dependent with the C-face to be significantly more reactive than the Si-face. Schottky barrier values of above metals are listed in Table 1.

Au forms Schottky contact to SiC with a high barrier height. S. Yoshida et al.²³ produced good quality Schottky barrier contacts by evaporating Au onto chemically etched surfaces of n-type 3C-SiC epilayers grown by CVD, and obtained the barrier height of 1.15±0.15 eV and 1.11±0.03 eV by the capacitance and photoresponse measurements, respectively. Au Schottky contacts on 3C-SiC remained unaltered by a one hour heat treatment at 300°C in argon. The contacts were rectifying after further heating at 500°C for 90 minutes. However, after heated at 700°C the contacts degraded and showed ohmic behavior. A gradual outdiffusion of Si was observed by AES, which became more prominent at high annealing temperatures.²⁴ Surface preparation is very important in the formation of Au Schottky barriers on SiC. V.A. Dmitriev et al.²⁵ pointed out that for bulk crystals the damaged layer can exceed 10 micron and all damaged surface must be removed before metal deposition. After sufficient etching, the final value of the barrier height of Au contacts to n-type 6H-SiC for both bulk material and epitaxial layers was determined to be approximately 2 eV.

E-beam-deposited Pt contacts on n-type 3C-SiC exhibited superior thermal stability when subjected to short annealing cycles at temperatures as high as 800°C. When thermally treated in the range of 450-800°C a combination of silicide and carbide was believed to occur at the Pt/SiC interface while improvement of the rectifying characteristics was simultaneously observed. Interfacial reaction was dominated by the diffusion of Pt into the SiC layer. As the annealing temperature increased, the barrier height increased from 0.95 to 1.35 eV. The lowest value of the ideality factor was measured at 1.5 after 450°C annealing. It seems that PtSi_x is a promising metallization on 3C-SiC for high-temperature applications²⁶. Pt deposited on 6H-SiC also formed Schottky contacts with low leakage current and low ideality factors. Its barrier height was found to be 1.06 eV determined from current-voltage measurements These Pt contacts were annealed from 450°C to 750°C in 100°C increments for 20 minutes at each temperature. The characteristics remained similar to those before annealing. Throughout the annealing series the ideality factors and leakage current remained low and the Schottky barrier heights increased with anneal temperature to 1.26 eV⁶. A high-voltage 400V 6H-SiC Schottky barrier diodes was fabricated using e-beam deposited Pt. These high-voltage Schottky barrier diodes were reported to have a low forward voltage drop (1.1 V for a J_F of 100 A/cm²), small reverse leakage, and excellent switching characteristics²⁷.

A metal contact to SiC can demonstrate either Schottky or ohmic characteristics, depending on the annealing conditions. H. Daimon et al.²⁸ reported that Al contacts on n-type 3C-SiC showed ohmic behavior stable up to 400°C, but showed distinct rectifying characteristics with annealing at 900°C. On the contrary, Al contacts on p-type 3C-SiC clearly changed from non-ohmic to ohmic with annealing at 900°C. Ni is a metal that demonstrates similar characteristics. A.J. Steckl et al.^29,30 developed a Ni metallization process to fabricate both rectifying and ohmic contact to n-type 3C-SiC by controlling the annealing temperature. The Ni Schottky diodes they fabricated provided high breakdown voltage (170V) for 3C-SiC. The Ni Schottky junction maintained a stable rectifying characteristics and a high breakdown voltage for annealing temperature as high as 600°C. Annealing at 800°C replaced the rectifying behavior with a linear, low resistance (ohmic) characteristics. The mechanism of the thermally-induced rectifying-to-ohmic transition was attributed to the formation of Ni silicides during high temperature annealing, which was confirmed by x-ray diffraction and Auger analysis.

Co contacts to n-type 6H-SiC were studied by N. Lundberg and M. Östling³¹. The Schottky barrier height was found to be 0.79 eV for the as-deposited contact. Excellent rectifying behavior was demonstrated up to 700°C. Consecutive annealing from 300 to 800°C increased the barrier height from 0.8 to 1.3 eV. Heat treatments at 900°C changed the contacts into an ohmic behavior. RBS, AES, and XRD studies showed that Co reacted with SiC and formed Co₂Si at elevated temperatures. Upon annealing at 900°C, CoSi started to form and produced a rough interface, which resulted in a drastic

Table 1. Schottky contacts to SiC
Contact		Annealing	SBH	Year
Metallization	SiC	[°C]	[eV]	Publshed	Ref.
Au	3C, n-type		1.1-1.15	1985	23
Al, Al-Si	3C, n-yupe	900	rectifying	1986	28
Au	3C	as-deposited	1.2	1987	24
Pt	3C, n-type	as-deposited	0.95	1989	26
PtSi_x	"	800	1.35	"	"
Pd	3C, n-type	as-deposited	0.95	1990	7
Au	"	"	0.78	"	"
Co	"	"	0.69	"	"
Ti	"	"	0.53	"	"
Ag	"	"	0.4	"	"
Tb	"	"	0.35	"	"
Al	"	"	0.16	"	"
TaSi₂	6H, n-type, C-face	as-deposited	1.8	1992	32
"	6H, n-type, Si-face	"	1.2	"	"
MoSi₂	6H, n-type, C-face	"	1	"	"
"	6H, n-type, Si-face	"	1	"	"
Ni-Mo	6H, n-type, C-face	"	1.8	"	"
"	6H, n-type, Si-face	"	0.9	"	"
"	6H, n-type, C-face	825°C-2 min	1.2	"	"
"	6H, n-type, Si-face	825°C-2 min	0.9	"	"
Ni	6H, n-type, C-face	as-deposited	2.2	"	"
"	6H, n-type, Si-face	"	1.5	"	"
Au	6H, n-type	as-deposited	2	1992	25
Ti	6H, n-type, Si-face	as-deposited	0.88	1992	33
"	6H, n-type, Si-face	700°C-60 min	1.04	"	"
Pd	6H, n-type, C-face	as-deposited	1.6	1992	8
"	6H, n-type, Si-face	"	1.11	"	"
Au	6H, n-type, C-face	"	1.14	"	"
"	6H, n-type, Si-face	"	1.4	"	"
Ag	6H, n-type, C-face	"	1.1	"	"
"	6H, n-type, Si-face	"	0.92	"	"
Mn	6H, n-type, Si-face	"	0.81	"	"
Al	6H, n-type, C-face	"	0.84	"	"
"	6H, n-type, Si-face	"	0.3	"	"
Mg	6H, n-type, C-face	"	0.33	"	"
"	6H, n-type, Si-face	"	0.3	"	"
Pt	6H, n-type	as-deposited	rectifying	1992	42
Ti	6H, n-type, C-face	as-deposited	1.0	1993	22
"	"	400°C	0.98	"	"
"	6H, n-type, Si-face	as-deposited	0.73	"	"
"	"	400°C	0.97	"	"
Ni	6H, n-type, C-face	as-deposited	1.59	"	"
"	"	400°C	1.66	"	"
"	6H, n-type, Si-face	as-deposited	1.24	"	"
Tanle 1. Schottky contacts to SiC (continued)
Ni	6H, n-type, Si-face	400°C	1.25	1993	22
"	"	600°	1.39	"	"
Al	6H, n-type, C-face	as-deposited	0.84	"	"
"	"	600°C	1.66	"	"
"	6H, n-type, Si-face	as-deposited	0.3	"	"
"	"	600°	1.12	"	"
Ni	3C, n-type	as-deposited	rectifying	1993	23
Ti	6H, n-type, Si-face	as-deposited	0.85	1993	6
Pt	"	"	1.02	"	"
Hf	"	"	0.97	"	"
Co	6H, n-type, Si-face	as-deposited	0.79	1993	31
"	"	400°C	0.90	"	"
"	"	600°C	1.08	"	"
"	"	800°C	1.30	"	"
Ni	3C, n-type	as-deposited	rectifying	1994	29

change of the electric properties and modified the rectifying contact into an ohmic behavior.

Silicides of TaSi₂ and MoSi₂ were deposited on n-type 6H-SiC by RF sputtering. Electric measurements of as-deposited TaSi₂ films revealed them to be rectifying with Schottky barrier heights of 1.8 and 1.2 eV on the C- and Si-faces, respectively. Reverse leakage currents were about 10^-5 A at -10V. The as-deposited MoSi₂ contacts were Schottky with a barrier height of 1.0 eV for both C- and Si-face. After annealing at 925°C for 2 minutes, the rectifying characteristics of both TaSi₂ and MoSi₂ contacts were deteriorated³². Schottky contacts to SiC were also studied for various metals such as Pd^7,8, Ti^6,7,22,33, Ag^7,8, Tb⁷, Mn⁸, Mg⁸, and Hf⁶. Table 1 summarized recent studies of Schottky contacts to SiC.

7. Ohmic contacts on SiC

A good ohmic contact on SiC is supposed to possess a low contact resistivity, good stability during high temperature operation, strong adhesion between the contact metal and the SiC, smooth surface morphology, low metal sheet resistance, simple fabrication process, and wide process window. Generally, it is difficult to satisfy all these requirements by using only one metal and this has led to the development of multilayer metallizations which seek to obtain the optimum metal system satisfying the above demands. In order to obtain ohmic contact with low contact resistivity, metals possessing low work function for contact on n-type SiC (high work function for contact on p-type SiC) and low Schottky barrier height should be chosen. However, because of the complexity of the Schottky barrier formed between metal and semiconductor, contact resistance can not yet be calculated or predicted by the first principle, but must be determined by experiments. For the purpose of ensuring high temperature stability, transition metals with high melting points, such as W, Ta, Mo, Ti, Ni, are widely used for ohmic contacts on SiC.

On the other hand, the doping level in semiconductor strongly influences carrier transport through the metal-semiconductor interface. Though increasing doping level does not decrease the barrier height, it promotes tunneling process and hence increases current flow. Therefore, an effective approach to decreasing contact resistance is to employ heavily doped semiconductors. When the doping level is extremely high, the depletion region can be very narrow which results in that only very thin heavily doped layer on the SiC surface is required. Figure 5 shows the dependence of contact resistivity on doping level for Al-Ti ohmic contacts on p-type 6H-SiC³⁴.

Figure 5. Contact resistivity versus doping level for Al-Ti ohmic contacts on p-type 6H-SiC (After J. Crofton et al.³⁴)

After annealing metal/SiC contacts at high temperatures, silicides and/or carbides may be formed. It has been found that a very simple linear correlation exists between Schottky barrier heights and heats of formation for transition metal silicides on n-type silicon³⁵. Unfortunately, there is no sufficient information available yet for silicides and carbides contacts to SiC. One would expect that a similar trend may exist. In other words, silicides (or carbides) possessing high negative value of heat of formation would probably have low Schottky barrier height. With this in mind, one can choose metals that form with SiC stable silicides and/or carbides possessing high negative value of heat of formation in hopes of decreasing contact resistance.

It is obvious that from the ideal metal-semiconductor contact model that when the work function of the metal is less than that of the n-type semiconductor or when the work function of the metal is greater than that of the p-type semiconductor, the contacts would be ohmic. In practice, quite a few ohmic contacts were reported in the as-deposited conditions. H. Daimon et al.²⁸ found that Al contact on n-type 3C-SiC was ohmic after deposition. R.C. Glass et al.³⁶ used low energy ion-assisted reactive evaporation to deposit TiN (work function » 3.74 eV) onto on 6H-SiC with the Si terminated (0001) surface (work function = 4.8 eV) and obtained good ohmic contact. This may be resulted from the fact that TiN has smaller work function than that of SiC, or from the formation of an amorphous Si-N interface layer between the TiN and the SiC which was involved in the ion-assisted reactive evaporation³⁷.

Ohmic contact was formed by J.S. Shor et al.³⁸ in the as-deposited Ti/Pt films on n-type b-SiC. The contact resistivity ranged from 2.5x10^-4 to 9x10^-5 ohm-cm². A one hour anneal at 650°C caused the contact resistivity to decrease by roughly a factor of two. However, after two hours at 650°C, most of the Ti/Pt contacts failed. C. Jacob et al.³⁹ obtained ohmic characteristics in W and Mo contacts on 3C- n-type SiC. The contact resistance of W/SiC was about 0.8 ohm-cm² before annealing and 0.66 ohm-cm² for the sample annealed at 900°C for 30 minutes. The Mo/SiC contact had a high contact resistance of 1.8 ohm-cm² in the as-deposited condition. After annealing at 900°C for 30 minutes the contact resistance dropped to 0.25 ohm-cm². Mo, Ta, Ti, and Zr contacts on n- and p-type 6H-SiC were also reported to display ohmic characteristics in the as-deposited state on degenerate epilayers⁴⁰.

Though ohmic contacts on SiC can be obtained in as-deposited condition, ohmic contacts with low contact resistivity and good thermal stability are usually developed by high temperature annealing in which silicides and/or carbides are formed that may decrease the Schottky barrier height and therefore decrease the contact resistance. Metals with high melting points and high chemical stability, such as W, Mo, Ti, Ni, Cr, have been widely used for the annealed ohmic contacts.

W was found to be both physically and chemically stable in contact with n-type 3C-SiC at temperatures up to approximately 900°C^39,41. The AES data indicated that there was a thin layer of WC and WSi₂ formed during the deposition process, however, no additional reaction was observed after annealing at 850°C for 30 minutes in a ultra high vacuum. The electrical measurements indicated that the W/SiC contact was ohmic and unaffected by vacuum annealing at temperatures up to 900°C. The contact resistance was found to be about 0.24 ohm-cm² at 23°C, dropping to 0.08 ohm-cm² at 900°C.

The specific contact resistance of W contact on n-type 3C-SiC obtained by M. I. Chaudhry et al.¹¹ prior to heat treatment was of the order of 1.5x10^-2 ohm-cm². As the sample were annealed, the contact resistance decreased to 6.1x10^-3 ohm-cm² during a 30 min period of annealing at 300°C. This decrease was attributed to the dissolution of the natural oxide at the W/SiC interface during subsequent annealing at 300°C. P.G. McMullin et at.⁴² reported a low contact resistivity of 8x10^-4 ohm-cm² for W/Au contact to n-type 3C-SiC after annealed the contact at 800°C for one hour. The W/Au contact also demonstrated good thermal stability when subjected to thermal cycles at 600°C for approximately 80 hours. A similar W/Pt contact to n-type 3C-SiC illustrated a contact resistivity of 1.4x10^-4 ohm-cm² after annealed at 650°C for 8 hours³⁸.

The contact resistivity of ohmic contacts to 6H-SiC is a crystal-face sensitive property. M.G. Rastegaeva et al.⁴³ reported that the specific contact resistance values of W contacts to C-faced n-type 6H-SiC were 2 to 2.5 times greater than those of the same contacts to Si-faced SiC for the same doping level of 3x10¹⁸/cm³. The respective resistance values were 2x10^-3 and 7x10^-4 ohm-cm². W is also used in many other contact metallization systems as a constituent part^44,45, a top layer^46-48, or a diffusion barrier layer of the metallization^26,49.

Mo forms ohmic contacts to n-type 3C- and 6H-SiC^39,40. Mo reacted with b-SiC to form MoSi₂ after annealing at 1150°C for 15 minutes. After annealing at 1200°C, Mo₅Si₃ also appeared and the amount of Mo₅Si₃ was increased with increasing the annealing time⁵⁰. After annealing at 970°C for 15 minutes, the contact resistivity of Mo/3C-SiC contact was 4x10^-2 ohm-cm². The Mo/b-SiC contact showed good thermal stability. The contact resistivity did not change after heat treated at 1200°C for 60 minutes.

Ni is an important element to form thermally stable and low contact resistant ohmic contact to n-type SiC. The Schottky barrier height of Ni contact to SiC is high and it forms good rectifying contacts to both 3C- and 6H-SiC. However, after high temperature annealing, the contacts changed from rectifying to ohmic characteristics. It was found that Ni did not react with b-SiC below 580°C. When the annealing temperatures were above 610°C, Ni began to react with 3C-SiC and formed polycrystalline Ni₂Si⁵⁰. Other nickel silicides such as NiSi₂ and Ni₅Si₂ were also reported³⁰. The contact resistivity of 2.8x10^-2 ohm-cm² for the Ni contact on 3C-SiC was obtained after annealed at 700°C for 15 minutes⁵⁰. Similar result was obtained for Ni contact to n-type 6H-SiC after annealing at 950°C for 5 minutes⁴⁴. It was reported that after annealing the Ni/SiC contact at 1050°C for 5 minutes, almost all deposited Ni reacted with SiC and formed Ni₂Si, which was identified by x-ray diffraction. At the same time, low contact resistivity of 10^-3 to 10^-4 ohm-cm² was obtained⁴⁶. Therefore, the low contact resistivity of the Ni contact to SiC was attributed to the formation of Ni₂Si. It is obvious, however, that this Ni₂Si was formed by consuming Silicon in the SiC substrate and thus, the composition of the SiC at the metal/semiconductor interface would be shifted toward a silicon-depleted direction and isolated graphite carbon atoms would be left behind. These isolated graphite carbon atoms were believed to deteriorate the electric properties of the ohmic contact and the SiC substrate. In order to avoid this problem, a thin silicon layer can be deposited between the metal layer and the SiC substrate⁴⁶. Another approach is to employ a carbide-forming element to produce a stable carbide. Both Ni/Ti/W and Ni/Cr/W ohmic contacts to n-type 6H-and 4H-SiC demonstrated low contact resistivity. Long-term aging test revealed that Ni/Cr/W contacts illustrated excellent thermal stability: the contacts were stable after aged at 650°C for 2000 hours. AES chemical depth profiles and x-ray diffraction study indicated that both silicide and carbide were formed after aged at 1050°C for 5 minutes⁴⁸.

In addition to forming silicides or carbides by high temperature annealing, various compounds can be deposited or grown on SiC. TiSi₂ and WSi₂ were reported to be deposited on n-type 3C-SiC by co-sputtering intrinsic silicon and titanium or tungsten. After deposition, the contacts were rapid thermal annealed (RTA) at 1000°C for 10 seconds followed by annealing at 450°C for 10 minutes to form silicides. After RTA, the contact resistivity of the TiSi₂ and WSi₂ contacts were 1.4x10^-1 ohm-cm² and 3.7x10^-2 ohm-cm², respectively. The contact resistivity decreased to 1.1x10^-4 and 3.0x10^-4 ohm-cm² after annealed at 450°C for 10 minutes¹¹. Titanium nitride films were deposited onto the Si-faced n-type 6H-SiC by ion-assisted reactive evaporation in a dual electron beam evaporation system. The TiN contacts were ohmic in the as-deposited condition and little change was observed after annealing at 450°C and 550°C for 15 minutes^36,37. A.K. Chaddha et al.⁵¹ used CVD technique to epitaxially grow a TiC contact layer on n-type 6H-SiC epilayer. The contact resistivity of the TiC/SiC contacts were 1,30x10-5 ohm-cm2. The contacts were found to be thermally and chemically stable after annealing at 1400°C for 2 hours in hydrogen.

Because 3C-SiC has lower energy gap (Eg » 3.0 eV) than 6H-SiC (Eg » 2.3 eV), it may be easier to make low resistivity ohmic contacts to 3C-SiC than to 6H-SiC. V.A. Dmitriev et al.[152] utilized a unique technique to obtain low resistivity ohmic contacts to 6H-SiC. They grew thin 3C-SiC layers (< 2000Å) on 6H-SiC substrates by low pressure CVD followed by depositing Ni for n-type contacts or Al/Ti for p-type contacts. The contacts then were annealed using a RTA in a forming gas at 1000°C for 30 seconds for the n-type contacts and at 950°C for 2 minutes for the p-type contacts. The contact resistivity of Ni contacts to n-type 3C-SiC/6H-SiC grown on the Si face were < 1.7x10^-5 ohm-cm² and < 6x10^-5 ohm-cm² when 3C-SiC/6H-SiC was grown on the C face. As a comparison, the contact resistivity of Ni contact to 6H-SiC (without 3C-SiC layer) was 2x10^-4 ohm-cm². For Al/Ti contacts to the p-type 3C-SiC/6H-SiC, the contact resistivity was found to be 2-3x10^-5 ohm-cm². Recent advances in ohmic contacts to SiC are summarized in Table 2.

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