The Schottky limit and a charge neutrality level found on metal/6h-sic interfaces

Transcription

1 Surface Science ) L85±L81 Surface Science Letters The Schottky lim and a charge neutraly level found on metal/6h-sic interfaces Shiro Hara National Instute of Advanced Industrial Science and Technology AIST), AIST Tsukuba Central 2, Tsukuba, Ibaraki , Japan Received 18 October 2; accepted for publication 31 August 21 Abstract We report the Schottky lim and a charge neutraly level CNL) experimentally demonstrated at metal/6h- SiC 1) interfaces. An interface wh the Schottky lim was formed by dipping SiC surfaces in boiling pure water before metallization. The total densy of interface states, D, was a drastically small value of 4:6 1 1 states cm 2 /ev, indicating the densy of the metal induced gap states was less than this value. In contrast, at incompletely passivated interfaces whout the boiling water dipping process, a broad continuum of interface states wh D of 2: states cm 2 /ev was observed wh the CNL located at.797 ev from the conduction band minimum. The origin of these interface states was found to be in the disordered interface layers. Ó 21 Elsevier Science B.V. All rights reserved. Keywords: Schottky barrier; Metal±semiconductor interfaces; Interface states; Surface electronic phenomena work function, surface potential, surface states, etc.); Silicon carbide; Evaporation and sublimation; Oxidation; Etching Despe ve decades of innumerable studies, there is still much controversy about the mechanism of Schottky barrier formation at metal/semiconductor interfaces, especially in terms of the essential factors determining the barrier height [1]. Proposed factors are often categorized into intrinsic and extrinsic factors. A so-called intrinsic factor is the metal induced gap states MIGS) conceptually proposed by Heine [2]. Computational demonstrations of MIGS have been made [3] and a method for calculating the charge neutraly level CNL) that dominates the interface Fermi level has been proposed [4]. Extensive calculations have been done [5] and CNL uctuations have been discussed address: shiro-hara@aist.go.jp S. Hara). [6]. The long-range space-charge contribution to the interface charges has also been considered [7]. Some of the reasons for the controversy arise from the lack of experimental supports for the penetration depth and energy distribution of MIGS in the semiconductor, and for a direct measurement of CNL. To reveal the MIGS e ect experimentally, a reduction of the extrinsic factors is a prerequise. Typical extrinsic factors are defects as pointed out by Spicer et al. [8], disorder that induces gap states DIGS) [9], and interface geometry as typically shown in the NiSi 2 /Si 1 1 1) interface [1]. These factors show the importance of the formation of an abrupt interface. This recognion has led to experiments of low-temperature interface formation [11] and speci c inert metal selections [12,13]. Imperfect termination of the surface before /1/$ - see front matter Ó 21 Elsevier Science B.V. All rights reserved. PII: S )1596-5

2 L86 S. Hara / Surface Science ) L85±L81 the metal deposion is also one of extrinsic factors as shown in the studies of sulfur passivation on GaAs [14] and hydrogen passivation on Si [12]. In the above studies on the extrinsic factors, however, the general trend of the Schottky barrier height SBH) for various metals is unclear and indirect. Due to the incomplete and partial control of these extrinsic factors, the energy distribution of the interface states becomes complicated, thereby making the SBH unpredictable. Further, the MIGS e ect reduces the SBH controllabily, which hinders the detection of the extrinsic factors as individual e ects on the SBH. According to the controversial history, one can express the total densy of interface states D total as D total E ˆD MIGS E D EXT E ; 1 if the interaction between MIGS, D MIGS and extrinsic states D EXT is neglected. To observe one of these two factors, the other factor should be reduced. Here, we use 6H-SiC crystal wh a wide band gap that is predicted to have a weaker MIGS e ect than Si or GaAs but still predicted to have a remarkable e ect wh D MIGS of 1 14 states cm 2 /ev [15]. 1 Also, SiC is expected to form an abrupt interface after room-temperature metal deposion [16] because of s strong Si±C bonding. We also apply the monohydride termination technique, developed for the Si 1 1 1) surface using boiling water immersion [17], for the SiC surface. Using these experimental techniques, we form atomby-atom interface connection and we demonstrate the rst perfect control of the extrinsic factors and observe the negligible MIGS e ect in this wide band gap. Commercially available nrogen-doped n-type 6H-SiC 1) epaxial wafers wh a carrier concentration of cm 3 were used. All samples were degreased followed by dipping in 5% HF solution. This cleaning process is referred to as DHF treatment. Some of the degreased samples were RCA cleaned and subsequently thermally oxidized. The oxide layer wh a thickness of 1 nm was etched by dipping in 5% HF solution. We will refer to the sequence of the treatment as O/E treatment. Some of the O/E samples were immersed in boiling water at 98 C for 1 min. The sequence of this treatment will be referred to as BW treatment. All samples were rinsed in the deionized water after each HF process. The treated surfaces were characterized using low energy electron di raction LEED), Auger electron spectroscopy AES), and X-ray photoemission spectroscopy XPS). Metals were deposed onto the SiC surfaces after each surface treatment using an e-beam evaporator wh maximum working pressures below Torr. During deposion, there was no intentional sample heating. In our preliminary report, we used only Ti, Mo, and Ni as contact materials [18]. Here we add Pt and Al to obtain D. SBH were measured by I±V and C±V methods. 2 The slope parameter S / de ned as o/ b =o/ m is a parameter required to estimate D, where / b is the SBH and / m is the metal work function. In our work, the experimental values of S / obtained from Al, Ti, Mo, Ni and Pt electrodes are.18,.549, and.754 for DHF, O/E, and BW samples, respectively. Previously reported values of S / for 6H-SiC 1) epaxial lms are.14 [19] and.63 [2], which are similar to the values of our DHF and O/E treatments, respectively. S / for the BW sample is larger than the reported ones. In a discussion on S /, one should notice that the leakage current tends to reduce the I±V measured SBH [21], which might cause an underestimation of S / compared wh the real S /. Fig. 1 is the relation between the ideal factor n and / b wh Pt electrodes. / b is the at-band barrier height canceling the image force lowering. Since all the data plots in each treatment are distributed on each straight line, the lines are extrapolated to ideal unique 1 S X values reported in this paper can be converted to D using the formulas of S / ˆ e i = e i q 2 dd and S X b m b =@/ m =@X m ˆ AS /, where X m is the electronegativy of a metal. The conversion factor A m =@X m is Detailed experimental condions and the results of our investigation on the metal/sic interface will be described in a full paper reported by Teraji and Hara, which will be submted elsewhere.

3 S. Hara / Surface Science ) L85±L81 L87 Fig. 1. Ideal factor n vs. / b for Pt/6H-SiC 1) interfaces. DHF, O/E, BW indicate diluted HF, oxidation and etching, and boiling water dipping process, respectively. Each plot indicates n and / b of individual Pt electrodes. The inset shows typical I±V curves. Fig. 2. / m vs. / b plots. Barrier height controllabily is raised going from DHF to BW through O/E treatment. On the BW line, the slope parameter S / is.994, which is approximately in the Schottky lim condion. / exp is the observed CNL of these metal/sic interfaces. / MIGS is the predicted CNL of MIGS [25]. The inset shows the energy distribution of the interface states. Distributions outside of the hatched areas are unknown in our experiment. barrier heights / b at n ˆ 1, obeying thermionic emission current transport whout leakage current. / b for DHF, O/E, and BW electrodes are 1:11 :15 ev, 1:365 :24 ev, and 1:632 :7 ev, respectively. The plot concentration in DHF and the plot broadening along the BW line suggest that the DHF interface has a large amount of D to form a strong pinning and the pinning is weaker in the BW interface. By obtaining / b also from other metal electrodes, relations between / m and / b are plotted as shown in Fig. 2. S / using / b for the DHF, O/E and BW electrodes are :215 :5, :739 :79, and :994 :53, respectively. The value of.994 is near the Schottky lim of S / ˆ 1 having the maximum SBH controllabily. C±V measured barrier heights / CV b provide almost the same barrier heights as / b and S / ˆ 1:11 for the BW interfaces. See footnote 2.) This indicates that / CV b / b 6ˆ / b at least in our experiments. In other words, the C±V barrier heights coincide wh the I±V barrier heights when the interfaces have no leakage current. We estimated D from the equation S / ˆ e i = e i q 2 dd [22], where e i is the permtivy of the interfacial layer and d the width of the interface layer. We use the bilayer thickness.252 nm in the 6H-SiC 1) crystal as d, and use the permtivy in vacuum e as e i since no report was found on e i for metal/sic interfaces. D for the DHF, O/E and BW electrodes are 2:8 1 13,2:7 1 12, and 4:6 1 1 states cm 2 / ev, respectively, as indicated in the inset gure. Even for the lower error lim of S / ˆ :941 ˆ :994 :53, D has a relatively low value of 4: states cm 2 /ev. The correlation coe cients r for the DHF, O/E, and BW lines are.928,.983, and.996, respectively. These strong correlations suggest there is no remarkable interface reaction generating interface states between the SiC substrate and the various metals. It is important to know the line shape in the relation between / b and / m to understand the distribution D E as depicted in Fig. 3. A U-shaped distribution provides a curved line. A straight line is generated by a broad distribution. Local states provide a stepwise line. According to the MIGS model proposed by Terso [4], MIGS should have

4 L88 S. Hara / Surface Science ) L85±L81 Fig. 3. Types of energy distribution in interface states and corresponding correlations of / m vs. / b. a U-shaped energy distribution. It is believed that the MIGS are not the direct penetration states of the metal wave function but metal-induced states redistributed from the conduction and the valence band states where the total number of the states in the semiconductor is preserved [4]. In our observations, the data plots in Fig. 2 form straight lines at least in the observed individual barrier height ranges that are indicated by hatched areas in the inset. This broad feature of D E suggests that the origin of the interface states is di erent from the MIGS that should have the U-shaped D E. In general, energetic broadness in electron states is a typical feature of a disordered structure. Fig. 4 shows cross-sectional lattice images of DHF and BW interfaces by transmission electron microscopy TEM). The DHF interface has a disordered interface layer of 2 nm thickness. In the BW sample the interface is atomically abrupt and has the atom-by-atom commensurate epaxial connection wh the 112 6H-SiC == 11 1 Ti relation whout any evidence of a SiO 2 formation in our cross-sectional TEM observation and electron energy loss spectroscopy EELS) of the crosssectional sample. The lattice type of the Ti layer is the face-centred cubic fcc) lattice. The formation of the fcc-ti structure has previously been suggested only in an ultra-thin Ti lm wh a thickness of several angstroms on Al surfaces [23]. We have observed the growth of the fcc-ti layer wh a thickness of 1 nm, which is the rst report on this thin layer single-crystal formation [24]. The bulk stable phase of Ti is hexagonal close-packed hcp) wh a lattice mismatch of 4% for the 6H- SiC 1) crystal. The lattice constant a of the observed fcc-ti structure is.438 nm wh a lattice mismatch of.79% between the 112 6H-SiC and 11 1 Ti faces. The phase change at the interface Fig. 4. High-resolution cross-sectional TEM lattice images wh the electron acceleration at 2 ev incident from the [1 1 2 ] direction of a) DHF and b) BW interfaces.

5 S. Hara / Surface Science ) L85±L81 L89 occurs in preference to the incommensurate hcp- Ti/6H-SiC 1) interface formation. This commensurate connection is strong evidence of direct Ti chemical bonding to the SiC crystal whout a hydrogen terminator. In LEED observations BWtreated SiC surfaces exhib sharp 1 1 patterns whereas DHF-treated SiC surfaces show no diffraction spot indicating that the DHF surface is disordered. The disordered surface and interfaces in DHF samples is consistent wh the broad D E. The uni ed disorder-induced gap state DIGS) model has been proposed as a model to deal wh interface disorder [9]. In this model, similar to the branch point mechanism, D E is a U-shaped continuum because the origin of the DIGS is de ned as the penetration tails of the valence and conduction band states. The observed broad D E suggests that another type of disorder exists whose origin is di erent from the above mechanism. One of our important observations is the rst experimental nding of a CNL, / exp ) indicated in Fig. 2. CNL is the crical parameter to determine the amount of the interface charge, but has been discussed only for the MIGS theory [4±6]. Here, we extend the CNL phenomenon to a more general concept that is not restricted to a speci c origin like MIGS. The three lines intersect wh each other at the CNL wh / b of.797 ev and / m of 4.65 ev. The observation of one CNL for the three kinds of surface preparations indicates that the origin of D is the same for the three interfaces, but the amount of D is variable. This / exp is located far from the MIGS±CNL / MIGS at 1.43 ev estimated from previous experimental values of / b [2] and the value of / m of 5.4 ev assuming that / m is equal p to the Miedima's electronegativy, X SiC ˆ X Si X C ) [25]. This is plotted in Fig. 2. In the MIGS theory, the interface Fermi level should be pinned at the branch point E BP where the e ects of the conduction and valence band balance [4]. E BP for the 6H-SiC crystal is calculated to be 1.41 ev, which has almost the same value as the above estimation of / MIGS but absolutely di erent from / exp. This di erence supports the assertion that / exp is an extrinsic level, not the intrinsic level based on the branch point mechanism. The discussed disordered features give / exp ˆ / disorder in this system. In other words, our experimentally observed CNL is a disorder-related CNL, / disorder, not MIGS±CNL / MIGS. Considering the possibily of a multiple CNL system, another general model wh an expression, D total E ˆD BP E D EXT1 E D EXT2 E ; 2 is given, where each term on the right has an individual /, and D BP is D wh the CNL at E BP, which originates from the MIGS or DIGS theories based on the branch point mechanism. Our metal/ SiC interfaces have D total D BP D disorder wh D BP < D total ˆ 4:6 1 1 states cm 2 /ev for the BW interface in the observed SBH range of.45±1.66 ev. This indicates that D MIGS is also lower than 4:6 1 1 states cm 2 /ev. According to the typical MIGS theory of Ref. [4], the penetration length of the metal wave function or the semiconductor layer depth modi ed by the metal was calculated to be.15 nm for ZnS wh the band gap of 3.6 ev that is comparable value of 6H-SiC. Nevertheless, D calculated for ZnS is 1 14 [26]. At least, our nding of for the MIGS densy strongly suggests that the MIGS penetration depth for SiC is much lower than.15 nm. This implies that actual penetration depths for other semiconductors may have much lower than the calculated values [4] and the semiconductors may have D wh one or two order of magnude lower than the predictions if one can exclude the extrinsic interface states. From XPS measurements of the core level posion of C 1s, the surface potential barrier is.19 ev and the surface band bending is nearly at of.8 ev for the BW surface. Using the value of.8 ev the ionized total densy of the surface states R D s de is evaluated to be 6: states/ cm 2. Let us estimate the densy of the surface states D s by assuming that the surface CNL has the same origin as the interface CNL. The energy width of charged states between the CNL and the Fermi level posion is.67 ev ˆ :797 :19 ev). Thus, D s is estimated at 1: states cm 2 /ev. This value is one and a half orders of magnude higher than D, implying that the residual surface states are e ectively annihilated by

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