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Schottky barrier formation on InP(110) passivated with one monolayer of Sb
Applied Surface Science 56-58 (1992)325-329 North-Holland
~:
~
surface science
Schottky barrier formation on InP(110) passivated with one monolayer of Sb M a s a o Y a m a d a *, A n i t a K. W a h i , T o m K e n d c l e w i c z a n d W i l l i a m E. S p i c c r Stanford Electronics Laboraror&% Stanford Universily, Stanford, CA 9430~, USA Received 6 May lC~l: aceepled fur publication I 1 September 1991
To passivate InP(110) surfaces chemically and electrically, we have investigated a new method using one monolayer of Sb as an interlayer at metal-semiconductor interfaces. Prior work has established that one monolayer of Sb ideally terminates InP(ll0) by providing a surface which satisfies all chemical bonds and is lattice matched. The deposition of one monolayer of Sb followed by annealing near 300°Cgives nearly the flat band condition for both n- and p-lyge I.P, with a residual band bending of less than 0.2 eV. To make Schoaky barrier~ on these surfaces, we have deposited five metals (Au, Pd, As, Cu, and AI), We have found that (1) Au and Ag are non-~eactive and form hls clusters, and f2) Pd. Cu, and AI are reactive on these surfaces. Ahbough Schotthy barrier heights on n-inP without an Sb interlayer range from 0.3 to 0.55 eV, those with an Sb intuflayer range from 0.49 to 0.82 ¢V. By using Ihis method, wc have firsl succeeded in obtaining a lechnolo$icallyimportant large Schonky barrier height of about 0.g2 eV at Au/n-lnP(110) ime: ~'aces,
1, Introduction A major objective Gf surface and interface work on I I I - V compound semiconductors has been to find an ideal termination of the surface which leaves no interface states. T h e literature of the last decade shows that one monolayer (ML) of Sb may provide such a termination. Prior work has established that one monolayer of Sh ideally terminates InP(110) by providing a surface which satisfies all chemical bonds and is lattice matched [1]. We show that one can obtain nearly flat band conditions with such one monolayer of Sb on InP(ll0). Further, it is found that such termination followed by metal deposition can produce strong changes in Schottky barrier heights. InP has some superior properties that better suit it for high speed device operation than G a A s [2]. U p until now, however, InP has not been
* Visiting scientist from Fujitsu Ltd., Kawasaki 21I, Japan.
used widely in active devices by itself, but instead was used only as a substrate material for optoelectronic devices [3]. O n e of the largest reason preventing its practical use in such devices as metal-semiconductor F E T ( M E S F E T ) is the low Schottky harrier heights of about 0.45 e V (_+0.l eV) on n-tnP, which result in a large leakage current that is detrimental to the device operation [4]. T o overcome this problem, other approaches using metal-insulator-semlconductor (MIS) structures have been developed [5-8]. However, these have been suffering frGm a different problem, that is, drain current instabilities [9]. T h e other reason is difficulty in making a good ohmic contact on p-lnP becausc of the high Schottky barrier height. Controlling the surface Fermi level position and understanding the mechanisms responsible for the Fermi level pinning is one of the oldest problems in the field of semiconductors since the original Schottk'y model was proposed it0]. These days, two basic models, the defect model i l l ] and
0169-4332/92/$05.00 © 1992 - Elsevier Science Publishers B.V, All fights reserved
32fi
Af. Yamada ct aL / Schouky b~*rriL,r formado~a on InP( llO) I~assit'atcd with I ML Sb
the metal-induced gap states (MIGS) model [12] are believed to provide the mosl likely explanations, According to these models, by either reducing interface defect slate densities or by reducing the tunneling probability of the wavefunction from the metal to the semiconductor, the surface Fermi level may be controlled. In this paper, we investigate a novel method using an interlayer of one monolayer of Sb. Wc discribe phenomenologieal features for the Schottlo' barrier formation from the evolution of the surface Fermi level positions for increasing
metal coverage. 2. Experimental Photocmission experiments were performed with a monochromatlzod He discharge lamp t h e II. h~. = 40.8 eV). A standard ultra-high vacuum chamber (base pressure of 1 x 10 -I" Torr) equipped with a double pass cylindrical mirror analyzer was used. Atomically clean (110) surfaces were prepared by cleaving loP crystals in vacuum. Crystals used were n-type (Sn doped at 9,3 × l0 t7 atoms/era 3) and p-type (Zn d~ped at 2 X 10Is aloms/cm~). Metal coverngc was measured using a quartz crystal monitor, with ouc monolayer defined as the surface density of atoms on the (110) face of lop (8.2 × 10t4 atoms/era2). In order to form a one monolayer passivation layer of Sb, we first d e . ~ i t e d 2.0 ML Sb (assuming a sticking coefficient of unity), and then annealed at 3OWC for 10 rain to remove the excess coverage. Schotlky harriers were formed on all of these annealed Sb(l ML)/InP surfaces. The metals (Au, Pd, Ag, Cu, and AD were evaporated from tungsten coils, To measure surface Fermi level shifts, all photoemission spectra of the In 4d core level were deconvolvod using a computer curve-fittlng routine in order to separatu surface, bulk, and reacted components. The curve-fitting parameters for the In4d core level were spinorbit splitting of 0.86 eV, branching ratio of 1.5, Lorentzian width of 0.2 eV, and Gaussian width of 0.4 cV. The surface Fermi level shifts were obtained from shifts in the bulk component. The accura~' of this method in determining ohift,¢ in the surface Fermi level position is +0.05 eV.
3. Results and discussion
To passivalc InP surfaces chemically and elecIrically,in other words, to reduce interface defect state densities, we have found that the deposition of one monolayer of Sb followed by anncaling at around 300°C is useful [I]. Chemical passivation requires reducing structural disruption at interfaces that occurs duc to interfacial chemical rearlions and electrical passivalion requires reducing interface defect state densities. One well-known example of chemical passivalion is seen in the melalization which uses a thin buffer layer of TiN to prevent alloying AI with Si [13]. However. few examples are available for electrical passivation. In order to control the surface Fermi level position, both chemical and electrical passivation must be achieved as mentioned. Group V elements on the (110) surfaces of III-V semiconductor are considered as possible candidates for this purpose, especially for reducing the it.terrace defect stale densities, because one monolayer of Group V elements has bvcn found to form a zig-zag epitaxial structure with an abrupt interface and theoretically there are no interface states in the gap [14,15]. In order to get unpinned su-faces in practice, appropriate annealing is required. The deposition of two monolayer of Sb on lnP(ll0) results in a Fermi level position about 0.35 eV below ~he conduction band minimum (CBM) for n-lnP and 0.85 eV above the valence band maximum (VBM) for p-loP. However, wc have found that annealing above 200°C but below 350°C moves the Fermi level to nearly the flat band condition with a residual band bending of less than 0,2 eV for both n- and p-type lop as shown in fig. I. This suggests that the loP surfaces passivated with one monolayer of Sb gives a lower interface defect state density (that is better electrical passivatiou) than other terminations do. Therefore, the next important problem is to test if these annealed InP(ll0) surfaces can change the Sehottky barrier height. As for the MIGS responsible for the surface Fermi level pinning, we consider that the effect of the MIGS can be reduced or modified by the presence of an Sb interlayer, because one moonlayer of Sb is not metallic but semiconducting,
I~ Yomada el aL / Schottky barrier formation on InP(110) passivated with 1 ML Sb
327
CBM
a..cleaved
A S~n-lnP
Cu/a~'ndnP
_t.t
~ o.~ U-o8 VB Coverage (ML] w~
,o'o ~
2'o
'
8'00
'
4°°
Fig, 3. Surface Fermi level shifts for increasing coverage at
Pd. Cu, and AI/lnP interfaces with an Sb interlayer.
Annealing Temperature (%)
Fig. I. Surface Fermi positioas at Sb/tnP interfaces as a function of annealing temperature. and the penetration depth of the wavefunction is limited to a few ~ngstr6ms. If the M I G S is reduced by the Sb with a low interface defect state density, we can certainly move the surface Fermi level widely. If it is not so, but if the modification of the charge neutrality point due to the Sb is desirably controlled by the Sb, we have also an increased possibility of changing the usually observed pinning positions. A m o n g the metals we have used, Au [16] and Ag [1] are non-reactive and form big clusters or islands on InP surfaces passivated with one monolayer of Sh, while Pd, Cu, and AI are still reactive, judging from photoemission spectra of the l n 4 d core level. Fig. 2 (for non-reactive metals) and fig. 3 (for reactive metals) show the CBM ~-
surface Fermi level positions as a function of metal coverage for both n- and p,type InF. The Schottky barrier heights obtained at 10 M L coverage are summarized in table 1. Schottky barrier heights without an Sb interlayer are also indicated in table 1 [17-19]. It !~ found from the evolution of the surface Fermi level positions for increasing coverage that: (1) in non-reactive cases (i.e., Au and Ag), the surface Fermi level positions on p-fnP converge strongly at the same level about 0.52 e V above the V B M from the early stage of the Schottky barrier formation irrespective o f the difference in metal electronegativity, and those on n-loP slowly converge toward the same level as p-lnP; (2) in reactive cases (i.e., Pd, Cu, and AI), the surface Fermi level positions range from 0.6g to 0.86 e V above the V B M with decreasing electroncgatlvity. Table I Schottl~ barrier heights obtained at tO ML coverage by PEa with and without an Sb interlayer
Ag/Sb/n-InP
Electronegativity
o3
Au/SI3/p-lnP Ag/ablp-lnP
V8
' o cave=age (ML)
Fig. 2. Surface Fermi level shins for increasing coverage at Au and Ag/lnP interfaces with an Sb interlayer.
With an Sb Without an Sb interlayer interlayer n-InP p-lnP n-lnP p-lnP AU 2.4 0.82 0.52 0.42 0,86 Pd 2,2 0,62 0.68 0.dl 0.87 Ag 1.9 0.66 0.52 0.54 0,76 Cu 1.9 0.58 0.69 0.42 0,90 AI 1.5 0.49 0.80 0.33 0,98 All data without an Sb interlayer were cited from refs. [17-19] (by I - V measurements). The measurement error of the Sehonky barrier heights by PEg is :t:o.l eV.
328
M, Yamada et a~ / Schot:£?~b~rrierforma|ion
On the basis of these experimental studies, in order to explain the Fermi level pinning at m e t a l - | n P interfaces with Sb interlayers, we would like to propose the simplest defect model using only two dominant defect levels, that i~ a donor level at 0.55 eV ( + 0 . 0 5 eV) above the VBM and an accepter level at 0.9 eV ( + 0 . 0 5 eV) above the VBM. With our limited data base, we suggest how the data fan be explained. However, note that, at the best, we show consistency with the model and not uniqueness. The fact that in reactive cases (Al, Cu, and Pd) the pinning pusi. tions fall within this range suggests that the bartier heights are controlled by charge exchanges between metals and defects depending on the metal electsonegatlvlty, That is, high electronegativity metals (e.g. Pd alld Cu) tend re pin lower ill the gap, while low electronegativiw metals (e.g. AI) tend to pin higher in the gap. In non-reactive eases (Ag and An), since the surface Fermi level is pinned strongly at about 0.55 t:V above the VBM, the Sb interlayer seems to prohibit strongly the cwation of other defects except for donors with a level at 0.55 eV ( + 0 . 0 5 eV), In this ease, as for a reason why the donor level can pin n-lnP, it will be explained assuming either that the charge flow from the lnP to the metal due to the high eleetronngativity of Au and Ag compensates the donor level or that there are some aceeptor levels below the donor level [20]. We also suggest that phosphorus vacancies relate to the donor level at about 0.55 eV above the VBM because uf assc~imion of pinning here with loss of phosphorus [1,21]. The MIGS model cannot be comple*.ely dunle~d to explain the pinning unserved in non-reactive cases. There is a pussibility that the charge neutrali~ point changes by the existence of the Sb [2O].
4. Conclusions In order to comsol the surface Fermi level of InP(ll0), in other words to passivate I n P ( l l 0 ) surfaces chemically and electrically, we have proposed a new method using one monolayer of Sb as an interlayer at metal-semiconductor inter-
rm
InP~lI0) Immit'ated w$th I ML Sb
faces. The deposition of one monolayer of Sh followed by annealing at about 300~C gives nearly the flat band condition for both n- and p-type INF'. Schottky barriers formed on these annealed lnP(IIO) surfaces change strongly as compared with those on clean cleaved surfaces. Among the metals we have deposited, we have found that Au and Ag are non-reactive and form big elus~,ers, and Pd, Cu, and AI are reactive on these surfaces. A technologically important large Scbottky harrier height of 0.82 eV on n-lnP is obtained at A u / S b ( l M L ) / n - l n l ' ( l l O ) interfaces. Since this method using an Sb interlayer gives high Schottky barrier heights on n-lnP, we emphasize that it can promote the realization of electronic devices using lnP as the active layer itself, Finally, we propose a defect model to explain observed Fermi level pinning.
Acknowledgements Support for this work was provided by Fujitsu Ltd., Japan, and by D A R P A / O N R under Contract No. N00014-89-1083.
References lit M. Yamada, A.K. Wahl P.L Meissner, A. Herrera, T. Kendelewiczand W.E. Sp[cer,Appl. Phys. Len. 58tl991) 2243. [2] S.M, Sze, Physics of Semiconductor Dories (Wiley. New York, 1969). [3] R. Dingle, Semiconductors and Semimetals, Vol. 24 (Academic Press, New York, 1987)ch, 7. [41 D.L IJle. J. Va¢. Sci. Technol. B 2 (t984) 4%. [hi D.L Lile and D.A. Collins, AppL Phys. Leu. 28 (1976) 554. [6] K.P. Pande and G.G, Roberts. J. Va¢. Sei. Technol. 16 (1979) 1470, [7] H. Hasegawaand I". Sawada,Thin Solid Films 103(1983) 119. [8] G. Hollinger. R. BlancheL M. Gendw, C. Saminelli, R. Skheyta arid P. Viktorovltch,J, AppL Phys. 67 (1990) 4173. [9] J.F. Wager, SJ.T. Owen and SJ. Prasad, J. Electrochem. S~. 1340987) 160. IlO] W. Schottky, Phys. Z. 41 (Iq4fl)570. [Ill W.E. Spicer, L Lindau, P. Ske~lh, C.Y. Su and Palrick Chye, Phys. Rev. l~tt. 44 (1980)420.
M. Yamada el aL / Schottky ~rtqer fornlation on laP( I IO) passiz'ated with 1 ML Sb
[12] J. Tersoff. J, Vac. Sci, TechnoL B 3 (1985) 1157. [13] C.Y. Tiag and M. Wittmer, Thin Solid Films 96 (1982} 327. [14] P, Skeath, C.Y. Su, W.A~ Harrison, 1. Lindau and W.E, Spicer. Phys. Rcv. B 27 (1983)6246. [15[ C, Mailhiot, C.B, Duke and DJ. Chad/, Phys. Rev. B 3[ (1985) 2213. [16] M. Yamada, A.K. Wahl, T. Kend¢lewicz and W.E, Spicer, AppL Phys.Lell. 58 (1991)2701. [17] T. Kendelewicz, N. Nt,'s~man. R.S. List, I. Lmdau and W.E. Spicer, J, Va¢. 5ci. Technnl. B 3 (1985) 1206.
329
[18] N, Newman, W.E Spicer, T, Kendelewicz and I, Lindau, J. Vac. Scl. Techno|. B 4 (1986) 93L [19] N, Newman. M. van Schilfgaade and W.E. $picer, Phys. Rev. B 35 (1987) 629~. 12o] M. Yamada, A.K. Wahl, T. Kendelowicz and W,E, Spicer, Phys. Rev, B, submitted. 121j M. Yamada, CJ. Spindt. K.F.. Miyano, A, HerreraGomcz, 1". Kendelewiezand W.E. Spiccr,L Appl. Phy~,, submitted.
~:
~
surface science
Schottky barrier formation on InP(110) passivated with one monolayer of Sb M a s a o Y a m a d a *, A n i t a K. W a h i , T o m K e n d c l e w i c z a n d W i l l i a m E. S p i c c r Stanford Electronics Laboraror&% Stanford Universily, Stanford, CA 9430~, USA Received 6 May lC~l: aceepled fur publication I 1 September 1991
To passivate InP(110) surfaces chemically and electrically, we have investigated a new method using one monolayer of Sb as an interlayer at metal-semiconductor interfaces. Prior work has established that one monolayer of Sb ideally terminates InP(ll0) by providing a surface which satisfies all chemical bonds and is lattice matched. The deposition of one monolayer of Sb followed by annealing near 300°Cgives nearly the flat band condition for both n- and p-lyge I.P, with a residual band bending of less than 0.2 eV. To make Schoaky barrier~ on these surfaces, we have deposited five metals (Au, Pd, As, Cu, and AI), We have found that (1) Au and Ag are non-~eactive and form hls clusters, and f2) Pd. Cu, and AI are reactive on these surfaces. Ahbough Schotthy barrier heights on n-inP without an Sb interlayer range from 0.3 to 0.55 eV, those with an Sb intuflayer range from 0.49 to 0.82 ¢V. By using Ihis method, wc have firsl succeeded in obtaining a lechnolo$icallyimportant large Schonky barrier height of about 0.g2 eV at Au/n-lnP(110) ime: ~'aces,
1, Introduction A major objective Gf surface and interface work on I I I - V compound semiconductors has been to find an ideal termination of the surface which leaves no interface states. T h e literature of the last decade shows that one monolayer (ML) of Sb may provide such a termination. Prior work has established that one monolayer of Sh ideally terminates InP(110) by providing a surface which satisfies all chemical bonds and is lattice matched [1]. We show that one can obtain nearly flat band conditions with such one monolayer of Sb on InP(ll0). Further, it is found that such termination followed by metal deposition can produce strong changes in Schottky barrier heights. InP has some superior properties that better suit it for high speed device operation than G a A s [2]. U p until now, however, InP has not been
* Visiting scientist from Fujitsu Ltd., Kawasaki 21I, Japan.
used widely in active devices by itself, but instead was used only as a substrate material for optoelectronic devices [3]. O n e of the largest reason preventing its practical use in such devices as metal-semiconductor F E T ( M E S F E T ) is the low Schottky harrier heights of about 0.45 e V (_+0.l eV) on n-tnP, which result in a large leakage current that is detrimental to the device operation [4]. T o overcome this problem, other approaches using metal-insulator-semlconductor (MIS) structures have been developed [5-8]. However, these have been suffering frGm a different problem, that is, drain current instabilities [9]. T h e other reason is difficulty in making a good ohmic contact on p-lnP becausc of the high Schottky barrier height. Controlling the surface Fermi level position and understanding the mechanisms responsible for the Fermi level pinning is one of the oldest problems in the field of semiconductors since the original Schottk'y model was proposed it0]. These days, two basic models, the defect model i l l ] and
0169-4332/92/$05.00 © 1992 - Elsevier Science Publishers B.V, All fights reserved
32fi
Af. Yamada ct aL / Schouky b~*rriL,r formado~a on InP( llO) I~assit'atcd with I ML Sb
the metal-induced gap states (MIGS) model [12] are believed to provide the mosl likely explanations, According to these models, by either reducing interface defect slate densities or by reducing the tunneling probability of the wavefunction from the metal to the semiconductor, the surface Fermi level may be controlled. In this paper, we investigate a novel method using an interlayer of one monolayer of Sb. Wc discribe phenomenologieal features for the Schottlo' barrier formation from the evolution of the surface Fermi level positions for increasing
metal coverage. 2. Experimental Photocmission experiments were performed with a monochromatlzod He discharge lamp t h e II. h~. = 40.8 eV). A standard ultra-high vacuum chamber (base pressure of 1 x 10 -I" Torr) equipped with a double pass cylindrical mirror analyzer was used. Atomically clean (110) surfaces were prepared by cleaving loP crystals in vacuum. Crystals used were n-type (Sn doped at 9,3 × l0 t7 atoms/era 3) and p-type (Zn d~ped at 2 X 10Is aloms/cm~). Metal coverngc was measured using a quartz crystal monitor, with ouc monolayer defined as the surface density of atoms on the (110) face of lop (8.2 × 10t4 atoms/era2). In order to form a one monolayer passivation layer of Sb, we first d e . ~ i t e d 2.0 ML Sb (assuming a sticking coefficient of unity), and then annealed at 3OWC for 10 rain to remove the excess coverage. Schotlky harriers were formed on all of these annealed Sb(l ML)/InP surfaces. The metals (Au, Pd, Ag, Cu, and AD were evaporated from tungsten coils, To measure surface Fermi level shifts, all photoemission spectra of the In 4d core level were deconvolvod using a computer curve-fittlng routine in order to separatu surface, bulk, and reacted components. The curve-fitting parameters for the In4d core level were spinorbit splitting of 0.86 eV, branching ratio of 1.5, Lorentzian width of 0.2 eV, and Gaussian width of 0.4 cV. The surface Fermi level shifts were obtained from shifts in the bulk component. The accura~' of this method in determining ohift,¢ in the surface Fermi level position is +0.05 eV.
3. Results and discussion
To passivalc InP surfaces chemically and elecIrically,in other words, to reduce interface defect state densities, we have found that the deposition of one monolayer of Sb followed by anncaling at around 300°C is useful [I]. Chemical passivation requires reducing structural disruption at interfaces that occurs duc to interfacial chemical rearlions and electrical passivalion requires reducing interface defect state densities. One well-known example of chemical passivalion is seen in the melalization which uses a thin buffer layer of TiN to prevent alloying AI with Si [13]. However. few examples are available for electrical passivation. In order to control the surface Fermi level position, both chemical and electrical passivation must be achieved as mentioned. Group V elements on the (110) surfaces of III-V semiconductor are considered as possible candidates for this purpose, especially for reducing the it.terrace defect stale densities, because one monolayer of Group V elements has bvcn found to form a zig-zag epitaxial structure with an abrupt interface and theoretically there are no interface states in the gap [14,15]. In order to get unpinned su-faces in practice, appropriate annealing is required. The deposition of two monolayer of Sb on lnP(ll0) results in a Fermi level position about 0.35 eV below ~he conduction band minimum (CBM) for n-lnP and 0.85 eV above the valence band maximum (VBM) for p-loP. However, wc have found that annealing above 200°C but below 350°C moves the Fermi level to nearly the flat band condition with a residual band bending of less than 0,2 eV for both n- and p-type lop as shown in fig. I. This suggests that the loP surfaces passivated with one monolayer of Sb gives a lower interface defect state density (that is better electrical passivatiou) than other terminations do. Therefore, the next important problem is to test if these annealed InP(ll0) surfaces can change the Sehottky barrier height. As for the MIGS responsible for the surface Fermi level pinning, we consider that the effect of the MIGS can be reduced or modified by the presence of an Sb interlayer, because one moonlayer of Sb is not metallic but semiconducting,
I~ Yomada el aL / Schottky barrier formation on InP(110) passivated with 1 ML Sb
327
CBM
a..cleaved
A S~n-lnP
Cu/a~'ndnP
_t.t
~ o.~ U-o8 VB Coverage (ML] w~
,o'o ~
2'o
'
8'00
'
4°°
Fig, 3. Surface Fermi level shifts for increasing coverage at
Pd. Cu, and AI/lnP interfaces with an Sb interlayer.
Annealing Temperature (%)
Fig. I. Surface Fermi positioas at Sb/tnP interfaces as a function of annealing temperature. and the penetration depth of the wavefunction is limited to a few ~ngstr6ms. If the M I G S is reduced by the Sb with a low interface defect state density, we can certainly move the surface Fermi level widely. If it is not so, but if the modification of the charge neutrality point due to the Sb is desirably controlled by the Sb, we have also an increased possibility of changing the usually observed pinning positions. A m o n g the metals we have used, Au [16] and Ag [1] are non-reactive and form big clusters or islands on InP surfaces passivated with one monolayer of Sh, while Pd, Cu, and AI are still reactive, judging from photoemission spectra of the l n 4 d core level. Fig. 2 (for non-reactive metals) and fig. 3 (for reactive metals) show the CBM ~-
surface Fermi level positions as a function of metal coverage for both n- and p,type InF. The Schottky barrier heights obtained at 10 M L coverage are summarized in table 1. Schottky barrier heights without an Sb interlayer are also indicated in table 1 [17-19]. It !~ found from the evolution of the surface Fermi level positions for increasing coverage that: (1) in non-reactive cases (i.e., Au and Ag), the surface Fermi level positions on p-fnP converge strongly at the same level about 0.52 e V above the V B M from the early stage of the Schottky barrier formation irrespective o f the difference in metal electronegativity, and those on n-loP slowly converge toward the same level as p-lnP; (2) in reactive cases (i.e., Pd, Cu, and AI), the surface Fermi level positions range from 0.6g to 0.86 e V above the V B M with decreasing electroncgatlvity. Table I Schottl~ barrier heights obtained at tO ML coverage by PEa with and without an Sb interlayer
Ag/Sb/n-InP
Electronegativity
o3
Au/SI3/p-lnP Ag/ablp-lnP
V8
' o cave=age (ML)
Fig. 2. Surface Fermi level shins for increasing coverage at Au and Ag/lnP interfaces with an Sb interlayer.
With an Sb Without an Sb interlayer interlayer n-InP p-lnP n-lnP p-lnP AU 2.4 0.82 0.52 0.42 0,86 Pd 2,2 0,62 0.68 0.dl 0.87 Ag 1.9 0.66 0.52 0.54 0,76 Cu 1.9 0.58 0.69 0.42 0,90 AI 1.5 0.49 0.80 0.33 0,98 All data without an Sb interlayer were cited from refs. [17-19] (by I - V measurements). The measurement error of the Sehonky barrier heights by PEg is :t:o.l eV.
328
M, Yamada et a~ / Schot:£?~b~rrierforma|ion
On the basis of these experimental studies, in order to explain the Fermi level pinning at m e t a l - | n P interfaces with Sb interlayers, we would like to propose the simplest defect model using only two dominant defect levels, that i~ a donor level at 0.55 eV ( + 0 . 0 5 eV) above the VBM and an accepter level at 0.9 eV ( + 0 . 0 5 eV) above the VBM. With our limited data base, we suggest how the data fan be explained. However, note that, at the best, we show consistency with the model and not uniqueness. The fact that in reactive cases (Al, Cu, and Pd) the pinning pusi. tions fall within this range suggests that the bartier heights are controlled by charge exchanges between metals and defects depending on the metal electsonegatlvlty, That is, high electronegativity metals (e.g. Pd alld Cu) tend re pin lower ill the gap, while low electronegativiw metals (e.g. AI) tend to pin higher in the gap. In non-reactive eases (Ag and An), since the surface Fermi level is pinned strongly at about 0.55 t:V above the VBM, the Sb interlayer seems to prohibit strongly the cwation of other defects except for donors with a level at 0.55 eV ( + 0 . 0 5 eV), In this ease, as for a reason why the donor level can pin n-lnP, it will be explained assuming either that the charge flow from the lnP to the metal due to the high eleetronngativity of Au and Ag compensates the donor level or that there are some aceeptor levels below the donor level [20]. We also suggest that phosphorus vacancies relate to the donor level at about 0.55 eV above the VBM because uf assc~imion of pinning here with loss of phosphorus [1,21]. The MIGS model cannot be comple*.ely dunle~d to explain the pinning unserved in non-reactive cases. There is a pussibility that the charge neutrali~ point changes by the existence of the Sb [2O].
4. Conclusions In order to comsol the surface Fermi level of InP(ll0), in other words to passivate I n P ( l l 0 ) surfaces chemically and electrically, we have proposed a new method using one monolayer of Sb as an interlayer at metal-semiconductor inter-
rm
InP~lI0) Immit'ated w$th I ML Sb
faces. The deposition of one monolayer of Sh followed by annealing at about 300~C gives nearly the flat band condition for both n- and p-type INF'. Schottky barriers formed on these annealed lnP(IIO) surfaces change strongly as compared with those on clean cleaved surfaces. Among the metals we have deposited, we have found that Au and Ag are non-reactive and form big elus~,ers, and Pd, Cu, and AI are reactive on these surfaces. A technologically important large Scbottky harrier height of 0.82 eV on n-lnP is obtained at A u / S b ( l M L ) / n - l n l ' ( l l O ) interfaces. Since this method using an Sb interlayer gives high Schottky barrier heights on n-lnP, we emphasize that it can promote the realization of electronic devices using lnP as the active layer itself, Finally, we propose a defect model to explain observed Fermi level pinning.
Acknowledgements Support for this work was provided by Fujitsu Ltd., Japan, and by D A R P A / O N R under Contract No. N00014-89-1083.
References lit M. Yamada, A.K. Wahl P.L Meissner, A. Herrera, T. Kendelewiczand W.E. Sp[cer,Appl. Phys. Len. 58tl991) 2243. [2] S.M, Sze, Physics of Semiconductor Dories (Wiley. New York, 1969). [3] R. Dingle, Semiconductors and Semimetals, Vol. 24 (Academic Press, New York, 1987)ch, 7. [41 D.L IJle. J. Va¢. Sci. Technol. B 2 (t984) 4%. [hi D.L Lile and D.A. Collins, AppL Phys. Leu. 28 (1976) 554. [6] K.P. Pande and G.G, Roberts. J. Va¢. Sei. Technol. 16 (1979) 1470, [7] H. Hasegawaand I". Sawada,Thin Solid Films 103(1983) 119. [8] G. Hollinger. R. BlancheL M. Gendw, C. Saminelli, R. Skheyta arid P. Viktorovltch,J, AppL Phys. 67 (1990) 4173. [9] J.F. Wager, SJ.T. Owen and SJ. Prasad, J. Electrochem. S~. 1340987) 160. IlO] W. Schottky, Phys. Z. 41 (Iq4fl)570. [Ill W.E. Spicer, L Lindau, P. Ske~lh, C.Y. Su and Palrick Chye, Phys. Rev. l~tt. 44 (1980)420.
M. Yamada el aL / Schottky ~rtqer fornlation on laP( I IO) passiz'ated with 1 ML Sb
[12] J. Tersoff. J, Vac. Sci, TechnoL B 3 (1985) 1157. [13] C.Y. Tiag and M. Wittmer, Thin Solid Films 96 (1982} 327. [14] P, Skeath, C.Y. Su, W.A~ Harrison, 1. Lindau and W.E, Spicer. Phys. Rcv. B 27 (1983)6246. [15[ C, Mailhiot, C.B, Duke and DJ. Chad/, Phys. Rev. B 3[ (1985) 2213. [16] M. Yamada, A.K. Wahl, T. Kend¢lewicz and W.E, Spicer, AppL Phys.Lell. 58 (1991)2701. [17] T. Kendelewicz, N. Nt,'s~man. R.S. List, I. Lmdau and W.E. Spicer, J, Va¢. 5ci. Technnl. B 3 (1985) 1206.
329
[18] N, Newman, W.E Spicer, T, Kendelewicz and I, Lindau, J. Vac. Scl. Techno|. B 4 (1986) 93L [19] N, Newman. M. van Schilfgaade and W.E. $picer, Phys. Rev. B 35 (1987) 629~. 12o] M. Yamada, A.K. Wahl, T. Kendelowicz and W,E, Spicer, Phys. Rev, B, submitted. 121j M. Yamada, CJ. Spindt. K.F.. Miyano, A, HerreraGomcz, 1". Kendelewiezand W.E. Spiccr,L Appl. Phy~,, submitted.