- Email: [email protected]
Titanium-silicon Schottky barrier diodes
Solid-State
Electronics
Pergamon Press 1970. Vol. 12, pp. 403-414.
TITANIUM-SILICON
SCHOTTKY
Printed in Great Britain
BARRIER DIODES
A. M. COWLEY HP Associates, 620 Page Mill Road, Palo Alto, California, 94304, U.S.A. (Received
11 July 1969; in revisedform
14 August
1969)
Abstract-Schottky barriers have been formed by vacuum evaporation of titanium onto chemically cleaned n- and p-type silicon. The barrier heights of the contacts were found to 0.50 and 0.61 eV for n- and p-type barriers, respectively. The barrier heights were determined from measurements of the 1 /C” vs. V characteristics, the reverse saturation current density, and the activation energy of the reverse current. The effects of fabrication technique on diode properties are discussed. It is found that p-type diodes can be fabricated using standard oxide passivation techniques, without severe degradation of most diode properties ; n-type diodes are severely degraded by the presence of silicon dioxide at the periphery of the diode, but this problem can be completely eliminated by Noise measurements are also presented for the p-type the use of a diffused p-type ‘guard-ring’. oxide-passivated diodes and for the n-type ‘guard-ring’ diodes; these diodes are found to have essentially ideal noise behavior above a few kHz. R&um6-Les barrieres Schottky ont ete produites par l’evaporation ii vide du titanium sur le silicium chimiquement propre de types n et p. Les hauteurs de barrier-e des contacts Btaient de 0,50 et 0,61 eV pour les types II etp respectivement. Les hauteurs de barribres ont ettc diterminbs des mesures des caracteristiques de l/C2 c. V, la densite de courant a saturation inverse et l’energie d’activation du courant inverse. Les effets de technique de fabrication sur les proprietes de diode sont discutes. On trouve que les diodes p peuvent &tre fabriquees en employant des techniques de passivation d’oxyde standard, sans deterioration serieuse des proprietes de diode; les diodes de type n sont sdrieusement affect&s par la presence de bioxide de silicium a la peripherie de la diode mais ce probleme peut &re completement elimine par l’emploi d’un anneau de garde diffid de type p. Des mesures de bruit des diodes de type p passivCs d’oxyde et des diodes de type n g am-reau de garde sont aussi present&es; celles-ci ont essentiellement un comportement de bruit ideal au-dessus de quelques kHz. Zusammenfassung-Durch Aufdampfen von Titan im Vakuum wurden auf chemisch vorgereinigten n- rmd p-Typ-Silizium-Kristallen Schottky-Kontakte hergestellt. Die Barrierenhijhen wurden zu 0,50 und 0,61 eV fti n- bzw. p-Typ bestimmt. Diese Werte wurden aus Messungen von l/C2 gegen V, der Siittigungsstromdichte im Sperrbereich und deren Aktivierungsenergie ermittelt. Einfhisse der Herstellungstechnik werden diskutiert. Ohne ernste Abnabme der Diodenqualitit kann bei p-Typ-Dioden die normale Passivierung durch Siliziumoxid zur Herstellung eingesetzt werden. n-Typ-Dioden dagegen werden durch Siliziumoxid an ihrer Peripherie stark geschiidigt. Dieser Einfluss kann aber vollst%ndig durch Eindiffision eines Schutzringes beseitigt werden. Rausch-Messungen werden fur die passivierten p-Typ-Dioden und fiir die Schutzringdioden aus n-Typ-Silizium wiedergegeben. Sie zeigen oberhalb von einigen kHz im wesentlichen ein ideales Rauschverhalten.
1. INTBODUCTION
THE work described in this paper was part of a broader investigation aimed at the development of low noise Schottky barrier diodes for microwave mixer and detector applications above 8 GHz. The titanium-silicon system was investigated in some
detail, and since this system has received little attention in the literature, it was thought worthwhile to present the results of this study. The barrier height, q5s, and saturation current density, J,, for titanium on chemically prepared n-type silicon are reported by ATALLA, in a
403
404
A. M. COWLEY
review article on the physics and current technology and applications of Schottky barriers. These data are presented in the form of a point on a graph of J, vs. 4s (Fig. 12 of Atalla), and were the result of an investigation of a large number of metalsilicon systems; no experimental details were given. In the present paper, the values of barrier height and saturation current density for titanium on both p- and n-type silicon are obtained; the values for n-type silicon are in reasonable agreement with those of Atalla. Most of the barrier height and saturation current density measurements reported in this paper were performed on diodes fabricated by evaporating titanium through a metal mask onto chemically cleaned silicon. This fabrication method is simple and quite adequate for these measurements. However, it has been pointed out by several workers in this field(2*3) that detailed comparison of measured Schottky barrier diode properties with simple rectification theories is often impossible because of the presence of serious leakage effects at the edges of the metal contacts. This problem is shown by these authors to be eliminated to a great extent on n-type diodes by the use of a diffused p-n junction ‘guard-ring’, which contains the periphery of the metal contact. This technique was used for some of the experiments in the present work, and titanium n-type silicon diodes were found to have reverse current-voltage characteristics which are in essentially perfect agreement with the simple image force theory of barrier lowering, using a relative dielectric constant of 12 for silicon.(4) Additional experiments were performed using simple oxide-passivated structures, in which the Schottky barriers were formed by depositing metal in a hole in thermally-grown silicon dioxide on the silicon wafer. This is a commonly used technique in the fabrication of microwave Schottky barrier diodes, and is necessary because of the extremely small geometries required in these diodes. Titanium diodes on n-type silicon were severely degraded using this technique, because of the presence of SiO, at the diode periphery. Degradation was not so severe for p-type diodes, and in fact, most parameters of interest for microwave applications were hardly affected at all by the presence of SiO, in these diodes. The electrical properties of the oxide passivated p-type diodes are compared to
oxide passivated n-type diodes of similar barrier height (nichrome); the p-type diodes were found to have more nearly ideal characteristics, and had much lower excess noise. Diode fabrication techniques are described in Section 2. Section 3 describes measurement techniques and results, and Section 4 is a discussion of the experimental results and their relevance to certain Schottky barrier diode applications. 2.
DIODE
FABRICATION
Diodes were fabricated by the vacuum evaporation of titanium onto chemically prepared silicon wafers. The titanium was obtained in the form of 0.030 in. dia. wire, and was stated to be 99.97 per cent pure by the supplier.(5) Evaporations were performed in an oil diffusion pumped vacuum system at an indicated pressure of -1 x 10m6 Torr. Tungsten wire filaments were used for the evaporations, and were cleaned and pre-outgassed at white heat in vacuum before use. The thickness of the titanium film was monitored during evaporation by means of a sheet resistance measurement; the films were evaporated to a sheet resistance of 1OQ per square. Film thicknesses were measured after evaporation with an interference microscope, and were found to be 1000 + 200A. Control of the duration of the evaporation was achieved by using a shutter between the filament and the silicon wafer; the shutter was opened after the titanium had wetted the filament and had begun to evaporate rapidly, and was closed when the sheet resistance reached 1OQ per square. Diodes formed on bare silicon surfaces were evaporated through a fine wire mesh@) in order to define the diode area. The diodes thus formed were squares with sides of about 37 pm.
/
SiOp
/
METAL
SILICON
/
lIl,/,.,l////l//i//,/,,~/,////,~////////i i FIG.
1.
OHMIC
CONTACT
Oxide-passivated Schottky structure.
barrier
diode
TITANIUM-SILICON
SCHOTTKY
Oxide passivated diodes had the structure shown in Fig. 1. The oxide was formed by thermal oxidation of the silicon wafers in steam at 1050°C. The wafers were oxidized for 30 minutes which resulted in approximately 4000 A of SiO,. Windows varying in diameter from 6pm to about 50pm were etched in the oxide using photoresist and a hydrofluoric acid etch. The wafers were cleaned in organic and inorganic solvents and rinsed in dilute hydrofluoric acid prior to evaporation of the metal. After evaporation of metal over the whole wafer, the individual diodes were etched out in a second photoresist step. The fabrication of guard-ring diodes was described in Ref. 3 ; the resulting structure is shown in Fig. 2. The guard ring was diffused to a
1,
BARRIER
DIODES
405
0.01 Q-cm) substrates to a thickness ranging from 1 to 7.5 pm. Except for the elimination of lapping and polishing, preparation of the epitaxial wafers for oxidation and/or metal deposition was the same as for the bulk wafers. 3. MEASUREMENTS (u)
Measurement techniques Barrier height measurements were obtained by three methods; (1) plotting the reciprocal square of the diode depletion capacitance (l/C’) vs. bias voltage(7); (2) calculation of barrier height from saturation current density using an assumed value of the Richardson constant; and (3) measurement of the activation energy of the diode reverse current. The l/C2 vs. V method has been described in some detail by GOODMAN.(~) Referring to the band
/
I/;CHOTTKY’ KEDRING BARRIER
n-TYPE
SILICON
//////////////////////////////////////. OHMIC
CONTACT
FIG. 2. Passivated Schottky barrier diode structure with guard-ring.
depth of 3 pm through rings cut into the oxide with photoresist. Inside and outside diameters of the ring were 45 pm and 65 pm, respectively. The active area of the Schottky barrier portion of the diodes was 1.2 x low5 cmm3. The bulk silicon wafers used in this work were cut from float-zone refined and Czochralski-grown ingots. The wafers were supplied by the Wacker Chemical Corporation. Wafer preparation consisted of mechanical lapping and polishing, followed by a chemical polish in a mixture of nitric, hydrofluoric and acetic acids. The wafers were then cleaned in organic and inorganic solvents and were rinsed in hydrofluoric acid prior to oxidation or metallization. Some of the diodes were fabricated on epitaxial wafers. The epitaxial layers were grown on low-resistivity (0.003 to
FIG. 3. Energy band diagram for metal/n-type conductor (Schottky barrier) contact.
semi-
diagram of Fig. 3 for a metal to n-type semiconductor contact, the intercept on the voltage axis of a plot of l/C2 vs. V is equal to V,o-KT/q, where I’,, is the da&ion potential. The barrier height, &o,is obtained as V,, + V,, where V, is the depth of the Fermi level below the conduction band. For ap-type semiconductor, V, is the height of the Fermi level above the valence band. The l/C2 vs. V method can be used only in cases where the contact has been formed on a uniformly doped semiconductor-the plot of l/C2 vs. V should then be a straight line. For titanium contacts on n-type silicon, we observed that l/C2 vs. V, even for uniform doping, was often not a straight line. This behavior is commonly attributed to the effects of an insulating layer between the metal and semiconductor.c7) A non-linear l/C2 vs. V plot cannot
A. M. COWLEY
406
be used reliably to determine barrier height in the absence of knowledge of the thickness and nature of the interfacial layer. The Schottky barrier current vs. voltage equation can be written I = Is(eqvikr - l),
(1)
where I, is the diode saturation current and I’is the applied voltage. For reverse voltages of about 4 kT/q (100 mV at 296°K) or greater, 1 is equal to -I,, in the absence of any leakage or other spurious effects. In the emission model(*) for a Schottky barrier, 1, is given by - q[&, --A$(
I, = AA*T2exp
VI
kT where A is the diode area, A* is the effective Richardson constant, and &o is the barrier height for zero field (Fig. 3). The quantity A+(v) is the image force correction to the barrier height, and is given byc4) A+(V) = ~$$(&o-G~)]“’
(3)
where ND,4 is the donor or acceptor density, la is the semiconductor dielectric constant, and I’,, is the diffusion potential of the Schottky barrier. The Richardson constant must be known in order to use (2) for determining the barrier height. For chemically prepared n-type silicon, A* is of the order of 100, from ATALLA( the surface preparations used in this work are essentially the same as those used to obtain the results quoted by Atalla. There has been no determination of A* for
similarly prepared P-type silicon contacts, to the author’s knowledge.7 However, in this work, it is found for the Ti-p-type silicon diodes that A* 21 100 gives reasonable agreement between &o determined from saturation current and &,o determined from l/C2 vs. V plots. The activation energy method of determining barrier height is based on (2). The quantity In (IS/T2) is plotted against l/T. If &, and A+(O) are not functions of T, then the slope of the plot is proportional to &A+(O). As CROWELL and SZE@) point out, however, the plot gives &oT(d&,/dT) - A+(O) for a temperature-dependent barrier height; this is also a well-known result from thermionic emission theory. If an accurate independent measurement of&, is available, then the activation energy measurement can, in principle, give the value of d$eo/dT. (6) n-type Si-Ti diodes (i) Barrier height measurements The results of barrier height determinations from l/C2 vs. Y data and from saturation current density measurements are shown in Table 1 for some typical diodes fabricated on n-type silicon. The measured saturation current density J, is corrected for the image force barrier lowering by multiplying by the factor exp( - qAq%/kT) and the t A value of 32 A/cm2-“K2 was given for A* by J. M. ANDREWS, JR. and M. P. LEPSEL.TER(IEEE Int. Electron Device Meeting, Washington, D.C., October 1968), for rhodium silicide on p-type silicon. However, it is not clear that this value applies in the present case because of the radically different method of forming the silicide contact.
Table 1. Barrier height measurements on titanium-n-type
Wafer No.
Fabrication method
Doping density (cmm3)
J EnI (A/cm?
A+(V = 0) (V)
silicon diodes
JS (A/cm”) (V)
13, 14c 17a MS-l 86~
Mesh Mesh Mesh Mesh Guard-ring
: A* = 100 A/ems-
‘Ka.
1.1 1.1 1.8 1 .o 5.0
x x x x x
10’6 101s 101s lo’s 101s
0.056 0.053 0.062 0.060 0.070
0.020 0.020 0.025 0.020 0.017
0.025 0.023 0.026 0.030 0.035
0.505 0.505 0.505 0.500 0.495
0.41-0.44 0.55-0.57 0.45-0.5s 0.54-0.57 -
TITANIUM-SILICON
SCHOTTKY
corrected value is denoted by J,. The barrier height is then determined as (KT/q)ln(A*P/J,). The values for J, are closely distributed around an average value of 0.028 A/cm2, leading to a value of 0.500~0.010 volts for the barrier height $so, for A* = 100 A/cm2-“K2. Values of&o determined from l/C2 vs. k’ plots were scattered between about 0.4OV and 0.6OV. As an example, wafers 13, and 14, were prepared on identical silicon slices; values of J, are in good agreement, but &o from l/C2 vs. P’ data differ by more than 100 mV. This general behavior is thought to arise from the presence of a nonreproducible and possibly non-uniform (patchy) interfacial layer with thickness of the order of monolayers between the titanium and the silicon. The l/C2 vs. I’ plots for the diodes also deviated somewhat from linearity in the reverse bias range of 0 to several volts; it was usually a matter of judgement as to the correct linear extrapolation of these plots to the voltage axis. In other experiments using the same silicon slices, wafer preparation, and measurement equipment, it was verified that linear l/C2 vs. V plots were obtained for other metal contacts, e.g. nichrome. This rules out the possibility of measurement error and/or nonuniform semiconductor doping, and would suggest that this behavior is peculiar to the titaniumsilicon system. Following TURNER and RHODERICK,‘~~) the titanium-silicon diodes were subjected to storage and accelerated aging cycles. For n-type diodes, the effect of several months storage and of 30 min heating at 200°C on J, was essentially negligible; the largest change was a decrease of about 20 per cent after two months of storage in nitrogen. The effect of heating and storage on the l/C2 vs. V plots varied; the plots remained non-linear in all cases, and both decreases and increases were observed in the intercept on the V-axis. No consistent trend was noted in these changes. In view of the lack of a reliable l/C2 vs. V determination of barrier height, it was desirable to have an independent measurement to corroborate the value of &o obtained from saturation current density measurements. Accordingly, the activation energy of the reverse current for diodes from wafer E-86, was measured between 230 and 350°K; the results on a typical diode are shown in Fig. 4. The activation energy at a
BARRIER
DIODES
407
reverse bias of -O.lV was 0.50 eV; the error in this determination is estimated to be less than + .OlO eV. The activation energies for - 1V and - 5V were also measured, and were 0.50 eV and 0.49 eV, respectively. These latter values are consistent, within the estimated error, with the
lOOO/
T
(OK-‘)
FIG. 4. Activation energy plots for reverse current of Ti/n-type
silicon contact with guard-ring.
additional barrier lowering relative to v = - O.lV calculated from (3). The image force lowering for V = -O.lV is -O.O2V, so &o- T(d&o/dT) is 0.52+ .Ol eV. If $so is taken to be the average of the values computed from J, in Table 1, 0.50+ .Ol eV, then (d&o/dT) could be between zero and N - 1 x 10e5 eV/“K at 300°K; this is less temperature dependence than observed for n-type silicongold barriers by CROWELL,et aZ.(ll) An apparent lack of temperature dependence has been reported previously for n-type silicon-chromium barriers.‘12) (ii) Voltage-current
characteristics
The voltage-current characteristic was measured at 296°K for guard-ring diodes from wafer E-86,; reverse characteristic for a typical diode is shown in Fig. 5. Below about 20 V, the reverse current is in essentially exact agreement with image force lowering theory.(4) Above 20 V, an excess current component, presumably leakage, is observed. Avalanche breakdown for this diode was clearly visible at 90 V and about 100 PA; the breakdown for these diodes is controlled by surface effects at the edge of the p-n junction guard-ring.
408
A. M. COWLEY
E-86-6 Ti GUARD-RING
DIODE
A=1 2~10~crn~ N,=5~lO’~crii~ T=297”K i
,a’L 0
1.0
2.0
(v+vsa+)
3.0 l/4
FIG. 5. Reverse current of Ti/n-type silicon guard-ring diode. Solid line is the reverse current calculated from (2) and (3).
The forward current-voltage characteristic of a Schottky barrier is usually determined by plotting In I vs. forward bias voltage, and then fitting this data to an equation of the form I = Iso(e svinw _
1)
where n is the diode ‘ideality factor’, and is usually adjusted empirically to make the equation fit the data; I,, is the saturation current obtained by extrapolating the plot to Y = 0. This technique could not be used effectively with the guard-ring diodes because the series resistance of the diodes (N60LR) produced significant voltage drop at currents only a decade or so greater than the saturation current. An alternative to this technique is to measure the diode a.c. resistance, given by R = R,+-p
nkT q(I+ IS)
where R, is the series resistance and the right-hand term is the dynamic barrier resistance, obtained by differentiating (4). By (5), R vs. (1+1,)-l should be a straight line with slope nkT/q and intercept R,. This measurement was performed on guardring diodes and the result for a typical diode is shown in Fig. 6. The value of n determined from
FIG. 6. -Alternating current resistance of Ti/n-type silicon guard-ring diode versus reciprocal current. Indicated values of n and R, are 1.03 and 58 Q, respectively.
the plot is 1.03, which agrees exactly with the value of n calculated from the image force effect.t The measurement of R was performed at a frequency of 10 kHz, and is accurate to within several tenths of a percent. (iii) Noise measurements
The noise properties of a biased Schottky barrier diode have been discussed in previous papers.(3J2) It was shown in Ref. 12 that the noise temperature ratio t, of the barrier is given by nr,o+(n-l)I
1 tB=Z
i l+
nr,, + .z
(6)
where I is the bias current and the other terms are as defined in (4). For a practical diode, (6) must be t Following CROWELL and SZE,(~~) 12 is given by n = l/[l -(d4a/dV)J, where 4B is the zero-field barrier height ~$*aminus barrier lowering effects. If the imageforce lowering, A$, is considered to be the sole source of barrier lowering, then 12 = l/[l +(dA$/dV)J and from (3), we have (dA+/dV) = -2. [A+(V)/(V,,-V-kT/q)] where I, is the applied forward bias voltage. A forward voltage of 0.15 V was used to calculate n for this diode; this was the midpoint of the voltage range used to obtain the data in Fig. 6.
TITANIUM-SILICON
corrected
for the series resistance
t=
SCHOTTKY
R, as follows
Rds + Rs R,fR,
(7)
*
The quantity R, is the dynamic barrier resistance nkZ’/q(l+l,). The physical significance of noise temperature ratio is that it is the available noise power of the diode divided by kTB where T is the ambient temperature in degrees Kelvin, and B is the measurement bandwidth. The measured noise temperature ratio for a typical guard-ring diode is shown in Fig. 7 as a function of frequency, with IO
z Y’ 2
DIODES
,
I SILICON
409
the diameter of the oxide window was 22 pm. These diodes invariably had extremely poor rectification characteristics; the reverse current below 1 V was typically several orders of magnitude greater than the nominal saturation current calculated from Table 1. The general appearance of the reverse v-1 characteristic was ‘leaky’, i.e. the saturating behavior usually associated with diode reverse I/-I characteristics was essentially absent, and the reverse current tended instead to increase very rapidly with reverse voltage. This behavior for passivated diodes on n-type silicon has been studied in some detail in our laboratories,(3) as well
I TITANIUM
Q 5
BARRIER
GUARD-RING
DIODE
E 86 B
5-
I
I n= 1.03 R,=940n i Rs=60n
Y
I Lo5
-_.---b--_
THEORETICAL:
MEASUREMENT
FREQUENCY
FIG. 7. Noise temperature guard-ring
t = 0.56
-
Hz
ratio of T&type diode vs. frequency.
the bias adjusted for a total diode dynamic resistance of 1OOOQ.The value calculated from (6) and (7) is shown for comparison, and the agreement is essentially exact above 10 kHz. Below 10 kHz excess noise having an approximate f-r spectrum is also present. This behavior is similar to that of the Mo-Si guard-ring diodes described in Ref. 3. The ‘noise corner frequency’, fN,defined as the frequency where the excess noise component is equal to the ‘white’ component calculated from (6) and (7), is a few hundred Hertz for the diode of Fig. 7. (iv) Oxide-Passivated diodes
Titanium diodes having the structure of Fig. 1 were fabricated on wafers of n-type silicon with nominal resistivity ranging from 0.3 to 0.7 Q-cm;
silicon
as by other workers,(2J3) and there is general agreement that the ‘leaky’ or ‘soft’ appearance of the reverse characteristics is due to leakage currents at the periphery of the contacts. The problem appears to be more pronounced for titanium contacts, compared to other metals such as Ni, Cr, MO, Au and various silicides; this is apparently associated with the somewhat lower barrier height for titanium compared to the other metals. The leakage is sufficient in the titanium diodes to make them useless for most diode applications. The noise temperature ratio for these diodes was also measured at 1.0 kHz for forward bias currents ranging from several tens of microamperes up to N 1.5mA;valuesbetween400and800wereobserved essentially independent of bias current. This is several orders of magnitude more noise than for
410
A. M. COWLEY
the diode of Fig. 7 and the diodes described in References 3 and 12, but is fairly typical for simple oxide-passivated diodes on n-type silicon, independent of the contact metal. As pointed out in Ref. 3, this excess is also associated with the periphery of the contact, where the Si, SiO, and metal come together, and it is almost completely eliminated by the use of the p-n junction guard-ring, as demonstrated in Fig. 7. diodes
(c) p-type Si-Ti (i) Barrier height measurements
Barrier height measurements were performed on p-type silicon-titanium diodes fabricated using the ‘mesh’ technique. The properties of p-type silicon-titanium diodes were found to be quite sensitive to heating of the diodes during and after the titanium evaporation. Specifically, the diode characteristics showed an annealing behavior with mild heating; the zero-bias capacitance of the diodes would increase by a factor of 2 or 3 at 100°C after a few minutes and the saturation current would typically double. Further heating at temperatures up to -300°C produced no further change, and the change was irreversible. A 2OO”C, S-min heat treatment was subsequently adopted as a standard part of the fabrication process. Diodes tested before treatment were found to have quite non-reproducible characteristics; in particular, the l/C2 vs. V plots were non-linear, and the values of C, and I, for diodes of the same nominal area varied considerably over a given wafer. The latter effect is assumed to arise from a partial, non-
uniform annealing of the diodes due to heating during the titanium evaporation. After annealing at 100°C or 200°C however, the diode properties became very uniform, and the l/C2 vs. I’ plots were linear. The mechanisms for this annealing behavior can only be guessed at. The behavior of the diode capacitance strongly suggests that an interfacial layer exists after the titanium is evaporated, and that this layer is somehow removed by heating. Mechanisms consistent with this hypothesis would be the reaction of the titanium film with the monolayers of oxygen and/or fluoride ions present on the silicon prior to evaporation, or simply a rearrangement of the titanium atoms during heating, leading to a more intimate metal-semiconductor contact. The data presented in Table 2 were taken on six wafers of Ti-p-type silicon diodes which had been subjected to 200°C 5 min annealing. Wafers 18,, 20,, 21, and 94 were epitaxial wafers with non-uniform impurity density; as discussed in Section II, the l/C2 vs. V plots for such wafers cannot be used for determining barrier height. The nominal surface doping estimated from the zero-bias capacitance was used to calculate the image force lowering; since A$ is only weakly dependent on doping, this calculation is expected to be sufficiently accurate for this purpose. The zero-field saturation current density averages about 4.0 x 10m4 A/cm2; a Richardson constant of 100 A/cm2-OK2 gives $no = 0.61 V, consistent with the values of &o from the two l/C2 vs. I’ measurements.
Table 2. Barrier height measurements on titanium/p -type silicon diodes
No.
Fabrication method
1% 2% 21B 22, z-1 94
Mesh Mesh Mesh Mesh Mesh Mesh
Wafer
t A* = 100 A/cm2-‘K2.
Doping density (cme3)
1.0 x 2.0 x 7.0 x 3.0 x 2.5 x “ig.
1016 1016 1016 1017 101’ (9)
J 8nl
(A/cm’)
6.65 x10-4 1.20x10-3 1.43 x10-3 3.70x10-3 1.25~10-~ 1.2 x10-3
A$(v=O) (V)
J, (A/cm’)
+BO~(JS) 44/C2 vs. V (V) (Y)
0.022 0.025 0.037
2.8x10-* 4.3x10-4 3.4x10-4
0.62 0.61 0.615
0.048
5.7 x 10 -4
0.60
0.027 0.033
4.3x10-4 3.3x10-4
0.61 0.615
0.60
0.62 -
TITANIUM-SILICON
SCHOTTKY
(ii) Oxide-passivated diodes
Titanium diodes having the structure of Fig. 1 were also fabricated on p-type silicon. In contrast to the n-type passivated diodes described in Section 3b, diodes fabricated on 0.1-0.8 Q-cm p-type silicon were not seriously degraded by the SiO, passivating film. The most noticeable effect of the passivating film was a degradation of the reverse current-voltage characteristic at higher voltages ; a general ‘softening’ of the reverse current was observed above 5 or 10 V, depending on the resistivity of the silicon. For resistivity higher than about 1 Q-cm, reverse currents many orders of magnitude higher than the nominal Schottky barrier diode saturation current were often observed; the current had the general appearance of a large saturation current, and was accompanied by an increase in the capacitance of the contact. Both current and capacitance were strongly dependent upon the intensity of light incident on the wafer during the measurement. This behavior is evidently due to inversion of the silicon surface around the contact by the fixed positive charge in the passivating SiO, layer. The same phenomenon has been observed by YU and SNOW for platinum-silicide diodes on n-type silicon; they induced an inversion layer around the contact by using a strongly negativelybiased guard-ring over the oxide surrounding the contact. It has been found in the present case, from MOS C-v measurements(l*) on the SiO, layers, that the oxide charge (typically +2-3 x 1011 cm- 2, is indeed sufficient to invert the surface of p-type silicon in the range of 1 Q-cm or higher. The remainder of the section will be devoted to the discussion of passivated diodes formed on O.l0.8 Q-cm p-type silicon. Of particular interest will be the comparison of these diodes with diodes of similar barrier height (nichrome) on n-type silicon, fabricated in exactly the same way. (1) Current-voltage characteristics. The currentvoltage characteristic for the passivated diodes followed a dependence of the form of (4), with n ranging from 1.04 to 1.08 for doping levels in the 5 x 10” cmm3 range. The calculated value of n is between 1.02 and 1.03. Diodes fabricated on the same silicon slices by rinsing off the oxide with hydrofluoric acid and evaporating the titanium through a mesh, as described in Section 2, had characteristics virtually identical to those of the 2
BARRIER
DIODES
411
passivated diodes. This would suggest that the departure of n from ideality is an inherent property of this particular method of forming the titaniumsilicon interface, rather than an edge effect. Saturation current densities of passivated diodes and mesh diodes fabricated on the same wafer were also identical, and were in agreement with the values listed in Table 2. Most of the passivated diodes fabricated during this study were designed as microwave video detectors at 10 GHz. The diodes were fabricated with an active area of 5 x lo-’ cm2 on thin (-1~) epitaxial silicon which was grown on a heavily-doped (N 0.003 R-cm) substrate. Doping profiles for these epitaxial layers were measured using the capacitance-voltage technique;(15) the layers were found to be graded, with the more lightly-doped material being near the surface. The grading was found to be due mostly to the depletion of boron from the surface during the steam oxidation,(16) as was verified by profiling wafers before and after oxidation. Reverse current for these small diodes increased more rapidly with voltage than image force lowering theory predicts, based on (3). This is partly due to the non-uniform doping in this case, but measurements on mesh diodes fabricated on uniformly doped material show a qualitatively similar disagreement with (3) for the reverse direction. Below 5 V or so, the diodes exhibited good rectification characteristics. At 5 or 6 V, the diodes began to conduct heavily, with 10 PA typically being reached between 6 and 8 V. The true avalanche breakdown voltage for these wafers, as determined from mesh diodes, was between 12 and 14 V. (2) Edge effects-comparison with NiCr-n-type siZicon diodes. Several experiments were performed to investigate the importance of leakage currents at the periphery of the passivated diodes. It has been well established at this laboratory, and confirmed by Yu and SNOW, that for diodes on n-type silicon, these edge effects dominate the current-voltage characteristics for reverse bias and for low forward bias. The results of one set of experiments, in which diodes having diameters ranging from 13 to 50 pm were fabricated simultaneously on the same silicon wafer, are shown for NiCr/n-type Si diodes in Fig. 8. The reverse current at -0.1 V and the forward current at
412
A. M. COWLEY
FWD.
t
/SLOPE
CONTACT
CURRENT
= I
DIAMETER-pm
CONTACT
DIAMETER-pm
FIG. 8. Forward and reverse current for NiCr/n-type silicon oxide-passivated diodes vs. diode diameter.
FIG. 9. Forward and reverse current for Ti/P-type silicon oxide-passivated diodes vs. diode diameter.
+0.3 V are shown plotted against diode diameter. The reverse currents depend linearly on diameter showing that most of the current is flowing at the edges of the diodes. The forward current has a slope lying between 1 and 2, showing that while a significant current flows in the whole area of the diode, the current density is higher at the edges, These diodes were fabricated on 0.4 Q-cm, 4 pm n on n+ epitaxial silicon slices. The data were averaged over many diodes of each size on the slice. This data shows that in the range of diode diameters important for most practical applications, the n-type passivated diodes are dominated by effects at the edges of the diodes. This phenomenon is thought to be the result of accumulation of the n-type silicon surface by the positive charge in the oxide surrounding the diodes.(2s13) The accumulation of negative charge in turn causes the depletionlayer field at the edge of the diode to be much higher than in the remainder of the diode, and this gives rise to increased current flow due to enhanced image-force lowering and tunneling through the barrier.(13) The experiment was repeated for titanium diodes on 0.8 Q-cm, 7.5 pm p-p+ epitaxial silicon wafers. The results are shown in Fig. 9. Here it is
seen that the currents are proportional to the diode area, and hence, edge leakage is not an important factor in the current flow mechanism for these diodes. These diodes were fabricated in exactly the same fashion as the NiCr/n-type diodes, using SiO, films grown in the same oxidation furnace. It is suggested that the edge effects are not present in the p-type diodes because the surface of the silicon around the diodes is depleted in this case for resistivities below 1 R-cm. The depletion of the surface around the Schottky barrier diodes does not give rise to high fields at the diode periphery, as does accumulation in the n-type case, and it is conceivable that the depletion layer field at the diode periphery is actually reduced for the p-type diodes. Further observations on p-type and n-type diodes which seem to be related to these effects are described below. (3) Activation energy of reverse current for n- and p-type diodes. The activation energy of the reverse current at -0.1 V was measured for Ti/p-type Si and NiCr/n-type Si diodes, and typical results are shown in Fig. 10. The activation energy for the Ti/p-type diodes is about 0.60 eV; the image force lowering in this case is 0.030 V, so this measurement would be consistent with a barrier height of
TITANIUM-SILICON
0 NiCr/N-TYPE
SCHOTTKY
BARRIER
DIODES
413
The parameter fNis the ‘noise corner frequency’ and is a measure of the severity of the excess component of the noise. For the data of Fig. 11, an fN of -3 kHz is indicated for the Ti/p-type diode while the NiCr/n-type diode has an fN of -2 MHz. The p-type diode shown here is quite
Si (N~=l.5x10’6cm~3;
I-
I
Q NiCr/N-TYPE
(ND~7w10’6cni’ A=l$cm*)
0 Ti/P-TYPE
.Ol
3.0
3.5 (OK-‘)
y
4.0
FIG. 10. Activation energy of the reverse current at -0.1 V for typical oxide-passivated Ti/p-type silicon and NiCr/n-type silicon diodes. 0.63 V, which is reasonable in light of the values for &o listed in Table 2. In contrast to the Ti/p-type Si diodes, the activation energy for the NiCr/n-type Si diodes is much less than the barrier height (&o 3: 0.65 eV) and, in general, the temperature dependence of the reverse current cannot be described by a single activation energy. The low values of activation energy are evidently associated with currents at the edges of the diodes, possibly tunneling current in combination with the normal thermionic current. (4) Noise temperature ratio. As a final comparison between the n- andp-type oxide-passivated diodes, the noise temperature ratio as a function of frequency is shown in Fig. 11, for a bias current of about 30pA, corresponding to a total diode dynamic reistance of 1OOOQ. The n-type diode shows a large component of excess noise having an f-l spectrum, while the p-type diode shows essentially the expected shot noise for an ideal Schottky barrier (t = OS), together with a much smaller component of excess noise also having an f-’ spectrum. For diodes having this type of spectrum, an equation can be fitted to the data, having the form tLz0.5
(
I+f”.
f >
(N*.~xIO’~C~~~;
FREQUENCY - Hz
FIG. 11. Noise temperature ratio vs. frequency for Ti/p-type and NiCr/n-type silicon diodes for similar semiconductor doping and diode area. typical. The n-type diode is worse than typical; the noise corner of these diodes varies widely, from as low as 50 kHz to as high as or higher than the value shown here. The difference in noise behavior for the two types of diodes is evidently associated with the edge effects discussed above. 4. DISCUSSION The results presented here provide the information essential for the design of Schottky barrier diodes using the titanium-silicon system, namely the barrier height and saturation current density. In addition, the effects of a thermally-grown passivating SiO, layer have been investigated, and it has been found in the case of p-type diodes that the essential characteristics of the contact are not seriously altered by the presence of SiO,, in contrast to contacts on n-type silicon with any metal. This property has important ramifications for the fabrication of microwave detector and Doppler mixer diodes, where low noise at video frequencies is important, and where the use of a guard-ring is undesirable because of its additional capacitance.
414
A. M. COWLEY
The lower barrier height of the titanium/n-tvne silicon system compared to the more commonlyused metals such as Au, Ni, Cr, MO, etc., suggests its use in current-limiting applications, where reverse saturation currents of tens of hundreds of microamperes are often desired without the necessity for extremely large-area devices.(l’) Use of the guard-ring would make such a device completely comp&ble with standard monolithic integrated circuit techniques. The lower barrier height on n-type silicon also reduces the voltage drop at a given forward current compared to the other metals; this property is sometimes useful in computer logic circuit design because it reduces power dissipation and voltage offsets. Again, the guard-ring is desirable in order to be able to monolithically integrate the diode. The different behavior of SiO,-passivated Ti/p-type and NiCr/n-type diodes can be understood reasonably well in terms of the model suggested by Yu and SNOW, i.e. accumulation of the n-type surface and depletion of the p-type surface by the thermally-grown oxide. This would suggest that the properties of the n-type diodes could be improved by a change in oxide properties; other things being equal, it is normally preferable to fabricate diodes on n-type silicon due to the higher mobility for electrons. The sum of the barrier heights of a given metal on p- and n-type samples of a given semiconductor should equal the bandgap of the semiconductor. This is evidently the case for the titanium-silicon contacts studied here, and has also been verified for Au, NiCr, MO, Cr and Al by the present author (unpublished). I
,I
Acknowledaement-The
author would like to exnress his
appreciation to a number of persons who assisted in this
work. Mrs. M. BRYAN fabricated most of the diodes, and Messrs. R. C. PATTERSON and M. R. GAVETTE
performed many of the measurements and calculations.
The epitaxial wafers were grown by Messrs. R. R. HANEY and J. E. LANGLEY. The supervision and advice of R. A. ZETTLER during the course of the work are also
greatly appreciated.
REFERENCES Symp., 1. M. M. ATALLA, Proc. 1966 Microelectron. Munich, Germany, pp. 123-157. 2. M. P. LEPSELTER and S. M. SZE, Bell Syst. tech. J. 47, 195 (1968). 3. R. A. ZETTLER and A. M. COWLEY, IEEE Trans. Electron Devices m-16, 58 (1969). (1964). 4. S. M. SZE, et al., J. appl. Phys. 35.2534 5. Electronic Space Products, Inc., Los Angeles, Calif. 6. Buckbee Mears Co., St. Paul, Minn. 7. A. M. GOODMAN. J. aabl. Phvs. 34. 329 (1963). 8. H. K. HENISCH, .Rect&ing Semiconductor Contacts, pp. 196-201. Oxford University Press, New York (1957). Electron. 9. C. R. CROWELL and S. M. SZE, Solid-St. 9, 1035 (1966). 10. M. J. TURNER and E. H. RHODERICK, Solid-St. Electron. 11,291 (1968). Il. C. R. CROWELL, et al., Appl. Phys. Lett. 4,91 (1964). 12. A. M. COWLEY and R. A. ZETTLJXR,IEEE Trans. Electron Dewices ED-15, 761 (1968). 13. A. Y. C. Yu and E. H. SNOW. J. at&. Phvs. 39. 3008 (1968). al., Solid-St. Electron. 8,145 (1965). 14. A. S. GRovqet 15. C. 0. THOMAS, et al., J. electrochem. Sot. 109, 1055 (1962). 16. A. S. GROVE, et al., J. appl. Phys. 35, 2695 (1964). 17. R. A. ZETTL.ER and R. W. SOSHEA. IEEE Trans. Electron Devices (Abstract) ED-13; 817 (1966). 18. C. A. MEAD and W. G. SPITTER, Phys. Rev. Lett. 10,471 (1963).
Electronics
Pergamon Press 1970. Vol. 12, pp. 403-414.
TITANIUM-SILICON
SCHOTTKY
Printed in Great Britain
BARRIER DIODES
A. M. COWLEY HP Associates, 620 Page Mill Road, Palo Alto, California, 94304, U.S.A. (Received
11 July 1969; in revisedform
14 August
1969)
Abstract-Schottky barriers have been formed by vacuum evaporation of titanium onto chemically cleaned n- and p-type silicon. The barrier heights of the contacts were found to 0.50 and 0.61 eV for n- and p-type barriers, respectively. The barrier heights were determined from measurements of the 1 /C” vs. V characteristics, the reverse saturation current density, and the activation energy of the reverse current. The effects of fabrication technique on diode properties are discussed. It is found that p-type diodes can be fabricated using standard oxide passivation techniques, without severe degradation of most diode properties ; n-type diodes are severely degraded by the presence of silicon dioxide at the periphery of the diode, but this problem can be completely eliminated by Noise measurements are also presented for the p-type the use of a diffused p-type ‘guard-ring’. oxide-passivated diodes and for the n-type ‘guard-ring’ diodes; these diodes are found to have essentially ideal noise behavior above a few kHz. R&um6-Les barrieres Schottky ont ete produites par l’evaporation ii vide du titanium sur le silicium chimiquement propre de types n et p. Les hauteurs de barrier-e des contacts Btaient de 0,50 et 0,61 eV pour les types II etp respectivement. Les hauteurs de barribres ont ettc diterminbs des mesures des caracteristiques de l/C2 c. V, la densite de courant a saturation inverse et l’energie d’activation du courant inverse. Les effets de technique de fabrication sur les proprietes de diode sont discutes. On trouve que les diodes p peuvent &tre fabriquees en employant des techniques de passivation d’oxyde standard, sans deterioration serieuse des proprietes de diode; les diodes de type n sont sdrieusement affect&s par la presence de bioxide de silicium a la peripherie de la diode mais ce probleme peut &re completement elimine par l’emploi d’un anneau de garde diffid de type p. Des mesures de bruit des diodes de type p passivCs d’oxyde et des diodes de type n g am-reau de garde sont aussi present&es; celles-ci ont essentiellement un comportement de bruit ideal au-dessus de quelques kHz. Zusammenfassung-Durch Aufdampfen von Titan im Vakuum wurden auf chemisch vorgereinigten n- rmd p-Typ-Silizium-Kristallen Schottky-Kontakte hergestellt. Die Barrierenhijhen wurden zu 0,50 und 0,61 eV fti n- bzw. p-Typ bestimmt. Diese Werte wurden aus Messungen von l/C2 gegen V, der Siittigungsstromdichte im Sperrbereich und deren Aktivierungsenergie ermittelt. Einfhisse der Herstellungstechnik werden diskutiert. Ohne ernste Abnabme der Diodenqualitit kann bei p-Typ-Dioden die normale Passivierung durch Siliziumoxid zur Herstellung eingesetzt werden. n-Typ-Dioden dagegen werden durch Siliziumoxid an ihrer Peripherie stark geschiidigt. Dieser Einfluss kann aber vollst%ndig durch Eindiffision eines Schutzringes beseitigt werden. Rausch-Messungen werden fur die passivierten p-Typ-Dioden und fiir die Schutzringdioden aus n-Typ-Silizium wiedergegeben. Sie zeigen oberhalb von einigen kHz im wesentlichen ein ideales Rauschverhalten.
1. INTBODUCTION
THE work described in this paper was part of a broader investigation aimed at the development of low noise Schottky barrier diodes for microwave mixer and detector applications above 8 GHz. The titanium-silicon system was investigated in some
detail, and since this system has received little attention in the literature, it was thought worthwhile to present the results of this study. The barrier height, q5s, and saturation current density, J,, for titanium on chemically prepared n-type silicon are reported by ATALLA, in a
403
404
A. M. COWLEY
review article on the physics and current technology and applications of Schottky barriers. These data are presented in the form of a point on a graph of J, vs. 4s (Fig. 12 of Atalla), and were the result of an investigation of a large number of metalsilicon systems; no experimental details were given. In the present paper, the values of barrier height and saturation current density for titanium on both p- and n-type silicon are obtained; the values for n-type silicon are in reasonable agreement with those of Atalla. Most of the barrier height and saturation current density measurements reported in this paper were performed on diodes fabricated by evaporating titanium through a metal mask onto chemically cleaned silicon. This fabrication method is simple and quite adequate for these measurements. However, it has been pointed out by several workers in this field(2*3) that detailed comparison of measured Schottky barrier diode properties with simple rectification theories is often impossible because of the presence of serious leakage effects at the edges of the metal contacts. This problem is shown by these authors to be eliminated to a great extent on n-type diodes by the use of a diffused p-n junction ‘guard-ring’, which contains the periphery of the metal contact. This technique was used for some of the experiments in the present work, and titanium n-type silicon diodes were found to have reverse current-voltage characteristics which are in essentially perfect agreement with the simple image force theory of barrier lowering, using a relative dielectric constant of 12 for silicon.(4) Additional experiments were performed using simple oxide-passivated structures, in which the Schottky barriers were formed by depositing metal in a hole in thermally-grown silicon dioxide on the silicon wafer. This is a commonly used technique in the fabrication of microwave Schottky barrier diodes, and is necessary because of the extremely small geometries required in these diodes. Titanium diodes on n-type silicon were severely degraded using this technique, because of the presence of SiO, at the diode periphery. Degradation was not so severe for p-type diodes, and in fact, most parameters of interest for microwave applications were hardly affected at all by the presence of SiO, in these diodes. The electrical properties of the oxide passivated p-type diodes are compared to
oxide passivated n-type diodes of similar barrier height (nichrome); the p-type diodes were found to have more nearly ideal characteristics, and had much lower excess noise. Diode fabrication techniques are described in Section 2. Section 3 describes measurement techniques and results, and Section 4 is a discussion of the experimental results and their relevance to certain Schottky barrier diode applications. 2.
DIODE
FABRICATION
Diodes were fabricated by the vacuum evaporation of titanium onto chemically prepared silicon wafers. The titanium was obtained in the form of 0.030 in. dia. wire, and was stated to be 99.97 per cent pure by the supplier.(5) Evaporations were performed in an oil diffusion pumped vacuum system at an indicated pressure of -1 x 10m6 Torr. Tungsten wire filaments were used for the evaporations, and were cleaned and pre-outgassed at white heat in vacuum before use. The thickness of the titanium film was monitored during evaporation by means of a sheet resistance measurement; the films were evaporated to a sheet resistance of 1OQ per square. Film thicknesses were measured after evaporation with an interference microscope, and were found to be 1000 + 200A. Control of the duration of the evaporation was achieved by using a shutter between the filament and the silicon wafer; the shutter was opened after the titanium had wetted the filament and had begun to evaporate rapidly, and was closed when the sheet resistance reached 1OQ per square. Diodes formed on bare silicon surfaces were evaporated through a fine wire mesh@) in order to define the diode area. The diodes thus formed were squares with sides of about 37 pm.
/
SiOp
/
METAL
SILICON
/
lIl,/,.,l////l//i//,/,,~/,////,~////////i i FIG.
1.
OHMIC
CONTACT
Oxide-passivated Schottky structure.
barrier
diode
TITANIUM-SILICON
SCHOTTKY
Oxide passivated diodes had the structure shown in Fig. 1. The oxide was formed by thermal oxidation of the silicon wafers in steam at 1050°C. The wafers were oxidized for 30 minutes which resulted in approximately 4000 A of SiO,. Windows varying in diameter from 6pm to about 50pm were etched in the oxide using photoresist and a hydrofluoric acid etch. The wafers were cleaned in organic and inorganic solvents and rinsed in dilute hydrofluoric acid prior to evaporation of the metal. After evaporation of metal over the whole wafer, the individual diodes were etched out in a second photoresist step. The fabrication of guard-ring diodes was described in Ref. 3 ; the resulting structure is shown in Fig. 2. The guard ring was diffused to a
1,
BARRIER
DIODES
405
0.01 Q-cm) substrates to a thickness ranging from 1 to 7.5 pm. Except for the elimination of lapping and polishing, preparation of the epitaxial wafers for oxidation and/or metal deposition was the same as for the bulk wafers. 3. MEASUREMENTS (u)
Measurement techniques Barrier height measurements were obtained by three methods; (1) plotting the reciprocal square of the diode depletion capacitance (l/C’) vs. bias voltage(7); (2) calculation of barrier height from saturation current density using an assumed value of the Richardson constant; and (3) measurement of the activation energy of the diode reverse current. The l/C2 vs. V method has been described in some detail by GOODMAN.(~) Referring to the band
/
I/;CHOTTKY’ KEDRING BARRIER
n-TYPE
SILICON
//////////////////////////////////////. OHMIC
CONTACT
FIG. 2. Passivated Schottky barrier diode structure with guard-ring.
depth of 3 pm through rings cut into the oxide with photoresist. Inside and outside diameters of the ring were 45 pm and 65 pm, respectively. The active area of the Schottky barrier portion of the diodes was 1.2 x low5 cmm3. The bulk silicon wafers used in this work were cut from float-zone refined and Czochralski-grown ingots. The wafers were supplied by the Wacker Chemical Corporation. Wafer preparation consisted of mechanical lapping and polishing, followed by a chemical polish in a mixture of nitric, hydrofluoric and acetic acids. The wafers were then cleaned in organic and inorganic solvents and were rinsed in hydrofluoric acid prior to oxidation or metallization. Some of the diodes were fabricated on epitaxial wafers. The epitaxial layers were grown on low-resistivity (0.003 to
FIG. 3. Energy band diagram for metal/n-type conductor (Schottky barrier) contact.
semi-
diagram of Fig. 3 for a metal to n-type semiconductor contact, the intercept on the voltage axis of a plot of l/C2 vs. V is equal to V,o-KT/q, where I’,, is the da&ion potential. The barrier height, &o,is obtained as V,, + V,, where V, is the depth of the Fermi level below the conduction band. For ap-type semiconductor, V, is the height of the Fermi level above the valence band. The l/C2 vs. V method can be used only in cases where the contact has been formed on a uniformly doped semiconductor-the plot of l/C2 vs. V should then be a straight line. For titanium contacts on n-type silicon, we observed that l/C2 vs. V, even for uniform doping, was often not a straight line. This behavior is commonly attributed to the effects of an insulating layer between the metal and semiconductor.c7) A non-linear l/C2 vs. V plot cannot
A. M. COWLEY
406
be used reliably to determine barrier height in the absence of knowledge of the thickness and nature of the interfacial layer. The Schottky barrier current vs. voltage equation can be written I = Is(eqvikr - l),
(1)
where I, is the diode saturation current and I’is the applied voltage. For reverse voltages of about 4 kT/q (100 mV at 296°K) or greater, 1 is equal to -I,, in the absence of any leakage or other spurious effects. In the emission model(*) for a Schottky barrier, 1, is given by - q[&, --A$(
I, = AA*T2exp
VI
kT where A is the diode area, A* is the effective Richardson constant, and &o is the barrier height for zero field (Fig. 3). The quantity A+(v) is the image force correction to the barrier height, and is given byc4) A+(V) = ~$$(&o-G~)]“’
(3)
where ND,4 is the donor or acceptor density, la is the semiconductor dielectric constant, and I’,, is the diffusion potential of the Schottky barrier. The Richardson constant must be known in order to use (2) for determining the barrier height. For chemically prepared n-type silicon, A* is of the order of 100, from ATALLA( the surface preparations used in this work are essentially the same as those used to obtain the results quoted by Atalla. There has been no determination of A* for
similarly prepared P-type silicon contacts, to the author’s knowledge.7 However, in this work, it is found for the Ti-p-type silicon diodes that A* 21 100 gives reasonable agreement between &o determined from saturation current and &,o determined from l/C2 vs. V plots. The activation energy method of determining barrier height is based on (2). The quantity In (IS/T2) is plotted against l/T. If &, and A+(O) are not functions of T, then the slope of the plot is proportional to &A+(O). As CROWELL and SZE@) point out, however, the plot gives &oT(d&,/dT) - A+(O) for a temperature-dependent barrier height; this is also a well-known result from thermionic emission theory. If an accurate independent measurement of&, is available, then the activation energy measurement can, in principle, give the value of d$eo/dT. (6) n-type Si-Ti diodes (i) Barrier height measurements The results of barrier height determinations from l/C2 vs. Y data and from saturation current density measurements are shown in Table 1 for some typical diodes fabricated on n-type silicon. The measured saturation current density J, is corrected for the image force barrier lowering by multiplying by the factor exp( - qAq%/kT) and the t A value of 32 A/cm2-“K2 was given for A* by J. M. ANDREWS, JR. and M. P. LEPSEL.TER(IEEE Int. Electron Device Meeting, Washington, D.C., October 1968), for rhodium silicide on p-type silicon. However, it is not clear that this value applies in the present case because of the radically different method of forming the silicide contact.
Table 1. Barrier height measurements on titanium-n-type
Wafer No.
Fabrication method
Doping density (cmm3)
J EnI (A/cm?
A+(V = 0) (V)
silicon diodes
JS (A/cm”) (V)
13, 14c 17a MS-l 86~
Mesh Mesh Mesh Mesh Guard-ring
: A* = 100 A/ems-
‘Ka.
1.1 1.1 1.8 1 .o 5.0
x x x x x
10’6 101s 101s lo’s 101s
0.056 0.053 0.062 0.060 0.070
0.020 0.020 0.025 0.020 0.017
0.025 0.023 0.026 0.030 0.035
0.505 0.505 0.505 0.500 0.495
0.41-0.44 0.55-0.57 0.45-0.5s 0.54-0.57 -
TITANIUM-SILICON
SCHOTTKY
corrected value is denoted by J,. The barrier height is then determined as (KT/q)ln(A*P/J,). The values for J, are closely distributed around an average value of 0.028 A/cm2, leading to a value of 0.500~0.010 volts for the barrier height $so, for A* = 100 A/cm2-“K2. Values of&o determined from l/C2 vs. k’ plots were scattered between about 0.4OV and 0.6OV. As an example, wafers 13, and 14, were prepared on identical silicon slices; values of J, are in good agreement, but &o from l/C2 vs. P’ data differ by more than 100 mV. This general behavior is thought to arise from the presence of a nonreproducible and possibly non-uniform (patchy) interfacial layer with thickness of the order of monolayers between the titanium and the silicon. The l/C2 vs. I’ plots for the diodes also deviated somewhat from linearity in the reverse bias range of 0 to several volts; it was usually a matter of judgement as to the correct linear extrapolation of these plots to the voltage axis. In other experiments using the same silicon slices, wafer preparation, and measurement equipment, it was verified that linear l/C2 vs. V plots were obtained for other metal contacts, e.g. nichrome. This rules out the possibility of measurement error and/or nonuniform semiconductor doping, and would suggest that this behavior is peculiar to the titaniumsilicon system. Following TURNER and RHODERICK,‘~~) the titanium-silicon diodes were subjected to storage and accelerated aging cycles. For n-type diodes, the effect of several months storage and of 30 min heating at 200°C on J, was essentially negligible; the largest change was a decrease of about 20 per cent after two months of storage in nitrogen. The effect of heating and storage on the l/C2 vs. V plots varied; the plots remained non-linear in all cases, and both decreases and increases were observed in the intercept on the V-axis. No consistent trend was noted in these changes. In view of the lack of a reliable l/C2 vs. V determination of barrier height, it was desirable to have an independent measurement to corroborate the value of &o obtained from saturation current density measurements. Accordingly, the activation energy of the reverse current for diodes from wafer E-86, was measured between 230 and 350°K; the results on a typical diode are shown in Fig. 4. The activation energy at a
BARRIER
DIODES
407
reverse bias of -O.lV was 0.50 eV; the error in this determination is estimated to be less than + .OlO eV. The activation energies for - 1V and - 5V were also measured, and were 0.50 eV and 0.49 eV, respectively. These latter values are consistent, within the estimated error, with the
lOOO/
T
(OK-‘)
FIG. 4. Activation energy plots for reverse current of Ti/n-type
silicon contact with guard-ring.
additional barrier lowering relative to v = - O.lV calculated from (3). The image force lowering for V = -O.lV is -O.O2V, so &o- T(d&o/dT) is 0.52+ .Ol eV. If $so is taken to be the average of the values computed from J, in Table 1, 0.50+ .Ol eV, then (d&o/dT) could be between zero and N - 1 x 10e5 eV/“K at 300°K; this is less temperature dependence than observed for n-type silicongold barriers by CROWELL,et aZ.(ll) An apparent lack of temperature dependence has been reported previously for n-type silicon-chromium barriers.‘12) (ii) Voltage-current
characteristics
The voltage-current characteristic was measured at 296°K for guard-ring diodes from wafer E-86,; reverse characteristic for a typical diode is shown in Fig. 5. Below about 20 V, the reverse current is in essentially exact agreement with image force lowering theory.(4) Above 20 V, an excess current component, presumably leakage, is observed. Avalanche breakdown for this diode was clearly visible at 90 V and about 100 PA; the breakdown for these diodes is controlled by surface effects at the edge of the p-n junction guard-ring.
408
A. M. COWLEY
E-86-6 Ti GUARD-RING
DIODE
A=1 2~10~crn~ N,=5~lO’~crii~ T=297”K i
,a’L 0
1.0
2.0
(v+vsa+)
3.0 l/4
FIG. 5. Reverse current of Ti/n-type silicon guard-ring diode. Solid line is the reverse current calculated from (2) and (3).
The forward current-voltage characteristic of a Schottky barrier is usually determined by plotting In I vs. forward bias voltage, and then fitting this data to an equation of the form I = Iso(e svinw _
1)
where n is the diode ‘ideality factor’, and is usually adjusted empirically to make the equation fit the data; I,, is the saturation current obtained by extrapolating the plot to Y = 0. This technique could not be used effectively with the guard-ring diodes because the series resistance of the diodes (N60LR) produced significant voltage drop at currents only a decade or so greater than the saturation current. An alternative to this technique is to measure the diode a.c. resistance, given by R = R,+-p
nkT q(I+ IS)
where R, is the series resistance and the right-hand term is the dynamic barrier resistance, obtained by differentiating (4). By (5), R vs. (1+1,)-l should be a straight line with slope nkT/q and intercept R,. This measurement was performed on guardring diodes and the result for a typical diode is shown in Fig. 6. The value of n determined from
FIG. 6. -Alternating current resistance of Ti/n-type silicon guard-ring diode versus reciprocal current. Indicated values of n and R, are 1.03 and 58 Q, respectively.
the plot is 1.03, which agrees exactly with the value of n calculated from the image force effect.t The measurement of R was performed at a frequency of 10 kHz, and is accurate to within several tenths of a percent. (iii) Noise measurements
The noise properties of a biased Schottky barrier diode have been discussed in previous papers.(3J2) It was shown in Ref. 12 that the noise temperature ratio t, of the barrier is given by nr,o+(n-l)I
1 tB=Z
i l+
nr,, + .z
(6)
where I is the bias current and the other terms are as defined in (4). For a practical diode, (6) must be t Following CROWELL and SZE,(~~) 12 is given by n = l/[l -(d4a/dV)J, where 4B is the zero-field barrier height ~$*aminus barrier lowering effects. If the imageforce lowering, A$, is considered to be the sole source of barrier lowering, then 12 = l/[l +(dA$/dV)J and from (3), we have (dA+/dV) = -2. [A+(V)/(V,,-V-kT/q)] where I, is the applied forward bias voltage. A forward voltage of 0.15 V was used to calculate n for this diode; this was the midpoint of the voltage range used to obtain the data in Fig. 6.
TITANIUM-SILICON
corrected
for the series resistance
t=
SCHOTTKY
R, as follows
Rds + Rs R,fR,
(7)
*
The quantity R, is the dynamic barrier resistance nkZ’/q(l+l,). The physical significance of noise temperature ratio is that it is the available noise power of the diode divided by kTB where T is the ambient temperature in degrees Kelvin, and B is the measurement bandwidth. The measured noise temperature ratio for a typical guard-ring diode is shown in Fig. 7 as a function of frequency, with IO
z Y’ 2
DIODES
,
I SILICON
409
the diameter of the oxide window was 22 pm. These diodes invariably had extremely poor rectification characteristics; the reverse current below 1 V was typically several orders of magnitude greater than the nominal saturation current calculated from Table 1. The general appearance of the reverse v-1 characteristic was ‘leaky’, i.e. the saturating behavior usually associated with diode reverse I/-I characteristics was essentially absent, and the reverse current tended instead to increase very rapidly with reverse voltage. This behavior for passivated diodes on n-type silicon has been studied in some detail in our laboratories,(3) as well
I TITANIUM
Q 5
BARRIER
GUARD-RING
DIODE
E 86 B
5-
I
I n= 1.03 R,=940n i Rs=60n
Y
I Lo5
-_.---b--_
THEORETICAL:
MEASUREMENT
FREQUENCY
FIG. 7. Noise temperature guard-ring
t = 0.56
-
Hz
ratio of T&type diode vs. frequency.
the bias adjusted for a total diode dynamic resistance of 1OOOQ.The value calculated from (6) and (7) is shown for comparison, and the agreement is essentially exact above 10 kHz. Below 10 kHz excess noise having an approximate f-r spectrum is also present. This behavior is similar to that of the Mo-Si guard-ring diodes described in Ref. 3. The ‘noise corner frequency’, fN,defined as the frequency where the excess noise component is equal to the ‘white’ component calculated from (6) and (7), is a few hundred Hertz for the diode of Fig. 7. (iv) Oxide-Passivated diodes
Titanium diodes having the structure of Fig. 1 were fabricated on wafers of n-type silicon with nominal resistivity ranging from 0.3 to 0.7 Q-cm;
silicon
as by other workers,(2J3) and there is general agreement that the ‘leaky’ or ‘soft’ appearance of the reverse characteristics is due to leakage currents at the periphery of the contacts. The problem appears to be more pronounced for titanium contacts, compared to other metals such as Ni, Cr, MO, Au and various silicides; this is apparently associated with the somewhat lower barrier height for titanium compared to the other metals. The leakage is sufficient in the titanium diodes to make them useless for most diode applications. The noise temperature ratio for these diodes was also measured at 1.0 kHz for forward bias currents ranging from several tens of microamperes up to N 1.5mA;valuesbetween400and800wereobserved essentially independent of bias current. This is several orders of magnitude more noise than for
410
A. M. COWLEY
the diode of Fig. 7 and the diodes described in References 3 and 12, but is fairly typical for simple oxide-passivated diodes on n-type silicon, independent of the contact metal. As pointed out in Ref. 3, this excess is also associated with the periphery of the contact, where the Si, SiO, and metal come together, and it is almost completely eliminated by the use of the p-n junction guard-ring, as demonstrated in Fig. 7. diodes
(c) p-type Si-Ti (i) Barrier height measurements
Barrier height measurements were performed on p-type silicon-titanium diodes fabricated using the ‘mesh’ technique. The properties of p-type silicon-titanium diodes were found to be quite sensitive to heating of the diodes during and after the titanium evaporation. Specifically, the diode characteristics showed an annealing behavior with mild heating; the zero-bias capacitance of the diodes would increase by a factor of 2 or 3 at 100°C after a few minutes and the saturation current would typically double. Further heating at temperatures up to -300°C produced no further change, and the change was irreversible. A 2OO”C, S-min heat treatment was subsequently adopted as a standard part of the fabrication process. Diodes tested before treatment were found to have quite non-reproducible characteristics; in particular, the l/C2 vs. V plots were non-linear, and the values of C, and I, for diodes of the same nominal area varied considerably over a given wafer. The latter effect is assumed to arise from a partial, non-
uniform annealing of the diodes due to heating during the titanium evaporation. After annealing at 100°C or 200°C however, the diode properties became very uniform, and the l/C2 vs. I’ plots were linear. The mechanisms for this annealing behavior can only be guessed at. The behavior of the diode capacitance strongly suggests that an interfacial layer exists after the titanium is evaporated, and that this layer is somehow removed by heating. Mechanisms consistent with this hypothesis would be the reaction of the titanium film with the monolayers of oxygen and/or fluoride ions present on the silicon prior to evaporation, or simply a rearrangement of the titanium atoms during heating, leading to a more intimate metal-semiconductor contact. The data presented in Table 2 were taken on six wafers of Ti-p-type silicon diodes which had been subjected to 200°C 5 min annealing. Wafers 18,, 20,, 21, and 94 were epitaxial wafers with non-uniform impurity density; as discussed in Section II, the l/C2 vs. V plots for such wafers cannot be used for determining barrier height. The nominal surface doping estimated from the zero-bias capacitance was used to calculate the image force lowering; since A$ is only weakly dependent on doping, this calculation is expected to be sufficiently accurate for this purpose. The zero-field saturation current density averages about 4.0 x 10m4 A/cm2; a Richardson constant of 100 A/cm2-OK2 gives $no = 0.61 V, consistent with the values of &o from the two l/C2 vs. I’ measurements.
Table 2. Barrier height measurements on titanium/p -type silicon diodes
No.
Fabrication method
1% 2% 21B 22, z-1 94
Mesh Mesh Mesh Mesh Mesh Mesh
Wafer
t A* = 100 A/cm2-‘K2.
Doping density (cme3)
1.0 x 2.0 x 7.0 x 3.0 x 2.5 x “ig.
1016 1016 1016 1017 101’ (9)
J 8nl
(A/cm’)
6.65 x10-4 1.20x10-3 1.43 x10-3 3.70x10-3 1.25~10-~ 1.2 x10-3
A$(v=O) (V)
J, (A/cm’)
+BO~(JS) 44/C2 vs. V (V) (Y)
0.022 0.025 0.037
2.8x10-* 4.3x10-4 3.4x10-4
0.62 0.61 0.615
0.048
5.7 x 10 -4
0.60
0.027 0.033
4.3x10-4 3.3x10-4
0.61 0.615
0.60
0.62 -
TITANIUM-SILICON
SCHOTTKY
(ii) Oxide-passivated diodes
Titanium diodes having the structure of Fig. 1 were also fabricated on p-type silicon. In contrast to the n-type passivated diodes described in Section 3b, diodes fabricated on 0.1-0.8 Q-cm p-type silicon were not seriously degraded by the SiO, passivating film. The most noticeable effect of the passivating film was a degradation of the reverse current-voltage characteristic at higher voltages ; a general ‘softening’ of the reverse current was observed above 5 or 10 V, depending on the resistivity of the silicon. For resistivity higher than about 1 Q-cm, reverse currents many orders of magnitude higher than the nominal Schottky barrier diode saturation current were often observed; the current had the general appearance of a large saturation current, and was accompanied by an increase in the capacitance of the contact. Both current and capacitance were strongly dependent upon the intensity of light incident on the wafer during the measurement. This behavior is evidently due to inversion of the silicon surface around the contact by the fixed positive charge in the passivating SiO, layer. The same phenomenon has been observed by YU and SNOW for platinum-silicide diodes on n-type silicon; they induced an inversion layer around the contact by using a strongly negativelybiased guard-ring over the oxide surrounding the contact. It has been found in the present case, from MOS C-v measurements(l*) on the SiO, layers, that the oxide charge (typically +2-3 x 1011 cm- 2, is indeed sufficient to invert the surface of p-type silicon in the range of 1 Q-cm or higher. The remainder of the section will be devoted to the discussion of passivated diodes formed on O.l0.8 Q-cm p-type silicon. Of particular interest will be the comparison of these diodes with diodes of similar barrier height (nichrome) on n-type silicon, fabricated in exactly the same way. (1) Current-voltage characteristics. The currentvoltage characteristic for the passivated diodes followed a dependence of the form of (4), with n ranging from 1.04 to 1.08 for doping levels in the 5 x 10” cmm3 range. The calculated value of n is between 1.02 and 1.03. Diodes fabricated on the same silicon slices by rinsing off the oxide with hydrofluoric acid and evaporating the titanium through a mesh, as described in Section 2, had characteristics virtually identical to those of the 2
BARRIER
DIODES
411
passivated diodes. This would suggest that the departure of n from ideality is an inherent property of this particular method of forming the titaniumsilicon interface, rather than an edge effect. Saturation current densities of passivated diodes and mesh diodes fabricated on the same wafer were also identical, and were in agreement with the values listed in Table 2. Most of the passivated diodes fabricated during this study were designed as microwave video detectors at 10 GHz. The diodes were fabricated with an active area of 5 x lo-’ cm2 on thin (-1~) epitaxial silicon which was grown on a heavily-doped (N 0.003 R-cm) substrate. Doping profiles for these epitaxial layers were measured using the capacitance-voltage technique;(15) the layers were found to be graded, with the more lightly-doped material being near the surface. The grading was found to be due mostly to the depletion of boron from the surface during the steam oxidation,(16) as was verified by profiling wafers before and after oxidation. Reverse current for these small diodes increased more rapidly with voltage than image force lowering theory predicts, based on (3). This is partly due to the non-uniform doping in this case, but measurements on mesh diodes fabricated on uniformly doped material show a qualitatively similar disagreement with (3) for the reverse direction. Below 5 V or so, the diodes exhibited good rectification characteristics. At 5 or 6 V, the diodes began to conduct heavily, with 10 PA typically being reached between 6 and 8 V. The true avalanche breakdown voltage for these wafers, as determined from mesh diodes, was between 12 and 14 V. (2) Edge effects-comparison with NiCr-n-type siZicon diodes. Several experiments were performed to investigate the importance of leakage currents at the periphery of the passivated diodes. It has been well established at this laboratory, and confirmed by Yu and SNOW, that for diodes on n-type silicon, these edge effects dominate the current-voltage characteristics for reverse bias and for low forward bias. The results of one set of experiments, in which diodes having diameters ranging from 13 to 50 pm were fabricated simultaneously on the same silicon wafer, are shown for NiCr/n-type Si diodes in Fig. 8. The reverse current at -0.1 V and the forward current at
412
A. M. COWLEY
FWD.
t
/SLOPE
CONTACT
CURRENT
= I
DIAMETER-pm
CONTACT
DIAMETER-pm
FIG. 8. Forward and reverse current for NiCr/n-type silicon oxide-passivated diodes vs. diode diameter.
FIG. 9. Forward and reverse current for Ti/P-type silicon oxide-passivated diodes vs. diode diameter.
+0.3 V are shown plotted against diode diameter. The reverse currents depend linearly on diameter showing that most of the current is flowing at the edges of the diodes. The forward current has a slope lying between 1 and 2, showing that while a significant current flows in the whole area of the diode, the current density is higher at the edges, These diodes were fabricated on 0.4 Q-cm, 4 pm n on n+ epitaxial silicon slices. The data were averaged over many diodes of each size on the slice. This data shows that in the range of diode diameters important for most practical applications, the n-type passivated diodes are dominated by effects at the edges of the diodes. This phenomenon is thought to be the result of accumulation of the n-type silicon surface by the positive charge in the oxide surrounding the diodes.(2s13) The accumulation of negative charge in turn causes the depletionlayer field at the edge of the diode to be much higher than in the remainder of the diode, and this gives rise to increased current flow due to enhanced image-force lowering and tunneling through the barrier.(13) The experiment was repeated for titanium diodes on 0.8 Q-cm, 7.5 pm p-p+ epitaxial silicon wafers. The results are shown in Fig. 9. Here it is
seen that the currents are proportional to the diode area, and hence, edge leakage is not an important factor in the current flow mechanism for these diodes. These diodes were fabricated in exactly the same fashion as the NiCr/n-type diodes, using SiO, films grown in the same oxidation furnace. It is suggested that the edge effects are not present in the p-type diodes because the surface of the silicon around the diodes is depleted in this case for resistivities below 1 R-cm. The depletion of the surface around the Schottky barrier diodes does not give rise to high fields at the diode periphery, as does accumulation in the n-type case, and it is conceivable that the depletion layer field at the diode periphery is actually reduced for the p-type diodes. Further observations on p-type and n-type diodes which seem to be related to these effects are described below. (3) Activation energy of reverse current for n- and p-type diodes. The activation energy of the reverse current at -0.1 V was measured for Ti/p-type Si and NiCr/n-type Si diodes, and typical results are shown in Fig. 10. The activation energy for the Ti/p-type diodes is about 0.60 eV; the image force lowering in this case is 0.030 V, so this measurement would be consistent with a barrier height of
TITANIUM-SILICON
0 NiCr/N-TYPE
SCHOTTKY
BARRIER
DIODES
413
The parameter fNis the ‘noise corner frequency’ and is a measure of the severity of the excess component of the noise. For the data of Fig. 11, an fN of -3 kHz is indicated for the Ti/p-type diode while the NiCr/n-type diode has an fN of -2 MHz. The p-type diode shown here is quite
Si (N~=l.5x10’6cm~3;
I-
I
Q NiCr/N-TYPE
(ND~7w10’6cni’ A=l$cm*)
0 Ti/P-TYPE
.Ol
3.0
3.5 (OK-‘)
y
4.0
FIG. 10. Activation energy of the reverse current at -0.1 V for typical oxide-passivated Ti/p-type silicon and NiCr/n-type silicon diodes. 0.63 V, which is reasonable in light of the values for &o listed in Table 2. In contrast to the Ti/p-type Si diodes, the activation energy for the NiCr/n-type Si diodes is much less than the barrier height (&o 3: 0.65 eV) and, in general, the temperature dependence of the reverse current cannot be described by a single activation energy. The low values of activation energy are evidently associated with currents at the edges of the diodes, possibly tunneling current in combination with the normal thermionic current. (4) Noise temperature ratio. As a final comparison between the n- andp-type oxide-passivated diodes, the noise temperature ratio as a function of frequency is shown in Fig. 11, for a bias current of about 30pA, corresponding to a total diode dynamic reistance of 1OOOQ. The n-type diode shows a large component of excess noise having an f-l spectrum, while the p-type diode shows essentially the expected shot noise for an ideal Schottky barrier (t = OS), together with a much smaller component of excess noise also having an f-’ spectrum. For diodes having this type of spectrum, an equation can be fitted to the data, having the form tLz0.5
(
I+f”.
f >
(N*.~xIO’~C~~~;
FREQUENCY - Hz
FIG. 11. Noise temperature ratio vs. frequency for Ti/p-type and NiCr/n-type silicon diodes for similar semiconductor doping and diode area. typical. The n-type diode is worse than typical; the noise corner of these diodes varies widely, from as low as 50 kHz to as high as or higher than the value shown here. The difference in noise behavior for the two types of diodes is evidently associated with the edge effects discussed above. 4. DISCUSSION The results presented here provide the information essential for the design of Schottky barrier diodes using the titanium-silicon system, namely the barrier height and saturation current density. In addition, the effects of a thermally-grown passivating SiO, layer have been investigated, and it has been found in the case of p-type diodes that the essential characteristics of the contact are not seriously altered by the presence of SiO,, in contrast to contacts on n-type silicon with any metal. This property has important ramifications for the fabrication of microwave detector and Doppler mixer diodes, where low noise at video frequencies is important, and where the use of a guard-ring is undesirable because of its additional capacitance.
414
A. M. COWLEY
The lower barrier height of the titanium/n-tvne silicon system compared to the more commonlyused metals such as Au, Ni, Cr, MO, etc., suggests its use in current-limiting applications, where reverse saturation currents of tens of hundreds of microamperes are often desired without the necessity for extremely large-area devices.(l’) Use of the guard-ring would make such a device completely comp&ble with standard monolithic integrated circuit techniques. The lower barrier height on n-type silicon also reduces the voltage drop at a given forward current compared to the other metals; this property is sometimes useful in computer logic circuit design because it reduces power dissipation and voltage offsets. Again, the guard-ring is desirable in order to be able to monolithically integrate the diode. The different behavior of SiO,-passivated Ti/p-type and NiCr/n-type diodes can be understood reasonably well in terms of the model suggested by Yu and SNOW, i.e. accumulation of the n-type surface and depletion of the p-type surface by the thermally-grown oxide. This would suggest that the properties of the n-type diodes could be improved by a change in oxide properties; other things being equal, it is normally preferable to fabricate diodes on n-type silicon due to the higher mobility for electrons. The sum of the barrier heights of a given metal on p- and n-type samples of a given semiconductor should equal the bandgap of the semiconductor. This is evidently the case for the titanium-silicon contacts studied here, and has also been verified for Au, NiCr, MO, Cr and Al by the present author (unpublished). I
,I
Acknowledaement-The
author would like to exnress his
appreciation to a number of persons who assisted in this
work. Mrs. M. BRYAN fabricated most of the diodes, and Messrs. R. C. PATTERSON and M. R. GAVETTE
performed many of the measurements and calculations.
The epitaxial wafers were grown by Messrs. R. R. HANEY and J. E. LANGLEY. The supervision and advice of R. A. ZETTLER during the course of the work are also
greatly appreciated.
REFERENCES Symp., 1. M. M. ATALLA, Proc. 1966 Microelectron. Munich, Germany, pp. 123-157. 2. M. P. LEPSELTER and S. M. SZE, Bell Syst. tech. J. 47, 195 (1968). 3. R. A. ZETTLER and A. M. COWLEY, IEEE Trans. Electron Devices m-16, 58 (1969). (1964). 4. S. M. SZE, et al., J. appl. Phys. 35.2534 5. Electronic Space Products, Inc., Los Angeles, Calif. 6. Buckbee Mears Co., St. Paul, Minn. 7. A. M. GOODMAN. J. aabl. Phvs. 34. 329 (1963). 8. H. K. HENISCH, .Rect&ing Semiconductor Contacts, pp. 196-201. Oxford University Press, New York (1957). Electron. 9. C. R. CROWELL and S. M. SZE, Solid-St. 9, 1035 (1966). 10. M. J. TURNER and E. H. RHODERICK, Solid-St. Electron. 11,291 (1968). Il. C. R. CROWELL, et al., Appl. Phys. Lett. 4,91 (1964). 12. A. M. COWLEY and R. A. ZETTLJXR,IEEE Trans. Electron Dewices ED-15, 761 (1968). 13. A. Y. C. Yu and E. H. SNOW. J. at&. Phvs. 39. 3008 (1968). al., Solid-St. Electron. 8,145 (1965). 14. A. S. GRovqet 15. C. 0. THOMAS, et al., J. electrochem. Sot. 109, 1055 (1962). 16. A. S. GROVE, et al., J. appl. Phys. 35, 2695 (1964). 17. R. A. ZETTL.ER and R. W. SOSHEA. IEEE Trans. Electron Devices (Abstract) ED-13; 817 (1966). 18. C. A. MEAD and W. G. SPITTER, Phys. Rev. Lett. 10,471 (1963).