n-type InP Schottky contacts with an interfacial layer

Solid-State Electronics Vol. 34, No. 5, pp. 527-531, 1991 Printed in Great Britain. All rights tescrvcd

0038-1101/91 s3.00 +o.oa

Copyright 0 1991 pergamon F+nxsplc

A NEW METHOD TO FABRICATE Au/n-TYPE SCHOTTKY CONTACTS WITH AN INTERFACIAL

InP LAYER

K. HAX-I-XXI and Y. Tonn Department of Electrical Engineering and Electronics, Toyohashi Univeristy of Technology, Toyohashi 441, Japan (Received 17 September 1990)

Abetract-A method to fabricate Au-n-type InP Schottky contacts with an interfacial layer has been developed. The interfacial layer is formed by deposition of a P,O layer and reaction of this layer with the InP substrate. The current-voltage and capacitance-voltage c&racteristics are measured at various temperatures. Excellent rectification is found in the fabricated contacts. The observed reverse currents are very low. The ideality factors are around 1.15. Apparent barrier heights q&, are evaluated from the extrapolated forward saturation current Z,. They are found to be sufficiently high. The typical value of q& at room temperature is obtained as 0.88 eV. The Richardson plot ln(Z,/T*) vs l/T is well fitted in a

straight line, where T is the temperature. From the Richardson plot, the true barrier height is estimated to be 0.41 eV. The effects of the interfacial layer are also discussed.

found in these Schottky contacts. The effects of the interfacial layer are also discussed.

1. INTRODUCTION

Indium phosphide has received much attention on account of its possible application to microwave field-effect transistors (FET’s) [ l-41, laser diodes[s8], photodiodes[9,10], solar cells[l l-141 and so on. The peak electron drift-velocity and breakdown electricfield of InP are higher than those of GaAs. These facts yield advantages of InP over GaAs for the fabrication of low-noise and high-power Schottky gate FET’s. However, a serious drawback of InP is the low Schottky barrier height. This increases the Schottky gate leakage current, and the device performance is degraded. Many attempts to increase the barrier heights of n-type InP Schottky contacts have been performed by forming an interfacial layer between the gate metal and semiconductor. For example, Gleason er a1.[15] exposed the sample surface to an oxygen plasma prior to the gate metal deposition. Wada et al.[ 161oxidized the sample in HNO, . Kamimura et a/.[1 71 formed an oxide layer dipping the sample in bromine water. Lee and Anderson[l8] also formed chemically grown oxides using a boiling aqueous solution of NH,OH and H202. The properties and effects of such chemically grown InP oxides have been studied from various points of view[19-221. Further, the thermal oxidation method was applied to p-type InP Schottky contacts by Eberspacher et a1.[23]. The purpose of the present paper is to develop a new method to fabricate Au/n-type InP Schottky contacts with an interfacial layer. The interfacial layer is formed by deposition of a PXO, layer and reaction of this layer with the InP substrate. The electrical characteristics of the fabricated Schottky contacts are investigated. Excellent rectification is 521

2. EXPERIMENTAL Undoped n-type InP wafers of (100) orientation were employed in this work. The carrier concentration was about 8 x lO”~m-~. The wafers were mechanically polished to a mirror-like surface and etched with 1% bromine methanol for 10 min. Ohmic contacts were formed by evaporation of Au/Sri (50 wt%) in a vacuum of 1O-6 Torr, and annealing in a hydrogen atmosphere at 450°C for 8 min. The InP surface for the gate electrode was again lightly etched with a 0.2% bromine methanol. Immediately after the etching, the sample was placed in the deposition apparatus shown in Fig. 1. In the quartz tube of the apparatus, P,O, powder was heated to 300°C and vaporized as PXO, molecules in a nitrogen atmosphere. The temperature of the sample was kept to be 200°C. After the PXO, vapor spread uniformly in the quartz tube, a PXO,,layer was deposited on the InP substrate for about 30 s. The sample was then cooled in the apparatus until its temperature became room temperature. Further, the sample was transferred to a quartz tube furnace. A reaction between the PXO, deposited layer and InP substrate was performed in an oxygen atmosphere at 250°C for 20 min, and then in a nitrogen atmosphere at 250°C for 60min. The thickness of the obtained layer was about 60A. The Schottky gate electrode was formed by evaporation of Au in a vacuum of 10e6 Torr. The shape of the Au electrode was a circular dot with a diameter of 1 mm. Further, the sample was annealed in a nitrogen atmosphere at 150°C for 30 min.

528

K. I

1N,

HA-RI

GAS

and Y.

TORII

Here A* is the effective Richardson constant, S the area of the contact, q+80 the barrier height, 6 the thickness of the interfacial layer which electrons tunnel through, x the mean tunneling

u

u

Fig. 1. Schematic diagram of the deposition apparatus.

In the method developed above, thick interfacial layers were also obtained, when the deposition time was long. The interfacial layers were little dissolved by inorganic acids, while they were dissolved by alkali hydrides. For the interfacial layers 50-1000 A thick, refractive indices n’ were measured ellipsometrically at 6328 A and evaluated as 1.58-l 60. These values of n’ were nearly equal to the value for InPO,, 1.6[18]. 3. RESULTS AND

DISCUSSION

Figure 2 shows the current-voltage (Z-Y) characteristics of a typical Schottky contact with an interfacial layer measured at 290 K. The observed reverse current is very low, while the forward current increases rapidly with increasing bias voltage. Excellent rectification is found in Fig. 2. The forward current in a Schottky contact with an interfacial layer for V > 3kT/q is given by[24]:

where n is the ideality factor, k the Boltzmann constant and T the temperature. The current Z, is the extrapolated saturation current and expressed as: Z, = A *ST2 exp

1O-6 2

lo-’

-

-f

(2mx)%+.xp(

I

FORWARD

-@).

(2)

barrier, and m the tunneling effective mass. The effective Richardson constant A * is 9.4 A cm- 2 K - 2 for n-type InP Schottky contacts. In the usual analyses of the experimental data on Schottky contacts, the barrier height is determined from the extrapolated saturation current Z,. However, this is an apparent barrier height, when an interfacial layer is present. The apparent barrier height is given by: q&, = kT ln(A *ST2/Zs). Substituting

eqn (2) into eqn (3), we have: 2 q&, = q&,,, + - (2mX)“26kT. h

(4)

From the forward Z-V curve in Fig. 2 we obtain q&, = 0.88 eV and n = 1.15 at 290 K. In order to understand more about the properties of the fabricated Schottky contacts, the Z-V characteristics were measured at different temperatures. The reverse leakage currents were found to be extremely low at low temperatures. They were less than 10 pA at - 1 V below 270 K. Figure 3 shows the reverse Z-V characteristics of a typical Schottky contact with an interfacial layer measured at several temperatures above 290 K. The reverse leakage currents increase with increasing temperature. However, the leakage currents shown in Fig. 3 are much lower than those observed in conventional n-type InP Schottky contacts[25-301. The reverse current density is evaluated as 1.3 x lo-*A/cm2 at -2V and 295 K. This value is smaller than those in n-type InP Schottky contacts with an interfacial layer so far reportedI 15-2 11. Figure 4 shows the forward Z-V characteristics of a typical Schottky contact with an interfacial layer

:

.*’

(3)

“320K . ...**** . . .* . . 3.‘?5...* . l.** . . . . 2’95K . .

l

._

l

10

-12

.

0.0

,

,

,

0.2

0.4

APPLIED

,

,

,

0.6

VOLTAGE

:

-

, 0.8

(VI

Fig. 2. Current-voltage characteristics of a typical Au/ntype InP Schottky contact with an interfacial layer at 290 K.

0.0

-0.2 APPLIED

-0.4

-0.6

VOLTAGE

-0.8 (V)

Fig. 3. Reverse current-voltage characteristics of a typical Au/n-type InP Schottky contact with an interfacial layer.

Au/n-type Schottky contacts

529

0.9 . 5: 0.0 z! $ n

0.7

lo+

%

0.6 :,[a

g l&O LL

0.5

lo-”

100

200

300

TEMPERATURE

lR'*

‘” 0.0

0.1

0.2 0.3 0.4 0.5

APPLIED

VOLTAGE

(V)

(K)

Pig. 6. Temperature dependence of the apparent barrier height q& for a typical Au/n-type InP Schottky contact with an interfacial layer.

Fig. 4. Forward current-voltage characteristics of a typical Au/n-type InP Schottky contact with an interfacial layer.

n-type InP Schottky contacts[2>32]. Such difference in q&, results from the fact that both q&, and 2(2m~)‘~S/h depend on the process in forming the interfacial layer. The Richardson plot ln(Z,/T*) vs l/T is useful in the accurate evaluations of q&,,, and 2(2m#%/h. From eqn (2), we have: conventional

measured at various temperatures. The ideality factor n is evaluated at each temperature and shown in Fig. 5. An obvious temperature dependence of n is not found. Despite of the presence of the interfacial layer, n is fairly close to unity, i.e. n GX1.15. These results imply that excess currents such as recombination currents are hardly present at forward biases. An increase of n will be observed in a low temperature region, if such excess currents are appreciably present. The apparent barrier height q&, is obtained at each temperature and shown in Fig. 6. A linear temperature dependence of q&, is observed as theoretically expected from eqn (4). The slope of the q& vs T plot is proportional to the electron tunneling factor 2(2m~)“%3/fi. It is well known that when a metal contact is evaporated on a chemically etched semiconductor surface, the metal and semiconductor are not in intimate contact. There inevitably exists a thin interfacial layer between the metal and semiconductor. Therefore, a thin interfacial layer is present even in a conventional InP Schottky contact without any intentional interfacial layer. For conventional n-type InP Schottky contacts, q& is around 0.5 eV at room temperature[25-321. In the present work, the typical value of q& at room temperature is 0.88 eV and about 0.4 eV larger than the values for

In $ = - e

- i (2m#%

+ ln(A *S).

(5)

The slope of the Richardson plot is proportional to q&, . The tunneling factor 2(2m~)“*6 /h is determined from the extrapolated intercept on the ln(Z,/T*) axis of this plot. Figure 7 shows the Richardson plot obtained from the Z-V data in Fig. 4. The Richardson plot is well fitted in a straight line as expected from eqn (5). The barrier height q&, is evaluated as 0.41 eV. The electron tunneling factor 2(2m~)%/h is estimated to be 18.3. The capacitance-voltage (C-V) characteristics were measured at a frequency of 100 kHz over the temperature range 130-3 10 K. The l/C* vs V plots of a typical Schottky contact with an interfacial layer are shown in Fig. 8. We analyze the C-V data based I ’ 1

I ’ I,,

’

c

K 1.20 e

_

2 k

-

z 3 a x

1.15

.

. . .

.”

.

-

. 1.10

-

1.05

M 100

.*

200 TEMPERATURE

-

300

( K)

Fig. 5. Ideality factor vs temperature for a typical Au/ntype InP Schottky contact with an interfacial layer.

345676 10001

T

(K-‘1

Fig. 7. Richardson plot of ln(I,/Tz) vs l/T for a typical Au/n-type InP Schottky contact with an interfacial layer.

K. HATMRIand Y. TORI]

530

APPLIED

VOLTAGE

(‘4)

Fig. 8. Capacitance-voltage characteristics of a typical Au/n-type InP Schottky contact with an interfacial layer.

effective thickness of the interfacial layer 616: as 20-24A. The thickness of the interfacial layer is about 60 A. Therefore, we have 6: x 2.7 at 100 kHz. In conclusion, we have developed a new method to fabricate Au/n-type InP Schottky contacts with an interfacial layer. The interfacial layer is formed by deposition of a PXO,, layer and reaction of this layer is about 60 A. The refractive index of the layer measured ellipsometrically at 6328 A is evaluated as 1.58-1.60, and nearly equal to that of InP0,[18]. Excellent rectification is found in the fabricated Schottky contacts. In spite of the presence of the interfacial layer, the ideality factors are little increased from unity. They are estimated to be about 1.15. The reverse leakage currents are found to be lower than those in n-type InP Schottky contacts with an interfacial layer so far reported[l5-211. The method developed in this work will be very useful to fabricate the gates of InP Schottky gate FETs.

upon the theoretical treatment developed in a previous work[33]. The l/C2 vs Vrelation is expressed as:

REFERENCES 1. J. S. Barrera and R. J. Archer, IEEE Devices ED-22, 1023 (1975).

Trans. Electron

2. S. Loualische,

H. L’Haridon, A. Le Corre, D. Lecrosnier, M. Salvi and P. N. Favennec, Appl. Phys. Len.

52, 540 (1988).

3. Y. Iwase, F. Arai and T. Sugano, Appl. Phys. Lett. 53, where co is the permittivity of free space, 6: the relative permittivity of the interfacial layer, 6: that of the semiconductor, Nn the donor concentration, and qV, the energy of the Fermi level measured from the conduction band edge in the neutral region. The value of c: is 12.35 for InP. The value of l/C* at a given voltage decreases with increasing temperature as expected from eqn (6). The donor concentration No is determined from the slope of the l/C2 vs V curve: No= -2[q&,d$(;~]-‘ .

(7)

From the C-V data in Fig. 8, we have N,, x 8.1 x lOi cme3. The intercept on the voltage axis of the l/C2 vs V curve is obtained as:

Solving eqn (8) with respect to 6/c:,

we have:

565 (1988).

4. S. J. Kim, J. Jeong, G. P. Vella-Coleiro and P. R. Smith, IEEE

Trans. Electron Device Lett.

EDL-11, 57 (1990).

5. N. K. Dutta, T. Wessel, T. Cella and R. L. Brown, Appl. Phys. Lett. 47, 981 (1985). 6. W. T. Tsang, Appl. Phys. L&t. 50, 63 (1987). 7. M. Razeghi, R. Blondeau, M. Krakowski, B. de Cremoux, J. P. Duchemin, F. Lozes, M. Martinot and M. A. Bensoussan, Appl. Phys. Lett. 50,230 (1987). 8. Z. L. Liau and J. N. Walpole, Appl. Phys. Len. 50, 528 (1987).

9. S. Miura, H. Kuwatsuka, T. Mikawa and 0. Wada, Appl. Phys. Lat. 49, 1522 (1986). 10. D. Gershoni, H. Temkin and M. B. Panish, Appl. Phys. IAt. 53, 1294 (1988). 11. A. Yamamoto, M. Yamaguchi and C. Uemura, Appl. Phys. Letf. 44, 611 (1984). 12. T. J. Coutts and S. Naseem, Appl. Phys. Lett. 46, 164 (1985). 13. M. B. Spitzer, C. J. Keavney, S. M. Vernon and V. E. Haven, Appl. Phys. Lett. 51, 364 (1987). 14. C. J. Kleavney and M. B. Spitzer, Appl. Phys. Lett. 52, 1439 (1988). 15. E. R. Gleason, H. B. Dietrich, R. L. Henny, E. D. Cohen and M. L. Bark, Appl. Phys. Lett. 32,578 (1978). 16. 0. Wada, A. Majerfeld and P. N. Robson, Solid-St. Electron.

25, 381 (1982).

17. K. Kamimura, T. Suzuki and A. Kunioka, J. uppl. Phys. 51, 4905 (1980).

The potential V,

=

F

V, is given by ln

2 =y D

ln

[;@F)“‘]. (10)

Here Nc is the effective density of states in the conduction band. The parameter m* is electron effective mass and given by O.O78m, for InP, where m, is the free electron mass. Substituting the values of No, 9s~ V, and V,, into eqn (9), we obtain the

18. Y. S. Lee and W. A. Anderson, J. appl. Phys. 65,405l (1989). 19. H. Lim. G. Saenes and G. Bastide. J. awl. _. Phvs. _ 53, 7450 (1482).

-

20. K. Hattori and Y. Izumi, J. appl. Phys. 53,6906 (1982). 21. H. Ricard. G. Couturier, A. Chaouki and A. S. Barriere, J. appl. Phys. 62, 3857 (1987). 22. A. Guivarc’h, H. L’Haridon, G. Pelous, G. Hollinger and P. Pertosa, J. uppl. Phys. 55, 1139 (1984). 23. C. Eberspacher, A. L. Fahrenbruch and R. H. Bube, J. appl. Phys. 58, 1876 (1985).

Au/n-type Schottky contacts

531

24. H. C. Card and E. H. Rhoderick, J. Phys. D 4, 1589 29. N. Newman, T. Kendelewicz, L. Bowman and W. E. Spicer, Appl. Phys. ktr. 46, 1176 (1985). (1971). 25. G. G. Roberts and K. P. Pande, .I. Phys. D 10, 1323 30. C. J. Sa and L. G. Meiners, Appl. Phys. L&t. 48, 1796 (1986). (1977). 26. D. V. Morgan, M. J. Howes and W. J. Devlin, J. Phys. 31. B. Tuck, G. Eftekhari and D. M. de Cogan, J. Phys. D 15, 457 (1982). D 11, 1341 (1978). 27. B. Tuck and B. R. Hayes-Gill, Physica Status Solidi CiOA, 32. E. Hdkelek and G. Y. Robinson, Appl. Phys. Mt. 40, 426 (1982). 215 (1980). 28. L. J. Brillson, C. F. Brucker, A. D. Katnani, N. G. Stoffel 33. K. Hattori, M. Yuito and T. Amakusa, Physica Status Solidi 73A, 157 (1982). and G. Margaritondo, Appl. Phys. Lat. 30,784 (1981).