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Electrical inhomogeneity in alloyed AuGeNi contact formed on GaAs
Solid-State Electronics Vol. 33, No. 8, pp. 999-1003, 1990 Printed in Great Britain.All rights reserved
0038-1101/90 $3.00+ 0.00 Copyright © 1990PergamonPress pie
ELECTRICAL INHOMOGENEITY IN ALLOYED AuGe-Ni CONTACT F O R M E D ON GaAs M. KAMADA,T. SUZUKI,K. TAIRAand M. ARAI Sony Corporation Research Center, 174 Fujitsuka-cho, Hodogaya-Ku, Yokohama, Japan 240 (Received 29 June 1989; in revised form 20 February 1990)
A~tract--Electrical properties of alloyed AuGe-Ni contacts to p-type GaAs were investigated and analyzed as a means to investigate the nature of alloyed AuGe-Ni contacts to n-type GaAs. Experimental results on the p-type GaAs were analyzed with a parallel-contact model: the contact is composed of two kinds of areas with different properties. The barrier height of the alloyed junction obtained from the current-voltage relation was 0.64 eV, while that obtained from the capacitance-voltage relation was 1.I eV. We explain the discrepancy by assuming that the contact area is composed of about 84% n +p junction with barrier height of 1.42 eV and about 16% Schottky junction with barrier height of 0.65 eV. We conclude that the AuGe Ni contacts to n-GaAs also have the mixed structure, composed of about 84% of n +n contact and about 16% of the Schottky contact.
I. INTRODUCTION Ohmic contacts to semiconductor layers are essential for semiconductor devices. Alloyed AuGe-Ni metals are most commonly used for ohmic contacts to n-type GaAs layers[l] because of their low contact resistance and good surface morphology. However, there has been no satisfactory understanding what factor determines the ohmic contact resistance. Braslau[2] reported a very important result, that the ohmic contact resistance to n-GaAs layers is proportional to the resistivity of the n-GaAs layers. From this observation, he concluded that the ohmic contact resistance to n-GaAs layers is determined by a spreading resistance, which is proportional to the resistivity of n-GaAs layers. He attributed this relationship to an inhomogeneous structure of the interface between the alloyed metals and the semiconductors. In fact, the inhomogeneous structure was observed in the alloyed Au/Ni/AuGe/GaAs system by Kuan et al.[31] using transmission electron microscopy (TEM). They suggested that the contact interface consisted of a "good" contact region (Ni2GeAs/GaAs) with a heavily doped n+GaAs layer formed by the diffusion of Ge and of a "poor" contact region (Au/GaAs), i.e. a metal-GaAs Schottky junction. In this way, the TEM result obtained by Kuan et al. support Braslau's model; however, there has been no evidence that the contact is electrically inhomogeneous. In this paper, we are going to report on electrical properties of alloyed AuGe-Ni contacts to p-type GaAs, which support the Braslau model of the alloyed AuGe-Ni contact to n-type GaAs. We fabricated alloyed AuGe-Ni junctions, to ptype GaAs layers. The "good" contact region, 999
and the "poor" contact region in Kuan's experiment correspond to n ÷GaAs-pGaAs junction (n ÷p junction), and metal-p GaAs junction (Schottky junction) in our experiment, respectively, and the whole contact is considered to be their mixture. The current-voltage ( I - V ) characteristics and the capacitance-voltage ( C - V ) characteristics for the alloyed junction provide useful information about the constituent regions. We believe that our result can be applied to alloyed AuGe-Ni contacts to n-GaAs and that the contacts are a mixture of ohmic regions and Schottky regions. 2. EXPERIMENTAL
The substrate used in this experiment was Zndoped p-type GaAs, which was grown by the horizontal Bridgman method. The carrier density and the hole mobility evaluated at room temperature by Hall measurements were 6.8 x 1017cm-3 and 175 cm2/Vs, respectively. The cross-sectional view of the sample is schematically shown in Fig. 1. The sample was prepared in the following way: (1) A p-type ohmic metal (AuZn) was evaporated on the back side of the wafer. (2) The wafer was first heat-treated at 420°C in H2/N2 (H2 4%) atmosphere to make ohmic contact. (3) AuGe (190nm, Ge 12% in weight)-Ni (52 nm), which is commonly used as an ohmic metal for n-type GaAs layers, was evaporated using a resistance heat source. The metal was lifted off using chlorobenzene-treated photoresist, resulting in a circular pattern with a diameter of 100 or 333 pm.
1000
M. KAMADAet al. -2
Ni AuGe
( 5 2 nm) (190 nm)
p-type p=6.8x1017
-3
-4
GoAs
/
cm-3
/
(a)Before
-5
AuZn
//,/'~ ~
Fig. 1. The cross-sectional view of the sample. (4) The wafer was again heat-treated at 450°C in a H2/N 2 (H2 4%) atmosphere to form an alloyed junction of the A u G e - N i metal to the p-type GaAs layer. This is a standard alloying condition for making an ohmic contact to an n-type GaAs layer in our laboratory. A low ohmic contact resistivity below 1 x 10 - 6 ~ cm 2 is reproducibly obtained when the same metal ( A u G ~ N i ) is used for ohmic contact to n-type GaAs layers.
3. E X P E R I M E N T A L R E S U L T S
[
/ -8
-9
100/.~m ot
R.T.
-40
-11
I
0.1
I
0.2
3.1. I - V characteristics
Figure 2 shows a typical I - V characteristic of a 100 #m diameter diode. Data corresponding to the sample before and after the second heat-treatment are drawn in the same figure. The former is an AuGe-p-type GaAs Schottky characteristic and the latter is an alloyed junction characteristic. In both cases, the I - V characteristics showed rectification. After the second heat-treatment, the current decreased by about two orders. The n-values before and after the second heat-treatment were 1.26 and 1.22, respectively. The barrier height was calculated from the current density extrapolated to a bias of 0 V using the thermionic emission model, in which the Richardson constant (A *) was assumed to be 74.4 Acm -2 K-214]. The barrier heights before and after the second heat-treatment were 0.54 and 0.64eV, respectively. The decrease in current density after the second heat-treatment corresponds to the increase in barrier height. We also measured the activation energy of the current for the diode after the second heat treatment. Figure 3 shows the I - V characteristics of the 333 #m diameter sample at various temperatures ranging from 243 to 375 K. In Fig. 4 the currents are plotted as a function of the inverse temperature. The activation energy was evaluated from the slope in Fig. 4. Figure 5 shows the relation between the activation energy and the applied bias. Reduction in the activation energy was observed under both the forward and the reverse
/ Reverse
-6,
I
0.3
I
0.4
I
0.5
V (V)
Fig. 2. The 1-Vcharacteristics of a 100 #m diameter diode. There are two data points, one corresponding to the sample before the second heat-treatment (a), and to the sample after the second heat-treatment (b). bias. The barrier height of the alloyed junction should correspond to the activation energy at the bias of 0V. We obtained a barrier height of 0.62eV, which is very close to the barrier height of 0.64eV obtained by using thermionic emission model. 3.2. C - V characteristics
The value of 1/C 2 is plotted as a function of the reverse bias in Fig. 6. The diode was 333 #m in diameter, and the frequency in the measurements was 1 MHz. The relation is linear as shown in Fig. 6. From the slope and the extrapolation of the line, the acceptor density in the p-type GaAs layer and the barrier height of the alloyed junction was evaluated. The barrier height was 1.1 eV, which was much higher than the 0.64eV obtained by the I - V measurement. This barrier height is too low to be the barrier height of an n+p junction and too high to be the barrier height of a Schottky junction. However, the acceptor density of 7.5 × 1 0 1 7 c m - 3 obtained by C - V measurement is very close to 6.8 × 1017cm 3 obtained by Hall measurement, which confirms the validity of the evaluation.
Inhomogeneity of AuGe-Ni contacts on GaAs
1001
1.0
Forward
Reverse
/
0.5 = o .1
i -0.5
0.5
0
V (V)
Fig. 5. The relation between the activation energy and the applied bias. 4. P A R A L L E L C O N T A C T M O D E L
• "0
0.1
0.2
O.5
0.4
0.5
v (V)
Fig. 3. The I - V characteristics of a 333/~m diameter diode at various temperatures ranging from 243 and 373 K. -2
In order to explain the discrepancy between the barrier heights obtained in the two experimental evaluations, we applied a parallel-contact model which assumes a mixed structure made of two regions which have different barrier heights. This structure is supported by Kuan's T E M result. The parallel contact model was originally developed by Ohdomari et aL[5]. The interface is not smooth but rough, as can be recognized from Kuan's results. However, we assume a smooth interface for simplicity. The parameters to be used are summarized in Table 1. The current through the sample (SI) is the sum of the current through the low-barrier-height region and the current through the high-barrierheight region.
S I = S, I l + S 2 I 2 .
-3
(1)
In the thermionic emision model, the currents are expressed as follows:
-4
I = A *T 2 exp(-qq~u_ v ) / k T ) e x p ( q V / n k T )
-5
(2)
11 = A * T 2 exp(-- q (tb I - A d p l ) / k T ) e x p ( q V / n k T ) (3)
-
I 2 = A *T 2 exp(-- q (~b2 - A d p 2 ) / k T ) e x p ( q V / n k T )
~0.2V
(4)
--8
Aq~l and A~b2 are the image-force lowering of each barrier, and the values are given by the following equations:
-0.1V
-9
- O.02V -10
0.002
3 3 3 p m ~b
I
0.004
0.005 1/T
I
Adpl = [( q3 Na/8n2¢~) (dpI -- ( E f - Ev )/q - k T/q )] TM (5)
0.005
( K -1)
Fig. 4. The relation between the current and the inverse temperature.
Adp:~ = [(q3 Na/8X2£3 ) (c~2 - ( E f :- E v ) / q
- k T / q )] TM
(6)
1002
M. KAMADAet al. xlO 5
/
4
q~c_v;,1.1
(ev)
q~l = 0.TeV q~2 " 1.4eV
/
S1/S = 0 0.2 0.4 0.6
6
o.e
1.0
%
1 O'
/
333p.m ~ I 0
-1
I 1
0
V (V) Fig. 6. The relation between I/C 2 a n d reverse bias.
qdPu- v) = - k T /q ln[SJS {exp(-q (~l - Ackl)/k T) - - e x p ( - q (q~2-- AdP2)/kT)} + exp( - q (q~2- Adp2)/kT)]
(7)
The capacitance of the sample is expressed as the parallel combination of the capacitance corresponding to the low-barrier-height region and the capacitance corresponding to the high-barrier-height region. S C = $1 Cl + $2 C2
(8)
{4), - v - k T / q
- ( E f - Ev)/q }-,,,2 C2 = ~
(9)
{02 - V - k T / q
-(Ef-Ev)/q}
1/2.
I -1
I 0
I 1
V (V)
The image-force lowering ranges from 0.06 to 0.08 eV in this experiment. By combining eqns (1)-(4), the following equation can be deduced:
c~ = ~
-
(10)
Examples of the calculated relation between the applied bias and 1/C 2 are shown in Fig. 7. In this calculation, we assumed q~bI and qq~2 to be 0.7 and 1.4 eV, respectively. The values of S J S were Table I. Parameters used in parallel contact model
Fig. 7. A n examples of the calculated relation between the applied bias a n d 1/C 2. qYPL a n d qq~2 were a s s u m e d to be 0.7 a n d 1.4 eV, respectively. The values o f S~/S are 1, 0.8, 0.6, 0.4, 0.2 and 0.
1, 0.8, 0.6, 0.4, 0.2 and 0. The relations were not exactly linear except when S,/S was assumed to be 1 or 0. However, the relation was always linear in the reverse-bias region, in which the C - V relation was measured. The lines in the reverse-bias region were extrapolated to the forward-bias region in order to calculate qq~(~. In Fig. 8, the calculated relation between the barrier height ( q ~ ( ~ and qq~u ~)) and the log(SJS) is shown. In this calculation q~bI and qq~2 were assumed to be 0.7 and 1.4eV, respectively. The barrier height obtained from the I - V measurement should always be lower than the barrier height obtained from C - V measurements, and a large difference is expected even when the ratio S~/S is small, as can be seen in Fig. 8. This is the result of the exponential behavior of the current-voltage characteristics of the junction and would explain the obtained experimental result. The current density through the low-barrier region was calculated to be about 1014 times that of the current density through the high-barrier region as a result of a 0.7 eV energy difference between the two barrier heights. This indicates that the current through the sample comes
S S, $2 1
the total area of the sample (S = S= + $2) the area of the low barrier region the area of the high barrier region the current density measured the current density through the low barrier region 6 the current density through the high barrier region C the capacitace per unit area measured the capacitace per unit area of the low barrier region C~ the capacitace per unit area of the high barrier region C2 the barrier height evaluated by I - V measurements q(alt v) the barrier height evaluated by C - V measurements qqSic v) qck, the barrier height of the low barrier region the barrier height of the high barrier region qck2 qA~bl, qA~2 the image force lowering V applied bias A* Richardson constant (74.4 Acm 2 K 2) energy difference between the Fermi energy and the Ef-Ev valence band top (0.062 eV) permittivity of GaAs (12.9) ~s N~ accepter density in p-type GaAs
1.5
1.0 '~
q ~(I-V)
8 0.5
--10
I
I
--8
-6
I
-4
I
i
-2
0
Log ($I / S ) Fig. 8. The relation between the log(Sj/S) and the barrier height to be measured. In this calculation, q~b] a n d q4)2 were a s s u m e d to be 0.7 a n d 1.4 eV, respectively.
1003
Inhomogeneity of AuGe-Ni contacts on GaAs mainly through the low barrier region, and explains why a very small area of the low barrier height region contributes to effective lowering of the barrier height evaluated by I - V measurement. When the parallel contact model is applied to our experimental result, there are 3 unknown parameters, i.e. qq~l, q~b~and SI/S. Three experimental values are required to evaluate the three parameters. Unfortunately, we can obtain only two parameters, q ~ and qC~c ~. However, q~2 can be assumed to equal the band gap energy of GaAs, 1.42 eV, because q~b2 corresponds to the built-in voltage of n +p junction according to Kuan's result. By making this assumption we can evaluate the two unknown parameters, q~bj and SI/S on the parallel contact model, by combining the values of qq~{~ and q~b~o. The values obtained through this procedure are, q~bI = 0.65 eV
SI/S = 0.16. Figure 9 shows the relation between the barrier heights (qdp~c_~ and q~b~o) and the log(S~/S) calculated using the value of q~b~. The result shows that 16% of the area of the alloyed junction is a low barrier region and 84% is a high barrier region. The 0.65 eV barrier height of the low barrier region is a reasonable value for a Schottky junction, lnfact, the apparent barrier height observed in I - V measurement is q(q~ - A~b~),which is 0.59 eV and very close to the 0.54 eV barrier height obtained from the I - V measurement before the second heat-treatment. When the alloyed metal system is formed on n-type GaAs layers, the region should also be a Schottky junction. This means that there is a Schottky junction region in an alloyed ohmic contact formed on an n-type GaAs layer. The high- and the low-barrier region in our experiment correspond to the ohmiccontact region (the "good" contact region) and the Schottky barrier region (the "poor" contact region) in the alloyed ohmic contact to the n-type GaAs layer. Our result is consistent with the model proposed by Braslau[2] and the TEM results of Kuan et aL[31], which suggests that the alloyed ohmic contact consists of an ohmic contact and a Schottky contact. 5. SUMMARY
This experiment is the first attempt to study ohmic contacts to n-type GaAs through study of alloyed junctions on p-type GaAs layers. We alloyed AuGe-Ni, which is commonly used for ohmic contacts to n-type GaAs, to p-type GaAs layers and studied both the I - V and C - V characteristics of the alloyed junctions. The barrier height obtained
SSE 33/&--B
~.5
._o, ~ I.o := 0.5 Qol
I
I
I I IIIII
I
0.1
t
t
I I I I11
1
$~/S Fig. 9. The relation between the log(SjS) and the barrier heights (q~&c-o and q~bt/_r~)calculated using the parameter q~l of 0.65 eV. by I - V measurement was 0.64 eV, which is close to the Schottky barrier height. On the other hand, the barrier height obtained by C - V measurements was 1.1 eV, which is too high for a Schottky barrier height and too low for a n +p junction. This discrepancy can be explained by adopting a parallel-contact model in which a mixture of two different regions is assumed. By assuming that one region was assumed to be n ÷p junction with a barrier height of 1.42 eV, the barrier height and the area ratio of the other region were deduced to be 0.65 eV and 16%. The low barrier region would be a Schottky contact. From this result we can reasonably infer that the low barrier region would correspond to a Schottky junction region in the AuGe-Ni alloyed ohmic contact to an n-type GaAs. These results are consistent with the mixed structure composed of Au and Ni2 GeAs contact regions observed by Kuan et al. and the inhomogeneous model proposed by Braslau. Therefore we conclude from the electrical evaluation of the alloyed junction that the alloyed AuGe-Ni ohmic contact to an n-type GaAs is electrically inhomogenous and composed of an n +n region and a Schottky region.
Acknowledgements--The authors wish to thank Professor N. Watanabe, Kanagawa University, for helpful discussions and Dr M. Kikuchi for his support and K. Okumura for the device processing.
REFERENCES
I. M. Ogawa, J. appl. Phys. 51, 406 (1980). 2. N. Braslau, J, Vac. Sci. Technol. 19, 803 (1981). 3. T. S. Kuan, P. E. Batson. T. N. Jackson, H. Rupprecht and E. L. Wilkie, J. appl. Phys. 54, 6952 (1983). 4. S. M. Sze, Physics of Semiconductor Devices, 2nd edn, p. 257. Wiley, New York (1981). 5. I. Ohdomari and K. N. Tu, J. appl. Phys. 51, 3735 (1980).
0038-1101/90 $3.00+ 0.00 Copyright © 1990PergamonPress pie
ELECTRICAL INHOMOGENEITY IN ALLOYED AuGe-Ni CONTACT F O R M E D ON GaAs M. KAMADA,T. SUZUKI,K. TAIRAand M. ARAI Sony Corporation Research Center, 174 Fujitsuka-cho, Hodogaya-Ku, Yokohama, Japan 240 (Received 29 June 1989; in revised form 20 February 1990)
A~tract--Electrical properties of alloyed AuGe-Ni contacts to p-type GaAs were investigated and analyzed as a means to investigate the nature of alloyed AuGe-Ni contacts to n-type GaAs. Experimental results on the p-type GaAs were analyzed with a parallel-contact model: the contact is composed of two kinds of areas with different properties. The barrier height of the alloyed junction obtained from the current-voltage relation was 0.64 eV, while that obtained from the capacitance-voltage relation was 1.I eV. We explain the discrepancy by assuming that the contact area is composed of about 84% n +p junction with barrier height of 1.42 eV and about 16% Schottky junction with barrier height of 0.65 eV. We conclude that the AuGe Ni contacts to n-GaAs also have the mixed structure, composed of about 84% of n +n contact and about 16% of the Schottky contact.
I. INTRODUCTION Ohmic contacts to semiconductor layers are essential for semiconductor devices. Alloyed AuGe-Ni metals are most commonly used for ohmic contacts to n-type GaAs layers[l] because of their low contact resistance and good surface morphology. However, there has been no satisfactory understanding what factor determines the ohmic contact resistance. Braslau[2] reported a very important result, that the ohmic contact resistance to n-GaAs layers is proportional to the resistivity of the n-GaAs layers. From this observation, he concluded that the ohmic contact resistance to n-GaAs layers is determined by a spreading resistance, which is proportional to the resistivity of n-GaAs layers. He attributed this relationship to an inhomogeneous structure of the interface between the alloyed metals and the semiconductors. In fact, the inhomogeneous structure was observed in the alloyed Au/Ni/AuGe/GaAs system by Kuan et al.[31] using transmission electron microscopy (TEM). They suggested that the contact interface consisted of a "good" contact region (Ni2GeAs/GaAs) with a heavily doped n+GaAs layer formed by the diffusion of Ge and of a "poor" contact region (Au/GaAs), i.e. a metal-GaAs Schottky junction. In this way, the TEM result obtained by Kuan et al. support Braslau's model; however, there has been no evidence that the contact is electrically inhomogeneous. In this paper, we are going to report on electrical properties of alloyed AuGe-Ni contacts to p-type GaAs, which support the Braslau model of the alloyed AuGe-Ni contact to n-type GaAs. We fabricated alloyed AuGe-Ni junctions, to ptype GaAs layers. The "good" contact region, 999
and the "poor" contact region in Kuan's experiment correspond to n ÷GaAs-pGaAs junction (n ÷p junction), and metal-p GaAs junction (Schottky junction) in our experiment, respectively, and the whole contact is considered to be their mixture. The current-voltage ( I - V ) characteristics and the capacitance-voltage ( C - V ) characteristics for the alloyed junction provide useful information about the constituent regions. We believe that our result can be applied to alloyed AuGe-Ni contacts to n-GaAs and that the contacts are a mixture of ohmic regions and Schottky regions. 2. EXPERIMENTAL
The substrate used in this experiment was Zndoped p-type GaAs, which was grown by the horizontal Bridgman method. The carrier density and the hole mobility evaluated at room temperature by Hall measurements were 6.8 x 1017cm-3 and 175 cm2/Vs, respectively. The cross-sectional view of the sample is schematically shown in Fig. 1. The sample was prepared in the following way: (1) A p-type ohmic metal (AuZn) was evaporated on the back side of the wafer. (2) The wafer was first heat-treated at 420°C in H2/N2 (H2 4%) atmosphere to make ohmic contact. (3) AuGe (190nm, Ge 12% in weight)-Ni (52 nm), which is commonly used as an ohmic metal for n-type GaAs layers, was evaporated using a resistance heat source. The metal was lifted off using chlorobenzene-treated photoresist, resulting in a circular pattern with a diameter of 100 or 333 pm.
1000
M. KAMADAet al. -2
Ni AuGe
( 5 2 nm) (190 nm)
p-type p=6.8x1017
-3
-4
GoAs
/
cm-3
/
(a)Before
-5
AuZn
//,/'~ ~
Fig. 1. The cross-sectional view of the sample. (4) The wafer was again heat-treated at 450°C in a H2/N 2 (H2 4%) atmosphere to form an alloyed junction of the A u G e - N i metal to the p-type GaAs layer. This is a standard alloying condition for making an ohmic contact to an n-type GaAs layer in our laboratory. A low ohmic contact resistivity below 1 x 10 - 6 ~ cm 2 is reproducibly obtained when the same metal ( A u G ~ N i ) is used for ohmic contact to n-type GaAs layers.
3. E X P E R I M E N T A L R E S U L T S
[
/ -8
-9
100/.~m ot
R.T.
-40
-11
I
0.1
I
0.2
3.1. I - V characteristics
Figure 2 shows a typical I - V characteristic of a 100 #m diameter diode. Data corresponding to the sample before and after the second heat-treatment are drawn in the same figure. The former is an AuGe-p-type GaAs Schottky characteristic and the latter is an alloyed junction characteristic. In both cases, the I - V characteristics showed rectification. After the second heat-treatment, the current decreased by about two orders. The n-values before and after the second heat-treatment were 1.26 and 1.22, respectively. The barrier height was calculated from the current density extrapolated to a bias of 0 V using the thermionic emission model, in which the Richardson constant (A *) was assumed to be 74.4 Acm -2 K-214]. The barrier heights before and after the second heat-treatment were 0.54 and 0.64eV, respectively. The decrease in current density after the second heat-treatment corresponds to the increase in barrier height. We also measured the activation energy of the current for the diode after the second heat treatment. Figure 3 shows the I - V characteristics of the 333 #m diameter sample at various temperatures ranging from 243 to 375 K. In Fig. 4 the currents are plotted as a function of the inverse temperature. The activation energy was evaluated from the slope in Fig. 4. Figure 5 shows the relation between the activation energy and the applied bias. Reduction in the activation energy was observed under both the forward and the reverse
/ Reverse
-6,
I
0.3
I
0.4
I
0.5
V (V)
Fig. 2. The 1-Vcharacteristics of a 100 #m diameter diode. There are two data points, one corresponding to the sample before the second heat-treatment (a), and to the sample after the second heat-treatment (b). bias. The barrier height of the alloyed junction should correspond to the activation energy at the bias of 0V. We obtained a barrier height of 0.62eV, which is very close to the barrier height of 0.64eV obtained by using thermionic emission model. 3.2. C - V characteristics
The value of 1/C 2 is plotted as a function of the reverse bias in Fig. 6. The diode was 333 #m in diameter, and the frequency in the measurements was 1 MHz. The relation is linear as shown in Fig. 6. From the slope and the extrapolation of the line, the acceptor density in the p-type GaAs layer and the barrier height of the alloyed junction was evaluated. The barrier height was 1.1 eV, which was much higher than the 0.64eV obtained by the I - V measurement. This barrier height is too low to be the barrier height of an n+p junction and too high to be the barrier height of a Schottky junction. However, the acceptor density of 7.5 × 1 0 1 7 c m - 3 obtained by C - V measurement is very close to 6.8 × 1017cm 3 obtained by Hall measurement, which confirms the validity of the evaluation.
Inhomogeneity of AuGe-Ni contacts on GaAs
1001
1.0
Forward
Reverse
/
0.5 = o .1
i -0.5
0.5
0
V (V)
Fig. 5. The relation between the activation energy and the applied bias. 4. P A R A L L E L C O N T A C T M O D E L
• "0
0.1
0.2
O.5
0.4
0.5
v (V)
Fig. 3. The I - V characteristics of a 333/~m diameter diode at various temperatures ranging from 243 and 373 K. -2
In order to explain the discrepancy between the barrier heights obtained in the two experimental evaluations, we applied a parallel-contact model which assumes a mixed structure made of two regions which have different barrier heights. This structure is supported by Kuan's T E M result. The parallel contact model was originally developed by Ohdomari et aL[5]. The interface is not smooth but rough, as can be recognized from Kuan's results. However, we assume a smooth interface for simplicity. The parameters to be used are summarized in Table 1. The current through the sample (SI) is the sum of the current through the low-barrier-height region and the current through the high-barrierheight region.
S I = S, I l + S 2 I 2 .
-3
(1)
In the thermionic emision model, the currents are expressed as follows:
-4
I = A *T 2 exp(-qq~u_ v ) / k T ) e x p ( q V / n k T )
-5
(2)
11 = A * T 2 exp(-- q (tb I - A d p l ) / k T ) e x p ( q V / n k T ) (3)
-
I 2 = A *T 2 exp(-- q (~b2 - A d p 2 ) / k T ) e x p ( q V / n k T )
~0.2V
(4)
--8
Aq~l and A~b2 are the image-force lowering of each barrier, and the values are given by the following equations:
-0.1V
-9
- O.02V -10
0.002
3 3 3 p m ~b
I
0.004
0.005 1/T
I
Adpl = [( q3 Na/8n2¢~) (dpI -- ( E f - Ev )/q - k T/q )] TM (5)
0.005
( K -1)
Fig. 4. The relation between the current and the inverse temperature.
Adp:~ = [(q3 Na/8X2£3 ) (c~2 - ( E f :- E v ) / q
- k T / q )] TM
(6)
1002
M. KAMADAet al. xlO 5
/
4
q~c_v;,1.1
(ev)
q~l = 0.TeV q~2 " 1.4eV
/
S1/S = 0 0.2 0.4 0.6
6
o.e
1.0
%
1 O'
/
333p.m ~ I 0
-1
I 1
0
V (V) Fig. 6. The relation between I/C 2 a n d reverse bias.
qdPu- v) = - k T /q ln[SJS {exp(-q (~l - Ackl)/k T) - - e x p ( - q (q~2-- AdP2)/kT)} + exp( - q (q~2- Adp2)/kT)]
(7)
The capacitance of the sample is expressed as the parallel combination of the capacitance corresponding to the low-barrier-height region and the capacitance corresponding to the high-barrier-height region. S C = $1 Cl + $2 C2
(8)
{4), - v - k T / q
- ( E f - Ev)/q }-,,,2 C2 = ~
(9)
{02 - V - k T / q
-(Ef-Ev)/q}
1/2.
I -1
I 0
I 1
V (V)
The image-force lowering ranges from 0.06 to 0.08 eV in this experiment. By combining eqns (1)-(4), the following equation can be deduced:
c~ = ~
-
(10)
Examples of the calculated relation between the applied bias and 1/C 2 are shown in Fig. 7. In this calculation, we assumed q~bI and qq~2 to be 0.7 and 1.4 eV, respectively. The values of S J S were Table I. Parameters used in parallel contact model
Fig. 7. A n examples of the calculated relation between the applied bias a n d 1/C 2. qYPL a n d qq~2 were a s s u m e d to be 0.7 a n d 1.4 eV, respectively. The values o f S~/S are 1, 0.8, 0.6, 0.4, 0.2 and 0.
1, 0.8, 0.6, 0.4, 0.2 and 0. The relations were not exactly linear except when S,/S was assumed to be 1 or 0. However, the relation was always linear in the reverse-bias region, in which the C - V relation was measured. The lines in the reverse-bias region were extrapolated to the forward-bias region in order to calculate qq~(~. In Fig. 8, the calculated relation between the barrier height ( q ~ ( ~ and qq~u ~)) and the log(SJS) is shown. In this calculation q~bI and qq~2 were assumed to be 0.7 and 1.4eV, respectively. The barrier height obtained from the I - V measurement should always be lower than the barrier height obtained from C - V measurements, and a large difference is expected even when the ratio S~/S is small, as can be seen in Fig. 8. This is the result of the exponential behavior of the current-voltage characteristics of the junction and would explain the obtained experimental result. The current density through the low-barrier region was calculated to be about 1014 times that of the current density through the high-barrier region as a result of a 0.7 eV energy difference between the two barrier heights. This indicates that the current through the sample comes
S S, $2 1
the total area of the sample (S = S= + $2) the area of the low barrier region the area of the high barrier region the current density measured the current density through the low barrier region 6 the current density through the high barrier region C the capacitace per unit area measured the capacitace per unit area of the low barrier region C~ the capacitace per unit area of the high barrier region C2 the barrier height evaluated by I - V measurements q(alt v) the barrier height evaluated by C - V measurements qqSic v) qck, the barrier height of the low barrier region the barrier height of the high barrier region qck2 qA~bl, qA~2 the image force lowering V applied bias A* Richardson constant (74.4 Acm 2 K 2) energy difference between the Fermi energy and the Ef-Ev valence band top (0.062 eV) permittivity of GaAs (12.9) ~s N~ accepter density in p-type GaAs
1.5
1.0 '~
q ~(I-V)
8 0.5
--10
I
I
--8
-6
I
-4
I
i
-2
0
Log ($I / S ) Fig. 8. The relation between the log(Sj/S) and the barrier height to be measured. In this calculation, q~b] a n d q4)2 were a s s u m e d to be 0.7 a n d 1.4 eV, respectively.
1003
Inhomogeneity of AuGe-Ni contacts on GaAs mainly through the low barrier region, and explains why a very small area of the low barrier height region contributes to effective lowering of the barrier height evaluated by I - V measurement. When the parallel contact model is applied to our experimental result, there are 3 unknown parameters, i.e. qq~l, q~b~and SI/S. Three experimental values are required to evaluate the three parameters. Unfortunately, we can obtain only two parameters, q ~ and qC~c ~. However, q~2 can be assumed to equal the band gap energy of GaAs, 1.42 eV, because q~b2 corresponds to the built-in voltage of n +p junction according to Kuan's result. By making this assumption we can evaluate the two unknown parameters, q~bj and SI/S on the parallel contact model, by combining the values of qq~{~ and q~b~o. The values obtained through this procedure are, q~bI = 0.65 eV
SI/S = 0.16. Figure 9 shows the relation between the barrier heights (qdp~c_~ and q~b~o) and the log(S~/S) calculated using the value of q~b~. The result shows that 16% of the area of the alloyed junction is a low barrier region and 84% is a high barrier region. The 0.65 eV barrier height of the low barrier region is a reasonable value for a Schottky junction, lnfact, the apparent barrier height observed in I - V measurement is q(q~ - A~b~),which is 0.59 eV and very close to the 0.54 eV barrier height obtained from the I - V measurement before the second heat-treatment. When the alloyed metal system is formed on n-type GaAs layers, the region should also be a Schottky junction. This means that there is a Schottky junction region in an alloyed ohmic contact formed on an n-type GaAs layer. The high- and the low-barrier region in our experiment correspond to the ohmiccontact region (the "good" contact region) and the Schottky barrier region (the "poor" contact region) in the alloyed ohmic contact to the n-type GaAs layer. Our result is consistent with the model proposed by Braslau[2] and the TEM results of Kuan et aL[31], which suggests that the alloyed ohmic contact consists of an ohmic contact and a Schottky contact. 5. SUMMARY
This experiment is the first attempt to study ohmic contacts to n-type GaAs through study of alloyed junctions on p-type GaAs layers. We alloyed AuGe-Ni, which is commonly used for ohmic contacts to n-type GaAs, to p-type GaAs layers and studied both the I - V and C - V characteristics of the alloyed junctions. The barrier height obtained
SSE 33/&--B
~.5
._o, ~ I.o := 0.5 Qol
I
I
I I IIIII
I
0.1
t
t
I I I I11
1
$~/S Fig. 9. The relation between the log(SjS) and the barrier heights (q~&c-o and q~bt/_r~)calculated using the parameter q~l of 0.65 eV. by I - V measurement was 0.64 eV, which is close to the Schottky barrier height. On the other hand, the barrier height obtained by C - V measurements was 1.1 eV, which is too high for a Schottky barrier height and too low for a n +p junction. This discrepancy can be explained by adopting a parallel-contact model in which a mixture of two different regions is assumed. By assuming that one region was assumed to be n ÷p junction with a barrier height of 1.42 eV, the barrier height and the area ratio of the other region were deduced to be 0.65 eV and 16%. The low barrier region would be a Schottky contact. From this result we can reasonably infer that the low barrier region would correspond to a Schottky junction region in the AuGe-Ni alloyed ohmic contact to an n-type GaAs. These results are consistent with the mixed structure composed of Au and Ni2 GeAs contact regions observed by Kuan et al. and the inhomogeneous model proposed by Braslau. Therefore we conclude from the electrical evaluation of the alloyed junction that the alloyed AuGe-Ni ohmic contact to an n-type GaAs is electrically inhomogenous and composed of an n +n region and a Schottky region.
Acknowledgements--The authors wish to thank Professor N. Watanabe, Kanagawa University, for helpful discussions and Dr M. Kikuchi for his support and K. Okumura for the device processing.
REFERENCES
I. M. Ogawa, J. appl. Phys. 51, 406 (1980). 2. N. Braslau, J, Vac. Sci. Technol. 19, 803 (1981). 3. T. S. Kuan, P. E. Batson. T. N. Jackson, H. Rupprecht and E. L. Wilkie, J. appl. Phys. 54, 6952 (1983). 4. S. M. Sze, Physics of Semiconductor Devices, 2nd edn, p. 257. Wiley, New York (1981). 5. I. Ohdomari and K. N. Tu, J. appl. Phys. 51, 3735 (1980).