Structure of ohmic electrode for semiconductor by atomic layer doping

An ohmic contact electrode for a semiconductor device which has a low contact resistance and high stability. The ohmic contact electrode includes: a semiconductor substrate; an atomic doping layer developed on the semiconductor substrate wherein the atomic doping layer is formed by doping impurities such that an energy level of the layer is higher than a Fermi level; a semiconductor layer developed on the atomic doping layer wherein the semiconductor layer is formed of the same material as in the semiconductor substrate; a metal electrode formed on the semiconductor layer for establishing an electric connection with the semiconductor substrate; wherein the semiconductor layer has a thickness sufficient for carriers to transfer between the metal electrode and the atomic doping layer by tunneling through the semiconductor layer.

FIELD OF THE INVENTION 
This invention relates to a structure and method of an ohmic contact 
electrode for electric connecting semiconductor and metal, and more 
particularly, to an ohmic contact electrode structure for decreasing a 
contact resistance between semiconductor and metal with the use of an 
atomic doping layer and to a production process of such an ohmic contact 
having an atomic doping layer. 
BACKGROUND OF THE INVENTION 
In establishing an ideal ohmic contact surface between semiconductor and 
metal in a semiconductor device, it is required that (1) a voltage-current 
curve be linear and symmetrical in a contact area between the 
semiconductor and the metal, and (2) a contact resistance be sufficiently 
low. 
Electric current, which is directly related to the contact resistance in 
the contact surface, is dependent upon a height of Schottky barrier, 
temperature and a doping density in the contact area. To decrease the 
contact resistance, it is necessary to lower the Schottky barrier or 
increase an electron density or concentration in the neighborhood of the 
contact area. 
In the past, as means for decreasing the Schottky barrier, it is used a 
material having a low Schottky barrier or alloy materials to lower the 
Schottky barrier. As means for forming a high electron concentration layer 
in the neighborhood of the contact area, an epitaxial growth or diffusion 
control growth process is usually used. 
With the increase of an component density and reliability in semiconductor 
devices, it has become especially necessary to have low contact resistance 
and high temperature stabilities in the boundaries of semiconductor and 
metal in the semiconductor devices. 
As noted above, in the conventional technology for decreasing the Schottky 
barrier, it is used either materials having a low Schottky barrier or 
alloying materials which can lower the Schottky barrier. However, in the 
former, such materials suitable for decreasing the Schottky barrier are 
limited and are often incompatible with the semiconductor. In the latter, 
it is difficult to form an alloy layer of uniform depth or thickness, and 
ordinarily, a semiconductor device having such an alloy layer has a lesser 
thermal stability. 
Further, as noted above, a high electron concentration layer is formed in 
the neighborhood of the contact area by an epitaxial growth or diffusion 
control growth process. However, for the purpose of improving a 
productivity, there is a need of an easier and more stabilized method for 
forming an electrode in the semiconductor devices. 
SUMMARY OF THE INVENTION 
It is, therefore, an object of the present invention to provide a structure 
of ohmic contact electrode for a semiconductor device which has low 
contact resistance and high temperature stability. 
It is another object of the present invention to provide a structure of 
ohmic contact electrode for a semiconductor device which is easily be 
produced with high stability and thus with low cost and has high 
productivity. 
The ohmic contact electrode of the present invention is formed of a 
semiconductor substrate, a metal electrode provided on said semiconductor 
substrate, and an atomic doping layer formed in an area of the 
semiconductor substrate close to a boundary of the semiconductor substrate 
and the metal electrode. 
One aspect of the present invention is a structure of an ohmic contact 
electrode for a semiconductor device. The ohmic contact electrode of the 
present invention includes: a semiconductor substrate; an atomic doping 
layer developed on the semiconductor substrate wherein the atomic doping 
layer is formed by doping impurities such that an energy level of the 
layer is higher than a Fermi level; a semiconductor layer developed on the 
atomic doping layer wherein the semiconductor layer is formed of the same 
material as in the semiconductor substrate; a metal electrode formed on 
the semiconductor layer for establishing an electric connection with the 
semiconductor substrate; wherein the semiconductor layer has a thickness 
sufficient for carriers to transfer between the metal electrode and the 
atomic doping layer by tunneling through the semiconductor layer. 
Another aspect of the present invention is a method of producing an ohmic 
contact electrode for a semiconductor device. The method of the present 
invention includes the following steps of: forming a semiconductor 
substrate; developing an atomic doping layer on the semiconductor 
substrate wherein the atomic doping layer is formed by doping impurities 
such that an energy level of the layer is higher than a Fermi level; 
forming a semiconductor layer on the atomic doping layer wherein the 
semiconductor layer is the same material as the semiconductor substrate 
with thickness sufficient for carriers to tunnel therethrough; and, 
forming a metal electrode on the semiconductor layer for establishing an 
electric connection with the semiconductor substrate. 
According to the present invention, the atomic doping layer is formed in an 
area of the semiconductor closer to a junction of the semiconductor and 
the metal electrode. By this arrangement, the Schottky barrier between the 
semiconductor and the metal is lowered, which realizes an ideal ohmic 
contact electrode. The ohmic contact electrode of the present invention 
has lower contact resistance and higher contact stability. Since the 
contact characteristics of the ohmic contact electrode is directly 
dependent of the doping level and the position of the doping layer, 
production of the electrode can be made easily with high efficiency and 
low cost.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
An example of structure of the ohmic contact electrode in accordance with 
the present invention is shown FIG. 2 wherein an electrode formed by a 
metal layer is provided on semiconductor substrate. The energy levels in 
the boundary area of the metal layer and the semiconductor of FIG. 2 are 
shown in FIG. 1. 
In the present invention, in an area close to a boundary between a metal 
electrode 22 and semiconductor 20, impurities are doped to form an atomic 
layer doping to realize an ohmic contact having high stability and low 
contact resistance. This atomic layer doping is also called planar doping 
wherein impurities are doped in semiconductor crystals to establish 
electrical resistance. The impurities are doped in a common atomic layer 
of the semiconductor and, thus the impurities are distributed in a delta 
function manner. Thus, atomic layer doping is also called delta doping. A 
delta doping layer is a highly doped, very thin layer of several 
nanometer. 
The atomic layer doping will be performed by a molecular beam epitaxy or a 
vapor phase epitaxy. In the following example of this invention, the 
molecular beam epitaxy such as for growing gallium arsenide crystal is 
used, although other process of crystal growth is also available. 
A structure of a preferred embodiment of the present invention is shown in 
FIG. 2 wherein indium gallium phosphorus (InGaP) is used as the 
semiconductor 20, and titanium/gold (Ti/Au) is used as the metal electrode 
22. Further, in this example, silicon is used as impurities to be doped in 
the semiconductor 20. The process of forming the electrode structure of 
FIG. 2 is as follows: 
(1) Form a semiconductor substrate 28 through crystal growth of the indium 
gallium phosphorus (InGaP). In the molecular beam epitaxy (MBE), this 
process is performed by vapor exposure of each In, Ga and PH3 to grow the 
semiconductor substrate 28. 
(2) Temporarily cease the vapor exposure of In, Ga to postpone the crystal 
growth of the InGaP substrate 28 when the substrate has grown to several 
nanometer less than the predetermined final thickness. 
(3) Form an atomic doping layer on the InGaP semiconductor crystal by vapor 
exposing PH3 and the dopant silicon (Si) which is the impurities for the 
semiconductor. The degree of doping should be small so that the thickness 
of the doping layer 26 is less than one atomic layer, for example, about 
10.sup.13 -10.sup.14 /cm.sup.-2. 
(4) Resume the crystal growth of InGaP substrate to form a semiconductor 
layer 24 over the doping layer 26 for the thickness of several nanometer. 
This thickness d of InGaP is determined mainly by the relationship between 
the semiconductor substrate InGaP and the metal material Ti/Au. In this 
example, excellent contact characteristic is achieved at the thickness d=5 
nm (nanometer). In the other combination of materials, preferable results 
are expected where the thickness d is less than 10 nm. 
(5) Finally, form a metal electrode 22 of Ti/Au on the semiconductor layer 
24. The structure of ohmic contact electrode of the present invention is 
formed in the process as described above. 
FIG. 3 shows experimental results of ohmic contact characteristics and 
contact resistance in the electrode formed by the above process. Excellent 
ohmic contact characteristics is attained in a range 35 in FIG. 3 where a 
sheet carrier concentration is 2.times.10.sup.13 -2.times.10.sup.14 
cm.sup.-2. The contact resistance within this range 35 is 
5-6.times.10.sup.-6 cm.sup.2. The lowest specific resistance of 
5.times.10.sup.-6 cm.sup.2 is observed at a point 36 where the atomic 
layer doping carrier concentration in the layer 26 is 7.times.10.sup.13 
cm.sup.-2, which is equivalent to the resistance value attained in an 
alloy junction. This means that the thickness of the semiconductor layer 
between the atomic doping layer and the metal electrode is large enough 
for allowing sufficient tunneling for carriers. 
The doping level, i.e., the sheet carrier concentration 2.times.10.sup.13 
cm.sup.-2 is substantially the same as a point where a well bottom 12 in 
FIG. 1 reaches a Fermi level. This means that an optimum doping level can 
be determined by the sheet carrier concentration wherein the energy level 
of the well bottom 12 becomes equal to the Fermi level. 
In the foregoing example of the ohmic contact electrode, the indium gallium 
phosphorus (InGaP) is used as the semiconductor substrate, the 
titanium/gold (Ti/Au) is used as the metal electrode, and the silicon is 
used as the impurities to be doped in the semiconductor substrate. The 
other combinations of materials for the semiconductor substrate, the metal 
electrode and the impurities are also possible. For example, as the 
semiconductor substrate, P-type semiconductor, N-type semiconductor, 
germanium, or gallium arsenide are equally applicable. As the metal 
electrode materials, aluminum, platinum or tungsten are also applicable. 
Further, for the dopant, the impurities to be doped in the semiconductor 
substrate, tin, carbon, beryllium or zinc are also applicable. 
The electrode structure by the above combinations, the atomic doping layer 
should be formed within the range 35 where the energy level exceeds the 
Fermi level. This structure of electrode achieves an ohmic contact which 
has higher stability and lower contact resistance than the conventional 
one. In other words, any metal materials can be used as an electrode if 
the materials have electric conductivity and are compatible with the 
semiconductor substrate, and any impurities which can be doped in the 
semiconductor material to raise the energy level higher than the Fermi 
level can be used. There is no restriction in the semiconductor materials, 
thus, any semiconductor materials can be used in forming the electrode of 
the present invention. 
In the foregoing example, the atomic layer doping is achieved by using the 
crystal growth process such as the molecular epitaxy. Other doping process 
such as a vapor phase epitaxial growth process can also be used to form 
the atomic doping layer which has an energy level higher than the Fermi 
level, i.e. within the range 35 of FIG. 3. 
As has been foregoing, according to the present invention, the atomic 
doping layer is formed in an area of the semiconductor closer to a 
junction of the semiconductor and the metal electrode. By this 
arrangement, the Schottky barrier between the semiconductor and the metal 
is lowered, which establishes an ideal ohmic contact electrode. The ohmic 
contact electrode of the present invention has lower contact resistance 
and higher contact stability. Since the contact characteristics of the 
ohmic contact electrode is directly dependent of the doping level and the 
position of the doping layer, production of the electrode can be made with 
high efficiency.