A semiconductor device has one surface of P type conductivity material having a wide energy bandgap and a large crystal lattice parameter. Applied to the P type surface of the semiconductor device is a degenerate region of semiconductor material, preferably a group III-V semiconductor material, having a narrower energy bandgap. The degenerate region is doped with tin to increase the crystal lattice of the region to more closely approximate the crystal lattice of the one surface of the semiconductor device. The degenerate region is compensatingly doped with a P type conductivity modifier. An electrical contact is applied to one surface of the degenerate region forming an ohmic contact with the semiconductor device

Hawrylo, Frank Zygmunt

Kressel, Henry

NASA Technical Reports Server

United States Patent WI 
Hawrylo et al. 
[11% 4,024,569 
[451 May 17, 1977 
OTHER PUBLICATIONS 
[ 541 SEMICONDUCTOR OHMIC CONTACT 
[75] Inventors: 
[ 73 1 Assignee: 
[22] Filed: 
Frank Zymunt Hawrylo, Trenton; 
Henry ~ ~ - 1 ,  Elizabeth, both of 
N.J. Primary Examiner-Edward J. Wojciechowicz 
RCA Corporation, New York, N.Y. 
Jan. 8, 1975 
Jour. Of App. Physics-Vol. 39, N O .  9, Aug. 1968- 
YEH et al. “Diffusion of tin into silicon” 
Attorney, Agent, or Firm-Glenn H. Bruestle; George 
E. Haas; Daniel N. Calder 
1571 ABSTRACT 
A semiconductor device has one surface of P type 
conductivitv material having a wide energy bandgap 
[ 2 1 ] Appl. No.: 539,532 
[52] U.S. CI. ................................... 357171; 357116; 
357160; 357161; 357163; 357165; 357190 
1511 Int. CI.’ ................ HOlL 291161; HOlL 29/04; 
HOlL 29/167; HOlL 23/48 
[ 5 8 ]  Field of Search .................. 357171, 16, 60, 61, 
357163, 65, 90 
~ 5 6 1  References Cited 
UNITED STATES PATENTS 
and a largkcrystal lattice pGameter. Applied to the P 
type surface of the semiconductor device is a degener- 
ate region of semiconductor material, preferably a 
group 111-V semiconductor material, having a narrower 
energy bandgap. The degenerate region is doped with 
tin to increase the crystal latticle of the region to more 
closely approximate the crystal lattice of the one sur- 
face of the semiconductor device. The degenerate re- 
gion is compensatingly doped with a P type conductiv- 
ity modifier. An electrical contact is applied to one 
surface of the degenerate region forming an ohmic 
contact with the semiconductor device. 
3,632,436 1/1972 Denning .............................. 357/71 
3,684,930 8/1972 Collins et al. ....................... 357/71 . 11 Claims, 1 Drawing Figure 
14 16 
https://ntrs.nasa.gov/search.jsp?R=20080006893 2019-08-30T03:10:13+00:00Z
U.S. Patent May 17, 1977 4,0243 69 
14 16 
4,024,569 
1 4 3 
SEMICONDUCTOR OHMIC CONTACT 
BACKGROUND OF THE INVENTION 
DESCRIPTON OF THE INVENTION 
The present invention will be described using a semi- 
conductor laser. However, the present invention may 
The invention described herein was made in the per- 5 be employed as an ohmic contact on other semicon- 
formance of work under a NASA contract and is sub- ductor devices. With reference to the Figure of the 
ject to the provisions of Section 305 of the National drawing, a semiconductor laser 10 comprises a right 
Aeronautics and Space Act of 1958, Public Law parallelepiped body 12 formed of a single crystalline 
85-568 (72 Stat. 435; 42 U.S.C. 2457). semiconductor material. The body 12 has two end 
The present invention relates to ohmic contacts on 10 surfaces 14 and opposed first and second surfaces 16 
wide bandgap P type semiconductor material having a and 18 respectively. 
relatively large crystal lattice. The body 12 includes a highly doped N type substrate 
One ofthe basic problems in manufacturing semicon- 22 extending along the first surface 16. A first epitaxial 
ductor devices lies in attaching electrical leads to the layer 24 of N type conductivity is contiguous With the 
seimconductor material. These metal to semiconductor 15 substrate 22. An N second epitaxial layer 26 is on 
contacts, commonly called ohmic contacts, must not the first layer 24. A third epitaxial layer 28 of p type 
add significant resistance to the on which conductivity material is on t h e  second epitaxial layer 
they are used, and should not 'alter the equilibrium 26. The third epitaxial layer 28 is Of a material having 
carrier densities within the semiconductor so as to a energy bandgap width than the materia1 of 
change the characteristics of the device. ~~d to semi- 20 the second epitaxial layer 26 and forms the recombina- 
conductor contacts may form barriers which tion region of the laser 10. Extending along the second 
renders them unacceptable for use in this area. men a surface 18 is a fourth epitaxial layer 30 which is also 
highly doped or degenerate semiconductor region is contiguous with the third epitaxial layer 28. The fourth 
employed to act as an interface between the metal and 
semiconductor material. This region is doped so that 25 
a semiconductor-semiconductor contact, have rela- 
layer 30 is Of 
having an 
A degenerate region 32 
type conductivity and is Of a 
bandgap which is greater 
Of semiconductor 
the two resulting contacts, a metal-semiconductor and 
tively linear resistivity. 
Satisfactory ohmic contacts are particularly difficult 
to form on P type semiconductor material having an 
energy bandgap in excess of 1.6 Ev. Under these cir- 
cumstances, the degenerate semiconductor region usu- 
ally has a narrower energy bandgap than the adjacent 
than about 
material, preferably from the 111-V compound family, 
is applied to the second surface 18. The energy band- 
30 gap of the degenerate region 32 is less than 1.6 Ex. The 
crystal lattice parameter of the fourth layer 30 is rela- 
tively large as compared to the semiconductor material 
of the degenerate region 32. Tin is used to dope the 
degenerate region 32. This tin doping enlarges the semiconductor 
to which the metal 'Ontact layer can be thus providing a better surface 35 crystal lattice parameter of the degenerate region 32 so that it more closely approximates the lattice parameter How- ' ever, this creates additional problems with respect to of the fourth layer 30. The degenerate region 32 is 
lattices of the two semiconductor regions differ sub- 4o degenerate region 32 should have higher conductivity 
efficiency in the semiconductor and may even cause 
the semiconductor material to crack at the semicon- 
ductor interface thereby physically damaging the de- 45 surface 16. 
vice. 
the matching Of the crystal lattices Of the wide energy 
bandgap and the degenerate region* If the 
compensatingly doped with a p type conductivity modi- 
fier to produce a highly P type conductivity region. The 
than the fourth layer 30. A first electrical contact layer stantially, stresses will occur at the interface between 
the two regions. These stresses may in a loss Of 34 is applied to the surface of the degenerate region 32 
in a manner so as to form a good ohmic contact, A 
second electrical contact layer 20 is applied to the first 
Semiconductor injection lasers, such as the one 
shown in the drawings are particularly representative of 
devices on which this invention may be employed. For 
A semiconductor device has one surface composed example, the semiconductor laser 10 may have a body 
of a p type conductivity material. The conduc- 50 12 composed of several layers of gallium arsenide and 
tivitY material has a wide energy bandgap and a large alloys of aluminum gallium arsenide. The fourth epitax- 
crystal lattice. TO the surface of the semi-conductor ial layer 30 may have an aluminum conc-ntration of 
device, a degenerate region of semiconductor material about 20% in which case it has an energy bandgap of 
is applied. The material of the degenerate region has a about 1.68 Ev. and an X-ray lattice parameter of about 
narrower energy bandgap and a higher P type conduc- 55 5.6548 a. The degenerate region 32 is formed of gal- 
tivity as compared to the surface of the semiconductor lium arsenide with approximately a one atomic percent 
device. The degenerate region is doped with tin to concentration of tin and is compensated highly P type 
enlarge the crystal lattice of the region to more closely with a doping of zinc. The cvstal lattice parameter of 
approximate the crystal lattice of the P type conductiv- the degenerate region 32 in this example is about 5.654 
ity material at the surface of the semiconductor device. 60 A. 
Since tin is an N type dopant, a P type conductivity The laser body 12 and the degenerate region 32 may 
modifier is employed to compensate the doping of the be grown by liquid phase epitaxy similar to that de- 
degenerate region. An electrical contact layer is ap- scribed in U.S. Pat. No. 3,565,702 issued on Feb. 23, 
plied to the surface of the degenerate region. 197 1 to H. Nelson. When using this method, the fourth 
SUMMARY OF THE INVENTION 
65 layer 30 in the example may be grown in a refractory 
furnace boat using a charge of 2 grams of gallium, 280 
milligrams of gallium arsenide, 6 milligrams of alumi- 
BRIEF DESCRIPTION OF THE DRAWINGS 
The figure of the drawing shows a semiconductor 
device embodying the present invention numiand a P Gpe dopant such as 25 milligrams of zinc. 
4,024,569 
3 4 
The charge which is used to grow the degenerate region 
32 uses tin as a solvent in which gallium arsenide and a 
P type dopant are dissolved. The specific charge con- 
centration to form this degenerate region 32 may con- 
sist of 2 grams of tin, 360 milligrams of gallium arse- 5 
nide, and 35 milligrams of zinc. The relatively small 
concentration of zinc in the solution as compared with 
the concentration of tin is sufficient to produce a Semi- 
conductor region which has a high P type conductivity. 
enlarge the crystal lattice parameter of the degen- 
erate region and being highly compensated with a P 
type conductivity modifier; and 
an electrical contact on the surface of the degenerate 
region. 
2. The device as in claim 1 wherein the degenerate 
region has a crystal lattice closely approximating the 
crystal lattice of the p type conductivity material. 
3. The device as in claim 1 wherein the degenerate 
The improved ohmic contact which is formed on the 10 region has a greater p type conductivity than said p 
P type surface has several advantages over the previous type conductivity material. 
ohmic contacts. The presence of the degenerate region 
32 which has high conductivity and a narrow energy of tin in the degenerate layer is about 1 atomic percent. 
bandgap provides a surface to which a good metal- wherein the P type con- 
semiconductor contact can be made. In addition the 15 ductivity material at the one surface of the body has an 
presence of tin as a dopant in the degenerate region 32 energy which is greater than -6 Ev. 
6. The device as in claim 5 wherein the degenerate expands the region's crystal lattice parameter. With reference to the above example, the lattice parameter region of semiconductor material has an energy band- of gallium arsenide is 5.6533 A, whereas the lattice gap less than 1.6 Ev. parameter of gallium arsenide doped with one atomic 20 
7. The device as in claim 1 wherein the degenerate percent of tin is enlarged to about 5,654 A. The en- 
more closely approximates the lattice size of the third 
duces the strain at the interface of the degenerate re- 25 region is 
of cracking or separation of the semiconductor mate- 
rial at the interface. 
4. me device as in claim 1 wherein the 
5. The device as in claim 
larged lattice parameter of the degenerate region 32 
layer 30 (5.6548 A). This improved lattice match-re- 
@on 32 and the fourth layer 30 reducing the possibility 
region is Of group 'I1-' semiconductor ma- 
terial* 
8. The device as in the degenerate 
the t Y p  con- 
Of gallium arsenide* 
9. The device as in claim 8 
ductivity modifier is zinc. 
10. The device as in claim 9 wherein the P type con- 
ductivity material is an alloy of aluminum gallium arse- 
11. The device as in claim 1 wherein said device 
consists of a semiconductor injection laser, said laser 
composed of a plurality of layers of gallium arsenide 
and alloys of aluminum gallium arsenide; and 
said degenerate region is composed of gallium arse- 
nide doped with a small amount of tin and compen- 
satedly doped P type with zinc. 
We claim: 
1. A simiconductor device comprising: 
a body with one surface of P type conductivity mate- 
rial, siad P type conductivity material having a wide 
energy bandgap and a large crystal lattice; 
a degenerate region of semiconductor material on 
the one surface, the material of said degenerate 35 
region having a relatively narrow energy bandgap 
with respect to said P type conductivity material, 
30 nide. 
said degenerate region being doped with tin to * * * * *  
40 
45 
50 
55 
60 
65 


Semiconductor ohmic contact

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