Method for cleaning a semiconductor surface

An improved method for cleaning a group III-nitride-based semiconductor surface prior to depositing electrodes or growing additional layers of semiconductor. In a cleaning method according to the present invention, the surface of the semiconductor is brought into contact with an etchant solution that includes hydrofluoric acid. The etching step is preferably carried out at a HF concentration greater than 5% and at a temperature between 10 to 100.degree. C. in an inert atmosphere. The etchant solution may also include other acids. Group III-nitride semiconductor devices cleaned in this manner require lower driving voltages than devices cleaned with prior art methods.

FIELD OF THE INVENTION 
The present invention relates to methods for cleaning semiconductor 
surfaces, and more particularly, to a method for cleaning a group 
III-nitride semiconductor surface before forming electrodes thereon or 
before homoepitaxial growth of additional semiconductor layers thereon. 
BACKGROUND OF THE INVENTION 
Group III-nitride semiconductors such as GaN are useful in fabricating 
light emitting elements that emit in the blue region of the optical 
spectrum. These elements include light emitting diodes and laser diodes. 
The light emitting elements are typically fabricated by creating a p-n 
diode structure on substrate. The diode is constructed from layers of 
group III-nitride semiconducting materials that are homoepitaxially grown. 
After the appropriate layers are grown and etched back to provide access 
to the bottom layer of the p-n diode, electrodes are formed on the p-type 
and n-type layers to provide the electrical connections for driving the 
light emitting element. 
The electrodes are formed by depositing metals on the semiconductor 
surface. If the surface is not cleaned to remove any oxide that has formed 
on the surface, the contact resistance between an electrode and the 
semiconductor becomes large. This large contact resistance increases the 
voltage needed to drive the light emitting element. Furthermore, the drive 
voltage from device to device is not consistent because the level of oxide 
present is highly variable. 
Similarly, if the semiconductor surface is not cleaned prior to 
homoepitaxial growth, the homoepitaxial growth will not occur properly. As 
a result, high performance light-emitting elements cannot be formed. 
The cleaning systems developed for other semiconductor systems do not work 
adequately for group III-nitride semiconductors. For example, cleaning 
systems for GaAs-based semiconductors or indium phosphide-based 
semiconductors use sulfuric acid or ammonia based etchants. However, these 
etchant systems when applied to group III-nitride semiconductors do not 
adequately clean the surface. Consequently, when electrodes are formed on 
a GaN-based semiconductor after using these prior art methods, the current 
and voltage characteristics between the electrode and the GaN 
semiconductor exhibited a large Schottky barrier. To overcome this 
barrier, an increase in the driving voltage is needed. 
Similarly, if these prior art cleaning systems are used before 
homoepitaxial growth on a conventional GaN semiconductor, the resulting 
crystallinity is poorer than that observed in GaAs system devices. 
Broadly, it is the object of the present invention to provide an improved 
method for cleaning the surface of group III-nitride semiconductors. 
It is a further object of the present invention to provide a cleaning 
method that decreases the drive voltage for the resultant semiconductor 
device. 
These and other objects of the present invention will become apparent to 
those skilled in the art from the following detailed description of the 
invention and the accompanying drawings. 
SUMMARY OF THE INVENTION 
The present invention is an improved method for cleaning a group 
III-nitride-based semiconductor surface prior to depositing electrodes or 
growing additional layers of semiconductor. In a cleaning method according 
to the present invention, the surface of the semiconductor is brought into 
contact with an etchant solution that includes hydrofluoric acid. The 
etching step is preferably carried out at a HF concentration greater than 
5% and at a temperature between 10 to 100.degree. C. in an inert 
atmosphere. The etchant solution may also include other acids.

DETAILED DESCRIPTION OF THE INVENTION 
The present invention is based on the experimental observation that 
hydrofluoric acid etchant systems such as those used for forming mirror 
surfaces on a silicon-based semiconductor act as an effective etchant for 
group III-nitride semiconductors. The method of the present invention can 
be applied to group III-nitride semiconductors such as semiconductors 
based on GaN, AlN, AlGaN, and GaInN. 
The hydrofluoric acid system etchant of the present invention utilizes 
hydrofluoric acid or an acid mixture containing hydrofluoric acid. For 
example, the hydrofluoric acid may be mixed with nitric acid or 
hydrochloric acid. The manner in which a group III-nitride semiconductor 
surface is cleaned by the method of the present invention involves four 
steps. First, the surface of the semiconductor is cleaned with an organic 
solvent-based solution. The preferred organic solvents are 
trichloroethylene, acetone, methanol, or isopropanol. These compounds may 
be used independently or in combinations of two or more. Cleaning methods 
based on such organic solvents are well known in the semiconductor arts, 
and hence, will not be discussed in detail here. 
Next, the semiconductor surface is rinsed with water. Purified water 
generated by ion exchange or distillation is preferred. Such rinsing 
procedures are well known in the semiconductor arts, and hence, will not 
be discussed in detail here. 
Third, the semiconductor is then subjected to a hydrofluoric acid-based 
etchant. The hydrofluoric acid is preferably applied in an aqueous 
solution at a concentration greater than 5%. However, the concentration is 
not critical. If the concentration is too low, the time needed to clean 
the surface becomes too long. 
As noted above, the hydrofluoric acid may be applied in combination with 
one or more additional acids. The ratio of the acids in the mixture and 
the concentration of the other acid are not particularly critical. In the 
preferred embodiment of the present invention, the hydrofluoric acid is 
present in the above discussed concentration range. 
The temperature at which the etchant is applied is also not particularly 
critical. In general, a temperature range from 10 to 100.degree. C. is 
preferred. If the temperature of the hydrofluoric acid system etchant is 
too low, the time required to achieve the desired degree of cleanliness 
becomes too long to provide a cost-effective process. The preferred 
treatment time is from about 30 seconds to 60 minutes. 
The treatment may be carried out in air at room temperature and atmospheric 
pressure. However, to prevent re-growth of an oxide film on the surface of 
the semiconductor, the etching operation is preferably carried out in the 
presence of an inert gas, such as nitrogen, and at a lower temperature. 
In the preferred embodiment of the present invention, the etching operation 
is carried out by immersing the semiconductor in the etchant solution in a 
suitable container. In principle, the etchant can also be applied spraying 
for the aforementioned treatment time. However, achieving uniform delivery 
of the solution by spraying with a nozzle over the entire surface is 
difficult. Similarly, the etchant may be applied by placing the 
semiconductor in an elongated container through which the etchant flows at 
a predetermined rate over the semiconductor. However, the length of the 
container and control of the flow rate of the hydrofluoric acid etchant 
become critical. Finally, these alternative methods are more difficult to 
implement in an airtight environment. Accordingly, the immersion method 
discussed above is preferred. 
After subjecting the semiconductor to the etchant, the semiconductor is 
rinsed with water as discussed above. The semiconductor is then dried. The 
drying removes the deionized water remaining on the surface of the 
semiconductor and any minute amounts of hydrofluoric acid that were not 
removed by the water rinse. The drying preferably takes place at a 
temperature between 80 to 200.degree. C. in an inert atmosphere such as 
nitrogen. The inert atmosphere prevents the re-growth of the oxide film. 
EXAMPLES OF ETCHING CONDITIONS 
To illustrate the improvement provided by the present invention, a p-type 
semiconductor (2-inch diameter) grown on a sapphire substrate was used as 
the group III-nitride semiconductor. The semiconductor's surface was 
cleaned as outlined below. Two similar substrates were used as controls. 
One was also cleaned using HCl with the same process, the other was 
processed without any etchant. All of the samples were plated with nickel 
electrodes and the current versus voltage curve for each sample was 
measured. 
The process steps were carried out in an airtight system in a nitrogen 
atmosphere. GaN-based semiconductors were utilized in the first set of 
tests. The GaN semiconductors were boiled for 5 minutes in 
trichloroethylene, immersed in methanol, and then ultrasonically agitated 
for 5 minutes. The substrates were then washed by flowing deionized water 
at 25.degree. C. over the entire surface of the sample at a flow rate of 1 
liter/minute. 
Next the samples were treated with the various etchants. The substrate that 
was treated with the etchant of the present invention was immersed for 30 
seconds in a container filled with a 50% HF aqueous solution of 
hydrofluoric acid. The controls were either not treated or treated by 
immersion in a 35% aqueous solution of HCl for 30 seconds. 
The remaining steps were the same for all samples. The samples were washed 
by flowing deionized water at 25.degree. C. over the entire surface of the 
semiconductor at a flow rate of 1 liter/minute for 5 minutes. The samples 
were then dried in an electric furnace at 110.degree. C. for 10 minutes. 
Nickel electrodes were then deposited on one side of the semiconductor. 
The voltage versus current curve for each of the semiconductor elements was 
then measured. The results are shown in FIG. 1. As can be seen from the 
figure, the sample treated by the method of the present invention (Curve 
1) exhibited substantially lower drive voltages compared to the control 
treated with HCl (Curve 3) or the control that was not etched (Curve 2). 
The above-described test has been repeated with other group III-nitride 
semiconductor materials such as AlGaN and GaInN with similar results. In 
each case, the driving voltages obtained using the method of the present 
invention were significantly less than those obtained using HCl or no 
etchant. 
The use of other etchant protocols does not alter the results obtained with 
the method of the present invention provided the etching procedure 
includes an etching step in HF. For example, a GaN substrate was processed 
as described above except that the etching step consisted of immersing the 
sample for 5 minutes in a container filled with aqua regia and then 
immersing the sample for 10 minutes in a container filled with a 50% 
aqueous hydrofluoric acid solution. The aqua regia consisted of 60% nitric 
acid and 35% hydrochloric acid. This semiconductor element had essentially 
the same driving voltage curve as shown in FIG. 1 for HF etchant described 
above. 
Various modifications to the present invention will become apparent to 
those skilled in the art from the foregoing description and accompanying 
drawings. Accordingly, the present invention is to be limited solely by 
the scope of the following claims.