In order to keep the resistive losses in solar cells to a minimum, it is often necessary for the ohmic contacts to be heat treated to lower the metal-semiconductor contact resistivity to acceptable values. Sintering of the contacts, however can result in extensive mechanical damage of the semiconductor surface under the metallization. An investigation of the detailed mechanisms involved in the process of contact formation during heat treatment may control the structural damage incurred by the semiconductor surface to acceptable levels, while achieving the desired values of contact resistivity for the ohmic contacts. The reaction kinetics of sintered gold contacts to InP were determined. It was found that the Au-InP interaction involves three consecutive stages marked by distinct color changes observed on the surface of the Au, and that each stage is governed by a different mechanism. A detailed description of these mechanisms and options to control them are presented

Fatemi, Navid S.

Weizer, Victor G.

English

NASA Technical Reports Server

N91-19203 !
Semiconductor Structural Damage Attendant to
Contact Formation in III-V Solar Cells
Navid S. Fatemi* and Victor G. Weizer
NASA Lewis Research Center
Cleveland, OH
In order to keep the resistive losses in solar cells to a minimum, it is often nec-
essary for the ohmic contacts to be heat treated to lower the metal- semiconductor
contact resistivity to acceptable values. Sintering of the contacts, however can result
in extensive mechanical damage of the semiconductor surface under the metallization.
An investigation of the detailed mechanisms involved in the process of contact forma-
tion during heat treatment may enable us to control the structural damage incurred
by the semiconductor surface to acceptable levels, while achieving the desired values
of contact resistivity for the ohmic contacts. In this work, we have determined the
reaction kinetics of sintered gold contacts to InP. We have found that the Au-InP in-
teraction involves three consecutive stages marked by distinct color changes observed
on the surface of the Au, and that each stage is governed by a different mechanism. A
detailed description of these mechanisms and options to control them are presented.
Introduction
Gold and gold-based contact systems are extensively used as metallization mate-
rials for III-V solar cells. In some cases, acceptably low metal-semiconductor contact
resistivity is achieved to highly doped emitters by as-deposited contacts. For in-
stance, as-deposited Au contacts on InP and Au-Zn contacts on GaAs heavily doped
substrates can produce specific contact resistivity (Re) values in the 10 .4 ohm-cm 2
range [refs. 1-2]. Although this contact resistivity range may be adequate for one
sun operation of solar cells, it falls short of the 10 .6 ohm-cm 2 values often required
for concentrator applications. There are also concerns about the stability of these
contacts with aging, for no stable compounds are present at the metal-semiconductor
interface. The most prominent advantage of the as-deposited contacts is that virtually
no structural damage is suffered by the semiconductor under the metallization.
On the other hand, much lower R c values are readily obtained by heat treating Au-
based contacts to variously doped III-V semiconductors. Rc values in the 10-6and
10 .7 ohm-cm 2 range have been measured by Au-based systems to InP and GaAs
substrates [refs. 2-6]. Heat treated contacts also tend to be more stable with aging
[ref. 6]. The greatest disadvantage of the heat treating process is the appearance of
severe pitting at the metal-semiconductor interface during contact formation. The
electrical and mechanical changes which occur during the heat treatment process
*Sverdrup Technology Inc., Lewis Research Center Group.
270
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however, may not necessarily be related. For example, encapsulation of Au-Ge-Ni
and Au-Zn contacts with SiO2 prior to heat treatment can not only slightly reduce
the metal-GaAs interaction rate but at the same time produce lower Rc values [refs.
7-9].
The purpose of our research program is to understand contact formation in III-V
semiconductor materials, and use that knowledge, to devise a contact system and/or
a processing scheme by which desired R_: values are achieved with minimal structural
damage to the semiconductor. In this paper, our focus has been on the investigation
of the mechanisms involved in contact formation in the Au-InP system.
The Kinetics of the Au-InP Interaction
The Au-InP interaction consists of three consecutive stages of reaction, each gov-
erned by a different rate limiting mechanism. The three stages are: 1. Au-to-Au(In)
(saturated solid solution), 2. Au(In)-to-AuaIn, and 3. AuaIn-to-Auln2 transitions.
In addition, spreading of Au contacts occurs concurrent with the third stage of the
reaction. In what follows, a discussion of the mechanisms operating in each stage, as
well as in the Au spreading phenomenon, and means to control them are presented.
STAGE I. At this early stage of the reaction In is dissolved into the Au lattice via
a dissociative diffusion mechanism until the saturation limit of 10% atomic is reached.
Rapid interstitial diffusion of In from the InP lattice toward the surface of the Au is
controlled by the rate of vacancy generation at the free surface of the metallization.
Consequently, the condition of the Au surface can dictate the reaction rate at the Au-
InP interface. The reaction suppressing effect of the capping of the contacts with SiO2
before heat treatment is due to the suppression of vacancy generation rate at that
surface. Conversely, surface abrasion of the Au tends to enhance the reaction rate at
the metal-semiconductor interface due to the fact that regions of lattice disorder in
the metallization are a greater source of vacancies than the undisturbed regions. We
have previously demonstrated the effect of surface damage and capping of the contacts
on the metal-semiconductor reaction rates in both the Au-GaAs and Au-InP systems
[refs. 10-11].
STAGE II. The conversion of Au(In) into Au3In occurs via nucleation and grain
growth. The reaction proceeds via an exchange or kickout mechanism by which
the substitutional Au atoms are displaced by interstitial In atoms, forcing the Au
atoms to become interstitial. These interstitial Au atoms, in turn diffuse to the Au-
InP interface, filling the vacancies left there by the out-diffusing In atoms. There
they form the compound Au2Pa with the unbound P. Au2Pa and AuaIn are formed
simultaneously with the phosphide located directly underneath the AuaIn nucleation
sites.
Since stage II is governed t)3, the kickout mechanism, it. was expected that capping
of the contacts would have no effect on the reaction rate. To verify this, we deposited a
271
saturated solid solution of In in Au (10% atomic) on a number of (100) oriented p-type
InP substrates doped to about 2.5 x 1016 cm -a and capped half of each sample with
600 _ layer of SiO2. Indeed after heat treating these samples at various temperatures,
no difference in the reaction rates of capped vs. uncapped areas could be detected.
It should be noted that much structural damage is done to the InP surface during
this stage of tile interaction.
Since the compound Au3In is pink in color, its nucleation and grain growth is
easily distinguished in the gold colored Au(In) matrix. Upon isothermal annealing
of the Au contacts, after an initial incubation period, the growth (i.e., the area) of
tile Au3In grains increases linearly with time. Repeating such isothermal annealing
experiments at different temperatures and measuring the time required (in the linear
region) for the metallization to convert to 100% Au3In, allows us to measure the acti-
vation energy (Ea) for stage II. The results, shown in figure 1, indicate an activation
energy of about 2.6 eV.
It should be mentioned that because tile Au3In grain growth is linear with time all
the way" to completion, and not, proportional to the circumference of the grains as they
grow in size, the rate limiting mechanism with an E_ of 2.6 eV is not, the activation
energy for tile localized kickout mechanism. In fact, the rate limiting mechanism in
this case is the rate of In release from the InP substrate, which also would have to
have a slower rate than the kickout mechanism.
Although for the data points of figure 1, we had used Au(In) contacts, repeating
the same experiment for samples with pure Au as the contacting metal, we essentially
obtained the same activation energy (2.8 eV) for stage II. Also since the capping of
the pure Au contacts suppresses the reaction rate for stage I, and therefore delays
the emergence of stage II, repeating tile same isothermal annealing experiments with
half-capped Au samples enabled us to extract the activation energy for stage I. Using
the delay times caused by capping of the contacts in the growth of Au3In grains and
plotting them as a function of temperature, we obtained the data points in figure
2. Although there is much scatter in the data points, the least square fit to the
points renders an E_ value of about 1.1 eV. This is slightly lower than the gold self-
diffusion activation energy of 1.8 eV expected for the reaction rate for Stage I. But
our calculations based on tile In concentration profiles at and near the At, surface
obtained by Auger electron Spectroscopy (AES) show that surface effects can lower
this activation energy to the vicinity of 1 eV. A thorough discussion of the kinetics
of stage I and stage II is presented in an upcoming publication [ref. 12].
STAGE III. Much like the reaction in stage II, the conversion of Au3In to AuIn2
occurs via nucleation and grain growth. We believe that similar to stage II, the
reaction rate in this stage is also governed by the kickout mechanism, given the
localized nature of the interaction. One marked difference here is that the compound
Au2P3 which was created at the Au-InP interface in stage II, does not form in stage III.
In other words, no Au2P3 was observed to grow under the AuIn 2 nucleation sites when
272
a layer of Au-In alloy with slightly lower In concentrationthan the Au3In compound
wasdepositedon InP and heat treated to stageIII nucleation. However,if the Au2P3
compound is already present at the metal-InP interface (leftover from stage II), it
is observedto grow further under the AuIn2 nucleation sitesat temperatures below
approximately 431°C . At temperaturesabove431°C, the phosphide layer seemsto
remain unchangedfrom its original form when created during stage II.
As with the marked color change from gold to pink occurring during stage II,
the AuIn2 grains are silver in color and are easily distinguished from the pink AuaIn
background matrix. Using the same procedure described for measuring the activation
energy for stage II, we find that there are two distinct activation energies present for
stage III with their intersection point at about 431°C.
As shown in figure 3, for temperatures above 431°C , Ea has a value of 12.1
eV, and for temperatures below 431°C, Ea increases to 28.5 eV. The measurement
of such abnormally high activation energies is perhaps the result of a multi-step
reaction process, with each step having a much lower activation energy. Similarly a
high activation energy of 8.6 + 3 eV has been reported by Elias, et al [ref. 13] while
investigating a phenomenon known as gold spreading in the Au-InP system. The
results of our findings on this spreading phenomenon and its relationship with stage
III are presented in the following section.
Lateral Spreading of Au Contacts
The lateral spreading of Au dots beyond the metallization boundaries on the sur-
face of InP upon heat treatment was first observed by Keramidas, et al [ref. 14].
Silver colored spread regions were observed to grow faster in certain crystallographic
directions. Later, Elias, et al [ref. 13], reported on the characteristics of the spreading
phenomenon and determined the activation energy of the process. In our investiga-
tions, we have observed that the lateral spreading of the contacts occurs concurrent
with stage III of the Au-InP interaction. In other words, the region on the Au dot
adjacent to the spread area has to be transformed to AuIn 2 before the spreading can
start.
Lateral spreading of a Au dot (2000 _,) heat treated at 434°C for 2.5 hours is
shown in figure 4a. An analysis of the rate of the growth front indicates that the
rapid rate of spreading is about four orders of magnitude faster than what would
be expected for a common vacancy dependent diffusion process. The only diffusion
mechanism capable of reconciling such a high growth rate is the interstitial transport
of Au to, and In from, the spread front. EDAX analysis has revealed the presence of
significant amounts of both In and Au in the spread region.
The activation energy that Elias, et al [ref. 13], measured for the spreading of
the Au contacts (8.6 -4-3 eV) is close to the E_ of 12.1 eV that we measured for stage
273
III for temperaturesgreater than 431°C. This is an indication that perhapssimilar
mechanismsoperate in both processes.
The Au dot of figure 4a is shown in figure 4b after removing the metallization
by a thiourea basedchemicaletchant. As shown in the figure, the gold phosphide
(Au2Pa) is presentonly in the dot regionand not in the spreadarea. Also apparent
from the enlarged view of figure 4b in figure 4c is the large amount of InP which
has disappearedfrom the growth area. This volume loss along with the absence
of the phosphidelayer in the spreadarea suggestthat the P atoms made available
after In interstitial transport toward the Au dot, had to volitalize before having an
opportunity to form the compoundAu2Pawith the interstitial Au atoms.
From the aboveargument,it becomesclear that in order to prevent the spreading
process,the lossof P shouldbe inhibited. To test this, we cappeda few Au dots and
their adjacentInP surfacewith a 600/_ layerof SiO2and heat treated them alongwith
a few uncappedsamples. Indeed, capping prevented the spreadingof the contacts.
It wasobservedthat the spreadfront is able to moveunder the cap for a distanceof
about one micron beforestopping. This one micron representsthe P escapelength.
The effectof cappingon suppressingthe spreadingprocessis shownin figure 5.
Another method to prevent spreadingis to deposit, instead of pure Au, Au-In
alloyshaving In concentrationsin excessof 25% (i.e., Aualn) on InP. Suchalloyswill
not only inhibit the spreadingprocess,but they will also suppresslhe interaction
rate of the metal-InP to a large degree. As mentioned earlier, no Au2Pa compound
is formed during the interaction of thesealloys with InP, for the formation of the
phosphide is strictly a stage II phenomenon.The presence of Au2Pa at the Au-InP
interface slows down the reaction to such an extent that it is easier for the reaction to
proceed to a phosphide free location (i.e., outside of the Au dot on the InP surface)
where the reaction rate is much faster. But in the absence of the phosphide layer the
reaction can continue underneath the Au dot, and there is no need for the reaction
to move outside of the metallization region.
Figures 6a and 6b are comparisons of the structural damage incurred by the
semiconductor by heat treating a pure Au and a Au-In alloy with nearly 25_ In
concentration samples on InP at 434°C for 2.,5 hours with all the reaction products
removed. As shown in these figures, the Au-In alloy sample has reduced the pitting
of the InP surface to a great extent compared to the pure Au sample.
Summary and Conclusions
The purpose of our research program has been to identify the mechanisms involved
in the process of contact formation in metal-semiconductor interactions. Fundamental
understanding of these mechanisms should enable us to manipulate them in such a
way as to satisfy our requirements of a nearly ideal metal-semiconductor contact
system (i.e., low Rc values plus minimum structural damage to the semiconduclor).
274
In this paper, we have presentedthe results of an investigation of the mechanisms
involved in the contact formation processin the Au-InP system.
We have shownthat the Au-InP interaction entails three consecutivestagesof
reactions. StageI of this interaction operatesvia a dissociativediffusion mechanism,
and the vacancygeneration rate on the surfaceof the metallization is rate limiting
with an activation energyof about 1eV. Capping of the metal surfaceor deposition
of a Au(In) layer instead of pure Au suppressestile reaction rate at this stage.
StageII operatesvia anexchangeor kickout mechanism.The reaction is localized,
and it is controlled by the rate of In releasefrom the InP substratewith an activation
energy of about 2.6 eV. Extensive pitting of the semiconductorsurface is observed
after this stagegoesto completion. Depositionof the alloy Aualn in placeof pure Au
inhibits the reaction at this stage.
StageIII reaction is alsolocalizedin nature and is likely'to operatevia the kickout
mechanism. There are two activation energiesobservedfor this stage: 12.1 eV for
temperatures greater than 431°C, and 28.5 eV for temperatures below 431°C. It is
likely that such high activation energy values are the result of many steps involved
for the reaction to go to completion, with each step having much lower activation
energy. The resultant activation energy would then be a complicated sum of these
smaller activation energies.
Also concurrent with stage III, a phenomenon known as spreading of Au is ob-
served. Fast interstitial movements of Au and In control the rate of reaction for this
process. We have shown that capping of the InP surface near the spread region in-
hibits the process by preventing the escape of P from the InP surface. In addition, in
contrast to pure Au, it was demonstrated that Au-In alloys (In concentration >25%)
show no sign of spreading.
We have shown that the mechanical damage to the semiconductor incurred by
heat treatment can be reduced considerably through deposition of Au-In alloys (
_>25% In) on InP in place of pure Au. We have also shown that capping of the
contacts suppresses the reaction rate. Our future thrust is to compare the electrical
characteristics of Au-In alloys with that of pure Au, and also to correlate the electrical
behavior of heat treated contacts (unary, binary, and ternary systems) with their
metallurgical behavior.
275
References
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Chapter 5, 293.
276
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277
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Semiconductor structural damage attendant to contact formation in III-V solar cells

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