Ge concentration in regrown GaAs for ohmic contacts

Article abstract of DOI:10.1063/1.120519

Dissociation and solid phase epitaxial regrowth of GaAs in Ti/Ge/Ni/GaAs were investigated using the transmission electron microscope (TEM) and energy dispersive spectroscopy (EDS) of x-rays with spatial resolution approaching 2 nm. A ternary Ni_2.4GaAs phase, ～ 130 nm thick, was formed by 300°C in situ anneals of 65 nm Ni film on GaAs. After this in situ anneal, films of 30 nm Ge and 20 nm Ti were deposited in sequence. The EDS analysis showed that Ni_2.4GaAs transformed into Ni-As and Ni-Ga binaries after annealing at 500°C for 5 min, while ～ 30 nm of GaAs regrew by solid phase epitaxial regrowth from decomposition of the binary phases. High spatial resolution microanalysis allowed detection of ～ 1 X 10²⁰ cm^-3 Ge in the regrown GaAs. This confirms that Ge is incorporated into GaAs during regrowth for ohmic contact formation.

Full text of DOI:10.1063/1.120519

Ge concentration in regrown GaAs for ohmic contacts
T. J. Kim, P. H. Holloway, and E. A. Kenik
Citation: Applied Physics Letters 71, 3835 (1997); doi: 10.1063/1.120519
View online: http://dx.doi.org/10.1063/1.120519
View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/71/26?ver=pdfcov
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Ge concentration in regrown GaAs for ohmic contacts
T. J. Kim and P. H. Holloway
Department of Materials Science and Engineering, University of Florida, Gainesville, Florida 32611
E. A. Kenik
Metal and Ceramic Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831-6376
͑Received 16 June 1997; accepted for publication 23 October 1997͒
Dissociation and solid phase epitaxial regrowth of GaAs in Ti/Ge/Ni/GaAs were investigated using
the transmission electron microscope ͑TEM͒ and energy dispersive spectroscopy ͑EDS͒ of x-rays
with spatial resolution approaching 2 nm. A ternary Ni2.4GaAs phase, ϳ130 nm thick, was formed
by 300 °C in situ anneals of 65 nm Ni film on GaAs. After this in situ anneal, films of 30 nm Ge
and 20 nm Ti were deposited in sequence. The EDS analysis showed that Ni2.4GaAs transformed
into Ni–As and Ni–Ga binaries after annealing at 500 °C for 5 min, while ϳ30 nm of GaAs regrew
by solid phase epitaxial regrowth from decomposition of the binary phases. High spatial resolution
microanalysis allowed detection of ϳ1ϫ1020 cmϪ3 Ge in the regrown GaAs. This confirms that Ge
is incorporated into GaAs during regrowth for ohmic contact formation. © 1997 American
Institute of Physics. ͓S0003-6951͑97͒00452-X͔
Ohmic contacts to GaAs and GaAs-based compound
semiconductors are produced by Au–Ge–Ni metallizations
which were first introduced in 1967.1 To improve the perfor-
mance of Au–Ge–Ni metallizations and to better understand
the mechanism of ohmic contact formation, single element/
GaAs and bielements/GaAs metallizations have been exten-
sively studied.2–9 As a consequence, solid-phase epitaxial re-
growth has been demonstrated for Ge/Pd/GaAs,2,3
Si/Ni/GaAs,4 Ge/Ni/GaAs,5 Al/Ni/GaAs,6 and Au/GaAs7–9
metallizations.
In solid-phase epitaxial regrowth, a metallic element,
such as Ni reacts with GaAs to produce interfacial NixGaAs
typically below 400 °C.10,11 This interfacial NixGaAs may be
decomposed by Au and Ge, resulting in the epitaxial re-
growth of Ge-doped GaAs. As a result of regrowth, the con-
tact property switches from rectifying to ohmic. Decomposi-
tion of the ternary phase with regrowth of GaAs can be
driven by the lower free energy of formation of Ni-Ge com-
pounds or the Ni–Au solid solution. Incorporation of Ge was
suggested to form nϩ-GaAs which allows tunneling of elec-
trons across the metal/GaAs barrier for ohmic contacts.5 For
understanding ohmic contact formation, the solid-phase re-
growth model is the most recent development and is also
being applied to ohmic contacts to InP.12
Even though solid-phase epitaxial regrowth has been
demonstrated, the mechanism and quantity of dopant incor-
poration into regrown GaAs has not been directly demon-
strated. Backside secondary ion mass spectroscopy ͑SIMS͒
data support incorporation of Ge into the GaAs interface re-
gion, although the dopant concentration could not be quanti-
fied as a function of distance from the interface.13 The pur-
pose of this study was to determine the concentration of Ge
in regrown GaAs. Another focus of this study was the evo-
lution of interfacial phases such as NixGaAs,14–16 using an in
situ annealing scheme for Ni films on GaAs.17
All the GaAs used in this study was n-type, which was
bulk doped with 2ϫ1018 cmϪ3 Si. The samples were ultra-
sonically cleaned in trichlroethane, acetone, and finally
methanol followed by N2 blow drying. The GaAs substrate
was then cleaned in HCl:H2O ͑1:1 by volume͒ for 60 s to
remove the native oxide and mounted on the sample holder.
After immediate loading and pumping down to ϳ1
ϫ10Ϫ6 Torr, the samples were radiatively heated to
ϳ610 °C for 15 s to desorb residual oxides. When the base
pressure reached mid-10Ϫ7 Torr or lower, 65 nm of the Ni
film was electron beam evaporated onto GaAs at room tem-
perature. After evaporation of the Ni film, the sample was in
situ annealed at 300 °C for 15 min without breaking vacuum.
During annealing, the Ni film reacted with GaAs to form an
interfacial layer of ternary Ni2.4GaAs. After this in situ an-
neal and without breaking vacuum, ϳ30 nm thick Ge and
ϳ20 nm thick Ti layers were sequentially evaporated. Ti was
used to drive decomposition of the interfacial Ni2.4GaAs
since NiTix has a lower free energy than NiGe or Ni2.4GaAs.
After deposition of the Ge and Ti layers, samples were
ex situ annealed in 1–2ϫ10Ϫ6 Torr vacuum at 500 °C for 5
min. Both deposited and vacuum annealed samples were
electrically characterized ex situ by their current–voltage
(I–V) characteristics at room temperature. The I–V data
were obtained by measuring the current flow between two
dots with a diameter of 0.5 mm, which were defined by a
tantalum shadow mask. Cross-sectional TEM images were
examined using a JEOL 200CX or 4000FX. High spatial
resolution x-ray EDS microanalysis was performed in a Phil-
ips EM400/FEG analytical electron microscope equipped
with a field emission gun and an EDAX 9100 energy disper-
sive x-ray spectrometer. Composition was measured in the
scanning transmission electron microscope mode with a
beam diameter of ϳ1.5 nm ͑full width, half-maximum͒. A
low background, cooled holder (Ϫ130 °C) was used to
minimize contamination under the focused probe. The EDS
spectrum was corrected for the normal ‘‘in-hole’’ counts as-
sociated with fluorescence of the entire specimen area by
uncollimated radiation. Convergent electron beam diffraction
patterns were used to precisely locate the interphase bound-
ary by monitoring reflections from both phases during analy-
sis. The primary beam position was repeatedly checked dur-
ing EDS acquisition to minimize any specimen drift. The
Ga/As k factor was calculated as 0.815 from measurements
of the GaAs substrate,18 whereas k factors for Ti, Ni, and Ge
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Appl. Phys. Lett. 71 (26), 29 December 1997 0003-6951/97/71(26)/3835/3/$10.00 © 1997 American Institute of Physics 3835
131.230.73.202 On: Mon, 22 Dec 2014 18:20:50

TABLE I. Concentration of elements measured in Ni2.4GaAs.
Distance ͑nm͒
Ni ͑at %͒
Ga ͑at %͒
As ͑at %͒
Ga/Ni
As/Ni
10
20
30
40
55.0
55.8
56.6
56.5
22.7
22.7
23.7
23.7
22.2
21.3
19.5
19.5
0.41
0.41
0.42
0.42
0.40
0.38
0.34
0.34
top surface of the ϳ130 nm thick Ni2.4GaAs layer. The
Ga/As ratios were 1.02Ϯ0.02 in the substrate GaAs and
1.09Ϯ0.03 in Ni2.4GaAs, respectively. Several studies have
shown that the stoichiometry of the NixGaAs phase varied
from 2 to 4 in NiAs-type hexagonal structure.14–16,19 The
Ga/Ni ratio was 0.42 while the As/Ni ratio was lower ͑0.40–
0.34͒, especially further from the interface, suggesting that
As loss was occurring at the closer the surface.
Figure 2 is a cross-section bright field image after an-
nealing the Ti/Ge/Ni2.4GaAs/GaAs structure at 500 °C for 5
min, showing two metal layers above the GaAs substrate
͑marked top-T, and bottom-B͒. This contact structure exhib-
ited linear I–V characteristic on the 10 mA/1V per division
scale after annealing. The top metal layer ͑marked as T in
Fig. 2͒ contained Ni, Ti, and As from SIMS analysis, and
was ϳ50 nm thick. The bottom ͑previously identified as
Ni2.4GaAs and marked as B͒ layer was ϳ100 nm thick, in-
dicating that on average, 30 nm of GaAs regrew as a result of
the annealing at 500 °C for 5 min. In Fig. 2, regrown GaAs
͑marked R͒ and substrate GaAs is demarcated by a dark solid
line, which corresponds to the original location of
Ni2.4GaAs/GaAs interface before annealing at 500 °C. An
open arrow indicates a trace of the decomposition of the
bottom layer. As shown in Fig. 2, grains with bright ͑a͒ and
dark ͑b͒ contrast are distinguishable along the interface with
the GaAs.
The composition of regrown GaAs and new phases re-
sulting from the reaction at 500 °C between Ni2.4GaAs and
Ge/Ti films were measured by high spatial resolution EDS.
Figures 3͑a͒ and 3͑b͒ show line scan composition data mea-
sured across the interface between the regrown GaAs and a
bright grain ͑similar to a in Fig. 2͒, or a dark grain ͑similar to
b in Fig. 2͒, respectively. The interface is at zero with the
GaAs substrate being at the negative distances. Since
ϳ30 nm of Ni2.4GaAs was decomposed resulting in the re-
growth of GaAs, the location at Ϫ30 nm would be the ap-
proximate location of the original interface between GaAs
and Ni2.4GaAs before any decomposition.
FIG. 1. Bright-field cross-section TEM micrograph of Ni2.4GaAs on GaAs.
relative to As were calculated as 0.489, 0.610, and 0.863
from a standardless quantification program.
Figure 1 shows the cross-section bright field image of a
Ni2.4GaAs layer before Ge and Ti deposition. The ϳ130 nm
thick Ni2.4GaAs layer resulted from the in situ annealing of
the 65 nm Ni film at 300°C for 15 min. The diffraction
pattern from the ternary Ni2.4GaAs taken along the 110
͓
͔
GaAs
zone axis revealed a NiAs-type hexagonal structure, which
was identical to that in Fig. 2 in Sand et al.19 When deposi-
tion of ϳ30 nm Ge and ϳ20 nm Ti layers onto the
Ni2.4GaAs were completed, the Ge layer was amorphous and
the Ti layer consisted of randomly oriented polycrystalline
grains. The as-deposited structure ͑i.e., Ti/Ge/Ni2.4GaAs/
GaAs͒ was a rectifying contact with a barrier height of
ϳ0.9 V.
The composition of the Ni2.4GaAs phase is summarized
in Table I. The distances in Table I are measured from the
metal/GaAs interface. The larger distances are closer to the
As shown in Fig. 3͑a͒, the new phase exhibiting the
bright contrast was a NiAs phase containing minor amounts
of Ga and Ge. The average composition measured from a
NiAs grain was Ni1.3As͑Ni1.3AsGa0.3Ge0.1͒ determined by
using two sets of data acquired at 10 nm into the Ni1.3As
grain. Figure 3͑b͒ shows the data for the phase with dark
contrast. The overall composition was very uniform over 30
nm distance, from 5 to 30 nm. By averaging the data be-
tween 10 and 30 nm, the composition of the dark phase was
determined as Ni2.1Ga͑Ni2.1GaGe0.5As0.2͒. The results show
that Ni2.4GaAs laterally separated into distinct Ni–As and
Ni–Ga binary phases as predicted in the equilibrium Ni–
Ga–As phase diagram.11,15 Thus, the 30 nm regrown GaAs
FIG. 2. Bright-field cross-section TEM micrograph of GaAs/Ni2.4GaAs/
Ge/Ti after vacuum annealed at 500 °C for 5 min.
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3836 Appl. Phys. Lett., Vol. 71, No. 26, 29 December 1997 Kim, Holloway, and Kenik
131.230.73.202 On: Mon, 22 Dec 2014 18:20:50

MMFϭ0.08 for Ge. Since the Ge concentration ͑0.34 at %
ϳ1ϫ1020 cm3͒ is four times larger than the error, the mea-
sured Ge concentration is statistically valid. Even though the
Ge concentration was measured to be ϳ1ϫ1020 cm3, free
carrier concentration is likely to be lower because of the
compensating nature in heavily doped GaAs.20
The concentration of Ge detected within Ϫ5 nm of the
interface ranged from 0.5 to 1.3 at. %, which corresponds to
a concentration of 2–5ϫ1020 cmϪ3. For these measure-
ments, beam broadening effects have not been quantified.
The regrown GaAs also contained ϳ2at. % Ni and Ni-
containing precipitates. The effects of Ni in the regrown
GaAs upon electron transport is also unclear. However, it is
clear that Ge is a critical element responsible for the forma-
tion of ohmic contacts.17
In summary, the in situ annealing scheme adopted en-
abled the determination of the extent of solid phase epitaxial
regrowth of GaAs. Electron diffraction and high resolution
EDS analysis revealed that the in situ annealing of Ni/GaAs
formed ϳ130 nm of Ni2.4GaAs which transformed into
Ni1.3As and Ni2.1Ga binaries upon subsequent annealing at
500 °C for 5 min. Films of Ti and Ge caused the decompo-
sition of the binaries, resulting in Ge being incorporated into
regrown GaAs. At least ϳ1ϫ1020 cmϪ3 of Ge was incorpo-
rated into ϳ30 nm regrown GaAs layers. The contacts with
Ge in regrown GaAs exhibited ohmic behavior.
This work was supported by ONR Grant No. N00014-
92-J-1895, and by the Department of Energy, under Contract
No. DE-AC05-96OR22464 with Lockheed Martin Energy
Research Corp., and through the SHaRE Program under
Contract No. DE-AC05-OR00033 with Oak Ridge Associ-
ated Universities.
FIG. 3. ͑a͒ High resolution EDS analysis across the regrown GaAs/Ni1.3As
interface. GaAs/Ni2.4GaAs/Ge/Ti after vacuum annealed at 500 °C for 5
min. ͑b͒ High resolution EDS analysis across the regrown GaAs/Ni2.1Ga
interface. GaAs/Ni2.4GaAs/Ge/Ti after vacuum annealed at 500 °C for 5
min.
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that for the estimated foil thickness ͑less than 80 nm͒ the
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valid minimum Ge concentration for the given approximate
conditions:
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1/2
ϭ3•͑␤͒ •͓͑Ge in wt %͒/͑counts of Ge signal͔͒
where ␤ is the background noise in counts ͑count per sec-
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