InGaP/GaAs HBT implantation leakage current and electrical breakdown
Transcript of InGaP/GaAs HBT implantation leakage current and electrical breakdown
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doi:10.1016/j.m
Materials Science in Semiconductor Processing 7 (2004) 63–68
InGaP/GaAs HBT implantation leakage current andelectrical breakdown
Hong Shen*, A.M. Arrale, Peter Dai, Shiban Tiku, Ravi Ramanathan
Skyworks Solutions Inc., 2427 Hillcrest Drive, Newbury Park, Canada CA 91320
Abstract
Implantation of HBT by helium ions has been an accepted means for achieving the desired low leakage currents. In
the past, two implant steps of doses 1E+14 and 4E+13 ions/cm2, at energies 200 and 400KeV, respectively, were
shown to adequately isolate the HBT. The present work shows a medium dose (4E+13 ions/cm2) of the low-energy
implant can result in an isolation resistance an order of magnitude higher than that of a high-dose implant. As a result,
the leakage current is reduced. However, the breakdown voltage is also lower compared to its value at higher dose.
Tunneling breakdown has been confirmed as being the major contributor to the observed breakdown, by its negative
temperature coefficient. In addition, infrared (IR) emission imaging also showed that impact ionization may be playing
a role for breakdown at higher electrical field. It is proposed that large As vacancies generated by helium ions makes the
GaAs epitaxial layer more p-type compared to the highly doped n+-GaAs, where the collector contact is built on, thus
causing it to display a weak diode-like behavior. Finally, the temperature and dose dependences of both the leakage
current and the breakdown voltage will be discussed in the context of damage accumulation and junction
characteristics.
r 2004 Elsevier Ltd. All rights reserved.
Keywords: GaAs; HBT; Helium; Implant; Leakage; Breakdown; Tunneling; Impact ionization
1. Introduction
Device isolation is always a critical process step in the
fabrication of III–V compound semiconductor devices,
and good isolation is essential for better performance
and reliability. Device-to-device isolation can be
achieved by mesa creation, trench fill dielectric deposi-
tion or ion-implantation technologies [1]. In recent
years, the continuous shrinkage in sizes of the III–V
compound semiconductor devices and their higher
packing density has made the ion implant-induced
electrical isolation more and more attractive, due to
the advantages of its lateral selectivity, preserved surface
planarity [1], reduced parasitic capacitances, reduced
surface recombination currents and improved heat
ing author. Fax: +1-805-480-4212.
ess: [email protected] (H. Shen).
e front matter r 2004 Elsevier Ltd. All rights reserve
ssp.2004.05.003
spreading capabilities [2]. The most commonly used
implantation method is to bombard the substrate
surface with light species such as H+, He+, B+, so that
damage-related deep level states in the material can be
created in thick epitaxial structures [3,4]. The implanta-
tion damage is thought to create vacancies that
significantly reduce the semiconductor conductivity by
trapping charge carriers. At normal device operation
temperatures all these vacancy deep level states are not
thermally activated and conductivity of the semicon-
ductor layer is significantly reduced [5]. The effectiveness
of electrical isolation is dependent on many process
parameters such as mass of the implant ion, its energy
and dose, target temperature during implantation and
damage accumulation [6–8]. Post-implant anneal is
important to reduce hopping conduction and reduce
leakage current [6–8]. For highly doped layers used in
GaAs HBT, multiple implants are often required.
d.
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1.00E-08
1.00E-07
1.00E-06
1.00E-05
1.00E-04
0 20 40 60 80 100 120 140 160
Temperature,oC
Leak
age
Cur
rent
, A
Fig. 1. Temperature dependence on RL–AA leakage current
tested at 7V. He+ dose for (K): 1E+14 ions/cm2; for (J):
4E+13 ions/cm2.
H. Shen et al. / Materials Science in Semiconductor Processing 7 (2004) 63–6864
Furthermore, implant dose beyond optimum value will
often result in poorer electrical isolation due to the
mechanism that caused damage in the crystal will start
to allow carrier hopping in between adjacent sites [9].
Higher dose will also result in longer implantation time
and poorer throughput. Photoresist removal will also be
more difficult because of the heat generated by the
implantation [10].
In this work, the leakage current between the
tantalum nitride (TaN) resistor (RL) and active area
(AA) was identified as the single highest leakage source
in the InGaP/GaAs HBT power amplifier operation.
Since the TaN resistor is fabricated directly on GaAs,
the only viable way to lower the leakage current is to
increase the resistivity of the implantation layer so that
the overall leakage current can be reduced.
0.00E+00
2.00E+03
4.00E+03
6.00E+03
8.00E+03
1.00E+04
1.20E+04
1.40E+04
1.60E+04
0 20 40 60 80 100 120 140 160
Temperature,oC
Res
istiv
ity,Ω
-cm
Fig. 2. Temperature dependence on RL–AA resistivity. He+
dose for (K): 1E+14 ions/cm2; for (J): 4E+13 ions/cm2.
2. Experimental
Following base pedestal etch for partial isolation of
active device, a standard photoresist mask was used to
protect the active devices during ion implantation.
Energetic ions generated by medium current implanter
were used to damage the exposed areas of the wafer. The
ion species, energy, dose and the numbers of implant
steps were dictated by the depth of the epi-structure to
be isolated as well as the magnitude of the resulting
isolation resistance. In the present work, two implant
steps were performed using helium ions whose energies
were 400 and 200KeV to achieve desirable profile. The
implant dose of interest to this study is the dose of the
second implant step, which mainly produces the isola-
tion near the surface. The high-energy dose was
optimized in previous studies [11].
The implanted wafers went through rapid thermal
anneal at 360C for 30 s for contact alloying, to repair
the damage and achieve higher implant resistivity. Comb
and serpentine structures were used to test leakage
current between resistor, RL (implant) and active area,
AA (collector contact). The same structures were also
used to measure the breakdown voltage. The leakage
current and the breakdown voltage were acquired in
large voltage and temperature ranges in order to explore
the breakdown mechanism. Thermal and IR images of
these structures under bias were also taken to gain
understanding on the breakdown mechanism.
3. Results and discussions
3.1. Temperature effects on leakage current
The RL–AA leakage current goes up when tested at
higher temperatures, as shown in Fig. 1. At room
temperature, the leakage current for the reduced dose of
4E+13 ions/cm2 is almost an order of magnitude lower
than that of the nominal dose of 1E+14 ions/cm2
implant. However, at a temperature of 150C their
values are much closer to each other. Both of these two
dose levels already pass the so-called ‘‘threshold’’
implant dose where isolation resistance decreases with
an increased dose [10,12]. At such high implant dose
there are many carrier traps in the materials capable of
promoting hopping conduction. This will result in lower
implant layer resistivity and higher leakage current as
the dose increases. Based on this theory, there are
perhaps less carrier traps, thus lower hopping conduc-
tion, in the 4E+13 ions/cm2 implant layer than that of
the 1E+14 ions/cm2 implant. However, as the tempera-
ture increases, traps caused by the lower dose will move
more freely and line up more easily in the crystal to
provide carrier hopping channel so that its leakage
current approaches that of the higher-dose implant.
Similarly, a plot of resistivity of these two dose levels,
shown in Fig. 2, clearly indicates that at lower
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-1.00E-06
-5.00E-07
0.00E+00
5.00E-07
1.00E-06
1.50E-06
2.00E-06
-8 -6 -4 -2 0 2 4 6 8
Voltage, V
Leak
age
Cur
rent
, A
Fig. 4. I–V curve measurements for implants with different
doses, tested at room temperature. (K): 1E+14 ions/cm2. (’):
4.4E+13 ions/cm2. (&): 4E+13 ions/cm2. ( ): 3.7E+13 ions/cm2.
0.00E+00
2.00E-03
4.00E-03
6.00E-03
8.00E-03
1.00E-02
1.20E-02
0.0 5.0 10.0 15.0 20.0 25.0 30.0
Voltage, V/µm
Cur
rent
, A
Fig. 5. Breakdown voltage measurements for implants with
different doses, tested at 150C. (K): 1E+14 ions/cm2. (’):
4.4E+13 ions/cm2. (&):4E+13 ions/cm2. ( ): 3.7E+13 ions/cm2.
H. Shen et al. / Materials Science in Semiconductor Processing 7 (2004) 63–68 65
temperature the lower-dose implant is much more
resistive than the higher-dose implant. However, at
higher temperature, where the carrier traps line up in the
crystal the low-dose implant resistivity becomes identical
to the higher dose one.
3.2. Temperature effects on breakdown voltage
Breakdown voltage decreases as the temperature
increases, as shown in Fig. 3. For both implants the
breakdown voltage was reduced by at least 2V/mm whentemperature was increased from 25C to 150C. At
lower temperature due to the large band gap of GaAs,
high electrical field is required to break down the
semiconductor layer [12]. For example, at 25C the
4E+13 ions/cm2 implant can withstand 17.4V/mm(6.9mA) electrical field without breakdown and the I–
V curve was still reversible. At an electrical field higher
than 17.4V/mm the current goes up sharply and the
device is physically damaged. On the other hand, at
higher temperature the band gap of GaAs becomes
smaller and the electrical field required to breakdown
the semiconductor layer is lower. At 150C the same
device can only withstand 14.7V/mm voltage (2.8mA), adecrease of 2.7V/mm from the 25C value, before its
physical breakdown.
3.3. Dose effects on leakage and breakdown
Leakage current versus voltage plot for various doses
is shown in Fig. 4. With a decrease in the implant dose
the leakage current decreases, indicating that all these
dose levels have passed the threshold implant dose. At
higher dose however, more crystal damage is generated
by the implanted ions. The resistivity of the layer drops
due to the hopping conduction of carriers. Interestingly
these implants show higher leakage currents at +7V
0.00E+00
2.00E-03
4.00E-03
6.00E-03
8.00E-03
1.00E-02
1.20E-02
0 5 10 15 20 25 30
Voltage, V/µm
Cur
rent
, A
Fig. 3. Implant breakdown voltage measurements. (K):
1E+14 ions/cm2, 25C. (’): 1E+14 ions/cm2, 150C. (J):
4E+13 ions/cm2, 25C. (&): 4E+13 ions/cm2, 150C.
compared to 7V. The RL–AA leakage measurementsshowed that there was a weak junction between the
implanted area and active area so that the measurements
were polarity dependant. This phenomenon will be
elaborated more in the following sections.
Fig. 5 shows the breakdown measurements of these
implants at various doses. It can be seen clearly that the
breakdown voltage decreases as the dose decreases. The
same phenomena were already shown once in Fig. 3.
Breakdown is typically a local phenomenon that
requires the electrical field between carrier traps to be
high enough to initiate the impact ionization. The higher
breakdown voltage at higher doses is due to the fact that
with higher dose there are more carrier traps generated
in the substrate crystal and hopping conduction
becomes easier with more traps lining up to each other.
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-0.015
-0.01
-0.005
0
0.005
0.01
0.015
-60 -50 -40 -30 -20 -10 0 10 20 30 40
Voltage, V
Cur
rent
, A
Fig. 6. I–V curve measurements for implants with 4E+13 ions/
cm2 dose, tested at 150C.
H. Shen et al. / Materials Science in Semiconductor Processing 7 (2004) 63–6866
The leakage current increases so that the electrical field
cannot build up high enough to break down the
material.
3.4. RL–AA junction behavior
As described before, the RL–AA showed a junction
behavior so that its leakage current is dependent on the
polarity of the measurements, as shown in Fig. 4. In the
power amplifier design, the RL is built on the implanted
layer, which is the lightly doped n-GaAs collector
layer. Implanted ions create damage by knocking off
either Ga ions or As ions in the crystal. In addition to
their recombination with carriers, He+ ions will also
create deep energy states that trap carriers, thus making
the implanted layer more p-type. The collector contact,
on the other hand, is built on the heavily doped n+-
GaAs. A weak p–n junction was formed between these
two layers.
Using forward bias mode, with the positive polarity
on the RL relative to the active area, electrons are
injected to the junction, and the depletion region
becomes thinner. The turn on voltage is about 22V.
This is high compared to 1–2V turn on voltage for a
normal p–n junction. Unlike normal p–n junctions, this
RL–AA junction width is much larger with a 2.0 mmspace between RL and AA and its turn-on voltage is
much higher. The high resistivity of the implant under
the RL is also contributing to the high turn-on voltage
of the RL–AA junction. The resistivity is in the order of
1000–10 000O cm, which hardly exists in any otherdiodes. The leakage current in this forward bias mode
is also higher compared to the reverse bias mode due to
the electron pulling caused by the recombination of
carriers at the junction interface.
Under the reversed bias, which is the positive polarity
on the AA relative to the resistor, electrons were pulled
away from the junction, and the junction itself under-
goes an increase in depletion. The leakage current is
lower and breakdown voltage is much higher (48V), as
shown in Fig. 6. This breakdown voltage (25.2V/mm) isconsidered the true RL–AA breakdown voltage since
the circuits are always reverse-biased under normal HBT
PA operating conditions.
3.5. Breakdown mechanism
There are many breakdown mechanisms, thermal,
impact ionization, tunneling, or more than one mechan-
isms combined, for semiconductors. In an attempt to
identify the RL–AA breakdown mechanism, the circuit
was forward-biased under a current just below its
breakdown current, and thermal image of the device
was taken. The device was packaged on a circuit board
in order to carry out the experiments. Noticeably the
voltage applied was higher than the breakdown voltage
at wafer level, due to the reason that there might be
some leakage between the device and the laminated
circuit board itself. The temperature in the circuit did
not rise more than 3C, indicating that the RL–AA
breakdown is not due to temperature-related process
and the thermal-induced breakdown mechanism can be
ruled out.
If the impact ionization-related breakdown is in-
volved, there should be photons emitting from carrier
recombination from the device at breakdown. The
infrared emission image of the same RL–AA structure
clearly shows that photons were being emitted from the
implant layer under high forward bias. The same
phenomenon was found also under reversed bias. The
intensities of both forward-biased and reversed-biased
tests are similar. This evidence indicates that impact
ionization was one of the mechanisms contributing to
the RL–AA breakdown, in which the electrical field in
between carrier traps is high enough to promote carrier
avalanche that physically damage the structure. For
impact ionization, the breakdown voltage normally
increases as the temperature increases, due to the
reduced mean free paths of the ions [13]. This is in
contradiction with the results shown in Fig. 3, where the
observed phenomenon was the opposite, suggesting that
there has to be another mechanism involved.
The other mechanism that contributes to the break-
down is the tunneling effect. In the tunneling break-
down, the breakdown voltage decreases as temperature
increases as shown in Fig. 3, and the leakage current
increases with the square root of the electric field as
shown in Fig. 7. Both characteristics are the signatures
of tunneling mechanism.
The following equation for calculating the tunneling
breakdown of GaAs has been obtained from Ref. [13]
Jt ¼
ffiffiffiffiffiffiffiffiffi2m
pq3eV
4p2_2E1=2gexp
4ffiffiffiffiffiffiffiffiffi2m
pE3=2g
3qe_; ð1Þ
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1.00E-07
1.00E-06
1.00E-05
1.00E-04
1.00E-03
1.00E-02
1.00E-01
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5
Sqrt Electrical Field , (100 V/cm)1/2
Leak
age
Cur
rent
, A
Fig. 7. I-sqrt(V) curve measurements for implants with
4E+13 ions/cm2 dose, tested at 150C.
0.00E+00
1.00E+08
2.00E+08
3.00E+08
4.00E+08
5.00E+08
6.00E+08
7.00E+08
8.00E+08
9.00E+08
1.00E+09
5 10 15 20 25 30 35 40 45
Applied Voltage, V
Cur
rent
Den
sity
, A/m
2
Fig. 8. Breakdown voltage comparison between theoretical
calculation and experimental data for implant dose of
4E+13 ions/cm2. (K): experimental at 25C; (m): experimental
at 150C; (—): theoretical at 25C; (– –): theoretical at 150C.
H. Shen et al. / Materials Science in Semiconductor Processing 7 (2004) 63–68 67
where Eg is the band gap of GaAs, which is 1.42 eV at
room temperature. The temperature dependence of the
band gap can be described in the following equation:
EgðTÞ ¼ Egð0Þ aT2
ðT þ bÞ; ð2Þ
where T is the temperature. For GaAs, Eg(0) is 1.519, ais 5.405E-14, and b is 204. In Eq. (1), e is the electricfield, V is the applied voltage, q is the electron charge,
1.60218E-19 C, _ equals to h/2p, which is 1.05458E-34 J-s, and m is the effective mass of the carrier, 0.067me forelectrons, and 0.082me for holes, in which me is the
electron rest mass, 0.91095E-30 kg. Jt is the current
density with a unit of A/m2. The electric field in the RL–
AA junction can be approximated as
emax ¼2ðFi þ V Þ
xd; ð3Þ
Fi is the built-in potential. The estimated concentra-tion of the implant is in a range of 1E+17 ions/cm3 so
that Fi is approximately 1.31 eV. The depletion width xdwas carefully chosen to be 0.59mm for 25C and 0.54mmfor 150C to simplify the calculation. Finally, in order to
calculate the current density, the total area of the RL–
AA junction is estimated as 5E-12m2, based on the test
structure, to facilitate the calculation. Fig. 8 shows the
correlation between theoretical calculation and experi-
mental results. There are some errors in the calculation;
for example, the total area of the junction is difficult to
estimate because of the non-uniform current distribution
in the junction. The band gap energy had to be estimated
as well since the real junction temperature was not
known, even though the experimental temperature was
set at 25C and 150C. The depletion width is also an
estimated value that could be in error. Considering these
difficulties in the calculation the correlation in Fig. 8 is
fairly good. The close match in shape of the curves
between the experimental data and theoretical data
confirms that the RL–AA breakdown is predominantly
a tunneling breakdown. The theoretical calculation at
150C also shows a decrease of about 3V/mm in
breakdown voltage compared to the one at 25C, which
is also a close match to the experimental value of 2.7V/
mm.
4. Conclusions
By carefully optimizing the implant dose the RL–AA
leakage current can be reduced. The breakdown voltage
decreases as the leakage current decreases, due to the
fact that the doses experimented have exceeded the
threshold dose. Increasing in dose at these levels will
largely damage the crystal and bring up the density of
carrier traps in the substrate to promote more hopping
conduction. However, due to the higher leakage current,
the required electrical field between carrier traps
required to break down the material will also be higher.
The implant does not change the GaAs layer into a
complete insulator but more or less change it into a
weak junction. The leakage current from this junction is
lower under reversed bias and its breakdown voltage is
also higher, compared to the situation under forward
bias. This is beneficial for the HBT PA under normal
operating conditions. High electrical field will conse-
quently promote the impact ionization, combined with
tunneling, and thus break down the GaAs material. At
that point the device is physically damaged.
References
[1] Rao MV. High-energy (MeV) ion implantation and its
device applications in GaAs and InP. IEEE Trans Electron
Dev 1993;40:1053–66.
ARTICLE IN PRESSH. Shen et al. / Materials Science in Semiconductor Processing 7 (2004) 63–6868
[2] Pearton SJ. Ion implant doping and isolation of III–V
semiconductors. Nucl Instrum Meth 1991;B59:970–7.
[3] Pearton SJ, Hobson W, Abernathy CR. Ion implantation
processing of GaAs and related compounds. Mater Res
Soc Symp Proc 1998;147:261–72.
[4] Pearton SJ. Use of MeV O+ ion implantation for isolation
of GaAs/AlGaAs heterojunction bipolar transistors.
J Appl Phys 1991;71:4949–54.
[5] Gramlich S, Nebauer E, Sebastian J, Beister G. Damage
profile of He implantation in AlGaAs laser diode material
detected by photoluminescence. Electron Lett 2001;37:
463–4.
[6] Knights AP, Hutchinson S, Sealy BJ, Simpson PJ. Carrier
removal in n-type GaAs layers by oxygen implantation
analyzed by positron annihilation spectroscopy, High
Perform Electron Dev Microwave Optoelectron Appl
1997;243–8.
[7] Ahmed S, Too P, Sealy BJ, Gwilliam R. Proton implanta-
tion for effective electrical isolation of InP, InGaAs and
GaAs: role of variable doses and implant temperature.
Indium Phosphide and Related Materials Conference
2002;225–8.
[8] Ahmed S, Gwilliam R, Sealy BJ. An effective electrical
isolation scheme by oxygen implantation-effect of damage
accumulation and target temperature. Electron Dev
Microwave Optoelectron Appl 2001;43–8.
[9] Short KT, Pearton SJ. Implant isolation of GaAs.
J Electrochem Soc 1988;135:2835–40.
[10] Teng SJJ, Wu CS, Hou LD, Wang DC. Implant isolation
of InGaAs/GaAs pseudomorphic high-electron mobility
transistor structure using boron. Elecron Lett 1994;30:
1539–40.
[11] Arrale AM, Bal P, Tiku S. Compound semiconductor
manufacturing expo. Tech Dig 2002;57.
[12] Ahmed S, Sealy BJ, Gwilliam R. Dose dependence
of proton-isolated n-type GaAs layers implanted
at room temperature and 200C. Electron Lett 2002;38:
250–2.
[13] Sze SM. Physics of semiconductor devices. 2nd ed. New
York: Wiley-Interscience; 1981. p. 97–108.