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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: hong.shen@skyworksinc.com (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.

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