Hatano Y., Katsumura Y., Mozumder A. (Eds.) Charged Particle and Photon Interactions with Matter - Recent Advances, Applications, and Interfaces

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840 Charged Particle and Photon Interactions with Matter

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841

Radiation Effects on

Semiconductors

and Polymers

for

Space Applications

Takeshi Ohshima

Japan Atomic Energy Agency

Takasaki,

Japan

Shinobu Onoda

Japan Atomic Energy Agency

Takasaki,

Japan

Yugo Kimoto

Japan Aerospace Exploration Agency

Tsukuba,

Japan

30.1 introduCtion

Technology for space exploration and development constantly progresses. Many space missions

have been completed and even more are planned. For example, the space station project has been

realized; as a result, one can stay beyond the bounds of the earth for long periods. The uses and

benets from man-made satellites are expansive, including weather forecast, broadcast, resource

Contents

30.1 Introduction .......................................................................................................................... 841

30.2 Space

Solar Cells ..................................................................................................................842

30.2.1

Radiation

Degradation of Solar Cells in Space ........................................................842

30.2.2

Basic

Mechanism of Radiation Degradation of Solar Cells .....................................844

30.2.3

Space

Solar Cells ......................................................................................................847

30.2.4

Radiation Degradation of Triple-Junction Solar Cells .............................................849

30.2.5 Prediction

Methodology of Triple-Junction Solar Cell Degradation........................ 851

30.3

Electronic

Devices................................................................................................................ 853

30.3.1

Overview

of Single-Event Effects in Space.............................................................. 853

30.3.2

Generation

of Charge and Ion Tracks in Semiconductors........................................854

30.3.3

Transient Currents Occurring Subsequent to Ion Track Generation........................ 857

30.3.4 Modeling

of Transient Voltage Pulse in Logic Circuits ...........................................859

30.4

Polymers ...............................................................................................................................863

30.4.1 General

Radiation Effects on Polymers....................................................................863

30.4.2

Ground

Evaluation Facility.......................................................................................868

30.4.3

Space Materials Exposure Experiment ....................................................................870

References...................................................................................................................................... 873

842 Charged Particle and Photon Interactions with Matter

exploration, broadband communication, global positioning, and so on. Missions for deep space as

well

as near the sun are also planned by the United States, Europe, and Japan.

Many

materials and devices that support missions with high reliability are used for space appli-

cations. Since the reliability and durability of such materials and devices directly relate to the life-

time of space applications, it is important to understand the characteristics of such materials and

devices before their launch. Ground testing is critical in the analysis and understanding of these

materials. In space, many kinds of high-energy radiations exist. For example, protons and heavy

ions with high energies come from the sun and from outside of our galaxy. High concentrations of

protons and electrons are trapped by the magnetic eld of the earth (Van Allen belts). Secondary

radiations such as x-rays and neutrons are also produced by the interaction between highly energetic

particles and space applications. In addition, highly reactive species such as atomic oxygen exist

and attack the surface of space-borne materials. Since the properties of materials and devices are

affected by their incidence, irradiation experiments on the ground must be carried out to predict

their lifetime in a space environment. In this chapter, radiation effects on semiconductor devices

and polymers are described from the point of view of space applications. First, the degradation of

the electrical performance of space solar cells is considered, and then, radiation effects on electronic

devices

are discussed. Finally, polymers for space applications are introduced.

30.2 spaCe solar Cells

30.2.1 radiation degradation of Solar cellS in Space

Three major effects, single-event effects (SEEs), total ionizing dose (TID) effects, and displacement

damage effects are observed in semiconductor devices by the incidence of charged particles. SEEs

are malfunctions of electronic devices such as large scale integration (LSIs) and power devices. There

are both nondestructive and destructive SEE failures. More comprehensive details of SEEs will be

described in Section 30.3. The TID effect only affects metal-insulator-semiconductor (MIS) struc-

tures, such as metal-oxide-semiconductor (MOS) devices, because of electron-hole pair generated

in insulator layers due to irradiation. If the TID effect occurs, the electrical characteristics of MIS

devices gradually degrade with increasing dose of radiations, until a destructive malfunction occurs.

When charged particles are introduced into semiconductors, atoms at lattice sites are scattered

into non-lattice sites (knock-on effects); as a result, vacancies are created in semiconductors. In

fact, many kinds of residual defects such as divacancies, vacancy clusters, and vacancy-impurity

complexes are created by irradiation because generated vacancies thermally migrate and transform

into more stable defects. Since such defects act as scattering/recombination centers to free carriers,

electrical properties of semiconductors are degraded by damage generation due to charged particle

irradiation. This degradation is called the displacement damage effect. Similar to the TID effect,

the degradation behavior of semiconductors by displacement damage effect worsens with increasing

charged

particle uence until the fatal malfunction nally occurs.

Displacement

damage is the most dominant effect for space solar cells since the electrical perfor-

mance is degraded by crystal damage due to proton and electron irradiations. Therefore, electron/

proton irradiation experiments using accelerators are conducted on the ground to understand the

decrease of power generation of solar cells during missions. For the evaluation on the ground, the

electrical performance of solar cells is generally measured at a separate facility before/after pro-

ton irradiation experiments (sequential technique). Additional photovoltaic characterization such as

spectral response and photo luminescence are also performed because these measurements can be

done independently from irradiation experiments. However, it is important to note that solar cells

often become radioactive after high-energy (such as 10MeV) proton irradiation. In this case, it is

difcult to handle irradiated solar cells for the measurement of the post-irradiation electrical per-

formance. Thus, we must wait for radioactivity

the solar cells to subside before the evaluation of

Radiation Effects on Semiconductors and Polymers for Space Applications 843

radiation degradation can be completed. In an effort to increase the turnaround of samples, evalua-

tion techniques that can measure the electrical performance of solar cells during irradiation experi-

ments have been developed (Ohshima et al., 1996). Figure 30.1 shows a vacuum chamber installed at

Japan Atomic Energy Agency (JAEA), Takasaki, for space solar cell experiments. Since the irradia-

tion chamber has an AM0 solar simulator, the degradation behavior of the electrical performance

of solar cells can be evaluated during proton irradiation experiments (simultaneous technique). In

addition, the degradation behavior of solar cells under low temperature irradiation has become a

topic of interest, specically for missions to Saturn and Jupiter. To tackle such varied missions,

a sample holder with a temperature control system was installed in the chamber (Ohshima et al.,

2005; Harris et al., 2008). Figure 30.2 shows the comparison of remaining factors of the electrical

performance (short current circuit, I

, open circuit voltage, V

, and maximum power, P

MAX

) of

space

triple-junction solar cells between RT and 175

irradiations (Ohshima et al., 2005).

additiontogroundexperiments,on-orbit ight demonstrationshavealsobeenconducted(Imaizumi

et al., 2005). The satellite named MDS-1 (Mission Demonstration-test Satellite No. 1) was launched in

2002. The MDS-1 was placed in a geosynchronous transfer orbit (GTO) with apogee of 36,000km and

perigee of 500km. Typical solar cells designed for terrestrial applications were mounted on the MDS-1,

and their on-orbit electrical performance was measured. A GTO was selected for this ight experiment

because it crosses the Van Allen radiation belts which consist of high-energy charged particles, mainly

electrons and protons. Thus, solar cells were subjected to high concentrations of electrons and protons

for a short time. Figure 30.3 shows the remaining factor of I

for solar cells on the MDS-1 after launch

(Imaizumi et al., 2005). The I

for all cell types, except Cu(In,Ga)Se

(CIGS), decreased within days

after launch as shown in Figure 30.3. Since MDS-1 cuts across the Van Allen belts, the degradation

behavior for solar cells without CIGS can be explained in terms of heavy damage created by proton and

electron irradiations. For CIGS solar cells, it has been reported that electrical performance recovers due

to the annealing of defects even at room temperature (Kawakita et al., 2002). Therefore, the apparent

lack of degradation for CIGS solar cells on MDS-1 is attributed to on-orbit defect annealing.

Figure 30.1 Photograph of a vacuum chamber for space solar cell evaluation installed at Japan Atomic

Energy

Agency (JAEA), Takasaki. The chamber is connected with a beam line of the AVF Cyclotron.

844 Charged Particle and Photon Interactions with Matter

30.2.2 baSic MechaniSM of radiation degradation of Solar cellS

Solar cells generate electric power when photo-induced minority carriers in a base layer migrate

and reach a top layer before recombination. Thus, the diffusion length (lifetime) of minority car-

riers is one of the most important parameters for the solar cell performance. As mentioned above,

charged particles create defects in semiconductors. Since some of the radiation-induced defects act

as recombination centers, a decrease in diffusion length (lifetime) of minority carriers due to irra-

diation is the most dominant degradation effect in solar cells. In this region, experimental results

can

be described by the following semi-empirical equation (Tada et al., 1982),

L L

= − Φ

























(30.1)

where

and L

are the minority carrier diffusion length after and before irradiation, respectively

and Φ are the damage coefcient of minority carrier and uence

Figure 30.4 shows the remaining factors of triple-junction solar cells irradiated with protons of vari-

ous energies (Sumita et al., 2003). As shown in the gure, the tting lines calculated using the semi-

empirical equation are in agreement with experimental results. From the fundamental point of view,

radiation-induced defects in new materials are intensively investigated because many candidates for

high efciency solar cells have been proposed and properties of defects in such new materials are

not

yet fully understood (Walters et al., 2001; Dharmarasu et al., 2002; Soga et al., 2003).

the second irradiation effect, majority carrier removal occurs in high-uence regions, and the

reduction

of majority carriers can be empirically expressed as

1.0

0.8

175 K

MAX

0.6

0.4

0.2

10 MeV proton fluence (cm

–2

)

Remaining factors

InGaP/GaAs/Ge triple-junction solar cells

Figure 30.2 Comparison of remaining factors of the electrical performance (short current circuit, I

open circuit voltage, V

, and maximum power, P

MAX

) of space InGaP/GaAs/Ge triple-junction solar cells

between RT and 175K irradiations. (Reprinted from Ohshima, T. et al., Evaluation of the electrical charac-

teristics of III-V compounds solar cells irradiated with proton at low temperature, Proceedings of 31st IEEE

Photovoltaic Specialists Conference,

Lake Buena Vista, FL, 2005, pp. 806–809. With permission.)

Radiation Effects on Semiconductors and Polymers for Space Applications 845

1.0

0.9

0.8

0.7

0.6

1.0

0.9

0.8

0.7

0.6

1.0

0.9

0.8

0.7

0.6

1.0

0.9

0.8

0.7

0.6

Remaining factor of I

1.0

0.9

0.8

0.7

0.6

Remaining factor of I

1.0

0.9

0.8

0.7

0.6

Remaining factor of I

1.0

0.9

0.8

0.7

0.6

Remaining factor of I

1.0

0.9

0.8

0.7

0.6

Remaining factor of I

1 10

(a)

(c)

(e)

(g) (h)

(f)

(d)

(b)

100

MET (days after launch)

1000

1 10 100

MET (days after launch)

1000

1 10 100

MET (days after launch)

1000

1 10 100

MET (days after launch)

1000

1 10 100

MET (days after launch)

1000

1 10 100 1000

CG=500 μm

CG=100 μm

CG=500 μm

CG=100 μm

CG=500 μm

CG=100 μm

CG=500 μm

CG=100 μm

CG=500 μm

CG=100 μm

CG=500 μm

CG=100 μm

CG=500 μm

CG=100 μm

CG=500 μm

CG=100 μm

Figure 30.3 Remaining factor of I

for various solar cells on the MDS-1 after launch. (a) U poly-crystalline

silicon cell. (b) Poly-crystalline silicon cell. (c) N-type base single crystal silicon cell. (d) InGaP/GaAs dual-

junction tandem cell. (e) Large-area CIGS cell. (f) High-efciency CIGS cell. (g) Single crystal silicon space

cell (2 Ω cm). (h) Single crystal silicon space cell (10 Ω cm). (Reprinted from Imaizumi, M. et al., Prog.

Photovolt: Res. Appl.,

13, 93, 2005. With permission. John Wiley & Sons, Ltd.)

846 Charged Particle and Photon Interactions with Matter

p p R p p

= −

−













0 C 0

or exp ,Φ =

(30.2)

where

and p

are majority carriers after and before irradiations, respectively

is a “carrier removal rate”

The carrier concentration in the base layer of solar cells decreases, when the carrier removal effect

appears. Since the depletion layer length in a solar cell depends on the difference of carrier concen-

tration between the top and base layers, the decrease in carrier concentration in the base layer causes

the depletion layer to extend. As a result, the number of minority carriers that can reach the top layer

from the base layer increases. Thus, this indicates that the I

increases with increasing uence. Of

course, this very unique increase in I

is a temporary behavior, and with further increase of uence,

the value of I

suddenly drops to almost zero because the base layer becomes intrinsic or type con-

version occurs in the base layer; as a result, no pn junction exists (Matsuura et al., 2006). Figure 30.5

shows the remaining factor of I

for Si solar cells irradiated with either 10MeV protons or 1 MeV

electrons as a function of uence. In a low uence region, the value of I

gradually decreases with

increasing uence. This decrease suggests that the dominant effect is the decrease in the diffusion

length of minority carriers. On the other hand, in the higher uence region, the increase in I

before

sudden dropping down is observed. This behavior can be explained in terms of majority carrier

removal. The excellent agreement of the above-mentioned mechanism with experimental results has

been

reported in the literature (Ohshima et al., 1996; Yamaguchi et al., 1996).

The

decreases in minority carrier diffusion length and majority carrier concentration are basi-

cally the main degradation effects of solar cells due to irradiation. However, in some cases, the

recovery of the electrical performance is also observed. For amorphous Si, the value of photocon-

ductivity becomes higher than the initial value by irradiation (Amekura et al., 2000). The electrical

1.0

0.9

0.8

0.7

0.6

0.5

Normalized I

degradation

0.4

0.3

(a)

1.0

0.9

0.8

0.7

0.6

Normalized V

degradation

(b)

0.03 MeV

0.10 MeV

0.25 MeV

0.2 MeV

10.0 MeV

0.03 MeV

0.10 MeV

0.25 MeV

0.2 MeV

10.0 MeV

1.0

0.8

0.6

0.4

0.2

Normalized P

MAX

degradation

(c)

MAX

Proton fluence (p/cm

)

Proton fluence (p/cm

)

Proton fluence (p/cm

)

0.03 MeV

0.10 MeV

0.25 MeV

0.2 MeV

10.0 MeV

Figure 30.4 Remaining factors of I

, V

, and P

MAX

for InGaP/GaAs/Ge triple-junction solar cells irradi-

ated with protons with various energies as a function of proton uence. Symbols and lines represent experi-

mental and tting results, respectively. (Reprinted from Sumita, T. et al., Nucl. Instrum. Methods B, 206, 448,

2003.

With permission.)

Radiation Effects on Semiconductors and Polymers for Space Applications 847

properties of CIGS irradiated with charged particles recovered even close to room temperature

annealing (Kawakita et al., 2002). For InP and related materials, the recovery of the electrical prop-

erties by minority carrier injection has been reported (Yamaguchi et al., 1995, 1997). These are very

unique behaviors. However, the details of defect annealing/creation have not yet been fully claried

these materials. Further investigation by many researchers is currently underway.

30.2.3 Space Solar cellS

A brief history of space photovoltaics will be described before discussing the current status. Early

on, simple single pn junction solar cells based on crystalline Si were used. The efciency of Si

solar cells installed on Vanguard I, launched in 1954, was 7%–8% under AM0 illumination. With

improving device fabrication processes, the design of Si space solar cells was modied, resulting

in a conversion efciency increase. A back surface eld (BSF), which is a p

layer, was fabricated

behind a p-type base layer to reduce the surface recombination of photo-induced carriers. To avoid

heating solar cells under illumination at long wavelengths, a back surface reector (BSR) layer

which consists of a thin metal layer was applied to space solar cells. In addition to these modica-

tions, inverse pyramid shaped textures creating a non reective surface (NRS) were fabricated at

the surface of the Si solar cells (Katsu et al., 1994). Then, the conversion efciency of Si solar cells

increased up to 18% under AM0. Single-junction-GaAs-based solar cells were also applied to space

applications because of their relatively high conversion efciency (18%–20%) (Matsuda et al., 1994).

From the point of view of high radiation resistance, InP and related materials have been pro-

posed for space solar cells (Yamaguchi et al., 1984, 1995; Yamaguchi, 2001). Figure 30.6 shows the

remaining factor of P

MAX

for InP, GaAs, and Si solar cells as a function of 1MeV electron uence

(Yamaguchi, 2001). InP solar cells obviously have higher radiation resistance than GaAs and Si

solar cells. In addition, depending on irradiation conditions, InP solar cells show different radia-

tion degradation. From DLTS studies, the variety of the degradation behaviors for InP solar cells is

interpreted in terms of the reduction of a major radiation-induced center, which acts as a hole trap

by minority carrier injection (Yamaguchi et al., 1986). Although the reason of this reduction has

not yet been fully claried, a reformation of defects in InP by energy transfer from injected carriers

has been proposed, most likely because the migration energy of vacancies at both In and P lattice

sites are lower than another III-V compounds such as GaAs (Yamaguchi, 2001). Further investiga-

tion is necessary to more fully understand the detailed mechanism of this minority carrier injection

100

10 MeV-proton

1 MeV-electron

Fluence (cm

–2

)

Remaining factor of I

Figure 30.5 Remaining factor of I

for Si solar cells irradiated with either 10MeV protons or 1 MeV

electrons

as a function of uence. Symbols and lines represent experimental and tting results, respectively.

848 Charged Particle and Photon Interactions with Matter

effect. The radiation resistance of InP in space was observed with no signicant degradation of

for about 1000 days after launch (in fact, a small increase by the injection effect was reported.)

(Weinberg,

1991).

Multi-junction

solar cells based on III-V compounds are the main streams for space applica-

tions nowadays because they combine high conversion efciency and high radiation resistance.

Figure 30.7 shows the schematic cross section of a typical InGaP/GaAs/Ge triple-junction solar

cell (3J solar cell). The 3J solar cells are grown on Ge substrates by an organometallic vapor phase

epitaxy (OMVPE). As shown in Figure 30.7, the 3J solar cells consist of three solar cells (InGaP,

GaAs, and Ge sub cells) with tunnel junction diodes located between each sub cell. Since each sub

cell can absorb light at specic wavelengths as shown in Figure 30.8, very high efciency ∼28% can

be achieved with AM0 illumination. In addition, new compounds such as AlInGaP and AlGaAs are

utilized in the top and middle sub cells of state-of-the-art 3J solar cells, to further increase in the

conversion efciency (Takamoto et al., 2005). Nitride-based top cells have also been investigated in

attempt to improve conversion efciency (Dimroth et al., 2005). Furthermore, using an inverted

metamorphic (IMM) approach to overcome lattice mismatch issues, very high efciency 3J solar

cells

have been achieved (Geisz et al., 2008).

InGaP (top cell)

Tunnel junction

Anti-reflection coating

GaAs (middle cell)

Ge (substrate)

Figure 30.7 Schematic cross section of InGaP/GaAs/Ge 3 junction solar cells.

0.9

0.8

0.7

0.6

0.5

2 5 2 5

GaAs cell

InP cell

Si cell

Dark

Results obtained from GaAs

Results obtained from Si solar cells

Light illumination (70 mW/cm

)

Light illumination (10 mW/cm

)

Forward bias (5 mA/cm

)

InP cells

1 MeV electron fluence φ (cm

–2

)

Normalized maximum power P

Figure 30.6 Remaining factor of P

MAX

for InP, GaAs, Si solar cells as a function of 1MeV electron uence.

For InP solar cells, results obtained from irradiation under various conditions are depicted. (Reprinted from

Yamaguchi, M., Sol. Energy Mater. Sol. Cells, 68, 31, 2001. With permission.)

Radiation Effects on Semiconductors and Polymers for Space Applications 849

The thin-lm solar cells based on new materials such as CIGS and amorphous Si (s-Si) have been

studied for space applications (Kawakita et al., 2002; Granata et al., 2005). These solar cells are

thought to be very promising candidates because light weight and exible paddles can be designed

to take advantage of their unique characteristics. The technologies for these solar cells are improved

day-by-day, and mass production for terrestrial application has already started. However, as men-

tioned above, since these solar cells show unique recovery behaviors, the mechanism of radiation

effects should be revealed to predict accurate lifetime in a space environment before commercial

use

for space applications.

30.2.4 radiation degradation of triple-junction Solar cellS

As mentioned above, the degradation of solar cells in space is caused by the decreases in minority

carrier diffusion length and majority carrier concentration due to charged particle irradiation. The

decrease in these parameters can be scaled using K

and R

, respectively, and these values depend

on the material. Thus, in the case of multi-junction solar cells, each sub cell has its own K

as well

as R

values, and as a result, the degradation behavior of a sub cell is different from that of another.

This suggests that the degradation behavior of multi-junction solar cells is not simple like that of a

single

junction solar cell because the radiation resistance is different between component sub cells.

For V

, the degradation behavior of multi-junction solar cells is the sum of the degradation of each

sub cell connected in series. On the other hand, the degradation behavior of I

for multi-junction

solar cells is equal to the degradation behavior of a sub cell with the lowest I

since the I

for

multi-junction solar cells is limited by a sub cell with lowest I

(current-limiting cell). Figure 30.9

shows the relative damage coefcient of V

for InGaP/GaAs/Ge 3J solar cells as a function of the

energy of irradiated protons. The relative damage coefcient is dened as the degradation value

normalized by the degradation value due to 10MeV proton (or 1MeV electron) irradiation. For

example, the relative damage coefcient of 10 means that the decrease in the electrical performance

is 10 times larger than that in the case of 10MeV at the same uence. In Figure 30.9, the peaks

are observed around 0.03, 0.25, and 2MeV. This indicates that the degradation becomes worse by

proton irradiation at these energies. Calculations using a Monte Carlo code, SRIM (Ziegler et al.,

2008), show that the projection range (penetration depth) of protons with these energies corresponds

to the position of pn junction of sub cells. Since protons create heavy damage at the tail end of their

projection range, the three peaks can be interpreted in terms of the heavy damage creation in each

sub cell by protons with these energies.

100

0 500 1000

Wavelength (nm)

QE (%)

1500

GaAs

InGaP

2000

AM0 (μW/cm

/nm)

100

150

200

250

300

Figure 30.8 Quantum efciency (QE) of InGaP/GaAs/Ge 3J solar cells as a function of wave length.

Thespectrum

of AM0 is also shown in the gure.