**5.5. Superior radiation resistance of AlInGaP solar cells**

above the valence band in p-InGaP by Deep Level Transient Spectroscopy (DLTS). Further‐ more, an evidence of a large minority carrier capture cross section for this hole trap has been obtained by double-carrier pulse DLTS which demonstrate the important role of this trap as an recombination center. The one aim of present study was to clarify the mechanism involved in minority-carrier injection-enhanced annealing of the radiation-induced defect H2 in p-

 In this section, we present the direct observation of minority-carrier injection annealing of the dominant 1 MeV electron irradiation-induced hole trap labeled H2 located at 0.50-0.55 eV above the valence band in p-InGaP by Deep Level Transient Spectroscopy (DLTS). Furthermore, an evidence of a large minority carrier capture cross section for this hole trap has been obtained by double-carrier pulse DLTS which demonstrate the important role of this trap as an recombination center. The one aim of present study was to clarify the mechanism involved in minority-carrier injection-

0.5 min

p-InGaP emission rate=1005 s-1 0 min

> Injection anneals 100mA/cm<sup>2</sup>

 at 25 <sup>o</sup> C

5 min

20 min

10 min

The important result of this study is the influence of minority-carrier injection on the annealing kinetics of dominant hole level H2 [21]. In order to clarify the recovery of the solar cells properties following minority-carrier injection annealing, we carried out a systematic study of the variation of the concentration of the hole level H2 using a constant amplitude of

The important result of this study is the influence of minority-carrier injection on the annealing kinetics of dominant hole level H2 [21]. In order to clarify the recovery of the solar cells properties following minority-carrier injection annealing, we carried out a systematic study of the variation of the concentration of the hole level H2 using a constant amplitude of forward

Temperature (K)

Figure 10. Changes of the DLTS spectrum of trap H2 with various time of injection at 25<sup>o</sup>

**Figure 10.** Changes of the DLTS spectrum of trap H2 with various time of injection at 25o

10-7

InGaP, determined from solar cells property ( Jsc or L) and for H2 trap observed by DLTS [21].

10-6

Thermal anneals (∆E= 1.68± 0.14eV) by DLTS

10-5

10-4

Annealing Rate ( s-1)

high temperature side appear to anneal relatively slowly

10-3

10-2

by minority-carrier injection-enhanced processes, determined from DLTS.

short-circuit current density *Jsc* of the solar cells according to the following:

10-1

200 250 300 350

DLTS scans taken after different forward bias injection steps show a pronounced reduction in the H2 amplitude as shown in Figure. 10, which is correlated with a recovery of the maximum power output of the solar cells. It should be noted that the H2 peak; is rather broad and after pronounced reduction following forward bias, exhibits a double structure, indicating that this peak consists of more than one closely spaced peaks; one on the high temperature side appear to anneal rela-

carrier emission DLTS scans taken after different forward bias injection steps show a pro‐ nounced reduction in the H2 amplitude as shown in Figure. 10, which is correlated with a recovery of the maximum power output of the solar cells. It should be noted that the H2 peak; is rather broad and after pronounced reduction following forward bias, exhibits a double structure, indicating that this peak consists of more than one closely spaced peaks; one on the

Figure 11 presents the temperature dependence of the annealing rate *A\** of the hole trap H2

A comparison is given with the injection-enhanced annealing rates estimated by changes in

Figure 11 presents the temperature dependence of the annealing rate A\* of the hole trap H2 by minority-carrier

Injection anneals (∆E = 0.51±0.09 eV) by DLTS

> Injection anneals (∆E= 0.54±0.11 eV ) by solar cell propetry

1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0

1000/T (K-1 )

Figure. 11. Temperature dependence of the thermal and injection annealing rates of the radiation-induced defects in p-

) at various temperatures for 0.5, 1, 2, 5, 10 and 20 min. The majority-carrier emission

) at various temperatures for 0.5, 1, 2, 5, 10 and 20 min. The majority-

C with an injection

forward bias injection (0.1 A/cm<sup>2</sup>

density of 0.1 A cm-2 [21].

tively slowly

5.3 Superior Radiation Resistance of InGaP Solar Cells

enhanced annealing of the radiation-induced defect H2 in p-InGaP [22-27].

DLTS Signal (a.u.)

C with an injection density of

InGaP [22-27].

forward bias injection (0.1 A/cm<sup>2</sup>

density of 0.1 A cm-2 [21].

0.1 A cm-2 [21].

injection-enhanced processes, determined from DLTS.

bias injection (0.1 A/cm2

tively slowly

5.3 Superior Radiation Resistance of InGaP Solar Cells

216 Solar Cells - New Approaches and Reviews

enhanced annealing of the radiation-induced defect H2 in p-InGaP [22-27].

DLTS Signal (a.u.)

Figure 12 presents the temperature dependence of the annealing rate *A* of the trap H1, in p-AlInGaP determined by DLTS. The annealing activation energy of electron irradiation-induced defect H1 in p-AlInGaP is evaluated to be 0.50eV. A comparison is provided with the injectionenhanced annealing rates estimated for the defect H2 in p-InGaP. It is to be noted that the minority carrier injection annealing properties of the defect H2 in p-InGaP observed in the previous 1 MeV electron study are almost the same as those observed in the present study of H1 defect in p-AlInGaP after 1 MeV electron irradiation, which identified that the defect H1 −p InGaP solar cells.

5.4 Superior Radiation Resistance of AlInGaP Solar Cells

density Jsc of the solar cells according to the following:

ty-carrier lifetime in n<sup>+</sup>

solar cell structures.

observed in p-AlInGaP has the same nature of the defect H2 previously observed in P-InGaP and H4 in InP [28,29]. electron study are almost the same as those observed in the present study of H1 defect in p-AlInGaP after 1 MeV electron irradiation, which identified that the defect H1 observed in p-AlInGaP has the same nature of the defect H2 pre-

noted that the minority carrier injection annealing properties of the defect H2 in p-InGaP observed in the previous 1 MeV

Figure 12 presents the temperature dependence of the annealing rate A of the trap H1, in p-AlInGaP determined by

A comparison is given with the injection-enhanced annealing rates estimated by changes in short-circuit current

22 0

− −

scsc scI

JJ J JJJ

φ

22 0

[ ] [ ])( )( <sup>~</sup> )(

2

where suffixes 0, φ, and I correspond to before and after irradiation, and after injection, respectively. The important re-

 A close agreement between activation energy for recovery of radiation-induced defects, determined by solar cell properties and for the hole traps H2, demonstrates that this trap controls the minority-carrier lifetime. This result demonstrates that the dominant majority hole level H2 (EV+0.5–0.55 eV) is the recombination center, which governs the minori-

φ

<sup>−</sup> <sup>=</sup> (30)

2

)(

φ

I

T TI

N N

22 0 2

22 0 2

−

LL L LL L

φ

I

φ sascI sc

sult of this study is the direct relationship between the annealing rates, the solar cells properties and the H2 trap.

The recovery of the radiation damage due to minority carrier injection under forward bias is thought to be caused by an energy release mechanism in which enhancement is induced by the energy released when a minority carrier is trapped on the defect site. According to this mechanism, a change of charge states due to capture of carriers can result in electron phonon coupling. That is, vibration relaxation occurs, which may activate various reactions of the defect such as its migration or destruction, and ultimately decays to heat the lattice. The detailed analysis of this mechanism was previously studied in case of defects in InP, GaAs, and InGaP by the authors and others. viously observed in P-InGaP and H4 in InP [28,29]. The recovery of the radiation damage due to minority carrier injection under forward bias is thought to be caused by an energy release mechanism in which enhancement is induced by the energy released when a minority carrier is trapped on the defect site. According to this mechanism, a change of charge states due to capture of carriers can result in electron phonon coupling. That is, vibration relaxation occurs, which may activate various reactions of the defect such as its migration or destruction, and ultimately decays to heat the lattice. The detailed analysis of this mechanism was previously studied in case of defects in InP, GaAs, and InGaP by the authors and others.

induced defect H1 (E**Figure 12.** V+0.37eV) in p-AlInGaP and H2 (E Comparison of the temperature dependence of injection annealing rates of the radiation-induced defect H1 <sup>V</sup>+0.55eV) in p-InGaP [29]. (EV+0.37eV) in p-AlInGaP and H2 (EV+0.55eV) in p-InGaP [29].

Figure. 12. Comparison of the temperature dependence of injection annealing rates of the radiation-

#### 5.5 Self-annihilation of electron irradiation induced defects in InAsXP1-X/InP multiquantum well solar cells **5.6. Self-annihilation of electron irradiation induced defects in InAsXP1-X/InP multiquantum well solar cells**

In this study, the authors demonstrated the direct observation of majority and minority carrier defects in InAsxP1 x/InP diodes and solar cells structures before, and after 1MeV electron irradiation by double-correlation deep level transient spectroscopy (DDLTS) in order to further evaluate the potential use of this material for space applications [30]. In order to electrically characterize radiation-induced deep center in InAsxP1-x/InP quantum well structure, the DDLTS technique is used to explore the recombination characteristics of deep levels in InAsxP1-x/InP multiquantum well In this study, the authors demonstrated the direct observation of majority and minority carrier defects in InAsxP1-x/InP diodes and solar cells structures before, and after 1MeV electron irradiation by double-correlation deep level transient spectroscopy (DDLTS) in order to further evaluate the potential use of this material for space applications [30].

The activation energy and apparent capture cross sections are determined to be 0.65 eV and 4.3 x 10-14 cm<sup>2</sup> parent high capture cross section of E1, suggests that this level might act as a strong recombination center. In order to electrically characterize radiation-induced deep center in InAsxP1-x/InP quantum well structure, the DDLTS technique is used to explore the recombination characteristics of deep levels in InAsxP1-x/InP multiquantum well solar cell structures.

. The ap-

We observed an unexpected and interesting reduction in the strength of the peak E1 in our deep level spectrum recorded subsequent to a room temperature storage of the irradiated device. Consequently, we carried out a detailed study of the room temperature isothermal annealing effects and carefully monitored the various The activation energy and apparent capture cross sections are determined to be 0.65 eV and 4.3 x 10-14 cm2 . The apparent high capture cross section of E1, suggests that this level might act as a strong recombination center.

We observed an unexpected and interesting reduction in the strength of the peak E1 in our deep level spectrum recorded subsequent to a room temperature storage of the irradiated device. Consequently, we carried out a detailed study of the room temperature isothermal annealing effects and carefully monitored the various deep-level peaks in our spectra as a function of sample storage time at room temperature (25o C). The dashed DLTS curve of Figure 13 represents a spectrum recorded after 90 days storage at room temperature and the signifi‐ cant reduction in the intensity of the E1 peak. deep-level peaks in our spectra as a function of sample storage time at room temperature (25<sup>o</sup> C). The dashed DLTS curve of Figure 13 represents a spectrum recorded after 90 days storage at room temperature and the significant reduction in the intensity of the E1 peak.

Figure. 13. Room temperature annealing effects: (a) after irradiation (b) after 90 days storage at room temperature following irradiation [30]. **Figure 13.** Room temperature annealing effects: (a) after irradiation (b) after 90 days storage at room temperature fol‐ lowing irradiation [30].

The activation energy and apparent capture cross sections are determined to be 0.65eV and 4.3 x 10-14 cm<sup>2</sup> apparent capture cross section of E1 is very high, which indicates that it may act as a strong recombination center. However, serendipitously long duration room temperature storage of the device yielded a total annihilation of E1. Although the detailed mechanisms at play are not fully understood, the serendipitous findings reported here clearly demonstrate the fact that insertion of QWs in the intrinsic region of an InP p-i-n solar cell results in a more radiation tolerant devices. The activation energy and apparent capture cross sections are determined to be 0.65eV and 4.3 x 10-14 cm2 . The apparent capture cross section of E1 is very high, which indicates that it may act as a strong recombination center. However, serendipitously long duration room temper‐ ature storage of the device yielded a total annihilation of E1. Although the detailed mechanisms at play are not fully understood, the serendipitous findings reported here clearly demonstrate the fact that insertion of QWs in the intrinsic region of an InP p-i-n solar cell results in a more radiation tolerant devices.

. The

#### DLTS is an effective spectroscopy technique for processing transient (capacitance or current) from deep levels. This technique has proved to be an instrumental in determining most of the properties of the defects such as structure, intro-**6. Summary**

their correlation with the solar cell parameters.

. The ap-

tration of 10<sup>9</sup>

1. Summary

observed in p-AlInGaP has the same nature of the defect H2 previously observed in P-InGaP

Figure 12 presents the temperature dependence of the annealing rate A of the trap H1, in p-AlInGaP determined by DLTS. The annealing activation energy of electron irradiation-induced defect H1 in p-AlInGaP is evaluated to be 0.50eV. A comparison is provided with the injection-enhanced annealing rates estimated for the defect H2 in p-InGaP. It is to be noted that the minority carrier injection annealing properties of the defect H2 in p-InGaP observed in the previous 1 MeV electron study are almost the same as those observed in the present study of H1 defect in p-AlInGaP after 1 MeV electron irradiation, which identified that the defect H1 observed in p-AlInGaP has the same nature of the defect H2 pre-

A comparison is given with the injection-enhanced annealing rates estimated by changes in short-circuit current

22 0

− −

scsc scI

JJ J JJJ

φ

22 0

[ ] [ ])( )( <sup>~</sup> )(

2

where suffixes 0, φ, and I correspond to before and after irradiation, and after injection, respectively. The important re-

 A close agreement between activation energy for recovery of radiation-induced defects, determined by solar cell properties and for the hole traps H2, demonstrates that this trap controls the minority-carrier lifetime. This result demonstrates that the dominant majority hole level H2 (EV+0.5–0.55 eV) is the recombination center, which governs the minori-

φ

<sup>−</sup> <sup>=</sup> (30)

2

)(

φ

I

T TI

N N

22 0 2

22 0 2

−

LL L LL L

φ

I

φ sascI sc

sult of this study is the direct relationship between the annealing rates, the solar cells properties and the H2 trap.

The recovery of the radiation damage due to minority carrier injection under forward bias is thought to be caused by an energy release mechanism in which enhancement is induced by the energy released when a minority carrier is trapped on the defect site. According to this mechanism, a change of charge states due to capture of carriers can result in electron phonon coupling. That is, vibration relaxation occurs, which may activate various reactions of the defect such as its migration or destruction, and ultimately decays to heat the lattice. The detailed analysis of this mechanism was previously studied in case of defects in InP, GaAs,

The recovery of the radiation damage due to minority carrier injection under forward bias is thought to be caused by an energy release mechanism in which enhancement is induced by the energy released when a minority carrier is trapped on the defect site. According to this mechanism, a change of charge states due to capture of carriers can result in electron phonon coupling. That is, vibration relaxation occurs, which may activate various reactions of the defect such as its migration or destruction, and ultimately decays to heat the lattice. The detailed analysis of this mechanism was

5.5 Self-annihilation of electron irradiation induced defects in InAsXP1-X/InP multiquantum well solar cells

Figure. 12. Comparison of the temperature dependence of injection annealing rates of the radiation-

The activation energy and apparent capture cross sections are determined to be 0.65 eV and 4.3 x 10-14 cm<sup>2</sup>

parent high capture cross section of E1, suggests that this level might act as a strong recombination center.

deep levels in InAsxP1-x/InP multiquantum well solar cell structures.

evaluate the potential use of this material for space applications [30].

In this study, the authors demonstrated the direct observation of majority and minority carrier defects in InAsxP1-

**5.6. Self-annihilation of electron irradiation induced defects in InAsXP1-X/InP multiquantum**

In this study, the authors demonstrated the direct observation of majority and minority carrier defects in InAsxP1-x/InP diodes and solar cells structures before, and after 1MeV electron irradiation by double-correlation deep level transient spectroscopy (DDLTS) in order to further

234567

)

1000/T(K-1

Injection anneals: H2 (p-InGaP)

(∆E = 0.51±0.09 eV)

by DLTS

induced defect H1 (E**Figure 12.** V+0.37eV) in p-AlInGaP and H2 (E Comparison of the temperature dependence of injection annealing rates of the radiation-induced defect H1 <sup>V</sup>+0.55eV) in p-InGaP [29].

Injection anneals: H1 (p-AlInGaP) (∆E = 0.50±0.05 eV) by DLTS

x/InP diodes and solar cells structures before, and after 1MeV electron irradiation by double-correlation deep level transient spectroscopy (DDLTS) in order to further evaluate the potential use of this material for space applications [30].

 In order to electrically characterize radiation-induced deep center in InAsxP1-x/InP quantum well structure, the DDLTS technique is used to explore the recombination characteristics of deep levels in InAsxP1-x/InP multiquantum well

We observed an unexpected and interesting reduction in the strength of the peak E1 in our deep level spectrum recorded subsequent to a room temperature storage of the irradiated device. Consequently, we carried out a detailed study of the room temperature isothermal annealing effects and carefully monitored the various

. The apparent high capture cross section of E1, suggests that this level might act

The activation energy and apparent capture cross sections are determined to be 0.65 eV and

In order to electrically characterize radiation-induced deep center in InAsxP1-x/InP quantum well structure, the DDLTS technique is used to explore the recombination characteristics of

and H4 in InP [28,29].

218 Solar Cells - New Approaches and Reviews

−p InGaP solar cells.

5.4 Superior Radiation Resistance of AlInGaP Solar Cells

viously observed in P-InGaP and H4 in InP [28,29].

density Jsc of the solar cells according to the following:

ty-carrier lifetime in n<sup>+</sup>

solar cell structures.

and InGaP by the authors and others.

Annealing rates (S-1

)

previously studied in case of defects in InP, GaAs, and InGaP by the authors and others.

1x10-12 1x10-11 1x10-10 1x10-9 1x10-8 1x10-7 1x10-6 1x10-5 1x10-4 1x10-3 1x10-2

(EV+0.37eV) in p-AlInGaP and H2 (EV+0.55eV) in p-InGaP [29].

**well solar cells**

4.3 x 10-14 cm2

as a strong recombination center.

be used to characterize defects using various kinds of space charge based devices such as Schottky barrier diodes, and p-n junction to quantum well based complex devices. In addition, sensitivity of the DLTS for detecting defects in concen cm-3 is superior to any other characterization technique. In this chapter we have reviewed the extensive work done by the authors, on the electronic properties of the recombination and compensator centers in Si and III-V compound materials for space and terrestrial solar cells. DLTS is an effective spectroscopy technique for processing transient (capacitance or current) from deep levels. This technique has proved to be an instrumental in determining most of the properties of the defects such as structure, introduction rates, introduction mechanism, thermal stability of the defects etc. DLTS is particularly attractive because it can be used to

duction rates, introduction mechanism, thermal stability of the defects etc. DLTS is particularly attractive because it can

Deep level transient spectroscopy (DLTS) is the best technique for monitoring and characterizing deep levels introduced intentionally or occurring naturally in semiconductor materials and complete devices. DLTS has the advantage over all the techniques used to-date in that it fulfils almost all the requirements for a complete characterization of a deep center and their correlation with the device properties. In particular the method can determine the activation energy of a deep level, its capture cross-section and concentration and can distinguish between traps and recombination centers. In this chapter we provide an overview of the extensive R & D work that has been carried out by the authors on the identification of the recombination and compensator centers in Si and III-V compound materials for space solar cells. In addition, we present an overview of key problems that remain in the understanding of the role of the point defects and

characterize defects using various kinds of space charge based devices such as Schottky barrier diodes, and p-n junction to quantum well based complex devices. In addition, sensitivity of the DLTS for detecting defects in concentration of 109 cm-3 is superior to any other characteri‐ zation technique. In this chapter we have reviewed the extensive work done by the authors, on the electronic properties of the recombination and compensator centers in Si and III-V compound materials for space and terrestrial solar cells.

Deep level transient spectroscopy (DLTS) is the best technique for monitoring and character‐ izing deep levels introduced intentionally or occurring naturally in semiconductor materials and complete devices. DLTS has the advantage over all the techniques used to-date in that it fulfils almost all the requirements for a complete characterization of a deep center and their correlation with the device properties. In particular the method can determine the activation energy of a deep level, its capture cross-section and concentration and can distinguish between traps and recombination centers.

In this chapter we provide an overview of the extensive R & D work that has been carried out by the authors on the identification of the recombination and compensator centers in Si and III-V compound materials for space solar cells. In addition, we present an overview of key problems that remain in the understanding of the role of the point defects and their correlation with the solar cell parameters.
