- Email: [email protected]
Ge space solar cells
Nuclear Instruments and Methods in Physics Research B 370 (2016) 59–62
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Temperature-dependent photoluminescence analysis of 1-MeV electron irradiation-induced nonradiative recombination centers in GaAs/Ge space solar cells Yi Tiancheng, Xiao Pengfei, Zheng Yong, Tang Juan, Wang Rong ⇑ Key Laboratory of Beam Technology and Materials Modification of Ministry of Education, College of Nuclear Science and Technology, Beijing Normal University, Beijing 100875, People’s Republic of China Beijing Radiation Center, Beijing 100875, People’s Republic of China
a r t i c l e
i n f o
Article history: Received 17 July 2015 Received in revised form 6 January 2016 Accepted 6 January 2016
Keywords: Photoluminescence Nonradiative recombination center Thermal activation energy
a b s t r a c t The effects of irradiation of 1-MeV electrons on p+–n GaAs/Ge solar cells have been investigated by temperature-dependent photoluminescence (PL) measurements in the temperature range of 10–290 K. The temperature dependence of the PL peak energy agrees well with the Varnish relation, and the thermal quenching of the total integrated PL intensity is well explained by the thermal quenching theory. Meanwhile, the thermal quenching of temperature-dependent PL confirmed that there are two nonradiative recombination centers in the solar cells, and the thermal activation energies of these centers are determined by Arrhenius plots of the total integrated PL intensity. Furthermore, the nonradiative recombination center, as a primary defect, is identified as the H3 hole trap located at Ev + 0.71 eV at room temperature and the H2 hole trap located at Ev + 0.41 eV in the temperature range of 100–200 K, by comparing the thermal activation and ionization energies of the defects. Ó 2016 Elsevier B.V. All rights reserved.
1. Introduction In our recent work [1] that aimed to identify the nonradiative recombination center of the electron-irradiated GaAs/Ge solar cells among all irradiation-induced defects and recognize the causes of the electron irradiation-induced degradation of the GaAs/Ge solar cells, electron irradiation-induced defects produced in GaAs/Ge space solar cells have been analyzed with photoluminescence (PL) measurements at room temperature, whose results show that the nonradiative recombination center (or the minority carrier capture center) introduced by the electron irradiation probably corresponds to the H3 hole trap located at Ev + 0.71 eV. In fact, temperature dependence of defect-related PL measurements better identify the nonradiative recombination center than the room-temperature PL measurements, because the temperature dependence of PL intensity and position can provide valuable information on the nonradiative recombination centers and help identify them among all irradiation-induced defects. With increasing temperature, the nonradiative recombination centers will be ⇑ Corresponding author at: Key Laboratory of Beam Technology and Materials Modification of Ministry of Education, College of Nuclear Science and Technology, Beijing Normal University, Beijing 100875, People’s Republic of China. E-mail address: [email protected] (W. Rong). http://dx.doi.org/10.1016/j.nimb.2016.01.010 0168-583X/Ó 2016 Elsevier B.V. All rights reserved.
activated, thereby altering the intensity of the PL by few orders of magnitude [2]. Therefore, in this study, we used temperaturedependent PL measurements to recognize the nonradiative recombination centers introduced by 1-MeV-electron irradiation in GaAs/Ge space solar cells.
2. Experiments and results The samples used for PL measurements are p+–n GaAs/Ge solar cells fabricated by metal–organic chemical vapor deposition (MOCVD). The detailed structure of the solar cells is shown in Ref. [3]. The solar cells were irradiated with electron beams with energy of 1 MeV, with fluences ranging up to 3 1015 cm2. In PL measurements, a 532-nm (2.3-eV) emission laser line (power of 100 mW and beam diameter of 3.0 mm) is used as a typical excitation source. The excitation energy is significantly larger than the band gap of GaAs and is strongly absorbed (absorption coefficient, a 105 cm1) [4]. PL spectra from the n-type base layer of GaAs/Ge solar cells were collected by a lens and then transferred to a grating monochromator with 600-grooves/mm grating blazed at 750 nm. The output signal from the monochromator was detected by a Si photodetector and the luminescence was chopped to provide a reference frequency for the lock-in amplifier.
Y. Tiancheng et al. / Nuclear Instruments and Methods in Physics Research B 370 (2016) 59–62
3. Analysis of the temperature dependence PL spectra Fig. 3 shows the temperature dependence of the band-edgerelated PL peak shift. The solid line in the figure shows that the measured data agree well with the Varnish equation [7]:
aT ; bþT 2
Eg ðTÞ ¼ Eg ð0Þ
ð1Þ
GaAs/Ge solar cell 1 MeV electron irradiated 15
ϕ = 3 × 10 cm
1.49 eV
-2
×1
30 K 1.48 eV
PL intensity (a.u.)
In order to measure temperature dependence of the PL spectra, a closed-cycle cryogenic refrigerator (ARS-4HW) equipped with a digital thermometer controller (Lake Shore, 355 Temperature Controller) was used to control the temperature from 10 to 290 K, with a temperature stability of 0.1 K or more. Fig. 1 shows the PL spectra of the 1-MeV-electron-irradiated and -nonirradiated GaAs/Ge solar cells at room temperature. It is evident from the figure that the emission band is the broad peak feature centered at 1.42 eV with the full-width at half-maximum of approximately 45 meV. Furthermore, as the band gap of GaAs is approximately 1.42 eV at room temperature, it is obvious that the GaAs PL spectra peak at 1.42 eV be attributed to near-bandedge emission and band-to-band recombination to dominate the spectra in this region at room temperature. It can also be observed from Fig. 1 that the PL intensity has a fast degradation after irradiation with 1-MeV electrons, and it further increases with increasing fluence. Essentially, the degradation of solar cells induced by electron irradiation is directly proportional to the concentration of the defects, which acts as nonradiative recombination centers for low-irradiation fluences (typically < 1016 cm2) [5]. Temperature-dependent PL spectra of GaAs/Ge solar cells irradiated with 1-MeV electrons with fluence of 3 1015 cm2 are shown in Fig. 2. It is evident from the figure that, at temperatures <30 K, a near-band-edge emission consisting of the exciton emission at 1.49 eV is present [6]. As the temperature increases to 100, 160, 230, and 290 K, the photon energy of the PL peak decreases by about 10, 20, 50, and 70 meV, respectively, compared with that at 30 K. It can also be observed from Fig. 2 that the position of the PL peak shifts toward the left-hand side and the PL intensity decreases with increasing temperature.
×5
100 K 1.47 eV
×10
160 K 1.44 eV
×20
230 K 1.42 eV 290 K
×40 1.25
1.30
1.35
1.40
1.45
1.50
1.55
1.60
1.65
Photon Energy (eV) Fig. 2. Temperature dependence of PL spectra of GaAs/Ge solar cells irradiated with 1-MeV electrons with fluence of 3 1015 cm2 and temperature in the range of 10– 290 K. The peak positions are marked by arrows.
1.50 GaAs/Ge solar cell 1 MeV electrons irradiated ϕ = 3 × 1015 cm-2
1.49
Band-edge related PL-peak (eV)
60
1.48 1.47 1.46 1.45 1.44 1.43 1.42 GaAs
Eg(0)/eV
/ eV·K -1
/K
1.49
6.1 × 10 -4
436
1.41
where Eg (0) is 1.49 eV, the energy gap at temperature T = 0 K and the parameters a and b equal 6.1 104 eV K1 and 436 K, respectively. Obviously, the measured data agree well with the Varnish equation.
0
50
100
150
200
250
300
Temperature (K) Fig. 3. Temperature dependence of the band-edge-related PL peak shift. The solid line shows the good agreement of the measured data (squares) with the Varnish equation Eq. (1).
5
1 MeV electrons irradiated GaAs/Ge solar cell
Unirradiation 3×1013 cm-2 3×1015 cm-2
PL intensity (a.u.)
4
Measured PL intensity of GaAs/Ge solar cells irradiated with 1-MeV electrons with fluence of 3 1015 cm2 is plotted against inverse temperature, that is, an Arrhenius plot, in Fig. 4. As shown in the figure, the data seem to exhibit three different exponential regions, indicating three different thermally activated, nonradiative recombination levels. Obviously, the data below the temperature of 50 K correspond to the shallowest defect. However, such a defect is not expected to play a significant role in the operation of the cell, as the defects closer to mid-gap are expected to control the solar cell performance. Thus, the shallowest defect level was ignored and the data were analyzed in terms of two thermally activated processes a and b. The efficiency of emission (g) was approximated by [8]:
3
2
1
0 1.30
1.35
1.40
1.45
1.50
1.55
Photon Energy (eV) Fig. 1. Room-temperature PL spectra of GaAs/Ge solar cells irradiated with 1-MeV electrons with fluence ranging from 0 to 3 1015 cm2.
g ¼ 1 þ ja exp
Ea kB T
1 Eb þ jb exp ; kB T
ð2Þ
where r is the ratio of the radiative to nonradiative recombination lifetimes at T = 300 K and E is the thermal activation energy
Y. Tiancheng et al. / Nuclear Instruments and Methods in Physics Research B 370 (2016) 59–62
Temperature (K) 200
10
50
PL Intensity (a.u.)
100
Eb = 0.41 eV κb = 109 10
GaAs/Ge solar cell 1 MeV electron irradiated ϕ = 3 × 1015 cm-2
-1
Ea = 0.71 eV κa = 1014
Measured data Fit to Eq.(2)
61
center. Moreover, the b center could be identified with the DLTS peak labeled H2 (Ev + 0.41 eV) by comparing the thermal activation and ionization energies. The value of rb is several orders of magnitude smaller than that of ra. Hence, the defect H2 is hardly detected by electroluminescence and PL measurements at room temperature, and the defect H3 is thermal activated. The PL efficiency is governed by the 0.71 eV defect level that is tentatively associated with the defect H3. Therefore, it suggests that, at room temperature, the performance of 1-MeV-electronirradiated GaAs/Ge solar cells is also controlled by this nonradiative recombination center. On the contrary, from 100 to 200 K, the nonradiative recombination center with a thermal activation energy of 0.41 eV, which seems to be associated with the defect H2, governed the emission efficiency. Therefore, in this temperature range, the performance of GaAs/Ge solar cells is controlled by the defect H2, as the defect H3 is not thermal activated.
10-2 10
20
30
40
50
60
70
80
90
100
1000/T (K -1) Fig. 4. Arrhenius plot of temperature dependence of PL intensity with the solid line representing the fit for the measured data (circles) by Eq. (2) with two nonradiative recombination mechanisms.
measured from the minority carrier band, that is, the valence band for the present n-type samples. The fit, shown as a solid line in Fig. 4, yielded Ea = 0.71 eV, Eb = 0.41 eV, ra = 1014, and rb = 109. 4. Identification of nonradiative recombination centers The hole traps of GaAs material induced by 1-MeV-electron irradiation have been extensively studied, [9] and their ionization energies deduced from deep-level transient spectroscopy (DLTS) data are summarized in Table 1. The table shows the detection of four defects (H0, H1, H2, and H3) in the GaAs material induced by the irradiation. Of them, the defects H0 and H1 are not deep enough in the gap to be ascribed to a recombination center [10] and control the performance of solar cells [8]. H3 is as attributed to a nonradiative recombination center induced by electron irradiation in GaAs by electroluminescence measurements [11] and PL measurements [1]. However, the defect H2 is not analyzed by the aforementioned methods at room temperature. From the multicenter model [2] of an n-type semiconductor PL quenching, Schön [12] and Klasens [13] suggested that the thermal quenching of PL from radiative defects is attributed to the nonradiative transitions. Furthermore, thermal activation energy in this model equals ionization energy of this nonradiative defect, [14] that is, Ea (or Eb) = Et. Therefore, ionization energies for the corresponding nonradiative recombination centers are determined from the activation energies of the thermal quenching. We thus conclude that a dominant nonradiative recombination center is the one at room temperature and that lies at 0.71 eV above the valence band. This defect could be identified with the radiation-induced, minority carrier trap labeled H3 (Ev + 0.71 eV) that is characterized by the DLTS studies of 1-MeVelectron-irradiated GaAs [9]. The high value of ra indicates that the defect H3 is a highly efficient nonradiative recombination
Table 1 Characteristics of the hole traps in GaAs induced by 1-MeV-electron irradiation obtained by DLTS [1]. Et – ionization energy of the defect; T0 – DLTS peak temperature. Defects
H0
H1
H2
H3
Et (eV) T0 (K)
0.06 50
0.29 150
0.41 190
0.71 340
5. Conclusion Temperature-dependent PL of p+–n GaAs/Ge solar cells irradiated by 1-MeV electrons has been studied in detail over a wide range of temperatures from 10 to 290 K. The Varnish relation was used to analyze the temperature dependence of the PL peak energy shift, and the Arrhenius plots of the total integrated PL intensity were used to determine the thermal activation energy of the defects induced by 1-MeV-electron irradiation. By comparing the ionization energy and thermal activation energy of the defects, nonradiative recombination center, as a primary defect, was identified as the H3 hole trap located at Ev + 0.71 eV at room temperature and the H2 hole trap located at Ev + 0.41 eV in the temperature range of 100–200 K. It has been clearly demonstrated that the temperature dependence of PL measurements can provide more information on the nonradiative recombination centers, such as H3 and H2 hole traps, which control the performance degradation of the electron-irradiated GaAs/Ge solar cells. The results can be potentially applied to understand the causes of the electron irradiation-induced degradation of the GaAs/Ge solar cells and improve the irradiation tolerance of the solar cells in space radiation environments. Acknowledgments This study was supported by the National Natural Science Foundation of China under Grant Nos. 10675023, 11075018, and 11375028, the Specialized Research Fund for the Doctoral Program of Higher Education (20120003110011), and Fundamental Research Funds for the Central Universities.
Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.nimb.2016.01. 010. References [1] Lu Ming, Wang Rong, Yang Kui, Yi Tiancheng, Photoluminescence analysis of electron irradiation-induced defects in GaAs/Ge space solar cells, Nucl. Instr. Meth. Phys. Res. B 312 (2013) 137–140. [2] M.A. Reshchikov, Temperature dependence of defect-related photoluminescence in III–V and II–VI semiconductors, J. Appl. Phys. 115 (2014). 012010-15. [3] Wang Rong, Guo Zengliang, Zhang Xinghui, et al., 5–20 MeV proton irradiation effects on GaAs/Ge solar cells for space use, Solar Energy Mater. Sol. Cells 77 (2003) 351–357. [4] B.E. Anspaugh, Solar Cell Radiation Handbook, JPL Publication, 1996.
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[5] M. Mbarki, G.C. Sun, J.C. Bourgoin, Prediction of solar cell degradation in space from the electron–proton equivalence, Semicond. Sci. Technol. 19 (2004) 1081–1085. [6] S.I. Maximenko, S.R. Messenger, C.D. Cress, J.A. Freitas, et al., Application of CL/ EBIC-SEM techniques for characterization of radiation effects in multijunction solar cells, IEEE Trans. Nucl. Sci. 57 (6) (2010) 3095–3100. [7] P. Varshni, Temperature dependence of the energy gap in semiconductors, Physica 34 (1) (1967) 149–154. [8] M.J. Romero, D. Araujo, R. Garcıa, et al., Spatial distribution of radiationinduced defects in p+–n InGaP solar cells, Appl. Phys. Lett. 74 (25) (1999) 3812–3814. [9] D. Pons, J.C. Bourgoin, Irradiation-induced defects in GaAs, Solid State Phys. 18 (1985) 3839–3871.
[10] J.C. Bourgoin, M. Zazoui, Irradiation-induced degradation in solar cell: characterization of recombination centres, Semicond. Sci. Technol. 17 (2002) 453–460. [11] M. Zazoui, M. Mbarki, A. Zin Aldin, et al., Analysis of multijunction solar cell degradation in space and irradiation induced recombination centers, J. Appl. Phys. 93 (2003) 5080–5084. [12] M. Schön, Zum Leuchtmechanismus der Kristallphosphore, Z. Phys. 119 (1942) 463–471. [13] H.A. Klasens, Transfer of energy between centres zinc sulphide phosphors, Nature 158 (1946) 306–307. [14] M.A. Reshchiko, R.Y. Korotkov, Analysis of the temperature and excitation intensity dependencies of photoluminescence in undoped GaN films, Phys. Rev. B 64 (2001). 115205-11.
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Temperature-dependent photoluminescence analysis of 1-MeV electron irradiation-induced nonradiative recombination centers in GaAs/Ge space solar cells Yi Tiancheng, Xiao Pengfei, Zheng Yong, Tang Juan, Wang Rong ⇑ Key Laboratory of Beam Technology and Materials Modification of Ministry of Education, College of Nuclear Science and Technology, Beijing Normal University, Beijing 100875, People’s Republic of China Beijing Radiation Center, Beijing 100875, People’s Republic of China
a r t i c l e
i n f o
Article history: Received 17 July 2015 Received in revised form 6 January 2016 Accepted 6 January 2016
Keywords: Photoluminescence Nonradiative recombination center Thermal activation energy
a b s t r a c t The effects of irradiation of 1-MeV electrons on p+–n GaAs/Ge solar cells have been investigated by temperature-dependent photoluminescence (PL) measurements in the temperature range of 10–290 K. The temperature dependence of the PL peak energy agrees well with the Varnish relation, and the thermal quenching of the total integrated PL intensity is well explained by the thermal quenching theory. Meanwhile, the thermal quenching of temperature-dependent PL confirmed that there are two nonradiative recombination centers in the solar cells, and the thermal activation energies of these centers are determined by Arrhenius plots of the total integrated PL intensity. Furthermore, the nonradiative recombination center, as a primary defect, is identified as the H3 hole trap located at Ev + 0.71 eV at room temperature and the H2 hole trap located at Ev + 0.41 eV in the temperature range of 100–200 K, by comparing the thermal activation and ionization energies of the defects. Ó 2016 Elsevier B.V. All rights reserved.
1. Introduction In our recent work [1] that aimed to identify the nonradiative recombination center of the electron-irradiated GaAs/Ge solar cells among all irradiation-induced defects and recognize the causes of the electron irradiation-induced degradation of the GaAs/Ge solar cells, electron irradiation-induced defects produced in GaAs/Ge space solar cells have been analyzed with photoluminescence (PL) measurements at room temperature, whose results show that the nonradiative recombination center (or the minority carrier capture center) introduced by the electron irradiation probably corresponds to the H3 hole trap located at Ev + 0.71 eV. In fact, temperature dependence of defect-related PL measurements better identify the nonradiative recombination center than the room-temperature PL measurements, because the temperature dependence of PL intensity and position can provide valuable information on the nonradiative recombination centers and help identify them among all irradiation-induced defects. With increasing temperature, the nonradiative recombination centers will be ⇑ Corresponding author at: Key Laboratory of Beam Technology and Materials Modification of Ministry of Education, College of Nuclear Science and Technology, Beijing Normal University, Beijing 100875, People’s Republic of China. E-mail address: [email protected] (W. Rong). http://dx.doi.org/10.1016/j.nimb.2016.01.010 0168-583X/Ó 2016 Elsevier B.V. All rights reserved.
activated, thereby altering the intensity of the PL by few orders of magnitude [2]. Therefore, in this study, we used temperaturedependent PL measurements to recognize the nonradiative recombination centers introduced by 1-MeV-electron irradiation in GaAs/Ge space solar cells.
2. Experiments and results The samples used for PL measurements are p+–n GaAs/Ge solar cells fabricated by metal–organic chemical vapor deposition (MOCVD). The detailed structure of the solar cells is shown in Ref. [3]. The solar cells were irradiated with electron beams with energy of 1 MeV, with fluences ranging up to 3 1015 cm2. In PL measurements, a 532-nm (2.3-eV) emission laser line (power of 100 mW and beam diameter of 3.0 mm) is used as a typical excitation source. The excitation energy is significantly larger than the band gap of GaAs and is strongly absorbed (absorption coefficient, a 105 cm1) [4]. PL spectra from the n-type base layer of GaAs/Ge solar cells were collected by a lens and then transferred to a grating monochromator with 600-grooves/mm grating blazed at 750 nm. The output signal from the monochromator was detected by a Si photodetector and the luminescence was chopped to provide a reference frequency for the lock-in amplifier.
Y. Tiancheng et al. / Nuclear Instruments and Methods in Physics Research B 370 (2016) 59–62
3. Analysis of the temperature dependence PL spectra Fig. 3 shows the temperature dependence of the band-edgerelated PL peak shift. The solid line in the figure shows that the measured data agree well with the Varnish equation [7]:
aT ; bþT 2
Eg ðTÞ ¼ Eg ð0Þ
ð1Þ
GaAs/Ge solar cell 1 MeV electron irradiated 15
ϕ = 3 × 10 cm
1.49 eV
-2
×1
30 K 1.48 eV
PL intensity (a.u.)
In order to measure temperature dependence of the PL spectra, a closed-cycle cryogenic refrigerator (ARS-4HW) equipped with a digital thermometer controller (Lake Shore, 355 Temperature Controller) was used to control the temperature from 10 to 290 K, with a temperature stability of 0.1 K or more. Fig. 1 shows the PL spectra of the 1-MeV-electron-irradiated and -nonirradiated GaAs/Ge solar cells at room temperature. It is evident from the figure that the emission band is the broad peak feature centered at 1.42 eV with the full-width at half-maximum of approximately 45 meV. Furthermore, as the band gap of GaAs is approximately 1.42 eV at room temperature, it is obvious that the GaAs PL spectra peak at 1.42 eV be attributed to near-bandedge emission and band-to-band recombination to dominate the spectra in this region at room temperature. It can also be observed from Fig. 1 that the PL intensity has a fast degradation after irradiation with 1-MeV electrons, and it further increases with increasing fluence. Essentially, the degradation of solar cells induced by electron irradiation is directly proportional to the concentration of the defects, which acts as nonradiative recombination centers for low-irradiation fluences (typically < 1016 cm2) [5]. Temperature-dependent PL spectra of GaAs/Ge solar cells irradiated with 1-MeV electrons with fluence of 3 1015 cm2 are shown in Fig. 2. It is evident from the figure that, at temperatures <30 K, a near-band-edge emission consisting of the exciton emission at 1.49 eV is present [6]. As the temperature increases to 100, 160, 230, and 290 K, the photon energy of the PL peak decreases by about 10, 20, 50, and 70 meV, respectively, compared with that at 30 K. It can also be observed from Fig. 2 that the position of the PL peak shifts toward the left-hand side and the PL intensity decreases with increasing temperature.
×5
100 K 1.47 eV
×10
160 K 1.44 eV
×20
230 K 1.42 eV 290 K
×40 1.25
1.30
1.35
1.40
1.45
1.50
1.55
1.60
1.65
Photon Energy (eV) Fig. 2. Temperature dependence of PL spectra of GaAs/Ge solar cells irradiated with 1-MeV electrons with fluence of 3 1015 cm2 and temperature in the range of 10– 290 K. The peak positions are marked by arrows.
1.50 GaAs/Ge solar cell 1 MeV electrons irradiated ϕ = 3 × 1015 cm-2
1.49
Band-edge related PL-peak (eV)
60
1.48 1.47 1.46 1.45 1.44 1.43 1.42 GaAs
Eg(0)/eV
/ eV·K -1
/K
1.49
6.1 × 10 -4
436
1.41
where Eg (0) is 1.49 eV, the energy gap at temperature T = 0 K and the parameters a and b equal 6.1 104 eV K1 and 436 K, respectively. Obviously, the measured data agree well with the Varnish equation.
0
50
100
150
200
250
300
Temperature (K) Fig. 3. Temperature dependence of the band-edge-related PL peak shift. The solid line shows the good agreement of the measured data (squares) with the Varnish equation Eq. (1).
5
1 MeV electrons irradiated GaAs/Ge solar cell
Unirradiation 3×1013 cm-2 3×1015 cm-2
PL intensity (a.u.)
4
Measured PL intensity of GaAs/Ge solar cells irradiated with 1-MeV electrons with fluence of 3 1015 cm2 is plotted against inverse temperature, that is, an Arrhenius plot, in Fig. 4. As shown in the figure, the data seem to exhibit three different exponential regions, indicating three different thermally activated, nonradiative recombination levels. Obviously, the data below the temperature of 50 K correspond to the shallowest defect. However, such a defect is not expected to play a significant role in the operation of the cell, as the defects closer to mid-gap are expected to control the solar cell performance. Thus, the shallowest defect level was ignored and the data were analyzed in terms of two thermally activated processes a and b. The efficiency of emission (g) was approximated by [8]:
3
2
1
0 1.30
1.35
1.40
1.45
1.50
1.55
Photon Energy (eV) Fig. 1. Room-temperature PL spectra of GaAs/Ge solar cells irradiated with 1-MeV electrons with fluence ranging from 0 to 3 1015 cm2.
g ¼ 1 þ ja exp
Ea kB T
1 Eb þ jb exp ; kB T
ð2Þ
where r is the ratio of the radiative to nonradiative recombination lifetimes at T = 300 K and E is the thermal activation energy
Y. Tiancheng et al. / Nuclear Instruments and Methods in Physics Research B 370 (2016) 59–62
Temperature (K) 200
10
50
PL Intensity (a.u.)
100
Eb = 0.41 eV κb = 109 10
GaAs/Ge solar cell 1 MeV electron irradiated ϕ = 3 × 1015 cm-2
-1
Ea = 0.71 eV κa = 1014
Measured data Fit to Eq.(2)
61
center. Moreover, the b center could be identified with the DLTS peak labeled H2 (Ev + 0.41 eV) by comparing the thermal activation and ionization energies. The value of rb is several orders of magnitude smaller than that of ra. Hence, the defect H2 is hardly detected by electroluminescence and PL measurements at room temperature, and the defect H3 is thermal activated. The PL efficiency is governed by the 0.71 eV defect level that is tentatively associated with the defect H3. Therefore, it suggests that, at room temperature, the performance of 1-MeV-electronirradiated GaAs/Ge solar cells is also controlled by this nonradiative recombination center. On the contrary, from 100 to 200 K, the nonradiative recombination center with a thermal activation energy of 0.41 eV, which seems to be associated with the defect H2, governed the emission efficiency. Therefore, in this temperature range, the performance of GaAs/Ge solar cells is controlled by the defect H2, as the defect H3 is not thermal activated.
10-2 10
20
30
40
50
60
70
80
90
100
1000/T (K -1) Fig. 4. Arrhenius plot of temperature dependence of PL intensity with the solid line representing the fit for the measured data (circles) by Eq. (2) with two nonradiative recombination mechanisms.
measured from the minority carrier band, that is, the valence band for the present n-type samples. The fit, shown as a solid line in Fig. 4, yielded Ea = 0.71 eV, Eb = 0.41 eV, ra = 1014, and rb = 109. 4. Identification of nonradiative recombination centers The hole traps of GaAs material induced by 1-MeV-electron irradiation have been extensively studied, [9] and their ionization energies deduced from deep-level transient spectroscopy (DLTS) data are summarized in Table 1. The table shows the detection of four defects (H0, H1, H2, and H3) in the GaAs material induced by the irradiation. Of them, the defects H0 and H1 are not deep enough in the gap to be ascribed to a recombination center [10] and control the performance of solar cells [8]. H3 is as attributed to a nonradiative recombination center induced by electron irradiation in GaAs by electroluminescence measurements [11] and PL measurements [1]. However, the defect H2 is not analyzed by the aforementioned methods at room temperature. From the multicenter model [2] of an n-type semiconductor PL quenching, Schön [12] and Klasens [13] suggested that the thermal quenching of PL from radiative defects is attributed to the nonradiative transitions. Furthermore, thermal activation energy in this model equals ionization energy of this nonradiative defect, [14] that is, Ea (or Eb) = Et. Therefore, ionization energies for the corresponding nonradiative recombination centers are determined from the activation energies of the thermal quenching. We thus conclude that a dominant nonradiative recombination center is the one at room temperature and that lies at 0.71 eV above the valence band. This defect could be identified with the radiation-induced, minority carrier trap labeled H3 (Ev + 0.71 eV) that is characterized by the DLTS studies of 1-MeVelectron-irradiated GaAs [9]. The high value of ra indicates that the defect H3 is a highly efficient nonradiative recombination
Table 1 Characteristics of the hole traps in GaAs induced by 1-MeV-electron irradiation obtained by DLTS [1]. Et – ionization energy of the defect; T0 – DLTS peak temperature. Defects
H0
H1
H2
H3
Et (eV) T0 (K)
0.06 50
0.29 150
0.41 190
0.71 340
5. Conclusion Temperature-dependent PL of p+–n GaAs/Ge solar cells irradiated by 1-MeV electrons has been studied in detail over a wide range of temperatures from 10 to 290 K. The Varnish relation was used to analyze the temperature dependence of the PL peak energy shift, and the Arrhenius plots of the total integrated PL intensity were used to determine the thermal activation energy of the defects induced by 1-MeV-electron irradiation. By comparing the ionization energy and thermal activation energy of the defects, nonradiative recombination center, as a primary defect, was identified as the H3 hole trap located at Ev + 0.71 eV at room temperature and the H2 hole trap located at Ev + 0.41 eV in the temperature range of 100–200 K. It has been clearly demonstrated that the temperature dependence of PL measurements can provide more information on the nonradiative recombination centers, such as H3 and H2 hole traps, which control the performance degradation of the electron-irradiated GaAs/Ge solar cells. The results can be potentially applied to understand the causes of the electron irradiation-induced degradation of the GaAs/Ge solar cells and improve the irradiation tolerance of the solar cells in space radiation environments. Acknowledgments This study was supported by the National Natural Science Foundation of China under Grant Nos. 10675023, 11075018, and 11375028, the Specialized Research Fund for the Doctoral Program of Higher Education (20120003110011), and Fundamental Research Funds for the Central Universities.
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