E-mail address:wangr@https://www.doczj.com/doc/738422139.html,(W.Rong).
0927-0248/02/$-see front matter r2002Elsevier Science B.V.All rights reserved.
PII:S0927-0248(02)00354-9
completely understood yet.In order to use the GaAs/Ge solar cell to space mission more ef?ciently,it is essential to predict its irradiation tolerance and clarify the mechanism for proton irradiation damage.
In this paper,we compare the electrical properties of GaAs/Ge space solar cells due to the proton irradiation with different energies (5,10and 20MeV)and various ?uences from 1?109to 7?1013cm à2,and discuss the relationship between the irradiation effects and the damage defect levels induced by proton irradiation.
2.Experiment
GaAs/Ge space solar cells (20mm ?20mm)used for this study were fabricated by metalorganic chemical vapor deposition (MOCVD)technique.Fig.1shows the structure of p +–n–n +GaAs/Ge solar cells.The carrier concentrations of n-base layer and p +-emitter layer are B 1.5?1017and B 3?1018cm à3,respectively.The junction depth is about 0.5m m.The beginning of life (BOL)ef?ciency of the cells with antire?ective (AR)coating was about 18.8%.
High-energy (5,10and 20MeV)proton scanning irradiation at room temperature was carried out using a HI-13tandem accelerator in the China Institute of Atomic Energy.The scanning uniformity was maintained within 74%.The irradiation ?uence ranged from 1?109to 7?1013cm à2.The AM0properties of the cells before and after irradiation were measured (251C)using an ORIEL solar simulator (made in USA)with illumination at 136.7mW cm à2.Carrier trapping centers in GaAs/Ge solar cells induced by proton irradiation were measured by depth-level transient spectroscopy (DLTS)method (V R ?à1V,V P ?0V,t ?1:656ms).
AR COATING FRONT CONTACT
Al Ga As P GaAs
n GaAs
n GaAs
0.04 0.050.50
3
1 2EMITTER
BASE BUFFER
SUBSTRATE
BACK CONTACT
~(X=0.80 0.85)~~
μm
μm
μm μm
Ge
1-x
x MOCVD GaAs/Ge Solar Cell
+
+
Fig.1.Schematic diagrams of GaAs/Ge cells used in the study.
W.Rong et al./Solar Energy Materials &Solar Cells 77(2003)351–357
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3.Results and discussion
3.1.Irradiation effects
Fig.2shows the changes in normalized short circuit current(I sc),normalized open circuit voltage(V oc)and normalized maximum power output(P max)for GaAs/Ge solar cells as a function of proton irradiation?uence with different energies.It can be seen that at?uences up to3?1011cmà2,the initial I sc reduces to92%,95%and 97%,the initial V oc reduces to93%,95%and97%,the P max reduces to79%,82% and87%,correspondence to5,10and20MeV proton irradiation,respectively;at a higher irradiation?uence,up to3?1012cmà2,the I sc is80%,86%and90%,the V oc is82%,85%and88%,the P max is60%,64%and67%,respectively;at a?uence up to5?1013cmà2,the P max decreases to26%,30%and36%corresponding to5,10 and20MeV proton irradiation,respectively.Also,it was known from the?tness curves in Fig.2(c)that the?uences for the initial P max reduced to75%are6?1011, 8?1011and1.7?1012cmà2corresponding to5,10and20MeV proton irradiation, respectively.It is noticeable in Fig.2that the degradation rates of electric parameters are proven to decrease with the increase in proton irradiation energy;Fig.3shows the degradation rates of P max as a function of proton energy with different irradiation?uences(3?1011,3?1012and5?1013cmà2).Obviously,the degrada-tion rates of electric parameter P max decrease with increase of proton irradiation energy;that is to say,the higher the energy of proton irradiation,the lesser the degradation rates of I sc;V oc and P max at the same proton?uences.
3.2.Analysis with the results simulated by the Monto Carlo Code TRIM95
The above irradiation effect curves can be explained with the results of the range, stopping power and displacement rates of proton in GaAs simulated by the Monto Carlo Code TRIM95[3].The simulated results are listed in Table1.The ranges, stopping powers and displacement rates of protons with5,10and20MeV in GaAs are139,427and1370m m,0.0420,0.0260and0.0158MeV cm2mgà1,and0.41,0.22 and0.13Disp m mà1,respectively.The projectile ranges calculated are much larger than the active region thickness;thus the high-energy protons penetrate beyond the region of major activity in the cell,and the defect pro?le is relatively uniform along the depth.The degradation under irradiation with5–20MeV protons may be thought to mainly cause the degradation of minority-carrier diffusion length in the p+-GaAs emitter and the n-GaAs base layer,as expressed by[4]
De1=L2T?1=L2
f à1=L2
?K L f;
where L is the minority-carrier diffusion length,suf?xes0and f indicate before and after irradiation,respectively,K L is the damage coef?cient for minority-carrier diffusion length,and f is the irradiation?uence.
From the data of stopping powers and displacement rates of protons with5,10 and20MeV in GaAs in Table1,we know that the stopping power and the displacement rates in the cell decrease signi?cantly as the energy of the proton W.Rong et al./Solar Energy Materials&Solar Cells77(2003)351–357353
1E91E101E111E121E131E141E15
20
40
60
80
100
(a)
1E91E101E111E121E131E141E15
40
60
80
100
(b)
1E9
1E10
1E11
1E12
1E13
1E141E15
020
40
6080
100
(c)
Proton fluence (p/cm 2
)
N o r m a l i z e d P m a x (%)
N o r m a l i z e d V o c (%)
N o r m a l i z e d I s c (%)
Fig.2.Changes in properties of GaAs/Ge solar cells as a function of proton irradiation ?uence with different energies:(a)normalized short circuit current (I sc ),(b)normalized open circuit voltage (V oc ),(c)normalized maximum power output (P max ).
W.Rong et al./Solar Energy Materials &Solar Cells 77(2003)351–357
354
increase.The decrease in the stopping power and the displacement rates near the p–n junction in the cell cause less elastic collision displacement damage.With increase in proton energy,the lower displacement damage gives rise to the lesser disruption of the periodic lattice structure,resulting in a degradation of the minority-carrier diffusion length.Therefore,the degradation rates of the cell’s performance decrease as the energy of the proton increases.
4.DLTS tests
The proton irradiation-induced vacancies,interstitial atoms or complexes formed with the impurities would form some electron and hole traps in the p +-GaAs emitter and the n-GaAs base layer,leading to the degradation of the minority-carrier diffusion length [5,6].This argument is supported by the DLTS tests shown in Fig.4.We have found an electron trap in the irradiation sample,which has no detectable
Table 1
Range,stopping power and displacement rates of protons with 5,10and 20MeV in GaAs simulated by the Monte Carlo Code TRIM95Proton energy (MeV)Range (m m)Stopping power (MeV cm 2mg à1)Displacement rates (Disp m m à1)51390.04200.41104270.02600.2220
1370
0.0158
0.13
5
10
15
20
20
40
60
80
proton energy (MeV)
P M a x d e g r a d a t i o n r a t e s (%)
Fig.3.Degradation rates of maximum power output (P max )of GaAs/Ge solar cells as a function of proton energy at different ?uences.
W.Rong et al./Solar Energy Materials &Solar Cells 77(2003)351–357355
traps before irradiation.This defect level lies below the bottom of the conduction band Ec by about 0.41eV,with a capture cross section of 1.65?10à17cm 2and different defect concentration.The irradiation with the 5,10and 20MeV protons of 5?1013cm à2generates the deep energy level with concentrations of 8.45?1016,1.99?1016and 2.76?1015cm à3,respectively.It is obvious that the deep energy level is responsible for the degradation of the solar cell’s performance,and its concentration decreases with the increasing of proton energy.This is consistent with the analytical results of TRIM95.
In conclusion,we get the variation behavior of electric property of GaAs/Ge solar cells irradiated with 5–20MeV protons at a ?uence ranging from 1?109to 7?1013cm à2.It is shown that the electric properties of GaAs/Ge solar cells degrade as proton ?uences increase,and the higher the energy of the protons,the lesser the degradation rates of the electric properties.Electron trap Ec à0.41eV was mainly the irradiation-induced defect and its concentration decreases with the increasing of proton energy.In addition,the data from the irradiation effect curves can be used to predict the irradiation behavior of GaAs/Ge solar cells in space environment.
Acknowledgements
The authors would like to thank Prof.Zhu Shengyun of the China Institute of Atomic Energy for his help in irradiation experiments,and Prof.Xiang Xianbi,Prof.Lu Liwu and Ms.Zhang Yanhua of the Institute of Semiconductors,
Chinese
Fig.4.DLTS spectrum in GaAs/Ge cells irradiated with various energies (5,10and 20MeV)of protons at a ?uence of 5?1013cm à2.
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W.Rong et al./Solar Energy Materials&Solar Cells77(2003)351–357357 Academy of Science,for fruitful discussions and their help in the DLTS measurements.This work was supported by the Foundation of Ministry of Education,China and Initiative Foundation of Science and Technology,Beijing.
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