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Displacement Damage Correlation of Proton and Silicon Ion Radiation in GaAs Jeffrey H. Warner, Scott R. Messenger, Robert J. Walters, and Geoffrey P. Summers
Abstract—We present results of displacement damage correlation between 2 MeV protons and 22 MeV silicon ion irradiation damage in p+ n GaAs solar cells. The radiation induced degradation of the photovoltaic response correlates well in terms of displacement damage dose. Index Terms—Damage correlation, displacement damage, GaAs solar cells, ion irradiation, nonionizing energy loss (NIEL), proton irradiation, radiation damage, SRIM.
I. INTRODUCTION
T
HE SPACE environment consists of many different types of charged particles varying over a wide energy range. The dominant particles in the space environment are electrons or protons or a combination of the two depending on the orbit. Exposure to these charged particles typically degrades the electrical performance of semiconductor devices. The ability to predict how these devices respond in a particle radiation environment is of utmost important in predicting the expected mission lifetime. When energetic particles penetrate a semiconductor material they transfer energy by collisions with the host atoms of the crystal lattice. Most of the energy is transferred through ionization of the target atoms, where electrons absorb the energy and are promoted to excited energy levels. The remaining portion of the energy is transferred through nonionizing events where the target atom is displaced. The displaced atom is commonly referred to as the primary knock-on atom (PKA). If the recoil energy of the PKA is large enough, the PKA can, in turn, produce secondary displacements creating a cascade of displacements and potentially defect clusters. Spacecraft microelectronics can be sensitive to the ionizing or nonionizing (or both) dose deposited by the irradiation. Solar cells and most optoelectronic devices, being minority carrier devices, are most sensitive to the nonionizing deposited dose. The rate at which the kinetic energy of the irradiating particle is transferred to a material to produce atomic displacements is called nonionizing energy loss (NIEL) [1], [2]. The NIEL calculation involves an integration over energy of the product of: 1) the differential cross section for atomic displacements, of which Manuscript received July 8, 2005; revised December 19, 2005. J. H. Warner and G. P. Summers are with the Naval Research Laboratory, Washington, DC 20375 USA, and also with the Physics Department, University of Maryland Baltimore County, Baltimore, MD 21250 USA (e-mail:
[email protected];
[email protected];
[email protected]). S. R. Messenger is with the SFA, Inc., Largo, MD 20774 USA (e-mail:
[email protected]). R. J. Walters is with the Naval Research Laboratory, Washington, DC 20375 USA (e-mail:
[email protected]). Digital Object Identifier 10.1109/TNS.2005.860737
there are three energy regimes to consider (Coulombic, nuclear elastic, and nuclear inelastic); 2) the energy of the recoiling atom (i.e., that of the primary knock-on atom, PKA); and 3) the Lindhard partition factor, which takes into account that only some of the energy of the recoil will actually go into producing displacements. The limits of integration are from the threshold for atomic displacements (on the order of 10 eV) to the maximum energy which can be transferred by the incident particle to host atoms. The amount of nonionizing radiation dose deposited by the irradiating particle is referred to as displacement damage [3], which is calculated by multiplying the particle dose fluence data by the appropriate NIEL value for the given irradiating particle and energy and target material as shown in (1) (1) is the NIEL value for particle , is the fluence where level for particle . For devices that are sensitive to displacement damage effects, the energy dependence of the damage coefficients (DCs) for a given observable of the device (e.g., minority carrier diffusion length, LED light output intensity, or solar cell maximum power output) usually follows the energy dependence of the NIEL. This is the case because, to a good approximation, the defect introduction rates induced by many different particles over a wide range of energies are proportional to the nonionizing energy loss of the primary particle and the energetic nuclear recoils it produces within the material. Furthermore, the degradation of the measured physical observable due to irradiation by different particles at different energies can be correlated in terms of displacement damage dose. This has been shown for electrons and protons incident upon Si, InP, GaAs, and InGaP/InGaAs/Ge solar cells, as well as many other devices [4]–[8]. Where a lack of experimental data exists, NIEL can be particularly useful for analyzing displacement damage in devices. This may be of particular interest in the case of displacement damage effects due to heavy ions. NIEL has been calculated for different incident particles on various target materials, but there is a limited data base for correlating experimental data with theoretical calculations between heavy ions and protons with NIEL [1], [2], [9]. The correlation of proton and heavy ion displacement damage effects is the main objective of the present research. This paper presents an experimental data set describing the photovoltaic response of GaAs solar cells as a function of particle fluence following heavy ion irradiation (22 MeV Silicon ions) and 2 MeV proton irradiation. The displacement damage dose methodology is used to correlate the measured degradation data.
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Fig. 1. Energy loss rate as a function of particle energy for protons and silicon ions incident upon GaAs. The nonionizing curves (solid lines) are referred to as the Coulombic NIEL. The NIEL calculation includes Coulombic interactions only and a screened potential. The dashed line and dashed-dotted line are the stopping powers (ionizing curves) for silicon and protons incident upon GaAs, respectively. The threshold energy for atomic displacement used for Ga and As was 10 eV.
II. EXPERIMENTAL DETAILS The samples used in this study were 0.5 0.5 cm p n GaAs solar cells grown on n-type GaAs substrates by Molecular Beam Epitaxy (MBE). A detailed description of the solar cells is given in [10]. The wafers were grown and processed at NRL. The devices were cleaved from a single wafer to eliminate possible wafer-to-wafer variations and any effects that may occur from irradiating devices with different structures. Several samples were mounted on a stainless steel plate for irradiation. The irradiations were performed at room temperature with 2 MeV protons up to fluences ranging from p /cm up to p /cm and 22 MeV Si ions up to fluences ranging from Si /cm up to Si /cm using the NRL Pelletron Facility. Illuminated current-voltage measurements were performed under 1 sun (1367 W/m ), AM0 (at 25 C) conditions using an Oriel 1000 W solar simulator. The average pre-irradiation values for the photocurrent, photovoltage, and efficiency for the solar cells were 24.13 mA/cm , 0.96 V, and 13.39%, respectively. The solar cells did not have an antireflective coating, which resulted in relatively lower photocurrent density. III. RESULTS NIEL is a quantity that describes the rate of energy loss due to atomic displacements as a particle traverses a material. NIEL is a calculated quantity that takes into account the various interactions of an incident particle with a target atom/material. NIEL has been calculated for different incident particles on various target materials and two such examples for particles on GaAs are shown in Fig. 1. The solid lines represent the calculated NIEL for protons and silicon ions incident upon GaAs corresponding to displacement damage effects (nonionizing) where only Coulombic interactions have been included [2]. NIEL is an analogue to ionization energy loss or stopping power, which is
also shown in the figure represented by the dashed-dotted and dashed lines for protons and silicon ions incident upon GaAs. The stopping powers were calculated using the Monte Carlo code called SRIM [11]. There are limitations using the NIEL approach as discussed in [12]. Using NIEL to correlate damage works well when the particle energy is essentially constant over the active region of the device. As the irradiating particle traverses the target material and transfers its kinetic energy to the target atoms, the particle energy decreases. In other words, the particle decelerates as it passes through the material. Therefore, particles have a finite range in a material. If the particle energy decreases sufficiently, then the NIEL of the particle can change significantly, which can lead to a nonuniform damage profile within the material. In such cases, care must be taken in the damage correlation in terms of NIEL since the NIEL calculated for the incident particle energy will not be representative of the damage produced by the decelerated particle. This issue has been addressed by Messenger et al. using NIEL values derived from SRIM Monte Carlo calculations for protons incident upon silicon, GaAs, and InGaP/GaAs/Ge solar cells [13]–[15]. GaAs solar cells have a large solar photon absorpm) tion coefficient, so the active region is typically thin ( and near the device surface. Therefore, protons with energies of MeV or greater will pass through the active region with negligible energy loss. For the present study, the active region extends approximately from 0 to 2.5 m. The 2 MeV protons m, which is more than sufficient to prohave a range of duce a uniform damage track. More care must be taken in considering the Si ion irradiation since there should be greater deceleration within the material due to the higher stopping powers. That is, we must consider the damage track structure of Si ions as related to the dimensions of the device to ensure uniform damage deposition. The SRIM-derived NIEL as a function of depth in GaAs for these two particles are shown in Fig. 2. The solid line represents that
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Fig. 2. SRIM-derived NIEL as a function of penetration depth derived from SRIM data for 2 MeV protons and 22 MeV silicon ions on GaAs. The top and bottom abscissa axes represent depth scales for 22 MeV Si and 2 MeV proton simulations, respectively, indicated by the arrows.
Fig. 3. Normalized photocurrent as a function of particle fluence for 2 MeV protons and 22 MeV silicon ion irradiations on GaAs solar cells. The solar illumination conditions are AM0, 1sun, and 25 C.
for protons on GaAs and the dotted line represents that for silicon ions on GaAs. Since NIEL is proportional to the amount of energy deposited into atomic displacements, Fig. 2 depicts the damage track structures of the particles in the material. These data show the damage track to be approximately constant over the active region of the device for both irradiating particles. The range of 22 MeV Si ions is approximately 7 m. Fig. 3 shows the degradation of the photocurrent as a function of proton fluence normalized to the pre-irradiation values. The solid circles and hollow diamonds represent the normalized photocurrent following 22 MeV Si ion irradiation and 2 MeV proton irradiation, respectively. The normalized photovoltage and maximum power (not shown here) display similar behavior. One feature of this plot is that for a given photocurrent degradation level (e.g., 0.8% or 80% remaining photocurrent) the proton orders of magnitude as data is shifted to a higher fluence by
compared to the Si ion data. This indicates that, as expected, irradiation by Si ions results in significantly more damage in the GaAs. This result is also suggested by the NIEL calculations for these particles on GaAs as shown in Fig. 1. The result of correlating the data in Fig. 3 in terms of displacement damage dose is shown in Fig. 4. This is calculated by multiplying the fluence data in Fig. 3 by the associated Coulombic NIEL value for each particle (see Fig. 1). For the case of 2 MeV protons in GaAs, the fluence data were multiplied by the 2 MeV MeVcm /g). We see that, proton NIEL value in GaAs ( within the uncertainty of the photocurrent measurement, the data collapse to a single damage curve. A similar correlation was observed for the photovoltage and maximum power. Given this correlation, this degradation can be considered to be characteristic for this solar cell structure and can be used to predict the device response to irradiation by other ions at other energies.
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Fig. 4. Normalized photocurrent as a function of displacement damage dose for 2 MeV protons and 22 MeV silicon ion irradiations on GaAs solar cells. The Coulombic NIEL was used for the correlation.
IV. DISCUSSION The present results demonstrate how analysis of solar cell raallows direct correladiation degradation data in terms of tion of data measured after a wide range of particles and energies. In the present case, the correlation was achieved over nearly three orders of magnitude in particle fluence. It follows, methodology, knowledge of the degrathen, that using the dation characteristic of a device for a given irradiating particle, e.g., 2 MeV protons on GaAs, allows prediction of the device response to any other particle irradiation using the calculated NIEL value. Using this approach, one can accurately estimate the fluence values to use for an irradiation experiment despite lack of data for that specific irradiation. Indeed, the 22 MeV Si ion fluence range for the present experiment was chosen in just this fashion based on existing proton irradiation data. This conclusion extends to analysis of device response to irradiation by a spectrum of particles as encountered in the space environment since by integrating the particle spectrum with the NIEL, the en. vironment can be reduced an equivalent value of An important note regarding damage correlation with different irradiating particles is that for proton irradiation in GaAs, the energy dependence of the relative damage coefficients (RDCs) vary linearly with NIEL while, for electron irradiation, the energy dependence of the RDCs varies from linear to quadratic depending on the polarity of the GaAs material [4]. Therefore, care must be taken when correlating electron and proton damage. However, for the present experiment, the of the Si ion with the proton direct correlation in terms of irradiation data suggests that the Si ion damage coefficients vary linearly with NIEL. The dependence of the proton damage coefficients on the NIEL for several GaAs optoelectronic devices (including solar cells) has been shown to be nonlinear for energies above 10 MeV [10]. For protons energies above 10 MeV, nuclear interactions become significant, while the interactions are strictly Coulombic for lower energies. The average recoil energies generated by Coulombic interactions are in the range of a few hundred eV, and the PKAs have ranges of only a few tenths of Angstroms, thereby resulting in simple point defects. However,
recoils produced by nuclear interactions can have energies in the MeV range and with ranges of m. In the present study, the interactions are almost exclusively Coulombic and the average recoil energies of the 2 MeV proeV and eV, respectively. tons and 22 MeV Si ions is Although the recoil energy for the Si ion is larger, the successful indicates that these correlation obtained directly in terms of particles create the same type of damage, but the Si ions create the damage at an increased rate. On a separate topic, there is one notable feature shown in Fig. 1 worth discussing. Considering the ionizing and nonionizing curves for protons, the energy dependence of these two curves are MeV. Because approximately the same for energies of this, it is difficult to distinguish between ionizing and nonionizing effects when analyzing device degradation data taken after different energy proton irradiations. For example, the amount of degradation measured after 10 and 3 MeV proton irradiations will roughly scale with a 1/E dependence, but it is not possible to determine if this is due to ionizing or nonionizing effects. However, this is in contrast to the results for silicon ions incident upon GaAs if the energy is chosen appropriately. For enerMeV, the silicon ion NIEL on GaAs continues to gies decrease as the proton energy increases while the ionizing contribution increases. There is a clear difference in the behavior of these curves which would allow ionizing and nonionizing effect to be clearly differentiated when using displacement damage dose method as long as two or more Si ion irradiations were performed at different energies. An alternative approach to distinguish between ionizing and nonionizing damage mechanisms would be to correlate the damage caused by two different particles at a given energy such that the ratios of the ionizing and nonionzing stopping powers are clearly different, which is the case in this experiment. For 2 MeV protons and 22 MeV Si ions, the ratio of the stopping and , respectively, powers and NIEL values are separation. The damage correlation results a factor of shown in Fig. 4 confirm that the primary degrading mechanisms in crystalline GaAs solar cells is displacement damage effects.
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Using ionizing stopping powers to do the correlation would not result in a single damage curve. It is interesting to note that J. R. Srour et al. irradiated amorphous silicon solar cells with protons and the data suggested evidence of ionizing radiation effects strongly governed the solar cell performance, which they confirmed using X-ray irradiation data [16]. Using Si ions could be an alternative method for performing this experiment. It is worth noting that as the Si ions exit the active region (corm), the energy has decreased by responding to a depth of a factor of about 2. Referring to the SRIM-derived NIEL versus depth data of Fig. 3, it can be calculated that this energy loss deposition within the will result in roughly 30% additional active region. This may be considered to be significant in terms of the damage correlation calculations. However, considering that the correlation has been achieved over 3 orders of magnitude in fluence and that the overall uncertainty in the fluence values, is %, a shift in the values values, and hence of 30% would not significantly alter the present results. V. CONCLUSION The data presented here supports the theory that the displacement damage induced by proton and silicon irradiation can be modeled based on NIEL. The good agreement for correlating the 2 MeV proton and 22 MeV silicon ion data on these GaAs solar cells show the usefulness of this method for correlating damage caused by different particles. A further benefit from this radiation experiment is the distinguishability between ionizing and displacement damage effects, thereby verifying that displacement damage is the primary degradation mechanism in GaAs solar cells. The data correlation shows that a characteristic degradation curve can be generated for these devices for any ion based on the 2 MeV proton or 22 MeV silicon ion data without performing additional radiation experiments. The only requirement is that the irradiating particle have sufficient energy to produce a uniform damage track over the active region of the device. The noninform damage case can be quantified, but detailed analyzes of the damage track structure need be studied. REFERENCES [1] S. R. Messenger, E. A. Burke, M. A. Xapsos, G. P. Summers, R. J. Walters, I. Jun, and T. Jordan, “NIEL for heavy ions: An analytical approach,” IEEE Trans. Nucl. Sci., vol. 50, no. 3, pp. 1919–1923, Jun. 2003.
[2] I. Jun, M. A. Xapsos, S. R. Messenger, E. A. Burke, R. J. Walters, G. P. Summers, and T. Jordan, “Proton nonionizing energy loss (NIEL) for device applications,” IEEE Trans. Nucl. Sci., vol. 50, no. 3, pp. 1924–1928, Jun. 2003. [3] S. R. Messenger, G. P. Summers, E. A. Burke, R. J. Walters, and M. A. Xapsos, “Modeling solar cell degradation in space: A comparison of the NRL displacement damage dose and the JPL equivalent fluence approaches,” Progr. Photovolt.: Res. Appl., vol. 9, pp. 103–121, 2001. [4] G. P. Summers, E. A. Burke, P. Shapiro, S. R. Messenger, and R. J. Walters, “Damage correlations in semiconductors exposed to gamma, electron, and proton radiation,” IEEE Trans. Nucl. Sci., vol. 40, no. 3, pp. 1372–1379, Jun. 1993. [5] S. R. Messenger, E. A. Burke, G. P. Summers, M. A. Xapsos, R. J. Walters, E. M. Jackson, and B. D. Weaver, “Nonionizing energy loss (NIEL) for heavy ions,” IEEE Trans. Nucl. Sci., vol. 46, no. 5, pp. 1595–1602, Oct. 1999. and Errata to “Nonionizing Energy Loss (NIEL) for Heavy Ions”. [6] S. R. Messenger, E. A. Burke, G. P. Summers, and R. J. Walters, “Application of displacement damage analysis to low-energy protons on silicon devices,” IEEE Trans. Nucl. Sci., vol. 49, no. 6, pp. 2690–2694, Dec. 2002. [7] S. R. Messenger, E. A. Burke, R. J. Walters, J. H. Warner, and G. P. Summers, “Using SRIM to calculate the relative damage coefficients for solar cells,” Progr. Photovolt.: Res. Appl., vol. 13, pp. 115–123, 2005. [8] S. M. Khanna, H. C. Lui, P. H. Wilson, L. Li, and M. Buchanan, “High energy proton and alpha radiation effects on GaAs/AlGaAs quantum well infrared photodetectors,” IEEE Trans. Nucl. Sci., vol. 43, no. 6, pp. 3012–3018, Dec. 1996. [9] C. Carlone, M. Parenteau, and S. M. Khanna, “Gigaelectron-volt heavy ion irradiation of gallium arsenide,” JAP, vol. 83, pp. 5164–5170, 1998. [10] J. H. Warner, R. J. Walters, S. R. Messenger, G. P. Summers, S. Khanna, D. Estan, L. Erhardt, and A. Houdayer, “High-energy proton irradiation effects in GaAs devices,” IEEE Trans. Nucl. Sci., vol. 51, no. 6, pp. 2887–2895, Dec. 2004. [11] J. F. Ziegler, J. B. Biersack, and U. Littmark, The Stopping and Range of Ions in Solids. New York: Pergammon, 1985, vol. 1. [12] S. R. Messenger, E. A. Burke, G. P. Summers, and R. J. Walters, “Limits to the application of NIEL for damage correlation,” IEEE Trans. Nucl. Sci., vol. 51, no. 6, pp. 3201–3206, Dec. 2004. [13] S. R. Messenger, E. A. Burke, G. P. Summers, M. A. Xapsos, R. J. Walters, E. M. Jackson, and B. D. Weaver, “Nonionizing energy loss (NIEL) for heavy ions,” IEEE Trans. Nucl. Sci., vol. 46, no. 6, pp. 1595–1602, Dec. 1999. [14] S. R. Messenger, E. A. Burke, G. P. Summers, and R. J. Walters, “Application of displacement damage analysis to low-energy protons on silicon devices,” IEEE Trans. Nucl. Sci., vol. 49, no. 6, pp. 2690–2694, Dec. 2002. [15] S. R. Messenger, E. A. Burke, R. J. Walters, J. H. Warner, and G. P. Summers, “Using SRIM to calculate the relative damage coefficients for solar cells,” Progr. Photovolt: Res. Appl., vol. 13, pp. 115–123, 2005. [16] J. R. Srour, G. J. Vendura Jr., D. H. Lo, C. M. C. Toporow, M. Dooley, R. P. Nakano, and E. E. King, “Damage mechanisms in radiation-tolerant amorphous silicon solar cells,” IEEE Trans. Nucl. Sci., vol. 45, no. 6, pp. 2624–2631, Dec. 1998.
