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Proton Energy Dependence of the Light Output in Gallium Nitride Light-Emitting Diodes Shyam M. Khanna, Diego Estan, Lorne S. Erhardt, Alain Houdayer, Cosmo Carlone, Anca Ionascut-Nedelcescu, Scott R. Messenger, Robert J. Walters, Geoffrey P. Summers, Jeffrey H. Warner, and Insoo Jun
Abstract—Gallium nitride (GaN)-based blue-emitting diodes (CREE Model C430-DH85) were irradiated at room temperature with protons in the energy range 2 to 115 MeV at fluences varying from 1 1011 to 1 1015 cm 2 . Light output degradation curves were obtained for each energy and the damage constant ( ) associated with these curves was determined according to the theory of Rose and Barnes. For proton energies less than varies inversely with the proton energy ( ). At 10 MeV, higher energies, is consistently above the 1 relationship. The change in nature of the energy dependence is attributed to nuclear interactions. Nonionizing energy loss calculations for the case of protons on GaN are presented. Good agreement between theory and experiment is obtained. Index Terms—Blue LEDs, energy dependence, gallium nitride, light emission degradation, non-ionizing energy loss (NIEL), optoelectronics, proton, quantum-well light-emitting diodes, radiation damage.
I. INTRODUCTION
G
ALLIUM NITRIDE (GaN) is a direct, wide-gap (3.36 eV) semiconductor with high potential for use in high-power high-frequency devices capable of operating at relatively high temperatures. Its use will also be important for electrooptical devices in the visible spectral range extending to UV. In addition to the above properties, GaN has demonstrated a high degree of radiation hardness. Khanna et al. [1] presented the first explicit evidence of high radiation tolerance of GaN by investigating the radiation damage caused by 2 MeV protons on gallium nitride films through low temperature photoluminescence spectroscopy measurements. Gaudreau et al. [2]
Manuscript received March 25, 2004; revised May 19, 2004. This work was supported in part by the U.S. Office of Naval Research. S. M. Khanna, D. Estan, and L. S. Erhardt are with the Defense Research Establishment Ottawa, Ottawa, ON K1A 0Z4, Canada (e-mail:
[email protected]). A. Houdayer is with the Physics Department, University of Montreal, Montreal, Canada (e-mail:
[email protected]). C. Carlone and A. Ionascut-Nedelcescu are with the Physics Department, University of Sherbrooke, Sherbrooke, Canada (e-mail:
[email protected]). S. R. Messenger is with SFA Inc., Largo, MD 20774 USA (e-mail:
[email protected]). R. J. Walters is with the U.S. Naval Research Laboratory, Washington, DC 20375 USA (e-mail:
[email protected]). G. P. Summers is with the U.S. Naval Research Laboratory, Washington, DC 20375 USA, and also with the University of Maryland Baltimore County, Baltimore, MD 21250 USA (e-mail:
[email protected]). J. H. Warner is with the U.S. Naval Research Laboratory, Washington, DC 20375 USA (e-mail:
[email protected]). I. Jun is with the Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109 USA (e-mail:
[email protected]). Digital Object Identifier 10.1109/TNS.2004.835097
measured the transport properties of a two dimensional gas formed at the AlGaN/GaN interface as well as the degradation of the light output of GaN-based light-emitting diodes (LEDs) [3]. These two works confirmed the relatively high radiation tolerance of GaN-based devices. Ionascut-Nedelcescu et al. (the [4] measured the displacement threshold energy minimum energy required to transfer a lattice atom from its equilibrium position to an interstitial one) of Ga (19.4 eV) in GaN. Although there are other factors which influence the damage characteristics of a particular material or device, in general, the larger the value of , the more resistant to radiation is the material. For example, was measured for C in SiC by B. Lehmann et al. [5] to be 21.8 eV. Thus GaN compares and subsequent radiation hardness. with SiC with respect to All of the previous works on GaN have been performed at a proton energy of 2 MeV. However, the space environment contains a wide range of high energy particles including electrons, protons and heavy ions in the keV to GeV energy range, whose permanent damage on electronic components needs to be quantified. The present paper concentrates on the permanent damage due to proton irradiation at energies above 2 MeV. To assess the permanent damage created by these particles, nonionizing energy loss (NIEL) calculations have been invoked in the past for several technologies with some success. The energy dependence of the damage constants is often compared with the energy dependence of NIEL. The agreement or disagreement of such an energy dependence for a given physical observable can greatly aid in the understanding of damage mechanisms in a given device. In the present work, we follow this approach and measure the degradation of the light output of a GaN-based blue light emitting diodes as a function of proton energy. In this paper, we present experimental results for the proton energy dependence of light output degradation of GaN quantum-well light-emitting diodes (QW LEDs) over the energy range of 2–115 MeV. It is shown that experimental light output degradation data at different energies can be correlated and be reduced to a single characteristic light output degradation curve for these GaN-based QW LEDs. Calculated values of the nonionizing energy loss in GaN are also presented and experimental results for the light output degradation from these QW LEDs are compared to proton energy dependence of the calculated NIEL. It is shown that the experimental light output degradation curve agrees well with the calculated NIEL curve MeV. At higher energies, the experimental data up to departs from Coulombic NIEL curve and appears to follow the total NIEL curve. The significance of these results is discussed.
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Fig. 1. Schematic diagram of the gallium nitride-based light emitting diode.
These are the first known results on the energy dependence of radiation damage in GaN. Finally, proton energy dependence and radiation hardness of the light output of GaN and GaAs QW LEDs are compared. II. EXPERIMENTAL A. Samples The GaN LEDs (Super-Blue LED C430-DH85) used in this experiment were manufactured by CREE Research Inc. (4600 Silicon Drive, NC 27703 USA) . A schematic diagram of the device structure is shown in Fig. 1. According to the information supplied by the manufacturer, the device was grown on a 250 m thick substrate with the active layer located 25 m below the m square. Actop surface. The device area is 200 m cording to manufacturer’s specifications, this results in a current density of about 2.5 A/cm at a measuring dc current of 1 mA. The devices were mounted on TO-18 headers without the encapsulating lens and were wire-bound to the posts. These LEDs are designed for use in high ambient light conditions with a typical output power of 55 mW and a 430 nm peak wavelength at a current of 20 mA (manufacturer’s specifications). In contrast, the operating current in the present work was set at a low value of 0.3 mA to minimize any possible injection annealing effects. All of these devices were prepared from the central part of the same processed GaN wafer. Devices from this same wafer have been investigated extensively in a separate study published previously [3]. B. Irradiation Procedure The University of Montreal 6 MV Tandem Van de Graaff accelerator was used to perform proton irradiation in vacuum for energies up to 10 MeV. Proton irradiations at 50 and 115 MeV were done in air using the cyclotron at TRIUMF in Vancouver, Canada. All irradiations at the University of Montreal and at TRIUMF were done at room temperature. The method used to irradiate the diodes at the University of Montreal and at TRIUMF have been described previously [6], [7]. We include key details of the irradiation procedure here for completeness.
For the present paper, the proton beam at the University of mm) for irMontreal was slightly defocused ( radiation and rastered over a region slightly larger than a tantalum collimator having an opening of 1.2 1.2 cm, which was located just in front of the target. The LED was mounted in vacuum on a target holder located directly in the center of the %. irradiation chamber. The dosimetry is estimated to be For 50 and 115 MeV irradiations at TRIUMF, the beam had a Gaussian profile with a FWHM of about 11 mm. At the Unito versity of Montreal, beam fluxes ranged from p/cm /s (beam currents – nA) over a 1 cm area. At TRIUMF, beam fluxes ranged from to p/cm /s (beam currents – nA) over a 1 cm area. With such low values of beam currents, there was insignificant sample heating and annealing due to heat or current injection. All irradiations at each proton energy were done on the same device to minimize errors due to device-to-device variations. For this purpose, the devices were irradiated incrementally at a given energy from an unirradiated state up to the highest fluence with electrical and optical measurements being performed at each fluence step. C. Device Characterization Details about the device characterization have been published previously [6], [7]. We provide here pertinent details for completeness of the present paper. For all measurements described in this paper, both the LED light emission and electrical characteristics were measured under computer control at each irradiation step immediately (within 30 to 60 s) after cessation of irradiation. The LED light emission intensity was measured through a silicon-based CCD camera. For device measurements for proton energies up to 10 MeV conducted at the University of Montreal, the measurements were taken without removing the sample from the radiation chamber and without any adjustment of the light emission measuring instrumentation. For measurements at 50 and 115 MeV in air at TRIUMF, the samples were removed from the radiation chamber after each irradiation step and mounted reproducibly in a measurement fixture. In this case also, the device characterization was completed within 30–60 seconds at each radiation step after cessation of the proton beam. The LED operating current for light emission was set at 0.3 mA for all measurements. III. RESULTS A. Raw Data at different energies from The normalized light output these LEDs taken at a dc current of 0.3 mA versus the fluence is shown in Fig. 2. Here is the light output after irradiation at a given fluence value and is the value for the unirradiated device. The nominal incident proton energies investigated in this work are 2, 3, 5, 10, 50, and 115 MeV. As mentioned earlier, since we have used low proton fluxes and have completed – seconds following irradiaour measurements within tion, we expect insignificant sample annealing due to sample heating and/or current injection. The sample-to-sample variaat a fixed fluence and tion in the normalized light output
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Fig. 2. GaN LED light output as a function of fluence normalized to the preirradiation value. There is one degradation curve for each energy studied in this paper.
proton energy was observed to range from % for low degra% for high degradation values. dation values to as high as As expected, for each energy, the normalized light output decreases as the fluence increases because the irradiation introduces defects which may act as either radiative or nonradiative recombination centers. However as the incident proton energy is increased, the degradation curves are noticed to shift to the right in proton fluence, thereby indicating that higher energies do less damage for the same proton fluence. This is in qualitative agreement with NIEL calculations [8]–[10]. The data corresponding to LED degradation due to particle irradiation at any given particle energy are fitted using the equation developed by Rose and Barnes [11]
TABLE I DAMAGE CONSTANTS FOR GaN LED PROTON IRRADIATION CALCULATED WITH (1) USING n = 0:32. THE FIRST COLUMN GIVES THE NOMINAL INCIDENT ENERGY, WHILE THE SECOND GIVES THE ENERGY AT THE ACTIVE LAYER. FOR 3 MeV, THE DATA AND THE FIT ARE SHOWN IN FIG. 3. THE THIRD COLUMN GIVES THE A VALUES OBTAINED FROM THE FIT. IN THE FOURTH COLUMN THEY ARE NORMALIZED TO THE 10 MeV VALUE
(1) where is the LED light output measured after irradiation, is the pre-irradiation value, is a fitting parameter defined as the damage constant, is the particle fluence in units of particles/cm , and is another fitting parameter. Fig. 3 shows degradation data taken at 3 MeV with the corresponding fit using (1). The calculated values of and for the 3 MeV data set are cm and 0.32, respectively. According to the Rose-Barnes theory, this value of implies that the recombination mechanism is predominantly space charge limited. This value of was also obtained from the fits of the other data sets. The values of , however, did change significantly with proton energy. We, therefore, can use the variability in to produce damage constants relative to a given reference energy (usually 10 MeV). This analysis procedure is similar to that used in [6] for the proton radiation damage energy dependence of GaAs QW LED light output data. Table I gives the fitted values of for different proton energies using a constant parameter for all energies used in our GaN QW LED value of measurements. To show that all the data can be fitted by (1), a characteristic damage curve for light output data at all energies
can be drawn, as shown in Fig. 4. In this figure, the normalized light output data at a given energy are plotted as a function of effective 10 MeV proton fluence. That is, for a given proton energy, the effective 10-MeV proton fluence for other proton energies is obtained by multiplying the experimental value of (10 MeV) proton fluence at that step with the factor for that energy as given in Table I. Fig. 4 shows that the energy dependence of the GaN QW LED light output damage constants is well correlated at all energies and the degradation of the light output can be represented well by (1). Furthermore, the shape of the degradation curves is the same regardless of proton energy. This was also observed for GaAs QW devices [6] and leads us to believe that no additional degradation mechanism is operative at very high radiation fluences. B. NIEL Calculations Proton NIEL calculations on GaN have been performed and are shown in Fig. 5. Refer to [8]–[10] for details of the calculation. Fig. 5 shows NIEL calculation results for two cases: 1) assuming that only Coulombic interactions occur, and
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Fig. 3. The GaN light output data from 3-MeV protons and the fit according to (1).
Fig. 4. GaN light output degradation data from Fig. 2 plotted as a function of effective 10-MeV proton fluence. The data at each energy were transformed into effective 10 MeV proton fluence data using the relative damage coefficients as discussed later.
2) where both Coulombic and nuclear interactions are included. It is shown in Fig. 5 that nuclear interactions start to dominate the NIEL calculation at proton energies above 10 MeV. The variation of damage constant with the proton energy at the active region is shown in Fig. 6. Normalized NIEL values (to 10 MeV) are also plotted in this figure in order to make comparisons with the experimental results. In Fig. 6, the parameters have been normalized to the value at 10 MeV such that the value is equal to unity at 10 MeV. This new parameter is called the “relative damage coefficient” (RDC). One must be careful here to use the proton energy which hits the active region of the device. Since the active regions of these devices are located 25 m below the surface, significant slowing down of the protons can occur. Proton track simulations were, therefore, performed on these GaN structures using the well known Monte Carlo code SRIM [12]. The results showed that, after traversing the window
layer having 25 m of GaN, the 10, 5, 3, and 2 MeV protons were reduced to 9.6, 4.2, 1.8, and 0.05 MeV, respectively. Returning to Fig. 6, we notice that the 1.8, 4.2, and 9.6 MeV points dependence. The lowest fall on a straight line following the energy data point (0.05 MeV) on Fig. 6 is shown to lie to the line, thereby showing less damage than expected left of the based on the physical models. We attribute this observation to the fact that protons are slowed down significantly in these devices by the time they reach the active region. This results in a nonuniform spectrum of energies being produced in the active region, which has been addressed on silicon devices using NIEL formalisms [13]. The NIEL curves presented in Fig. 6 would apply for monoenergetic protons incident on the active region of the device. The formalism presented in [13] uses an actual SRIM damage profile for a particular device structure to modify the NIEL calculation to account for such geometry considera-
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Fig. 5. GaN proton NIEL calculations as a function of proton energy. The dashed line represents the NIEL calculation where only Coulombic interactions are used. The solid line includes nuclear interactions as well. See [8]–[10] for details of the NIEL calculations.
Fig. 6. The RDCs, which are the A parameters in Table I, all divided by the A value at 10 MeV, as a function of energy.
tions. The 50 and 115 MeV data presented in Fig. 6 is noticed to curve. This apparent deviation of the RDCs lie above the at higher energies has previously been observed for some physical observables in GaAs-based devices and can be explained by NIEL calculations [6]. The relative increase in the energy dependence of the damage constants at high energies seems to be attributed to the effect of nuclear interactions on the damage mechanism for a given parameter. The basic premise in [6] was that the RDCs from those parameters that were most strongly affected by changes in the depletion region characteristics, like the recombination or generation current, appeared to follow the total NIEL while the RDCs for those parameters most strongly affected by changes in the bulk material characteristics, like minority carrier lifetimes, tracked just the Coulombic portion of the NIEL. The similarity between the GaN data (Fig. 6) and
that of GaAs data in [6], suggest that nuclear interactions are also affecting the light output RDCs occurring in GaN devices at energies above 10 MeV. IV. DISCUSSION By comparing the collapsed degradation data (Fig. 4) with the similar data for GaAs diodes ([6, Fig. 4 ]), it is deduced that GaN technology is more radiation hard than that based on GaAs. These results are summarized in Fig. 7 for 10 MeV proton irradiations. For QW GaN LED technology, the normalized light output is reduced to 10% of its initial value at a 10 MeV proton p/cm . For QW GaAs LED technology, fluence of about p/cm . Thus, the associated 10 MeV fluence is about QW GaN based diodes are about two orders of magnitude harder
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Fig. 7. A comparison of LED light output radiation response from 10 MeV protons of GaAs and GaN technologies.
than QW GaAs diodes. This difference is also observed with the photoluminescence data on thin films [1] and also on electrical measurements on 2-D gas structures [2]. Fig. 7 also shows that QW technology is superior to bulk GaAs (homojunction) technology, with the associated damage being several additional orders of magnitude higher. Rose and Barnes presented a theory of the fluence dependence of the light output [11]. According to this theory, the and , value of the constant , which varies between suggests which charge current mechanism takes place in the , the current is predomdevice. At the lower limit inantly due to space charge recombination, and at the other , the current is dominated by diffusion. In the limit GaN diodes studied here, the electrons in the two-dimensional N interface tunnel (2-D) electron gas formed at the n-Al Ga N active layer and recombine with holes across the In Ga N interface. The in the 2-D hole gas formed in the p-Al Ga nanometer thickness of the active layer suggests recombination being quantum in nature. We are not aware of the value which would be predicted by quantum theory, but a space charge limitation is possible. Hence , the observed value of near 0.3 is reasonable. On the other hand, the temperature independence of the slope of the current–voltage curves [3] excludes temperature dependent diffusion and, consistently, we do not observe close to 0.6. It is noted that the light output degradation from GaN QW LEDs follows the Coulombic NIEL curve up to about 10 MeV. At higher energies, it deviates from this curve. Nuclear contributions make the light output data closer to the total NIEL values. These results are thus similar to those observed in the light output data from GaAs QW LEDs [6] and GaAs bulk LEDs [14]. GaN compares in radiation hardness with SiC and is harder than other semiconductors such as GaAs [4]. Radiation-hard materials withstand higher dose by definition. As harder materials are developed (e.g., diamond technology) even higher doses will be supportable. However, the concentration of de-
fects due to nuclear reactions will increase. A comparison of the measured RDCs and the NIEL calculations show that for GaN, nuclear interactions contribute to light output degradation at higher energies. V. SUMMARY GaN-based LEDs have been irradiated with protons in the energy range 2–115 MeV. The damage constant associated with the degradation of the light output has been measured. For energies less than 10 MeV, the degradation depends inversely with the energy thereby following NIEL predictions. The deviation at higher energies is attributed to contributions due to nuclear interactions. REFERENCES [1] S. M. Khanna, J. Webb, H. Tang, A. J. Houdayer, and C. Carlone, “2 MeV proton radiation damage studies of gallium nitride films through low temperature photoluminescence spectroscopy measurements,” IEEE Trans. Nucl. Sci., vol. 47, pp. 2322–2328, 2000. [2] F. Gaudreau, P. Fournier, C. Carlone, S. M. Khanna, H. Tang, J. Webb, and A. Houdayer, “Transport properties of proton irradiated gallium nitride based two-dimensional electron gas system,” IEEE Trans. Nucl. Sci., vol. 49, pp. 2702–2707, 2002. [3] F. Gaudreau, C. Carlone, A. Houdayer, and S. M. Khanna, “Spectral properties of proton irradiated gallium nitride blue diodes,” IEEE Trans. Nucl. Sci., vol. 48, pp. 1778–1784, 2001. [4] A. Ionascut-Nedelcescu, C. Carlone, A. Houdayer, J. H. von Bardeleben, J. L. Cantin, and S. Raymond, “Radiation hardness of gallium nitride,” IEEE Trans. Nucl. Sci., vol. 49, pp. 2733–2738, 2002. [5] B. Lehmann, M. A. Brière, D. Brauning, and A. L. Barry, “Threshold energy in GaAs determined by electrical and optical investigations,” in ESA SP-313, 1991, pp. 287–292. [6] R. J. Walters, S. R. Messenger, G. P. Summers, E. A. Burke, S. M. Khanna, D. Estan, L. S. Erhardt, H. C. Liu, M. Gao, M. Buchanan, A. J. SpringThorpe, A. Houdayer, and C. Carlone, “Correlation of proton radiation damage in InGaAs-GaAs quantum-well light-emitting diodes,” IEEE Trans. Nucl. Sci., vol. 48, pp. 1773–1778, 2001. [7] S. M. Khanna, D. Estan, H. C. Liu, M. Gao, M. Buchanan, and A. J. SpringThorpe, “1–15 MeV proton and alpha particle radiation effects on GaAs quantum well light emitting diodes,” IEEE Trans. Nucl. Sci., vol. 47, pp. 2508–2514, 2000.
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[8] 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 radiations,” IEEE Trans. Nucl. Sci., vol. 40, pp. 1372–1379, 1993. [9] S. R. Messenger, E. A. Burke, M. A. Xapsos, G. P. Summers, R. J. Walters, I. Jun, and T. M. Jordan, “NIEL for heavy ions: An analytical approach,” IEEE Trans. Nucl. Sci., vol. 50, pp. 1919–1923, 2003. [10] 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, pp. 1924–1928, 2003. [11] B. H. Rose and C. E. Barnes, “Proton damage effects on light emitting diodes,” J. Appl. Phys., vol. 53, pp. 1772–1780, 1982.
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[12] J. F. Ziegler, J. P. Biersack, and U. Littmart, The Stopping and Range of Ions in Solids. New York: Pergammon, 1985, vol. 1. (SRIM is freely available at http://www.srim.org. The current SRIM version is 2003.20). [13] S. R. Messenger, E. A. Burke, G. P. Summers, and R. J. Walters, “Application of displacement damage dose analysis to low-energy protons on silicon devices,” IEEE Trans. Nucl. Sci., vol. 49, pp. 2690–2694, 2002. [14] J. H. Warner, R. J. Walters, S. R. Messenger, G. P. Summers, S. M. Khanna, D. Estan, L. S. Erhardt, and A. Houdayer, “High energy proton radiation effects in GaAs devices,” IEEE Trans. Nucl. Sci., vol. 51, pp. 2887–2895, Oct. 2004.
