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GaN LEDs
Journal of Crystal Growth ∎ (∎∎∎∎) ∎∎∎–∎∎∎
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Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Low-frequency noise in diagnostics of power blue InGaN/GaN LEDs A.E. Chernyakov a, M.E. Levinshtein b, N.A. Talnishnikh b, E.I. Shabunina b,n, N.M. Shmidt b a b
Science and Technology Center of Microelectronics and Submicrometer Heterostructures, RAS, St. Petersburg 194021, Russia Ioffe Physical Technical Institute, Polytekhnicheskaya 26, 194021 St. Petersburg, Russia
art ic l e i nf o
a b s t r a c t
Keywords: A1. Nanostructures B1. Nitrides B2. Semiconducting III–V materials B3. Light emitting diodes
Low-frequency noise measurements, combined with the conventional techniques for the study of InGaN/ GaN-based LEDs, make it possible to separate the contribution of conductive paths associated with the extended defect system (EDS) and point defects (PD) to non-radiative recombination processes. These measurements also can reveal physical mechanisms leading to the unpredictable failure of LEDs, such as non-uniform current distribution, local overheating, and presence of local InGaN regions with a reduced band gap width Eg. & 2014 Elsevier B.V. All rights reserved.
1. Introduction High-power blue InGaN/GaN light emitting diodes (LEDs) form the basis of the solid-state lighting techniques. In spite of the remarkable progress in the technology of light-emitting devices, the physical mechanisms responsible for the non-radiative recombination and the unpredictable failure of some of these diodes after a comparatively short operation time have not been elucidated so far. It is noteworthy that the external quantum efficiencies of these diodes and LEDs with a long service life are nearly the same. Preliminary studies have shown that these mechanisms seem to be related to numerous forms of the nano-structural arrangement (NA) in the LEDs and the nano-scale phase separation (NPS) in InGaN. Both NA and NPS strongly depend on growth conditions. The nature of NA is determined by coalesced crystallites appearing in the course of 3D or 2D growth. The NA results in that the extended defect system (EDS) piercing the LED active region is formed. The EDS includes, in particular, threading dislocations, dislocation clusters, and dislocation boundaries. It seems that low-frequency noise (LFN) measurements could be very useful for obtaining additional information on the properties of EDS. The LFN technique is known to be an effective tool providing comprehensive information about not only the generation–recombination (G–R) noise caused by point defects (PD), but also charge fluctuations of surface states, presence of several barriers with close energy levels, nano-material disorder or local field fluctuations [1]. Moreover, the behavior of the defects can be examined in a wide range of current densities.
n
Corresponding author. Tel.: þ 7 812 2927193. E-mail address: [email protected] (E.I. Shabunina).
In this paper, we present our results demonstrating the efficiency of the LFN technique for analysis of the physical mechanisms responsible for the non-radiative recombination and the unpredictable failure of LEDs.
2. Experimental All the results were obtained on InGaN/GaN LEDs with external quantum efficiency (η) of about 45–50% at wavelengths of 450– 460 nm. The spectral voltage—[SV(j)] and current—[SI(j)] noise density dependences were measured in the frequency range 1 Hz–10 kHz and at current densities of 10 4–50 A/cm2. Along with the measurements of the η(j) dependences, all LEDs were classified according to their leakage currents (LC) (Fig. 1). It has been shown recently that the LC at forward biases Uo2 V is a very important characteristic of the electrical properties of EDS. The lower the LC, the smaller the contribution of extended defects and the lower the extent of nano-structural disorder [2].
3. Results and discussion Fig. 2 shows the dependences of the spectral current-noise densities on the forward current density for LEDs with the LCs in Fig. 1. It is seen that the larger LC, the higher the noise level, especially in at current densities in the range j r10 2 A/cm2, in which the non-radiative recombination prevails. It is known that just EDS makes the main contribution to the LC in this range [2,3]. Hence, we can conclude that the EDS is also responsible for the non-radiative recombination at small current densities. A highly characteristic feature of the SI(j) dependences of the diodes with high noise level (and high LC) is the part of SI(j) curves
0022-0248/$ - see front matter & 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jcrysgro.2013.11.097
Please cite this article as: A.E. Chernyakov, et al., Journal of Crystal Growth (2014), http://dx.doi.org/10.1016/j.jcrysgro.2013.11.097i
2
A.E. Chernyakov et al. / Journal of Crystal Growth ∎ (∎∎∎∎) ∎∎∎–∎∎∎
Fig. 1. Reverse current–voltage characteristics of LEDs with drastically different LC values.
Fig. 2. Dependences of spectral current-noise densities on the forward current density for LEDs with the LCs in Fig. 1. T ¼ 298 K. The frequency of analysis is 1.22 Hz. Curves 1–3 correspond to curves 1–3 in Fig. 1. The vertical lines show the onset of light emission and current density values jmax corresponding to the maximum external quantum efficiencies η.
in which the noise decreases with j increasing within the range 10 2 A/cm2 rjrjmax (curves 2 and 3 in Fig. 2). (Spectral noise densities in this part of the curves follow the law SI 1/j, rather than SI 1/j2 earlier reported for SiC diodes [4]). It is noteworthy that these decreasing parts of the SI(j) curves are observed at j410 2 А/cm2, when the radiative recombination starts to prevail [5]. In InGaN/GaN LEDs with the minimum LC (curve 1 in Fig. 1), SI depends on j rather weakly at j 410 2 А/cm2. This circumstance further confirms that EDS makes an important contribution to the noise and non-radiative recombination, but its relative contribution decreases with increasing current density and non-radiative recombination through PD takes place at 10 2–10 A/cm2. Fig. 3 shows the SI(j) dependences of LEDs with the minimum LC (curve 1 in Fig. 1) at 77 K and 300 K. It can be seen in Fig. 3 that lowering the temperature results in that the noise level decreases at small current densities (j o10 2 A/cm2) and increases at comparatively high jZ 10 2 A/cm2. The decrease in the noise level at small current densities is very typical of excess tunnel currents flowing through shunts piercing the LED active region [6]. An increase in the noise level at j 410 2 A/cm2, where the radiative recombination prevails, has been observed for quantum well InGaAs/GaAs structures [6]. However, it is necessary to note that this effect is much stronger in the InGaN/GaN structures under study. It can be considered that this is due to the strong fluctuations in the InGaN/GaN compound composition, always observed
Fig. 3. Current-density dependences of the spectral current-noise densities for LEDs with LC in Fig. 1 (curve 1) at T¼ 77 and 298 K. The frequency of analysis is 1.22 Hz.
Fig. 4. Current-density dependences of the noise level for two types of LEDs with small LC but very different forward I–U characteristics shown in the insert.
in electroluminescence spectra at temperatures below 200 K [7]. The results we obtained suggest that this phenomenon is related to the Fowler–Nordheim-assisted lateral tunneling of carriers discussed in Ref [6]. A small part of the diodes with low LC shows a very high forward current at low forward biases (inset in Fig. 4, curve 1). It can be seen in inset in Fig. 4 that the current density at a bias U¼1 V in diodes of this kind is three orders of amplitude higher than that in the conventional diodes (curve 2 in the inset). According to the data presented in Ref. [8], such a high forward current is due to the strong fluctuations of the In content in InGaN and to the presence of small local regions with a reduced band gap width Eg. The inhomogeneous nature of the flowing current is confirmed by the SI(j) dependence (curve 1 in Fig. 4). As seen in Fig. 4, there is a portion of the SI(j) dependence (curve 1) in which SI j4 at a medium current density j 10 2 A/cm2. The faster increase in the noise density than that according to the law SI j2.5 shows that new defects are generated by the flowing current [9] and there are local current filaments with a very high current density. This fact earnestly confirms that a highly inhomogeneous current flows in blue InGaN/GaN LEDs. It has been shown previously [10] that diodes of this kind may suffer an unpredictable failure after 10–1000 h of aging (at a current density of 35 A/cm2 and p–n junction temperature of 100 1C.) This failure is due to the re-distribution of In between the
Please cite this article as: A.E. Chernyakov, et al., Journal of Crystal Growth (2014), http://dx.doi.org/10.1016/j.jcrysgro.2013.11.097i
A.E. Chernyakov et al. / Journal of Crystal Growth ∎ (∎∎∎∎) ∎∎∎–∎∎∎
Fig. 5. Dependences of spectral voltage-noise densities Sv on the forward current density for LEDs. Curves 1 and 2 correspond to curves 1 and 2 in Fig. 4.
local regions of the InGaN compound with non-equilibrium In concentration. Additional evidence in favor of the inhomogeneous current flow in InGaN/GaN LEDs with rather strongly inhomogeneous InGaN compound can be obtained from noise spectra of voltage fluctuations (Fig. 5). It is known that the spectral noise density of voltage fluctuations SV at a homogeneous current flow is inversely proportional to j: SV 1/j [11]. It can be seen in Fig. 5 that, for diodes with a high level of the compound inhomogeneity (curve 1), the departure from the 1/j law is extremely strong. Moreover, in some portion of the SV(j) dependence, SV grows with increasing j. 4. Conclusion It was demonstrated that, together with point defects, the extended defect system (EDS) essentially contributes to nonradiative recombination processes in InGaN/GaN LEDs at
3
comparatively small current densities. The relative contribution of EDS decreases with increasing current density. It is shown that the low-frequency nose technique can be effectively used to study reasons for the unpredictable failure of InGaN/GaN LEDs. This technique, in particular, makes it possible to find diodes with an extremely inhomogeneous current flow. The appearance of portions in the SI(I) dependences, in which SI I4 at a medium current density j 10 2 A/cm2, reveals a very inhomogeneous current flow caused by the inhomogeneity of the In distribution in InGaN compound and the by the presence of small local regions of InGaN with a reduced band gap width Eg. The very inhomogeneous current flow, which results in overheating of local areas, causes migration of In ions between the local regions of an InGaN compound with the non-equilibrium In concentration [12]. The ion migration and the inhomogeneous defect generation are very important reasons for the unpredictable failures of blue InGaN/ GaN LEDs.
References [1] N.V. Dyakonova et al., Physics and Techniques of Semiconductors. 25(12) (1991), p.2065. [2] A.V. Kamanin, et al., Phys. Status Solidi C 3 (2006) 2129. [3] M. Albrecht, T. Schulz, J. Weyher, B. Lucznik, P. Prystawko, In: Proceedings of the Eighth International Workshop BIAMS. 1 (2006), p.28. [4] S.L. Rumyantsev, M.S. Shur, M.E. Levinshtein, A.D. Dmitriev, D. Veksler, J. W. Palmour, M.K. Das, B.A. Hull., J. Appl. Phys 100 (2006) 064505. [5] S. Sawyer, S.L. Rumyantsev, M.S. Shur, N. Pala, Yu. Bilenko, J.P. Zhang, X. Hu, A. Lunev, J. Deng, R. Gaska., J. Appl. Phys 100 (2006) 034504. [6] Lech Dobrzanski, J. Appl. Phys. 96 (2004) 4135. [7] K. Kazlauskas, M.A. Khan, M.S Shur, Appl. Phys. Lett. 83 (2003) 3722. [8] E.F. Schubert, Light-emitting Diodes, second ed., Cambridge University Press, UK, 2006. [9] G.P. Zhigal’skii, Phys.—Usp 173 (2003) 465 (in Russian). [10] A.E. Chernyakov, M.E. Levinshtein, P.V. Petrov, N.M. Shmidt, E.I. Shabunina, A.L. Zakheim., Microelectron. Reliab. 52 (9–10) (2012) 2180–2183. [11] T.G.M. Kleinpenning, Physica 98B (1980) 293. [12] S.K. Jeon, J.G. Lee, E.H. Park, J.H. Jang, J.G. Lim, S.K. Kim, J.S. Park., Appl. Phys. Lett. 94 (2009) 131106.
Please cite this article as: A.E. Chernyakov, et al., Journal of Crystal Growth (2014), http://dx.doi.org/10.1016/j.jcrysgro.2013.11.097i
Contents lists available at ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Low-frequency noise in diagnostics of power blue InGaN/GaN LEDs A.E. Chernyakov a, M.E. Levinshtein b, N.A. Talnishnikh b, E.I. Shabunina b,n, N.M. Shmidt b a b
Science and Technology Center of Microelectronics and Submicrometer Heterostructures, RAS, St. Petersburg 194021, Russia Ioffe Physical Technical Institute, Polytekhnicheskaya 26, 194021 St. Petersburg, Russia
art ic l e i nf o
a b s t r a c t
Keywords: A1. Nanostructures B1. Nitrides B2. Semiconducting III–V materials B3. Light emitting diodes
Low-frequency noise measurements, combined with the conventional techniques for the study of InGaN/ GaN-based LEDs, make it possible to separate the contribution of conductive paths associated with the extended defect system (EDS) and point defects (PD) to non-radiative recombination processes. These measurements also can reveal physical mechanisms leading to the unpredictable failure of LEDs, such as non-uniform current distribution, local overheating, and presence of local InGaN regions with a reduced band gap width Eg. & 2014 Elsevier B.V. All rights reserved.
1. Introduction High-power blue InGaN/GaN light emitting diodes (LEDs) form the basis of the solid-state lighting techniques. In spite of the remarkable progress in the technology of light-emitting devices, the physical mechanisms responsible for the non-radiative recombination and the unpredictable failure of some of these diodes after a comparatively short operation time have not been elucidated so far. It is noteworthy that the external quantum efficiencies of these diodes and LEDs with a long service life are nearly the same. Preliminary studies have shown that these mechanisms seem to be related to numerous forms of the nano-structural arrangement (NA) in the LEDs and the nano-scale phase separation (NPS) in InGaN. Both NA and NPS strongly depend on growth conditions. The nature of NA is determined by coalesced crystallites appearing in the course of 3D or 2D growth. The NA results in that the extended defect system (EDS) piercing the LED active region is formed. The EDS includes, in particular, threading dislocations, dislocation clusters, and dislocation boundaries. It seems that low-frequency noise (LFN) measurements could be very useful for obtaining additional information on the properties of EDS. The LFN technique is known to be an effective tool providing comprehensive information about not only the generation–recombination (G–R) noise caused by point defects (PD), but also charge fluctuations of surface states, presence of several barriers with close energy levels, nano-material disorder or local field fluctuations [1]. Moreover, the behavior of the defects can be examined in a wide range of current densities.
n
Corresponding author. Tel.: þ 7 812 2927193. E-mail address: [email protected] (E.I. Shabunina).
In this paper, we present our results demonstrating the efficiency of the LFN technique for analysis of the physical mechanisms responsible for the non-radiative recombination and the unpredictable failure of LEDs.
2. Experimental All the results were obtained on InGaN/GaN LEDs with external quantum efficiency (η) of about 45–50% at wavelengths of 450– 460 nm. The spectral voltage—[SV(j)] and current—[SI(j)] noise density dependences were measured in the frequency range 1 Hz–10 kHz and at current densities of 10 4–50 A/cm2. Along with the measurements of the η(j) dependences, all LEDs were classified according to their leakage currents (LC) (Fig. 1). It has been shown recently that the LC at forward biases Uo2 V is a very important characteristic of the electrical properties of EDS. The lower the LC, the smaller the contribution of extended defects and the lower the extent of nano-structural disorder [2].
3. Results and discussion Fig. 2 shows the dependences of the spectral current-noise densities on the forward current density for LEDs with the LCs in Fig. 1. It is seen that the larger LC, the higher the noise level, especially in at current densities in the range j r10 2 A/cm2, in which the non-radiative recombination prevails. It is known that just EDS makes the main contribution to the LC in this range [2,3]. Hence, we can conclude that the EDS is also responsible for the non-radiative recombination at small current densities. A highly characteristic feature of the SI(j) dependences of the diodes with high noise level (and high LC) is the part of SI(j) curves
0022-0248/$ - see front matter & 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jcrysgro.2013.11.097
Please cite this article as: A.E. Chernyakov, et al., Journal of Crystal Growth (2014), http://dx.doi.org/10.1016/j.jcrysgro.2013.11.097i
2
A.E. Chernyakov et al. / Journal of Crystal Growth ∎ (∎∎∎∎) ∎∎∎–∎∎∎
Fig. 1. Reverse current–voltage characteristics of LEDs with drastically different LC values.
Fig. 2. Dependences of spectral current-noise densities on the forward current density for LEDs with the LCs in Fig. 1. T ¼ 298 K. The frequency of analysis is 1.22 Hz. Curves 1–3 correspond to curves 1–3 in Fig. 1. The vertical lines show the onset of light emission and current density values jmax corresponding to the maximum external quantum efficiencies η.
in which the noise decreases with j increasing within the range 10 2 A/cm2 rjrjmax (curves 2 and 3 in Fig. 2). (Spectral noise densities in this part of the curves follow the law SI 1/j, rather than SI 1/j2 earlier reported for SiC diodes [4]). It is noteworthy that these decreasing parts of the SI(j) curves are observed at j410 2 А/cm2, when the radiative recombination starts to prevail [5]. In InGaN/GaN LEDs with the minimum LC (curve 1 in Fig. 1), SI depends on j rather weakly at j 410 2 А/cm2. This circumstance further confirms that EDS makes an important contribution to the noise and non-radiative recombination, but its relative contribution decreases with increasing current density and non-radiative recombination through PD takes place at 10 2–10 A/cm2. Fig. 3 shows the SI(j) dependences of LEDs with the minimum LC (curve 1 in Fig. 1) at 77 K and 300 K. It can be seen in Fig. 3 that lowering the temperature results in that the noise level decreases at small current densities (j o10 2 A/cm2) and increases at comparatively high jZ 10 2 A/cm2. The decrease in the noise level at small current densities is very typical of excess tunnel currents flowing through shunts piercing the LED active region [6]. An increase in the noise level at j 410 2 A/cm2, where the radiative recombination prevails, has been observed for quantum well InGaAs/GaAs structures [6]. However, it is necessary to note that this effect is much stronger in the InGaN/GaN structures under study. It can be considered that this is due to the strong fluctuations in the InGaN/GaN compound composition, always observed
Fig. 3. Current-density dependences of the spectral current-noise densities for LEDs with LC in Fig. 1 (curve 1) at T¼ 77 and 298 K. The frequency of analysis is 1.22 Hz.
Fig. 4. Current-density dependences of the noise level for two types of LEDs with small LC but very different forward I–U characteristics shown in the insert.
in electroluminescence spectra at temperatures below 200 K [7]. The results we obtained suggest that this phenomenon is related to the Fowler–Nordheim-assisted lateral tunneling of carriers discussed in Ref [6]. A small part of the diodes with low LC shows a very high forward current at low forward biases (inset in Fig. 4, curve 1). It can be seen in inset in Fig. 4 that the current density at a bias U¼1 V in diodes of this kind is three orders of amplitude higher than that in the conventional diodes (curve 2 in the inset). According to the data presented in Ref. [8], such a high forward current is due to the strong fluctuations of the In content in InGaN and to the presence of small local regions with a reduced band gap width Eg. The inhomogeneous nature of the flowing current is confirmed by the SI(j) dependence (curve 1 in Fig. 4). As seen in Fig. 4, there is a portion of the SI(j) dependence (curve 1) in which SI j4 at a medium current density j 10 2 A/cm2. The faster increase in the noise density than that according to the law SI j2.5 shows that new defects are generated by the flowing current [9] and there are local current filaments with a very high current density. This fact earnestly confirms that a highly inhomogeneous current flows in blue InGaN/GaN LEDs. It has been shown previously [10] that diodes of this kind may suffer an unpredictable failure after 10–1000 h of aging (at a current density of 35 A/cm2 and p–n junction temperature of 100 1C.) This failure is due to the re-distribution of In between the
Please cite this article as: A.E. Chernyakov, et al., Journal of Crystal Growth (2014), http://dx.doi.org/10.1016/j.jcrysgro.2013.11.097i
A.E. Chernyakov et al. / Journal of Crystal Growth ∎ (∎∎∎∎) ∎∎∎–∎∎∎
Fig. 5. Dependences of spectral voltage-noise densities Sv on the forward current density for LEDs. Curves 1 and 2 correspond to curves 1 and 2 in Fig. 4.
local regions of the InGaN compound with non-equilibrium In concentration. Additional evidence in favor of the inhomogeneous current flow in InGaN/GaN LEDs with rather strongly inhomogeneous InGaN compound can be obtained from noise spectra of voltage fluctuations (Fig. 5). It is known that the spectral noise density of voltage fluctuations SV at a homogeneous current flow is inversely proportional to j: SV 1/j [11]. It can be seen in Fig. 5 that, for diodes with a high level of the compound inhomogeneity (curve 1), the departure from the 1/j law is extremely strong. Moreover, in some portion of the SV(j) dependence, SV grows with increasing j. 4. Conclusion It was demonstrated that, together with point defects, the extended defect system (EDS) essentially contributes to nonradiative recombination processes in InGaN/GaN LEDs at
3
comparatively small current densities. The relative contribution of EDS decreases with increasing current density. It is shown that the low-frequency nose technique can be effectively used to study reasons for the unpredictable failure of InGaN/GaN LEDs. This technique, in particular, makes it possible to find diodes with an extremely inhomogeneous current flow. The appearance of portions in the SI(I) dependences, in which SI I4 at a medium current density j 10 2 A/cm2, reveals a very inhomogeneous current flow caused by the inhomogeneity of the In distribution in InGaN compound and the by the presence of small local regions of InGaN with a reduced band gap width Eg. The very inhomogeneous current flow, which results in overheating of local areas, causes migration of In ions between the local regions of an InGaN compound with the non-equilibrium In concentration [12]. The ion migration and the inhomogeneous defect generation are very important reasons for the unpredictable failures of blue InGaN/ GaN LEDs.
References [1] N.V. Dyakonova et al., Physics and Techniques of Semiconductors. 25(12) (1991), p.2065. [2] A.V. Kamanin, et al., Phys. Status Solidi C 3 (2006) 2129. [3] M. Albrecht, T. Schulz, J. Weyher, B. Lucznik, P. Prystawko, In: Proceedings of the Eighth International Workshop BIAMS. 1 (2006), p.28. [4] S.L. Rumyantsev, M.S. Shur, M.E. Levinshtein, A.D. Dmitriev, D. Veksler, J. W. Palmour, M.K. Das, B.A. Hull., J. Appl. Phys 100 (2006) 064505. [5] S. Sawyer, S.L. Rumyantsev, M.S. Shur, N. Pala, Yu. Bilenko, J.P. Zhang, X. Hu, A. Lunev, J. Deng, R. Gaska., J. Appl. Phys 100 (2006) 034504. [6] Lech Dobrzanski, J. Appl. Phys. 96 (2004) 4135. [7] K. Kazlauskas, M.A. Khan, M.S Shur, Appl. Phys. Lett. 83 (2003) 3722. [8] E.F. Schubert, Light-emitting Diodes, second ed., Cambridge University Press, UK, 2006. [9] G.P. Zhigal’skii, Phys.—Usp 173 (2003) 465 (in Russian). [10] A.E. Chernyakov, M.E. Levinshtein, P.V. Petrov, N.M. Shmidt, E.I. Shabunina, A.L. Zakheim., Microelectron. Reliab. 52 (9–10) (2012) 2180–2183. [11] T.G.M. Kleinpenning, Physica 98B (1980) 293. [12] S.K. Jeon, J.G. Lee, E.H. Park, J.H. Jang, J.G. Lim, S.K. Kim, J.S. Park., Appl. Phys. Lett. 94 (2009) 131106.
Please cite this article as: A.E. Chernyakov, et al., Journal of Crystal Growth (2014), http://dx.doi.org/10.1016/j.jcrysgro.2013.11.097i