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Phys. Status Solidi B 250, No. 4, 698–702 (2013) / DOI 10.1002/pssb.201200514

Optical properties of InGaN-based LEDs investigated using hydrostatic pressure dependent techniques

B. G. Crutchley, I. P. Marko and S. J. Sweeney

Advanced Technology Institute and Department of Physics, University of Surrey, Guildford, Surrey, GU27XH, U.K.

J. Pal and M. A. Migliorato School of Electrical and Electronic Engineering, University of Manchester, United Kingdom

Abstract

High pressure electroluminescence measurements were carried out on blue and green emitting InGaN-based LEDs. The weak pressure coefficient of the peak emission energy of the LEDs is found to with increasing injection current. Such behaviour is consistent with an enhancement of the piezoelectric fields under high pressure which becomes screened at high currents. A subsequent increase in the quantum confined Stark effect (QCSE) is expected to cause a reduction of the light output power as pressure is applied at a fixed low current value (10.4Acm-2). A similar proportional reduction of light output power is observed for a fixed high current (260Acm -2) which suggests that the non-radiative loss process which occurs in InGaN-based devices is relatively insensitive to pressure. Such behaviour is shown to be consistent with a defect-related recombination process occurring at high currents. Introduction

Strong advances in the field of nitride semiconductors have led to the improvement of InGaN light emitters in recent years. Solid state lighting, large and portable displays, back lighting, traffic signal and automotive applications are a few of the applications which have caused considerable interest in developing InGaN-based LEDs [1]. Additionally, the improved performance of InGaN-based laser diodes over the past few years has led to commercially-viable applications such as blu-ray players and pico-projection systems. Continuing improvements in the performance of the InGaN emitters are required in order to achieve the market targets for InGaN-based emitters [2]. A phenomenon which limits the performance of the devices is the much published issue of efficiency droop [3, 4]. Figure 1 shows an example of such an effect for a blue LED where it is observed that the efficiency peaks at a low current and reduces as there is a further increase of the injection current. Despite intense investigations there is little agreement on the non-radiative recombination process which causes droop in InGaN emitters. Auger recombination[5, 6], carrier leakage [3, 7] and a defect- related loss mechanism [8, 9] have all been proposed as the cause of efficiency droop. One proposed method of alleviating the efficiency droop is to produce LEDs grown along the m-plane which are free of the high internal fields and polarization fields which occur in conventional c-plane LEDs [10]. However, using this growth method only provides a marginal improvement to the overall light output performance [10].

In this study the use of a high hydrostatic pressure technique is employed to investigate the electroluminescence properties of blue and green InGaN-based LEDs. Previous high hydrostatic pressure studies have shown that the application of pressure will result in an increase of the internal polarization fields as pressure is applied [11, 12]. We therefore study the influence of an increasing internal polarization field on the optoelectronic properties using in a single device which therefore eliminates the influence of varying growth conditions.

Figure 1- The normalized efficiency as a function of current for a blue LED at a pressure of 10kbar.

Experiment

Commercially-available blue and green polar multi quantum well (MQW) LEDs were examined in this study. The quantum well widths of both LEDs were 3nms and the barrier width was 13nm and 16nm for the blue and green LEDs, respectively, as measured from cleaved-edge transmission electron microscopy. The dimensions of both LED chips were ~310μm2. The estimated indium contents of 14% (providing emission of ~460nm) and 26% (providing emission ~505nm) for the blue and green LEDs, respectively, were obtained from calculations of Moses et al. [13]. A deencapsulation process was carried out on the LEDs before the bare chips where mounted onto TO-headers in order for the measurements to be taken. A helium compressor system that is capable of generating pressures of up to 10kbar was connected to a pressure cell where the LEDs were separately inserted. Electrical access is provided in one side of the cell and the emitted light from the LEDs is collected through a sapphire window which is found on the other side of the cell. Further details of the system can be found in Ref [14]. Pulsed measurements using 2μs pulses at 10kHz frequency were used in the measurements in order to reduce the effects of Joule heating. The spectral studies were performed with a HR2000 spectrometer and the light output power was measured by the use of a calibrated silicon-based light power meter.

Results and discussion

The pressure coefficients in this study are determined by measuring the peak emission energy as a function of pressure at a fixed current. Figure 2 shows that the electroluminescence (EL) emission peak energy has an approximately linear dependence on the hydrostatic pressure for both LEDs over this pressure range.

Figure 2- The EL peak emission energy as a function of pressure for a blue LED (blue squares) and a green LED (green gircles) at a current of 5mA.

The pressure coefficients at 5.20Acm-2 for both the blue (2.14meV/kbar) and green (1.21±0.02meV/kbar) InGaN-based LEDs are smaller than the pressure coefficients which have been determined by photoluminescence studies on the binaries GaN (4meV/kbar) and InN (2.7meV/kbar). Our results are consistent with previous studies which have shown, in agreement with theoretical models, that an increase of the piezoelectric field as applied pressure is always expected to reduce the pressure coefficient. With increasing current the pressure coefficient increases and can be attributed to carrier screening effects which reduce the influence of the internal polarization fields, as shown in Figure 3.

Figure 3- The pressure coefficient as a function of current density for a blue LED (blue squares) and a green LED (green circles). We find that the strong blueshift as a function of current, which has been attributed to the screening of the internal field, increases as pressure is applied due to a larger current required to screen the stronger internal field. Further evidence that there is an increase in the strength of the internal field is provided by the increased applied voltage required to obtain a fixed current as pressure is applied, as shown in Figure 4. The higher applied voltage required is due to the fact that there is a reduction of the injection efficiency for stronger internal fields [15].

Figure 4- The voltage required in order to obtain a current of 10mA for a blue LED (blue squares) and a green LED (green circles).

The application of high hydrostatic pressure will induce strain due to the different compressibilities of the well and barrier region[16]. We use the relationship for the pressure-induced strain of Thomas et al. [17] with bulk modulus values of BGaN=210GPa [18], BInGaN(blue)=200.2GPa and

BInGaN(green)=191.8GPa where the Bulk modulus values for the InGaN wells are obtained using Vegard’s law with BInN=140 [19]. The relative change in strain is shown in Figure 5.

Figure 5-The proportional change of the hydrostatic compressive strain within the LEDs as a function of pressure.

The application of pressure is therefore found to increase the polarization strength of the well and barrier as a function of pressure. Figure 6 shows the calculated change in polarization in the wells and barriers for the blue and green LEDs as hydrostatic pressure is applied. For the QW, this is in addition to the natural strain originating from pseudomorphic growth. Non linear piezoelectric coefficients from Pal et al. [20] are used in this calculation, as opposed to the linear theory of Bernardini et al. (F. Bernardini and V. Fiorentini, Appl. Phys. Lett. 80, 4145 (2002).)

Figure 6-Calculated polarization dependence on pressure for the barrier (upper graph) and well (lower graph) for a blue LED (blue squares) and a green LED (green circles)

The change in the polarization strengths between the barrier and well will cause the internal fields to change as determined by the relationship,

where Lb,w are the widths of the barrier and well, Pb,w are the polarizations of the barrier and well, and εb,w is the dielectric constants of the barrier and well. The strength of the internal field as a function of pressure can be calculated as shown in Figure 8.

We have used b=10 and obtain w(blue)=10.7 and w(green)=11.4 using Vegard’s law with InN=15.3 [20]. Figure 7 shows the internal field strength is expected to increase from 2.47MVcm -1 to 2.56MVcm-1 (~4% increase) and from 4.54MVcm-1 to 4.70MVcm-1 (~4% increase) for the blue and green LED, respectively, as pressure is increased from 2kbar to 10kbar.

Figure 7- The calculated internal field of the QW as a function of pressure for a blue LED (blue squares) and a green LED (green circles)

The enhancement of the piezoelectric field as pressure is applied will enhance the quantum confined Stark effect (QCSE) causing a reduction to the radiative recombination rate. This is due to the fact that there is a strong reduction to the matrix element in Fermi’s golden rule as the strength of the internal field is increased due to the larger separation of the electron and hole wavefunction overlap [21, 22]. This is indeed observed in our measurements where there is a reduction of the light output power as a function of pressure at a fixed low current (10.4Acm -2) where the radiative recombination is expected to dominate (Figure 8).

Figure 8- The measured normalized electroluminescence light output power as a function of pressure for the blue LED (a) and the green LED (b) at a current of 10.4Acm-2

We observe a reduction of ~4% in the light output power as a function of pressure for both LEDs at a low current of 5mA where radiative recombination is expected to dominate. This observation can be explained by the reduced radiative recombination rate which is expected to show a strong reduction with an increase in the strength of the internal field strength (due to being proportional to the square of the electron and hole wavefunction overlap) as pressure is applied.

Interestingly, Figure 9 shows that there is a similar reduction of the light output power as a function of pressure at a fixed high current value of 260Acm -2 where the non-radiative recombination process responsible for efficiency droop is expected to dominate (see Fig. 1). This observation implies that the non-radiative recombination process which causes efficiency droop is pressure insensitive. The reduced radiative recombination rate due to the increased QCSE at high pressures can therefore be explained as the cause of the reduced light output power over the entire current range. Figure 9- The measured normalized electroluminescence light output power as a function of pressure for the blue LED (a) and the green LED (b) at a current of 260Acm-2

We will now consider the affect that pressure is expected to have on the candidate droop-causing mechanisms. Previous studies have found that the Auger recombination rate is expected to be reduced as pressure is applied [14, 23] and is therefore unlikely to cause the reduction of the light output power as a function of pressure (see Fig.9). However, the enhancement of the internal fields may cause a possible enhancement to the Auger recombination rate (Cn 3) as found in previous studies for devices which consist of stronger internal fields [24]. One would expect a stronger reduction of the light output power at high currents if this is the case. As we do not observe this, it is unlikely that Auger recombination is the cause of efficiency droop. Similarly, the carrier leakage rate has also been shown to be increase as the internal field strength increases [25] and is therefore not consistent with the observed pressure-insensitive behaviour. The results however, can be explained by using a defect-related loss process taking place in InGaN-based LEDs at high currents which is approximately constant as a function of pressure. In line with the calculations of Vaschenko et al. [11], the effects of carrier localization is not expected to be significantly changed as a function of pressure. Therefore at all pressures there is expected to be a similar proportion of defect-related recombination due to reduced carrier localization effects at high currents that causes the efficiency droop effect which is shown in Figure 1. The influence of indium clustering is also expected to contribute to a further reduction of the pressure coefficient as described in Ref [26]. The weaker pressure coefficient of the green LED in comparison with the blue LED (see Fig. 2) is expected to be due to enhanced indium clustering effects due to its higher indium content. This observation is in agreement with calculations of Gorczyca et al. [26] where it has been shown that there will be a strong bowing of the pressure coefficient as indium content is increased. This is attributed to a stronger influence of indium inhomogeneities for structures as the indium content is increased up to 50%.

Conclusion

The electroluminescence of commercial blue and green LEDs was studied as a function of current and high hydrostatic pressure. The weak pressure coefficients of such devices are found to be caused by an enhancement of the electric field as pressure is applied. Calculations show that the piezoelectric field will increase from 2.44MVcm-2 to 2.52MVcm-2 and from 4.60MVcm-2 to 4.71MVcm- 2 for the blue and green LEDs, respectively, as pressure is applied from 2kbar to 10kbar. Such increases of the piezoelectric field was found to cause a reduction in the light output power as pressure is applied due to the reduced radiative recombination rate which results from an increased QCSE. It is expected that the relatively constant reduction in light output power over the entire current range as pressure is increased is due to the reduced radiative recombination rate alone and that the non-radiative loss mechanism responsible for efficiency droop is not significantly changed as there is an increase in pressure. These results imply that the loss process which causes efficiency droop as current is increased in InGaN-based emitters may be explained by an increase in the defect related recombination as carriers populate higher energy trap states.

[1] J. K. Kim and E. F. Schubert, "Transcending the replacement paradigm of solid-state lighting," Opt. Express, vol. 16, pp. 21835-21842, 2008. [2] http://www.semiconductor-today.com/news_items/2009/APRIL/INSTAT_080409.htm. [3] M.-H. Kim, M. F. Schubert, Q. Dai, J. K. Kim, E. F. Schubert, J. Piprek, and Y. Park, "Origin of efficiency droop in GaN-based light-emitting diodes," Applied Physics Letters, vol. 91, p. 183507, 2007. [4] J. Piprek, "Efficiency droop in nitride-based light-emitting diodes," physica status solidi (a), vol. 207, pp. 2217-2225, 2010. [5] Y. C. Shen, G. O. Mueller, S. Watanabe, N. F. Gardner, A. Munkholm, and M. R. Krames, "Auger recombination in InGaN measured by photoluminescence," Applied Physics Letters, vol. 91, pp. 141101-3, 2007. [6] N. F. Gardner, G. O. Muller, Y. C. Shen, G. Chen, S. Watanabe, W. Gotz, and M. R. Krames, "Blue-emitting InGaN--GaN double-heterostructure light-emitting diodes reaching maximum quantum efficiency above 200 A/cm[sup 2]," Applied Physics Letters, vol. 91, pp. 243506-3, 2007. [7] M. F. Schubert, S. Chhajed, J. K. Kim, E. F. Schubert, D. D. Koleske, M. H. Crawford, S. R. Lee, A. J. Fischer, G. Thaler, and M. A. Banas, "Effect of dislocation density on efficiency droop in GaInN/GaN light-emitting diodes," Applied Physics Letters, vol. 91, p. 231114, 2007. [8] N. I. Bochkareva, V. V. Voronenkov, R. I. Gorbunov, A. S. Zubrilov, Y. S. Lelikov, P. E. Latyshev, Y. T. Rebane, A. I. Tsyuk, and Y. G. Shreter, "Defect-related tunneling mechanism of efficiency droop in III-nitride light-emitting diodes," Applied Physics Letters, vol. 96, p. 133502, 2010. [9] J. Hader, J. V. Moloney, and S. W. Koch, "Density-activated defect recombination as a possible explanation for the efficiency droop in GaN-based diodes," Applied Physics Letters, vol. 96, p. 221106, 2010. [10] X. Li, X. Ni, H. Y. Liu, F. Zhang, S. Liu, J. Lee, V. Avrutin, Ü. Özgür, T. Paskova, G. Mulholland, K. R. Evans, and H. Morkoç, "On the reduction of efficiency loss in polar c -plane and non-polar m -plane InGaN light emitting diodes," physica status solidi (c), vol. 8, pp. 1560-1563, 2011. [11] G. Vaschenko, D. Patel, C. S. Menoni, N. F. Gardner, J. Sun, W. Götz, C. N. Tomé, and B. Clausen, "Pressure Dependence of Piezoelectric Field in InGaN/GaN Quantum Wells," physica status solidi (b), vol. 228, pp. 73-76, 2001. [12] G. Staszczak, T. Suski, A. Khachapuridze, P. Perlin, M. Funato, and Y. Kawakami, "Different behavior of semipolar and polar InGaN/GaN quantum wells: Pressure studies of photoluminescence," physica status solidi (a), vol. 208, pp. 1526-1528, 2011. [13] P. G. Moses and C. G. V. d. Walle, "Band bowing and band alignment in InGaN alloys," Applied Physics Letters, vol. 96, p. 021908, 2010. [14] S. R. Jin, S. J. Sweeney, C. N. Ahmad, A. R. Adams, and B. N. Murdin, "Radiative and Auger recombination in 1.3 mu m InGaAsP and 1.5 mu m InGaAs quantum-well lasers measured under high pressure at low and room temperatures," Applied Physics Letters, vol. 85, pp. 357-359, 2004. [15] M. F. Schubert, J. Xu, J. K. Kim, E. F. Schubert, M. H. Kim, S. Yoon, S. M. Lee, C. Sone, T. Sakong, and Y. Park, "Polarization-matched GaInN/AlGaInN multi-quantum-well light- emitting diodes with reduced efficiency droop," Applied Physics Letters, vol. 93, pp. 041102- 3, 2008. [16] J. A. Tuchman and I. P. Herman, "General trends in changing epilayer strains through the application of hydrostatic pressure," Physical Review B, vol. 45, pp. 11929-11935, 1992. [17] R. J. Thomas, M. S. Boley, H. R. Chandrasekhar, M. Chandrasekhar, C. Parks, A. K. Ramdas, J. Han, M. Kobayashi, and R. L. Gunshor, "Raman and modulated-reflectivity spectra of a strained pseudomorphic ZnTe epilayer on InAs under pressure," Physical Review B, vol. 49, pp. 2181-2184, 1994. [18] A. Polian, M. Grimsditch, and I. Grzegory, "Elastic constants of gallium nitride," Journal of Applied Physics, vol. 79, pp. 3343-3344, 1996. [19] Z. A., L. M.E., R. S.L., and S. M.S., Properties of Advanced SemiconductorMaterials GaN, AlN, InN, BN, SiC, SiGe. New York, 2001. [20] J. Pal, G. Tse, V. Haxha, M. A. Migliorato, and S. Tomić, "Second-order piezoelectricity in wurtzite III-N semiconductors," Physical Review B, vol. 84, p. 085211, 2011. [21] M. B. Nardelli, K. Rapcewicz, and J. Bernholc, "Polarization field effects on the electron-hole recombination dynamics in In[sub 0.2]Ga[sub 0.8]N/In[sub 1 - x]Ga[sub x]N multiple quantum wells," Applied Physics Letters, vol. 71, pp. 3135-3137, 1997. [22] E. Berkowicz, D. Gershoni, G. Bahir, A. C. Abare, S. P. DenBaars, and L. A. Coldren, "Optical Spectroscopy of InGaN/GaN Quantum Wells," physica status solidi (b), vol. 216, pp. 291-300, 1999. [23] R. Fehse, S. J. Sweeney, A. R. Adams, E. P. O'Reilly, A. Y. Egorov, H. Riechert, and S. Illek, "Insights into carrier recombination processes in 1.3 μm GaInNAs-based semiconductor lasers attained using high pressure," Electronics Letters, vol. 37, pp. 92-93, 2001. [24] M. Zhang, P. Bhattacharya, J. Singh, and J. Hinckley, "Direct measurement of auger recombination in In[sub 0.1]Ga[sub 0.9]N/GaN quantum wells and its impact on the efficiency of In[sub 0.1]Ga[sub 0.9]N/GaN multiple quantum well light emitting diodes," Applied Physics Letters, vol. 95, p. 201108, 2009. [25] J. Piprek, R. Farrell, S. DenBaars, and S. Nakamura, "Effects of built-in polarization on InGaN- GaN vertical-cavity surface-emitting lasers," Photonics Technology Letters, IEEE, vol. 18, pp. 7-9, 2006. [26] I. Gorczyca, T. Suski, A. Kamińska, G. Staszczak, H. P. D. Schenk, N. E. Christensen, and A. Svane, "In-clustering effects in InAlN and InGaN revealed by high pressure studies," physica status solidi (a), vol. 207, pp. 1369-1371, 2010.