The effect of junction temperature on the

Journal of Luminescence 132 (2012) 429–433

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The effect of junction temperature on the optoelectrical properties of InGaN/GaN multiple quantum well light-emitting diodes Jen-Cheng Wang a, Chia-Hui Fang a, Ya-Fen Wu b, Wei-Jen Chen a, Da-Chuan Kuo a, Ping-Lin Fan c, Joe-Air Jiang d, Tzer-En Nee a,n a

Graduate Institute of Electro-Optical Engineering and Department of Electronic Engineering, Chang Gung University, Kwei-Shan, Tao-Yuan 333, Taiwan, Republic of China Department of Electronic Engineering, Ming Chi University of Technology, Taishan Dist., New Taipei City 243, Taiwan, Republic of China c Department of Digital Technology Design and Graduate School of Toy and Game Design, National Taipei University of Education, Taipei 106, Taiwan, Republic of China d Department of Bio-Industrial Mechatronics Engineering, National Taiwan University, Taipei 106, Taiwan, Republic of China b

a r t i c l e i n f o

abstract

Article history: Received 17 December 2010 Received in revised form 29 August 2011 Accepted 2 September 2011 Available online 8 September 2011

Thermal effects on the optoelectrical characteristics of green InGaN/GaN multiple quantum well (MQW) light-emitting diodes (LEDs) have been investigated in detail for a broad temperature range, from 30 1C to 100 1C. The current-dependent electroluminescence (EL) spectra, current–voltage (I–V) curves and luminescence intensity–current (L–I) characteristics of green InGaN/GaN MQW LEDs have been measured to characterize the thermal-related effects on the optoelectrical properties of the InGaN/GaN MQW LEDs. The experimental results show that both the forward voltages decreased with a slope of 3.7 mV/K and the emission peak wavelength increased with a slope of þ 0.02 nm/K with increasing temperature, indicating a change in the contact resistance between the metal and GaN layers and the existence of a band gap shrinkage effect. The junction temperature estimated from the forward voltage and the emission peak shift varied from 25.6 to 14.5 1C and from 22.4 to 35.6 1C, respectively. At the same time, the carrier temperature decreased from 371.2 to 348.1 1C as estimated from the slope of high-energy side of the emission spectra. With increasing injection current, there was found to be a strong current-dependent blueshift of 0.15 nm/mA in the emission peak wavelength of the EL spectra. This could be attributed to not only the stronger band-filling effect but also the enhanced quantum confinement effect that resulted from the piezoelectric polarization and spontaneous polarization in InGaN/GaN heterostructures. We also demonstrate a helpful and easy way to measure and calculate the junction temperature of InGaN/GaN MQW LEDs. & 2011 Elsevier B.V. All rights reserved.

Keywords: Gallium nitride (GaN) Multiple quantum well (MQW) Light-emitting diode (LED) Junction temperature Heterostructure

1. Introduction InGaN-based multiple quantum wells (MQW) are the best known nitride heterostructures for the fabrication of light-emitting diodes (LEDs) and laser diodes (LDs) [1,2]. Since the InGaN bandgap ranges from 0.7 eV for InN to 3.4 eV for GaN, the diodes operate in visible and ultraviolet spectral region [3]. Furthermore, rapid progress in growth of InGaN-based LEDs should mitigate some of these problems. There are some important issues related to the fabrication of InGaN/GaN MQW green LEDs that affect the optoelectrical properties of the devices. These include the phase separation [4], surface morphology [5] and V-shaped defects [6]. When the optoelectronic devices are operated in dissimilar categories, the environmental temperature generally fluctuates, essentially causing some critical thermal-related effects. The effects of temperature variation on the characteristics of LEDs inherently lead to

n

Corresponding author. Tel.: þ886 3 2118800; fax: þ 886 3 2118507. E-mail address: [email protected] (T.-E. Nee).

0022-2313/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2011.09.001

deterioration in the light output power constancy. In addition, the thermal effect of an LED affects the peak wavelength, output power intensity, internal efficiency, device reliability and lifetime. There is higher resistivity found in self-heating process, which changes the wavelength and forward voltage [7,8]. Furthermore, the migration of dislocations as well as the diffusion of impurities is strongly dependent on the temperature. Clearly the thermal effects on LED performances are a very important technical issue. The junction temperature is a critical parameter influencing the internal efficiency, output power and reliability of a device. Hence, it is very important to accurately evaluate the junction temperature and thermal resistance in the LED. Todoroki et al. did apply micro-Raman spectroscopy to estimate the junction temperature of laser diodes [9]. However, a sophisticated experimental setup is needed for Raman microscopy and its accuracy is limited. Abdelkader et al. used the threshold voltage method [10] and Epperlein proposed the utilization of photothermal reflectance microscopy (PRM) [11] for estimation of the junction temperature of laser diodes. However, neither of these methods are applicable to LEDs. Indirect

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J.-C. Wang et al. / Journal of Luminescence 132 (2012) 429–433

techniques such as electroluminescence (EL) [12] and photoluminescence (PL) [13] have also been used for junction-temperature measurements of laser diodes. However, the EL and PL methods also offer only limited accuracy. Gu and Narendran proposed a noncontact method [14] based on the emission peak ratio that has been demonstrated for use with a white dichromatic LED source but it cannot be used for monochromatic LEDs. There have been a variety of techniques and theoretical methods used to investigate the junction temperature of LEDs [7,8,15,16]. It is very reasonable to suppose that the junction temperature of an LED will fluctuate with the ambient temperature due to significant heating caused by the pulsed current of a high duty cycle or dc forward current. However, a pulsed current with a low duty cycle can be used to minimize the device self-heating effect and moderate the difference between the ambient temperature and junction temperature. Xi et al. successfully modeled the current–voltage (I–V) characteristics of such devices [8]. They estimated the junction temperature of the individual rectifying junctions within a p–n junction diode by taking into account the temperature dependence of the energy gap and the temperature coefficient of the diode resistance. Xi’s model is essentially suitable for analysis of the multi-nanostructure behavior of heterostructure devices consisting of atomic scaled hetero-layers. In this study, we investigate the correlations of junction temperature and carrier temperature existing between the electrical and optical characteristics of InGaN/GaN MQW LEDs by examining the evolution of the current–voltage (I–V) characteristic curves and luminescence spectra over the temperature range from 30 1C to 100 1C. An expression for the forward voltage temperature coefficient is derived that takes into account all relevant contributions to the temperature dependence of the forward voltage including the intrinsic carrier concentration, the band-gap energy and the effective density of states. The experimental results also reveal that the carrier transport process was essentially responsible for the influence on the optoelectrical characteristics of InGaN/GaN MQW LEDs under the injecting current.

2. Background In order to realize the relation between the junction temperature and the forward voltage (Vf) we use the Shockley equation [17,18] eVf 1 , ð1Þ Jf ¼ Js exp nideal kT where Js is the saturation current density; k is Boltzmann’s constant and nideal is the ideality factor. The Js can be expressed as [18] 2sffiffiffiffiffiffiffiffiffiffiffiffiffi sffiffiffiffiffiffiffiffiffiffiffiffi3 Dp n2i 5 Dn n2i , ð2Þ Js ¼ e4 þ tn ND tp NA where Dn and Dp are the diffusion constants of electrons and holes, respectively; tn and tp are the minority carrier lifetimes of electrons and holes and ND and NA are the concentrations of electrons in the donor or acceptor energy state, respectively. In terms of phonon scattering, the diffusion constants decrease with temperature, according to the T 1/2 dependence, so the carrier lifetime will change at different temperatures. The intrinsic carrier concentration ni can be expressed as [18] pffiffiffiffiffiffiffiffiffiffiffiffi Eg , ð3Þ ni ¼ NC NV exp 2kT where Nc and Nv are the effective density of states at the conduction band and valence band edges, respectively. These

are given by [18] 3=2 2pmde kT NC ¼ 2 MC , h2

ð4Þ

2pmdh kT NV ¼ 2 , h2

ð5Þ

where mde and mdh are the density-of-state effective mass for electrons and holes, respectively; h is Planck’s constant and MC is the number of equivalent minima in the conduction band. The derivative of the junction voltage with respect to the junction temperature can be written as [8,15] dVf Jf d nideal kT ln ¼ : ð6Þ dT e dT Js If Vf 4 4kT/e and all parameters are taken into account, the derivative of the junction voltage with the junction temperature can be written as [8] dVf eVf Eg 1 dEg 3k ¼ þ : e dT e dT eT

ð7Þ

The temperature dependence of the diffusion constants and lifetimes would only have a little influence on the temperature coefficient, so we neglect these contributions. For nondegenerate doping concentrations and considering Varshni’s law, the equation can be written as [8] dVf k ND NA aTðT þ 2bÞ 3k ð8Þ ln , e e dT NC NV eðT þ bÞ2 where a and b are the Varshni’s parameters. This equation is a useful expression of the temperature coefficient of the junction voltage. The forward voltage method is based on the calibration measurement and the actual junction-temperature measurement. In the experimental results, the relation of Vf versus T is very close to linear and can be fitted by the equation Vf ¼ A þBTo ,

ð9Þ

where To is ambient temperature and A and B are the fitting parameters. After using the calibration measurement, the junction temperatures for different currents can be found by Tj ¼

ðVf AÞ : B

ð10Þ

In the calibration measurement, the junction temperature can also be deduced by the shift in the emission peak energy. On the other hand, the high-energy slope follows the proportionality [15,16] hn Ipexp , ð11Þ kTc where Tc is the carrier temperature; k is Boltzmann’s constant and hn is the photon energy. In summary, it is easy to estimate the junction temperature of InGaN/GaN MQW LEDs using the three methods based on the p–n junction recombination mechanism. These methods offer a simple approach with a low calculation burden, which can be used to directly determine the junction temperature of InGaN/GaN MQW LEDs from their I–V characteristic curve and electroluminescence spectra. This ensures that junction temperatures are estimated in practical applications. It is expected that using this method, we can investigate the temperature effect on the optoelectrical characteristics of InGaN/GaN MQW LEDs.


3. Experiments InGaN/GaN multiple quantum well heterostructures were grown by metal organic vapor phase epitaxy (MOVPE) on c-plane sapphire substrates. The Ga, In and N sources were trimethylgallium (TMGa), trimethyindium (TMIn) and ammonia (NH3) with N2 carrier gas, respectively. The wafer consisted of a 25 nm buffer layer, a 4 mm-thick n-GaN layer, a four-period undoped InxGa1 xN (2 nm)/GaN (12 nm) (0.24ox o0.26) multiple quantum well (MQW) active layer, followed by a 100 nm-thick p-GaN contact layer. During the growth of In rich QW herterostructures, only TMIn and NH3 were supplied as precursors and at the same time, N2 carrier gas was used. After the InGaN alloy growth, the TMIn and TMGa were turned off right away. Only NH3 and N2 carrier gas flowed during the interruption time. In order to control the quality of InGaN/GaN MQW heterostructures, the growth interruption time was controlled to be 30 s. The GaN barrier layer was grown at the same temperature as the QW. Silane (SiH4) and bis(cyclopentadienyl)magnesium (Cp2Mg) were used as the n-type and p-type dopants, respectively. The doping levels were nominally 5 1018 and 1 1017 cm 3. The In composition in the MQW structures was examined through X-ray diffraction (XRD) measurement (results not shown here). The n-type and p-type electrodes of Ti–Al and Ni–Au, respectively, were produced by the standard photolithography processes. Both electrodes were treated by rapid thermal processing in a furnace to ensure better ohmic properties. The overall dimensions of the LED chips were maintained at 350 350 mm2. For temperature-dependent EL measurement, the samples were mounted on a closed-cycle He cryostat that was carried out using a Keithley 2430 current source over a wide temperature range (30–100 1C). The electroluminescence (EL) signals were detected by a Si detector with a 0.5 m monochromator employing a standard lock-in technique. The luminescence intensity–current (L–I) characteristics of the InGaN/GaN MQW LEDs were measured by a Si detector. Their forward voltages were extracted from the current–voltage (I–V) curves measured by the Keithley 2430 under DC current condition for a broad temperature range, from 30 1C to 100 1C.

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4.3 to 3.4 a.u. and the forward voltage decreased from 4.3 V to 3.9 V for an injection current of 20 mA. This phenomenon is due to the self-heating process in the junctions, which changes the electrical and optical properties [7]. Similar results have been observed for nitride-based LEDs by several groups [19,20]. In order to realize the thermal effect on InGaN/GaN MQW LEDs, we further investigated the EL spectra and current–voltage characteristics of these devices in depth. Fig. 2 shows the EL spectra of InGaN/GaN MQW LEDs with increasing DC injection currents from 0.5 to 100 mA at an ambient temperature of 30 1C. The spectra show a strong blueshift, from 521.4 nm to 506.9 nm as the injection current increases. This can be attributed not only to the stronger band-filling effect but also the enhanced quantum confinement effect resulting from the piezoelectric polarization and spontaneous polarization in the InGaN/GaN heterostructures [21–23]. In addition, the red shift is not resolved. Thus, the piezoelectric effect or the quantum confined Stark effect (QCSE) is not obvious. It is possible to assume that the junction temperature of the LED does not increase so easily because electrons have been confined in the MQW because of radiative recombination due to the strong quantum confinement effect. During the process of I–V characteristic measurement, the injection current is increased from 0.5 to 100 mA. The forward voltage versus ambient temperature of InGaN/GaN MQW LEDs subject to different injection currents is shown in Fig. 3. The forward voltage shift is determined to have a slope of 3.7 mV/K at an injection current of 1 mA and þ24.6 mV/mA with an ambient temperature of 30 1C. The temperature coefficient of the forward voltage at low currents is 3.7 mV/K, which is slightly larger in magnitude than the theoretical result of 1.74 mV/K [24]. The difference between the theoretical and

4. Results and discussion The thermal effects on the electrical and optical properties of LEDs under a direct current were ascertained by measuring the junction temperature with the forward voltage method. Fig. 1 shows the luminescence intensity–current–voltage (L–I–V) curves for InGaN/GaN MQW LEDs at different ambient temperatures of 30 1C and 100 1C. The curves indicate that when the temperature increased to 100 1C, the luminescence intensity decreased from

Fig. 1. Luminescence intensity–current–voltage (L–I–V) characteristics of InGaN/ GaN multiple quantum well light-emitting diodes under ambient temperatures of 30 1C and 100 1C.

Fig. 2. Electroluminescence spectra of InGaN/GaN multiple quantum well lightemitting diodes with increasing DC injection currents from 0.5 to 100 mA.

Fig. 3. Experimental forward voltage versus ambient temperature for InGaN/GaN multiple quantum well light-emitting diodes under different DC injection currents from 0.5 to 100 mA.

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experimental coefficients is attributed to the resistive contributions of the neutral regions, exhibiting a higher doping activation at elevated temperatures. A higher doping activation increases the conductivity of the neutral regions, thereby decreasing Vf. The difference could be due to a decrease in the resistivity of the p-type GaN layer, which is caused by more complete acceptor activation at the elevated temperature. This phenomenon could be enhanced by the poor thermal conductivity of the epoxy, so that the generated heat would stay in the chip [7]. The shift in forward voltage with increasing temperature is due to the contact resistance between the metal and GaN layers and the band gap shrinkage effect. In addition, based on the Shockley equation, the forward voltage increases with the injection current [25]. Fig. 4 shows the experimental emission peak shift versus ambient temperature for InGaN/GaN green MQW LEDs under different DC injection currents from 0.5 to 100 mA. The experimental emission peak shifts with the ambient temperature. The data for InGaN/GaN multiple quantum well green light-emitting diodes under different injection currents (from 0.5 to 100 mA) are shown. The peak wavelengths decrease with increasing injection current when a constant ambient temperature is maintained, as shown in Fig. 4. At an ambient temperature of 30 1C, this curve is almost linear with a slope estimated to be 0.15 nm/mA. The blueshift in the emission peak wavelength is due to the stronger band-filling effect as well as the enhanced quantum confinement effect, which result from the piezoelectric polarization and the spontaneous polarization in the InGaN/GaN heterostructures. Furthermore, the shift in the emission peak wavelength to longer wavelengths is almost linear (with a slope of þ 0.02 nm/K) for a constant injection current of 1 mA, due to Varshni’s law.

Fig. 4. The shift in the experimental emission peak versus ambient temperature for InGaN/GaN multiple quantum well light-emitting diodes under different DC injection currents from 0.5 to 100 mA.

In order to determine the thermal effect in InGaN/GaN MQW LEDs, we can compare the junction temperatures estimated by the forward voltage and emission peak shift techniques, as shown in Fig. 5. The carrier temperatures are also shown in this figure. The carrier temperatures are estimated from the high-energy side of the EL spectra, where the carrier distribution can be approximated by the Boltzmann distribution. The carrier temperature, estimated from the slope of the high-energy side in the EL spectra, decreased from 371.2 to 348.1 1C while the injection current increased. In Fig. 5, it can be seen that the junction temperature measured by the forward voltage method decreased from 25.6 to 14.5 1C. This is not only due to the obvious quantum confinement and band filling effect, but also the filling of the space charge regions or the non-radiative recombination centers, which decreased the carrier temperature. As can be seen in Fig. 5, the junction temperature measured by the emission peak shift method increased from 22.4 to 35.6 1C while the measured injection current increased, indicating that most of the carriers are subjected to radiative recombination in MQWs and that the number of carriers increased as the injection current increased. This phenomenon is consistent with the carrier temperature. These results indicate that the optical and electrical effects of the junction temperature are different.

5. Conclusion Junction temperatures have different effects in the optical and electrical characteristics of InGaN/GaN MQW green LEDs. Due to an increase in the injection current, more carriers will be confined in the quantum wells leading to radiative recombination; only a few carriers will be subject to nonradiative recombination in the space charge regions due to the strong quantum confinement effect. As a consequence, the junction temperature, measured from the emission peak shift, decreased from 25.6 to 14.5 1C and increased from 22.4 to 35.6 1C, as extracted by the forward voltage method. At the same time, the carrier temperature decreased from 371.2 to 348.1 1C as estimated from the slope of the high-energy side of the emission spectra. The stronger band-filling effect as well as the enhanced quantum confinement effect occurred as a result of the piezoelectric polarization and the spontaneous polarization in InGaN/GaN heterostructures; the shift in the emission peak increased with a slope of þ0.02 nm/K and that in the EL spectra decreased with a slope of 0.15 nm/mA. The forward voltages decreased with a slope of 3.7 mV/K and increased with a slope of þ24.6 mV/mA indicating a change in the contact resistance between the metal and GaN layers and the band gap shrinkage effect. We demonstrated an easy way to measure and calculate the junction temperature of InGaN/GaN MQW LEDs using the emission peak shift method or the forward voltage method.

Acknowledgments The authors would like to acknowledge support from the staff of GALOIS (Group of Abel and Lie Operations In Sciences) and the QUEST Laboratory (Quantum Electro-optical Science and Technology Laboratory), Graduate Institute of Electro-Optical Engineering and Department of Electronic Engineering, Chang Gung University, Taiwan. This study was financially supported by the National Science Council of the Republic of China under Contract Nos. NSC 97-2112-M-182-002-MY3 and 100–2112-M-182-003-MY2. References Fig. 5. Junction temperatures and carrier temperature as a function of the DC injection current for InGaN/GaN multiple quantum well light-emitting diodes at an ambient temperature of 30 1C.

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