- Email: [email protected]
Homoepitaxially grown GaN-based light-emitting diodes with peak emission at 405 nm
ARTICLE IN PRESS
Journal of Crystal Growth 269 (2004) 242–248
Homoepitaxially grown GaN-based light-emitting diodes with peak emission at 405 nm X.A. Caoa,*, C.H. Yanb, M.P. D’Evelyna, S.F. LeBoeufa, J.W. Kretchmera, R. Klingera, S.D. Arthura, D.W. Merfelda a
GE Global Research Center, STL, 1 Research Circle, Niskayuna, NY 12309, USA b AXT Optoelectronics, El Monte, CA 91731, USA Received 2 March 2004; accepted 14 May 2004 Available online 20 July 2004
Abstract Near-ultraviolet InGaN/GaN multiple-quantum-well light-emitting diodes (LEDs) grown homoepitaxially on a free standing GaN substrate exhibited improved microstructural, electrical and optical properties compared to similar devices grown on sapphire. In contrast to strong tunneling behaviors in the LEDs on sapphire, thermally activated currents were observed at both forward and reverse biases in the LEDs on GaN. At low injection currents, the quantum efficiency of the LEDs on GaN was much higher, suggesting reduced nonradative recombination centers. At high driving currents, the homoepitaxial LEDs greatly outperformed the devices on sapphire as a result of improved material quality, heat dissipation and current spreading. These results show the promise of homoepitaxially grown GaN-based LEDs for solid-state lighting applications. r 2004 Elsevier B.V. All rights reserved. PACS: 73.40.Kp; 78.60.Fi; 81.15.Gh; 85.60.Jb Keywords: A3. Homoepitaxy; B1.GaN; B1. InGaN; B3. Light emitting diode; B3. Electroluminescence
1. Introduction GaN-based light-emitting diodes (LEDs) grown heteroepitaxially on sapphire or SiC substrates suffer from a high density of threading dislocations and excess strain [1–9]. These problems can be largely overcome by using a high-quality bulk *Corresponding author. Tel.: +1-5183874361; fax: +15183875997. E-mail address: [email protected] (X.A. Cao).
GaN substrate. The procedures for epitaxial growth of LED structures on GaN are greatly simplified because additional steps such as surface nitridation and deposition of a low-temperature buffer layer which are necessary in heteroepitaxy are not needed. The defects and strain which arise from the mismatch in lattice constant and thermal expansion coefficient of the epitaxial layers and substrate are substantially reduced. In addition, LEDs grown on an electrically conductive GaN substrate can be fabricated in a vertical configura-
0022-0248/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2004.05.065
ARTICLE IN PRESS X.A. Cao et al. / Journal of Crystal Growth 269 (2004) 242–248
tion, eliminating current crowding as occurs in lateral LEDs on sapphire and enabling highcurrent operation. The recent development of hydride-vaporphase-epitaxy (HVPE) growth of bulk-like GaN has led to remarkable improvement in crystal quality and size. Large (up to 200 ) free-standing crack-free GaN substrates are now commercially available [10,11]. Yasan et al. [3] demonstrated 340 nm AlGaN LEDs on a HVPE grown GaN substrate, and the device produced an output power one order of magnitude higher than similar LEDs grown on sapphire. It is generally believed, however, that defect reduction will have less impact on the optical performance of InGaN/ GaN LEDs due to strong carrier localization effects in InGaN alloys. Previous work done by Mukai et al. [4] showed that blue and green InGaN single-quantum-well LEDs grown on epitaxially laterally overgrown GaN and sapphire had similar external quantum efficiencies. In this work, we report on structural, electrical and optical characterization of near UV InGaN/ GaN multiple-quantum-well (MQW) LEDs grown on HVPE GaN and sapphire substrates. LEDs with a peak wavelength near 405 nm, when used in conjunction with multiband phosphors, can produce white light and are currently receiving much attention due to their promise for solid-state lighting applications. We observe a greatly improved performance of the homoepitaxially grown LEDs compared to their counterparts on sapphire, particularly at high currents, confirming the advantages of bulk GaN substrates for epitaxy of low-In InGaN-based light-emitting devices.
2. Experimental procedure InGaN/GaN MQW LEDs with an identical structure were grown on two different substrates— (i) a 200 sapphire substrate with a 100 nm GaN buffer layer, and (ii) a free-standing 1 1 cm2 HVPE GaN wafer with a polished Ga-face—using metalorganic chemical vapor deposition. The GaN substrate was unintentionally doped with a carrier concentration of B7 1017 cm3. The LED structure consisted of a 10-period InGaN/GaN MQW
243
active layer sandwiched by a 2 mm n-GaN layer (SiB5 1018 cm3) and a 0.1 mm p-type AlGaN cladding layer, and a 0.2 mm p-GaN contact layer (Mg B1 1019 cm3). The nominal indium mole fraction in the InGaN MQWs was B0.10. The microstrutural properties of the LEDs were characterized using atomic force microscopy (AFM) and transmission electron microscopy (TEM). Top-emitting LEDs with a size of 300 300 mm2 were fabricated using standard photolithography and dry etch techniques [12]. Ni/Au (5 nm/6 nm) was deposited as the semitransparent p-type contact. Ti/Al metallization was formed on the plasma-etched n-GaN to produce conventional lateral-structure LEDs on sapphire, whereas a fullarea Ti/Al contact was formed on the N-face of the GaN substrate as the n-type electrode of LEDs with a vertical configuration. The electrical and optical characteristics of the LEDs were measured using a Keithley 238 current source measurement unit and a silicon photodiode-array fiber-optic spectrometer.
3. Results and discussion Fig. 1 illustrates typical 10 10 mm2 AFM images of the as-grown LED samples. The LED on sapphire exhibits a surface density of B1 108 cm2 so-called V-defects [13] with a size in the range of 50–200 nm. In contrast, the surface of the LED on GaN is free of visible defects. In addition to micro-scale morphological features consisting of long valleys, atomic step structure with terraces of B100 nm can be seen. The rms roughness of the LED structures on sapphire and GaN are 1.7 and 0.7 nm, respectively. The smoother surface of the homoepitaxial LED may be indicative of very abrupt InGaN/GaN and AlGaN/GaN heterostructural interfaces. TEM measurements showed that the density of threading dislocations reaching the active region in the LED on sapphire was B2–4 109 cm2. A small portion of the dislocations were terminated by V-defects in the MQWs or the p-GaN layer. The V-defects and the associated dislocations, which in most cases were found to have mixed or
ARTICLE IN PRESS 244
X.A. Cao et al. / Journal of Crystal Growth 269 (2004) 242–248
Current (A)
the density of dislocations with screw and mixed character. Fig. 2 shows the forward I2V characteristics of the LED on sapphire measured at increasing temperatures. At low and moderate forward biases, two main exponential segments with different slopes can be distinguished for the LED on sapphire (Fig 2(a)). The I2V characteristics vary as I ¼ I0 expðqV =EÞ; where the energy parameter E is nearly temperature-independent, and has values of 190 and 70 meV in the voltage ranges 0–2.0 V and 2.0–2.8 V, respectively. No realistic ideality factors can be extracted. These behaviors are characteristic of tunneling current in a semiconductor diode [6,7]. At high bias (>3.0 V), the I2V behavior is dominated by series resistance. The forward I2V characteristics of the LED on GaN also divide into two distinct linear sections with different slopes (see Fig. 2(b)). However, the slopes appear to be a function of temperature. At low injection levels tunneling may
pure screw character, may behave as leakage current paths connected across the p–n junctions [5]. As expected, the dislocation density in the homoepitaxially grown LED was much lower. The actual value, however, was too small to be determined by cross-sectional TEM. Earlier work suggested that the dislocation density in homoepitaxially grown GaN was similar to that in the original GaN substrate [14]. The dislocations in the as-received GaN wafers were found to be terminated by surface pits after the final chemical mechanical polish, and the density was B2 107 cm2. The absence of V-defects in the LED on GaN suggests a substantial reduction in
Current (A)
Fig. 1. Typical 10 10 mm2 AFM images of the LEDs grown on (a) sapphire and (b) GaN. The gray scale is 15 nm.
10-2 10-3 10-4 10-5 10-6 10-7 10-8 10-9 10-10 10-11 10-12 10-2 10-3 10-4 10-5 10-6 10-7 10-8 10-9 10-10 10-11 10-12 0.0
(a) 0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
o
25 C 50 oC o 100 C 150 oC 200 oC
(b)
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
Forward Voltage (V) Fig. 2. Temperature-dependent forward I2V characteristics of the LEDs grown on sapphire and GaN.
ARTICLE IN PRESS X.A. Cao et al. / Journal of Crystal Growth 269 (2004) 242–248
still dominate, but the temperature dependence of the slope suggests the involvement of thermally activated currents. As the injection level increases, diffusion and recombination currents start to dominate over the tunneling component. At forward bias >2.6 V the current can be described by the conventional Shockley model as I ¼ I0 expðqV =1:5 kTÞ until series resistance in the diode dominates. The dominance of diffusionrecombination current reflects the high material quality of the homoepitaxial LED where defectassisted tunneling current is greatly suppressed. The temperature-dependent reverse I2V characteristics of the LEDs are shown in Fig. 3. The current in the LED on sapphire is much higher than conventional diffusion and generation currents, which are immeasurably small in GaNbased p–n junctions. The strong field-dependence but low-temperature sensitivity of the leakage current is indicative of carrier tunneling. In sharp contrast, the LED on GaN shows a dramatic
10-4
(a)
10-5
Current (A)
10
-6
10-7 10-8 10-9 10-10 10-11
reduction in reverse current by more than six orders of magnitude, consistent with much lower surface and bulk defect densities in this LED. The strong dependence of the remaining leakage current on temperature indicates the presence of a thermally generated current. In particular, the sudden jump in the low-bias current between 100 C and 150 C may arise from thermal ionization of carriers from deep traps and trap-assisted tunneling process. An activation energy of B0.6 eV is extracted from the log ðIÞF1=T plot at 12.5 V, where the change of the current as a function of temperature is roughly an exponential function. The vertical geometry of the LEDs on GaN significantly simplifies the procedures for chip processing and packaging. More importantly, it eliminates or reduces long lateral current paths and current crowding which inevitably exist in a lateral device [12]. Fig. 4 compares the forward I V characteristics of a typical lateral LED on sapphire and a vertical LED on GaN. The turn-on voltages of the LEDs are similar B3.0 V. The series resistances Rs of the lateral and vertical LEDs are 14.2 and 7.9 O, respectively. The smaller Rs of the vertical LED reflects much more uniform current spreading over the chip cross section, allowing device operation at much higher current densities. Fig. 5 shows the electroluminescence (EL) spectra of the LEDs at increasing pump currents.
10-12
0.25 -20
10-6
Current (A)
-15
-10
-5
25 oC o 50 C 100 oC 150 oC o 200 C
10-5 10-7 10-8
(b)
Forward Current (A)
10
-4
245
10-9 10-10 10-11
Vertical LED on GaN Lateral LED on sapphire
0.20
0.15
0.10
0.05
10-12 -20
-15
-10
-5
0
Reverse Voltage (V) Fig. 3. Temperature-dependent reverse I2V characteristics of the LEDs grown on sapphire and GaN.
0.00 0
1
2
3
4
5
6
Forward Voltage (V) Fig. 4. Forward I2V characteristics of the lateral LED on sapphire and vertical LED on GaN.
ARTICLE IN PRESS X.A. Cao et al. / Journal of Crystal Growth 269 (2004) 242–248
246
Light Intensity (a.u.)
LED/sapphire
LED/GaN
mA 300 mA
200
300
100
200 100
x 10
10 x 10
10
x 100
2
x 100
1
0.5
x 1000
0.1
x 10000
0.01
x 100000
0.001
x 1000
0.1
x 40000 360
380
400
420
440
Wavelength (nm)
460 360
380
400
420
440
460
Wavelength (nm)
Fig. 5. Electroluminescence spectra of the LEDs on sapphire and GaN at different injection currents.
The presence of Fabry–Perot interference fringes is a characteristic feature for LEDs grown on sapphire. As expected, they are absent in the homoepitaxial LED. The peak wavelength of the LED on GaN is 405.5 nm, whereas it is B5 nm longer on sapphire. This discrepancy may be attributed to the slightly different growth temperatures of the InGaN MQWs due to differences in substrate thermal conductivity and thermal coupling of the substrates to the susceptors. Note that the slightly higher growth temperature in the homoepitaxial LED may also contribute to the suppression of V-defects by enhancing the rates of Ga diffusion and incorporation on the off-axis facets [8]. A blue shift in emission energy with increasing current, which is commonly observed in blue and green LEDs, is not seen in either of the LEDs tested in this work, indicating limited piezoelectric field and band-filling effects in the active regions [15]. The latter supports the hypothesis that the density of localized states in the near UV LEDs is much lower due to the small In content. At high currents (>200 mA), the LED on sapphire exhibits a redshift in peak wavelength, indicative of severe joule heating due to the poor
thermal conductivity of sapphire. Another interesting feature in Fig. 5 is that the spectra of the homoepitaxial LED are narrower (FWHM B16 nm). This may be attributed in part to smoother heterointerfaces and thus smaller thickness fluctuations of the InGaN/GaN MQWs. Fig. 6(a) presents the L I characteristics of the LEDs measured in cw injection mode. The same data is plotted on a log–log scale in Fig. 6(b). The light output in the homoepitaxial LED increases steadily with increasing current, and the dependence is nearly linear over the entire current range. In contrast, the light output of the LED on sapphire shows a superlinear increase at low currents, but saturates at B200 mA. At 0.2, 20 and 200 mA the output powers of the LED on GaN are 33 , 45% and 89% higher than the LED on sapphire, respectively. The remarkable improvement of the light emission at low injection levels can be ascribed to reduced nonradiative recombination in the active region. At high currents, the defect states are saturated. The superior performance of the LED on GaN in this case is largely due the improved current spreading and heat dissipation. In addition, inefficient carrier
ARTICLE IN PRESS X.A. Cao et al. / Journal of Crystal Growth 269 (2004) 242–248
LED on sapphire LED on GaN
80
40
0 0 (a)
50
100
150
200
250
300
Forward Current (mA)
Light Output (a.u.)
102 101 100 10-1 10-2 10-3 10-4 10-3 (b)
10-2
10-1
100
101
102
Forward Current (mA)
Fig. 6. L I characteristics of the LEDs on sapphire and GaN on (a) a linear scale and (b) a log–log scale.
confinement in the MQWs due to the poor material quality may also contribute to the early saturation of the output power in the LED on sapphire. To compare the performance of homoepitaxial LEDs with different In contents in the active layers, blue LEDs with a similar structure were also grown on free-standing GaN. The devices exhibited improved light output only at high currents compared to their counterparts on sapphire. This finding suggests that microstructural defects have a much smaller adverse effect on the performance of the blue LEDs. It is generally accepted that large In-related localization effects are responsible for the high radiative efficiencies in blue and green LEDs despite the presence of a high density of dislocations. However, the degree of the localization effects was found to decrease rapidly with decreasing In content in the active layer [9]. Our results confirm that high quality materials, as
yielded by homoepitaxy, are crucial for realizing efficient low-In and In-free light-emitting devices. The external quantum efficiency of the LEDs, which is defined as the number of extracted photons per injected carrier, can be determined by measuring the total output power of packaged devices. In order to estimate the internal quantum efficiencies (Zi ), the light extraction efficiencies of the LEDs, which is the ratio of extracted light to the total light generated inside the MQWs, must be identified. The absorption coefficient of the HVPE GaN at 405 nm was found to be B12 cm1 as determined by transmission measurements, whereas sapphire was essentially transparent. As a result, a considerable amount of downward light in the homoepitaxial LED is absorbed by the substrate. The chip-to-air light extraction efficiencies calculated using ray-tracing simulation are 8.2% and 11.5% for the LEDs on GaN and sapphire, respectively. The deduced Zi of the nearUV LEDs at different injection currents are shown in Fig. 7. At low currents, Zi for both devices increases rapidly with increasing current, indicating enhanced radiative recombination. The efficiency of the homoepitaxial LED reaches its maximum value of 33.7% in the range of 20– 50 mA, twice as high as the device on sapphire. As the current increases further, Zi decreases due to the occurrence of current overflow. Note that the 10 0
Internal Quantum Efficiency
Light Output (a.u.)
120
247
10 -1
10 -2 LED on sapphire LED on GaN 10 -3 10 -3
10 -2
10 -1 10 0 10 1 Forward Current (mA)
10 2
Fig. 7. Internal quantum efficiency of the LEDs as a function of injection current plotted on a log–log scale.
ARTICLE IN PRESS 248
X.A. Cao et al. / Journal of Crystal Growth 269 (2004) 242–248
homoepitaxial LED exhibits a much slower decrease in efficiency at high currents, and therefore is much better suited for high power operation. The device operated well at 300 mA in cw mode, corresponding to a current density of B500 A/cm2, which is one order of magnitude higher than the standard rating for commercially available blue and green LEDs. Further improvement in quantum efficiency is expected by optimizing the LED structure and by further improving the bulk GaN quality. It is also important to improve LED chip design for high light extraction efficiency. One critical issue is to enhance the optical transparency of the bulk GaN to minimize parasitic light absorption. In summary, we have demonstrated that homoepitaxy of InGaN/GaN MQW LEDs yielded greatly improved material quality. The near UV LED grown on free-standing GaN showed a marked reduction in reverse leakage current and forward tunneling current compared to identical devices grown on conventional sapphire substrates. At a typical pump current of 20 mA, an increase in internal quantum efficiency of over 100% was achieved. The homoepitaxial LED with a vertical geometry has proven to be capable of operating at much higher current densities due to the improved heat dissipation and current spreading. These results offer strong support for the viability of bulk GaN substrates for the growth of GaN-based light-emitting devices.
Acknowledgements This work was performed with partial support from the US Department of Commerce, National Institute of Standards and Technology, Advanced
Technology Program, Cooperative Agreement No. 70NANB9H3020.
References [1] J.S. Speck, S.J. Rosner, Physica B 273–274 (1999) 24. [2] T. Deguchi, K. Sekiguchi, A. Nakamura, T. Sota, R. Matsuo, S. Chichibu, S. Nakamura, Jpn. J. Appl. Phys. (Part 2) 38 (1999) L914. [3] A. Yasan, R. McClintock, K. Mayes, S.R. Darvish, H. Zhang, P. Kung, M. Razeghi, S.K. Lee, J.Y. Han, Appl. Phys. Lett. 81 (2002) 2151. [4] T. Mukai, S. Nakamura, Jpn. J. Appl. Phys. 38 (1999) 5735. [5] X.A. Cao, J.M. Teetsov, F. Shahedipour, S.D. Arthur, J. Crystal Growth 264 (2004) 172. [6] P.G. Eliseev, P. Perlin, J. Furioli, P. Sartori, J. Mu, M. Osinski, J. Electron. Mater. 26 (1997) 311. [7] X.A. Cao, E.B. Stokes, P. Sandvik, S.F. LeBoeuf, J. Kretchmer, D. Walker, IEEE Electron. Dev. Lett. 23 (2002) 535. [8] D.I. Florescu, S.M. Ting, J.C. Ramer, D.S. Lee, V.N. Merai, A. Parkeh, D. Lu, E.A. Armour, L. Chernyak, Appl. Phys. Lett. 83 (2003) 33. [9] X.A. Cao, S.F. LeBoeuf, L.B. Rowland, C.H. Yan, H. Liu, Appl. Phys. Lett. 82 (2003) 3614. [10] R.P. Vaudo, X. Xu, C. Lario, A.D. Salant, J.S. Flynn, G.R. Brandes, Phys. Solidi (A) 194 (2002) 494. [11] M.K. Kelly, R.P. Vaudo, V.M. Phanse, L. Gorgens, O. Ambacher, M. Stutzmann, Jpn. J. Appl. Phys. (Part 2) 38 (1999) L217. [12] X.A. Cao, E.B. Stokes, P. Sandvik, N. Taskar, J. Kretchmer, D. Walker, Solid State Electron. 46 (2002) 1235. [13] X.H. Wu, C.R. Elsass, A. Abare, M. Mack, S. Keller, P.M. Petroff, S.P. DenBaars, J.S. Speck, S.J. Rosner, Appl. Phys. Lett. 72 (1998) 692. [14] C.R. Miskys, M.K. Kelly, O. Ambacher, G. MartinezCriado, M. Stutzmann, Appl. Phys. Lett. 77 (2000) 1858. [15] E. Kuokstis, J.W. Yang, G. Simin, M.A. Khan, R. Gaska, M.S. Shur, Appl. Phys. Lett. 80 (2002) 977.
Journal of Crystal Growth 269 (2004) 242–248
Homoepitaxially grown GaN-based light-emitting diodes with peak emission at 405 nm X.A. Caoa,*, C.H. Yanb, M.P. D’Evelyna, S.F. LeBoeufa, J.W. Kretchmera, R. Klingera, S.D. Arthura, D.W. Merfelda a
GE Global Research Center, STL, 1 Research Circle, Niskayuna, NY 12309, USA b AXT Optoelectronics, El Monte, CA 91731, USA Received 2 March 2004; accepted 14 May 2004 Available online 20 July 2004
Abstract Near-ultraviolet InGaN/GaN multiple-quantum-well light-emitting diodes (LEDs) grown homoepitaxially on a free standing GaN substrate exhibited improved microstructural, electrical and optical properties compared to similar devices grown on sapphire. In contrast to strong tunneling behaviors in the LEDs on sapphire, thermally activated currents were observed at both forward and reverse biases in the LEDs on GaN. At low injection currents, the quantum efficiency of the LEDs on GaN was much higher, suggesting reduced nonradative recombination centers. At high driving currents, the homoepitaxial LEDs greatly outperformed the devices on sapphire as a result of improved material quality, heat dissipation and current spreading. These results show the promise of homoepitaxially grown GaN-based LEDs for solid-state lighting applications. r 2004 Elsevier B.V. All rights reserved. PACS: 73.40.Kp; 78.60.Fi; 81.15.Gh; 85.60.Jb Keywords: A3. Homoepitaxy; B1.GaN; B1. InGaN; B3. Light emitting diode; B3. Electroluminescence
1. Introduction GaN-based light-emitting diodes (LEDs) grown heteroepitaxially on sapphire or SiC substrates suffer from a high density of threading dislocations and excess strain [1–9]. These problems can be largely overcome by using a high-quality bulk *Corresponding author. Tel.: +1-5183874361; fax: +15183875997. E-mail address: [email protected] (X.A. Cao).
GaN substrate. The procedures for epitaxial growth of LED structures on GaN are greatly simplified because additional steps such as surface nitridation and deposition of a low-temperature buffer layer which are necessary in heteroepitaxy are not needed. The defects and strain which arise from the mismatch in lattice constant and thermal expansion coefficient of the epitaxial layers and substrate are substantially reduced. In addition, LEDs grown on an electrically conductive GaN substrate can be fabricated in a vertical configura-
0022-0248/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2004.05.065
ARTICLE IN PRESS X.A. Cao et al. / Journal of Crystal Growth 269 (2004) 242–248
tion, eliminating current crowding as occurs in lateral LEDs on sapphire and enabling highcurrent operation. The recent development of hydride-vaporphase-epitaxy (HVPE) growth of bulk-like GaN has led to remarkable improvement in crystal quality and size. Large (up to 200 ) free-standing crack-free GaN substrates are now commercially available [10,11]. Yasan et al. [3] demonstrated 340 nm AlGaN LEDs on a HVPE grown GaN substrate, and the device produced an output power one order of magnitude higher than similar LEDs grown on sapphire. It is generally believed, however, that defect reduction will have less impact on the optical performance of InGaN/ GaN LEDs due to strong carrier localization effects in InGaN alloys. Previous work done by Mukai et al. [4] showed that blue and green InGaN single-quantum-well LEDs grown on epitaxially laterally overgrown GaN and sapphire had similar external quantum efficiencies. In this work, we report on structural, electrical and optical characterization of near UV InGaN/ GaN multiple-quantum-well (MQW) LEDs grown on HVPE GaN and sapphire substrates. LEDs with a peak wavelength near 405 nm, when used in conjunction with multiband phosphors, can produce white light and are currently receiving much attention due to their promise for solid-state lighting applications. We observe a greatly improved performance of the homoepitaxially grown LEDs compared to their counterparts on sapphire, particularly at high currents, confirming the advantages of bulk GaN substrates for epitaxy of low-In InGaN-based light-emitting devices.
2. Experimental procedure InGaN/GaN MQW LEDs with an identical structure were grown on two different substrates— (i) a 200 sapphire substrate with a 100 nm GaN buffer layer, and (ii) a free-standing 1 1 cm2 HVPE GaN wafer with a polished Ga-face—using metalorganic chemical vapor deposition. The GaN substrate was unintentionally doped with a carrier concentration of B7 1017 cm3. The LED structure consisted of a 10-period InGaN/GaN MQW
243
active layer sandwiched by a 2 mm n-GaN layer (SiB5 1018 cm3) and a 0.1 mm p-type AlGaN cladding layer, and a 0.2 mm p-GaN contact layer (Mg B1 1019 cm3). The nominal indium mole fraction in the InGaN MQWs was B0.10. The microstrutural properties of the LEDs were characterized using atomic force microscopy (AFM) and transmission electron microscopy (TEM). Top-emitting LEDs with a size of 300 300 mm2 were fabricated using standard photolithography and dry etch techniques [12]. Ni/Au (5 nm/6 nm) was deposited as the semitransparent p-type contact. Ti/Al metallization was formed on the plasma-etched n-GaN to produce conventional lateral-structure LEDs on sapphire, whereas a fullarea Ti/Al contact was formed on the N-face of the GaN substrate as the n-type electrode of LEDs with a vertical configuration. The electrical and optical characteristics of the LEDs were measured using a Keithley 238 current source measurement unit and a silicon photodiode-array fiber-optic spectrometer.
3. Results and discussion Fig. 1 illustrates typical 10 10 mm2 AFM images of the as-grown LED samples. The LED on sapphire exhibits a surface density of B1 108 cm2 so-called V-defects [13] with a size in the range of 50–200 nm. In contrast, the surface of the LED on GaN is free of visible defects. In addition to micro-scale morphological features consisting of long valleys, atomic step structure with terraces of B100 nm can be seen. The rms roughness of the LED structures on sapphire and GaN are 1.7 and 0.7 nm, respectively. The smoother surface of the homoepitaxial LED may be indicative of very abrupt InGaN/GaN and AlGaN/GaN heterostructural interfaces. TEM measurements showed that the density of threading dislocations reaching the active region in the LED on sapphire was B2–4 109 cm2. A small portion of the dislocations were terminated by V-defects in the MQWs or the p-GaN layer. The V-defects and the associated dislocations, which in most cases were found to have mixed or
ARTICLE IN PRESS 244
X.A. Cao et al. / Journal of Crystal Growth 269 (2004) 242–248
Current (A)
the density of dislocations with screw and mixed character. Fig. 2 shows the forward I2V characteristics of the LED on sapphire measured at increasing temperatures. At low and moderate forward biases, two main exponential segments with different slopes can be distinguished for the LED on sapphire (Fig 2(a)). The I2V characteristics vary as I ¼ I0 expðqV =EÞ; where the energy parameter E is nearly temperature-independent, and has values of 190 and 70 meV in the voltage ranges 0–2.0 V and 2.0–2.8 V, respectively. No realistic ideality factors can be extracted. These behaviors are characteristic of tunneling current in a semiconductor diode [6,7]. At high bias (>3.0 V), the I2V behavior is dominated by series resistance. The forward I2V characteristics of the LED on GaN also divide into two distinct linear sections with different slopes (see Fig. 2(b)). However, the slopes appear to be a function of temperature. At low injection levels tunneling may
pure screw character, may behave as leakage current paths connected across the p–n junctions [5]. As expected, the dislocation density in the homoepitaxially grown LED was much lower. The actual value, however, was too small to be determined by cross-sectional TEM. Earlier work suggested that the dislocation density in homoepitaxially grown GaN was similar to that in the original GaN substrate [14]. The dislocations in the as-received GaN wafers were found to be terminated by surface pits after the final chemical mechanical polish, and the density was B2 107 cm2. The absence of V-defects in the LED on GaN suggests a substantial reduction in
Current (A)
Fig. 1. Typical 10 10 mm2 AFM images of the LEDs grown on (a) sapphire and (b) GaN. The gray scale is 15 nm.
10-2 10-3 10-4 10-5 10-6 10-7 10-8 10-9 10-10 10-11 10-12 10-2 10-3 10-4 10-5 10-6 10-7 10-8 10-9 10-10 10-11 10-12 0.0
(a) 0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
o
25 C 50 oC o 100 C 150 oC 200 oC
(b)
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
Forward Voltage (V) Fig. 2. Temperature-dependent forward I2V characteristics of the LEDs grown on sapphire and GaN.
ARTICLE IN PRESS X.A. Cao et al. / Journal of Crystal Growth 269 (2004) 242–248
still dominate, but the temperature dependence of the slope suggests the involvement of thermally activated currents. As the injection level increases, diffusion and recombination currents start to dominate over the tunneling component. At forward bias >2.6 V the current can be described by the conventional Shockley model as I ¼ I0 expðqV =1:5 kTÞ until series resistance in the diode dominates. The dominance of diffusionrecombination current reflects the high material quality of the homoepitaxial LED where defectassisted tunneling current is greatly suppressed. The temperature-dependent reverse I2V characteristics of the LEDs are shown in Fig. 3. The current in the LED on sapphire is much higher than conventional diffusion and generation currents, which are immeasurably small in GaNbased p–n junctions. The strong field-dependence but low-temperature sensitivity of the leakage current is indicative of carrier tunneling. In sharp contrast, the LED on GaN shows a dramatic
10-4
(a)
10-5
Current (A)
10
-6
10-7 10-8 10-9 10-10 10-11
reduction in reverse current by more than six orders of magnitude, consistent with much lower surface and bulk defect densities in this LED. The strong dependence of the remaining leakage current on temperature indicates the presence of a thermally generated current. In particular, the sudden jump in the low-bias current between 100 C and 150 C may arise from thermal ionization of carriers from deep traps and trap-assisted tunneling process. An activation energy of B0.6 eV is extracted from the log ðIÞF1=T plot at 12.5 V, where the change of the current as a function of temperature is roughly an exponential function. The vertical geometry of the LEDs on GaN significantly simplifies the procedures for chip processing and packaging. More importantly, it eliminates or reduces long lateral current paths and current crowding which inevitably exist in a lateral device [12]. Fig. 4 compares the forward I V characteristics of a typical lateral LED on sapphire and a vertical LED on GaN. The turn-on voltages of the LEDs are similar B3.0 V. The series resistances Rs of the lateral and vertical LEDs are 14.2 and 7.9 O, respectively. The smaller Rs of the vertical LED reflects much more uniform current spreading over the chip cross section, allowing device operation at much higher current densities. Fig. 5 shows the electroluminescence (EL) spectra of the LEDs at increasing pump currents.
10-12
0.25 -20
10-6
Current (A)
-15
-10
-5
25 oC o 50 C 100 oC 150 oC o 200 C
10-5 10-7 10-8
(b)
Forward Current (A)
10
-4
245
10-9 10-10 10-11
Vertical LED on GaN Lateral LED on sapphire
0.20
0.15
0.10
0.05
10-12 -20
-15
-10
-5
0
Reverse Voltage (V) Fig. 3. Temperature-dependent reverse I2V characteristics of the LEDs grown on sapphire and GaN.
0.00 0
1
2
3
4
5
6
Forward Voltage (V) Fig. 4. Forward I2V characteristics of the lateral LED on sapphire and vertical LED on GaN.
ARTICLE IN PRESS X.A. Cao et al. / Journal of Crystal Growth 269 (2004) 242–248
246
Light Intensity (a.u.)
LED/sapphire
LED/GaN
mA 300 mA
200
300
100
200 100
x 10
10 x 10
10
x 100
2
x 100
1
0.5
x 1000
0.1
x 10000
0.01
x 100000
0.001
x 1000
0.1
x 40000 360
380
400
420
440
Wavelength (nm)
460 360
380
400
420
440
460
Wavelength (nm)
Fig. 5. Electroluminescence spectra of the LEDs on sapphire and GaN at different injection currents.
The presence of Fabry–Perot interference fringes is a characteristic feature for LEDs grown on sapphire. As expected, they are absent in the homoepitaxial LED. The peak wavelength of the LED on GaN is 405.5 nm, whereas it is B5 nm longer on sapphire. This discrepancy may be attributed to the slightly different growth temperatures of the InGaN MQWs due to differences in substrate thermal conductivity and thermal coupling of the substrates to the susceptors. Note that the slightly higher growth temperature in the homoepitaxial LED may also contribute to the suppression of V-defects by enhancing the rates of Ga diffusion and incorporation on the off-axis facets [8]. A blue shift in emission energy with increasing current, which is commonly observed in blue and green LEDs, is not seen in either of the LEDs tested in this work, indicating limited piezoelectric field and band-filling effects in the active regions [15]. The latter supports the hypothesis that the density of localized states in the near UV LEDs is much lower due to the small In content. At high currents (>200 mA), the LED on sapphire exhibits a redshift in peak wavelength, indicative of severe joule heating due to the poor
thermal conductivity of sapphire. Another interesting feature in Fig. 5 is that the spectra of the homoepitaxial LED are narrower (FWHM B16 nm). This may be attributed in part to smoother heterointerfaces and thus smaller thickness fluctuations of the InGaN/GaN MQWs. Fig. 6(a) presents the L I characteristics of the LEDs measured in cw injection mode. The same data is plotted on a log–log scale in Fig. 6(b). The light output in the homoepitaxial LED increases steadily with increasing current, and the dependence is nearly linear over the entire current range. In contrast, the light output of the LED on sapphire shows a superlinear increase at low currents, but saturates at B200 mA. At 0.2, 20 and 200 mA the output powers of the LED on GaN are 33 , 45% and 89% higher than the LED on sapphire, respectively. The remarkable improvement of the light emission at low injection levels can be ascribed to reduced nonradiative recombination in the active region. At high currents, the defect states are saturated. The superior performance of the LED on GaN in this case is largely due the improved current spreading and heat dissipation. In addition, inefficient carrier
ARTICLE IN PRESS X.A. Cao et al. / Journal of Crystal Growth 269 (2004) 242–248
LED on sapphire LED on GaN
80
40
0 0 (a)
50
100
150
200
250
300
Forward Current (mA)
Light Output (a.u.)
102 101 100 10-1 10-2 10-3 10-4 10-3 (b)
10-2
10-1
100
101
102
Forward Current (mA)
Fig. 6. L I characteristics of the LEDs on sapphire and GaN on (a) a linear scale and (b) a log–log scale.
confinement in the MQWs due to the poor material quality may also contribute to the early saturation of the output power in the LED on sapphire. To compare the performance of homoepitaxial LEDs with different In contents in the active layers, blue LEDs with a similar structure were also grown on free-standing GaN. The devices exhibited improved light output only at high currents compared to their counterparts on sapphire. This finding suggests that microstructural defects have a much smaller adverse effect on the performance of the blue LEDs. It is generally accepted that large In-related localization effects are responsible for the high radiative efficiencies in blue and green LEDs despite the presence of a high density of dislocations. However, the degree of the localization effects was found to decrease rapidly with decreasing In content in the active layer [9]. Our results confirm that high quality materials, as
yielded by homoepitaxy, are crucial for realizing efficient low-In and In-free light-emitting devices. The external quantum efficiency of the LEDs, which is defined as the number of extracted photons per injected carrier, can be determined by measuring the total output power of packaged devices. In order to estimate the internal quantum efficiencies (Zi ), the light extraction efficiencies of the LEDs, which is the ratio of extracted light to the total light generated inside the MQWs, must be identified. The absorption coefficient of the HVPE GaN at 405 nm was found to be B12 cm1 as determined by transmission measurements, whereas sapphire was essentially transparent. As a result, a considerable amount of downward light in the homoepitaxial LED is absorbed by the substrate. The chip-to-air light extraction efficiencies calculated using ray-tracing simulation are 8.2% and 11.5% for the LEDs on GaN and sapphire, respectively. The deduced Zi of the nearUV LEDs at different injection currents are shown in Fig. 7. At low currents, Zi for both devices increases rapidly with increasing current, indicating enhanced radiative recombination. The efficiency of the homoepitaxial LED reaches its maximum value of 33.7% in the range of 20– 50 mA, twice as high as the device on sapphire. As the current increases further, Zi decreases due to the occurrence of current overflow. Note that the 10 0
Internal Quantum Efficiency
Light Output (a.u.)
120
247
10 -1
10 -2 LED on sapphire LED on GaN 10 -3 10 -3
10 -2
10 -1 10 0 10 1 Forward Current (mA)
10 2
Fig. 7. Internal quantum efficiency of the LEDs as a function of injection current plotted on a log–log scale.
ARTICLE IN PRESS 248
X.A. Cao et al. / Journal of Crystal Growth 269 (2004) 242–248
homoepitaxial LED exhibits a much slower decrease in efficiency at high currents, and therefore is much better suited for high power operation. The device operated well at 300 mA in cw mode, corresponding to a current density of B500 A/cm2, which is one order of magnitude higher than the standard rating for commercially available blue and green LEDs. Further improvement in quantum efficiency is expected by optimizing the LED structure and by further improving the bulk GaN quality. It is also important to improve LED chip design for high light extraction efficiency. One critical issue is to enhance the optical transparency of the bulk GaN to minimize parasitic light absorption. In summary, we have demonstrated that homoepitaxy of InGaN/GaN MQW LEDs yielded greatly improved material quality. The near UV LED grown on free-standing GaN showed a marked reduction in reverse leakage current and forward tunneling current compared to identical devices grown on conventional sapphire substrates. At a typical pump current of 20 mA, an increase in internal quantum efficiency of over 100% was achieved. The homoepitaxial LED with a vertical geometry has proven to be capable of operating at much higher current densities due to the improved heat dissipation and current spreading. These results offer strong support for the viability of bulk GaN substrates for the growth of GaN-based light-emitting devices.
Acknowledgements This work was performed with partial support from the US Department of Commerce, National Institute of Standards and Technology, Advanced
Technology Program, Cooperative Agreement No. 70NANB9H3020.
References [1] J.S. Speck, S.J. Rosner, Physica B 273–274 (1999) 24. [2] T. Deguchi, K. Sekiguchi, A. Nakamura, T. Sota, R. Matsuo, S. Chichibu, S. Nakamura, Jpn. J. Appl. Phys. (Part 2) 38 (1999) L914. [3] A. Yasan, R. McClintock, K. Mayes, S.R. Darvish, H. Zhang, P. Kung, M. Razeghi, S.K. Lee, J.Y. Han, Appl. Phys. Lett. 81 (2002) 2151. [4] T. Mukai, S. Nakamura, Jpn. J. Appl. Phys. 38 (1999) 5735. [5] X.A. Cao, J.M. Teetsov, F. Shahedipour, S.D. Arthur, J. Crystal Growth 264 (2004) 172. [6] P.G. Eliseev, P. Perlin, J. Furioli, P. Sartori, J. Mu, M. Osinski, J. Electron. Mater. 26 (1997) 311. [7] X.A. Cao, E.B. Stokes, P. Sandvik, S.F. LeBoeuf, J. Kretchmer, D. Walker, IEEE Electron. Dev. Lett. 23 (2002) 535. [8] D.I. Florescu, S.M. Ting, J.C. Ramer, D.S. Lee, V.N. Merai, A. Parkeh, D. Lu, E.A. Armour, L. Chernyak, Appl. Phys. Lett. 83 (2003) 33. [9] X.A. Cao, S.F. LeBoeuf, L.B. Rowland, C.H. Yan, H. Liu, Appl. Phys. Lett. 82 (2003) 3614. [10] R.P. Vaudo, X. Xu, C. Lario, A.D. Salant, J.S. Flynn, G.R. Brandes, Phys. Solidi (A) 194 (2002) 494. [11] M.K. Kelly, R.P. Vaudo, V.M. Phanse, L. Gorgens, O. Ambacher, M. Stutzmann, Jpn. J. Appl. Phys. (Part 2) 38 (1999) L217. [12] X.A. Cao, E.B. Stokes, P. Sandvik, N. Taskar, J. Kretchmer, D. Walker, Solid State Electron. 46 (2002) 1235. [13] X.H. Wu, C.R. Elsass, A. Abare, M. Mack, S. Keller, P.M. Petroff, S.P. DenBaars, J.S. Speck, S.J. Rosner, Appl. Phys. Lett. 72 (1998) 692. [14] C.R. Miskys, M.K. Kelly, O. Ambacher, G. MartinezCriado, M. Stutzmann, Appl. Phys. Lett. 77 (2000) 1858. [15] E. Kuokstis, J.W. Yang, G. Simin, M.A. Khan, R. Gaska, M.S. Shur, Appl. Phys. Lett. 80 (2002) 977.