- Email: [email protected]
GaN short-period superlattice
Journal of Crystal Growth 315 (2011) 267–271
Contents lists available at ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Single quantum well deep-green LEDs with buried InGaN/GaN short-period superlattice W.V. Lundin a,b,n, A.E. Nikolaev a,b, A.V. Sakharov a,b, E.E. Zavarin a,b, G.A. Valkovskiy a, M.A. Yagovkina a, S.O. Usov b, N.V. Kryzhanovskaya a,b, V.S. Sizov a,b, P.N. Brunkov a, A.L. Zakgeim b, A.E. Cherniakov b, N.A. Cherkashin c, M.J. Hytch c, E.V. Yakovlev d, D.S. Bazarevskiy d, M.M. Rozhavskaya b, A.F. Tsatsulnikov a,b a
Ioffe Physico-Technical Institute of the Russian Academy of Science, Politechnicheskaya 26, St. Petersburg 194021, Russia Scientific and Technological Center for Microelectronics and Submicron Heterosctructures of the Russian Academy of Science, Politechnicheskaya 26, St. Petersburg 194021, Russia CEMES/CNRS, 29 rue Jeanne Marvig, 31055 Toulouse, France d STR Group—Soft-Impact Ltd., P.O. Box 89, St. Petersburg 194156, Russia b c
a r t i c l e in fo
abstract
Available online 22 September 2010
In spite of the great progress in III-N technology, LEDs with wavelength 4 530 nm still exhibit low efficiency compared to blue and short-wavelength-green LEDs. Here we report on significant improvement of deep-green LED properties by modifications of the structure design. The combination of InGaN/GaN superlattice followed by low-temperature GaN is the key element to increase the electroluminescence efficiency for deep-green LED. Various techniques were employed to clarify the correlation between structure properties, growth regimes and design. Modification of the defect structure of the GaN buffer by InGaN layers appears to be mostly responsible for the observed effect. LEDs processed and assembled in a standard flip-chip geometry with Ni–Ag p-contact demonstrate external quantum efficiencies of 8–20% in the 560–530 nm range. & 2010 Elsevier B.V. All rights reserved.
Keywords: A1. Nanostructures A3. MOCVD B1. Nitrides B3. LEDs
1. Introduction Light-emitting diodes (LEDs) emitting in the 540–570 nm range are very important for the development of new generations of multi-color light emitters including smart white RGB light sources for solid-state lighting (SSL) [1]. However, in spite of the great progress in III-N technology, LEDs with peak wavelength above 530 nm still suffer from low efficiency compared to blue and short-wavelength-green LEDs [2]. Few effects are responsible for the strong deterioration of InGaN/GaN LED efficiency with wavelength increase. Significant lattice mismatch between GaN and InGaN with indium content high enough for long-wavelength emission results in the formation of additional dislocations in the active region [3]. The wellknown tendency of InGaN to phase separation, fruitful in blue LED structures due to the formation of localized centers increasing EL efficiency, with indium content increase can result in the formation of large dislocated clusters [4]. Quantum confined Stark effect (QCSE) as well as Auger processes [5] also become
n
Corresponding author at: Ioffe Physico-Technical Institute of the Russian Academy of Science, Politechnicheskaya 26, St. Petersburg 194021, Russia. Tel./fax: +7 812 2973182. E-mail address: [email protected] (W.V. Lundin). 0022-0248/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2010.09.043
stronger as the In content in InGaN increases. In spite of some gradual improvement in green LED properties, it seems that the ideas used to improve the properties of blue LEDs have not led to high-performance green LEDs. QCSE may be reduced or totally eliminated by changing the growth plane and significant progress in green laser diodes was achieved that way [6,7]. However, non-polar or semi-polar GaN substrates are still small and too expensive for LED production and non- or semi-polar GaN grown on other substrates is too imperfect for high-efficiency device applications. It was reported that utilization of c-plane low-dislocationdensity GaN substrates suppresses the deterioration of InGaN QW properties with increase in the indium content [8], which allowed the fabrication of green laser diodes [9] but again such substrates are too expensive for LEDs. Thus, a new way to form deep-green LED structures should be investigated. Here we report on significant improvement in deepgreen LED properties by modifications of structure design.
2. Experiment The structures were grown in an AIX2000HT 6 200 planetary reactor on (0 0 0 1) sapphire substrates, utilizing a conventional
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low-temperature GaN nucleation layer technique. Hydrogen was used as the carrier gas for the growth of undoped and n-doped GaN buffer layers, while a nitrogen–hydrogen 1:1 mixture or pure nitrogen were used as carrier gas for the growth of p-doped layers. Indium-containing layers were grown using nitrogen as a carrier gas, but hydrogen also was used during the formation of these layers as described later. First, 2.5 mm of GaN buffer layer were grown at 800 mbar reactor pressure, then the pressure was reduced to 400 mbar for the subsequent part of the n-GaN buffer. The InGaN/GaN SL was formed at 200 mbar, and the rest of the structure was grown at 300 mbar. Deep-green LED structures in this study (Fig. 1) were composed of 5 mm GaN buffer grown at 1190 1C, 24-nm-thick InGaN/GaN superlattice (SL) grown at 910 1C, n-GaN barrier grown at the same temperature as the SL (LT GaN), InGaN QW grown at 810–835 1C, undoped upper GaN barrier grown at a temperature 70 1C higher than that used for the QW, 15 nm p-AlGaN, and 120 nm p-GaN grown at 1000 1C. The temperatures mentioned were measured by a pyrometer through a bottom lightpipe and correspond to the lower surface of the main susceptor. The InGaN QW growth temperature was adjusted for the desired emission wavelength. The number of periods in the InGaN/GaN SL, thickness of LT-GaN and upper GaN barrier were varied during structure design optimization as described later. The InGaN/GaN SLs in the investigated structures were formed using a InGaN-conversion technique [10–12] by alternating growth of In0.1Ga0.9N and growth interruption (GI) where the alkyl precursors were switched off and the carrier gas composition was changed from pure N2 to a mixture of N2:H2 ¼7:3. In this procedure, the indium concentration on the surface is governed by an interplay between InGaN decomposition during the GI, indium segregation, desorption, and incorporation into InGaN during subsequent growth. Addition of hydrogen to the reactor ambient during GI increases the rate of indium atoms release from In0.1Ga0.9N, but not very significantly due to relatively low InGaN indium content. At the same time, in the presence of hydrogen not only indium, but also gallium atoms are released from the bulk crystal to the surface, but in contrast to indium, gallium cannot evaporate at this low temperature. Thus Ga adatoms migrate over the surface and re-incorporate in the crystal. All of the processes together lead to conversion of InGaN to GaN in the very upper layer. If the growth-GI process is repeated several times, an SL structure is formed as confirmed by TEM (see Fig. 1) and HRXRD. In most of the structures investigated, the number of periods of the SL was 12, each InGaN growth time was 60 s and the duration of the GI was 20 s. Under these conditions the SL period is 2 nm, which is practically equal to the thickness of individual InGaN layers grown between GIs,
Fig. 1. TEM image of the active region of deep-green LED.
and the InN mole fraction periodically changes from the initial 10% to nearly zero. The structures grown were investigated using photo- and electroluminescence spectroscopy, high-resolution X-ray diffractometry (HRXRD) and atomic force microscopy (AFM). Weakbeam dark field transmission electron microscopy (WBDF TEM) imaging was used for dislocation structure characterization. Geometric phase analysis (GPA) [13] of high-resolution TEM (HRTEM) images obtained at the SACTEM-Toulouse microscope [14] was used for 2D mapping of the distribution of the strain perpendicular to a surface in the heterostructures. The effect of free surfaces of a TEM lamella on strain relaxation was estimated by finite element simulations. Afterward, the indium composition within InGaN layers was deduced using Vegard’s law. Special precautions taken for the sample preparation and regarding the conditions used for imaging allowed to avoid the InGaN deterioration under an electron beam, which was described in [15]. The LEDs fabricated from the grown structures were processed in a flip-chip geometry with Ni–Ag p-contact (no special techniques of light extraction improvement such as roughening or lift-off were used), and investigated after encapsulation using an Optronic Laboratories OL770-LED system with an OL ISA-670 integrating sphere.
3. Optimization of deep-green LED structure To minimize the formation of new misfit dislocations, single QW structures were used. During structure optimization it was discovered that incorporation of InGaN/GaN SL and LT GaN barrier below the QW results in a 15–30 times increase in EL efficiency in comparison with a simple structure with QW grown directly on top of the high-temperature n-GaN layer (Fig. 2). It should be stressed that InGaN/GaN SL and LT GaN barriers improve LED
Fig. 2. Dependence of external quantum efficiency on current for LED structures of various designs emitting at 550 nm (on-wafer probing). 1: QW grown on high-temperature GaN buffer; 2: QW grown on LT GaN; 3,4: QW grown on 12-period InGaN/GaN SL capped with LT-GaN; 5: QW grown on 3-period InGaN/ GaN SL capped with LT-GaN. 1–3: p-GaN grown with H2+ N2 carrier gas; 4,5: p-GaN grown with N2 carrier gas.
W.V. Lundin et al. / Journal of Crystal Growth 315 (2011) 267–271
properties only if implemented together and have a small effect if used alone (at least in our growth conditions). Further optimization includes adjustment of the thicknesses of the GaN layers adjacent to the QW. The dependence of LED efficiency on thickness of LT-GaN between SL and QW is very strong at low thickness and nearly saturates around 10–15 nm, but some rise may be observed until 20–25 nm. The role of this LT-GaN seems to be the same as in previously reported blue LEDs [10]—just to prevent injected holes overflow from QW to InGaN/ GaN SL. The optimal value of undoped GaN barrier above the QW is about 5 nm. Too high a thickness results in carrier injection deterioration, while at too low a thickness of this layer, chemical and thermal decomposition of the InGaN QW occurs during p-AlGaN growth. All structures described above were grown using an N2:H2 mixture as a carrier gas for all p-doped materials in accordance with our blue-LED fabrication technology. The next step of
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deep-green LED optimization was done using pure nitrogen instead of an N2:H2 mixture as a carrier gas during p-GaN growth: this results in a noticeable increase in EL efficiency (Fig. 2, curve 4). These structures were processed and assembled in a standard flip-chip geometry with 0.156 mm2 p–n junction area and Ni–Ag p-contact. External efficiencies of 8–20% in the 560–530 nm range were achieved (see Fig. 3). All structures described earlier contain the same 12-fold InGaN/GaN SL with 2 nm period. In the next step we have investigated the influence of SL design on LED properties. The total SL thickness was fixed at 24 nm while the number of periods was varied by changing the number of GIs from 1 to 24. The sum of the GI durations was kept constant. The best results were obtained for a 3-period SL (Fig. 2, curve 5). The dependence of maximal external quantum efficiency on emission wavelength for the series of samples described above is summarized in Fig. 4. It can be observed that the best results were achieved by the combination of a 3-period SL and growth of p-GaN in hydrogen-free ambient over the whole wavelength range investigated. At the same time, a 24-period SL also gives better results than a 12-period SL at around 560 nm.
4. Discussion
Fig. 3. Dependence of external quantum efficiency on current for LEDs with p-GaN grown using N2 carrier gas and 12-period InGaN/GaN SL.
Fig. 4. Dependence of external quantum efficiency on emission wavelength for the series of LED structures.
Summarizing experimental results, two major effects were observed: very strong improvement in LED efficiency by InGaN/ GaN SL incorporation and noticeable improvement in efficiency by p-GaN growth under hydrogen-free ambient. The nature of both the effects is unclear yet. Some hypotheses and speculations are presented below. The possible reasons for the advantage of H2-free ambient for p-GaN formation for deep-green LEDs are as follows: first of all, as it was shown in [11,16], in hydrogen-free ambient the growth rate anisotropy of Mg-doped GaN is less pronounced and, even more importantly, realization of the p-type conductivity in GaN:Mg is insensitive to the growth direction. These facts come from less equilibrium-like GaN growth conditions in the absence of hydrogen. A deep-green LED structure is very close to the onset of strain relaxation and thus some locally imperfect regions may be formed in the structure. Hydrogen-free growth conditions result in suppression of p-GaN nonuniformity caused by these local disturbances of material properties. The second reason (which appears less credible) is the possible decomposition of InGaN by hydrogen penetrating through the AlGaN barrier during p-GaN layer growth. It should be mentioned that investigation of the effect of p-GaN growth temperature on deep-green LED properties has shown that the influence of H2 on the LED properties cannot be explained in terms of surface temperature dependence on the carrier gas composition. Changing the carrier gas also did not affect the p-GaN morphology and thus the effect cannot be explained in terms of light extraction efficiency. While the role of LT GaN between SL and QW is more or less clear, the effect of the SL itself on LED properties should be clarified. Implementation of the SL in an LED structure may result in: reduction of dislocation density, strain relaxation resulting in metamorphic growth of the active region, and modification of the surface morphology beneath the QW. The first two assumptions can be rejected after analysis of HRXRD reciprocal space mapping and TEM data. The last assumption was clarified during investigation of the influence of the SL design on LED properties. In Fig. 5 one can see AFM images of three structures with InGaN/GaN SLs covered with LT GaN (just beneath the active QW of a full LED). The morphologies of these structures are very different: for the 12- and 24-period SL, the surfaces look more or less 2D like high-temperature GaN while for
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Fig. 5. AFM images of 24-nm-thick InGaN/GaN SLs capped with LT-GaN. (a) 3-period SL fabricated with 80 s GIs, (b) 12-period SL fabricated with 20 s GIs, (c) 24-period SL fabricated with 10 s GIs.
the 3-period SL with thick InGaN layers it is clearly of a 3D type. At the same time, there is no clear correlation between surface morphology and LED properties. In the 555–570 nm range, the efficiency of LEDs with 3- and 24-period SL differs by less than 20% while for the structure with 12-period SL the efficiency is much lower in spite of the very similar morphologies with 12- and 24-period SLs. However, in the 545–555 nm range, LEDs with 24-period and 12-period SLs show a similar efficiency, which is much lower than for an LED with 3-period SL. Moreover, an increase in efficiency obtained by implementation of any of the SLs used in the deep-green LED structure is much stronger than any further improvement by SL formation process modification. So the surface morphology at least is not the main reason for the effect. Further investigations are needed for separating the effects of SL design and surface morphology on EL efficiency. As all hypotheses listed above have been rejected, we have to assume some other material properties’ modification by the InGaN/GaN SL. GPA of HRTEM images of deep-green LEDs reveals that if an InGaN QW is grown directly on top of high-temperature GaN alloy composition fluctuations having a lateral size of about 100 nm are very pronounced while no such nonuniformity is observed in the structures grown with InGaN/GaN SL overgrown by LT GaN. Thus, the origin of the differences should be looked for at the scale of tens to hundreds of nanometers. The dislocation density in our GaN buffer layers, as revealed by TEM measurements, is about 5 6 108 cm 2 without any manifestation of mosaic structure with domain boundaries formed with dislocations. At the same time HRXRD reciprocal space mapping revealed a pronounced mosaic structure for our GaN buffer layers. The characteristic domain size in this mosaic structure (70–100 nm) is well below the dislocation-to-dislocation distance. So, the defect structure is not governed only by dislocations. GPA of HRTEM images has revealed strong nonuniformity of the SLs in the lateral direction at the nanometer scale (see Fig. 6). So, these structures are not classical short-period SLs, but layered nanocomposites, which is confirmed by optical investigations as well [12]. It looks feasible that the material with such kind of structure can suppress the influence of GaN buffer domain boundaries on InGaN QW formation, promoting for it a uniform composition. Although the mechanism for this process is unclear at present, the above speculations at the moment appear to be the only explanation we can propose. Further investigations are needed for clarification.
Fig. 6. GPA of HRTEM: distribution of the strain perpendicular to the surface eGaN zz with respect to GaN in the active region of a deep-green LED with 12-period InGaN/ GaN SL. A higher positive value of strain corresponds to higher indium content.
5. Conclusions Summarizing the results we conclude that using an InGaN/GaN SL fabricated by the InGaN conversion technique and capping it with low-temperature GaN as a buffer for InGaN QW results in a strong increase in the efficiency of LEDs emitting in the deep-green range. The main reason for this effect seems to originate from the modification of the defect structure of the grown material at the lateral scale of about 100 nm. Although the details of this mechanism are not entirely clear, the significant improvement in the deep-green LED properties is supported by a large number of experiments. Deep-green LEDs with external efficiencies of 8–20% in the 560–530 nm range were fabricated. Acknowledgements The work was supported by a governmental contract No. 02.523.12.3017 from 14.08.2008, federal educational agency (‘‘Technology of monolithic polychromatic white light emitter’’),
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Presidium of the RAS (Basic Research Program No. 27), and Russian Foundation for Basic Research. Facilities of regional JRC Material science and characterization in advanced technologies were used. Processing and assembling of LEDs by SvetlanaOptoelectronics company is very much appreciated for. References [1] A. Zukauskas, M. Shur, R. Gaska, Introduction to Solid-State Lighting, John Wiley & Sons, New York, 2002. [2] D.F. Feezell, M.C. Schmidt, S.P. DenBaars, S. Nakamura, MRS Bull. V 34 (5) (2009) 318–323. [3] D.M. Graham, P. Dawson, M.J. Godfrey, M.J. Kappers, P.M.F.J. Costa, M.E. Vickers, R. Datta, C.J. Humphreys, E.J. Thrush, High quantum efficiency InGaN/GaN structures emitting at 540 nm, Phys. Status Solidi (c) 3 (2006) 1970–1973. [4] Y.-H. Cho, S.K. Lee, H.S. Kwack, J.Y. Kim, K.S. Lim, H.M. Kim, T.W. Kang, S.N. Lee, M.S. Seon, O.H. Nam, Y.J. Park, Appl. Phys. Lett. 83 (13) (2003) 2578–2580. [5] K.T. Delaney, P. Rinke, C.G. Van de Walle, Appl. Phys. Lett. 94 (2009) 191109. [6] H. Asamizu, M. Saito, K. Fujito, J.S. Speck, S.P. DenBaars, S. Nakamura, Appl. Phys. Express 2 (2009) 021002.
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[7] K. Okamoto, J. Kashiwagi, T. Tanaka, M. Kubota, Appl. Phys. Lett. 94 (2009) 071105. [8] A. Avramescu, M. Schillgalies, D. Queren, S. Lutgen, G. Bruederl, U. Strauss, IC-MOVPE XIV, Metz, 1–6 June, 2008, abstracts, We-A1.3, p. 219. ¨ [9] A. Avramescu, T. Lermer, J. Muller, Ch. Eichler, G. Bruederl, M. Sabathil, S. Lutgen, U. Strauss, Appl. Phys. Express 3 (2010) 061003. [10] A.F. Tsatsulnikov, W.V. Lundin, A.V. Sakharov, E.E. Zavarin, S.O. Usov, A.E. Nikolaev, N.A. Cherkashin, B.Ya. Ber, D.Yu. Kazantsev, M.N. Mizerov, Hee Seok Park, M. Hytch, F. Hue, Semiconductors 44 (1) (2010) 93–97. [11] W.V. Lundin, E.E. Zavarin, M.A. Sinitsyn, A.E. Nikolaev, A.V. Sakharov, A.F. Tsatsulnikov, E.V. Yakovlev, R.A. Talalaev, A.V. Lobanova, A.S. Segal, EWMOVPE 13, Ulm, 7–10 June 2009, Booklet of Extended abstracts, Inv 2. [12] N.V. Kryzhanovskaya, W.V. Lundin, A.E. Nikolaev, A.F. Tsatsul’nikov, A.V. Sakharov, M.M. Pavlov, N.A. Cherkachin, M.J. Hytch, G.A. Valkovsky, M.A. Yagovkina, S.O. Usov, Semiconductors 44 (6) (2010) 828–834. [13] M.J. Hytch, E. Snoeck, R. Kilaas, Ultramicroscopy 74 (1998) 131. ¨ ¨ E. Snoeck, in: P.W. Hawkes (Ed.), Aberration [14] F. Houdellier, M. Hytch, F. Hue, Correction in TEM, vol. 225, 2008 Chapter 6. [15] T.M. Smeeton, M.J. Kappers, J.S. Barnard, M.E. Vickers, C.J. Humphreys, Appl. Phys. Lett. 83 (2003) 5419. [16] E.V. Yakovlev, R.A. Talalaev, A.V. Kondratyev, A.S. Segal, A.V. Lobanova, W.V. Lundin, E.E. Zavarin, M.A. Sinitsyn, A.F. Tsatsulnikov, A.E. Nikolaev, J. Cryst. Growth 310 (2008) 4862–4866.
Contents lists available at ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Single quantum well deep-green LEDs with buried InGaN/GaN short-period superlattice W.V. Lundin a,b,n, A.E. Nikolaev a,b, A.V. Sakharov a,b, E.E. Zavarin a,b, G.A. Valkovskiy a, M.A. Yagovkina a, S.O. Usov b, N.V. Kryzhanovskaya a,b, V.S. Sizov a,b, P.N. Brunkov a, A.L. Zakgeim b, A.E. Cherniakov b, N.A. Cherkashin c, M.J. Hytch c, E.V. Yakovlev d, D.S. Bazarevskiy d, M.M. Rozhavskaya b, A.F. Tsatsulnikov a,b a
Ioffe Physico-Technical Institute of the Russian Academy of Science, Politechnicheskaya 26, St. Petersburg 194021, Russia Scientific and Technological Center for Microelectronics and Submicron Heterosctructures of the Russian Academy of Science, Politechnicheskaya 26, St. Petersburg 194021, Russia CEMES/CNRS, 29 rue Jeanne Marvig, 31055 Toulouse, France d STR Group—Soft-Impact Ltd., P.O. Box 89, St. Petersburg 194156, Russia b c
a r t i c l e in fo
abstract
Available online 22 September 2010
In spite of the great progress in III-N technology, LEDs with wavelength 4 530 nm still exhibit low efficiency compared to blue and short-wavelength-green LEDs. Here we report on significant improvement of deep-green LED properties by modifications of the structure design. The combination of InGaN/GaN superlattice followed by low-temperature GaN is the key element to increase the electroluminescence efficiency for deep-green LED. Various techniques were employed to clarify the correlation between structure properties, growth regimes and design. Modification of the defect structure of the GaN buffer by InGaN layers appears to be mostly responsible for the observed effect. LEDs processed and assembled in a standard flip-chip geometry with Ni–Ag p-contact demonstrate external quantum efficiencies of 8–20% in the 560–530 nm range. & 2010 Elsevier B.V. All rights reserved.
Keywords: A1. Nanostructures A3. MOCVD B1. Nitrides B3. LEDs
1. Introduction Light-emitting diodes (LEDs) emitting in the 540–570 nm range are very important for the development of new generations of multi-color light emitters including smart white RGB light sources for solid-state lighting (SSL) [1]. However, in spite of the great progress in III-N technology, LEDs with peak wavelength above 530 nm still suffer from low efficiency compared to blue and short-wavelength-green LEDs [2]. Few effects are responsible for the strong deterioration of InGaN/GaN LED efficiency with wavelength increase. Significant lattice mismatch between GaN and InGaN with indium content high enough for long-wavelength emission results in the formation of additional dislocations in the active region [3]. The wellknown tendency of InGaN to phase separation, fruitful in blue LED structures due to the formation of localized centers increasing EL efficiency, with indium content increase can result in the formation of large dislocated clusters [4]. Quantum confined Stark effect (QCSE) as well as Auger processes [5] also become
n
Corresponding author at: Ioffe Physico-Technical Institute of the Russian Academy of Science, Politechnicheskaya 26, St. Petersburg 194021, Russia. Tel./fax: +7 812 2973182. E-mail address: [email protected] (W.V. Lundin). 0022-0248/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2010.09.043
stronger as the In content in InGaN increases. In spite of some gradual improvement in green LED properties, it seems that the ideas used to improve the properties of blue LEDs have not led to high-performance green LEDs. QCSE may be reduced or totally eliminated by changing the growth plane and significant progress in green laser diodes was achieved that way [6,7]. However, non-polar or semi-polar GaN substrates are still small and too expensive for LED production and non- or semi-polar GaN grown on other substrates is too imperfect for high-efficiency device applications. It was reported that utilization of c-plane low-dislocationdensity GaN substrates suppresses the deterioration of InGaN QW properties with increase in the indium content [8], which allowed the fabrication of green laser diodes [9] but again such substrates are too expensive for LEDs. Thus, a new way to form deep-green LED structures should be investigated. Here we report on significant improvement in deepgreen LED properties by modifications of structure design.
2. Experiment The structures were grown in an AIX2000HT 6 200 planetary reactor on (0 0 0 1) sapphire substrates, utilizing a conventional
268
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low-temperature GaN nucleation layer technique. Hydrogen was used as the carrier gas for the growth of undoped and n-doped GaN buffer layers, while a nitrogen–hydrogen 1:1 mixture or pure nitrogen were used as carrier gas for the growth of p-doped layers. Indium-containing layers were grown using nitrogen as a carrier gas, but hydrogen also was used during the formation of these layers as described later. First, 2.5 mm of GaN buffer layer were grown at 800 mbar reactor pressure, then the pressure was reduced to 400 mbar for the subsequent part of the n-GaN buffer. The InGaN/GaN SL was formed at 200 mbar, and the rest of the structure was grown at 300 mbar. Deep-green LED structures in this study (Fig. 1) were composed of 5 mm GaN buffer grown at 1190 1C, 24-nm-thick InGaN/GaN superlattice (SL) grown at 910 1C, n-GaN barrier grown at the same temperature as the SL (LT GaN), InGaN QW grown at 810–835 1C, undoped upper GaN barrier grown at a temperature 70 1C higher than that used for the QW, 15 nm p-AlGaN, and 120 nm p-GaN grown at 1000 1C. The temperatures mentioned were measured by a pyrometer through a bottom lightpipe and correspond to the lower surface of the main susceptor. The InGaN QW growth temperature was adjusted for the desired emission wavelength. The number of periods in the InGaN/GaN SL, thickness of LT-GaN and upper GaN barrier were varied during structure design optimization as described later. The InGaN/GaN SLs in the investigated structures were formed using a InGaN-conversion technique [10–12] by alternating growth of In0.1Ga0.9N and growth interruption (GI) where the alkyl precursors were switched off and the carrier gas composition was changed from pure N2 to a mixture of N2:H2 ¼7:3. In this procedure, the indium concentration on the surface is governed by an interplay between InGaN decomposition during the GI, indium segregation, desorption, and incorporation into InGaN during subsequent growth. Addition of hydrogen to the reactor ambient during GI increases the rate of indium atoms release from In0.1Ga0.9N, but not very significantly due to relatively low InGaN indium content. At the same time, in the presence of hydrogen not only indium, but also gallium atoms are released from the bulk crystal to the surface, but in contrast to indium, gallium cannot evaporate at this low temperature. Thus Ga adatoms migrate over the surface and re-incorporate in the crystal. All of the processes together lead to conversion of InGaN to GaN in the very upper layer. If the growth-GI process is repeated several times, an SL structure is formed as confirmed by TEM (see Fig. 1) and HRXRD. In most of the structures investigated, the number of periods of the SL was 12, each InGaN growth time was 60 s and the duration of the GI was 20 s. Under these conditions the SL period is 2 nm, which is practically equal to the thickness of individual InGaN layers grown between GIs,
Fig. 1. TEM image of the active region of deep-green LED.
and the InN mole fraction periodically changes from the initial 10% to nearly zero. The structures grown were investigated using photo- and electroluminescence spectroscopy, high-resolution X-ray diffractometry (HRXRD) and atomic force microscopy (AFM). Weakbeam dark field transmission electron microscopy (WBDF TEM) imaging was used for dislocation structure characterization. Geometric phase analysis (GPA) [13] of high-resolution TEM (HRTEM) images obtained at the SACTEM-Toulouse microscope [14] was used for 2D mapping of the distribution of the strain perpendicular to a surface in the heterostructures. The effect of free surfaces of a TEM lamella on strain relaxation was estimated by finite element simulations. Afterward, the indium composition within InGaN layers was deduced using Vegard’s law. Special precautions taken for the sample preparation and regarding the conditions used for imaging allowed to avoid the InGaN deterioration under an electron beam, which was described in [15]. The LEDs fabricated from the grown structures were processed in a flip-chip geometry with Ni–Ag p-contact (no special techniques of light extraction improvement such as roughening or lift-off were used), and investigated after encapsulation using an Optronic Laboratories OL770-LED system with an OL ISA-670 integrating sphere.
3. Optimization of deep-green LED structure To minimize the formation of new misfit dislocations, single QW structures were used. During structure optimization it was discovered that incorporation of InGaN/GaN SL and LT GaN barrier below the QW results in a 15–30 times increase in EL efficiency in comparison with a simple structure with QW grown directly on top of the high-temperature n-GaN layer (Fig. 2). It should be stressed that InGaN/GaN SL and LT GaN barriers improve LED
Fig. 2. Dependence of external quantum efficiency on current for LED structures of various designs emitting at 550 nm (on-wafer probing). 1: QW grown on high-temperature GaN buffer; 2: QW grown on LT GaN; 3,4: QW grown on 12-period InGaN/GaN SL capped with LT-GaN; 5: QW grown on 3-period InGaN/ GaN SL capped with LT-GaN. 1–3: p-GaN grown with H2+ N2 carrier gas; 4,5: p-GaN grown with N2 carrier gas.
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properties only if implemented together and have a small effect if used alone (at least in our growth conditions). Further optimization includes adjustment of the thicknesses of the GaN layers adjacent to the QW. The dependence of LED efficiency on thickness of LT-GaN between SL and QW is very strong at low thickness and nearly saturates around 10–15 nm, but some rise may be observed until 20–25 nm. The role of this LT-GaN seems to be the same as in previously reported blue LEDs [10]—just to prevent injected holes overflow from QW to InGaN/ GaN SL. The optimal value of undoped GaN barrier above the QW is about 5 nm. Too high a thickness results in carrier injection deterioration, while at too low a thickness of this layer, chemical and thermal decomposition of the InGaN QW occurs during p-AlGaN growth. All structures described above were grown using an N2:H2 mixture as a carrier gas for all p-doped materials in accordance with our blue-LED fabrication technology. The next step of
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deep-green LED optimization was done using pure nitrogen instead of an N2:H2 mixture as a carrier gas during p-GaN growth: this results in a noticeable increase in EL efficiency (Fig. 2, curve 4). These structures were processed and assembled in a standard flip-chip geometry with 0.156 mm2 p–n junction area and Ni–Ag p-contact. External efficiencies of 8–20% in the 560–530 nm range were achieved (see Fig. 3). All structures described earlier contain the same 12-fold InGaN/GaN SL with 2 nm period. In the next step we have investigated the influence of SL design on LED properties. The total SL thickness was fixed at 24 nm while the number of periods was varied by changing the number of GIs from 1 to 24. The sum of the GI durations was kept constant. The best results were obtained for a 3-period SL (Fig. 2, curve 5). The dependence of maximal external quantum efficiency on emission wavelength for the series of samples described above is summarized in Fig. 4. It can be observed that the best results were achieved by the combination of a 3-period SL and growth of p-GaN in hydrogen-free ambient over the whole wavelength range investigated. At the same time, a 24-period SL also gives better results than a 12-period SL at around 560 nm.
4. Discussion
Fig. 3. Dependence of external quantum efficiency on current for LEDs with p-GaN grown using N2 carrier gas and 12-period InGaN/GaN SL.
Fig. 4. Dependence of external quantum efficiency on emission wavelength for the series of LED structures.
Summarizing experimental results, two major effects were observed: very strong improvement in LED efficiency by InGaN/ GaN SL incorporation and noticeable improvement in efficiency by p-GaN growth under hydrogen-free ambient. The nature of both the effects is unclear yet. Some hypotheses and speculations are presented below. The possible reasons for the advantage of H2-free ambient for p-GaN formation for deep-green LEDs are as follows: first of all, as it was shown in [11,16], in hydrogen-free ambient the growth rate anisotropy of Mg-doped GaN is less pronounced and, even more importantly, realization of the p-type conductivity in GaN:Mg is insensitive to the growth direction. These facts come from less equilibrium-like GaN growth conditions in the absence of hydrogen. A deep-green LED structure is very close to the onset of strain relaxation and thus some locally imperfect regions may be formed in the structure. Hydrogen-free growth conditions result in suppression of p-GaN nonuniformity caused by these local disturbances of material properties. The second reason (which appears less credible) is the possible decomposition of InGaN by hydrogen penetrating through the AlGaN barrier during p-GaN layer growth. It should be mentioned that investigation of the effect of p-GaN growth temperature on deep-green LED properties has shown that the influence of H2 on the LED properties cannot be explained in terms of surface temperature dependence on the carrier gas composition. Changing the carrier gas also did not affect the p-GaN morphology and thus the effect cannot be explained in terms of light extraction efficiency. While the role of LT GaN between SL and QW is more or less clear, the effect of the SL itself on LED properties should be clarified. Implementation of the SL in an LED structure may result in: reduction of dislocation density, strain relaxation resulting in metamorphic growth of the active region, and modification of the surface morphology beneath the QW. The first two assumptions can be rejected after analysis of HRXRD reciprocal space mapping and TEM data. The last assumption was clarified during investigation of the influence of the SL design on LED properties. In Fig. 5 one can see AFM images of three structures with InGaN/GaN SLs covered with LT GaN (just beneath the active QW of a full LED). The morphologies of these structures are very different: for the 12- and 24-period SL, the surfaces look more or less 2D like high-temperature GaN while for
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Fig. 5. AFM images of 24-nm-thick InGaN/GaN SLs capped with LT-GaN. (a) 3-period SL fabricated with 80 s GIs, (b) 12-period SL fabricated with 20 s GIs, (c) 24-period SL fabricated with 10 s GIs.
the 3-period SL with thick InGaN layers it is clearly of a 3D type. At the same time, there is no clear correlation between surface morphology and LED properties. In the 555–570 nm range, the efficiency of LEDs with 3- and 24-period SL differs by less than 20% while for the structure with 12-period SL the efficiency is much lower in spite of the very similar morphologies with 12- and 24-period SLs. However, in the 545–555 nm range, LEDs with 24-period and 12-period SLs show a similar efficiency, which is much lower than for an LED with 3-period SL. Moreover, an increase in efficiency obtained by implementation of any of the SLs used in the deep-green LED structure is much stronger than any further improvement by SL formation process modification. So the surface morphology at least is not the main reason for the effect. Further investigations are needed for separating the effects of SL design and surface morphology on EL efficiency. As all hypotheses listed above have been rejected, we have to assume some other material properties’ modification by the InGaN/GaN SL. GPA of HRTEM images of deep-green LEDs reveals that if an InGaN QW is grown directly on top of high-temperature GaN alloy composition fluctuations having a lateral size of about 100 nm are very pronounced while no such nonuniformity is observed in the structures grown with InGaN/GaN SL overgrown by LT GaN. Thus, the origin of the differences should be looked for at the scale of tens to hundreds of nanometers. The dislocation density in our GaN buffer layers, as revealed by TEM measurements, is about 5 6 108 cm 2 without any manifestation of mosaic structure with domain boundaries formed with dislocations. At the same time HRXRD reciprocal space mapping revealed a pronounced mosaic structure for our GaN buffer layers. The characteristic domain size in this mosaic structure (70–100 nm) is well below the dislocation-to-dislocation distance. So, the defect structure is not governed only by dislocations. GPA of HRTEM images has revealed strong nonuniformity of the SLs in the lateral direction at the nanometer scale (see Fig. 6). So, these structures are not classical short-period SLs, but layered nanocomposites, which is confirmed by optical investigations as well [12]. It looks feasible that the material with such kind of structure can suppress the influence of GaN buffer domain boundaries on InGaN QW formation, promoting for it a uniform composition. Although the mechanism for this process is unclear at present, the above speculations at the moment appear to be the only explanation we can propose. Further investigations are needed for clarification.
Fig. 6. GPA of HRTEM: distribution of the strain perpendicular to the surface eGaN zz with respect to GaN in the active region of a deep-green LED with 12-period InGaN/ GaN SL. A higher positive value of strain corresponds to higher indium content.
5. Conclusions Summarizing the results we conclude that using an InGaN/GaN SL fabricated by the InGaN conversion technique and capping it with low-temperature GaN as a buffer for InGaN QW results in a strong increase in the efficiency of LEDs emitting in the deep-green range. The main reason for this effect seems to originate from the modification of the defect structure of the grown material at the lateral scale of about 100 nm. Although the details of this mechanism are not entirely clear, the significant improvement in the deep-green LED properties is supported by a large number of experiments. Deep-green LEDs with external efficiencies of 8–20% in the 560–530 nm range were fabricated. Acknowledgements The work was supported by a governmental contract No. 02.523.12.3017 from 14.08.2008, federal educational agency (‘‘Technology of monolithic polychromatic white light emitter’’),
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