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Fabrication and properties of GaN-based lasers
ARTICLE IN PRESS Journal of Crystal Growth 310 (2008) 3979– 3982
Contents lists available at ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Fabrication and properties of GaN-based lasers P. Perlin a,b,, T. S´wietlik a, L. Marona a, R. Czernecki a,b, T. Suski a, M. Leszczyn´ski a,b, I. Grzegory a,b, S. Krukowski a, G. Nowak a, G. Kamler a, A. Czerwinski c, M. Plusa c, M. Bednarek d, J. Rybin´ski d, S. Porowski a a
Institute of High Pressure Physics, "UNIPRESS", Sokolowska 29/37, 01-142 Warsaw, Poland TopGaN Ltd., Warsaw, Poland c Institute of Electron Technology, Warsaw, Poland d Firefighting School, Warsaw, Poland b
a r t i c l e in fo
abstract
Available online 7 June 2008
In this paper we will discuss the application of almost dislocation-free, high-pressure grown gallium nitride bulk crystals as a substrate for the epitaxy of GaN/InGaN/AlGaN laser diode structures. We show that these laser diodes may have very low dislocation densities (even down to 8 104 cm2). These dislocations appear during the growth of the laser structure as a result of the combined effect of strain and perturbation of the atomic step flow. We show also that the lifetime of these devices seems to be dependent on operating current via the increasing junction temperature. & 2008 Elsevier B.V. All rights reserved.
PACS: 73.40.Kp 61.72.Ff 42.55.Ah 42.55.Px 42.60.By 42.60.Lh Keywords: A3. Metal organic chemic vapor deposition B2. Semiconducting III–V materials B3. Laser diodes
1. Introduction Ten years after the first demonstration of the InGaN laser diode (LD) by Nakamura [1], the Japanese industry finally succeeded in launching a mass production of violet, 405 nm, LDs for optical data storage applications (BlueRay and HD DVD). It is also envisioned that nitride devices may become a base for a new branch of consumer electronics—laser TV. For these devices a longer emission wavelength, 450–460 nm, is required. The production volume necessary to meet the total demand of these two markets can be estimated at hundreds of millions of LDs per year. In order to meet this demand, the further development of the production of bulk GaN substrate crystals must be achieved. As for today, the dominating method of GaN substrate production is the hydride vapor-phase epitaxy–dislocation elimination by epitaxial growth with inverse-pyramidal pits (HVPE–DEEP) method introduced by Sumitomo Electronic [2,3]. The GaN crystals obtained by this method have an average density of dislocations in the order of 105–106 cm2. Though the DEEP method obviously dominates in the mass production of GaN substrates,
Corresponding author at: Institute of High Pressure Physics, "UNIPRESS", Polish Academy of Sciences, Sokolowska 29/37, 01-142 Warsaw, Poland. Tel.: +48 22 888 00 76; fax: +48 22 877 3598. E-mail address: [email protected] (P. Perlin).
0022-0248/$ - see front matter & 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2008.06.010
alternative methods like the amonothermal [4] method or the highpressure synthesis method [5] may offer advantages for nitride lasers industry, especially in terms of higher wafer quality. Indeed, the density of dislocations of the high-pressure grown crystals is lower than 100 cm2 [5]. In this paper we would like to give the reader a short review of the challenges related to the production of laser structures on very low dislocation density, high-pressure grown bulk GaN crystals. We will discuss the quality of GaN/ InGaN/AlGaN LDs grown on such crystals, demonstrating typical defects and their distribution. The final part of the paper is devoted to reliability studies of these devices. We will show the mechanisms which may influence the lifetime of nitride LDs, focusing on the high operating temperature of the junction as the driving force for device degradation. 2. Substrate crystals We synthesized our crystals using the high nitrogen pressure solution [5] method, which is a temperature gradient growth method based on the direct reaction between gallium and nitrogen at high-temperature and high-nitrogen pressure. The dominating morphological form of GaN crystals grown by the high-pressure method is a thin hexagonal platelet (Fig. 1). The large hexagonal surfaces of the platelets always corresponds
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0 0.0
Fig. 1. High-pressure grown GaN crystals.
Device voltage (V)
P. Perlin et al. / Journal of Crystal Growth 310 (2008) 3979–3982
Optical power (mW)
3980
0 0.2
0.4 Current (A)
0.6
0.8
Fig. 2. Current–voltage and light–current characteristics of a laser diode. The dimensions of the device are 5 500 mm2. All characteristics are measured at room temperatures.
to the {0 0 0 1} polar crystallographic planes of the wurzite structure. The side facets of the crystals are the semi-polar {1 0 1¯ 1} and non-polar {1 0 1¯ 0} planes. The maximum lateral size of the platelets for 100–150 h processes is up to 3 cm2, whereas the thickness is about 100 mm. These crystals become epi-ready substrates after preparation by mechano-chemical polishing process of its gallium polar surface. The surface misorientation with respect to the wurtzite c-plane is typically 0.1–0.31. The latter value is related to the accuracy of preliminary mechanical polishing of the gallium nitride platelets. Such GaN substrates are highly conductive n-type material due to unintentionally introduced oxygen donors. The dislocation density usually does not exceed 102 cm2. Sometimes macrodefects like carbon inclusions may also appear.
3. Laser structure The LD epitaxial layers are deposited by metalorganic chemical vapor deposition (MOCVD) in a vertical-flow reactor. Besides NH3 as a source of nitrogen, typical metalorganic precursors like: trimethyl- and triethyl-gallium, trimethyl-aluminum, and trimethyl-indium were used. Dopants were delivered from other hydrides (monosilane SiH4 or disilane Si2H6) or solid source for Mg (biscyclopentadienyl-magnesium) for n- or p-type, respectively. The carrier gas is nitrogen, especially for indium-containing layers and/or hydrogen, which is important for p-type layers. The n-type side layers are grown at about 1020–1070 1C, the active region at above 800 1C, while the electron blocking layer and the other Mg-doped layers are grown at temperatures about 120–200 1C higher. The devices are processed as ridge-waveguide, oxide-isolated lasers. The stripe width is typically 5 mm and the resonator length is 500 or 1000 mm. The mesa is etched by reactive ion etching, down to the middle of the upper AlGaN cladding layer (0.3 mm thick). The active region consists of 3, 3.5nm thick, undoped In0.09Ga0.91N quantum wells (QWs) separated by 8–15 nm, silicon-doped In0.02Ga0.98N barriers. The electron blocking layer (EBL) is formed by a 200-A˚-thick Mg-doped Al0.2Ga0.8N layer. Typical parameters for LDs operated in the continuous wave (CW) regime are the following: threshold current density, 4–7 kA/cm2; threshold voltage, 6–10 V; emission wavelength, 405–420 nm; maximum optical power, 215 mW (Fig. 2). The total differential efficiency varies between 0.4 and 1.2 W/A. Completely processed LDs are cleaved into individual
Fig. 3. Cathodoluminescence, scanning electron microscope picture of a laser diode. Black points represent the dislocations present in the structure. The photograph was taken at a wavelength of 410 nm.
chips. The devices are mounted p-side down on a diamond submount, next on a copper heat sink.
4. Dislocation density and structure morphology As we mentioned before, the density of dislocations in the GaN substrate crystal is very low, usually not exceeding 102 cm2. In Fig. 3, we show a cathodoluminescence image of a laser structure (plain view picture taken at the emission wavelength of the InGaN quantum wells). The dislocations are visible in this picture as dark spots. The density of dislocations in our LDs, revealed by cathodoluminescence and also by selective etching [6], is 8 104 up to 106 cm2 (around 5 105 cm2 in the case of the presented microphotograph). The study of these dislocations and their evolution in the LD structure is very interesting because their appearance and distribution is completely masked for lowerquality substrates (sapphire, SiC) by the high number of dislocations existing in such substrates. Interestingly, the dislocations are not distributed randomly, but are rather grouped along the lines, which mark the borders between the regions of slightly different local crystallographic orientation (as can be observed by an atomic force microscope). At this point we would like to remind
ARTICLE IN PRESS P. Perlin et al. / Journal of Crystal Growth 310 (2008) 3979–3982
3981
prominent fluctuations of the electroluminescence emitted from various regions of the laser stripe. These inhomogeneities can be traced back to the same growth instabilities, which are supposed to be responsible for the appearance of new dislocations in the structure. Concluding this part, it seems to be clear that the appearance of new dislocations is due to the combination of high tensile strain occurring in the AlGaN layers and the perturbation in the step flow during the growth of the layers. Comparing these results with data typical for gallium arsenide devices (around 104 dislocations per centimeter square), one can conclude that nitride devices have at least one order of magnitude higher dislocation densities. This situation may be improved by better strain engineering, and tighter control of the growth process and by choosing a proper substrate miscut.
5. Device temperature and degradation
Fig. 4. Etch pits as revealed by selective KOH etching of a nitride laser structure.
Wide bandgap semiconductors, because of their high density of states at the bottom of the conduction band, tend to have high transparency (point at which the gain becomes positive) levels exceeding 1019 cm3. The value of this latter parameter is almost ten times higher than the respective value in GaAs [7]. This latter fact in conjunction with the simple statement that in case of nitrides the operation voltage is almost twice larger, makes the total electric power supplied at the LD threshold almost 20 times larger than for the high-quality gallium arsenide devices. For example, the electrical power density of GaN lasers is in the range of 15–50 kW/cm2 (the minimum value for the best commercial lasers—3.5 kA/cm2 and 4.2 V is close to 14.7 k W/cm2). The dissipation of such a high amount of heat presents a big challenge for the laser design and packaging technology, even though the use of high thermal conductivity GaN substrate, somehow mitigates the situation. Fig. 6 shows the infrared (thermogram) photograph of a standard 5.6 mm laser package. The diode is CWoperated at 425 mA on a thermoelectric cooler stabilized at 18 1C.
Fig. 5. Optical microscope picture of the inhomogeneous emission of light from the laser stripe.
the reader that the density of dislocations in the substrate is 2–3 orders of magnitude lower and cannot be correlated with what we observe in the laser structure. Fig. 4 presents examples of etch pits, revealed by the selective etching on our laser structures. One can easily see that they initiate at very different depths. The deep pit corresponds to the lower AlGaN cladding and the shallower one to the region of quantum wells or/and p-type electron blocking layer. Usually, shallow pits are more abundant than deep ones. The dislocations mentioned before decorate border lines, and separate also regions of very different electrical and optical properties. Fig. 5 shows the optical microphotograph of a positively biased LD. The photo is registered from the back of the chip, through the transparent GaN substrate. One can see
Fig. 6. Infrared thermogram of the nitride laser diode operating in a 5.6 package. The laser chip is mounted p-down on the diamond heatspreader.
ARTICLE IN PRESS 3982
P. Perlin et al. / Journal of Crystal Growth 310 (2008) 3979–3982
The maximum chip temperature reaches about 87 1C. We observe some slight heat accumulation at the diamond–Cu interface, which indicates the need of improvement in eutectic soldering. The high density of electrical power also leads to difficulty of operating very wide-stripe devices. For instance we did not yet succeed in operating our 50 mm stripe devices in the continuous wave regime, although they showed excellent laser properties while driven with short (50 ns) current pulses. This problem is related, most likely, to insufficient lateral heat spreading in the case of wide stripes. Similar observation was previously reported by the Sony group [8]. The existence of very hot regions in the nitride structures has one obvious consequence—a faster degradation rate. Since it is well known that most of the degradation mechanisms are thermally activated with an activation energy of about 0.3–0.5 eV, we can expect a strong dependence of the degradation rate on the device operating current. To check this hypothesis we performed current dependent measurements of the LD degradation. Indeed we observed a fast increase of the degradation rate with current, which followed quite closely the following equation: dIIthr ¼ CeEa =kðT RT þaDIÞ dt
(1)
Here d (which may be expressed as a time derivative of the threshold current) is the degradation rate, Ea is the degradation activation energy, TRT room temperature expressed in Kelvins and DI is the driving current. This equation can be easily derived assuming that the degradation rate is thermally activated with an energy Ea. Experimentally determined constants are: Ea ¼ 0.42 eV and a ¼ 200 K/A [9]. While the value of the degradation activation energy is in good agreement with our previous results (also with those reported by other groups) [10,11], the calculated device temperature is much higher than estimated from the infrared thermographs or from the electroluminescence peak position. This may mean that the maximum temperature of the device is higher than that estimated by the previously mentioned methods. Most likely, the facet regions (or/and the central parts of the laser stripe) get substantially hotter than the average temperature of the device.
6. Summary LDs fabricated on high-pressure synthesized, gallium nitride substrates are characterized by low defect densities (down to few times 104 cm2). These dislocations are generated because of the strain existing in the structures and growth instabilities occurring during the flow of the atomic steps. Better strain engineering and new growth strategies may lower the dislocation density down to the 104 cm2 level. Temperature seems to be a main driving mechanism for device degradation.
Acknowledgment The financial help from the European Structural Fund and the Polish Ministry of Science through the project: ‘‘Single mode, high-power nitride-based laser diodes’’ is acknowledged. References [1] S. Nakamura, M. Senoh, S. Nagahama, N. Iwasa, T. Yamada, T. Matsushita, H. Kiyoku, Y. Sugimotu, Jpn. J. Appl. Phys. 35 (1996) L74. [2] Y. Kumagai, H. Murakami, H. Seki, A. Koukitu, J. Crystal Growth 246 (2002) 215. [3] K. Motoki, T. Okahisa, S. Nakahata, N. Matsumoto, H. Kimura, H. Kasai, K. Takemoto, K. Uematsu, M. Ueno, Y. Kumagai, A. Koukitu, H. Seki, J. Crystal Growth 237 (2002) 912. [4] T. Hashimoto, F. Wu, J.S. Speck, S. Nakamura, Jpn. J. Appl. Phys. 46 (2007) L889. [5] I. Grzegory, M. Boc´kowski, S. Porowski, GaN bulk substrates grown under pressure from solution, in: P. Capper (Ed.), Bulk Crystal Growth of Electronic, Optical and Optoelectronic Materials, Wiley, New York, 2005, p. 173. [6] G. Kamler, J. Smalc, M. Wozniak, J.L. Weyher, R. Czernecki, G. Targowski, M. Leszczynski, I. Grzegory, S. Porowski, J. Crystal Growth 293 (1) (2006) 18. [7] L.A. Coldren, S.W. Corzine, in: Diode Lasers and Photonic Integrated Circuits, Wiley Series in Microwave and Optical Engineering, New York, 1995. [8] S. Goto, M. Ohta, Y. Yabuki, Y. Hoshina, K. Naganuma, K. Tamamura, T. Hashizu, M. Ikeda, Phys. Status Solidi A 200 (2003) 122. [9] L. Marona, P. Wis´niewski, M. Leszczyn´ski, I. Grzegory, T. Suski, S. Porowski, R. Czernecki, A. Czerwinski, M. Pluska, J. Ratajczak, P. Perlin, in: H. Morkoc- , W. Cole Litton, J. Chyi, Y. Nanishi, E. Yoon (Eds.), SPIE Proceedings, vol. 6894, Gallium Nitride Materials and Devices III, 68940R, 2008. [10] L. Marona, P. Wisniewski, P. Prystawko, I. Grzegory, T. Suski, S. Porowski, P. Perlin, M. Leszczynski, R. Czernecki, Appl. Phys. Lett. 88 (2006) 201111. [11] M. Kneissl, D. Bour, L. Romano, Ch. Van de Walle, J. Northrup, W. Wong, D. Treat, M. Teepe, T. Schmidt, N. Johnson, Appl. Phys. Lett. 77 (2000) 1931.
Contents lists available at ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Fabrication and properties of GaN-based lasers P. Perlin a,b,, T. S´wietlik a, L. Marona a, R. Czernecki a,b, T. Suski a, M. Leszczyn´ski a,b, I. Grzegory a,b, S. Krukowski a, G. Nowak a, G. Kamler a, A. Czerwinski c, M. Plusa c, M. Bednarek d, J. Rybin´ski d, S. Porowski a a
Institute of High Pressure Physics, "UNIPRESS", Sokolowska 29/37, 01-142 Warsaw, Poland TopGaN Ltd., Warsaw, Poland c Institute of Electron Technology, Warsaw, Poland d Firefighting School, Warsaw, Poland b
a r t i c l e in fo
abstract
Available online 7 June 2008
In this paper we will discuss the application of almost dislocation-free, high-pressure grown gallium nitride bulk crystals as a substrate for the epitaxy of GaN/InGaN/AlGaN laser diode structures. We show that these laser diodes may have very low dislocation densities (even down to 8 104 cm2). These dislocations appear during the growth of the laser structure as a result of the combined effect of strain and perturbation of the atomic step flow. We show also that the lifetime of these devices seems to be dependent on operating current via the increasing junction temperature. & 2008 Elsevier B.V. All rights reserved.
PACS: 73.40.Kp 61.72.Ff 42.55.Ah 42.55.Px 42.60.By 42.60.Lh Keywords: A3. Metal organic chemic vapor deposition B2. Semiconducting III–V materials B3. Laser diodes
1. Introduction Ten years after the first demonstration of the InGaN laser diode (LD) by Nakamura [1], the Japanese industry finally succeeded in launching a mass production of violet, 405 nm, LDs for optical data storage applications (BlueRay and HD DVD). It is also envisioned that nitride devices may become a base for a new branch of consumer electronics—laser TV. For these devices a longer emission wavelength, 450–460 nm, is required. The production volume necessary to meet the total demand of these two markets can be estimated at hundreds of millions of LDs per year. In order to meet this demand, the further development of the production of bulk GaN substrate crystals must be achieved. As for today, the dominating method of GaN substrate production is the hydride vapor-phase epitaxy–dislocation elimination by epitaxial growth with inverse-pyramidal pits (HVPE–DEEP) method introduced by Sumitomo Electronic [2,3]. The GaN crystals obtained by this method have an average density of dislocations in the order of 105–106 cm2. Though the DEEP method obviously dominates in the mass production of GaN substrates,
Corresponding author at: Institute of High Pressure Physics, "UNIPRESS", Polish Academy of Sciences, Sokolowska 29/37, 01-142 Warsaw, Poland. Tel.: +48 22 888 00 76; fax: +48 22 877 3598. E-mail address: [email protected] (P. Perlin).
0022-0248/$ - see front matter & 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2008.06.010
alternative methods like the amonothermal [4] method or the highpressure synthesis method [5] may offer advantages for nitride lasers industry, especially in terms of higher wafer quality. Indeed, the density of dislocations of the high-pressure grown crystals is lower than 100 cm2 [5]. In this paper we would like to give the reader a short review of the challenges related to the production of laser structures on very low dislocation density, high-pressure grown bulk GaN crystals. We will discuss the quality of GaN/ InGaN/AlGaN LDs grown on such crystals, demonstrating typical defects and their distribution. The final part of the paper is devoted to reliability studies of these devices. We will show the mechanisms which may influence the lifetime of nitride LDs, focusing on the high operating temperature of the junction as the driving force for device degradation. 2. Substrate crystals We synthesized our crystals using the high nitrogen pressure solution [5] method, which is a temperature gradient growth method based on the direct reaction between gallium and nitrogen at high-temperature and high-nitrogen pressure. The dominating morphological form of GaN crystals grown by the high-pressure method is a thin hexagonal platelet (Fig. 1). The large hexagonal surfaces of the platelets always corresponds
ARTICLE IN PRESS
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10
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8
150
6
100
4
50
2
0 0.0
Fig. 1. High-pressure grown GaN crystals.
Device voltage (V)
P. Perlin et al. / Journal of Crystal Growth 310 (2008) 3979–3982
Optical power (mW)
3980
0 0.2
0.4 Current (A)
0.6
0.8
Fig. 2. Current–voltage and light–current characteristics of a laser diode. The dimensions of the device are 5 500 mm2. All characteristics are measured at room temperatures.
to the {0 0 0 1} polar crystallographic planes of the wurzite structure. The side facets of the crystals are the semi-polar {1 0 1¯ 1} and non-polar {1 0 1¯ 0} planes. The maximum lateral size of the platelets for 100–150 h processes is up to 3 cm2, whereas the thickness is about 100 mm. These crystals become epi-ready substrates after preparation by mechano-chemical polishing process of its gallium polar surface. The surface misorientation with respect to the wurtzite c-plane is typically 0.1–0.31. The latter value is related to the accuracy of preliminary mechanical polishing of the gallium nitride platelets. Such GaN substrates are highly conductive n-type material due to unintentionally introduced oxygen donors. The dislocation density usually does not exceed 102 cm2. Sometimes macrodefects like carbon inclusions may also appear.
3. Laser structure The LD epitaxial layers are deposited by metalorganic chemical vapor deposition (MOCVD) in a vertical-flow reactor. Besides NH3 as a source of nitrogen, typical metalorganic precursors like: trimethyl- and triethyl-gallium, trimethyl-aluminum, and trimethyl-indium were used. Dopants were delivered from other hydrides (monosilane SiH4 or disilane Si2H6) or solid source for Mg (biscyclopentadienyl-magnesium) for n- or p-type, respectively. The carrier gas is nitrogen, especially for indium-containing layers and/or hydrogen, which is important for p-type layers. The n-type side layers are grown at about 1020–1070 1C, the active region at above 800 1C, while the electron blocking layer and the other Mg-doped layers are grown at temperatures about 120–200 1C higher. The devices are processed as ridge-waveguide, oxide-isolated lasers. The stripe width is typically 5 mm and the resonator length is 500 or 1000 mm. The mesa is etched by reactive ion etching, down to the middle of the upper AlGaN cladding layer (0.3 mm thick). The active region consists of 3, 3.5nm thick, undoped In0.09Ga0.91N quantum wells (QWs) separated by 8–15 nm, silicon-doped In0.02Ga0.98N barriers. The electron blocking layer (EBL) is formed by a 200-A˚-thick Mg-doped Al0.2Ga0.8N layer. Typical parameters for LDs operated in the continuous wave (CW) regime are the following: threshold current density, 4–7 kA/cm2; threshold voltage, 6–10 V; emission wavelength, 405–420 nm; maximum optical power, 215 mW (Fig. 2). The total differential efficiency varies between 0.4 and 1.2 W/A. Completely processed LDs are cleaved into individual
Fig. 3. Cathodoluminescence, scanning electron microscope picture of a laser diode. Black points represent the dislocations present in the structure. The photograph was taken at a wavelength of 410 nm.
chips. The devices are mounted p-side down on a diamond submount, next on a copper heat sink.
4. Dislocation density and structure morphology As we mentioned before, the density of dislocations in the GaN substrate crystal is very low, usually not exceeding 102 cm2. In Fig. 3, we show a cathodoluminescence image of a laser structure (plain view picture taken at the emission wavelength of the InGaN quantum wells). The dislocations are visible in this picture as dark spots. The density of dislocations in our LDs, revealed by cathodoluminescence and also by selective etching [6], is 8 104 up to 106 cm2 (around 5 105 cm2 in the case of the presented microphotograph). The study of these dislocations and their evolution in the LD structure is very interesting because their appearance and distribution is completely masked for lowerquality substrates (sapphire, SiC) by the high number of dislocations existing in such substrates. Interestingly, the dislocations are not distributed randomly, but are rather grouped along the lines, which mark the borders between the regions of slightly different local crystallographic orientation (as can be observed by an atomic force microscope). At this point we would like to remind
ARTICLE IN PRESS P. Perlin et al. / Journal of Crystal Growth 310 (2008) 3979–3982
3981
prominent fluctuations of the electroluminescence emitted from various regions of the laser stripe. These inhomogeneities can be traced back to the same growth instabilities, which are supposed to be responsible for the appearance of new dislocations in the structure. Concluding this part, it seems to be clear that the appearance of new dislocations is due to the combination of high tensile strain occurring in the AlGaN layers and the perturbation in the step flow during the growth of the layers. Comparing these results with data typical for gallium arsenide devices (around 104 dislocations per centimeter square), one can conclude that nitride devices have at least one order of magnitude higher dislocation densities. This situation may be improved by better strain engineering, and tighter control of the growth process and by choosing a proper substrate miscut.
5. Device temperature and degradation
Fig. 4. Etch pits as revealed by selective KOH etching of a nitride laser structure.
Wide bandgap semiconductors, because of their high density of states at the bottom of the conduction band, tend to have high transparency (point at which the gain becomes positive) levels exceeding 1019 cm3. The value of this latter parameter is almost ten times higher than the respective value in GaAs [7]. This latter fact in conjunction with the simple statement that in case of nitrides the operation voltage is almost twice larger, makes the total electric power supplied at the LD threshold almost 20 times larger than for the high-quality gallium arsenide devices. For example, the electrical power density of GaN lasers is in the range of 15–50 kW/cm2 (the minimum value for the best commercial lasers—3.5 kA/cm2 and 4.2 V is close to 14.7 k W/cm2). The dissipation of such a high amount of heat presents a big challenge for the laser design and packaging technology, even though the use of high thermal conductivity GaN substrate, somehow mitigates the situation. Fig. 6 shows the infrared (thermogram) photograph of a standard 5.6 mm laser package. The diode is CWoperated at 425 mA on a thermoelectric cooler stabilized at 18 1C.
Fig. 5. Optical microscope picture of the inhomogeneous emission of light from the laser stripe.
the reader that the density of dislocations in the substrate is 2–3 orders of magnitude lower and cannot be correlated with what we observe in the laser structure. Fig. 4 presents examples of etch pits, revealed by the selective etching on our laser structures. One can easily see that they initiate at very different depths. The deep pit corresponds to the lower AlGaN cladding and the shallower one to the region of quantum wells or/and p-type electron blocking layer. Usually, shallow pits are more abundant than deep ones. The dislocations mentioned before decorate border lines, and separate also regions of very different electrical and optical properties. Fig. 5 shows the optical microphotograph of a positively biased LD. The photo is registered from the back of the chip, through the transparent GaN substrate. One can see
Fig. 6. Infrared thermogram of the nitride laser diode operating in a 5.6 package. The laser chip is mounted p-down on the diamond heatspreader.
ARTICLE IN PRESS 3982
P. Perlin et al. / Journal of Crystal Growth 310 (2008) 3979–3982
The maximum chip temperature reaches about 87 1C. We observe some slight heat accumulation at the diamond–Cu interface, which indicates the need of improvement in eutectic soldering. The high density of electrical power also leads to difficulty of operating very wide-stripe devices. For instance we did not yet succeed in operating our 50 mm stripe devices in the continuous wave regime, although they showed excellent laser properties while driven with short (50 ns) current pulses. This problem is related, most likely, to insufficient lateral heat spreading in the case of wide stripes. Similar observation was previously reported by the Sony group [8]. The existence of very hot regions in the nitride structures has one obvious consequence—a faster degradation rate. Since it is well known that most of the degradation mechanisms are thermally activated with an activation energy of about 0.3–0.5 eV, we can expect a strong dependence of the degradation rate on the device operating current. To check this hypothesis we performed current dependent measurements of the LD degradation. Indeed we observed a fast increase of the degradation rate with current, which followed quite closely the following equation: dIIthr ¼ CeEa =kðT RT þaDIÞ dt
(1)
Here d (which may be expressed as a time derivative of the threshold current) is the degradation rate, Ea is the degradation activation energy, TRT room temperature expressed in Kelvins and DI is the driving current. This equation can be easily derived assuming that the degradation rate is thermally activated with an energy Ea. Experimentally determined constants are: Ea ¼ 0.42 eV and a ¼ 200 K/A [9]. While the value of the degradation activation energy is in good agreement with our previous results (also with those reported by other groups) [10,11], the calculated device temperature is much higher than estimated from the infrared thermographs or from the electroluminescence peak position. This may mean that the maximum temperature of the device is higher than that estimated by the previously mentioned methods. Most likely, the facet regions (or/and the central parts of the laser stripe) get substantially hotter than the average temperature of the device.
6. Summary LDs fabricated on high-pressure synthesized, gallium nitride substrates are characterized by low defect densities (down to few times 104 cm2). These dislocations are generated because of the strain existing in the structures and growth instabilities occurring during the flow of the atomic steps. Better strain engineering and new growth strategies may lower the dislocation density down to the 104 cm2 level. Temperature seems to be a main driving mechanism for device degradation.
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