- Email: [email protected]
Device performance of ultra-violet emitting diodes grown by MBE
Journal of Crystal Growth 189/190 (1998) 782—785
Device performance of ultra-violet emitting diodes grown by MBE Markus Mayer*, Arthur Pelzmann, Christoph Kirchner, Markus Schauler, Franz Eberhard, Markus Kamp, Peter Unger, Karl Joachim Ebeling Department of Optoelectronics, University of Ulm, Albert-Einstein-Allee 45, D-89069 Ulm, Germany
Abstract Ultra-violet emitting diodes with electroluminescence linewidths as narrow as 8 nm have been grown by molecular beam epitaxy. Double-heterostructure pn- and homotype pin-diodes reveal single-peak emission at wavelengths around 371 nm. The dependence of the optical output power on the forward current indicates light generation by diffusion current. Turn-on and reverse breakdown voltages are 4—5 V and above 30 V, respectively. For the first time, the metalorganic precursor MCp Mg is employed as p-dopant in nitride molecular beam epitaxy. ( 1998 Elsevier Science 2 B.V. All rights reserved. Keywords: LED; UVED; MBE; NH ; MCp Mg; GaN 3 2
1. Introduction Emerging new applications for LEDs include luminescence conversion, where pn-junctions are used to pump phosphors or organic dyes being encapsulated in the LED housing. Here, the light generated by electron—hole recombination is partly absorbed and re-emitted at lower energies [1,2]. By this down-conversion any color, including white light, can be achieved from blue or ultra-violet (UV) emitting pn-junctions. UV emitting diodes
* Corresponding author. Tel.: #49 731 502 6050; fax: #49 731 502 6049; e-mail: [email protected].
(UVEDs), however, can also be employed for optical analyses of organic molecules with their numerous absorption bands in this spectral region [3]. Focusing on the above applications, we have realized ultra-violet emitting diodes employing homotype pin- and double-hetero (DH) pn-structures.
2. Structure of UVEDs The schematics of the employed homotype and DH UVEDs are illustrated in Fig. 1. The homotype pin-diode consists of 800 nm Sidoped n-GaN, followed by a 50 nm intrinsic GaN
0022-0248/98/$19.00 ( 1998 Elsevier Science B.V. All rights reserved. PII S 0 0 2 2 - 0 2 4 8 ( 9 8 ) 0 0 2 9 3 - 0
M. Mayer et al. / Journal of Crystal Growth 189/190 (1998) 782–785
783
Fig. 1. Schematics of homotype and double-heterostructure ultra violet emitting diodes.
recombination layer and succeeded by 300 nm Mg doped p-GaN. The active layer of the DH pn-diode consists of 50 nm n-GaN, embedded in 150 nm nand p-doped AlGaN cladding layers for carrier confinement. The electroluminescence (EL) is detected through the backside, i.e. through the n-GaN and the transparent sapphire substrate. The UVEDs are grown by gas source molecular beam epitaxy using on surface cracking of ammonia [4]. Elemental Al and Si are supplied by conventional effusion cells, whereas Ga is evaporated from an EPI Sumo effusion cell and MCp Mg is used as Mg-source. 2 Details of the growth procedure are described elsewhere [5]. The homotype devices are grown directly on c-plane sapphire using a GaN nucleation layer, whereas a 2 lm thick GaN template grown by MOVPE is used for the DH-structures. Chemically assisted ion-beam etching (CAIBE) is used for dry etching of 150 lm]150 lm mesas. Ti/Au and Ni/Au metallization are used for n- and p-contacts.
3. Properties of UVEDs The room temperature EL of a homotype pinjunction is shown in Fig. 2 for different forward currents, revealing single-peak emission around 371 nm with linewidths as narrow as 12 nm. The linewidth compares favorable to results from homotype LEDs reported by Nakamura, showing linewidths of 55 nm, however the UVED’s output
power is significantly lower [6]. The near-bandedge emission peaks at 370 nm for low currents and slightly shifts towards 374 nm for higher currents. The energetic position is in agreement with electroluminescence, obtained from undoped GaN [7] indicating that electron—hole recombination takes place in the active intrinsic layer. Blue emission at 430 nm from transitions related to the Mg level in p-GaN [6] is not observable. Clearly visible in Fig. 2 is the internal absorption within the n-doped GaN layer, leading to the abrupt decrease in luminescence on the high energy side of the spectra. To confirm the influence of absorption within the Si doped n-GaN layer, for comparison, a transmittance spectrum of a Si doped n-GaN sample is included in Fig. 2. On the high-energy side of the spectra the transmittance and the electroluminescence curves fall together, indicating that the original generated luminescence is partly absorpted, thus determining intensity and line shape. Fig. 3 depicts the EL spectra obtained from the DH-UVEDs featuring single peak emission at 371 nm with a further reduced linewidth of 8 nm. Due to the Si-doping in the GaN recombination region the emission takes place at a lower energy than in the case of the pin-UVED. Therefore, the DH device shows no wavelength shift with increasing forward current and significant internal absorption occurs only at high currents. At 50 mA, the originally generated emission can be extrapolated with the plausible assumption that the absorption at the low-energy side of the EL spectrum is negligible
784
M. Mayer et al. / Journal of Crystal Growth 189/190 (1998) 782–785
Fig. 2. Electroluminescence of a pin-UVED with near-bandedge emission from 370 to 374 nm and linewidths as narrow as 12 nm (300 K).
design, avoiding internal optical absorption, is expected to distinctly improve the device performance. Further progress can be achieved by replacing GaN with AlGaN containing a small amount of Al. Fig. 5 shows the optical output power P versus forward current I of a pin-diode, as measured and as obtained by the Lorentzian curve fit. The extrapolated intensities are included to evaluate the influence of the internal absorption on the slope depicted in Fig. 5. According to simple LED theory, the dependence of the optical output power P versus forward current I should follow a power law: PJIm. The exponent m is predicted to be 1 or
Fig. 3. Electroluminescence of a double-heterostructure UVED with near-bandedge emission at 371 nm and linewidths as narrow as 8 nm (300 K).
Fig. 4. Electroluminescence of a UVED, assumed light generation in the active layer and the transmission of n-GaN.
and that the generated emission has a Lorentzian shape, as depicted in Fig. 4. The maximum of the Lorentzian curve peaks at the bandedge of GaN, between 364 and 366 nm, which is identical to the PL wavelength of n-doped GaN at 300 K. The originally generated intensity is 6 times higher than the measured one and the linewidth is still as narrow as 12 nm. Clearly, the Lorentzian curve reveals that internal absorption within the UVEDs limits to a high degree the luminescence intensity and the device efficiency. The above shown internal absorption is also present in some EL spectra shown by Nakamura [6] and Amano [7], featuring also an abrupt decay at the high-energy side. A slightly modified vertical
Fig. 5. Optical output power P versus forward current I, as obtained from the Lorentzian curves and as measured. The Lorentzian fit is performed to exclude any influence of the omnipresent absorption on the slope of the optical output power.
M. Mayer et al. / Journal of Crystal Growth 189/190 (1998) 782–785
2 for light generation by diffusion currents or space-charge recombination currents, respectively. The slope obtained from the Lorentzian fit is 1.2, whereas the measured intensity yields 1.1, both indicating the predominance of recombination by diffusion currents. Current-voltage characteristics of the homotype pin UVEDs reveal series resistances, turn-on and reverse breakdown voltages of 250 ), 4—5 V and beyond 30 V, respectively. In the case of the DH UVEDs, the electrical characteristics indicate 400 ), 6—8 V and beyond 10 V. We conclude that the electrical properties of the UVEDs are still subject of improvement by increasing p-conductivity and a reduced contact resistance.
4. Conclusion Single-peak electroluminescence is achieved with homotype pin- and double-heterostructure pndiodes. Homotype devices emit at 370 nm for low currents and slightly shift to 374 nm for higher currents with a linewidth of 12 nm. Double-heterostructure UVEDs reveal emission at 371 nm with a linewidth as narrow as 8 nm. The electroluminescence spectra show an abrupt decrease at the high-
785
energy sides caused by internal optical absorption within the structure. The power current dependence indicates electron—hole recombination being dominated by diffusion current. The narrow electroluminescence is encouraging for the presented homotype and double-heterostructure UVEDs and shows the potential of the metalorganic precursor MCp Mg as p-dopant in MBE. Employing a verti2 cal design, with reduced internal absorption, is expected to improve the device efficiency and the optical output power. References [1] P. Schlotter, R. Schmidt, J. Schneider, Appl. Phys. A 64 (1997) 417. [2] S. Nakamura, G. Fasol, The Blue Laser Diode, GaN based Light Emitters and Lasers, Springer, Berlin, 1997. [3] H.H. Perkampus, UV-VIS Atlas of Organic Compounds, VCH Publishers, Weinheim, 1992. [4] M. Kamp, M. Mayer, A. Pelzmann, K.J. Ebeling, Mater. Res. Soc. Proc. 449 (1997) 161. [5] M. Mayer, A. Pelzmann, M. Schauler, F. Eberhard, C. Kirchner, M. Kamp, P. Unger, K.J. Ebeling, Electron. Lett., submitted. [6] S. Nakamura, T. Mukai, M. Senoh, Jpn. J. Appl. Phys. 30 (1991) 1998. [7] H. Amano, M. Kito, K. Hiramatsu, I. Akasaki, Jpn. J. Appl. Phys. 28 (1989) L2112.
Device performance of ultra-violet emitting diodes grown by MBE Markus Mayer*, Arthur Pelzmann, Christoph Kirchner, Markus Schauler, Franz Eberhard, Markus Kamp, Peter Unger, Karl Joachim Ebeling Department of Optoelectronics, University of Ulm, Albert-Einstein-Allee 45, D-89069 Ulm, Germany
Abstract Ultra-violet emitting diodes with electroluminescence linewidths as narrow as 8 nm have been grown by molecular beam epitaxy. Double-heterostructure pn- and homotype pin-diodes reveal single-peak emission at wavelengths around 371 nm. The dependence of the optical output power on the forward current indicates light generation by diffusion current. Turn-on and reverse breakdown voltages are 4—5 V and above 30 V, respectively. For the first time, the metalorganic precursor MCp Mg is employed as p-dopant in nitride molecular beam epitaxy. ( 1998 Elsevier Science 2 B.V. All rights reserved. Keywords: LED; UVED; MBE; NH ; MCp Mg; GaN 3 2
1. Introduction Emerging new applications for LEDs include luminescence conversion, where pn-junctions are used to pump phosphors or organic dyes being encapsulated in the LED housing. Here, the light generated by electron—hole recombination is partly absorbed and re-emitted at lower energies [1,2]. By this down-conversion any color, including white light, can be achieved from blue or ultra-violet (UV) emitting pn-junctions. UV emitting diodes
* Corresponding author. Tel.: #49 731 502 6050; fax: #49 731 502 6049; e-mail: [email protected].
(UVEDs), however, can also be employed for optical analyses of organic molecules with their numerous absorption bands in this spectral region [3]. Focusing on the above applications, we have realized ultra-violet emitting diodes employing homotype pin- and double-hetero (DH) pn-structures.
2. Structure of UVEDs The schematics of the employed homotype and DH UVEDs are illustrated in Fig. 1. The homotype pin-diode consists of 800 nm Sidoped n-GaN, followed by a 50 nm intrinsic GaN
0022-0248/98/$19.00 ( 1998 Elsevier Science B.V. All rights reserved. PII S 0 0 2 2 - 0 2 4 8 ( 9 8 ) 0 0 2 9 3 - 0
M. Mayer et al. / Journal of Crystal Growth 189/190 (1998) 782–785
783
Fig. 1. Schematics of homotype and double-heterostructure ultra violet emitting diodes.
recombination layer and succeeded by 300 nm Mg doped p-GaN. The active layer of the DH pn-diode consists of 50 nm n-GaN, embedded in 150 nm nand p-doped AlGaN cladding layers for carrier confinement. The electroluminescence (EL) is detected through the backside, i.e. through the n-GaN and the transparent sapphire substrate. The UVEDs are grown by gas source molecular beam epitaxy using on surface cracking of ammonia [4]. Elemental Al and Si are supplied by conventional effusion cells, whereas Ga is evaporated from an EPI Sumo effusion cell and MCp Mg is used as Mg-source. 2 Details of the growth procedure are described elsewhere [5]. The homotype devices are grown directly on c-plane sapphire using a GaN nucleation layer, whereas a 2 lm thick GaN template grown by MOVPE is used for the DH-structures. Chemically assisted ion-beam etching (CAIBE) is used for dry etching of 150 lm]150 lm mesas. Ti/Au and Ni/Au metallization are used for n- and p-contacts.
3. Properties of UVEDs The room temperature EL of a homotype pinjunction is shown in Fig. 2 for different forward currents, revealing single-peak emission around 371 nm with linewidths as narrow as 12 nm. The linewidth compares favorable to results from homotype LEDs reported by Nakamura, showing linewidths of 55 nm, however the UVED’s output
power is significantly lower [6]. The near-bandedge emission peaks at 370 nm for low currents and slightly shifts towards 374 nm for higher currents. The energetic position is in agreement with electroluminescence, obtained from undoped GaN [7] indicating that electron—hole recombination takes place in the active intrinsic layer. Blue emission at 430 nm from transitions related to the Mg level in p-GaN [6] is not observable. Clearly visible in Fig. 2 is the internal absorption within the n-doped GaN layer, leading to the abrupt decrease in luminescence on the high energy side of the spectra. To confirm the influence of absorption within the Si doped n-GaN layer, for comparison, a transmittance spectrum of a Si doped n-GaN sample is included in Fig. 2. On the high-energy side of the spectra the transmittance and the electroluminescence curves fall together, indicating that the original generated luminescence is partly absorpted, thus determining intensity and line shape. Fig. 3 depicts the EL spectra obtained from the DH-UVEDs featuring single peak emission at 371 nm with a further reduced linewidth of 8 nm. Due to the Si-doping in the GaN recombination region the emission takes place at a lower energy than in the case of the pin-UVED. Therefore, the DH device shows no wavelength shift with increasing forward current and significant internal absorption occurs only at high currents. At 50 mA, the originally generated emission can be extrapolated with the plausible assumption that the absorption at the low-energy side of the EL spectrum is negligible
784
M. Mayer et al. / Journal of Crystal Growth 189/190 (1998) 782–785
Fig. 2. Electroluminescence of a pin-UVED with near-bandedge emission from 370 to 374 nm and linewidths as narrow as 12 nm (300 K).
design, avoiding internal optical absorption, is expected to distinctly improve the device performance. Further progress can be achieved by replacing GaN with AlGaN containing a small amount of Al. Fig. 5 shows the optical output power P versus forward current I of a pin-diode, as measured and as obtained by the Lorentzian curve fit. The extrapolated intensities are included to evaluate the influence of the internal absorption on the slope depicted in Fig. 5. According to simple LED theory, the dependence of the optical output power P versus forward current I should follow a power law: PJIm. The exponent m is predicted to be 1 or
Fig. 3. Electroluminescence of a double-heterostructure UVED with near-bandedge emission at 371 nm and linewidths as narrow as 8 nm (300 K).
Fig. 4. Electroluminescence of a UVED, assumed light generation in the active layer and the transmission of n-GaN.
and that the generated emission has a Lorentzian shape, as depicted in Fig. 4. The maximum of the Lorentzian curve peaks at the bandedge of GaN, between 364 and 366 nm, which is identical to the PL wavelength of n-doped GaN at 300 K. The originally generated intensity is 6 times higher than the measured one and the linewidth is still as narrow as 12 nm. Clearly, the Lorentzian curve reveals that internal absorption within the UVEDs limits to a high degree the luminescence intensity and the device efficiency. The above shown internal absorption is also present in some EL spectra shown by Nakamura [6] and Amano [7], featuring also an abrupt decay at the high-energy side. A slightly modified vertical
Fig. 5. Optical output power P versus forward current I, as obtained from the Lorentzian curves and as measured. The Lorentzian fit is performed to exclude any influence of the omnipresent absorption on the slope of the optical output power.
M. Mayer et al. / Journal of Crystal Growth 189/190 (1998) 782–785
2 for light generation by diffusion currents or space-charge recombination currents, respectively. The slope obtained from the Lorentzian fit is 1.2, whereas the measured intensity yields 1.1, both indicating the predominance of recombination by diffusion currents. Current-voltage characteristics of the homotype pin UVEDs reveal series resistances, turn-on and reverse breakdown voltages of 250 ), 4—5 V and beyond 30 V, respectively. In the case of the DH UVEDs, the electrical characteristics indicate 400 ), 6—8 V and beyond 10 V. We conclude that the electrical properties of the UVEDs are still subject of improvement by increasing p-conductivity and a reduced contact resistance.
4. Conclusion Single-peak electroluminescence is achieved with homotype pin- and double-heterostructure pndiodes. Homotype devices emit at 370 nm for low currents and slightly shift to 374 nm for higher currents with a linewidth of 12 nm. Double-heterostructure UVEDs reveal emission at 371 nm with a linewidth as narrow as 8 nm. The electroluminescence spectra show an abrupt decrease at the high-
785
energy sides caused by internal optical absorption within the structure. The power current dependence indicates electron—hole recombination being dominated by diffusion current. The narrow electroluminescence is encouraging for the presented homotype and double-heterostructure UVEDs and shows the potential of the metalorganic precursor MCp Mg as p-dopant in MBE. Employing a verti2 cal design, with reduced internal absorption, is expected to improve the device efficiency and the optical output power. References [1] P. Schlotter, R. Schmidt, J. Schneider, Appl. Phys. A 64 (1997) 417. [2] S. Nakamura, G. Fasol, The Blue Laser Diode, GaN based Light Emitters and Lasers, Springer, Berlin, 1997. [3] H.H. Perkampus, UV-VIS Atlas of Organic Compounds, VCH Publishers, Weinheim, 1992. [4] M. Kamp, M. Mayer, A. Pelzmann, K.J. Ebeling, Mater. Res. Soc. Proc. 449 (1997) 161. [5] M. Mayer, A. Pelzmann, M. Schauler, F. Eberhard, C. Kirchner, M. Kamp, P. Unger, K.J. Ebeling, Electron. Lett., submitted. [6] S. Nakamura, T. Mukai, M. Senoh, Jpn. J. Appl. Phys. 30 (1991) 1998. [7] H. Amano, M. Kito, K. Hiramatsu, I. Akasaki, Jpn. J. Appl. Phys. 28 (1989) L2112.