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Blue and green electroluminescence from GaNInGaN heterostructures
Journal
ELSEVIER
of Crystal
Growth
175i176 (1997) 122-124
Blue and green electroluminescence heterostructures R. Averbeck*, Sirens
H. Tews, A. Graber,
AG, Corporate Research and Dewlopment.
from GaN/InGaN H. Riechert
D-HI 730 Munich, Gemzat~~~
Abstract GaN,‘InGaN pn-junctions were grown by molecular beam epitaxy. Depending or green (513 nm) electroluminescence was observed at room temperature.
on the In content
bright blue (470 nm)
PACS: 42.80; 73.40.L; 68.55; 78.60.F
All commercially available GaN based light emitting diodes (LEDs) are presently fabricated by metalorganic vapour phase epitaxy [l, 21. Up to now only GaN homojunctions [3] or parts of an LED structure [4] have been grown by molecular beam epitaxy (MBE). Our paper presents the first report of bright blue and green electroluminescence from GaN/InGaN heterostructures grown entirely by MBE. GaN and InGaN layers were grown by MBE on c-plane oriented sapphire substrates using elemental sources for Ga and In and an RF plasma source for nitrogen. All layer sequences were started with a thin GaN buffer layer grown at 800 C. The growth temperatures of GaN and InGaN were 720 C and 700 C, respectively. We realized InGaN
*Corresponding
author.
0022-0248:97!$17.00 Copyright PII SOO22-0248(96)01009-3
layers with In contents exceeding 40%, as determined by X-ray diffraction. Nominally undoped GaN layers show room temperature hall mobilites of over 2.50 cm’/(V s) and an electron density of about 2 x 10” cm-3. Conventional effusion cells with Si and Mg were used for n- and p-type doping. No annealing or other treatment was required to activate the dopants. As-grown GaN : Mg yields a hole concentration of 2.5 x 10” cme3 at a hole mobility of 10 cm2/(V s). Fig. 1 shows the LED test structure used for this study. It consists of 1000 nm GaN : Si followed by 30 nm of undoped InGaN and 200 nm GaN : Mg. Circular Ti/Au contacts with 200 pm diameter were evaporated onto the wafer after growth. Applying a DC voltage between two contacts yields a symmetric I-I/-curve of two pn-junctions with opposite polarity connected in series. At voltages below 5 V no significant current is measured due to the high sheet resistivity of the thin GaN : Mg
(: 1997 Elsevier Science B.V. All rights reserved
R. At’erbeck et al. /Journal
qfCgxta1
layer. At voltages above 10 V leakage through the reverse biased diode leads to a superlinear increase of the current and electroluminescence is observed under the contact of the forward biased diode.
200nm GaN:Mg 30nm InGaN
lpm GaN:Si
Fig. I. Layer sequence study.
of the LED test structure
10
I
used in this
I
I
1
Electroluminescence 300 K
I 400
450 Wavelength
Fig. 2. Blue electroluminescence
123
Fig. 2 presents the electroluminescence spectrum of an LED test structure with an In content of 27% according to X-ray diffraction. It was taken at a current of 20 mA at room temperature. Intensive blue light is observed, visible even during daylight. The peak position of 470 nm (2.64 eV) coincides with the photoluminescence signal. From the measured In content of 27% one would expect a bandgap of 2.80 eV [l] showing that an approximately 160 meV deep level is involved in the optical transition. This may be due to Mg impurities since Mg is used for the p-type doping [S]. The electroluminescence spectrum displayed in Fig. 3 was taken from a sample with an In content of 40%. The emission wavelength of 5 13 nm (2.42 eV) yields bright green light. The peak position is also in accordance with photoluminescence measurements and lies about 160 meV below the expected bandgap of 2.58 eV [l]. In conclusion we have shown that MBE is a viable technique to grow GaN/InGaN heterostructures for LED applications. High In incorporation due to low growth temperatures and efficient ptype doping without post growth treatment are clear advantages compared to currently used growth techniques.
I
Ei-
0 350
Growth 175/I 76 (1997) 122- 124
from a GaNiInoZ,Ga
500
550
600
(nm) 0.73N pn-structure
at room temperature
124
R. Auerheck et al. ! Journal ofCyvsta1 Growth 1751176 (1997) 122-124
Electroluminescence 300 K
a t
Wavelength Fig. 3. Green electroluminescence
(nm)
from a GaN/In,.,,GaO.,r,N
References [1] S. Nakamura, J. Vat. Sci. Technol. A 13 (1995) 705. [2] Cree Research Inc., V.A. Dmitriev, reported at the 1st European GaN workshop, Rigi, Switzerland (1996) unpublished. [3] R.J. Molnar, R. Singh and T.D. Moustakas, Appl. Phys. Lett. 66 (1995) 268.
pn-structure
at room temperature,
[4] H. Sakai, T. Koide, H. Suzuki. M. Yamaguchi. S. Yamasaki. M. Koike, H. Amano and I. Akasaki, Jpn. J. Appl. Phys. 34 (1995) 1429. [S] W. Gotz, N.M. Johnson, J. Walker, D.P. Bour, H. Amano and 1. Akasaki, Appl. Phys. Lett. 67 (1995) 2666.
ELSEVIER
of Crystal
Growth
175i176 (1997) 122-124
Blue and green electroluminescence heterostructures R. Averbeck*, Sirens
H. Tews, A. Graber,
AG, Corporate Research and Dewlopment.
from GaN/InGaN H. Riechert
D-HI 730 Munich, Gemzat~~~
Abstract GaN,‘InGaN pn-junctions were grown by molecular beam epitaxy. Depending or green (513 nm) electroluminescence was observed at room temperature.
on the In content
bright blue (470 nm)
PACS: 42.80; 73.40.L; 68.55; 78.60.F
All commercially available GaN based light emitting diodes (LEDs) are presently fabricated by metalorganic vapour phase epitaxy [l, 21. Up to now only GaN homojunctions [3] or parts of an LED structure [4] have been grown by molecular beam epitaxy (MBE). Our paper presents the first report of bright blue and green electroluminescence from GaN/InGaN heterostructures grown entirely by MBE. GaN and InGaN layers were grown by MBE on c-plane oriented sapphire substrates using elemental sources for Ga and In and an RF plasma source for nitrogen. All layer sequences were started with a thin GaN buffer layer grown at 800 C. The growth temperatures of GaN and InGaN were 720 C and 700 C, respectively. We realized InGaN
*Corresponding
author.
0022-0248:97!$17.00 Copyright PII SOO22-0248(96)01009-3
layers with In contents exceeding 40%, as determined by X-ray diffraction. Nominally undoped GaN layers show room temperature hall mobilites of over 2.50 cm’/(V s) and an electron density of about 2 x 10” cm-3. Conventional effusion cells with Si and Mg were used for n- and p-type doping. No annealing or other treatment was required to activate the dopants. As-grown GaN : Mg yields a hole concentration of 2.5 x 10” cme3 at a hole mobility of 10 cm2/(V s). Fig. 1 shows the LED test structure used for this study. It consists of 1000 nm GaN : Si followed by 30 nm of undoped InGaN and 200 nm GaN : Mg. Circular Ti/Au contacts with 200 pm diameter were evaporated onto the wafer after growth. Applying a DC voltage between two contacts yields a symmetric I-I/-curve of two pn-junctions with opposite polarity connected in series. At voltages below 5 V no significant current is measured due to the high sheet resistivity of the thin GaN : Mg
(: 1997 Elsevier Science B.V. All rights reserved
R. At’erbeck et al. /Journal
qfCgxta1
layer. At voltages above 10 V leakage through the reverse biased diode leads to a superlinear increase of the current and electroluminescence is observed under the contact of the forward biased diode.
200nm GaN:Mg 30nm InGaN
lpm GaN:Si
Fig. I. Layer sequence study.
of the LED test structure
10
I
used in this
I
I
1
Electroluminescence 300 K
I 400
450 Wavelength
Fig. 2. Blue electroluminescence
123
Fig. 2 presents the electroluminescence spectrum of an LED test structure with an In content of 27% according to X-ray diffraction. It was taken at a current of 20 mA at room temperature. Intensive blue light is observed, visible even during daylight. The peak position of 470 nm (2.64 eV) coincides with the photoluminescence signal. From the measured In content of 27% one would expect a bandgap of 2.80 eV [l] showing that an approximately 160 meV deep level is involved in the optical transition. This may be due to Mg impurities since Mg is used for the p-type doping [S]. The electroluminescence spectrum displayed in Fig. 3 was taken from a sample with an In content of 40%. The emission wavelength of 5 13 nm (2.42 eV) yields bright green light. The peak position is also in accordance with photoluminescence measurements and lies about 160 meV below the expected bandgap of 2.58 eV [l]. In conclusion we have shown that MBE is a viable technique to grow GaN/InGaN heterostructures for LED applications. High In incorporation due to low growth temperatures and efficient ptype doping without post growth treatment are clear advantages compared to currently used growth techniques.
I
Ei-
0 350
Growth 175/I 76 (1997) 122- 124
from a GaNiInoZ,Ga
500
550
600
(nm) 0.73N pn-structure
at room temperature
124
R. Auerheck et al. ! Journal ofCyvsta1 Growth 1751176 (1997) 122-124
Electroluminescence 300 K
a t
Wavelength Fig. 3. Green electroluminescence
(nm)
from a GaN/In,.,,GaO.,r,N
References [1] S. Nakamura, J. Vat. Sci. Technol. A 13 (1995) 705. [2] Cree Research Inc., V.A. Dmitriev, reported at the 1st European GaN workshop, Rigi, Switzerland (1996) unpublished. [3] R.J. Molnar, R. Singh and T.D. Moustakas, Appl. Phys. Lett. 66 (1995) 268.
pn-structure
at room temperature,
[4] H. Sakai, T. Koide, H. Suzuki. M. Yamaguchi. S. Yamasaki. M. Koike, H. Amano and I. Akasaki, Jpn. J. Appl. Phys. 34 (1995) 1429. [S] W. Gotz, N.M. Johnson, J. Walker, D.P. Bour, H. Amano and 1. Akasaki, Appl. Phys. Lett. 67 (1995) 2666.