1) Researchers fabricated ultraviolet light emitting diodes that operate at a wavelength of 346 nm, the shortest reported at the time. 2) This was achieved by using a strain-reduced multi-quantum well structure of AlGaN/AlGaN to reduce the piezoelectric field effects that deteriorate emission efficiency at short wavelengths. 3) Electroluminescence measurements showed a dominant emission peak at 346 nm with a narrow full width at half maximum of 5 nm, demonstrating efficient band-edge emission at this short wavelength.

1. T. Nishida and N. Kobayashi: 346 nm Emission from AlGaN Multi-QW Diodes 45
phys. stat. sol. (a) 176, 45 (1999)
Subject classification: 78.66.Fd; 77.65.Ly; 78.55.Cr; S7.14; S7.15
346 nm Emission from AlGaN Multi-Quantum-Well
Light Emitting Diode
T. Nishida 1 ) and N. Kobayashi
NTT Basic Research Laboratories, 3-1, Morinosato-wakamiya, Atsugi-shi,
Kanagawa Pref., 243-0198, Japan
(Received July 4, 1999)
To realize short wavelength light emitting diodes, nitride quantum structures are studied. Control
of the piezoelectric field and thickness design of wurtzite nitride quantum wells are important for
band edge emission in the short wavelength region. By reducing the strain between the AlGaN
well and the barrier layers of multi-quantum wells, ultraviolet light emitting diodes operating at
the wavelength of 346 nm were successfully fabricated.
1. Introduction
Nitride semiconductors have been intensively studied for short wavelength emitters. By
introducing InGaN active layers, light emitting diodes (LEDs) and laser diodes (LDs)
have been successfully operated at wavelengths longer than 370 nm [1, 2]. For future
applications for highly dense optical storage and chemical analysis, further deep-UV
emitters are desired. The bandgap wavelength of GaN is about 360 nm. Although
AlGaN material has a shorter bandgap wavelength, it usually has poor optical quality.
To overcome this disadvantage, the introduction of quantum well (QW) structure con-
sisting of GaN and AlGaN is promising [3]. However, these nitrides have heavy elec-
tron masses. Therefore, a very flat and abrupt hetero-interface is required for the for-
mation of quantum structures for deep-UV emitters. In previous reports [4, 5] we
showed that a very flat hetero-interface can be achieved by metalorganic vapor phase
epitaxy (MOVPE) on on-axis 6H-SiC (0001)Si substrate. Besides the heavy electron
masses, wurtzite nitrides are piezoactive and their hetero-structures suffer from a large
strain due to the lattice mismatch [6 to 8]. Therefore, their hetero-structures show a
significant piezoelectric effect, resulting in the deterioration of emission efficiency due
to the spatially indirect transition.
First, we evaluate the room temperature optical characteristics of a conventional
GaN/AlGaN QW. Next, we report a short wavelength (346 nm) LED built by utilizing
strain-reduced multi-quantum-well (MQW) structures.
2. Photoluminescence Spectra of GaN/AlGaN Quantum Wells
Fig. 1 shows the change of photoluminescence (PL) intensity of GaNaAl0X15 Ga0X85 N
QWs at various temperatures. The well thicknesses are 1.2, 2.4, and 4.8 nm, as shown in
1
) Corresponding author; Tel.: +81-46-240-3174; Fax: +81-46-240-4729;
e-mail: tnishida@will.brl.ntt.co.jp

2. 46 T. Nishida and N. Kobayashi
Fig. 1. Photoluminescence (PL) intensity of GaNaAl0X15 Ga0X85 N QWs at various temperatures. Well
thicknesses are 1.2, 2.4, and 4.8 nm. Each intensity is normalized by the PL intensity of the GaN
layer
the inset. Each intensity is normalized by the PL intensity of the GaN layer. At low
temperature, both the 1.2 nm thick well and the 2.4 nm thick well show a luminescence
enhancement due to quantum confinement. On the other hand, the 4.8 nm thick well
shows poor luminescence intensity. This is due to the piezoelectric effect, which results
in a spatially indirect transition [6, 7]. By increasing the temperature, the PL intensity
of the narrow QW (1.2 nm thick well) is drastically reduced. This is perhaps due to
carrier relaxation to non-radiative recombination centers by thermalization. From the
Fig. 2. Photoluminescence spectra of GaNaAl0X15 Ga0X85 N QWs. The Al contents in the barrier
layers are (a) 15% and (b) 22%. The emission efficiency deteriorates at wavelengths shorter than
350 nm

3. 346 nm Emission from AlGaN Multi-QW Diodes 47
viewpoint of device application, room-temperature performance is very important.
Fig. 2 shows the room-temperature photoluminescence (PL) spectra of GaN/AlGaN
quantum wells (QWs). The aluminium contents in the barrier layers are (a) 15% and
(b) 22%. The PL peak wavelengths of 4.8 nm thick wells become longer than that of
bulk GaN. This tendency is much more pronounced for QWs consisting of barriers with
a higher Al molar fraction. This means that the larger strain due to the difference of
the Al molar fraction between well layer and barrier layer causes a significant piezo-
electric field, resulting in the longer emission wavelength and poorer emission effi-
ciency. Therefore, it is difficult to shorten the emission wavelength simply by narrowing
the well thickness and/or by increasing the Al molar fraction in the barrier layers. As
shown in Fig. 2, luminescence enhancement by utilizing quantum structures is achieved
only for the 2.4 nm thick GaNaAl0X15 Ga0X85 N QW at 354 nm.
3. 346 nm Emission from AlGaN/AlGaN Light Emitting Diode
The emission efficiency at wavelengths shorter than 350 nm can be improved by sup-
pressing the piezoelectric field. This can be done by reducing the strain between
the well layers and the barrier layers, i.e. by increasing the Al content in the well
layers. Fig. 3 shows the device structure we fabricated. The 400 nm thick Si-doped
Al0X12 Ga0X88 N layer is directly grown on the SiC substrate. The active region consists of
2 nm thick Al0X08 Ga0X92 N well layers and 2 nm thick Al0X12 Ga0X88 N barrier layers. Next,
we grew 400 nm thick Mg-doped Al0X12 Ga0X88 N and 10 nm thick GaN. The 15 nm thick
Ni is deposited on the p-GaN contact layer as a semi-transparent window. To achieve
electric contact, we formed an Au pad electrode on Ni and Au/Ti on the backside of
SiC substrate.
Fig. 4 shows the electroluminescence (EL) spectra as a function of injection current.
The semitransparent window area is 4 Â 10À5 cm2. The dominant emission wavelength
is 346 nm. In spite of the simple junction structure without a separate carrier confine-
ment structure and without modulation doping, clear band edge emission is observed.
This is the shortest wavelength ever reported. The FWHM of the emission peak is as
narrow as 5 nm and emission holds the same emission wavelength with an injection current
of less than 800 A/cm2 . We attribute this narrow and stable luminescence feature to the
well-regulated heterostructures achiewed by flat nitride growth on SiC substrate [4, 5, 8].
Fig. 3. Schematic drawing of the structure of the LED sample. The active region consists of an
AlGaN alloy well and barrier to reduce the piezoelectric effect. The window area is 4 Â 10À5 cm2

4. 48 T. Nishida and N. Kobayashi: 346 nm Emission from AlGaN Multi-QW Diodes
Fig. 4. Electroluminescence spectra of UV-LED. The emission peak is at 346 nm with the injection
current density of less than 800 A/cm2
4. Conclusion
In conclusion, we studied emission efficiency and wavelength dependence on strain be-
tween well and barriers by photoluminescence. We proposed a strain-reduced
Alx GaN1Àx NaAly Ga1Ày N QW structure for short wavelength emitters, and successfully
fabricated a UV LED operating at the wavelength of 346 nm.
References
[1] I. Akasaki, H. Amano, K. Itoh, N. Koide, and K. Manabe, Inst. Phys. Conf. Ser. 129, 851
(1993).
[2] S. Nakamura and G. Fasol, The Blue Laser Diode, Springer-Verlag, Berlin 1997.
[3] J. Han, M. H. Crawford, R. J. Shul, J. J. Figiel, M. Banas, L. Zhang, Y. K. Song, H. Zhou, and
A. V. Nurmikko, Appl. Phys. Lett. 73, 1688 (1998).
[4] T. Nishida, T. Akasaka, and N. Kobayashi, Jpn. J. Appl. Phys. 37, L459 (1998).
[5] T. Nishida, N. Maeda, T. Akasaka, and N. Kobayashi, J. Cryst. Growth 195, 41 (1998).
[6] C. W. Wetzel, H. Amano, I. Akasaki, T. Saki, J. W. Ager, E. R. Weber, E. E. Haller, and
B. K. Meyer, Mater. Res. Soc. Symp. Proc. 482, 489 (1998).
[7] Jin Seo Im, H. Kollmer, J. Off, A. Sohmer, F. Scholz, and A. Hangleiter, Phys. Rev. B 57,
R9435 (1998).
[8] T. Nishida, M. Kumagai, N. Meda, and N. Kobayashi, 2nd Internat. Symp. Blue Laser and Light
Emitting Diodes, Chiba 1998 (Paper Tu-08).