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. Controlof the piezoelectric field and thickness design of wurtzite nitride quantum wells are important forband edge emission in the short wavelength region. By reducing the strain between the AlGaNwell and the barrier layers of multi-quantum wells, ultraviolet light emitting diodes operating atthe wavelength of 346 nm were successfully fabricated. 1. IntroductionNitride semiconductors have been intensively studied for short wavelength emitters. Byintroducing InGaN active layers, light emitting diodes (LEDs) and laser diodes (LDs)have been successfully operated at wavelengths longer than 370 nm [1, 2]. For futureapplications for highly dense optical storage and chemical analysis, further deep-UVemitters are desired. The bandgap wavelength of GaN is about 360 nm. AlthoughAlGaN 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 . 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] weshowed that a very flat hetero-interface can be achieved by metalorganic vapor phaseepitaxy (MOVPE) on on-axis 6H-SiC (0001)Si substrate. Besides the heavy electronmasses, wurtzite nitrides are piezoactive and their hetero-structures suffer from a largestrain due to the lattice mismatch [6 to 8]. Therefore, their hetero-structures show asignificant piezoelectric effect, resulting in the deterioration of emission efficiency dueto the spatially indirect transition. First, we evaluate the room temperature optical characteristics of a conventionalGaN/AlGaN QW. Next, we report a short wavelength (346 nm) LED built by utilizingstrain-reduced multi-quantum-well (MQW) structures. 2. Photoluminescence Spectra of GaN/AlGaN Quantum WellsFig. 1 shows the change of photoluminescence (PL) intensity of GaNaAl0X15 Ga0X85 NQWs 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: email@example.com
46 T. Nishida and N. KobayashiFig. 1. Photoluminescence (PL) intensity of GaNaAl0X15 Ga0X85 N QWs at various temperatures. Wellthicknesses are 1.2, 2.4, and 4.8 nm. Each intensity is normalized by the PL intensity of the GaNlayerthe inset. Each intensity is normalized by the PL intensity of the GaN layer. At lowtemperature, both the 1.2 nm thick well and the 2.4 nm thick well show a luminescenceenhancement due to quantum confinement. On the other hand, the 4.8 nm thick wellshows poor luminescence intensity. This is due to the piezoelectric effect, which resultsin a spatially indirect transition [6, 7]. By increasing the temperature, the PL intensityof the narrow QW (1.2 nm thick well) is drastically reduced. This is perhaps due tocarrier relaxation to non-radiative recombination centers by thermalization. From theFig. 2. Photoluminescence spectra of GaNaAl0X15 Ga0X85 N QWs. The Al contents in the barrierlayers are (a) 15% and (b) 22%. The emission efficiency deteriorates at wavelengths shorter than350 nm
346 nm Emission from AlGaN Multi-QW Diodes 47viewpoint of device application, room-temperature performance is very important.Fig. 2 shows the room-temperature photoluminescence (PL) spectra of GaN/AlGaNquantum 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 ofbulk GaN. This tendency is much more pronounced for QWs consisting of barriers witha higher Al molar fraction. This means that the larger strain due to the difference ofthe 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 narrowingthe well thickness and/or by increasing the Al molar fraction in the barrier layers. Asshown in Fig. 2, luminescence enhancement by utilizing quantum structures is achievedonly for the 2.4 nm thick GaNaAl0X15 Ga0X85 N QW at 354 nm. 3. 346 nm Emission from AlGaN/AlGaN Light Emitting DiodeThe 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 betweenthe well layers and the barrier layers, i.e. by increasing the Al content in the welllayers. Fig. 3 shows the device structure we fabricated. The 400 nm thick Si-dopedAl0X12 Ga0X88 N layer is directly grown on the SiC substrate. The active region consists of2 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 thickNi is deposited on the p-GaN contact layer as a semi-transparent window. To achieveelectric contact, we formed an Au pad electrode on Ni and Au/Ti on the backside ofSiC 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 wavelengthis 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 asnarrow as 5 nm and emission holds the same emission wavelength with an injection currentof less than 800 A/cm2 . We attribute this narrow and stable luminescence feature to thewell-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 anAlGaN alloy well and barrier to reduce the piezoelectric effect. The window area is 4 Â 10À5 cm2
48 T. Nishida and N. Kobayashi: 346 nm Emission from AlGaN Multi-QW DiodesFig. 4. Electroluminescence spectra of UV-LED. The emission peak is at 346 nm with the injectioncurrent density of less than 800 A/cm2 4. ConclusionIn conclusion, we studied emission efficiency and wavelength dependence on strain be-tween well and barriers by photoluminescence. We proposed a strain-reducedAlx GaN1Àx NaAly Ga1Ày N QW structure for short wavelength emitters, and successfullyfabricated a UV LED operating at the wavelength of 346 nm. References I. Akasaki, H. Amano, K. Itoh, N. Koide, and K. Manabe, Inst. Phys. Conf. Ser. 129, 851 (1993). S. Nakamura and G. Fasol, The Blue Laser Diode, Springer-Verlag, Berlin 1997. 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). T. Nishida, T. Akasaka, and N. Kobayashi, Jpn. J. Appl. Phys. 37, L459 (1998). T. Nishida, N. Maeda, T. Akasaka, and N. Kobayashi, J. Cryst. Growth 195, 41 (1998). 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). Jin Seo Im, H. Kollmer, J. Off, A. Sohmer, F. Scholz, and A. Hangleiter, Phys. Rev. B 57, R9435 (1998). T. Nishida, M. Kumagai, N. Meda, and N. Kobayashi, 2nd Internat. Symp. Blue Laser and Light Emitting Diodes, Chiba 1998 (Paper Tu-08).