phys. stat. sol. (a) 188, No. 1, 187–190 (2001) AlGaN Resonant Tunneling Diodes Grown by rf-MBE A. Kikuchi1), R. Bannai, and K. Kishino Department of Electrical and Electronics Engineering, Sophia University, 7-1, Kioi-cho, Chiyoda-ku, Tokyo 102-8554, Japan (Received July 4, 2001; accepted July 12, 2001) Subject classification: 73.40.Kp; S7.14AlGaN resonant tunneling diodes (RTD) have been successfully grown by molecular beam epi-taxy. Two kinds of RTD samples were fabricated; one is a double barrier type RTD which has1 nm AlN barriers and a 0.75 nm GaN well, and the other is a superlattice barrier type RTD whichhas six 1 nm AlN barriers and five 1 nm GaN wells. The negative differential resistance (NDR)effect in the current–voltage characteristics was clearly observed at room temperature. For thedouble barrier RTD sample, the NDR was observed at 2.4 V with a peak current density of930 mA/cm2 and a peak-to-valley ratio of 3.1. For the superlattice barrier RTD sample, the NDRwas observed at 1.6 V. The peak current density and peak-to-valley ratio were 142 A/cm2 and 9.7,respectively.Introduction The III-nitride material system has many attractive properties such as alarge bandgap energy, large bandgap discontinuity, high peak electron velocity, highsaturation electron velocity and higher thermal stability. Many studies on AlGaN basedelectrical devices have been reported on HEMT, FET, HBT, etc. From the point ofview of large bandgap discontinuity of ~1.95 eV (for AlN/GaN system), various quan-tum effect device applications are expected; such as resonant tunneling diodes (RTD), intersubband transition near infrared optical devices  and so on. The RTD isattractive for high-frequency functional device applications. The large bandgap disconti-nuity requires monolayer-order thickness controllability and a smooth interface for theepitaxial growth. In this respect, molecular beam epitaxy (MBE) seems to be a suitablegrowth technique to realize a fine control of the nitride heterostructures. In fact, re-cently, very high two-dimensional electron gas mobility over 50000 cm2/Vs [3, 4] andnear infrared intersubband transition  have been demonstrated by MBE grownAlGaN/GaN heterostructure. In this paper, we will descrive the first successful growth of AlN/GaN RTD by MBEusing rf-plasma nitrogen source (rf-MBE). Two kinds of RTDs with different AlN bar-rier structures were adopted. One is conventional double barrier (DB) type, and theother is superlattice barrier (SLB) type. The current–voltage characteristics showedclear negative differential resistance (NDR) in both samples at room temperature. Thestructural dependence of peak current density and peak-to-vally current ratio of NDRwill be described.Epitaxial Growth and Device Processing The AlN/GaN RTD structures were grownby rf-MBE on 4 mm thick n-type GaN templates which were grown on (0001) sapphiresubstrates by MOCVD. The active nitrogen was supplied through an RF-plasma source 1 ) Corresponding author; Phone: +81 3 3238 3323; Fax: +81 3 3238 3321;e-mail: email@example.com # WILEY-VCH Verlag Berlin GmbH, 13086 Berlin, 2001 0031-8965/01/18811-0187 $ 17.50þ.50/0
188 A. Kikuchi et al.: AlGaN Resonant Tunneling Diodes Grown by rf-MBEwith high purity nitrogen gas (99.9999%). Prior to the rf-MBE growth, the backsides ofthe GaN templates were coated with Ti to enhance heat absorption and mounted onIn-free Mo blocks. Prior to the MBE growth, the GaN templates were exposed to Gabeam at 300 oC, then the substrate temperature was ramped to 700 oC and the activenitrogen was irradiated to the substrate surface. In the growth process, the substratetemperature was set to be 720 oC. The AlN layers were grown by migration enhancedepitaxy (Al and nitrogen were alternatively supplied)  and the GaN layers weregrown by the shutter control method (Ga was continuously supplied and nitrogen wasperiodically interrupted) . The detailed structure of the two samples, DB-RTD andSLB-RTD, will be shown in the following sections. For the electrical measurements, the samples were processed into mesa-type devicestructure. 80 mm square Ti (35 nm)/Al(130 nm) electrodes were formed through elec-tron beam evaporation and conventional lift-off technique. Then 700 nm height mesastructures were formed by atom beam etching using the electrodes as masks. The etch-ing was carried out under the following conditions: Ar/CH4/H2 ¼ 10/10/80 mixed gasflow with a total pressure of 4.0 Â 10 – Torr, bias voltage of 0.8 kV and a substrate –3 otemperature of 400 C. The etching rate was 15.5 nm/min. Finally, Ti (35 nm)/Al(130 nm) lower electrodes in the width of 80 mm were formed with the same process astop electrode formation.Double Barrier RTD The first RTD structure is of conventional double barrier typeas shown in Fig. 1. In this sample, five layers of 8 nm thick high-temperature grownAlN multiple interlayers (HT-AlN-MIL) separated by 40 nm thick GaN layers wereinserted between the MOCVD-GaN template and RTD structure to reduce threadingdislocations [6, 8, 9]. As a result of dislocation reduction by HT-AlN-MIL, we haverealized fine step flow GaN growth by rf-MBE on MOCVD-GaN template. On the HT-AlN-MIL, DB-RTD structure was grown as a sequence of 800 nm thick Si-doped GaNlayer, 2 nm thick undoped GaN, 1 nm (4 ML) thick AlN barrier, 0.75 nm (3 ML) thick Fig. 1 Fig. 2Fig. 1 Schematic diagram of AlN/GaN double barrier RTD grown on MOCVD-GaN templateFig. 2. Current–voltage characteristics of AlN/GaN double barrier RTD measured in 12.5 mVsteps at room temperature
phys. stat. sol. (a) 188, No. 1 (2001) 189undoped GaN, 1 nm (4 ML) thick AlN barrier, 2 nm thick undoped GaN and 400 nmthick Si-doped GaN layer. Here, the Si-doping level was estimated to be 8 Â 1017 cm – .–3 The current–voltage characteristics were measured with a single sweep mode from0 V to forward and reverse direction in 12.5 mV steps at room temperature. Figure 2shows the typical current–voltage characterristics of the DB-RTD. The NDR was ob-served around 2.4 V with a peak current of 60 mA. The corresponding current densitywas estimated as 930 mA/cm2. In this sample, the peak-to-valley current ratio was 3.1.By solving the Schrodinger equation using square potential profile, the resonant energy ¨level E1 was estimated to be at 800 meV from the GaN conduction band edge whichcorresponds to the NDR voltage (%2E1/e) of 1.6 V. The difference of NDR voltagebetween experiment and calculation may be caused by relatively high series resistanceand rectification property of the contact due to the damage induced by the atom beametching process or thickness reduction of GaN well layer to 2 ML. The hysteretic prop-erty originated from high series resistance was not confirmed here, because of the sin-gle sweep mode measurement. Almost all samples showed asymmetrical current–volt-age characteristics, that is, NDR was observed only for the single side in themeasurement range. The asymmetric I–V characteristics may be originated from twopossible reasons, one is a piezoelectric field induced asymmetrical potential profile andanother is a rectification property in the contact. The later is not an essential problem,and it can be avoided by optimization of the device processes. The origin of the asym-metric property will be clarified by the precise theoretical simulation or optimization ofdevice processing.Superlattice Barrier RTD Figure 3 shows the schematic diagram of the superlatticebarrier RTD, which consists of six AlN barrier layers. This structure was directly grownon the MOCVD-GaN template, a sequence of 800 nm thick Si-doped GaN, six layers of1 nm (4 ML) thick AlN barrier separated by 1 nm (4 ML) thick Si-doped GaN welllayers and 400 nm thick Si-doped GaN top layer. Here, the doping level of the n-GaNwas estimated to be 2 Â 1018 cm – , which was 2.5 times higher than for DB-RTD case. –3 Fig. 3 Fig. 4Fig. 3. Schematic diagram of AlN/GaN superlattice barrier RTD grown on MOCVD-GaN templateFig. 4. Current–voltage characteristics of AlN/GaN superlattice barrier RTD measured in 26.7 mVsteps at room temperature
190 A. Kikuchi et al.: AlGaN Resonant Tunneling Diodes Grown by rf-MBE The typical current–voltage characteristics at room temperature is shown in Fig. 4.The measurement was carried out in 26.7 mV steps. NDR was clearly observed atabout 1.6 V. This smaller NDR voltage compared to DB-RTD may arise from the low-er resonant energy level estimated to be 0.55 eV. A maximum peak-to-valley currentratio of 9.7 was obtained. The peak current was 9 mA, which corresponds to 142 A/cm2in the current density. This superior characteristics may be brought about by the in-crease of doping level and introduction of superlattice barriers. The transport mecha-nism  in the superlattice barrier is still under investigation and will be described infurther studies.Conclusions We have demonstrated room temperature NDR effects in AlN/GaN reso-nant tunneling diodes grown by rf-MBE for the first time. Two kinds of RTD structure,double barrier type RTD and superlattice barrier type RTD, were adopted. For theDB-RTD, the NDR was observed at 2.4 V with a peak current density of 930 mA /cm2and a peak-to-valley ratio of 3.1. For the superlattice barrier RTD, the NDR was ob-served at 1.6 V. The peak current density and a peak-to-valley ratio were 142 A/cm2and 9.7, respectively. These results may open a new application field for AlGaN materi-al systems such as high-speed functional electronic devices and near infrared quantumcascade devices.Acknowledgements This work was supported by the ‘‘Research for the Future” pro-gram of the Japan Society for the Promotion of Science No. JSPS-RFTF97P00102, andpartly by science research grant-in-aid No. 13750315 from the Ministry of Education,Culture, Sports, Science and Technology. The authors would like to acknowledge Prof.T. Waho for his valuable discussion, Associate Prof. K. Shimomura for his permission toutilize the experimental equipments, and Mr. K. Serizawa for his valuable help on de-vice fabrication and measurement. References  L. L. Chang, L. Esaki, and R. Tsu, Appl. Phys. Lett. 24, 593 (1974).  J. Faist, F. Capasso, D. L. Sivco, C. Sirtori, A. L. Hutchinson, and A. Y. Cho, Science 264, 553 (1994).  I. P. Smorchkova, C. R. Elsass, J. P. Ibbetson, R. Vetury, B. Heying, P. Fini, E. Haus, S. P. DenBaars, J. S. Speck, and U. K. Mishra, J. Appl. Phys. 86, 4520 (1999).  M. J. Manfra, L. N. Pfeiffer, K. W. West, H. L. Stormer, K. W. Baldwin, J. W. P. Hsu, D. V. Lang, and R. J. Molnar, Appl. Phys. Lett. 77, 2889 (2000).  C. Gmachl, H. M. Ng, S.-N. G. Chu, and A. Y. Cho, Appl. Phys. Lett. 77, 3722 (2000).  D. Sugihara, A. Kikuchi, K. Kusakabe, S. Nakamura, Y. Toyoura, T. Yamada, and K. Kishino, Jpn. J. Appl. Phys. 39, L197 (2000).  A. Kikuchi, M. Yoshizawa, M. Mori, N. Fujita, K. Kushi, H. Sasamoto, and K. Kishino, J. Cryst. Growth 189/190, 109 (1998).  A. Kikuchi, T. Yamada, S. Nakamura, K. Kusakabe, D. Sugihara, and K. Kishino, Jpn. J. Appl. Phys. 39, L330 (2000).  A. Kikuchi, T. Yamada, S. Nakamura, K. Kusakabe, D. Sugihara, and K. Kishino, Mater. Sci. Eng. B 82, 12 (2001). F. Capasso, K. Mohammed, and A. Y. Cho, IEEE J. Quantum Electron. 22, 1853 (1986).