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2001-Characterizations of recessed gate AlGaN


IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 48, NO. 3, MARCH 2001

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Characterizations of Recessed Gate AlGaN/GaN HEMTs on Sapphire
Takashi Egawa, Guang-Yuan Zhao, Hiroyasu Ishikawa, Masayoshi Umeno, Member, IEEE, and Takashi Jimbo
Abstract—A recessed gate high electron mobility transistor (HEMT) has been fabricated with AlGaN/GaN heterostructure on a sapphire substrate using metalorganic chemical vapor deposition. Capacitance–voltage (C–V) and Shubnikov-de Haas measurements have shown the formation of two-dimensional (2-D) electron gas (2DEG) at Al0 11 Ga0 89 N/GaN heterointerface. A 2DEG mobility 12 000 cm2 /V-s with a sheet carrier density 2 8 1012 cm 2 was measured on Al0 11 Ga0 89 N/GaN heterostructure at 8.9K. The recessed gate Al0 26 Ga0 74 N/GaN HEMT structure showed maximum extrinsic transconductance 181 mS/mm and drain-source current 1120 mA/mm for a gate length 1.5 m at 25 C. The device exhibited stable operation characteristics at 350 C for long time (500 h). No interfacial change has been observed at metal/AlGaN interface even after 350 C for 500 h treatment. The threshold voltage of device does not depend very much on operating temperature (25 to 350 C). Index Terms—AlGaN/GaN HEMT, recessed gate, Shubnikov-de-Haas, 2DEG.
Fig. 1. Cross-sectional structure of the recessed gate Al Ga N/GaN HEMT grown on sapphire by MOCVD. The mesa isolation and the gate recess etch were formed by use of reactive ion etching in a BCl plasma.

I. INTRODUCTION HE NECESSITY for high-power and high-frequency solid state amplifiers is rapidly increasing with the development of wireless communications. Chow et al. [1] reported that wide bandgap compound semiconductors formed the most attractive alternative over Si and GaAs because of inherent material advantages and its own figures of merit have been improved using wide bandgap compound semiconN/GaN high electron mobility transistors ductors. Al Ga (HEMTs) have been demonstrated for devices operating under high-power, high-frequency, and high-temperature conditions due to their large sheet carrier density, small gate leakage current and large breakdown voltage [2]–[5]. Enhancement of two-dimensional electron gas (2DEG) mobility and reduction of parasitic source resistance are necessary for the fabrication N/GaN HEMT’s. However, of high-performance Al Ga N/GaN heterostructures grown on sapphire subAl Ga strates exhibited lesser 2DEG mobilities than those on SiC substrates because of 13.8% lattice mismatch between sapphire and GaN [6]. To minimize the parasitic source resistance, the recessed gate process has been applied for the GaN-based FETs. Binari et al. [7] and Burm et al. [8] have reported that the
Manuscript received April 17, 2000; revised November 20, 2000. This research is a part of the Public Participation Program for Frequency Resources Development 1999–2000 by the Ministry of Posts and Telecommunications. the review of this paper was arranged by T. Egawa, G.-Y. Zhao, H. Ishikawa, and M. Umeno are with the Research Center for Micro-Structure Devices, Nagoya Institute of Technology, Nagoya 466-8555, Japan (e-mail: egawa@mothra.elcom.nitech.ac.jp). T. Jimbo is with the Department of Environmental Technology and Urban Planning, Nagoya Institute of Technology, Nagoya 466-8555, Japan. Publisher Item Identifier S 0018-9383(01)02487-X.

T

transconductance 41 mS/mm for the GaN metal-semiconductor field-effect transistor (MESFET) with the gate length ( ) 1.3 m and 45 mS/mm for the Al Ga N/GaN HEMT with m using the recessed gate process, respectively. Recently, we have shown the transconductance of 146 mS/mm and high-temperature characteristics for the recessed gate m [9]. Further Al Ga N/GaN HEMT with N/GaN HEMTs are advances in the performance of Al Ga expected to occur with improvements in 2DEG mobility at the N/GaN heterointerface with low source resistance. In Al Ga this study we show the transconductance as high as 181 mS/mm and stable operation at high temperature for the recessed gate Al Ga N/GaN HEMT on sapphire substrate. II. EXPERIMENTAL PROCEDURE Samples were grown on (0001) sapphire substrates by metalorganic chemical vapor deposition (MOCVD) at atmospheric pressure using two-step growth technique. Fig. 1 shows the cross-sectional structure of the Al Ga N/GaN HEMT grown on a sapphire substrate by MOCVD. The epitaxial layers for the Al Ga N/GaN HEMT structure consist of a 30-nm-thick GaN nucleation layer, a 2.5- m-thick undoped GaN layer, a 10-nm-thick Al Ga N spacer layer, a 20-nm-thick n -Al Ga N layer with Si doped to cm and a 20-nm-thick n -GaN layer with Si doped to cm . For the growth of undoped GaN layer, the flow rates of NH and TMG were 5 l/min and 69 mol/min, respectively. The flow rates of NH , TMG, and TMA were 5 l/min, 29.5 mol/min, and 5.2 mol/min, respectively, for

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IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 48, NO. 3, MARCH 2001

spectively. The threshold voltage was determined from the relationship between square root of drain–source current ( ) and ). The source resistance ( ) measurement was gate bias ( followed by [13]. The – measurements were performed on the Schottky contact of 300- m diameter grown on the same wafer as the HEMT device. For comparison, a GaN MESFET with the gate length of 2.1 m and the width of 15 m was also fabricated on the sapphire substrate. The details of epilayer structures and fabrication process steps described elsewhere [14]. The high-temperature characteristics and reliability tests of the fabricated devices were conducted in N atmosphere. To check the interfacial property of the Schottky contact Au/Ti/Pt/AlGaN/GaN, Auger electron spectroscopy (AES) was performed using JEOL JAMP-7800 Auger microprobe. III. RESULTS AND DISCUSSION Fig. 2 shows the electron mobility and sheet carrier density of Al Ga /GaN heterostructure as a function of temperature. Above 100K, the electron mobility decreased rapidly and the sheet carrier density increased weakly with an increase of sample temperature. At low temperatures, where the ionized impurity scattering would be expected to dominate, the electron mobility and the sheet carrier density values were independent of the temperature. The values of electron mobility and the sheet carrier density of Al Ga /GaN heterostructure were cm at 300K, and 12 000 cm /Vs 740 cm /Vs and cm at 8.9K, respectively. The value of sheet and cm has been obtained carrier density as high as from AlGaN/GaN heterointerface due to the piezoelectric effect [10], [12], [15]. The sheet carrier density ( cm ) is relatively low at 300K because that the barrier layer (Al Ga N) has low Al content [10]. However, the sheet carrier density of the AlGaN/GaN heterostructure is much higher than the value of AlGaAs/GaAs system ( cm ) [16]. The high value of sheet carrier density is due to the piezoelectric effect [10], [12], [15]. Shubnikov-de Haas (SdH) measurement was used to confirm the presence of 2DEG at the Al Ga /GaN heterointerface. The inset of ) as a function of Fig. 2 shows the magnetoresistance ( magnetic field at 4.2K. Oscillations are present for fields of 3.8 T and the minima of the oscillations decrease as the field increases, which indicates the presence of the 2DEG at the Al Ga N/GaN heterointerface. The sheet carrier density cm . As shown calculated from the SdH data was later, the sharp increase in the carrier concentration from the – measurement also shows the presence of the 2DEG at the heterointerface. These results are confirming the formation of 2DEG at the Al Ga N/GaN heterointerface. – characteristic of the 1.5- m-gate Fig. 3 shows the ranging from length Al Ga N/GaN HEMT for the to V at 25 C. The value of sheet carrier density cm ) for the Al Ga N/GaN HEMT struc( ture is higher than the Al Ga N/GaN heterostructure cm ). The increase of sheet carrier density for ( HEMT structure is due to the piezoelectric effect [10], [12], ) and [15]. The maximum extrinsic transconductance ( ) as high as 181 maximum drain–source current (

Fig. 2. 2DEG mobility and sheet carrier density as a function of temperature for the Al Ga N/GaN heterostructure on sapphire grown by MOCVD. Inset shows the magnetoresistance R at 4.2K as a function of magnetic field for the Al Ga N/GaN heterostructure.

the growth of Al Ga N layers. The Al-content in the Al Ga N layer was determined by double crystal x-ray rocking curve measurement ( -scan). N/GaN hetThe characteristics of 2DEG at the Al Ga N barrier layer [10]–[12]. erointerface depend on Al Ga The 200-nm-thick Al Ga N layer on the 2- m-thick GaN layer has been used for low temperature Hall effect measurement. The Hall effect measurements were performed on Al Ga N/GaN heterostructure using Van der Pauw Hall method at 0.4 T magnetic field. The magnetic field dependence of the Al Ga N/GaN heterostructure resistance was measured at 4.2K using a superconducting magnetic field 6 T. After the growth of Al Ga N/GaN HEMT structure, the mesa isolation was formed using reactive ion etching (RIE) in a BCl plasma at the RF power of 10 W and a chamber presnm/min for the GaN layer. sure of 3 Pa. The etch rate was The drain-source ohmic contacts were obtained with Ti/Al (25 nm/150 nm) annealed at 900 C for 60 s. The gate recess was nm and the width performed to yield the etch depth of m using RIE technique under the same conditions as of the mesa isolation. The gate metallization was done by vacuum evaporation of Pt/Ti/Au (10 nm/40 nm/100 nm). Separate photolithographic steps were used for the etch and gate metal deposition. The Al Ga N/GaN HEMTs with the gate width of 15 m have the gate lengths of 1.5 and 2.1 m. The channel opening (source to drain distance) was 10 m. The gate metal was placed on the center of the gate recessed region. The distance from the gate metal to the recess edge was 2 m. Annealing to remove the surface damage was not done in the fabrication of the recessed gate Al Ga N/GaN HEMT. The current–voltage ( – ) and capacitance–voltage ( – ) characteristics of the devices were measured using HP 4145B semiconductor parameter analyzer and HP 4284A LCR meter, re-

EGAWA et al.: RECESSED GATE AlGaN/GaN HEMTs ON SAPPHIRE

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Fig. 3. I –V characteristic for the recessed gate Al Ga N/GaN HEMT with L 1:5 m at 25 C. The gate bias V was changed from 1.5 to 6:5 V in steps of 2 V.

0

=

Fig. 4. g Ga Al

and I as a function of temperature for the recessed gate N/GaN HEMT with L = 2:1 m.

0

mS/mm and 1120 mA/mm were obtained for the recessed m, regate Al Ga N/GaN HEMT with spectively. The specific contact resistance was measured to -cm using transmission line model. The be – characteristic at negative slope was observed in the V. The presence of the negative slope is attributed to the self-heating effect because the device structure was grown on sapphire substrate [17]. The negative slope can be suppressed using SiC substrate since the thermal conductivity of SiC is more than an order of magnitude larger than that of sapphire substrate. To confirm the advantages of the recessed gate process, the nonrecessed device was also fabricated and of 146 mS/mm the characteristics were compared. The of 6.1 -mm were obtained for the recessed gate and the m. On the other hand, the nonrecessed device with of 93 mS/mm and the of gate device showed the 20 -mm. The source resistance of the recessed-gate device was 1/3 lower than the value of the nonrecessed AlGaN/GaN HEMT. Furthermore, the Al Ga N/GaN HEMT with m was tested at high temperatures to study the performance of the device at high temperatures. The and as a function of temperature are shown in decreases rapidly from 146 mS/mm (25 Fig. 4. The C) to 81 mS/mm (200 C), and then decreases gradually to 62 also decreases with the increase mS/mm for 350 C. The of temperature. The reduction of transconductance at high temperature is probably due to the reduction of 2DEG mobility. Fig. 5 shows the threshold voltage of GaN MESFET and Al Ga N/GaN HEMT as a function of temperature. The V threshold voltage of GaN MESFET decreases from V (300 C). On the other hand, the threshold (25 C) to voltages of the recessed gate Al Ga N/GaN HEMT were and V at 25 and 350 C, respectively. Note that the temperature dependence of the threshold voltage is very weak

Fig. 5. Dependence of threshold voltage on temperature for the GaN MESFET and Al Ga N/GaN HEMT on sapphire.

for the Al Ga N/GaN HEMT in comparison to the GaN MESFET. The – measurements at 1 MHz were performed on the GaN-based MESFET and HEMT to study the dependence of threshold voltage on temperature. Fig. 6(a) and (b) show the carrier concentration profiles for the MESFET and HEMT at 25 and 300 C, respectively. For the GaN-based MESFET, the carrier profile at 300 C spreads in the low electron concentration level compared with that at 25 C [Fig. 6(a)]. There was a shift of threshold voltage for GaN-based MESFET as shown in Fig. 5. On the other hand, for the HEMT device shown in Fig. 6(b), the electrons confine into the Al Ga N/GaN heterointerface and the profile remains unchanged up to the depth 90 nm even at 300 C, which probably leads to the temperature independence of the threshold voltage for the Al Ga N/GaN

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IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 48, NO. 3, MARCH 2001

(a) Fig. 7. Aging result of the Al V and V of V

= +1

= 15 V at 350

Ga

N/GaN HEMT under the conditions C in N atmosphere.

(b) Fig. 6. Carrier concentration profiles measured by C –V method at 25 and 300 C for (a) GaN MESFET and (b) Al Ga N/GaN HEMT structures.

HEMT. The spike in the electron concentration at nm corresponds to the position of the Al Ga N/GaN heterointerface indicating the majority of carriers are confined to this region. The maximum electron concentration of the recessed gate cm . The temperature indepenHEMT is as high as dence of threshold voltage may be also due to the high sheet cm ) of the recessed gate HEMT. carrier density ( We have performed the life test on our devices C to investigate the reliability of the at 350 Al Ga N/GaN HEMT. Fig. 7 shows the aging result of the Al Ga N/GaN HEMT under the conditions V and V at 350 C in N of atmosphere. The device exhibited the stable operation with mS/mm and mA/mm over 500 h increased to A at V, at 350 C. The V, and 350 C although the was as small as negligible at 25 C. After aging test at 350 C for 500

h, the – characteristic was measured again at 25 C. The device showed the initial – characteristics, which indicates that not much deterioration was observed at the metal/semiconductor interface. Aktas et al. reported that the nonrecessed AlGaN/GaN HEMT was able to recover the original characteristics when cooled down to room temperature from 300 C [18]. In order to investigate whether or not the Schottky metal diffuses into the AlGaN layer after aging, the interfaces were analyzed using AES for the devices before and after the aging test at 350 C for 500 h. Compared with the initial AES profiles, the Schottky electrode metals such as Pt, Ti, and Au did not diffuse into the AlGaN layer beneath the Schottky contact. The metal/semiconductor interfaces are still abrupt even after the aging test. The similar stable operation at high temperature has also been reported for the nonrecessed GaN-based MESFET [19]. It has been reported that the AlGaAs/GaAs HEMT cannot operate above 250 C due to the degradation of ohmic or gate metallization [20]. Compared with the GaAs-based FET, the GaN-based FET is promising for the high-temperature device applications. IV. CONCLUSION We have improved the characteristics of the MOCVD-grown Al Ga N/GaN HEMT’s on sapphire using the process of recessed gate. The – and SdH measurements have indicated the formation of the 2DEG at the Al Ga N/GaN heterointerface. The 2DEG mobility and the sheet carrier density in the Al Ga N/GaN heterostructure were 740 cm at 300K, and 12 000 cm /V-s cm /V-s and cm at 8.9K, respectively. The recessed gate and Al Ga N/GaN HEMT showed the large transconductance of 181 mS/mm and the high drain-source current level of 1120 mA/mm for the gate length of 1.5 m. There were no interfacial changes at Pt/Ti/Au–AlGaN interfaces even after 350 C for 500 h treatment. The device exhibited the stable operation at

EGAWA et al.: RECESSED GATE AlGaN/GaN HEMTs ON SAPPHIRE

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350 C for 500 h with the temperature independence of the threshold voltage.

ACKNOWLEDGMENT The authors would like to acknowledge Dr. S. Arulkumaran for useful discussions and Dr. N. Akutsu for SdH measurement.

REFERENCES
[1] T. P. Chow and R. Tyagi, “Wide bandgap compound semiconductors for superior high-voltage unipolar power devices,” IEEE Trans. Electron Devices, vol. 41, pp. 1481–1483, Aug. 1994. [2] M. A. Khan et al., “Microwave operation of GaN/AlGaN-doped channel heterostructure field effect transistors,” IEEE Electron Device Lett., vol. 17, pp. 325–337, July 1996. [3] Y.-F. Wu et al., “High Al-content AlGaN/GaN MODFET’s for ultrahigh performance,” IEEE Electron Device Lett., vol. 19, pp. 50–53, Feb. 1998. [4] C.-H. Chen et al., “High-transconductance self-aligned AlGaN/GaN modulation-doped field-effect transistors with regrown ohmic contacts,” Appl. Phys. Lett., vol. 73, pp. 3147–3149, Nov. 1998. [5] R. Li et al., “An Al Ga N/GaN undoped channel heterostructure field effect transistor with F of 107 GHz,” IEEE Electron Device Lett., vol. 20, pp. 323–325, July 1999. [6] J. M. Redwing et al., “Two-dimensional electron gas properties of AlGaN/GaN heterostructures grown on 6H-SiC and sapphire substrates,” Appl. Phys. Lett., vol. 69, pp. 963–965, Aug. 1996. [7] S. C. Binari et al., “GaN FET’s for microwave and high-temperature applications,” Solid-State Electron., vol. 41, pp. 177–180, 1997. [8] J. Burm et al., “Recessed gate GaN MODFETs,” Solid-State Electron., vol. 41, no. 2, pp. 247–250, 1997. [9] T. Egawa, H. Ishikawa, M. Umeno, and T. Jimbo, “Recessed gate AlGaN/GaN modulation-doped field-effect transistors on sapphire,” Appl. Phys. Lett., vol. 76, pp. 121–123, Jan. 2000. [10] O. Ambacher et al., “Two-dimensional electron gases induced by spontaneous and piezoelectric polarization charges in N- and Ga-face AlGaN/GaN heterostructures,” J. Appl. Phys., vol. 85, pp. 3222–3233, Mar. 1999. [11] G.-Y. Zhao et al., “High-mobility AlGaN/GaN heterostructures grown on sapphire by metal-organic chemical vapor deposition,” Jpn. J. Appl. Phys., vol. 39, pp. 1035–1038, Mar. 2000. [12] J. P. Ibbetson et al., “Polarization effects, surface states, and the source of electrons in AlGaN/GaN heterostructure field effect transistors,” Appl. Phys. Lett., vol. 77, pp. 250–252, July 2000. [13] R. E. Williams, Gallium Arsenide Processing Techniques. Dedham, MA: Artech House, 1984. [14] T. Egawa et al., “Characteristics of a GaN metal semiconductor field-effect transistor grown on a sapphire substrate by metalorganic chemical vapor deposition,” Jpn. J. Appl. Phys., vol. 38, pp. 2630–2633, Apr. 1999. [15] E. T. Yu et al., “Measurement of piezoelectrically induced charge in GaN/AlGaN heterostructure field-effect transistors,” Appl. Phys. Lett., vol. 71, no. 19, pp. 2794–2796, Nov. 1997. [16] S. Hiyamizu and T. Mimura, “High mobility electrons in selectively doped GaAs/n–AlGaAs heterostructures grown by MBE and their application to high-speed devices,” J. Crystal Growth, vol. 56, pp. 455–463, 1982. [17] R. Gaska, A. Osinsky, J. W. Yang, and M. S. Shur, “Self-heating in highpower AlGaN-GaN HFET’s,” IEEE Electron Device Lett., vol. 19, no. 3, pp. 89–91, Mar. 1998. [18] O. Aktas, Z. F. Fan, S. N. Mohammad, A. E. Botchkarev, and H. Morkoc, “High temperature characteristics of AlGaN/GaN modulation doped field-effect transistors,” Appl. Phys. Lett., vol. 69, pp. 3872–3874, Dec. 1996. [19] S. Yoshida and J. Suzuki, “Reliability of metal semiconductor field-effect transistor using GaN at high temperature,” J. Appl. Phys., vol. 84, pp. 2940–2942, Sept. 1998. [20] D. W. Langer, A. Ezis, and A. K. Rai, “Structure and lateral diffusion of ohmic contacts in AlGaAs/GaAs high electron mobility transistors and GaAs devices,” J. Vac. Sci. Technol., vol. B5, pp. 1030–1032, July/Aug. 1987.

Takashi Egawa received the B.E. and M.E. degrees in electronics from Nagoya Institute of Technology in 1980 and 1982, respectively, and the D.E. degree in electrical and computer engineering from Nagoya Institute of Technology, Japan, in 1991. From 1982 to 1988, he was engaged in research on high-speed GaAs LSI in Oki Ltd., Tokyo, Japan. In 1991, he joined Nagoya Institute of Technology as a Research Associate. He became an Associate Professor in 1993, a Professor in 1999 at Research Center for Micro-Structure Devices, Nagoya Institute of Technology. Fields of his current interest are heteroepitaxy of GaN and GaAs by MOCVD and its application to electronic and optical devices. Dr. Egawa is a member of the Japan Society of Applied Physics and the IEE of Japan. He received the the Kodaira Memorial Prize from IEE Japan with in 1991.

Guang-Yuan Zhao received the B.S. degree in physics from Inner Mongolia University, Huhot, China in 1986, the M.S. degree in physics from Chinese Academy of Sciences, Changchun, China, in 1991, and D.E. degree in electrical and computer engineering from Nagoya Institute of Technology, Japan, in 1998. He joined Nagoya Institute of Technology in 1998. His present activities are in the area of properties of GaN-based quantum structures.

Hiroyasu Ishikawa received the B.E. degree in electronic engineering and the M.E. degree in electrical engineering from Shibaura Institute of Technology, Japan, in 1993 and 1995, respectively, and the D.E. degree in electrical and computer engineering from Nagoya Institute of Technology, Japan, in 1998. He joined Nagoya Institute of Technology as a Research Associate in 1998. His interests are heteroepitaxy of GaN by MOCVD and its application to electronic and optical devices. Dr. Ishikawa is a member of the Japan Society of Applied Physics.

Masayoshi Umeno (M’72) graduated from Nagoya Institute of Technology in 1960 and received M.E. degree from Tokyo Institute of Technology in 1962. He became a Research Associate in 1962, and an Assistant Professor in 1969 at Nagoya University. He became a Professor at Nagoya Institute of Technology in 1978. From 1990 to 1996, he was Director of the Center for Cooperative Research. From 1993 to 1996, he was a Director of Research Center for Micro-Structure Devices. From 1996 to 1998, he was a Vice President of Nagoya Institute of Technology. He is now a Director of Research Center for Micro-Structure Devices. He has studied the interactions of semiconductor photo-devices, especially photon drag photodetectors, semiconductor lasers, photo-transistors, high efficiency solar cells, image sensors, crystal growth of various semiconductors for semiconductor devices. His current research interests include multilayer thin-film crystal growth by MOCVD, heteroepitaxy on Si and their application to optical integrated circuits, and 3-D circuits such as artificial retina and parallel processing of image. Dr. Umeno is a member of Institute of TV Engineers of Japan, the Society of Instrumentation Control Engineering (Japan), the Japan Society of Applied Physics, and the Physical Society of Japan,. He won the Yonezawa prize in 1967 and the Award from IEE Japan with the Kodaira Memorial Prize in 1991.

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IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 48, NO. 3, MARCH 2001

Takashi Jimbo received the B.E. and M.E. degrees in electronics and the D.E. degree in electrical engineering from Nagoya University, Nagoya, Japan, in 1970, 1972, and 1978, respectively. He joined Nagoya University as a Research Associate in 1975. He was with the City College of New York studying nonlinear optics from 1985 to 1986. He transferred to Nagoya Institute of Technology in 1987. He became a Professor in 1993 and shifted from the Research Center for Micro-Structure Devices to the Department of Environmental Technology and Urban Planning in 1997. He is studying semiconductor opto-electronics including the development of lasers and solar cells. Dr. Jimbo is a member of the Physical Society of Japan, the Japan Society of Applied Physics, the Laser Society of Japan, the Japan Society of Infrared Science, the Institute of Electronics, Information, and Communicaton Engineers (Japan), the Institute of Electtrical Engineering of Japan, the Japan Society of Medical Electronics and Bio-Engineering, and the Society of Instrumentation Control Engineering (Japan).


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