Selected Papers: Advanced Materials and Their

Novel Nitride Semiconductor Devices
Toshiki Makimoto†
Abstract
Nitride semiconductors are best known as materials for blue light emitting diodes, but they haveunique characteristics applicable to other devices. This paper describes three such novel nitride devices
that NTT Basic Research Laboratories has developed: (1) a nitride heterojunction bipolar transistor,
which shows a very-high power density of 270,000 W/cm2, (2) a nitride field emission display, which ispromising for an efficient, reliable, and bright flat panel display, and (3) a nitride surface emitting laser
diode, which should lead to micro optical devices and optoelectronic integrated circuits.

1. Introduction
Over the last five decades, research on the conventional semiconductors Si, GaAs, and InP has
advanced steadily and these semiconductors havebeen applied to various optical and electronic
devices. In contrast, in the early stages of research on
the nitride semiconductors GaN, InGaN, and AlGaN,
progress was slow because these semiconductors
were very difficult to grow. However, the situation
changed in 1986 with the development of a growthmethod that produced high-quality nitride semiconductors [1]. Since then, this research has accelerated.
The bandgaps of nitride semiconductors are wider
than those of the conventional semiconductors, sothey are suitable materials for short-wavelength optical devices such as blue light emitting diodes (LEDs)[2], [3]. In addition to their wide bandgaps, nitride
semiconductors have several unique characteristics,
such as high crystal hardness, a high breakdown field,
high saturation velocity, and low electron affinity.
These characteristics make them applicable to otherdevices in addition to simple blue LEDs, so NTT
Basic Research Laboratories has taken advantage ofthese unique characteristics to develop novel nitride
devices. This paper describes three of them: a nitride
heterojunction bipolar transistor (HBT), a nitride† NTT Basic Research Laboratories
Atsugi-shi, 243-0198, Japan
E-mail: makimoto@nttbrl.jp

12

field emission display, (FED), and a nitride surface
emitting laser diode (SELD).
2. Nitride heterojunction bipolar transistor forvery-high power operation
High-power radio-frequency (RF) devices are necessary for the base station transmitters of cellular
phone systems, satellite transmitters, and high-definition television (HDTV) transmitters and for the
power modules of phase-array radars and surveillance and traffic control radars. However, it has notbeen possible to design and fabricate solid-state transistors that can yield RF output power of the order ofthe hundreds of watts that would be necessary tocompete with microwave vacuum tubes. This is mainly because Si, GaAs, and InP have low breakdown
fields. Since nitride semiconductors have high breakdown fields, these materials are suitable for highpower electronic devices that can operate at high voltages.
As an electronic device an HBT has several advantages over a field effect transistor (FET), such as highcurrent density and good threshold voltage uniformity. Considering these characteristics in terms of bothmaterials and device type, an HBT composed of
nitride semiconductors should exhibit a very highpower density. However, it is difficult to fabricate
good nitride HBTs because their structure is rather
complicated.
At other research institutions, AlGaN/GaN HBTs
NTT Technical Review

Applications to Future Functional Devices

have been fabricated using a p-GaN base [4]-[7], but
their current gains were low. In contrast, GaN/InGaN
HBTs using a p-InGaN base have been developed by
NTT Basic Research Laboratories [8]-[10]. Figure 1
illustrates their typical structure, which consists of an
n-GaN emitter, a p-InGaN base, and an n-GaN collector. In bipolar transistors, the current flows fromthe emitter to the collector; that is, the collector current is controlled by the base current, and the current
gain is defined as the change in collector current
divided by the change in base current. The p-InGaN
base shows higher conductivity than a conventional
p-GaN base [11], which means that it is easy to control the collector current. In addition, the collector is
GaN instead of InGaN. This double heterojunction

Emitter electrodestructure is preferable for high-power operation. Figure 2 shows an example of the common-emitter current-voltage (I-V) characteristics. The offset voltage,i.e., the turn-on voltage of the collector current, is lessthan 1 V, which is one-fifth that of other HBTs [10].
Figure 3 shows the trend of current gain for nitride
HBTs. The maximum current gain of our HBTexceeds 2000, which is 100 times the previous record.
Research on nitride HBTs started in 1998. At thattime, the current gain of the first nitride HBT was just
3. But, in 2003, as a result of considerable effort dedicated to increasing the current gain, NTT Basic
Research Laboratories reported a record-high current
gain of 3000 for a GaN/InGaN HBT.
Figure 4 shows breakdown fields of HBTs com-

50

Base
electrodeIb = 5 µA/stepn-GaN emitter

40

p-InGaN base
Graded InGaN
Collector
electroden-GaN collector
n-GaN sub-collector
AlN bufferCollector current (mA)Regrown p-InGaN

30

20

10

SiC substrate0

Fig. 1. Illustration of a typical HBT structure, which
consists of an n-GaN emitter, a p-InGaN base, andan n-GaN collector.
10,0000

5
10
15Collector-emitter voltage (V)20

Fig. 2. Example of common emitter I-V characteristics ofthe HBT developed by NTT Basic ResearchLaboratories.

NTT
[10]

Current gain

1000

Si/SiGe HBT[12]

InGaP/GaAs HBT

100

NTT
[8]

[7]

[13]

InP/InGaAs HBT

[6]

[14]
NTT [9]

10
Nitride HBT[4],[5]

0
1
1998

2000

2002

2004

Year

Fig. 3. Trend of current gain for nitride HBTs. Numbers inbrackets are references.

Vol. 2 No. 6 June 2004

1
2
Breakdown field (MV/cm)

Fig. 4. Breakdown fields of HBTs composed of Si/SiGe[12], InGaP/GaAs [13], and InP/InGaAs [14]semiconductor material systems. Numbers inbrackets are references.

13

Selected Papers

posed of Si/SiGe [12], InGaP/GaAs [13], and
InP/InGaAs [14] semiconductor material systems.
The breakdown field of the nitride HBT is muchhigher than those of HBTs composed of other materials. This high breakdown field is comparable to thetheoretically expected value [15] and it results fromthe wide bandgap of the nitride semiconductors. The
maximum operating voltage is limited by the productof this breakdown field and the collector thickness, so
a high breakdown field at a fixed collector thickness
is necessary for high-power operation. On the other
hand, we have reported that the maximum collector
current of a GaN/InGaN HBT is proportional to theemitter area and increases with a slope of 7000 A/cm2[16]. This means that a nitride HBT with a large emitter area operates at high voltage with high current,
resulting in a very-high power electronic device. Up
to now, we have obtained a maximum DC power of
10 W and corresponding power density as high as
270,000 W/cm2 [16].
Research on nitride HBTs has only just started andtheir characteristics have been getting better. Higher
power will be easily obtainable by increasing theemitter area in GaN/InGaN HBTs because the collector current is proportional to the emitter area. Further
investigations of high-frequency characteristics anddevice reliability are necessary to determine theirsuitability for practical use.
3. Nitride field emission for an efficient, reliable,
and bright flat panel display
There is strong demand for a flat panel display thatis efficient, reliable, and bright. Field emission,
which is a phenomenon where electrons are emitted
from a semiconductor surface into a vacuum by anelectric field, is applicable to such a display. For thispurpose, we need high-performance electron fieldemitters. Since aluminum nitride (AlN) has a widebandgap of 6.2 eV, its electron affinity is expected tobe nearly zero. Therefore, AlN is a promising material for field emitters because electrons in materials
with nearly zero electron affinity can be easily
extracted from the surface into a vacuum by a small
electric field. However, the reported field emission
current for AlN is still too low and the turn-on electric field too high for device application [17], [18].
This is probably because the electron concentrationin the undoped AlN samples is so low that the electrons necessary for field emission cannot be efficiently supplied to the surface.
Recently, NTT Basic Research Laboratories has
14reported that heavy Si doping is very effective in
improving the field emission properties of AlN [19].
As the Si doping concentration increases, the turn-on
voltage decreases, resulting in an increase in the fieldemission current. Two factors that contribute to thefield emission properties of the heavily Si-doped AlN[20]-[23] are the formation of a Si impurity band
below the conduction band and nearly zero electron
affinity. A third one is the field enhancement afforded by the nano-scale pyramid structure formed by Si
doping. The tip of the pyramid decreases the effectiveenergy barrier of field emission and thereby enhances
field emission efficiency. Figure 5 illustrates fieldemission from the pyramid tips. Electrons are easily
extracted from the tip of a pyramid by the electricfield.
An AlN FED device was fabricated to investigate
its luminescence characteristics. As illustrated in Fig.
6, it has a vertical triode structure consisting of heavily Si-doped AlN, a mesh grid, and a phosphor-coated anode screen. The heavily Si-doped AlN was set
on the cathode electrode. The mesh grid was separat-

Anode

e
AlN
pyramidseeeCathodeFig. 5. Illustration of field emission from the tips of AlN
pyramids.

Glass

IA
VA

A

Phosphor (green)
Anode
IG
A

Grid
Glass
spacer

AlN

VG

IC
A

CathodeFig. 6. Illustration of the AlN FED device developed by
NTT Basic Research Laboratories.

NTT Technical Review

Selected Papers

ed from the AlN surface by a glass spacer and acted
as an extraction electrode. The distance between the
AlN surface and the grid was about 100 µm. The glass
plate was coated with P22-type phosphor for green
light, one standard for color cathode ray tubes. A thin
Al layer evaporated onto the phosphor-coated glass
was used as an anode and positioned about 5 mmfrom the AlN surface. The field-emitted electrons
were accelerated by the anode voltage and passed
through the Al thin layer. Then, they excited the phosphor, and the luminescence emitted from the phosphor was observed through the glass.
Figure 7 shows photographs of the FED (a) beforeand (b) during operation. Green luminescence was
emitted from the center of the phosphor screen. Scattered light from the surrounding area was also
observed. The luminescence area (6 mmφ) on thephosphor screen is almost the same as the open areaof the mesh grid. Uniform and bright luminescence
was observed over the entire field emission area. Figure 8 shows brightness as a function of the applied

electric field strength (EG) [24]-[26]. The brightness
increased exponentially with increasing E G andreached 200 cd/m2 at EG = 22 V/µm. The maximum
brightness was 300 cd/m2. This is high enough fordisplay application because conventional displays
have values around 200 cd/m2.
The FED operated in DC mode without a feedback
circuit at least for 80 minutes. No significant degradation of the field emission current was observed.
During operation, the brightness and uniformity of
the luminescence from the phosphor were maintained, indicating that the FED with the AlN fieldemitter has stable properties. The observed high stability of the field emission current is ascribed to asmall change in the emission site density and a low
rate of field evaporation of surface atoms, both ofwhich originate from the high physical and chemical
stability of the AlN surface afforded by the strong AlN bond. This is one of the advantages of AlN-based
field emitters.
AlN field emission is also expected to find application in electron beam sources for electron-beam lithography, electron microscopy, and X-ray generation.
While high stability of the field emission current is
one of advantages of AlN-based field emitters, theyneed higher electric fields than carbon nanotube fieldemitters because the threshold electric field is closely
related to the condition of the AlN surface. Further
research is necessary to investigate the relationshipbetween the surface condition and field emission
characteristics for practical use.

10 mm
300
(a) Before operation

Brightness (cd/m2)

VA = 6 kV(b) During operation

Fig. 7. Photographs of the FED (a) before and (b) during
operation.

Vol. 2 No. 6 June 2004

200

100

0
10

15
20
Electric field EG (V/µm)

25

Fig. 8. Brightness as a function of the applied electricfield strength EG.

15

Selected Papers

4. Nitride microstructure for a surface emitting
laser diodeNTT Basic Research Laboratories has shown that it
is possible to fabricate semiconductor microstructures using the unique characteristics of crystal
growth [27]-[30]. These structures will lead to micro
optical devices and optoelectronic integrated circuits.
As one such microdevice, NTT Basic Research Laboratories has proposed and successfully fabricated a
new type of a nitride surface emitting laser diode(SELD).
A conventional SELD emits a laser beam verticallyfrom the substrate surface [31], so it enables the fabrication of a two-dimensional laser array, integrationof SELDs with other components, and on-wafer testing compatible to LSI; it also offers high fiber-coupling efficiency. A microlaser array using these
SELDs is expected to raise device performance andlead to devices with novel functions. Nitride SELDs
have shorter lasing wavelengths than those of conventional semiconductors, such as GaAs and InP, sothey are preferable for DVDs and laser printers. A
laser array composed of nitride SELDs will allow
high-density DVDs to operate at very high speed; itwill also let laser printers work at very high speedwith higher quality. A conventional SELD consists ofan active layer sandwiched by two reflectors parallelto the surface. For nitride semiconductors, however, it

has been difficult to fabricate reflectors having both
high reflectivity and high electrical conductivity [32],so there have been no reports of current-injected lasing of nitride SELDs until now. Below, I describe thefirst current-injected lasing of a nitride SELD.
Figure 9 illustrates the new SELD, which consistsof a horizontal cavity laser diode (HCLD) with two
Fabry-Perot cavity mirrors and an outer micromirror.
Both of the Fabry-Perot cavity mirrors are vertical to
the surface, while the outer micromirror is inclined toit [33]. The outer micromirror is coated with an Al
film to increase its reflectivity to over 90% for blueultraviolet light. A laser beam is emitted from the
HCLD and is directed upward by the outer micromirror. The angle between the inclined outer micromirror
and the surface is 58°, so the laser beam is emitted at
an angle of 64° from the surface as shown in Fig. 9.
Since these mirrors are formed by crystal growth,
they are atomically smooth and have little angular
misalignment from the crystallographic viewpoint[34], [35]. Mirrors fabricated by dry-etching, on theother hand, suffer from angular misalignment andetching damage [36]. Note also that the two kinds ofmirrors are simultaneously fabricated in one growthstep, which means that the SELD fabrication process
is simple. Figure 10 shows a bird’s-eye-view scanning electron micrograph of the fabricated SELD. It
shows that an HCLD with smooth mirrors was fabricated under the optimal crystal growth conditions.

LD

Top view

Laser beam
directed upward

Outer micromirror

LD
64°
Side view

Fabry-Perotcavity mirror

Fig. 9. Illustration of the new type SELD developed by NTT Basic Research Laboratories. The SELD consists of an
HCLD with two Fabry-Perot cavity mirrors and an outer micromirror.

16

NTT Technical Review

Selected Papers

The first current-injected lasing of the nitride SELD
was observed at a wavelength of 405 nm. Figure 11
shows emission spectra obtained below (a) and above(b) the lasing threshold (Jth) at room temperature. The
spectrum width is clearly narrower in spectrum (b).
Non-linear behavior was also observed around Jth in
the light output versus current (L-I) curves [34], [35].
No longitudinal mode was observed in the lasing
spectra because of both the long cavity length of 500µm and the low wavelength resolution (> 1 nm) of themeasurement system.
Growth facet

Outer
micromirror(Al film)p-typeelectrode

Fabry-perot
cavity mirror
TrenchFig. 10. Scanning electron micrograph showing abird’s-eye view of the fabricated SELD.

Intensity (arb. unit)

Pulsedat RT

For nitride SELDs to become practical, we must
decrease their threshold current. Therefore, it is necessary to investigate how device characteristics areaffected by surface conditions before regrowth.
5. Summary
The novel nitride devices developed by NTT Basic
Research Laboratories utilize the unique characteristics of nitride semiconductors. This paper described
three of them: a heterojunction bipolar transistor(HBT), a field emission display (FED), and a surface
emitting laser diode (SELD). The nitride HBT hasvery-high power density, which is preferable for thevarious transmitters and the power modules of phasearray radars. The nitride FED will make it possible tobuild efficient, reliable, and bright flat panel displays.
AlN field emission is also expected to find application in the electron beam sources. Nitride SELDs canbe arrayed on a chip. The nitride SELD arrays willlead to the development of high-speed high-density
DVDs and fast high-quality laser printers. Furthermore, semiconductor microstructures fabricated by
crystal growth will allow us to connect lasers towaveguides and optoelectronic integrated circuits,
which is expected to boost optical device performance and lead to devices with new functions.
These devices developed by NTT Basic ResearchLaboratories are still at the first stages of research.
For nitride HBTs, high-frequency characteristics anddevice reliability should be investigated. For AlN
FEDs and nitride SELDs, further research is neededon the influence of surface conditions on device characteristics. Nevertheless, I believe that some of thesedevices will be commercialized in the future and that
this will lead to the creation of new services that will
give the world economy a big boost.
References[1](b) Above Jth[2](a) Below Jth

[3]
350

400

450

Wavelength (nm)Fig. 11. Emission spectra of SELD obtained (a) below and(b) above the lasing threshold (J th ) at room
temperature (RT).

Vol. 2 No. 6 June 2004

[4]

[5]

H. Amano, N. Sawaki, I. Akasaki, and Y. Toyoda, “Metalorganic
vapor phase epitaxial growth of a high quality GaN film using an AlN
buffer layer,” Appl. Phys. Lett., Vol. 48, No. 5, pp. 353-355, 1986.
H. Amano, M. Kito, K. Hiramatsu, and I. Akasaki, “P-Type Conduction in Mg-Doped GaN Treated with Low-Energy Electron Beam
Irradiation (LEEBI),” Jpn. J. Appl. Phys., Vol. 28, No. 12, pp. L2112L2114, 1989.
S. Nakamura, T. Mukai, and M. Senoh, “High-Power GaN P-N Junction Blue-Light-Emitting Diodes,” Jpn. J. Appl. Phys., Vol. 30, No.
12A, pp. L1998-L2001, 1991.
L. S. McCarthy, P. Kozodoy, S. P. Denbaars, M. Rodwell, and U. K.
Mishra, “First Demonstration of an AlGaN/GaN Heterojunction
Bipolar Transistor,” 25th International Symposium on Compound
Semiconductors (ISCS 1998), We2A-1, Nara, Japan, Oct. 1998.
F. Ren, C. R. Abernathy, J. M. Van Hove, P. P. Chow, R. Hickman, J.
J. Klaasen, R. F. Kopf, H. Cho, K. B. Jung, J. R. La Roche, R. G. Wil-

17

Selected Papers

son, J. Han, R. J. Shul, A. G. Baca, and S. J. Pearton, “300°C
GaN/AlGaN Heterojunction Bipolar Transistor,” MRS Internet J.
Nitride Semicond., Vol. 3, No. 41, 1998.
J. J. Huang, M. Hattendorf, M. Feng, D. J. H. Lambert, B. S. Shelton,
M. M. Wong, U. Chowdhury, T. G. Zhu, H. K. Kwon, and R. D.
Dupuis, “Graded-emitter AlGaN/GaN heterojunction bipolar transistors,” Electron. Lett., Vol. 36, No. 14, pp. 1239-1240, 2000.
[7] H. Xing, P. Chavarkar, R. Sharma, J. G. Champlain, S. Keller, U. K.
Mishra, S. P. DenBaars, “High voltage operation (> 330 V) of
AlGaN/GaN heterojunction bipolar transistors,” 28th International
Symposium on Compound Semiconductors (ISCS 2001), ThM-7,
Tokyo, Japan, Oct. 2001.
[8] T. Makimoto, K. Kumakura, and N. Kobayashi, “High current gains
obtained by InGaN/GaN double heterojunction bipolar transistors
with p-InGaN base,” Appl. Phys. Lett., Vol. 79, No. 3, pp. 380-381,
2001.
[9] T. Makimoto, K. Kumakura, and N. Kobayashi, “N-AlGaN/pInGaN/n-GaN Heterojunction Bipolar Transistors for High PowerOperation,” physica staus solidi (c) Vol. 0, No. 1, pp. 95-98, 2002.
[10] T. Makimoto, K. Kumakura, and N. Kobayashi, “High current gain (>2000) of GaN/InGaN double heterojunction bipolar transistor usingbase regrowth of p-InGaN,” Appl. Phys. Lett., Vol. 83, No. 5, pp.
1035-1037, 2003.
[11] K. Kumakura, T. Makimoto, and N. Kobayashi, “Activation Energy
and Electrical Activity of Mg in Mg-Doped InxGa1–xN (x < 0.2),” Jpn.
J. Appl. Phys., Vol. 39, No. 4B, pp. L337-L339, 2000.
[12] T. Tatsumi, H. Hirayama, and N. Aizaki, “Si/Ge0.3Si0.7/Si heterojunction bipolar transistor made with Si molecular beam epitaxy,” Appl.
Phys. Lett., Vol. 52, No. 11, pp. 895-897, 1988.
[13] J. -I. Song, C. Caneau, W. -P. Hong, and K. B. Chough, “Characterisation of GaInP/GaAs double heterojunction bipolar transistors withdifferent collector designs,” Electron. Lett., Vol. 29, No. 21, pp. 18811883, 1993.
[14] K. Kurishima, H. Nakajima, T. Kobayashi, Y. Matsuoka, and T.
Ishibashi, “Fabrication and characterization of high-performance
InP/InGaAs double-heterojunction bipolar transistors,” IEEE Trans.
Electron Devices, Vol. 41, No. 8, pp. 1319-1326, 1994.
[15] J. C. Zolper, “A review of junction field effect transistors for hightemperature and high-power electronics,” Solid-State Electron., Vol.
42, No. 12, pp. 2153-2156, 1998.
[16] T. Makimoto, Y. Yamauchi, and K. Kumakura, “High-Power Characteristics of GaN/InGaN Double Heterojunction Bipolar Transistors
with a Regrown p-InGaN Base Layer,” Appl. Phys. Lett., Vol. 84, No.
11, pp. 1964-1966, 2004.
[17] A. T. Sowers, J. A. Christman, M. D. Bremser, B. L. Ward, R. F.
Davis, and R. J. Nemanich, “Thin films of aluminum nitride and aluminum gallium nitride for cold cathode applications,” Appl. Phys.
Lett., Vol. 71, No. 16, pp. 2289-2291, 1997.
[18] E. M. Forsythe, J. A. Sprague, B. A. Khan, S. Metha, D. A. Smith, I.
H. Murzin, B. Ahern, D. W. Weyburne, and G. S. Tompa, “Study of
IBAD Deposited AlN Films for Vacuum Diode Electron Emission,”
Mater. Res. Soc. Symp. Proc., Vol. 449, pp. 1233-1238, 1997.
[19] M. Kasu and N. Kobayashi, “Field-emission characteristics and large
current density of heavily Si-doped AlN and AlxGa1–xN (0.38 < x < 1),”
Appl. Phys. Lett., Vol. 79, No. 22, pp. 3642-3644, 2001.
[20] M. Kasu and N. Kobayashi, “Spontaneous ridge-structure formationand large field emission of heavily Si-doped AlN,” Appl. Phys. Lett.,Vol. 78, No. 13, pp. 1835-1837, 2001.
[21] Y. Taniyasu, M. Kasu, and N. Kobayashi, “Intentional control of ntype conduction for Si-doped AlN and AlxGa1–xN (0.42 < x < 1),”
Appl. Phys. Lett., Vol. 81, No. 7, pp. 1255-1257, 2002.
[22] M. Kasu and N. Kobayashi, “Spontaneous Ridge Formation and Its
Effect on Field Emission of Heavily Si-Doped AlN,” physica staus
solidi (a), Vol. 188, No. 2, pp. 779-782, 2001.
[23] Y. Taniyasu, M. Kasu, and N. Kobayashi, “Lattice parameters of
wurtzite Al1-xSixN ternary alloys,” Appl. Phys. Lett., Vol. 79, No. 26,
pp. 4351-4353, 2001.
[24] Y. Taniyasu, M. Kasu, T. Makimoto, and N. Kobayashi, “Triode-type
basic display structure using Si-doped AlN field emitters,” physica[6]

18

status solidi (a), Vol. 200, No. 1, pp. 199-201, 2003.
[25] Y. Taniyasu, M. Kasu, T. Makimoto, N. Kobayashi, “Field-EmissionDisplay Basic Structure Using Si-Doped AlN,” 16th International
Vacuum Microelectronics Conference (IVMC 2003), P2-28, Osaka,
Japan, Jul. 2003.
[26] Y. Taniyasu, M. Kasu, and T. Makimoto, “Field emission properties ofheavily Si-doped AlN in triode-type display structure,” Appl. Phys.
Lett., Vol. 84, No. 12, pp. 2115-2117, 2004.
[27] S. Ando, N. Kobayashi, and H. Ando, “Selective area metalorganic
chemical vapor deposition growth for hexagonal-facet lasers,” J.
Cryst. Growth, Vol. 145, No. 1-4, pp. 302-307, 1994.
[28] S. Ando, N. Kobayashi, H. Ando, and Y. Horikoshi, “Hexagonal facet
laser with optical waveguides grown by flow-rate modulation epitaxy,” J. Cryst. Growth, Vol. 170, No. 1-4, pp. 719-724, 1997.
[29] T. Akasaka, Y. Kobayashi, S. Ando, and N. Kobayashi, “GaN hexagonal microprisms with smooth vertical facets fabricated by selective
metalorganic vapor phase epitaxy,” Appl. Phys. Lett., Vol. 71, No. 15,
pp. 2196-2198, 1997.
[30] T. Akasaka, S. Ando, T. Nishida, H. Saito, and N. Kobayashi, “Roomtemperature lasing of InGaN multiquantum-well hexagonal microfacet lasers by current injection,” Appl. Phys. Lett., Vol. 79, No. 10,
pp. 1414-1416, 2001.
[31] K. Iga, S. Ishikawa, S. Ohkouchi, and T. Nishimura, “Room-temperature pulsed oscillation of GaAlAs/GaAs surface emitting injection
laser,” Appl. Phys. Lett., Vol. 45, No. 4, pp. 348-350, 1984.
[32] T. Someya and Y. Arakawa, “Highly reflective GaN/Al0.34Ga0.66N
quarter-wave reflectors grown by metal organic chemical vapor deposition,” Appl. Phys. Lett., Vol. 73, No. 25, pp. 3653-3655, 1998.
[33] T. Akasaka, S. Ando, T. Nishida, T. Saitoh, and N. Kobayashi, “Current-confining structure of InGaN hexagonal microfacet lasers byselective incorporation of Mg during selective-area MOVPE,” J.
Cryst. Growth, Vol. 248, pp. 537-541, 2003.
[34] T. Akasaka, T. Nishida, T. Makimoto, and N. Kobayashi, “InGaNbased horizontal cavity surface emitting laser diode with selectively
grown cavity and outer micromirrors,” 2003 Solid State Devices andMaterials (SSDM 2003), F-4-2, Tokyo, Japan, Sep. 2003.
[35] T. Akasaka, T. Nishida, and T. Makimoto, “An InGaN-based horizontal cavity surface emitting laser diode,” Phys. Lett., Vol. 84, No.
20, pp. 4104-4106, May 2004.
[36] S. Nakamura, M. Senoh, S. Nagahama, N. Iwasa, T. Yamada, T. Matsushita, Y. Sugimoto, and H. Kiyoku, “InGaN-Based Multi-Quantum-Well-Structure Laser Diodes,” Jpn. J. Appl. Phys., Vol. 35, No.
1B, pp. L74-L76, 1996.

Toshiki Makimoto
Distinguished Technical Member, SeniorResearch Scientist, and Group Leader, Thin-Film
Materials Research Group, Materials Science
Laboratory, NTT Basic Research Laboratories.
He received the B.E., M.S., and Ph.D. degrees
in electrical engineering from the University ofTokyo in 1983, 1985, and 1993, respectively. He
joined NTT Basic Research Laboratories in
1985. He was a visiting researcher at the University of California, Santa Barbara, USA during
1993-1994. Since 1985, he has engaged in epitaxial growth and device fabrication of III-V
compound semiconductors. His current interests
are epitaxial growth of nitride semiconductors
and nitride semiconductor devices. He is an associate editor of Japan Society Applied Physics
International. He is a member of the Japan Society of Applied Physics, the Institute of Electronics, Information and Communication Engineers,
and the Materials Research Society.

NTT Technical Review