fascinated journeys into blue light · fascinated journeys into blue light . contents 1....

Fascinated Journeys into Blue Light

CONTENTS

1. Introduction

2. Creation of GaN single crystal with excellent quality

3. Development of GaN p-n junction Blue LEDs and Laser diodes

4. Summary

Isamu AKASAKI Meijo University and Nagoya University

1/20

2/20

Elec

tron

ener

gy

(Internal) photo-electric effect spontaneous emission

Valence band

Light

Conduction band electron

(Positive)

hole Ene

rgy

band

gap

Eg

Excitation light

Conservation of energy

［A］ Energy bandgap Eg： >2.6 eV (< 480 nm) (Wide bandgap semiconductors) ［B］ Energy band structure： Direct-transition type for conservation of electron momentum

Blue Light-Emitting Devices (LED, Laser diode)

(radiative recombination)

1. Introduction

[1] High-quality single crystal [2] p-n junction

+ + - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -

+ + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + +

-

- -

- -

+ + + +

n type p type

Depletion layer

electron hole

High-performance Blue LED and Laser diode

Light emission

Eg ＋－

（~3V） Electric current p-n junction

hole Elec

tron

ener

gy

Applied forward voltage

1. Introduction 3/20

p-n junction LED

Candidate materials for Blue Light-Emitters in 1960s-’70s


ZnSe GaN [A] Energy gap (Eg) 2.7 eV 3.4 eV

[B] Energy band structure direct direct

[1] Crystal growth straightforward too difficult

Substrate GaAs sapphire

Lattice mismatch 0.26 % 16 %

[2] p-n junction not realized at that time

Number of researchers many few

Physical & chemical stability low high

Chose GaN in 1973 at Matsushita Research Institute Tokyo (MRIT) because of toughness, wider direct Eg, and non-toxicity

Candidate materials for Blue Light-Emitters in 1960s-’70s


ZnSe GaN [A] Energy gap (Eg) 2.7 eV 3.4 eV

[B] Energy band structure direct direct

[1] Crystal growth straightforward too difficult

Substrate GaAs sapphire

Lattice mismatch 0.26 % 16 %

[2] p-n junction not realized at that time

Number of researchers many few

Physical & chemical stability low high

GaN MIS Blue LED by HVPE

MIS Blue LED

1978

MIS Blue LED, not p-n junction LED

1. Introduction

at MRIT

sapphire

SiO2 film

n-GaN i-GaN

Ohmic electrode

n-GaN (30 μm)

i-GaN (~1μm)

Epoxy dome

hν

The brightest Blue LED at that time, however, still weak and high operating voltage

First as-grown highly n-type cathode

6/20

Started growth of GaN by MBE in 1973, and by HVPE in 1975

Tiny but high-quality crystallites embedded in HVPE-grown crystals

Recognized the great potential of GaN

Made up my mind to go back to the beginning; i.e. Crystal Growth in 1978

High-quality tiny crystallites

Potential of GaN

100μm

1. Introduction

Surface of GaN grown on sapphire by HVPE (1975-78)

7/20

at MRIT

Crystal growth methods for GaN Hydride Vapor Phase Epitaxy (HVPE) H. P. Maruska and J. J. Tietjen: (1969).

GaCl (g) + NH3 (g) = GaN (s) + HCl (g) + H2 (g)

Issues: Susceptible to reverse reactions, Too fast growth rate

Molecular Beam Epitaxy (MBE) I. Akasaki: (1974) (unpublished).

Ga (g) + NH3 (g) = GaN (s) + H2 (g)

Issues: Prone to nitrogen deficiency, Slow growth rate (at that time)

Metalorganic Vapor Phase Epitaxy (MOVPE), (MOCVD) H. M. Manasevit et al: (1971).

Ga(CH3)3 (g) + NH3 (g) GaN (s) + 3CH4 (g)

Advantages: • No reverse reactions • Easy to control growth rate, alloy (AlGaN, GaInN) composition,

and impurity-doping

Decided to adopt MOVPE (1979)

23


at MRIT

8/20

Mixing TMG (TMA) with NH3 just before the reactor inlet, and (1) High speed gas flow (2) Substrate inclined at a 45-degree angle

110 cm/sec (1)

Inclined substrate (2)

sapphire Y. Koide

(1985)

First MOVPE system (Handmade)


Exhaust

Uniform growth, but not specular surface, still poor material quality

Improvements in MOVPE reactor and growth condition (1) (2) Started anew to MOVPE since 1981 at Nagoya University

0.7~20cm/sec substrate susceptor

RF coil

guide

NH3+H2 TMG+TMA+H2

delivery tube TMG+(TMA)+H2

NH3+H2

Exhaust

Reactor

Bird-view SEM image

Suppressed the convective gas stream, and the adduct formation GaN

sapphire

9/20

Reactor design changed

10 μm

by H. Amano

Growth on a highly-mismatched substrate

Homoepitaxy Heteroepitaxy

defects

Lattice matching Huge lattice-mismatch

Interfacial energy σ

Growth on the same substrate

GaN

GaN but not available

A

A

A

B

For epitaxial growth, it is considered to be gospel to have a lattice matching: (e. g. Si on Si, GaAs on GaAs)

GaN

sapphire

mismatch ~ 16%


Lattice

10/20

TMG(TMA)+H2

NH3+H2

delivery tube

substrate

susceptor

Exhaust

RF coil

Direct growth (Common method)

newly-developed

LT-buffer layer

425 cm/sec

Key technologies:

(1) Much higher-speed gas flow (425 cm/sec)

(2) Substrate inclined at a 45-degree angle

(3) Deposition of thin AlN buffer layer at about 500 oC,

before the growth of GaN single crystal at about 1000 oC

(1)

(3)

(3) Innovation in MOVPE growth method (1985)

Low-temperature (LT-) buffer layer Sapphire(0001)

LT-buffer

LT-buffer (20~50 nm) ~500 oC

GaN High-quality GaN ~ 1000 oC

H. Amano


(2)

11/20

Creation of high-quality GaN (1985) Until 1985

(b)

GaN grown by HVPE GaN grown by MOVPE using LT-buffer Since the late 1985

Many cracks, pits Crack-free, pit-free Rough surface Specular surface Dislocations: > 1011 cm-2 Dislocations: 108-109 cm-2

Free electron conc. >1019 cm-3 Free electron conc. < 1016 cm-3

Electron mobility: ~20 cm2/V･s Electron mobility: ~700 cm2/V･s Weak luminescence Intense luminescence

Crystal quality, electrical property, and luminescence property were dramatically improved at the same time

GaN grown by MOVPE

100μm


GaN island crystal

Sapphire

2mm

12/20


Incr

ease

GaN

thic

knes

s

LT-buffer layer Sapphire

Mixture both amorphous & fine crystallites of AlN

GaN

LT-buffer layer Sapphire

Lateral growth of GaN dominates

Surface (SEM) images Growth model

Growth model of GaN using LT-buffer layer

(1) As-deposited LT-AlN buffer layer

(2) 5 min GaN growth




GaN island

Growth model

GaN island Sapphire

K. Hiramatsu

Direct growth for 60 min. (No LT-buffer)

Surface (SEM) images

13/20

LT-buffer layerSapphire

GaN layer of excellent quality

1μm

Realization of p-type GaN, AlGaN, and GaInN

1988 Found greatly enhanced blue emission of Zn doped GaN by electron irradiation (LEEBI)

Achieved the first p-type GaN

1989 Doped Mg using CP2Mg and electron irradiation

High-quality Mg-doped GaN subjected to LEEBI

1991 p-type AlGaN

1995 p-type GaInN M. Kito

H. Amano

1986 High-quality GaN using LT-buffer layer (Low residual impurities) Basic Technology

(C5H5)2Mg

3. Development of GaN p-n junction Blue LEDs and Laser diodes 14/20

The world’s first GaN p-n junction blue LED (1989)

I-V curves

p-n LED MIS LED

GaN p-n junction Blue LED

Voltage

Cur

rent

H. Amano M. Kito

3. Development of GaN p-n junction Blue LEDs and Laser diodes 15/20

Achieved conductivity control of n-type GaN

SiH4 flow rate [sccm]

Elec

tron

con

cent

ratio

n [c

m-3

]

1016

1017

1018

1019

1 10 100

1989 Doped Si into high-quality GaN using SiH4

n-type AlGaN 1991

Conductivity control of n-type GaN, AlGaN 1986 High-quality GaN using LT-buffer layer

(Low residual impurities) Basic Technology


Allowed the use of heterostructure and quantum well in the design of more efficient p-n junction light-emitting structures

16/20

UV (376 nm) Laser diode (1996)

Stimulated emission by optical pumping (1990)

EL in

tens

ity

AlGaN/GaN/GaInN

・

・

Wavelength (nm)3600

0

370 380 390

1.5kA/cm2

~ x 50

Current (mA)

Inte

grat

ed li

ght i

nten

sity

(arb

. uni

ts)

3.0kA/cm2

GaN-based laser

EL in

tens

ity (a

rb. u

nits

)

Stimulated emission by current injection (1995)

By current injection

3.43.3 3.5Photon energy (eV)

Emis

sion

inte

nsity

Room temp.


On the basis of the technologies of LT-buffer layer and p-n junction heterostructures, GaN-based lasers were achieved

Wavelength (nm) 300 400 500 600 700

AlGaN/GaN/GaInN quantum well

Room temp.

17/20

388 nm

Number of Papers Related to Nitrides (1965-2012)

1970 1980 1990 2000 20100

1000

2000

3000

Num

ber o

f Pap

ers

Year

Web of Science Keywords; AlN, GaN,InN,AlGaN, InGaN

High-quality GaN using LT-buffer

(1986)

Distinguished Important Achievements 1969 Single crystal GaN (H. P. Maruska et al.)

1971 MIS-type Blue LED (J. I. Pankove et al.)

1986 High-quality GaN single crystal grown with LT-buffer layer by MOVPE

1989 GaN p-n junction Blue LED

1989 Conductivity control of p- and n-type GaN

1990 Room temperature UV stimulated emission from GaN by optical pumping

1990 Growth of GaInN (T. Matsuoka et al.)

1991 p-type GaN by thermal annealing (S. Nakamura et al.)

1993 GaInN double hetero-junction Blue LED (S. Nakamura et al.)

1995 Stimulated emission from GaInN/GaN quantum wells by current injection

1996 Violet laser diode (S. Nakamura et al.) 1996 UV laser diode

3409 papers(2012)

Red characters: Akasaki and Amano group

4. Summary 18/20

p-n junction Blue LED

(1989)

While many researchers abandoned the development of GaN Blue LED, I have been fascinated with the research on GaN-based semiconductors, since 1973.

Through persistent efforts, with the collaboration of Hiroshi Amano, Yasuo Koide, and many students/ coresearchers over many years, we invented high-quality GaN single crystal in 1986, and GaN p-n junction Blue LED in 1989.

GaN-based photonic & electronic devices are environmentally-sound, robust, and energy-saving, which benefit humanity.

4. Summary 19/20

20/20

Acknowledgements With the most generous cooperation of M. Hashimoto, Y. Ohki, H. Kobayasi, Y. Toyoda, M. Ohshima, N. Mazda

Matsushita Research Institute Tokyo, Inc. (1964−1981) N. Sawaki, K. Hiramatsu, Y. Koide, H. Amano, M. Kito, T. Kozawa

Nagoya University (1981−1992) H. Amano, S. Kamiyama, T. Takeuchi, M. Iwaya

Meijo University (1992− ) B. Monemar

Linköping and Lund University TOYODA GOSEI CO., LTD, & TOYOTA CENTRAL R&D LABS.,INC. (1987 − )

MITI Project (Blue LED,1975−1978) Grants-in-Aid for Scientific Research (MEXT, JSPS) (1981− ) JST Project (Blue LED,1987−1990) (Violet laser diode,1993−1999) JSPS Research for the Future Program (1996−2001) MEXT High-Tech Research Center Project (1996−2004)

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