Improvement of Characteristic

Temperature for AlGaInP Laser

Diodes

Presenter: Hsiu-Fen Chen ( 陳秀芬 )

Authors: Man-Fang Huang ( 黃滿芳 ), Meng-Lun Tsai ( 蔡孟倫 ), Yen-Kuang Kuo ( 郭艷光 )

Institute of PhotonicsNational Chunghua University of Education

國立彰化師範大學 光電科技研究所

2 Paper 5628-21

Content

Introduction

Requirement of DVD Laser Diodes

Disadvantage of DVD Laser Diodes

Approach for High Temperature Operation

Theoretical Analysis

Experimental Results

Conclusion

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Introduction AlGaInP laser diodes (LDs) are widely used in DVD-ROM,

DVD-R/RW and DVD player.

However, the requirement for the operation temperature of the AlGaInP LD has been increased from 70 ºC in the past to the more recent 100 ºC, especially for the outdoor applications such as portable players, computers or vehicle-used player.

The main reason to prevent AlGaInP LD from high operation temperature is the electron overflow from active region to p-cladding.

This study will focus on how to minimize the leakage current and improve the operation temperature for AlGaInP LD.

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Specification of DVD LD

Refer to the web site: http://sharp-world.com/products/device/lineup/opto/laser-diode

AlGaInP LD with high operation temperature for outdoor application is still under development.

Application DVD-ROM/player DVD-R/RW DVD Vedio

Wavelength (nm) 654 658 788/654 dual

Power (mW) Up to 10 60 (x4) 7

OperationTemperature( )℃

-10 ~ 70 (indoor)-10 ~ 100 (outdoor)*

-10 ~ 70 -10 ~ 70

FFP(⊥) 29 17 29

Aspect Ratio ~3 1.7 ~ 3

Mode Pattern Single transverse mode

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Disadvantage of AlGaInP LD

Small conduction band offset (Ec=0.27 eV) Result in bad electron confinement

Increase leakage current over p-cladding layer Low Zn-doping concentration when Al is increased

Increase leakage current over p-cladding Large thermal resistivity (14~19 Kcm/W)

Cause heat dissipation problem Increase threshold and operation currents

Ec

Leakage current

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Approach for High Operation Temperature

Utilize strained multiple quantum well (strained-MQW) to reduce threshold current

Optimize quantum well numbers to minimize electron overflow

Increase P-doping concentration to reduce leakage current

Utilize multiple quantum barrier(MQB) to block overflow electrons

Our Work

Optimize barrier/confining layer composition along with quantum well numbers

Change confining layer structure (or SCH) to graded-index separate confinement hetero-structure (GRIN-SCH) to enhance carrier confinement.

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Strain-Induced Effect

Strain-induced effect Split of HH band from LH band density of states in valence band Ith

HH

LH

E

HH

LH

E

Band mixing

Unstrained

Strained Density of states

Efv

Ev

Density of states

Efv

Ev

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Wavelength Design

For DVD application, A compressive strain of 0.5% (In0.55Ga0.45P) and a well

width of 5 nm is used for the well region

600

620

640

660

680

700

20 40 60 80 100 120

Wav

elen

gth

(nm

)

Well Width (

0.42

0.4

0.46

0.5

0.55

0.6

Ga=

Optimized

(angstrom)

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Well Number vs. Operation Temperature

Well number carrier overflow characteristic temperature (T0)

However, Threshold Current There is a trade-off

3 wells 4 wells2 wells ElectronOverflow

0

2

4

6

8

10

12

14

16

0 20 40 60 80 100

20

30

40

50

60

70

Pow

er

(mW

)

Current(mA)

0

2

4

6

8

10

12

14

16

0 20 40 60 80 100

Pow

er

(mW

)

Current(mA)

20

70

0

2

4

6

8

10

12

14

16

0 20 40 60 80 100

Pow

er

(mW

)

Current(mA)

20

70

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Barrier height =Eg - p - ( n + p ) and p decreases by increasing p-doping [~ kT ln(p/Nv)]

Therefore, doping concentration of the p-cladding layer quasi-Fermi level (p) conduction barrier height leakage current

P-doped Concentration vs Leakage Current

n

p

Ec

Ev

p

gE

active p-claddingn-cladding

barrier heightJrJnr ~ Jo exp(-barrier height/kT)

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P-doping Concentration Effect

0

10

20

30

40

50

60

70

50 100 150 200 250

Current (mA)

Powe

r (m

W)

80

90

100

110

120

290 300 310 320 330 340

CW, 5 m x 1500 m, p-side down

Thre

shol

d Cu

rrent

(mA

)

Temperature (K)

To=150 K

T0= 100 K

Use Mg as p-type dopant Carrier Concentration up to 1~2 x 1018 cm-3

(1) External differential efficiency ~ 90 %

(2) Characteristic temperature > 100 K

(Man-Fang Huang et al, IEDMS, Vol. B, 1996)

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Multi-quantum Barrier (MQB)

Multi-quantum barrier (MQB) in p-cladding Bragg reflector for electrons reduce electron overflow High operation temperature (IEEE QE-29, p.1844, 1993)

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Some Issues In reality, diffusion of p-dopants causes reliability issue Un-dope the “p-type” cladding layer for more than one thous

and angstroms P-doping concentration at the interface between the active lay

er and p-cladding layer cannot be too high Leakage current cannot be ignored

Control for MQB thickness accuracy and uniformity is NOT easy Inaccuracy in MQB thickness may cause the increase in leakage

current. Careful control of thickness of the MQB is critical to obtain hig

h-performance LDs

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Study Goal Theoretical analysis is done using LASTIP software Key parameters including

Quantum barrier composition Quantum well number Confining layer structure

The optimization of the structure is based on constant emission wavelength and far-field pattern. =654 nm (lasing)

or 645 nm (spontaneous) =29° single transverse mode

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Laser Diode Structure

Cavity length = 450 m; Ridge width =5m No facet coating

n-GaAs (0.3m, 1x1018 cm-3)n-(Al0.7Ga0.3)InP (1.3 m, 1x1018 cm-3)

(AlxGa1-x)InP confining layer (undoped)5 nm Ga0.45InP well

n-GaAs substrate (200 m, 1x1018 cm-3)

(AlxGa1-x)InP barrier

(AlxGa1-x)InP confining layer (undoped)p-(Al0.7Ga0.3)InP (0.17m, 1x1018 cm-3)

p-(Al0.7Ga0.3)InP (1.13m, 1x1018 cm-3)

P-InGaP (0.05m, 5x1018 cm-3)P-GaAs(0.1m, 1x1019 cm-3)

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Far Field Pattern and Optical Confinement

Perpendicular

FWHM=29

0

0.2

0.4

0.6

0.8

1

-80 -60 -40 -20 0 20 40 60 80

Far F

ield In

tensity

(a.u.

)

Angle (degree)

To achieve a constant vertical emission angle of 29o

The optical confinement factor is about 0.3.

FFP

Parallel

FWHM = 9.2°

Perpendicular

FWHM = 29°

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Effect of Barrier Composition and Quantum Well Number

At RT, 4QW & x=0.4 demonstrates the lowest threshold current However, at 80℃, 5QW & x=0.5 shows the lowest threshold current Al increases threshold current increases as well

20

30

40

50

60

70

0.35 0.4 0.45 0.5 0.55 0.6 0.65

QW=4QW=5QW=6

Thr

esho

ld C

urre

nt (

mA

)

Barrier Al Composition

80 oC

20 oC

[x in (AlxGa1-x)InP]

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Effect of Barrier Composition

x=0.4 in the (AlxGa1-x)InP barrier layer

a lower quantum barrier uniform stimulated emission rates x=0.4 in the (AlxGa1-x)InP confining layer

a higher cladding barrier lower electron overflowTherefore, a high average stimulated emission rate is achieved

1.6

1.8

2.0

2.2

2.4

0

0.7

1.4

2.1

1.5 1.6 1.7 1.8

Sti

mul

ated

Em

issi

on R

ate

(1028

cm-3

/s)

Ene

rgy

(eV

)

Distance (m)

(a) QW=4, Al=0.4

Ec

Fn

SER 1.6

1.8

2.0

2.2

2.4

0

0.7

1.4

2.1

1.5 1.6 1.7 1.8

Ec

Fn

(b) QW=4, Al=0.6

SER

Sti

mul

ated

Em

issi

on R

ate

(1028

cm-3

/s)

Ene

rgy

(eV

)

Distance (m)

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Leakage Current

5QW the leakage currents are smaller than those of 4QW

small difference among various aluminum compositions. Al=0.6 has a higher leakage current due to smaller confining barrier

0

2

4

6

8

10 20 30 40 50 60 70

Al=0.4Al=0.5Al=0.6

Lea

kage

Cur

rent

(m

A)

Current (mA)

MQW=4

MQW=5

80 oC

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Simulated Characteristic Temperature

The threshold current of 4-QW LD is increased faster than 5-QW LD 6-QW has similar temperature characteristics; however, the threshold current is

too large 5-QW with x=0.5 is a better choice for high operation temperature application

7

7.2

7.4

7.6

7.8

8

10 20 30 40 50 60 70 80 90

4QW5QW6QW

T0 = 105 K

T0 = 112 K

T0 = 78 K

Temperature (oC)

ln (

J th)

(A/c

m2 )

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Experimental Characteristic Temperature

7

7.2

7.4

7.6

7.8

8

10 20 30 40 50 60 70 80 90

4QW

5QW

T0 = 79 K

T0 = 111 K

Temperature (oC)

ln (

J th)

(A/c

m2 )

There is a crossover point between 4QW and 5QW A characteristic temperature of as high as 110 K is

obtained for 5-QW

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Different Confining Structures

GRIN-SCH is widely used and generally combined with SQW ∵ A reduction in the density of states in the optical confinement region Threshold current can be reduced

Mostly, linear-GRIN-SCH is employed

However, we will demonstrate that parabolic-GRIN-SCH shows a better choice for AlGaInP LD in terms of high operation temperature

STEP-SCH

GRIN-SCH

LinearParabolic

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Active Region Structure

The optimization is based on a fixed far-field pattern (FFP).

The confining layer thicknesses are different for different confining structures.

Confining layer thicknesses for different QW number and GRIN-SCH combinations are given as follows:

Step-SCHLinear-GRIN-

SCHParabolic-GRIN-SCH

3 55 - -

4 47 55.5 52.2

5 38.7 - 41.6

6 32.4 - -

Spacer (nm)QW#

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Band Diagrams

Step-SCH A dip at the interface between the n-cladding and the confining layer Some of carriers are confined in this dip

GRIN-SCH No dip better carrier injection Graded confining structure carrier distribution is non-uniform

1.2 1.25 1.3 1.35 1.4 1.45 1.5 1.55

Epitaxial Growth Direction (m)

Ener

gy (e

V)

Quasi Fermi Level

Light Hole (red line)

Heavy Hole (blue line)

n-side p-side

1.2 1.25 1.3 1.35 1.4 1.45 1.5 1.55

Ener

gy (e

V)

Epitaxial Growth Direction (m)

Quasi Fermi Level

Light Hole (red line)

Heavy Hole (blue line)

n-side p-side

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Carrier Distribution

Electrons accumulate in the n-cladding/confining interface for SCH Injection efficiency is poor GRIN-SCH has non-uniform electron distribution in the confining

region Less electron overflow in the p-cladding Better carrier confinement

10

12

14

16

18

20

1.25 1.3 1.35 1.4 1.45 1.5

80 oC

GRIN-SCH4

SCH4

Distance (m)

Ele

ctro

n C

once

ntra

tion

(lo

g) (

cm-3

)

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Stimulated Emission Rate

Stimulated emission rates (SER) at 20 ºC are almost the same for different GRIN-SCHs.

At 80 ºC, SERs become more uniform among different quantum wells du

e to increase in thermionic transport Parabolic-GRIN-SCH has higher SERs than linear-GRIN-SCH

0.6

0.8

1

1.2

1.4

1.6

1.33 1.34 1.35 1.36 1.37 1.38 1.39

GRIN-x

GRIN-x2

SCH

Stim

ulat

ed E

mis

sion

Rat

e (1

028

cm-3

/s)

Distance (m)

(a) 20 oC

0

0.2

0.4

0.6

0.8

1

1.33 1.34 1.35 1.36 1.37 1.38 1.39

GRIN-x

GRIN-x2

SCH

Stim

ulat

ed E

mis

sion

Rat

e (1

028cm

-3/s

)

Distance (m)

(b) 80 oC

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Leakage Current for GRIN-SCH

Linear-GRIN-SCH shows higher leakage current than parabolic-GRIN-SCH.

Parabolic-GRIN-SCH has better carrier confinement.

0

0.05

0.1

0.15

0.2

0.25

0 10 20 30 40 50 60 70

GRIN-x-4QW

GRIN-x2-4QW

Current (mA)

Lea

kage

Cur

rent

(m

A)

(a) 20 oC

0

2

4

6

8

10

10 20 30 40 50 60 70

GRIN-x-4QW

GRIN-x2-4QW

Current (mA)

Lea

kage

Cur

rent

(m

A)

(b) 80 oC

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Experimental Results

Threshold Current GRIN-SCH is lower than Step-SCH Characteristic Temperature GRIN-SCH-4QW is similar to SCH-5QW GRIN-SCH-4QW is the best choice for lower threshold current and

higher operation temperature

7.2

7.4

7.6

7.8

8

8.2

20 30 40 50 60 70 80 90 100

Data 3

SCH-4

GRIN-SCH-x2-4SCH-5

GRIN-SCH-x2-5

ln(J

th)

(A/c

m2 )

Temperature (oC)

29 Paper 5628-21

Conclusions

We have done the optimization for the AlGaInP LD under the same waveguide confinement.

The simulation results suggest that five quantum wells are good enough to inhibit the electron overflow.

We theoretically show that the parabolic GRIN-SCH has a better carrier injection and smaller overflow than other SCH.

Experimental results show that LD with GRIN-SCH-4QW demonstrates the best performance. The characteristic temperature can be as high as 110K.

This work is supported by the National Science Council of the Republic of China, Taiwan, under grant NSC-92-2218-E-018-002.

For questions, please contact Prof. Man-Fang Huang at mfhuang@cc.ncue.edu.tw