Extremely Low Threshold 1.3-µm InAsP
n-Type Modulation Doped MQW Lasers
by Hitoshi Shimizu *, Kouji Kumada *, Nobumitsu Yamanaka *,Norihiro Iwai *, Tomokazu Mukaihara * and Akihiko Kasukawa *A semiconductor laser with a low threshold current emitting at 1.3 µm wavelength
is a key component for access networks and high-density parallel optical interconnections. A very low CW threshold current of 0.9 mA has been obtained at room temperature
for lasers grown by gas-source molecular-beam epitaxy, which is the lowest value grown by any
kind of molecular-beam epitaxy in the long wavelength region. Both the reduction of the threshold current and the carrier lifetime lead to the reduction of the turn-on delay time by about 30%.
The 1.3 µm InAsP strained MQW lasers using n-type modulation doping with very low power consumption and small turn-on delay time are very attractive for laser array application in high density parallel optical interconnection systems.

ABSTRACT1.

INTRODUCTION

A semiconductor laser with a low threshold current emitting at 1.3 µm wavelength is a key device for access networks and high-density parallel optical interconnections.
The strained-layer quantum well (SL-QW) lasers have an
improved laser performance, such as low threshold current, high output power, high relaxation oscillation frequency and so on. These improvements result from low
internal loss and high differential gain due to the changeof valence band structure 1). So far, GaInAsP/InP material
systems have been used in 1.3 µm SL-QW lasers for optical communication systems. On the other hand, InAsP/InP
system is another candidate for this wavelength 2), 3). The
large conduction band offset of ∆Ec=0.7 ∆Eg reported forthis material system 4) makes it possible to realize a strong
electron confinement in the wells, which is expected toresult in a lower threshold current, higher differential gain
and higher characteristic temperature (T0). However, inorder to adjust the wavelength to 1.3 µm range, InAsP
wells have to have a large compressive strain(ε=1.4~1.6%), therefore gas-source molecular-beam epitaxy (GSMBE) is favorable as the growth method of InAsP
material because the low temperature growth of GSMBE
increases the critical thickness.
On the other hand, modulation doped MQW (MD-MQW)
laser, which consists of undoped wells and doped barriers, has been proposed for further improvement of thelaser performance 5). So far, low threshold lasers havebeen reported for n-type MD-MQW lasers grown by both
MOCVD (Se-dope) 6), 7) and GSMBE (Si-dope) 8), 9). Optical*

Semiconductor R&D Center, Yokohama R&D Laboratories,
R & D Div.

gain g is proportional to (fc-fv), where fc (fv) is Fermi-Diracfunction for electrons (holes). This term can be divided
into two terms, which are expressed asg~(f c -f v )=f c (1-f v )-f v (1-f c )...(1)

Here, the first term fc(1-fv) shows the stimulated emission term and the second term shows the stimulated
absorption term. As fc approaches unity with respect to ntype MD-MQW laser, the absorption term becomes zero,
which leads to low threshold current density. However, thedoping profile needs to be sharp in this structure. Due tothese points, we think that GSMBE is a suitable growthmethod for this type of laser.
In this paper, we investigate systematically the effect ofn-type modulation doping on the characteristics of 1.3 µm
InAsP MQW lasers grown by GSMBE for the first time. In
section 2, we discuss the structure of n-type MD-MQW
lasers and the results of the threshold current densities. In
section 3, we discuss fabrication of ridge lasers andextract the basic laser parameters such as gain coefficient, internal loss, and characteristic temperature of ntype MD-MQW laser and undoped MQW laser. In section
4, we present fabrication of buried heterostructure (BH)
lasers and show the excellent performance of the n-type
modulation doped MQW lasers.

2.

FABRICATION OF MQW LASERS ANDEVALUATION OF BASIC MATERIALQUALITY

We have grown 1.3 µm InAsP/GaInAsP MQW lasers on
n-InP substrates by GSMBE in which group III is the solid
source and group V is AsH3 and PH3 and the dopant is Si

149

for n-type and Be for p-type. As shown in Figure 1, thelasers consisted of n-, p-InP cladding, compressively
strained MQW active layer, and 120 nm thick GaInAsP
SCH layer (λg=1.1 µm) located on both sides of MQW.
The MQW active region consists of 1.45% compressively
strained three InAs0.45P0.55 quantum wells (each 8 nm
thick) separated by lattice-matched GaInAsP barrier layers (each 10 nm thick) of the same composition as SCH.
The 7 nm thick center region of each barrier layer isdoped with silicon. The optimum growth temperature is
515°C for InP cladding layer and 455°C for SCH-MQW
layer and the optimum modulation doping density was
1x1018 cm-3 as described in another paper 8),10).
Figure 2 shows the dependence of Jth on inverse cavity
for undoped MQW lasers and MD-MQW lasers(ND=1x1018 cm-3) grown at optimum growth condition. The
threshold current density at an infinite cavity Jth∞ was
reduced by about 30% as compared with undoped MQWlasers and the extremely low threshold current density of
250 A/cm 2 was obtained for 1200 mm-long MD-MQW
lasers. The threshold current density per well was comparable to the previously reported n-type MD-MQW lasers
grown by MOCVD 6),7). The reduction of Jth of MD-MQW
laser could be due to the reduction of the threshold carrier
density caused by the reduction of the transparent carrier
density 5),6).

Selectively n-doped region(ND=1x1018 cm-3)

n-InP

7 nm
120 nmp-InP

120 nmLASING CHARACTERISTICS OF RIDGELASERSFor the estimation of the laser characteristics, we fabricated ridge-lasers with 20 µm width for MD-MQW lasers (ND
of 1x1018 cm-3) and undoped MQW lasers. To fabricate
ridge waveguide, p + -GaInAs contact layer and p-InP
cladding layer were removed by conventional wet chemical etching with stripe geometry oxide mask.
The external differential quantum efficiency ηd is associated to the internal loss α i and the internal efficiency ηi,which is determined by the product of internal quantum
efficiency and current injection efficiency, as

α iL
1
1
=
(1+
)
ln(1/R)ηdηiwhere: L is the cavity length and R is the average facet
reflectivity.
Figure 3 shows the inverse external differential quantum
efficiency versus cavity length for devices with cleaved
facets. From the relationship in (2),α i and ηi were estimated. The obtained internal efficiencies were almost 100%for both types of lasers, and the internal loss was 6.0 cm-1
for the undoped laser and 4.6 cm-1 for MD-MQW laser.
The reduction of internal loss of MD-MQW laser was
explained by the reduction of intervalence band absorption loss caused by the reduction of the threshold carrier
densities 6). Semilogarithmic approximation for optical gain
g per well is given by

g=G 0 ln(

η iJ)
N w J tr...(3)Jth can be then expressed as follows.
InAsP wells(+1.45% strain)
8 nm

10 nm

J th =(Nw=3)

Schematic band diagram of 1.3 µm InAsP/GaInAsP
n-type MD-MQW lasers

N w J tr1exp
( α i+ α m)
ηi
N w Γ wG 0{...(4)

3.0
Broad contact laser

W=20 µm, C/C
25˚C

2.5

undope MQW
αi=6.0 cm-1

1/ηb

Jth (A/cm2)

2.0
undoped MQW laser
MD-MQW laser(ND=1x1018 cm-3)

ηi=100%1.5
1.0

n-MD MQW
αi=4.6 cm-1

0.5
100

0.0
0

10

20

30

40

0

1/L (cm-1)150}

where: Γw and αm denote the optical confinement factor
per well and mirror loss, respectively.

1000

Figure 2...(2)

where : J is the injected current density, Jtr is the transparent current density, and G0 is the gain coefficient.

GaInAsP( λ g=1.1 µm)

Figure 13.

Jth dependence of on inverse cavity length inundoped MQW lasers and MD-MQW

500

1000

1500

Cavity length (µm)

Figure 3

Inverse differential quantum efficiency versus cavity
length for ridge lasers with 20 µm width

Furukawa Review, No. 19. 2000

The optical confinement factor per well is theoreticallyestimated by solving the Maxwell's equation for multilayer
waveguide with TE mode, which is 0.0149 per well. G0
estimated from the results of ridge lasers was 720 cm-1 for
undoped MQW lasers and 650 cm-1 for MD-MQW lasers,
which indicates that G0 does not decrease significantly byusing n-type MD-MQW lasers. The value of G 0 for
undoped MQW lasers is comparable to the previously
reported value on undoped InAsP MQW laser (Nw=3) .

20

Power (mW)

T=20˚CI th=28 mA
S d=0.66 W/An-MD MQW
Ridge laser
L=300 µm
32%/96%
Pulse∆T=10K

10

70˚C

(a)

T0=66K(20-70˚C)

0
0

50

100

150

Current (mA)

Power (mW)20

T=20˚CI th=36 mA
S d=0.62 W/A

T=70˚C
T0=70K(20-70˚C)

10

4.

undoped MQW
Ridge laser
L=300 µm
32%/96%
Pulse∆T=10K

(b)

0
0

50

100

150

Current (mA)

Figure 4

Temperature dependence of light output versus current characteristics at pulse operation for MD-MQW
ridge laser (a) and undoped MQW ridge laser (b) with
300 µm-long cavity and 20 µm width (cleaved front
facet, Rr=96% coated)10Output Power (arb. units)

The temperature dependence of light output power versus injected current characteristics for ridge lasers were
investigated. Figure 4 shows the temperature dependenceof output power versus current characteristics at pulse
operation for MD-MQW lasers and undoped MQW lasers
on 300 µm-long device with high reflective coatings (96%)
on the rear facet. The threshold current of MD-MQW
lasers was as low as 28 mA at 20°C and was reduced byabout 20% as compared with undoped MQW laser. The
obtained slope efficiency was as high as 0.66 W/A for MDMQW laser, and that of undoped MQW laser was 0.62
W/A. The characteristic temperature T0 was 66 K for MDMQW and 70 K for undoped MQW laser in the range of
20°C to 70°C. No significant degradation of T 0 was
observed for MD-MQW laser as compared with undoped
MQW lasers, which agrees with the results of G0.
To reduce the threshold current of ridge lasers, we fabricated short cavity lasers with high reflection (HR) coatings
on both facets (Rf/Rr=90%/96%), because Jth of n-type
MD-MQW laser becomes very low at the low mirror loss
region. As shown in Figure 5, a very low threshold currentof 11.5 mA was obtained for MD-MQW lasers with 200µm-long cavity although the width of ridge lasers were 20µm, from which we can expect submilliampere levels forthe threshold current if we fabricate 1.5 µm-wide BH
lasers.

W =20 µm
HR(90%)/HR(96%)
L=200 µm

LASING CHARACTERISTICS OF BH
LASERS

Buried heterostructure (BH) lasers with p- and n-InP
blocking layers were fabricated for the GSMBE-grown
MD-MQW epitaxitial wafers (ND=1x1018 cm-3) by MOCVD
as shown in Figure 6. The typical width of the active
region W is 1.6 µm. The trench structure was fabricated toreduce the leakage current and the intrinsic capacitance.
The cavity length dependence of the threshold currents
as a parameter of the facet reflectivity for MD-MQW lasers(ND=1x1018 cm-3) under room-temperature pulsed condition is shown in Figure 7. The lines are theoretical values
which are calculated by the following equation using theparameters extracted from the threshold characteristics ofthe ridge lasers mentioned in section 3.

20 µm

p-electrode5

MD-MQW laser
n =1x1018cm-3
I th=11.5 mA

p+-GaInAs

SCH-MQW

SiNx
undoped laser
I th=13.5 mA

p-InP
n-InP
p-InP

0
0

10

20

30

Current (mA)

Figure 5

Light output versus current characteristics of MDMQW ridge laser (ND=1x1018cm-3) and undoped MQWridge laser with 20 µm-width and 200 µm-long cavity(Rf=90%, Rr=96% coated). The inset shows theschematic sectional plan of ridge lasers.

SCH-MQW

n-InP sub.
n-electrode

Figure 6

Schematic diagram of 1.3-µm InAsP MQW buried hetero-structure lasers151

Ith=WL

NwJtr
1
1
1
exp
(αi+
) ln(
)
ηi
NwΓwΓlG0
2L
R fR r{}...(5)

where: Γl is the lateral confinement factor and Rf (Rr) is
the reflectivity on the front (rear) facet. Γl was
assumed to be unity because Γw and W are relatively large.
Open circles, crosses, and squares show the experimental results of lasers with cleaved facets, 32%/96%,
and 90%/96%, respectively. The averaged threshold currents of 300 µm, 600 µm and 900 µm long devices withcleaved facets were 7 mA, 6.3 mA, and 7.5 mA, respectively. The threshold currents for devices with short cavity
and HR coating agree with calculations, on the other
hand, discrepancies between calculations and experiments are observed at the large mirror loss region and thelong cavity region. The discrepancies are considered to beleakage current due to imperfect current blocking in theburied heterostructure, which becomes larger in devices
with longer cavity and higher threshold current density.
A CW light output power versus current characteristics
at 25°C is shown in Figure 8 for a 150 µm-long device
with HR coatings of Rf/Rr=90%/96% along with the inset
of lasing spectrum operating at 2 mA. A very low CW

threshold current of 0.9 mA, corresponding to the threshold current density of 375 A/cm 2 , was obtained eventhough the laser is not bonded on a heat sink. So far, afew studies have been done on submilliampere operationin 1.3 µm range at room temperature (0.4 mA 11), 0.58 mA 12),0.98 mA 13), 0.80 mA 14)), and all of those were grown by
MOCVD using strained quantum well layers. This is thelowest value ever reported for long wavelength lasers
using n-type modulation doping 6) and the lowest grown by
any kind of MBE in the long wavelength 3) to our best
knowledge. Note that a lower threshold current can be
expected in a device with shorter cavity and narrower
stripe width.
The temperature dependence of light output power versus injected current characteristics for BH lasers with MDMQW ( N D =1x10 18 cm -3 ) were investigated. Figure 9shows the temperature dependence of output power versus current characteristics at DC operation for MD-MQW
lasers on 300 µm-long device with HR coating (96%) on
the rear facet. The threshold current at 20°C was as low
as 3.2 mA and the slope efficiency was as high as 0.70
W/A. The threshold current at 70°C was 8.8 mA and theslope efficiency was 0.57 W/A. The characteristic temperature was 48 K in the range of 20°C to 70°C. This low T0value compared with the results of ridge lasers is probably
102032%/32%1032%/96%n-MD MQW laser
L=300 µm, CL/96%
DC, ∆T=10K

8Power (mW)15

I th (mA)

32%/32%32%/96%90%/96%

BH laser
W =1.6 µm
25˚C

90%/96%

5

T=20˚CI th=3.2 mA
Sd=0.70 W/A

6

4

2

T0=48K(@20~70˚C)

0

0
0

200

400

600

800

1000

0

5

Threshold current versus cavity length of MD-MQW
BH lasers (ND=1x1018cm-3) with facet reflectivity as aparameter

I =2 mA

6
4
2
0
-2
1320

1322

1324

Wavelength (nm)

1

2

3

Current (mA)

Figure 8

152

25

30

250

MD-MQW laser
L=1200 µm
AR(5%)/HR(96%)
25˚C, CW

200
150
100

I th=8.6 mA
PMAX=300 mW

50
0
0

20

Temperature dependence of output power versus
current characteristics for MD-MQW BH laser with
300 µm-long cavity (cleaved front facet, Rr=96% coated) under DC operation

Output power (mW)5

Figure 9

300

10
8

15

350

L=150 µm, HR(90%)/HR(96%)
25˚C, CW
I th=0.9 mA
Peak intensity (a. u.)

Output power (arb. units)

10

10

Current (mA)

Cavity length (µm)

Figure 7

T=70˚CI th=8.8 mA
Sd=0.57 W/ALight output power versus current characteristics of
150 µm-long MD-MQW BH lasers with HR coatings(Rf/Rr=90%/96% ) with the inset of the lasing spectrum operating at 2 mA

0
0

200

400

600

800

1000

Current (mA)

Figure 10 Light output power versus current characteristics of1200 µm-long MD-MQW BH lasers with AR/HR coatedfacets (Rf=5%, Rr=96% coated)

Furukawa Review, No. 19. 2000

1.5
undoped MQW
τs=1.79 ns

Turn-on delay time (ns)Turn-on delay time (ns)3

L=300 µm
CL/CL
25˚C

2

1
MD-MQWτs=1.20 ns

MD-MQW
L=200 µm
32%/96%25˚C
I th=3.1 mA1

τs=1.38 ns

0.5τd=244 ps@Ip=20 mA
0

0
1

10

1

10

I p / (I p-I th)

I p/(I p-I th)

Figure 11 Turn-on delay time versus normalized bias level Ip/(IpIth) for MD-MQW BH laser and undoped MQW BH
laser with 300 µm-long cavity and cleaved facets. The
lasers were zero-biased

Figure 13 Turn-on delay time versus normalized bias level Ip/(IpIth) for MD-MQW BH laser of 200 µm-long cavity(cleaved front facet, Rr=96% coated). The lasers werezero-biased

Turn-on delay time (ns)3
BH laser
L=300 µm, CL/CL
25˚C

τ d = τ s ln{I p /(I p -I th )...(6)

2

where: Ip is the height of the pulse current and τs is thecarrier lifetime.

undoped MQW
laser
1

MD-MQW laser(ND=1x1018 cm-3)
0
0

5

10

15

20

25

30

I p (mA)

Figure 12 Turn-on delay time versus height of pulse current for
MD-MQW BH lasers and undoped MQW BH lasers
with 300 µm-long cavity and cleaved facets. The
lasers were zero-biased

governed by the T0 of leakage current in the blocking
layer.
Further, we evaluated high output power characteristics
of MD-MQW lasers because internal loss of MD-MQW
lasers becomes smaller than that of undoped MQWlasers. Figure 10 shows the light output power versus
injected current characteristics of 1200 µm-long MD-MQW
laser with AR coating (5%) on the front facet and HR coating (96%) on the rear facet. The lasers were operated
under DC operation at 25°C. The threshold current was
8.6 mA and the obtained maximum output power was 300
mW. The saturation of output power was probably limitedby the increase of the temperature of the active region.
The FWHM of the far-field patterns (FFP's) parallel andperpendicular to the junction plane at output power of 100
mW were 22° and 32°, respectively.
The other advantage of low threshold lasers is todecrease the skew in laser array. The turn-on delay timeof the semiconductor laser under zero-bias operation isaffected by its threshold current as 15)

N-type MD-MQW lasers have been predicted todecrease the carrier lifetime because of their higher total
spontaneous emission rate with the increased doping density, which results in further reduction of turn-on delay
time. All measurements were carried out under zero-bias
condition at 25°C. Figure 11 shows the relationshipbetween the turn-on delay time and the normalized bias
level Ip/(Ip-Ith) for MD-MQW BH laser and undoped MQW
BH laser with 300 µm-long cavity and cleaved facets. The
carrier lifetimes of 1.79 ns for undoped MQW and 1.20 ns
for MD-MQW lasers were estimated from the slope in
Figure 11. The carrier lifetimes were reduced by about
33%. The reduction of both the threshold current and thecarrier lifetime using n-type MD-MQW leads to the reduction of the turn-on delay time 6).
Figure 12 shows the relationship between turn-on delay
time and the height of the pulse current Ip for MD-MQW
BH lasers and undoped MQW BH lasers. The turn-on
delay time of MD-MQW laser was reduced by about 30%
as compared with undoped MQW lasers at the height of apulse current of 20 mA.
Figure 13 shows the relationship between the turn-on
delay time and the normalized bias level Ip/(Ip-Ith) for MDMQW BH laser of 200 µm-long cavity with HR coating(96%) on the rear facet. The threshold current was 3.1 mA
at 25°C and the lasers were zero-biased. The obtained
turn-on delay time was 244 ps at a pulse current height of
20 mA.
The 1.3 µm InAsP strained MQW lasers using n-type
modulation doping with very low power consumption andsmall turn-on delay time grown by gas-source MBE arevery attractive for laser array application in high density
parallel optical interconnection systems.

153

5.

CONCLUSIONS

We investigated the effect of n-type modulation doping on
the performance of 1.3 µm InAsP strained MQW lasers
grown by gas-source MBE. A very low CW threshold current of 0.9 mA has been obtained in 1.3 µm InAsP n-type
modulation doped MQW laser, at room temperature,
which is the lowest grown value among all kinds of molecular beam epitaxy in the long wavelength region. The carrier lifetime was also reduced by about 33% by using ntype MD-MQW lasers. Both the reduction of the thresholdcurrent and the carrier lifetime caused the reduction of theturn-on delay time by about 30%. The high power performance of MD-MQW lasers was also demonstrated. We
confirmed the low threshold current, the high efficiency
and the short turn-on delay time by using n-type modulation doped MQW lasers.

ACKNOWLEDGMENTS
The authors would like to thank Mr. J. Kikawa and Mr. T.
Ninomiya for their encouragement throughout this study.
We also acknowledge Dr. N. Yokouchi for his assistancein the theory of x-ray simulation and the calculation of the
optical confinement factors. Further, we acknowledge Dr.
S. Yoshida, Dr. Y. Hiratani, and Dr. J. Yoshida for helpful
discussions on GSMBE growth.

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