Gas-Source MBE Growth of a Long-Wavelength Material
and Its Application to Semiconductor Lasers
by Hitoshi Shimizu * , Kouji Kumada * , Nobumitsu Yamanaka * ,Norihiro Iwai * , Tomokazu Mukaihara * and Akihiko Kasukawa *Work is under way on the development of Group III-V optical and electronic devices

ABSTRACT using gas-source molecular-beam epitaxy (GSMBE), an extension of molecularbeam epitaxy in which the Group V are gas-source (AsH3, PH3). High uniformity of composition andthickness has been achieved by optimizing the position and conditions of growth. In considering theapplication of GSMBE to 1.3µm MQW lasers, n-type modulation-doped MQW lasers using InAsP
multi-quantum wells were studied, since GSMBE is suited to the doping of extremely restricted
regions. Using a laser selectively n-doped to 1 x 1018cm-3 at optimized growth temperature, with acavity length of 1200µm, the extremely low threshold current density Jth of 250 A/cm2 was achieved.
This has for the first time confirmed the superiority of GSMBE for selective n-doping with respect tothreshold current density.

1. INTRODUCTION
Studies on gas-source molecular-beam epitaxy, or
GSMBE, in which one or more of the sources for MBE is agas, are being actively carried out in Europe and the UnitedStates. The main advantages of MBE are the abruptnessof heterojunctions and impurity distributions, uniformity oflayer thickness and composition, safety, since it is an ultra-high vacuum process, and the fact that growth can bemonitored in situ. GSMBE, while retaining these advantages, makes possible simultaneous handling of As andP, which is a problem in conventional MBE, and improved
duty factors since sources are supplied from outside. The
use of gases means that safety is slightly compromised
relative to MBE, but since it consumes only about 1/20th
as much AsH3 and PH3 as MOCVD, it offers an excellent
trade-off between safety and environmental friendliness.
At present only Group V materials are used in gas form,
but this will eventually be possible for Group III too, making possible selective growth, lateral growth1), etc., rendering GSMBE technology indispensable to the future realization of optoelectronic ICs.
In introducing this GSMBE technique in which Group III
elements are supplied in metallic form, but Group V in theform of AsH3 and PH3 gas, we have confirmed the basic
performance of the Group V cracking cell, and studied thefundamental aspects of GaInAsP uniformity on 2-inch
wafers and MQW laser growth conditions. In terms of laser applications, we have also studied 1.3-µm n-type modu* 1st Reseach Sec., Semiconductor Device Research
Dept., Yokohama Research Lab., R&D Div.

lation-doped InAsP MQW lasers, which show great promise as an optical source for access networks and in laser
arrays. This paper describes GSMBE growth of GaInAsP--a long-wavelength material--and discusses InAsP MQW
lasers.

2. UNIFORMITY OF THICKNESS
Wafer Position (θ) Dependence of Thickness
Distribution in AlAs/GaAs
First the wafer positionθ at which uniformity is maximized
was determined using GaAs and AlAs, which as binary
compounds are the most fundamental.
When crystals are grown with the center of the wafer
rotated to the peak position of the molecular beam of eachof the sources (center of the chamber), the growth rate at
the center of the wafer increases, creating a convex profile in which the layer is thicker at the center and thinner
toward the edges. It is therefore necessary that growth becarried out with the center of the manipulator offset fromthe peak position. Accordingly in these experiments, theposition of the angle of wafer retention (θ) was adjusted
equivalently to find the point of maximum uniformity.
The experiments were conducted by growing alternate
layers of AlAs and GaAs on 2-inch GaAs wafers, moving
the position ofθ to 80 through 95° (where 95° is the point
at which the wafer is oriented directly underneath), andobserving changes in thickness distribution by scanningelectron microscopy (SEM).
As Figure 1 shows, thickness distribution was poorest
2.1

61

when θ was 90-95°, whereas at 84-87°, a relatively goodresult of ±2% or less was obtained for both AlAs and GaAs.
Since wafer thickness was measured by SEM, accuracies better than ±2% could not be effectively measured,
but this method is used to find trends relating to the pointof optimum uniformity. Thickness distribution was calculated as being ±1/2 {(maximum value - minimum value) /average value}. If, on the other hand, θ is decreased, thegrowth rate for AlAs was reduced, and that for GaAs tended
to increase slightly. This is because moving theθ of thewafer resulted in a decrease in the angle of incidence of
the Al cell molecular beam to the wafer.
Thus from the standpoint of both thickness distribution
and growth rate, the growth position of θ = 86° gave agood thickness distribution for both AlAs and GaAs, withvalues of ±1.9% and ±1.3% respectively.
2.2

Distributions of Thickness, Composition andPhotoluminescence Wavelength for InP, Ternary
Lattice- Matched In1-xGaxAs and 1.3 µm Quaternary In1- xGaxAsyP1-y (1.3Q)
Experiments for growth uniformity were carried out at
growth rates (GR) of 0.3 µm/h for InP, 0.5 µm/h for In1-xGaxAs
and 0.4 µm/h for 1.3Q. In all cases the growth temperature was 515°C and the chamber pressure in the order of
4-5 x 10-5 torr. Thickness distribution was measured by the
α step difference due to selective etching. Table 1 summarizes the results obtained.
Comments regarding the main properties are as follows:
1) The thickness distribution for InGaAs was ±1.1% andthe photoluminescence wavelength distribution (except
for 5 mm at the edge) was ±0.5 nm, which were satisfactory.
2) The thickness distribution for 1.3 µm bulk InGaAsP
was ±1.6% and the photoluminescence wavelength distribution (except for 5 mm at the edge) was ±1 through±3 nm, which were satisfactory.

Thickness distribution in 2-inch wafer (±%)

8
AlAs
6
GaAs
4

2

0
75

80

85

90

95

100

Growth position (˚)

Figure 1

62

Thickness distribution vs. growth position in 2inch wafers

Table 1

3.

Uniformity in 2-inch wafers grown by GSMBE

InP thickness distribution±2.5%

InGaAs bulk thickness distribution±1.1%

InGaAs bulk wavelength distribution±0.5nm

1.3Q bulk thickness distribution±1.6%

1.3Q bulk wavelength distribution±2nm

TEMPERATURE DEPENDENCE OF MQWLASER GROWTH

The growth temperature dependency was investigated for1.3-µm compressively-strained InAsP/GaInAsP MQW lasers (nondoped barriers). InAsP MQW lasers have a large
conduction band offset ∆Ec, have attracted attention as amaterial for high-temperature operation lasers, and arebeing actively developed outside. GSMBE, being a lowtemperature growth technique, is considered suitable forthe growth of InAsP MQW lasers which have high-strain(1.4 to 1.6%).
Using GSMBE, we grew a quantum well layer of compressively strained InAsyP1-y 7-8 nm thick, a barrier layerof lattice-matched GaInAsP 9-10 nm thick (having a wavelength of 1.1 µm, abbreviated 1.1Q), an optical confinement (SCH) layer of 1.1Q 120 nm thick, a lower clad layerof n-InP 0.3 µm thick doped with 5 x 1017cm-3 Si, and an
upper clad layer of p-InP 0.1 µm thick doped with 5 x
1017cm-3 Be. Growth temperature for the InP clad layers
was 515°C constant, but was varied to several points between 415 and 515°C for the SCH-MQW layer to investigate the influence of growth temperature.
The photoluminescence (PL) wavelength from both the
MQW and SCH layers was lengthened by a decrease in
growth temperature. To investigate the reason for thislengthening, composition and thickness were calculated
for each of the layers. The composition of the 1.1Q opticalconfinement and barrier layers was determined by X-ray
and PL wavelength, and the composition and thickness of
the MQW by X-ray analysis and quantum level, after carrying out PL and X-ray analysis, and TEM measurements.
Figure 2a) shows the growth temperature dependenceof composition (x, y) in InAsP/GaInAsP MQW layers. The
Group III constituent does not vary greatly, but the amountof As in the Group V constituent increased at lower growthtemperature.
Figure 2b) shows the growth temperature dependenceof thickness in InAsP/GaInAsP MQW layers. The rate ofchange of the barrier layer is small, but that of the welllayer is great, so that growth at lower temperature resultsin greater thickness. This is attributed to increased growthrate due to an increase in the rate of capture of As and P
at lower temperatures. The smaller change in barrier layer
growth rate is due to the fact that it includes Ga, and since
the bonding strength of Ga to P and Ga to As is greaterthan that of In to P or In to As, there is less chance of there-desorption of impinging molecules2).
Figure 3a) shows the growth temperature Tg dependence

Furukawa Review, No. 18. 1999

1200

0.5
well
Jth(@L=600 m)(A/cm 2 )(a)Composition (x,y) of MQWy (InAs y P1-y )0.4
barriery(1.1Q)
0.3
In 1-x Ga x As y P1-y

1000
L=600 m C/C
800

600

400

(1.1Q)
0.2

200
405

barrierx(1.1Q)
0.1
405

425

445

465
485
Tg (˚C)a) Of composition

445
485
Tg (˚C)a) Of threshold current density Jth

525

20

505

(b)

525

1.3 m InAsP MQW laser∆λ (±nm)10
Barrier thickness

10

Thickness (nm)

9

8

0
405

Well thickness

445

485
Tg (˚C)b) Of wavelength distribution in 2-inch wafers

525

7

Figure 36405

Figure 2

425

445

465
485
Tg (˚C)b) Of layer thickness

505

525

Growth temperature Tg dependence in InAsP/GaInAsP MQW layersof threshold current density Jth in InAsP MQW lasers, at a
fixed cavity length L of 600 µm. The rise in Jth at the hightemperature side is due to deterioration in crystal properties due to the decrease in critical thickness as growthtemperature increases. The rise in Jth at the low-temperature side is due to the fact that the atoms lack in migration,
resulting in the proliferation of nonradiative recombination
centers. Thus the threshold current density is at the minimum around 445°C, and as a result of this it was possible
to reduce Jth from 450 to 325 A/cm2 (at L = 600 µm) by
growing the clad layers at 515°C and growing the opticalconfinement and MQW layers at around 445°C. These results are an improvement over the Jth of 1.3-µm lasers
made by MOCVD3). Figure 3b) shows the growth temperature Tg dependence of wavelength distribution in 2-inch
InAsP MQW laser wafers (except for 5 mm at the waferedge). At a growth temperature of 515°C, there was a large
distribution of ±18 nm, but at 445°C or below, the distribution was ±3 nm or less, a comparatively good value. Heretoo, a growth temperature of around 445°C proves suitable.

Growth temperature Tg dependence in InAsP
MQW lasers

4. LOW-THRESHOLD 1.3µm InAsP n-TYPE
MD-MQW LASERS
4.1

The Need for Low-Threshold Lasers and thePrinciple of the MD-MQW
The crucial component of high-density parallel optical interconnection systems is a semiconductor laser with anemitting wavelength of 1.3 µm. The need to reduce discrepancies in delay time (skew) and reduce power consumption make it essential that a lower threshold current
be achieved. One attractive method for reducing threshold current density is the n-type modulation doped MQW
structure, which has an undoped quantum well layer andan n-doped barrier layer. This is because it has been theoretically demonstrated that the use of this MD-MQW structure will greatly reduce the transparent carrier density4).
There have already been several reports of n-type MD
1.3µm GaInAsP MQW lasers using MOCVD technology5)~6), but none for that wavelength band using MBE or
GSMBE. Since GSMBE makes it possible to achieve anabrupt doping profile, it is considered suited to the growing of MD-MQWs, and a study has accordingly been conducted on the use of GSMBE in growing n-type modulation doped 1.3µm InAsP MQW lasers.
4.2 Fabrication and Evaluation of MD-MQW Lasers
A 1.3µm InAsP/GaInAsP n-type MD-MQW laser (see Figure 4) was grown on an n-InP substrate. The laser wascomposed of n-InP and p-InP clad layers, a compressively
strained MQW layer, and SCH layers of GaInAsP (1.1Q)
120 nm thick on both sides of the MQW. The MQW active
layer was composed of a 1.45% compressively strained

63

InAs0.45P0.55 quantum well layer 8 nm thick and a barrier
layer of lattice-matched GaInAsP on InP having the same
composition as the SCH layer and measuring 10 nm. There
were 3 wells. The center 7 nm of the barrier layers were ndoped, with doping density ND varied from 1 to 5 x 1018cm-3.
The dopant used was silicon.
Figure 5 shows the silicon doping profile near the MQWlayer measured by secondary ion mass spectroscopy(SIMS). The doping density of the barrier layers was 1 x
1018cm-3. As can be seen, the doping profile is extremely
abrupt, with most of the silicon atoms remaining within
the barrier layer. To evaluate optical properties, measurements of room-temperature photoluminescence weremade for all the types of lasers grown at varying doping
concentrations. For lasers with undoped barrier layers, it
was possible to obtain higher photoluminescence intensities with a full-width half-maximum value of only 32 meV,
but as the doping concentration of the barrier layer in-

p-InP

0.4 m

17

(p=5×100.12 mΦ -1.1Q

8nm

Φ -InAsP

10nm

Φ -1.1Q
Φ -InAsP

8nm
10nm

-3

cm )
1.5nm

1.5nm

7nm

Φ -1.1Q

8nm

Φ -InAsP

0.12 mΦ -1.1Q
selectively
n- doped region

n-InP
0.6 m

17

(n=5x10

-3

cm )

n-InP
Substrate

Figure 4

Schematic structure of 1.3µm InAsP/GaInAsP n- type MD-MQW lasers grownby GSMBE

creases, the peak intensity drops abruptly and the FWHM
value increases, even to 130 meV in lasers doped to 5 x
1018cm-3.
Figure 6 shows the dependence of threshold current
density on inverse cavity length during growth at a constant temperature of 515°C with ND as a parameter. In
devices with cleaved facets and a cavity length of 1000µm
or more, a decrease in threshold current density was observed in 1 x 1018cm-3 doped MD lasers compared to
nondoped barrier lasers. The minimum threshold current
density was in lasers doped to 1 x 1018cm-3, with 330 A/
cm2 obtained at a cavity length of 1200 µm.
Further, optimization of the growth temperature of MDMQW lasers was then carried out with ND constant at 1 x
1018cm-3, as shown in Figure 7. The growth mode at which
minimum threshold current density was achieved consistedof growing the clad layer at 515°C, the SCH-MQW layer at
455°C, with a threshold current density of only 250 A/cm2--a world-class value--achieved at a cavity length of
1200µm. An estimate was also made of the threshold current density at infinite cavity length Jth∞.
Figure 8 shows the relationship Jth∞ per well and ND for
two kinds of growth temperature. The broken line shows
extrapolation from results obtained from lasers grown at aconstant temperature of 515°C. The minimum Jth∞ per well,for lasers doped to 1 x 1018cm-3, was 57 A/cm2/well, a result that is just as good as that reported for an n-type MD
laser made by MOCVD8).
In addition, with respect to nondoped barrier lasers and1 x 1018cm-3 doped MD-MQW lasers, stripe lasers having
a width of 20µm were fabricated and their properties were
evaluated. The differential quantum efficiency of devices
with cleaved facets having a cavity length of 300µm wasgood, at 42%/facet for nondoped barrier lasers and 44%
for MD-MQW lasers. Based on the cavity length dependence of efficiency, internal loss was estimated to be 6.0
cm-1 for nondoped barrier lasers and 4.6 cm-1 for MD lasers, and quantum well efficiency to be 100% for both.
The lower internal loss for MD lasers is attributed to a
reduction in intervalence band absorprion caused by a
10000

1.5E2

Tg=515˚C constant

P2

1.4E4

Ga+P

1.2E4
1.0E4
8.0E3

1.0E2

6.0E3

1000

4.0E3

5.0E1
Si
0.0E0
4000

n=5x1018cm-3doped ( )Jth (A/cm2)

SECONDARY ION COUNTS

2.0E2

4200

4400

4600

4800

5000

undoped ( )

2.0E3

0.0E0
5200

n=1x1018cm-3doped ( )100TIME (sec)Figure 5

64

Silicon doping profile, together with Ga and P
element profiles near the quantum well active
layer, measured by SIMS; doping density ofthe barrier layers was 1 x 1018cm-3

Figure 601020
1/L (cm-1)

30

40

Dependence of threshold current density Jth
on inverse cavity length 1/L in 1.3µm MDMQW lasers (ND = 1 x 1018cm-3)

Furukawa Review, No. 18. 1999

10

10000

W=20 mHR(90%)/HR(96%)

laser

L=200 m18

Output Power (arb. units)

1.3 m InAsP MD-MQW-3

Jth (A/cm2)

n=1x10 cm

Tg=515 C
constant

1000

Tg=515˚C (Cladding)455˚C (SCH-MQW)
10

30

20

n=1×1018cm-3
Ith=11.5mA

SCH-MQW

undopedlaser
Ith=13.5mA
0

10

Figure 9

Dependence of threshold current density Jth
on inverse cavity length 1/L in 1.3µm MDMQW lasers (ND= 1 x 1018cm-3) at selected
growth temperatures Tg

20

30

Current (mA)

40

1/L (cm-1)

Figure 7

5

0

100
0

20 m
MD-laserLight output power vs. current in 1.3µm MDMQW lasers (ND= 1 x 1018cm-3) and undoped
MQW lasers

5. CONCLUSION
150

Jth ∞ per well (A/cm2)

Tg=515˚C constant100

50

Tg=515˚C (Cladding)455˚C (SCH-MQW)

0
0

2

4

6

Doping density in barrier layers (x1018cm-3)

Figure 8

Dependence of threshold current density Jthat infinite cavity length on doping density inbarrier layers of 1.3µm MD-MQW; broken line
is extrapolated from data for 515°C constant
growth lasers

decrease in threshold carrier density. The characteristic
temperature T0 in the range of 20 to 70°C in devices withcleaved facets and a cavity length of 1200µm were 75 K
for both types of lasers.
Further, with respect to stripe lasers with a width of 20µm,
devices were fabricated with a cavity length of 200µm andwith highly reflective coatings on both facets (90%/96%)(see Figure 9), and comparisons with nondoped barrier
lasers were carried out. In MD-MQW lasers, low-threshold operation (at a threshold current of 11.5 mA) was confirmed. This suggests that if an ideal buried heterostructure
laser with a width of 1µm can be fabricated, we may expect to achieve threshold currents of as low as 0.6 mA,
opening the way for applications in laser array devices.

Work is proceeding on the development of Group III-V
optical devices and electronic devices by means of gassource molecular beam epitaxy (GSMBE), a variant of
MBE technology in which the Group V sources are in theform of the gases AsH3 and PH3. A major advantage of
GSMBE is high uniformity, and by optimizing the growth
position and growth conditions it was possible to achieve
excellent uniformity in composition and thickness. With aview to using the advantages of GSMBE technology inthe growing of 1.3µm MQW lasers, which show promise
as optical sources for access networks and laser arraydevices, work is going forward on n-type modulation-doped
MQW lasers in which the quantum well layer is nondoped,
and only the barrier layer is n-doped. With a 1 x 1018cm-3
selectively n-doped laser grown under optimized temperature, extremely low threshold current density Jth--250 A/
cm2 at a cavity length of 1200µm--were achieved. It waspossible to confirm the superiority of n-type modulation
doping by GSMBE in terms of threshold current density.

REFERENCES

1) M. Wachter et al.: J. Crystal Growth, 164(1996), 302
2) H. Seki et al.: J. of Crystal Growth, 78(1986), 342
3) A. Kasukawa et al.: IEEE J. of Quantum Electronics, 29(1993),
1528
4) K. Uomi : J. J. Appl. Phys., 29(1990), 81
5) T. Yamamoto et al.: Photo. Tech. Lett., 6(1994),1165
6) K. Nakahara et al.: IEEE of Selected Topics in Quantum Electronics, 3(1997), 166
Manuscript received on July 13, 1998.

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