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Short wavelength (visible) quantum well lasers grown by molecular beam epitaxy
Physica 129B (1985) 465-468 North-Holland, Amsterdam
SHORT WAVELENGTH
465
(VISIBLE) QUANTUM WELL LASERS GROWN BY MOLECULAR BEAM EPITAXY
P. BLOOD, E.D. FLETCHER,
K. WOODBRIDGE
Philips Research Laboratories,
and P.J. HULYER
Redhill,
Surrey, RHI 5HA, England.
We have fabricated AIGaAs multiple quantum well lasers from a variety of structures grown by molecular beam epitaxy with the objective of achieving operation at a short wavelength with GaAs wells. A series of structures with well widths from 55~ down to 13~ gave pulsed room temperature laser operation at wavelengths from 837nm to 707nm. All the devices operated at longer wavelengths than that calculated for the n=l(e-hh) transition, though from measurements of their electroluminescence spectra at currents as low as 7% of threshold we find no evidence for changes in the sub-band separation at high injection. The threshold current density of a simple broad area MQW device with 160~ wide GaAs wells operating at ~S80nm was 1.2kA cm -2 a~d an analysis of the threshold current density and losses in these devices suggests that interface optical scattering is small.
2. LAYER GROWTH
I. INTRODUCTION The use of quantum size effects to modify the operating wavelength
of semiconductor
lasers has been extensively especially
investigated,
in the GaAs-AIGaAs
system.
High
The structures were grown by Molecular Beam Epitaxy in a laboratory
built system 8 using Be
and Si as the dopants.
The substrate
temperature
for growth of the device structure
quality current injection quantum well lasers
was 700°C and the growth rate for GaAs was
have been demonstrated
about 1~m hr -I.
by both molecular
epitaxy (MBE)I, 2 and metalorganlc vapour deposition
beam
chemical
(MOCVD)3, 4 using GaAs well
The substrate was rotated
during growth at 120 rpm, giving at least one complete rotation per monolayer deposited,
widths down to about 60~ producing emission as
ensure good lateral uniformity.
short as 820nm.
used to determine
Recently,
Hersee et al 5 have
to
The methods
the well width have been
reported growth of injection structures with
described prevlously 9.
wells nominally as thin as 20~ but emission
A1 in the wells because the measured cut off
wavelengths
of the AI beam by the shutter occurs in a time
apparently
suggest the wells are
much thicker than the nominal values. reductions
Further
in wavelength have only been
achieved by using relatively wide AIGaAs wells
We believe there is no
shorter than the monolayer deposition
time,
and A1 diffusion at the growth temperature
is
negligible.
where the wavelength shortening due to the quantum size effect is small 6,7.
In this work
we have studied the growth and device characteristics
of current injection multiple
3. DEVICE RESULTS The dependence of the emission wavelength of a set of MQW lasers as a function of the
quantum well (MQW) lasers with narrow (~55~)
GaAs well width is shown in Figure I.
GaAs wells in which we have achieved laser
sample details are given in reference 9.
emission down to 707nm utilising
device with wells only 13~ wide operates at a
size effect alone.
the quantum
We have also investigated
wavelength of 707nm.
The The
Note also that the
the factors affecting the threshold current of
operating wavelength of the device with 25A
devices with 160~ wide GaAs wells.
wells and 40A barriers appears to lengthen
0378-4363/85/$03.30 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
46b
P. B l o o d ct al. ,' Short warelc'ngth (risibh,) qu~mmm ~t'¢'ll h~s¢'rs
absorption data measured on the same sample9.0ne possibility
850
is that the sub-band
separation is reduced at high injection i'effective gap shrinkage") eLectroluminescence
p.
800
E
show no shift tn peak wavelength
• 10 wells • 20 wells
• 40, ~, barriers oa Calculation
2,
unlikely.
photoexcitation of similar 60 well MQW isotype samples give edge photolumineseence
at a
18nm longer than the surface
luminescence II.
' ~ X b '~0 45
for currents
[th so this e×planation Is
We have observed that spot
wavelength m 700
b.t
spectra from the N) × 55A
well device ot Figure l, given in Figure ~t,
down to 0.07
g 7.5O
Levels
A similar effect has been
reported by Tarncha e t a [ 12 and shown to be due to ceabsorption.
65C
o
l Well
L
2o
3'0
width,
i
The threshold current of these devices is
do
the valnes have b e e n
abs
55£ M a W 154)
Lz,(,&)
FIGURE I Plot of emission wavelength as a function of well width for MQW lasers. The I0 well devices are simple MQW structures with about 34% AI in the barriers; The 20 well devices have GRIN regions with 60% AI in the outer cladding. Except for the device indicated, all devices have 804 wide barriers. The calculated circles give the wavelength of the n=l(e-hh) transition calculated for each device.
compared with the other devices with 80~ barriers,
shown in Figure 3:
suggesting some coupling between the
®
-
__
/
\ ~
I/ ~\
0 951,, -
i,Y " J," I/ \\
0.48Z.
254 wide wells with the thinner barriers. An important feature of these results is
0211,
that all devices operate at longer wavelengths than that calculated for the n=l(e-hh) 0 0 7 1 ,.
transition using the measured well widths and barrier compositions 9.
The participation of
800
85O Wavelength t n m )
LO phonons in the emission process I0 is unlikely because the wavelength shift varies from sample to sample.
Furthermore the effect
cannot be due to error in the assignment of the well width, graded barriers or fluctuations in composition (as suggested by Hersee et al 5) because we observe differences between laser emission wavelengths and
FIGURE Z Electroluminescence spectra from a 10 x 55A GaAs MQW laser at various drive currents. Also shown are the wavelengths of laser emission and the calculated n=l(e-hh) transition, and the positions of the MQW absorption peaks (Figure 2 ref. 9). The latter differs from the calculation by the exciton binding energy.
467
P. Blood et al. / Short wavelength (visible) quantum well lasers
optical loss ~i ~ 2cm-l"
20
x 10 w e l l , no GRIN 20 w e l l , GRIN
o
The loss obtained
from external differential efficiency (qD) data is about 10cm -I.
16
At threshold,
free-carrler scattering, ~fc, in "bulk" material is about 10cm -I, contributing a loss
12
Jth (kA cm -2)
8
Fsfc ~ 3.5cm -I in these devices.
The loss in
the cladding and barrier regions,
(l-r)~fc , is
about 1.6cm -I suggesting an overall free carrier loss of about 5cm -I.
Comparison of
our data with these numbers suggests that despite the number of layers in the structure I
I
O
I
10
I
I
I
2'0 3JO 4'0 Well w i d t h (,~)
I
5'0
60
the optical loss due to scattering at interfaces is small, at the most about 5cm -I. The temperature dependence of the threshold
FIGURE 3 Threshold current density of 50um oxide stripe MQW lasers as a function of well width (all the devices are those of Figure i) with no correction for current spreading.
current in these 160~ MQW devices in the range 30-60=C can be characterlsed by a T o ~ 200K, which is larger than obtained by Tsang 13. From a plot of nD-i versus device length for 50~m stripe devices we have also been able
calculated for 250~m long 50~m wide oxide
to estimate the internal differential
stripe devices with no corrections for current
efficiency q i above threshold.
spreading.
obtained for the 8 x 160~ GaAs well devices is
Although these data represent a
variety of MQW structures they show the
The value
0.37 by a fit to all data points, and from the
advantages of using a graded refractive index
best values of qD at each length we obtain
(GRIN) structure and the strong increase in
qi = 0.57.
threshold current for narrow wells.
For stripe devices circulating
The
latter may be due to both a decrease in the
8 x 1 6 0 A wells, 60~, b a r r i e r s , 33°/° A[
capture probability for carriers into the
x
wells and a decrease in the optical
3
confinement factor. We have also grown a set of simple MQW lasers with 8 160A GaAs wells and 33% AI in the barriers.
These lasers emit at about
Jtn
(kA cm -2) 2
880nm, with a broad area (250~m x 150~m) threshold current density of 1.2kAcm -2, as also reported by Tsang 13 for a similar structure.
A plot of threshold current
density versus reciprocal device length for
o
~o
~ 4~0 ' L-I (cm -I)
6~0
~
8~0
50um oxide stripe devices is shown in Figure 4.
The equivalent broad area threshold
current density at infinite length is about 850Acm -2 and using gain current data of Kasemset et a114 we calculate an internal
FIGURE 4 Threshold current density as a function of reciprocal length for a set of 50Bm oxide stripe devices having a simple 8 x 160~ GaAs MQW active region.
P. Blood el al. / Short wa~,elength (l,isible) qualttum well laser~
468
modes should not affect the measurement
of n i
and the non-uniform current distribution under the stripe should not reduce the measured efficiency to much less than 80% 15 .
I. W.T.
Tsang, Appl. Phys. Lett. 38 (1981
204.
In these
simple devices without separate confinement regions the injected carriers may leak into the cladding regions on either side of the MQW structure.
REFERENCES
According to Tang et a116 the
probability of an electron being captured into a 160A wide well is at least 0.5 and so the
2. W.T. Tsang, Appl. Phys. Lett. 39 ([98[ 786. 3. S.D. Hersee, M. Baldy, P. Assenat, B. de Cremoux and J.P. Duchemin, Elec. Lett. 18 (1982) 870. 4. D. Kasemset, C.S. Hong, N.B. Patel and P.D. Dapkus, Appl. Phys, Lett. 41 (1982) 912.
fraction transmitted across 8 wells will only be (I-0.5) 8 = 0.004.
However the capture
probability for holes will be less than for electrons and so there is the possibility of holes leaking through to the N-cladding.
In
addition there may be spontaneous or
5. S.D. Hersee, B. de Cremoux and J.P. Duchemin, Appl. Phys. Lett. 44 ([984) 476. 6. R.D. Burnham, C. LindstrSm, T.L. Pao[L, D.R. Scrifes, W. Streifer and N. Holonyak Jnr., Appl. Phys. Lett. 42 (1983) 937.
non-radiative recombination in the barrier 7. W.T. Tsang and J.A. Ditzenberger, Phys. Lett. 39 (1981) 193.
regions between the wells.
4. SUMMARY We have prepared multiple quantum well lasers by molecular beam epitaxy with GaAs wells as thin as 13~ operating in the visible part of the spectrum at 707nm.
The threshold
currents of these devices increase with decreasing well width.
An analysis of the
Appl.
8. K. Woodbridge, P. Dawson, J.P. Gowers and C.T. Foxon, J. Vac. Sci. Technol. B2 (1984) 163. 9. K. Woodbridge, P. Blood, E.D. Fletcher and P.J. Hulyer, Appl. Phys. Lett. 45 (1984) 16. 10. J.J. Coleman, P.D. Dapkus, D.R. Clarke, M.D. Camras and N. Holonyak Jnr., Appl. Phys. Lett. 39 (1981) 864.
threshold currents of simple MQW devices with 8 x 160A GaAs wells suggests that the total internal losses including interface optical scattering are small.
ACKNOWLEDGEMENTS We thank G. Duggan for calculation of the transition wavelengths, P. Kershaw for help
I[. P, Dawson, unpublished. L2. S. Tarucha, Y. Horikoshi and H. Okamoto, Jap. J. Appl. Phys. 22 (1983) L482. 13. W.T. Tsang, Appl. Phys. Lett. 39 (1981) 786. 14. D. Kasemset, C-S. Hong, N.B. Patel and P.D. Dapkus, IEEE Journ. Quantum Electrics QE-19 (1983) 1025.
with the measurements and G. Duggan, P. Dawson and C.T. Foxon for helpful discussions.
15. B.W. Hakki, Journ. AppL. Phys. 46 (1975) 292. 16. J.Y. Tang, K. Hess, N. Hoionyak Jnr., J.J. Coleman and P.D. Dapkus, Journ. Appl. Phys. 53 (1982) 604~.
SHORT WAVELENGTH
465
(VISIBLE) QUANTUM WELL LASERS GROWN BY MOLECULAR BEAM EPITAXY
P. BLOOD, E.D. FLETCHER,
K. WOODBRIDGE
Philips Research Laboratories,
and P.J. HULYER
Redhill,
Surrey, RHI 5HA, England.
We have fabricated AIGaAs multiple quantum well lasers from a variety of structures grown by molecular beam epitaxy with the objective of achieving operation at a short wavelength with GaAs wells. A series of structures with well widths from 55~ down to 13~ gave pulsed room temperature laser operation at wavelengths from 837nm to 707nm. All the devices operated at longer wavelengths than that calculated for the n=l(e-hh) transition, though from measurements of their electroluminescence spectra at currents as low as 7% of threshold we find no evidence for changes in the sub-band separation at high injection. The threshold current density of a simple broad area MQW device with 160~ wide GaAs wells operating at ~S80nm was 1.2kA cm -2 a~d an analysis of the threshold current density and losses in these devices suggests that interface optical scattering is small.
2. LAYER GROWTH
I. INTRODUCTION The use of quantum size effects to modify the operating wavelength
of semiconductor
lasers has been extensively especially
investigated,
in the GaAs-AIGaAs
system.
High
The structures were grown by Molecular Beam Epitaxy in a laboratory
built system 8 using Be
and Si as the dopants.
The substrate
temperature
for growth of the device structure
quality current injection quantum well lasers
was 700°C and the growth rate for GaAs was
have been demonstrated
about 1~m hr -I.
by both molecular
epitaxy (MBE)I, 2 and metalorganlc vapour deposition
beam
chemical
(MOCVD)3, 4 using GaAs well
The substrate was rotated
during growth at 120 rpm, giving at least one complete rotation per monolayer deposited,
widths down to about 60~ producing emission as
ensure good lateral uniformity.
short as 820nm.
used to determine
Recently,
Hersee et al 5 have
to
The methods
the well width have been
reported growth of injection structures with
described prevlously 9.
wells nominally as thin as 20~ but emission
A1 in the wells because the measured cut off
wavelengths
of the AI beam by the shutter occurs in a time
apparently
suggest the wells are
much thicker than the nominal values. reductions
Further
in wavelength have only been
achieved by using relatively wide AIGaAs wells
We believe there is no
shorter than the monolayer deposition
time,
and A1 diffusion at the growth temperature
is
negligible.
where the wavelength shortening due to the quantum size effect is small 6,7.
In this work
we have studied the growth and device characteristics
of current injection multiple
3. DEVICE RESULTS The dependence of the emission wavelength of a set of MQW lasers as a function of the
quantum well (MQW) lasers with narrow (~55~)
GaAs well width is shown in Figure I.
GaAs wells in which we have achieved laser
sample details are given in reference 9.
emission down to 707nm utilising
device with wells only 13~ wide operates at a
size effect alone.
the quantum
We have also investigated
wavelength of 707nm.
The The
Note also that the
the factors affecting the threshold current of
operating wavelength of the device with 25A
devices with 160~ wide GaAs wells.
wells and 40A barriers appears to lengthen
0378-4363/85/$03.30 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
46b
P. B l o o d ct al. ,' Short warelc'ngth (risibh,) qu~mmm ~t'¢'ll h~s¢'rs
absorption data measured on the same sample9.0ne possibility
850
is that the sub-band
separation is reduced at high injection i'effective gap shrinkage") eLectroluminescence
p.
800
E
show no shift tn peak wavelength
• 10 wells • 20 wells
• 40, ~, barriers oa Calculation
2,
unlikely.
photoexcitation of similar 60 well MQW isotype samples give edge photolumineseence
at a
18nm longer than the surface
luminescence II.
' ~ X b '~0 45
for currents
[th so this e×planation Is
We have observed that spot
wavelength m 700
b.t
spectra from the N) × 55A
well device ot Figure l, given in Figure ~t,
down to 0.07
g 7.5O
Levels
A similar effect has been
reported by Tarncha e t a [ 12 and shown to be due to ceabsorption.
65C
o
l Well
L
2o
3'0
width,
i
The threshold current of these devices is
do
the valnes have b e e n
abs
55£ M a W 154)
Lz,(,&)
FIGURE I Plot of emission wavelength as a function of well width for MQW lasers. The I0 well devices are simple MQW structures with about 34% AI in the barriers; The 20 well devices have GRIN regions with 60% AI in the outer cladding. Except for the device indicated, all devices have 804 wide barriers. The calculated circles give the wavelength of the n=l(e-hh) transition calculated for each device.
compared with the other devices with 80~ barriers,
shown in Figure 3:
suggesting some coupling between the
®
-
__
/
\ ~
I/ ~\
0 951,, -
i,Y " J," I/ \\
0.48Z.
254 wide wells with the thinner barriers. An important feature of these results is
0211,
that all devices operate at longer wavelengths than that calculated for the n=l(e-hh) 0 0 7 1 ,.
transition using the measured well widths and barrier compositions 9.
The participation of
800
85O Wavelength t n m )
LO phonons in the emission process I0 is unlikely because the wavelength shift varies from sample to sample.
Furthermore the effect
cannot be due to error in the assignment of the well width, graded barriers or fluctuations in composition (as suggested by Hersee et al 5) because we observe differences between laser emission wavelengths and
FIGURE Z Electroluminescence spectra from a 10 x 55A GaAs MQW laser at various drive currents. Also shown are the wavelengths of laser emission and the calculated n=l(e-hh) transition, and the positions of the MQW absorption peaks (Figure 2 ref. 9). The latter differs from the calculation by the exciton binding energy.
467
P. Blood et al. / Short wavelength (visible) quantum well lasers
optical loss ~i ~ 2cm-l"
20
x 10 w e l l , no GRIN 20 w e l l , GRIN
o
The loss obtained
from external differential efficiency (qD) data is about 10cm -I.
16
At threshold,
free-carrler scattering, ~fc, in "bulk" material is about 10cm -I, contributing a loss
12
Jth (kA cm -2)
8
Fsfc ~ 3.5cm -I in these devices.
The loss in
the cladding and barrier regions,
(l-r)~fc , is
about 1.6cm -I suggesting an overall free carrier loss of about 5cm -I.
Comparison of
our data with these numbers suggests that despite the number of layers in the structure I
I
O
I
10
I
I
I
2'0 3JO 4'0 Well w i d t h (,~)
I
5'0
60
the optical loss due to scattering at interfaces is small, at the most about 5cm -I. The temperature dependence of the threshold
FIGURE 3 Threshold current density of 50um oxide stripe MQW lasers as a function of well width (all the devices are those of Figure i) with no correction for current spreading.
current in these 160~ MQW devices in the range 30-60=C can be characterlsed by a T o ~ 200K, which is larger than obtained by Tsang 13. From a plot of nD-i versus device length for 50~m stripe devices we have also been able
calculated for 250~m long 50~m wide oxide
to estimate the internal differential
stripe devices with no corrections for current
efficiency q i above threshold.
spreading.
obtained for the 8 x 160~ GaAs well devices is
Although these data represent a
variety of MQW structures they show the
The value
0.37 by a fit to all data points, and from the
advantages of using a graded refractive index
best values of qD at each length we obtain
(GRIN) structure and the strong increase in
qi = 0.57.
threshold current for narrow wells.
For stripe devices circulating
The
latter may be due to both a decrease in the
8 x 1 6 0 A wells, 60~, b a r r i e r s , 33°/° A[
capture probability for carriers into the
x
wells and a decrease in the optical
3
confinement factor. We have also grown a set of simple MQW lasers with 8 160A GaAs wells and 33% AI in the barriers.
These lasers emit at about
Jtn
(kA cm -2) 2
880nm, with a broad area (250~m x 150~m) threshold current density of 1.2kAcm -2, as also reported by Tsang 13 for a similar structure.
A plot of threshold current
density versus reciprocal device length for
o
~o
~ 4~0 ' L-I (cm -I)
6~0
~
8~0
50um oxide stripe devices is shown in Figure 4.
The equivalent broad area threshold
current density at infinite length is about 850Acm -2 and using gain current data of Kasemset et a114 we calculate an internal
FIGURE 4 Threshold current density as a function of reciprocal length for a set of 50Bm oxide stripe devices having a simple 8 x 160~ GaAs MQW active region.
P. Blood el al. / Short wa~,elength (l,isible) qualttum well laser~
468
modes should not affect the measurement
of n i
and the non-uniform current distribution under the stripe should not reduce the measured efficiency to much less than 80% 15 .
I. W.T.
Tsang, Appl. Phys. Lett. 38 (1981
204.
In these
simple devices without separate confinement regions the injected carriers may leak into the cladding regions on either side of the MQW structure.
REFERENCES
According to Tang et a116 the
probability of an electron being captured into a 160A wide well is at least 0.5 and so the
2. W.T. Tsang, Appl. Phys. Lett. 39 ([98[ 786. 3. S.D. Hersee, M. Baldy, P. Assenat, B. de Cremoux and J.P. Duchemin, Elec. Lett. 18 (1982) 870. 4. D. Kasemset, C.S. Hong, N.B. Patel and P.D. Dapkus, Appl. Phys, Lett. 41 (1982) 912.
fraction transmitted across 8 wells will only be (I-0.5) 8 = 0.004.
However the capture
probability for holes will be less than for electrons and so there is the possibility of holes leaking through to the N-cladding.
In
addition there may be spontaneous or
5. S.D. Hersee, B. de Cremoux and J.P. Duchemin, Appl. Phys. Lett. 44 ([984) 476. 6. R.D. Burnham, C. LindstrSm, T.L. Pao[L, D.R. Scrifes, W. Streifer and N. Holonyak Jnr., Appl. Phys. Lett. 42 (1983) 937.
non-radiative recombination in the barrier 7. W.T. Tsang and J.A. Ditzenberger, Phys. Lett. 39 (1981) 193.
regions between the wells.
4. SUMMARY We have prepared multiple quantum well lasers by molecular beam epitaxy with GaAs wells as thin as 13~ operating in the visible part of the spectrum at 707nm.
The threshold
currents of these devices increase with decreasing well width.
An analysis of the
Appl.
8. K. Woodbridge, P. Dawson, J.P. Gowers and C.T. Foxon, J. Vac. Sci. Technol. B2 (1984) 163. 9. K. Woodbridge, P. Blood, E.D. Fletcher and P.J. Hulyer, Appl. Phys. Lett. 45 (1984) 16. 10. J.J. Coleman, P.D. Dapkus, D.R. Clarke, M.D. Camras and N. Holonyak Jnr., Appl. Phys. Lett. 39 (1981) 864.
threshold currents of simple MQW devices with 8 x 160A GaAs wells suggests that the total internal losses including interface optical scattering are small.
ACKNOWLEDGEMENTS We thank G. Duggan for calculation of the transition wavelengths, P. Kershaw for help
I[. P, Dawson, unpublished. L2. S. Tarucha, Y. Horikoshi and H. Okamoto, Jap. J. Appl. Phys. 22 (1983) L482. 13. W.T. Tsang, Appl. Phys. Lett. 39 (1981) 786. 14. D. Kasemset, C-S. Hong, N.B. Patel and P.D. Dapkus, IEEE Journ. Quantum Electrics QE-19 (1983) 1025.
with the measurements and G. Duggan, P. Dawson and C.T. Foxon for helpful discussions.
15. B.W. Hakki, Journ. AppL. Phys. 46 (1975) 292. 16. J.Y. Tang, K. Hess, N. Hoionyak Jnr., J.J. Coleman and P.D. Dapkus, Journ. Appl. Phys. 53 (1982) 604~.