- Email: [email protected]
MOVPE-grown (AlGa)As double-barrier multiquantum well (DBMQW) laser diode with low vertical beam divergence
,. . . . . . . .
C R Y S T A L G R O W T H
ELSEVIER
Journal of Crystal Growth 170 (1997) 408-412
MOVPE-grown (AlGa) As double-barrier multiquantum well (DBMQW) laser diode with low vertical beam divergence Andrzej Mal~g a,*, W~odzimierz Strupifiski b a Institute of Electron Technology, A I. Lotnik6w 3 2 / 4 6 , P-02 668 Warsaw, Poland b Institute of Electronic Materials Technology, Ul. W61czyfiska 133, P-O1 919 Warsaw, Poland
Abstract We present a (AlGa)As laser diode based on a MOVPE-grown heterostructure modified toward reduction of the vertical (perpendicular to junction plane) light beam divergence. Insertion of thin, wide-gap barrier layers at the interfaces between the MQW active region and the cladding layers allows for separate controlling the carrier and optical confinements. According to theoretical modeling, the antiguiding influence of the barriers on the primary waveguiding properties of a MQW structure causes a weakening of the optical confinement and thereby a reduction of the vertical beam divergence. This occurs however without weakening of the carrier confinement and an excessive increase of threshold current density. As a result, the beam divergence of 17-13 ° has been experimentally achieved (depending on construction details), for devices manufactured from the heterostructure theoretically expected to give 12°. MOVPE growth conditions of this modified "double-barrier multiquantum well" (DBMQW) heterostructure are described. The light-current characteristics presented show threshold current densities of 0.9-2.1 kA cm -2 for laser cavity lengths of 0.75-0.25 mm, respectively and good quantum efficiencies.
1. Introduction
In many applications of semiconductor laser diodes (LDs), their wide radiation beam is one of the major problems. A typical beam divergence in the fundamental transverse mode is 5 - 3 0 ° in the LDs junction plane and 3 0 - 6 0 ° perpendicularly to this plane. Then reduction of the second one is essential if a low beam divergence emitter is needed. The problem is difficult in edge-emitting LDs because of contradictory requirements of low vertical beam divergence angle ( O ± ) and low threshold current density (Jth)- A large size spot on the laser facet is
* Corresponding author. Fax: +48 22 470631.
necessary to get a low (9±, which means weak optical confinement but simultaneously weak carrier confinement resulting in high Jth in conventional LDs. One possible solution to resolve this contradiction is heterostructure modification by insertion of additional sets of layers [1-6] aimed at the separate control of carrier and light confinements and thereby the inhibition of an excessive Jth increase connected with O . reduction. Actually, as a low O± requires the optical confinement to be weak, some Jth increase is unavoidable [2,3,5,6], even though the carrier confinement remains unaffected. We need to find the optimized solution in terms of a minimum O ± at an acceptable increase of Jth. This paper presents an (AlGa)As LD based on a MOVPE-grown
0022-0248/97/$17.00 Copyright © 1997 Elsevier Science B.V. All rights reserved PII S0022-0248(96)005 65-9
409
A. Malqg, W. Strupihski / Journal of Crystal Growth 170 (1997) 408-412
modified heterostructure with separate confinements controlled by thin, wide-gap barriers inserted at the interfaces of the active region and the cladding layers. The influence of the barriers on the heterostructure properties and the MOVPE (AlGa)As growth details are presented in Section 2. In Section 3 the theoretical expectations and experimental electrooptical characteristics of so obtained (AlGa)As LDs are shown to be in good agreement, giving low-6)± devices. Such a structure will be further termed a double-barrier multiquantum well (DBMQW) heterostructure (by analogy to earlier the DBDH structure [1 ]).
2. Heterostructure concept and MOVPE growth characteristics The heterostructure under consideration is shown in Fig. 1. The inserted barriers of thickness d b and dg thick graded (for smoothing-out band discontinuities) layers are doped in order to facilitate majority carriers transport. The role of the barriers is twofold. First, they improve the carrier confinement and the A1 content (x b) of 0.55 is sufficient for this goal [1]. Next, the influence of the barriers on the optical confinement can be considered as a competition of the index guiding and antiguiding effects at both interfaces of barrier layers. Predominance of antiguiding weakens the original waveguiding properties of the MQW region. It can be controlled by the barrier
/
DBMQW
Active Region
0,6
Xb
( A l G a ) A s Heterostructure
"
0 0.2 d~ = 7 nm dd= 4 nm d b = d g - l O nm d . = 30 nm
0.8 0.7
.=. 0.6 ~) O
0.5 0.4
p+
I
[ 3.5
p
db dadd dg 3.6
3.7
Distance [w~n]
n 3E1 / (Si)
5E1 7 (Zn)
n+ ~
318
doping type
0.3 0.2 0.1
0.0
cap
p - cladding
dc
active region
n - cladding
buffer layer
dc
Distance from Surface
[p,m]
Fig. l. Al-content profile of the investigated DBMQW (AlGa)As heterostructure and other importantdetails of the structure design.
banier MQW
region
barrier
Fig. 2. TEM and SEM microphotographsof the MOVPE-grown (AlGa)As DBMQW heterostructure under investigation.
thickness (db +dg) and height (Xb) as described elsewhere [7,8]. (AlGa)As DBMQW heterostructures were grown using an automatic controlled LP-MOVPE Aixtron system, model 200 R & D on a 2 inch (100) oriented GaAs substrate. The quartz reactor is of the horizontal type. A graphite susceptor coated with silicon carbide with IR heating enables the rotation of the substrate during the growth. The sources were Pdpurified hydrogen, 100% arsine (ASH3), trimethylgallium (TMGa), trimethylaluminum (TMAI), 2% silane (Sill 4) in H 2 and dimethylzinc (DMZn). A pressure in the range 20-100 mbar, a V / I I I ratio of 250 and a temperature of 680-720°C were applied for all experiments. Vent/run balancing was used during the entire growth in order not to introduce any flow oscillations around the composition of (AlxGa I ,)AsOne of the main problems in performing the proposed laser structure were large thickness differences among quantum wells (7 rim), interwell and outer inserted barriers (4 and 10 nm, respectively) and cladding layers (3.5 pLm). Optimization of the growth rate was necessary to obtain a heterostructure containing very thin and relatively thick layers during a reasonable time, with abrupt interfaces and thickness nonuniformity within 1-2 monolayers. The maximal growth rate for two-dimensional deposition was found basing on measurements of electrical parameters (Hall method), PL intensity and quantum
410
A. Malqg, W. Strupifiski / Journal oJ"Crystal Growth 170 (1997) 408 412
DBMQW
LD
~ .
--n-oro,,,o -I .....
TEo, TEo,
dg=t0nm dla=2Onm ~. = 0.86p,m
3 65 ' E 3.60
da = 7 nm dd:,nm db= I 0 nm
~> 3.55
7';:~,.%~r,
3.60
/ R = o.~
345
•
--
.~
3 35
t,
3"35J"/
// ,'/
i7
.....¢./
_....=~:i
I I ' '
l
,"7 \"
i
V,, \ ,
""~---
-
,>" 1.o D B M Q W
i deg deg
LD /"~
'~ 0.8
•
c --
0.6
04 ~, Z
N 0.4 o
Z
0.2
o.o
0.0
n- 3.30 3.25
|
-30 Distance from S u r f a c e
J
[
0.2
,~. ....
i
- - TEo. dg = 10 nm, Oj.= 11.8 ..... TEo, dg = 20 nm, e.L= 10.3
"~ ~ 0 . 4 0.6 '10
"~:~ °:' °:' 0:6"
--\',
i
=2Ohm
....
~.',
i
LlO
I ,'7 I t ' / °° , o > , 3.5oi .,'Y fJL ~'. , 0.6 0.8
(I)' //
/ O. = 7 cm-1
3.,0
-'rE d
/ ] j
i
. . .~ .
TEo Mode Intens,y Distributio,, ~;d.'g'~onol -
-20
,
,
,
,
,
-1 0
0
10
20
30
Angle [deg]
[p.m]
Fig. 3. Calculated fundamental transverse mode intensity profiles for two DBMQW structures (see Fig. 1) with graded barrier interface thicknesses of d~ = 10 and 20 nm. Details of the structure and assumed parameters of the laser cavity are given. The refractive index profile (including the free carriers contribution) for the DBMQW structure is shown for the d~ = 10 nm case.
well thickness (in the range 4 - 8 ML). Perfection of interfaces with regard to the chemical composition was optimized by growth interruption. Strong emphasis was put on diminishing the oxygen incorporation into the layers. TEM and SEM microphotographs of a so obtained heterostructure is shown in Fig. 2. Good interface quality and agreement between the nominal and actual thicknesses of all layers is seen.
Fig. 4. Calculated far-field intensity distributions perpendicular to the junction plane for the transverse mode profiles as in Fig. 2.
20 nm, respectively. A small difference in these O ± values is a result of the limitation by the cladding layers thickness [7]. According to the logarithmic gain-current model for M Q W lasers [9], the calculated Jth is 1.43 kA cm -2 for the laser cavity length L = 0.5 mm and Jth = 0.95 k A cm -2 for L = 1 mm, both for 4 - Q W structure of d a = 7 nm [9] and dg = 10 nm. These relatively high Jth values are due to the low F value characteristic for the D B M Q W heterostructure. Instead, high quantum efficiencies can be expected. Proton-isolated, 50 ~ m wide-stripe laser devices have been prepared, using sputtered CrPt p-contacts and evaporated AuGeNi n-contacts on n-GaAs substrates.
3. DBMQW laser diode: modeling and performance The influence of the theoretical barrier thickness on the TE 0 mode distribution is shown in Fig. 3. A very wide mode profile (due to weak guiding) is seen, but its further widening due to the increase of dg (clearly visible in the inset) is limited by the thickness of the cladding layers (dc = 3 I~m) [7]. The directional characteristics of the D B M Q W LDs are seen in Fig. 4, where the two far-field (FF) intensity distributions correspond to the mode profiles of Fig. 3. The calculated beam divergence F W H M values (denoted O l ) are 11.8 ° and 10.3 ° and the cumulative confinement factors (for all quantum wells) F are 0.027 and 0.021 for d g = 10 and
2.0
"~ O
o...
' . . . . . . . . . . . . . .
' ....
' ....
1.8 1.6
D B M Q W LDs pulsed 200 ns, 5 kHz,
1"4
W=50gm
' ....
/ /
'
R ~ / /
L=
1.2 1.0
~
=
= 0.5 • mm
0.8
0.6
0.75 mrn
.,..., 0.4 O 0.2 0.0
0.0
0.5
1.0
1.5
2.0
Diode Current
2.5
3.0
[A]
Fig. 5. A family of pulsed light-current characteristics for a set of DBMQW LDs of various cavity lengths (L). Stripe width W = 50 IJ,m.
A. Malqg, W. Strupihski /Journal of Crystal Growth 170 (1997) 408-412
,
558M :>,
8K2 H3
~-,,~.
1.0
Q~
_~ 0.8
P : o.sw,
.....
= 0.85 A =1.2A
P = 0.7 W, I= 1.55 A
L = 0.5 mm
W = C0 gm
ff
i~
"o
.~ 0.6
-40 -30
-20
-10
0
Angle
10
20
30 40
[deg]
Fig. 6. CCD camera recorded set of intensity distributions perpendicular to the junction plane (QFF patterns) of the light beam emitted under a pulse regime (200 ns, 5 kHz) at various optical power levels by one DBMQW LD.
411
of the DBMQW LDs recorded with a CCD camera (quasi far-field " Q F F " patterns [8]) are shown in Figs. 6 and 7. In Fig. 6 a set of QFF patterns for optical power levels ranging from 0.3 to 0.7 W (yet possible to record with one set of filters) emitted by the same LD is shown. The corresponding O 1 range of 14.5-16.1 ° indicates good beam stability but differs from the theoretically expected 12° for this particular DBMQW structure design. Fig. 7 shows that a lower O , of 12.9 ° (close to the theoretical value) has been obtained for longer laser cavities. This effect can be explained by the growing role of gain-guiding for short cavities in the presence of the weak index-guiding.
4. Conclusion A family of pulsed light-current ( L - I ) characteristics of the manufactured DBMQW LDs with uncoated mirrors for three laser cavity lengths (L) are shown in Fig. 5. The measured /th ( J t h ) values are 0.26 (2.1), 0.3 (1.2), 0.35 (0.93) A (kA cm 2) and external quantum efficiencies (from one mirror) r/ are 0.64, 0.57, 0.51 W / A for L = 0.25, 0.5, 0.75 mm, respectively. Note that the Jth values are below theoretical expectations, while the Jth dependence on L generally agrees with the logarithmic gain-current model. The rather weak dependence of 7/ on L is a proof of a small cavity loss and good internal quantum efficiency. Normalized far-field (FF) beam intensity distributions in the plane perpendicular to the junction plane
1.0
~
'~ 0.8
Popt : 0.7 W
_E
W = 50 gm
0.6
/ ..... k = 0.5 mm ~ L=O.75mm
~,
/!
~!!:=
,¢')
I:;,
ft'
1 7.2
e,:l 6.4, og 1 2.9
Acknowledgements The authors wish to thank Mr. Jacek Ratajczak (lET) for TEM and SEM characterizations of heterostructures.
FWHM (deg) ~ , ! O.L I ~il)
= 0.4
Z
A modified DBMQW (AlGa)As laser heterostructure has been grown by MOVPE. The indication of the growth perfection is the agreement of the theoretically predicted and experimental characteristics of laser diodes based on this heterostructure. From the experimental devices presented the case of L of 0.75 mm is the most interesting because of the lowest O 1 (12.9 ° - close to theoretical 12°) and the lowest Jth of 0.93 kA c m - : while ~7 is still high (0.51 W / A from one uncovered facet). The Jth value below theoretical predictions can come from the uncertainty in the threshold model parameters for QW lasers, or from nonexact modeling. In general, the present 0 1 values are close to the lowest reported [3], but without any side maxima. Relatively high Jth are compensated by high quantum efficiency.
0.2 00
-40 -30
-20
-10
Angle
0 10 [deg]
20
30 40
Fig. 7. CCD camera recorded QFF patterns of the pulsed light beam emitted under constant optical power level by DBMQW LDs of various cavity lengths (L).
References [1] W.T. Tsang, Appl. Phys. Lett. 38 (1981) 835. [2] M.C. Wu, Y.K. Chen, M. Hong, J.P. Mannaerts, M.A. Chin and A.M. Sergent, Appl. Phys. Lett. 59 (1991) 1046.
412
A. Mal(lg, W. Strupifiski/Journal of Co'stal Growth 170 (1997) 408-412
[3] Y.C. Chen, R.G. Waters and R.J. Dalby, Electron. Lett. 26 (1990) 1348. [4] J. Yemmyo and M. Sugo, Electron Lett. 31 (1995) 642. [5] T.M. Cockerill, J. Honig, T.A. DeTemple and J.J. Coleman, Appl. Phys. Lett. 59 (1991) 2694. [6] S.T. Yen and C.P. Lee, IEEE J. Quantum. Electron. 32 (1996) 4.
[7] A. Malog and B. Mroziewicz, J. Lightwave Technol. 14 (1996) 1514. [8] A. Mal~g and W. Strupifiski, Electron Technol. 29 (1996) 176. [9] J.Z. Wilcox, G.L. Peterson, S. Ou, J.J. Yang and M. Jansen, J. Appl. Phys. 64 (1988) 6564.
C R Y S T A L G R O W T H
ELSEVIER
Journal of Crystal Growth 170 (1997) 408-412
MOVPE-grown (AlGa) As double-barrier multiquantum well (DBMQW) laser diode with low vertical beam divergence Andrzej Mal~g a,*, W~odzimierz Strupifiski b a Institute of Electron Technology, A I. Lotnik6w 3 2 / 4 6 , P-02 668 Warsaw, Poland b Institute of Electronic Materials Technology, Ul. W61czyfiska 133, P-O1 919 Warsaw, Poland
Abstract We present a (AlGa)As laser diode based on a MOVPE-grown heterostructure modified toward reduction of the vertical (perpendicular to junction plane) light beam divergence. Insertion of thin, wide-gap barrier layers at the interfaces between the MQW active region and the cladding layers allows for separate controlling the carrier and optical confinements. According to theoretical modeling, the antiguiding influence of the barriers on the primary waveguiding properties of a MQW structure causes a weakening of the optical confinement and thereby a reduction of the vertical beam divergence. This occurs however without weakening of the carrier confinement and an excessive increase of threshold current density. As a result, the beam divergence of 17-13 ° has been experimentally achieved (depending on construction details), for devices manufactured from the heterostructure theoretically expected to give 12°. MOVPE growth conditions of this modified "double-barrier multiquantum well" (DBMQW) heterostructure are described. The light-current characteristics presented show threshold current densities of 0.9-2.1 kA cm -2 for laser cavity lengths of 0.75-0.25 mm, respectively and good quantum efficiencies.
1. Introduction
In many applications of semiconductor laser diodes (LDs), their wide radiation beam is one of the major problems. A typical beam divergence in the fundamental transverse mode is 5 - 3 0 ° in the LDs junction plane and 3 0 - 6 0 ° perpendicularly to this plane. Then reduction of the second one is essential if a low beam divergence emitter is needed. The problem is difficult in edge-emitting LDs because of contradictory requirements of low vertical beam divergence angle ( O ± ) and low threshold current density (Jth)- A large size spot on the laser facet is
* Corresponding author. Fax: +48 22 470631.
necessary to get a low (9±, which means weak optical confinement but simultaneously weak carrier confinement resulting in high Jth in conventional LDs. One possible solution to resolve this contradiction is heterostructure modification by insertion of additional sets of layers [1-6] aimed at the separate control of carrier and light confinements and thereby the inhibition of an excessive Jth increase connected with O . reduction. Actually, as a low O± requires the optical confinement to be weak, some Jth increase is unavoidable [2,3,5,6], even though the carrier confinement remains unaffected. We need to find the optimized solution in terms of a minimum O ± at an acceptable increase of Jth. This paper presents an (AlGa)As LD based on a MOVPE-grown
0022-0248/97/$17.00 Copyright © 1997 Elsevier Science B.V. All rights reserved PII S0022-0248(96)005 65-9
409
A. Malqg, W. Strupihski / Journal of Crystal Growth 170 (1997) 408-412
modified heterostructure with separate confinements controlled by thin, wide-gap barriers inserted at the interfaces of the active region and the cladding layers. The influence of the barriers on the heterostructure properties and the MOVPE (AlGa)As growth details are presented in Section 2. In Section 3 the theoretical expectations and experimental electrooptical characteristics of so obtained (AlGa)As LDs are shown to be in good agreement, giving low-6)± devices. Such a structure will be further termed a double-barrier multiquantum well (DBMQW) heterostructure (by analogy to earlier the DBDH structure [1 ]).
2. Heterostructure concept and MOVPE growth characteristics The heterostructure under consideration is shown in Fig. 1. The inserted barriers of thickness d b and dg thick graded (for smoothing-out band discontinuities) layers are doped in order to facilitate majority carriers transport. The role of the barriers is twofold. First, they improve the carrier confinement and the A1 content (x b) of 0.55 is sufficient for this goal [1]. Next, the influence of the barriers on the optical confinement can be considered as a competition of the index guiding and antiguiding effects at both interfaces of barrier layers. Predominance of antiguiding weakens the original waveguiding properties of the MQW region. It can be controlled by the barrier
/
DBMQW
Active Region
0,6
Xb
( A l G a ) A s Heterostructure
"
0 0.2 d~ = 7 nm dd= 4 nm d b = d g - l O nm d . = 30 nm
0.8 0.7
.=. 0.6 ~) O
0.5 0.4
p+
I
[ 3.5
p
db dadd dg 3.6
3.7
Distance [w~n]
n 3E1 / (Si)
5E1 7 (Zn)
n+ ~
318
doping type
0.3 0.2 0.1
0.0
cap
p - cladding
dc
active region
n - cladding
buffer layer
dc
Distance from Surface
[p,m]
Fig. l. Al-content profile of the investigated DBMQW (AlGa)As heterostructure and other importantdetails of the structure design.
banier MQW
region
barrier
Fig. 2. TEM and SEM microphotographsof the MOVPE-grown (AlGa)As DBMQW heterostructure under investigation.
thickness (db +dg) and height (Xb) as described elsewhere [7,8]. (AlGa)As DBMQW heterostructures were grown using an automatic controlled LP-MOVPE Aixtron system, model 200 R & D on a 2 inch (100) oriented GaAs substrate. The quartz reactor is of the horizontal type. A graphite susceptor coated with silicon carbide with IR heating enables the rotation of the substrate during the growth. The sources were Pdpurified hydrogen, 100% arsine (ASH3), trimethylgallium (TMGa), trimethylaluminum (TMAI), 2% silane (Sill 4) in H 2 and dimethylzinc (DMZn). A pressure in the range 20-100 mbar, a V / I I I ratio of 250 and a temperature of 680-720°C were applied for all experiments. Vent/run balancing was used during the entire growth in order not to introduce any flow oscillations around the composition of (AlxGa I ,)AsOne of the main problems in performing the proposed laser structure were large thickness differences among quantum wells (7 rim), interwell and outer inserted barriers (4 and 10 nm, respectively) and cladding layers (3.5 pLm). Optimization of the growth rate was necessary to obtain a heterostructure containing very thin and relatively thick layers during a reasonable time, with abrupt interfaces and thickness nonuniformity within 1-2 monolayers. The maximal growth rate for two-dimensional deposition was found basing on measurements of electrical parameters (Hall method), PL intensity and quantum
410
A. Malqg, W. Strupifiski / Journal oJ"Crystal Growth 170 (1997) 408 412
DBMQW
LD
~ .
--n-oro,,,o
TEo, TEo,
dg=t0nm dla=2Onm ~. = 0.86p,m
3 65 ' E 3.60
da = 7 nm dd:,nm db= I 0 nm
~> 3.55
7';:~,.%~r,
3.60
/ R = o.~
345
•
--
.~
3 35
t,
3"35J"/
// ,'/
i7
.....¢./
_....=~:i
I I ' '
l
,"7 \"
i
V,, \ ,
""~---
-
,>" 1.o D B M Q W
i deg deg
LD /"~
'~ 0.8
•
c --
0.6
04 ~, Z
N 0.4 o
Z
0.2
o.o
0.0
n- 3.30 3.25
|
-30 Distance from S u r f a c e
J
[
0.2
,~. ....
i
- - TEo. dg = 10 nm, Oj.= 11.8 ..... TEo, dg = 20 nm, e.L= 10.3
"~ ~ 0 . 4 0.6 '10
"~:~ °:' °:' 0:6"
--\',
i
=2Ohm
....
~.',
i
LlO
I ,'7 I t ' / °° , o > , 3.5oi .,'Y fJL ~'. , 0.6 0.8
(I)' //
/ O. = 7 cm-1
3.,0
-'rE d
/ ] j
i
. . .~ .
TEo Mode Intens,y Distributio,, ~;d.'g'~onol -
-20
,
,
,
,
,
-1 0
0
10
20
30
Angle [deg]
[p.m]
Fig. 3. Calculated fundamental transverse mode intensity profiles for two DBMQW structures (see Fig. 1) with graded barrier interface thicknesses of d~ = 10 and 20 nm. Details of the structure and assumed parameters of the laser cavity are given. The refractive index profile (including the free carriers contribution) for the DBMQW structure is shown for the d~ = 10 nm case.
well thickness (in the range 4 - 8 ML). Perfection of interfaces with regard to the chemical composition was optimized by growth interruption. Strong emphasis was put on diminishing the oxygen incorporation into the layers. TEM and SEM microphotographs of a so obtained heterostructure is shown in Fig. 2. Good interface quality and agreement between the nominal and actual thicknesses of all layers is seen.
Fig. 4. Calculated far-field intensity distributions perpendicular to the junction plane for the transverse mode profiles as in Fig. 2.
20 nm, respectively. A small difference in these O ± values is a result of the limitation by the cladding layers thickness [7]. According to the logarithmic gain-current model for M Q W lasers [9], the calculated Jth is 1.43 kA cm -2 for the laser cavity length L = 0.5 mm and Jth = 0.95 k A cm -2 for L = 1 mm, both for 4 - Q W structure of d a = 7 nm [9] and dg = 10 nm. These relatively high Jth values are due to the low F value characteristic for the D B M Q W heterostructure. Instead, high quantum efficiencies can be expected. Proton-isolated, 50 ~ m wide-stripe laser devices have been prepared, using sputtered CrPt p-contacts and evaporated AuGeNi n-contacts on n-GaAs substrates.
3. DBMQW laser diode: modeling and performance The influence of the theoretical barrier thickness on the TE 0 mode distribution is shown in Fig. 3. A very wide mode profile (due to weak guiding) is seen, but its further widening due to the increase of dg (clearly visible in the inset) is limited by the thickness of the cladding layers (dc = 3 I~m) [7]. The directional characteristics of the D B M Q W LDs are seen in Fig. 4, where the two far-field (FF) intensity distributions correspond to the mode profiles of Fig. 3. The calculated beam divergence F W H M values (denoted O l ) are 11.8 ° and 10.3 ° and the cumulative confinement factors (for all quantum wells) F are 0.027 and 0.021 for d g = 10 and
2.0
"~ O
o...
' . . . . . . . . . . . . . .
' ....
' ....
1.8 1.6
D B M Q W LDs pulsed 200 ns, 5 kHz,
1"4
W=50gm
' ....
/ /
'
R ~ / /
L=
1.2 1.0
~
=
= 0.5 • mm
0.8
0.6
0.75 mrn
.,..., 0.4 O 0.2 0.0
0.0
0.5
1.0
1.5
2.0
Diode Current
2.5
3.0
[A]
Fig. 5. A family of pulsed light-current characteristics for a set of DBMQW LDs of various cavity lengths (L). Stripe width W = 50 IJ,m.
A. Malqg, W. Strupihski /Journal of Crystal Growth 170 (1997) 408-412
,
558M :>,
8K2 H3
~-,,~.
1.0
Q~
_~ 0.8
P : o.sw,
.....
= 0.85 A =1.2A
P = 0.7 W, I= 1.55 A
L = 0.5 mm
W = C0 gm
ff
i~
"o
.~ 0.6
-40 -30
-20
-10
0
Angle
10
20
30 40
[deg]
Fig. 6. CCD camera recorded set of intensity distributions perpendicular to the junction plane (QFF patterns) of the light beam emitted under a pulse regime (200 ns, 5 kHz) at various optical power levels by one DBMQW LD.
411
of the DBMQW LDs recorded with a CCD camera (quasi far-field " Q F F " patterns [8]) are shown in Figs. 6 and 7. In Fig. 6 a set of QFF patterns for optical power levels ranging from 0.3 to 0.7 W (yet possible to record with one set of filters) emitted by the same LD is shown. The corresponding O 1 range of 14.5-16.1 ° indicates good beam stability but differs from the theoretically expected 12° for this particular DBMQW structure design. Fig. 7 shows that a lower O , of 12.9 ° (close to the theoretical value) has been obtained for longer laser cavities. This effect can be explained by the growing role of gain-guiding for short cavities in the presence of the weak index-guiding.
4. Conclusion A family of pulsed light-current ( L - I ) characteristics of the manufactured DBMQW LDs with uncoated mirrors for three laser cavity lengths (L) are shown in Fig. 5. The measured /th ( J t h ) values are 0.26 (2.1), 0.3 (1.2), 0.35 (0.93) A (kA cm 2) and external quantum efficiencies (from one mirror) r/ are 0.64, 0.57, 0.51 W / A for L = 0.25, 0.5, 0.75 mm, respectively. Note that the Jth values are below theoretical expectations, while the Jth dependence on L generally agrees with the logarithmic gain-current model. The rather weak dependence of 7/ on L is a proof of a small cavity loss and good internal quantum efficiency. Normalized far-field (FF) beam intensity distributions in the plane perpendicular to the junction plane
1.0
~
'~ 0.8
Popt : 0.7 W
_E
W = 50 gm
0.6
/ ..... k = 0.5 mm ~ L=O.75mm
~,
/!
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ft'
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Acknowledgements The authors wish to thank Mr. Jacek Ratajczak (lET) for TEM and SEM characterizations of heterostructures.
FWHM (deg) ~ , ! O.L I ~il)
= 0.4
Z
A modified DBMQW (AlGa)As laser heterostructure has been grown by MOVPE. The indication of the growth perfection is the agreement of the theoretically predicted and experimental characteristics of laser diodes based on this heterostructure. From the experimental devices presented the case of L of 0.75 mm is the most interesting because of the lowest O 1 (12.9 ° - close to theoretical 12°) and the lowest Jth of 0.93 kA c m - : while ~7 is still high (0.51 W / A from one uncovered facet). The Jth value below theoretical predictions can come from the uncertainty in the threshold model parameters for QW lasers, or from nonexact modeling. In general, the present 0 1 values are close to the lowest reported [3], but without any side maxima. Relatively high Jth are compensated by high quantum efficiency.
0.2 00
-40 -30
-20
-10
Angle
0 10 [deg]
20
30 40
Fig. 7. CCD camera recorded QFF patterns of the pulsed light beam emitted under constant optical power level by DBMQW LDs of various cavity lengths (L).
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412
A. Mal(lg, W. Strupifiski/Journal of Co'stal Growth 170 (1997) 408-412
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