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MBE growth of high-power InAsSbInAlAsSb quantum-well diode lasers emitting at 3.5 μm
ORYIITAL OJlIOWTH
Journal of Crystal Growth 175/176 (1997)825-832
ELSEVIER
MBE growth of high-power InAsSb/InA1AsSb quantum-well diode lasers emitting at 3.5 gm G.W. Turner*, M.J. Manfra, H.K. Choi, M.K. Connors Lincoln Laboratory, Massachusetts Institute of Technology, Lexington, Massachusetts 02173-9108, USA
Abstract
Molecular beam epitaxy (MBE) has been employed for the growth of strained quantum-well laser structures on InAs substrates. These lasers consist of compressively strained InAsSb wells, tensile-strained InA1AsSb barriers, and latticematched A1AsSb cladding layers. Broad-stripe lasers, with emission at wavelengths between 3.2 and 3.55 lam, have exhibited cw power of 215 mW/facet at 80 K, pulsed threshold current density as low as 30 A/cm 2 at 80 K, characteristic temperatures (To) between 30 and 40 K, and maximum pulsed operating temperature of 225 K. Ridge-waveguide lasers have cw threshold current of 12 mA at 100 K, and a maximum cw operating temperature of 175 K. In this paper we will present some of the key issues regarding the MBE growth of such high-power lasers on InAs and discuss future directions for improved device performance. PACS: 68.55.Bd; 42.55.Px
1. Introduction
A number of different approaches are actively being investigated with the goal of obtaining highperformance mid-infrared (3-5 I~m) diode lasers that are capable of high-power cw operation at room temperature. These include laser structures based on conventional electron-hole recombination in quantum-well (QW) and superlattice active regions of either Type I [1] or Type II I-2] band alignment, and quantum cascade structures which are based on unipolar intersubband transitions I-3].
* Corresponding author.
All of these approaches attempt to overcome limitations, such as nonradiative Auger recombination, that are increasingly important for roomtemperature operation of long-wavelength diode lasers. We have previously reported Type I multiple-quantum-well (MQW) diode lasers, grown by molecular beam epitaxy (MBE) on GaSb substrates, consisting of active regions of compressively strained InAsSb wells and tensile-strained InA1AsSb barriers, and lattice-matched A1AsSb cladding layers. Such lasers emitted at 3.9 rtm and operated cw up to 128 K I-4]. In this paper, we describe our investigation of similar strained M Q W diode lasers, based on active regions of InAsSb wells and InA1AsSb barriers, and A1AsSb cladding layers, but grown by MBE on
0022-0248/97/$17.00 Copyright © 1997 Published by ElsevierScienceB.V. All rights reserved PII S0022-0248(96)0 1 030- 5
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G.W. Turner et al. / Journal of Crystal Growth 175/176 (1997) 825-832
InAs substrates. These laser structures are grown with epitaxial compositions adjusted to (a) achieve the desired strains and Type I band alignments in the active region materials and (b) maintain lattice matching of the cladding layers to the InAs substrate. The reasons for investigating such QW structures on InAs substrates will be explained and the resultant performance of high-power, mid-infrared diode lasers will be discussed. We will also describe some of the MBE growth issues which were found to influence the characteristics of these longwavelength diode lasers.
2. Experimental procedure The InAsSb QW lasers were grown in a solidsource EPI Gen II 3 in MBE system, equipped with two Sb4 sources, an EPI valved As cracking source used to provide As2 flux, and conventional ion pumps. InAs(100) n-type substrates were prepared by either (a) solvent cleaning and chemical etching in a 4: 1 : 7 H2SO4 : H202 : H20 solution, followed by In-bonding for typical substrate material, or (b) directly loaded into a solderless block containing an A1203 backing plate for 2 in epi-ready substrate material. After careful UHV outgassing and oxide desorption of the substrates in an As2 flux, the
following layers were grown: n+-InAs buffer, ~ 2.0 ~tm n-A1Aso.16Sbo.84 lower cladding, MQW active region consisting of 150 A comporessively strained InAso.935Sbo.o65 wells and 300 A tensilestrained Ino.ssAlo.15Aso.9Sbo.1 barriers, ~ 2.0 rtm p-A1Aso.16Sb0.84 upper cladding, and 500A p÷GaSb cap layer. These values give approximately 0.45% strain in the wells and 0.27% strain in the barriers, with the strain-thickness product in wells and barriers set to approximately the same value in an attempt to achieve a strain-balanced active region. A cross-section of the laser structure is shown in Fig. 1. Be was used as the p-type dopant and GaTe was the source for Te as an n-type dopant. Hall measurements of nominally undoped InAsSb and InA1AsSb layers grown on semi-insulating GaAs substrates yielded n-type conductivity, as was observed for growth of the related alloys on GaSb. Since the active regions used for this work were undoped, the p-n junction was formed at the active-layer/upper-cladding interface. The two Sb4 effusion cells allowed some optimization of the III/V ratio for each alloy, when used in conjunction with the valved As2 source. The buffer and cladding layers were grown at ~ 0.8-0.9 ~tm/h, and the QW active regions were grown at slightly lower growth rate. The substrate temperature was controlled at 520°C for the lower cladding, 430°C for the QW
p+ GaSb P AIAS0.16Sb0.84
CAP
2.0 um
.........
n AIASo.16Sbo.84
2.0 pm
InAs BUFFER
0.5 pm
v.~
InAs SUBSTRATE
Fig. 1. Cross section of InAs Q W laser structure.
v. iT
-~.o
boo]
G.W. Turner et al. / Journal of Crystal Growth 175/176 (1997) 825-832
active region, and reduced to 490°C for the upper cladding and GaSb cap layer. As will be shown below, this reduction in upper cladding temperature was necessary to prevent degradation of the active regions. As previously reported [5], the necessity of precise control of composition, strain, and lattice parameter required a number of test growths and calibration procedures. After dismounting of the In-bonded samples, the In-alloyed region on the back side of the InAs wafer was removed by etching in HNO3, making sure that the wafer surface was protected during the etching process. Samples grown on the 2 in epiready substrates did not require this additional step, as there was no apparent degradation of the back side of the InAs substrate at the growth temperatures used. The surfaces of the InAs substrates were characterized by atomic force microscopy (AFM), and the epitaxial layers were characterized by photoluminescence spectroscopy, doublecrystal X-ray diffraction (DCXRD), and Auger microprobe chemical analysis. The complete MBEgrown laser wafers exhibited excellent surface morphology, and especially for the epi-ready 2 in substrates, very good yields of laser diodes with consistent characteristics were obtained across the entire wafer. To evaluate laser performance, broad-stripe lasers 100-250 gm wide were fabricated by using SiO2 patterning. For both n and p contacts, nonalloyed Ti/Pt/Au was used. Ridge-waveguide lasers, 8 gm wide, were fabricated from some wafers using reactive ion etching in a BCla/Ar plasma to define the ridges. Lasers were mounted junctionside up on Cu heat sinks using In, and loaded into a dewar for low-temperature I - V and L - I measurements.
3. Results
Because of the desire to grow complete laser structures on large-area, nonbonded substrates, epi-ready InAs material was evaluated by AFM. Fig. 2 shows a comparison between conventional InAs material and epi-ready material from two different vendors, with the respective samples taken directly from the vendor's packaging to the AFM
827
apparatus. The remarkable smoothness of the epi-ready InAs surface was also confirmed by a comparison of reflection high-energy electron diffraction images obtained before oxide desorption in the growth chamber. In the AFM image of the epi-ready samples there is some evidence of atomic structure on the as-received surface, and this structure was determined to be crystallographically oriented by AFM scanning in different directions. A key reason for the consideration of the growth of strained InAsSb/InAIAsSb QW active regions on InAs substrates is the presence of a large predicted miscibility gap for the InA1AsSb barrier material [6]. As shown in Fig. 3, changing from GaSb to InAs substrates increases the predicted region of stable growth for InA1AsSb from a maximum A1 content of less than 6% to approximately 15%. Increasing the A1 content in the InA1AsSb barrier material is important to improve both the conduction and the valence band offset in the active region, for improved carrier confinement and enhanced laser performance [7]. Although these theoretical predictions of regions of stable growth for the InA1AsSb alloy are strictly valid for equilibrium growth conditions, our previous experience with the nonequilibrium, MBE growth of InAsSb/InAIAsSb QWs on GaSb substrates confirmed the practical difficulty of increasing the AI content in this barrier alloy, while still maintaining high-quality QWs. As an indication of the quality of InAso.9asSbo.o6s/Ino.asAlo. 1sAso.9Sbo.1 strained MQWs that can be grown on InAs substrates by MBE, Fig. 4 shows a comparison of a 10-well test structure grown on an InAs buffer layer. The excellent agreement between the actual DCXRD data and the simulated diffraction curve shows high structural perfection in the MQW test structure. For these QW structures, the best results were obtained with an As2 interruption at each well/barrier interface. While the use of InAs substrates permits the growth of high-quality MQW active regions, the growth of the A1Aso.16Sbo.s4 lattice-matched cladding region becomes more difficult, since this alloy now contains ~ 2X as much arsenic as was the case for the lattice-matched cladding alloy grown on GaSb, and is near a region of
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G.W.. Turner et al. / Journal of Crystal Growth 175/176 (1997) 825-832
VENDOR A (standard surface)
rn/DlV Im/DlV IJm
VENDOR B (epi-ready surface)
pm
t.
rn/DlV ,=v.uvv nm/DIV
Fig. 2. Atomic force micrograph of conventional and epi-ready substrates.
alloy miscibility [-8]. In spite of this difficulty, highquality n- and p-type A1AsSb alloys were grown on InAs. In addition, as was found with the growth on similar structures on GaSb, the transition from Sb-dominated growth in the lower cladding to Asdominated growth in the active regions (and the reverse transition to the upper cladding) was best achieved by enhancing Sb-like interfaces and suppression of As-like interfaces in these transition regions. This observation has also been reported for the growth of InAs/A1Sb quantum wells [9-]. A more challenging problem with the growth of complete MQW laser structures on InAs was the degradation of the active region caused by the overgrowth of the upper cladding layer at the increased growth temperature necessary for the good-quality material. To investigate this effect, a MQW test structure, similar to that in Fig. 4, was
furnace annealed in a sealed ampoule for various times, and DCXRD scans were taken after the annealing. Fig. 5 shows the results of these experiments, where annealing at 530°C for 3 h (a typical growth time for the upper cladding) completely obliterated the satellite structure, while annealing at 510°C for 3 h resulted in less degradation of the satellite structure. A growth temperature of 490°C was chosen as a reasonable compromise between optimizing upper cladding layer electrical and optical quality and active layer degradation. Broad-stripe diode lasers fabricated from these structures showed emission near 3.4 lam, with pulsed current density as low as 30 A/cm 2 at 80 K, and characteristic temperatures in the range 3040 K. By measuring devices with cavity lengths of 500, 1000 and 1500 lam the internal quantum efficiency and internal loss coefficient are estimated to be
G.W. Turner et al. / Journal of Crystal Growth 175/176 (1997) 825-832
829
InSb
AISb
J
LATTICE MATCHED TO GaSb
~//nAs
AIAs LATrlCE MATCHED TO InAs
Fig. 3. Calculated alloy stability curves for In-A1-As-Sb system (from Ref. 6). The two dotted lines represent alloys matched to GaSb and InAs.
SIMULATION
lO,
{11~o~A'no.,A'o,,A'o..o~O.lO x 10
i
137 A InAso.93Sbo.o7 309 A Ino.M AIo.lSASo.9oSbo.lo InAs
103 _-DATA SIMULATION
I
-3000
-2000
0 1000 -1000 DIFFRACTION ANGLE (s)
2000
Fig. 4. D C X R D scan and simulation of InAsSb/InAlAsSb Q W test structure grown on InAs.
830
G.W. Turner et al. / Journal of Crystal Growth 175/176 (1997) 825-832
106
105
530"C
104 U) Z I11 I- 103
13h
510"C 13h
Z
CONTROL
102
-2111111
I
I
I
-1000
0
1000
DIFFRACTION A N G L E (S)
Fig. 5. DCXRD comparison of annealed QW test structures: (a) control sample, (b) after annealing at 510°C for 3 h, and (c) after annealing at 530°C for 3 h.
2,~
I
I
I
I
1
W=250pm 200
T=80K
_ L = 1500 pm
~ ~ 1 0 0
A
~
g ,,=, ~o 400 o. o
~
~
~
~
120
_
i ~ 140
-
150
1
2
I 3 4 CURRENT (A)
I S
Fig. 6. CW output power versus current for QW broad-area device at various temperatures.
63% and 9 cm- 1, respectively. Fig. 6 shows the cw power versus current of a 1500 pm device at several temperatures. At 80 K the maximum cw output power is 215 mW/facet, limited by junction heating. The maximum output power decreases with temperature, and at 150 K it is ~ 35 mW/facet. The operating voltage at the maximum power is ~ 4 V at 80 K. This surprisingly large operating voltage (more than 10 times the photon energy) is caused by the presence of large internal heterobarriers, as seen in the band offset diagram, Fig. 7. Substantially higher output power would be expected if these large operating voltages can be reduced by incorporating graded interfaces in the laser structure. Lasers with different active layer QW structures have been evaluated and emission wavelengths at 80 K of 3.21-3.38 pm have been observed. For a laser with 8 0 K emission at 3.21 pro, emission wavelength versus temperature is
G.W. Turner et al. / Journal of Crystal Growth 175/176 (1997) 825-832
T=0K
AIAs0.16Sb0.84 (EG~ 1.05) I t/~Ec' - 1.00
AEc - 1.22
831
i. . . . . .
}
InAs (EG- 0.42) LIEv - 0.01"(] I"
c~ 2pm
AEc" -
0.22
Ino.85A10.15As0.90Sb0.10 InAllo.93Sbo.07 (EG- 0.64) (EG- 0.36) I} ASv" - 0.07 " ..... I},~ev' - 0.01 I'l'~
200 - 300 A - - - - ~ - 100- 150 A
"1
Fig. 7. B a n d offset d i a g r a m o f I n A s S b / I n A l A s S b Q W l a s e r o n InAs,
3.6
A
E
=.
"I"
3,5
Fz LU .J I11
z
o_
3.4
W
3.3 50
I
I
I
100
150
200
TEMPERATURE
250
(K)
Fig. 8. Emission wavelength versus temperature for broad-area QW laser. shown in Fig. 8. At low temperatures the wavelength shifts at ,-, 1 nm/K, gradually increasing to ~ 2 n m / K at 220 K. Ridge-waveguide lasers 8 pm wide exhibited C W threshold current of 12 mA at 100 K and m a x i m u m cw operating temperature of 175 K. Additional details on laser performance are reported elsewhere [10].
4. Discussion
Impressive diode laser performance has been obtained at ,,- 3.5 pm with strained InAsSb/InAIAsSb M Q W structures grown on InAs. The use of InAs substrates has enabled the growth of InAIAsSb barriers with increased AI content, which in turn
832
G. W. Turner et al. /Journal of Crystal Growth 175/176 (1997) 825-832
lead to the development of improved Q W active regions. The use of InAs substrates, however, further complicated the growth of lattice-matched A1AsSb cladding layers. Although we have not fully optimized the present laser structures, we are confident that by incorporating graded regions to reduce heterobarrier offsets, we can obtain improved performance with reduced operating voltage. With these complex laser structures, careful attention to the growth issues such as strain and lattice matching, thermal effects, and chemistry at interfaces is required for optimum performance.
5. Summary We have demonstrated that MBE growth on InAs substrates is capable of producing high-performance InAsSb/InA1AsSb Q W diode lasers with emission near 3.5 gm. Further improvements in device structures should lead to even higher performance lasers.
Acknowledgements The authors would like to thank D.L. Spears for helpful discussions, D.R. Calawa, J.W. Chludzinski,
M.K. Connors, L.M. Eriksen, L. Krohn, W.L. McGilvary and P.M. Nitishin for technical assistance, and K.J. Challberg for manuscript editing. The authors would like to acknowledge R. Dawson and Y.-H. Zhang for the suggestion to evaluate epi-ready InAs wafers. This work was sponsored by the Phillips Laboratory, Department of the Air Force.
References [1] G.W.Turner, H.K. Choi and H.Q. Le, J. Vac. Sci. Technol. B 13 (1995) 699. [2] T.C.Hasenberg, D.H. Chow, A.R. Kost, R.H. Miles and L. West, Electron. Lett. 31 (1995) 275. [3] J. Faist, F. Capasso, D.L. Sivco,C. Sirtori, A.L. Hutchinson and A.Y. Cho, Science 264 (1994) 553. [41 H.K. Choi and G.W. Turner, J. Appl. Phys. Lett. 67 (1995) 332. [5"1 G.W. Turner, H.K. Choi, D.R. Calawa, J.V. Pantano and J.W. Chludzinski, J. Vac. Sci. Technol. B 12 (1994) 1266. [6"1 K. Onabe, NEC Res. Dev. 72 (1984) 1. [7"1 H.K. Choi and G.W. Turner, Phys. Scr., to be published. [.8"1 H.C. Casey and M.B. Panish, in: Heterostructure Lasers, Part B (AcademicPress, New York, 1978) p. 25. [9"1 G. Tuttle, H. Kroemer and J.H. English, J. Appl. Phys. 67 (1990) 3032. [10"1 H.K. Choi, G.W. Turner, M.J. Manfra and M.K. Connors, J. Appl. Phys. Lett 68 (1996)2936;H.K. Choi, G.W.Turner and M.J. Manfra, Electron. Lett. 32 (1996) 1296.
Journal of Crystal Growth 175/176 (1997)825-832
ELSEVIER
MBE growth of high-power InAsSb/InA1AsSb quantum-well diode lasers emitting at 3.5 gm G.W. Turner*, M.J. Manfra, H.K. Choi, M.K. Connors Lincoln Laboratory, Massachusetts Institute of Technology, Lexington, Massachusetts 02173-9108, USA
Abstract
Molecular beam epitaxy (MBE) has been employed for the growth of strained quantum-well laser structures on InAs substrates. These lasers consist of compressively strained InAsSb wells, tensile-strained InA1AsSb barriers, and latticematched A1AsSb cladding layers. Broad-stripe lasers, with emission at wavelengths between 3.2 and 3.55 lam, have exhibited cw power of 215 mW/facet at 80 K, pulsed threshold current density as low as 30 A/cm 2 at 80 K, characteristic temperatures (To) between 30 and 40 K, and maximum pulsed operating temperature of 225 K. Ridge-waveguide lasers have cw threshold current of 12 mA at 100 K, and a maximum cw operating temperature of 175 K. In this paper we will present some of the key issues regarding the MBE growth of such high-power lasers on InAs and discuss future directions for improved device performance. PACS: 68.55.Bd; 42.55.Px
1. Introduction
A number of different approaches are actively being investigated with the goal of obtaining highperformance mid-infrared (3-5 I~m) diode lasers that are capable of high-power cw operation at room temperature. These include laser structures based on conventional electron-hole recombination in quantum-well (QW) and superlattice active regions of either Type I [1] or Type II I-2] band alignment, and quantum cascade structures which are based on unipolar intersubband transitions I-3].
* Corresponding author.
All of these approaches attempt to overcome limitations, such as nonradiative Auger recombination, that are increasingly important for roomtemperature operation of long-wavelength diode lasers. We have previously reported Type I multiple-quantum-well (MQW) diode lasers, grown by molecular beam epitaxy (MBE) on GaSb substrates, consisting of active regions of compressively strained InAsSb wells and tensile-strained InA1AsSb barriers, and lattice-matched A1AsSb cladding layers. Such lasers emitted at 3.9 rtm and operated cw up to 128 K I-4]. In this paper, we describe our investigation of similar strained M Q W diode lasers, based on active regions of InAsSb wells and InA1AsSb barriers, and A1AsSb cladding layers, but grown by MBE on
0022-0248/97/$17.00 Copyright © 1997 Published by ElsevierScienceB.V. All rights reserved PII S0022-0248(96)0 1 030- 5
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G.W. Turner et al. / Journal of Crystal Growth 175/176 (1997) 825-832
InAs substrates. These laser structures are grown with epitaxial compositions adjusted to (a) achieve the desired strains and Type I band alignments in the active region materials and (b) maintain lattice matching of the cladding layers to the InAs substrate. The reasons for investigating such QW structures on InAs substrates will be explained and the resultant performance of high-power, mid-infrared diode lasers will be discussed. We will also describe some of the MBE growth issues which were found to influence the characteristics of these longwavelength diode lasers.
2. Experimental procedure The InAsSb QW lasers were grown in a solidsource EPI Gen II 3 in MBE system, equipped with two Sb4 sources, an EPI valved As cracking source used to provide As2 flux, and conventional ion pumps. InAs(100) n-type substrates were prepared by either (a) solvent cleaning and chemical etching in a 4: 1 : 7 H2SO4 : H202 : H20 solution, followed by In-bonding for typical substrate material, or (b) directly loaded into a solderless block containing an A1203 backing plate for 2 in epi-ready substrate material. After careful UHV outgassing and oxide desorption of the substrates in an As2 flux, the
following layers were grown: n+-InAs buffer, ~ 2.0 ~tm n-A1Aso.16Sbo.84 lower cladding, MQW active region consisting of 150 A comporessively strained InAso.935Sbo.o65 wells and 300 A tensilestrained Ino.ssAlo.15Aso.9Sbo.1 barriers, ~ 2.0 rtm p-A1Aso.16Sb0.84 upper cladding, and 500A p÷GaSb cap layer. These values give approximately 0.45% strain in the wells and 0.27% strain in the barriers, with the strain-thickness product in wells and barriers set to approximately the same value in an attempt to achieve a strain-balanced active region. A cross-section of the laser structure is shown in Fig. 1. Be was used as the p-type dopant and GaTe was the source for Te as an n-type dopant. Hall measurements of nominally undoped InAsSb and InA1AsSb layers grown on semi-insulating GaAs substrates yielded n-type conductivity, as was observed for growth of the related alloys on GaSb. Since the active regions used for this work were undoped, the p-n junction was formed at the active-layer/upper-cladding interface. The two Sb4 effusion cells allowed some optimization of the III/V ratio for each alloy, when used in conjunction with the valved As2 source. The buffer and cladding layers were grown at ~ 0.8-0.9 ~tm/h, and the QW active regions were grown at slightly lower growth rate. The substrate temperature was controlled at 520°C for the lower cladding, 430°C for the QW
p+ GaSb P AIAS0.16Sb0.84
CAP
2.0 um
.........
n AIASo.16Sbo.84
2.0 pm
InAs BUFFER
0.5 pm
v.~
InAs SUBSTRATE
Fig. 1. Cross section of InAs Q W laser structure.
v. iT
-~.o
boo]
G.W. Turner et al. / Journal of Crystal Growth 175/176 (1997) 825-832
active region, and reduced to 490°C for the upper cladding and GaSb cap layer. As will be shown below, this reduction in upper cladding temperature was necessary to prevent degradation of the active regions. As previously reported [5], the necessity of precise control of composition, strain, and lattice parameter required a number of test growths and calibration procedures. After dismounting of the In-bonded samples, the In-alloyed region on the back side of the InAs wafer was removed by etching in HNO3, making sure that the wafer surface was protected during the etching process. Samples grown on the 2 in epiready substrates did not require this additional step, as there was no apparent degradation of the back side of the InAs substrate at the growth temperatures used. The surfaces of the InAs substrates were characterized by atomic force microscopy (AFM), and the epitaxial layers were characterized by photoluminescence spectroscopy, doublecrystal X-ray diffraction (DCXRD), and Auger microprobe chemical analysis. The complete MBEgrown laser wafers exhibited excellent surface morphology, and especially for the epi-ready 2 in substrates, very good yields of laser diodes with consistent characteristics were obtained across the entire wafer. To evaluate laser performance, broad-stripe lasers 100-250 gm wide were fabricated by using SiO2 patterning. For both n and p contacts, nonalloyed Ti/Pt/Au was used. Ridge-waveguide lasers, 8 gm wide, were fabricated from some wafers using reactive ion etching in a BCla/Ar plasma to define the ridges. Lasers were mounted junctionside up on Cu heat sinks using In, and loaded into a dewar for low-temperature I - V and L - I measurements.
3. Results
Because of the desire to grow complete laser structures on large-area, nonbonded substrates, epi-ready InAs material was evaluated by AFM. Fig. 2 shows a comparison between conventional InAs material and epi-ready material from two different vendors, with the respective samples taken directly from the vendor's packaging to the AFM
827
apparatus. The remarkable smoothness of the epi-ready InAs surface was also confirmed by a comparison of reflection high-energy electron diffraction images obtained before oxide desorption in the growth chamber. In the AFM image of the epi-ready samples there is some evidence of atomic structure on the as-received surface, and this structure was determined to be crystallographically oriented by AFM scanning in different directions. A key reason for the consideration of the growth of strained InAsSb/InAIAsSb QW active regions on InAs substrates is the presence of a large predicted miscibility gap for the InA1AsSb barrier material [6]. As shown in Fig. 3, changing from GaSb to InAs substrates increases the predicted region of stable growth for InA1AsSb from a maximum A1 content of less than 6% to approximately 15%. Increasing the A1 content in the InA1AsSb barrier material is important to improve both the conduction and the valence band offset in the active region, for improved carrier confinement and enhanced laser performance [7]. Although these theoretical predictions of regions of stable growth for the InA1AsSb alloy are strictly valid for equilibrium growth conditions, our previous experience with the nonequilibrium, MBE growth of InAsSb/InAIAsSb QWs on GaSb substrates confirmed the practical difficulty of increasing the AI content in this barrier alloy, while still maintaining high-quality QWs. As an indication of the quality of InAso.9asSbo.o6s/Ino.asAlo. 1sAso.9Sbo.1 strained MQWs that can be grown on InAs substrates by MBE, Fig. 4 shows a comparison of a 10-well test structure grown on an InAs buffer layer. The excellent agreement between the actual DCXRD data and the simulated diffraction curve shows high structural perfection in the MQW test structure. For these QW structures, the best results were obtained with an As2 interruption at each well/barrier interface. While the use of InAs substrates permits the growth of high-quality MQW active regions, the growth of the A1Aso.16Sbo.s4 lattice-matched cladding region becomes more difficult, since this alloy now contains ~ 2X as much arsenic as was the case for the lattice-matched cladding alloy grown on GaSb, and is near a region of
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G.W.. Turner et al. / Journal of Crystal Growth 175/176 (1997) 825-832
VENDOR A (standard surface)
rn/DlV Im/DlV IJm
VENDOR B (epi-ready surface)
pm
t.
rn/DlV ,=v.uvv nm/DIV
Fig. 2. Atomic force micrograph of conventional and epi-ready substrates.
alloy miscibility [-8]. In spite of this difficulty, highquality n- and p-type A1AsSb alloys were grown on InAs. In addition, as was found with the growth on similar structures on GaSb, the transition from Sb-dominated growth in the lower cladding to Asdominated growth in the active regions (and the reverse transition to the upper cladding) was best achieved by enhancing Sb-like interfaces and suppression of As-like interfaces in these transition regions. This observation has also been reported for the growth of InAs/A1Sb quantum wells [9-]. A more challenging problem with the growth of complete MQW laser structures on InAs was the degradation of the active region caused by the overgrowth of the upper cladding layer at the increased growth temperature necessary for the good-quality material. To investigate this effect, a MQW test structure, similar to that in Fig. 4, was
furnace annealed in a sealed ampoule for various times, and DCXRD scans were taken after the annealing. Fig. 5 shows the results of these experiments, where annealing at 530°C for 3 h (a typical growth time for the upper cladding) completely obliterated the satellite structure, while annealing at 510°C for 3 h resulted in less degradation of the satellite structure. A growth temperature of 490°C was chosen as a reasonable compromise between optimizing upper cladding layer electrical and optical quality and active layer degradation. Broad-stripe diode lasers fabricated from these structures showed emission near 3.4 lam, with pulsed current density as low as 30 A/cm 2 at 80 K, and characteristic temperatures in the range 3040 K. By measuring devices with cavity lengths of 500, 1000 and 1500 lam the internal quantum efficiency and internal loss coefficient are estimated to be
G.W. Turner et al. / Journal of Crystal Growth 175/176 (1997) 825-832
829
InSb
AISb
J
LATTICE MATCHED TO GaSb
~//nAs
AIAs LATrlCE MATCHED TO InAs
Fig. 3. Calculated alloy stability curves for In-A1-As-Sb system (from Ref. 6). The two dotted lines represent alloys matched to GaSb and InAs.
SIMULATION
lO,
{11~o~A'no.,A'o,,A'o..o~O.lO x 10
i
137 A InAso.93Sbo.o7 309 A Ino.M AIo.lSASo.9oSbo.lo InAs
103 _-DATA SIMULATION
I
-3000
-2000
0 1000 -1000 DIFFRACTION ANGLE (s)
2000
Fig. 4. D C X R D scan and simulation of InAsSb/InAlAsSb Q W test structure grown on InAs.
830
G.W. Turner et al. / Journal of Crystal Growth 175/176 (1997) 825-832
106
105
530"C
104 U) Z I11 I- 103
13h
510"C 13h
Z
CONTROL
102
-2111111
I
I
I
-1000
0
1000
DIFFRACTION A N G L E (S)
Fig. 5. DCXRD comparison of annealed QW test structures: (a) control sample, (b) after annealing at 510°C for 3 h, and (c) after annealing at 530°C for 3 h.
2,~
I
I
I
I
1
W=250pm 200
T=80K
_ L = 1500 pm
~ ~ 1 0 0
A
~
g ,,=, ~o 400 o. o
~
~
~
~
120
_
i ~ 140
-
150
1
2
I 3 4 CURRENT (A)
I S
Fig. 6. CW output power versus current for QW broad-area device at various temperatures.
63% and 9 cm- 1, respectively. Fig. 6 shows the cw power versus current of a 1500 pm device at several temperatures. At 80 K the maximum cw output power is 215 mW/facet, limited by junction heating. The maximum output power decreases with temperature, and at 150 K it is ~ 35 mW/facet. The operating voltage at the maximum power is ~ 4 V at 80 K. This surprisingly large operating voltage (more than 10 times the photon energy) is caused by the presence of large internal heterobarriers, as seen in the band offset diagram, Fig. 7. Substantially higher output power would be expected if these large operating voltages can be reduced by incorporating graded interfaces in the laser structure. Lasers with different active layer QW structures have been evaluated and emission wavelengths at 80 K of 3.21-3.38 pm have been observed. For a laser with 8 0 K emission at 3.21 pro, emission wavelength versus temperature is
G.W. Turner et al. / Journal of Crystal Growth 175/176 (1997) 825-832
T=0K
AIAs0.16Sb0.84 (EG~ 1.05) I t/~Ec' - 1.00
AEc - 1.22
831
i. . . . . .
}
InAs (EG- 0.42) LIEv - 0.01"(] I"
c~ 2pm
AEc" -
0.22
Ino.85A10.15As0.90Sb0.10 InAllo.93Sbo.07 (EG- 0.64) (EG- 0.36) I} ASv" - 0.07 " ..... I},~ev' - 0.01 I'l'~
200 - 300 A - - - - ~ - 100- 150 A
"1
Fig. 7. B a n d offset d i a g r a m o f I n A s S b / I n A l A s S b Q W l a s e r o n InAs,
3.6
A
E
=.
"I"
3,5
Fz LU .J I11
z
o_
3.4
W
3.3 50
I
I
I
100
150
200
TEMPERATURE
250
(K)
Fig. 8. Emission wavelength versus temperature for broad-area QW laser. shown in Fig. 8. At low temperatures the wavelength shifts at ,-, 1 nm/K, gradually increasing to ~ 2 n m / K at 220 K. Ridge-waveguide lasers 8 pm wide exhibited C W threshold current of 12 mA at 100 K and m a x i m u m cw operating temperature of 175 K. Additional details on laser performance are reported elsewhere [10].
4. Discussion
Impressive diode laser performance has been obtained at ,,- 3.5 pm with strained InAsSb/InAIAsSb M Q W structures grown on InAs. The use of InAs substrates has enabled the growth of InAIAsSb barriers with increased AI content, which in turn
832
G. W. Turner et al. /Journal of Crystal Growth 175/176 (1997) 825-832
lead to the development of improved Q W active regions. The use of InAs substrates, however, further complicated the growth of lattice-matched A1AsSb cladding layers. Although we have not fully optimized the present laser structures, we are confident that by incorporating graded regions to reduce heterobarrier offsets, we can obtain improved performance with reduced operating voltage. With these complex laser structures, careful attention to the growth issues such as strain and lattice matching, thermal effects, and chemistry at interfaces is required for optimum performance.
5. Summary We have demonstrated that MBE growth on InAs substrates is capable of producing high-performance InAsSb/InA1AsSb Q W diode lasers with emission near 3.5 gm. Further improvements in device structures should lead to even higher performance lasers.
Acknowledgements The authors would like to thank D.L. Spears for helpful discussions, D.R. Calawa, J.W. Chludzinski,
M.K. Connors, L.M. Eriksen, L. Krohn, W.L. McGilvary and P.M. Nitishin for technical assistance, and K.J. Challberg for manuscript editing. The authors would like to acknowledge R. Dawson and Y.-H. Zhang for the suggestion to evaluate epi-ready InAs wafers. This work was sponsored by the Phillips Laboratory, Department of the Air Force.
References [1] G.W.Turner, H.K. Choi and H.Q. Le, J. Vac. Sci. Technol. B 13 (1995) 699. [2] T.C.Hasenberg, D.H. Chow, A.R. Kost, R.H. Miles and L. West, Electron. Lett. 31 (1995) 275. [3] J. Faist, F. Capasso, D.L. Sivco,C. Sirtori, A.L. Hutchinson and A.Y. Cho, Science 264 (1994) 553. [41 H.K. Choi and G.W. Turner, J. Appl. Phys. Lett. 67 (1995) 332. [5"1 G.W. Turner, H.K. Choi, D.R. Calawa, J.V. Pantano and J.W. Chludzinski, J. Vac. Sci. Technol. B 12 (1994) 1266. [6"1 K. Onabe, NEC Res. Dev. 72 (1984) 1. [7"1 H.K. Choi and G.W. Turner, Phys. Scr., to be published. [.8"1 H.C. Casey and M.B. Panish, in: Heterostructure Lasers, Part B (AcademicPress, New York, 1978) p. 25. [9"1 G. Tuttle, H. Kroemer and J.H. English, J. Appl. Phys. 67 (1990) 3032. [10"1 H.K. Choi, G.W. Turner, M.J. Manfra and M.K. Connors, J. Appl. Phys. Lett 68 (1996)2936;H.K. Choi, G.W.Turner and M.J. Manfra, Electron. Lett. 32 (1996) 1296.