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AlAsSb distributed Bragg mirrors on InP
Superlattices and Microstructures, Vol. 31, No. 6, 2002 doi:10.1006/spmi.2002.1046 Available online at http://www.idealibrary.com on
A comparative study of AlGaAsSb/AlAsSb distributed Bragg mirrors on InP D. O. T OGINHO F ILHO , I. F. L. D IAS , J. L. D UARTE , S. A. L OURENÇO , L. C. P OÇAS , E. L AURETO Departamento de Física, Universidade Estadual de Londrina, CP6001, CEP 86051-970 Londrina-Paraná, Brazil
J. C. H ARMAND CNRS URA 250, Laboratoire CDP, 196 rue H. Ravera, 92220 Bagneux, France (Received 23 October 2001)
AlGaAsSb/AlAsSb Bragg mirrors lattice matched on InP with six pairs of layers were grown by molecular beam epitaxy. The effect of Te doping on the electrical and optical properties of the Bragg mirrors, and the presence of digital alloy gradient layers between the ternary and quaternary layers are analysed. The presence of digital alloy layers at the interfaces reduces the electrical resistance through the perpendicular direction of the Bragg mirror, without significantly affecting the reflectivity. c 2002 Elsevier Science Ltd. All rights reserved.
Key words: semiconductors, electrical conductance, reflectance, Bragg mirror.
1. Introduction The distributed Bragg mirror (DBR) is a key component in optoelectronic devices such as vertical-cavity surface emitting lasers (VCSEL). In VCSEL devices, a mirror with reflectivity exceeding 99% and low electrical resistance is required for n-type and p-type doped Bragg stacks [1]. High reflectivity is achieved by using material systems with a high refractive index contrast (1n). However, material systems with high 1n also present a high band discontinuity (1Eg) which is not favourable to the current flow through the heterointerfaces. A condition for low electrical resistance is a low discontinuity in the conduction band (1Ec) for the n-type Bragg stack and in the valence band (1Ev) for the p-type Bragg stack. To overcome these contradictory conditions (high 1Eg for high reflectivities and low 1Eg for low electrical resistance), high doping levels are generally used in the quarterwave Bragg stack to lower the electrical resistance. On the other hand, high doping levels induce free carrier absorption which lowers reflectivity. A compromise between the high reflectivity condition and the lower electrical resistance can be expected by adjusting the doping level. Material systems with low 1Ec and 1Ev can decrease the electrical resistance but demand a great number of quarterwave pairs. Thus, thick structures can bring difficulties to both growth and processing technologies. An alternative method to reduce growth complexity involves preparing modified heterointerfaces including composition grading and/or modulation doping [2–4]. Finally, to build a DBR, a material system lattice matched on InP is highly recommended. 0749–6036/02/060277 + 07
$35.00/0
c 2002 Elsevier Science Ltd. All rights reserved.
278
Superlattices and Microstructures, Vol. 31, No. 6, 2002
It has not been established yet which semiconductor systems are the most suitable for DBRs in a 1.3–1.55 µm VCSELs field. Several III–V semiconductor materials which satisfy some of the DBRs manufacturing conditions have been experimented on InP substrates. Material systems such as InGaAsP/InP [5, 6], AlGaInAs/AlInAs [7, 8] and AlGaAsSb/AlAsSb [9–13] have been used in light-emitting devices. Among these systems, the antimonide alloys exhibit the highest refractive index contrast [14, 15]. The high refractive index contrast of the AlGaAsSb/AlAsSb layers in the Bragg stacks makes it possible to obtain a high reflectivity for a small number of quarterwave layers (20.5 pairs versus 41.5 pairs for the InGaAsP/InP system [10]). At the same time, the AlGaAsSb/AlAsSb system presents a higher voltage drop/pair than other systems such as AlGaInAs/AlInAs and InGaAsP/InP [10]. However, it is important to emphasize that the maximum reflectivity of Te-doped AlGaAsSb/AlAsSb Bragg mirrors is comparable to the reflectivity of an undoped mirror with the same structure [16]. Studies of the AlGaAsSb/AlAsSb Bragg mirrors on InP founded in the literature emphasize the growth of alloys by molecular beam epitaxy (MBE) [12–14, 16, 18, 19], and the analysis of the optical [12, 14, 18, 19], structural [12, 14] and electrical [9, 16] properties of Bragg mirrors. More recent studies have also described the VCSELs processing and test based on the Bragg mirrors [8, 20–22]. However, these studies dealt with Bragg mirrors with an abrupt interface between the ternary and quaternary alloy. In this paper we present a comparative study carried out on the electrical and optical characteristics of undoped and n-type doped Bragg mirrors prepared with the AlGaAsSb/AlAsSb system lattice matched on InP designed for 1.55 µm. A digital alloy gradient interface was prepared in one sample to find an alternative that could contribute to the current flow through the heterointerfaces.
2. Experimental details Te-doped AlGaAsSb/AlAsSb Bragg mirrors were grown by MBE on a (100) InP-n substrate for the perpendicular measurements and an undoped AlGaAsSb/AlAsSb Bragg mirror was grown on a (100) InP:Fe substrate. Bragg mirrors consist of 6.5 pairs of AlGaAsSb/AlAsSb layers with 1115 and 1308 Å (undoped) and 1083 and 1271 Å (Te doped) nominal thickness, respectively. Quaternary and ternary layers from the 17Q36 and 17Q44 samples were prepared with lower thickness to take into consideration the effects of Te doping on the refractive index of these materials. Material compositions were chosen in order to achieve transparency at 1.55 µm. The quaternary and ternary layers were Te doped at the ∼3.0 × 1018 cm−3 and ∼1.8 × 1018 cm−3 doping level, respectively. Te is the most common n-type dopant found in antimonides, and high doping levels can be obtained by using an Sb2 Te3 source. Carriers in the Bragg mirrors were determined by Hall measurements, at room temperature, using the Van der Paw method. The n-type doped Bragg mirrors were metallized with Ti/Au ohmic contacts evaporated on the top and on the bottom of the sample. These metallized samples were patterned with 100 × 100 µm2 square mesas etched by an H2 SO4 :H2 O2 :H2 O solution. After being etched, the actual contact area was determined by scanning electron microscopy. The substrate underside was thinned to 150 µm to decrease the substrate contribution to the electrical measurements. The current–voltage (I –V ) dc characteristics are made by transmission line method (TLM) measurements. Photoluminescence was made at a temperature of 2 K using the 441.6 nm line of a He–Cd laser as an excitation source. The spectral analysis of luminescence was carried out by a 0.5 m monochromator and detected by a liquid N2 cooled Ge photodetector, using a standard lock-in technique. Experimental reflectivity was measured with a Fourier transform infrared spectrometer (FTIR) in the so-called VW configuration [17]. This configuration provided an absolute measurement of the reflectivity and the real shape of the reflectivity spectrum with quantitative values at a 0.2% level of accuracy. Details of the sample parameters such as alloy composition, nominal layer thickness, doping levels, I–V and reflectivity characteristics can be seen in Table 1.
Superlattices and Microstructures, Vol. 31, No. 6, 2002
279
Table 1: Parameters of samples, I –V and reflectivity characteristics.
Sample
Materials
Alloy Doping Alloy Layer Drop pair Reflectivity Stop composition level gradient thickness (mV) % Band x y (1018 cm−3 ) (Å) (µm)
17Q29 Alx Ga1−x As y Sb1−y 0.12 0.51 AlAs y Sb1−y — 0.56
—
No
1115 1308
—
0.910
0.39
17Q36 Alx Ga1−x As y Sb1−y 0.12 0.51 AlAs y Sb1−y — 0.56
3.0 1.8
Yes
1083 1271
62
0.885
0.37
17Q44 Alx Ga1−x As y Sb1−y 0.12 0.51 AlAs y Sb1−y — 0.56
3.0 1.8
No
1083 1271
138
0.905
0.37
Fig. 1. Energy band diagram of one period of the AlGaAsSb/AlAsSb system with simple heterointerface (upper) and AlGaAsSb/AlAsSb with digital alloy gradient heterointerfaces (lower) are presented.
3. Results and discussions The energy band diagram for one period AlGaAsSb/AlAsSb Bragg mirrors, with simple heterointerfaces, and with digital alloy gradient interfaces, is shown in Fig. 1. The thickness of the barrier (AlAsSb) and quantum wells (AlGaAsSb) forming the digital alloy gradient interface is chosen to improve the flow of carriers through the heterointerface between the AlGaAsSb and AlAsSb layers. Figure 2 shows the photoluminescence spectrum obtained at 2 K for all samples. The non-doped sample 17Q29 shows a strong transition at 929 meV with a narrow full width half maximum (FWHM) of 11 meV.
280
Superlattices and Microstructures, Vol. 31, No. 6, 2002 0.928 eV
T = 2K
Intensity (a.u.)
17Q29 17Q36 17Q44 10X
5X
0.75
0.80
0.85
0.90
0.95
1.00
1.05
Energy (eV) Fig. 2. Photoluminescence spectrum from the quaternary cap layer AlGaAsSb of the Bragg mirror, at 2 K.
400
400
17Q36 17Q44
Current (mA)
200
200
0
0
-200
-200
-400
-400
-1500
-1000
-500
0
500
1000
1500
Voltage (mV) Fig. 3. I –V characteristics for the AlGaAsSb/AlAsSb simple heterointerfaces and AlGaAsSb/AlAsSb with digital alloy gradient.
This transition is due to the excitonic recombination in quaternary layers. The Te-doped samples 17Q44 (simple heterointerfaces) and 17Q36 (digital alloy gradient heterointerfaces) show broad bands whose line shape can be adjusted by three gaussian curves (not shown here). The positions of PL peaks are 899, 931 and 953 meV for sample 17Q44 and 931, 960 and 985 meV for sample 17Q36. Results from this study also show that the recombination mechanisms are the same in the two Te-doped samples since the energy
Superlattices and Microstructures, Vol. 31, No. 6, 2002
281 Wavelength (µm)
2.2
2
1.8
1.6 1.56λ m
1.0
1.4 1.50λ m
1.2
1.46λ m
17Q29 17Q36 17Q44
Reflectivity
0.8
0.6
0.4
0.2
0.6
0.7
0.8
0.9
1.0
1.1
Energy (eV) Fig. 4. Experimental reflectivity due to energy for the AlGaAsSb/AlAsSb Bragg mirror.
differences, relative intensity, and the FWHM of gaussian curves are similar. This discussion, however, is not the objective of this work, and a more systematic analysis of this subject will be carried out in a future paper [23]. The current versus voltage characteristics, perpendicular to the interface, including contributions from the contact and substrate resistances, in the two Te-doped samples, are shown in Fig. 3. The voltage drop per period is listed in Table 1. At a current density of 1 KA cm−2 a voltage drop per period of 138 mV in the sample with simple interfaces and of 62 mV in the sample with digital alloy gradient heterointerface was observed. Next, the presence of a digital alloy heterointerfaces reduces the voltage drop per period by a factor of two. The serial resistance found in DBR is due mainly to the potential barriers in the ternary–quaternary layers [9]. The transport properties in AlGaAsSb/AlAsSb interfaces are dominated by the 0-X barrier height due to different characteristics in the lowest conduction band valleys of the AlGaAsSb (direct 0-valley) and AlAsSb (indirect X-valley) layers. The conduction band offset between AlGaAsSb and AlAsSb materials is approximately 0.467 eV (0-X) and an important contribution of the tunnelling current in these heterointerfaces was expected [10]. Then the quantum well structure forming the digital alloy gradient heterointerface supported the flow of carriers through the interfaces between the different semiconductor materials via tunnelling mechanisms. However, it is necessary to get the best conditions in the growth parameters to obtain a smaller voltage drop per period. If, on the one hand, the quite large offset between the AlGaAsSb and AlAsSb materials increases the serial resistance, on the other hand, it increases the refractive index to a high value [10]. This high refractive index contrast provides a DBR with high reflectivity and wide stopband with a relatively low number of quarterwave layers. The experimental reflectivity measurements are shown in Fig. 4. Results also show that, even for a relatively small number of periods, it is possible to obtain a relatively high reflectivity and a wide stopband. The highest reflectivity and widest stopband were obtained for the non-doped Bragg mirror.
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Superlattices and Microstructures, Vol. 31, No. 6, 2002
The reflectivity decreases in the Te-doped sample with simple interface and is lower in the Te-doped sample with a digital alloy gradient interface. This decrease in reflectivity is associated with the increase in absorption due to the Te presence and the disturbance introduced in the reflection of the light by the digital alloy gradient heterointerfaces with quantum wells. Displacement in the reflectivity maximum observed in other samples demonstrates that they were not obtained under optimized growth conditions for doped samples.
4. Conclusions Results from this study show that the system AlGaAsSb/AlAsSb should be considered in the preparation of Bragg mirrors for VCSELs. However, a more systematic work in the preparation of samples to obtain optimized growth conditions is needed. Acknowledgements—We acknowledge the financial support by the Brazilian agencies CAPES, CNPq and FBB.
References [1] T. Anan, H. Shimomura, and S. Sugou, Electron. Lett. 30, 2138 (1994). [2] M. Sugimoto, H. Kosaka, K. Kurihara, I. Ogura, T. Numai, and K. Kasahara, Electron. Lett. 28, 385 (1992). [3] T. E. Sale (ed.), Vertical Cavity Surface Emitting Lasers (Research Studies Press Ltd, John Wiley & Sons, 1995). [4] M. G. Peters, B. J. Thibeault, D. B. Young, J. W. Scott, F. H. Peters, A. C. Gossard, and L. A. Coldren, Appl. Phys. Lett. 63, 3411 (1993). [5] P. Salet, P. Pagnod-Rossiaux, F. Gaborit, A. Plais, and J. Jacquet, Electron. Lett. 33, 1145 (1997). [6] Y. Ohiso, R. Iga, K. Kishi, and C. Amano, Electron. Lett. 36, 39 (2000). [7] I. F. L. Dias, J. L. Duarte, E. Laureto, R. V. Gelamo, E. A. Menezes, and J. C. Harmand, Superlatt. Microstruct. 28, 29 (2000). [8] G. Almuneau, E. Hall, S. Nakagawa, J. K. Kim, D. Lofgreen, O. Sjolund, C. Luo, D. R. Clarke, J. H. English, and L. A. Coldren, J. Vac. Sci. Technol. B18, 1601 (2000). [9] I. F. L. Dias, B. Nabet, A. Kohl, and J. C. Harmand, Electron. Lett. 33, 716 (1997). [10] I. F. L. Dias, B. Nabet, A. Kohl, J. L. Benchimol, and J. C. Harmand, IEEE Photonics Technol. Lett. 10, 763 (1998). [11] G. Almuneau, E. Hall, T. Mukaihara, S. Nakagawa, C. Y. Luo, D. R. Clarke, and L. A. Coldren, Photonics Technol. Lett. 12, 1322 (2000). [12] F. Genty, G. Almuneau, N. Bertru, L. Chusseau, P. Grech, D. Cot, and J. Jacquet, J. Cryst. Growth 183, 15 (1998). [13] G. Almuneau, E. Hall, S. Mathis, and L. A. Coldren, J. Cryst. Growth 208, 113 (2000). [14] B. Lambert, Y. Toudic, Y. Rouillard, M. Gauneau, M. Baudet, F. Alard, I. Valiente, and J. C. Simon, Appl. Phys. Lett. 66, 442 (1995). [15] O. Blum, I. J. Fritz, L. R. Dawson, A. J. Howard, T. J. Headley, J. F. Klem, and T. J. Drummond, Appl. Phys. Lett. 66, 329 (1995). [16] J. C. Harmand, A. Kohl, H. Juhel, and G. LeRoux, J. Cryst. Growth 175, 372 (1997). [17] J. M. Palmer, The measurements of transmission, absorption, emission, and reflexion, Handbook of Optics Part-II (McGraw-Hill, New York, 1995). [18] O. Blum, I. J. Fritz, L. R. Dawson, and T. J. Drummond, Electron. Lett. 31, 1247 (1995). [19] J. C. Harmand, F. Jeannès, G. Le Roux, and M. Juhel, Electron. Lett. 31, 1689 (1995). [20] J. C. Harmand et al., J. Cryst. Growth 201/202, 837 (2000).
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[21] E. Hall, S. Nagakawa, G. Almuneau, J. K. Kin, and L. A. Coldren, Electron. Lett. 36, 1465 (2000). [22] S. Nagakawa, E. Hall, G. Almuneau, J. K. Kim, D. A. Buell, H. Kroemer, and L. A. Coldren, IEEE J. Selected Topics Quantum Electron. 7, 224 (2001). [23] D. O. Toginho Filho and I. F. L. Dias (in press).
A comparative study of AlGaAsSb/AlAsSb distributed Bragg mirrors on InP D. O. T OGINHO F ILHO , I. F. L. D IAS , J. L. D UARTE , S. A. L OURENÇO , L. C. P OÇAS , E. L AURETO Departamento de Física, Universidade Estadual de Londrina, CP6001, CEP 86051-970 Londrina-Paraná, Brazil
J. C. H ARMAND CNRS URA 250, Laboratoire CDP, 196 rue H. Ravera, 92220 Bagneux, France (Received 23 October 2001)
AlGaAsSb/AlAsSb Bragg mirrors lattice matched on InP with six pairs of layers were grown by molecular beam epitaxy. The effect of Te doping on the electrical and optical properties of the Bragg mirrors, and the presence of digital alloy gradient layers between the ternary and quaternary layers are analysed. The presence of digital alloy layers at the interfaces reduces the electrical resistance through the perpendicular direction of the Bragg mirror, without significantly affecting the reflectivity. c 2002 Elsevier Science Ltd. All rights reserved.
Key words: semiconductors, electrical conductance, reflectance, Bragg mirror.
1. Introduction The distributed Bragg mirror (DBR) is a key component in optoelectronic devices such as vertical-cavity surface emitting lasers (VCSEL). In VCSEL devices, a mirror with reflectivity exceeding 99% and low electrical resistance is required for n-type and p-type doped Bragg stacks [1]. High reflectivity is achieved by using material systems with a high refractive index contrast (1n). However, material systems with high 1n also present a high band discontinuity (1Eg) which is not favourable to the current flow through the heterointerfaces. A condition for low electrical resistance is a low discontinuity in the conduction band (1Ec) for the n-type Bragg stack and in the valence band (1Ev) for the p-type Bragg stack. To overcome these contradictory conditions (high 1Eg for high reflectivities and low 1Eg for low electrical resistance), high doping levels are generally used in the quarterwave Bragg stack to lower the electrical resistance. On the other hand, high doping levels induce free carrier absorption which lowers reflectivity. A compromise between the high reflectivity condition and the lower electrical resistance can be expected by adjusting the doping level. Material systems with low 1Ec and 1Ev can decrease the electrical resistance but demand a great number of quarterwave pairs. Thus, thick structures can bring difficulties to both growth and processing technologies. An alternative method to reduce growth complexity involves preparing modified heterointerfaces including composition grading and/or modulation doping [2–4]. Finally, to build a DBR, a material system lattice matched on InP is highly recommended. 0749–6036/02/060277 + 07
$35.00/0
c 2002 Elsevier Science Ltd. All rights reserved.
278
Superlattices and Microstructures, Vol. 31, No. 6, 2002
It has not been established yet which semiconductor systems are the most suitable for DBRs in a 1.3–1.55 µm VCSELs field. Several III–V semiconductor materials which satisfy some of the DBRs manufacturing conditions have been experimented on InP substrates. Material systems such as InGaAsP/InP [5, 6], AlGaInAs/AlInAs [7, 8] and AlGaAsSb/AlAsSb [9–13] have been used in light-emitting devices. Among these systems, the antimonide alloys exhibit the highest refractive index contrast [14, 15]. The high refractive index contrast of the AlGaAsSb/AlAsSb layers in the Bragg stacks makes it possible to obtain a high reflectivity for a small number of quarterwave layers (20.5 pairs versus 41.5 pairs for the InGaAsP/InP system [10]). At the same time, the AlGaAsSb/AlAsSb system presents a higher voltage drop/pair than other systems such as AlGaInAs/AlInAs and InGaAsP/InP [10]. However, it is important to emphasize that the maximum reflectivity of Te-doped AlGaAsSb/AlAsSb Bragg mirrors is comparable to the reflectivity of an undoped mirror with the same structure [16]. Studies of the AlGaAsSb/AlAsSb Bragg mirrors on InP founded in the literature emphasize the growth of alloys by molecular beam epitaxy (MBE) [12–14, 16, 18, 19], and the analysis of the optical [12, 14, 18, 19], structural [12, 14] and electrical [9, 16] properties of Bragg mirrors. More recent studies have also described the VCSELs processing and test based on the Bragg mirrors [8, 20–22]. However, these studies dealt with Bragg mirrors with an abrupt interface between the ternary and quaternary alloy. In this paper we present a comparative study carried out on the electrical and optical characteristics of undoped and n-type doped Bragg mirrors prepared with the AlGaAsSb/AlAsSb system lattice matched on InP designed for 1.55 µm. A digital alloy gradient interface was prepared in one sample to find an alternative that could contribute to the current flow through the heterointerfaces.
2. Experimental details Te-doped AlGaAsSb/AlAsSb Bragg mirrors were grown by MBE on a (100) InP-n substrate for the perpendicular measurements and an undoped AlGaAsSb/AlAsSb Bragg mirror was grown on a (100) InP:Fe substrate. Bragg mirrors consist of 6.5 pairs of AlGaAsSb/AlAsSb layers with 1115 and 1308 Å (undoped) and 1083 and 1271 Å (Te doped) nominal thickness, respectively. Quaternary and ternary layers from the 17Q36 and 17Q44 samples were prepared with lower thickness to take into consideration the effects of Te doping on the refractive index of these materials. Material compositions were chosen in order to achieve transparency at 1.55 µm. The quaternary and ternary layers were Te doped at the ∼3.0 × 1018 cm−3 and ∼1.8 × 1018 cm−3 doping level, respectively. Te is the most common n-type dopant found in antimonides, and high doping levels can be obtained by using an Sb2 Te3 source. Carriers in the Bragg mirrors were determined by Hall measurements, at room temperature, using the Van der Paw method. The n-type doped Bragg mirrors were metallized with Ti/Au ohmic contacts evaporated on the top and on the bottom of the sample. These metallized samples were patterned with 100 × 100 µm2 square mesas etched by an H2 SO4 :H2 O2 :H2 O solution. After being etched, the actual contact area was determined by scanning electron microscopy. The substrate underside was thinned to 150 µm to decrease the substrate contribution to the electrical measurements. The current–voltage (I –V ) dc characteristics are made by transmission line method (TLM) measurements. Photoluminescence was made at a temperature of 2 K using the 441.6 nm line of a He–Cd laser as an excitation source. The spectral analysis of luminescence was carried out by a 0.5 m monochromator and detected by a liquid N2 cooled Ge photodetector, using a standard lock-in technique. Experimental reflectivity was measured with a Fourier transform infrared spectrometer (FTIR) in the so-called VW configuration [17]. This configuration provided an absolute measurement of the reflectivity and the real shape of the reflectivity spectrum with quantitative values at a 0.2% level of accuracy. Details of the sample parameters such as alloy composition, nominal layer thickness, doping levels, I–V and reflectivity characteristics can be seen in Table 1.
Superlattices and Microstructures, Vol. 31, No. 6, 2002
279
Table 1: Parameters of samples, I –V and reflectivity characteristics.
Sample
Materials
Alloy Doping Alloy Layer Drop pair Reflectivity Stop composition level gradient thickness (mV) % Band x y (1018 cm−3 ) (Å) (µm)
17Q29 Alx Ga1−x As y Sb1−y 0.12 0.51 AlAs y Sb1−y — 0.56
—
No
1115 1308
—
0.910
0.39
17Q36 Alx Ga1−x As y Sb1−y 0.12 0.51 AlAs y Sb1−y — 0.56
3.0 1.8
Yes
1083 1271
62
0.885
0.37
17Q44 Alx Ga1−x As y Sb1−y 0.12 0.51 AlAs y Sb1−y — 0.56
3.0 1.8
No
1083 1271
138
0.905
0.37
Fig. 1. Energy band diagram of one period of the AlGaAsSb/AlAsSb system with simple heterointerface (upper) and AlGaAsSb/AlAsSb with digital alloy gradient heterointerfaces (lower) are presented.
3. Results and discussions The energy band diagram for one period AlGaAsSb/AlAsSb Bragg mirrors, with simple heterointerfaces, and with digital alloy gradient interfaces, is shown in Fig. 1. The thickness of the barrier (AlAsSb) and quantum wells (AlGaAsSb) forming the digital alloy gradient interface is chosen to improve the flow of carriers through the heterointerface between the AlGaAsSb and AlAsSb layers. Figure 2 shows the photoluminescence spectrum obtained at 2 K for all samples. The non-doped sample 17Q29 shows a strong transition at 929 meV with a narrow full width half maximum (FWHM) of 11 meV.
280
Superlattices and Microstructures, Vol. 31, No. 6, 2002 0.928 eV
T = 2K
Intensity (a.u.)
17Q29 17Q36 17Q44 10X
5X
0.75
0.80
0.85
0.90
0.95
1.00
1.05
Energy (eV) Fig. 2. Photoluminescence spectrum from the quaternary cap layer AlGaAsSb of the Bragg mirror, at 2 K.
400
400
17Q36 17Q44
Current (mA)
200
200
0
0
-200
-200
-400
-400
-1500
-1000
-500
0
500
1000
1500
Voltage (mV) Fig. 3. I –V characteristics for the AlGaAsSb/AlAsSb simple heterointerfaces and AlGaAsSb/AlAsSb with digital alloy gradient.
This transition is due to the excitonic recombination in quaternary layers. The Te-doped samples 17Q44 (simple heterointerfaces) and 17Q36 (digital alloy gradient heterointerfaces) show broad bands whose line shape can be adjusted by three gaussian curves (not shown here). The positions of PL peaks are 899, 931 and 953 meV for sample 17Q44 and 931, 960 and 985 meV for sample 17Q36. Results from this study also show that the recombination mechanisms are the same in the two Te-doped samples since the energy
Superlattices and Microstructures, Vol. 31, No. 6, 2002
281 Wavelength (µm)
2.2
2
1.8
1.6 1.56λ m
1.0
1.4 1.50λ m
1.2
1.46λ m
17Q29 17Q36 17Q44
Reflectivity
0.8
0.6
0.4
0.2
0.6
0.7
0.8
0.9
1.0
1.1
Energy (eV) Fig. 4. Experimental reflectivity due to energy for the AlGaAsSb/AlAsSb Bragg mirror.
differences, relative intensity, and the FWHM of gaussian curves are similar. This discussion, however, is not the objective of this work, and a more systematic analysis of this subject will be carried out in a future paper [23]. The current versus voltage characteristics, perpendicular to the interface, including contributions from the contact and substrate resistances, in the two Te-doped samples, are shown in Fig. 3. The voltage drop per period is listed in Table 1. At a current density of 1 KA cm−2 a voltage drop per period of 138 mV in the sample with simple interfaces and of 62 mV in the sample with digital alloy gradient heterointerface was observed. Next, the presence of a digital alloy heterointerfaces reduces the voltage drop per period by a factor of two. The serial resistance found in DBR is due mainly to the potential barriers in the ternary–quaternary layers [9]. The transport properties in AlGaAsSb/AlAsSb interfaces are dominated by the 0-X barrier height due to different characteristics in the lowest conduction band valleys of the AlGaAsSb (direct 0-valley) and AlAsSb (indirect X-valley) layers. The conduction band offset between AlGaAsSb and AlAsSb materials is approximately 0.467 eV (0-X) and an important contribution of the tunnelling current in these heterointerfaces was expected [10]. Then the quantum well structure forming the digital alloy gradient heterointerface supported the flow of carriers through the interfaces between the different semiconductor materials via tunnelling mechanisms. However, it is necessary to get the best conditions in the growth parameters to obtain a smaller voltage drop per period. If, on the one hand, the quite large offset between the AlGaAsSb and AlAsSb materials increases the serial resistance, on the other hand, it increases the refractive index to a high value [10]. This high refractive index contrast provides a DBR with high reflectivity and wide stopband with a relatively low number of quarterwave layers. The experimental reflectivity measurements are shown in Fig. 4. Results also show that, even for a relatively small number of periods, it is possible to obtain a relatively high reflectivity and a wide stopband. The highest reflectivity and widest stopband were obtained for the non-doped Bragg mirror.
282
Superlattices and Microstructures, Vol. 31, No. 6, 2002
The reflectivity decreases in the Te-doped sample with simple interface and is lower in the Te-doped sample with a digital alloy gradient interface. This decrease in reflectivity is associated with the increase in absorption due to the Te presence and the disturbance introduced in the reflection of the light by the digital alloy gradient heterointerfaces with quantum wells. Displacement in the reflectivity maximum observed in other samples demonstrates that they were not obtained under optimized growth conditions for doped samples.
4. Conclusions Results from this study show that the system AlGaAsSb/AlAsSb should be considered in the preparation of Bragg mirrors for VCSELs. However, a more systematic work in the preparation of samples to obtain optimized growth conditions is needed. Acknowledgements—We acknowledge the financial support by the Brazilian agencies CAPES, CNPq and FBB.
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