- Email: [email protected]
Optimization of MOVPE growth for 650 nm-emitting VCSELs
Journal of Crystal Growth 221 (2000) 663 }667
Optimization of MOVPE growth for 650 nm-emitting VCSELs A. Bhattacharya, M. Zorn*, A. Oster, M. Nasarek, H. Wenzel, J. Sebastian, M. Weyers, G. TraK nkle Ferdinand-Braun-Institut fu( r Ho( chstfrequenztechnik, Albert-Einstein-Str. 11, D-12489 Berlin, Germany
Abstract This paper reports on the optimization of the growth of visible-wavelength vertical-cavity surface-emitting laser (VCSEL) diodes by metalorganic vapour-phase epitaxy (MOVPE). The VCSEL structure has an GaInP/AlGaInP quantum well active zone (AZ) sandwiched between AlGaAs/AlAs distributed Bragg re#ectors (DBRs). We present results on the optimization of the DBR re#ectivity and the electrical resistance of the p-DBR and discuss the switching sequence at the AZ to p-DBR interface which is critical due to the change of the group V component. Using these optimized parameters 640}655 nm emitting VCSELs could be demonstrated, with a minimum threshold current density of 2.8 kA/cm at 654 nm. 2000 Elsevier Science B.V. All rights reserved. PACS: 81.15.Gh; 81.05.Ea; 42.55.Px; 78.20.Ci; 78.66.Fd Keywords: MOVPE; Red VCSEL; Bragg re#ector
1. Introduction Vertical-cavity surface-emitting lasers (VCSELs) o!er distinct advantages over conventional edge emitting lasers, such as circular beam pro"les, dynamic single-frequency operation, ease of twodimensional array fabrication and possibility of wafer-scale testing [1]. While high-performance near-IR (In)GaAs/AlGaAs VCSELs are commercially available, heterostructure design for highperformance AlGaInP-based visible-wavelength devices (630}670 nm) is, however, considerably Present address: Solid States Electronics Group, Tata Institute of Fundamental Research, Homi Bhabha road, Mumbai400 005, India. * Corresponding author. Tel.: #49-30-6392-2676; fax: #4939-6392-2685. E-mail addresses: [email protected] (A. Bhattacharya), [email protected] (M. Zorn).
more challenging thus making visible VCSELs the focus of intensive research. High-performance visible-wavelength VCSELs (j&650 nm) would impact emerging technologies such as plastic}"berbased communication, high-density optical storage systems, and high-de"nition laser printing [2]. Visible VCSELs, however, raise particularly di$cult MOVPE growth issues. Due to the large number of distributed Bragg re#ector mirror pairs the layer stack is extremely thick (&8}9 lm). Maintaining tight control over the layer thickness and growth rate stability for the relatively long growth period is vital. Furthermore, integrating the AlGaInP-based optical cavity with the AlGaAs-based DBRs is often challenging, not only physically due to the large reversed band o!sets, but also from a crystal growth viewpoint [3]. This paper presents results on the in#uence of parameters like substrate misorientation, growth interrupts and doping on
0022-0248/00/$ - see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 0 0 ) 0 0 7 9 6 - X
664
A. Bhattacharya et al. / Journal of Crystal Growth 221 (2000) 663}667
the performance of high-re#ectivity Al Ga As/ V \V AlAs-based Bragg mirrors and VCSEL structures.
resistance (R "d
2. Experimental procedure
3. Results and discussion
All the epitaxial layer structures discussed in this work were grown in an Aixtron-200 low-pressure MOVPE system. The sources used include trimethylgallium (TMGa), trimethylaluminium (TMAl), trimethylindium (TMIn), arsine (AsH ) and phosphine (PH ). Dopants used were Si from Si H for n-type, and Zn from diethylzinc (DEZn) or intrinsically incorporated C by growth at low V/III ratios for p-type, respectively. The growth pressure was 70 mbar and typical growth temperature 7703C with epi-ready n> GaAs substrates being used. While a range of substrate misorientations was used to characterize the growth of the DBR layers, the VCSEL device structures were grown on (1 0 0) substrates misoriented 63 towards 111A. The AlGaAs/AlAs DBR mirrors were grown without any interrupts at the interfaces with growth rates of &10 As /s for AlGaAs and &5 As /s for AlAs. This was found to result in the best structural quality as evidenced from the superlattice X-ray rocking curves. Re#ectance spectra for the DBR mirrors and VCSEL structures were obtained relative to a standard dielectric mirror. The measured absolute re#ectivity was accurate to 0.5%. This was su$cient to quantify the position of relevant re#ectance features like position and width of the DBR mirror band across the wafer. For device characterization, the VCSEL layers were processed into mesa structures of 23}67 lm diameter using standard photolithography and dry-etching techniques. The mesa top has a 6-lmwide metallized ring that serves as the p-side contact and also de"nes the light emission window. To evaluate the electrical characteristics of individual DBR mirrors similar mesa structures were used, however without any window in the top metallization. Voltage/current and power/ current characteristics are measured at room temperature under pulsed conditions (pulsewidth : 500 ns, duty cycle : 1 : 500). The series resistance of the p-DBRs is characterized by the di!erential
Fig. 1 shows the schematic structure of a typical visible VCSEL. A central one-wavelength-thick optical cavity contains compressively-strained GaInP quantum wells (QW) embedded within an (Al Ga ) In P spacer layer. The cavity is surrounded by quarter-wave Al Ga As/AlAs V \V distributed Bragg re#ector (DBR) mirrors (55; n-type, 35;p-type) to ensure longitudinal con"nement of the laser "eld. At j"650 nm an Al concentration x*0.5 is necessary in the high-index layer for low absorption thus reducing the index contrast ratio and requiring a large number of DBR periods to achieve the necessary high re#ectivities. The active region was independently optimized via the growth of edge-emitting QW lasers with AlInP cladding layers. Such lasers demonstrated transparency current densities of 225, 170 and 160 A/cm/well for 1-, 2- and 3-QW lasers, respectively, similar to values reported in the literature [4]. 3.1. Distributed Bragg reyectors For the VCSEL shown in Fig. 1 the lower nDBR with 55 Al Ga As/AlAs pairs has a
Fig. 1. Schematic cross section of the VCSEL structure discussed in this paper.
A. Bhattacharya et al. / Journal of Crystal Growth 221 (2000) 663}667
Fig. 2. Dependence of DBR re#ectivity and surface roughness (measured by AFM) on number of Al Ga As/AlAs pairs for exact (1 0 0) and samples misoriented 63 towards 111A. For comparison the calculated re#ectivity is shown also.
calculated peak re#ectivity of more than 99.9%, while the upper p-DBR, through which the light is emitted, has 35 pairs corresponding to a re#ectivity of about 99.8%. Fig. 2 shows the measured peak re#ectivity data for a series of Al Ga As/AlAs based Bragg mirrors centered around the 650 nm wavelength region di!ering only in the number of mirror pairs but otherwise grown under identical conditions. Growth on exact (1 0 0)$0.53 oriented substrates, typically used for AlGaAs-based edgeemitting lasers, shows a deterioration of mirror quality with increasing number of mirror pairs. This manifests itself as a saturation in the peak re#ectivity with even 40 pair DBRs having only &95% re#ectivity. This is also seen in an increase in surface roughness and can be further correlated to broadening of satellite peaks in the X-ray rocking curve of the superlattices [5]. Growth on substrates misoriented 63 towards 11 1 12A o!er better performance, with the measured re#ectivity almost matching the calculated values. This is fortuitous, as the 63-misoriented substrates are also better suited for the AlGaInP-based active region [6]. The electrical resistance of the DBR mirrors is another important issue for VCSEL design. The large band o!sets at the heterobarriers between high- and low-index DBR layers form potential barriers for charge carriers. Due to the large e!ective mass of holes, this problem is more severe in the p-type DBR, and particularly so in visible
665
VCSELs with a large number of DBR pairs. Lowresistance DBR mirrors can be achieved by the use of alloy grading [7] at interfaces to #atten out barrier spikes at abrupt heterojunctions and by the additional doping at interfaces. In this work, the interfaces between the DBR layers are graded over a thickness of 10 nm. We have also compared ptype DBRs doped with carbon and zinc. While zinc is usually used as a p-type dopant for bulk AlGaAs layers carbon is a preferred dopant for the DBR structures due to its lower di!usivity in AlGaAs. For the high Al containing AlGaAs layers (50}100% Al) in our structure intrinsic carbon doping by a reduction in the V/III ratio is su$cient for p-doping in the range of 10}10 cm\ even at 7703C growth temperature. Fig. 3 shows the dependence of the di!erential resistance on the size of the mesas for three di!erent p-DBRs: (a) Zn-doped with abrupt interfaces, (b) C-doped with abrupt interfaces, and (c) C-doped with graded interfaces. As can be seen from the "gure the di!erential resistance decreases from 16 ) at a mesa size of 57 lm for the Zn-doped p-DBR (p"1;10 cm\) to 8 ) for the C-doped one (p"3;10 cm\ AlAs, p"7;10 cm\ AlGaAs) with graded interfaces. This decrease in resistivity of the p-DBRs is re#ected in the decrease in operating voltage for the complete VCSEL structures discussed later (see Fig. 5), where the best results are from devices employing C-doped p-DBRs with graded interfaces.
Fig. 3. Dependence of di!erential resistance on mesa size for p-DBRs with di!erent doping and interface grading.
666
A. Bhattacharya et al. / Journal of Crystal Growth 221 (2000) 663}667
Fig. 4. SEM pictures of VCSELs grown with di!erent switching sequences: (a) with standard switching sequence and (b) with improved switching sequence.
3.2. AlGaInP/AlGaAs interfaces The AlGaInP/AlGaAs interfaces between the DBR and the cavity are critical for optimal device performance. The complete group V switchover required here as well as problems due to In-segregation at the AlGaInP-to-AlGaAs interface can lead to the formation of defects and graded interface layers [8]. In our 1j cavity design the optical "eld has an antinode at the DBR/cavity interface and defects here can seriously impact device performance. We have investigated the in#uence of the residual strain as well as that of various switching sequences at the cavity/DBR interface. The critical interface is the cavity to upper DBR one. Here, a long growth interrupt (*0.5 s) under PH stabilization followed by a gas exchange leads to signi"cant In carryover into the upper layer (Fig. 4a). These In-rich regions disrupt the crystalline perfection of the upper DBR mirror. In the visible VCSEL structure this problem is further aggravated as the AlGaInP cavity is grown over relatively highly strained thick n-DBR layers. AlGaAs layers grown over bulk, lattice-matched AlGaInP are not so sensitive to a growth interrupt at the interface. Fig. 4b shows the specular surface morphology attained using optimal switching, where the TMAl, TMGa and As H are injected
into the reactor immediately after the growth of the AlGaInP layer. 3.3. Device structures VCSEL structures were processed into simple dry-etched mesa structures to achieve transverse optical and electrical con"nement. Fig. 5 shows the room-temperature threshold current density J as a function of wavelength for 57 lm mesas devices at various positions on the wafer. The
Fig. 5. Dependence of threshold current density, J , on wavelength for VCSELs with di!erent p-DBRs.
A. Bhattacharya et al. / Journal of Crystal Growth 221 (2000) 663}667
results presented here are for the three p-DBR doping pro"les discussed earlier: (a) Zn doping, (b) C doping with abrupt interfaces and (c) C doping with graded interfaces. As there is a relative shift across the wafer between the DBR stop-band and gain peak wavelength (&640 nm) de"ned by the MQW active region, devices of various wavelengths are available, e.g. from 653 nm at the wafer center down to 639 nm near the wafer edge in case (b). The sharp increase in the threshold current density with decreasing wavelength re#ects the current leakage out of the QW region due to the poorer con"nement at shorter wavelengths. As can be seen from the "gure, the C-doped VCSEL with graded interfaces has the lowest J , with values of 4 kA/cm at 650 nm, and 2.8 kA/cm at 654 nm. These values are reasonable given that the devices were fabricated by a simple mesa etch. Further improvements are expected from devices using selective oxidation for current con"nement. This has been shown to signi"cantly reduce the threshold current density compared to air-post or implanted devices [2].
4. Conclusion The optimization of the MOVPE growth for &650-nm-emitting VCSELs is discussed. High-re#ectivity AlGaAs/AlAs DBR mirrors could be grown on samples misoriented 63 towards 111A. In constrast, mirrors grown on exact (1 0 0) surfaces show a degradation in re#ectivity which can be correlated to a rougher surface morphology. The electrical resistance of the p-DBR mirror is minimum for carbon-doped mirrors with graded
667
regions between AlGaAs/AlAs layers. The switching sequence at the group V change between the active zone and the p-DBR is also critical to prevent defects at the interface. With these optimized conditions dry-etched, air-post VCSELs with a J of 2.8 kA/cm at 654 nm could be demonstrated.
Acknowledgements The authors would like to thank O. Fink for technical assistance with the MOVPE system. One of the authors (AB) is grateful to the Alexandervon-Humboldt foundation for a research fellowship. The work was supported by the Deutsche Forschungsgemeinschaft (DFG) under Contract Tr 357.
References [1] J.L. Jewell, J.P. Harbison, A. Scherer, Y.H. Lee, L.T. Florez, IEEE J. Quant. Electron. QE-27 (1991) 1332. [2] W.W. Chow, K.D. Choquette, M.H. Crawford, K.L. Lear, G.R. Hadley, IEEE J. Quant. Electron. QE-33 (1997) 1810. [3] R.P. Schneider Jr., J.A. Lott, K.L. Lear, K.D. Choquette, M.H. Crawford, S.P. Kilcoyne, J.J. Fiegel, J. Crystal Growth 145 (1994) 838. [4] D.P. Bour, R.S. Geels, D.W. Treat, T.L. Paoli, F. Ponce, R.L. Thornton, B.S. Krusor, R.D. Bringans, D.F. Welch, IEEE J. Quant. Electron. QE-30 (1994) 593. [5] M. Nasarek, Diplomarbeit, Humboldt Universitaet, Berlin, 2000. [6] R.P. Schneider Jr., R.P. Bryan, J.A. Lott, E.D. Jones, G.R. Olbright, J. Crystal Growth 124 (1992) 763. [7] K.L. Lear, R.P. Schneider Jr., Appl. Phys. Lett. 62 (1996) 605. [8] F. Bugge, G. Beister, G. Erbert, S. Gramlich, K. Vogel, U. Zeimer, M. Weyers, Inst. Phys. Conf. Ser. 155 (1997) 573.
Optimization of MOVPE growth for 650 nm-emitting VCSELs A. Bhattacharya, M. Zorn*, A. Oster, M. Nasarek, H. Wenzel, J. Sebastian, M. Weyers, G. TraK nkle Ferdinand-Braun-Institut fu( r Ho( chstfrequenztechnik, Albert-Einstein-Str. 11, D-12489 Berlin, Germany
Abstract This paper reports on the optimization of the growth of visible-wavelength vertical-cavity surface-emitting laser (VCSEL) diodes by metalorganic vapour-phase epitaxy (MOVPE). The VCSEL structure has an GaInP/AlGaInP quantum well active zone (AZ) sandwiched between AlGaAs/AlAs distributed Bragg re#ectors (DBRs). We present results on the optimization of the DBR re#ectivity and the electrical resistance of the p-DBR and discuss the switching sequence at the AZ to p-DBR interface which is critical due to the change of the group V component. Using these optimized parameters 640}655 nm emitting VCSELs could be demonstrated, with a minimum threshold current density of 2.8 kA/cm at 654 nm. 2000 Elsevier Science B.V. All rights reserved. PACS: 81.15.Gh; 81.05.Ea; 42.55.Px; 78.20.Ci; 78.66.Fd Keywords: MOVPE; Red VCSEL; Bragg re#ector
1. Introduction Vertical-cavity surface-emitting lasers (VCSELs) o!er distinct advantages over conventional edge emitting lasers, such as circular beam pro"les, dynamic single-frequency operation, ease of twodimensional array fabrication and possibility of wafer-scale testing [1]. While high-performance near-IR (In)GaAs/AlGaAs VCSELs are commercially available, heterostructure design for highperformance AlGaInP-based visible-wavelength devices (630}670 nm) is, however, considerably Present address: Solid States Electronics Group, Tata Institute of Fundamental Research, Homi Bhabha road, Mumbai400 005, India. * Corresponding author. Tel.: #49-30-6392-2676; fax: #4939-6392-2685. E-mail addresses: [email protected] (A. Bhattacharya), [email protected] (M. Zorn).
more challenging thus making visible VCSELs the focus of intensive research. High-performance visible-wavelength VCSELs (j&650 nm) would impact emerging technologies such as plastic}"berbased communication, high-density optical storage systems, and high-de"nition laser printing [2]. Visible VCSELs, however, raise particularly di$cult MOVPE growth issues. Due to the large number of distributed Bragg re#ector mirror pairs the layer stack is extremely thick (&8}9 lm). Maintaining tight control over the layer thickness and growth rate stability for the relatively long growth period is vital. Furthermore, integrating the AlGaInP-based optical cavity with the AlGaAs-based DBRs is often challenging, not only physically due to the large reversed band o!sets, but also from a crystal growth viewpoint [3]. This paper presents results on the in#uence of parameters like substrate misorientation, growth interrupts and doping on
0022-0248/00/$ - see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 0 0 ) 0 0 7 9 6 - X
664
A. Bhattacharya et al. / Journal of Crystal Growth 221 (2000) 663}667
the performance of high-re#ectivity Al Ga As/ V \V AlAs-based Bragg mirrors and VCSEL structures.
resistance (R "d
2. Experimental procedure
3. Results and discussion
All the epitaxial layer structures discussed in this work were grown in an Aixtron-200 low-pressure MOVPE system. The sources used include trimethylgallium (TMGa), trimethylaluminium (TMAl), trimethylindium (TMIn), arsine (AsH ) and phosphine (PH ). Dopants used were Si from Si H for n-type, and Zn from diethylzinc (DEZn) or intrinsically incorporated C by growth at low V/III ratios for p-type, respectively. The growth pressure was 70 mbar and typical growth temperature 7703C with epi-ready n> GaAs substrates being used. While a range of substrate misorientations was used to characterize the growth of the DBR layers, the VCSEL device structures were grown on (1 0 0) substrates misoriented 63 towards 111A. The AlGaAs/AlAs DBR mirrors were grown without any interrupts at the interfaces with growth rates of &10 As /s for AlGaAs and &5 As /s for AlAs. This was found to result in the best structural quality as evidenced from the superlattice X-ray rocking curves. Re#ectance spectra for the DBR mirrors and VCSEL structures were obtained relative to a standard dielectric mirror. The measured absolute re#ectivity was accurate to 0.5%. This was su$cient to quantify the position of relevant re#ectance features like position and width of the DBR mirror band across the wafer. For device characterization, the VCSEL layers were processed into mesa structures of 23}67 lm diameter using standard photolithography and dry-etching techniques. The mesa top has a 6-lmwide metallized ring that serves as the p-side contact and also de"nes the light emission window. To evaluate the electrical characteristics of individual DBR mirrors similar mesa structures were used, however without any window in the top metallization. Voltage/current and power/ current characteristics are measured at room temperature under pulsed conditions (pulsewidth : 500 ns, duty cycle : 1 : 500). The series resistance of the p-DBRs is characterized by the di!erential
Fig. 1 shows the schematic structure of a typical visible VCSEL. A central one-wavelength-thick optical cavity contains compressively-strained GaInP quantum wells (QW) embedded within an (Al Ga ) In P spacer layer. The cavity is surrounded by quarter-wave Al Ga As/AlAs V \V distributed Bragg re#ector (DBR) mirrors (55; n-type, 35;p-type) to ensure longitudinal con"nement of the laser "eld. At j"650 nm an Al concentration x*0.5 is necessary in the high-index layer for low absorption thus reducing the index contrast ratio and requiring a large number of DBR periods to achieve the necessary high re#ectivities. The active region was independently optimized via the growth of edge-emitting QW lasers with AlInP cladding layers. Such lasers demonstrated transparency current densities of 225, 170 and 160 A/cm/well for 1-, 2- and 3-QW lasers, respectively, similar to values reported in the literature [4]. 3.1. Distributed Bragg reyectors For the VCSEL shown in Fig. 1 the lower nDBR with 55 Al Ga As/AlAs pairs has a
Fig. 1. Schematic cross section of the VCSEL structure discussed in this paper.
A. Bhattacharya et al. / Journal of Crystal Growth 221 (2000) 663}667
Fig. 2. Dependence of DBR re#ectivity and surface roughness (measured by AFM) on number of Al Ga As/AlAs pairs for exact (1 0 0) and samples misoriented 63 towards 111A. For comparison the calculated re#ectivity is shown also.
calculated peak re#ectivity of more than 99.9%, while the upper p-DBR, through which the light is emitted, has 35 pairs corresponding to a re#ectivity of about 99.8%. Fig. 2 shows the measured peak re#ectivity data for a series of Al Ga As/AlAs based Bragg mirrors centered around the 650 nm wavelength region di!ering only in the number of mirror pairs but otherwise grown under identical conditions. Growth on exact (1 0 0)$0.53 oriented substrates, typically used for AlGaAs-based edgeemitting lasers, shows a deterioration of mirror quality with increasing number of mirror pairs. This manifests itself as a saturation in the peak re#ectivity with even 40 pair DBRs having only &95% re#ectivity. This is also seen in an increase in surface roughness and can be further correlated to broadening of satellite peaks in the X-ray rocking curve of the superlattices [5]. Growth on substrates misoriented 63 towards 11 1 12A o!er better performance, with the measured re#ectivity almost matching the calculated values. This is fortuitous, as the 63-misoriented substrates are also better suited for the AlGaInP-based active region [6]. The electrical resistance of the DBR mirrors is another important issue for VCSEL design. The large band o!sets at the heterobarriers between high- and low-index DBR layers form potential barriers for charge carriers. Due to the large e!ective mass of holes, this problem is more severe in the p-type DBR, and particularly so in visible
665
VCSELs with a large number of DBR pairs. Lowresistance DBR mirrors can be achieved by the use of alloy grading [7] at interfaces to #atten out barrier spikes at abrupt heterojunctions and by the additional doping at interfaces. In this work, the interfaces between the DBR layers are graded over a thickness of 10 nm. We have also compared ptype DBRs doped with carbon and zinc. While zinc is usually used as a p-type dopant for bulk AlGaAs layers carbon is a preferred dopant for the DBR structures due to its lower di!usivity in AlGaAs. For the high Al containing AlGaAs layers (50}100% Al) in our structure intrinsic carbon doping by a reduction in the V/III ratio is su$cient for p-doping in the range of 10}10 cm\ even at 7703C growth temperature. Fig. 3 shows the dependence of the di!erential resistance on the size of the mesas for three di!erent p-DBRs: (a) Zn-doped with abrupt interfaces, (b) C-doped with abrupt interfaces, and (c) C-doped with graded interfaces. As can be seen from the "gure the di!erential resistance decreases from 16 ) at a mesa size of 57 lm for the Zn-doped p-DBR (p"1;10 cm\) to 8 ) for the C-doped one (p"3;10 cm\ AlAs, p"7;10 cm\ AlGaAs) with graded interfaces. This decrease in resistivity of the p-DBRs is re#ected in the decrease in operating voltage for the complete VCSEL structures discussed later (see Fig. 5), where the best results are from devices employing C-doped p-DBRs with graded interfaces.
Fig. 3. Dependence of di!erential resistance on mesa size for p-DBRs with di!erent doping and interface grading.
666
A. Bhattacharya et al. / Journal of Crystal Growth 221 (2000) 663}667
Fig. 4. SEM pictures of VCSELs grown with di!erent switching sequences: (a) with standard switching sequence and (b) with improved switching sequence.
3.2. AlGaInP/AlGaAs interfaces The AlGaInP/AlGaAs interfaces between the DBR and the cavity are critical for optimal device performance. The complete group V switchover required here as well as problems due to In-segregation at the AlGaInP-to-AlGaAs interface can lead to the formation of defects and graded interface layers [8]. In our 1j cavity design the optical "eld has an antinode at the DBR/cavity interface and defects here can seriously impact device performance. We have investigated the in#uence of the residual strain as well as that of various switching sequences at the cavity/DBR interface. The critical interface is the cavity to upper DBR one. Here, a long growth interrupt (*0.5 s) under PH stabilization followed by a gas exchange leads to signi"cant In carryover into the upper layer (Fig. 4a). These In-rich regions disrupt the crystalline perfection of the upper DBR mirror. In the visible VCSEL structure this problem is further aggravated as the AlGaInP cavity is grown over relatively highly strained thick n-DBR layers. AlGaAs layers grown over bulk, lattice-matched AlGaInP are not so sensitive to a growth interrupt at the interface. Fig. 4b shows the specular surface morphology attained using optimal switching, where the TMAl, TMGa and As H are injected
into the reactor immediately after the growth of the AlGaInP layer. 3.3. Device structures VCSEL structures were processed into simple dry-etched mesa structures to achieve transverse optical and electrical con"nement. Fig. 5 shows the room-temperature threshold current density J as a function of wavelength for 57 lm mesas devices at various positions on the wafer. The
Fig. 5. Dependence of threshold current density, J , on wavelength for VCSELs with di!erent p-DBRs.
A. Bhattacharya et al. / Journal of Crystal Growth 221 (2000) 663}667
results presented here are for the three p-DBR doping pro"les discussed earlier: (a) Zn doping, (b) C doping with abrupt interfaces and (c) C doping with graded interfaces. As there is a relative shift across the wafer between the DBR stop-band and gain peak wavelength (&640 nm) de"ned by the MQW active region, devices of various wavelengths are available, e.g. from 653 nm at the wafer center down to 639 nm near the wafer edge in case (b). The sharp increase in the threshold current density with decreasing wavelength re#ects the current leakage out of the QW region due to the poorer con"nement at shorter wavelengths. As can be seen from the "gure, the C-doped VCSEL with graded interfaces has the lowest J , with values of 4 kA/cm at 650 nm, and 2.8 kA/cm at 654 nm. These values are reasonable given that the devices were fabricated by a simple mesa etch. Further improvements are expected from devices using selective oxidation for current con"nement. This has been shown to signi"cantly reduce the threshold current density compared to air-post or implanted devices [2].
4. Conclusion The optimization of the MOVPE growth for &650-nm-emitting VCSELs is discussed. High-re#ectivity AlGaAs/AlAs DBR mirrors could be grown on samples misoriented 63 towards 111A. In constrast, mirrors grown on exact (1 0 0) surfaces show a degradation in re#ectivity which can be correlated to a rougher surface morphology. The electrical resistance of the p-DBR mirror is minimum for carbon-doped mirrors with graded
667
regions between AlGaAs/AlAs layers. The switching sequence at the group V change between the active zone and the p-DBR is also critical to prevent defects at the interface. With these optimized conditions dry-etched, air-post VCSELs with a J of 2.8 kA/cm at 654 nm could be demonstrated.
Acknowledgements The authors would like to thank O. Fink for technical assistance with the MOVPE system. One of the authors (AB) is grateful to the Alexandervon-Humboldt foundation for a research fellowship. The work was supported by the Deutsche Forschungsgemeinschaft (DFG) under Contract Tr 357.
References [1] J.L. Jewell, J.P. Harbison, A. Scherer, Y.H. Lee, L.T. Florez, IEEE J. Quant. Electron. QE-27 (1991) 1332. [2] W.W. Chow, K.D. Choquette, M.H. Crawford, K.L. Lear, G.R. Hadley, IEEE J. Quant. Electron. QE-33 (1997) 1810. [3] R.P. Schneider Jr., J.A. Lott, K.L. Lear, K.D. Choquette, M.H. Crawford, S.P. Kilcoyne, J.J. Fiegel, J. Crystal Growth 145 (1994) 838. [4] D.P. Bour, R.S. Geels, D.W. Treat, T.L. Paoli, F. Ponce, R.L. Thornton, B.S. Krusor, R.D. Bringans, D.F. Welch, IEEE J. Quant. Electron. QE-30 (1994) 593. [5] M. Nasarek, Diplomarbeit, Humboldt Universitaet, Berlin, 2000. [6] R.P. Schneider Jr., R.P. Bryan, J.A. Lott, E.D. Jones, G.R. Olbright, J. Crystal Growth 124 (1992) 763. [7] K.L. Lear, R.P. Schneider Jr., Appl. Phys. Lett. 62 (1996) 605. [8] F. Bugge, G. Beister, G. Erbert, S. Gramlich, K. Vogel, U. Zeimer, M. Weyers, Inst. Phys. Conf. Ser. 155 (1997) 573.