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Fabrication of low-threshold red VCSELs
Materials Science in Semiconductor Processing 3 (2000) 517–521
Fabrication of low-threshold red VCSELs T.M. Calvert*, J.D. Lambkin, B. Corbett, A.F. Phillips, G.M. Crean National Microelectronics Research Centre, University College Cork, Cork, Ireland
Abstract Red vertical cavity lasers (VCSELs) are ideally suited as optical sources for plastic optical fibre networks. These networks will form the communication backbone of future automobiles and aircraft replacing the current copper networks. However VCSELs at these wavelengths are difficult to realise due to the lower refractive index offsets and unsuitable band structure alignments. The Esprit BREDSELS project has addressed the design, growth and fabrication of low threshold red VCSELs. This paper reports the fabrication of record low threshold (200 mA at 208C) VCSELs at a wavelength of 665 nm. This device performance has been achieved through the use of selective oxidation techniques. The devices operate CW to 508C. The trade off between low threshold and high power will be discussed. # 2001 Elsevier Science Ltd. All rights reserved. Keywords: VCSEL; Visible; Selective oxidation
1. Introduction The structure of a VCSEL gives the device many advantages over edge emitting lasers. In general VCSELs are characterised by single longitudinal mode operation, circularly symmetric Gaussian beam profiles with divergence of less than 108 and low-threshold currents. A major advantage of these devices is the ability to test devices on wafer avoiding the expensive process of facet cleaving and packaging. As AlGaInP VCSELs are pushed to even shorter wavelengths they increasingly suffer from high threshold currents, low powers and poor thermal stability. This behaviour is caused by several factors. The top p-doped AlGaAs DBR mirror presents a high series resistance due to the large valence band offset and also causes a significant degree of self absorption of the lasing emission due to the band tailing in the heavily doped Bragg layers. These factors give rise to strong Ohmic heating and poor thermal impedance, in addition the bonding of the laser with the p mirror up further increases the thermal impedance of the packaged laser. The active region material (InGaP/AlGaInP) gives rise
*Corresponding author. E-mail address: [email protected] (T.M. Calvert).
to further problems due the relatively small band edge discontinuities associated with this material resulting in high leakage currents and low-quantum efficiencies particularly at shorter wavelengths. Up to the early 1990’s transverse optical and electrical confinement in VCSELs was either achieved using etched mesa index guided structures or proton-implanted gain-guided structures. However the introduction of selective oxidation has brought about considerable improvements in VCSEL performance. Selective oxidation is carried out by etching a mesa into the structure exposing the edges of a high Al fraction AlGaAs layer. By exposing this layer to water vapour at 380–4308C the high-fraction AlGaAs is converted into its native oxide forming a current aperture around the active region. These current apertures provide welldefined current paths with strong index guiding and avoids the problems of sidewall non-radiative recombination emission or implantation damage associated with air post or proton-implanted devices, respectively.
2. VCSEL structure and fabrication A schematic diagram of the VCSEL structure used in this study is shown in Fig. 1. The active region contains four quantum wells at the centre of a 1 l cavity. The top
1369-8001/00/$ - see front matter # 2001 Elsevier Science Ltd. All rights reserved. PII: S 1 3 6 9 - 8 0 0 1 ( 0 0 ) 0 0 0 7 7 - 9
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T.M. Calvert et al. / Materials Science in Semiconductor Processing 3 (2000) 517–521
bond tracks to each VCSEL the sidewall profile of the mesa needs to be gentle enough to allow the metal tracks to run up to the contact rings. The depth of the p-type stack rules out reactive ion etching as this results in a sidewall with too steep a profile. After the etching, the high Al fraction AlGaAs layer in the p-DBR was selectively oxidised in an N2/H2O atmosphere at 4008C. This oxidation process reduces the current aperture to 6 mm for the single mode low-threshold devices. The surface was passivated with a SiN film and the contact rings and apertures were opened up. The apertures were protected by resist and the devices patterned for bond pads and tracks. These were deposited by e-beam evaporation. A five level Au/Ge/Au/Ni/Au contact was evaporated on the backside of the GaAs substrate as the n-type contact. Finally the devices were annealed at 4208C for 10 min to form the Ohmic contacts. Fig. 1. Device structure.
3. Selective oxidation The selective oxidation process involves etching the p-type stack to reveal the high Al fraction AlGaAs layer
Fig. 2. Selective oxidation test structure.
distributed Bragg reflector (DBR) consists of 35 pairs of Al0.95Ga0.05As/Al0.5Ga0.5As carbon doped p-type quarter-wave stacks with a Al0.98Ga0.02As layer as the selective oxidation layer. The bottom n-type Bragg mirror consists of 44.5 pairs of Al0.95Ga0.05As/Al0.5Ga0.5As quarter-wave stacks. A graded AlGaAs layer was used between the high and low contrast mirrors. The material for this device was grown by Epitaxial Products International. To fabricate the VCSEL devices, Ti–Pt–Au annular contacts were deposited onto the material by e-beam evaporation as the p-ohmic contact. A mesa structure was formed by wet etching through the p-type stack using a selective etch that stops on the active region. The etchant used for defining the mesas was 1 : 1 : 1 : 1 H3PO4 : H2SO4 : H2O2 : H2O which selectively etches AlGaAs but not AlGaInP. Due to the need for
Fig. 3. Oxidation calibration curve using test structure.
Fig. 4. Variation of oxidation depth dependent on layer doping.
T.M. Calvert et al. / Materials Science in Semiconductor Processing 3 (2000) 517–521
which is then oxidised by heating in steam, formed by bubbling nitrogen gas through hot water. Considerable care is needed to achieve a reproducible result. Our system consists of a water bubbler immersed in a silicon oil bath that is maintained at the set temperature by means of a feedback thermometer. Nitrogen is feed through the bubbler by means of a flow controller and into a low mass furnace, where the sample is oxidised. Calibration of the system is carried out by oxidising a test structure prior to oxidation of the device. This test
519
structure, as shown in Fig. 2, contains three layers of different AlGaAs and doping concentrations. Fig. 3 shows a calibration graph of oxidation distance against oxidation time for samples containing 99% AlGaAs and AlAs at a furnace temperature of 4008C and water temperatures of 85 and 958C. It has been observed during these tests that not only does the oxidation rate depend on the usual factors (percentage of Al in the sample, nitrogen flow rate, furnace and water bath temperature) but also on the doping concentration in the
Fig. 5. LI measurements on a single-moded device (Ith ¼ 0:2 mA).
Fig. 6. Spectral measurements of a single-moded device as a function of drive current.
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T.M. Calvert et al. / Materials Science in Semiconductor Processing 3 (2000) 517–521
oxidation layer. The SEM micrograph in Fig. 4 shows this effect. This micrograph shows the various oxidation depths of the layers in the test structure. Note that while
‘‘a’’ is of the same composition as ‘‘b’’ (both 98% AlGaAs) it has a higher doping and has not oxidised in as far as the lower doped layer. The same is true for ‘‘c’’ and ‘‘d’’ which are 99% AlGaAs layers, again the higher doped layer has not oxidised in as far as the lower doped layer. This effect has been observed in VCSEL structures containing multiple high Al fraction AlGaAs layers as well. Work is currently under way to fully characterise this effect.
4. Results
Fig. 7. Confinement of the optical mode by the current aperture in a single-moded device. (a) At threshold; (b) above threshold (1:3 Ith ).
Electrical and optical power measurements made on devices with small current confining apertures (6 mm) have shown them to be single moded with peak powers of 0.2 mW at 208C. Typical LI measurements of a singlemoded device at temperatures between 5 and 508C is shown in Fig. 5. These devices have good wavelength stability as a function of drive current (Fig. 6). The width of the current confining aperture created by the selective oxidation process is crucial in determining if the device will have a single transverse mode or if it will be multi moded. Optical micrographs, shown in Fig. 7, taken of the single moded device at threshold and just above threshold show the mode being confined by the current aperture (6 mm). To determine the importance of the current aperture in the mode confinement a second batch of devices was oxidised for a shorter time to give devices with a current aperture of 10 mm. LI and spectral measurements on these devices revealed them to be multi-moded with an optical output power over twice that of the single-moded devices (0.5 mW as against 0.2 mW at 208C). A typical LI of the multi-moded
Fig. 8. LI measurements on a multi-moded device (Ith ¼ 0:5 mA) at 208C.
T.M. Calvert et al. / Materials Science in Semiconductor Processing 3 (2000) 517–521
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Fig. 9. Spectral measurements on a multimode device (Ith ¼ 0:5 mA) as a function of drive current.
devices is shown in Fig. 8. Spectral measurements (Fig. 9) show the presence of the modes above threshold (0.5 mA).
5. Conclusions We have successfully fabricated single-moded visible VCSELs emitting at 665 nm with thresholds of 0.2 mA at room temperature, the lowest thresholds reported to date at this wavelength. These devices work CW up to 508C. Powers in excess of 0.5 mW have been achieved for devices with thresholds below 1 mA. For comparison Choquette et al. [1] have produced visible VCSELs emitting at 670 nm with thresholds of 0.6 mA at room temperature. The depth of selective oxidation determines the optical mode properties of the device, VCSELs with current apertures of 6 mm being singlemoded while devices with apertures greater than this being multi-moded. This results in a trade off between low threshold and high power. For a device to have a low threshold it must have a small and well confined
current aperture which results in a single moded device and therefore a lower optical output power than highthreshold devices with several transverse modes and thus more power. We have also shown that the selective oxidation rate is dependent on the doping profile in the oxidation layer with oxidation occurring faster in layers with a low (2.5e17) doping concentration and slower in layers containing a higher (1e18) doping concentration.
Acknowledgements This work was supported by the EU long term research scheme project 23455 (BREDSELS). The authors would like to thank the project partners for useful discussion.
Reference [1] Choquette KD, Schneider RP, Hagerott Crawford M, Geib KM, Figiel JJ. CLEO ’95 CPD5-1, Baltimore.
Fabrication of low-threshold red VCSELs T.M. Calvert*, J.D. Lambkin, B. Corbett, A.F. Phillips, G.M. Crean National Microelectronics Research Centre, University College Cork, Cork, Ireland
Abstract Red vertical cavity lasers (VCSELs) are ideally suited as optical sources for plastic optical fibre networks. These networks will form the communication backbone of future automobiles and aircraft replacing the current copper networks. However VCSELs at these wavelengths are difficult to realise due to the lower refractive index offsets and unsuitable band structure alignments. The Esprit BREDSELS project has addressed the design, growth and fabrication of low threshold red VCSELs. This paper reports the fabrication of record low threshold (200 mA at 208C) VCSELs at a wavelength of 665 nm. This device performance has been achieved through the use of selective oxidation techniques. The devices operate CW to 508C. The trade off between low threshold and high power will be discussed. # 2001 Elsevier Science Ltd. All rights reserved. Keywords: VCSEL; Visible; Selective oxidation
1. Introduction The structure of a VCSEL gives the device many advantages over edge emitting lasers. In general VCSELs are characterised by single longitudinal mode operation, circularly symmetric Gaussian beam profiles with divergence of less than 108 and low-threshold currents. A major advantage of these devices is the ability to test devices on wafer avoiding the expensive process of facet cleaving and packaging. As AlGaInP VCSELs are pushed to even shorter wavelengths they increasingly suffer from high threshold currents, low powers and poor thermal stability. This behaviour is caused by several factors. The top p-doped AlGaAs DBR mirror presents a high series resistance due to the large valence band offset and also causes a significant degree of self absorption of the lasing emission due to the band tailing in the heavily doped Bragg layers. These factors give rise to strong Ohmic heating and poor thermal impedance, in addition the bonding of the laser with the p mirror up further increases the thermal impedance of the packaged laser. The active region material (InGaP/AlGaInP) gives rise
*Corresponding author. E-mail address: [email protected] (T.M. Calvert).
to further problems due the relatively small band edge discontinuities associated with this material resulting in high leakage currents and low-quantum efficiencies particularly at shorter wavelengths. Up to the early 1990’s transverse optical and electrical confinement in VCSELs was either achieved using etched mesa index guided structures or proton-implanted gain-guided structures. However the introduction of selective oxidation has brought about considerable improvements in VCSEL performance. Selective oxidation is carried out by etching a mesa into the structure exposing the edges of a high Al fraction AlGaAs layer. By exposing this layer to water vapour at 380–4308C the high-fraction AlGaAs is converted into its native oxide forming a current aperture around the active region. These current apertures provide welldefined current paths with strong index guiding and avoids the problems of sidewall non-radiative recombination emission or implantation damage associated with air post or proton-implanted devices, respectively.
2. VCSEL structure and fabrication A schematic diagram of the VCSEL structure used in this study is shown in Fig. 1. The active region contains four quantum wells at the centre of a 1 l cavity. The top
1369-8001/00/$ - see front matter # 2001 Elsevier Science Ltd. All rights reserved. PII: S 1 3 6 9 - 8 0 0 1 ( 0 0 ) 0 0 0 7 7 - 9
518
T.M. Calvert et al. / Materials Science in Semiconductor Processing 3 (2000) 517–521
bond tracks to each VCSEL the sidewall profile of the mesa needs to be gentle enough to allow the metal tracks to run up to the contact rings. The depth of the p-type stack rules out reactive ion etching as this results in a sidewall with too steep a profile. After the etching, the high Al fraction AlGaAs layer in the p-DBR was selectively oxidised in an N2/H2O atmosphere at 4008C. This oxidation process reduces the current aperture to 6 mm for the single mode low-threshold devices. The surface was passivated with a SiN film and the contact rings and apertures were opened up. The apertures were protected by resist and the devices patterned for bond pads and tracks. These were deposited by e-beam evaporation. A five level Au/Ge/Au/Ni/Au contact was evaporated on the backside of the GaAs substrate as the n-type contact. Finally the devices were annealed at 4208C for 10 min to form the Ohmic contacts. Fig. 1. Device structure.
3. Selective oxidation The selective oxidation process involves etching the p-type stack to reveal the high Al fraction AlGaAs layer
Fig. 2. Selective oxidation test structure.
distributed Bragg reflector (DBR) consists of 35 pairs of Al0.95Ga0.05As/Al0.5Ga0.5As carbon doped p-type quarter-wave stacks with a Al0.98Ga0.02As layer as the selective oxidation layer. The bottom n-type Bragg mirror consists of 44.5 pairs of Al0.95Ga0.05As/Al0.5Ga0.5As quarter-wave stacks. A graded AlGaAs layer was used between the high and low contrast mirrors. The material for this device was grown by Epitaxial Products International. To fabricate the VCSEL devices, Ti–Pt–Au annular contacts were deposited onto the material by e-beam evaporation as the p-ohmic contact. A mesa structure was formed by wet etching through the p-type stack using a selective etch that stops on the active region. The etchant used for defining the mesas was 1 : 1 : 1 : 1 H3PO4 : H2SO4 : H2O2 : H2O which selectively etches AlGaAs but not AlGaInP. Due to the need for
Fig. 3. Oxidation calibration curve using test structure.
Fig. 4. Variation of oxidation depth dependent on layer doping.
T.M. Calvert et al. / Materials Science in Semiconductor Processing 3 (2000) 517–521
which is then oxidised by heating in steam, formed by bubbling nitrogen gas through hot water. Considerable care is needed to achieve a reproducible result. Our system consists of a water bubbler immersed in a silicon oil bath that is maintained at the set temperature by means of a feedback thermometer. Nitrogen is feed through the bubbler by means of a flow controller and into a low mass furnace, where the sample is oxidised. Calibration of the system is carried out by oxidising a test structure prior to oxidation of the device. This test
519
structure, as shown in Fig. 2, contains three layers of different AlGaAs and doping concentrations. Fig. 3 shows a calibration graph of oxidation distance against oxidation time for samples containing 99% AlGaAs and AlAs at a furnace temperature of 4008C and water temperatures of 85 and 958C. It has been observed during these tests that not only does the oxidation rate depend on the usual factors (percentage of Al in the sample, nitrogen flow rate, furnace and water bath temperature) but also on the doping concentration in the
Fig. 5. LI measurements on a single-moded device (Ith ¼ 0:2 mA).
Fig. 6. Spectral measurements of a single-moded device as a function of drive current.
520
T.M. Calvert et al. / Materials Science in Semiconductor Processing 3 (2000) 517–521
oxidation layer. The SEM micrograph in Fig. 4 shows this effect. This micrograph shows the various oxidation depths of the layers in the test structure. Note that while
‘‘a’’ is of the same composition as ‘‘b’’ (both 98% AlGaAs) it has a higher doping and has not oxidised in as far as the lower doped layer. The same is true for ‘‘c’’ and ‘‘d’’ which are 99% AlGaAs layers, again the higher doped layer has not oxidised in as far as the lower doped layer. This effect has been observed in VCSEL structures containing multiple high Al fraction AlGaAs layers as well. Work is currently under way to fully characterise this effect.
4. Results
Fig. 7. Confinement of the optical mode by the current aperture in a single-moded device. (a) At threshold; (b) above threshold (1:3 Ith ).
Electrical and optical power measurements made on devices with small current confining apertures (6 mm) have shown them to be single moded with peak powers of 0.2 mW at 208C. Typical LI measurements of a singlemoded device at temperatures between 5 and 508C is shown in Fig. 5. These devices have good wavelength stability as a function of drive current (Fig. 6). The width of the current confining aperture created by the selective oxidation process is crucial in determining if the device will have a single transverse mode or if it will be multi moded. Optical micrographs, shown in Fig. 7, taken of the single moded device at threshold and just above threshold show the mode being confined by the current aperture (6 mm). To determine the importance of the current aperture in the mode confinement a second batch of devices was oxidised for a shorter time to give devices with a current aperture of 10 mm. LI and spectral measurements on these devices revealed them to be multi-moded with an optical output power over twice that of the single-moded devices (0.5 mW as against 0.2 mW at 208C). A typical LI of the multi-moded
Fig. 8. LI measurements on a multi-moded device (Ith ¼ 0:5 mA) at 208C.
T.M. Calvert et al. / Materials Science in Semiconductor Processing 3 (2000) 517–521
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Fig. 9. Spectral measurements on a multimode device (Ith ¼ 0:5 mA) as a function of drive current.
devices is shown in Fig. 8. Spectral measurements (Fig. 9) show the presence of the modes above threshold (0.5 mA).
5. Conclusions We have successfully fabricated single-moded visible VCSELs emitting at 665 nm with thresholds of 0.2 mA at room temperature, the lowest thresholds reported to date at this wavelength. These devices work CW up to 508C. Powers in excess of 0.5 mW have been achieved for devices with thresholds below 1 mA. For comparison Choquette et al. [1] have produced visible VCSELs emitting at 670 nm with thresholds of 0.6 mA at room temperature. The depth of selective oxidation determines the optical mode properties of the device, VCSELs with current apertures of 6 mm being singlemoded while devices with apertures greater than this being multi-moded. This results in a trade off between low threshold and high power. For a device to have a low threshold it must have a small and well confined
current aperture which results in a single moded device and therefore a lower optical output power than highthreshold devices with several transverse modes and thus more power. We have also shown that the selective oxidation rate is dependent on the doping profile in the oxidation layer with oxidation occurring faster in layers with a low (2.5e17) doping concentration and slower in layers containing a higher (1e18) doping concentration.
Acknowledgements This work was supported by the EU long term research scheme project 23455 (BREDSELS). The authors would like to thank the project partners for useful discussion.
Reference [1] Choquette KD, Schneider RP, Hagerott Crawford M, Geib KM, Figiel JJ. CLEO ’95 CPD5-1, Baltimore.