| 1. |
- Bergmann, Michael Alexander, 1989, et al.
(författare)
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Towards ultraviolet and blue microcavity lasers
- 2018
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Ingår i: Northen Optics and Photonics conference. - 9789163964886 ; 2018
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Konferensbidrag (refereegranskat)abstract
- The development of III-nitride-based (Al,Ga,In(N)) microcavity lasers is a challenging task. Significant progress in recent years has resulted in realizations of electrically pumped devices with optical output power in the mW-range and with threshold current densities below 20 kA/cm2. However, to become practical, the lifetime and power conversion efficiency of these devices must be improved. Among the challenges are achieving transverse optical mode confinement, highreflectivity mirrors and control over the resonator length. We will highlight our theoretical work on transverse optical mode confinement, emphasising the overwhelming risk of ending up with an optically anti-guided cavity, and its consequences such as very high optical losses that easily could double the threshold gain for lasing. We will show some anti-guided cavities with reasonable threshold gain and built-in modal discrimination. However, all anti-guided cavities are very sensitive to temperature effects and small structural changes in the cavity caused by fabrication imperfections. We have explored electrically conductive distributed Bragg reflectors (DBRs) in both AlN/GaN and ZnO/GaN. The AlN/GaN DBRs were grown with different strain-compensating interlayers, and the DBR without interlayers had the lowest vertical resistivity with a specific series resistance of 0.044 cmfor eight DBRpairs. In the ZnO/GaN DBR, the measured resistance was dominated by lateral and contact contributions, setting a lower measurable limit of ~10 for three DBR-pairs. Numerical simulations show the importance of having in-plane strained layers in the ZnO/GaN DBR, since that leads to cancellation of the spontaneous and piezoelectric polarization. This results in a dramatically reduced vertical resistance, potentially three orders of magnitude lower than what could be measured. cm An alternative to an epitaxially grown DBR is a dielectric DBR, which offers high reflectivity over a broader wavelength range, relaxing the requirements on resonator length control. To deposit a dielectric DBR on the bottom side of the cavity, the sample must first be bonded to a carrier wafer before the substrate can be removed. We used thermocompression gold-gold bonding to successfully bond the laser structure to a Si carrier wafer. The subsequent substrate removal is a challenging process due to the chemical inertness of the III-nitride-based materials. A doping-dependent electrochemical etch technique was used, which allows for the selective removal of a sacrificial (n-doped) layer between the cavity and the substrate. This resulted in nm-precise cavity lift-off with a low root-mean-square surface roughness down to 0.3 nm. Thus, the process is suitable for the fabrication of high-quality optical devices such as microcavity lasers. In addition, the technique offers a new alternative to create III-nitridebased optical resonators, mechanical resonators, thin film LEDs and transistors.
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| 2. |
- Chang, Tsu Chi, et al.
(författare)
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Electrically Injected GaN-Based Vertical-Cavity Surface-Emitting Lasers with TiO2 High-Index-Contrast Grating Reflectors
- 2020
-
Ingår i: ACS Photonics. - : American Chemical Society (ACS). - 2330-4022. ; 7:4, s. 861-866
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Tidskriftsartikel (refereegranskat)abstract
- We demonstrate the first electrically injected GaN-based vertical-cavity surface-emitting lasers (VCSELs) with a TiO2 high-index-contrast grating (HCG) as the top mirror. Replacing the top distributed Bragg reflector (DBR) with an HCG offers substantial thickness reduction, polarization-pinning, and setting of the resonance wavelength by the grating parameters. Conventional HCGs are usually suspended in the low refractive index material, such as air, in order to create the largest refractive index contrast. However, the mechanical stability of such structures can be questioned and creating free-hanging GaN-membrane on top of GaN is problematic. We have therefore fabricated TiO2-HCGs resting directly on GaN without an air-gap. No DBR layers are used below the HCG to boost the reflectivity. A VCSEL with an aperture diameter of 10 μm shows a threshold current of 25 mA under pulsed operation at room temperature. The lasing modes locate around 400 nm and are transversely electrically -polarized with a line width of 0.5 nm. The full-width half-maximum beam divergence is 10°. This demonstration of a TiO2-HCG VCSEL offers a new route to achieve polarization pinning and could also allow additional benefits such as postgrowth setting of the resonance wavelength.
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| 3. |
- Chang, Tsu-Chi, et al.
(författare)
-
GaN-based vertical-cavity surface-emitting laser incorporating a TiO2 high-index-contrast grating
- 2020
-
Ingår i: Proceedings of SPIE - The International Society for Optical Engineering. - : SPIE. - 0277-786X .- 1996-756X. ; 11280
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Konferensbidrag (refereegranskat)abstract
- We demonstrate the first electrically injected GaN-based VCSEL with a TiO2 high-contrast grating (HCG) as the top mirror. The TiO2-HCG rested directly on the n-GaN without an airgap for mechanical stability. A VCSEL with an aperture diameter of 10 mu m had a threshold current of 25 mA under pulsed operation at room temperature. Multiple longitudinal modes coexist around 400 nm, each TM-polarized with a linewidth of 0.5 nm (spectral resolution limited). This first demonstration of a TiO2-HCG VCSEL offers a new route to achieve polarization pinning and could also allow additional benefits such as post-growth setting of resonance wavelength.
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| 4. |
- Chang, Tsu-Chi, 1990, et al.
(författare)
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GaN vertical-cavity surface-emitting laser with a high-contrast grating reflector
- 2018
-
Ingår i: Proceedings of SPIE - The International Society for Optical Engineering. - : SPIE. - 0277-786X .- 1996-756X. ; 10542
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Konferensbidrag (refereegranskat)abstract
- We report a GaN-based VCSEL with a high-contrast grating (HCG) as the top mirror. The HCG consisted of TiO2 and rested directly on the n-GaN without an airgap or the use of any DBR layers to boost the reflectivity. The full VCSEL structure was optically pumped at room temperature and showed a lasing threshold of approximately 0.69MW/cm2 and a lasing wavelength at 369.1 nm. This first demonstration of lasing in a HCG GaN-based VCSEL opens up the possibility to explore all the potential benefits of HCGs in the blue and ultraviolet spectral regime.
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| 5. |
- Chang, Tsu Chi, et al.
(författare)
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Lasing Action in GaN-Based VCSELs with top High-Contrast Grating Reflectors
- 2018
-
Ingår i: Conference Digest - IEEE International Semiconductor Laser Conference. - 0899-9406. ; 2018-September, s. 191-192
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Konferensbidrag (refereegranskat)abstract
- We report a GaN-based VCSEL with a high-contrast grating (HCG) as the top mirror. The HCG consisted of TiO2 and rested directly on the n-GaN without an airgap or the use of any DBR layers to boost the reflectivity. The full VCSEL structure was optically pumped at room temperature and showed a lasing threshold of approximately 0.99 MW/cm2 and a lasing wavelength at 369.8 nm. The demonstration of HCG GaN-based VCSEL opens up the possibility to explore all the potential benefits of HCGs in the blue and ultraviolet spectral regime.
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| 6. |
- Haglund, Åsa, 1976, et al.
(författare)
-
Are blue and ultraviolet VCSELs a reality or just a dream?
- 2020
-
Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- Blue-emitting vertical-cavity surface-emitting lasers (VCSELs) with high optical output powers and decent threshold current densities have recently been demonstrated, but the power efficiency is still below 10%. We will give an overview of the challenges to realize high-performing devices and summarize state-of-the-art results, including our schemes for optically guided devices that are used in the best blue VCSELs today. In addition, we will present our method to simultaneously achieve high-reflectivity mirrors and good control over the cavity length for ultraviolet VCSELs. This has resulted in, to the best of our knowledge, the shortest VCSEL emission wavelength around 310 nm.
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| 7. |
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| 8. |
- Haglund, Åsa, 1976, et al.
(författare)
-
Blue and ultraviolet vertical-cavity surface-emitting lasers
- 2019
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Ingår i: Optics InfoBase Conference Papers. - 2162-2701. ; Part F129-CLEO_SI 2019
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Konferensbidrag (refereegranskat)abstract
- We will summarize state-of-the-art results in III-nitride-based vertical-cavity surface-emitting lasers (VCSELs) for blue and ultraviolet emission, including our schemes for optically guided devices and our approach for UV-VCSELs with double dielectric distributed Bragg reflectors.
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| 9. |
- Haglund, Åsa, 1976, et al.
(författare)
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GaN-based VCSELs
- 2015
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Ingår i: VI Workshop on Physics and Technology of Semiconductor Lasers.
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Konferensbidrag (refereegranskat)abstract
- The Vertical-Cavity Surface-Emitting Laser (VCSEL) is an established optical source in short-distance optical communication links, computer mice and tailored infrared power heating systems. Its low power consumption, easy integration into two-dimensional arrays, and low-cost manufacturing also make this type of semiconductor laser suitable for application in areas such as high-resolution printing, bio-medical and general lighting. However, these applications require emission wavelengths in the blue-UV instead of the established infrared regime, which can be achieved by using GaN-based instead of GaAs-based materials. The development of GaN-based VCSELs have shown to be challenging, and so far only a handful research groups have demonstrated lasing from such electrically pumped devices [1-6]. The presented performance is typically orders of magnitudes lower compared to that from electrically driven GaAs-based VCSELs. Some of the challenges are to achieve efficient transverse current spreading, transverse optical mode confinement, high-reflectivity mirrors and resonator length control. This talk will summarize the different strategies to solve these issues in electrically pumped GaN-VCSELs together with state-of-the-art results. We will highlight our work on combined transverse current and optical mode confinement, where we show that many structures used for current confinement result in unintentionally optically anti-guided resonators. Such resonators can have a very high optical loss, which easily doubles the threshold gain for lasing [7]. We will also present an alternative to the use of distributed Bragg reflectors as high-reflectivity mirrors, namely a TiO2/air high contrast gratings (HCGs). Fabricated HCGs of this type show a high reflectivity (>95%) over a 25 nm wavelength span, which is in excellent agreement to the reflectivity spectrum predicted by numerical simulations assuming an ideal HCG geometry [8]. References [1] T.-C. Lu, et al., Applied Physics Letters, 92, 14, (2008).[2] Y. Higuchi, et al., Applied physics express, 1, 12, 121102, (2008).[3] G. Cosendey, et al., Applied Physics Letters, 101, 15, (2012).[4] C. Holder, et al., Applied Physics Express, 5, 092104, (2012).[5] T. Onishi, et al., IEEE J. of Quantum Electronics, 48, 9,1107–1112, (2012).[6] W.-J. Liu, et al., Applied Physics Letters, 104, 251116 (2014).[7] E. Hashemi, et al., Optics Express, vol. 22 1, p. 411-426, (2014).[8] E. Hashemi, et al., Proceedings of SPIE, (0277-786X), vol. 9372 (2015).
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| 10. |
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| 11. |
- Haglund, Åsa, 1976, et al.
(författare)
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Progress and challenges in electrically pumped GaN-based VCSELs
- 2016
-
Ingår i: Proceedings of SPIE - The International Society for Optical Engineering. - : SPIE. - 0277-786X .- 1996-756X. ; 9892
-
Konferensbidrag (refereegranskat)abstract
- The Vertical-Cavity Surface-Emitting Laser (VCSEL) is an established optical source in short-distance optical communication links, computer mice and tailored infrared power heating systems. Its low power consumption, easy integration into two-dimensional arrays, and low-cost manufacturing also make this type of semiconductor laser suitable for application in areas such as high-resolution printing, medical applications, and general lighting. However, these applications require emission wavelengths in the blue-UV instead of the established infrared regime, which can be achieved by using GaN-based instead of GaAs-based materials. The development of GaN-based VCSELs is challenging, but during recent years several groups have managed to demonstrate electrically pumped GaN-based VCSELs with close to 1 mW of optical output power and threshold current densities between 3-16 kA/cm2. The performance is limited by challenges such as achieving high-reflectivity mirrors, vertical and lateral carrier confinement, efficient lateral current spreading, accurate cavity length control and lateral optical mode confinement. This paper summarizes different strategies to solve these issues in electrically pumped GaN-VCSELs together with state-of-the-art results. We will highlight our work on combined transverse current and optical mode confinement, where we show that many structures used for current confinement result in unintentionally optically anti-guided resonators. Such resonators can have a very high optical loss, which easily doubles the threshold gain for lasing. We will also present an alternative to the use of distributed Bragg reflectors as high-reflectivity mirrors, namely TiO2/air high contrast gratings (HCGs). Fabricated HCGs of this type show a high reflectivity (>95%) over a 25 nm wavelength span.
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| 12. |
- Haglund, Åsa, 1976, et al.
(författare)
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Recent progress for blue VCSELs and challenges to move to UV
- 2020
-
Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- We will summarize state-of-the-art results in III-nitride-based vertical-cavity surface-emitting lasers (VCSELs) and the main challenges in going from blue to ultraviolet-emitting devices. In particular we will highlight how to reduce threshold current densities by using optically guided cavities and our method to simultaneously achieve high-reflectivity mirrors and good control over the cavity length for AlGaN-based ultraviolet VCSELs. The latter has resulted in the shortest emission wavelength from a VCSEL at around 310 nm. This claim of lasing is supported by a nonlinear increase in output power versus pump power and a simultaneous beam width and spectral narrowing around threshold.
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| 13. |
- Hashemi, Seyed Ehsan, 1986, et al.
(författare)
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Air-suspended TiO2-based HCG reflectors for visible spectral range
- 2015
-
Ingår i: Proceedings of SPIE - The International Society for Optical Engineering. - : SPIE. - 0277-786X .- 1996-756X. ; 9372
-
Konferensbidrag (refereegranskat)abstract
- For GaN-based microcavity light emitters, such as vertical-cavity surface-emitting lasers (VCSELs) and resonant cavity light emitting diodes (RCLEDs) in the blue-green wavelength regime, achieving a high reflectivity wide bandwidth feedback mirror is truly challenging. The material properties of the III-nitride alloys are hardly compatible with the conventional distributed Bragg reflectors (DBRs) and the newly proposed high-contrast gratings (HCGs). Alternatively, at least for the top outcoupling mirror, dielectric materials offer more suitable material combinations not only for the DBRs but also for the HCGs. HCGs may offer advantages such as transverse mode and polarization control, a broader reflectivity spectrum than epitaxially grown DBRs, and the possibility to set the resonance wavelength after epitaxial growth by the grating parameters. In this work we have realized an air-suspended TiO2 grating with the help of a SiO2 sacrificial layer. The deposition processes for the dielectric layers were fine-tuned to minimize the residual stress. To achieve an accurate control of the grating duty cycle, a newly developed lift-off process, using hydrogen silesquioxan (HSQ) and sacrificial polymethyl-methacrylate (PMMA) resists, was applied to deposit the hard mask, providing sub-10 nm resolution. The finally obtained TiO2/air HCGs were characterized in a micro-reflectance measurement setup. A peak power reflectivity in excess of 95% was achieved for TM polarization at the center wavelength of 435 nm, with a reflectivity stopband width of about 80 nm (FWHM). The measured HCG reflectance spectra were compared to corresponding simulations obtained from rigorous coupled-wave analysis and very good agreement was found.
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| 14. |
- Hashemi, Seyed Ehsan, 1986, et al.
(författare)
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Analysis of structurally sensitive loss in GaN-based VCSEL cavities and its effect on modal discrimination
- 2014
-
Ingår i: Optics Express. - 1094-4087 .- 1094-4087. ; 22:1, s. 411-426
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Tidskriftsartikel (refereegranskat)abstract
- Lateral loss causes optical energy to leave the laser cavity in the transverse, lateral, direction, and is sometimes neglected to simplify the numerical simulations. However, in contrast to outcoupling and absorption losses, we show that the lateral loss can change drastically with only nanometer-sized changes of the cavity structure, from being virtually zero to becoming the major source of cavity loss, since the cavity becomes antiguiding. This can be explained as the opening of a channel of efficient resonant lateral leakage of optical power at a certain oblique propagation angle. A number of different realizations of current apertures and top mirror designs in GaN-based VCSEL cavities, which have been suggested for realization of microcavity lasers emitting in the blue wavelength range, are simulated. Many of these are shown to lead to unintentional antiguiding, which can more than double the threshold gain for lasing. Notably, for strong enough antiguiding the resonant lateral leakage decreases so that the threshold gain values might again be tolerable. This regime has been suggested for robust single-mode operation since earlier predictions, building on analogies with slab waveguides, hinted at a very strong suppression of higher order modes. However, our simulations indicate that for the VCSEL cavities the derived formulas grossly overestimate the modal discrimination.
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| 15. |
- Hashemi, Seyed Ehsan, 1986, et al.
(författare)
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Effect of compositional interlayers on the vertical electrical conductivity of Si-doped AlN/GaN distributed Bragg reflectors grown on SiC''
- 2017
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Ingår i: Applied Physics Express. - 1882-0786 .- 1882-0778. ; 10:5, s. 055501-
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Tidskriftsartikel (refereegranskat)abstract
- We have investigated the effect of strain-compensating interlayers on the vertical electrical conductivity of Si-doped AlN/GaN distributed Bragg reflectors (DBRs). Samples with 10.5 mirror pairs were grown through plasma-assisted molecular beam epitaxy on SiC. Room-temperature current–voltage characteristics were measured vertically in mesas through 8 of the 10.5 pairs. The sample with no interlayers yields a mean specific series resistance of 0.044 Ω cm2 at low current densities, while three samples with 5/5-Å-thick, 2/2-nm-thick, and graded interlayers have resistivities between 0.16 and 0.34 Ω cm2. Thus, interlayers impair vertical current transport, and they must be designed carefully when developing conductive DBRs.
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| 16. |
- Hashemi, Seyed Ehsan, 1986, et al.
(författare)
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Engineering the Lateral Optical Guiding in Gallium Nitride (GaN)-based Vertical-Cavity Surface-Emitting Laser (VCSEL) Cavities to Reach the Lowest Threshold Gain
- 2013
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Ingår i: Japanese Journal of Applied Physics. - 1347-4065 .- 0021-4922. ; 52:8 PART 2, s. 08JG04 -
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Tidskriftsartikel (refereegranskat)abstract
- In order to improve the current injection in GaN-based blue vertical-cavity surface-emitting lasers (VCSELs) a dielectric aperture is generally used in combination with an indium–tin-oxide (ITO) layer on the top intracavity p-contact layer. The most straightforward way to realize this introduces a depression of the structure near the optical axis and we show, by using a two-dimensional (2D) effective index method and a three-dimensional (3D) coupled-cavity beam propagation method, that this typically results in optically anti-guided structures with associated high optical losses and thus very high threshold gains. Remarkably, the threshold gain reduces with increased negative guiding, which is due to improved lateral confinement and reduction of lateral leakage. Still, moderately positively guided designs should be preferred to avoid the detrimental effect of lateral leakage and high diffraction loss. To ensure positive index guiding, we propose to planarize the structure or introduce an elevation near the optical axis by additional processing, with an associated reduction in threshold material gain from 6000 to 2000 cm-1 for the studied structures.
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| 17. |
- Hashemi, Seyed Ehsan, 1986, et al.
(författare)
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Engineering the transverse optical guiding in GaN-based VCSELs to avoid detrimental optical loss
- 2012
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Ingår i: International Workshop on Nitride Semiconductors 2012, October 14-19, 2012, Sapporo, Japan, p. ThP-OD-33.
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- In order to improve the current injection in GaN-based blue vertical cavity surface emitting lasers (VCSELs) a dielectric aperture is generally used in combination with an indium-tin-oxide (ITO) layer on the top intracavity p-contact layer. The most straightforward way to realize this introduces a depression of the structure near the optical axis and we show, by using a 2D effective index method and a 3D coupled-cavity beam propagation method, that this typically results in optically anti-guided structures with associated high optical losses and thus very high threshold gains. Remarkably, the threshold gain reduces with increased negative guiding, which is due to improved lateral confinement and reduction of lateral leakage. Still, moderately positively guided designs should be preferred to avoid the detrimental effect of lateral leakage and high diffraction loss. To ensure positive index guiding, we propose to planarize the structure or introduce an elevation near the optical axis by additional processing, with an associated reduction in threshold material gain from 6000 cm-1 to 2000 cm-1 for the studied structures.
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| 18. |
- Hashemi, Seyed Ehsan, 1986
(författare)
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Optical Guiding and Feedback in Gallium Nitride Based Vertical Cavity Surface Emitting Lasers
- 2015
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The gallium-nitride (GaN) semiconductor material has been the core of the revolutionary breakthroughs during the last two decades in the lighting industry, by enabling manufacturing of efficient blue light emitting diodes (LEDs), for which the 2014 Nobel prize in physics was awarded. The GaN technology has further led to violet edge-emitting lasers (EELs), enabling the Blu-ray disk technology, and also to the commercialization of directly green emitting EELs. A natural next step is the realization of GaN-based vertical-cavity surface-emitting lasers (VCSELs), which has proved to be a challenging task. The first electrically injected GaN-based VCSEL was announced in 2008, more than a decade after the first reports on its EEL counterpart. Still today only four groups in the world have demonstrated lasing under continuous-wave operation in a blue VCSEL.Some of the major challenges to realize GaN-based VCSELs are the lack of two lattice matched materials for forming high reflectivity distributed Bragg reflectors (DBRs), the poor current spreading capabilities in p-doped GaN, and the difficulty to achieve current and optical confinement. In this work we have addressed those issues. We have developed the novel concept of TiO2/air high contrast gratings (HCGs) to achieve high reflectivity over a broad wavelength range. The HCGs show a high reflectivity (>95%) over a 25 nm wavelength span, and a very good agreement between simulated and measured reflectivity spectra has been achieved. By using our in-house developed VCSEL simulation tools we have studied existing GaN-based VCSEL designs and shown that the approach taken by most groups to confine the current to the center of the device (transverse direction), yields an optical resonator that is weakly antiguiding with very high optical losses and thereby high threshold currents. These anti-guided devices have total losses that are typically 100-200% higher than in our newly proposed structures, in which current confinement and optically guiding can be achieved simultaneously. These structures have already been implemented by two of the world's leading groups in the area.
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| 19. |
- Hashemi, Seyed Ehsan, 1986
(författare)
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Optical Guiding and Feedback Structures in GaN-Based Vertical-Cavity Surface-Emitting Lasers
- 2016
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The Vertical-Cavity Surface-Emitting Laser (VCSEL) made from GaAs-based materials is an established infrared light source, and is produced in large volumes for short-distance optical links, computer mice, and thermal processing systems. A wide range of applications, such as high-resolution printing, general lighting, and biomedical diagnosis and treatment could benefit from using a VCSEL with blue or ultraviolet emission. A promising solution is the use of GaN-based materials, here only a few research groups have so far demonstrated electrically pumped GaN VCSELs with optical output powers in the milliwatt-range and threshold current densities 95 % over a 25 nm wavelength span.
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| 20. |
- Hashemi, Seyed Ehsan, 1986, et al.
(författare)
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Thermal lensing effects on lateral leakage in GaN-based vertical-cavity surface-emitting laser cavities
- 2017
-
Ingår i: Optics Express. - 1094-4087 .- 1094-4087. ; 25:9, s. 9556-9568
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Tidskriftsartikel (refereegranskat)abstract
- Lateral leakage of light has been identified as a detrimental loss source in many suggested and experimentally realized GaN-based VCSELs. In the present work we include thermal effects to realistically account for the substantial Joule heating in these devices. In contrast to what could be expected from the previous results, the induced thermal lensing does not make antiguided cavities more positively guided, so that they approach the unguided regime with extremely high lateral leakage. Rather, thermal lensing strongly suppresses lateral leakage for both antiguided and guided cavities. This is explained in terms of lowered launch of power from the central part of the cavity and/or lower total internal reflection in the peripheral part; the former effect is active in all cavities whereas the latter only contributes to the very strongly reduced leakage in weakly antiguided cavities. Thermal lensing suppresses lateral leakage both for the fundamental and the first higher order mode, but a strong modal discrimination is still achieved for the antiguided cavities. Thus, strongly antiguided cavities could be used to achieve single-mode devices, but at the cost of slightly higher threshold gain and stronger temperature dependent performance characteristics.
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| 21. |
- Hashemi, Seyed Ehsan, 1986, et al.
(författare)
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TiO2 membrane high-contrast grating reflectors for vertical-cavity light-emitters in the visible wavelength regime
- 2015
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Ingår i: Journal of Vacuum Science and Technology B: Nanotechnology and Microelectronics. - : American Vacuum Society. - 2166-2746 .- 2166-2754. ; 33:5
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Tidskriftsartikel (refereegranskat)abstract
- © 2015 American Vacuum Society. In this work, the authors describe a novel route to achieve a high reflectivity, wide bandwidth feedback mirror for GaN-based vertical-cavity light emitters; using air-suspended high contrast gratings in TiO2, with SiO2 as a sacrificial layer. The TiO2 film deposition and the etching processes are developed to yield grating bars without bending, and with near-ideal rectangular cross-sections. Measured optical reflectivity spectra of the fabricated high contrast gratings show very good agreement with simulations, with a high reflectivity of >95% over a 25 nm wavelength span centered around 435 nm for the transverse-magnetic polarization.
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| 22. |
- Hashemi, Seyed Ehsan, 1986, et al.
(författare)
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Triggering of guiding and antiguiding effects in GaN-based VCSELs
- 2014
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Ingår i: Proceedings of SPIE - The International Society for Optical Engineering. - : SPIE. - 0277-786X .- 1996-756X. ; 9001
-
Konferensbidrag (refereegranskat)abstract
- We show numerically that many recently proposed GaN-based VCSEL cavities, with DBR mirrors deposited onto the current aperture, balance dangerously close to the border between the guided and antiguided regime. A guided cavity is often preferred because of its lower optical loss, but a strongly antiguided cavity offers built-in modal discrimination favoring single fundamental mode operation. We show that very small changes in the VCSEL structure are sufficient to strongly change the guiding character of the VCSEL cavity, and that thermal lensing caused by device self-heating under operation can dramatically reduce the optical loss but not the modal discrimination in the antiguided cavities.
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| 23. |
- Hjort, Filip, 1991, et al.
(författare)
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Electrically conductive ZnO/GaN distributed Bragg reflectors grown by hybrid plasma-assisted molecular beam epitaxy
- 2017
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Ingår i: Proceedings of SPIE - The International Society for Optical Engineering. - : SPIE. - 0277-786X .- 1996-756X. - 9781510606494 ; 10104, s. 1010413-1
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Konferensbidrag (refereegranskat)abstract
- III-nitride-based vertical-cavity surface-emitting lasers have so far used intracavity contacting schemes since electrically conductive distributed Bragg reflectors (DBRs) have been difficult to achieve. A promising material combination for conductive DBRs is ZnO/GaN due to the small conduction band offset and ease of n-type doping. In addition, this combination offers a small lattice mismatch and high refractive index contrast, which could yield a mirror with a broad stopband and a high peak reflectivity using less than 20 DBR-pairs. A crack-free ZnO/GaN DBR was grown by hybrid plasma-assisted molecular beam epitaxy. The ZnO layers were approximately 20 nm thick and had an electron concentration of 1×1019 cm-3, while the GaN layers were 80-110 nm thick with an electron concentration of 1.8×1018 cm-3. In order to measure the resistance, mesa structures were formed by dry etching through the top 3 DBR-pairs and depositing non-annealed Al contacts on the GaN-layers at the top and next to the mesas. The measured specific series resistance was dominated by the lateral and contact contributions and gave an upper limit of ~10-3Ωcm2 for the vertical resistance. Simulations show that the ZnO electron concentration and the cancellation of piezoelectric and spontaneous polarization in strained ZnO have a large impact on the vertical resistance and that it could be orders of magnitudes lower than what was measured. This is the first report on electrically conductive ZnO/GaN DBRs and the upper limit of the resistance reported here is close to the lowest values reported for III-nitride-based DBRs.
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| 24. |
- Hjort, Filip, 1991, et al.
(författare)
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Vertical Electrical Conductivity of ZnO/GaN Multilayers for Application in Distributed Bragg Reflectors
- 2018
-
Ingår i: IEEE Journal of Quantum Electronics. - 0018-9197 .- 1558-1713. ; 54:4
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Tidskriftsartikel (refereegranskat)abstract
- We have demonstrated an electrically conductive ZnO/GaN multilayer structure using hybrid plasma-assisted molecular beam epitaxy. Electrical I-V characteristics were measured through the top three pairs of a six pair ZnO/GaN sample. The total measured resistance was dominated by lateral and contact resistances, setting an upper limit of similar to 10(-4) Omega.cm(2) for the vertical specific series resistance of the stack. A strong contribution to the low resistance is the cancellation of spontaneous and piezoelectric polarization that occurs in the in- plane strained ZnO/GaN sample, as shown by electrical simulations. In addition, the simulations show that the actual vertical resistance of the sample could in fact be three orders of magnitude lower and that ZnO/GaN structures with thicknesses fulfilling the Bragg condition should have similar resistance. Our results suggest that ZnO/GaN distributed Bragg reflectors (DBRs) are a promising alternative to pure III-nitride DBRs in GaN-based vertical-cavity surface-emitting lasers.
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