Compared to the maturity of today’s blue laser diodes, which exhibit high efficiencies, low threshold currents, and long lifetimes, deep-ultraviolet (<280 nm) lasers have essentially just been born. We have only recently witnessed the first deep-UV, continuous-wave edge-emitting lasers operating at room temperature under electrical injection. And more complex laser structures in the deep-UV, such as vertical-cavity surface-emitting lasers and photonic crystal surface emitting lasers, are even further behind, having only been demonstrated under pulsed optical pumping. Among the many difficulties in transitioning from blue to deep-UV are the problems of efficient electrical injection, creation of optical waveguides and cavities in materials with low refractive index contrast, and high material defect densities. The question is, are these problems fundamental limitations to the technology, or just temporary growing pains to be overcome with hard work and persistence as we push lasers deeper into unseen wavelengths and frontiers?
Electrochemical etching of III-nitride-materials is a fast-developing research field. This method is used to selectively porosify or completely etch such materials and thereby opens up a new design space for both photonic and electronic devices. Here we will focus on complete lateral electrochemical etching for substrate removal to realise thin-film vertical-cavity surface-emitting lasers (VCSELs) and light emitting diodes (LEDs). Key challenges that will be addressed are how to achieve etched surfaces as smooth as the as-grown material and how to protect fully processed and highly doped device structures such as tunnel junctions, during substrate removal.
We will give an overview of state-of-the-art results and challenges to achieve high-performing III-nitride vertical-cavity surface-emitting lasers (VCSELs), with a particular focus on the requirements to push the emission wavelength into the ultraviolet (UV). Our method to simultaneously achieve high-reflectivity mirrors and good cavity length control by electrochemical etching enabled the world’s first UV-B VCSEL. The use of dielectric mirrors yielded lasers with a very temperature-stable emission wavelength thanks to the negative thermo-optic coefficient of the mirrors. We have used the same etch methodology to also lift-off fully processed LEDs from their growth substrate to improve the light extraction efficiency.
We will give an overview of different concepts to increase the light extraction efficiency (LEE) of ultraviolet (UV) light-emitting diodes (LEDs) with a focus on thin-film flip-chip (TFFC) devices. Optical simulations show that a TFFC design can greatly improve the LEE with a transparent p-side, reflective contacts, and optimized surface roughening. We will demonstrate UVB-emitting TFFC LEDs based on our fabrication platform for AlGaN thin films with high aluminum content. The fabrication is compatible with a standard LED process and uses substrate removal based on selective electrochemical etching as the key enabling technology.
In recent years, there has been tremendous improvement in the performance of blue-emitting vertical-cavity surface-emitting lasers (VCSELs) and they are now on the cusp of commercialization. We will summarize state-of-the-art results and outline the main challenges in extending the emission wavelength into the ultraviolet (UV). Our method to simultaneously achieve high-reflectivity mirrors and good cavity length control by selective electrochemical etching has been essential to demonstrate the world’s first UV-B VCSEL. The use of dielectric mirrors, where one material has a negative thermo-optical coefficient, counteracts the inherent red-shift of the resonance wavelength, enabling a temperature-stable emission.
We will give an overview of the progress in ultraviolet-emitting vertical-cavity surface-emitting lasers (VCSELs) and their potential applications in areas such as disinfection and medical therapy. This includes our demonstration of the shortest wavelength VCSEL, emitting at 310 nm under optical pumping, and a detailed analysis of its filamentary lasing characteristics. The UVB-emitting AlGaN-based VCSEL was realized by substrate removal using electrochemical etching, enabling the use of two high-reflectivity dielectric distributed Bragg reflectors. The potential of using this or alternative methods to push the emission to shorter wavelengths will be examined as well as concepts to realize electrically injected devices.
We here demonstrate thin-film flip-chip (TFFC) ultraviolet-B light-emitting diodes (LEDs) fabricated by a standard LED process and followed by a substrate removal based on selective electrochemical etching of an n-doped multilayered Al0.11Ga0.89N/Al0.37Ga0.63N sacrificial layer. The integration of the LEDs to a Si carrier using thermocompression bonding allowed roughening of the N-polar AlGaN side of the TFFC LEDs using TMAH-etching, which increased the light extraction efficiency by approximately 45% without negatively affecting the I-V-characteristics. This resulted in an optical output power of 0.47 mW at 10 mA for an LED with a p-contact area of 0.03 mm2.
III-nitride membranes offer novel device designs in photonics, electronics and optomechanics. However, substrate removal often leads to a rough back surface, which degrades device performance. Here, we demonstrate GaN membranes with atomically smooth etched surfaces by electrochemical lift-off, through the implementation of a built-in polarization field in the sacrificial layer. This leads to a faster reduction in the sacrificial layer free carrier density during etching and thus an abrupter etch stop, reducing the root-mean-square roughness down to 0.4 nm over 5×5 µm2. These results open interesting perspectives on high-quality optical cavities and waveguides in the ultraviolet and visible.
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 μm 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.
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.
The use of a high-contrast grating (HCG) as the top mirror in a vertical-cavity surface-emitting laser (VCSEL) allows for setting the resonance wavelength by the grating parameters in a post-epitaxial growth fabrication process. Using this technique, we demonstrate electrically driven multi-wavelength VCSEL arrays at ~980 nm wavelength. The VCSELs are GaAs-based and the suspended GaAs HCGs were fabricated using electron-beam lithography, dry etching and selective removal of an InGaP sacrificial layer. The air-coupled cavity design enabled 4-channel arrays with 5 nm wavelength spacing and sub-mA threshold currents thanks to the high HCG reflectance.
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.
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.
We present a GaAs-based VCSEL structure, BCB bonded to a Si3N4 waveguide circuit, where one DBR is substituted by
a free-standing Si3N4 high-contrast-grating (HCG) reflector realized in the Si3N4 waveguide layer. This design enables
solutions for on-chip spectroscopic sensing, and the dense integration of 850-nm WDM data communication transmitters
where individual channel wavelengths are set by varying the HCG parameters. RCWA shows that a 300nm-thick Si3N4
HCG with 800nm period and 40% duty cycle reflects strongly (<99%) over a 75nm wavelength range around 850nm. A
design with a standing-optical-field minimum at the III-V/airgap interface maximizes the HCG’s influence on the
VCSEL wavelength, allowing for a 15-nm-wide wavelength setting range with low threshold gain (<1000 cm-1).
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.
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.
A simple and low-cost technology for tunable vertical-cavity surface-emitting lasers (VCSELs) with curved movable
micromirror is presented. The micro-electro-mechanical system (MEMS) is integrated with the active optical
component (so-called half-VCSEL) by means of surface-micromachining using a reflown photoresist droplet as
sacrificial layer. The technology is demonstrated for electrically pumped, short-wavelength (850 nm) tunable
VCSELs. Fabricated devices with 10 μm oxide aperture are singlemode with sidemode suppression >35 dB,
tunable over 24 nm with output power up to 0.5mW, and have a beam divergence angle <6 °. An improved
high-speed design with reduced parasitic capacitance enables direct modulation with 3dB-bandwidths up to
6GHz and error-free data transmission at 5Gbit/s. The modulation response of the MEMS under electrothermal
actuation has a bandwidth of 400 Hz corresponding to switching times of about 10ms. The thermal
crosstalk between MEMS and half-VCSEL is negligible and not degrading the device performance. With these
characteristics the integrated MEMS-tunable VCSELs are basically suitable for use in reconfigurable optical
interconnects and ready for test in a prototype system. Schemes for improving output power, tuning speed, and
modulation bandwidth are briefly discussed.
We have reduced the spectral width of high speed oxide confined 850 nm VCSELs using a shallow surface relief for
suppression of higher order transverse modes. The surface relief acts as a mode filter by introducing a spatially varying
and therefore mode selective loss. The VCSEL employs multiple oxide layers for reduced capacitance which leads to a
strong index guiding and a large spectral width in the absence of a mode filter. With an appropriate choice of surface
relief parameters, the RMS spectral width for a 5 μm oxide aperture VCSEL is reduced from 0.6 to 0.3 nm. The small
signal modulation bandwidth is 19 GHz. Due to reduced effects of chromatic and modal fiber dispersion, the maximum
error-free (bit-error-rate < 10-12) transmission distance at 25 Gb/s over OM3+ fiber is increased from 100 to 500 m.
This paper presents a review of recent work on high speed tunable and fixed wavelength vertical cavity surface emitting
lasers (VCSELs) at Chalmers University of Technology. All VCSELs are GaAs-based, employ an oxide aperture for
current and/or optical confinement, and emit around 850 nm. With proper active region and cavity designs, and
techniques for reducing capacitance and thermal impedance, our fixed wavelength VCSELs have reached a modulation
bandwidth of 23 GHz, which has enabled error-free 40 Gbps back-to-back transmission and 35 Gbps transmission over
100 m of multimode fiber. A MEMS-technology for wafer scale integration of tunable high speed VCSELs has also been
developed. A tuning range of 24 nm and a modulation bandwidth of 6 GHz have been achieved, enabling error-free
back-to-back transmission at 5 Gbps.
We report the investigation of the state of polarization (SOP) of a tunable vertical-cavity surface-emitting laser
(VCSEL) operating near 850 nm with a mode-hop free single-mode tuning range of about 12 nm and an amplitude
modulation bandwidth of about 5 GHz. In addition, the effect of a sub-wavelength grating on the device and
its influence on the polarization stability and polarization switching has been investigated. The VCSEL with an
integrated sub-wavelength grating shows a stable SOP with a polarization mode suppression ratio (PMSR) more
than 35 dB during the tuning.
We present an empirical thermal model for VCSELs based on extraction of temperature dependence of macroscopic VCSEL
parameters from CW measurements. We apply our model to two, oxide-confined, 850-nm VCSELs, fabricated with
a 9-μm inner-aperture diameter and optimized for high-speed operation. We demonstrate that for both these devices, the
power dissipation due to linear heat sources dominates the total self-heating. We further show that reducing photon lifetime
down to 2 ps drastically reduces absorption heating and improves device static performance by delaying the onset
of thermal rollover. The new thermal model can identify the mechanisms limiting the thermal performance and help in
formulating the design strategies to ameliorate them.
The impedance characteristics and the effects of photon lifetime reduction on the performance of high-speed 850 nm
VCSELs are investigated. Through S11 measurements and equivalent circuit modeling we show that the parasitic mesa
capacitance can be significantly reduced by using multiple oxide layers. By performing a shallow surface etch (25 -
55 nm) on the fabricated VCSELs, we are able to reduce the photon lifetime by up to 80% and thereby significantly
improve both static and dynamic properties of the VCSELs. By optimizing the photon lifetime we are able to enhance
the 3dB modulation bandwidth of 7 μm oxide aperture VCSELs from 15 GHz to 23 GHz and finally demonstrate errorfree
transmission at up to 40 Gbit/s.
Widely tunable vertical cavity surface emitting lasers (VCSEL) are of high interest for optical communications,
gas spectroscopy and fiber-Bragg-grating measurements. In this paper we present tunable VCSEL operating at
wavelength around 850 nm and 1550 nm with tuning ranges up to 20 nm and 76 nm respectively. The first versions
of VCSEL operating at 1550 nm with 76 nm tuning range and an output power of 1.3mW were not designed for
high speed modulation, but for applications where only stable continious tuning is essential (e.g. gas sensing).
The next step was the design of non tunable VCSEL showing high speed modulation frequencies of 10 GHz with
side mode supression ratios beyond 50 dB. The latest version of these devices show record output powers of
6.7mW at 20 °C and 3mW at 80 °C. The emphasis of our present and future work lies on the combination of
both technologies. The tunable VCSEL operating in the 850 nm-region reaches a modulation
bandwidth of 5.5GHz with an output power of 0.8mW.
We have explored the possibility to extend the data transmission rate for standard 850-nm GaAs-based VCSELs beyond
the 10 Gbit/s limit of today's commercially available directly-modulated devices. By sophisticated tailoring of the design
for high-speed performance we demonstrate that 10 Gb/s is far from the upper limit. For example, the thermal
conductivity of the bottom mirror is improved by the use of binary compounds, and the electrical parasitics are kept at a
minimum by incorporating a large diameter double layered oxide aperture in the design. We also show that the intrinsic
high speed performance is significantly improved by replacing the traditional GaAs QWs with strained InGaAs QWs in
the active region. The best overall performance is achieved for a device with a 9 μm diameter oxide aperture, having in
a threshold current of 0.6 mA, a maximum output power of 9 mW, a thermal resistance of 1.9 °C/mW, and a differential
resistance of 80 Ω. The measured 3dB bandwidth exceeds 20 GHz, and we experimentally demonstrate that the device is
capable of error-free transmission (BER<10-12) under direct modulation at a record-high bit-rate of 32 Gb/s over 50 m of
OM3 fiber at room temperature, and at 25 Gb/s over 100 m of OM3 fiber at 85 °C. We also demonstrate transmission at
40 Gb/s over 200 m of OM3+ fiber at room temperature using a subcarrier multiplexing scheme with a spectrally
efficient 16 QAM modulation format. All transmission results were obtained with the VCSEL biased at current densities
between 11-14 kA/cm2, which is close to the 10 kA/cm2 industry benchmark for reliability. Finally, we show that by a
further reduction of the oxide capacitance and by reducing the photon lifetime using a shallow surface etch, a record
bandwidth of 23 GHz for 850 nm VCSELs can be reached.
The design of an oxide confined 850 nm VCSEL has been engineered for high speed operation at low current density.
Strained InGaAs/AlGaAs QWs, with a careful choice of In and Al concentrations based on rigorous band structure and
gain calculations, were used to increase differential gain and reduce threshold carrier density. Various measures,
including multiple oxide layers and a binary compound in the lower distributed Bragg reflector, were implemented for
reducing capacitance and thermal impedance. Modulation bandwidths > 20 GHz at 25°C and > 15 GHz at 85°C were
obtained. At room temperature, the bandwidth was found to be limited primarily by the still relatively large oxide
capacitance, while at 85°C the bandwidth was also limited by the thermal saturation of the resonance frequency.
Transmission up to 32 Gb/s (on-off keying) over multimode fiber was successfully demonstrated with the VCSEL biased
at a current density of only 11 kA/cm2. In addition, using a more spectrally efficient modulation format (16 QAM subcarrier
multiplexing), transmission at 40 Gb/s over 200 m multimode fiber was demonstrated.
We have performed an extensive experimental study of the high-speed digital modulation characteristics of BCB-planarized oxide confined 850 nm VCSELs. In particular, we have compared the performance of single- and multimode VCSELs intended for high capacity free space optical interconnects. The digital modulation characteristics were evaluated by recording eye diagrams from 2 to 12 Gbit/s and measuring bit-error-rates (BER) at 10 Gbit/s. The single-mode VCSELs produce open and symmetric eyes at all bit rates and this behavior is maintained under large variations in bias current and modulation depth. For the multimode VCSELs, symmetric eyes can only be achieved under certain bias and modulation conditions. Both VCSELs allow for error free transmission at 10 Gbit/s. The receiver sensitivity at a BER of 1-9 under optimum bias and modulation conditions was -12.9 and -13.4 dBm for the single-mode and multimode VCSELs, respectively. The single-mode VCSELs produce near Gaussian beams ideal for free space optical interconnects where well defined and stable beams are needed for high efficiency and low cross-talk. The multimode VCSELs, on the other hand, have beam characteristics that are unpredictable and dependent on bias current, and are therefore less favorable for free space optical interconnects.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.