Ahmad N. Al-Omari

Polyimide-planarized vertical-cavity surface-emitting lasers with 17.0-GHz bandwidth

High-speed oxide-confined polyimide-planarized 850-nm vertical-cavity surface-emitting lasers exhibit -3-dB modulation bandwidths up to 17.0 GHz. The devices are fabricated using a reproducible, simple process incorporating polyimide with good adhesion that does not require implantation or semiinsulating substrates to achieve low capacitance.

Low thermal resistance high-speed top-emitting 980-nm VCSELs

Increasing copper plated heatsink radii from 0 to 4 mum greater than the mesa in vertical-cavity surface-emitting lasers (VCSELs) reduced the measured thermal resistance for a range of device sizes to values 50% lower than previously reported over a range of device sizes. For a 9-mum diameter oxide aperture, the larger heatsink increases output power and bandwidth by 131% and 40%, respectively. The lasers exhibit a 3-dB modulation frequency bandwidth up to 9.8 GHz at 10.5 kA/cm2. The functional dependence of thermal resistance on oxide aperture diameter indicates the importance of lateral heat flow to mesa sidewalls

VCSELs with a self-aligned contact and copper-plated heatsink

Top-emitting, 850-nm vertical-cavity surface-emitting lasers with a self-aligned top contact and evaporated gold or plated copper heatsink were fabricated and characterized. Thermal resistance was reduced by 44%, and output power and bandwidth were increased by 38% and 12%, respectively. The fabricated devices exhibit a 3-dB modulation frequency bandwidth up to 16.3 GHz at 10 kA/cm/sup 2/.

Progress and issues for high-speed vertical cavity surface emitting lasers

Extrinsic electrical, thermal, and optical issues rather than intrinsic factors currently constrain the maximum bandwidth of directly modulated vertical cavity surface emitting lasers (VCSELs). Intrinsic limits based on resonance frequency, damping, and K-factor analysis are summarized. Previous reports are used to compare parasitic circuit values and electrical 3dB bandwidths and thermal resistances. A correlation between multimode operation and junction heating with bandwidth saturation is presented. The extrinsic factors motivate modified bottom-emitting structures with no electrical pads, small mesas, copper plated heatsinks, and uniform current injection. Selected results on high speed quantum well and quantum dot VCSELs at 850 nm, 980 nm, and 1070 nm are reviewed including small-signal 3dB frequencies up to 21.5 GHz and bit rates up to 30 Gb/s.

Temperature Dependence of 980-nm Oxide-Confined VCSEL Dynamics

Temperature dependence of 980-nm vertical-cavity surface-emitting laser (VCSEL) dynamics is reported. Resonance frequency and damping factor of a 7-m active diameter laser have been measured using noise spectra. The primary cause of reduced bandwidth with increasing temperature was reduced photon density. Differential gain is relatively constant for temperatures from 10 C to 70 C, and damping analysis showed there was also no significant change in -factor as a function of temperature.

Fabrication, performance and parasitic parameter extraction of 850 nm high-speed vertical-cavity lasers

High-speed, oxide-confined, polyimide-planarized 850 nm vertical-cavity surface emitting lasers (VCSELs) with different aperture sizes were fabricated and characterized. Comprehensive small signal measurements and analysis were conducted. The VCSELs exhibit intrinsic, parasitic and thermal maximum bandwidth limitations of 42.3 GHz, 21.5 GHz and 17.6 GHz, respectively. Devices with a 10 μm oxide aperture exhibited a maximum modulation bandwidth of 15.3 GHz, limited by thermal effects, and a modulation current efficiency factor (MCEF) of 9.2 GHz mA −1/2 . A VCSEL equivalent circuit model which incorporates the frequency dependence of the polyimide dielectric permittivity and loss is presented. A genetic algorithm (GA) was utilized to extract the parasitic circuit elements from measured microwave reflection coefficients (S11) over a frequency range of 50 MHz to 20 GHz for different device sizes at different bias currents. Good agreement between extracted and calculated parasitic circuit element values was obtained. Several modifications to the device’s fabrication steps and structure are suggested to further improve the device’s output power and modulation bandwidth. (Some figures in this article are in colour only in the electronic version)

High-speed polyimide-planarized vertical-cavity surface-emitting lasers

Oxide-confined, polyimide-planarized 850 nm vertical cavity surface emitting lasers (VCSELs) with excellent high-speed performance were fabricated and characterized. The reproducible, simple process provides good metal adhesion to photodefined polyimide offering low capacitance without implantation or semi-insulating substrates. Microwave measurements are used to extract parameters for a physically based equivalent circuit for the VCSEL.

Novel VCSEL structure with reduced constraints on modulation bandwidth

A novel high-speed, bottom-emitting, flip-chipped vertical cavity surface emitting laser (VCSEL) structure with reduced constraints on modulation bandwidth is designed based on high-speed, fabrication and characterization. Results from top-emitting devices are reported. Benefits of the proposed structure are presented.

Investigations on the high temperature compatibility of various adhesion layers for platinum

In this paper we report on the high temperature compatibility of various adhesion layers for plat inum (Pt ) thin films. We investigated different adhesion layers, such as titanium (Ti), tantalum (Ta), aluminium nitride (AlN), aluminium oxide (Al2O3) and titanium oxide (TiO2). All films were deposited on SiO2/Si substrate by using the sputter technique. After deposition the films were annealed in air at 800°C for different time lengths up to 16 h ours. After annealing, Al2O3 and TiO2 showed a dense oxide layer between Pt and SiO2/Si and they seem to be suitable as adhesion layers for Pt at high temperatures. AlN is not suitable as adhesion layer for Pt at high temperatures. Ti and Ta are also not suitable for high temperatures, diffusing strongly into Pt layers and leading to the format ion of oxide precipitates (TiOx or TaOx) in the Pt grain boundaries. In addition, the format ion of Pt-crystallites (hillocks) on the surface was common in all the films.

Pressure dependence of the quality factor of piezoelectrically driven AlN/Si-microcantilevers

In this work, the fabrication process of piezoelectric AlN cantilevers is presented. The cantilevers were electrically characterized in a vacuum chamber offering the possibility to close-loop control the back pressure from atmospheric conditions down to 5x10-3 mbar. The quality factor (Q factor) is an important figure of merit to evaluate the performance of micro-resonators. In particular, two different modes were detected and analyzed. The first bending mode detected at 19.5 kHz has a quality factor of 470 at atmospheric pressure which increases continuously to 985 at 1x10-1 mbar. The corresponding resonant frequency shifted from 19.500 kHz at atmospheric pressure to 19.573 kHz at 5 mbar. Below this pressure level, the resonance frequency stays unaffected within the measurement accuracy. The second bending mode detected at 117.264 kHz exhibits a quality factor of about 570 at atmospheric pressure increasing continuously to 1275 at 1x10-1 mbar. In agreement with the other resonant frequency under investigation the corresponding resonant frequency decreased from 117.264 kHz at atmospheric pressure to 117.630 kHz at 5 mbar.