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Convergence of magnetics and plasmonics through semiconductors

Final Report Summary - COMPASS (Convergence of magnetics and plasmonics through semiconductors)

In the last 5 years we have witnessed a remarkable change in the way we communicate and hence how we organise our data. Our personal web pages, pictures, videos and what we watch is increasingly being stored in the ‘cloud’ which in practice means in vast data centres. These data centres now consume several percent of all the electricity being produced. A large number of them are remotely located in countries with cold climates to manage the temperature and minimise the energy consumption. The associated explosion in the use of the internet has led to a huge increase in total amount of data being stored in the form of magnetic bits on a hard disk drive. The storage capacity shipped in 2010, for example, corresponded to 400 exabytes where an exabyte is 10 to the power of 18. In 2020 it is expected that 7,000 exabytes will be produced. In order to prevent the data centres becoming even larger and consuming more energy we need to keep the storage racks of the same size and denser magnetic memories by increasing the amount of data being stored per unit area. A technology termed Heat Assisted Magnetic Recording (HAMR) is what the magnetic industry believe is the most promising solution to achieving a storage density of a Terabit per square inch and greater. This is a highly technical, multidisciplinary and important challenge which the COMPASS (Convergence of magnetics and plasmonics through semiconductors) project addressed.

As the critical dimension used to store a bit of information becomes less than 50 nanometres a ‘harder’ magnetic material is required which cannot be switched by a magnetic field alone. A heating pulse has to be applied simultaneously. The HAMR technology is based on a laser producing high optical power light which has to be coupled efficiently into a waveguide and delivered to a nanoscale plasmonic antenna which radiates the electromagnetic energy into the magnetic media where the information is being stored and heats the material beyond its Curie temperature. Ideally, for cost and efficiency reasons, the laser should be integrated on the read-write head which imposes severe challenges for a semiconductor laser as the foot-print for the laser is restricted and the laser has to work at an environmental temperature of 60 degrees. The magnetic industry is typically not an expert with photonics and vice versa and hence the collaboration between Seagate – the leading manufacturer of read-write heads in Europe and Tyndall who have 25 years of experience with semiconductor photonics. Particularly the project investigated (1) lasers customised for high temperature operating conditions, (2) lasers with etched facets, (3) the thermo-mechanical aspects of the integrated lasers (4) underlying degradation of lasers (5) system design & integration. These goals were achieved by transfer of personnel between Seagate and Tyndall and by bringing in an expert in plasmonics.

The results of the project are summarised below and more details are in the published papers referenced at the project website:

Lasers customised for high temperature operating conditions: We performed a comprehensive design of experiment optimisation for the epitaxial layer structure of AlGaAs based 840 nm lasers for operation at high temperature (100 °C) using Technology Computer-Aided Design (TCAD). This wavelength and temperature is representative of needs of HAMR. We optimised the waveguide thickness, Al content, doping level, and quantum well thickness. The resultant laser design was grown and the fabricated ridge waveguides were optimised for carrier injection and, at 100 °C, the lasers achieved a total power output of 28 mW at a current of 50 mA, a total slope efficiency 0.84 W/A with a corresponding wall-plug efficiency of 33%. This laser is state of the art and meets the needs for the application.

Lasers with etched facets: A plasma etching process was optimised to produce very smooth mirrors that can be used for an on-chip laser cavity. This step frees the need of cleaving the facets and is simpler to implement than a distributed feedback scheme. As a result it is possible to design a scheme where the laser can be removed from its native substrate using a selective undercutting and then moved to a new substrate (eg the ceramic wafer for the read-write head) in a process called transfer printing. Here we demonstrated the transfer transfer of the fabricated lasers onto a silicon substrate. At 60°C with 100 mA drive current and a voltage of 2.1 V, a 390 μm long laser on a silicon substrate has a total power of > 40mW in continuous wave operation. Such lasers can be the light source for energising the plasmonic transducers for HAMR.

Thermo-mechanical aspects: 2-dimensional temperature distributions of thin-film edge-emitting GaAs 3-μm wide ridge lasers transfer-bonded to substrates with different thermal conductivities, k, were simulated. The thermal resistance, Rth, was compared with a simplified steady-state analytic expression. The effect of laser cavity length, thickness of dielectric passivation layer, contact metal layer thickness and submount material were investigated in order to reduce the thermal resistance of the laser. The simulations show the importance in reducing the GaAs substrate thickness especially for short cavity lengths. The Rth of a 200 μm long laser with the substrate fully removed is 37 K/W, compared with 230 K/W for the laser with a 100 μm thick GaAs substrate. Increased p-contact metal thickness and reduced dielectric layer thickness further reduce Rth. If alumina (k=1.35 W/mK) is used as a submount, its thickness above the heat sink should be minimised to decrease the junction temperature. A 10 μm thick Si submount (k=150 W/mK) above a perfect heat sink provides an acceptably low Rth.

Underlying degradation of lasers: Techniques to passivation the laser facet were investigated. Different chemical treatments and surface protection layers were applied. The lasers were stressed for different times and the change in device characteristics monitored. The facets were examined by transmission electron microscopy to reveal the nature of the changes and identify different defect generation mechanisms.

System design & integration: We proposed a new strategy to deliver light to the near-field transducer (NFT) of a HAMR head using a Mach-Zehnder Interferometer (MZI) waveguide arrangement. The transducer is essentially a nanoscale antenna and has a particular radiation pattern. Our design offers great flexibility in the coupling of light to the NFT in order to match the radiation pattern of the antenna and thus to optimize the energy transfer from the laser light to the plasmonic energy on the NFT. Furthermore, it allows excitation of particular surface plasmon resonances of the transducer (quadrupole or higher) by controlling the coupling angle and the phase of the two beams of light. The optimum phase shift between the transverse electric waveguide modes incident on either side of NFT can be achieved either statically, by making one of the MZI arms longer/shorter compared to the other one, or dynamically, by changing the mode effective index of the MZI waveguide arm through electro-optic or thermo-optic modulation. Another advantage of the design is that the magnetic-write pole can be easily integrated on the same chip, preferably above the center of the MZI, between both arms of the interferometer and away from the propagating mode thus avoiding blocking of the intended light path. We also propose a “droplet” shaped NFT which takes full advantage of the MZI coupling arrangement. It leads to better impedance match of the NFT with the recording media and consequently better coupling of the power. The transducer design allows he confinement of the light in a spot size much smaller than present state-of-the-art lollipop transducer integrated in planar geometry.

The COMPASS project has laid important groundwork for the implementation of HAMR in upcoming magnetic read-write heads. HAMR technology is now being implemented in the Seagate manufacturing plant and has lead to employment of Tyndall graduates. The next stage is to further integrate the laser and the plasmonic transducer. A deep exchange of knowledge between the disciplines of magnetics and photonics has taken place and a long term collaboration between Seagate and Tyndall has developed with follow on projects.