technology VCSELs
placing bottom-emitting devices with electrical fanout on one side, and using the other side for optics. Such a design requires a transparent substrate, thereby preventing the use of 850 nm VCSELs, the standard source for multi-mode fiber links providing data transmission over hundreds of meters.
980 nm VCSELs are suitable, due to transparency of the GaAs substrate at this wavelength, and they have one big benefit over their shorter wavelength siblings: They enable the construction of distributed Bragg mirrors with binary layers, rather than more complex ternary variants that degrade the thermal conductivity of the chip. Thanks to this advantage, devices built with binary mirrors can operate at much higher ambient temperatures, such as those found when lasers are placed in uncooled dense arrays or integrated on top of a high-performance silicon CPU or memory.
Record-performing VCSEL devices Taking these considerations into account, out team of researchers at the Center of Nanophotonics at the Technical University of Berlin has focused on 980-nm VCSEL devices of ultimate bandwidth under direct modulation. The VCSEL is a well-established device manufactured in very high-volumes, so if our efforts are to be beneficial outside the lab, they must employ processes used in industry. To that end, we fabricate full three-inch VCSEL wafers with a very high production yield. Our lines of enquiry for improving VCSEL performance are transferred into the mask layout, which features systematically varying device design parameters. After processing in our own class-10 cleanroom dedicated to the development of III-Vs, a robot automatically measures the performance characteristics of thousands of devices.
We use home-built software to handle the vast amount of data generated by these measurements. This enables us to unveil the best chip design from statistical evaluation of measured data. The winning formula is verified with data-transmission and system experiments.
A great strength of this approach is that it enables rapid progress, by exploiting advantages of techniques used in industry and academia. We are able to carry out every step in-house, from simulation to design, epitaxial growth, device processing and a wide range of characterization techniques, including high-speed data- transmission experiments. This is an enviable suite of facilities that many groups do not have.
Efforts in our lab have recently led to tremendous improvements in the performance of 980 nm VCSELs over a wide temperature range. Devices fabricated from a single VCSEL wafer have broken the record for the highest speed, enabled 40 Gbit/s operation at temperatures up to 40 °C, and set a new benchmark for energy efficiency at bit rates beyond 30 Gbit/s (see Figures 1 and 2).
Steps that we took to hit these record-breaking speeds
with our 980 nm VCSELs included a reduction in the cavity length to half the wavelength of light, and a decrease in the reflectivity of the out-coupling mirror to cut photon lifetime and realize higher modulation speeds. We used oxide aperture layers made from 20 nm-thick layers of Al0.98
As that were positioned very close to the active region to reduce parasitic capacitance. These apertures were positioned very close to the active region to avoid damping of the resonance frequency by carrier transport.
Ga0.02
We produced a range of 980 nm VCSELs with oxide- aperture diameters from 1 µm to 10 µm. Designs had six different active media, between three and seven quantum wells with widths of 4 nm to 6 nm, and barriers with and without phosphorous. Unlike our previous generation of VCSELs, these lasers had an abrupt interface, rather than a graded one, around the active region. This change improved carrier confinement at high temperatures.
Testing this portfolio of devices revealed that VCSELs with aperture diameters between 5 µm and 7 µm could combine high output powers with high modulation bandwidths. Typical characteristics for a 6 µm diameter oxide aperture VCSEL were a threshold current of 0.9 mA at 20 °C, a peak power of 8 mW at a rollover current of 22 mA, and continuous wave operation up to 200 °C.
VCSELs with a 5 µm aperture delivered the most impressive performance at low temperatures. Butt- coupled to a 3 m multi-mode fiber, these devices
Fig 3.
Figure 1. Summary of the results achieved on high-speed temperature- stable VCSELs at TU Berlin in 2011
compared to previously published results from various groups
June 2012
www.compoundsemiconductor.net 33
Fig.2.
A successful VCSEL research effort must combine expertise in material growth and device fabrication expertise with dedicated characterizati on capabilities, such as
measurement robots that can candle thousands of devices
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72 |
Page 73 |
Page 74 |
Page 75 |
Page 76 |
Page 77 |
Page 78 |
Page 79 |
Page 80 |
Page 81 |
Page 82 |
Page 83 |
Page 84 |
Page 85 |
Page 86 |
Page 87 |
Page 88 |
Page 89 |
Page 90 |
Page 91 |
Page 92 |
Page 93 |
Page 94 |
Page 95 |
Page 96 |
Page 97 |
Page 98 |
Page 99 |
Page 100 |
Page 101 |
Page 102 |
Page 103 |
Page 104 |
Page 105