Researchers grow a lateral-junction VCSEL

KYOTO-Vertical p-n junction laser diodes have electrons and holes injected in the active layer through higher bandgap layers that are used to create optical and carrier confinement. The carriers thus have unnecessary excess energy that must be dissipated before carriers become available for radiative recombination. The time required to do this, called the relaxation time, can limit the modulation bandwidth of conventional quantum-well semiconductor lasers.

Aug 1st, 1999
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KYOTO-Vertical p-n junction laser diodes have electrons and holes injected in the active layer through higher bandgap layers that are used to create optical and carrier confinement. The carriers thus have unnecessary excess energy that must be dissipated before carriers become available for radiative recombination. The time required to do this, called the relaxation time, can limit the modulation bandwidth of conventional quantum-well semiconductor lasers.

Researchers at the Adaptive Communications Research Laboratories of the Advanced Telecommunications Research Institute (ATR) think they have a unique solution to this problem-forming a lateral p-n junction via epitaxial growth of silicon-doped gallium-arsenide layers on a patterned gallium-arsenide (n11) A substrate.1

Pablo Vaccaro and colleagues Hajime Ohnishi and Kazuhisa Fujita used molecular-beam epitaxy to grow a vertical-cavity surface-emitting laser (VCSEL). Vaccaro reports that the lateral p-n junction allows direct injection of electrons and holes into the indium gallium arsenide (InGaAs) active layer of the VCSEL, without first flowing them through the higher-bandgap material of the distributed Bragg reflectors. The resulting carriers have little excess energy, thus a shorter relaxation time (see figure).

In the VCSEL structure, the active layer is a double quantum well of InGaAs. The structure requires distributed Bragg reflectors with just a few periods to obtain the required high reflectance because there is a large refractive-index difference between the oxidized aluminum arsenide layers and unoxidized aluminum gallium arsenide layers.


The p-n junction of the lateral-junction VCSEL structure is at the intersection between the mesa sidewall and the flat region. Side view (top) illustrates how the etched mesa exposes the edge of the silicon-doped GaAs layers inside the optical cavity where the electrical contacts are attached.
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Another reported benefit of lateral p-n junctions is the capability to reduce the applied bias required to inject a given current, which can improve power efficiency of the device. Lateral junctions also can be orders of magnitude smaller in size than vertical junctions, reducing capacitance of the device.

When testing the device, the researchers measured a threshold current for laser emission at approximately 2.3 mA. The laser emits only in pulsed mode at room temperature because a high series resistance of 1400 W prevents continuous-wave operation. This resistance originates mostly in the p-type silicon-doped GaAs layer. (According to Vaccaro, enhancing device performance will require both a higher doping level and thicker silicon-doped GaAs layers.)

Above threshold, the light-emission spectrum has a single peak at 942 nm with FWHM of 0.15 nm. This linewidth is reportedly quite narrow for a VCSEL with an optical-cavity length of only one wavelength.

In essence, the ATR researchers have proposed a new way to solve the relaxation time problem without further complicating the growth process. Vaccaro notes that the device can be grown on a semi-insulating substrate that facilitates the fabrication of arrays and integrated circuits with coplanar contacts. In addition, the 2.62-µm total thickness of the structure is much smaller than that of conventional designs, which means reduced epitaxial growth time and simplified device processing. The lateral-junction concept also works for fabricating edge-emitting laser diodes and light-emitting diodes.2

Paula M. Noaker

REFERENCES

  1. P. Vaccaro et al., Appl. Phys. Lett. 74, 3854 (June 21, 1999).
  2. P. Vaccaro et al., Appl. Phys. Lett. 72, 818 (Feb. 16, 1998).

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