For fiber-based optical data transmission, opto-electronic components are reaching their intrinsic speed limits.The next generation of telecommunications components will require ultra-high frequency detectors that do not suffer from parasitic capacitance due to space-charge effects. Given the physical constraints in terms of operation wavelength, a technology taking advantage of both a resonant absorption process and optical non-linearities would be ideal.

Such a process occurs between the bound levels of a quantum well (QW) fabricated from ultra-thin semiconductor layers. Intersubband (ISB) absorption, as it is called, is not only highly efficient but has the additional advantage of ultra-fast recovery time. In the past, it has been used in compound semiconductor systems (e.g., GaAs/AlGaAs and InGaAs/InAlAs) for fabricating photoconductive quantum-well infrared photo-detectors (QWIPs) in the mid-infrared (mid-IR) range.¹ However, in order to access the shorter near-IR wavelengths, a different class of materials, with considerably larger conduction-band discontinuity, must be employed.

In our work, we stack and interleave thin gallium nitride quantum well (GaN QW) and aluminum nitride (AlN) barrier layers. This combination offers a conduction band discontinuity of nearly 2eV, which is sufficiently large to accommodate ISB transitions in the near-IR range. In order to avoid dark current noise, a photovoltaic detection mechanism is used for this work. As Figure 1 shows, electrons undergo a small lateral displacement (along the growth direction) when being excited into the upper-bound level of a QW. This displacement results in the formation of an electrical dipole that polarizes the surrounding material. Added up across the entire quantum well and repeated several times, such polarization results in an appreciable photoinduced voltage. This mechanism, first described in 1989, is also called resonant optical rectification.²

Figure 1. Asymmetric quantum well in gallium nitride/aluminum nitride (GaN/AlN) superlattice.

Our photodetector structures are epitaxially grown on c-face sapphire substrates using plasma enhanced molecular beam epitaxy. The active region consists of a 40-period superlattice with 1.5nm thick Si-doped GaN QW layers and 1.5nm undoped AlN barriers. This superlattice is deposited on a 1μm AlN buffer and covered with a 100nm AlN cap layer. Processed photodetectors are entirely planar and consist of a pair of evaporated metal contacts: a dark reference contact and an illuminated signal contact with a size of 100μm². The photovoltage produced by illumination is then measured between the reference and the signal contact.³

Due to the ISB nature of the involved optical transition, a spectrally narrow response results with a center wavelength of 1.55μm and a relative linewidth on the order of 10%. Although maximum performance occurs at a temperature of 200K, room temperature operation with a maximum response of 10V/W could also be observed.

For high frequency testing, we illuminated the detector with a directly modulated 1.55μm laser diode coupled to an optical fiber. The detector signal was amplified with a single low-noise amplifier and measured in a spectrum analyzer. As Figure 2 shows, the maximum frequency for which a signal could be seen was 2.94GHz.⁴ Because no special care was taken to reduce parasitic capacitance and impedance mismatch, this value does not constitute the intrinsic speed limit of this novel optoelectronic component. In addition, it shows that the wide bandgap semiconductor GaN is not only suitable for applications in the ultraviolet wavelength region, but also has considerable potential as a material for future optical data transmission systems.

Figure 2. Frequency response of the detector. The inset shows the signal at the maximum frequency for which a signal could be seen.

In sum, the use of an optically non-linear effect in ultra-thin layers of a strongly piezo- and pyro-electric semiconductor has resulted in the demonstration of a high-speed photo-detector for future multi-gbit/s data rate telecom applications. We have demonstrated the high-speed operation of such a device up to frequencies in the lower GHz range. In future experiments, we will optimize these detectors and reduce the parasitic effects in the amplifier electronic circuit. This should eventually result in components operating beyond 40 GHz.

We would like to acknowledge the financial support of the European project "NitWave" (Strep #004170), the Professorship Program of the Swiss National Science Foundation (SNSF), and the National Center of Competence in Research Quantum Photonics (NCCR QP), also of the SNSF.