The characteristics of intersubband transitions in III-nitride quantum wells are promising for detectors and all-optical
switches through a high intrinsic speed (~1 THz), and can also provide a high optical saturation power and a desired
small negative chirp parameter in electroabsorption modulators. The high LO-phonon energy allows to improve the
operating temperature of THz emitters. Recent achievements and prospects for intersubband III-nitride photonic devices,
mainly for λ=1.55 μm, are briefly reviewed. Further, means to enhance material quality by achieving crack-free growth
of GaN/AlN multiple-quantum-well (MQW) structures, and by employing intersubband transitions in multiple-quantum-disk
(MQD) structures incorporated into dislocation free GaN nanocolumns are discussed. We investigate the occurrence
of cracks in MBE-grown GaN/AlN MQWs on GaN MOVPE templates with respect to the buffer layer, the number of
QWs and the temperature reduction rate after growth. Intersubband absorption in GaN/AlN MQDs in the wavelength
range 1.38-1.72 μm is demonstrated in three samples grown on Si(111).
We have used MBE to grow MQW structures on MOVPE GaN/sapphire templates. The MQW devices are intended for high speed intersubband electroabsorption modulator devices operating at 1.55-&mgr;m. The GaN/AlN multiple quantum well material was systematically studied regarding the surface morphology, structural characterization and optical property by atomic force microscopy, X-ray diffraction and Fourier transform infrared spectroscopy, respectively. The intersubband resonance energy was also calculated considering many-body effects in n-type doped structures. The multiple quantum well structure showed superior performance in terms of linewidth when grown on GaN templates as compared on sapphire. GaN quantum well and AlN barriers with a thickness of 3.3 and 4.2 nm respectively resulted in FWHM of the intersubband absorption peak as low as 93 meV at an absorption energy of 700 meV. This is promising for intersubband modulator applications.
Electron tunneling between two dimensional electron systems of different electron concentration in magnetic field normal to the layers have been studied. Parallel 2DES were separated from each other by Al0.3Ga0.7As barrier and from highly doped n+-GaAs contact regions by undoped GaAs-layers. In our samples each of the 2DES and adjacent contact region were in thermodynamic equilibrium provided by free carrier exchange. The measured I-V dependencies demonstrated pronounced resonant features arising when ground states of the 2DES's
were adjusted by external bias. The magnetic field shifted resonant peak position. The shift on the voltage scale was linear versus magnetic field, but exhibited discontinuity in particular magnetic fields. The Landau levels pinning in the 2DES's by chemical potential in the contact region explains the experimental findings.
The excitation dependent carrier recombination lifetime in periodically (delta) -doped strained InGaAs/GaAs multiple quantum well structures has been investigated both experimentally and theoretically. Experimentally, we find more than six orders of magnitude increase in the lifetime over that for undoped material due to the spatial separation of photogenerated carriers. This results in strong photo-optic effects and optical nonlinearities. Calculations, on the other hand, predict intrinsic recombination lifetimes in the periodically (delta) -doped material far above those found experimentally. Using secondary ion mass spectroscopy, transmission electron microscopy, cathodoluminescence imaging, and electron beam induced absorption modulation imaging we find evidence for misfit dislocation related recombination mechanisms that limit the carrier lifetime in the strained quantum well material.
The critical layer thickness (CLT), based on the transition from two dimensional to three dimensional (3D) growth, was investigated by the use of photoluminescence in highly strained InxGa1-xAs/GaAs (0.36
The critical layer thickness (CLT) for InxGaixAs/GaAs has been studied by photoluminescence transmission electron microscopy and reflection high energy electron diffraction with the composition x and growth temperatures as parameters. The obtained values of CLT have been compared with theoretical models. None of them coincide with the experimental results. For x 0. 3 the CLT varies rapidly with x while the dependence is slow for x 3. Here the CLT is small typically a few monolayers. The CLT is governed by dislocation generation for x 0. 3 and by the onset of three dimensional growth for x 3.
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