We study four compressively strained GaInSb/AlGaInSb type I multi quantum-well (QW) laser
structures grown on GaAs, with increasingly strained QWs, aimed at emitting at ~4μm. This
wavelength region is highly important for applications such a free space communication,
biomedical imaging and trace gas sensing. The structures are analysed using photoluminescence,
photo-modulated reflectance and, at room temperature, using our novel, recently developed Fourier
transform infrared surface photo-voltage spectroscopy technique (FTIR-SPS). Neither
photoluminescence nor photo-modulated reflectance managed to give any characterisation
information at room temperature or such detailed information even at low temperatures. However,
FTIR-SPS clearly yielded a full set of transitions for all four samples including not only the barrier
bandgap, but also the QW ground state transition, from which the device operating wavelengths can
be inferred, and up to five excited state QW transitions. The full set of measured transition energies
are then compared closely with those predicted by an 8-band k.p model which takes account of the
band anisotropy and strain. There is generally a good agreement between the QW transitions
predicted by the model and those measured experimentally, but there is also a strong indication that
the current literature values for the AlGaInSb bandgap seem to be in considerable error for the
present alloy compositions. The FTIR-SPS technique gives information of great importance when
designing future devices to emit in this wavelength region.
Using a least squares technique we fit the measured normal-incidence reflectivity spectra of resonant cavity light emitting diode (RCLED) structures, using six fitting parameters-the thicknesses of the AlAs and AlGaAs layers in the top and bottom Bragg stacks, the thicknesses of the cavity region, and the Al concentration in the AlGaAs components of both Bragg stacks. We find that the fitting procedure indicates growth errors in these thicknesses and in the Al concentrations, and, in particular, gives a best fit when the Al concentration in the AlGaAs component of the Bragg mirrors is typically 61±1% instead of the intended 50%. Furthermore, the fitting reveals that the spatial period of the upper Bragg stack is typically 4% less than that in the lower stack, in these growth runs, a finding which is confirmed by a detailed analysis of scanning electron microscopy images of cleaved pieces of the RCLED wafers. This fitting method provides a useful and non-destructive tool to determine as-grown thicknesses and compositions of complex multilayer heterostructures which are otherwise difficult to ascertain.
The vertical-emitting devices, resonant-cavity LEDs (RCLEDs) and vertical-cavity surface-emitting lasers (VCSELs)
are key components in a broad range of applications including optical communications. However, the complexity of these
multi-layer structures causes significant difficulties in their non-destructive characterisation at the pre-fabrication stage, and
they have defied analysis by conventional optical techniques, such as photo-luminescence (PL). Fortunately, a
complementary spectroscopy, modulated reflectance (MR), provides a viable alternative. MR is a simple technique in
which the reflection spectrum of a semiconductor is periodically externally perturbed - most usefully using a
mechanically-chopped laser beam, i.e. photo-modulated reflectance (PR). This yields sharp derivative-like spectra which
are replete with features from ground-state, and, in contrast to PL, many other possible higher-energy optical transitions.
This detailed information enables the deduction of material parameters crucial to efficient device operation, such as
compositions, layer thicknesses, in-built electric fields and band line-ups. PR is truly non-destructive because samples
need no special mounting, can be studied in air at room-temperature, and can be full-sized pre-fabrication wafers. At
Surrey we have pioneered the application and interpretation of MR to the assessment of VCSELs and RCLEDs, and here
we discuss the advances that we have made, which have attracted interest from the growth industry.
This paper presents results that have emerged from the European funded ESPRIT Project, Bright Red Surface Emitting Lasers (BREDSELS-23455). The project's main objective has been to develop arrays of Vertical Cavity Surface Emitting Lasers (VCSEL's) emitting in the region of 650 nm. These VCSEL arrays, suitably coupled to plastic fiber ribbon, are potentially ideal sources for high-speed plastic optical fiber networks. Linear 1 X 8 VCSEL arrays have been fabricated from wafers grown in multi-wafer MOVPE reactors. Individual VCSELs are shown to generate a peak room temperature power of 2 mW at 674 nm and are capable of operating continuous wave to a temperature of 60 degrees Celsius. The use of selective oxidation in the fabrication process is found to be essential in terms of providing effective heat sinking to the active region, while free carrier absorption is found to be a significant loss mechanism. A detailed description of the device results including modal behavior is presented along with the initial results from the plastic fiber ribbon module.
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