CHARACTERIZATION AND DEVELOPMENT OF SEMICONDUCTOR CASCADE DEVICES

Interband cascade lasers (ICLs) are efficient source of mid-infrared light for many applications that require low power consumption and continuous wave (cw) operation at room temperature. In the last decade, remarkable progress has been made in developing ICLs on GaSb substrates, mainly in the 3 to 4μm wavelength region, but room temperature (RT) cw operation at the longer wavelength region beyond 6μm has not been achieved. Based on the characterization of earlier grown ICLs, InAs-based plasmon-waveguide ICLs were designed and fabricated to achieve improved performance near or beyond 6μm. These lasers were extensively characterized, investigated and analyzed in terms of various performance features and compared to other state-of-the-art lasers. One laser demonstrated a threshold current density (Jth) as low as 333A/cm2 at 300K for emission at 6003nm. This Jth is the lowest ever reported for a mid-infrared semiconductor laser in this wavelength range. These ICLs lased at temperatures up to 293K in cw mode and up to 357K in pulsed mode. A narrow-ridge laser operated in cw at 293K with a threshold input power as low as 0.66W, and was able to generate 3mW/facet output power at 280K, without accounting for beam divergence loss. These results are very encouraging examples of efficient ICLs at long wavelengths with low power consumption. To investigate the quality of the laser beam of these InAs-based ICLs, the far-field patterns were studied and analyzed for both broad-area and narrow-ridge geometries. In the growth direction, near diffraction-limited single-mode beams were obtained. As expected, multi-mode lasing was observed in the lateral direction because the stripe widths were much longer than the lasing wavelength. Also, as expected, these multi-modes were found to be dependent on applied bias current. These far-field profiles were compared to simulations and reasonable agreement was obtained. Beam propagation factors were plotted against the laser stripe sizes and the values were found close to unity along the growth direction -indicating better beam quality, and below 18 along lateral direction -indicating room for improvement. This methodology can be applied to previously fabricated ICLs to better understand their beam optics. In a related research project, single mode operation of ICLs was obtained by designing and fabricating a cleaved-coupled-cavity (CCC) laser. In our case, rather than cleaving to fabricate two cavities, we successfully demonstrated two ion-milling recipes, using a focused ion beam (FIB) to fabricate our CCC lasers. One recipe separated the two sections of CCC lasers electrically and the other recipe milled a narrow slot (air-gap) into the mesa. Based on the optical power from the fabricated cavities, the quality of the milled facet was found to be comparable to that of an as-cleaved facet. This CCC laser demonstrated a single mode operation at ~3109nm at room temperature with a reasonable side-mode-suppression-ratio (SMSR) of 24dB. A continuous tuning range of ~1.1nm and quasi-continuous tuning range of 10.3nm was obtained, which are substantially smaller than that for a state-of-the-art distributed feedback or Bragg reflector type laser; however, this result is encouraging considering its simple operation and low power consumption. Additionally, the applications of the interband cascade devices were extended to high-speed operation. A multi-stage interband cascade infrared photodetector (ICIP) of 20μm×20μm size demonstrated a 1.3GHz operational bandwidth at RT, which is much higher than that for a single stage ICIP. This result clearly shows the advantage of using the cascade architecture in ICIPs. Some important parameters, such as device capacitance, were extracted by fitting the experimental data. The bonding pad capacitance was identified as one of the major limiting factors towards obtaining high-speed operation. The bandwidths of different ICIPs were found dependent on their sizes, which showed that their performance was limited by external circuit parasitic, and not by their intrinsic carrier transit time. This indicates the possibility of further improvement. The quality of high-speed data transmission was investigated by establishing a one-meter free-space optical (FSO) link. Clear and open eye-diagrams along with a strong signal level were observed for multi-stage ICPs up to 50Mb/s (limit of the instrumentation), which indicates better transmission quality with the possibility to increase the link without need for a signal amplifier. In the final research project, carrier transport mechanisms were investigated in interband (IB) and intersubband (ISB) based optoelectronic devices by estimating the reverse saturation current density (J0) from corresponding current-voltage (I-V) plots and by developing a semi-empirical model that was able to relate J0 to device resistances. Two distinct trend lines were observed; the line corresponding to IB-based devices stayed more than an order of magnitude below that of ISB-based devices when J0 vs. ΔE (Energy separation) data were plotted for a large wavelength range. This plot has a significant meaning from a device performance perspective; it indicates the intrinsic advantages that interband cascade devices possess over quantum cascade devices in terms of carrier lifetime