Fundamental studies of nanoscale phenomena in ultraviolet photonic materials and devices

Femtosecond time-resolved and continuous wave optical techniques have been used to study fundamental nanoscale materials issues in III-nitride semiconductors relevant to the realization of high quality ultraviolet light emitters and photodetectors. It is demonstrated that compositional fluctuations in AlGaN active regions grown by plasma-assisted MBE can be employed to create nanoscale spatial localization that enhances the luminescence efficiency and PL lifetime (300-400 ps) despite high defect density (>1010cm-2) by inhibiting movement of carriers to nonradiative sites. Significant enhancement of this phenomenon has been obtained in a DH LED structure grown on a lower defect density (mid-109cm-2) AlGaN template, with PL lifetime increased by nearly a factor of two, corresponding to a defect density in the mid-107 cm-2 range, and only a 3.3 times drop in PL intensity when the temperature is raised from 12 K to room temperature, suggesting up to ~ 30% internal quantum efficiency. Femtosecond, time-resolved electroabsorption measurements of nanoscale high field transport in an AlGaN/GaN heterojunction p-i-n diode show an onset of velocity overshoot at an electric field of ~105 kV/cm for transport in the c-direction of wurzite GaN. Theoretical Monte Carlo calculations employing a full band structure indicate that at fields below ~300 kV/cm this velocity overshoot is associated primarily with band nonparabolicity in the Γ valley related to a negative electron effective mass. In addition, these calculations show that similar behavior is not expected for transport in the basal plane until much higher fields are attained, with important implications for the design of high power, high frequency electronics and avalanche photodetectors.

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