posted on 2016-04-04, 00:00authored byS.M. Moududul Islam
Semiconductor based deep ultraviolet (<280 nm) optoelectronics has versatile applications in current state of art industrial/research facility. But as shorter wavelengths are approached, a number of fundamental challenges are encountered which hinder the realization of energy efficient deep-UV optical emission. Though optical devices in the entire UV spectral window have been demonstrated by using III-N semiconductors and their alloys, the overall efficiency of such deep UV emitters is still well below 10%. In this work, we investigate novel heterostructure designs to overcome these basic challenges. Instead of the conventional ternary AlGaN alloy quantum wells in the active region of the devices, we explore the use of ultra-thin binary GaN quantum structures. With binary GaN we investigate the effect of the quantum structures on deep-UV emission properties related to compositional fluctuations in ternary alloy wells with a goal to obtain reproducible emission wavelengths in the deep-UV window. Remarkably, extreme quantum confinement with 1-2 monolayer thick GaN quantum wells and quantum dots results deep-UV emission down to 219 nm (5.66 eV), which is more than 2.0 eV above the bulk bandgap of GaN. These wavelengths are the shortest reported till date in such semiconductor heterostructures. The internal quantum efficiencies of the dots and wells are compared in the deep UV regime and dots are found to be 2X more efficient than wells for ~220 nm emission. In an attempt to reduce threading dislocations through the light emitting heterostructure to improve the radiative efficiency, epitaxial growth on high quality bulk substrates has been explored. Necessary conditions for defect free crystal growth by MBE has been identified and 150% efficiency enhancement was achieved with bulk substrates for 280 nm emission. The experimental results were also compared with theoretical calculations showing decent agreement. To improve the injection efficiency, Polarization-induced doping and tunnel-injection assisted deep-UV LEDs were then fabricated from the Molecular Beam Epitaxy grown heterostructures. Deep-UV emission down to 232 nm was successfully demonstrated. A low turn-on voltage and temperature-independent behavior was achieved as a proof of concept for the effectiveness of polarization-induced doping. Negative-differential-resistance (NDR) phenomena was observed consistently for active regions with these ultrathin GaN quantum structures, signifying Esaki-type superlattice tunneling in the active region. A 250 nm deep UV LED has been demonstrated on the bulk substrates by MBE showing improved rectification properties. Future work will comprise of exploring low leakage deep-UV LED designs on bulk substrates including tunnel junctions to improve metal contacts. Also deep-UV LASERs will be designed and fabricated.