Energy band engineering using polarization induced interface charges in MOCVD grown III-nitride heterojunction devices.

摘要

Characteristics of III-nitride based heterojunction devices are greatly influenced by the presence of high density of polarization induced interface charges. Research undertaken in the current doctoral thesis demonstrates the effect of presence of one, three and six sheets of polarization induced charges in three different III-nitride based devices, namely in a photocathode, a high electron mobility transistor (HEMT) and a hyperspectral detector structure. Through a systematic set of experiments and theoretical modeling an in-depth study of the interaction between multiple sheets of polarization induced charges and their impact on energy band profile was undertaken. Various device designs were studied and optimized using device simulations. Subsequently device structures were grown using metallorganic chemical vapor deposition (MOCVD). Growth conditions for III-nitride epilayers were optimized for pressure, temperature and V/III ratio. Devices were fabricated using photolithography and e-beam evaporation. Novel GaN and GaN/AlGaN photocathode structures were developed. First demonstration of effective negative electron affinity (ENEA) in a GaN photocathode without the use of Cs was made. Effect of polarization induced surface charges on photoemission characteristics was successfully explained using simulated energy band diagrams. AlGaN/GaN/AlGaN/SiO2 based back barrier HEMT structures were developed in which bandgap, thin film thicknesses and polarization induced charge density were engineered to demonstrate Normally OFF operation along with the ability to engineer turn ON voltage of the device. Further, AlGaN based tunable hyperspectral detector pixel with 6-heterojunctions, for application in wavelength spectrometry from UV to IR part of the spectrum, was developed. The novel device design used in the hyperspectral detector utilized voltage tunable internal photoemission (IPE) barriers to measure the energy of the incident photon. Detailed IPE measurements were performed on the device structures to distinguish between the hot electron and hot hole photoemission events, originating in both the semiconductor and the metal contact. Presence of high polarization induced electric fields was observed to result in counter intuitive device characteristics that were successfully explained by developing a systematic set of device structures and corresponding simulated energy band diagrams. Voltage controlled emission threshold variation from 3.9 eV to 4.5 eV was successfully demonstrated.