Polarization Engineering for Novel III-V Nitride Device Structures

III-V nitride semiconductors have attracted considerable attention for opto-electronic and electronic devices as they are direct band gap semiconductors spanning a wide range of band gaps from 0.7 eV (InN, IR), through 3.4 eV (GaN, UV) to 6.2 eV (AlN, deep UV) and possess built-in polarization with spontaneous and strain-induced polarization components. Polarization in III-V nitrides has been exploited to achieve interband tunneling but the tunnel current is still low due to large barrier height in GaN/AlN/GaN tunnel diode structures. In this work a novel tunnel diode structure exploiting the N-face growth of III-V nitride semiconductors is discussed. By growing along N-face and using InGaN which has a smaller band gap and a high built-in polar- ization Ìâåøeld in the barrier, it is possible to obtain higher interband tunnel current densities as the barrier height will be reduced considerably. Simulations using the WKB approximation show an increase in tunnel current densities by four orders of magnitude. Such tunnel diodes can be used in multi-junction solar cells, multi color light emitters (multi color LEDs) and ohmic contact to p-type wide band gap semiconductors. N-face growth can also help in solving the p-type doping problem for wide band gap nitrides. By compositionally grading GaN to AlGaN along N-face, efficient polarization-induced p-type doping can be obtained. The polarization-induced doping can be used in UV-LEDs for eÌâå±cient injection of holes into the active layers. GaN quantum well polarization doped UV-LED structures were grown using MBE and characterized using RHEED, XRD and AFM. The IV curves obtained were rectifying and the EL spectra at various current levels showed a peak emission wavelength of 382 nm. In addition a shoulder extending to energies higher than GaN band gap (3.4 eV) was observed in the EL spectra signifying recombination occuring in high band gap AlGaN layers. Electroluminescence from AlGaN regions indicates that AlxGa1-xN QWs and AlyGa1-yN barriers can now be used to obtain higher emission intensities for higher energy photons.