Polarization-Engineered N-face III-V Nitride Quantum Well LEDs

摘要

III-V nitride semiconductors 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). This makes them uniquely suited for fabricating visible and UV LEDs [1]. UV LEDs have applications in water purification, microscopy and chemical analysis. However, wide band gap nitrides suffer from poor p-type doping owing to large activation energy of Mg acceptor dopant (E_A～200 meV for GaN [2] and 650 meV for AlN [3]). This results in low thermal activation of holes at room temperature and causes low p-type conductivity. III-V nitrides also exhibit large built-in polarization field with spontaneous and strain induced piezoelectric components [4]. The polarization has recently been exploited to demonstrate N-face AlGaN/GaN p-n heterojunctions with improved p-type conductivities and electroluminescence [5]. In this work, we demonstrate that incorporating quantum wells (QWs) into the active regions improves electroluminescence (EL). Simultaneously, a number of advantages of N-face structures emerge from the point of view of polarization-engineering. Optical device structures grown along the N-face orientation offer a number of unexplored features. Fig 1 shows the simulated energy band diagrams for GaN QWs incorporated between AlGaN barriers on Ga-face and N-face GaN substrate at a forward bias of 3V. The polarization field in the barriers for Gaface structure points in the opposite direction to that in the N-face structure. The polarization field opposes electron/hole injection into the quantum wells from the n- and p-regions for the Ga-face structures but assists this process in the N-face structures. Traditional electron blocking layers are sharp AlGaN/GaN heterojunction, as shown in Fig 2(a). The unavoidable valence band offset also blocks hole injection. To use polarization induced p-type doping on Ga-face LED structures requires grading down to p-type InGaN which is detrimental due to the increase in absorption of emitted light, and the loss of electron blocking layer. N-face structures solve these problems simultaneously as shown in the energy band diagrams in Fig 2(b). Specifically, the polarization-induced p-doped cap layer acts simultaneously as an electron blocking layer while removing any barriers from the path of holes.