GaN-based photon conversion devices with vertical-conduction structure

Abstract

The sun provides energy to the Earth which allows humans to survive dynamically and civilizations to develop and prosper Electricity is the key to rapid progress of science and industries Optoelectronic phenomena involve transformation of photons and electrons which is one of the most important discoveries in modern physics Photons and electrons are often applied in general lighting displays sensors lasers illumination sources and solar cells The III-nitride semiconductor material which has a wide direct bandgap and excellent stability has significantly progressed in the last two decades This dissertation studies GaN-based photon conversion devices with a vertical-conduction structure and excellent thermal dissipation and light extraction characteristics The first part of this study focuses on photon-recycling light-emitting diodes (PRLEDs) First green PRLED has a high potential in achieving high green light efficiency under high current density Green InGaN/GaN multiple quantum well (MQW) converter structures with near-ultraviolet (n-UV) pumping source are epitaxially grown on a sapphire substrate simultaneously Green quantum wells (QWs) are pumped with n-UV light to reemit low-energy photons when light emitting diodes (LEDs) are electrically driven with a forward current Efficiency droop is potentially insignificant compared with direct green LEDs because of increased active layer effective volume in optically pumped green LEDs Compared with direct green LEDs light emitting green PRLEDs are no longer limited in the QWs that are nearest to the p-type region which can cause severe Auger recombination and carrier overflow losses Second a trichromatic GaN-based LED that emits n-UV blue and green peaks is combined with single red phosphor to generate white light with a high color rendering index (CRI) and a low correlated color temperature (CCT) Similar to the structure of green PRLEDs blue and green InGaN/GaN MQWs are pumped with n-UV light to reemit low-energy photons when the trichromatic PRLEDs are electrically driven with a forward current The emission spectrum includes three peaks at approximately 405 468 and 537 nm Furthermore the chips are combined with red phosphor to generate white light with a high CRI of 92 at 2900 K color temperature Third a new microrod-like structure of photon-recycling MQW is proposed which is different from previous film-type PRLEDs A safety UV-LED using an indicative microrod structure is demonstrated through this structure to ensure that the invisible and hazardous high photonic energy UV light can be sensed by using the excited eye-sensible green indicator light Different areas of photon-recycling green MQW could be defined through the vertical chip process and the ratio of green and UV light can be changed effectively A microrod structure is suitable to indicate a UV-range with high efficiency and stable conversion within a wide driving current range The second part of this dissertation focuses on photovoltaic and demonstrates InGaN/GaN-based photovoltaic using a metal reflector sandwiched between GaN-based epitaxial layers and a silicon substrate The photovoltaic could increase the effective thickness of an absorption layer Given the high thermal conductivity of a structure solar cells do not show power conversion efficiency (PCE) degradation even under very high concentrations of sunlight With 300-sun illumination PCE is enhanced by approximately 33% compared with 1-sun illumination Vertical GaN-based solar cell provides a great opportunity for future applications of highly concentrated solar system New devices will probably be invented in the near future based on the photon converting technology and such inventions will benefit humans

Date of Award	2015 Aug 3
Original language	English
Supervisor	Jinn-Kong Sheu (Supervisor)