| Abstract Scope |
High-power GaN-based vertical light-emitting diodes (VLEDs) are considerably important for solid state lighting application. One of the main barriers to the realization of solid-state light is the formation of high-quality ohmic contacts having low-resistance and excellent reliability. For conventional lateral LEDs, n-type ohmic contacts are easily formed on Ga-face n-GaN using either Ti- or V-based schemes. However, for n-type side-up VLEDs (where the n-GaN has N-polarity), n-type ohmic contacts were shown to be difficult to form. For example, when Ti/Al contacts to n-GaN were annealed at temperatures exceeding 500°C, Ga-face n-type contact showed ohmic behaviours with a contact resistivity of ~10<sup>-5</SUP> Ωcm<sup>2</SUP>. However, N-face n-contacts produced non-ohmic behaviours. Furthermore, Ti/Al contacts to N-face n-GaN thin films prepared by a laser lift-off process yielded significantly degraded electrical properties when annealed at temperatures above 300°C. The degradation was attributed to the absence of polarization-induced two-dimensional electron gas caused by the formation of interfacial AlN on p-GaN. In particular, unlike lateral LEDs, the fabrication of vertical LED structures requires relatively low processing temperatures not to damage host substrates formed by wafer bonding or electroplating. Thus, in this work, in order to develop high-quality n-contacts to N-face n-GaN for high-performance vertical LEDs by avoiding damaging the host substrates, we introduced a laser-annealing process. Ti/Al/Pt contacts were first laser-annealed at 400 − 600 mJ/cm<sup>2</SUP> in air by means of KrF excimer laser (248nm) prior to rapid-thermal annealing at 250°C. The electrical properties of the Ti/Al/Pt contacts to N-face n-type GaN were investigated. It is shown that all the Ti/Al/Pt contacts exhibit ohmic behaviors before annealing. Unlike the contacts without laser annealing, the laser-treated samples produce ohmic behavior after annealing at 250°C for 1 − 2 min. It is further shown that the laser-treated samples give almost constant total resistance although annealed at 300°C for 60 min in an oven. However, the untreated samples become gradually degraded with increasing annealing time. To understand the ohmic formation mechanisms, X-ray photoemission spectroscopy examination were performed on the samples before and after laser-annealing. For untreated samples, the Ga 2p core level shifts toward the lower binding energy side upon annealing at 250°C. For the laser-annealed samples followed by rapid-thermal annealing at 250°C, however, the Ga 2p core level remains almost unchanged after annealing. Furthermore, measurements showed that the normalized N/Ga atomic ratios are 1.04± 0.02 and 1.00± 0.01 for the samples without and with laser annealing, respectively. Based on the x-ray photoemission spectroscopy, Auger electron spectroscopy, and TEM results, the laser-annealing effect on the electrical characteristics of n-contacts to N-face n-type GaN layers are described and discussed. |