Study On N-polar N-type Ohmic Contact And Aging Characteristics Of GaN-based LED

Recently, GaN-based wide band gap semiconductors have attracted considerable interests due their excellent optoelectric characteristics and attractive applications in solid-state lighting. Commercial GaN-based LEDs are usually grown by MOCVD on sapphire or SiC substrates. Compared these substrates, Si has the advantage of lower price, good thermal conductivity and mature technology. Furthermore, GaN grown on Si substrate can be easily transferred to other substrates and fabricated vertical structure LEDs, which are effective in resolving current crowding, high forward voltage drop and poor heat dissipation, when operated under high current compared with conventional lateral LEDs. In addition, Vertical structure LED chips with N-polar n-GaN upturned favored roughening and improve optoelectric performance. Unlike lateral structure LEDs, vertical LEDs require formation of high quality Ohmic contacts on surfaces of both N-polar n-GaN and Ga-polar p-GaN, It is, however, shown that the formation of high-thermal stable ohmic contact on N-polar n-GaN is difficult. In this theis, N-polar n-type ohmic contact of GaN-based LED on Si substrate was studied firstly, secondly the aging characteristics of high-performance GaN-based vertical structure blue LED on Si substrate was studied. The main results of our research are as following:1. For smooth N-polar n-GaN surface with dilute HCl cleaning, as-deposited Ti/Al bilayer metals, Both low contact resistivity and high stability ohmic contact can be obtained.2. Effects of surface treatments on ohmic contact to roughened N-polar n-GaN was studied, The results show Ar plasma treatment and dilute HCl cleaning can reduce the contact resistivity of GaN/Al/Ti/Au and achieve stable operating voltage during aging. XPS results indicates the approach above is not only able to decrease the height of Schottky Barrier of sample surface, and produce a large number of N vacancies in the interface between GaN and Al/Ti/Au.Besides that, The active O atoms could be eliminated completely after the anneal.3. Effects of AlN buffer layer to N-polar n-type ohmic contact of GaN-based LED on Si substrate was studied For the first time. The result shows the exsiting of AlN buffer layer is the key to forming high-thermal stable Ohmic Contact for GaN-based vertical structure LED on Si substrate. The result was attributed to the fact that the existence of AlN prevents the out-diffusion of N and Ga atoms, which might causes N vacancies annihilable and the generation of Ga vacancies near the n-GaN surface region, respectively (N vacancies and Ga vacancies in GaN play the role of donor and acceptor, respectively.).4.Through Circular Transmission Line Methods (CTLM), the nonalloy ohmic contact resistivity of Ti/Al, Al contacts to n-GaN was calculate,ρc-10-5Ω·cm2, futhermore, anneal temperature has no significant effect to ohmic contact resistivity, which shows contact with good stability, the results fully satisfy the demand for high-power LED.5. The characteristics of Ti/Al,Al/Ti/Au,Cr/Pt/Au�'�AuGeNi/Ni/Al/Ni/Au contacts to n-GaN were studied, The results show Al/Ti/Au and Ti/Al contacts can achieved low ohmic contact resistivity and stable operating voltage in the process of aging.6.The optoelectric performance of high-power GaN-based blue LED chips on Si substrate were investigated after aging under 900mA at room temperature for 1044h, and no obvious luminous decay was found, and external quantum efficiency(EQE) almost shows no change and a slight increase for high injection currents and low currents, respectively. The change of domainate wavelength measured at 350mA was less than 0.2nm. This excellent characteristics was attributed to the high quality crystal, excellent heat condctivity of vertical structure chips and the reasonable structure of chip structure. In conclusion, high-power GaN-based vertical structure blue LED chips on Si substrate have excellent reliability.