Study of GaN Light Emitting Devices Fabricated by Laser Lift-off Technique

• Main Authors: Chen-Fu Chu, 朱振甫
• Other Authors: Shing-Chung Wang
• Format: Others
• Language: en_US
• Published: 2003
• Online Access: http://ndltd.ncl.edu.tw/handle/57175946587143102217

博士 === 國立交通大學 === 光電工程所 === 91 === The GaN-based wide band gap semiconductors have been employed for blue light emitting diodes (LEDs) and laser diodes. These devices were grown heteroepitaxially onto dissimilar substrates such as sapphire and SiC because of difficulties in the growth of bulk GaN. However, due to the poor electrical and thermal conductivity of sapphire substrate, the device process steps are relatively complicated compared with other compound semiconductor optoelectronic devices. Therefore, fabrication of GaN-based light emitting devices on electrically and thermally conducting substrate by separating sapphire substrate is most desirable. Several techniques were used to achieve this process including, metallization and wafer bonding, lift-off, and layer transfer. In this thesis, we report the research results on the fabrication of free standing GaN LEDs on conductivity substrate. The establishment of laser lift-off (LLO) conditions for freestanding GaN thin film was presented. By combing the LLO process, new p-type ohmic contact metallization, and wafer bonding techniques, the performance of freestanding LLO-LEDs on copper substrate with p-side up and p-side down configuration, and a large-area-emission LEDs were demonstrated. We established the LLO conditions using the undoped GaN. The rate of removable of GaN or etching rate as a function of laser fluence was investigated using the KrF excimer laser at wavelength of l=248 nm with pulse width of 25 ns. The experiment value of threshold laser fluence for removable of GaN layer was estimated of about 0.3 J/cm^2, which is in agreement with the simulated threshold laser fluence of 0.3 J/cm^2 for GaN material. Successful separation of undoped GaN thin film from sapphire substrate was achieved by using the pulsed laser directly passing through the transparent sapphire substrate. The incident laser fluence was set at 0.6 J/cm^2 by taking account into the attenuation and reflection of sapphire and GaN. Characterization of LLO GaN sample using scanning electro microscopy, atomic force microscopy, x-ray rocking curve, and photoluminescence showed that a no major degradation after LLO. For the fabrication of freestanding GaN LEDs, several major considerations and technical approaches were discussed and described. First, the incident laser fluence was modified to a value of 0.8 J/cm^2, corresponding to a laser fluence of about 0.5 J/cm^2 at the interface of GaN/sapphire to complete interfacial decomposition for obtaining the accomplished device thin film. Second, electrical contacts for GaN LEDs by using the new p-type ohmic contact metallization of Ni/Pd/Au were studied. The ‘as-grown p-type GaN’ samples were grown by metalorganic chemical vapor deposition (MOCVD) and the p-GaN layer was annealed in external fluence. The ‘as-grown p-type GaN’ samples were deposited with Ni (20 nm)/Pd (20 nm)/Au (100 nm) and then alloyed in air, nitrogen and oxygen ambient conditions at different annealing temperatures ranging from 350℃ to 650℃ to optimize the alloy condition. Linear I-V ohmic characteristics were observed with the specific contact resistance as low as 1.1x10^-4 ohm-cm^2 for the samples alloyed at 550℃ in oxygen atmosphere for 5 min. Conventionally, the p-GaN layer of GaN-based light emitting devices was in-situ annealed in MOCVD reactor namely the ‘in-situ p-type GaN’. The in-situ annealed Mg-doped p-GaN samples were re-annealed in external furnace with and without cap in higher pressure N2-ambient. Both the hole carrier concentration and the intensity of the Mg-related p-GaN spectrum was increased after the re-annealing process. The specific contact resistance of the re-annealed samples was reduced by two orders of magnitude from 10^-2 ohm-cm^2 to 10^-4 ohm-cm^2. The increasing in hole carrier concentration and the creation of Ga vacancies could be responsible for the reduction of the contact resistance. The GaN light emitting diodes (LEDs) fabricated with re-annealed GaN LED wafer also show the improvement of the turn on voltage at 20 mA and the reducing of average dynamic resistances. Third, the Cu metal was selected for the fabrication of GaN-LLO-LEDs due to it has good electrical and thermal conductivity. Fourth, both soft In bonding process and strong Ni bonding process were investigated and compared for the fabrication of GaN LLO-LEDs. For the soft In bonding, In has relatively soft and low melting temperature (Tm=156℃) that can adherent with different metals and GaN easily to simplify the fabrication process steps of GaN LLO-LEDs. For the strong Ni bonding process, Ni is a good bonding substance compared with In to limit the electrical properties after the bonding process. Ni is a very useful metal to prevent the GaN layer during the inductively coupled plasma reactive ion etching. Finally, two types of GaN LLO-LEDs can be fabricated. We fabricated the InGaN LED structures on sapphire substrate and transferred to Cu substrate by LLO process into two different configurations, namely p-side up LLO-LEDs on Cu and p-side down LLO-LEDs on Cu. The GaN LLO-LEDs on Cu with p-side up and p-side down configurations were fabricated by two different transfer processes and their performance were compared. For fabrication of p-side up LLO-LEDs on Cu, the regular p-side up GaN LEDs on sapphire substrate was first fabricated and then transferred to a supported glass carrier by LLO process. The lift-off film with LED devices was then double transferred to Cu substrate by soft In metal bonding. Although the I-V characteristics showed the same behavior before and after LLO, the light output power of p-side up LLO-LEDs on Cu was reduced to be about 50% compared with the regular GaN LEDs on sapphire. The relatively uneven and rough surface of the bonding interface could affect the light output. However, a high operation current up to 400 mA for the p-side up LLO-LEDs on Cu and about 1.5 times larger heat dissipation capacity for the p-side up LLO-LEDs on Cu compared to the regular LEDs on sapphire was demonstrated. For the fabrication of the p-side down LLO-LEDs on Cu, the LED wafer sample was transferred to Cu substrate by soft In bonding and LLO process. The LEDs was fabricated on Cu substrate. For the p-side down LLO-LEDs on Cu, an n-side up configuration without semitransparent metal contact providing a better current spreading in n-GaN layer was demonstrated. The I-V characteristics showed higher operation voltage; nevertheless, a high operation current up to 400 mA for the p-side down LLO-LEDs on Cu was also obtained. The light output power of p-side down LLO-LEDs on Cu showed a nearly 4-fold increase over the regular LLO-LEDs on sapphire. About 4 times larger heat dissipation capacity for the p-side down LLO-LEDs on Cu compared to the regular LEDs on sapphire was obtained. Several issues of soft indium bonding were improved using strong Ni bonding process. The bonding substance metal of Ni protects the Ni/Pd/Au p-contact metallization scheme for metal bonding process and acts as a mesa for ICP/RIE etching. The strong metal bonding process can provide a pure Ar atmosphere to minimize oxidation of the bonding metals during 400℃ annealing. The p-side down LLO-LEDs on Cu fabricated by strong Ni bonding process showed better L-I-V characteristics compared with the soft In bonding process. In order to scale up the light output power, a large sample size with 800 um× 800 um of p-side down LLO-LEDs on Cu was also fabricated by strong Ni bonding process. The active light emission area of the large size sample increased with increasing current. About 1.8-2.8 times higher light output power for the large size sample compared to the regular sample size with 300um× 300um was demonstrated. The LLO process should be also applicable to other GaN-based optoelectronics devices and in particular suitable for high light output power, high operation current, high heat dissipation capacity, and large area emission.