| Summary: | 碩士 === 國立成功大學 === 光電科學與工程研究所 === 98 === In this essay, dual-beam focused ion beam system (DB-FIB) was used to finish dry etching process on the GaN-based LED devices which were growth on c-plane sapphire substrate by metal-organic chemical vapor deposition (MOCVD). By using this technique, we have fabricated the sub-micrum meter scale LEDs, and measured their electric property and light emission characteristics.
Due to the multi quantum wells (MQWs) of blue LED is formed by InGaN/GaN heterojuction, the lattice mismatch between GaN and InGaN generates naturally built-in piezoelectric polarization and leads to band tilt. The band tilt can abate the overlap of electron and hole wavefunction, and it results in the decreasing of luminescent efficiency. To lower the stress induced piezoelectric polarization, decreasing LED size will be a possible opportunity to release the crystal strain and increase the internal quantum efficiency.
In our experiment, we have already fabricated sub-micro meter scale GaN-based LED by DB-FIB. The scale of fabricated linewidth minimizes to 500nm and still maintains its LED property. It is obvious that after FIB etching the leakage current increases. According to the temperature-dependent (from 77K to 300K) of current-voltage characteristics, the quench of leakage current in low temperature might stem from the defect assist recombination (or call Shockley-Read-Hall recombination). On the other hand, LED specimen which were after process of FIB etching have larger surface-to-volume ratio, meaning that the surface recombination current is more significant.
When mention to optical properties, we can find that the peak wavelength of FIB-damage specimen show blueshift while LED device operate in low current density. This is the evident of stain release by scale down. Besides, strain release can reduce quantum confine stark effect made the luminescent wavelength almost not blueshift with the current increases, and it also promote the internal quantum efficiency. The full width of half maximum (FWHM) in the electric luminescent spectrum, FWHM of the LED device after FIB etching decreases with increasing current density. FIB damage could increase the leakage current and decrease the resistance, so that can decrease the heat generation, too.
There are two distinct yellow luminescent (YL) peaks in our LED devices after ion beam etching, which are not shown in the reference (un-etched) samples. A lot of papers have already studies this phenomenon, the YL is generally caused by GaN material defect. By using SRIM (stopping range of ions matter) 2008 simulate Ga ions incident into GaN, the result reveals there are a lot of vacancies generated (330 vacancy per 1000 Ga ion incident), especially Ga vacancies. We also make a study of Ga vacancy in GaN and draft the position of defect state in GaN band diagram.
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