GaN-on-Si blue/white LEDs: epitaxy, chip, and package

The dream of epitaxially integrating III-nitride semiconductors on large diameter silicon is being fulfilled through the joint R&D efforts of academia and industry, which is driven by the great potential of Ga N-onsilicon technology in improving the efficiency yet at a much reduced manufacturing cost for solid state lighting and power electronics. It is very challenging to grow high quality Ga N on Si substrates because of the huge mismatch in the coefficient of thermal expansion(CTE) and the large mismatch in lattice constant between Ga N and silicon, often causing a micro-crack network and a high density of threading dislocations(TDs) in the Ga N film.Al-composition graded Al Ga N/Al N buffer layers have been utilized to not only build up a compressive strain during the high temperature growth for compensating the tensile stress generated during the cool down, but also filter out the TDs to achieve crack-free high-quality n-Ga N film on Si substrates, with an X-ray rocking curve linewidth below 300 arcsec for both(0002) and(10N12) diffractions. Upon the Ga N-on-Si templates, prior to the deposition of p-Al Ga N and p-Ga N layers, high quality In Ga N/Ga N multiple quantum wells(MQWs) are overgrown with well-engineered V-defects intentionally incorporated to shield the TDs as non-radiative recombination centers and to enhance the hole injection into the MQWs through the via-like structures. The as-grown Ga N-on-Si LED wafers are processed into vertical structure thin film LED chips with a reflective p-electrode and the N-face surface roughened after the removal of the epitaxial Si(111) substrates, to enhance the light extraction efficiency. We have commercialized Ga N-on-Si LEDs with an average efficacy of 150–160 lm/W for 1mm~2 LED chips at an injection current of 350 m A, which have passed the 10000-h LM80 reliability test. The as-produced Ga N-on-Si LEDs featured with a single-side uniform emission and a nearly Lambertian distribution can adopt the wafer-level phosphor coating procedure, and are suitable for directional lighting, camera flash, streetlighting, automotive headlamps, and otherlighting applications.     

Reflection of the transitional electromagnetic wave by inhomogeneous ionized media having a peak of electron density

The reflection coefficient of an inhomogeneous ionized medium having a peak in electron density is analysed. The result shows the existence of peak influences for the phase term of non-peak linearly varying electron density in the medium. The characteristic of the reflection coefficient phase delay versus frequency is related to the electron density at a peak point and the position of the electron peak. Using this theory we can obtain the electron density distribution map in the inhomogeneous ionized medium. To verify the theory the reflection coefficient is compared with the results from the finite difference time domain method; both results give a good agreement.     

Over 1000 V/30 mA operation GaN-on-Si MOSFETs fabricated on Si substrates

We demonstrated the operation of GaN-on-Si metal-oxide-semiconductor field effect transistors (MOSFETs) for power electronics components. The interface states at SiO₂/GaN were successfully improved by annealing at 800 °C for 30 min in N₂ ambient. The interface state density was less than 1 × 10¹¹ cm^-2 eV⁻¹ at E_c − 0.4 eV. The n⁺ contact layers as the source and drain regions as well as the reduced surface field (RESURF) zone were formed using a Si ion implantation technique with the activation annealing at 1200 °C for 10 s in rapid thermal annealing (RTA). As a result, we achieved an over 1000 V and 30 mA operation on GaN-on-Si MOSFETs. The threshold voltage was +2.6 V. It was found that the breakdown voltage depended upon the RESURF length and nitride based epi-layer thickness. In addition, we discussed the comparison of each performance of GaN-on-Si with -sapphire devices.