To reduce the $/lm for GaN-based LEDs, most LED makers are adopting flip-chip based Chip Scale Packaging (CSP) technology. However, it is difficult to realize true wafer-level (WL) CSP technology with conventional sapphire substrates caused by “blue light leak” issue. On the other hand, thin-film flip-chip technology practically eliminates blue light leak and allows a simpler process of phosphor coating and dicing.
To maximize the advantages of WL-CSP, large diameter process is necessary for it to be cost effective. In this sense, 8 inch Si wafer is the best candidate. GaN-on-Si based LEDs, however, have seen little growth in the lighting market due to several issues that hamper the efficiency and reliability related to the quality of GaN films grown on Si. However, we have overcome all drawbacks including yield, device reliability and LM-80 by proprietary stress-managed buffer and optimized LED epitaxial structure. In addition to the comparable efficiency and reliability performance, we discovered several other advantages of using 8 inch Si substrates such as the wavelength uniformity, low thermal droop and low compressive strain of MQW.
Wafer-level chip scale package (WL-CSP) on silicon substrates has its own set of advantages such as relatively easier to texture the GaN or phosphor surface at the wafer level to maximize photon extraction efficiency and multi-layer phosphor coating to further push the efficiency upwards.
We model carrier-density-dependent radiative and non-radiative recombination rates in an InGaN/GaN quantum well structure containing a V-pit and a threading dislocation. It is known that the threading dislocation acts as the nonradiative recombination center, leading to the reduction of carriers which can participate in the radiative recombination. On the other hand, the quantum well structure grown on the sidewalls of V-pit, formed by the strain relying on In/Ga contents and connected with threading dislocation directed along the polar direction, plays a role of energy barriers to prevent quantum well in-plane charge carriers from flowing to the non-radiative recombination center, i.e., the threading dislocation. Therefore, such V-pits can enhance the internal quantum efficiency in the InGaN/GaN quantum well light emitting diode (LED). However, the explicit model of the V-pit and the threading dislocation coupled to three dimensional electronic states has rarely been studied. We take into account those defects by including their potentials in a system Hamiltonian. It can describe the electronic states of in-plane quantum well, in which a V-pit and a threading dislocation are positioned. Here we show that charged carriers are more distributed away from the threading dislocation by having the V-pit, and it leads to the reduction of carrier losses to the non-radiative recombination and hence the enhancement of radiative recombination rate. Their effects on the recombination rates depend on injected carrier densities. We also discuss mid-gap defect states, which may be generated due to the threading dislocation and the V-pit.
We have grown LED structures on top of a robust n-type GaN template on 8-inch diameter silicon
substrates achieving both a low dislocation density and a 7 um-thick template without crack even at a
sufficient Si doping condition. Such high crystalline quality of n-GaN templates on Si were obtained by
optimizing combination of stress compensation layers and dislocation reduction layers. Wafer bowing of LED
structures were well controlled and measured below 20 μm and the warpage of LED on Si substrate was
found to strongly depend on initial bowing of 8-inch Si substrates. The full-width at half-maximum (FWHM)
values of GaN (0002) and (10-12) ω-rocking curves of LED samples grown on 8-inch Si substrates were 220
and 320 arcsec. The difference between minimum and maximum of FWHM GaN (0002) was 40 arcsec. The
dislocation densities were measured about 2~3×108/cm2 by atomic force microscopy (AFM) after in-situ SiH4
and NH3 treatment. The measured quasi internal quantum efficiency of 8-inch InGaN/GaN LED was ~ 90 %
with excitation power and temperature-dependent photoluminescence method. Under the un-encapsulated
measurement condition of vertical InGaN/GaN LED grown on 8-inch Si substrate, the overall output power of
the 1.4×1.4 mm2 chips representing a median performance exceeded 484 mW with the forward voltage of 3.2
V at the driving current of 350 mA.
Highly efficient InGaN/GaN LEDs grown on 4- and 8-inch silicon substrates comparable to those on sapphire
substrates have been successfully demonstrated. High crystalline quality of n-GaN templates on Si were obtained by
optimizing combination of stress compensation layers and dislocation reduction layers. The full-width at half-maximum
(FWHM) values of GaN (0002) and (10-12) ω-rocking curves of n-GaN templates on 4-inch Si substrates were 205 and
290 arcsec and those on 8-inch Si substrate were 220 and 320 arcsec, respectively. The dislocation densities were
measured about 2~3×108/cm2 by atomic force microscopy (AFM) after in-situ SiH4 and NH3 treatment. Under the unencapsulated
measurement condition of vertical InGaN/GaN LED grown on 4-inch Si substrate, the overall output power
of the 1.4×1.4 mm2 chips representing a median performance exceeded 504 mW with the forward voltage of 3.2 V at the
driving current of 350 mA. These are the best values among the reported values of blue LEDs grown on Si substrates.
The measured internal quantum efficiency was 90 % at injection current of 350 mA. The efficiency droops of vertical
LED chips on Si between the maximum efficiency and the efficiency measured at 1A (56.69 A/cm2) input current was
We report on a 1060 nm single transverse mode operation of an end-pumped vertical external cavity surface emitting laser (VECSEL). End-pumping scheme is enabled by capillary bonding of a VECSEL chip with a diamond heat spreader followed by a GaAs substrate removal by selective wet etching. The VECSEL structure is consisted of 10 periods of resonant periodic gain with an 8 nm InGaAs single quantum well at the antinodes of the standing wave optical field and a 35 pair AlAs/AlGaAs bottom distributed Brag reflector (DBR). Optical pump efficiency through the bottom mirror is enhanced by a modified DBR structure with a reduced reflectance in 808 nm pump wavelength region. A low threshold pump density of 433 W/cm2 and over 45 W/W optical to optical conversion efficiency are achieved with reflectivity of 94 % output coupler at the heat spreader temperature of 20°C. The laser operates in a circular TEM00 mode (M2<1.5) up to 7 W, and maximum power of 9.1 W is limited by our pump laser power.
We have optimized a resonant gain structure of a 920 nm vertical external cavity surface emitting laser. We found that a long saturated carrier lifetime in shallow quantum well (QW) under a high injection level restricts the laser performance. An insertion of non-absorbing laser in the middle of barrier layers with multi QWs is effective to reduce the saturated carrier lifetime and, therefore, to enhance the laser performance. With the optimized laser structure, which has 10 periods of triple In0.09Ga0.91 As QWs located at the anti-standing wave optical field with Al0.3Ga0.7As non-absorbing layers in the middle of GaAs barrier, we achieved 4.9 W operation at 920nm. Subsequently blue laser was achieved by employing an intra-cavity frequency doubling crystal LBO. As a result, we demonstrated 2 W single transverse mode operation in blue (460 nm) with a 20 W pump laser power. The conversion efficiency from 808 nm pump laser to the blue laser is measured to be 10 %.