Enqing Guo

Efficiency droop in InGaN/GaN multiple-quantum-well blue light-emitting diodes grown on free-standing GaN substrate

In this work, the dislocation-related efficiency droop in InGaN/GaN blue light-emitting diodes (LEDs) was investigated by comparing the external quantum efficiency (EQE) of GaN grown on c-plane sapphire and free-standing GaN substrate over a wide range of operation conditions. The values of A, B, and C coefficients had been iteratively obtained by fitting quantum efficiency in the rate equation model. Analysis revealed that threading dislocation density was strongly related to the decrease in EQE of InGaN LEDs at elevated currents by introducing a number of acceptor-like levels with the energy EA lying within the band gap.

Effects of light extraction efficiency to the efficiency droop of InGaN-based light-emitting diodes

Light extraction efficiency (LEE) droop as an important factor contributing to the efficiency droop of InGaN-based light-emitting diodes (LEDs) has been demonstrated and investigated in detail. The LEE droop effect is induced by the spatial dependence of the extraction efficiency of photons inside of the LED devices and the aggravating crowding effect of the injection electrons around n-type electrodes as injection current increases. A current blocking layer is introduced to alleviate the LEE droop effect. And the light output power of the LEDs is also improved by 43% at an injection current of 350 mA.

Partially sandwiched graphene as transparent conductive layer for InGaN-based vertical light emitting diodes

InGaN-based vertical structure light emitting diodes (VLEDs) with multi-layer graphene transparent electrodes with higher optical output have been fabricated and tested. High temperature annealing introduced inter-diffusion of metal atoms and Ga atoms and generated the partially sandwiched graphene structure, which contributed to the performance improvement of VLEDs.

Interface and transport properties of GaN/graphene junction in GaN-based LEDs

A normalized circular transmission line method pattern with uniform interface area was developed to obtain contact resistances of p-, u-, n-GaN/graphene contacts (p, u and n represent p-type doped, unintentionally doped and n-type doped, respectively) and N-polar u-, n-GaN/graphene contacts in GaN-based LEDs. The resistances of the graphene/GaN contacts were mainly determined by the work function gap and the carrier concentration in GaN. Annealing caused diffusion of metal atoms and significantly influenced the interface transport properties.

Improved transport properties of graphene/GaN junctions in GaN-based vertical light emitting diodes by acid doping

Electrical characteristics of p-, n-GaN/graphene junctions before and after nitric acid doping have been investigated. Acid treatment can significantly improve the junction conductance in both cases, which is advantageous for the light emitting diode (LED) to reduce the operating voltage. GaN-based vertical LEDs incorporating graphene as transparent electrodes are further assembled and tested, showing significant improvement in forward electrical characteristics and light output power upon acid modification.

Stimulated emission at 288 nm from silicon-doped AlGaN-based multiple-quantum-well laser.

We demonstrated stimulated emission at 288 nm from a silicon-doped AlGaN-based multiple-quantum-well (MQW) ultraviolet (UV) laser grown on sapphire. The optical pumping threshold energy density of the UV laser was 64 mJ/cm2, while lasing behavior was not observed in undoped AlGaN MQWs. This means silicon doping could effectively reduce the lasing threshold of UV lasers, and the mechanism was studied showing that the silicon-doped AlGaN MQWs had a 41% higher internal quantum efficiency (IQE) compared with the undoped one. The transmission electron microscopy characterization showed that silicon doping explicitly improved the crystallographic quality of MQWs. Calculation of the polarization charge in the MQWs further revealed that the advantage of better structure quality outweighed the reduction of internal polarization field by Si doping for the IQE enhancement and successful stimulated emission.

InGaN-based vertical light-emitting diodes with acid-modified graphene transparent conductor and highly reflective membrane current blocking layer

Vertical light-emitting diodes (VLEDs) with a highly reflective membrane (HRM) as current blocking layer (CBL) and a graphene transparent conductive layer (TCL) have been fabricated and characterized. High reflectance of HRM and high transmittance of graphene ensure less loss of the optical power. The VLEDs show improved optical output and less efficiency droop, thanks to the current spreading effect of HRM CBL and graphene TCL by preventing current crowding under the top electrode and thus increasing the internal and external quantum efficiency. With further acid modification, forward electrical characteristics of VLEDs are improved.

Interface and Transport Properties of Metallization Contacts to Flat and Wet-Etching Roughed N-Polar n-Type GaN

The electrical characteristics of metallization contacts to flat (F-sample, without wet-etching roughed) and wet-etching roughed (R-sample) N-polar (Nitrogen-polar) n-GaN have been investigated. R-sample shows higher contact resistance (Rc) to Al/Ti/Au (~2.5 × 10(-5) Ω·cm(2)) and higher Schottky barriers height (SBH, ~0.386 eV) to Ni/Au, compared with that of F-sample (~1.3 × 10(-6) Ω·cm(2), ~0.154 eV). Reasons accounting for this discrepancy has been detail investigated and discussed: for R-sample, wet-etching process caused surface state and spontaneous polarization variation will degraded its electrical characteristics. Metal on R-sample shows smoother morphology, however, the effect of metal deposition state on electrical characteristics is negligible. Metallization contact area for both samples has also been further considered. Electrical characteristics of metallization contact to both samples show degradation upon annealing. The VLED chip (1 mm × 1 mm), which was fabricated on the basis of a hybrid scheme, coupling the advantage of F- and R-sample, shows the lowest forward voltage (2.75 V@350 mA) and the highest light output power.

Electroless Silver Plating Reflectors to Boost the Performance of Vertical Light-Emitting Diodes

In this paper, we experimentally demonstrated a novel approach known as electroless silver plating (ESP) for the fabrication of Ag-only reflectors on vertical light-emitting diodes (VLEDs). The Ag reflector that was obtained through the ESP approach shows a higher reflectance of 83% and a lower contact resistivity of 6 * 10-3 Ω · cm2 with p-GaN, compared with that fabricated through the conventional electron beam (EB) approach, which shows a reflectance of 75% and a contact resistivity of 6.2 * 10-2 Ω · cm2, respectively. Improvements in the light output intensity-current (L-I) and current-voltage (I-V) results in VLEDs, which originated from the high reflectance and low-contact resistivity of the incorporated ESP-Ag reflector. Our result opens up an alternative and promising way to substitute the traditional EB approach for Ag-based reflector fabrication and, thus, is practically meaningful in real device fabrication, such as with VLEDs.

High-Performance Nitride Vertical Light-Emitting Diodes Based on Cu Electroplating Technical Route

GaN-based vertical-geometry light-emitting diodes (V-LEDs) are considered as ideal candidates for future high-power and high-efficiency lighting devices. However, the optoelectrical performance, yield, and stability of V-LEDs are still inferior than the conventional lateral LEDs (L-LEDs) due to some fabrication process hurdles, such as the p-type reflective electrode, nitrogen-polar (N-polar) n-type contact, surface texturization, insufficient current diffusion, and the foreign conductive substrate transfer. Here, we present a minireview of our long-term efforts on V-LEDs fabrication based on Cu electroplating foreign substrate transfer technical route. The electroplating approach shows its inherent advantages of moderation, flexibility, and reliability, yet still hampered by some technical hurdles specifically for V-LEDs fabrication: nonuniformity, easily thermal deformation, difficult to dice out. Since the foreign substrate transfer process is the most critical step, here, first, we introduced our efforts on Cu electroplating manipulation in detail: optimized the polishing and grinding process, Cu:W pseudoalloy and Cu/Ni bilayer electroplating, hybrid wet-etching plus dry laser scribing, and fully wet-etching approaches. Then, we summarized our investigation results for the p-type reflective electrode, nitrogen-polar (N-polar) n-type contact deposition, surface texturization, and current diffusion boost design. Based on the rationally manipulated process, the assembled V-LEDs show excellent optoelectrical performance: record-low forward voltage (VF, 2.75 V at 350 mA, 3.04 V at 1000 mA, 1 mm2), low reverse leakage current (I R, 0.1-0.25 μA at -10 V), high lumen efficiency (115 lm/W at 350 mA), low thermal resistance (RT, 1.58 °C/W for V-LEDs chip and 12.06 °C/W for V-LEDs lamp), high yield (>90%), and long-term stability.