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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.

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 ( $V_{F}$ , 2.75 V at 350 mA, 3.04 V at 1000 mA, 1 mm2), low reverse leakage current ( $IR$ , 0.1– $0.25~\mu \text{A}$ 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.

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.

In this work, we first proposed an innovation on GaN p-type reflective electrode in flip-chip LEDs (FCLEDs) using a novel method so called electroless silver plating (ESP). In our ESP method, we use chemical way to form a perfect Ag reflector, differ from traditional fabrication of p-type reflective electrode using electron beam deposition (EB). Under our investigation, the Ag reflective electrode formed by ESP possesses better optical and electric characteristics. Compared with EB, the reflectance of electrode by ESP over the whole visible light spectrum was 93%, higher than EB samples, which only show 82%. Otherwise, ohmic contact characteristic between metal and GaN also shows the same rule as reflectance. ESP samples exhibit better specific contact resistance than EB samples, they're 6.5 ∗ 10−3Ω · cm2 and 6 ∗ 10−2Ω· cm2, respectively. Moreover, the fantastic results of I-V as well as L-I measurement likewise verify electroless silver plating to be a potential manufacturing technology of p-type reflective electrode.

The emission from a light emitting diode (LED) that is emitted under the metal electrode cannot escape into free space. A current blocking layer (CBL) is used to address this issue by forcing the current to flow laterally under the electrode reducing the emission absorbed and hence increasing the overall efficiency of the LED. In this paper a new method to fabricate Schottky and isolating CBLs in GaN LED are investigated. Optical and electrical measurements of these vertical LEDs with and without CBL show different light output powers at identical current densities. The results of this study indicate that CBLs could also be used to suppress the efficiency droop effect for GaN LEDs.

Low resistance and thermally stable n-type contact to N-polar GaN is essentially important for vertical light emitting diodes (VLEDs) . In this paper, the difference of the contacts deposited on wet-etching roughed and un-roughed surface was investigated. It was found that VLEDs with contacts deposited on roughed surface exhibits lower leakage currents yet higher operating voltage. Based on this, a new scheme for depositing metallization contacts has been developed. VLEDs fabricated based on this scheme shows extremely low operating voltage (2.75V@350mA), low reverse leakage current (0.015μA@-5V). There is no optical output compromise and aging test further confirmed its stability.

In this experiment, a flip-chip light-emitting diode with photonic crystal was fabricated at the interface of p-GaN and Ag reflector via nanospheres lithography technique. In this structure, photonic crystal could couple with the guide-light efficiently by reason of the little distance between photonic crystal and active region. The light output power of light emitting diode with embedded photonic crystal was 1.42 times larger than that of planar flip-chip light-emitting diode. Moreover, the embedded photonic crystal structure makes the far-field divergence angle decreased by 18° without spectra shift. The three-dimensional finite difference time domain simulation results show that photonic crystal could improve the light extraction, and enhance the light absorption caused by Ag reflector simultaneously, because of the roughed surface. The depth of photonic crystal is the key parameter affecting the light extraction and absorption. Light extraction efficiency increases with the depth photonic crystal structure rapidly, and reaches the maximum at the depth 80 nm, beyond which light extraction decrease drastically.

A Ni/Ag based metallization contact to p-type GaN (p-GaN), free of a subsequent annealing process, was optimized to couple excellent optical reflectance (as high as 90% at 460 nm) and electrical characteristics (specific contact resistivity as low as 2.1 × 10−5 Ω cm2) simultaneously. Vertical light emitting diode devices with optimized Ni/Ag/Pt/Au contacts were fabricated, and they exhibited extremely low forward voltage (2.75 V@350 mA, approaching its corresponding theoretical limit) and light output power was improved by 35%. Chip-on-wafer mapping tests revealed significantly high yield and excellent uniformity across the wafer. X-ray diffraction results indicated improved crystalline quality and more favourable crystal orientation of Ni [1 1 1]∥Ag [1 1 1]∥GaN [0 0 0 2] at high deposition temperature. Auger electron spectroscopy profiles were used to examine the inter-diffusion of Ni/Ag atoms and Ga atoms in p-GaN, which led to the formation of a Ni/Ag–Ga alloy interface and the improvement of hole concentration in p-GaN. As a result, the chips' reliability and Ni/Ag-p-GaN contact characteristics were improved.

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.

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.

In this work, the advantages of the p-InGaN/AlGaN electron blocking layer (EBL) for InGaN/GaN light-emitting diodes (LEDs) were studied numerically and experimentally. The LEDs with p-InGaN/AlGaN EBL exhibited better optical performance over a wide range of carrier concentration due to the enhancement of holes' injection and electrons' confinement. The values of A, B, C, and D coefficients had been iteratively obtained by fitting quantum efficiency in the modified rate equation model. The analysis indicated that the improvement in the device properties could be attributed to the relatively small band gap and p-type doping of InGaN insertion layer.

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.

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.

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.