Xinbo Zou

Performance improvement of GaN-based light-emitting diodes grown on patterned Si substrate transferred to copper.

LEDs on Si offer excellent potential of low cost manufacturing for solid state lighting and display, taking advantage of the well-developed IC technologies of silicon. In this paper, we report how the performance of LEDs grown on Si can be improved. Multiple quantum well InGaN LED structure was grown on patterned silicon substrates and circular LEDs 160 µm in radius were processed. Fabricated LEDs were then transferred to an electroplated copper substrate with a reflective mirror inserted by a double-flip transfer process, to improve the light extraction efficiency and heat dissipation. The light output power of LEDs on copper increased by ~80% after the transfer. The operating current before the onset of light output power saturation also increased by 25% because of the good thermal conductivity of copper. The light output power of packaged LEDs on copper was 6.5 mW under 20 mA current injection and as high as 14 mW driven at 55 mA.

Fully Vertical GaN p-i-n Diodes Using GaN-on-Si Epilayers

Using GaN-on-Si epilayers, for the first time, fully vertical p-i-n diodes are demonstrated after Si substrate removal, transfer, and n-electrode formation at the top of the device. After SiO2 sidewall passivation, the vertical p-i-n diodes, with n-GaN facing up, exhibit VON of 3.35 V at 1 A/cm2, a low differential ON-resistance of 3.3 mΩcm2 at 300 A/cm2, and a breakdown voltage of 350 V. The corresponding Baligas figure of merit is 37.0 MW/cm2, a very good value for GaN-based p-i-n rectifiers grown on Si substrates. The results indicate that fully vertical rectifiers using GaN-on-Si epilayers have great potential in achieving cost-effective GaN devices for high-power and high-voltage applications.

InGaN-based light-emitting diodes grown and fabricated on nanopatterned Si substrates

InGaN-based light-emitting diodes (LEDs) were grown and fabricated on nanoscale patterned Si (111) substrates (NPSi). Using anodized aluminum oxide as the etch mask, the NPSi was prepared with an average nanopore diameter of 150 nm and interpore distance of 120 nm. LEDs grown on NPSi exhibit relaxed tensile stress relative to the ones grown on microscale patterned Si (111) substrates (MPSi). Nanoheteroepitaxial lateral overgrowth was significantly promoted on NPSi, which led to extensive dislocation bending and annihilation. The devices made on NPSi exhibit lower leakage current and higher light output power as compared with those on MPSi.

Transfer of GaN-Based Light-Emitting Diodes From Silicon Growth Substrate to Copper

III-nitride light-emitting diodes (LEDs) grown on Si (111) substrates have the potential of low-cost manufacturing for solid-state lighting and display, by taking advantage of the well-developed IC technologies of silicon. In this letter, LEDs grown on silicon substrates were transferred onto copper substrates, to maximize light extraction and heat dissipation. On Si substrates, 300 × 300 ¿m2 multiple quantum well InGaN LEDs were first grown and processed. The top surface of the fabricated devices was then temporarily bonded to a sapphire wafer and the Si substrate was chemically etched. Ti/Al/Ti/Au layers were deposited on the backside of LEDs. An 80-¿m-thick copper layer was electroplated and the temporary bonding was removed, resulting in LEDs on copper substrate. The optical output power of LEDs on copper increased by ~ 70% as compared to that of the LEDs on silicon. The improved performance was attributed to the removal of the light-absorbing Si substrate and the good thermal conductivity of copper.

High-Performance Green and Yellow LEDs Grown on

High-performance GaN-based green and yellow light-emitting diodes (LEDs) are grown on SiO2 nanorod patterned GaN/Si templates by metalorganic chemical vapor deposition. The high-density SiO2 nanorods are prepared by nonlithographic HCl-treated indium tin oxide and dry etching. The dislocation density of GaN is significantly reduced by nanoscale epitaxial lateral overgrowth. In addition to the much improved green LED (505 and 530 nm) results, the fabricated yellow (565 nm) InGaN/GaN-based multiquantum well (MQW) LEDs on Si substrates are demonstrated for the first time. High-quality GaN buffer and localized states in MQWs are correlated to obtaining high-efficiency long-wavelength emission in our devices.

{\rm SiO}_{2}

This letter reports the breakdown ruggedness of GaN-based quasi-vertical p-i-n diodes on Si for the first time. With a 2μm-thick drift layer, the 0.08-mm2 devices can sustain a surge current up to 0.73 A, and maximum sink in energy of 3 mJ using an unclamped inductive switching test setup. No parametric drift nor device degradation was found after repetitive avalanche test consisting of multiple 50 000 breakdown events with a frequency of 1 kHz. At elevated temperatures, the breakdown voltage exhibits little temperature dependence, which could be explained by a trap-assisted space-charge-limited current conduction mechanism. With good ruggedness quality under repetitive test and at elevated temperatures, the quasi-vertical GaN p-i-n diodes on Si show great potential in achieving cost-effective rectifiers for high-voltage applications.

Nanorod Patterned GaN/Si Templates

In this letter, monolithic integration of InGaN/GaN light emitting diodes (LEDs) with vertical metal-oxide-semiconductor field effect transistor (VMOSFET) drivers have been proposed and demonstrated. The VMOSFET was achieved by simply regrowing a p- and n-GaN bilayer on top of a standard LED structure. After fabrication, the VMOSFET is connected with the LED through the conductive n-GaN layer, with no need of extra metal interconnections. The junction-based VMOSFET is inherently an enhancement-mode (E-mode) device with a threshold voltage of 1.6 V. By controlling the gate bias of the VMOSFET, the light intensity emitted from the integrated VMOSFET-LED device could be well modulated, which shows great potential for various applications, including solid-state lighting, micro-displays, and visible light communications.

Breakdown Ruggedness of Quasi-Vertical GaN-Based p-i-n Diodes on Si Substrates

We report the growth and characterization of InAlGaAs/InAlAs multiquantum wells (MQWs) emitting at ~1310 -nm grown on silicon by organometallic vapor phase epitaxy. Compared with the same structure grown on a reference planar InP substrate, photoluminescence of the MQWs on Si shows both comparable line widths and internal quantum efficiencies at room temperature. A specially engineered InP buffer with interlayers on a nanopatterned silicon substrate was used. Cross-sectional transmission electron microscopy reveals effective dislocation filtering by the three strained InGaAs interlayers. The high-quality quantum-well structure grown on the InP-on-Si template suggests great potential of integrating III-V photonic devices on the Si platform.

Monolithic integration of enhancement-mode vertical driving transistorson a standard InGaN/GaN light emitting diode structure

We report on the reduction of off-state leakage current in AlGaN/GaN high electron mobility transistors (HEMTs) by a two-step process combining pre-gate surface treatment and post-gate annealing (PGA), which suppressed the two leakage paths, namely, lateral surface leakage and vertical tunneling leakage, separately. The lateral surface leakage current, which was mainly induced by the high-density trap states generated during the device isolation etching process, was significantly reduced by a low power O2-plasma and HCl surface treatment process. The PGA process reduced the vertical tunneling leakage current by improving the Schottky contact quality of the transistor gate. Consequently, the device off-state leakage current was decreased by about 7 orders of magnitude and no degradation was introduced to the on-state performance, leading to a high on/off current ratio of 1010 and steep subthreshold slope (SS) of 62 mV/dec. The origin and leakage suppression mechanisms are also investigated and discussed in detail.

InAlGaAs/InAlAs MQWs on Si Substrate

Vertical-injection light-emitting diodes (VLEDs) were fabricated and demonstrated on mechanically rigid and flexible substrates using GaN-on-Si epilayers and a cost-effective Au-free Cu/Sn bonding method. With a mirror layer between the VLEDs and the receptor Si(100) carrier, 500 μm × 500 μm VLEDs emit up to 134-mW optical power at a drive current of 300 mA. The peak wall-plug efficiency was 23% at 1-A/cm2 current injection density. Due to excellent heat dissipation of the metal and Si carrier, the VLED junction temperature was measured to be only 47.5°C at a working current density of 350 mA/mm2 and increased by 0.27°C per 1-mA current increment. After release from the rigid substrate, the 40-μm-thick Cu/Sn/Cu bonding layer can also work as a handling substrate to paste LED thin films onto flexible substrates, including plastic and paper. The self-contained VLED structure exhibited the original I-V characteristics and high brightness on various substrates. The results attest to the feasibility of using GaN-on-Si epilayers and Cu/Sn/Cu bonding for a wide range of applications, including low-cost solid-state lighting and flexible illuminations.