Shengli Qi

Study on the formation of dodecagonal pyramid on nitrogen polar GaN surface etched by hot H3PO4

Hot phosphor acid (H3PO4) etching is presented to form a roughened surface with dodecagonal pyramids on laser lift-off N face GaN grown by metalorganic chemical vapor deposition. A detailed analysis of time evolution of surface morphology is described as a function of etching temperature. The activation energy of the H3PO4 etching process is 1.25 eV, indicating the process is reaction-limited scheme. And it is found that the oblique angle between the facets and the base plane increases as the temperature increases. Thermodynamics and kinetics related factors of the formation mechanism of the dodecagonal pyramid are also discussed. The light output power of a vertical injection light-emitting-diode (LED) with proper roughened surface shows about 2.5 fold increase compared with that of LED without roughened surface.

Properties of GaN-based light-emitting diode thin film chips fabricated by laser lift-off and transferred to Cu

Conventional GaN-based light-emitting diodes (LEDs) on sapphire substrates and laser lift-off (LLO) lateral current structure GaN LED thin film chips on Cu substrates have been fabricated and their properties are compared. It is found that after the LLO process, the reverse bias leakage current obviously increases and equivalent parallel resistance decreases two orders accordingly. From analyses of I–V curves the fact that tunneling behavior dominates under the reverse bias is confirmed, and the LLO process aids more defects to become tunneling active whereas the similar ideality factors and equivalent series resistances of LLO-LEDs on Cu and conventional LEDs on sapphire suggest that the LLO process does not much damage the electrical characteristics at a forward bias. The analyses of L–I curves reveal that the LLO process induces more nonradiation centers. However, the LLO-LEDs show superior performance under large injection current. The LLO-LEDs have 1.8 times greater maximum output power and 2.5 times higher current operation capabilities than the conventional LEDs within 300 mA for the good thermal conductivity of Cu.

Polarization modification in InGaN/GaN multiple quantum wells by symmetrical thin low temperature-GaN layers

Light emitting diodes (LEDs) using InGaN/GaN quantum wells (QWs) with thin low temperature GaN (LT-GaN) layers bounding each InGaN layer are grown by metal-organic vapor phase epitaxy. The light output power of such LEDs increases by a factor of 2 at a drive current density of 35 A/cm2 compared to that from reference LEDs without the LT-GaN. The blueshift in the emission wavelength is 5.2 nm when the current density increases from 3 to 50 A/cm2, which is much smaller than the shift 8.1 nm from reference LEDs. Moreover, the efficiency droop at high current injection is also reduced by 28%, and current density at which peak efficiency is observed increases from 1 to 2 A/cm2. High resolution transmission electron microscopy of the QWs bounded with LT-GaN shows higher quality and less strain compared to the reference samples. The better performance of LEDs incorporating the LT-GaN layers is attributed to suppressed polarization from piezoelectric fields.

Luminescent properties in the strain adjusted phosphor-free GaN based white light-emitting diode

A kind of phosphor-free GaN based white light-emitting diode was fabricated with a strain adjusting InGaN interlayer. The origin of the strain adjusted white luminescent properties was studied with cathodoluminescence, asymmetrically reciprocal space mapping with high resolution x-ray diffraction, and scanning electron microscopy. The yellow and blue components of the electroluminescence spectrum were attributed to the high indium core and the adjacent indium depleted region in the inverted pyramidal pits on the device surface, respectively. These pits existed at the end of the dislocations induced by the strain relaxation process of the InGaN interlayer.

GaN-based LEDs with a high light extraction composite surface structure fabricated by a modified YAG laser lift-off technology and the patterned sapphire substrates

Vertical structure LEDs have been fabricated with a novel light extraction composite surface structure composed of a micron grating and nano-structure. The composite surface structure was generated by using a modified YAG laser lift-off technique, separating the wafers from cone-shaped patterned sapphire substrates. LEDs thus fabricated showed the light output power increase about 1.7–2.5 times when compared with conventional vertical structure LEDs grown on plane sapphire substrates. A three-dimensional finite difference time domain method was used to simulate this new kind of LED device. It was determined that nano-structures in composite surface patterns play a key role in the improvement of light extraction efficiency of LEDs.

Flexible vertical structure GaN-based light emitting diodes on an AuSn substrate

By combining the metal bonding/debonding and laser lift off techniques, a new approach to fabricating flexible vertical structure GaN-based light emitting diodes (F-LEDs) has been developed. The performance of these F-LEDs under different bending radii was investigated in detail.

Changing oblique angles of pyramid facets fabricated by wet etching of N polar GaN

Wet etching was performed on N polar GaN, which was fabricated by laser lift-off from a sapphire substrate. Dodecagonal pyramids appeared on the N-polar GaN surface after immersion into hot H3PO4 solution even if it had been etched previously with hot KOH solution. According to the symmetry of the space group of C6v4-P6mc, the oblique angle and crystallographic plane indices of the pyramid facets were obtained. It was observed that the oblique angles of the etched facets decreased from the tip to the base of the pyramids. The etching rate was fast when the etching temperature was above 130 °C, and the oblique angle at the base was reduced. The enhancement of light output with increasing etching temperature has been confirmed. The polarization charges on the different facets were assigned to a kinetics-limited process by the special behavior of the hot H3PO4 etching.