Ji-Hao Cheng

Improved crystal quality and performance of GaN-based light-emitting diodes by decreasing the slanted angle of patterned sapphire

Periodic triangle pyramidal array patterned sapphire substrates (PSSs) with various slanted angles were fabricated by wet etching. It was found beside normal wurtzite GaN, zinc blende GaN was found on the sidewall surfaces of PSS. The crystal quality and performance of PSS-LEDs improved with decrease in slanted angle from 57.4° to 31.6°. This is because most of the growth of GaN was initiated from c-planes. As the growth time increased, GaN epilayers on the bottom c-plane covered these pyramids by lateral growth causing the threading dislocation to bend toward the pyramids.

Effects of laser sources on the reverse-bias leakages of laser lift-off GaN-based light-emitting diodes

The KrF pulsed excimer laser (248nm) and the frequency-tripled neodymium doped yttrium aluminum garnet laser (355nm) have been used to separate GaN thin films from sapphire substrates and transfer to bond other substrate. However, these processes would increase the dislocation density, resulting in an increase of the leakage current. In this study, the effects of these two laser sources on the reverse-bias leakages of InGaN–GaN light-emitting diodes were studied.

Effects of Laser Sources on Damage Mechanisms and Reverse-Bias Leakages of Laser Lift-Off GaN-Based LEDs

In this study, three types of LEDs were investigated. Samples designated as “CV-LED” were conventional LEDs without any laser treatment. Samples designated as “KrF-LED” were CV-LEDs treated with KrF pulsed excimer laser, while “YAG-LEDs” were CV-LEDs treated with Nd:YAG laser. The basic fabrication processes of these LEDs were almost the same. 7 The LED structures were grown by low pressure metallorganic chemical vapor deposition. The structures comprised a 5 nm thick Si-doped n + -InGaN layer, a 400 nm thick Mg-doped p-GaN layer, an InGaN‐GaN multiple quantum well MQW ,a2 m thick Si-doped n-GaN layer, a 2 m thick undoped-GaN layer film, and a buffer layer on the sapphire substrate. For the CV-LED, the device mesa with a chip size of 350 350 m was defined by an inductively coupled plasma which removed the Mg-doped GaN and MQW until the Si-doped GaN was exposed. Then, the indium tin oxide ITO layer was deposited on the n + -InGaN layer using an E-beam coater to form a p-side contact layer and a current spreading layer. The Cr/Au layer was deposited onto the ITO layer to form the p- and n-side electrodes. The fabrication processes of KrF-LED and YAG-LED are shown in Fig. 1. Before the LLO process, the CV-LED wafer was bonded to a host substrate covered with an adhesive/glue layer, and the sapphire substrate of KrF-LED was polished using a diamond paper. The KrF laser beam spot size of 700 700 m four chips were lifted off in etch pulse was scanned without overlap from pulse to pulse. The 355 nm Nd:YAG LLO system was constructed by a series of lens, the laser beam was near circle shape, and the lateral energy of the converged laser beam was a Gaussian distribution. The Nd:YAG laser beam spot size of 500 m was scanned with a 20% overlap from pulse to pulse. The pulse length of the KrF laser was 35 ns, which was longer than that of the YAG laser 5n s. The energy densities of the KrF laser were in the range between 700 and 1000 mJ/cm 2 , while those of the YAG laser were in the range between 100 and 400 mJ. These wafers were then bonded to a sapphire substrate with an adhesive layer at 200°C for 60 min with a comprehensive load of 10 kg/cm 2 . The host substrate and glue layer were subsequently removed.

First-principles analysis of interfacial nanoscaled oxide layers of bonded N- and P-type GaAs wafers

First-principles analysis is applied in relating microstructures with properties of interfacial nanoscaled oxide layers of bonded N- and P-type GaAs wafers. Using high-resolution transmission electron microscope results, the detailed atomic arrangements of materials specimen can be obtained and fed into the first-principles calculations. Therefore, the corresponding electronic structure and associated property can be reliably derived to identify responsible microstructural features. The electrical performance is found to be closely related to the variation of nanosized interface morphology and types of wafers.

Nanoscaled interfacial oxide layers of bonded n- and p-type GaAs wafers

This work examined in detail the electrical characteristics and microstructures of in- and antiphase bonded interfaces for both n- and p-type GaAs wafers treated at 500 and 600°C, respectively. The n-GaAs wafers did not bond directly to itself but instead via an amorphous oxide layer at 500°C. These temperatures are lower than most other works. The nonlinear behavior of the current versus the voltage is related to the potential barrier formed at the continuous oxide interface. Both experimental observation and first-principles calculations confirm the existence of this barrier. The higher interface energy for the antiphase bonding tends to stabilize the interfacial oxide layer. The evolution of interfacial layers occurred much faster for the p-type wafers than for n-type wafers. Electrical performance was found to be closely related to the variation of nanosized interface morphology.

Anomalous electrical performance of nanoscaled interfacial oxides for bonded n-GaAs wafers

Electrical performance was found to be closely related to the variation of nanosized interface morphology in previous studies. This work investigated in detail the microstructural development of in- and anti-phase bonded interfaces for n-type (100) GaAs wafers treated at 500, 600, 700 and 850°C. The interfacial energy of anti-phase bonding is higher than that of in-phase bonding based on the first-principles calculations. The higher interface energy tends to stabilize the interfacial oxide layer. The continuous interfacial oxide layer observed below 700°C can deteriorate the electrical property due to its insulating property. However, the existence of nanoscaled oxide at anti-phase bonded interfaces can improve the electrical conductivity at 700°C. This is due to the suppression of the evaporation of As atom by the interfacial nanoscaled oxides based on the analysis of autocorrelation function and energy dispersive x-ray spectroscopy.

Effect of Laser Sources on the Structural Damage Mechanisms and Reverse-bias Leakages of Laser Lift-off GaN-based LEDs

Sapphire substrate has been used for high temperature GaN growths. However, the performance of GaN device was limited to the high series resistance and the poor heat dissipation caused by its insulating nature. In order to solve such problems, the laser-lift off technique has been developed for removal of GaN-based device layers , and transfer to the better heat and electronic conductivity substrate. Two kinds of LED were investigated in this study, one is KrF Excimer laser system at 248nm and another is 355nm frequency-tripled Nd YAG laser system. Figure 1(a) and 1(b) are the cross-section transmission electron microscopy (TEM) image of KrF-LED and YAG-LED. The screw dislocation density in the bulk region (which penetrate the MQW) of YAG-LLO LED was 2.9 10 cm, which was 10 times higher than that of the Excimer-LLO LED (3.75 10 cm). As a result, the leakage current of YAG-LED (1.65×10 nA) was 10,000 times higher than that of the KrF-LED (0.17 nA). Figure 1(c) and 1(d) is the enlargement of the part in the LLO interface region. We found that there are lots of dislocations located at 200nm away from LLO interface in KrF-LED, which was different from YAG-LED. Inverse fast Fourier transform (IFFT) image using a diffraction spots long (0002) direction of part of 200nm around the interface (Fig. 1(e) and 1(f)) show sequential dislocations with have the same center in KrF-LED (the white dashed line) but not observed in YAG-LED. We believe those phenomenon was due to the laser pulse time and the difference absorption edge between the GaN and laser wave length. This is because Nd YAG has thicker absorption depth, which may generate more screw dislocation density during the rapid thermal decomposition.