Yu-Cheng Kao

Pulsed laser deposition of hexagonal GaN-on-Si(100) template for MOCVD applications

Growth of hexagonal GaN on Si(100) templates via pulsed laser deposition (PLD) was investigated for the further development of GaN-on-Si technology. The evolution of the GaN growth mechanism at various growth times was monitored by SEM and TEM, which indicated that the GaN growth mode changes gradually from island growth to layer growth as the growth time increases up to 2 hours. Moreover, the high-temperature operation (1000 °C) of the PLD meant no significant GaN meltback occurred on the GaN template surface. The completed GaN templates were subjected to MOCVD treatment to regrow a GaN layer. The results of X-ray diffraction analysis and photoluminescence measurements show not only the reliability of the GaN template, but also the promise of the PLD technique for the development of GaN-on-Si technology.

Thin-film vertical-type AlGaInP LEDs fabricated by epitaxial lift-off process via the patterned design of Cu substrate.

In this study, the thin-film vertical-type AlGaInP LEDs on Cu substrates were fabricated. By performing the epitaxial lift-off (ELO) process, the LED device can be transferred from GaAs to Cu substrate. Then the GaAs substrate was separated and the ELO-LED was completed. To overcome the drawback of crack formation in the epilayer during the ELO process, various patterned Cu substrates were designed. Moreover, the finite element method was used to simulate the stress distribution in the LED sample during the ELO process. From the simulation results, an optimum structure of patterned Cu substrate was obtained since its maximum stress can be confined to the chip edges and the stress was decreased significantly during the ELO process, resulting in an apparent reduction of crack generation after separating the GaAs substrate. This optimum patterned Cu substrate was employed for the fabrication of ELO-LED. In addition, the chemical etching process was also used to etch the GaAs substrate, and this device transferred to Cu substrate was denoted as CE-LED. Based on the measurements of device performances, the forward voltages (@350 mA) of the CE-LED and ELO-LED were measured to be 2.20 and 2.29 V, while the output powers (@350 mA) of these two devices were 49.9 and 48.2 mW, respectively. Furthermore, the surface temperatures (@350 mA) of these two samples were 46.9-48.3 and 45.2-47.0 °C, respectively. Obviously, the device characteristics of the ELO-LED are very similar to those of the CE-LED. It confirms that the design of patterned Cu substrate is very helpful to obtain the thin-film vertical-type AlGaInP LEDs. Additionally, via the ELO process, the separated GaAs substrate can be reused for production cost down.

High separation rate of epitaxial lift-off using hydrophilic solvent for III–V solar cell and reusable applications

Through the epitaxial lift-off (ELO) process using the HF solutions mixed with hydrophilic substances consisting of acetone (ACE), isopropanol (IPA), and methanol (MA), the separation rate for the GaAs substrate and the III-V solar cell can be improved significantly. Especially for the use of HF:ACE etchant, a extremely high lateral etching rate (14.3 μm/min) of the AlAs sacrificial layer can be achieved, as compared with that employing the pure HF etchant (3.6 μm/min). In addition, by conducting the ELO technique, the GaAs substrate was reused for four times, indicating its high potential for the reusable applications.

Effects of buffer layers and front electrode angles on the performance of In 0.16 Ga 0.84 As solar cells

Three buffer layers, that included 2-step, step-graded, and linear-graded layers, were designed to improve the performance of In_0.16Ga_0.84As solar cell. The In₀₁₆Ga₀₈₄As solar cells fabricated on these three buffer layers were denoted as 2S-cell, S-cell, and L-cell, respectively. The efficiencies of these three devices were 9.8%, 14.4%, and 16.3%, respectively. Obviously, the linear-graded buffer layer is most helpful to enhance the efficiency of In0.16Ga0.84As solar cell. Additionally, the front electrode angle of In_0.16Ga_0.84As solar cell was varied from 0° to 90°. When the angles were fixed at 0° and 90°, the devices possessed higher efficiencies of 16.3% and 16.6%, respectively.

Colloidal quantum dot enhanced dual-junction tandem solar cells

In this work, we demonstrate the the enhancement of two type hybrid QD dual-junction solar cell. The green QD520 sample, shows better performance than blue QD450 sample in current density and power conversion efficiency. The enhancement of short-circuit current and power conversion efficiency for green QD450 sample are 4.43% and 3.77% and the blue QD520 samples are 9.44% and 5.78%. For short wavelength, the enhancement can mainly be attributed to the photon downshift effect and antireflection, for long wavelength, it only the antireflection effect.

Separation-rate improvement of epitaxial lift-off for III-V solar cells

Thin-film III-V semiconductor solar cells have a number of advantages compared with other types of solar cells. For example, tuning the bandgap of III-V compound materials to match the solar spectrum gives the resulting solar cells unsurpassed conversion efficiencies. The virtues of these devices notwithstanding, the semiconductor substrate used in fabricating them is expensive, which adds to their cost. A method known as epitaxial lift-off (ELO) enables substrate reuse, which enhances affordability.1 However, the technique relies on hydrofluoric acid (HF) solution, a popular chemical etchant, and long-term exposure to the etchant increases the surface roughness of either the epilayer (i.e., the thin film containing the device) or the substrate. This roughness in turn hinders both substrate reuse and the performance of the solar cells. To solve this problem, various chemical fluids have been proposed to clean the substrate and modify the surface structure.2 But chemical cleaning is difficult to control because it is isotropic (that is, it etches at the same rate in every direction). A gallium arsenide (GaAs) solar cell on a (100) GaAs substrate consists of a 0.2 m-thick GaAs buffer layer, a 0.2 m-thick indium gallium phosphide (InGaP) etching stop layer, a 3 mthick GaAs buffer layer, a 20nm-thick aluminum arsenide (AlAs) sacrificial layer, and a 2.6 m-thick GaAs device epilayer. For ELO to be practical, etching time needs to be fast. However, arsine (AsH3) bubbles formed during the ELO process are known to obstruct the etching slits and prevent the AlAs sacrificial layer from reacting with the HF solution. Previous research3 established that oxygen is required for chemical etching of AlAs in HF solution. Blocking of the etching slits by the AsH3 bubbles Figure 1. (a) Lateral etching rate for the aluminum arsenide (AlAs) sacrificial layer during solar cell fabrication using various hydrofluoric acid (HF) solution mixtures. (b) Photograph of the sample. H2O: Water. ACE: Acetone. IPA: Isopropanol. MA: Methanol.

The growth of hexagonal GaN-on-Si(100) using pulsed laser deposition

The growth of hexagonal GaN-on-Si(100) sample prepared by pulsed laser deposition (PLD) was employed in the development of GaN-on-Si technology. In contrast to common GaN-on-sapphire and GaN-on-Si(111) technologies, the use of the GaN film on Si(100) by PLD provides low-cost and large-area single crystalline GaN template for GaN applications, via a single growth process without any interlayer or interruption layer. The evolution of GaN growth mechanism on Si(100) substrate with various growth times is established by SEM and TEM data, which indicated that the growth mode of the GaN films gradually changes from island growth to layer growth when the growth time increases up to 2hrs. Moreover, no significant GaN meltback was found on the GaN sample surface due to the high-temperature operation of PLD. The GaN sample was subjected to MOCVD treatment to regrow a GaN layer. The results of X-ray diffraction analysis and photoluminescence measurement show the reliability of the PLD-GaN sample and are promising for the development of the GaN-on-Si technology using PLD technique.

Improvement in etching rate for epilayer lift-off with surfactant

In this study, the GaAs epilayer is quickly separated from GaAs substrate by epitaxial lift-off (ELO) process with mixture etchant solution. The HF solution mixes with surfactant as mixture etchant solution to etch AlAs sacrificial layer for the selective wet etching of AlAs sacrificial layer. Addiction surfactants etchant significantly enhance the etching rate in the hydrofluoric acid etching solution. It is because surfactant provides hydrophilicity to change the contact angle with enhances the fluid properties of the mixture etchant between GaAs epilayer and GaAs substrate. Arsine gas was released from the etchant solution because the critical reaction product in semiconductor etching is dissolved arsine gas. Arsine gas forms a bubble, which easily displaces the etchant solution, before the AlAs layer was undercut. The results showed that acetone and hydrofluoric acid ratio of about 1:1 for the fastest etching rate of 13.2 μm / min. The etching rate increases about 4 times compared with pure hydrofluoric acid, moreover can shorten the separation time about 70% of GaAs epilayer with GaAs substrate. The results indicate that etching ratio and stability are improved by mixture etchant solution. It is not only saving the epilayer and the etching solution exposure time, but also reducing the damage to the epilayer structure.