Sin-Liang Ou

Pulsed laser deposition of gallium oxide films for high performance solar-blind photodetectors

Monoclinic gallium oxide thin films were grown on (0001) sapphire at various substrate temperatures ranging from 400 to 1000 °C by pulsed laser deposition using a KrF excimer laser. The structural, optical and compositional properties of the films were analyzed by using x-ray diffraction, transmission electron microscopy, optical transmittance, and Rutherford backscattering spectroscopy. As the substrate temperature was increased to 800 °C, the gallium oxide film possesses single crystalline phase with a preferred growth orientation of (−201) plane and higher crystal quality than those at the other temperatures. Optical transmittance measurements reveal the films grown at 600-1000 °C exhibit a clear absorption edge at the deep ultraviolet region around 250 nm wavelength. Based on the results of Rutherford backscattering spectroscopy, the O/Ga ratio of gallium oxide film increased gradually with increasing substrate temperature. When the substrate temperature was raised to 800-1000 °C, the film composition was close to the formation of Ga2O3, indicating the O vacancies and defects were reduced. Furthermore, the films grown at 600 and 800 °C were chosen to fabricate solar-blind metal-semiconductor-metal photodetectors. At an applied bias of 5 V, the photodetector prepared with 800 °C-grown film has a lower dark current of 1.2 × 10−11 A and a higher responsivity of 0.903 A/W (at a wavelength of 250 nm) than those with 600 °C-grown films. The better device performance is ascribed to the higher crystal quality and fewer O vacancies in the 800 °C-grown film. Moreover, the results indicate the gallium oxide films presented in this study have high potential for deep ultraviolet photodetector applications.

Performance of GaN-based light-emitting diodes fabricated using GaN epilayers grown on silicon substrates

Light extraction of GaN-based light-emitting diodes grown on Si(111) substrate (GaN-on-Si based LEDs) is presented in this study. Three different designs of GaN-on-Si based LEDs with the lateral structure, lateral structure on mirror/Si(100) substrate, and vertical structure on mirror/Si(100) substrate were epitaxially grown by metalorganic chemical vapor deposition and fabricated using chemical lift-off and double-transfer techniques. Current-voltage, light output power, far-field radiation patterns, and electroluminescence characteristics of these three LEDs were discussed. At an injection current of 700 mA, the output powers of LEDs with the lateral structure on mirror/Si(100) substrate and vertical structure on mirror/Si(100) substrate were measured to be 155.07 and 261.07 mW, respectively. The output powers of these two LEDs had 70.63% and 187.26% enhancement compared to that of LED with the lateral structure, respectively. The result indicated this vertical structure LED was useful in improving the light extraction due to an enhancement in light scattering efficiency while the high-reflection mirror and diffuse surfaces were employed.

Highly Ultraviolet-Transparent ZnO:Al Conducting Layers by Pulsed Laser Deposition

The Zn-rich ZnO:Al (AZO) thin films were grown on c-plane sapphire substrates by pulsed laser deposition using Ar atmosphere. The effects of substrate temperature on the characteristics of AZO films were investigated in terms of x-ray diffraction, x-ray photoelectron spectroscopy, transmission electron microscopy, optical transmittance and Hall measurements. From the temperature-dependent Hall measurements, all the as-deposited AZO films exhibit the metal-like conductivity behavior. It was found that the band gap of AZO films increased from 3.45 to 3.8 eV when the substrate temperature decreased from 700 to 100°C. The AZO film deposited at 100°C shows low resistivity of 233 μΩ cm and high transmittance in both visible and UV regions. The high transmittance of AZO films with large Zn content could be due to the formation of Zn nanoparticles, which is referred to surface plasmon resonance. Especially, the transmittance of AZO film (200 nm thick) can reach 91% at a wavelength of 365 nm, showing high potential in UV device applications.

Thin Film GaN LEDs Using a Patterned Oxide Sacrificial Layer by Chemical Lift-Off Process

A high-quality GaN-based vertical light-emitting diode (LED) was successfully fabricated and transferred to an electroplated Cu substrate using strip-patterned silicon dioxide (SiO2) as a sacrificial layer in a chemical lift-off (CLO) process. The SiO2 strip patterns not only provide the sacrificial structure during the detachment process, but also improve the quality of GaN epilayers through epitaxial lateral overgrowth. Compared with conventional LEDs, the CLO-LEDs have a higher output power and a lower forward voltage. The CLO-LED has a decrease in forward voltage of 0.42 V (at 20 mA) as compared with the conventional LED. In addition, at a drive current of 350 mA, the output power of CLO-LEDs is enhanced ~ 2.2 fold, compared with that of conventional LEDs.

Effect of diamond like carbon layer on heat dissipation and optoelectronic performance of vertical-type InGaN light emitting diodes

This study reports the transfer of InGaN light-emitting diodes (LEDs) embedded with and without a diamond like carbon (DLC) layer to Si substrates. It also investigates the heat dissipation and output power performance after the addition of the DLC layer. The LED device with a DLC layer had a lower thermal resistance (13.2 K/W) and surface temperature (55.51-65.34 °C at 700 mA) than that without a DLC layer. This likely resulted from the fast heat dissipation of the DLC layer in both vertical and horizontal directions. The LED device with a DLC layer achieved a 20.8% improvement in output power.

Transparent Conductive Oxide Films Embedded with Plasmonic Nanostructure for Light-Emitting Diode Applications

In this study, a spin coating process in which the grating structure comprises an Ag nanoparticle layer coated on a p-GaN top layer of InGaN/GaN light-emitting diode (LED) was developed. Various sizes of plasmonic nanoparticles embedded in a transparent conductive layer were clearly observed after the deposition of indium tin oxide (ITO). The plasmonic nanostructure enhanced the light extraction efficiency of blue LED. Output power was 1.8 times the magnitude of that of conventional LEDs operating at 350 mA, but retained nearly the same current-voltage characteristic. Unlike in previous research on surface-plasmon-enhanced LEDs, the metallic nanoparticles were consistently deposited over the surface area. However, according to microstructural observation, ITO layer mixed with Ag-based nanoparticles was distributed at a distance of approximately 150 nm from the interface of ITO/p-GaN. Device performance can be improved substantially by using the three-dimensional distribution of Ag-based nanoparticles in the transparent conductive layer, which scatters the propagating light randomly and is coupled between the localized surface plasmon and incident light internally trapped in the LED structure through total internal reflection.

Structural, Surface Morphology and Optical Properties of ZnS Films by Chemical Bath Deposition at Various Zn/S Molar Ratios

In this study, ZnS thin films were prepared on glass substrates by chemical bath deposition at various Zn/S molar ratios from 1/50 to 1/150. The effects of Zn/S molar ratio in precursor on the characteristics of ZnS films were demonstrated by X-ray diffraction, scanning electron microscopy, optical transmittance, X-ray photoelectron spectroscopy, and Fourier transform infrared spectrometry. It was found that more voids were formed in the ZnS film prepared using the precursor with Zn/S molar ratio of 1/50, and the other ZnS films showed the denser structure as the molar ratio was decreased from 1/75 to 1/150. From the analyses of chemical bonding states, the ZnS phase was indeed formed in these films. Moreover, the ZnO and Zn(OH)2 also appeared due to the water absorption on film surface during deposition. This would be helpful to the junction in cell device. With changing the Zn/S molar ratio from 1/75 to 1/150, the ZnS films demonstrate high transmittance of 75–88% in the visible region, indicating the films are potentially useful in photovoltaic applications.

Growth and characterization of Ga 2 O 3 on sapphire substrates for UVsensor applications

The β-Ga2O3 films were grown on (0001) sapphire at 500 °C by metal organic chemical vapor deposition. In the analysis of crystal structure, we found that the (-201) oriented single crystal β-Ga2O3 epilayer can be obtained under low chamber pressure of 15 torr. Moreover, a metal-semiconductor-metal solar-blind deep ultraviolet photodetector was fabricated with the β-Ga2O3 epilayer. As the bias voltage is 5 V, the photodetector exhibits a relatively low dark current about 0.2 pA, which induced by the highly resistive nature of the β-Ga2O3 thin films. From the responsivity result, it can be observed that photodetector shows a maximum responsivity at 260 nm, revealing the β-Ga2O3 photodetector was really solar-blind. The responsivity of the photodetector was as high as 20.1 A/W with an applied bias of 5 V and an incident light wavelength of 260 nm. The improved performance is attributed to the high quality of β-Ga2O3 epilayer.

Analysis of the Thickness Effect of Undoped Electron-Blocking Layer in Ultraviolet LEDs

In this paper, the thickness effect of an undoped Al0.23Ga0.77N inserted in the Mg-doped Al0.23Ga0.77N electron-blocking layer (EBL) on the characteristics of the ultraviolet light-emitting diodes (UV-LEDs) was analyzed. The results of secondary-ion-mass spectrometry clearly show that the concentration of Mg back-diffusion from the p-GaN and the Mg-doped EBL decreases with the thickness of an undoped EBL. The radiative recombination rate in the multiple quantum wells (MQWs) can be obviously enhanced after inserting an undoped EBL from the simulated results, and the sample with a 6-nm-thick undoped EBL has the highest radiative recombination rate. It is worth noticing that the hole carrier concentration in the MQWs increases with increasing the thickness of an undoped EBL. The UV-LED possesses 400% enhancement in the output power (at 20 mA) by inserting a 6-nm-thick undoped EBL. The significant enhancement is ascribed to the decrease of nonradiative recombination defects formed by Mg atoms and the increase of hole concentration in the MQWs.

A visualization of threading dislocations formation and dynamics in mosaic growth of GaN-based light emitting diode epitaxial layers on (0001) sapphire

A clear visualization of the origin and characteristics of threading dislocations (TDs) of GaN-based light emitting diode epitaxial layers on (0001) sapphire substrates have been carried out. Special experimental set up and chemical etchant along with field emission scanning electron microscopy are employed to study the dynamics of GaN TDs at different growth stages. Cross-sectional transmission electron microscopy analysis visualized the formation of edge TDs is arising from extension of coalescences at boundaries of different tilting-twining nucleation grains “mosaic growth.” Etch pits as representatives of edge TDs are in agreement with previous theoretical models and analyses of TDs core position and characteristics.