Yi Hsuan Wang

Resonant cavity light‐emitting diode

A novel concept of a light‐emitting diode (LED) is proposed and demonstrated in which the active region of the device is placed in a resonant optical cavity. As a consequence, the optical emission from the active region is restricted to the modes of the cavity. Resonant cavity light‐emitting diodes (RCLED) have higher spectral purity and higher emission intensity as compared to conventional light emitting diodes. Results on a top‐emitting RCLED structure with AlAs/AlxGa1−xAs quarter wave mirrors grown by molecular beam epitaxy are presented. The experimental emission linewidth is 17 meV (0.65 kT) at room temperature. The top‐emission intensity is a factor of 1.7 higher as compared to conventional LEDs.

Electrically gain-switched vertical-cavity surface-emitting lasers

The authors report electrical gain-switching of a packaged vertical-cavity surface-emitting laser (VCSEL). Pulse durations as short as 24 ps at repetition rates up to 2 GHz were obtained from a four-quantum-well GaAs/AlGaAs VCSEL, which emits 0.8 mW continuous wave power in a single mode at room temperature and has a current threshold of 5 mA. Simultaneous measurement of the optical spectrum showed an almost transform limited linewidth indicating ultralow chirp. Optical pumping with subpicosecond pulses of the same packaged devices, held at a constant electrical bias, yielded 22 ps pulses, in good agreement with the electrical pumping. Simple calculations show that the pulse duration obtained by gain-switching is limited by the design constraints necessary to operate the VCSEL continuous wave at room temperature with low-threshold current, high-quantum efficiency, and reasonable output power.<<ETX>>

A Simple Passivation Technique for AlGaN/GaN Ultraviolet Schottky Barrier Photodetector

This letter demonstrates and investigates AlGaN/GaN ultraviolet photodetector (UV-PD) with a simple passivation process. The hydrogen peroxide (H₂O₂) oxidation technique is adopted to complete the passivation. The results of chemical analysis suggest that the Al and Ga dangling bonds react with the oxygen atoms. This passivation process effectively reduces the side-wall surface states, which also suppress the dark current to 11 pA. In addition, the photo response and the UV to visible rejection ratio of the PD with H₂O₂ passivation process are enhanced to 8.1×10^-3 A/W and 2.3×10³ when the PD is biased at -10 V. The noise equivalent power and the detectivity are determined to be 1.63×10^-8 W and 1.33×10⁸ cmHz^0.5W^-1. The simple passivation technique improves the AlGaN/GaN UV PD performances effectively.

GaAs/AlGaAs multiple quantum well GRIN-SCH vertical cavity surface emitting laser diodes

Vertical cavity surface emitting lasers (VCSELs) with GaAs/AlGaAs multiple quantum well (20 wells) graded-index separate-confinement-heterostructure (GRIN-SCH) active regions are discussed. The VCSEL structures, which also contained two Al/sub x/Ga/sub 1-x/As/Al/sub y/Ga/sub 1-y/As distributed Bragg reflectors, were grown by molecular beam epitaxy and had a threshold current and current density at room temperature pulsed excitation of 16 mA and 14 kA/cm/sup 2/, respectively, near 0.85- mu m wavelength. Both single-longitudinal and fundamental transverse mode emission characteristics were observed with a light output greater than 3 mW and a slope efficiency of 0.12 mW/mA.<<ETX>>

Al 2 O 3 Passivation Layer for InGaN/GaN LED Deposited by Ultrasonic Spray Pyrolysis

This letter utilizes ultrasonic spray pyrolysis (USP) to deposit Al₂O₃ as the passivation layer of GaN light-emitting diodes (LEDs). The composition of Al₂O₃ is analyzed by X-ray photoelectron spectroscopy. The refractive index and transmittance of USP-grown Al₂O₃ are investigated. The thickness of Al₂O₃ is determined to be 70 nm, which is close to the optimum from antireflection theorem calculation for the wavelength of 455 nm. The coverage quality and thickness are confirmed by transmission electron microscopy. The light output power (LOP) of USP-grown-Al₂O₃-passivated InGaN/GaN LED is improved from 321 mW to 347 mW compared with an LED without passivation. The forward voltage (V_F) and series resistance (R_s) are not subject to significant variations but the shunt resistance (R_SH) is increased after Al₂O₃ passivation. LED chips with Al₂O₃ and SiO₂ passivation are packaged as LED lamps. Devices passivated by USP-grown Al₂O₃ show slightly better LOP performance than those passivated by plasma-enhanced chemical vapor deposition-grown SiO₂.

Suppression of Dark Current on AlGaN/GaN Metal–Semiconductor–Metal Photodetectors

This paper proposes the H₂O₂ oxidation technique to grow Al₂O₃ as the surface passivation layer of the metal-semiconductor-metal ultraviolet (UV) photodetector (PD). The dark current of the H₂O₂-grown-Al₂O₃-passivated PD was reduced from 104 to 4.43 nA. The surface leakage of the PD was reduced from 65.7 nA/1.67 μA to 0.46 nA/0.5 nA in the dark/under illumination. It was found that ~35% photocurrent results from the surface leakage. The surface leakages in the H₂O₂-grown-Al₂O₃ passivated and the plasma-enhanced chemical vapor deposition (PECVD)-grown-SiO₂ passivated PDs were reduced significantly. Although the photocurrent and the photoresponsivity of the oxide-passivated PDs were lower than those of the unpassivated one, the dark current, UV-to-visible rejection ratio, noise equivalent power, and the detectivity of the oxide-passivated PDs were better than those of the unpassivated one. In addition, the performances of the H₂O₂-grown-Al₂O₃ passivated PD were better than those of the PECVD-grown-SiO₂ passivated one.

Fabrication AlGaN/GaN MIS UV Photodetector by H 2 O 2 Oxidation

This letter demonstrates and investigates AlGaN/GaN metal-insulator-semiconductor ultraviolet photodetector (MIS-UV-PD) with a cost-effective wet oxidation technique. The H₂O₂ oxidation technique is adopted to grow an insulator layer. The material qualitative and semiquantitative analyses of the H₂O₂-grown aluminum oxide are studied by energy dispersive X-ray spectroscopy The performances of the present MIS-UV-PD with different H₂O₂ treatment time are also investigated. The MIS-PD with 5-min H₂O₂ treatment time has the optimum performances. The dark current is suppressed from 4.23 nA to 5.15 pA at -10 V. The responsivity and the UV to visible rejection ratio are enhanced to 1.03×10^-2 A/W and 3.38×10⁵. Moreover, the noise equivalent power and detectivity are determined to be 4.8 × 10^-11 W and 4.52 × 10¹⁰ cmHz^0.5W^-1. This cost-effective oxidation technique provides a simple approach to fabricate AlGaN/GaN MIS-UV-PD and its performances are also improved.

Growing Al 2 O 3 by Ultrasonic Spray Pyrolysis for Al 2 O 3 /AlGaN/GaN Metal-Insulator-Semiconductor Ultraviolet Photodetectors

This paper proposed Al₂O₃ deposition by ultrasonic spray pyrolysis (USP) method as an insulator layer for Al₂O₃/AlGaN/GaN metal-insulator-semiconductor ultraviolet photodetector (MIS-UV-PD) applications. The composition of USP-grown Al₂O₃ was confirmed by X-ray photoelectron spectroscopy. The refractive index and transmittance characteristics of USP-grown Al₂O₃ were also characterized. The Al₂O₃/AlGaN/GaN MIS-UV-PD performances with different Al₂O₃ thickness (30, 20, and 15 nm) were investigated. The responsivity was 1.3 × 10^-3/7.5 × 10^-3/0.83 A/W, UV-to-visible rejection ratio was 2.34 × 10³/1.37 × 10⁴/3.18 × 10⁵, and the detectivity was 2.78 × 10⁸/1.26 × 10⁹/1.17 × 10¹¹ cmHz^0.5W^-1 for the MIS-UV-PD with 30-/20-/15-nm-thick Al₂O₃. It was found that the performances of MIS-UV-PD with 15-nm Al₂O₃ as the insulator layer are much better than the MIS-UV-PD with 20and 30-nm Al₂O₃.

Vertical cavity top-surface emitting lasers with thin Ag mirrors and hybrid reflectors

Abstract Three types of vertical cavity surface emitting lasers based on GaAs/AlGaAs have been grown by molecular beam epitaxy (MBE). The laser structures with top emission have been evolved from a simple double heterostructure (DH) with n-type distributed Bragg reflectors (DBR) as the bottom mirror and a semitransparent Ag as the top mirror, to a DH structure with a hybrid reflector consisting of a thin metal and a few pairs of p-DBR as the top mirror, and to a 3-QW structure again with a hybrid reflector as the top mirror. MBE growth of these three device structures is discussed, particularly on the in-situ growth of metals on GaAs. Devices have been fabricated and measured. A theoretical model has been used to gain an understanding of the parameters in each design. The MBE growth techniques have been improved to modify the heterostructural interface in the DBR to reduce the series resistance. The laser performance has been improved from a high threshold current in the first structure to a much lower threshold current in the second structure and finally to a cw room temperature operation with low threshold currents in the third structure.

Study of AuAgFe/AlGaAs Schottky diodes fabricated byin situ molecular beam epitaxy

This study investigated single magnetic crystal Au/Ag/Fe layers directly grown (in situ) on (001) AlxGa1 −x As Schottky diodes by molecular beam epitaxy. The barrier height was found to increase with increasing Al content to a maximum of 0.934 eV for Au/Ag/Fe/AlAs, which can endure the highest operational temperatures. Annealing caused interdiffusion and the formation of some compound phases. Results of current-voltage (I-V) and capacitance-voltage (C-V) measurements revealed that increasing the doping concentration with increasing annealing temperature causes the Au/Ag/Fe/AlxGa1 −xAs effective barrier height to decrease. The mechanisms are discussed.