Kai-Feng Huang

Determination of built-in field by applying fast Fourier transform to the photoreflectance of surface-intrinsic n+-type doped GaAs

Photoreflectance spectroscopy of surface-intrinsic n+-doped (s-i-n+) GaAs has been measured at various power densities (Ppu) of a pump beam. Many Franz–Keldysh oscillations (FKOs) were observed above the band-gap energy, which will enable the electric-field strength (F) to be determined from the periods of the FKOs. Field F thus obtained is subject to photovoltaic effects. In order to reduce the photovoltaic effects from the pump beam, Ppu was kept below 10 μW/cm2 in the previous experiments. Here, we demonstrate that the built-in field can be determined at a larger Ppu by using fast Fourier transform techniques.

Instability in surface emitting lasers due to external optical feedback

Instability in the output of dc‐biased surface emitting lasers due to an external cavity effect was observed. The output power spectrum exhibited multiple peaks with spacing corresponding to exactly the round‐trip delay in silica fibers with length ranging from 2 m to 2 km. The magnitude of the peaks was enhanced in the spectral region centered at the laser relaxation frequency. With increased feedback, the background of the output spectrum was found to increase, indicating the presence of optical chaos. Numerical simulation based on the rate equation analysis was found to agree with the experiment, indicating the surface emitting lasers are well described by the rate equation and are susceptible to feedback as the edge emitting lasers.

Improved Output Power of Nitride-Based Light-Emitting Diodes With Convex-Patterned Sapphire Substrates

Nitride-based light-emitting diodes (LEDs) grown on different convex-patterned sapphire substrates are proposed and fabricated. The electrical and optical properties of these LEDs are discussed in detail. It is found that the LED with a ball-shape patterned sapphire substrate has the best crystalline quality and I-V characteristic. On the other hand, on reducing total internal reflection, the LED with hexagonal-shape patterned sapphire substrate (HPSS) has the best performance on output power and external quantum efficiency. In comparison to the LED without patterned substrate, the LED with HPSS has 175% and 165% enhancement on output power and external quantum efficiency, respectively.

UV enhanced oxygen response resistance ratio of ZnO prepared by thermally oxidized Zn on sapphire substrate

ZnO thin film was fabricated by thermally oxidized Zn at 600°C for 1 h. A surface containing nanostructured dumbbell and lines was observed by scanning electron microscope (SEM). The ZnO resistor device was formed after the following Ti/Au metallization. The device resistance was characterized at different oxygen pressure environment in the dark and under ultraviolet (UV) light illumination coming from the mercury lamp with a short pass filter. The resistance increases with the increase of oxygen pressure. The resistance decreases and response increases with the increase of light intensity. Models considering the barrier height variation caused by the adsorbed oxygen related species were used to explain these results. The UV light illumination technology shows an effective method to enhance the detection response for this ZnO resistor oxygen sensor.

Study of InGaN-Based Light-Emitting Diodes on a Roughened Backside GaN Substrate by a Chemical Wet-Etching Process

The InGaN-based light-emitting diodes (LEDs) with a roughened backside on the N-face surface of GaN substrate were fabricated through a chemical wet-etching process to increase light-extraction efficiency. The stable crystallographic etching planes were formed as the GaN {101̅1̅} planes. When the near-ultraviolet and blue LED were operated as a forward current of 20 mA, the output power of LEDs was improved from 13.2 and 19.9 mW to 25.6 and 24.0 mW, respectively. The different enhanced ratio is attributed to the different transmittance as a function of wavelength is caused from hexagonal pyramid on N-face GaN substrate after wet-etching process.

RELIABILITY OF PROTON IMPLANTED VERTICAL CAVITY SURFACE EMITTING LASERS

Reliable gain-guided vertical cavity surface emitting lasers were fabricated by the proton implantation technique at the 0.85 and 0.98 μm emission wavelengths. The device failure mode was found to be mainly the gradual degradation mode, manifested in the gradual reduction of output power and gradual increase of diode series resistance. Continuous wave operation of the 0.85 μm GaAs/AlGaAs quantum well lasers for 12,000 hours was seen at 25 and 50°C at 15 mA (~ 4 Ith). Extrapolation from accelerated aging test performed at higher temperatures yields a 25°C mean-time-to-failure of 1−2.5×105 hours at 15 mA. For the 0.98 μm InGaAs/GaAs strained quantum well lasers, a much slower degradation rate at high temperatures was seen in the 0.85 μm lasers. Secondary ion mass spectrometry revealed that the implant proton depth is short enough that the active region of these lasers is free from proton bombardment. The near field emission pattern of the aged lasers by cathodo-luminescence and electro-luminescence microscopies did not contain dark spots and line features.

Reliability studies of gain-guided 0.85 /spl mu/m GaAs/AlGaAs quantum well surface emitting lasers

A reliability study of gain-guided 0.85 /spl mu/m GaAs/AlGaAs quantum well surface emitting lasers is reported for the first time. 32 lasers were randomly selected to operate at 25 or 50 C with bias currents up to 15 mA, about 4 times the threshold values. The power outputs of the 32 lasers showed no noticeable degradation after 2000-3000 hours of operation.<<ETX>>

Effects of Doping Ratio and Thermal Annealing on Structural and Electrical Properties of Boron-Doped ZnO Thin Films by Spray Pyrolysis

Boron-doped zinc oxide (BZO) thin films have been fabricated by spray pyrolysis on a glass substrate. The morphology and electrical properties of the thin films were investigated. X-ray diffraction (XRD) and scanning electron microscopy (SEM) analyses were performed. It was found that [B]/[Zn] ratio altered both the microstructure and concentration of the BZO thin films. The film grain size was reduced by increasing the [B]/[Zn] ratio. The highest Hall mobility was 3.65 cm2 V-1 s-1 for the undoped ZnO thin film, and the highest carrier concentration of 1.0×1019 cm-3 was achieved for the as-deposited BZO thin film with [B]/[Zn] = 1.5 at. %. Conductivity was determined at different measurement temperatures and shallow donors provided the dominate conduction mechanism for the as-deposited BZO thin films. After 600 °C annealing, shallow level reduction and donors with a high activation energy of 129±6 meV in the BZO thin films were characterized, and the shallow donors that dominate the carrier concentration for the as-deposited spray-pyrolized BZO thin film were eliminated.

Effect of modulating field on photoreflectance simulated by electroreflectance

Photoreflectance (PR) of surface-intrinsic-n+ (s-i-n+) type doped GaAs has been simulated by electroreflectance (ER). The simulated spectra of the s-i-n+ sample have exhibited many Franz–Keldysh oscillations, which enable the electric field (F) to be determined. It is known that F’s determined from PR are subjected to photovoltaic effect and the measured F is close to Fbi−δF/2 when the modulating field, δF≪Fbi, where Fbi is the built-in field of the sample and δF is the modulating field. In this work, we have investigated the relation between the measured F and δF not only for the region where δF≪Fbi holds, but also for a whole range of δF. In order to determine the magnitude of δF, we have used ER to simulate PR, that is, the measurements of ER under a forward bias, which is set to be equal to δF/2.

Indium-nitrogen codoped zinc oxide thin film deposited by ultrasonic spray pyrolysis on n-(111) Si substrate: the effect of film thickness

Indium-nitrogen codoped zinc oxide (INZO) thin films were fabricated by spray pyrolysis deposition technique on n-(111) Si substrate with different film thicknesses at 450°C using a precursor containing zinc acetate, ammonium acetate, and indium nitrate with 1 : 3 : 0.05 at.% concentration. The morphology and structure studies were carried out by scanning electron microscopy (SEM) and X-ray diffraction (XRD). The grain size of the films increased when increasing the film thickness. From XRD spectra, polycrystalline ZnO structure can be observed and the preferred orientation behavior varied from (002) to (101) as the film thickness increased. The concentration and mobility were investigated by Hall effect measurement. the p-type films with a hole mobility around 3 cm2 V-1s-1 and hole concentration around 3×1019 cm-3 can be achieved with film thickness less than 385 nm. The ntype conduction with concentration 1 × 1020 cm-3 is observed for film with thickness 1089 nm. The defect states were characterized by photoluminescence. With temperature-dependent conductivity analysis, acceptor state with activation energy 0.139 eV dominate the p type conduction for thin INZO film. And the Zn-related shallow donors with activation energy 0.029 eV dominate the ntype conduction for the thick INZO film.