Chia-Ming Lee

Dependence of composition fluctuation on indium content in InGaN/GaN multiple quantum wells

The information on the variations of indium composition, aggregation size, and quantum-well width is crucially important for understanding the optical properties and, hence, fabricating efficient light-emitting devices. Our results showed that spinodal decomposition could occur in InGaN/GaN multiple quantum wells with indium content in the range of 15%–25% (grown with metal–organic chemical-vapor deposition). A lower nominal indium content led to a better confinement of indium-rich clusters within InGaN quantum wells. The InGaN/GaN interfaces became more diffusive, and indium-rich aggregates extended into GaN barriers with increasing indium content. It was also observed that indium-rich precipitates with diameter ranging from 5 to 12 nm preferred aggregating near V-shaped defects.

AlN/GaN double-barrier resonant tunneling diodes grown by rf-plasma-assisted molecular-beam epitaxy

AlN/GaN double-barrier resonant tunneling diodes (DB–RTDs) were fabricated on (0001) Al2O3 substrates by molecular-beam epitaxy, using a rf-plasma nitrogen source. The AlN/GaN DB–RTDs were designed to have a 3-ML-thick GaN quantum well and 4-ML-thick AlN barrier layers sandwiched by Si-doped n-type GaN contact layers. The current–voltage characteristics of mesa diode samples showed clear negative differential resistance (NDR) at room temperature. The NDR was observed at 2.4 V with a peak current of 2.9 mA, which corresponds to 180 A/cm2. A peak-to-valley current ratio as high as 32 was obtained.

Interdiffusion of In and Ga in InGaN/GaN multiple quantum wells

Thermal stability of InxGa1-xN/GaN multiple quantum wells with InN mole fraction of ∼0.23 and ∼0.30 was investigated by postgrowth thermal annealing. Low temperature photoluminescence spectroscopy was employed to determine the temperature dependence of the interdiffusion coefficient of In and Ga in InGaN/GaN quantum wells. The interdiffusion process is characterized by a single activation energy of about 3.4±0.5 eV and governed by vacancy-controlled second-nearest-neighbor hopping. Due to composition inhomogeneity, lower diffusivity is observed at the early stage of thermal annealing.

Luminescence efficiency of InGaN multiple-quantum-well ultravioletlight-emitting diodes

The electroluminescence efficiency of In0.06Ga0.94N∕GaN multiple-quantum-well UV light-emitting diodes (LEDs) with emission wavelength of 400nm has been investigated and compared with blue (470nm) LEDs. Based on their injection current-dependent characteristics under dc and pulsed operation, it can be concluded that carrier overflow is the dominant factor that affects the external quantum efficiency of UVLED before thermal effects take over. It is experimentally shown that increasing the number of quantum wells is necessary to alleviate the carrier overflow issue and improve the luminescence efficiency of the UVLEDs.

Temperature dependence of the radiative recombination zone in InGaN/GaN multiple quantum well light-emitting diodes

The temperature dependence of the radiative recombination zone in InGaN/GaN multiple quantum well light-emitting diodes is investigated. From the electroluminescence spectra measured at various temperatures, it is found that there are two peaks at about 400 and 460 nm, which can be assigned as Mg-related and quantum well transitions, respectively. The behavior of these two peaks with temperature is modeled by the two rate equation. Based on this model, we deduce the activation energy of Mg in GaN films to be about 126 meV, which is consistent with reported results obtained by other techniques.

Light Output Enhancement of InGaN Light-Emitting Diodes Grown on Masklessly Etched Sapphire Substrates

A maskless wet-etching method is used to prepare patterned sapphire substrates for enhancing the output power of InGaN light-emitting diodes (LEDs). Blue LEDs grown on the patterned sapphire substrates exhibit an output power of 24.9 mW, which is 19.4% higher than that of the devices grown on flat substrates. The uniformity of the optical and electrical properties of LEDs across a 2-in wafer is slightly improved as well.

High-brightness inverted InGaN-GaN multiple-quantum-well light-emitting diodes without a transparent conductive layer

Unlike the conventional layer structure of an InGaN-GaN multiple-quantum-well light-emitting diode (LED), an LED with reversed p-type and n-type layer sequence, and an n+/p+ tunnel junction has been investigated. When operated at 20 mA, the output power of the inverted LED is almost twice that of the conventional LED. Since the structures of these two LEDs are alike when analyzed by X-ray diffraction, the improvement in the light intensity could be attributed to the elimination of the absorption/reflection by the transparent conductive layer and/or some quality improvement of p-type GaN in the inverted LED.

Effect of composition inhomogeneity on the photoluminescence of InGaN/GaN multiple quantum wells upon thermal annealing

The optical properties of thermally annealed InGaN/GaN multiple quantum wells were investigated by low-temperature photoluminescence measurements. It is found that the photoluminescence peak exhibits a redshift followed by a blueshift as the annealing time is increased. In contrast, the assigned photoluminescence peak from an In-rich dot-like structure shows a monotonic blueshift with more annealing time. Transmission electron microscopic observation confirms that the density of dot-like structures is reduced after thermal annealing, indicating that phase separation does not take place in these samples. Instead, in-plane and out-plane outdiffusion of dot-like structures is proposed to account for the spectral shift with more annealing time. Based on this diffusion model, a quantized state transition in the quantum well along with the composition inhomogeneity and piezoelectric field is considered to be the dominant luminescence mechanism.

InGaN-GaN MQW LEDs with current blocking layer formed by selective activation

Selective activation technique was used to define a semi-insulating current-blocking layer underneath the p-pad of InGaN-GaN multiple-quantum-well light-emitting diodes (LEDs). The output power of the LEDs at 20 mA was increased 10% because less current was injected underneath the opaque p-pad.

Stimulated emission study of InGaN/GaN multiple quantum well structures

We report the study results of an InGaN/GaN multiple quantum well structure with a nominal indium content of 25%. The high-resolution transmission electron microscopy and x-ray diffraction show clear indium aggregation and phase separation. Stimulated emission data always show two major peaks in spectrum. The long- (short-) wavelength peak is assigned to the recombination of localized state carriers (free carriers). At low temperatures or optical pump levels, the localized-state recombination dominates the stimulated emission; however, at high temperatures or pump levels, the free-carrier recombination becomes dominant. The peak position corresponding to localized states changes little in spectrum as temperature or pump level varies. This result is attributed to carrier overflow, strain relaxation, and carrier shielding in increasing temperature or carrier supply.