G. M. Yang

Lateral wet oxidation of AlxGa1−xAs‐GaAs depending on its structures

Data are presented demonstrating that the lateral wet oxidation of Al(Ga)As layer is strongly influenced by its thicknesses and heterointerface structures as well as Al compositions. The oxidation length decreases rapidly with decreasing AlAs thickness in the range of <80 nm and oxidation nearly stops at a thickness of ∼11 nm. Also, the oxidation rate of AlxGa1−xAs decreases quickly with decreasing Al composition, providing a high degree of oxidation selectivity. AlGaAs layers on both sides of AlAs layer reduce the lateral oxidation rate which is enhanced by the stress induced by oxidized AlAs.

Hole transport in Mg-doped GaN epilayers grown by metalorganic chemical vapor deposition

A two-band model involving the heavy- and light-hole bands was adopted to analyze the temperature-dependent Hall effect measured on Mg-doped p-type GaN epilayers. At 300 K, the hole concentration was determined to be nearly twice the Hall concentration estimated from the measured Hall coefficient, meanwhile the Hall mobility of heavy hole turned out to be only half of the measured one. It is shown that the scattering by space charge and acoustic deformation potential is anomalously enhanced in Mg-doped GaN, and that the light hole affects conspicuously the observed transport parameters.

AlGaN∕GaN metal-oxide-semiconductor heterostructure field-effect transistor with oxidized Ni as a gate insulator

We fabricated the AlGaN∕GaN metal-oxide-semiconductor heterostructure field-effect transistor (MOSHFET) using the oxidized Ni(NiO) as a gate oxide and compared electrical properties of this device with those of a conventional AlGaN∕GaN heterostructure field-effect transistor (HFET). NiO was prepared by oxidation of Ni metal of 100A at 600°C for 5min in air ambient. For HFET and MOSHFET with a gate length of 1.2μm, the maximum drain currents were about 800mA∕mm and the maximum transconductances were 136 and 105mS∕mm, respectively. As the oxidation temperature of Ni increased from 300 to 600°C the gate leakage current decreased dramatically due to the formation of insulating NiO. The gate leakage current for the MOSHFET with the oxidized NiO at 600°C was about four orders of magnitude smaller than that of the HFET. Based on the dc characteristics, NiO as a gate oxide is comparable with other gate oxides.

GaN microcavity structure with dielectric distributed Bragg reflectors fabricated by using a wet-chemical etching of a (111) Si substrate

GaN microcavity structure with SiO2∕ZrO2 dielectric distributed Bragg reflectors was fabricated by means of transferring an InGaN∕GaN multiple quantum well (QW) structure from the (111) Si substrate onto a sapphire carrier and wet-chemical etching of the substrate. A dip in the reflectivity spectrum of the microcavity structure is observed at a wavelength of 411nm indicating the cavity resonance mode. Also, the strong influence of the cavity on the QW photoluminescence has been observed. A sharp emission spectrum, with a linewidth of 3.5nm, occurs at a wavelength of 411nm coincided with the position of the cavity resonance mode.

Thermal distributions of surface states causing the current collapse in unpassivated AlGaN∕GaN heterostructure field-effect transistors

The dc characteristics of the AlGaN∕GaN heterostructure field-effect transistors were examined at temperatures ranging from 25 to 260 °C under white light illumination. Drain current collapse measured was defined by the difference of drain current between light on and light off at Vgs=1V and Vds=5V. The surface-passivated device showed no drain current collapse, but the unpassivated device showed severe drain current collapse at 25 °C. Drain current and drain current collapse with an increase in temperature reduced, which resulted from the reduction of the electron mobility or saturation velocity and the thermal activation of the trapped electrons, respectively. Eventually, drain current collapse disappeared completely above 250 °C. The behavior of the temperature-dependent drain current collapse showed that the surface states for trapping electrons were continuously distributed with the temperature not having specific energy states.

Buried Tunnel Contact Junctions in GaN‐Based Light‐Emitting Diodes

InGaN/GaN multiple-quantum-well light-emitting diode structures utilizing tunnel contact junctions have been demonstrated. The heavily doped p + /n + GaN tunnel junctions are located in the upper cladding layers of conventional devices allowing n-type GaN instead of p-type GaN as a top contact layer. Accordingly, metal Ohmic contacts are performed at the same time on the upper and the lower contact layers. The reverse-biased thin tunnel contact junction provides lateral electron current spreading along the n + layer without the use of a semi-transparent electrode and spatially uniform luminescence exhibiting an improved radiative efficiency.

Allowable Substrate Bias for the Etching of n‐GaN in Photo‐Enhanced Electrochemical Etching

Etching characteristics for n-GaN on bias voltage were examined in various chemical solutions including the solutions not considered as etchants, n-GaN was not etched in HNO 3 and CH 3 COOH solutions during photo-enhanced electrochemical (PEC) etching under UV illumination of 90 mW/cm 2 . It was successfully etched in 1% CH 3 COOH solution by applying a bias larger than 0.6 V and the etch rate was increased to 23 nm/min at 1.5 V. Also, etching of n-GaN began to appear at 0.4 V in 0.05% HNO 3 solution. On the contrary, n-GaN was etched at a rate of 12 nm/ min in 0.04M KOH solution and 6 nm/min in 0.01M H 3 PO 4 solution even though the bias was not applied and -0.4 and -0.3 V were needed to stop the etching of n-GaN, respectively. The increase of etch rate on the reverse bias was shown for all the examined solutions and the critical bias for n-GaN etching was not varied on the UV illumination intensity.

Enhanced optical output power by the silver localized surface plasmon coupling through side facets of micro-hole patterned InGaN/GaN light-emitting diodes

Light extraction efficiency of GaN-based light emitting diodes were significantly enhanced using silver nanostructures incorporated in periodic micro-hole patterned multi quantum wells (MQWs). Our results show an enhancement of 60% in the wall-plug efficiency at an injection current of 100 mA when Ag nano-particles were deposited on side facet of MQWs passivated with SiO2. This improvement can be attributed to an increase in the spontaneous emission rate through resonance coupling between localized surface plasmons in Ag nano-particles and the excitons in MQWs.

Effect of Well Thickness on GaN/AlGaN Separate Confinement Heterostructure Emission

We investigated the effects of well thickness on spontaneous and stimulated emission (SE) in GaN/AlGaN separate confinement heterostructures (SCHs), grown by low pressure metal organic chemical vapor deposition. The SCH wells are unstrained and lattice matched to a GaN buffer layer. Our series of SCHs had GaN well thicknesses of 3, 5, 9, and 15 nm. We explain the spontaneous emission peak energy positions of the SCHs in terms of spontaneous and strain-induced piezoelectric polarizations. At 10 K, the carrier lifetime was found to be lowest for a 3 nm well, and the SE threshold was lowest for a 5 nm well. We show that the screening of the piezoelectric field and the electron-hole separation are strongly dependent on the well thickness and have a profound effect on the optical properties of the GaN/AlGaN SCHs. The implications of this study on the development of near- and deep-ultraviolet light emitters are discussed.

Highly uniform and reproducible 850 nm vertical-cavity surface-emitting lasers grown by MOCVD

850 nm vertical-cavity surface-emitting lasers (VCSELs) were grown by metalorganic chemical vapor deposition (MOCVD). High uniformity of the Fabry-Perot cavity wavelength for VCSEL materials of /spl plusmn/0.15 % across a 3-in diameter wafer was achieved. Also, /spl plusmn/0.1 % Al composition uniformity of Al/sub 0.98/Ga/sub 0.02/As was obtained.