C. J. Youn

Wide-band gap oxide alloy: BeZnO

A wide-band gap oxide alloy, BeZnO, is proposed and studied in this letter. The BeZnO films were deposited on sapphire substrates by our hybrid beam deposition growth method. The value of the energy band gap of BeZnO can be efficiently engineered to vary from the ZnO band gap (3.4 eV) to that of BeO (10.6 eV). BeZnO can be used for fabricating films and heterostructures of ZnO-based electronic and photonic devices and for other applications. Changes in the measured energy band gap and lattice constant values with Be content are described for BeZnO alloys.

Excitonic ultraviolet lasing in ZnO-based light emitting devices

The authors have fabricated ultraviolet (UV) laser diodes based on ZnO∕BeZnO films. The devices have p-n heterojunction structures with a multiple quantum well (MQW) active layer sandwiched between guide-confinement layers. The MQW active layer comprises undoped ZnO and BeZnO, while the two guide-confinement layers were As-doped p-type ZnO∕BeZnO and Ga-doped n-type BeZnO∕ZnO films, respectively. The exciton binding energy in the MQW region is exceptionally large (263meV). Exciton-related lasing was observed by optically pumping the MQWs. ZnO∕BeZnO-based diodes showed laser action by current injection at room temperature. The lasing mechanism is inelastic exciton-exciton collision.

ZnO devices: Photodiodes and p-type field-effect transistors

The potential use of ZnO-based photonic and electronic devices has been demonstrated by the fabrication of prototype ultraviolet (UV) photodetector and field-effect transistor (FET) devices that contain films of p-type ZnO with arsenic as the p-type dopant. These p-type films have high crystalline quality and show long-term stability. The ZnO UV photodetectors are based on p-n junctions. The FETs are made with metal-semiconductor Schottky contacts on p-type ZnO and are normally off (enhancement) devices. The spectral and electrical characteristics of these devices are presented and explained.

Codoping characteristics of Zn with Mg in GaN

The doping characteristics of Mg–Zn codoped GaN films grown by metalorganic chemical vapor deposition are investigated. By means of the concept of Mg–Zn codoping technique, we have grown p-GaN showing a low electrical resistivity (0.72 Ω cm) and a high hole concentration (8.5×1017 cm−3) without structural degradation of the film. It is thought that the codoping of Zn atoms with Mg raises the Mg activation ratio by reducing the hydrogen solubility in p-GaN. In addition, the measured specific contact resistance of Mg–Zn codoped GaN film is 5.0×10−4 Ω cm2, which is one order of magnitude lower than that of Mg doped only GaN film (1.9×10−3 Ω cm2).

Optical properties of BeZnO layers studied by photoluminescence spectroscopy

Epitaxial BeZnO layers, the candidates for active barrier layers composed of the ZnO/BexZn1−xO quantum well structure, were successfully grown using the hybrid beam deposition method. The Be rate of x in the grown BexZn1−xO layer was estimated to be 0.11. The optical properties caused by the thermal quenching phenomenon of BeZnO, an ultraviolet light emitting material, were studied using the temperature dependence of photoluminescence spectra. Emission of the neutral donor-bound exciton shows a tendency to quench the intensity and extend the spectral width with increasing temperatures. This tendency is related to the phonons generated by the lattice vibration of the host material in BeZnO. These findings led us to conclude that the phonons may be participating in the quenching process.

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.

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.

The characterization of ZnSeGaAs epilayers grown by hot wall epitaxy

Abstract ZnSe GaAs , ZnSe : Zn GaAs , ZnSe : Zn GaAs (annealed in Zn vapor) and ZnSe GaAs (annealed in Se vapor) epilayers on (100) GaAs substrates have been grown by hot wall epitaxy under various growth conditions. X-ray double crystal rocking curves and photoluminescence measurements confirmed the good quality of the grown epilayers. The X-ray rocking curves show a FWHM value for the 0.37 μm thick ZnSe epilayer of 161 arcsec and a lattice mismatch between the (100) ZnSe epilayer and the (100) GaAs substrate of 0.27%. Photoluminescence studies indicate that the origin of the I 2 peak at 2.794 eV is associated with V Se at a neutral donor. It is also confirmed that the binding energy of the bound exciton (D 0 ,X), E b BX , is 14 meV and the binding energy E D , is 70 meV.

Point-defect study from low-temperature photoluminescence of CdGa2Se4 layers through the postannealing in various ambient

The CdGa2Se4 layers were grown by the hot-wall epitaxy method. From the absorption measurement, the band-gap variation in CdGa2Se4 was well interpreted using Varshni’s equation. After the postannealing in various ambient, the behavior of point defects in CdGa2Se4 was investigated by measuring photoluminescence (PL). Point defects originating from VCd, VSe, Cdint, and/or Seint were classified as donor or acceptor types. Thus, the Ga in CdGa2Se4 did not form native defects because the Ga existed in the form of stable bonds in CdGa2Se4. Based on these PL results, we schemed out a band diagram of the recombination process in CdGa2Se4.

Temperature-dependent study of photocurrent signal in CdGa2Se4 layers

The photoconductive CdGa2Se4 layer has been investigated using photocurrent (PC) spectroscopy as a function of temperature. Three peaks corresponding to the band-to-band transitions were observed in the PC spectra for all temperature ranges. From the relations of peak position and temperature, the temperature dependence of the band-gap energy is precisely discussed. Also, contrary to our expectation, the PC intensities decreased with decreasing temperatures. From the relation of log Jph versus 1/T, where Jph is the PC density, two dominant levels by the exponential variation in the PC with varying temperature were observed, one at high temperatures and the other at low temperatures.