Zinc oxide (ZnO) is a material of great interest for short-wavelength optoelectronic applications due to its wide band gap
(3.37 eV) and high exciton binding energy (60 meV). Due to the difficulty in stable p-type doping of ZnO, other p-type
materials such as gallium nitride (GaN) have been used to form heterojunctions with ZnO. p-GaN/n-ZnO heterojunction
devices, in particular light-emitting diodes (LED) have been extensively studied. There was a huge variety of electronic
properties and emission colors on the reported devices. It is due to the different energy alignment at the interface caused
by different properties of the GaN layer and ZnO counterpart in the junction. Attempts have been made on modifying the
heterojunction by various methods, such as introducing a dielectric interlayer and post-growth surface treatment, and
changing the growth methods of ZnO. In this study, heterojunction LED devices with p-GaN and ZnO nanorods array
are demonstrated. The ZnO nanorods were grown by a solution method. The ZnO nanorods were exposed to different
kinds of plasma treatments (such as nitrogen and oxygen) after the growth. It was found that the treatment could cause
significant change on the optical properties of the ZnO nanorods, as well as the electronic properties and light emissions
of the resultant LED devices.
We report investigation of SnS van der Waals epitaxies (vdWEs) grown by molecular beam epitaxy (MBE) technique.
Experimental results demonstrate an indirect bandgap of ~1 eV and a direct bandgap of ~1.25 eV. Substantial
improvement in the crystallinity for the SnS thin films is accomplished by using graphene as the buffer layer. Using this
novel growth technique we observed significant lowering in the rocking curve FWHM of the SnS films. Crystallite size
in the range of 2-3 μm is observed which represents a significant improvement over the existing results. The absorption coefficient, α, is found to be of the order of 104 cm-1 which demonstrates sharp cutoff as a function of energy for films grown using graphene buffer layers indicating low concentration of localized states in the bandgap. Hole mobility as high as 81 cm2V-1s-1 is observed for SnS films on graphene/GaAs(100) substrates. The improvements in the physical properties of the films are attributed to the unique layered structure and chemically saturated bonds at the SnS/graphene interface. As a result, the interaction between the SnS thin films and the graphene buffer layer is dominated by a weak vdW force and structural defects at the interface, such as dangling bonds or dislocations, are substantially reduced.
We investigated the influence of the growth method, growth conditions, and post-growth treatments on the ZnO nanorod
properties and the performance of heterojunction light emitting diodes (LEDs) based on ZnO nanorods. Due to small
lattice mismatch between GaN and ZnO, we will mainly consider p-GaN/n-ZnO nanorod heterojunctions. The influence
of p-GaN substrate and the influence of growth method and growth conditions used for ZnO nanorods on the LED
performance will be discussed.
In this paper we report systematic reliability studies of GaN UV detectors exposed to high power UV radiation. GaN
epitaxial layers are deposited by rf plasma-assisted molecular beam epitaxy (MBE) utilizing a double buffer layer
structure. Our studies show that the optimal buffer layer structure consists of a conventional AlN high-temperature
buffer layer (HTBL) and an 800 nm thick GaN intermediate temperature buffer layer (ITBL) deposited at 690°C. Two
types of devices are being investigated. Type I devices were fabricated on the optimal double buffer layer structure. Type
II devices have only a conventional AlN buffer layer. Flicker noise measurement is used to monitor the degradation of
the device due to optical stress. In addition, I-V and responsivity measurements were also performed. The experimental
results are consistent with each other which show that the degradation of the devices arises from the generation of
crystalline defects at the Schottky junction due to the exposure of the devices to the high power UV radiation. Both types
of devices demonstrate degradation in their optoelectronic properties. However, while type I devices general exhibit
gradual and slow degradations type II devices exhibit catastrophic breakdowns in the device characteristics. Our
experimental data show that visible-blind UV detectors fabricated on the optimized double buffer layer structure indicate
significant improvements in the radiation hardness of the devices.