Michael A. Mastro

Large-area transparent conductive few-layer graphene electrode in GaN-based ultra-violet light-emitting diodes

We report on the development of a large-area few-layer graphene (FLG)—based transparent conductive electrode as a current spreading layer for GaN-based ultraviolet (UV) light-emitting diodes (LEDs). Large-area FLG was deposited on Cu using the chemical vapor deposition (CVD) method and subsequently transferred to the surface of the UV LED. UV light at a peak of 372 nm was emitted through the FLG-based transparent conductive electrode. The current spreading effects of FLG were clearly evident in both the optical images of electroluminescence (EL) and current-voltage (I-V) characteristics. Degradation of the FLG-based transparent conductive electrode could be induced by high power operation. Our results indicate that a large-area FLG-based electrode on GaN offers excellent current spreading and ultra-violet transparency properties when compared to the standard optoelectronic indium tin oxide (ITO) contact layer.

Surface depletion effects in semiconducting nanowires

The impact of surface depletion on the electronic properties of semiconductor nanowires (NWs) is explored both theoretically and experimentally. The impact of dopant concentration, surface barrier height, and NW radius on surface depletion and extracted material properties are determined by solving Poisson’s equation for the cylindrical system. The theoretical results reveal a size-dependent systematic error in carrier concentration extraction, which is verified through experiment. Interrogation of GaN NWs with radii from 15 to 70 nm exposed an error that reaches over an order of magnitude for the samples studied. These data compared favorably to an analytical treatment assuming physically reasonable material properties. While this manuscript focuses on GaN, the systematic error discussed will be present for any semiconducting NW, which exhibits surface band bending and therefore influences the behavior and characterization of a wide range of semiconducting nanoelements.

Assessment of GaN Surface Pretreatment for Atomic Layer Deposited High-k Dielectrics

We report the effects of GaN surface pretreatments on the material and electrical properties of Al2O3 dielectric deposited by atomic layer deposition (ALD). A layer of Al2O3 was deposited at different temperatures on metal organic chemical vapor deposition grown n-GaN that was treated with either H2O2:H2SO4 (1:5, piranha), HCl:H2O (1:1, HCl), or HF:H2O (1:1, HF) prior to Al2O3 deposition. The Al2O3 layers on piranha- and HF-treated GaN were observed to be uniformly smooth. The piranha pretreatment resulted in the lowest hysteresis. Pretreatment of the GaN surface with piranha removes carbon and hydroxylates the surface, resulting in better quality ALD Al2O3.

Self-Assembled Monolayers of Alkylphosphonic Acid on GaN Substrates

In this paper we describe the formation and characterization of self-assembled monolayers of octadecylphosphonic acid (ODPA) on epitaxial (0001) GaN films on sapphire. By immersing the substrate in its toluene solution, ODPA strongly adsorbed onto UV/O 3-treated GaN to give a hydrophobic surface. Spectroscopic ellipsometry verified the formation of a well-packed monolayer of ODPA on the GaN substrate. In contrast, adsorption of other primarily substituted hydrocarbons (C n H 2 n+1 X; n = 16-18; X = -COOH, -NH 2, -SH, and -OH) offered less hydrophobic surfaces, reflecting their weaker interaction with the GaN substrate surfaces. A UV/O 3-treated N-polar GaN had a high affinity to the -COOH group in addition to ODPA, possibly reflecting the basic properties of the surface. These observations suggested that the molecular adsorption was primarily based on hydrogen bond interactions between the surface oxide layer on the GaN substrate and the polar functional groups of the molecules. The as-prepared ODPA monolayers were desorbed from the GaN substrates by soaking in an aqueous solution, particularly in a basic solution. However, ODPA monolayers heated at 160 degrees C exhibited suppressed desorption in acidic and neutral aqueous solution maybe due to covalent bond formation between ODPA and the surface. X-ray photoelectron spectroscopy provided insight into the effect of the UV/O 3 treatment on the surface composition of the GaN substrate and also the ODPA monolayer formation. These results demonstrate that the surface of a GaN substrate can be tailored with organic molecules having an alkylphosphonic acid moiety for future sensor and device applications.

Cu 2 ZnSnS 4 polycrystalline thin films with large densely packed grains prepared by sol-gel method

Cu2ZnSnS4 (CZTS) was obtained from a sol-gel precursor which consists of copper chloride, zinc chloride, tin chloride, and thiourea. CZTS thin films were prepared by spin- coating the sol-gel precursor followed by annealing in a nitrogen atmosphere. The morphology, composition, and structure of the absorber layer were studied by scanning electron microscopy, energy dispersive spectroscopy, x-ray diffraction, and Raman scattering. The optical measure- ment shows the bandgap of these films is ∼1.51 eV, and the optical absorption coefficient is on the order of 10 4 cm −1 . CZTS solar cells with a structure of low-alkali glass/Mo/CZTS/CdS/i- ZnO/ZnO:Al/Al grid were tentatively fabricated. The best solar cell showed a short-circuit current density of 5.06 mA/cm 2 , an open-circuit voltage of 358 mV, a fill factor of 34.66%, and an efficiency of 0.63% under AM1.5 (100 mW/cm 2 ) illumination. These results demonstrate the CZTS thin films were successfully deposited by a cheap sol-gel technique. C 2011 Society of

Transparent conductive graphene electrode in GaN-based ultra-violet light emitting diodes

We report a graphene-based transparent conductive electrode for use in ultraviolet (UV) GaN light emitting diodes (LEDs). A few-layer graphene (FLG) layer was mechanically deposited. UV light at a peak wavelength of 368 nm was successfully emitted by the FLG layer as transparent contact to p-GaN. The emission of UV light through the thin graphene layer was brighter than through the thick graphene layer. The thickness of the graphene layer was characterized by micro-Raman spectroscopy. Our results indicate that this novel graphene-based transparent conductive electrode holds great promise for use in UV optoelectronics for which conventional ITO is less transparent than graphene.

Nondestructive analysis of threading dislocations in GaN by electron channeling contrast imaging

Threading dislocations in metal-organic chemical-vapor grown GaN films were imaged nondestructively by the electron channeling contrast imaging (ECCI) technique. Comparisons between ECCI and cross-sectional transmission electron microscopy indicated that pure edge dislocations can be imaged in GaN by ECCI. Total threading dislocation densities were measured by ECCI for various GaN films on engineered 4H-SiC surfaces and ranged from 107to109cm−2. A comparison between the ultraviolet electroluminescent output measured at 380nm and the total dislocation density as measured by ECCI revealed an inverse logarithmic dependence.

Substrate-Dependent Effects on the Response of AlGaN/GaN HEMTs to 2-MeV Proton Irradiation

AlGaN/GaN high electron mobility transistors grown on Si, SiC, and sapphire substrates were exposed to 2-MeV proton irradiation in incremental fluences up to 6 × 1014 cm-2. The devices were characterized initially and after each irradiation by Hall and dc I-V measurements to probe the mechanisms associated with radiation-induced degradation and failure. It was determined that defects created at the AlGaN/GaN interface introduce scattering centers near the two-dimensional electron gas (2DEG), which result in degraded mobility. Additionally, charged traps in the structure serve to screen the 2DEG resulting in reduced sheet carrier density. These two effects are responsible for degraded I-V behavior, including reduced saturation current and transconductance, increased ON-resistance, and positive threshold voltage shift. Interestingly, the sample with the most pre-existing defects was the most tolerant of radiation-induced damage.

Effect of front and back gates on β-Ga2O3 nano-belt field-effect transistors

Field effect transistors (FETs) using SiO2 and Al2O3 as the gate oxides for the back and front sides, respectively, were fabricated on exfoliated two-dimensional (2D) β-Ga2O3 nano-belts transferred to a SiO2/Si substrate. The mechanical exfoliation and transfer process produced nano-belts with smooth surface morphologies and a uniform low defect density interface with the SiO2/Si substrate. The depletion mode nanobelt transistors exhibited better channel modulation with both front and back gates operational compared to either front or back-gating alone. The maximum transconductance was ∼4.4 mS mm−1 with front and back-gating and ∼3.7 mS mm−1 with front-gating only and a maximum drain source current density of 60 mA mm−1 was achieved at a drain-source voltage of 10 V. The FETs had on/off ratios of ∼105 at 25 °C with gate-source current densities of ∼2 × 10−3 mA mm−1 at a gate voltage of −30 V. The device characteristics were stable over more than a month for storage in air ambient and the results show the ...

Lowered dislocation densities in uniform GaN layers grown on step-free (0001) 4H-SiC mesa surfaces

We report that very low threading dislocation densities (8×107∕cm2) were achieved in uniform GaN layers grown by metalorganic chemical vapor deposition on (0001) 4H-SiC mesa surfaces 50μm×50μm in area that were completely free of steps. Transmission electron microscopy (TEM) indicated that all observable GaN film threading dislocations were of edge type. TEM analysis of the defect structure of the nucleation layer (aluminum nitride, AlN) revealed a lack of c-component dislocations, and the clean annihilation of lateral, a-type dislocations within the first 200 nm of growth, with no lateral dislocations developing threading arms. These results indicate that the elimination of steps on the initial (0001) 4H-SiC growth surface may play an important role in the removal of mixed and c-type dislocations in subsequently grown AlN and GaN heteroepitaxial layers.