Goutam Koley

Effect of an AlN buffer layer on the epitaxial growth of InN by molecular-beam epitaxy

The effect of an AlN buffer layer on the epitaxial growth of InN by molecular-beam epitaxy (MBE) is studied. Using an AlN buffer layer can significantly improve the structural and electrical properties of InN. With increasing thickness of the AlN buffer layer, the Hall mobility of InN will monotonically increase while the electron carrier concentration decreases. The surface morphology of the film also improves. A Hall mobility of more than 800 cm2/V s with a carrier concentration of 2–3×1018 cm−3 at room temperature can be routinely obtained on ∼0.1 μm InN film. More importantly, it is found that under optimum growth conditions, by using an AlN buffer layer, InN films with comparable quality can be achieved by the conventional MBE technique compared to InN grown by migration-enhanced epitaxy. Increasing InN thickness also increases Hall mobility.

Slow transients observed in AlGaN/GaN HFETs: effects of SiN/sub x/ passivation and UV illumination

Very slow drain current and surface potential transients have been observed in AlGaN/GaN heterostructure field effect transistors that are subjected to high bias stress. Simultaneous measurements of drain current and surface potential indicate that large change in surface potential after stress is responsible for the reduction in drain current in these devices. Measurements of surface potential profile from the gate edge toward the drain as a function of time indicate that surface potential changes occur mostly near the gate. It is proposed that the surface potential changes are caused by electrons which tunnel from the gate under high bias stress and get trapped at the surface states near the gate. Passivation of the surface with SiN/sub x/ reduces the transient magnitudes to a large extent. This correlates with a large improvement in microwave power performance in these devices after passivation. UV illumination of these devices totally eliminates the drain current and surface potential transients.

Surface potential measurements on GaN and AlGaN/GaN heterostructures by scanning Kelvin probe microscopy

Surface potentials on GaN epilayers and Al0.35Ga0.65N/GaN heterostructures have been studied by scanning Kelvin probe microscopy (SKPM) in conjunction with noncontact atomic force microscopy. The dependence of the surface potential on doping in GaN films, as well as the variation of surface potential with Al0.35Ga0.65N barrier layer thickness has been investigated. The bare surface barrier height (BSBH), as measured by SKPM, is observed to decrease from ∼1. 40±0.1 eV to ∼0.60±0.1 eV with increasing doping in the GaN epilayers. Schottky barrier height calculated from the measurements of BSBH on n-GaN agrees very well with results from previous studies. We have also estimated the surface state density for GaN based on the measured values of BSBH. The semiconductor “work function” at the Al0.35Ga0.65N surface (in heterostructure samples) is observed to decrease by ∼0.60 eV with increase in barrier layer thickness from ∼50 to ∼440 A. A simple model considering the presence of a uniform density of charged acce...

Modification of Indium Tin Oxide for Improved Hole Injection in Organic Light Emitting Diodes

Modification of indium tin oxide (ITO) electrode interface for improved hole injection in organic light emitting diodes (OLED) was investigated. The injection efficiency measurements were carried out to characterize contact between ITO and the organic semiconductor triphenyldiamine (TDP) layer. Coating of ITO with self-assembled ultrathin platinum (Pt) films as interface modification layers, dramatically enhances OLED efficiency and contact with TDP becomes nearly ohmic. The surface morphology of ITO electrodes was investigated by atomic force microscopy (AFM).

Cantilever effects on the measurement of electrostatic potentials by scanning Kelvin probe microscopy

Scanning Kelvin probe microscopy (SKPM) is a unique way to measure electrostatic potentials for small geometries. It has numerous applications including characterization of integrated circuits and nanoscale devices. SKPM is attractive because of the quantitative nature of the measurements. In this work, we have examined one of the principal sources of measurement error, the cantilever (which holds the probe tip). The accuracy of measurements of electrostatic potentials on closely spaced regions biased differently is reduced due to a large capacitance gradient associated with the cantilever. However, it is observed that the accuracy of measurements increases as the tip–sample distance is decreased because the capacitance gradient of the tip becomes proportionally larger relative to that of the cantilever. It is further observed that longer tips with smaller cantilever areas measure the electrostatic potentials more accurately as the capacitance gradient of the cantilever is reduced. Scanning probe tips are parametrized by a factor R, which indicates their suitability for SKPM measurements.

On the origin of the two-dimensional electron gas at the AlGaN∕GaN heterostructure interface

Bare surface barrier heights (BSBHs) of AlGaN∕GaN heterostructures, with varying AlGaN layer thickness and ∼35% Al alloy composition, have been measured using UV laser induced transients. The BSBH has been observed to vary with AlGaN thickness, before it saturates beyond a critical thickness, and the variation found to be very similar to that of the two-dimensional electron gas (2DEG) density at the interface. Such a trend can be explained by considering the presence of surface donor states distributed in the band gap. The density of the surface donor states has been calculated from the variation of the surface barrier with the 2DEG density, and found to be almost constant at ∼1.6×1013cm−2eV−1 in the energy range of ∼1.0–1.8eV from the conduction band. After SiNx passivation of the surface, the BSBH reduces, and sheet charge density increases, indicating the presence of positive charges in the passivation layer.

InN nanowire based sensors

High quality InN nanowires (NWs) were grown from nanoscale catalyst patterns by vapor-liquid-solid mechanism. The nanowires bend spontaneously or get deflected from other nanowires at multiples of 30deg forming nano-networks. Smooth and planar NWs used to fabricate field effect transistors (FET) exhibited excellent drain current modulation in a back-gated geometry. The mobility calculated from the I-V characteristics is 36 cm2/Vs, while the carrier concentration is 4.8times1018 cm-3. The NW FET based nanosensor demonstrated high sensitivity to trace NO2 due to a thin In2O3 shell layer present around the InN core. The adsorption of the NO2 molecules reduces the density of the carriers confined at the InN/In2O3 interface, thus reducing the drain current. The change in drain current for a single NW based FET resulted in a very high sensitivity of 45 ppb in ambient conditions. Planar InN nano-networks can potentially offer a much improved sensitivity to trace NO2 in ambient conditions.

Scanning Kelvin probe microscopy characterization of dislocations in III-nitrides grown by metalorganic chemical vapor deposition

Scanning Kelvin probe microscopy has been used in conjunction with noncontact atomic force microscopy for characterizing dislocations in n-GaN and Al0.35Ga0.65N/GaN heterostructures. The surface potential variations around the dislocations present in the Al0.35Ga0.65N/GaN heterostructure have been observed to be 0.1–0.2 V with full width at half maximums (FWHMs) of 100–200 nm. On the other hand, n-GaN shows potential variations of 0.3–0.5 V having FWHMs of 20–50 nm. The dislocations (present in densities of ∼109 cm−2) have been found to be negatively charged for both n-GaN and Al0.35Ga0.65N/GaN heterostructure samples.

Tunable Reverse‐Biased Graphene/Silicon Heterojunction Schottky Diode Sensor

A new chemical sensor based on reverse-biased graphene/Si heterojunction diode has been developed that exhibits extremely high bias-dependent molecular detection sensitivity and low operating power. The device takes advantage of graphenes atomically thin nature, which enables molecular adsorption on its surface to directly alter graphene/Si interface barrier height, thus affecting the junction current exponentially when operated in reverse bias and resulting in ultrahigh sensitivity. By operating the device in reverse bias, the work function of graphene, and hence the barrier height at the graphene/Si heterointerface, can be controlled by the bias magnitude, leading to a wide tunability of the molecular detection sensitivity. Such sensitivity control is also possible by carefully selecting the graphene/Si heterojunction Schottky barrier height. Compared to a conventional graphene amperometric sensor fabricated on the same chip, the proposed sensor demonstrated 13 times higher sensitivity for NO₂ and 3 times higher for NH₃ in ambient conditions, while consuming ∼500 times less power for same magnitude of applied voltage bias. The sensing mechanism based on heterojunction Schottky barrier height change has been confirmed using capacitance-voltage measurements.

Fabrication and characterization of pre-aligned gallium nitride nanowire field-effect transistors

We report on the fabrication of gallium nitride (GaN) nanowire field-effect transistors (FETs) with both bottom-gate and top-gate structures, with very high yield using a unique pre-alignment process. The catalyst positions were chosen to be aligned with the source/drain position, and Ni catalysts with a diameter of 200 nm were deposited selectively at these pre-determined positions. Electrostatic analysis was performed for the bottom-gate devices to estimate the nanowires electrical characteristics. Comparison of the bottom-gate and the top-gate structures indicated that better performance, in terms of saturation and breakdown characteristics, can be obtained using the top-gate structure. For the top-gate nanowire FETs, temperature-dependent characteristics were investigated up to the current saturation regime, and memory effects were observed at room temperature.