Syed Mukulika Dinara

Effects of threading dislocations on drain current dispersion and slow transients in unpassivated AlGaN/GaN/Si heterostructure field-effect transistors

Current transient analysis combined with response to pulsed bias drives have been used to explore the possibilities of threading dislocations affecting the current dispersion characteristics of AlGaN/GaN heterostructure field-effect transistors (HFETs). A growth strategy is developed to modulate the dislocation density among the heterostructures grown on silicon by plasma-assisted molecular-beam epitaxy. Slow pulsed I-V measurements show severe compressions and appear to be significantly dependent on the threading dislocation density. By analyzing the corresponding slow detrapping process, a deep-level trap with emission time constant in the order of seconds was identified as the cause. Among the specimens, both in the epilayers and at the surface, the number of dislocations was found to have a notable influence on the spatial distribution of deep-level trap density. The observations confirm that the commonly observed degraded frequency performance among AlGaN/GaN HFETs in the form of DC-radio frequency dispersions can at least partly be correlated with threading dislocation density.

Enhancement of two dimensional electron gas concentrations due to Si3N4 passivation on Al0.3Ga0.7N/GaN heterostructure: strain and interface capacitance analysis

Enhancement of two dimensional electron gas (2DEG) concentrations at Al0.3Ga0.7N/GaN hetero interface after a-Si3N4 (SiN) passivation has been investigated from non-destructive High Resolution X-ray Diffraction (HRXRD) analysis, depletion depth and capacitance-voltage (C-V) profile measurement. The crystalline quality and strained in-plane lattice parameters of Al0.3Ga0.7N and GaN were evaluated from double axis (002) symmetric (ω-2θ) diffraction scan and double axis (105) asymmetric reciprocal space mapping (DA RSM) which revealed that the tensile strain of the Al0.3Ga0.7N layer increased by 15.6% after SiN passivation. In accordance with the predictions from theoretical solution of Schrodinger-Poisson’s equations, both electrochemical capacitance voltage (ECV) depletion depth profile and C-V characteristics analyses were performed which implied effective 9.5% increase in 2DEG carrier density after passivation. The enhancement of polarization charges results from increased tensile strain in the Al0.3Ga0.7N layer and also due to the decreased surface states at the interface of SiN/Al0.3Ga0.7N layer, effectively improving the carrier confinement at the interface.

Reverse bias leakage current mechanism of AlGaN/InGaN/GaN heterostructure

The reverse bias leakage current mechanism of AlGaN/InGaN/GaN heterostructure is investigated by current-voltage measurement in temperature range from 298 K to 423 K. The Higher electric field across the AlGaN barrier layer of AlGaN/InGaN/GaN double heterostructure due to higher polarization charge is found to be responsible for strong Fowler-Nordheim (FN) tunnelling in the electric field higher than 3.66 MV/cm. For electric field less than 3.56 MV/cm, the reverse bias leakage current is also found to follow the trap assisted Frenkel-Poole (FP) emission in low negative bias region. Analysis of reverse FP emission yielded the barrier height of trap energy level of 0.34 eV with respect to Fermi level.

Fowler–Nordheim Tunnelling Contribution in AlGaN/GaN on Si (111) Schottky Current

ABSTRACT AlGaN/GaN heterojunction with Schottky metal contact can be modelled with two back-to-back diodes. The forward-biased diode between metal and AlGaN barrier acts at the onset of current with positive bias. Fowler– Nordheim tunnelling is mainly responsible for the electron transport at the low positive bias level. Downward energy band bending of AlGaN barrier with further positive voltage reduces the tunnelling probability due to lowering of the barrier height of the first diode, causing a dramatic change in the current.

Analysis of strain induced carrier confinement with varying passivation thickness of the Al0.3Ga0.7N/GaN heterostructure with graded AlxGa1-xN buffer on Si (111) substrate

In this study, the authors demonstrate the strain induced piezoelectric charge effect on carrier confinement at the Al0.3Ga0.7N/GaN heterointerface with varying passivation (Si3N4) thicknesses. The graded Al0.2Ga0.8N/Al0.1Ga0.9N buffer on the Si (111) substrate grown by plasma assisted molecular beam epitaxy reduces the dislocation density of the GaN layer, which significantly improves the carrier concentration at the Al0.3Ga0.7N/GaN interface. The carrier confinement as well as the two-dimensional electron gas (2DEG) density with varying passivation thicknesses has been investigated through high resolution x-ray diffraction (HRXRD) followed by strain analysis and capacitance–voltage (C-V) measurements. As per the HRXRD strain analysis, the 2DEG density was predicted to increase about 5%, 7.9%, and 10% after Si3N4 passivation of 20, 30, and 40 nm, respectively. This enhancement in carrier density (2DEGs) was then validated by C-V characteristics for the same Si3N4 variation. After passivation, the induced...

Quantitative investigation into the source of current slump in AlGaN/GaN HEMT on both Si (111) and sapphire: Self-heating and trapping

We have experimentally studied trapping and self-heating effect in terms of current slump in AlGaN/GaN HEMT grown and identically processed on Silicon (111) and Sapphire (0001) substrates. Different responses have been observed through DC characterization of different duty cycle (100%, 50%, 5% and 0.5%) of pulses at drain end. Effect of self-heating is more in case of HEMT on Sapphire due to its comparative poor thermal conductivity whereas trapped charges have strong contribution in current drop of HEMT on Si (111) due to larger lattice as well as thermal expansion coefficient mismatched epitaxy between GaN and Si (111). These results have been compared among substrates that lead us to find out optimal source of current slump quantitatively between traps and self-heating.

Structural, optical, and transport properties of AlGaN/GaN and AlGaN/InGaN heterostructure on sapphire grown by plasma assisted molecular beam epitaxy

The effects of indium on the strain states, surface morphologies, and polarization induced charges (and in turn on sheet carrier concentrations) at the interfacial channel layers of AlGaN/GaN and AlGaN/InGaN specimens have been investigated. Room temperature Raman spectroscopy was performed to explain the residual strains by the optical phonon frequency shift from their bulk values, and also to analyze the quality of the GaN and InGaN channel layer by the full width at half maxima of E2high and A1(LO) (longitudinal-optical) phonon lines. The strain state was also analyzed by room temperature photoluminescence (PL) spectra by observing the direct excitonic transition from Γ6v →Γ1c (A, B) for valance band to conduction band. The surface morphology was observed from atomic force microscopy imaging inferring pit densities of 4.4×107 cm−2 and 6×107 cm−2 for 5×5 μm area scans. In addition, the screw and edge type defect density were 3.28×107 and 5.85×109; 2.40×108 and 4.91×109 cm−2 as calculated from high resol...

Strain Effects on Band Structure of Wurtzite InGaN/GaN Quantum Well on Si Substrate

Heteroepitaxial crystal growth of InGaN/GaN Quantum Well (QW) on Si substrate is necessary for the development of visible and ultraviolet (UV) Light Emitting Diodes (LEDs). In this work we accumulate the efforts that have been made to focus the analysis of strain effects on the band structure of GaN quantum well systems on Si substrate due to its potential application. We limit this investigation on the effect of pseudomorphic GaN quantum wells on Si substrate grown at different GaN planes. It is observed that the deformation potential constants and anisotropic strain separate Heavy Hole (HH) and Light Hole (LH) band and eventually making LH the top most bands.

Comparative study on hydrostatic strain, stress and dislocation density of Al0.3Ga0.7N/GaN heterostructure before and after a-Si3N4 passivation

The hydrostatic strain, stress and dislocation densities were comparatively analyzed before and after passivation of amorphous silicon nitride (a-Si3N4) layer on Al0.3Ga0.7N/GaN heterostructure by nondestructive high resolution x-ray diffraction (HRXRD) technique. The crystalline quality, in-plane and out-of plane strain were evaluated from triple-axis (TA) (ω-2θ) diffraction profile across the (002) reflection plane and double-axis (DA) (ω-2θ) glancing incidence (GI) diffraction profile across (105) reflection plane. The hydrostatic strain and stress of Al0.3Ga0.7N barrier layer were increased significantly after passivation and both are tensile in nature. The dislocation density of GaN was also analyzed and no significant change was observed after passivation of the heterostructure. The crystalline quality was not degraded after passivation on the heterostructure confirmed by the full-width-half-maximum (FWHM) analysis.

Comprehensive Analytical Modeling of N-polar GaN/AlGaN Insulated Gate HEMTs with and without Polarization Neutralization Layer

Electrical characteristics of N-polar GaN/AlxGa1-xN insulated gate (MIS) HEMTs are theoretically analyzed. Threshold voltage models are developed for both the generalized and the polarization neutralized structures. Based on these developments, the drain current and transconductance are formulated after incorporation of an appropriate Monte-Carlo simulation based mobility model for GaN. Non-idealities such as imaging charges in the interface/buffer, source drain resistances, and velocity saturation are taken into account in the present model. The analytical results on the transport characteristics of the device are compared with experimentally measured data and are in close agreement.