Nadeemullah A. Mahadik

Homoepitaxial growth of β-Ga2O3 thin films by low pressure chemical vapor deposition

This paper presents the homoepitaxial growth of phase pure (010) β-Ga2O3 thin films on (010) β-Ga2O3 substrate by low pressure chemical vapor deposition. The effects of growth temperature on the surface morphology and crystal quality of the thin films were systematically investigated. The thin films were synthesized using high purity metallic gallium (Ga) and oxygen (O2) as precursors for gallium and oxygen, respectively. The surface morphology and structural properties of the thin films were characterized by atomic force microscopy, X-ray diffraction, and high resolution transmission electron microscopy. Material characterization indicates the growth temperature played an important role in controlling both surface morphology and crystal quality of the β-Ga2O3 thin films. The smallest root-mean-square surface roughness of ∼7 nm was for thin films grown at a temperature of 950 °C, whereas the highest growth rate (∼1.3 μm/h) with a fixed oxygen flow rate was obtained for the epitaxial layers grown at 850 °C.

Microwave annealing of Mg-implanted and in situ Be-doped GaN

An ultrafast microwave annealing method, different from conventional thermal annealing, is used to activate Mg-implants in GaN layer. The x-ray diffraction measurements indicated complete disappearance of the defect sublattice peak, introduced by the implantation process for single-energy Mg-implantation, when the annealing was performed at ≥1400 °C for 15 s. An increase in the intensity of Mg-acceptor related luminescence peak (at 3.26 eV) in the photoluminescence spectra confirms the Mg-acceptor activation in single-energy Mg-implanted GaN. In case of multiple-energy implantation, the implant generated defects persisted even after 1500 °C/15 s annealing, resulting in no net Mg-acceptor activation of the Mg-implant. The Mg-implant is relatively thermally stable and the sample surface roughness is 6 nm after 1500 °C/15 s annealing, using a 600 nm thick AlN cap. In situ Be-doped GaN films, after 1300 °C/5 s annealing have shown Be out-diffusion into the AlN layer and also in-diffusion toward the GaN/SiC in...

Epitaxial growth of AlN films via plasma-assisted atomic layer epitaxy

Thin AlN layers were grown at 200–650 °C by plasma assisted atomic layer epitaxy (PA-ALE) simultaneously on Si(111), sapphire (112¯0), and GaN/sapphire substrates. The AlN growth on Si(111) is self-limited for trimethyaluminum (TMA) pulse of length > 0.04 s, using a 10 s purge. However, the AlN nucleation on GaN/sapphire is non-uniform and has a bimodal island size distribution for TMA pulse of ≤0.03 s. The growth rate (GR) remains almost constant for Tg between 300 and 400 °C indicating ALE mode at those temperatures. The GR is increased by 20% at Tg = 500 °C. Spectroscopic ellipsometry (SE) measurement shows that the ALE AlN layers grown at Tg ≤ 400 °C have no clear band edge related features, however, the theoretically estimated band gap of 6.2 eV was measured for AlN grown at Tg ≥ 500 °C. X-ray diffraction measurements on 37 nm thick AlN films grown at optimized growth conditions (Tg = 500 °C, 10 s purge, 0.06 s TMA pulse) reveal that the ALE AlN on GaN/sapphire is (0002) oriented with rocking curve f...

Observation of stacking faults from basal plane dislocations in highly doped 4H-SiC epilayers

Stacking fault (SF) expansion from basal plane dislocations (BPDs) confined in highly doped 4H-SiC buffer layers is observed under high-power ultraviolet illumination (>1000 W/cm2). Once the SFs reach the active drift layers, grown above the buffer layers, they are seen to rapidly expand up to the sample surface where they can cause device degradation. BPD faulting in the buffer appears to have a carrier injection threshold. Carrier density simulations under various injection conditions and carrier lifetimes are used to establish the conditions of BPD faulting within the buffer layer that could prevent SF expansion into the drift layer.

Phase transition in Sr0.75Ba0.25NbO3 near the Curie temperature

Strontium barium niobate has the tungsten-bronze structure with a tetragonal unit cell, and it exhibits negative thermal expansion along the c axis between −120°C and room temperature while having a positive thermal expansion along the a axis for the same temperature range. At higher temperatures, close to the Curie temperature and above, the negative thermal expansion along the c axis changes to positive thermal expansion. The a axis lattice parameter as a function of temperature shows a change in slope at the Curie temperature. These results indicate the presence of a second-order phase transition near the Curie temperature.

Failure mechanism of THz GaAs photoconductive antenna

We investigated the failure mechanism of THz GaAs photoconductive antenna using high resolution x-ray diffraction topography. From these studies, it was found that grain boundaries are formed during the high frequency device operation. This results in the segregation of gold at the boundaries causing electromigration of the metal between the gold micro-strips. This disrupts the photocurrents from being produced by femtosecond laser thus preventing terahertz beam generation from the photoconductive antennae leading to device failure.

Surface strain and its impact on the electrical resistivity of GaN channel in AlGaN/GaN high electron mobility transistor

Localized strain in AlGaN/GaN high electron mobility transistor (HEMT) device structures was studied by high resolution x-ray diffraction and rocking curve measurements, and the results were compared with the corresponding channel sheet resistance measurements. The map of in-plane tensile strain on the HEMT wafer showed a near one-to-one correspondence with the electrical resistivity. The in-plane strain variation in the range of (2.295–3.539)×10−4 resulted in a corresponding sheet resistance variation between 345 and 411 Ω/◻.

Nucleation of in-grown stacking faults and dislocation half-loops in 4H-SiC epitaxy

Ultraviolet photoluminescence, transmission electron microscopy and KOH etching were used to characterize extended defects in 4H-SiC epilayers grown at high growth rates (18 μm/h). Layers exhibited high densities of in-grown stacking faults and dislocation half-loops. The stacking faults were 8H Shockley-type faults. The Burgers vector of the dislocation half-loops was in the (0001) basal plane. Both defects nucleate within the epilayer at early stages of growth. Defect nucleation is directly correlated with high initial growth rate and is not related to any defects/heterogeneities in the substrate or epilayer. Epilayer growth by nucleation of two-dimensional islands is proposed as a possible mechanism for the formation of both defects, through nucleation of faulted Si-C bilayers.

Mitigation of BPD by Pre-Epigrowth High Temperature Substrate Annealing

Basal Plane Dislocations (BPD) intersecting the SiC substrate surface were converted to threading edge dislocations (TED) by high temperature annealing of the substrates in the temperature range of 1750 °C – 1950 °C. Successively, epitaxial growth on annealed as well as non-annealed samples was performed, concurrently, to investigate the effect of the substrate annealing on BPD mitigation in the epilayers. For the 1950 °C/10min anneal, a 3x reduction in BPD density was observed. Additionally, surface roughness measured using atomic force microscopy revealed no degradation in surface morphology of the grown epilayers after annealing.

Basal Plane Dislocations from Inclusions in 4H-SiC Epitaxy

Suppression of basal plane dislocations (BPDs) from critical epitaxial drift layer has occurred mainly by converting BPDs in the substrate into threading edge dislocations before the BPDs enter the drift layer. As optimized epitaxial growth has produced drift layers free of BPDs originating from the substrate over a large fraction of the wafer, other sources of BPDs have become important. One source of BPDs introduced during epitaxial growth is from inclusions, which mainly consist of misoriented 4H-SiC. Inclusions are surrounded by a local cluster of BPDs and in thick, low-BPD epitaxy the outermost BPDs glide centimeters from the inclusion forming a much larger damaged area. The details of BPD migration from inclusions are discussed.