Shailesh Madisetti

Preparation of gallium nitride surfaces for atomic layer deposition of aluminum oxide

A combined wet and dry cleaning process for GaN(0001) has been investigated with XPS and DFT-MD modeling to determine the molecular-level mechanisms for cleaning and the subsequent nucleation of gate oxide atomic layer deposition (ALD). In situ XPS studies show that for the wet sulfur treatment on GaN(0001), sulfur desorbs at room temperature in vacuum prior to gate oxide deposition. Angle resolved depth profiling XPS post-ALD deposition shows that the a-Al2O3 gate oxide bonds directly to the GaN substrate leaving both the gallium surface atoms and the oxide interfacial atoms with XPS chemical shifts consistent with bulk-like charge. These results are in agreement with DFT calculations that predict the oxide/GaN(0001) interface will have bulk-like charges and a low density of band gap states. This passivation is consistent with the oxide restoring the surface gallium atoms to tetrahedral bonding by eliminating the gallium empty dangling bonds on bulk terminated GaN(0001).

Nitride passivation of the interface between high-k dielectrics and SiGe

In-situ direct ammonia (NH3) plasma nitridation has been used to passivate the Al2O3/SiGe interfaces with Si nitride and oxynitride. X-ray photoelectron spectroscopy of the buried Al2O3/SiGe interface shows that NH3 plasma pre-treatment should be performed at high temperatures (300 °C) to fully prevent Ge nitride and oxynitride formation at the interface and Ge out-diffusion into the oxide. C-V and I-V spectroscopy results show a lower density of interface traps and smaller gate leakage for samples with plasma nitridation at 300 °C.

Growth of strained InGaSb quantum wells for p-FET on Si: Defects, interfaces, and electrical properties

A study of heteroepitaxial molecular beam epitaxy growth of strained p-channel InGaSb quantum well (QW) on lattice mismatched Si (100) using Al(Ga)Sb metamorphic buffers is presented in this paper. The migration enhanced epitaxy (MEE) technique was employed for AlSb nucleation layer (NL) on Si and analyzed using atomic force microscopy and in-situ Auger electron spectroscopy techniques to optimize growth conditions for continuous 2D buffer layers and improve surface quality of subsequent layers. Growth-related defects (threading dislocations, microtwins, and antiphase boundaries) and their effect on surface morphology and electrical properties of the QWs are analyzed with scanning electron microscope and transmission electron microscopy and correlated to the NL properties. The baseline data for defect density in the layers and resultant surface morphology are presented. Room temperature p-channel Hall mobility of 660 cm2/V s at 3 × 1011 cm−2 sheet hole concentration is achieved in InGaSb QWs using an opti...

Interface trap density and mobility extraction in InGaAs buried quantum well metal-oxide-semiconductor field-effect-transistors by gated Hall method

In this work, we are using a gated Hall method for measurement of free carrier density and electron mobility in buried InGaAs quantum well metal-oxide-semiconductor field-effect-transistor channels. At room temperature, mobility over 8000 cm2/Vs is observed at ∼1.4 × 1012 cm−2. Temperature dependence of the electron mobility gives the evidence that remote Coulomb scattering dominates at electron density <2 × 1011 cm−2. Spectrum of the interface/border traps is quantified from comparison of Hall data with capacitance-voltage measurements or electrostatic modeling. Above the threshold voltage, gate control is strongly limited by fast traps that cannot be distinguished from free channel carriers just by capacitance-based methods and can be the reason for significant overestimation of channel density and underestimation of carrier mobility from transistor measurements.

Density-Functional Theory Molecular Dynamics Simulations and Experimental Characterization of a-Al₂O₃/SiGe Interfaces.

Density-functional theory molecular dynamics simulations were employed to investigate direct interfaces between a-Al2O3 and Si0.50Ge0.50 with Si- and Ge-terminations. The simulated stacks revealed mixed interfacial bonding. While Si-O and Ge-O bonds are unlikely to be problematic, bonding between Al and Si or Ge could result in metallic bond formation; however, the internal bonds of a-Al2O3 are sufficiently strong to allow just weak Al bonding to the SiGe surface thereby preventing formation of metallic-like states but leave dangling bonds. The oxide/SiGe band gaps were unpinned and close to the SiGe bulk band gap. The interfaces had SiGe dangling bonds, but they were sufficiently filled that they did not produce midgap states. Capacitance-voltage (C-V) spectroscopy and angle-resolved X-ray photoelectron spectroscopy experimentally confirmed formation of interfaces with low interface trap density via direct bonding between a-Al2O3 and SiGe.

Electrical properties related to growth defects in metamorphic GaSb films on Si

This paper reports on correlation of growth-related defects and electrical properties in GaSb films grown on different Si substrates using metamorphic buffers. Large lattice mismatch between GaSb and Si (∼11%) results in the formation of threading dislocations (TDs) and microtwins (MTs) along with antiphase domains due to the lack of inversion symmetry in III-Vs. The defect density profiles were analyzed using transmission electron microscopy and atomic force microscopy. The TD density of just below 108 cm−2 and MT density below 104 cm−1 were found in 2.1 μm thick structures, and were found to be four times higher than in similar GaSb structures on GaAs substrates. Hole density and mobility profiles were obtained using differential Hall method and show that dislocations (TDs or MT partials) generate about 25 acceptors/nm. Minimum midgap interface trap density values are similar in the metal-oxide-semiconductor structures prepared on GaAs and Si, ∼2 × 1012 cm2 eV−1.

Quantification of interface trap density above threshold voltage by gated hall method in InGaAs buried quantum well MOSFET

Low density of states (DOS) and typically high interface and border trap densities (D<;sub>it<;/sub>) in high mobility group III-V semiconductors provide difficulties in quantification of D<;sub>it<;/sub> near the conduction band edge. The trap response above the threshold voltage can be very fast, and conventional D<;sub>it<;/sub> extraction methods, based on capacitance/conductance response (CV methods) of MOS capacitors at frequencies <;1MHz, cannot distinguish conducting and trapped carriers. In addition, the CV methods have to deal with high dispersion in the accumulation region that makes it a difficult task to measure the true oxide capacitance C<;sub>ox<;/sub> value. Another implication of these properties of III-V interfaces is an ambiguity of determination of electron density in the MOSFET channel. Traditional evaluation of carrier density by integration of the C-V curve, gives significantly overestimated results even if corrected by D<;sub>it<;/sub>. It happens because the CV methods can distinguish free and trap carriers exclusively by their response kinetics, and therefore all trapped electrons responding faster than ~1μs are treated as free electrons. In this work, we are using a gated Hall method [1,2] which allows for direct measurement of free carrier density, and therefore, when combined with CV measurements or electrostatic modeling allows for accurate quantification of Dit spectrum. In addition, the former approach does not need knowledge of Cox.

Mobility and scattering mechanisms in buried InGaSb quantum well channels integrated with in-situ MBE grown gate oxide

InGaSb material family with its higher hole transport properties are potential candidates for group III-V CMOS circuits. Understanding of the dominant scattering mechanisms is crucial for the development of future high speed, low power device applications. We present Hall mobility data of p-type InGaSb quantum well (QW) channels and derive the dominant scattering mechanisms related to the interface and trapped charges that degrade mobility in these structures.

Improvement of the GaSb/Al 2 O 3 interface using a thin InAs surface layer

A major challenge in fabricating a high mobility (In)GaSb based transistor is reducing the density of trap states (D it ) at the III-V/high-k interface [1]. Unoccupied surface bonds react with ambient oxygen and create energy levels within the GaSb bandgap which limit Fermi level movement and degrade sub-threshold slope. InGaAs and other As-rich surfaces have been thoroughly investigated and exhibit significant trap state reduction with Ammonium Sulfide passivation and further native oxide reduction when exposed to TMA precursor used in ALD [2,3]. Interface engineering of GaSb is accomplished using an epitaxially grown 2nm InAs top layer. This GaSb/InAs gate stack has shown immense improvement in C-V characteristics and a significant reduction of D it at the III-V/high-k interface.