Multiple valley couplings in nanometer Si metal-oxide-semiconductor field-effect transistors

We investigate the couplings between different energy band valleys in a metal-oxide-semiconductor field-effect transistor (MOSFET) device using self-consistent calculations of million-atom Schrodinger-Poisson equations. Atomistic empirical pseudopotentials are used to describe the device Hamiltonian and the underlying bulk band structure. The MOSFET device is under nonequilibrium condition with a source-drain bias up to 2 V and a gate potential close to the threshold potential. We find that all the intervalley couplings are small, with the coupling constants less than 3 meV. As a result, the system eigenstates derived from different bulk valleys can be calculated separately. This will significantly reduce the simulation time because the diagonalization of the Hamiltonian matrix scales as the third power of the total number of basis functions. (C) 2008 American Institute of Physics.

Defect evolution and accompanied change of electrical properties during the GaN growth by metalorganic chemical vapor deposition

The defect evolution and its correlation with electrical properties of GaN films grown by metalorganic chemical vapor deposition are investigated. It is found that the dislocation density decreases gradually during the growth process, and the dislocation reduction rate in the island coalescence process is especially rapid. The changes in electron mobility of GaN with the increase of growth time are mainly dependent on the dislocations acting as scattering centers. Furthermore, the variation of carrier concentration in GaN may be related with the point defects and their clusters. The quality of GaN could be improved by suitably increasing the film thickness. (C) 2009 Elsevier B.V. All rights reserved.

Influence of penetrating V-pits on leakage current of GaN based p-i-n UV detector

The leakage mechanism of GaN-based p-i-n (p-AlGaN/i-GaN/n-GaN) UV detector has been investigated. With the same dislocation density, devices made from material with higher density of V-pits on surface produce larger leakage current. SEM images show that some V-pits penetrate into i-GaN layer, sometimes even the n-GaN layer. If p-ohmic contact metal (Ni/Au) deposits in the V-pits, Schottky contact would be formed at the interface of metal and i-GaN, or form ohmic contact at the interface of metal and n-GaN. The existence of parallel Schottky junction and ohmic contact resistance enhances the leakage current greatly.

Quantum mechanical simulation of nanosized metal-oxide-semiconductor field-effect transistor using empirical pseudopotentials: A comparison for charge density occupation methods

The atomistic pseudopotential quantum mechanical calculations are used to study the transport in million atom nanosized metal-oxide-semiconductor field-effect transistors. In the charge self-consistent calculation, the quantum mechanical eigenstates of closed systems instead of scattering states of open systems are calculated. The question of how to use these eigenstates to simulate a nonequilibrium system, and how to calculate the electric currents, is addressed. Two methods to occupy the electron eigenstates to yield the charge density in a nonequilibrium condition are tested and compared. One is a partition method and another is a quasi-Fermi level method. Two methods are also used to evaluate the current: one uses the ballistic and tunneling current approximation, another uses the drift-diffusion method. (C) 2009 American Institute of Physics. [doi:10.1063/1.3248262]

InGaN/GaN p-i-n Photodiodes Fabricated with Mg-Doped p-InGaN Layer

Mg-doped p-InGaN layers with In composition of about 10% are grown by metalorganic chemical vapor deposition (MOCVD). The effect of the annealing temperature on the p-type behavior of Mg-doped InGaN is studied. It is found that the hole concentration in p-InGaN increases with a rising annealing temperature in the range of 600 850 C, while the hole mobility remains nearly unchanged until the annealing temperature increases up to 750 C, after which it decreases. On the basis of conductive p-type InGaN growth, the p-In0.1Ga0.9N/i-In0.1Ga0.9N/n-GaN junction structure is grown and fabricated into photodiodes. The spectral responsivity of the InGaN/GaN p-i-n photodiodes shows that the peak responsivity at zero bias is in the wavelength range 350-400 nm.

The effect of single AlGaN interlayer on the structural properties of GaN epilayers grown on Si (111) substrates

High-quality and nearly crack-free GaN epitaxial layer was obtained by inserting a single AlGaN interlayer between GaN epilayer and high-temperature AlN buffer layer on Si (111) substrate by metalorganic chemical vapor deposition. This paper investigates the effect of AlGaN interlayer on the structural proper-ties of the resulting GaN epilayer. It confirms from the optical microscopy and Raman scattering spectroscopy that the AlGaN interlayer has a remarkable effect on introducing relative compressive strain to the top GaN layer and preventing the formation of cracks. X-ray diffraction and transmission electron microscopy analysis reveal that a significant reduction in both screw and edge threading dislocations is achieved in GaN epilayer by the insertion of AlGaN interlayer. The process of threading dislocation reduction in both AlGaN interlayer and GaN epilayer is demonstrated.

Suppression of exciton recombination in symmetric GaAs0.7Sb0.3/GaAs/GaAs0.7P0.3 coupled quantum wells induced by an in-plane magnetic field

Suppression of the exciton recombination in GaAs0.7Sb0.3/GaAs/GaAs0.7P0.3 coupled quantum well (CQW) induced by an external magnetic field is investigated theoretically. Unlike the usual electro-Stark effect, the exciton energy dispersion of an exciton is modified by an external in-plane magnetic field, the ground state of the magnetoexciton shifts from a zero in-plane center of mass (CM) momentum to a finite CM momentum, and the Lorentz force induces the spatial separation of electron and hole. Consequently, this effect renders the ground state of magnetoexciton stable against radiative recombination due to momentum conservation. This effect depends sensitively on the thickness and height of GaAs0.7Sb0.3 layer, therefore it could provide us useful infometion about the band alignment of CQW. (C) 2004 American Institute of Physics.

Surface plasmon modulated nano-aperture vertical-cavity surface-emitting laser

Surface plasmon modulated nano-aperture vertical-cavity surface-emitting lasers were fabricated from common 850 nm VCSELs. When the diameter of the aperture was 200 nm, and the period of grating was 400 nm, the maximum far-field output power reached 0.3mW at a driving current of 15 mA. The fabrication process was described and the beaming properties were studied via experimentally and theoretically.

Effects of the wave function localization in AlInGaN quaternary alloys

Using the first-principles band-structure method and the special quasirandom structures approach, the authors have investigated the band structure of random AlxInyGa1-x-yN quaternary alloys. They show that the wave functions of the band edge states are more localized on the InN sites. Consequently, the photoluminescence transition intensity in the alloy is higher than that in GaN. The valence band maximum state of the quaternary alloy is also higher than GaN with the same band gap, indicating that the alloy can be doped more easily as p-type. (c) 2007 American Institute of Physics.

Optical analysis of dislocation-related physical processes in GaN-based epilayers

In this paper, recent progresses in optical analysis of dislocation-related physical properties in GaN-based epilayers are surveyed with a brief review. The influence of dislocations on both near-band edge emission and yellow luminescence (YL) is examined either in a statistical way as a function of dislocation density or focused on individual dislocation lines with a high spatial resolution. Threading dislocations may introduce non-radiative recombination centers and enhance YL, but their effects are affected by the structural and chemical environment. The minority carrier diffusion length may be dependent on either dislocation density or impurity doping as confirmed by the result of photovoltaic spectra. The in situ optical monitoring of the strain evolution process is employed during GaN heteroepitaxy using an AIN interlayer. A typical transition of strain from compression to tension is observed and its correlation with the reduction and inclination of threading dislocation lines is revealed. (c) 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Electronic structure of Mn-doped ZnO quantum wires: A mean-field theory study

Based on the effective-mass model and the mean-field approximation, we investigate the energy levels of the electron and hole states of the Mn-doped ZnO quantum wires (x=0.0018) in the presence of the external magnetic field. It is found that either twofold degenerated electron or fourfold degenerated hole states split in the field. The splitting energy is about 100 times larger than those of undoped cases. There is a dark exciton effect when the radius R is smaller than 16.6 nm, and it is independent of the effective doped Mn concentration. The lowest state transitions split into six Zeeman components in the magnetic field, four sigma(+/-) and two pi polarized Zeeman components, their splittings depend on the Mn-doped concentration, and the order of pi and sigma(+/-) polarized Zeeman components is reversed for thin quantum wires (R < 2.3 nm) due to the quantum confinement effect.

The influence of V/III ratio in the initial growth stage on the properties of GaN epilayer deposited on low temperature AlN buffer layer

The V/III ratio in the initial growth stage of metalorganic chemical vapor deposition has an important influence on the quality of a GaN epilayer grown on a low-temperature AIN buffer layer and c-plane sapphire substrate. A weaker yellow luminescence, a narrower half-width of the X-ray diffraction peak, and a higher electron mobility result when a lower V/III ratio is taken. The intensity of in situ optical reflectivity measurements indicates that the film surface is rougher at the beginning of GaN growth, and a longer time is needed for the islands to coalesce and for a quasi-two dimensional mode growth to start. A comparison of front- and back-illuminated photoluminescence spectra confirms that many threading dislocations are bent during the initial stage, leading to a better structural quality of the GaN layer. (C) 2007 Elsevier B.V. All rights reserved.

Influence of defects in n(-)-GaN layer on the responsivity of Schottky barrier ultraviolet photodetectors

The influence of defects on the responsivity of GaN Schottky barrier ultraviolet photodetectors with n(-)-GaN/n(+)-GaN layer structures is investigated. It is found that employing undoped GaN instead of Si-doped GaN as the n(-)-GaN layer brings about a higher responsivity due to a lower Ga vacancy concentration. On the other hand, the dislocations may increase the recombination of electron-hole pairs and enhance the surface recombination in the photodetectors. Employing undoped GaN and reducing the dislocation density in the n(-)-GaN layer are necessary to improve the responsivity of Schottky barrier photodetectors. (c) 2007 American Institute of Physics.

Correlation between optical and structural properties of (Al,Ga)N layers grown by MOCVD

For both, (Al,Ga)N with low Al content grown on a GaN nucleation layer and (AI,Ga)N with high Al content gown on an AlN nucleation layer, the inhomogeneous distribution of the luminescence is linked to the distribution of defects, which may be due to inversion domains. In the former system, defect regions exhibit a much lower Al content than the nominal one leading to a splitting of the respective luminescence spectra. In the latter system, a domain-like growth is observed with a pyramidal surface morphology and non-radiative recombination within the domain boundaries. (c) 2007 WILEYNCH Verlag GmbH & Co. KGaA, Weinheim.

Effect of Al incorporation on the AlGaN growth by metalorganic chemical vapor deposition

The effect of Al incorporation on the AlGaN growth by metalorganic chemical vapor deposition is investigated. With the increase of trimethylalluminum (TMAl) flux, the crystal quality becomes worse, and the epilayer surface becomes rougher. An interesting phenomenon is that the growth rate of AlGaN decrease with increasing TMAl flux, which is opposite to the AlN growth rate dependence on the TMAl flux. All these effects are attributed to the different properties of At atoms due to the higher bond strength of Al-N compared with Ga-N, which lead to lower surface mobility and stronger competitive ability of Al atoms during the growth. The enhancement of the surface mobility of Al is especially important for improving the quality of AlGaN. (c) 2006 Elsevier B.V. All rights reserved.