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B Schaff

Bio: B Schaff is an academic researcher from Cornell University. The author has contributed to research in topics: Electric field & Electron mobility. The author has an hindex of 2, co-authored 2 publications receiving 952 citations.

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TL;DR: In this paper, it was shown that the macroscopic nonlinear pyroelectric polarization of wurtzite AlInN/GaN, InxGa1-xN and AlxIn1xN ternary compounds (large spontaneous polarization and piezoelectric coupling) dramatically affects the optical and electrical properties of multilayered Al(In)GaN/GAN hetero-, nanostructures and devices, due to the huge built-in electrostatic fields and bound interface charges caused by gradients in polarization at surfaces and heter
Abstract: The macroscopic nonlinear pyroelectric polarization of wurtzite AlxGa1-xN, InxGa1-xN and AlxIn1-xN ternary compounds (large spontaneous polarization and piezoelectric coupling) dramatically affects the optical and electrical properties of multilayered Al(In)GaN/GaN hetero-, nanostructures and devices, due to the huge built-in electrostatic fields and bound interface charges caused by gradients in polarization at surfaces and heterointerfaces. Models of polarization-induced effects in GaN-based devices so far have assumed that polarization in ternary nitride alloys can be calculated by a linear interpolation between the limiting values of the binary compounds. We present theoretical and experimental evidence that the macroscopic polarization in nitride alloys is a nonlinear function of strain and composition. We have applied these results to interpret experimental data obtained in a number of InGaN/GaN quantum wells?(QWs) as well as AlInN/GaN and AlGaN/GaN transistor structures. We find that the discrepancies between experiment and ab initio theory present so far are almost completely eliminated for the AlGaN/GaN-based heterostructures when the nonlinearity of polarization is accounted for. The realization of undoped lattice-matched AlInN/GaN heterostructures further allows us to prove the existence of a gradient in spontaneous polarization by the experimental observation of two-dimensional electron gases?(2DEGs). The confinement of 2DEGs in InGaN/GaN QWs in combination with the measured Stark shift of excitonic recombination is used to determine the polarization-induced electric fields in nanostructures. To facilitate inclusion of the predicted nonlinear polarization in future simulations, we give an explicit prescription to calculate polarization-induced electric fields and bound interface charges for arbitrary composition in each of the ternary III-N alloys. In addition, the theoretical and experimental results presented here allow a detailed comparison of the predicted electric fields and bound interface charges with the measured Stark shift and the sheet carrier concentration of polarization-induced 2DEGs. This comparison provides an insight into the reliability of the calculated nonlinear piezoelectric and spontaneous polarization of group III nitride ternary alloys.

975 citations

Journal ArticleDOI
TL;DR: In this paper, the effect of hot-phonon production on the observed momentum and energy relaxation of hot electrons is discussed within the framework of a theoretical model, which was originally developed for III-V material systems and has been adapted for a two-dimensional electron gas in GaN, in which phonon drift is neglected.
Abstract: We report the experimental studies of hot-electron energy and momentum relaxation in the steady state in GaN/AlGaN HEMT structures with a high two-dimensional electron density of n = 1.5×1013 cm-2. From the LO-phonon-scattering-limited component of the mobility we obtain for the LO phonon the energy of ω~90 meV and the momentum relaxation time of τm~4 fs. Drift velocity versus electric field characteristics obtained from the pulsed I-V measurements show that, at TL = 77 K, the drift velocity saturates at vd = 1.0×107 cm s-1 at electric fields in excess of E~7.5 kV cm-1, and at TL = 300 K it saturates at vd~5×106 cm s-1, at an electric field of around E~10 kV cm-1. Electron temperature as a function of applied electric field is obtained by comparing the measured electric field dependence of the mobility µE at a fixed lattice temperature, with the lattice temperature dependence of the mobility at a fixed low electric field. The electron energy loss rate is then determined from the electron temperature dependence of the power loss using the power balance equations. The effect of hot-phonon production on the observed momentum and energy relaxation of hot electrons is discussed within the framework of a theoretical model, which was originally developed for III-V material systems and has been adapted for a two-dimensional electron gas in GaN, and in which phonon drift is neglected.

48 citations


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TL;DR: In this paper, a comprehensive and up-to-date compilation of band parameters for all of the nitrogen-containing III-V semiconductors that have been investigated to date is presented.
Abstract: We present a comprehensive and up-to-date compilation of band parameters for all of the nitrogen-containing III–V semiconductors that have been investigated to date. The two main classes are: (1) “conventional” nitrides (wurtzite and zinc-blende GaN, InN, and AlN, along with their alloys) and (2) “dilute” nitrides (zinc-blende ternaries and quaternaries in which a relatively small fraction of N is added to a host III–V material, e.g., GaAsN and GaInAsN). As in our more general review of III–V semiconductor band parameters [I. Vurgaftman et al., J. Appl. Phys. 89, 5815 (2001)], complete and consistent parameter sets are recommended on the basis of a thorough and critical review of the existing literature. We tabulate the direct and indirect energy gaps, spin-orbit and crystal-field splittings, alloy bowing parameters, electron and hole effective masses, deformation potentials, elastic constants, piezoelectric and spontaneous polarization coefficients, as well as heterostructure band offsets. Temperature an...

2,525 citations

Journal ArticleDOI
TL;DR: In this article, the basic operation principle for MEMS with wide band gap semiconductors is described, and the first applications of SiC based MEMS are demonstrated, and innovative MEMS and NEMS devices are reviewed.
Abstract: With the increasing requirements for microelectromechanical systems (MEMS) regarding stability, miniaturization and integration, novel materials such as wide band gap semiconductors are attracting more attention. Polycrystalline SiC has first been implemented into Si micromachining techniques, mainly as etch stop and protective layers. However, the outstanding properties of wide band gap semiconductors offer many more possibilities for the implementation of new functionalities. Now, a variety of technologies for SiC and group III nitrides exist to fabricate fully wide band gap semiconductor based MEMS. In this paper we first review the basic technology (deposition and etching) for group III nitrides and SiC with a special focus on the fabrication of three-dimensional microstructures relevant for MEMS. The basic operation principle for MEMS with wide band gap semiconductors is described. Finally, the first applications of SiC based MEMS are demonstrated, and innovative MEMS and NEMS devices are reviewed.

352 citations

Journal ArticleDOI
TL;DR: In this paper, the structural and optical properties of lattice-matching AlInN layers to GaN have been investigated and their specific use to realize nearly strain-free structures for photonic and electronic applications has been discussed.
Abstract: We report on the current properties of Al1-x InxN (x approximate to 0.18) layers lattice- matched ( LM) to GaN and their specific use to realize nearly strain- free structures for photonic and electronic applications. Following a literature survey of the general properties of AlInN layers, structural and optical properties of thin state- of- the- art AlInN layers LM to GaN are described showing that despite improved structural properties these layers are still characterized by a typical background donor concentration of ( 1 - 5) x 10(18) cm(-3) and a large Stokes shift (similar to 800 meV) between luminescence and absorption edge. The use of these AlInN layers LM to GaN is then exemplified through the properties of GaN/ AlInN multiple quantum wells ( QWs) suitable for near- infrared intersubband applications. A built- in electric field of 3.64MVcm(-1) solely due to spontaneous polarization is deduced from photoluminescence measurements carried out on strain- free single QW heterostructures, a value in good agreement with that deduced from theoretical calculation. Other potentialities regarding optoelectronics are demonstrated through the successful realization of crack- free highly reflective AlInN/ GaN distributed Bragg reflectors ( R > 99%) and high quality factor microcavities ( Q > 2800) likely to be of high interest for short wavelength vertical light emitting devices and fundamental studies on the strong coupling regime between excitons and cavity photons. In this respect, room temperature ( RT) lasing of a LM AlInN/ GaN vertical cavity surface emitting laser under optical pumping is reported. A description of the selective lateral oxidation of AlInN layers for current confinement in nitride- based light emitting devices and the selective chemical etching of oxidized AlInN layers is also given. Finally, the characterization of LM AlInN/ GaN heterojunctions will reveal the potential of such a system for the fabrication of high electron mobility transistors through the report of a high two- dimensional electron gas sheet carrier density ( n(s) similar to 2.6 x 10(13) cm(-2)) combined with a RT mobility mu(e) similar to 1170 cm(2) V-1 s(-1) and a low sheet resistance, R similar to 210 Omega square.

313 citations

Journal ArticleDOI
TL;DR: In this article, the authors proposed that the key carrier transport process is emission of electrons from a trap state near the metal-semiconductor interface into a continuum of states associated with each conductive dislocation.
Abstract: Temperature-dependent current-voltage measurements combined with conductive atomic force microscopy and analytical modeling have been used to assess possible mechanisms of reverse-bias leakage current flow in Schottky diodes fabricated from GaN and Al0.25Ga0.75N∕GaN structures grown by molecular-beam epitaxy. Below 150K, leakage current is nearly independent of temperature, indicating that conduction is dominated by tunneling transport. At higher temperatures, leakage current in both GaN and Al0.25Ga0.75N∕GaN diode structures is well described by a Frenkel-Poole emission model. Based on the inferred emission barrier heights and the observation that room-temperature leakage current is dominated by the presence of highly conductive dislocations, it is suggested that the key carrier transport process is emission of electrons from a trap state near the metal-semiconductor interface into a continuum of states associated with each conductive dislocation. In this model for leakage current flow, the emission barr...

312 citations

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TL;DR: In this article, a staggered InGaN quantum well with step-function-like In content in the quantum well offers significantly improved radiative recombination rate and optical gain in comparison to the conventional type-I In-GaN QW.
Abstract: Staggered InGaN quantum wells (QWs) grown by metal-organic chemical vapor deposition are demonstrated as improved active region for visible light emitters. Theoretical studies indicate that InGaN QW with step-function-like In content in the quantum well offers significantly improved radiative recombination rate and optical gain in comparison to the conventional type-I InGaN QW. Experimental results of light emitting diode (LED) structure utilizing staggered InGaN QW show good agreement with theory. Polarization band engineering via staggered InGaN quantum well allows enhancement of radiative recombination rate, leading to the improvement of photoluminescence intensity and LED output power.

305 citations