J. M. Zavada

Hydrogen incorporation and diffusivity in plasma-exposed bulk ZnO

Hydrogen incorporation depths of >25 μm were obtained in bulk, single-crystal ZnO during exposure to 2H plasmas for 0.5 h at 300 °C, producing an estimated diffusivity of ∼8×10−10 cm2/V⋅s at this temperature. The activation energy for diffusion was 0.17±0.12 eV, indicating an interstitial mechanism. Subsequent annealing at 500–600 °C was sufficient to evolve all of the hydrogen out of the ZnO, at least to the sensitivity of secondary ion mass spectrometry (<5×1015 cm−3). The thermal stability of hydrogen retention is slightly greater when the hydrogen is incorporated by direct implantation relative to plasma exposure, due to trapping at residual damage in the former case.

Dominance of tunneling current and band filling in InGaN/AlGaN double heterostructure blue light‐emitting diodes

Measurement of the room temperature forward bias current‐voltage behavior of InGaN/AlGaN double heterostructure blue light‐emitting diodes demonstrates a significant departure from the usual Is exp(qV/ nkT) behavior where n is the ideality factor which varies between 1 and 2. The observed current‐voltage behavior at room temperature may be represented as I=2.7×10−11 exp(5.7V) which suggests a tunneling mechanism. Measurement of the electroluminescence for currents from 0.5 to 100 mA demonstrates that the emission peak shifts to higher energy while increasing in intensity. The shifting peak spectra is due to band filling, a process which results from the injection of holes via tunneling into an empty acceptor impurity band and vacant valence band tails. At currents near 100 mA, a non‐shifting band‐to‐band emission approaches the intensity of the shifting peak spectra. The active layer of these diodes is codoped with both the donor Si and the acceptor Zn.

Room temperature deposited indium zinc oxide thin film transistors

Depletion-mode indium zinc oxide (IZO) channel thin film transistors were fabricated on glass substrates from layers deposited at room temperature using rf magnetron sputtering. The threshold voltage was in the range from −5.5to−6.5V depending on gate dielectric (SiO2) thickness and the drain current on-to-off ratio was ∼105. The maximum field effect mobility in the channel was ∼4.5cm2V−1s−1, lower than the Hall mobility of ∼17cm2V−1s−1 in the same layers, suggesting a strong influence of scattering due to trapped charges at the SiO2-IZO interface. The low deposition and processing temperatures make these devices suitable for applications requiring flexible substrates.

Spin field effect transistor with a graphene channel

A spin field effect transistor (FET) is proposed by utilizing a graphene layer as the channel. Similar to the conventional spin FETs, the device involves spin injection and spin detection by ferromagnetic source and drain. Due to the negligible spin-orbit coupling in the carbon based materials, spin manipulation in the channel is achieved via electrical control of the electron exchange interaction with a ferromagnetic gate dielectric. Numerical estimates indicate the feasibility of the concept if the bias can induce a change in the exchange interaction energy of the order of meV. When nanoribbons are used for a finite channel width, those with armchair-type edges can maintain the device stability against the thermal dispersion.

First-principles analysis of electron-phonon interactions in graphene

The electron-phonon interaction in monolayer graphene is investigated using density-functional perturbation theory. The results indicate that the electron-phonon interaction strength is of comparable magnitude for all four in-plane phonon branches and must be considered simultaneously. Moreover, the calculated scattering rates suggest an acoustic-phonon contribution that is much weaker than previously thought, revealing an important role of optical phonons even at low energies. Accordingly it is predicted, in good agreement with a recent measurement, that the intrinsic mobility of graphene may be more than an order of magnitude larger than the already high values reported in suspended samples.

Luminescence properties of erbium in III–V compound semiconductors

Abstract Optoelectronic materials doped with Er atoms are receiving widespread attentions due to their impact on optical communication systems operating at 1.54 μm. Optical amplifiers based on Er-doped fibers have demonstrated major improvements in link distance, data rates and reduced needs for signal regeneration. III–V semiconductors doped with Er offer the prospect of very stable, temperature-insensitive, laser diodes emitting at 1.54 μm. This paper provides a review of the luminescence characteristics of III–V semiconductors doped with Er atoms. Aspects of Er incorporation in the III–V crystal host, photoluminescence properties, and prototype electroluminescent devices are addressed. Details of some of the first observations of photoluminescence of Er atoms in III–V nitride semiconductors, in particular GaN epilayers, are discussed. The GaN epilayers were optically excited using an argon-ion laser and spectra, centered at 1.54 μm, were observed at 6, 77 and 300 K. The spectra display many of the allowed transitions typical of the Er 3+ configuration and are nearly as intense at room temperature as at 77K. This result indicates that the wide bandgap III–V semiconductors may be ideal host materials for Er-doped electroluminescent devices.

Photoluminescence spectroscopy of erbium implanted gallium nitride

Results of a photoluminescence (PL) and photoluminescence excitation (PLE) study of Er implanted GaN are presented. Upon optical excitation at 325 and 488 nm, we observed strong 1.54 μm Er3+ PL which remained temperature stable from 15 to 550 K. At 550 K, the integrated PL intensity decreased by ∼10% for above gap excitation (λex=325 nm) and ∼50% for below gap excitation (λex=488 nm) relative to its value at 15 K. The excellent temperature stability makes GaN:Er very attractive for high temperature optoelectronic device applications. PLE measurements were conducted to gain insight into the Er3+ excitation mechanisms in the GaN host. The PLE results show that Er3+ can be excited continuously over a broad wavelength region spanning from 425 to 680 nm. In addition, sharp PLE features were observed at approximately 495, 525, 553, 651, and 980 nm. The PLE spectrum suggests that optically active Er3+ ions can be excited either through carrier-mediated processes involving defects in the host or through resonant ...

Thermal stability of ion-implanted hydrogen in ZnO

The evolution of implanted 2H profiles in single-crystal ZnO was examined as a function of annealing temperature (500–700 °C) by secondary ion mass spectrometry. The as-implanted profiles show a peak concentration of ∼2.7×1019 cm−3 at a depth of ∼0.96 μm for a dose of 1015 cm−2. Subsequent annealing causes outdiffusion of 2H from the ZnO, with the remaining hydrogen decorating the residual implant damage. Only 0.2% of the original dose is retained after annealing at 600 °C. Rutherford backscattering/channeling of samples implanted with 1H at a dose of 1016 cm−2 showed no change in backscattering yield near the ZnO surface, but did result in an increase near the end-of-range from 6.5% of the random level before 1H implantation to ∼7.8% after implantation. Results of both cathodoluminescence and photoluminescence studies show that even for a 1H dose of 1015 cm−2, the intensity of the near gap emission from ZnO is reduced more than 2 orders of magnitude from the values in unimplanted samples. This is due to ...

Wide bandgap GaN-based semiconductors for spintronics

Recent results on achieving ferromagnetism in transition-metal-doped GaN, A1N and related materials are discussed. The field of semiconductor spintronics seeks to exploit the spin of charge carrier ...

Thermal stability of implanted dopants in GaN

Results are reported of measurements of depth profiles and stability against redistribution with annealing up to 800 or 900 °C, for implanted Be, C, Mg, Si, S, Zn, Ge, and Se as dopants in GaN. The results confirm the high‐temperature stability of dopants in this material up to temperatures that vary from 600 to 900 °C. S redistributes for temperatures above 600 °C, and Zn and Se, for temperatures above 800 °C. All of the other elements are stable to 900 °C. These results indicate that direct implantation of dopants rather than masked diffusion will probably be necessary to define selective area doping of III–V nitride device structures based on these results for GaN.