Joshua B. Halpern

Heteroepitaxy of ZnO on GaN and its implications for fabrication of hybrid optoelectronic devices

ZnO thin films have been grown heteroepitaxially on epi-GaN/sapphire (0001) substrates. Rutherford backscattering spectroscopy, ion channeling, and high resolution transmission electron microscopy studies revealed high-quality epitaxial growth of ZnO on GaN with an atomically sharp interface. The x-ray diffraction and ion channeling measurements indicate near perfect alignment of the ZnO epilayers on GaN as compared to those grown directly on sapphire (0001). Low-temperature cathodoluminescence studies also indicate high optical quality of these films presumably due to the close lattice match and stacking order between ZnO and GaN. Lattice-matched epitaxy and good luminescence properties of ZnO/GaN heterostructures are thus promising for ultraviolet lasers. These heterostructures demonstrate the feasibility of integrating hybrid ZnO/GaN optoelectronic devices.

Growth of large-scale GaN nanowires and tubes by direct reaction of Ga with NH3

Large-scale wurtzite GaN nanowires and nanotubes were grown by direct reaction of metal gallium vapor with flowing ammonia in an 850–900 °C horizontal oven. The cylindrical structures were as long as 500 μm with diameters between 26 and ∼100 nm. Transmission electron microscopy, scanning electron microscopy, and x-ray diffraction were used to measure the size and structures of the samples. Preliminary results show that the size of the nanowires depends on the temperature and the NH3 flow rate. The growth mechanism is discussed briefly. The simple method presented here demonstrates that GaN nanowires can be grown without the use of a template or catalyst, as reported elsewhere.

Rotational line strengths in two- and three-photon transitions in diatomic molecules

Abstract Rotational line intensity factors are calculated for two- and three-photon excitation of diatomic molecules belonging to Hunds case (a) and intermediate (a)-(b) coupling schemes. The intensity factors compare well with experimentally measured normalized line intensities for two-photon transitions in the Lyman-Birge-Hopfield system of N2, the γ-bands of NO and the fourth-positive system of CO and for three-photon excitation of the fourth-positive system of CO.

Growth of GaN nanowires by direct reaction of Ga with NH3

Semiconducting, single crystal wurtzite GaN nanowires have been grown by direct reaction of metal Ga with NH3 in a tube furnace. This paper discusses the growth mechanism. Nanowires grow only between 8251C and 925 o C. Their diameters vary between 20 and 150 nm and depend directly on temperature and NH3 flow rate. Wires as long as 500mm have been fabricated; once wires have formed, their length increases directly with time in the reactor. There are three different stages in the process, each of which has its own mechanism. First, a nearly amorphous GaN matrix forms, followed by growth of hillocks of thin GaN platelets. Finally, nanowires emerge from the edges of the platelets in characteristic directions. This analysis can be used as a guide for controlling GaN wire diameters and lengths. Strategies for growth of thinner and thicker nanowires are suggested. Thicker cylindrical structures denoted as rods grow from the face of the platelets. Description of their growth mechanism requires further study. r 2001 Elsevier Science B.V. All rights reserved.

Room-temperature growth of AlN thin films by laser ablation

Excimer laser ablation of compressed AlN powder has been used to grow thin AlN films at room temperature on a variety of substrates. The films have a band gap of 6.15 eV as measured by UV absorption. Examination with a scanning electron microscope and an optical microscope shows that the films are smooth. The IR spectrum has an absorption characteristic of AlN. Growth rates are extremely rapid, exceeding 70 nm/min.

Vibrational spectroscopy of a transient species through time-resolved Fourier transform infrared emission spectroscopy: The vinyl radical

An approach for detecting the vibrational spectrum of transient species is demonstrated on the vinyl radical. Photodissociation of carefully chosen precursors at selected photolysis wavelengths produce highly vibrationally excited radicals. Infrared (IR) emission from these radicals is then measured by time-resolved Fourier transform spectroscopy with nanosecond time resolution. All nine vibrational bands of the vinyl radical, generated from four different precursors, are obtained and reported here for the first time.

Multiphoton ultraviolet photochemistry

Abstract Multiphoton photodissociation has been observed in C 2 N 2 , C 2 H 2 , C 2 H 4 , CH 3 OH, and C 2 H 5 OH. In all of these molecules fluorescence from small free radicals such as CN, CH, and OH has been observed. None of the observed fluorescence can result from the absorption of a single 193 nm photon. Arguments are presented which suggest that the observed results are best explained by invoking a sequential absorption scheme where the excited molecule absorbs a second laser photon rather than predissociating. Emissions have also been observed when H 2 O is photolyzed with the ArFlaser. In H 2 O the evidence suggest that this emission arises from collisional dissociation of two excited H 2 O molecules.

Coaxial measurement of the translational energy distribution of CS produced in the laser photolysis of CS2 at 193 nm

Carbon disulfide (CS2) photolysis was investigated in the gas phase using an argon fluoride (ArF) laser at 193 nm. The coaxial time‐of‐flight (TOF) distributions of CS radicals produced in the photolysis have been measured. Photochemical fragments have been observed with translational energies below 3 kcal/mol. The vibrational distribution of the CS fragments was also probed by laser induced fluorescence (LIF), and these measurements confirm that significant amounts of CS radicals are produced in vibrational levels greater than v‘=6. From a computer simulation of the experimental LIF data, a vibrational distribution was also obtained. Vibrational levels up to v‘=12 were found to be populated in a bimodal distribution, which peaks at v‘=4, and extends to v‘=12. There was a significant amount of rotational excitation of nascent CS produced in high vibrational levels of the ground state. The disjoint translational energy and CS vibrational energy distributions can be used to obtain an estimate of the S(3P) t...

The vinyl radical (Ã2A″←X̃2A′) spectrum between 530 and 415 nm measured by cavity ring-down spectroscopy

High resolution (0.008 nm) transient absorption spectra have been measured for the vinyl radical (C2H3) A 2A″←X 2A′ transition between 530 and 415 nm using cavity ring-down laser absorption spectroscopy. This is over 200 times better resolution than the only previous recorded spectrum of vinyl in this wavelength region by Hunziker, Kneppe, McLean, Siegbahn, and Wendt [Can J. Chem. 61, 993 (1983)]. With the improved resolution in the present study, new vibrational bands are detected, and some rotational structure of the bands is resolved. A calculation of the envelope of the origin band was carried out, using a model for a c-type transition of an asymmetric top. For a best fit to the data, the model requires a 1 cm−1 Lorentzian linewidth, which suggests a short-lived excited state.

Dissociation dynamics of acetylene Rydberg states as a function of excited state lifetime

The state selective photodissociation of acetylene, C2H2/C2D2, was studied in the wavelength range 121.2–132.2 nm by high resolution Rydberg atom time-of-flight measurements on the atomic fragment, H/D. In the wavelength region studied members of all four Rydberg series and the highly excited Ẽ valence state were state selectively excited using tunable vacuum-ultraviolet laser radiation. The lifetime of the excited states which were studied varied from 58 fs to more than 2 ps. Formation of the ethynyl radical in its X electronic ground state and its first electronically excited A state is observed with practically no indication of B state fragments. Two decay channels with different dissociation dynamics were also observed. In both channels the observed decay dynamics depended strongly on the excited state of the parent molecule. Further there are major differences between these two dissociation pathways with respect to the measured internal energy and angular distributions. In one channel the dissociat...