Search

We present a far-infrared transmission study on group-III nitride thin films. Cubic GaN and AlN layers and c-oriented wurtzite GaN, AlN, and AlxGa1-xN (x<0.3) layers were grown by molecular beam epitaxy on GaAs and Si(111) substrates, respectively. The Berreman effect allows us to observe simultaneously the transverse optic and the longitudinal optic phonons of both the cubic and the hexagonal films as transmission minima in the infrared spectra acquired with obliquely incident radiation. We discuss our results in terms of the relevant electromagnetic theory of infrared transmission in cubic and wurtzite thin films. We compare the infrared results with visible Raman-scattering measurements. In the case of films with low scattering volumes and/or low Raman efficiencies and also when the Raman signal of the substrate material obscures the weaker peaks from the nitride films, we find that the Berreman technique is particularly useful to complement Raman spectroscopy. [ABSTRACT FROM AUTHOR]

Examines the band gaps of both hexagonal and cubic thin films deposited by a modified molecular-beam-epitaxy process on gallium arsenide and gallium phosphide substrates. Significance of the method of supplying nitrogen to the growing film; Application of x-ray diffraction and arsenic beam; Factors that may cause errors in band-gap measurements; Structural properties of the film.

We have performed spatially resolved polar magneto-optical Kerr effect microscopy measurements on as-grown and annealed Ga0.95Mn0.05As thin films with tensile strain. We find that the films exhibit very strong perpendicular magnetic anisotropy which is increased upon annealing. During magnetic reversal, the domain walls propagate along the direction of surface ripples for the as-grown sample at low temperatures and along the [110] direction for the annealed sample. This indicates that the magnetic domain pattern during reversal is determined by a combination of magnetocrystalline anisotropy and a distribution of pinning sites along the surface ripples that can be altered by annealing. These mechanisms could lead to an effective method of controlling domain wall propagation. [ABSTRACT FROM AUTHOR]

The resistivity, temperature, and magnetic field dependence of the anomalous Hall effect in a series of metallic Ga[SUB1-x]Mn[SUBx]As thin films with x=0.015 to 0.08 is presented. A quadratic dependence of the anomalous Hall resistance on the resistivity is observed, with a magnitude which is in agreement with Berry phase theories of the anomalous Hall effect in dilute magnetic semiconductors. [ABSTRACT FROM AUTHOR]

In this letter, we study the optical properties of GaN1-xSbx thin films. Films with an Sb fraction up to 42% were synthesized by alternating GaN-GaSb layers at a constant temperature of 325 °C. The measured optical absorption data of the films are interpreted using a modified band anticrossing model that is applicable to highly mismatched alloys such as GaN1-xSbx in the entire composition range. The presented model allows us to more accurately determine the band gap as well as the band edges over the entire composition range thereby providing means for determining the composition for, e.g., efficient spontaneous photoelectrochemical cell applications. [ABSTRACT FROM AUTHOR]

We investigate the dependence of the magnetic and structural properties of (Ga,Mn)As/(Al,Ga,Mn)As bilayer films on the stoichiometry of the interface region. For films incorporating a thin As-deficient layer at the interface, the out-diffusion of interstitial Mn from the bottom layer is strongly suppressed, resulting in a large difference in TC and magnetic anisotropy between the two layers. X-ray reflectivity measurements show that the suppression of interstitial diffusion is correlated with an increased interface roughness. When the As-deficient interface layer is thicker than 2.5 nm, the in-plane uniaxial magnetic easy axis rotates from the [1-10] to the [110] crystalline axis. [ABSTRACT FROM AUTHOR]

Summary Thin films incorporating GaN, InGaN and AlGaN are presently arousing considerable excitement because of their suitability for UV and visible light-emitting diodes and laser diodes. However, because of the lattice mismatch between presently used substrates and epitaxial nitride thin films, the films are of variable quality. In this paper we describe our preliminary studies of nitride thin films using electron backscattered diffraction (EBSD). We show that the EBSD technique may be used to reveal the relative orientation of an epitaxial thin film with respect to its substrate (a 90° rotation between a GaN epitaxial thin film and its sapphire substrate is observed) and to determine its tilt (a GaN thin film was found to be tilted by 13 ± 1° towards [1010]GaN ), where the tilt is due to the inclination of the sapphire substrate (cut off-axis by 10° from (0001)sapphire towards (1010)sapphire ). We compare EBSD patterns obtained from As-doped GaN films grown by plasma-assisted molecular beam epitaxy (PA-MBE) with low and high As4 flux, respectively. Higher As4 flux results in sharper, better defined patterns, this observation is consistent with the improved surface morphology observed in AFM studies. Finally, we show that more detail can be discerned in EBSD patterns from GaN thin films when samples are cooled. [ABSTRACT FROM AUTHOR]

A detailed study of the effect of rapid thermal annealing in a N[sub 2] or Ar atmosphere on the properties of thin GaN layers grown by molecular-beam epitaxy on sapphire substrates was performed. After rapid thermal annealing, an enhancement of the crystal quality of such films was observed. Low-temperature photoluminescence measurements revealed a substantial increase in impurity recombination near the fundamental absorption edge after a rapid high-temperature anneal in a nitrogen atmosphere. A significant decrease in the impurity photoluminescence of the GaN films with protective SiO[sub 2] coatings was observed following the anneals. [ABSTRACT FROM AUTHOR]

Amorphous GaN1-xAsx layers with As content in the range of x=0.1 to 0.6 were grown by molecular beam epitaxy on Pyrex glass substrate. These alloys exhibit a wide range of band gap values from 2.2 to 1.3 eV. We found that the density of the amorphous films is ∼0.8–0.85 of their corresponding crystalline value. These amorphous films have smooth morphology, homogeneous composition, and sharp well defined optical absorption edges. The measured band gap values for the crystalline and amorphous GaN1-xAsx alloys are in excellent agreement with the predictions of the band anticrossing model. The high absorption coefficient of ∼105 cm-1 for the amorphous GaN1-xAsx films suggests that relatively thin films (on the order of 1 μm) are necessary for photovoltaic application. [ABSTRACT FROM AUTHOR]

We demonstrate that low concentrations of a secondary magnetic phase in (Ga,Mn)As thin films can enhance the coercivity by factors up to ∼100 without significantly degrading the Curie temperature or saturation magnetization. Magnetic measurements indicate that the secondary phase consists of MnAs nanoclusters, of average size ∼7 nm. This approach to controlling the coercivity while maintaining a high Curie temperature may be important for realizing ferromagnetic semiconductor based devices. [ABSTRACT FROM AUTHOR]

The electrical properties of p-type cubic (Ga,Mn)N films are reported. Hole concentrations above 1018 cm-3 at room temperature are observed. Activated behavior is observed down to 150 K, with an acceptor ionization energy of around 45–60 meV. The dependence of hole concentration and ionization energy on Mn concentration indicates that the shallow acceptor level is not simply due to unintentional codoping. Thermopower measurements on freestanding films, capacitance–voltage profilometry, and the dependence of conductivity on thickness and growth temperature, all show that this is not due to diffusion into the substrate. We therefore associate the p-type conductivity with the presence of the Mn in the cubic GaN films. [ABSTRACT FROM AUTHOR]

We show that by annealing Ga[sub 1-x]Mn[sub x]As thin films at temperatures significantly lower than in previous studies, and monitoring the resistivity during growth, an unprecedented high Curie temperature T[sub C] and conductivity can be obtained. T[sub C] is unambiguously determined to be 118 K for Mn concentration x = 0.05, 140 K for x = 0.06, and 120 K for x = 0.08. We also identify a clear correlation between T[sub C] and the room temperature conductivity. The results indicate that Curie temperatures significantly in excess of the current values are achievable with improvements in growth and post-growth annealing conditions. [ABSTRACT FROM AUTHOR]

We have used short-period all-binary (AlAs)(GaAs) superlattices with layers as thin as three monolayers to synthesize the barrier and cladding regions of GaAs quantum well lasers grown by molecular beam epitaxy. By studying the threshold current of single- and double-well devices as a function of cavity length and temperature, we conclude that the optical scattering losses are very low, that the gain-current characteristics are similar to alloy barrier devices, and that there is evidence for current leakage by recombination in the barriers. [ABSTRACT FROM AUTHOR]

Presents a corrected version of the article 'Study of GaN thin layers subjected to high-temperature rapid thermal annealing,' published in the October 1998 issue of 'Semiconductors.'