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Typically only dilute (up to ∼10%) highly mismatched alloys can be grown due to the large differences in atomic size and electronegativity of the host and the alloying elements. We have overcome the miscibility gap of the GaN1-xAsx system using low temperature molecular beam epitaxy. In the intermediate composition range (0.10 < x < 0.75), the resulting alloys are amorphous. To gain a better understanding of the amorphous structure, the local environment of the As and Ga atoms was investigated using extended x-ray absorption fine structure (EXAFS). The EXAFS analysis shows a high concentration of dangling bonds compared to the crystalline binary endpoint compounds of the alloy system. The disorder parameter was larger for amorphous films compared to crystalline references, but comparable with other amorphous semiconductors. By examining the Ga local environment, the dangling bond density and disorder associated with As-related and N-related bonds could be decoupled. The N-related bonds had a lower dangling bond density and lower disorder. [ABSTRACT FROM AUTHOR]

Recent work has shown that GaN1-xAsx can be grown across the entire composition range by low temperature molecular beam epitaxy with intermediate compositions being amorphous, but control of the electrical properties through doping is critical for functionalizing this material. Here we report the bipolar doping of GaN1-xAsx with high As content to conductivities above 4 S/cm at room temperature using Mg or Te. The carrier type was confirmed by thermopower measurements. Doping requires an increase in Ga flux during growth resulting in a mixed phase material of polycrystalline GaAs:N embedded in amorphous GaN1-xAsx. [ABSTRACT FROM AUTHOR]

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 present an experimental investigation of the magnetic, electrical, and structural properties of Ga0.94Mn0.06As1-yPy layers grown by molecular beam epitaxy on GaAs substrates for y≤0.3. X-ray diffraction measurements reveal that the layers are under tensile strain, which gives rise to a magnetic easy axis perpendicular to the plane of the layers. The strength of the magnetic anisotropy and the coercive field increases as the phosphorous concentration is increased. The resistivity of all samples shows metallic behavior with the resistivity increasing as y increases. These materials will be useful for studies of micromagnetic phenomena requiring metallic ferromagnetic material with perpendicular magnetic anisotropy. [ABSTRACT FROM AUTHOR]

Recent developments with group III nitrides suggest AlxGa1-xN based LEDs can be new alternative commercially viable deep ultra-violet light sources. Due to a significant difference in the lattice parameters of GaN and AlN, AlxGa1-xN substrates would be preferable to either GaN or AlN for ultraviolet device applications. We have studied the growth of free-standing wurtzite AlxGa1-xN bulk crystals by plasma-assisted molecular beam epitaxy (PA-MBE) using a novel RF plasma source. Thick wurtz-ite AlxGa1-xN films were grown by PA-MBE on 2-inch GaAs (111)B substrates and were removed from the GaAs substrate after growth to provide free standing AlxGa1-xN samples. Growth rates of AlxGa1-xN up to 3 µm/h have been demonstrated. Our novel high efficiency RF plasma source allowed us to achieve free-standing bulk AlxGa1-xN layers in a single day's growth, which makes our MBE bulk growth technique commercially viable. [ABSTRACT FROM AUTHOR]

We have studied the growth of GaN layers by plasma-assisted electroepitaxy (PAEE). We have used an RF-plasma source to produce high concentrations of N species in the Ga melt and a high DC electric current to transfer the N species from the Ga surface to the growth interface. We have achieved continuous GaN layers by PAEE at growth temperatures as low as ∼650oC. We have also investigated PAEE growth of GaN layers on GaN nanocolumn templates. (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [ABSTRACT FROM AUTHOR]

Ga 1- x Mn x As/(001) GaAs ( x &ThickSpace;=&ThickSpace;2.2,&ThickSpace;5.6,&ThickSpace;9 at.%) epitaxial layers grown by low-temperature MBE, exhibiting anisotropic magnetic properties, have been characterized using a range of transmission electron microscopy techniques and secondary-ion mass spectrometry. Banded contrast features on inclined planes are observed for the [110] projection of micron-thick samples with high Mn content, and are attributed to a compositional fluctuation in the interstitial Mn content. Precipitates attributed to tetragonally distorted MnAs are associated with the onset of stacking-fault formation during layer growth. The formation of an Mn–O layer at the surface of the samples is also observed, attributed to post-growth oxidation of an Mn surfactant layer. [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]

We demonstrate a strong dependence of the anisotropic magnetoresistance (AMR) on the direction of current with respect to the crystalline axes in the quaternary diluted ferromagnetic semiconductor Ga0.94Mn0.06As0.9Sb0.1. Dominant cubic and uniaxial symmetries are observed for current along the [formula] and [110] crystalline directions, respectively. In addition, an anomalous temperature dependence of the symmetry and magnitude of the AMR is observed. Phenomenological analysis reveals an important contribution of a crossed cubic crystalline/non-crystalline term, which is much larger than previously observed in (Ga,Mn)As films. Possible reasons for the enhancement of this term are discussed. [ABSTRACT FROM AUTHOR]

GaN1-xAsx alloys grown across the composition range by low temperature molecular beam epitaxy have great technological potential for photovoltaic applications owing to their strong absorption coefficient and wide tunability of band gap and band edges. We found that amorphous GaN1-xAsx alloys that are formed for the compositions x, in the range of x∼0.3-0.7 are stable up to 700 °C. This is surprising since growth of GaN1-xAsx above 400 °C results in phase segregation. At annealing temperatures higher than 700 °C the alloy phase segregates into GaAs:N and GaN:As. The relative size of the nanocrystals depends on the initial film composition and annealing conditions. [ABSTRACT FROM AUTHOR]

Through nonequilibrium low-temperature molecular beam epitaxy, we have grown GaN1-xBix alloys on sapphire substrates with x up to 0.11. The GaN1-xBix alloys are found to be amorphous with GaN crystals distributed throughout the film. A dramatic reduction in the optical band gap from 3.4 eV in GaN to as low as 1.2 eV for x∼0.11 was qualitatively explained by formation of a narrow band originating from anticrossing interaction between Bi localized states and the extended states of the GaN matrix. [ABSTRACT FROM AUTHOR]

We have investigated the effects of postgrowth low temperature annealing on the magnetic, electrical, and structural properties of (Ga0.94Mn0.06)(As0.9P0.1) layers grown by molecular beam epitaxy. By controlling the annealing time, we are able to tune the magnetic anisotropy between an easy axis in the plane for the as-grown samples, to an easy axis perpendicular to the plane for fully annealed samples. The increase in the carrier density, as a result of annealing, is found to be the primary reason for the sign reversal of the magnetic anisotropy, in qualitative agreement with theoretical predictions. [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 study the effects of growth temperature, Ga:As ratio, and postgrowth annealing procedure on the Curie temperature TC of (Ga,Mn)As layers grown by molecular beam epitaxy. We achieve the highest TC values for growth temperatures very close to the two-dimensional–three-dimensional phase boundary. The increase in TC, due to the removal of interstitial Mn by postgrowth annealing, is counteracted by a second process, which reduces TC and which is more effective at higher annealing temperatures. Our results show that it is necessary to optimize the growth parameters and postgrowth annealing procedure to obtain the highest TC. [ABSTRACT FROM AUTHOR]

Transmission electron microscopy is used to reveal threading defects in single crystal c-oriented GaN nanorods grown on (0001)sapphire by molecular beam epitaxy. The defects are shown to be planar faults lying on {1010} planes and bounded by opposite partial screw dislocations with Burgers vectors of 1/2<0001>. The faults nucleate, as dislocation half-loops, from points close to the GaN/(0001)sapphire interface. It is proposed that the spiral growth of the partial atomic step joining the emerging dislocations controls nanorod growth and accounts for the growth surface morphology. The significance of these defects for nanorod growth and applications is discussed. [ABSTRACT FROM AUTHOR]

Transmission and scanning electron microscopies are used to examine the epitaxial lateral overgrowth of GaN on GaN nanocolumns grown on AlN/(0001)sapphire by molecular beam epitaxy. Initially, N-rich growth gave a bimodal morphology consisting of defect-free Ga-polar nanocolumns emanating from a compact, highly defective N-polar layer. Under subsequent Ga-rich conditions, the nanocolumns grew laterally to produce continuous Ga-polar overlayers. Threading dislocation (TD) densities in the overlayer were in the range of 108–109 cm-2, up to two orders of magnitude less than in the N-polar underlayer. It is proposed that the change in polarity is a key factor controlling the reduction in TD density. [ABSTRACT FROM AUTHOR]

Zincblende p-type Ga1-xMnxN epilayers, grown with and without AlN/GaN buffer layers using plasma-assisted molecular beam epitaxy on (001) oriented GaAs substrates, have been investigated using a variety of complementary transmission electron microscopy techniques. The epilayers were found to contain a high anisotropic density of stacking faults and microtwins. MnAs inclusions were identified at the Ga1-xMnxN/(001)GaAs interface extending into the substrate. The use of AlN/GaN buffer layers was found to inhibit the formation of these inclusions. [ABSTRACT FROM AUTHOR]

We report direct phonon absorption experiments in n-type GaN epitaxial layers grown by molecular beam epitaxy. Nonequilibrium phonons with characteristic energies up to 15 meV (3.5 THz) are injected from a constantan heater. They propagate ballistically through the sapphire substrate, reach the GaN layer, and are absorbed by the degenerate three-dimensional electron gas. The phonon absorption is studied as a function of heater temperature through the effects of phonon induced changes of the device resistivity. The experimental results lead us to the conclusion that in low-mobility GaN, the momentum conservation cutoff for electron-phonon transitions is shifted to higher energy than predicted by standard theory. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Direct measurements of the desorption rate of gallium from GaAs and (Al,Ga)As during growth by molecular beam epitaxy at high temperatures have been made by modulated beam mass spectrometry. The activation energy for desorption is dependent upon the nature of the site from which the gallium is lost. From free gallium atoms not incorporated into the lattice, behavior similar to that encountered under equilibrium conditions for gallium over gallium or gallium over GaAs is observed. For gallium lost from the GaAs lattice the apparent activation energy is higher and is influenced by the arsenic flux reaching the surface. [ABSTRACT FROM AUTHOR]

The composition of InxGa1 − xN nanorods grown by molecular beam epitaxy with nominal x = 0.5 has been mapped by electron microscopy using Z-contrast imaging and x-ray microanalysis. This shows a coherent and highly strained core-shell structure with a near-atomically sharp boundary between a Ga-rich shell (x ∼ 0.3) and an In-rich core (x ∼ 0.7), which itself has In- and Ga-rich platelets alternating along the growth axis. It is proposed that the shell and core regions are lateral and vertical growth sectors, with the core structure determined by spinodal decomposition. [ABSTRACT FROM AUTHOR]

The authors have studied the growth of bulk, freestanding zinc-blende (cubic) GaN layers by plasma-assisted molecular beam epitaxy (PA-MBE). They have established that the best structural properties of freestanding zinc-blende GaN can be achieved with initiation under Ga-rich conditions but without Ga droplet formation. It is difficult to initiate the growth of zinc-blende GaN, but it is even more difficult to sustain the growth of the pure zinc-blende polytype in thick layers without any wurtzite inclusions. In order to grow high quality freestanding cubic GaN layers, it is necessary to maintain the same growth conditions for about 1 week. The best quality zinc-blende phase GaN was achieved in the first 10 μm of the GaN layers. The authors have produced zinc-blende GaN substrates from our thick bulk GaN layers and they used the side previously attached to the GaAs substrate as the episide of these zinc-blende GaN substrates. They have demonstrated the scalability of the process by growing zinc-blende GaN layers on 2 and 3 in. diameter wafers. The growth of freestanding bulk GaN layers has allowed them to refine the value for the lattice parameter of zinc-blende GaN as 4.510±0.005 Å. They have demonstrated that the PA-MBE process developed has allowed them to grow freestanding AlxGa1-xN wafers. [ABSTRACT FROM AUTHOR]

The authors have succeeded in growing GaN1-xAsx alloys over a large composition range (02 flux at low (as low as 100 °C) growth temperatures, which is much below the normal GaN growth temperature range. Using x-ray and transmission electron microscopy, they found that the GaNAs alloys with high As content x>0.17 are amorphous. Optical absorption measurements together with x-ray absorption and emission spectroscopy results reveal a continuous gradual decrease in band gap from ∼3.4 to <1 eV with increasing As content. The energy gap reaches its minimum of ∼0.8 eV at x∼0.8. The composition dependence of the band gap of the crystalline GaN1-xAsx alloys follows the prediction of the band anticrossing model (BAC). However, our measured band gap of amorphous GaN1-xAsx with 0.31-xAsx alloys have short-range ordering that resembles random crystalline GaN1-xAsx alloys. They have demonstrated the possibility of the growth of amorphous GaN1-xAsx layers with variable As content on glass substrates. [ABSTRACT FROM AUTHOR]

Mass spectrometer studies provide a powerful means to evaluate both the major species coming from a Knudsen source used for molecular beam epitaxy and also impurities, which may be detrimental to device performance. To distinguish signals from the source from those due to background gases in the ultrahigh vacuum system it is essential to have direct line of sight from the source to the ionizer of the mass spectrometer. For condensable species this criterion combined with a liquid nitrogen cryopanel is sufficient to ensure the species detected come from the source. However, for volatile species, such as arsenic, this is not the case and to adequately identify material from the source it is necessary to use modulated beam mass spectrometer studies as has previously been demonstrated in studies of the evaporation of GaAs [C. T. Foxon et al., J. Phys. Chem. Solids 34, 1693 (1973)]. The authors have recently constructed a new version of this apparatus using a Hiden HAL IV mass spectrometer, which has both pre and postfilters to improve transmission and sensitivity at high mass numbers. The new apparatus has several improvements compared to the original system used by one of the authors. In particular, in addition to the improved intrinsic sensitivity of the mass spectrometer, it is now possible to tune the electron ionization energy to a low enough value to prevent cracking of species within the ionizer of the mass spectrometer, thus simplifying the interpretation of the data. This new system has been used to study the performance of a Veeco Mark V arsenic valved cracker equipped with a variety of different baffles. In all cases, at a sufficiently high temperature of the hot (cracker) zone, complete decomposition of the arsenic from tetramers (As4) to dimers (As2) is achieved. Surprisingly, the source with an open tube achieves significant cracking at very low cracker temperatures. In this case the flux would not be sufficiently uniform over the area of a growth sample, so additional beam forming apertures or a series of baffle plates are typically used to alleviate this situation. With such apertures or baffles present, complete decomposition can also be achieved, but at a slightly higher operating temperature, suggesting some recombination takes place. In this article, the authors concentrate on baffle assemblies that will be familiar to users of most of the current models of cracker in service today. Aperture plates of various designs will be the subject of future work. [ABSTRACT FROM AUTHOR]

Phonon-drag imaging has been used to study the electron-phonon interaction in a low-mobility n-type gallium nitride (GaN) epilayer grown by molecular beam epitaxy on sapphire. Currents up to j=4.7x10[sup 3] A/m[sup 2] were induced by transverse acoustic phonon drag of electrons in this structure. Such currents are much bigger than predicted by the conventional theory of electron-phonon interaction. We explain this difference by the fact that, in low-mobility GaN, the 2k[sub F] cutoff for electron-phonon transitions is relaxed. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Arsenic doped GaN grown by molecular beam epitaxy has been studied by room temperature photoluminescence. In addition to the wurzite band edge transition, luminescence from the cubic phase and very strong blue emission at ∼2.6 eV are observed. The intensities of the blue and the cubic band edge emissions have a power law dependence on the As[sub 2] flux. The formation of the cubic phase has been explained by the initial formation of GaAs before substitution of the As by the more reactive N. The intensity of the blue emission at room temperature of the As doped samples is more than an order of magnitude stronger than the band edge emission in undoped samples. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We present a Raman-scattering study of optical phonons in zinc-blende (cubic) GaN for temperatures ranging from 80 to 750 K. The experiments were performed on high-quality, cubic GaN films grown by molecular-beam epitaxy on GaAs (001) substrates. The observed temperature dependence of the optical phonon frequencies and linewidths is analyzed in the framework of anharmonic decay theory, and possible decay channels are discussed in the light of density-functional-theory calculations. The longitudinal-optical (LO) mode relaxation is found to occur via asymmetric decay into acoustic phonons, with an appreciable contribution of higher-order processes. The transverse-optical mode linewidth shows a weak temperature dependence and its frequency downshift is primarily determined by the lattice thermal expansion. The LO phonon lifetime is derived from the observed Raman linewidth and an excellent agreement with previous theoretical predictions is found. [ABSTRACT FROM AUTHOR]

Our ab initio theory calculations predict a semiconducting band structure of I-Mn-V compounds. We demonstrate on LiMnAs that high-quality materials with group-I alkali metals in the crystal structure can be grown by molecular beam epitaxy. Optical measurements on the LiMnAs epilayers are consistent with the theoretical electronic structure. Our calculations also reproduce earlier reports of high antiferromagnetic ordering temperature and predict large, spin-orbit-coupling-induced magnetic anisotropy effects. We propose a strategy for employing antiferromagnetic semiconductors in high-temperature semiconductor spintronics. [ABSTRACT FROM AUTHOR]