C.R. Staddon

Growth front nucleation of rubrene thin films for high mobility organic transistors

We demonstrate a mode of thin film growth in which amorphous islands crystallize into highly oriented platelets. A cascade of crystallization is observed, in which platelets growing outward from a central nucleation point impinge on neighboring amorphous islands and provide a seed for further nucleation. Through control of growth parameters, it is possible to produce high quality thin films which are well suited to the formation of organic transistors. We demonstrate this through the fabrication of rubrene thin film transistors with high carrier mobility.

Intrinsic and extrinsic contributions to the lattice parameter of GaMnAs

We report on measurements of the crystal structure and hole density in a series of as-grown and annealed GaMnAs samples. The measured hole densities are used to obtain the fraction of incorporated Mn atoms occupying interstitial and substitutional sites. This allows us to make a direct comparison of the measured lattice parameters with recent density-functional theory (DFT) predictions. We find that the decrease in lattice constant observed on annealing is smaller than that predicted due to the out-diffusion of interstitial Mn during annealing. The measured lattice parameters after annealing are still significantly larger than that of GaAs even in samples with very low compensation. This indicates that the intrinsic lattice parameter of GaMnAs is significantly larger than that of GaAs, in contradiction to the DFT prediction.

Molecular beam epitaxy grown (Ga,Mn)(As,P) with perpendicular to plane magnetic easy axis

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 less than or equal to 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 increase as the phosphorous concentration is increased. The resistivity of all samples shows metallic behaviour 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.

Domain imaging and domain wall propagation in (Ga, Mn)As thin films with tensile strain

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.

Determination of the Mn concentration in GaMnAs

Three series of 1 µm thick Ga1−xMnxAs films with different Mn composition have been characterized using high-resolution x-ray diffraction (HRXRD). The results show that they are highly-crystalline and the growth is reproducible. The Mn compositions have also been measured by other popular methods: in situ ion gauge, x-ray fluorescence (XRF), electron probe microanalysis (EPMA) and secondary ion mass spectrometry (SIMS). The results show that the Mn concentrations measured by different methods are different and the difference between them becomes smaller with increasing Mn content.

Elasto-optical properties of zinc-blende (cubic) GaN measured by picosecond acoustics

Using the technique of picosecond acoustics, we measure the basic elastic and optical properties of a micrometre-thickness zinc-blende (cubic) GaN epitaxial film grown on GaAs. We provide low temperature values of the speed of sound, c11 elastic constant and refractive index. Our value of the elastic constant is in good agreement with the theoretical calculations for cubic GaN (Wright 1997 J. Appl. Phys. 82 2833).

The growth of high quality GaMnAs films by MBE

Using As2 as an arsenic source together with other appropriate growth and annealing procedures, we have improved the electrical and magnetic properties of GaMnAs films and obtained Curie temperatures as high as 159 K. For Mn concentrations up to 7%, we have demonstrated that the compensation previously observed in such films is mainly due to Mn interstitials, which can be removed by using appropriate annealing procedures for thin films. For the Mn concentrations >7%, we suggest that additional AsGa defects also cause compensation, which cannot be removed by our present annealing procedure.

The influence of arsenic incorporation on the optical properties of As-doped GaN films grown by molecular beam epitaxy using As4 molecules

We have studied the influence of the incorporation of As on the optical properties of As-doped GaN layers grown by plasma-assisted molecular beam epitaxy (PA-MBE) using arsenic tetramers. The doping level of arsenic was determined by secondary ion mass spectrometry. The arsenic concentration is uniform throughout the layers. There is a sub-linear dependence of the arsenic incorporation on the flux with a log-log slope of about 0.1. The photoluminescence from the As-doped GaN films consists of UV excitonic emission at 3.4 eV, UV emission at 3.2 eV and a strong blue band centred at 2.6 eV. The intensity of the blue band centred at 2.6 eV increases more rapidly with arsenic flux than the concentration of arsenic in the bulk, and has a log-log slope of about 0.49. This suggests an approximately fourth-power dependence of the intensity of the blue emission on the concentration of arsenic in the GaN films.

Photoenhanced wet chemical etching of MBE grown gallium nitride

Abstract Room-temperature photoenhanced wet chemical etching of MBE grown GaN has been studied during the processing of devices for a physics research programme. The process uses a 0.5 M KOH solution with illumination provided by a mercury arc lamp emitting at 365 nm. The maximum etch rate achieved was ∼45 nm min −1 , much lower than that for MOCVD material etched under similar conditions. Surface morphologies vary significantly for wafers etched under nominally identical conditions, from surfaces with whiskers to highly crystallographic faces.

Influence of internal fields on radiative and nonradiative processes in AlN/GaN superlattices

We have investigated the effect of the internal electric fields on radiative and nonradiative processes in a series of AlN/GaN superlattices. The results of measurements involving direct detection of phonons emitted as a result of nonradiative recombination and carrier energy relaxation are compared with time resolved photoluminescence studies. Using these complementary techniques we have observed directly the effect of free carrier screening on the radiative and nonradiative recombination processes in the superlattice samples. We find that at high excitation power, photoinjected carriers screen the strong internal fields, resulting in an enhanced radiative recombination. As the carriers recombine, the descreening effect results in an increase in recombination via nonradiative processes, which we observe directly as a delayed phonon signal in the time of flight measurements.