F. J. Sánchez

The effect of the III/V ratio and substrate temperature on the morphology and properties of GaN- and AlN-layers grown by molecular beam epitaxy on Si(1 1 1)

Abstract We have studied the effect of the III/V ratio and substrate temperature on the growth of GaN and A1N films on Si(1 1 1) substrates by molecular beam epitaxy, where active nitrogen was generated by a radio frequency plasma source. In the case of GaN, two distinct regimes of growth (Ga-rich and N-rich conditions) lead to different crystal morphologies and luminescence properties. Scanning electron micrographs of the cleaved edges of films grown under highly N-rich conditions reveal columnar features, while growth under Ga-excess results in compact layers. The lowtemperature photoluminescence associated with the N-rich films is dominated by intense and narrow exciton lines, with peaks having full-width at half-maximum of less than 2 meV, whereas the Ga-rich films exhibit weaker and broader emissions. For increasing substrate temperatures above 700°C, stoichiometry is reached at higher Ga/N ratios, pointing to an enhancement of Ga desorption characterized by an activation energy of 2.5 eV. A similar study of A1N films shows that the desorption of A1 in terms of growth rate is not relevant for the substrate temperature range studied (850–920°C). III/V ratios close to the stoichiometric value and substrate temperatures above 900°C lead to high-quality A1N layers on Si(l 1 1) substrates. Complete relaxation is reached, for both GaN and A1N, in films with thicknesses well below 1 μm.

Photoconductor gain mechanisms in GaN ultraviolet detectors

GaN photoconductive detectors have been fabricated on sapphire substrates by metal organic vapor phase epitaxy and gas-source molecular beam epitaxy on Si (111) substrates. The photodetectors showed high photoconductor gains, a very nonlinear response with illuminating power, and an intrinsic nonexponential photoconductance recovery process. A novel photoconductor gain mechanism is proposed to explain such results, based on a modulation of the conductive volume of the layer.

Analysis and modeling of AlxGa1−xN-based Schottky barrier photodiodes

Schottky barrier photovoltaic detectors have been fabricated on n-AlxGa1−xN(0⩽x⩽0.35) and p-GaN epitaxial layers grown on sapphire. Their characteristics have been analyzed and modeled, in order to determine the physical mechanisms that limit their performance. The influence of material properties on device parameters is discussed. Our analysis considers front and back illumination and distinguishes between devices fabricated on ideal high-quality material and state-of-the-art heteroepitaxial AlxGa1−xN. In the former case, low doping levels are advisable to achieve high responsivity and a sharp spectral cutoff. The epitaxial layer should be thin (<0.5 μm) to optimize the ultraviolet/visible contrast. In present devices fabricated on heteroepitaxial AlxGa1−xN, the responsivity is limited by the diffusion length. In this case, thick AlxGa1−xN layers are advisable, because the reduction in the dislocation density results in lower leakage currents, larger diffusion length, and higher responsivity. In order to...

The effect of Si doping on the defect structure of GaN/AlN/Si(111)

The effect of Si doping on the structural quality of wurtzite GaN layers grown by molecular beam epitaxy on AlN buffered (111) Si substrates is studied. The planar defect density in the grown GaN layer strongly increases with Si doping. The dislocation density at the free surface of GaN significantly decreases when Si doping overpasses a limit value. Si doping affects the misorientation of the subgrains that constitutes the mosaic structure of GaN. The increase of the planar defect density and out-plane misorientation angles of the GaN subgrains with Si doping explain the decrease of dislocations that reach the free surface of GaN. A redshift in the photoluminescence spectra together with a decrease in the c-axis lattice parameter as the Si doping increases point to an increase in the residual biaxial tensile strain in the GaN samples.

Exciton and donor - acceptor recombination in undoped GaN on Si(111)

The optical transitions in undoped, hexagonal GaN layers, grown on Si(111) by molecular beam epitaxy under nitrogen-rich conditions, have been studied by photoluminescence spectroscopy. Several intense excitonic emissions, of free and bound character, are detected as narrow as 1.7 meV at low temperature. The free A, B and C excitons, observed at 3.4786 eV, 3.484 eV and 3.503 eV, respectively, allow the determination of the crystal-field ( meV) and spin - orbit ( meV) splittings. The evolution of their energies with temperature has been analysed with two different fits, the gap shift proportional to ) and respectively. Information on the scattering processes is obtained from the peak broadening, which is due to exciton - phonon interactions. Both the free exciton energies and their temperature behaviour agree with those observed in bulk and homoepitaxial GaN, and therefore the studied GaN/Si layers are strain-free. Up to four extrinsic transitions at 3.4755 eV, 3.4714 eV, 3.456 eV and 3.450 eV have also been observed, and their assignment to bound excitons and donor to band transitions is discussed. Finally, a band at 3.41 - 3.42 eV is attributed to a donor-to-acceptor transition. This interpretation implies the presence of an acceptor lying at 70 meV above the valence band, shallower than those usually employed for p-type doping.

Growth kinetics and morphology of high quality AlN grown on Si(111) by plasma-assisted molecular beam epitaxy

AlN layers were grown on Si(111) substrates by plasma-assisted molecular beam epitaxy. Crystal quality was assessed by atomic force microscopy and high resolution x-ray diffraction. The III/V ratio and the growth temperature, rather than thickness and growth rate, are found to be critical parameters to achieve good quality AlN layers. III/V ratios close to stoichiometry, and high growth temperatures (⩾900 °C) lead to optimal AlN layers. The growth rate is barely modified when growth temperature changes from 780 to 920 °C, but the growth mode and surface roughness are strongly affected. Optimal AlN layers have full-widths at half-maximum values of 10 arcmin, and an average surface roughness of 48 A.

Reactive ion etching of GaN layers using

The characteristics of reactive ion etching of gallium nitride layers, using etching gas are investigated. The GaN etch rate is examined by varying the bias voltage and the flow rate of . For bias voltages in the range of 250 V to 400 V, the etch rate is found to increase with voltage, attaining a maximum rate of at 400 V. The rate also increases with increasing flow. The addition of an inert gas, Ar, or of a reactive gas, , is found to barely affect the etch rate. Surface morphology after etching is checked by atomic force microscopy and scanning electron microscopy, which show that the smoothness of the etched surface is comparable to that of the unetched, and the etched sidewall forms an angle of to the surface normal.

Experimental evidence for a Be shallow acceptor in GaN grown on Si(111) by molecular beam epitaxy

Be-doped GaN layers have been grown on Si(111) by molecular beam epitaxy. The relative Be concentration was measured by secondary ion mass spectroscopy analysis. Photoluminescence spectra have been taken under continuous wave and time-resolved conditions. A new emission at 3.384 eV, which is probably related to substitutional Be, is reported, together with its first and second order phonon replica. Clear blue-shifts are observed when increasing temperature and excitation power, suggesting that this emission is associated with a transition from a residual donor to the Be acceptor. From time-resolved spectra, a very slow and strongly non-exponential decay, as well as a red-shift of the peak energy position with time, confirm the donor-acceptor character of the Be-related emission. The estimated ionization energy of the acceptor is around 90 meV, so Be is the shallowest p-dopant ever reported in GaN.

Thermal stability of Pt- and Ni-based Schottky contacts on GaN and Al0.31Ga0.69N

In this work we analyse the performance of Pt- and Ni-based Schottky metallizations on AlxGa1−xN (x = 0, 0.31). An intermediate thin Ti layer is shown to enhance the thermal stability of Pt/Au, and leads to an increase of the Schottky barrier height. Pt/Ti/Au contacts on GaN provide a barrier height of 1.18 ± 0.07 eV, increasing up to 2.0 ± 0.1 eV on Al0.31Ga0.69N. Further improvement of Schottky contacts is achieved by surface passivation with plasma-enhanced chemical vapour deposited SiO2 or SixNy, which reduces the leakage current by two orders of magnitude and structural modifications in the metal due to thermal ageing.

Analysis of the Visible and UV Electroluminescence in Homojunction GaN LED's

The electrical and electroluminescent properties of MOVPE GaN p-n homojunctions have been analyzed as a function of temperature and bias. Electroluminescence is observed for V>3 V under dc and ac conditions. The main emission at low T is a donor-acceptor transition involving shallow acceptors, though it disappears at higher T due to the ionization of the acceptors and compensation by ionized donors. Room temperature dc and ac electroluminescence spectra evolve under increasing bias from a blue-shifting visible band involving deep states at the p-type side of the p-n junction, to a band-to-band UV recombination at high bias. In agreement, the superlinear dependence of light-current characteristics at low current injection becomes linear when the defects are saturated. Time analysis of the spectra vs pulse duration and duty cycle allows the determination of the visible radiative recombination and relaxation times associated to the Mg-related deep states, which are found to behave as acceptors lying 0.55 eV above the valence band. A simple 3-level model is able to explain the visible emission, which involves the conduction band (or shallow donor) and those deep acceptors in the p-layer. Optimum UV/visible ratio emission requires intense and relatively long pulses, with a high duty cycle to impede visible recombination.