F. B. Naranjo

Growth of III-nitrides on Si(1 1 1) by molecular beam epitaxy: Doping, optical, and electrical properties

The growth of high-quality III-nitrides by plasma-assisted molecular beam epitaxy on Si(1 1 1) substrates is addressed. A combination of optimized AIN buffer layers and a two-step growth process leads to GaN layers of high crystal quality (8 arcmin X-ray diffraction full-width at half-maximum) and flat surfaces (57 A rms). Low-temperature luminescence spectra, dominated by excitonic emissions at 3.465 ± 0.002 eV, reveal the presence of a biaxial tensile strain of thermal origin. AlGaN layers, grown within the alloy range 0.10 < x < 0.76, have flat surfaces and exhibit strong excitonic luminescence. Si-doping of GaN and AlGaN produces n-type films reaching electron densities up to 2 × 10 19 and 8 × 10 19 cm -3 , respectively. From photoluminescence and Hall data analysis a Si-donor ionization energy between 50 and 60 meV is derived in GaN. The exciton bound to Si neutral donors at 3.445 eV redshifts while the c-axis lattice parameter decreases as the Si-doping increases, indicating an enhancement of the biaxial tensile strain in the film. This strain increase is a consequence of a strong reduction of the density of dislocations reaching the free surface, due to a particular grain size and orientation governed by the presence of Si donors. Be-doping is also achieved on GaN giving the shallowest acceptor activation energy reported so far, around 90-100 meV. However, there is a severe limitation of the Be incorporation on substitutional sites, leading to the formation of complex, deep defects.

Strong localization in InGaN layers with high In content grown by molecular-beam epitaxy

The effect of the III/V ratio and growth temperature on the In incorporation has been studied in thick (>300 nm) InGaN layers, with In mole fractions from 19% to 37%, grown by molecular-beam epitaxy on sapphire and on GaN templates. Significant desorption of In occurs at growth temperatures above 550 °C. Symmetric and asymmetric reflections from high resolution X-ray diffraction reveals that the layers are not fully relaxed. A bowing parameter of 3.6 eV is calculated from optical absorption data, once corrected for strain-free band gap values. The increase of both, the absorption band-edge broadening and the photoluminescence full width at half maximum at room temperature with the In content, is discussed in terms of a strong In localization effect. This localization effect is further evidenced by the S-shaped temperature dependence of the emission energy.

Resonant-cavity InGaN multiple-quantum-well green light-emitting diode grown by molecular-beam epitaxy

A resonant-cavity InGaN/GaN multiple-quantum-well (MQW) light-emitting diode, incorporating a semitransparent Bragg reflector, has been grown by molecular-beam epitaxy. A 30% reflectivity AlGaN/GaN distributed Bragg reflector and a Ni coating were used as bottom and top mirrors, for backside emission. High-resolution x-ray diffraction data reveal high-quality interfaces in both the MQW and the Bragg mirror. Room-temperature electroluminescence is centered at 507 nm, with an estimated output power of 5 μW at 20 mA. The external efficiency is increased by a factor of 12 compared to similar structures without resonant cavity.

High visible rejection AlGaN photodetectors on Si(111) substrates

We report on the fabrication and characterization of Schottky barrier photodetectors based on Si-doped Al0.35Ga0.65N layers grown on Si(111) substrates, for solar UV-band monitoring (λ 104) and a responsivity of 5 mA/W at 257 nm are reached. The detector time response is limited by the resistance×capacitance product, with a minimum time constant of 20 ns in the zero-load-resistance limit. After photodiode voltage breakdown, the effect on the detector response is discussed.

Ultraviolet electroluminescence in GaN/AlGaN single-heterojunction light-emitting diodes grown on Si(111)

GaN:Mg/AlGaN single-heterojunction light-emitting diodes were grown on Si(111) substrates by plasma-assisted molecular-beam epitaxy. High-quality Mg-doped GaN layers with hole concentrations up to 1.2×1017 holes/cm3 and intense low-temperature photoluminescence, which increases in annealed samples, were obtained. Smooth AlGaN layers, with surface roughness below 5 nm, were used as buffer layers. Continuous-wave room-temperature ultraviolet electroluminescence was observed at 365 nm with a full width at half maximum of 8 nm. An estimated optical power output of 1.5 μW was achieved under 15 V/35 mA operation.

High-quality distributed Bragg reflectors based on AlxGa1−xN/GaN multilayers grown by molecular-beam epitaxy

Distributed Bragg reflectors based on AlxGa1−xN/GaN multilayer stacks have been grown by plasma-assisted molecular-beam epitaxy on GaN templates. The nominal Al composition ranged from 30% to 45%, and the layer thicknesses of the ten-period stack were designed for a target wavelength of 510 nm. Transmission electron microscopy data reveal periodic structures where (Al,Ga)N on the GaN interface is sharper than GaN on the (Al,Ga)N one. X-ray diffraction spectra fitted to a dynamic diffraction simulation model yield an estimate of the layer thicknesses, Al%, and lattice strain. Reflectivity values above 50% at 510 nm have been reproducibly achieved, in very good agreement with the results of the matrix-method simulation.

Resonant Raman scattering in strained and relaxed InGaN∕GaN multi-quantum wells

The effects of the composition and strain in InGaN∕GaN multi-quantum wells on their phonon frequencies have been determined using resonant Raman scattering in a wide energy range. In pseudomorphic quantum wells a strong compensation of both effects occurs, resulting in the InGaN A1LO phonon frequency being almost independent on In concentration. In relaxed quantum wells the A1LO frequency is clearly below the GaN value and depends on the excitation energy, as reported in thick films. This variation, together with the resonance profile, gives a direct estimate of the In concentration and its fluctuations.

Local vibrational modes of H complexes in Mg-doped GaN grown by molecular beam epitaxy

Local vibrational modes (LVM’s) in Mg-doped GaN grown by molecular beam epitaxy have been studied by Raman scattering. Besides Mg local vibrational modes, several local mode peaks associated with hydrogen complexes are observed in the region around 2200 cm−1 and peaks assigned to carbon-hydrogen complexes are also detected in the region around 2900 cm−1. These modes arise from the presence of unintentional carbon and hydrogen impurities in the sample, which was corroborated by secondary ion mass spectroscopy measurements. Raman scattering evidence of local vibrational modes of the C-H complex in GaN is reported. The behavior of the LVM’s associated with the hydrogen and carbon impurity complexes with annealing temperature is also reported.

Properties of Homoepitaxial and Heteroepitaxial GaN Layers Grown by Plasma-Assisted MBE

This work presents a comparative study of the growth by plasma-assisted molecular beam epitaxy (MBE) of GaN layers on four different substrates: Si(111), Al2O3(0001), GaN/Al2O3 and ELOG GaN/Al2O3 templates. Optimization of the growth parameters for the case of growth of GaN layers on silicon substrates leads to smooth films with surface roughness below 5 nm, intense low temperature photoluminescence (PL) (15 meV FWHM) and X-ray diffraction (XRD) values of 8.5 arcmin. (FWHM). The quality of the material clearly improves when growing on sapphire substrates obtaining intense low- and-room temperature PL (10 and 54 meV FWHM, respectively) and XRD values of 6.5 arcmin (FWHM). The best GaN epilayers (intense low-temperature PL emissions with FWHM of 4 meV) are obtained when growing homoepitaxially on high quality GaN/Al2O3 templates, reproducing the optical and structural properties of the template underneath. Finally, the dislocation density decreases drastically from (6 to 10) × 109 cm2 for the case of silicon substrate to 〈106 cm–2 for the GaN layers grown on the ELOG templates.