F. Calle

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.

High-responsivity submicron metal-semiconductor-metal ultraviolet detectors

Metal-semiconductor-metal ultraviolet (UV) detectors with finger width and pitch ranging from 0.5 to 4 μm have been fabricated on GaN. A superlinear enhancement of responsivity is observed when scaling down, in agreement with a model that includes optical absorption and the variation of the space-charge regions with bias. No degradation is found in terms of UV/visible contrast or photocurrent linearity.

Molecular beam epitaxy growth and doping of III-nitrides on Si(1 1 1): layer morphology and doping efficiency

Abstract Wurtzite GaN layers grown by plasma-assisted molecular beam epitaxy on Si(1xa01xa01) reveal strong morphology changes as a function of the III/V ratio. For nominally N-rich conditions, GaN nanocolumns are reproducibly grown with diameters ranging from 600 to 1500 A. These nanocolumns are fully relaxed from lattice and thermal strain, having a very good crystal quality characterized by strong and narrow (2 meV) photoluminescence excitonic lines at 3.472–3.478 eV. The nanocolumns generate from a reduced Ga adatoms diffusion due to the excess nitrogen (Ga-balling). Si-doping yields 2×10 19 and 8×10 19 electrons cm −3 in compact GaN and AlGaN (up to 45%) layers, respectively. In addition, Si-doping decreases the threading dislocation density while enhancing the layer biaxial tensile strain. P-type doping with Be, Mg and C is analyzed and compared. Carbon shows a low solubility according to theoretical predictions. Mg-doping is efficient leading to hole densities in the mid 10 17 cm −3 range. Be is the shallowest acceptor level (90–100 meV), but its efficiency is hampered by the generation of deep Be-related traps, most probably V Ga –Be i complexes, and by self-compensation by Be i . Positron annihilation spectroscopy results; the effect of Mg when codoping with Mg+Be, and the presence of a strong Be-related yellow luminescence back these assumptions.

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.

Conduction‐band engineering in piezoelectric [111] multiple quantum well p‐i‐n photodiodes

The influence of the design parameters on the conduction‐band profile and optoelectronic properties of [111]‐oriented InGaAs/GaAs p‐i(MQW)‐n diodes is presented. An analytical expression for the average electric field (AEF) in the p‐i‐n active region (MQWs within the intrinsic region) is obtained. The existence of two different potential envelopes, corresponding to a positive or to a negative sign of the AEF, and giving rise to clearly different optical and electronic properties, is demonstrated. In samples with negative AEF, as compared to structures with positive AEF, larger reverse voltages are needed to quench the photoluminescence and to enhance the p‐i‐n photocurrent. An analysis of both transition energies and intensities, versus bias, clearly indicates that in samples with a negative AEF carriers accumulate at the extremes of the active region, giving rise to a long‐range screening effect of the field in the wells.

Visible-blind ultraviolet photodetectors based on ZnMgBeSe Schottky barrier diodes

Planar geometry Schottky barrier photodiodes designed for visible-blind ultraviolet detection have been fabricated. They are based on ZnMgBeSe alloys grown by molecular-beam epitaxy. High crystalline quality is achieved, which leads to a high responsivity (0.17 A/W at 375 nm) and a sharp cutoff of more than three orders of magnitude. As attested by the linear variation of the photocurrent with the optical excitation, there is no internal gain mechanism. A detectivity of 2×1010u200amHz1/2u200aW−1 is obtained showing that low-noise devices with high sensitivity have been fabricated.

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.

In situ growth monitoring of distributed GaN–AlGaN Bragg reflectors by metalorganic vapor phase epitaxy

A series of distributed GaN-AlGaN Bragg reflectors (DBR) has been grown on Al2O3(0001) substrates by metalorganic vapor phase epitaxy. The growth of the GaN template as well as of the GaN–AlxGa1−xN quarter-wave stack has been monitored by laser reflectometry. The evolution of the in situ reflectivity as well as DBR reflection spectra are discussed as function of the AlxGa1−xN composition x.

MBE growth of GaN and AlGaN layers on Si(1 1 1) substrates: doping effects

Abstract High-quality GaN layers with 8 arcmin (X-ray diffraction full-width at half-maximum, XRD FWHM) and rms surface roughness of 57xa0A are grown on Si(1xa01xa01) substrates when using optimized AlN buffer layers. Si-doping produces n-type films reaching carrier concentrations up to 1.7×10 19 xa0cm −3 with mobilities of 100xa0cm 2 /Vxa0s. A reduction of the lattice parameter c together with a red shift in the photoluminescence (PL) emission is observed with increasing Si doping. The dislocation density observed by plan-view transmission electron microscopy (PVTEM) also decreases by close to one order of magnitude (from 5.3×10 9 to 8×10 8 xa0cm −2 ) when increasing the Si doping (from 1.1×10 17 to 6.0×10 18 xa0cm −3 ). AlGaN layers were grown with Al content ranging from 10% to 76% with XRD FWHM of 22 arcmin and intense low-temperature photoluminescence. N-type doping is achieved in AlGaN (40%) with Si, reaching electron concentrations of 8×10 19 xa0cm −3 .

Influence of the surface morphology on the relaxation of low-strained InxGa1-xAs linear buffer structures

Abstract The relaxation of low-strained InGaAs linear-graded buffer layers up to 30 and 60% In content grown on GaAs substrates is studied. Strain-limited designs allow to preserve bidimensional (2D) growth mode with a relaxation driven by dislocation multiplication for large thickness, as in the case of single layers. However, it is shown that low-strained linear buffer layers are not able to inhibit tridimensional (3D) growth mode. Under standard growth conditions (500°C), the surface morphology evolves from a near-flat one for samples up to x = 0.3, to a rough one at higher In content, characterised by saw-tooth morphology with well-defined facets. The development of the surface roughness changes the strain relaxation mechanism from dislocation multiplication inside the layer to surface elastic strain relaxation plus surface nucleation and a high density of threading dislocations. A constant surface strain in linear grades during all the dislocation-driven relaxation is proposed to allow the observed 2D to 3D transition. The influence of the grading rate, composition steps, and growth temperature on the surface morphology and the relaxation are analysed. Appropriate growth conditions and buffer design up to high Indium content allow to produce structures with improved structural and optical quality.