J. Serafińczuk

Nonpolar GaN substrates grown by ammonothermal method

In this letter, the authors demonstrate large size m-plane GaN substrates grown by ammonothermal method. These substrates have excellent structural quality. The concentration of threading dislocation density is below 5×104 cm−2 and the full width at half maximum for the symmetrical and asymmetrical peaks equals 16 and 19 arc sec, respectively. Also good optical quality, the energy gap-related transition is clearly observed at room temperature in photoluminescence and contactless electroreflectance spectra. GaN epilayers deposited on these substrates exhibit intrinsic narrow exciton lines which are very sensitive to the optical selection rules typical for hexagonal symmetry, proving truly nonpolar character of the material.

Influence of Annealing on Excitation of Terbium Luminescence in YAlO3 Films Deposited onto Porous Anodic Alumina

Terbium-doped yttrium-aluminum oxide films were synthesized by spin-on deposition on porous anodic alumina grown on a silicon wafer and annealed from 400 to 1100°C. An influence of the annealing temperature on the terbium photoluminescence (PL) was studied using two-dimensional PL excitation and time-resolved spectroscopy. Further, a comparison of thermal quenching data for the most intensive 5 D 4 → 7 F 5 luminescence band of Tb 3+ ions was performed for 10-300 K. From the resultant data, the mechanisms of Tb excitation and its dependence on the annealing conditions are proposed and discussed.

Photoreflectance study of exciton energies and linewidths for homoepitaxial and heteroepitaxial GaN layers

Photoreflectance has been applied to study the exciton energies and linewidths for hetero- and homoepitaxial GaN layers (i.e., GaN layers grown on sapphire and truly bulk GaN crystal obtained by ammonothermal method). In order to modulate the built-in electric field inside the samples and eliminate photoluminescence signal from photoreflectance spectra, the surface band bending was modulated by the laser light with the photon energy smaller than the energy gap of GaN, i.e., a 532 nm laser line. The Varshni-type and Bose–Einstein-type parameters that describe the temperature dependence of the exciton transition energies have been evaluated and compared for the two GaN epilayers. It has been concluded that the residual strain in the heteroepitaxial layer influences the exciton transition energy but does not influence the Varshni and Bose–Einstein parameters. It has been found that this strain significantly influences the exciton linewidth. The broadening parameter, which is associated with the temperature-i...

Structural and Optical Properties of Semipolar GaN Substrates Obtained by Ammonothermal Method

Large sizes (9×12 mm2) semipolar GaN substrates with the plane orientation of (2021) were obtained by using the ammonothermal method. Their structural quality was investigated by using the X-ray diffraction whereas the optical quality was probed by absorptionlike (contactless electroreflectance) and emissionlike (photoluminescence) techniques. It is shown that these substrates are characterized by very narrow X-ray diffractions peaks [17 and 21 arcsec for the (2021) and (1010) peak, respectively] and the GaN energy gap-related transition is clearly observed in contactless electroreflectance and photoluminescence spectra. A defect-related emission is also observed in photoluminescence spectra but its intensity is much weaker that the energy-gap related emission.

Photoreflectance of InN and InN:Mg layers: An evidence of Fermi level shift toward the valence band upon Mg doping in InN

Photoreflectance (PR) has been applied to study the energy-gap related absorption in undoped and Mg-doped InN layers. This technique, due to its sensitivity to built-in electric fields, enables studies of the Fermi level position inside an InN bulk. Narrow PR resonance at 0.68 eV has been observed in the unintentionally doped n-InN layer at 10 K. Due to Mg doping this resonance broadens drastically and exhibits the Franz–Keldysh oscillations typical for band-to-band absorption in a region with strong band bending. Such changes in PR spectra are evidences of a strong Fermi level shift toward the valence band due to Mg doping in the probed bulklike region of a sample.

Electronic band structure of compressively strained Ge1−xSnx with x < 0.11 studied by contactless electroreflectance

Contactless electroreflectance is applied to study direct optical transitions from the heavy hole, light hole, and spin-orbit split-off band to the conduction band in compressively strained Ge1−xSnx layers of various Sn concentrations at room temperature. It is shown that the energies of these transitions are in very good agreement with theoretical predictions, which take into account non-linear variation of bandgap and spin-orbit splitting plus the strain-related shifts obtained from the Bir-Pikus theory. The bowing parameter for the direct bandgap has been determined to be 1.8 ± 0.2 eV and agree with this one obtained within ab initio calculations, which is 1.97 eV (for indirect bandgap the bowing parameter is 0.26 eV).

Technology and characterisation of GaAsN/GaAs heterostructures for photodetector applications

The nitrogen-containing conventional AIIIBV semiconductor alloys, so-called diluted nitrides (AIIIBV-N), have been extensively studied recently. Unusual properties of these materials make them very promising for applications in lasers and very efficient multijunction solar cells. This work presents the technology and properties of undoped GaAs1-xNx/GaAs heterostructures used as active regions in the construction of metal-semiconductor-metal (MSM) photodetectors. The atmospheric pressure metal organic vapour phase epitaxy (APMOVPE) was applied for growing MSM test structures. Their structural and optical properties were examined using high resolution X-ray diffraction (HRXRD), photoluminescence (PL), and photoreflectance spectroscopy (PR). Chemical wet etching was applied for forming an active region and a multifinger Schottky metallization was used as MSM contacts. Dark and illuminated current-voltage characteristics were measured. Based on the obtained results, the main detector parameters as responsivity and spectral response were estimated

Atomic force microscopy of partially polished and epi-ready c-plane GaN substrates obtained by an ammonothermal method

In this paper, the propagation of scratches on the surfaces of c-plane GaN substrates due to slicing and polishing is studied through atomic force microscopy (AFM). For epi-ready substrates, the AFM images confirm a flat surface with atomic step roughness, while for partially polished GaN substrates, many scratches are visible in the AFM images. A Fourier analysis of the AFM images shows that the scratches propagate more easily along the {m-plane} and {a-plane} directions on a c-plane GaN surface. Most of these scratches are generated by the mechanical slicing of GaN crystals and/or non-optimal polishing conditions. A proper chemomechanical polishing process is able to remove the damaged material and obtain a flat surface with atomic step roughness. This observation is evidence for the anisotropy of mechanical properties of GaN crystals in the microscale range.

Influence of the AP MOVPE process parameters on properties of (In, Ga)(As, N)/ GaAs heterostructures for photovoltaic applications

GaAsN and InGaAsN semiconductor alloys with a small amount of nitrogen, so called dilute nitrides, are especially attractive for telecom lasers and very efficient multijunction solar cells applications. The epitaxial growth of these materials using MBE and MOVPE is a big challenge for technologists due to the large miscibility gap between GaAs and GaN. Additionally, elaboration of the growth process of quaternary alloys InGaAsN is more complicated than GaAsN epitaxy because a precise determination of their composition requires applying different examination methods and comparison of the obtained results. This work presents the influence of the growth parameters on the properties and alloy composition of the triple quantum wells 3×InGaAsN/GaAs grown by atmospheric pressure metal organic vapour phase epitaxy AP MOVPE. Dependence of the structural and optical parameters of the investigated heterostructures on the growth temperature and the nitrogen source concentration in the reactor atmosphere was analyzed. Material quality of the obtained InGaAsN quantum wells was studied using high resolution X-Ray diffraction HRXRD, contactless electro-reflectance spectroscopy CER, photoluminescence PL, secondary ion mass spectrometry SIMS, photocurrent PC and Raman RS spectroscopies, deep level transient spectroscopy DLTS and transmission electron microscopy TEM.

Structural Characterization of Doped Thick Gainnas Layers - Ambiguities and Challenges

Abstract GaInNAs alloys are mostly used as an active part of light sources for long wavelength telecom applications. Beside this, these materials are used as thin quantum wells (QWs), and a need is to grow thick layers of such semiconductor alloys for photodetectors and photovoltaic cells applications. However, structural characterization of the GaInNAs layers is hindered by non-homogeneity of the In and N distributions along the layer. In this work the challenges of the structural characterization of doped thick GaInNAs layers grown by atmospheric pressure metalorganic vapour phase epitaxy (APMOVPE) will be presented