R. Paszkiewicz

Contactless electromodulation spectroscopy of AlGaN∕GaN heterostructures with a two-dimensional electron gas: A comparison of photoreflectance and contactless electroreflectance

Photoreflectance (PR) and contactless electroreflectance (CER) spectroscopies have been applied to study optical transitions in undoped and Si-doped AlGaN∕GaN heterostructures at room temperature. Spectral features related to excitonic and band-to-band absorptions in GaN layer and band-to-band absorption in AlGaN layer have been resolved and analyzed. In addition, a broad spectral feature related to two-dimensional electron gas has been observed for the Si-doped heterostructure. It has been found that some of the mentioned optical transitions are not observed in CER spectra whereas they are very strong in PR spectra. This phenomenon is associated with different mechanisms of the modulation of built-in electric field in the investigated structure. A combination of PR and CER gives the possibility of a richer interpretation of both PR and CER spectra.

Single crystalline Sc2O3/Y2O3 heterostructures as novel engineered buffer approach for GaN integration on Si (111)

The preparation of GaN virtual substrates on Si wafers via buffer layers is intensively pursued for high power/high frequency electronics as well as optoelectronics applications. Here, GaN is integrated on the Si platform by a novel engineered bilayer oxide buffer, namely, Sc2O3/Y2O3, which gradually reduces the lattice misfit of ∼−17% between GaN and Si. Single crystalline GaN(0001)/Sc2O3(111)/Y2O3(111)/Si(111) heterostructures were prepared by molecular beam epitaxy and characterized ex situ by various techniques. Laboratory-based x-ray diffraction shows that the epitaxial Sc2O3 grows fully relaxed on the Y2O3/Si(111) support, creating a high quality template for subsequent GaN overgrowth. The high structural quality of the Sc2O3 film is demonstrated by the fact that the concentration of extended planar defects in the preferred {111} slip planes is below the detection limit of synchrotron based diffuse x-ray scattering studies. Transmission electron microscopy (TEM) analysis reveal that the full relaxat...

Structural and optical quality of GaN grown on Sc2O3/Y2O3/Si(111)

Thick (∼900 nm) GaN layers were grown by molecular beam epitaxy on cost-effective Sc2O3/Y2O3/Si(111) substrates and characterized by x-ray diffraction and photoluminescence. Samples grown in Ga-rich condition show superior structural and optical quality with reduced density of cubic GaN inclusions within the hexagonal matrix and a relatively strong photoluminescence emission at 3.45 eV at 10 K. Cubic inclusions are formed in the initial growth stage and their concentration is reduced with increasing film thickness and after rapid thermal annealing.

Two constant phase element behaviour of the admittance characteristics of GaAs metal-insulator-semiconductor structure with deep traps

Abstract The capacitance and the conductance vs. the frequency characteristics of Au/Pd/Ti–SiO2–(n) GaAs structures with and without (NH4)2Sx treated GaAs surface have been investigated. For normalized conductance Gm/ω characteristics, the peaks with the position dependent on the gate voltage have been observed for both types of structures and also the peaks with the position not dependent on the bias for the (NH4)2Sx treated samples. That behaviour has been attributed to the interface state contribution and the deep trap contribution, respectively. The equivalent circuit has been presented which allows for the description of the observed admittance peculiarities in a simple way. It contains: the insulator capacitance, the series resistance, the space charge capacitance and the two parallel branches including the constant phase elements (CPE1 and CPE2) connected in series with resistances. The values of parameters of the equivalent circuit elements at different gate voltages have been determined. These elements are attributed to the physical phenomena in different regions of the analysed system. The CPE1 (with parameter n1 ∼0.54) connected in series with resistance describes the electron processes in the SiO2-GaAs region leading to the frequency dispersion of the electrical characteristics of the investigated structures in the frequency range which depends on the bias, while CPE2 (with parameter n2 ∼0.89–0.95) connected in series with resistance models the deep traps admittance.

MOVPE GaN Grown on Alternative Substrates

The structure, morphology and optical properties of GaN films deposited by metalorganic vapour phase epitaxy (MOVPE) on alternative substrates: ZnO, NdGaO, YSZ (yttria stabilized zirconia). Scanning electron microscopy, X-ray diffraction and photoluminescence were used for the epitaxial layers characterisation. The obtained results have been compared to those of GaN layers grown on c-plane sapphire substrates. It was established that the most important step towards the realisation of device quality GaN material on alternative substrates is the first stage of the growth process.

Stress engineering in GaN structures grown on Si(111) substrates by SiN masking layer application

GaN layers without and with an in-situ SiN mask were grown by using metal organic vapor phase epitaxy for three different approaches used in GaN on silicon(111) growth, and the physical and optical properties of the GaN layers were studied. For each approach applied, GaN layers of 1.4 μm total thickness were grown, using silan SiH4 as Si source in order to grow SixNx masking layer. The optical micrographs, scanning electron microscope images, and atomic force microscope images of the grown samples revealed cracks for samples without SiN mask, and micropits, which were characteristic for the samples grown with SiN mask. In situ reflectance signal traces were studied showing a decrease of layer coalescence time and higher degree of 3D growth mode for samples with SiN masking layer. Stress measurements were conducted by two methods—by recording micro-Raman spectra and ex-situ curvature radius measurement—additionally PLs spectra were obtained revealing blueshift of PL peak positions with increasing stress. T...

Structure Characterization of (Al,Ga)N Epitaxial Layers by Means of X-Ray Diffractometry

The structural properties of (Al,Ga)N epitaxial layers were studied basing on X-ray diffractometric measurements. The examined layers were deposited by MOVPE on c-plane sapphire substrate. The measurements were performed on MRD-Philips diffractometer. Particularly, the density of lateral correlation length (coherence length) distribution, the most probable shape of the (Al,Ga)N/GaN blocks and twist as well as tilt mosaicities were described. The distribution of the crystallite block sizes was calculated from X-ray peak profile analysis. Twist and tilt mosaicities were examined using rocking curve mode and specially chosen configuration, where an edge of the sample was illuminated.

Contactless electroreflectance study of band bending for undoped, Si- and Mg-doped GaN layers and AlGaN/GaN transistor heterostructures

The surface band bending in undoped, Si-doped and Mg-doped GaN layers with Ga-face polarity as well as AlGaN/GaN heterostructures with Ga(Al)-face polarity has been investigated by room temperature contactless electroreflectance (CER) spectroscopy. The opposite phase of CER resonance (i.e., opposite band bendings) has been observed for n-type (undoped and Si-doped) and p-type (Mg-doped) GaN layers. It means that for thick GaN layers the surface band bending results not from crystal polarity but from the Fermi-level pinning at the surface and carrier type/concentration inside the layer. The crystal polarity can influence the surface band bending for thin (Al)GaN layers for which the screening phenomena can be neglected or are very weak. Such a situation is typical of AlGaN/GaN transistor structures where the thickness of AlGaN layer is below 40nm. In this case, the strong internal electric field in AlGaN layer is manifested in CER spectra by a resonance with a long period Franz-Keldysh oscillation.

Room temperature photoreflectance of different electron concentration GaN epitaxial layers

Abstract Wurtzite-structure GaN epilayers grown by metal-organic vapor phase epitaxy (MOVPE) on sapphire substrates were studied by photoreflectance (PR) spectroscopy performed at room temperature. Several nominally undoped samples with different electron concentrations (5×10 15 –5×10 18 cm −3 ), deposited under different growth conditions, were investigated. For low electron concentration we have observed three well-resolved excitonic transitions related to A, B, and C excitons. In this case, extremely small broadening of PR lines (few milielectronvolts) has been found. With the increase of carrier concentration in epilayers, we have observed an increase of broadening of the transitions. This has led to the disappearance of excitons in PR spectra and to the observations of one broad non-excitonic feature for highest electron concentration. Additionally, in some cases the Franz–Keldysh oscillations (FKO) have been also observed reflecting the existence of a surface built-in electric field in the epilayers, which do not destroy the excitons.

Fabrication of ohmic contact based on platinum to p-type compositionally graded AlGaAs layers

Novel metallization scheme was proposed for ohmic contact formation to compositionally graded p-type AlGaAs. A metal multilayers of Ti/Pt/Au, Pt/Ti/Pt/Au and Pt/Ti/Ni/Au were deposited by thermal evaporation using electron gun and resistance heater. The contacts were sequentially annealed by rapid thermall annealing system in N2 atmosphere at various temperatures (in the range from 350°C to 550°C). The duration of annealing step was 2 minutes. The as-deposited Pt/Ti/Pt/Au and Pt/Ti/Ni/Au multilayer metallizations had resistivities of 1.4·10-5 Ω·cm2 which have been gradually deteriorated after each subsequent annealing. The current-voltage characteristics of the ohmic contacts to compositionally graded p-type AlGaAs epitaxial layers were studied and discussed.