B. 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.

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...

Contactless electroreflectance evidence for reduction in the surface potential barrier in AlGaN/GaN heterostructures passivated by SiN layer

Contactless electroreflectance (CER) has been applied to study the AlGaN potential-barrier height in AlGaN/GaN heterostructures without and with a SiN passivation layer. In the case of an unpassivated structure, an AlGaN band-edge signal with a strong Franz–Keldysh oscillation (FKO) was observed. On the basis of the FKO period, the surface potential barrier has been determined to be ∼1.1 eV. For the SiN passivated structure, a broad CER signal without FKO appears at the AlGaN edge. This observation is associated with a decrease in the height of the surface potential barrier, i.e., a shift in the Fermi level position at the AlGaN surface toward the conduction band.

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.

Influence of crystallographic structure on electrical characteristics of (Al,Ga)N epitaxial layers grown by MOVPE method

The paper presents a study on the correlation between the material structure and electrical characteristics of nitride epitaxial layers grown by metalorganic vapour phase epitaxy (MOVPE) on sapphire substrate. Structural characterisation was performed by means of X-ray diffractometry. Distributions of crystallites lateral sizes as well as the lattice strains inside the blocks were studied. Twist and tilt mosaicities were examined using rocking curves and a specially chosen configuration, where an edge of the sample was illuminated. The grain sizes in the layers were calculated on the basis of peak profile analysis and solution of the Fredholm equations. The electrical properties of the nitride epitaxial layers were determined by impedance spectroscopy method. An attempt was made to correlate the layers of electrical and structural characteristics for better physical interpretation of the results and verification of epitaxial structures usefulness for device application.

MOVPE technology and characterisation of silicon δ-doped GaAs and AlxGa1−xAs

Abstract This work presents the investigation of MOVPE growth of silicon δ-doped GaAs and AlxGa1−xAs epilayers and different methods used for their characterisation. The influence of the growth temperature, SiH4 flow rate and AlxGa1−xAs composition on δ-doping characteristics is discussed. Properties of the Si δ-doped structures were examined using capacitance–voltage (C–V) measurements, photoreflectance spectroscopy, micro-photoluminescence, micro-Raman and photocurrent spectroscopies.