M. Tłaczała

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.

Photoreflectance study of δ-doped semiconductor layers by a fast Fourier transformation

Abstract Photoreflectance (PR) spectroscopy has been applied to the investigation of Si δ-doped GaAs, Al 0.35 Ga 0.65 As and AlAs layers grown by metal–organic vapor phase epitaxy (MOVPE) on GaAs substrates. The observation of Franz–Keldysh oscillations (FKO) and the application of fast Fourier transform (FFT) has allowed us to determine the internal electric field and, hence, the potential barrier between surface and δ-doped region of the layer. The FFT of the photoreflectance spectra has exhibited two separate heavy and light hole frequencies showing that the FKO in the PR signal are always the superposition of these two components.

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.

Investigation of deep energy levels in heterostructures based on GaN by DLTS

In this paper we report our results of DLTS investigations of deep-level defects in Schottky-gate AlGaN/GaN LP-MOVPE structures grown on sapphire substrate. The exact location of heterostructures interface below the surface (20 nm) was determined from the concentration profile to depletion region width dependence. The free charge carrier density was calculated (n<inf>2D</inf> = 4.75÷5.09×10¹⁶ m⁻²). Four deep energy levels have been identified from selected DLTFS spectra (activation energies: E1=E<inf>C</inf>−0.545 eV, E2=E<inf>C</inf>−0.599 eV, E3=E<inf>C</inf>−0.642 eV, and E4=EC-1,118 eV).

Field emission from GaN deposited on the (100) Si substrate

Abstract The field emission properties of a GaN on silicon have been investigated. A MOVPE method has been used to fabricate the GaN layer. Emission measurements were performed at the pressure below 10−5 Pa. The Fowler–Nordheim plot shows a linear relationship that indicates the emitted current to be of field emission origin. The effective work function estimated from the slope of FN plot is very low.

Measurement Systemwith Hall and a Four Point Probes for Characterization of Semiconductors

One of the biggest challenges of communication networks is the video transmission in real time. It requires high demands on the available network capacity and transport mechanisms. Availability of smart mobile devices with batteries, which keep the terminal working for several hours, caused an increased interest in the research of the deployment of video transmission in wireless transmission systems. The presented paper deals with the transmission of video encoded with H.264/AVC (Advanced Video Coding) video coding standard through wireless local area network (WLAN) using the programming environment OPNET Modeller (OM). The test network studied in this work was prepared by combining real and simulated networks, which allows interesting possibilities when working with the OM tools. Such an approach to working with OM allows a detailed video streaming analysis, because the video output was noticeably not only in the form of statistics, but we can see the real impact of transmission failures. Using the OM simulation environment allows to design the transmission systems, which would be difficult to establish in laboratory conditions.

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.

Wet thermal oxidation for GaAs, GaN and Metal/GaN device applications

Thermal wet oxidation of gallium arsenide GaAs (wafers) and gallium nitride GaN (layers from metalorganic vapor phase epitaxy MOVPE and hydride vapor phase epitaxy HVPE) was carried out in N2 as a main gas and H2O as an oxidizing agent. Materials parameters and surface morphology were studied by means x-ray diffraction, ellipsometry, photoreflectance PR, micro Raman spectroscopy, optical microscopy and atomic force microscopy AFM. The lack of materials parameters or their wide range, especially refractive index, dielectric constant and their dependence of oxides composition and structure constituted some problems during measurements. GaAs oxidation was more difficult as GaN oxidation, especially GaN from HVPE.

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.