Ladislav Harmatha

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).

Trap-assisted tunnelling current in MIM structures

A new model is presented of current transport in Metal Insulator Metal (MIM) structures by quantum mechanical tunnelling. In addition to direct tunnelling through an insulating layer, tunnelling via defects present in the insulating layer plays an important role. Examples of the influence of the material and thickness of the insulating layer, energy distribution of traps, and metal work functions are also provided.

Preparation of transparent conductive AZO thin films for solar cells

A study of the effect of technology parameters (sputtering power, substrate temperature and post-deposition annealing) on structural, electrical and optical properties of aluminium-doped zinc oxide (AZO) thin films was carried out. The optimal technology parameters of preparation were found to get necessary properties of AZO thin films for application in solar cells - the high figure of merit (F ges 4 %/Omega), low electrical sheet resistance (Rs les 10 Omega/square) and high optical transmittance (T ges 82%, including the glass substrate).

Charge injection and transport properties of an organic light-emitting diode

Summary The charge behavior of organic light emitting diode (OLED) is investigated by steady-state current–voltage technique and impedance spectroscopy at various temperatures to obtain activation energies of charge injection and transport processes. Good agreement of activation energies obtained by steady-state and frequency-domain was used to analyze their contributions to the charge injection and transport. We concluded that charge is injected into the OLED device mostly through the interfacial states at low voltage region, whereas the thermionic injection dominates in the high voltage region. This comparison of experimental techniques demonstrates their capabilities of identification of major bottleneck of charge injection and transport.

A new model of trap assisted band-to-band tunnelling

The paper describes a new approach to calculate currents in a PN diode based on the extension of the Shockley-Read-Hall recombination-generation model. Presented model is an alternative to Hurkx model of trap assisted tunnelling.

CAPACITANCE ANALYSIS OF THE STRUCTURES WITH THE a-Si:H(i)/c-Si(p) HETEROJUNCTION FOR SOLAR-CELL APPLICATIONS

Abstract In this paper we present the capacitance study of the intrinsic amorphous silicon/crystalline silicon heterostructure with the aim to gain insight on the heterointerface properties of a passivated silicon heterojunction solar cell. It is shown that due to the high density of defect states in the amorphous layer the structure has to be analyzed as a heterojunction. Using the analysis, the following values have been determined: conduction-band offset of 0.13 eV, electron affinity of 3.92 eV, and density of defect states in the intrinsic amorphous silicon being that of 4.14 X 1021m—3.

4H-SiC Diode with a RuOx and a RuWOx Schottky Contact Irradiated by Fast Electrons

The impact of radiation damage on the device performance of 4H-SiC Schottky diodes with a RuO2 and a RuWOx Schottky contacts, which were irradiated at room temperature with 9 MeV electrons (absorbed dose of 50 gy), is studied. No degradation is observed from measured capacitance-voltage curves. The radiation-induced decrease of the Schottky barrier height is observed from current-voltage measurements. Eight electron deep energy levels are observed before irradiation, and eighteen electron levels are induced after irradiation by a standard deep level transient spectroscopy

DLTFS study of InGaAs/AlInAs heterostructures grown on n-InP:S substrates

In this paper authors are presenting the first results of electrically active defect investigations in the three 5×UD-In0.53Ga0.47As/Al0.42In0.58As MQW structures grown on n-InP:S substrates by low pressure metal organic vapour phase epitaxy at same growth conditions but at different growth rates. The defect distribution is different for each sample. As thinner are the quantum well layers as more significant minority carrier responses are measured. Parameters of nine deep energy levels were identified by Deep Level Transient Fourier Spectroscopy method. The three of them T3 (~0.38 eV), T5 (~0.17 eV) and T7 (~0.32 eV) were identified in all investigated structures. We assume that level T1 (~0.19 eV) and T5 (0.17 eV) are with high probability the result of carrier emissions from QWs.

Electrophysical Properties of GaAs P–I–N Structures for Concentrator Solar Cell Applications

Abstract This paper is dedicated to electro-physical characterisation of a GaAs p-i-n structure grown for solar cell applications, which was carried out by light and dark current-voltage (I–V) and Deep Level Transient Fourier Spectroscopy (DLTFS) methods. The conversion efficiency and open-circuit voltage were determined from I–V measurement at 1 and 20× sun light concentrations. Three electron like defects TAn1, TAn2, TDn and one hole like defect TBp obtained by DLTFS measurements were confirmed. The origin of these defect states was stated as native GaAs impurities.

Electrically Active Defects In Solar Cells Based On Amorphous Silicon/Crystalline Silicon Heterojunction After Irradiation By Heavy Xe Ions

Abstract The contribution is focused on the diagnostics of structures with a heterojunction between amorphous and crystalline silicon prepared by HIT (Heterojunction with an Intrinsic Thin layer) technology. The samples were irradiated by Xe ions with energy 167 MeV and doses from 5 × 108 cm−2 to 5 × 1010 cm−2. Radiation defects induced in the bulk of Si and at the hydrogenated amorphous silicon and crystalline silicon (a-Si:H/c-Si) interface were identified by Deep Level Transient Spectroscopy (DLTS). Radiation induced A-centre traps, boron vacancy traps and different types of divacancies with a high value of activation energy were observed. With an increased fluence of heavy ions the nature and density of the radiation induced defects was changed.