In the present work a Metal / organic / inorganic semiconductor hybrid heterostructure
(Ag / DiMe−PTCDI / GaAs) was built under UHV conditions and characterised in situ. The
aim was to investigate the influence of the organic layer in the surface properties of
GaAs(100) and in the electrical response of organic−modified Ag / GaAs Schottky diodes.
The device was tested by combining surface−sensitive techniques (Photoemission
spectroscopy and NEXAFS) with electrical measurements (current−voltage,
capacitance−voltage, impedance and charge transient spectroscopies).
Core level examination by PES confirms removal of native oxide layers on sulphur
passivated (S−GaAs) and hydrogen plasma treated GaAs(100) (H+GaAs) surfaces.
Additional deposition of ultrathin layers of DiMe−PTCDI may lead to a reduction of the
surface defects density and thereby to an improvement of the electronic properties of GaAs.
The energy level alignment through the heterostructure was deduced by combining UPS and
I−V measurements. This allows fitting of the I−V characteristics with electron as majority
carriers injected over a barrier by thermionic emission as a primary event. For thin organic
layers (below 8 nm thickness) several techniques (UPS, I−V, C−V, QTS and AFM) show non
homogeneous layer growth, leading to formation of voids. The coverage of the H+GaAs
substrate as a function of the nominal thickness of DiMe−PTCDI was assessed via C−V
measurements assuming a voltage independent capacitance of the organic layer.
The frequency response of the device was evaluated through C−V and impedance
measurements in the range 1 kHz−1 MHz. The almost independent behaviour of the
capacitance in the measured frequency range confirmed the assumption of a near
geometrical capacitor, which was used for modelling the impedance with an equivalent circuit
of seven components. From there it was found a predominance of the space charge region
impedance, so that A.C. conduction can only takes place through the parallel conductance,
with a significant contribution of the back contact. Additionally a non linear behaviour of the
organic layer resistance probably due to the presence of traps was deduced. ( ) ω ' R
QTS measurements performed on the heterostructure showed the presence of two
relaxations induced by deposition of the organic layer. The first one is attributed to the
presence of a deep trap probably located at the metal / organic interface, while the second
one has very small activation energy ( ~ 20 meV) which are probably due to disorder at the
organic film. Those processes with small activation energies proved to be determinant for fitting the I−V characteristics of DiMe−PTCDI organic modified diodes using the expressions
of a trapped charge limited current regime TCLC. Such a model was the best analytical
approach found for fitting the I−V response. Further improving probably will involve
implementation of numerical calculations or additional considerations in the physics of the
device.
| Identifer | oai:union.ndltd.org:DRESDEN/oai:qucosa.de:swb:ch1-200601242 |
| Date | 10 August 2006 |
| Creators | Henry Alberto, Mendez Pinzon |
| Contributors | TU Chemnitz, Fakultät für Naturwissenschaften, Dr. Ilja Thurzo, Prof. Dr. Dr. h. c. Dietrich R.T. Zahn, Prof. Dr. Norbert Esser, Prof. Dr. Michael Hietschold |
| Publisher | Universitätsbibliothek Chemnitz |
| Source Sets | Hochschulschriftenserver (HSSS) der SLUB Dresden |
| Language | English |
| Detected Language | English |
| Type | doc-type:doctoralThesis |
| Format | application/pdf, text/plain, application/zip |
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