1.4. APPLIED PHYSICS

 

ANALYSIS OF InGaN HETEROEPITAXIAL FILMS &

Anni Seppälä, Tommy Ahlgren, Eero Rauhala, Ari Rinta-Möykky* and Pekka Laukkanen*

The III-V nitrides are widely studied materials due to their many applications as light sources, detectors and high power components. One interesting application of these large band-gap materials is a blue-light laser diode, which is based on a GaN/InGaN quantum well structure.

The structural properties and crystal quality of molecular beam epitaxy (MBE) grown InGaN heteroepitaxial films were studied. Films were grown on GaN buffer layer on Al2O3 substrate. The influence of different growth parameters (gas partial pressures and the growth temperature) on In content and quality of crystal structure in InGaN film and luminescence properties were measured with ion beam analysis methods at the Accelerator laboratory and with photoluminescence measurements at the Tampere University of Technology. The PL wavelengths correlate quite well with In concentrations measured with Rutherford backscattering. The lattice quality degraded as In concentration increased.

& This work has been partly funded by the Academy of Finland under the MACOMIO project.

* ORC, Tampere University of Technology, P.O.Box 692, 33101 Tampere

 

ANNEALING BEHAVIOUR OF ALUMINIUM-IMPLANTED InP

Vesa Kyllönen, Jyrki Räisänen*, Anni Seppälä, Tommy Ahlgren and Jari Likonen**

Annealing behaviour of implanted aluminium in n-type indium phosphide wafers was studied. Two different ion energies and doses (1015 and 1016 cm-2) were used in the implantations. The depth profiles of Al were measured with Secondary Ion Mass Spectrometry (SIMS) at Research Center of Finland. Aluminium concentration profiles for the high dose implanted samples were also checked with nuclear resonance broadening technique (NRB) at the Accelerator laboratory. It was found out that SIMS was not a proper technique for profiling high Al concentrations. Al concentrations over 4 x 1020 at./cm3 will result in decreased sputtering rate giving erroneous depth profiles.

The effect of different ion doses and crystal damage caused by implantation was also studied. Fast migration of aluminium via defects was observed in the samples implanted with 1016 ions/cm2.

* Department of Physics, Univ. Jyväskylä

** Technical Research Center of Finland, Chemical Technology, 02044 VTT

 

NON-EQUILIBRIUM PROPERTIES OF GaAs INTERATOMIC POTENTIALS

Kai Nordlund and A. Kuronen*

Ion irradiation can lead to local melting and other large distortions in a material. Thus it is very important that computer simulation models of irradiation phenomena can describe melting and structure changes well. We examined how GaAs interatomic potentials, which have been used to describe irradiation phenomena in the literature, behave on melting. We showed that the most commonly used GaAs potential [1] does not have the correct ground state structure, making it unsuitable for describing ion irradiation processes [2].

* Laboratory of Computational Engineering, P.O. Box 9400, FIN-02015 Helsinki University of Technology

1. M. Sayed, J.H. Jefferson, A.B. Walker and A.G. Gullis, Computer simulation of Atomic Displacements in Si, GaAs and AlAs, Nucl Instr Meth in Phys Res B 102 (1995) 232

2. K. Nordlund and A. Kuronen, Non-equilibrium properties of GaAs interatomic potentials, Nucl Instr Meth in Phys Res B 159 (1999) 183

STRAIN EFFECTS IN SiGe SURFACE CASCADES

Jura Tarus, Kai Nordlund and Juhani Keinonen

Development in epitaxial growth methods has enabled the synthesis of pseudomorphic SiGe alloys in semiconductor technology. Heterojunction structures have many benefits over conventional homojunction approaches. One of the advantages is that the bandgap can be altered by changing the proportion of the Ge content and strain caused by the lattice mismatch. SiGe is also faster than Si in terms of carrier mobility and thus may be the future substitute for pure silicon. We have performed a case study using strained Ge instead of SiGe.

By using the classical molecular dynamics technique we have simulated the effects of 5 keV Xe atoms impinging on the strained Ge(100)2x1 surface. We found that large adatom islands are formed on top of the amorphous zones created by the cascades. We also found that lattice atoms around the molten zone move radially inwards and thus cause strain relief in the sample.

During the collision cascade a liquid area is formed first and as the atomic volume of the liquid atoms is somewhat smaller than the atomic volumes at the surrounding lattice, the lattice atoms relax radially inwards to the soft liquid core causing the pressure in the liquid to rise. As the liquid cools, the transition to the amorphous phase further increases the pressure owing to its lower density causing formation of an adatom island on the surface. At the same time atoms below the amorphous zone are pushed deeper into the bulk and thus create strain in the direction perpendicular to the surface.

 

DIFFUSE X-RAY STREAKS FROM STACKING FAULTS IN Si ANALYZED BY ATOMISTIC SIMULATIONS

Kai Nordlund, U. Beck*, T.H. Metzger* and J. Patel**

Ion implantation of silicon and subsequent high-temperature annealing plays a central role in present-day semiconductor manufacturing. Although the overall development of the implantation damage production and annealing is relatively well understood, many details remain unclear.

For instance, understanding their properties of extrinsic stacking faults, which can form during post-implantation annealing of Si, is important for reliable control of semiconductor manufacturing processes. We demonstrated how grazing incidence X-ray scattering methods can be used as a nondestructive means for detecting extrinsic stacking faults in Si [1]. Atomistic analysis of diffuse intensity streaks is used to determine the size of the faults, the minimum size at which the streak pattern in the scattering will be visible, and the magnitude of atomic displacements in the center of the stacking fault.

* Sektion Physik and Center for NanoScience (CeNS), LMU München, Geschwister-Scholl-Platz 1, D-80539 München, Germany

** SSRL/SLAC, Stanford University, Stanford, CA 94309, USA

1. K. Nordlund, U. Beck, T.H. Metzger and J.R. Patel, Diffuse X-ray streaks from stacking faults in Si analyzed by atomistic simulations, Appl Phys Lett 76 (2000) 846

 

THE ELECTRONIC STOPPING FROM A 3D CHARGE DISTRIBUTION

Jussi Sillanpää, Kai Nordlund and Juhani Keinonen

Description of the slowing down of energetic ions penetrating matter is a long-standing problem of considerable theoretical and technological interest. Despite much intensive work during the last 80 years, the models describing the slowing down of an ion by collisions with electrons (electronic stopping) may still give results with errors of several tens of percents [1]. We present a model for the electronic stopping at low velocities (v < v0) [2]. By using molecular dynamics and calculating the electronic stopping from a 3D charge distribution without using any free parameters, we obtain accurate range distributions on a realistic physical basis. Our electronic stopping model is based on the Brandt-Kitagawa (BK) theory, in which the electronic stopping of a heavy ion is the electronic stopping of a proton scaled by the square of the effective charge. The model also includes a version of the Firsov model to describe the energy loss due to electron transfer between the ion and target atoms. We calculate the stoppings of different ions in silicon, for which both accurate electron distributions and experimental range distributions are available. We first test the model for hydrogen ions, to determine whether a basis exists for the scaling hypothesis, and then for heavier ions. The results are compared with experimental range profiles and show good agreement, much better than that achieved by using standard (nonlocal) electronic stopping models.

1. J. Sillanpää, E. Vainonen-Ahlgren, P. Haussalo and J. Keinonen, Stopping of 5-100 keV helium in molybdenum, chromium, copper and nickel, Nucl Instr and Meth in Phys Res B 142 (1998) 1-8

2. J. Sillanpää, K. Nordlund and J. Keinonen, The electronic stopping of silicon form a 3D charge distribution, accepted for publication in Phys Rev B

 

ACTIVATION ANALYSIS OF PROTON AND ALPHA-PARTICLE IRRADIATED POLY (VINYL FLUORIDE) THIN FILMS

M. Paronen*, Pertti Tikkanen and Eero Rauhala

Polymer electrolyte membrane fuel cells and the possible membrane materials are subject to intense investigation. We have shown previously that ion irradiated and subsequently sulfonated poly(vinyl fluoride) thin films have very promising properties in this respect. However, the induced radioactivity of the irradiated films has not been studied previously.

In this study we have used proton and alpha-particle beams to irradiate the thin films at energies of 2.5 to 5 MeV (protons) and 5 to 14 MeV (alphas). The induced (long-term) activity of the films consist mainly of 13-N (9.97 min) and 22-Na (2.605 a) which are both b+-emitters. The activities have been determined from the continuous b-spectra which were measured with a 700-┬Ám-thick PIPS detector in a close geometry. Preliminary analysis of the data shows that the level of the induced activity is very low. In most cases the activities are negligible in comparison to the regulated limits.

*Laboratory of Polymer Chemistry, P.O. Box 55, FIN-00014 University of Helsinki

 

LARGE ION BEAM ANALYSIS OF THIN FILMS PRODUCED BY ATOMIC LAYER DEPOSITION AND ELECTRODEPOSITION

Timo Sajavaara, Eero Rauhala, Kai Arstila, Juhani Keinonen, M. Ritala*, M. Leskelä*, M. Kemell*, R. Matero*, M. Vehkamäki*, A. Rahtu*, H. Saloniemi*, T. Hänninen*, M. Nieminen**, M. Putkonen**, L. Niinistö**, K. Kukli+, and V. Sammelselg+

Ion beam analysis was applied for the determination of major constituent and impurity concentrations and their depth profiles in a variety of thin film materials. To quantify light elements and impurities, elastic recoil detection analysis (ERDA) was used, and Rutherford backscattering spectrometry (RBS) was mainly utilised for the determination of depth profiles of heavier elements.

Atomic layer deposition (ALD) offers a well controlled gas phase method for the deposition of good conformality thin films. The search of new and better precursors is intense and requires continuous film chracterisation. Because of the use of organic precursors, the knowledge of hydrogen and carbon amounts are perhaps the most important. In addition to ALD grown films, electrodeposited CuInSe2 thin film is studied as it is a potential low cost absorber material for thin film solar cells.

* Laboratory of Inorganic Chemistry, Department of Chemistry, Univ. Helsinki

** Laboratory of Inorganic and Analytical Chemistry, Helsinki University of Technology

+ Institute of Experimental Physics and Technology, Univ. Tartu

 

MECHANICAL TESTING AND EVALUATION OF MATERIALS USED IN ORTHOPAEDIC SURGERY

Reijo Lappalainen, Panu Pekko, Sanna Lehti, Asko Anttila, S. Santavirta* and Y.T. Konttinen**

Polymers are commonly used biomaterials in orthopaedic surgery. In load bearing applications, e.g. in hip and knee joints, mechanical strength and wear are very relevant issues. For example, we have tested possibilities to reduce wear of two commonly used plastic materials polyethylene and polymethylmethacrylate (PMMA) by using amorphous diamond coating on the counterface material. PMMA is the most common bone cement used to fix implants and may wear out significantly due to micromotion between the implant and cement mantle. It turned out that both abrasive and adhesive wear were minimised by amorphous diamond coating. The wear rate of these polymers reduced by a factor of about 100-500 compared to uncoated metallic counterface material. Furthermore, the wear debris particles were small (submicron) and both sliding surfaces remained smooth. On the other hand, amorphous diamond coating offered sufficient initial stability for bone cement fixation (about 75 % of the value for metallic implants). Also in clinical use, this kind of a coating should decrease significantly the amount of wear particles released in surrounding tissues leading to aseptic loosening of an implant.

* Department of Orthopedics and Traumatology, Helsinki, University Central Hospital, Topeliuksenkatu 5, FIN-00260 Helsinki

** Department of Anatomy, P.O. Box 9, University of Helsinki, FIN-00014 Helsinki