A. Laakso

Influence of layer thickness on the formation of In vacancies in InN grown by molecular beam epitaxy

We have used a low-energy positron beam to identify In vacancies in InN layers grown on Al2O3 by molecular beam epitaxy. Their concentration decreases from ∼5×1018 to below 1016 cm−3 with increasing layer thickness (120–800 nm). The decrease in the vacancy concentration coincides with the increase in the electron Hall mobility, suggesting that In vacancies act as electron scattering centers.

Evaporation of gold thin films on mica : effect of evaporation parameters

We have evaporated gold on mica surfaces to produce flat surfaces to be used as substrates in STM experiments. We have experimented with different evaporation parameters to find the optimum values for them, since the parameters representing the results of majority of the published experiments did not yield good results for us. We found that the most important parameters were evaporation rate and substrate temperature. The respective optimum values found were 0.3 nm/s and 360°C. These values matched some published results, while conflicting with others. We will discuss various factors, which possibly cause this discrepancy.

Digital measurement of positron lifetime

A prototype of a digital positron lifetime apparatus is presented. We demonstrate that direct digitizing of detector pulses and subsequent simple analysis with software, can be used to replace the conventional analog electronics chain (constant-fraction discriminators, time-to-amplitude converter and multichannel analyzer). In this work, we use a fast digital oscilloscope. The quality of the lifetime data is shown to be as good as with a usual apparatus. For a pulsed positron lifetime beam the digital system is particularly suitable as no coincidence detection is needed and only one analog pulse has to be analyzed.

Vacancy defects in O-doped GaN grown by molecular-beam epitaxy: The role of growth polarity and stoichiometry

Positron annihilation spectroscopy is used to study vacancy defects in GaN grown by molecular-beam epitaxy due to different polar directions and varying stoichiometry conditions during oxygen doping. We show that Ga-polar material is free of compensating Ga vacancies up to [O]=1018 cm−3 in Ga stable growth, but high concentrations of VGa are formed in N-stable conditions. We also show that vacancy clusters are formed in N-polar material grown in Ga stable conditions, which may be related to the higher reactivity of the N-polar surface. These clusters have no apparent influence on the electrical properties of the material. We thus infer that their charge state is neutral.

Evaporation of silver thin films on mica

We have studied the effect of evaporation parameters on the growth of epitaxial silver films on mica. Our object was to grow films with large flat areas to be used as substrates in STM studies. We have found optimum parameters which result in extremely flat films which in the scale of several micrometers consist only of terraces several hundred nanometers wide and separated by monatomic steps; and a few deep pinholes averaging an areal density of roughly one pinhole per every square micrometer of film surface. Certain other parameter values produced exceedingly rough surfaces.

Positron lifetime beam for defect studies in thin epitaxial semiconductor structures

Positron annihilation spectroscopies are methods for direct identification of vacancy-type defects by measuring positron lifetime and Doppler broadening of annihilation radiation and providing information about open volume, concentration and atoms surrounding the defect. Both these techniques are easily applied to bulk samples. Only the Doppler broadening spectroscopy can be employed in thin epitaxial samples by utilizing low-energy positron beams. Here we describe the positron lifetime beam which will provide us with a method to measure lifetime in thin semiconductor layers.

Scattering Effects in a Positron Lifetime Beam Line

Electrons can be used as test particles of the pulsed positron lifetime beam. The beam should be designed so that electron scattering effects do not distort the timing spectrum. To study these effects we have developed a simulation program which takes into account the collisions of the particles with beam structures and calculates the final time distribution. This program has been used in the design of an improved chopper for positron lifetime beam.

HV Design of a Pulsed Lifetime Beam with a Grounded Sample

A pulsed positron lifetime beam designed for semiconductor studies is under construction at Helsinki University of Technology [1]. The target of the beam is kept at ground potential to simplify the use of the beam and facilitate sample manipulation while the source and pulsing components are floating. Due to this it is necessary to pay attention to the realization of the high voltage power supply configuration and the HV surge protection. Details of the measurement chamber and the acceleratordecelerator structure are presented.

Adsorption of HgCl2 molecules on Au (111) surfaces studied by scanning tunneling microscopy

We have studied the adsorption of HgCl2 on Au surfaces by scanning tunneling microscopy (STM). HgCl2 was found to form a stable, ordered overlayer on Au(111) surface in ambient conditions and it was imaged with molecular resolution. We suggest that the revealed structure is (7×7)R19.1°-2HgCl2. This model supposes that the HgCl2 molecule bends in the adsorption to have a 123° angle between Cl–Hg bonds. In addition, the adsorption capacity of Au(111) surface for HgCl2 was determined to be 2:7.