K. Saarinen

Contributions from gallium vacancies and carbon-related defects to the yellow luminescence in GaN

Carbon-doped GaN layers grown by molecular-beam epitaxy are studied with photoluminescence and positron annihilation spectroscopy. Semi-insulating layers doped with >1018 cm−3 carbon show a strong luminescence band centered at ∼2.2 eV (yellow luminescence). The absolute intensity of the 2.2 eV band is compared with the gallium vacancy concentration determined by positron annihilation spectroscopy. The results indicate that a high concentration of gallium vacancies is not necessary for yellow luminescence and that there is in fact a causal relationship between carbon and the 2.2 eV band. Markedly different deep-level ionization energies are found for the high-temperature quenching of the 2.2 eV photoluminescence in carbon-doped and reference samples. We propose that while the model of Neugebauer and Van de Walle [Appl. Phys. Lett. 69, 503 (1996)] applies for GaN of low carbon concentration, a different yellow luminescence mechanism is involved when the interstitial carbon concentration is comparable to or ...

Ga Vacancies as Dominant Intrinsic Acceptors in GaN Grown by Hydride Vapor Phase Epitaxy

Positron annihilation measurements show that negative Ga vacancies are the dominant acceptors in n-type gallium nitride grown by hydride vapor phase epitaxy. The concentration of Ga vacancies decreases, from more than 1019 to below 1016 cm−3, as the distance from the interface region increases from 1 to 300 μm. These concentrations are the same as the total acceptor densities determined in Hall experiments. The depth profile of O is similar to that of VGa, suggesting that the Ga vacancies are complexed with the oxygen impurities.

GALLIUM VACANCIES AND THE GROWTH STOICHIOMETRY OF GAN STUDIED BY POSITRON ANNIHILATION SPECTROSCOPY

We have applied positron spectroscopy to study the formation of vacancy defects in undoped n-type metal organic chemical vapor deposition grown GaN, where the stoichiometry was varied. Ga vacancies are found in all samples. Their concentration increases from 1016 to 1019 cm−3 when the V/III molar ratio increases from 1000 to 10 000. In nitrogen rich conditions Ga lattice sites are thus left empty and Ga vacancies are abundantly formed. The creation of Ga vacancies is accompanied by the decrease of free electron concentration from 1020 to 1016 cm−3, demonstrating their role as compensating centers.

Observation of defect complexes containing Ga vacancies in GaAsN

Positron annihilation spectroscopy was used to study GaAsN/GaAs epilayers. GaAsN layers were found to contain Ga vacancies in defect complexes. The density of the vacancy complexes increases rapidly to the order of 1018 cm−3 with increasing N composition and decreases after annealing at 700 °C. The anticorrelation of the vacancy concentration and the integrated photoluminescence intensity suggests that the Ga vacancy complexes act as nonradiative recombination centers.

Effect of growth polarity on vacancy defect and impurity incorporation in dislocation-free GaN

We have used positron annihilation, secondary ion mass spectrometry, and photoluminescence to study the point defects in GaN grown by hydride vapor phase epitaxy (HVPE) on GaN bulk crystals. The results show that N polar growth incorporates many more donor and acceptor type impurities and also Ga vacancies. Vacancy clusters with a positron lifetime τD=470±50ps were found near the N polar surfaces of both the HVPE GaN layers and bulk crystals.

THE INFLUENCE OF MG DOPING ON THE FORMATION OF GA VACANCIES AND NEGATIVE IONS IN GAN BULK CRYSTALS

Gallium vacancies and negative ions are observed in GaN bulk crystals by applying positron lifetime spectroscopy. The concentration of Ga vacancies decreases with increasing Mg doping, as expected from the behavior of the VGa formation energy as a function of the Fermi level. The concentration of negative ions correlates with that of Mg impurities determined by secondary ion mass spectrometry. We thus attribute the negative ions to MgGa−. The negative charge of Mg suggests that Mg doping converts n-type GaN to semi-insulating mainly due to the electrical compensation of ON+ donors by MgGa− acceptors.

Ionization levels of As vacancies in as-grown GaAs studied by positron-lifetime spectroscopy.

The properties of the native monovacancy defects are systematically investigated by positron-lifetime measurements in {ital n}-type GaAs with carrier concentrations of {ital n}=10{sup 15--}10{sup 18} cm{sup {minus}3}. The native defects present two ionization levels at {ital E}{sub {ital C}}{minus}30 meV and {ital E}{sub {ital C}}{minus}140 meV. The first corresponds to a charge transition 1{minus}{r arrow}0 and the second to 0{r arrow}1+. The transitions are attributed to ionizations of As vacancy, which may be isolated or part of a complex. In a simple identification of the defect with {ital V}{sub As}, the ionization level at {ital E}{sub {ital C}}{minus}30 meV is attributed to the transition {ital V}{sub As}{sup {minus}}{r arrow}{ital V}{sub As}{sup 0} and the ionization level at {ital E}{sub {ital C}}{minus}140 meV to the transition {ital V}{sub As}{sup 0}{r arrow}{ital V}{sub As}{sup +}. The results show further that the configuration of {ital V}{sub As}{sup {minus}} is strongly relaxed inwards compared to the structure of {ital V}{sub As}{sup 0}.

Dislocation-independent Mobility in Lattice-Mismatched Epitaxy: Application To GaN

Abstract Lattice-mismatched epitaxy produces a high concentration of dislocations ( N dis ) in the interface region, and this region is often highly conductive, due to donor ( N D ) decoration of the dislocations. Here we show that a simple postulate, N D = α ( N dis / c ), where c is the lattice constant and α a constant of order 1–2, predicts a nearly constant low-temperature mobility, independent of N dis . This prediction is experimentally verified in GaN grown on Al 2 O 3 , and is also applied to other mismatched systems.

Zinc vacancies in the heteroepitaxy of ZnO on sapphire: Influence of the substrate orientation and layer thickness

Positron annihilation spectroscopy has been used to study the vacancy-type defects produced in films grown by metalorganic chemical vapor deposition on different sapphire orientations. Zn vacancies are the defects controlling the positron annihilation spectra at room temperature. Close to the interface (<500nm) their concentration depends on the surface plane of sapphire over which the ZnO film has been grown. The Zn vacancy content in the film decreases with thickness, and above 1μm it is independent of the substrate orientation.

Chapter 5 Positron Annihilation Spectroscopy of Defects in Semiconductors

Publisher Summary This chapter discusses the positron annihilation spectroscopy of defects in semiconductors. There are many techniques to identify defects in semiconductors on an atomic scale. The role of positron annihilation is in its ability to detect vacancy-type defects. An energetic positron that has penetrated into a solid rapidly loses its energy and then lives a few hundred picoseconds in thermal equilibrium with the environment. The sensitivity of positron annihilation spectroscopy to vacancy-type defects is easy to understand. The free positron in a crystal lattice feels strong repulsion from positive ion cores. The main advantages of positron spectroscopy can be listed as follows: (1) the identification of vacancy-type defects is straightforward, (2) the technique is strongly supported by theory, because the annihilation characteristics can be calculated from first principles, and (3) positron annihilation can be applied to bulk crystals and thin layers of any electrical conduction type. The increase of the average positron lifetime under illumination indicates that some vacancies are converted to more efficient positron traps by capturing electrons.