OPTOELEKTRONIK AUF SILIZIUM – EINE HERAUSFORDERUNG FÜR DIE HALBLEITERPHYSIK

Derzeit vollzieht sich, noch nahezu unbemerkt, eine Revolution auf dem Beleuchtungssektor – die Ablösung der Glühbirne durch lichtemittierende (optoelektronische) Halbleiterbauelemente, die weißen Leuchtdioden (LEDs) auf der Basis von Galliumnitrid (GaN). Galliumnitrid wird bisher entweder auf Saphir- oder auf Siliziumkarbid-Substraten hergestellt. Beide Substratarten haben jedoch einige unerwünschte physikalische Eigenschaften und sind relativ teuer. Daher ist man seit Ende der 80er Jahre auf der Suche nach alternativen Substraten wie Silizium (Si). Auf Silizium ließe sich Galliumnitrid nicht nur großflächig und kostengünstig herstellen, es böte sich auch erstmals die faszinierende Möglichkeit der optoelektronischen Integration auf Silizium. In diesem Artikel berichten wir über die Forschungsaktivitäten der Abteilung Halbleiterepitaxie am Institut für Experimentelle Physik der Otto-von-Guericke-Universität Magdeburg auf diesem Gebiet, die bereits nach kurzer Zeit zu bauelementreifen Prototypen von blauen LEDs und Transistoren auf Silizium geführt haben

Electronic excitations stabilized by a degenerate electron gas in semiconductors

Excitons in semiconductors and insulators consist of fermionic subsystems, electrons and holes, whose attractive interaction facilitates bound quasiparticles with quasi-bosonic character. In the presence of a degenerate electron gas, such excitons dissociate due to free carrier screening. Despite their absence, we found pronounced emission traces in the below-band-edge region of bulk, germanium-doped GaN up to a temperature of 100 K, mimicking sharp spectral features at high free electron concentrations (3.4E19–8.9E19 cm−3). Our interpretation of the data suggests that a degenerate, three-dimensional electron gas stabilizes a novel class of quasiparticles, which we name collexons. These many-particle complexes are formed by exchange of electrons with the Fermi gas. The potential observation of collexons and their stabilization with rising doping concentration is enabled by high crystal quality due to the almost ideal substitution of host atoms with dopants.DFG, 43659573, SFB 787: Semiconductor Nanophotonics: Materials, Models, Device

Luminescence from two-dimensional electron gases in InAlN/GaN heterostructures with different In content

The luminescence properties of InxAl1−xN/GaN heterostructures are investigated systematically as a function of the In content (x = 0.067 − 0.208). The recombination between electrons confined in the two-dimensional electron gas and free holes in the GaN template is identified and analyzed. We find a systematic shift of the recombination with increasing In content from about 80 meV to only few meV below the GaN exciton emission. These results are compared with model calculations and can be attributed to the changing band profile and originating from the polarization gradient between InAlN and GaN

Reliable GaN-Based THz Gunn Diodes With Side-Contact and Field-Plate Technologies

For the first time, Gallium Nitride(GaN)-based Gunn diodes with side-contact and fieldplate technologies were fabricated and measured with reliable characteristics. A high negative differential resistance (NDR) region was characterised for the GaN Gunn effect using side-contact technology. The I-V measurement of the THz diode showed the ohmic and the Gunn effect region with high forward current of 0.65 A and high current drop of approximately 100 mA for a small ring diode width wd of 1.5µm with 600 nm effective diode height hd at a small threshold voltage of 8.5 V. This THz diode worked stable due to good passivation as protection from electro-migration and ionisation between the electrodes as well as a better heat sink to the GaN substrate and large side-contacts. The diodes can provide for this thickness a fundamental frequency in the range of 0.3 - 0.4 THz with reliable characteristics

Reduction of on-resistance and current crowding in quasi-vertical GaN power diodes

This paper studies the key parameters affecting on-resistance and current crowding in quasi-vertical GaN power devices by experiment and simulation. The current distribution in the drift region, n⁻-GaN, was found to be mainly determined by the sheet resistance of the current spreading layer, n⁺-GaN. The actual on-resistance of the drift region significantly depends on this current distribution rather than the intrinsic resistivity of the drift layer. As a result, the total specific on-resistance of quasi-vertical diodes shows a strong correlation with the device area and sheet resistance of the current spreading layer. By reducing the sheet resistance of the current spreading layer, the specific on-resistance of quasi-vertical GaN-on-Si power diodes has been reduced from ~10 mΩ x cm² to below 1 mΩ x cm². Design space of the specific on-resistance at different breakdown voltage levels has also been revealed in optimized quasi-vertical GaN power diodes

Accurate determination of polarization fields in (0 0 0 1) c-plane InAlN/GaN heterostructures with capacitance-voltage-measurements

In this paper the internal electric fields of nearly lattice matched InAlN/GaN heterostructures were determined. Pin-diodes containing InAlN/GaN heterostructures grown on (0 0 0 1) sapphire substrates by metalorganic vapour phase epitaxy were fabricated by standard lithography and metallization techniques. To determine the polarization fields in the InAlN quantum wells capacitance-voltage-measurements were performed on the pin-diodes. To reduce the measurement error, the heterostructure thicknesses were accurately determined by transmission electron microscopy. Large polarization fields, which correspond mainly to the spontaneous polarizations, for In0.15Al0.85N (5.9 +/- 0.8 MV cm(-1)), In0.18Al0.84N (5.4 +/- 0.9 MV cm(-1)) and In0.21Al0.79N (5.1 +/- 0.8 MV cm(-1)) quantum wells were observed. The results of the internal field strength and field direction are in excellent agreement with values predicted by theory and a CVM-based coupled Poisson/carrier transport simulation approach

Termination of hollow core nanopipes in GaN by an AlN interlayer

"Nanopipes associated to screw dislocations are studied by transmission electron microscopy in Si-doped GaN films grown on silicon substrates. The observations revealed that dislocations had an empty core and that an AlN interlayer is suited to block their propagation. The termination mechanism is discussed in terms of strain and kinetic growth factors, which may affect the creation and propagation of nanopipes. According to the observations, it is proposed that either step pinning or lateral overgrowth occurring at the proximity of the defect assists in capping the nanopipe.