Klaus Thonke

The boron acceptor in diamond

To date, the only dopant available for bulk diamond with good controllability is boron, which acts as an acceptor and can be incorporated in relatively high concentrations, allowing the design of devices for electronic applications. This paper summarizes data on doping procedures and on optical and electrical properties.

Polarization fields of III-nitrides grown in different crystal orientations

This article reviews the piezoelectric properties of III-nitrides with emphasis on GaN, InN, and their ternary alloys. After a short literature survey we concentrate on semipolar and nonpolar quantum wells grown on crystal planes other than the commonly used c plane ({0001}). The electrostatic field within a quantum well causes a quantum confined Stark effect and thus lowers the radiative transition probability as well as the transition energy. The basic impact of the quantum confined Stark effect on the optical properties of GaInN/GaN quantum wells is discussed in detail. Some routes to determine the magnitude of the electric field are described, and recent results are considered. The measured values are compared to published piezoelectric tensor elements.

Growth and Characterization of AlN and AlGaN Epitaxial Films on AlN Single Crystal Substrates

AlN and AlGaN epitaxial films were deposited by metal organic chemical vapor deposition on single crystal AlN substrates processed from AlN boules grown by physical vapor transport. Structural, chemical, and optical characterization demonstrated the high crystalline quality of the films and interfaces.

Direct observation of band‐edge luminescence and alloy luminescence from ultrametastable silicon‐germanium alloy layers

Ultrametastable silicon‐germanium (Si1−xGex) layers with a Ge content x in the range from about 20% to 27% were grown by Si‐MBE at temperatures far below 550 °C (325–450 °C). The thicknesses of the layers (up to 500 nm) exceed the equilibrium thickness by a factor of up to 50. We observe in the as‐grown samples without any annealing both the excitonic Si1−xGex band‐edge luminescence and a broad alloy luminescence of unknown origin. The two peaks have an energy difference of ≊144 meV and shift linearly with the Ge content. The alloy band luminescence disappears when strain relaxation sets on upon annealing at around 600 °C.

Bright semipolar GaInN∕GaN blue light emitting diode on side facets of selectively grown GaN stripes

The authors demonstrate the fabrication and evaluation of bright semipolar GaInN∕GaN blue light emitting diodes (LEDs). The structures are realized by growing five GaInN∕GaN quantum wells on the {11¯01} side facets of selectively grown n-GaN stripes with triangular shape running along the ⟨112¯0⟩ direction covered with a Mg-doped GaN top layer. The growth was done by metal organic vapor phase epitaxy using a conventional [0001] sapphire substrate. The devices have circular mesa structures with diameters between 70 and 140μm. Continuous wave on-wafer optical output powers as high as 700μW and 3mW could be achieved under dc conditions for 20 and 110mA, respectively. The current dependent blueshift of the peak emission wavelength caused by screening effects of the piezoelectric field was only 1.5nm for currents between 1 and 50mA. This is less than half the value measured on c-plane LEDs and confirms the reduced piezoelectric field in our LED structures.

Ultraviolet-emitting ZnO nanowhiskers prepared by a vapor transport process on prestructured surfaces with self-assembled polymers

ZnO wires were grown by a vapor–liquid–solid phase transport process. Self-assembled Au nano-clusters act as a catalyst or seed for the highly oriented growth of so-called ZnO whiskers on sapphire substrates by a vapor–liquid–solid phase transport process. The ZnO nanowires were more than 500 nm high and smaller than 30 nm in diameter. Low-temperature photoluminescence measurements reveal intense and detailed ultraviolet light emission near the opitical band gap of ZnO at 3.37 eV. The ZnO nanowires show almost no broad green photoluminescence emission band related to oxygen defects and only a weak signal due to donor–acceptor pair recombination. X-ray diffraction proves that the ZnO wires were grown c-plane oriented on an a-plane sapphire substrate with high crystal quality most likely because of a kind of self-purification during the growth process.

Piezoelectric fields in GaInN∕GaN quantum wells on different crystal facets

Direction and strength of piezoelectric built-in fields of GaInN quantum wells have been experimentally determined. The quantum wells have been grown either on the conventional {0001} crystal plane of GaN or on {11¯01} facets of selectively grown GaN stripes. The emission peak position of the electric-field-dependent photoluminescence can be modeled and yields value and sign of the piezoelectric field dependent on the strain of the quantum wells. On the semipolar {11¯01} facets, the quantum wells show a much weaker field (−0.1MV∕cm) compared to quantum wells grown on polar {0001} planes (−1.9MV∕cm), consistent with theoretic predictions.

Homoepitaxial growth of GaN by metalorganic vapor phase epitaxy: A benchmark for GaN technology

Carefully optimized low-pressure metalorganic vapor phase epitaxy is used for homoepitaxial growth on distinctively pretreated GaN bulk single crystal substrates. Thereby, outstanding structural and optical qualities of the material have been achieved, exhibiting photoluminescence linewidths for bound excitons as narrow as 95 μeV. These extremely sharp lines reveal fine structures, not reported for GaN. Additionally, all three free excitons as well as their excited states are visible in low-temperature photoluminescence at 2 K. These transitions are clearly identified by reflectance measurements. X-ray diffraction analysis of these layers reveal about 20 arcsec linewidth for the (0004) reflex using CuKα radiation.

Semiconductor nanostructures defined with self-organizing polymers

We describe a technique to create very small semiconductor nanostructures, with sizes far beyond the limit of conventional optical lithography processes, by the use self-assembling diblock copolymers as nanolithographic masks. Quantum structures with very high aspect ratio of 1:10 were fabricated by dry etching. In a first step, so-called diblock copolymer micelles were generated in a toluene solution. These micelles were loaded by a noble-metal salt. After dipping a substrate into this solution, a monolayer of ordered micelles is generated, covering almost the complete surface. After treatment in a hydrogen plasma all of the organic components are removed and only crystalline metal clusters of ≈12 nm size remain. This metal cluster mask can be used directly in a chlorine dry etching process to etch cylinders in GaAs and its alloys of In and Al. It is also possible to etch through a quantum well layer underneath the surface in order to produce quantum dots. The resulting nanostructures were investigated b...

Conductivity of single ZnO nanorods after Ga implantation in a focused-ion-beam system

ZnO nanorods were implanted with Ga+ ions in a combined scanning-electron-microscope/focused-ion-beam system with doses from 1011to1017cm−2. Electrical resistance measurements performed on single ZnO nanorods yield first an increase of the resistance due to defect formation which lowers the electron mobility. Implantation doses exceeding 1015cm−2 yield a strong decrease of the resistance to values significantly below the resistance before Ga+-ion implantation. Low specific resistivities of about 3×10−3Ωcm are reached without additional annealing treatment after high-dose implantation.