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Highlights

Spectral signatures of high-symmetry quantum dots

Quantum dots with at least three symmetry planes provide a very promising route for the generation of photons for quantum information applications. The great challenge to fabricate nanoscopic dots of high symmetry is complicated by the lack of characterization techniques able to resolve small symmetry breaking. Here, we present an approach for identifying and analyzing signatures of symmetry breaking in the optical spectra of quantum dots. Read more...

Single photons from a hexagonal micro-pyramid

Single photon sources are of interest for quantum information applications. Here we report on single photon emission from an InGaN quantum dot, formed on the apex of a site-controlled hexagonal GaN micro-pyramid. An approach to suppress uncorrelated emission from the pyramid base with a processed metal film is demonstrated to strongly enhance the signal-to-background ratio of the emission. Read more...

Brominated chemistry for chemical vapor deposition of electronic grade SiC

We present a direct comparison between chlorinated and brominated chemistry for chemical vapor deposition (CVD) of SiC. Addition of chlorine to the gas mixture reduces the time needed for growth of SiC layers for electronic devices by a factor ten. Addition of Cl enables formation of molecules with Si-Cl bonds which are stronger than Si-Si bonds preventing formation of silicon clusters. By using either HCl or HBr gas as additive we show that brominated chemistry leads to the same high material quality and control of material properties as chlorinated chemistry. Read more...

Coherent control of single spins in silicon carbide

Electron spins that can be prepared in arbitrary states are the basic elements for quantum spintronics, such as quantum computing and nanoscale sensing. Here we demonstrate that missing atoms in a silicon carbide crystal can host single spins that are accessible by optical spectroscopy, with long coherence times even at room temperature. These results expand the interest of silicon carbide into the areas of quantum processing and integrated spintronics. Read more...

Assessing properties of mixed crystal structures in thin films

When grown under non-equilibrium conditions, crystals or thin films of technologically important materials may contain domains with different crystal structures. Here we develop and demonstrate nondestructive structural and optical methods to determine the ratio between different crystal phases and study the free-charge carrier and vibrational properties of mixed-phase films. Our approach allows us to establish the elusive properties of cubic InN. Read more...

Cubic Silicon Carbide (3C-SiC) approaches quality of commercial hexagonal SiC

Cubic silicon carbide (3C-SiC) is an attractive material for a number of semiconductor applications. However, due to its metastable nature, it is very challenging to grow with a crystalline quality similar to the one obtained in commercially available hexagonal SiC substrates. We introduce a novel approach to grow high crystalline quality 3C-SiC in a reproducible and controllable way. Thick (up to 1 mm) 3C-SiC layers grown using our approach could be used as seeding layers in bulk growth or in homoepitaxial growth for fabricating various device structures. Read more...

Resolving the doping limitations in aluminum gallium nitride

We achieve better understanding of complex growth phenomena underlying the deposition of the ultimate wide band gap semiconductor AlN and high-Al-content AlGaN alloys; and related doping by silicon (Si). We contribute essential new knowledge in explaining the notorious sharp increase of resistivity of Si-doped high-Al-content AlGaN by corroborating aspects of material growth with electron paramagnetic resonance measurements. Read more...

Graphene on the cubic silicon carbide

We applied a high temperature process to grow graphene on cubic silicon carbide (3C-SiC). No buffer layer was observed for the graphene grown on the (001) plane. The cubic symmetry of 3C-SiC leads to a lack of spontaneous polarization, as confirmed by the mild n-doping observed in graphene grown on the (111) plane. We demonstrated from different aspects that 3C-SiC is a good substrate for growth of epitaxial graphene. The 3C-SiC samples were grown in a well-controlled process for high crystalline quality without foreign polytype inclusions. Read more...

Study of Ag-dopants on the structural and optical properties of ZnO nanorods

Zinc oxide (ZnO) is a semiconductor material promising for optoelectronics due to its efficient light emitting properties. However, the key challenge that must be solved before ZnO can be used as a LED material is to make it so-called p-type. Substitution of some Zn atoms with silver (Ag) was recently proposed as an approach for obtaining p-type ZnO. Here we investigate the structural and optical properties of ZnO nanorods doped with Ag atoms. We demonstrate that Ag promotes the generation crystal defects and significantly modifies the optical spectrum of ZnO. These results can be of high importance for further progress on p-type ZnO. Read more...

Micropyramid emits antibunched photons in the ultraviolet

Linearly polarized photons emitted one by one forms the basis for novel cryptography methods. Here we demonstrate that the photons emitted from a InGaN quantum dot grown on the apex of a GaN micropyramid exhibits single photon characteristics known as antibunching. Our experiments show that the quantum dot itself is a fast and close to perfect single photon emitter, but a superimposed background signal from the pyramid needs to be eliminated before utilization in polarization-based single photon applications. Read more...

High thermal stability quasi-free-standing graphene on silicon carbide
through Platinum functionalization

Graphene grown on silicon carbide (SiC) provide solutions for high frequency electronics operating at high temperature. However, a major obstacle is that the electrons are substantially slowed down due to the first carbon layer formed on the SiC. Here we report on quasi-free-standing graphene layers with potentially fast electrons even at very high temperatures (1200°C), achieved by letting Platinum penetrate into the graphene-SiC interface. Read more...

Resonant ionization of shallow donors in electric field

Semiconductors are non-conductive at low temperatures because the electrons freeze in their lowest energy state at impurities. However, a sudden rise of the conductivity can occur at very high electric fields due to resonant ionization. We discuss this effect using a simplified model of the energy states of donor impurities, and our results are similar to the predictions of more advanced models and in very good agreement with our experiment. Read more...

Layer-number determination in graphene on SiC by reflectance mapping

Graphene attracts much attention due to its exceptional properties for future electronics. Growth of graphene on silicon carbide is promising for large-scale device-ready production. A significant parameter characterizing the quality of the grown material is the number of layers. Here we report a simple, handy and affordable optical approach for precise number-of-layers determination of graphene based on the reflected power of a laser beam. Read more...

Nano-resolution reveals the true nature of graphene

Graphene grown on the basal planes of silicon carbide is considered a most promising route for carbon-based nano-electronics. Two nonequivalent faces of silicon carbide can be used for this purpose, the carbon-face and the silicon-face. It was claimed that these two faces result in graphene with fundamentally different electronic properties. Here we reveal the actual similarity between graphene layers on the two faces by experiments on a nanometer scale. Moreover, the apparent difference previously seen in standard experiments can now be explained as the collective effect of microscopic grains of graphene formed on the carbon-face. Read more...

Plasma chemistry gets a boost from pulsed power

In the plasma state, free electrons and ions open up new low temperature reaction pathways, enabling thin film deposition on sensitive materials such as plastics. We recently presented the concept of high power pulsed plasma enhanced chemical vapor deposition (HiPP-PECVD), which use plasmas thousands of times more electron rich than conventional PECVD. By using carbon films as model system, we now show that more film is deposited from the same amount of acetylene and power when the power is delivered as high power pulses. This is attributed to a more efficient plasma chemistry due to the increased number of electrons. Read more...

Aluminum tunes the electronic properties of graphene on silicon carbide

Graphene has unique and superior electronic properties. It can be grown on silicon carbide by a graphitization process, enabling development of single layer graphene based electronics for high temperatures and high voltages. Aluminum is a commonly used contact material for electronics, but its stability on graphene has not been investigated. We show that a distinct change in the electronic properties of graphene occurs for temperatures above 350 °C, when aluminum penetrates into the graphene-silicon carbide interface. Our results evidence the importance of temperature of the graphene device, when using aluminum as contact material. Read more...

Understanding magnesium impurities in gallium nitride

Light-emitting diodes and laser diodes rely on positive charge carriers known as holes. These holes are provided by impurities, and for gallium nitride is magnesium the only impurity that is useful for this purpose. The exact properties of the magnesium impurity have long been controversial, partly due to instability of its spectral features. We are able to spectrally identify isolated magnesium impurities as well as impurities near faults in the stacking sequence of atomic layers in the gallium nitride crystal. These experimental results are in conflict with recent theoretical predictions of magnesium in gallium nitride. Read more...

Micro-pyramids offer route to polarization controlled photon emitters

We have developed a concept for emission of strongly polarized light with good control of the linear polarization direction. The light is generated by quantum dots formed on top of elongated hexagonal micro-pyramids. Our results could find use in applications such as energy-efficient backlighting of displays and polarized single-photon sources for quantum communication. Read more…

World’s first isotope enriched silicon carbide
for energy efficient power electronics

Silicon carbide (SiC) is a material for energy efficient power electronics. Some of its properties - such as the thermal conductivity - are negatively affected by the natural occurrence of different isotopes of silicon and carbon. We have, for the first time, produced isotope-enriched SiC using only one isotope of each element. Preliminary measurements show an increase of at least 20 percent of the thermal conductivity, while theoreticians predict 25 percent increase. These results are highly relevant from both scientific and commercial points of view, and device manufacturers exhibit a great interest. Read more in Swedish...

Growth of high quality electronic grade silicon carbide using
methane as carbon source

Methane is hardly ever used for growing silicon carbide, but it is interesting to study to shed light on the differences in gas phase reactions using different hydrocarbons and the differences in surface reactions of the specie that reach the surface. We observe that methane plays a considerable role for the growth of silicon carbide and it has a much higher probability to stick to the surface than previously believed. We also show that high quality layers may be grown using methane if the carbon to silicon ratio is carefully tuned. Read more...

Energy relevance of semiconductor research

"The transition from silicon to silicon carbide will save significantly more energy than what all the renewable energy sources currently produce."

"For someone interested both in the environment and in technology is our area perfect..."

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Towards Terahertz technologies

Using a novel Optical Hall technique, we have been able to determine, for the first time, how fast the free electrons can travel in thin aluminum-rich AlGaN layers and how they scatter. We also studied stacks of graphene layers and could identify extremely mobile electrons in perfect and non-interacting layers. In addition, channels with slow and very slow electrons were identified, where the reduced mobility is caused by interactions among graphene layers as well as with the underlying substrate. These results are prerequisites for THz device design and operation. Read more…

Shortcomings of growth modelling of silicon carbide

Simulations provide a better understanding of the chemical vapor deposition process of silicon carbide (SiC). However, the current models have shortcomings prohibiting accurate simulations. In this work we point out that improvements are needed in both the chemical species' thermodynamic data and the gas phase reaction models, as well as in the surface reaction descriptions. For example, we show that the current surface reaction models do not provide realistic predictions, and we suggest that several hydrocarbon molecules may have higher reactivities with the SiC surface than previously accepted values. Read more...

Unpaired hole-particles probe quantum dot asymmetry

We demonstrate that two unpaired holes trapped within a nanometer sized quantum dot act as an efficient probe of the dot symmetry. Our finding enables highly accurate characterization of dot asymmetry, and it may therefore be used to locate quantum dots with the high symmetry needed to generate polarization-entangled photons required for applications within quantum cryptography and quantum computing. Read more...

First observation of the trion in a nitride-based quantum dot

Nitride-based quantum dots populated with two or four charge carriers have previously been studied. Rather surprisingly, no such quantum dots populated with odd number of charge carriers have been evidenced earlier. In our work, we investigate an indium gallium nitride quantum dot populated with three charge carriers, two electrons and one hole, forming a so-called negative trion. Our research improves the fundamental understanding of these quantum dots, which are highly potential for future applications, such as quantum cryptography. Read more…

Carbon vacancy in silicon carbide: Electron trap for lifetime control

Silicon carbide (SiC) is an attractive material for high-voltage power devices. High voltages require thick SiC layers and long electron lifetimes. In SiC is the electron lifetime limited by the Z1/Z2 center – a common defect that traps electrons. We have been able to experimentally identify the Z1/Z2 center and correlate it with carbon vacancies. Our results are further supported by calculations, which enable us to identify the Z1/Z2 center as the charged states of the carbon vacancy at two different sites in the SiC crystal. These findings demonstrate that the carbon vacancy is suitable for lifetime control. Read more…

Impact of residual carbon on electrical properties of
gallium nitride transistor structure

A very precise amount of carbon atoms should be incorporated into gallium nitride based high electron mobility transistors for optimal performance and reliability. We demonstrate that the carbon concentration effectively can be controlled by the temperature in the fabrication process, letting us reach excellent uniformity of the electrical properties with very high free electron density and high mobility. Our result is relevant for developing energy efficient high power and high frequency electronic devices.
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Defect engineering of quantum wells in a single semiconductor material

One of the most promising materials for optoelectronics is zinc oxide, with efficient blue and utraviolet light emission. A zinc oxide crystal consists of stacked layers of atoms, and two different stacking sequences known as wurtzite and zincblende can occur. Our study shows that faults in the stacking sequence in fact are minimal segments of zincblende inserted into a wurtzite crystal. A zincblende insertion traps and confines electrons, but not holes, acting as an electron quantum well. Once the insertions of zincblende are controllable, they can be used for tuning the electronic and optical properties of the material. Read more...

Low temperature buffer offers the possibility to produce
large-area gallium nitride substrates at lower cost

Gallium nitride (GaN) devices are often grown on foreign substrates such as sapphire or silicon carbide, which give rise to problems with defects and cracking. We have developed a technique to manufacture freestanding native GaN substrates. The technique uses a thin so-called buffer layer of GaN, grown on sapphire at low temperature. On this buffer layer, thick GaN is grown at standard temperature. Layers thicker than one millimeter spontaneously self-separate from the sapphire substrate. This low cost process has importance for the development of ultra-high brightness light emitting diodes. Read more...

Understanding graphene formation mechanisms on different
silicon carbide polytypes – a way to optimize graphene quality

We use a high temperature sublimation technique to grow a single layer of graphene on silicon carbide (SiC) substrates. The structural and electronic properties of the resulting graphene are dependent on the detailed characteristics of the surface on which it grows, which is different for different SiC polytypes. We have grown, for the first time, large area graphene with superior uniformity on the unconventional cubic polytype of SiC (3C). A comparison is made with results on two hexagonal polytypes. The present work contributes to understanding and quality control of graphene growth on SiC. Read more…


Responsible for this page: Fredrik Karlsson
Last updated: 11/12/15