research

nanowires

The progress in nanoscale growth of semiconductors has led to the first successful demonstration of nanodevices. These advances have generated an intense interest in the physical, chemical and material science communities for developing new materials, novel nanoscale characterization tools and new functionalities. Semiconductor nanowires have received increasing attention in recent years as one-dimensional structures and building blocks for nanodevices.Semiconductor nanowires with diameters ranging from a few to several hundred nanometers can be grown on a substrate using the vapor-liquid-solid (VLS) method. By modifying the growth conditions, lateral and longitudinal control over the nanowire size, composition, and doping can be achieved. Depending on the geometry, these nanowires may exhibit quantum confinement, light emission and lasing. Optimized light-emitting semiconductor nanowires are interesting for the development of light-emitting diodes (LEDs) provided they can be electrically excited. Due to the low dimensionality of nanowires, the out-coupling of light generated in nanowire based LEDs could be very efficient. Given their large surface-to-volume area, semiconductor nanowires can be used as sensitive optical (bio-)sensors.

In spite of the promising perspectives of nanowires for technological applications, little is known so far about the characteristics of light emitted by nanowires and its propagation through ensembles of nanowires. Semiconductor nanowires are structures with a large geometrical anisotropy, which influences strongly their optical properties. A deep understanding of the optical anisotropy of nanowires and of their emission characteristics is necessary for the development of nanowire based optical devices with an optimum performance.

In the group nanowire photonics we study the emission and scattering charectistics of indivual semiconductor nanowires and of ensembles of nanowires with different parameters (wire radius, length, semiconductor filling fraction). These investigations should lead to single nanowires with larger internal emission efficiency and to ensembles of nanowires with larger external efficiency and with controllable directionality and polarization of the emission. The large surface to volume ratio of nanowires and the strong sensitivity of their optical properties to the surroundings encourages the use of nanowires as sensors. Based on our investigations of the emission and scattering characteristics of nanowires, we define new concepts for sensitive optical (bio-)sensing using nanowires.

The nanowires that we investigate are grown at Philips Research by the group headed by dr. E.P.A.M. Bakkers using the vapor-liquid-solid (VLS) growth method in a metal-organic vapor phase epitaxy reactor. To this end colloidal gold particles (20-100 nm) are deposited on a substrate that is heated under a small partial pressure of metal-organic semiconductor precursor material. The semiconductor vapor diffuses over the substrate and is preferentially absorbed into the gold catalyst particles forming a gold-semiconductor eutectic. Upon saturation the semiconductor material is deposited epitaxially on the substrate forming a column or nanowire beneath the gold particle. With an increase in temperature and vapor pressure a second (lateral) growth step can be accomplished, where semiconductor material is then deposited on the walls of the wire.

We investigate three closely related optical properties of individual nanowires and of ensembles of nanowires: i) birefringence, ii) antireflection, and iii) anisotropic scattering.

1. Birefringence. Ensembles of nanowires with large volume fraction exhibit a large form birefringence, which may be even larger that the natural birefringence of materials such as quartz and calcite. Light propagating through the ensemble will “see” a different effective refractive index depending on its polarization state. We have recently demonstrated giant form birefringence in samples of nanowires by measuring the transmission contrast of light with different polarization propagating through dense samples of nanowires. Highly dense ensembles of GaP nanowires epitaxially grown on a GaP substrate exhibit a birefringence parameter (refractive index along the wire axis direction-refractive index along the perpendicular direction) up to 0.8. This is the strongest birefringent material to date.

2. Antireflection. Light travels as a straight beam in homogeneous media. The direction of the light beam changes when it arrives at an interface between two different media. A fraction of light is reflected into the first medium and the rest is refracted into the second medium. At large angles of incidence at an interface between air and a solid material such as a semiconductor, nearly 100 % of the light is reflected. Interfaces which reduce this reflection exist in nature. For example, the eyes of a moth are covered with tapered nanostructures, which increase the eye sight of the moth in the dark by allowing more light to enter the eyes. Inspired by these biostructures, we have developed a method which drastically reduces the reflection between air and a semiconducting substrate. This method consists of the growth of nanowires with different lengths or with the same length, but conically shaped. Using this method, a gradual change from air to semiconductor is achieved, which leads to an efficient coupling of light into the semiconductor and minimizes the reflection. These layers show a large reduction of the reflection over a broad range of colors and angles of incidence. The reduction of the reflection is of importance for different applications. A low reflection can not only increase the sensitivity of a light detector, it can also increase the efficiency of solar cells and LEDs.

3. Anisotropic scattering. Light emitted by a nanowire in an ensemble will be unavoidable scattered by other nanowires. A fraction of this light will be absorbed and lost. Due to the strong optical anisotropy, the scattering cross section of nanowires differs from light polarized along the wire axis or perpendicular to it. Scattering can be used to optimize some characteristics of the emitted light. Therefore, we investigate the scattering of the emitted light of nanowires in ensembles and the factors that determine the anisotropy of the scattering.