New paper accepted for Publication in Appl. Phys. Lett.
A paper entitled "Multicolor photodetector basaed on GaAs quantum rings grown by droplet epitaxy" has been accepted for publication in Applied Physics Letters (4/9/2009). The devices were fabricted and tested by Jiang Wu and Dali Shao. The quantum rings were grown by a droplet MBE technique. An AFM image of these quantum rings is shown in Fig. 1. The photoresponse spectra were investigated as a function of temperature and some of the results are shown in Fig. 2. The quantum rings exhibit a photovoltaic effect even at room temperature rendering them useful as solar cells.
Figure 2: Photoresponse spectra of a GaAs/Al0.3Ga0.7A multiple quantum rings device measured at different temperature for a bias voltage of 1.5 V. The spectra are due to interband transitions.
Undergraduate Research: Growth of Zinc Oxide nanorods
After working for over eighteen months, Shannen Adcock was able to grow high density ZnO rods using the chemical vapor deposition technique. Shannen started building the growth system while she was a freshman in the department of Electrical Engineering. The apparatus is basically an oven with two tubes that were kept under rough vacuum. The Zn powder was placed in a crucible in the middle of the furnace and the oxygen gas was injected using a mass flow controller. The ZnO rods were grown on Si substrate. The high density of the rods is evident in the Scanning Electron Microscope (SEM) image shown in Fig. 1. The hexagonal structure of the ZnO tubes is clearly shown in Fig. 2. The average diameter of the rods is about 200 nm.
Figure 1: An SEM image showing a large number of ZnO rods. The rods appear to be uniform in diameter with a length ranging between 10 and 20 micron. The scale bar is 5 micron.
Figure 2: SEM images of ZnO tube standing vertically on the substrate.
Figure 3: Some of the ZnO rods were observed to have grown smaller diameter extensions.
Many rods were observed to grow small diameter appendages as shown in Fig. 3. The length of these extensions is 30 micron or longer. Shannen is currently working on optimizing the growth conditions to produce high density uniform rods on other substrates such as SiC and sapphire. The ultimate goal is to harvest these nanorods and embed them in conductive polymer to form hybrid organic/inorganic photovoltaic devices. Shannen and other graduate students will be working on the optical and electrical characterization of these ZnO rods by embedding them into PMMA films to determine the band gap, phonon modes, and emission properties.Omar Manasreh
14 July 2008
Zinc Oxide Microcrystals
These images are taken for ZnO microcrystals grown by using chemical vapor deposition technique. The experimental set up and growth was made by Shannen Adcock, who is a junior student in the department of Electrical Engineering. 18 June 2008.
Cubic GaN/AlN multiple quantum well photodetectors were fabricated in collaboration with University of Paderborn-Germany. May 2008.
Fabrication of Photodetectors based on both interband and intersubband transitions in InAs/GaAs multiple quantum dots. May 2008.
Intersubband transitions in Cubic GaN/AlN superlattices have been observed recently in samples obtained from Paderborn university, Germany. May 2007.
New research is being conducted on organic/inorganic composites for photovoltaic applications. The emphasis is on semiconducotr nanocrystals and nanorods for inorganic materials and on conductive conjugate polymers for organic materials. May 2007.
New Book Published
A new textbook is published: Semiconductor Heterojunctions and Nanostructures, Omar Manasreh (McGraw-Hill, New York, 2005). A textbook for graduate courses (554 pages) on Semiconductor Nanostructures as well as for courses on the subjects of Solid State Electronics and Optoelectronic Properties of Semiconductors. August 2005.