The current interest is focused on the optoelectronic properties of III-V semiconductor nanostructures with the emphasis on new quantum structures for detectors and emitters. In particular, the research efforts are directed toward investigating quantum dots for infrared detectors, exploring the infrared applications of III-nitride quantum structures, designing multi-junction concentrators that contains diluted nitride heterojunctions for high efficiency and high performance photovoltaic devices, probing intersubband transitions for long wavelength infrared detectors and quantum cascade lasers, investigating impurities and dopant incorporation in nanostructures and heterojunctions, and exploring new quantum structures for novel devices. Various optical spectroscopy techniques are currently being used to probe interband, intersubband, intersublevel, and vibrational transitions in epitaxially grown quantum dots, wells, and heterojunctions.
Theoretical investigation of the band structure is one of our major research tasks. The construction of the band structure is one of the most fundamental properties of any FT Spectrometersemiconductor materials, heterostructures, quantum wells, or self-assembled quantum dots. It is the basis for understanding thermal, transport, optical, and electrical properties of these complex materials. Central to band gap engineering is the concept that, by spatially varying the composition and doping of the semiconductor over distances ranging from a few micrometers down to one monolayer, we tailor the electronic band structure in a nearly arbitrary and continuous way. Thus, semiconductor structures with new electronic and optical properties can be custom designed for specific applications. To implement band gap engineering of high performance electronic, optoelectronic, and photonic devices, precise control of composition, doping, and thickness of complex semiconductor heterostructure layers must be achieved by advanced thin-film growth techniques such as molecular beam epitaxy. The demand for more stringent structure requirements in high performance, high frequency devices has driven the semiconductor industry to rely on the understanding of basic properties of semiconductor materials and devices. Thus, advances in developing semiconductor quantum wells and dots are taking a central stage in our research activities.
The following synopses provide brief details on some of the current research.
Infrared Detectors based on Quantum Dots and Quantum Wells
Intersubband transitions in multiple quantum dot stacked array structures in material systems such as GaAs/GaP, InAs/InP. InAs/GaAs, InAs/AlAs, and InP/InGaP are the basis for infrared detectors. These transitions offer several advantages over bulk or quantum well devices. Our efforts are focused on the growth and doping of quantum dots, growth of multi-layers and embedded multi-layers of quantum dots, the effect of blocking barrier layer on dark current, controlling the shape and size of quantum dots, selection rules and electron-photon coupling, characterization and analysis of various quantum dots grown on different buffer layers, and device fabrication and testing. Recent investigation reveals that with the proper growth conditions, InGaAs quantum dots can be designed to cover the spectral range of 6 -14 µm.
Investigation of III-nitride Quantum Wells and Dots
III-nitride materials have attracted tremendous interest for their applications to ultraviolet, blue/green diode lasers and LEDs, high temperature electronics, high-density optical data storage, and electronics for aerospace and automobiles. While most of the applications of III-nitride materials lie in the visible and ultraviolet spectral region, there has been increasing interest in this class of materials for the infrared spectral region. This interest stems from the fact that the GaN/AlGaN system exhibits a large conduction band offset (up to 1.7 eV for AlN barrier) that allows one to optically design structures with intersubband transitions in the wavelength region spanning 0.7 – 30 µm. This research is focused on developing quantum well structures for both infrared and ultraviolet detectors. Our approach is based on the following:
• Study intersubband transitions in GaN/AlGaN and InGaN/AlGaN multiple quantum wells (MQWs).
• Study interband transitions in epitaxial thin films of GaN and AlGaN and near band edge absorption for ultraviolet (UV) detectors.
• Study dopant incorporation in various nitride semiconductors. This is very important for intersubband transitions since the quantum wells should be doped to produce two-dimensional electron gas.
• Study Schottky photodiodes, metal-semiconductor-metal photodiode, and p-n & p-i-n photodiodes using III-V nitride semiconductors.,
• Combine the MQWs stack with thin films of AlGaN to explore the possibility of obtaining IR and UV detector system that could operate under a single bias voltage.
Recent investigation is focused on the spontaneous polarization induced charge carriers in GaN/AlGaN multiple quantum wells. We were able to observe intersubband transitions in undoped GaN/AlGaN quantum wells. The significance of this observation is that detectors can now be fabricated from undoped quantum structures, which greatly simplify the growth without doping.
Development of Diluted Nitride Materials for Multijunction Solar Cells
There are two major research approaches that are currently pursued to produce PV devices with spectrophotometerhigh efficiencies:
• Polycrystalline thin-film tandem cells
• III-V multijunction concentrators
Our research efforts are directed toward exploring the III-V multijunction devices that can be integrated into high-flux systems. Thus, the objective of this research is to develop diluted nitride materials (mainly InGaAsN) with band gaps in the order of 1.0 eV that can be incorporated as a fourth junction in monolithic GaInP2/GaAs/ InGaAsN/Ge four-junction concentrator cell. One of the recent investigations was directed toward the determination of the carrier concentration in InGaAsN/GaAs single quantum wells using Raman scattering from longitudinal optical phonon – plasmon coupled mode. Raman spectra were fitted with a theoretical model based on the dielectric constant that contains the contribution from both phonon and plasmon.
Dopant Incorporation and Impurities Identification in Semiconductors Using Localized Vibrational Mode Spectroscopy
One of the most important aspects of recent investigations of heterojunctions is the ability to produce and control the n- and p- type materials. In the present research, we are investigating the identification of defects, impurities, dopants (including dopant incorporation) and irradiation induced defects (in particular proton irradiation) by using the localized vibrational modes (LVMs) spectroscopy. The LVM technique is a very powerful tool in identifying impurities and dopants in semiconductors. With this type of spectroscopy, we can provide useful information on whether the dopants are acceptors or donors by studying their LVMs properties using the high resolution optical absorption spectroscopy whether they are charged or neutral. With this method, one can identify the lattice site of the impurity atoms due to the fact that many III-V semiconductors consist of atoms that have two or more major isotopes, which give rise to fine structures in the impurity LVMs. It is also observed in III-V semiconductors that the LVMs are not affected by the charge state if the impurity is a shallow acceptor or donor. On the other hand, the LVMs are dramatically affected by the charge- state if the impurities or dopants are deep centers due to the formation of more complex defects. We are undertaking several initiatives to study various dopant incorporations in both bulk and III-V semiconductor quantum structures grown by different techniques.
Lasers and LEDs
We are currently investigating quantum cascade, edge emitting, and vertical cavity surface emitting lasers as well as light emitting diodes (LEDs) based on III-V semiconductor quantum structures. Quantum structures, which incorporate quantum dots, are investigated because they offer the following advantages:
• Reduced chirp
• Large increase in the differential gain
• Reduction of the threshold current
• Temperature insensitivity
Our recent collaboration with the Max-Born Institute (Dr. Jens Tomm), allows us to investigate the high resolution longitudinal modes in high power InGaAlAs/AlGaAs quantum well edge-emitting laser. From these longitudinal modes can experimentally determine the exact length of the device by taking the Fourier transform of the longitudinal mode spectra. The separation between the peaks in the Fourier transform spectrum is a direct measure of the device length. Our goal in this research is to produce diode lasers and detectors that both operate in the same spectral region. The investigation then will be extended to integrate the detectors and lasers on the same chip with optical interconnects.
Semiconductor Nanocrystals/Conductive Polymers Composites for Photovoltaic Device
Objective and Approach: The main objective of this research is to investigate the formation of hybrid semiconductor nanocrystals/conductive polymers composites for photovoltaic device applications. Furthermore, devices with simple structures are being fabricated and their dark and photocurrents will be measured. To accomplish this objective, we focus our investigation on the following tasks:
Investigate the suitability of various nanocrystals: CdS, CdSe, and CdTe nanocrystals are the most used semiconductor nanocrystals in composite devices at present. However, such devices are unlikely to have a commercial future due to the heavy cadmium element. For this reason, we propose to explore possibilities of using high quality III-V semiconductor nanocrystals and nanorods, mainly InP related compounds (InGaP) and oxide nanorods, such as ZnO and TiO2 . The choice of InP compounds is based on the fact that InP nanocrystals don’t contain any heavy metals. Additionally, InP is actually one of the best semiconductors in terms of bandgap matching for solar radiation as illustrated in Figure 1.
Incorporate metal (such as Au) nanoparticles or single wall carbon nanotubes (SWNT) in the composites: The SWNTs will be obtained commercially and/or from our collaborators, such as RIT, NRL, and NASA. Inclusion of SWNTs to the polymer/nanocrystal mixture can improve the electron transport problem. SWNTs are excellent electron transport materials, and their length far exceeds the longest quantum rod materials. In addition, improved electron conductivity can be achieved with much lower weight percent loading levels.
Another approach to harvesting the charge carriers is to decorate the semiconductor nanocrystals or nanorods with gold nanoparticles as illustrated in Fig. 2. The figure shows the nanocrystals or nanorods decorated by Au nanoparticles. The sketch in the upper right hand corner shows the band alignment and how the electrons are collected at the metal after being excited by an incident photon. The sketch on the lower right hand corner shows a possible process where the photo generated exciton is broken. The electrons then are collected at the cathode and the holes are collected at the anode through the conductive conjugate polymer.
The role of the metal nanocrystal is to enhance the charge separation at the nanocrystal surface. The introduction of metal nanoparticles will increase the photocurrent and photoelectrochemical reactions. The roles of metal nanoparticles and/or carbon nanotubes will be investigated as pertains to the energy conversion efficiency of the device.
The use of conjugated polymers: These polymers have been used in photovoltaic conversion since the early 1990’s. Photon absorption in the organic-based composites produces bound state excitons. Dissociation of these charge pairs can be accomplished by the potential difference across a polymer-metal junction, provided the excitons are near the interface. However, the dissociation can also be accomplished via electron accepting impurities. Thus, under illumination a preferential transfer of electrons to the acceptors leave holes to be preferentially transported through the conjugated polymer. This process is known as photoinduced charge transfer. Since the discovery of photoinduced charge transfer, a variety of acceptor materials have been introduced into donating conjugated polymers to produce photovoltaic devices. Several reaction schemes will be investigated, such as: I) Coupling of mercaptoacetic acid nanocrystals with carboxylic acid functionalized single-wall carbon nanotubes using ethylenediamine as a linker. The reaction proceeds via an EDC/s-NHS activation of the terminal carboxylic acid groups on both the nanocrystals and SWNTs. II) Coupling of nanocrystals with carboxylic acid functionalized SWNTs using aminoethanethiol (AET) as a linker. The reaction proceeds by ligand exchange with AET, followed by reacting EDC/s-NHS activated carboxylic acid groups on the SWNTs to form the amide covalent bond. III) Coupling of the nanocrystals with carbonyl chloride functionalized SWNTs using 4-aminothiophenol (4ATP) as a linker. The reaction proceeds by ligand exchange with 4ATP, followed by direct reaction between the amino groups on the quantum dot with carbonyl chloride groups on the SWNTs to form the amide covalent bond.
Metallization: The procedures for providing metallization in photovoltaic device rely on conventional processes used in the semiconductor industry. Using conventional processes will ease future technology transfer when appropriate. While the formation of metallic electrodes appears to be performed by routine methods (see Fig. 3), a low resistance metal-semiconductor contact is critical to device performance. Losses at the metallized electrodes should be minimized to take advantage of the charge separation properties of the composite material.
Uniqueness of the research: There are several reports in the literature showing that polymeric materials are used for solar cell, but with low energy conversion efficiency. There are reports on the carbon nanotubes and ZnO rods as being used as solar cells. The CdSe/polymer composites are also reported to be used as solar cells. However, our proposed research is focused on forming a composite made of conductive polymers, nanocrystals (mainly InP related materials), and nanorods/nanotubes. Forming metallization and packaging the device is one of the challenging problems we will be undertaking in this research. Furthermore, we expect to make advances in understanding the theoretical aspects of excitonic photovoltaic device, which include the exciton dissociation mechanisms, harvesting of the photo-generated charge carriers, multi-exciton generation processes, and the coupling of nanocrystals with nanotubes. Our facility contains state-of-the-art test equipment and a class 100 clean room. Finally, the organic/inorganic composites not only can be used for solar cells, but also can be used in spray painting on many military and space-based applications.
Growth of ZnO Nanorods
Zinc oxide attracted many research activities in recent years due to its direct band gap and the large exciton binding energy, which are both useful for electronic and optoelectronic applications. It has been grown by various techniques including molecular beam epitaxy, metalorganic chemical vapor depositions, and chemical vapor deposition in many shapes and sizes including thin films and nanorods. Many of the applications of ZnO materials are directed toward solar cells and photovoltaic devices, light emitting diodes and field effect transistors. The growth of Zinc oxide nanorods, by using chemical vapor deposition technique, is usually achieved by adding gold nanoparticles to the substrate, which act as a catalyst.
The growth apparatus is composed of a furnace, two quartz tubes, a mass flow controller (MFC) and a vacuum pump as shown in Fig. 4. Zinc powder was placed on a Si wafer in the center of the furnace while several silicon and sapphire substrates with gold nanoparticles were lined up in the inner quartz tube as shown in the figure. The chamber was evacuated to about one millitorr then flushed several times with nitrogen gas prior to the introduction of oxygen. The Zn powder temperature was raised to 575 oC then the oxygen was introduced through the MFC with a rate of 10 sccm. The growth time was 30 minutes and the substrates were removed after the furnace temperature was cooled to room temperature. The structural analysis was conducted using aa FEI Nova NanoLab 200 SEM and aa FEI Titan 80-300 S/TEM equipped with an image corrector. The optical absorption spectra were recorded by using a Cary 500 spectrometer in conjunction with a closed cycle refrigerator. The optical absorption spectra were obtained for the ZnO rods grown directly on sapphire. Similar results were obtained for rods removed from Si substrates and embedded into polymethyl methacrylate (PMMA) thick films.
SEM images of ZnO nanorods are shown in Fig. 5, which were grown on Si substrates with diameter gold nanoparticles. These rods are grown in a tetrahedron (tetrapod) shape as shown in the images with each branch exhibiting a hexagonal structure. This is an indication that the preferential growth is along the (0001) c-axis. Many of the tetrahedrons joined together to form a more complex structure. The end of each arm of these tetrahedrons exhibits a flat surface with the distinctive hexagonal structure. Several growth conditions were used to produce different shapes of ZnO nanorods as SEM images indicate.