Research:

Research at the MRC spans the full range from theoretical studies to cooperative university/industry ventures in product development.

The following are brief descriptions of the current programmatic research interests.

1.  Photovoltaic Materials and Device Research (Vikram Dalal)

MRC scientists and students have established a world-class laboratory for research in photovoltaic(solar) energy conversion materials and devices. The program focuses on growth of thin film electronic materials suitable for photovoltaic conversion, and on fabricating devices in them. Current materials of interest include amorphous Si and its alloys, and CdTe.

A-Si is deposited using a remote, reactive ECR plasma deposition, and after deposition, the films are characterized for their optical and electronic properties. The reactors can be used also to make doped layers and devices in these films. The devices are measured using solar simulators and other electronic measurement techniques, such as capacitance-frequency measurements and quantum efficiency measurements. This is an active research program, supported by National Renewable Energy Laboratory and by industry. ISU is one of the leaders in the world in this field.

Another part of the a-Si program is a theoretical effort to understand the microscopic origins of defects in a-Si , using molecular dynamic simulations to simulate local Si-Si and Si-H bonds in the material, and their statistical distributions. The simulations also try to model the movement of H in these materials in response to energetic inputs, and see how these movements and bond-rearrangements affect the electronic properties.

This theoretical effort is complemented by experimental work to measure the diffusion of H and D in a-Si , a-(Si,C) and a-(Si,Ge) materials. It is known that diffusion of H plays a critical role in defect creation in these materials, and by using Secondary Ion Mass Spectroscopy, one can determine the diffusion kinetics of H and D, and correlate them with the defect structure in the material.

CdTe is an important PV material. We are researching novel device structures in this material, including its hetero-junctions with (Zn,Cd)Te. The objective is to make thin film CdTe solar cells on a thin film substrate, such as stainless steel. CdTe and ZnTe films are deposited in a high vacuum, oil-free triode sputtering system. The electronic and optical properties of the individual layers are measured and then a device is made by sequential deposition of appropriate layers.

C. Semiconductor Process research

Production cost of solar electric conversion panels is the critical element which determines their commercial success. We work with a local company, Iowa Thin Film technologies, Inc.(ITFT), which is a spin-off from ISU, to improve the processing of solar cells. ITFT deposits these cells on polyimide substrates, in continuous deposition reactors using a roll-to-roll process. ISU scientists work with ITFT to understand the plasma processes that govern deposition, and on development of in-situ sensors for process controls and reliability.

2. Electronic Devices on Plastic Substrates

New applications, such as large area conformal displays, conformal electronic devices for biological applications, driver circuits for organic LED’s etc. require that high quality electronic devices be made on insulating substrates, sometimes even on plastic substrates. MRC scientists and students have developed several new technologies for depositing both amorphous and crystalline Si devices on plastic substrates. Using a rective ECR plasma beam process, we have been able to deposit thin films of crystalline Si directly on a plastic substrate without having to do any laser recrystallization. Recently, we produced a crystalline Si solar cell on polyimide substrates with very good properties. This development is likely to revolutionize semiconductor technology by allowing for multiple-device integration on insulating layers.

In addition to crystalline Si films on plastic substrates, we have made thin film transistors(TFT) in amorphous Si on polyimide substrates. Using these TFT devices, we have been able to make memory devices, such as EEPROM , on polyimide.

An active area of research in this field is the research on appropriate gate insulators deposited at low temperatures. Several novel approaches such as reactive ECR plasma oxidation are being tried to improve the quality and stability of the gate oxides.

3.  Photonic Band Gap (PBG) Materials (Gary Tuttle)

PBG materials are periodic arrays of dielectrics that exhibit stop bands for the propagation of electromagnetic waves. Considerable theoretical modeling on such structures is carried out using techniques developed at MRC and ISU, and structures are fabricated. To date structures with band gaps in the 12 GHz, 100 GHz, 350 GHz, and 500 GHz regions have been made. The MRC was the first to design and fabricate such structures with band gaps above 100 GHz. Notch filters, antennas, directional filters, millimeter­wave devices, and metallic PGB structures are being fabricated and tested. In addition, fabrication techniques for both higher­ and lower­frequency PBG materials are being developed.

4.  Molecular­Beam Epitaxy (MBE) (Gary Tuttle)

The MBE system is used in support of the PBG effort to grow material for high­frequency electronic devices to be used in current PBG crystals and to develop new fabrication schemes for yet higher frequency PBGs. Also, MBE material is grown as part of a collaboration with Ekmel Ozbay (former MRC scientist) at Bilkent University, Turkey. The system is set up to grow Sb­ and N­based semiconductors. Structures for long­wavelength surface­normal GaSb/AlSb optoelectronic devices have been grown and tested. An optimized process for growth of(Ga,In) N has been developed. Growth of specialized A1N structures for thin film resonators is under way, which includes investigation of unusual whisker growth for AlN that occurs under certain conditions.

5. Pulsed Laser Deposition (Alan Constant and David Cann)

Deposition and properties of meta-stable thin film materials for electronic, photovoltaic and magnetic applications: Growth of meta-stable materials using Pulsed Laser Deposition (PLD), reactive sputtering and Plasma Enhanced CVD.

A. The meta-stable Ge-C system:

Germanium has several properties that make it superior to Si. It has significantly higher electron and hole mobilities than Si. It also possesses a direct transition (G_25’®G_2’) only slightly higher in energy than the indirect bandgap G®L₁. This leads to Ge having a higher optical absorption coefficient. Two limitations exist with regard to the wide spread application of Ge in opto and microelectronics:

       1) Ge has small E_g, making devices fabricated from Ge susceptible to thermal noise.

         2) Ge has a larger lattice constant than Si, making it difficult to lattice match Ge to Si.   

         The Ge:C system may offer solutions. Band calculations predict that E_g should increase significantly for C > 10% and that Si lattice matching should occur at ~15% C. Calculations also indicate that the direct (G_25’®G_2’) transition could become the lower energy and dominant transition at C ~10%. Unfortunately, Ge:C has remained relatively unstudied because C and Ge are completely immiscible at thermal equilibrium. Attempts to grow meta-stable thin films of Ge_1-xC_x have met with only limited success - typically <5 % C. The goal of this study is to demonstrate thin film Ge_1-xC_x with an E_g (possibly direct) near 1 eV and a lattice parameter that is near that of Si. Photovoltaic devices employing Ge-C active layers are also being developed.

B. Giant magnetic moment meta-stable phases in the Fe-N system.

Sporadic reports over the years have suggested that a meta-stable ordered martensitic a”-Fe₁₆N₂ phase may possess a magnetic moment M_s in excess of 150% that of Fe and larger than any theoretically predicted M_s (Slater-Pauling Model). Researchers in the field have hotly contested the existence of this phase because results have been difficult to reproduce and M_s values have been derived from mixed phase assemblages in thin films. This study is employing PLD to produce thin film Fe:N to elucidate the phase space of this system and identify possible high M_s materials.  

C. Stabilized Li sulfides and oxy-sulfides glasses for application in Li battery electrodes.

Lithium sulfide (Li₂S) glasses are a highly desirable replacement material for current solid-state ionic conductors employed in Li batteries. These materials have far higher conductivities than corresponding oxide systems but are limited by the fact that they are unstable in atmosphere. This study is attempting to stabilize sulfide films by reactively sputtering them in N₂ and O₂ to form oxy and oxy-nitride sulfides that are more thermodynamically stable in the presence of oxygen and H₂O.

D. Ultra-hard Al-Mg-B based thin films deposited by PLD

AlMgB₁₄ (or BAM) has been identified as the second hardest material next to diamond. In this study we are depositing BAM thin film glasses as ultra-hard coatings for LIGA microdevices. A concurrent study is investigating the semiconducting properties of this material.

6.  Electro­Optical Studies of Semiconductors (Donald Wolford)

A laboratory for the compete characterization of semiconductor materials and interfaces is being reassembled at the MRC from its original location at IBM. A large variety of linear and non­linear optical spectra will be taken on samples to be made by MOCVD at the MRC, under a variety of conditions, including low temperatures and high pressures.

7.  Chemical and Biological Sensors (Ruth Shinar and Joe Shinar)

Novel fluorescence-based chemical and biological sensors are being developed that are structurally integrated with an organic light emitting device (OLED) excitation source. Examples include oxygen, hydrazine, and glucose sensors as well as immunoassays. While OLEDs and fluorescent chemical sensors are not novel, their integration is. The sensing component is fabricated on one side of a glass or plastic substrate, while the light-source required to excite the fluorescence (i.e., an OLED array) is fabricated on the other side. Such integrated sensors will be compact, battery-operated, inexpensive (eventually disposable), automated, and remotely-operated. This integration addresses the need for miniaturized devices in medical and environmental testing, high-throughput drug discovery, and detection of inorganic gases, volatile organic compounds (VOCs), pathogens, warfare agents, and in-vivo biological compounds and organisms. In developing the devices, we optimize the performance of the sensing components and the OLEDs, to maximize their dynamic range, sensitivity, and stability, while minimizing power consumption. The OLEDs are fabricated by Joe Shinar’s group.

8.  Acoustic Charge Transfer Devices (Clive Woods)

Acoustic charge transfer (ACT) devices use a surface-acoustic wave (SAW) in a III-V semiconductor sample to produce periodic travelling-wave electric fields, near to the surface of the material. These fields can be used to transport injected charge along a charge confinement layer, such as a quantum well, parallel to the crystal surface. The device may be 6mm or more in length and so a time delay of the order of several microseconds is introduced because the SAW velocity is around 3000m/s. The SAW drives the electrical charge between well-separated sets of Schottky and Ohmic contacts. The troughs of the potential wave successively sample the charge injected electrically (of much lower frequency than the SAW) and pass the sampled signal to the retrieval contacts. Signal-processing and filtering functions can be obtained by using appropriate arrangements of electrodes.

9.  Photonic Crystals (Kristen Constant and Wai Leung)

A Photonic Crystal (PC) is an artificially made structural material. The unique property of such material is that it prohibits propagation of electromagnetic waves in all directions. However, with proper designs we can manipulate electromagnetic waves with photonic crystals. In turn, it leads to the fabrication of photonic devices for optical processing. To see some photomicrographs of photonic crystals, click here.

10.  Nanocrystalline Group IV Materials and Devices (Vikram Dalal and Rana Biswas)

• Ames Lab Insider press release
• A powerpoint presentation on nano-crystalline silicon

Nanocrystalline Si:H is a fascinating new electronic material. The Si grains are small, ~10-20 nm, and the grain-boundaries are coated with H and very thin a-Si:H. The H distribution in the material can be estimated using Molecular Dynamic simulations, and is shown in Fig. 1. The simulations clearly show that most of the hydrogen is at the grain boundaries, with nothing inside the nano-grain. The presence of H and a-Si:H tissue at the grain boundaries passivates these boundaries, and allows for efficient transfer of minority and majority carriers across the grain. As a result, hole lifetimes of the order of microsecond can be obtained even in this nanocrystallione material. Electron mobilities are of the order of 30 cm²/V-sec and hole mobilities ~ 10 cm²/V-sec in as-grown materials, orders of magnitude higher than in amorphous materials. The hole diffusion lengths can be in excess of 1 micrometer. See Fig. 2. The nanocrystalline Si material is fundamentally stable, as opposed to amorphous materials. These outstanding properties allow one to make useful devices like solar cells and thin film transistors on plastic substrates in this material.

In addition to Si, one can also grow nano (Si,Ge):H, nano (Si,C):H and nano-(Ge,C):H materials. This versatility allows one to visualize many different classes of electo-optic devices in this new material system.

The materials are grown using plasma enhanced CVD in a reactive plasma system. Both VHF and ECR plasmas are used. Typical Raman spectra of the materials are shown in Fig.3, and x-ray diffraction in Fig. 4. Solar cell and TFT devices are being fabricated in these material systems.

Fig. 1 Hydrogen distribution in nanocrystalline Si:H modeled using Moelecualr Dynamic simulations

Fig. 2 Diffusion length measurement in nanocrystalline Si:H solar cells

Fig. 3 Raman spectrum of nanocrystalline Si film deposited using low pressure VHF plasma

Fig. 4 x-ray diffraction spectrum of nanocrystalline Si:H deposited on stainless steel substrate, showing the <111> peak dominating. The sharp peak at 45° is due to iron from the substrate.

An on-going collaboration between MRC and Ion-Optics Inc, (Waltham, MA) was very successful in modeling the infrared emission from periodic structures of trenches in a metal covered silicon wafer produced through MEMS.. When such a patterned surface is heated it emits in a narrow band at a wavelength close to the lattice spacing. The width and strength of the emission is controlled by the two-dimensional photonic lattice.

Spectroscopic gas sensors have greater sensitivity and stability than conventional electrochemical sensors. Spectroscopic sensors of toxic gases rely on the fact that each gas has a unique absorption line in the infrared portion of the spectrum, arising from the molecular stretching or rotational modes. For example CO has a sharp absorption at 4.7 m, NO absorbs at 5.3 m whereas nerve gases and toxic serin have absorption features near 10 m. Spectroscopic infrared gas sensors offer very high sensitivity for conclusive detection of individual species since each gas has unique absorption lines in the infrared spectrum. Spectroscopic sensors are lightweight, battery-powered, low-maintenance and low-cost- essential attributes for counter-terrorism applications.

The objective of this project is to design and develop a new generation of high-sensitivity infrared sensors, whose critical component is the tunable high power narrow-band infrared source. We develop an infrared source based on the sharp absorption of surface plasmons in a two-dimensional metallic photonic lattice, residing on a semiconductor substrate. We recently found that very narrow band emitters can be achieved utilizing the narrow absorption profile of surface plasmon (SP) modes in a metal-coated periodic lattice.

Fig. 5 Measured reflectance at room temperature of three patterned metal coated silicon wafers as a function of wavelength.  The solid curves correspond to theoretical TMM calculations for both the s and p polarizations at both 0 and 15. Square lattices are utilized.

Fig. 6 Emission spectra and the corresponding blackbody spectrum at 325 C.  The out of band emission is only 10% of the blackbody emission yet in band the emission is at the theoretical limit.

11.  Physics and Chemistry of Plasma Processing of Electronic Materials and Devices (Vikram Dalal)

Plasma processing is ubiquitous in the electronics industry. Thin films of Si based materials, Si oxides and nitrides and newer materials like Germanium-Carbon and Silicon-Carbon alloys are used extensively in both electronics and optical industries. The properties of the materials depend critically upon exactly how the film was deposited, and what happened at the interfaces between different films. In this research program , we investigate systematically the growth of thin films under different, well controlled plasma conditions so that we can correlate the film properties with plasma deposition chemistry. On-line optical emission spectroscopy, in-line mass spectrometry and movable Langmuir probes are used to characterize the plasma, and to measure properties such as plasma potential, electron and ion energies, and the density of radicals actually arriving at the substrate. The plasma characterization tools are movable and can be used to diagnose ECR plasma ( which operates at very low pressures), VHF plasma and RF glow discharge plasma. A surprising discovery has been that, contrary to the usual model of growth, SiH₃ is not the predominant radical involved in the growth of Si films. Rather, SiH radical may also play a major role in this process. Fig. 1 shows that results obtained using a mass-spectrometer in a low pressure, remote ECR silane plasma. This is a significant result, because it indicates that the properties of the films may also depend strongly upon the plasma conditions. We hope to extend this research to the growth of films from specific radicals in the future.

Fig. 1. Densities of different silane radicals measured using in-line mass-spectrometry in a remote ECR plasma. Note that the density of SiH is higher than that of SiH₃ at high powers.

Mission | History | Staff & Faculty | Students | Publications | Research | Facilities/Equipment | Shuttle Schedule

Back to the MRC Homepage

Revised: 23 September 2003 by D. Schmidt