Louis Guido - Academia.edu (original) (raw)

Papers by Louis Guido

2001 Annual Conference Proceedings

A group of ten faculty members from four departments and two colleges are developing an interdisc... more A group of ten faculty members from four departments and two colleges are developing an interdisciplinary undergraduate and graduate curriculum in the area of microelectronics, optoelectronics, and nanotechnology. Our goal is to introduce the concepts of microchip fabrication in the sophomore year and then develop an option or minor in microelectronics engineering, open to students from various branches of engineering and the sciences, that may be completed as part of the curriculum in each participating department. We have developed an introductory microchip fabrication laboratory in which students process 4-inch wafers to create working nMOSFETs and other devices. It is anticipated that this course will be required of all students in Computer Engineering, Electrical Engineering, Materials Science and Engineering, and Physics, and has only freshman chemistry as its prerequisite. Interested students who then select the microelectronics option then take a sequence of core courses in solid state physics and semiconductor processing technologies, and select electives from a menu including range of electronics materials processing courses, semiconductor devices, semiconductor manufacturing processes, optoelectronic devices, photonic and optical materials, and nanotechnology. We have designed our curriculum to take advantage of Virginia Tech's five-year bachelors/masters program for honors-level students and have revised our entire entry-level graduate program offerings to mesh with our undergraduate program. The advanced courses are designed such that students in the five-year option, as well as entering graduate students, can pursue courses in all areas of electronic materials and semiconductor manufacturing. We are working with several semiconductor manufacturing firms to develop a coop program that will allow students to work in state-of-the-art production facilities in partial fulfillment of the requirements of the option.

AIP Advances, 2020

Schottky diodes were formed by oxidizing Ru thin films deposited on n-type GaN at 400, 500, and 6... more Schottky diodes were formed by oxidizing Ru thin films deposited on n-type GaN at 400, 500, and 600 ○ C in normal laboratory air, and their electrical behavior was compared to that of a Ru/n-GaN reference device. The GaN epitaxial layers were grown via metalorganic chemical vapor deposition. The ruthenium films were deposited by electron beam evaporation. The Schottky barriers were characterized via current vs voltage (IV) and deep-level transient spectroscopy (DLTS) measurements between 70 and 400 K. The temperature dependent forward bias IV characteristics were fit, and the extracted temperature dependence of the effective barrier height for each device was shown to be caused by inhomogeneity at the metal/semiconductor interface. It was found that barrier inhomogeneity could be well described by a modified lognormal distribution. In reverse bias, it was shown that the low-energy tail of the barrier distribution is an important factor in determining leakage current. Favorable results occur for diodes oxidized at 400 and 500 ○ C, but raising the oxidation temperature to 600 ○ C results in a drastic increase in leakage current. DLTS measurements reveal one electron trap at EC − 0.57 eV in each of the samples. It was found that the concentration of this 0.57 eV trap increases substantially at 600 ○ C and that trap-assisted tunneling likely contributes an additional pathway for reverse leakage current.

IEEE Electron Device Letters, 2018

High-performance vertical GaN-based p-n junction diodes fabricated using bandgap selective photoe... more High-performance vertical GaN-based p-n junction diodes fabricated using bandgap selective photoelectrochemical etching-based epitaxial liftoff (ELO) from bulk GaN substrates are demonstrated. The epitaxial GaN layers and pseudomorphic InGaN release layer were grown by MOCVD on bulk GaN substrates. A comparison study was performed between devices after liftoff processing (after transfer to a Cu substrate) and nominally identical control devices on GaN substrates without the buried release layer or ELO-related processing. ELO and bonded devices exhibit nearly identical electrical performance and improved thermal performance, compared with the control devices on full-thickness GaN substrates. The breakdown voltage, ideality factor, and forward turn-ON performance were found to be nearly identical, indicating that the transfer process does not degrade the quality of the p-n junctions. The devices exhibit turn-ON voltages of 3.1 V at a current density of 100 A/cm 2 , with a specific ON-resistance (R ON) of 0.2-0.5 m • cm 2 at 5 V and a breakdown voltage (V br) of 1.3 kV. Both optical and electrical characterization techniques show that the thermal resistance of ELO devices bonded to a Cu carrier is approximately 30% lower than that for control devices on GaN substrates. Index Terms-GaN p-n junctions, epitaxial lift-off, thermal resistance. I. INTRODUCTION V ERTICAL GaN (and related III-N materials)-based devices are promising for power electronics due to both the exceptional properties of the III-N material system and the advantages of vertical device architectures [1]-[7]. However, vertical GaN-based device performance is often limited due to the use of lattice-mismatched foreign substrates, resulting in high dislocation densities as well as limited thermal conductivity for heat removal [8], [9].

Applied Physics Letters, 2017

The unique material properties of GaN and related III-N semiconductors, such as a high critical e... more The unique material properties of GaN and related III-N semiconductors, such as a high critical electrical field, large band gap, high saturation electron velocity, good electron mobility, and high thermal conductivity 1,2 , have made GaN and related materials one of the most promising material systems for high-performance optoelectronics as well as next-generation power electronics. Despite the inherent material advantages of GaN and numerous device demonstrations, the actual performance of many lateral GaN devices has fallen short of the ultimate performance expected from consideration of fundamental material parameters. These performance discrepancies, which include effects such as large ideality factors, higher-than-expected reverse saturation currents, and the inability to support avalanche currents in diodes 3 , 4 and the presence of surface-and/or buffer-related effects such as dynamic on-state resistance, current-collapse, and hysteresis in FETs, 5,6 have thus far limited the applications that can be addressed using GaN electronics. The use of lattice-mismatched non-native GaN substrates (driven by the high cost and limited availability of native GaN substrates) also results not only in large dislocation densities but also limited thermal conductance for through-substrate heat removal. 7,8 As is well known, power devices are typically thermally limited 9,10 so that the die size is set by power dissipation and thermal resistance considerations, rather than by current density limitations. Epitaxial lift-off (ELO) processing offers an alternative approach to address these issues.

Materials Science in Semiconductor Processing, 2016

Abstract GaInN/GaN solar cells made without p-type material are demonstrated using an oxidized Ni... more Abstract GaInN/GaN solar cells made without p-type material are demonstrated using an oxidized Ni/Au Schottky barrier design to collect photo-generated carriers. The best devices exhibit a short-circuit current density of 0.065 mA/cm2 with an open-circuit voltage of 0.4 V under AM0 (1-Sun) illumination. Preliminary computer simulations are in reasonable agreement with experimental results, giving a pathway to improve device performance via iterative redesign and testing.

1999 IEEE LEOS Annual Meeting Conference Proceedings. LEOS'99. 12th Annual Meeting. IEEE Lasers and Electro-Optics Society 1999 Annual Meeting (Cat. No.99CH37009)

ABSTRACT

Conference on Lasers and Electro-Optics (CLEO 2000). Technical Digest. Postconference Edition. TOPS Vol.39 (IEEE Cat. No.00CH37088), 2000

ABSTRACT

Technical Digest. Summaries of papers presented at the Quantum Electronics and Laser Science Conference. Postconference Technical Digest (IEEE Cat. No.01CH37172)

... Optics Hailin Wana, Univ. ... The microstructures have math-ematically defined shapes: r(Q) =... more ... Optics Hailin Wana, Univ. ... The microstructures have math-ematically defined shapes: r(Q) = ro[l + ~Cos(2Q)l for the quadrupole, r(Q) = ro/[ 1 +(( 1 + E ) ~ - 1) Cos(2Q)] ???* for the ellipse, and r(Q) = 1 + E(COS*Q + 3/2Cos4Q) for the quadrupole-octu-pole, with ro = 100 pm and E ...

Technical Digest. Summaries of papers presented at the Conference on Lasers and Electro-Optics. Postconference Edition. CLEO '99. Conference on Lasers and Electro-Optics (IEEE Cat. No.99CH37013)

ABSTRACT

MRS Proceedings, 1998

Arsenic-doped GaN films and GaNAs films have been synthesized by MOCVD. Samples were grown on sap... more Arsenic-doped GaN films and GaNAs films have been synthesized by MOCVD. Samples were grown on sapphire, GaN-coated sapphire, and GaAs substrates. Composition, structure, and phase distribution were characterized by EPMA, SIMS, XRD, and TEM. The arsenic content increases demonstrably as the growth temperature descreases from 1030 to 700°C. In the high temperature limit, high quality arsenic-doped GaN forms on GaN-coated sapphire. In the low temperature regime, nitrogen-rich GaNAs forms under some growth conditions, with a maximum arsenic mole fraction of 3%, and phase segregation in the form of GaAs precipitates occurs with an increase in arsine pressure, Preferential formation of the nitrogen-rich phase on GaN-coated sapphire suggests the presence of substrate-induced “composition pulling”.

We have investigated the early stages of evolution of highly strained 2-D InAs layers and 3-D InA... more We have investigated the early stages of evolution of highly strained 2-D InAs layers and 3-D InAs islands grown by metal-organic chemical vapor deposition (MOCVD) on (100) and (111)B GaAs substrates. The InAs epilayer / GaAs substrate combination has been chosen because the latticemismatch is severe (~7.2%), yet these materials are otherwise very similar. By examining InAs-on-GaAs composites instead of the more common InxGal.xAs alloy, we remove an additional degree of freedom (x) and thereby simplify data interpretation. A matrix of experiments is described in which the MOCVD growth parameters-susceptor temperature, TMIn flux, and AsH 3 flux-have been varied over a wide range. Scanning electron microscopy, atomic force microscopy, transmission electron microscopy, and electron microprobe analysis have been employed to observe the thin film surface morphology. In the case of 3-D growth, we have extracted activation energies and power-dependent exponents that characterize the nucleation process. As a consequence, optimized growth conditions have been identified for depositing 250 A thick (100) and (111)B oriented InAs layers with relatively smooth surfaces. Together with preliminary data on the strain relaxation of these layers, the above results on the evolution of thin InAs films indicate that the (111)B orientation is particularly promising for yielding lattice-mismatched films that are fully relaxed with only misfit dislocations at the epilayer / substrate interface.

MRS Proceedings, 2001

GaN and its related alloys are being widely developed for blue-ultraviolet emitting and detection... more GaN and its related alloys are being widely developed for blue-ultraviolet emitting and detection devices as well as high temperature, high power, and high frequency electronics. Despite the fast improvement in the growth of good quality GaN, a high concentration of deep level defects of yet unconfirmed origins are still found in GaN. For both optical and electronic devices, these deep carrier traps and/or recombination centers are very important and must therefore be understood. In the present work, deep level defects in GaN grown on sapphire substrates by metal organic chemical vapor deposition (MOCVD) have been investigated using Isothermal Capacitance Transient Spectroscopy (ICTS) and Current Voltage Temperature (IVT) measurements. Several deep level electron traps were characterized, obtaining the emission energy, concentration, and capture cross section from a fit of exponentials to the capacitance transients. ICTS was also used to reveal information about the capture kinetics...

Energy Harvesting and Storage: Materials, Devices, and Applications II, 2011

ABSTRACT Quantum structured solar cells seek to harness a wide spectrum of photons at high voltag... more ABSTRACT Quantum structured solar cells seek to harness a wide spectrum of photons at high voltages by embedding low energy-gap wells or dots within a high energy-gap matrix. Quantum well and quantum dot solar cells have the potential to deliver ultra-high power conversion efficiencies in single junction devices, efficiencies that in theory can approach 45% in un-concentrated sunlight over a wide range of environmental conditions. In this paper, we will briefly review the theoretical underpinnings of quantum well and quantum dot photovoltaic devices, and summarize recent experimental efforts developing quantum-structured solar cell devices. In a specific example, test devices utilizing radiation-hard, III-V nitride materials have been built using both bulk and multiple quantum well (MQW) structures. Photovoltaic devices with an InGaN MQW structure are shown to outperform devices employing a thicker, bulk InGaN layer. These results, along with the underlying theoretical foundations, suggest that quantum well and quantum dot structures can enhance the performance of photovoltaic devices for a variety of defense applications.

SPIE Proceedings, 2010

ABSTRACT Nitride semiconductors possess a number of unique material properties applicable to ener... more ABSTRACT Nitride semiconductors possess a number of unique material properties applicable to energy harvesting photovoltaic devices, including a large range of energy gaps, superior radiation resistance, and tolerance to high temperatures. We present here our experimental results related to the self-assembled InN quantum dots formed on Si substrates. We have been successful at synthesizing InN quantum dots using the metal-organic chemical vapor deposition (MOCVD) process. We demonstrate the synthesis of a high density of InN dots exhibiting excellent structural and optical properties. An unprecedented range of absorption energies, ranging from the infrared to the ultraviolet, can be obtained by embedding InN-based quantum dots in a wide band gap GaN barrier. The combination of energy-gaps accessible to III-V nitride materials may be used to reap the benefits of advance quantum dot device concepts involving hot carrier effects or multiple carrier generation processes.

Laser Resonators III, 2000

2-micrometers thick GaN microdisks with the following shapes: circular; square; and quadrupolar w... more 2-micrometers thick GaN microdisks with the following shapes: circular; square; and quadrupolar with various deformation amplitudes have been investigated by optical pumping. For circular microdisks, the minimum laser threshold was found when the pump is a ring-shaped 355 nm laser beam. An imaging technique is used to photograph (with a CCD camera) the sidewall of the microdisk at various angles in the horizontal plane. From the imaging results as a function of observation angle, it is possible to extract information about the laser output location along the sidewall and its far-field angular pattern. Image results for the quadrupoles (with optical pumping of the top face) suggest that the directionality of laser emission is associated with chaotic- orbits that emerge just outside the highest curvature edges. With focused pumping of the same quadrupole structures at the middle of the top face, only Fabry-Perot modes involving the two lowest curvature interfaces are observed.

2014 IEEE 40th Photovoltaic Specialist Conference (PVSC), 2014

Schottky barrier solar cells made from two different GaInN/GaN material structures combined with ... more Schottky barrier solar cells made from two different GaInN/GaN material structures combined with Ni/Au transparent conducting oxide films are demonstrated herein. The GaInN/GaN multiple quantum well structure has a short-circuit current density of 0.062 mA/cm2, open-circuit voltage of 0.468 V, and fill-factor of 69.8%. The GaInN/GaN double-heterostructure exhibits a 51% reduction in short-circuit current density, 47% lower open-circuit voltage, and 27% smaller fill-factor. Preliminary computer simulations indicate that a 10-fold increase in short-circuit current density should be possible for the GaInN/GaN multiple quantum well structure. The Ni/Au layer is responsible for some of this shortfall as its optical transparency varies from a low of 46.4% at 300 nm to a high of 76.8% at 500 nm. There is also evidence that photo-generated carriers are not being collected from the entire illuminated device area. The Ni/Au bi-layer has an electrical resistivity of 2.9 × 10-5 Ωcm, but it is very thin and no effort has been made to trade-off its electrical behavior against its optical properties. Work is now underway to increase the open-circuit voltage of these devices by adopting “barrier height enhancement” schemes.

Solar Energy Materials and Solar Cells, 1995

A simple graphical method is used to establish that the (AlxGa 1_x)0.65In0.35As semiconductor all... more A simple graphical method is used to establish that the (AlxGa 1_x)0.65In0.35As semiconductor alloy provides the range of energy-band gaps required to both maximize power conversion efficiency and achieve current-matching for two-terminal, multijunction solar cells. Within this framework, the development needs of a three-junction, monolithic solar cell with lattice-matched subceils and a strain-relieved Gaylnl_yAs/GaAs pseudo-substrate are discussed. The theoretical limiting efficiency of the proposed design is approximately 47.2% at 1 sun (AM 1.5 spectrum).

Journal of Electronic Materials, 1992

Data presented here demonstrate that strained-layer (111)B Alo.~Gao.85As-Ino.o4Gao.96As quantum w... more Data presented here demonstrate that strained-layer (111)B Alo.~Gao.85As-Ino.o4Gao.96As quantum wells exhibit unique optical properties when compared to otherwise identical (100) oriented strained layers. Photoluminescence measurements identify a strain-induced electric field of order 6.7 V~m-~ within the (lll)B quantum well that is not present for the (100) case. Photoluminescence excitation spectroscopy measurements show that the heavy-hole to light-hole energy band splitting is approximately 7 meV larger for the (lll)B structure than for the (100) structure.

2001 Annual Conference Proceedings

A group of ten faculty members from four departments and two colleges are developing an interdisc... more A group of ten faculty members from four departments and two colleges are developing an interdisciplinary undergraduate and graduate curriculum in the area of microelectronics, optoelectronics, and nanotechnology. Our goal is to introduce the concepts of microchip fabrication in the sophomore year and then develop an option or minor in microelectronics engineering, open to students from various branches of engineering and the sciences, that may be completed as part of the curriculum in each participating department. We have developed an introductory microchip fabrication laboratory in which students process 4-inch wafers to create working nMOSFETs and other devices. It is anticipated that this course will be required of all students in Computer Engineering, Electrical Engineering, Materials Science and Engineering, and Physics, and has only freshman chemistry as its prerequisite. Interested students who then select the microelectronics option then take a sequence of core courses in solid state physics and semiconductor processing technologies, and select electives from a menu including range of electronics materials processing courses, semiconductor devices, semiconductor manufacturing processes, optoelectronic devices, photonic and optical materials, and nanotechnology. We have designed our curriculum to take advantage of Virginia Tech's five-year bachelors/masters program for honors-level students and have revised our entire entry-level graduate program offerings to mesh with our undergraduate program. The advanced courses are designed such that students in the five-year option, as well as entering graduate students, can pursue courses in all areas of electronic materials and semiconductor manufacturing. We are working with several semiconductor manufacturing firms to develop a coop program that will allow students to work in state-of-the-art production facilities in partial fulfillment of the requirements of the option.

AIP Advances, 2020

Schottky diodes were formed by oxidizing Ru thin films deposited on n-type GaN at 400, 500, and 6... more Schottky diodes were formed by oxidizing Ru thin films deposited on n-type GaN at 400, 500, and 600 ○ C in normal laboratory air, and their electrical behavior was compared to that of a Ru/n-GaN reference device. The GaN epitaxial layers were grown via metalorganic chemical vapor deposition. The ruthenium films were deposited by electron beam evaporation. The Schottky barriers were characterized via current vs voltage (IV) and deep-level transient spectroscopy (DLTS) measurements between 70 and 400 K. The temperature dependent forward bias IV characteristics were fit, and the extracted temperature dependence of the effective barrier height for each device was shown to be caused by inhomogeneity at the metal/semiconductor interface. It was found that barrier inhomogeneity could be well described by a modified lognormal distribution. In reverse bias, it was shown that the low-energy tail of the barrier distribution is an important factor in determining leakage current. Favorable results occur for diodes oxidized at 400 and 500 ○ C, but raising the oxidation temperature to 600 ○ C results in a drastic increase in leakage current. DLTS measurements reveal one electron trap at EC − 0.57 eV in each of the samples. It was found that the concentration of this 0.57 eV trap increases substantially at 600 ○ C and that trap-assisted tunneling likely contributes an additional pathway for reverse leakage current.

IEEE Electron Device Letters, 2018

High-performance vertical GaN-based p-n junction diodes fabricated using bandgap selective photoe... more High-performance vertical GaN-based p-n junction diodes fabricated using bandgap selective photoelectrochemical etching-based epitaxial liftoff (ELO) from bulk GaN substrates are demonstrated. The epitaxial GaN layers and pseudomorphic InGaN release layer were grown by MOCVD on bulk GaN substrates. A comparison study was performed between devices after liftoff processing (after transfer to a Cu substrate) and nominally identical control devices on GaN substrates without the buried release layer or ELO-related processing. ELO and bonded devices exhibit nearly identical electrical performance and improved thermal performance, compared with the control devices on full-thickness GaN substrates. The breakdown voltage, ideality factor, and forward turn-ON performance were found to be nearly identical, indicating that the transfer process does not degrade the quality of the p-n junctions. The devices exhibit turn-ON voltages of 3.1 V at a current density of 100 A/cm 2 , with a specific ON-resistance (R ON) of 0.2-0.5 m • cm 2 at 5 V and a breakdown voltage (V br) of 1.3 kV. Both optical and electrical characterization techniques show that the thermal resistance of ELO devices bonded to a Cu carrier is approximately 30% lower than that for control devices on GaN substrates. Index Terms-GaN p-n junctions, epitaxial lift-off, thermal resistance. I. INTRODUCTION V ERTICAL GaN (and related III-N materials)-based devices are promising for power electronics due to both the exceptional properties of the III-N material system and the advantages of vertical device architectures [1]-[7]. However, vertical GaN-based device performance is often limited due to the use of lattice-mismatched foreign substrates, resulting in high dislocation densities as well as limited thermal conductivity for heat removal [8], [9].

Applied Physics Letters, 2017

The unique material properties of GaN and related III-N semiconductors, such as a high critical e... more The unique material properties of GaN and related III-N semiconductors, such as a high critical electrical field, large band gap, high saturation electron velocity, good electron mobility, and high thermal conductivity 1,2 , have made GaN and related materials one of the most promising material systems for high-performance optoelectronics as well as next-generation power electronics. Despite the inherent material advantages of GaN and numerous device demonstrations, the actual performance of many lateral GaN devices has fallen short of the ultimate performance expected from consideration of fundamental material parameters. These performance discrepancies, which include effects such as large ideality factors, higher-than-expected reverse saturation currents, and the inability to support avalanche currents in diodes 3 , 4 and the presence of surface-and/or buffer-related effects such as dynamic on-state resistance, current-collapse, and hysteresis in FETs, 5,6 have thus far limited the applications that can be addressed using GaN electronics. The use of lattice-mismatched non-native GaN substrates (driven by the high cost and limited availability of native GaN substrates) also results not only in large dislocation densities but also limited thermal conductance for through-substrate heat removal. 7,8 As is well known, power devices are typically thermally limited 9,10 so that the die size is set by power dissipation and thermal resistance considerations, rather than by current density limitations. Epitaxial lift-off (ELO) processing offers an alternative approach to address these issues.

Materials Science in Semiconductor Processing, 2016

Abstract GaInN/GaN solar cells made without p-type material are demonstrated using an oxidized Ni... more Abstract GaInN/GaN solar cells made without p-type material are demonstrated using an oxidized Ni/Au Schottky barrier design to collect photo-generated carriers. The best devices exhibit a short-circuit current density of 0.065 mA/cm2 with an open-circuit voltage of 0.4 V under AM0 (1-Sun) illumination. Preliminary computer simulations are in reasonable agreement with experimental results, giving a pathway to improve device performance via iterative redesign and testing.

1999 IEEE LEOS Annual Meeting Conference Proceedings. LEOS'99. 12th Annual Meeting. IEEE Lasers and Electro-Optics Society 1999 Annual Meeting (Cat. No.99CH37009)

ABSTRACT

Conference on Lasers and Electro-Optics (CLEO 2000). Technical Digest. Postconference Edition. TOPS Vol.39 (IEEE Cat. No.00CH37088), 2000

ABSTRACT

Technical Digest. Summaries of papers presented at the Quantum Electronics and Laser Science Conference. Postconference Technical Digest (IEEE Cat. No.01CH37172)

... Optics Hailin Wana, Univ. ... The microstructures have math-ematically defined shapes: r(Q) =... more ... Optics Hailin Wana, Univ. ... The microstructures have math-ematically defined shapes: r(Q) = ro[l + ~Cos(2Q)l for the quadrupole, r(Q) = ro/[ 1 +(( 1 + E ) ~ - 1) Cos(2Q)] ???* for the ellipse, and r(Q) = 1 + E(COS*Q + 3/2Cos4Q) for the quadrupole-octu-pole, with ro = 100 pm and E ...

Technical Digest. Summaries of papers presented at the Conference on Lasers and Electro-Optics. Postconference Edition. CLEO '99. Conference on Lasers and Electro-Optics (IEEE Cat. No.99CH37013)

ABSTRACT

MRS Proceedings, 1998

Arsenic-doped GaN films and GaNAs films have been synthesized by MOCVD. Samples were grown on sap... more Arsenic-doped GaN films and GaNAs films have been synthesized by MOCVD. Samples were grown on sapphire, GaN-coated sapphire, and GaAs substrates. Composition, structure, and phase distribution were characterized by EPMA, SIMS, XRD, and TEM. The arsenic content increases demonstrably as the growth temperature descreases from 1030 to 700°C. In the high temperature limit, high quality arsenic-doped GaN forms on GaN-coated sapphire. In the low temperature regime, nitrogen-rich GaNAs forms under some growth conditions, with a maximum arsenic mole fraction of 3%, and phase segregation in the form of GaAs precipitates occurs with an increase in arsine pressure, Preferential formation of the nitrogen-rich phase on GaN-coated sapphire suggests the presence of substrate-induced “composition pulling”.

We have investigated the early stages of evolution of highly strained 2-D InAs layers and 3-D InA... more We have investigated the early stages of evolution of highly strained 2-D InAs layers and 3-D InAs islands grown by metal-organic chemical vapor deposition (MOCVD) on (100) and (111)B GaAs substrates. The InAs epilayer / GaAs substrate combination has been chosen because the latticemismatch is severe (~7.2%), yet these materials are otherwise very similar. By examining InAs-on-GaAs composites instead of the more common InxGal.xAs alloy, we remove an additional degree of freedom (x) and thereby simplify data interpretation. A matrix of experiments is described in which the MOCVD growth parameters-susceptor temperature, TMIn flux, and AsH 3 flux-have been varied over a wide range. Scanning electron microscopy, atomic force microscopy, transmission electron microscopy, and electron microprobe analysis have been employed to observe the thin film surface morphology. In the case of 3-D growth, we have extracted activation energies and power-dependent exponents that characterize the nucleation process. As a consequence, optimized growth conditions have been identified for depositing 250 A thick (100) and (111)B oriented InAs layers with relatively smooth surfaces. Together with preliminary data on the strain relaxation of these layers, the above results on the evolution of thin InAs films indicate that the (111)B orientation is particularly promising for yielding lattice-mismatched films that are fully relaxed with only misfit dislocations at the epilayer / substrate interface.

MRS Proceedings, 2001

GaN and its related alloys are being widely developed for blue-ultraviolet emitting and detection... more GaN and its related alloys are being widely developed for blue-ultraviolet emitting and detection devices as well as high temperature, high power, and high frequency electronics. Despite the fast improvement in the growth of good quality GaN, a high concentration of deep level defects of yet unconfirmed origins are still found in GaN. For both optical and electronic devices, these deep carrier traps and/or recombination centers are very important and must therefore be understood. In the present work, deep level defects in GaN grown on sapphire substrates by metal organic chemical vapor deposition (MOCVD) have been investigated using Isothermal Capacitance Transient Spectroscopy (ICTS) and Current Voltage Temperature (IVT) measurements. Several deep level electron traps were characterized, obtaining the emission energy, concentration, and capture cross section from a fit of exponentials to the capacitance transients. ICTS was also used to reveal information about the capture kinetics...

Energy Harvesting and Storage: Materials, Devices, and Applications II, 2011

ABSTRACT Quantum structured solar cells seek to harness a wide spectrum of photons at high voltag... more ABSTRACT Quantum structured solar cells seek to harness a wide spectrum of photons at high voltages by embedding low energy-gap wells or dots within a high energy-gap matrix. Quantum well and quantum dot solar cells have the potential to deliver ultra-high power conversion efficiencies in single junction devices, efficiencies that in theory can approach 45% in un-concentrated sunlight over a wide range of environmental conditions. In this paper, we will briefly review the theoretical underpinnings of quantum well and quantum dot photovoltaic devices, and summarize recent experimental efforts developing quantum-structured solar cell devices. In a specific example, test devices utilizing radiation-hard, III-V nitride materials have been built using both bulk and multiple quantum well (MQW) structures. Photovoltaic devices with an InGaN MQW structure are shown to outperform devices employing a thicker, bulk InGaN layer. These results, along with the underlying theoretical foundations, suggest that quantum well and quantum dot structures can enhance the performance of photovoltaic devices for a variety of defense applications.

SPIE Proceedings, 2010

ABSTRACT Nitride semiconductors possess a number of unique material properties applicable to ener... more ABSTRACT Nitride semiconductors possess a number of unique material properties applicable to energy harvesting photovoltaic devices, including a large range of energy gaps, superior radiation resistance, and tolerance to high temperatures. We present here our experimental results related to the self-assembled InN quantum dots formed on Si substrates. We have been successful at synthesizing InN quantum dots using the metal-organic chemical vapor deposition (MOCVD) process. We demonstrate the synthesis of a high density of InN dots exhibiting excellent structural and optical properties. An unprecedented range of absorption energies, ranging from the infrared to the ultraviolet, can be obtained by embedding InN-based quantum dots in a wide band gap GaN barrier. The combination of energy-gaps accessible to III-V nitride materials may be used to reap the benefits of advance quantum dot device concepts involving hot carrier effects or multiple carrier generation processes.

Laser Resonators III, 2000

2-micrometers thick GaN microdisks with the following shapes: circular; square; and quadrupolar w... more 2-micrometers thick GaN microdisks with the following shapes: circular; square; and quadrupolar with various deformation amplitudes have been investigated by optical pumping. For circular microdisks, the minimum laser threshold was found when the pump is a ring-shaped 355 nm laser beam. An imaging technique is used to photograph (with a CCD camera) the sidewall of the microdisk at various angles in the horizontal plane. From the imaging results as a function of observation angle, it is possible to extract information about the laser output location along the sidewall and its far-field angular pattern. Image results for the quadrupoles (with optical pumping of the top face) suggest that the directionality of laser emission is associated with chaotic- orbits that emerge just outside the highest curvature edges. With focused pumping of the same quadrupole structures at the middle of the top face, only Fabry-Perot modes involving the two lowest curvature interfaces are observed.

2014 IEEE 40th Photovoltaic Specialist Conference (PVSC), 2014

Schottky barrier solar cells made from two different GaInN/GaN material structures combined with ... more Schottky barrier solar cells made from two different GaInN/GaN material structures combined with Ni/Au transparent conducting oxide films are demonstrated herein. The GaInN/GaN multiple quantum well structure has a short-circuit current density of 0.062 mA/cm2, open-circuit voltage of 0.468 V, and fill-factor of 69.8%. The GaInN/GaN double-heterostructure exhibits a 51% reduction in short-circuit current density, 47% lower open-circuit voltage, and 27% smaller fill-factor. Preliminary computer simulations indicate that a 10-fold increase in short-circuit current density should be possible for the GaInN/GaN multiple quantum well structure. The Ni/Au layer is responsible for some of this shortfall as its optical transparency varies from a low of 46.4% at 300 nm to a high of 76.8% at 500 nm. There is also evidence that photo-generated carriers are not being collected from the entire illuminated device area. The Ni/Au bi-layer has an electrical resistivity of 2.9 × 10-5 Ωcm, but it is very thin and no effort has been made to trade-off its electrical behavior against its optical properties. Work is now underway to increase the open-circuit voltage of these devices by adopting “barrier height enhancement” schemes.

Solar Energy Materials and Solar Cells, 1995

A simple graphical method is used to establish that the (AlxGa 1_x)0.65In0.35As semiconductor all... more A simple graphical method is used to establish that the (AlxGa 1_x)0.65In0.35As semiconductor alloy provides the range of energy-band gaps required to both maximize power conversion efficiency and achieve current-matching for two-terminal, multijunction solar cells. Within this framework, the development needs of a three-junction, monolithic solar cell with lattice-matched subceils and a strain-relieved Gaylnl_yAs/GaAs pseudo-substrate are discussed. The theoretical limiting efficiency of the proposed design is approximately 47.2% at 1 sun (AM 1.5 spectrum).

Journal of Electronic Materials, 1992

Data presented here demonstrate that strained-layer (111)B Alo.~Gao.85As-Ino.o4Gao.96As quantum w... more Data presented here demonstrate that strained-layer (111)B Alo.~Gao.85As-Ino.o4Gao.96As quantum wells exhibit unique optical properties when compared to otherwise identical (100) oriented strained layers. Photoluminescence measurements identify a strain-induced electric field of order 6.7 V~m-~ within the (lll)B quantum well that is not present for the (100) case. Photoluminescence excitation spectroscopy measurements show that the heavy-hole to light-hole energy band splitting is approximately 7 meV larger for the (lll)B structure than for the (100) structure.