C. Tu - Academia.edu (original) (raw)
Papers by C. Tu
California Univ San Diego Report, Nov 28, 1990
Abstract : In this contract we have investigated various approaches the InAs molar fraction in an... more Abstract : In this contract we have investigated various approaches the InAs molar fraction in an InGaAs-channel MODFET. Conventional solid-source MBE and hydride-source MBE were used. Key accomplishments included: Establishing a baseline A1GaAs/InGaAs pseudomorphic MODFET technology. Using an (InAs) 2 (GaAs) 2 superlattice grown by migration-enhanced epitaxy at 400 C as better luminescence properties and device performance than MBE-grown random-alloy In0. 4Ga0.6 As as the channel layer. Setting up a gas-source MBE system with elemental group-III and doping sources and arsine and phosphine. Establishing precisely the V/III atomic ratio on the growing surface during gas-source MBE by using the RHEED oscillation technique in the group-V-limited growth mode. Obtaining some preliminary results on modulation-doped, pseudomorphic InGaP/ InGaAs.
Physical Review Letters, 1989
We combine chemical lattice imaging with digital pattern recognition to map, at near-atomic resol... more We combine chemical lattice imaging with digital pattern recognition to map, at near-atomic resolution, the compositional change across GaAs/A1GaAs interfaces of the highest optical quality. These maps quantify the information content of each unit cell of the lattice image. Our results show that state-of-the-art GaAs/A1GaAs interfaces contain substantial atomic roughness on scales finer than suggested by optical measurements.
![Research paper thumbnail of Randgap engineering of GaInNP on GaAs[001] for electronic applications](https://attachments.academia-assets.com/91871938/thumbnails/1.jpg)
](16th IPRM. 2004 International Conference on Indium Phosphide and Related Materials, 2004.
Nitrogen incorporation dramatically reduces the Gal_Jn,P band gap. Using the red shift of the pho... more Nitrogen incorporation dramatically reduces the Gal_Jn,P band gap. Using the red shift of the photoluminescence spectrum of Gao.46Ino.54No.oosPo,~~/GaAs quantum wells, which indicates less quantum confinement, we estimate the valence band offset to be about 97% of the total band gap difference. N incorporation significantly reduces the free-electron concentration and mobility, and the free-electron concentration of G~. 4 8~0. 5 2~0 .~5 P 0. 9 9 5 decreases dramatically with high-temperature annealing (800 "C), from 4. 4~ 10'' to 8. 0~1 0 '~ cuie3. This is believed to be due to passivation of Si by N through the formation of SiN pairs. Therefore, GaInNP is more suitable to be a thin layer as the hole-blocking layer for tunnel-collector HBTs. The large valence-band discontinuity and large hole effective mass would block holes, while there would be no electron barrier at the basecol lector j uncti on.
Physical Review Letters, 1986
%e report on picosecond luminescence studies of GaAslA1GaAs quantum wells in the regime of the qu... more %e report on picosecond luminescence studies of GaAslA1GaAs quantum wells in the regime of the quantum-confined Stark effect. A drastic increase of the recombination lifetime is accompanied by a Stark shift of the photoluminescence of the lowest free exciton for electric fields perpendicular to the quantum-well layers. A consistent picture of the quantum-confined Stark effect is presented.
IEEE Photonics Technology Letters, 2000
Novel red light-emitting diodes (LEDs) based on GaN 0 011 P 0 989-GaP double-heterostructure (DH)... more Novel red light-emitting diodes (LEDs) based on GaN 0 011 P 0 989-GaP double-heterostructure (DH) directly grown on (100) GaP substrates have been fabricated for the first time. The samples were grown by gas-source molecular beam epitaxy with an RF nitrogen radical beam source to incorporate N in GaP. Compared to conventional GaAs-based AlGaInP red LED's, this novel LED structure eliminates the complicated steps of etching the light-absorption GaAs substrate and wafer-bonding to a transparent GaP substrate. Based on the uncoated devices made with the heterojunction bipolar transistor masks, the emission efficiency of the DH LEDs is 20 times stronger than that of a GaNP pn homojunction diode.
Journal of Physics: Condensed Matter, 2012
Effects of a longitudinal magnetic field on optical spin injection and detection in InAs/GaAs qua... more Effects of a longitudinal magnetic field on optical spin injection and detection in InAs/GaAs quantum dot (QD) structures are investigated by optical orientation spectroscopy. An increase in optical and spin polarization of the QDs is observed with increasing magnetic field in the range of 0-2 T, and is attributed to suppression of exciton spin depolarization within the QDs that is promoted by hyperfine interaction and anisotropic electron-hole exchange interaction. This leads to a corresponding enhancement in spin detection efficiency of the QDs by a factor of up to 2.5. At higher magnetic fields when these spin depolarization processes are quenched, electron spin polarization in anisotropic QD structures (such as double QDs that are preferably aligned along a specific crystallographic axis) still exhibits rather strong field dependence under non-resonant excitation. In contrast, such field dependence is practically absent in more "isotropic" QD structures (e.g. single QDs). We attribute the observed effect to stronger electron spin relaxation in the spin injectors (i.e. wetting layer and GaAs barriers) of the lower-symmetry QD structures, which also explains the lower spin injection efficiency observed in these structures.
Journal of Applied Physics, 2012
Physical Review B, 2006
Using microphotoluminescence ͑-PL͒, in dilute N GaAs 1−x N x alloys, we observe a PL band far abo... more Using microphotoluminescence ͑-PL͒, in dilute N GaAs 1−x N x alloys, we observe a PL band far above the bandgap E 0 with its peak energy following the so-called E + transition, but with contribution from perturbed GaAs host states in a broad spectral range ͑Ͼ100 meV͒. This finding is in sharp contrast to the general understanding that E + is associated with a well-defined conduction band level ͑either L 1c or N x ͒. Beyond this insight regarding the strong perturbation of the GaAs band structure caused by N incorporation, we demonstrate that a small amount of isoelectronic doping in conjunction with-PL allows direct observation of above-bandgap transitions that are not usually accessible by PL.
Applied Physics Letters, 2011
Applied Physics Letters, 1995
The effect of thermal annealing on the interface quality in undoped, AlAs/GaAs multiple quantum w... more The effect of thermal annealing on the interface quality in undoped, AlAs/GaAs multiple quantum well ͑MQW͒ structures grown at a low substrate temperature ͑310°C͒ by molecular beam epitaxy has been investigated using chemical lattice imaging and high resolution x-ray diffraction. The low-temperature-grown MQW is of high crystalline quality comparable to the standard-temperature-grown MQW. However, significant interface roughening and intermixing occurs at the quantum well heterointerface when the structures are annealed beyond 700°C. The effective activation energy for interdiffusion is estimated as 0.24Ϯ0.07 eV. The structural properties observed here suggest that the excess arsenic associated with the low-temperature growth substantially enhances the diffusion of column III vacancies across an interface, which leads directly to intermixing of Al and Ga.
Physical Review B, 2000
Quantum confinement in GaAs 1Ϫx N x /GaAs quantum wells (0.009ϽxϽ0.045) is studied using electror... more Quantum confinement in GaAs 1Ϫx N x /GaAs quantum wells (0.009ϽxϽ0.045) is studied using electroreflectance measurements. Formation of an impurity band due to heavy nitrogen doping and the quantum confinement of an electron belonging to such an impurity band have been demonstrated. The formation of an impurity band results in an unusual variation in the electron effective mass with nitrogen doping.
Applied Physics Letters, 2011
Recently discovered effect of spin-filtering and spin amplification in GaNAs enables us to reliab... more Recently discovered effect of spin-filtering and spin amplification in GaNAs enables us to reliably obtain detailed information on the degree of spin loss during optical spin injection across a semiconductor heterointerface at room temperature. Spin polarization of electrons injected from GaAs into GaNAs is found to be less than half of what is generated in GaNAs by optical orientation. We show that the observed reduced spin injection efficiency is not only due to spin relaxation in GaAs, but more importantly due to spin loss across the interface due to structural inversion asymmetry and probably also interfacial point defects.
Applied Physics B, 2012
Efficient upconversion of photoluminescence (PL) from donor bound excitons is revealed in bulk an... more Efficient upconversion of photoluminescence (PL) from donor bound excitons is revealed in bulk and nanorod ZnO. Based on excitation power dependent PL measurements performed with different energies of excitation photons, two-photon-absorption (TPA) and two-step TPA (TS-TPA) processes are concluded to be responsible for the upconversion. The TS-TPA process is found to occur via a defect/impurity (or defects/impurities) with an energy level (or levels) lying within 1.14-1.56 eV from one of the band edges, without involving photon recycling. One of the possible defect candidates could be V Zn. A sharp energy threshold, different from that for the corresponding one-photon absorption, is observed for the TPA process and is explained in terms of selection rules for the involved optical transitions.
The effects of diluted nitrogen impurities on the valence-and conduction-band states of GaP 1−x N... more The effects of diluted nitrogen impurities on the valence-and conduction-band states of GaP 1−x N x have been predicted and measured experimentally. The calculation uses state-of-the-art atomistic modeling: we use large supercells with screened pseudopotentials and consider several random realizations of the nitrogen configurations. These calculations agree with photoluminescence excitation ͑PLE͒ measurements performed for nitrogen concentrations x up to 0.035 and photon energies up to 1 eV above the GaP optical-absorption edge, as well as with published ellipsometry data. In particular, a predicted nitrogen-induced buildup of the L character near the valence-and conduction-band edges accounts for the surprising broad-absorption plateau observed in PLE between the X 1c and the ⌫ 1c critical points of GaP. Moreover, theory accounts quantitatively for the downward bowing of the indirect conduction-band edge and for the upward bowing of the direct transition with increasing nitrogen concentration. We review some of the controversies in the literature regarding the shifts in the conduction band with composition, and conclude that measured results at ultralow N concentration cannot be used to judge behavior at a higher concentration. In particular, we find that at the high concentrations of nitrogen studied here ͑ϳ1%͒ the conduction-band edge ͑CBE͒ is a hybridized state made from the original GaP X 1c band-edge state plus all cluster states. In this limit, the CBE plunges down in energy as the N concentration increases, in quantitative agreement with the measurements reported here. However, at ultralow nitrogen concentrations ͑Ͻ0.1% ͒, the CBE is the nearly unperturbed host X 1c , which does not sense the nitrogen cluster levels. Thus, this state does not move energetically as nitrogen is added and stays pinned in energy, in agreement with experimental results.
Advanced Energy Materials, 2015
ABSTRACT A photoanode protection strategy using a multifunctional NiOx coating is presented. The ... more ABSTRACT A photoanode protection strategy using a multifunctional NiOx coating is presented. The transparency/antireflectivity, low electrochromism, conduction of holes, corrosion protection, and active electrocatalysis for water-oxidation half-reaction are described.
Physical Review Letters, 1995
... The laser is formed by a strongly guided single-mode waveguide with a mode area of 0.02 Itm2 ... more ... The laser is formed by a strongly guided single-mode waveguide with a mode area of 0.02 Itm2 and phys-ical dimensions of 0.19 ,tm X 0.4 ,tm. ... 26, 1492 (1990); JR Snow, S.-X. Qian, and RKChang, Opt. Lett. ... Lett. 67, 437 (1991). [2] ST Ho, SL McCall, and RE Slusher, Opt. Lett. ...
IEEE Photonics Technology Letters, 2000
Low-loss InAsP-GaInP multiquantum-well electroabsorption waveguide modulators have been developed... more Low-loss InAsP-GaInP multiquantum-well electroabsorption waveguide modulators have been developed for transmitting microwaves as subcarriers over optical fibers. The fiberto-fiber insertion loss is only 5 dB at 1.32-m wavelength. The electrooptic slope efficiency of an 185-m-long 11-GHz bandwidth device is equivalent to a Mach-Zehnder modulator with a V of 2.2 V. The linearity performance was characterized for a test link without any form of amplification. A RF-to-RF link efficiency of 025.5 dB, noise figure of 27 dB and suboctave spurious-free dynamic range of 114 dB 1 Hz 4=5 have been achieved with 16-mW input optical carrier power. The measured 3-dB electrical bandwidth exceeds 20 GHz for a 90-m-long device.
California Univ San Diego Report, Nov 28, 1990
Abstract : In this contract we have investigated various approaches the InAs molar fraction in an... more Abstract : In this contract we have investigated various approaches the InAs molar fraction in an InGaAs-channel MODFET. Conventional solid-source MBE and hydride-source MBE were used. Key accomplishments included: Establishing a baseline A1GaAs/InGaAs pseudomorphic MODFET technology. Using an (InAs) 2 (GaAs) 2 superlattice grown by migration-enhanced epitaxy at 400 C as better luminescence properties and device performance than MBE-grown random-alloy In0. 4Ga0.6 As as the channel layer. Setting up a gas-source MBE system with elemental group-III and doping sources and arsine and phosphine. Establishing precisely the V/III atomic ratio on the growing surface during gas-source MBE by using the RHEED oscillation technique in the group-V-limited growth mode. Obtaining some preliminary results on modulation-doped, pseudomorphic InGaP/ InGaAs.
Physical Review Letters, 1989
We combine chemical lattice imaging with digital pattern recognition to map, at near-atomic resol... more We combine chemical lattice imaging with digital pattern recognition to map, at near-atomic resolution, the compositional change across GaAs/A1GaAs interfaces of the highest optical quality. These maps quantify the information content of each unit cell of the lattice image. Our results show that state-of-the-art GaAs/A1GaAs interfaces contain substantial atomic roughness on scales finer than suggested by optical measurements.
16th IPRM. 2004 International Conference on Indium Phosphide and Related Materials, 2004.
Nitrogen incorporation dramatically reduces the Gal_Jn,P band gap. Using the red shift of the pho... more Nitrogen incorporation dramatically reduces the Gal_Jn,P band gap. Using the red shift of the photoluminescence spectrum of Gao.46Ino.54No.oosPo,~~/GaAs quantum wells, which indicates less quantum confinement, we estimate the valence band offset to be about 97% of the total band gap difference. N incorporation significantly reduces the free-electron concentration and mobility, and the free-electron concentration of G~. 4 8~0. 5 2~0 .~5 P 0. 9 9 5 decreases dramatically with high-temperature annealing (800 "C), from 4. 4~ 10'' to 8. 0~1 0 '~ cuie3. This is believed to be due to passivation of Si by N through the formation of SiN pairs. Therefore, GaInNP is more suitable to be a thin layer as the hole-blocking layer for tunnel-collector HBTs. The large valence-band discontinuity and large hole effective mass would block holes, while there would be no electron barrier at the basecol lector j uncti on.
Physical Review Letters, 1986
%e report on picosecond luminescence studies of GaAslA1GaAs quantum wells in the regime of the qu... more %e report on picosecond luminescence studies of GaAslA1GaAs quantum wells in the regime of the quantum-confined Stark effect. A drastic increase of the recombination lifetime is accompanied by a Stark shift of the photoluminescence of the lowest free exciton for electric fields perpendicular to the quantum-well layers. A consistent picture of the quantum-confined Stark effect is presented.
IEEE Photonics Technology Letters, 2000
Novel red light-emitting diodes (LEDs) based on GaN 0 011 P 0 989-GaP double-heterostructure (DH)... more Novel red light-emitting diodes (LEDs) based on GaN 0 011 P 0 989-GaP double-heterostructure (DH) directly grown on (100) GaP substrates have been fabricated for the first time. The samples were grown by gas-source molecular beam epitaxy with an RF nitrogen radical beam source to incorporate N in GaP. Compared to conventional GaAs-based AlGaInP red LED's, this novel LED structure eliminates the complicated steps of etching the light-absorption GaAs substrate and wafer-bonding to a transparent GaP substrate. Based on the uncoated devices made with the heterojunction bipolar transistor masks, the emission efficiency of the DH LEDs is 20 times stronger than that of a GaNP pn homojunction diode.
Journal of Physics: Condensed Matter, 2012
Effects of a longitudinal magnetic field on optical spin injection and detection in InAs/GaAs qua... more Effects of a longitudinal magnetic field on optical spin injection and detection in InAs/GaAs quantum dot (QD) structures are investigated by optical orientation spectroscopy. An increase in optical and spin polarization of the QDs is observed with increasing magnetic field in the range of 0-2 T, and is attributed to suppression of exciton spin depolarization within the QDs that is promoted by hyperfine interaction and anisotropic electron-hole exchange interaction. This leads to a corresponding enhancement in spin detection efficiency of the QDs by a factor of up to 2.5. At higher magnetic fields when these spin depolarization processes are quenched, electron spin polarization in anisotropic QD structures (such as double QDs that are preferably aligned along a specific crystallographic axis) still exhibits rather strong field dependence under non-resonant excitation. In contrast, such field dependence is practically absent in more "isotropic" QD structures (e.g. single QDs). We attribute the observed effect to stronger electron spin relaxation in the spin injectors (i.e. wetting layer and GaAs barriers) of the lower-symmetry QD structures, which also explains the lower spin injection efficiency observed in these structures.
Journal of Applied Physics, 2012
Physical Review B, 2006
Using microphotoluminescence ͑-PL͒, in dilute N GaAs 1−x N x alloys, we observe a PL band far abo... more Using microphotoluminescence ͑-PL͒, in dilute N GaAs 1−x N x alloys, we observe a PL band far above the bandgap E 0 with its peak energy following the so-called E + transition, but with contribution from perturbed GaAs host states in a broad spectral range ͑Ͼ100 meV͒. This finding is in sharp contrast to the general understanding that E + is associated with a well-defined conduction band level ͑either L 1c or N x ͒. Beyond this insight regarding the strong perturbation of the GaAs band structure caused by N incorporation, we demonstrate that a small amount of isoelectronic doping in conjunction with-PL allows direct observation of above-bandgap transitions that are not usually accessible by PL.
Applied Physics Letters, 2011
Applied Physics Letters, 1995
The effect of thermal annealing on the interface quality in undoped, AlAs/GaAs multiple quantum w... more The effect of thermal annealing on the interface quality in undoped, AlAs/GaAs multiple quantum well ͑MQW͒ structures grown at a low substrate temperature ͑310°C͒ by molecular beam epitaxy has been investigated using chemical lattice imaging and high resolution x-ray diffraction. The low-temperature-grown MQW is of high crystalline quality comparable to the standard-temperature-grown MQW. However, significant interface roughening and intermixing occurs at the quantum well heterointerface when the structures are annealed beyond 700°C. The effective activation energy for interdiffusion is estimated as 0.24Ϯ0.07 eV. The structural properties observed here suggest that the excess arsenic associated with the low-temperature growth substantially enhances the diffusion of column III vacancies across an interface, which leads directly to intermixing of Al and Ga.
Physical Review B, 2000
Quantum confinement in GaAs 1Ϫx N x /GaAs quantum wells (0.009ϽxϽ0.045) is studied using electror... more Quantum confinement in GaAs 1Ϫx N x /GaAs quantum wells (0.009ϽxϽ0.045) is studied using electroreflectance measurements. Formation of an impurity band due to heavy nitrogen doping and the quantum confinement of an electron belonging to such an impurity band have been demonstrated. The formation of an impurity band results in an unusual variation in the electron effective mass with nitrogen doping.
Applied Physics Letters, 2011
Recently discovered effect of spin-filtering and spin amplification in GaNAs enables us to reliab... more Recently discovered effect of spin-filtering and spin amplification in GaNAs enables us to reliably obtain detailed information on the degree of spin loss during optical spin injection across a semiconductor heterointerface at room temperature. Spin polarization of electrons injected from GaAs into GaNAs is found to be less than half of what is generated in GaNAs by optical orientation. We show that the observed reduced spin injection efficiency is not only due to spin relaxation in GaAs, but more importantly due to spin loss across the interface due to structural inversion asymmetry and probably also interfacial point defects.
Applied Physics B, 2012
Efficient upconversion of photoluminescence (PL) from donor bound excitons is revealed in bulk an... more Efficient upconversion of photoluminescence (PL) from donor bound excitons is revealed in bulk and nanorod ZnO. Based on excitation power dependent PL measurements performed with different energies of excitation photons, two-photon-absorption (TPA) and two-step TPA (TS-TPA) processes are concluded to be responsible for the upconversion. The TS-TPA process is found to occur via a defect/impurity (or defects/impurities) with an energy level (or levels) lying within 1.14-1.56 eV from one of the band edges, without involving photon recycling. One of the possible defect candidates could be V Zn. A sharp energy threshold, different from that for the corresponding one-photon absorption, is observed for the TPA process and is explained in terms of selection rules for the involved optical transitions.
The effects of diluted nitrogen impurities on the valence-and conduction-band states of GaP 1−x N... more The effects of diluted nitrogen impurities on the valence-and conduction-band states of GaP 1−x N x have been predicted and measured experimentally. The calculation uses state-of-the-art atomistic modeling: we use large supercells with screened pseudopotentials and consider several random realizations of the nitrogen configurations. These calculations agree with photoluminescence excitation ͑PLE͒ measurements performed for nitrogen concentrations x up to 0.035 and photon energies up to 1 eV above the GaP optical-absorption edge, as well as with published ellipsometry data. In particular, a predicted nitrogen-induced buildup of the L character near the valence-and conduction-band edges accounts for the surprising broad-absorption plateau observed in PLE between the X 1c and the ⌫ 1c critical points of GaP. Moreover, theory accounts quantitatively for the downward bowing of the indirect conduction-band edge and for the upward bowing of the direct transition with increasing nitrogen concentration. We review some of the controversies in the literature regarding the shifts in the conduction band with composition, and conclude that measured results at ultralow N concentration cannot be used to judge behavior at a higher concentration. In particular, we find that at the high concentrations of nitrogen studied here ͑ϳ1%͒ the conduction-band edge ͑CBE͒ is a hybridized state made from the original GaP X 1c band-edge state plus all cluster states. In this limit, the CBE plunges down in energy as the N concentration increases, in quantitative agreement with the measurements reported here. However, at ultralow nitrogen concentrations ͑Ͻ0.1% ͒, the CBE is the nearly unperturbed host X 1c , which does not sense the nitrogen cluster levels. Thus, this state does not move energetically as nitrogen is added and stays pinned in energy, in agreement with experimental results.
Advanced Energy Materials, 2015
ABSTRACT A photoanode protection strategy using a multifunctional NiOx coating is presented. The ... more ABSTRACT A photoanode protection strategy using a multifunctional NiOx coating is presented. The transparency/antireflectivity, low electrochromism, conduction of holes, corrosion protection, and active electrocatalysis for water-oxidation half-reaction are described.
Physical Review Letters, 1995
... The laser is formed by a strongly guided single-mode waveguide with a mode area of 0.02 Itm2 ... more ... The laser is formed by a strongly guided single-mode waveguide with a mode area of 0.02 Itm2 and phys-ical dimensions of 0.19 ,tm X 0.4 ,tm. ... 26, 1492 (1990); JR Snow, S.-X. Qian, and RKChang, Opt. Lett. ... Lett. 67, 437 (1991). [2] ST Ho, SL McCall, and RE Slusher, Opt. Lett. ...
IEEE Photonics Technology Letters, 2000
Low-loss InAsP-GaInP multiquantum-well electroabsorption waveguide modulators have been developed... more Low-loss InAsP-GaInP multiquantum-well electroabsorption waveguide modulators have been developed for transmitting microwaves as subcarriers over optical fibers. The fiberto-fiber insertion loss is only 5 dB at 1.32-m wavelength. The electrooptic slope efficiency of an 185-m-long 11-GHz bandwidth device is equivalent to a Mach-Zehnder modulator with a V of 2.2 V. The linearity performance was characterized for a test link without any form of amplification. A RF-to-RF link efficiency of 025.5 dB, noise figure of 27 dB and suboctave spurious-free dynamic range of 114 dB 1 Hz 4=5 have been achieved with 16-mW input optical carrier power. The measured 3-dB electrical bandwidth exceeds 20 GHz for a 90-m-long device.