Hadis Morkoç - Academia.edu (original) (raw)

Papers by Hadis Morkoç

Research paper thumbnail of Comparative study of Ga- and N-polar GaN films grown on sapphire substrates by molecular beam epitaxy

Journal of Vacuum Science & Technology B: Microelectronics and Nanometer Structures Processing, Measurement, and Phenomena, 2002

We report the surface, structural, and optical properties of typical Ga- and N-polar GaN films gr... more We report the surface, structural, and optical properties of typical Ga- and N-polar GaN films grown on sapphire substrates by molecular beam epitaxy. The Ga-polar films were grown on AlN buffer while the N-polar films were grown on GaN buffer layers. Atomic force microscopy imaging shows that the as-grown and chemically etched Ga-polar films have a flat and pitted surface while the N-polar surface is rougher with isolated columns or islands. Transmission electron microscopy demonstrates a low density of inversion domains in the Ga-polar films, while a much higher density of inversion domains was observed in the N-polar films. X-ray diffraction curves show a narrower (002) peak for Ga-polar films than that for N-polar films. On the other hand, both Ga- and N-polar films show a similar width of (104) peak. Despite their rough surfaces, high density of inversion domains, and broader (002) x-ray diffraction peaks, N-polar films with low dislocation density were demonstrated. In additio...

Research paper thumbnail of Highly selective photoenhanced wet etching of GaN for defect investigation and device fabrication

MRS Proceedings, 2000

ABSTRACTPhotoenhanced electro-chemical (PEC) wet etching has been shown to be suitable for disloc... more ABSTRACTPhotoenhanced electro-chemical (PEC) wet etching has been shown to be suitable for dislocation-density estimation in n-GaN films as well as for GaN-based device fabrication. We report on PEC etching of n-GaN samples grown by MBE and HVPE methods in unstirred aqueous KOH solution under He-Cd laser illumination. Characterization of the etched samples was carried out using atomic force microscopy (AFM) in both cross-sectional and plan-view configurations and scanning electron microscopy (SEM). At moderate illumination densities, the SEM and AFM analyses reveal sub-100 nm scale threading vertical wires on the etched surfaces. The calculated density (∼1×10 9cm−2) is in agreement with dislocation density found by transmission electron microscopy. Using cross-sectional AFM, we find that these vertical wires are ∼1[.proportional]m high and are perpendicular to the sapphire surface. Applying a higher illumination density or an external voltage, we obtain a higher etch rate with a smo...

Research paper thumbnail of Defect investigation of GaN thin films etched by photo-electrochemical and hot wet etching by atomic force and transmission electron microscopy

MRS Proceedings, 2001

ABSTRACTThe availability of reliable and quick methods to investigate defects in GaN films is of ... more ABSTRACTThe availability of reliable and quick methods to investigate defects in GaN films is of great interest. Photo-electrochemical (PEC), and hot wet etching using both H3PO4 acid and molten KOH have been used to study structural defects in GaN layers grown by hydride vapor phase epitaxy and molecular beam epitaxy. The purpose of this work is to determine whether, and under what conditions, these different methods of investigation are consistent and to get to a more accurate estimation of the defect density. As-grown and etched surfaces were investigated by atomic force microscopy (AFM), and plan-view and cross-sectional transmission electron microscopy (TEM). Free-standing whisker-like features and hexagonal etch pits were formed on the etched sample surfaces by PEC and wet etching, respectively. Using plan-view AFM, we found the density of whiskers (8x108-1×109 cm−2) to be similar to the etch pit densities when etched in both H3PO4 and molten KOH under precise etching conditio...

Research paper thumbnail of Band bending near the surface in GaN as detected by a charge sensitive probe

MRS Proceedings, 2003

ABSTRACTWe measured the absolute value of the surface band bending in GaN layers grown by molecul... more ABSTRACTWe measured the absolute value of the surface band bending in GaN layers grown by molecular beam epitaxy with a charge sensitive surface microprobe. Surface potential measurements showed an upward band bending from 0.7 to 1.4 eV in undoped and Si-doped GaN. The samples stored in dark for one week showed an increase in band bending by up to 0.1 eV. The effect of ultraviolet (UV) exposure (with a lamp or a pulsed nitrogen laser) on band bending was also studied. Typically, the surface barrier decreased by about 0.2 – 0.5 eV under UV light. The barrier was restored very slowly (by a logarithmic law) in the dark at room temperature. These and other similar phenomena are tentatively attributed to thermionic transfer of free electrons from the bulk to the surface states. Photo-induced desorption of oxygen may also play a role in the observed effects.

Research paper thumbnail of Unusual Properties of the Red and Green Luminescence Bands in Ga-rich GaN

MRS Proceedings, 2004

ABSTRACTWe studied photoluminescence (PL) from deep-level defects in GaN grown under Garich condi... more ABSTRACTWe studied photoluminescence (PL) from deep-level defects in GaN grown under Garich conditions at relatively low temperatures (700–800°C) by molecular-beam epitaxy (MBE). The dominant features of PL spectrum are red and green bands peaking respectively at ∼1.8 and ∼2.35 eV. Both PL bands decay exponentially at low temperatures (15 – 100 K) after pulsed excitation. The characteristic lifetime for the red band decreases by almost two orders of magnitude from 110 to 2 μs with increasing temperature from 15 to 100 K, while its integrated intensity after each pulse remains nearly unchanged in the same temperature range due to an increase in the peak intensity in the time-resolved PL curve. The lifetime of the green band remained unchanged in this temperature range. We suggest that these PL bands are caused by transitions between excited and ground states of some deep defects rather than transitions involving a shallow donor, conduction or valence bands.

Research paper thumbnail of Epitaxy of highly optical efficient GaN on O and Zn face ZnO

MRS Proceedings, 2003

ABSTRACTZnO is a highly efficient photon emitter, has optical and piezoelectric properties that a... more ABSTRACTZnO is a highly efficient photon emitter, has optical and piezoelectric properties that are attractive for a variety of applications. Due to its stacking order and close lattice to GaN, it is also considered as a substrate material for GaN epitaxy. In the past the poor preparation of ZnO surface has been a major handicap to GaN epitaxy. However, proper treatment we developed recently can make both faces of ZnO smooth with atomic level terraces. Epitaxy of GaN on O-face and Zn-face ZnO by reactive molecular beam epitaxy was performed. We used low-temperature RF growth of GaN buffer layer on ZnO surface to protect it from both ammonia and Ga. No Ga2ZnO4, an oxide with the spinel structure formed due to reaction of ZnO with Ga, was found, in contrast to earlier reports. The low-temperature photoluminescence (PL) indicates that both faces of ZnO can provide a high quality GaN with high radiative efficiency. In previous research it has been reported that O-face ZnO is slightly be...

Research paper thumbnail of Effects of GaN passivation with SiO2 and SiNx studied by photoluminescence and surface potential electric force microscopy

MRS Proceedings, 2005

The influence of passivation with SiO2 and SiNx on optical properties and surface band bending in... more The influence of passivation with SiO2 and SiNx on optical properties and surface band bending in unintentionally doped GaN has been studied by steady-state photoluminescence (PL) and surface potential electric force microscopy (SP-EFM). For both types of passivation we observed a significant increase of PL intensity in air ambient at room temperature. The measured surface potential was the same for control and passivated samples within the experimental error. The value of the surface band-bending was determined as 1.0±0.2 eV in all cases. We suggest that the strong enhancement of PL is caused by reduction of contribution of the surface states to recombination of photogenerated carriers after passivation.

Research paper thumbnail of Photoluminescence Study of Defects in GaN Grown by Molecular Beam Epitaxy

MRS Proceedings, 2000

ABSTRACTDefect related photoluminescence (PL) in unintentionally doped GaN layers grown by molecu... more ABSTRACTDefect related photoluminescence (PL) in unintentionally doped GaN layers grown by molecular beam epitaxy (MBE) was studied. Under certain growth conditions, we observed new defect-related bands: a red band with a maximum at about 1.88 eV and a green band with a maximum at about 2.37 eV. The quenching of these bands with increasing temperature took place with an activation energy of about 120-140 meV at temperatures above 100 K. Moreover, the red band exhibited an increase of PL intensity with an activation energy of 1.2 meV in the range of 10-60 K. The observed behavior is explained by invoking a configuration coordinate model and that we speculate the defects to be partially nonradiative and related to Ga atoms.

Research paper thumbnail of High quantum efficiency of photoluminescence in GaN and ZnO

MRS Proceedings, 2005

The quantum efficiency (QE) of photoluminescence (PL) has been estimated in GaN and ZnO samples. ... more The quantum efficiency (QE) of photoluminescence (PL) has been estimated in GaN and ZnO samples. A Si-doped GaN layer grown by molecular beam epitaxy (MBE) exhibited the highest QE of about 90% at low temperatures. Recombination via the shallow donor-acceptor pair transitions dominated in this sample. In contrast, a bulk ZnO crystal with the QE of PL of about 85% contained almost no defect- or impurity-related PL signatures besides the emission attributed to free and bound excitons. The sources of radiative and nonradiative recombination in GaN and ZnO are discussed.

Research paper thumbnail of Photoluminescence and Excitation Spectra of Deep Defects in GaN

MRS Proceedings, 2001

ABSTRACTDeep defects responsible for broad bands in the red-to-green range of the photoluminescen... more ABSTRACTDeep defects responsible for broad bands in the red-to-green range of the photoluminescence (PL) spectrum of undoped and Si-doped GaN grown by molecular beam epitaxy (MBE) and hydride vapor phase epitaxy (HVPE) were studied by employing PL and PL excitation (PLE) methods. In HVPE grown samples, a red luminescence (RL) and a green luminescence (GL) bands were observed, respectively, at about 1.9 and 2.4 eV. Similar in positions but different in properties red and green bands (RL2 and GL2, respectively) dominated in the samples grown in Ga-rich conditions by MBE with radio frequency plasma as a nitrogen source (RF-MBE). A yellow luminescence (YL) with a maximum at about 2.2 eV dominated in the samples with ammonia used as a nitrogen source (NH3-MBE). It has been established from the variation of temperature, excitation intensity and excitation wavelength that the abovementioned five bands are related to different deep-level defects.

Research paper thumbnail of GaN based quantum dot heterostructures

MRS Proceedings, 2000

ABSTRACTGaN dots have been grown on c-plane sapphire and (111) Si substrates by reactive molecula... more ABSTRACTGaN dots have been grown on c-plane sapphire and (111) Si substrates by reactive molecular beam epitaxy. A new method involving two-dimensional growth followed by a controlled annealing during which dots are formed was employed. Due the dot nature and large dot density, relatively high luminescence efficiencies were obtained on both substrates. Single layer dots were used for AFM analysis whereas 30 layer dots were used for photoluminescence experiments. AlN barrier layers, some too thick for mechanical interaction, some thin enough for vertical coupling were used. Strong polarization effects lead to a sizeable red shift, which depends on the size of the dots.

Research paper thumbnail of GaN Quantum Dots on Sapphire and Si Substrates

Materials Science Forum, 2000

Research paper thumbnail of Effect of ambient on electrical and optical properties of GaN

SPIE Proceedings, 2009

Photoluminescence (PL) and surface photovoltage (SPV) of GaN layers were studied in vacuum and ai... more Photoluminescence (PL) and surface photovoltage (SPV) of GaN layers were studied in vacuum and air ambient. SPV transients were measured with two set-ups: traditional Kelvin probe attached to an optical cryostat and atomic force microscope in contact potential mode. It is found that upward band bending in GaN decreases from its dark value of about 0.9 eV to about 0.3

Research paper thumbnail of Kelvin probe measurements of p-type GaN

SPIE Proceedings, 2011

We have studied the surface photovoltage (SPV) for band-to-band illumination on a variety of p-ty... more We have studied the surface photovoltage (SPV) for band-to-band illumination on a variety of p-type (Mg-doped) GaN samples. In particular, differences in the steady-state and transient SPV have been investigated in air and vacuum for samples grown by hydride vapor phase epitaxy (HVPE) or metal-organic chemical vapor deposition (MOCVD). The SPV spectra for both samples behave in a similar manner,

Research paper thumbnail of 4115 Terahertz radiation from magnetoresistive Pr0.7Ca0.3MnO3 thin films Noriaki Kida, Masayoshi Tonouchi 4118 Picosecond Z-scan measurements on bulk GaN crystals

Research paper thumbnail of Characterization of GaN and In x Ga 1− x N films grown by MOCVD and MBE on free-standing GaN templates and quantum well structures

Journal of Materials Science-materials in Electronics, 2003

Structural and optical studies have been performed on GaN, InGaN layers, In0.08Ga0.92N/GaN hetero... more Structural and optical studies have been performed on GaN, InGaN layers, In0.08Ga0.92N/GaN heterostructures, In0.08Ga0.92N/In0.02Ga0.98N single and multiquantum wells grown by metal organic chemical vapor deposition (MOCVD) and GaN by molecular beam epitaxy (MBE) on GaN templates by using transmission electron microscopy (TEM), X-ray diffraction (XRD), and photoluminescence (PL). The layers are found to be high quality with low defect density,

Research paper thumbnail of Unstable luminescence in GaN and ZnO

Physica B: Condensed Matter, 2006

We report observations of unstable photoluminescence (PL) in GaN and ZnO. Beginning with GaN, a b... more We report observations of unstable photoluminescence (PL) in GaN and ZnO. Beginning with GaN, a broad blue band with a maximum at 3.02 eV and well-defined fine structure at its high-energy wing bleached under ultraviolet (UV) exposure at low temperatures. The bleaching of this band was accompanied by enhancement of a broad yellow band peaking at 2.2 eV. We also observed a very similar PL evolution in undoped bulk ZnO samples, where the intensity of the green band (at 2.6 eV) significantly decreased with concomitant enhancement of the yellow band (at 2.25 eV). The transformations are explained by recombination-enhanced defect reactions in these materials.

Research paper thumbnail of Luminescence from defects in GaN

Physica B: Condensed Matter, 2006

ABSTRACT

Research paper thumbnail of Investigation of defects and surface polarity in GaN using hot wet etching together with microscopy and diffraction techniques

Materials Science and Engineering: B, 2002

Research paper thumbnail of Effect of ambient on photoluminescence from GaN grown by molecular-beam epitaxy

Journal of Electronic Materials, 2003

We report the investigation of reversible-transient effects in photoluminescence (PL) of GaN laye... more We report the investigation of reversible-transient effects in photoluminescence (PL) of GaN layers in different measurement ambients including air, oxygen, nitrogen, and hydrogen. While the presence of air and oxygen led to similar amounts of significant decrease in overall luminescence, nitrogen and hydrogen produced a much smaller effect. Data on the different behaviors of the near-band-edge and the deep-level (yellow luminescence (YL)) spectral components of the luminescence with change from vacuum to the various ambient gases will be presented. A redshift of the deep-level luminescence band was noticed in oxygen or air ambient that has been attributed to a decrease of the band bending.

Research paper thumbnail of Comparative study of Ga- and N-polar GaN films grown on sapphire substrates by molecular beam epitaxy

Journal of Vacuum Science & Technology B: Microelectronics and Nanometer Structures Processing, Measurement, and Phenomena, 2002

We report the surface, structural, and optical properties of typical Ga- and N-polar GaN films gr... more We report the surface, structural, and optical properties of typical Ga- and N-polar GaN films grown on sapphire substrates by molecular beam epitaxy. The Ga-polar films were grown on AlN buffer while the N-polar films were grown on GaN buffer layers. Atomic force microscopy imaging shows that the as-grown and chemically etched Ga-polar films have a flat and pitted surface while the N-polar surface is rougher with isolated columns or islands. Transmission electron microscopy demonstrates a low density of inversion domains in the Ga-polar films, while a much higher density of inversion domains was observed in the N-polar films. X-ray diffraction curves show a narrower (002) peak for Ga-polar films than that for N-polar films. On the other hand, both Ga- and N-polar films show a similar width of (104) peak. Despite their rough surfaces, high density of inversion domains, and broader (002) x-ray diffraction peaks, N-polar films with low dislocation density were demonstrated. In additio...

Research paper thumbnail of Highly selective photoenhanced wet etching of GaN for defect investigation and device fabrication

MRS Proceedings, 2000

ABSTRACTPhotoenhanced electro-chemical (PEC) wet etching has been shown to be suitable for disloc... more ABSTRACTPhotoenhanced electro-chemical (PEC) wet etching has been shown to be suitable for dislocation-density estimation in n-GaN films as well as for GaN-based device fabrication. We report on PEC etching of n-GaN samples grown by MBE and HVPE methods in unstirred aqueous KOH solution under He-Cd laser illumination. Characterization of the etched samples was carried out using atomic force microscopy (AFM) in both cross-sectional and plan-view configurations and scanning electron microscopy (SEM). At moderate illumination densities, the SEM and AFM analyses reveal sub-100 nm scale threading vertical wires on the etched surfaces. The calculated density (∼1×10 9cm−2) is in agreement with dislocation density found by transmission electron microscopy. Using cross-sectional AFM, we find that these vertical wires are ∼1[.proportional]m high and are perpendicular to the sapphire surface. Applying a higher illumination density or an external voltage, we obtain a higher etch rate with a smo...

Research paper thumbnail of Defect investigation of GaN thin films etched by photo-electrochemical and hot wet etching by atomic force and transmission electron microscopy

MRS Proceedings, 2001

ABSTRACTThe availability of reliable and quick methods to investigate defects in GaN films is of ... more ABSTRACTThe availability of reliable and quick methods to investigate defects in GaN films is of great interest. Photo-electrochemical (PEC), and hot wet etching using both H3PO4 acid and molten KOH have been used to study structural defects in GaN layers grown by hydride vapor phase epitaxy and molecular beam epitaxy. The purpose of this work is to determine whether, and under what conditions, these different methods of investigation are consistent and to get to a more accurate estimation of the defect density. As-grown and etched surfaces were investigated by atomic force microscopy (AFM), and plan-view and cross-sectional transmission electron microscopy (TEM). Free-standing whisker-like features and hexagonal etch pits were formed on the etched sample surfaces by PEC and wet etching, respectively. Using plan-view AFM, we found the density of whiskers (8x108-1×109 cm−2) to be similar to the etch pit densities when etched in both H3PO4 and molten KOH under precise etching conditio...

Research paper thumbnail of Band bending near the surface in GaN as detected by a charge sensitive probe

MRS Proceedings, 2003

ABSTRACTWe measured the absolute value of the surface band bending in GaN layers grown by molecul... more ABSTRACTWe measured the absolute value of the surface band bending in GaN layers grown by molecular beam epitaxy with a charge sensitive surface microprobe. Surface potential measurements showed an upward band bending from 0.7 to 1.4 eV in undoped and Si-doped GaN. The samples stored in dark for one week showed an increase in band bending by up to 0.1 eV. The effect of ultraviolet (UV) exposure (with a lamp or a pulsed nitrogen laser) on band bending was also studied. Typically, the surface barrier decreased by about 0.2 – 0.5 eV under UV light. The barrier was restored very slowly (by a logarithmic law) in the dark at room temperature. These and other similar phenomena are tentatively attributed to thermionic transfer of free electrons from the bulk to the surface states. Photo-induced desorption of oxygen may also play a role in the observed effects.

Research paper thumbnail of Unusual Properties of the Red and Green Luminescence Bands in Ga-rich GaN

MRS Proceedings, 2004

ABSTRACTWe studied photoluminescence (PL) from deep-level defects in GaN grown under Garich condi... more ABSTRACTWe studied photoluminescence (PL) from deep-level defects in GaN grown under Garich conditions at relatively low temperatures (700–800°C) by molecular-beam epitaxy (MBE). The dominant features of PL spectrum are red and green bands peaking respectively at ∼1.8 and ∼2.35 eV. Both PL bands decay exponentially at low temperatures (15 – 100 K) after pulsed excitation. The characteristic lifetime for the red band decreases by almost two orders of magnitude from 110 to 2 μs with increasing temperature from 15 to 100 K, while its integrated intensity after each pulse remains nearly unchanged in the same temperature range due to an increase in the peak intensity in the time-resolved PL curve. The lifetime of the green band remained unchanged in this temperature range. We suggest that these PL bands are caused by transitions between excited and ground states of some deep defects rather than transitions involving a shallow donor, conduction or valence bands.

Research paper thumbnail of Epitaxy of highly optical efficient GaN on O and Zn face ZnO

MRS Proceedings, 2003

ABSTRACTZnO is a highly efficient photon emitter, has optical and piezoelectric properties that a... more ABSTRACTZnO is a highly efficient photon emitter, has optical and piezoelectric properties that are attractive for a variety of applications. Due to its stacking order and close lattice to GaN, it is also considered as a substrate material for GaN epitaxy. In the past the poor preparation of ZnO surface has been a major handicap to GaN epitaxy. However, proper treatment we developed recently can make both faces of ZnO smooth with atomic level terraces. Epitaxy of GaN on O-face and Zn-face ZnO by reactive molecular beam epitaxy was performed. We used low-temperature RF growth of GaN buffer layer on ZnO surface to protect it from both ammonia and Ga. No Ga2ZnO4, an oxide with the spinel structure formed due to reaction of ZnO with Ga, was found, in contrast to earlier reports. The low-temperature photoluminescence (PL) indicates that both faces of ZnO can provide a high quality GaN with high radiative efficiency. In previous research it has been reported that O-face ZnO is slightly be...

Research paper thumbnail of Effects of GaN passivation with SiO2 and SiNx studied by photoluminescence and surface potential electric force microscopy

MRS Proceedings, 2005

The influence of passivation with SiO2 and SiNx on optical properties and surface band bending in... more The influence of passivation with SiO2 and SiNx on optical properties and surface band bending in unintentionally doped GaN has been studied by steady-state photoluminescence (PL) and surface potential electric force microscopy (SP-EFM). For both types of passivation we observed a significant increase of PL intensity in air ambient at room temperature. The measured surface potential was the same for control and passivated samples within the experimental error. The value of the surface band-bending was determined as 1.0±0.2 eV in all cases. We suggest that the strong enhancement of PL is caused by reduction of contribution of the surface states to recombination of photogenerated carriers after passivation.

Research paper thumbnail of Photoluminescence Study of Defects in GaN Grown by Molecular Beam Epitaxy

MRS Proceedings, 2000

ABSTRACTDefect related photoluminescence (PL) in unintentionally doped GaN layers grown by molecu... more ABSTRACTDefect related photoluminescence (PL) in unintentionally doped GaN layers grown by molecular beam epitaxy (MBE) was studied. Under certain growth conditions, we observed new defect-related bands: a red band with a maximum at about 1.88 eV and a green band with a maximum at about 2.37 eV. The quenching of these bands with increasing temperature took place with an activation energy of about 120-140 meV at temperatures above 100 K. Moreover, the red band exhibited an increase of PL intensity with an activation energy of 1.2 meV in the range of 10-60 K. The observed behavior is explained by invoking a configuration coordinate model and that we speculate the defects to be partially nonradiative and related to Ga atoms.

Research paper thumbnail of High quantum efficiency of photoluminescence in GaN and ZnO

MRS Proceedings, 2005

The quantum efficiency (QE) of photoluminescence (PL) has been estimated in GaN and ZnO samples. ... more The quantum efficiency (QE) of photoluminescence (PL) has been estimated in GaN and ZnO samples. A Si-doped GaN layer grown by molecular beam epitaxy (MBE) exhibited the highest QE of about 90% at low temperatures. Recombination via the shallow donor-acceptor pair transitions dominated in this sample. In contrast, a bulk ZnO crystal with the QE of PL of about 85% contained almost no defect- or impurity-related PL signatures besides the emission attributed to free and bound excitons. The sources of radiative and nonradiative recombination in GaN and ZnO are discussed.

Research paper thumbnail of Photoluminescence and Excitation Spectra of Deep Defects in GaN

MRS Proceedings, 2001

ABSTRACTDeep defects responsible for broad bands in the red-to-green range of the photoluminescen... more ABSTRACTDeep defects responsible for broad bands in the red-to-green range of the photoluminescence (PL) spectrum of undoped and Si-doped GaN grown by molecular beam epitaxy (MBE) and hydride vapor phase epitaxy (HVPE) were studied by employing PL and PL excitation (PLE) methods. In HVPE grown samples, a red luminescence (RL) and a green luminescence (GL) bands were observed, respectively, at about 1.9 and 2.4 eV. Similar in positions but different in properties red and green bands (RL2 and GL2, respectively) dominated in the samples grown in Ga-rich conditions by MBE with radio frequency plasma as a nitrogen source (RF-MBE). A yellow luminescence (YL) with a maximum at about 2.2 eV dominated in the samples with ammonia used as a nitrogen source (NH3-MBE). It has been established from the variation of temperature, excitation intensity and excitation wavelength that the abovementioned five bands are related to different deep-level defects.

Research paper thumbnail of GaN based quantum dot heterostructures

MRS Proceedings, 2000

ABSTRACTGaN dots have been grown on c-plane sapphire and (111) Si substrates by reactive molecula... more ABSTRACTGaN dots have been grown on c-plane sapphire and (111) Si substrates by reactive molecular beam epitaxy. A new method involving two-dimensional growth followed by a controlled annealing during which dots are formed was employed. Due the dot nature and large dot density, relatively high luminescence efficiencies were obtained on both substrates. Single layer dots were used for AFM analysis whereas 30 layer dots were used for photoluminescence experiments. AlN barrier layers, some too thick for mechanical interaction, some thin enough for vertical coupling were used. Strong polarization effects lead to a sizeable red shift, which depends on the size of the dots.

Research paper thumbnail of GaN Quantum Dots on Sapphire and Si Substrates

Materials Science Forum, 2000

Research paper thumbnail of Effect of ambient on electrical and optical properties of GaN

SPIE Proceedings, 2009

Photoluminescence (PL) and surface photovoltage (SPV) of GaN layers were studied in vacuum and ai... more Photoluminescence (PL) and surface photovoltage (SPV) of GaN layers were studied in vacuum and air ambient. SPV transients were measured with two set-ups: traditional Kelvin probe attached to an optical cryostat and atomic force microscope in contact potential mode. It is found that upward band bending in GaN decreases from its dark value of about 0.9 eV to about 0.3

Research paper thumbnail of Kelvin probe measurements of p-type GaN

SPIE Proceedings, 2011

We have studied the surface photovoltage (SPV) for band-to-band illumination on a variety of p-ty... more We have studied the surface photovoltage (SPV) for band-to-band illumination on a variety of p-type (Mg-doped) GaN samples. In particular, differences in the steady-state and transient SPV have been investigated in air and vacuum for samples grown by hydride vapor phase epitaxy (HVPE) or metal-organic chemical vapor deposition (MOCVD). The SPV spectra for both samples behave in a similar manner,

Research paper thumbnail of 4115 Terahertz radiation from magnetoresistive Pr0.7Ca0.3MnO3 thin films Noriaki Kida, Masayoshi Tonouchi 4118 Picosecond Z-scan measurements on bulk GaN crystals

Research paper thumbnail of Characterization of GaN and In x Ga 1− x N films grown by MOCVD and MBE on free-standing GaN templates and quantum well structures

Journal of Materials Science-materials in Electronics, 2003

Structural and optical studies have been performed on GaN, InGaN layers, In0.08Ga0.92N/GaN hetero... more Structural and optical studies have been performed on GaN, InGaN layers, In0.08Ga0.92N/GaN heterostructures, In0.08Ga0.92N/In0.02Ga0.98N single and multiquantum wells grown by metal organic chemical vapor deposition (MOCVD) and GaN by molecular beam epitaxy (MBE) on GaN templates by using transmission electron microscopy (TEM), X-ray diffraction (XRD), and photoluminescence (PL). The layers are found to be high quality with low defect density,

Research paper thumbnail of Unstable luminescence in GaN and ZnO

Physica B: Condensed Matter, 2006

We report observations of unstable photoluminescence (PL) in GaN and ZnO. Beginning with GaN, a b... more We report observations of unstable photoluminescence (PL) in GaN and ZnO. Beginning with GaN, a broad blue band with a maximum at 3.02 eV and well-defined fine structure at its high-energy wing bleached under ultraviolet (UV) exposure at low temperatures. The bleaching of this band was accompanied by enhancement of a broad yellow band peaking at 2.2 eV. We also observed a very similar PL evolution in undoped bulk ZnO samples, where the intensity of the green band (at 2.6 eV) significantly decreased with concomitant enhancement of the yellow band (at 2.25 eV). The transformations are explained by recombination-enhanced defect reactions in these materials.

Research paper thumbnail of Luminescence from defects in GaN

Physica B: Condensed Matter, 2006

ABSTRACT

Research paper thumbnail of Investigation of defects and surface polarity in GaN using hot wet etching together with microscopy and diffraction techniques

Materials Science and Engineering: B, 2002

Research paper thumbnail of Effect of ambient on photoluminescence from GaN grown by molecular-beam epitaxy

Journal of Electronic Materials, 2003

We report the investigation of reversible-transient effects in photoluminescence (PL) of GaN laye... more We report the investigation of reversible-transient effects in photoluminescence (PL) of GaN layers in different measurement ambients including air, oxygen, nitrogen, and hydrogen. While the presence of air and oxygen led to similar amounts of significant decrease in overall luminescence, nitrogen and hydrogen produced a much smaller effect. Data on the different behaviors of the near-band-edge and the deep-level (yellow luminescence (YL)) spectral components of the luminescence with change from vacuum to the various ambient gases will be presented. A redshift of the deep-level luminescence band was noticed in oxygen or air ambient that has been attributed to a decrease of the band bending.